Nanocomposite Structures and Dispersions: Science and Nanotechnology - Fundamental Principles and Colloidal Particles

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STUDIES IN INTERFACE SCIENCE

Nanocomposite Structures and Dispersions Science and Nanotechnology – Fundamental Principles and Colloidal Particles

STUDIES IN INTERFACE SCIENCE

SERIES EDITORS

D. Möbius and R. Miller Vol. 1

Dynamics of Adsorption at Liquid Interfaces. Theory, Experiment, Application. By S.S. Dukhin, G. Kretzschmar and R. Miller Vol. 2 An Introduction to Dynamics of Colloids. By J.K.G. Dhont Vol. 3 Interfacial Tensiometry. By A.I. Rusanov and V.A. Prokhorov Vol. 4 New Developments in Construction and Functions of Organic Thin Films. Edited by T. Kajiyama and M. Aizawa Vol. 5 Foam and Foam Films. By D. Exerowa and P.M. Kruglyakov Vol. 6 Drops and Bubbles in Interfacial Research. Edited by D. Möbius and R. Miller Vol. 7 Proteins at Liquid Interfaces. Edited by D. Möbius and R. Miller Vol. 8 Dynamic Surface Tensiometry in Medicine. By V.M. Kazakov, O.V. Sinyachenko, V.B. Fainerman, U. Pison and R. Miller Vol. 9 Hydrophile-Lipophile Balance of Surfactants and Solid Particles. Physicochemical Aspects and Applications. By P.M. Kruglyakov Vol. 1o Particles at Fluid Interfaces and Membranes. Attachment of Colloid Particles and Proteins to Interfaces and Formation of Two-Dimensional Arrays. By P.A. Kralchevsky and K. Nagayama Vol. 11 Novel Methods to Study Interfacial Layers. By D. Möbius and R. Miller Vol. 12 Colloid and Surface Chemistry. By E.D. Shchukin, A.V. Pertsov, E.A. Amelina and A.S. Zelenev Vol. 13 Surfactants: Chemistry, Interfacial Properties, Applications. Edited by V.B. Fainerman, D. Möbius and R. Miller Vol. 14 Complex Wave Dynamics on Thin Films. By H.-C. Chang and E.A. Demekhin Vol. 15 Ultrasound for Characterizing Colloids. Particle Sizing, Zeta Potential, Rheology. By A.S. Dukhin and P.J. Goetz Vol. 16 Organized Monolayers and Assemblies: Structure, Processes and Function. Edited by D. Möbius and R. Miller Vol. 17 Introduction to Molecular-Microsimulation of Colloidal Dispersions. By A. Satoh Vol. 18 Transport Mediated by Electrified Interfaces: Studies in the linear, non-linear and far from equilibrium regimes. By R.C. Srivastava and R.P. Rastogi Vol. 19 Stable Gas-in-Liquid Emulsions: Production in Natural Waters and Artificial Media Second Edition By J.S. D’Arrigo Vol. 20 Interfacial Separation of Particles. By S. Lu, R.J. Pugh and E. Forssberg Vol. 21 Surface Activity in Drug Action. By R.C. Srivastava and A.N. Nagappa Vol. 22 Electrorheological Fluids: The Non-aqueous Suspensions. T. Hao

Nanocomposite Structures and Dispersions Science and Nanotechnology – Fundamental Principles and Colloidal Particles

I. Capek Polymer Institute, Slovak Academy of Sciences, Dúbravska cesta 9, Bratislava, Slovakia and Faculty of Industrial Technologies, Trencin University, Púchov, Slovakia

ELSEVIER Amsterdam – Boston – Heidelberg – London – New York – Oxford Paris – San Diego – San Francisco – Singapore – Sydney – Tokyo

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK

First edition 2006 Copyright © 2006 Elsevier B.V. All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material. Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN-13: ISBN-10:

978-0-444-52716-5 0-444-52716-8

For information on all Elsevier publications visit our website at books.elsevier.com Printed and bound in The Netherlands 06 07 08 09 10 10 9 8 7 6 5 4 3 2 1

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Contents

Preface

vii

Chapter 1

Nanotechnology and nanomaterials Content 1.1 Introduction 1.2 Nanoscience and nanotechnology 1.3 Nanoparticles – basic component of nanotechnology 1.4 Nanodevices, nanoelectronics 1.5 Industrial aspects of nanotechnology 1.6 Investigative tools 1.7 Nano-architectures References

1 1 1 7 18 24 39 48 53 59

Chapter 2

Preparation of polymer-based nanomaterials Content 2.1 Introduction 2.2 Solution/bulk polymerization 2.3 Emulsion polymerization 2.4 Microemulsion polymerization 2.4.1 Micelles and microemulsion 2.4.2 Microemulsion polymerization 2.5 Miniemulsion polymerization 2.6 Dispersion polymerization 2.7 Self-assemblies References

71 71 71 74 78 88 88 94 98 104 116 127

Chapter 3

Preparation of colloidal metal particles Content 3.1 Introduction 3.2 Bottom-up approach 3.2.1 Precipitation 3.2.2 Microemulsion 3.2.2.1 Inverse microemulsion 3.2.2.2 Inverse copolymer micelles 3.2.3 Other approaches 3.2.4 Bimetalic particles 3.2.5 Reducing agents and processes 3.2.6 Recipes for magnetic colloidal particles References

137 137 137 140 140 147 147 161 166 178 189 196 211

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Chapter 4

Index

Contents

Modification and passivation of colloidal particles Content 4.1 Introduction 4.2 Solvents and ligands 4.3 Ligand exchange 4.4 Particle growth techniques 4.5 Digestive ripening process 4.6 Deposition 4.7 Recipes for nanocomposite particles References

225 225 225 228 242 251 258 263 268 283 293

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Preface

“Nanocomposite structures and dispersions” I. Capek This book is concerned with the chemistry of the reaction approaches by which polymer and metal particles are synthesized. The book is published as Volume I of the NSD (Nanocomposite structures and dispersions) series. Although, the scope of this Volume is focused on metal particles, the basic knowledge of polymer colloids preparation is presented. This is due to the fact that the reaction media and stabilizers have the similar role for both polymer and metal colloids. In the book “Nanocomposite structures and dispersions” we have followed is to introduce the reader to the basic knowledge of nanoscience concerning of preparation of nonconventional colloidal particles and dispersions. The aim of this book is to summarize the fundamentals and mechanistic approaches in preparation and characterization of colloidal nanoparticles and dispersions so as to provide the readers a systematic and coherent picture of the field. The book introduces the reader the interesting field of nanoscience based on polymer and metal colloidal nanoparticles. The book places a special emphasis on polymer, inorganic and metal nanomaterials classified as nanoparticles, nanocrystals, nanorods, nanotubes, nanobelts, etc. Variety of synthetic approaches is described including emulsion, miniemulsion and microemulsion approaches, hogeneous and heterogeneous nucleation approaches under mild and high temperatures, . . . This book is not as a guide for the beginners, however, beginners might enjoy this book even if they lack a deep knowledge in the subject material. Therefore, this book would serve as a general introduction to people just entering the field and for the experts seeking for the information in other sub-fields. It is our intention that this book is intended to be mostly a comprehensive review. That makes it impossible for a book to cover all the aspects of the nanoparticles, nanomaterials and nanotechnology. Furthermore, the book is focused on both the organic and inorganic materials, and their hybrids. The most appropriate readers of this book would be experienced synthetic specialists. Scientists working in the area of nanoparticle synthesis will be able to intuitively understand the basic concepts. To enter deeply into some topics, good references are given at the end of each chapter. In synthesis of both organic and inorganic nanomaterials in the inverse micellar solutions under mild and high temperatures, the role of organic additives, such as emulsifiers, coemulsifiers, surface active additives, polymers, biopolymers, etc. in the preparation of polymer and metal particles and nanostructured materials is reviewed. In the synthesis of

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Preface

polymer and metal particles, one of the great challenges is to deal with the large surface to volume ratio. Therefore, the great attention is devoted to the surface modification and the physical chemistry of solid surfaces with introduction various synthetic techniques for various mini- and nanoparticles and nanomaterials. The fundamentals of homogeneous and heterogeneous nucleation are discussed more in detail. Particular attention is paid to the fundamentals for the control of particle size and chemical composition. The study is also focus on the studies of kinetically and thermodynamically stable nanoparticle solutions. As mentioned above, this book deals with nanoscience and nanotechnology from the viewpoint of a chemist. Therefore, it describes in detail in terms, definitions, theories, experiments, and techniques dealing with synthesis of polymer and metal particles. Chemical methods belong to a special category of bottom-up techniques. One of the bottom-up techniques is based on organized aggregation of organic precursors in the presence of stabilizers and costabilizers. As a partner to preparation of colloidal particles, the characterization is the next important topic in the corresponding research fields. Some sections in this book deal with analytical aspects of the field, such as XRD, NMR, TEM, SEM, XPS, . . . are briefly introduced. Regarding polymer colloid science, we can say that the discipline is at the stage at which polymer colloid application is being suppressed at the expense of seeking new technologies, materials and synthetic approaches. Advances are being expected in colloidal nanocomposites of polymers, metals and semiconductors. Specific examples include self-assembling superparamagnetic metal nanoparticles with special chemical, optical and mechanical properties. To optimize these high-performance nanocolloids and nanomaterials requires precise control of the particle size and composition at the nanosize and molecular level. The book contains four chapters. The first chapter (Nanotechnology and nanomaterials) begins by providing a basic background of the subject matter of nanoscience, nanotechnology, nanodevices, nanoarchitectures, nanocrystals, nanoparticles, etc. Nanomaterials have received much recent attention because they are expected to be used in various applications based on their excellent and unique optical, electrical, magnetic, catalytic, biological, or mechanical properties. Such properties originate from the finely tuned nanoarchitectures and nanostructures of these materials. However, the fabrication and analysis of nanomaterials remains challenging and, therefore, considerable and continuous efforts have been made to explore novel synthetic and analytical methods for nanoarchitectures and nanostructures. The fascinating world of these nanomaterials and their manifold applications becomes part of our life.

Preface

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In the second chapter (Preparation of polymer-based nanomaterials), we summarize and discuss the literature data concerning of polymer and polymer particle preparations. This includes the description of mechanism of the radical polymerization of unsaturated monomers by which polymer (latexes) dispersions are generated. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. The spherical shape of polymer particles and the uniformity of their size distribution are reviewed. The reaction mechanisms of polymer particle preparation in the micellar systems such as emulsion, miniemulsion and microemulsion polymerizations are described. The short section on radical polymerization mechanism is included. Furthermore, the formation of larger sized monodisperse polymer particles by the dispersion polymerization is reviewed as well as the assembling phenomena of polymer nanoparticles. The third chapter (Preparation of colloidal metal particles) then moves to the main coarse: metal particle synthesis. Numerous ways of particle preparation divided into two main groups are summarized, one including physical and the other-chemical approaches. In the first group, metallic nanoparticles are either assembled from atoms in the process of metal vaporization and subsequent condensation on various supports, or obtained through the treatment of the bigger particles in colloidal dispersions by means of colloidal mills, ultrasound, etc. In the second group reviewed the main chemical way is the reduction of metal ions in solution in conditions favoring the subsequent formation of small metal clusters or aggregates. The main disadvantages of chemical syntheses in liquid phase are their relatively low stability that requires the good knowledge of role organic stabilizers and thus complicates the structure and studies of the properties of the whole system. With respect to the mode of particle stabilization, chemical methods divided into two groups are discussed. The much attention is devoted to the particle formation governed by the thermodynamic or kinetic processes. In the former case, the synthetic process consists of supersaturation stage, nucleation and subsequent particle growth. In the kinetic process, the formation of nanoparticles is achieved by the limiting the amount of precursors available for the nucleation and the growth. Many approaches have been developed to prepare and stabilize nano-scale metal particles, nanocrystals in organic and aqueous media. These include mainly bottom-up approaches. Chapter 4 (Modification and passivation of colloidal particles) deals with passivation of metal and semiconductive colloids in situ reactions, such as chemical reductions, photoreductions, polymerizations or thermal decompositions. The preparation of stable organics- or stabilizer-protected particles is very important to permit studies the novel

x

Preface

properties of the nanospheres. A variety of preparation routes have been reported for the preparation of passivated nanosized metal, superparamagnetic, semiconductive, and semimetal particles and noble metal crystallites. Nanoparticles for various studies and applications must be uniform not only in size and shape, but also they must also have a controlled surface chemistry. Passivated metal and semimetal particles and clusters are of high intrinsic interest since they behave just like simple chemical compounds; they can be precipitated and redissolved without any apparent change in the properties. Encapsulation of metal particles and inorganic pigments into organic phase endows spheres with important properties that bare uncoated particles lack. Organic coatings on metal or inorganic particles enhance compatibility with organic ingredients, reduce susceptibility to leaching, and protect particle surfaces from oxidation. Consequently, passivation improves dispersibility, improves chemical stability, the colloidal stability in aqueous or organic media and reduces toxicity.

1

Chapter 1

Nanotechnology and nanomaterials 1.1. Introduction 1.2. Nanoscience and nanotechnology 1.3. Nanoparticles – basic component of nanotechnology 1.4. Nanodevices, nanoelectronics 1.5. Industrial aspects of nanotechnology 1.6. Investigative tools 1.7. Nano-architectures Abbreviations References

1.1. Introduction During the last decade, due to the emergence of a new generation of high-technology materials, the number of research groups involved in nanomaterials has increased exponentially. Nanomaterials are implicated in several domains such as chemistry, electronics, high-density magnetic recording media, sensors, biotechnology, etc. Nano-sized materials now emerged as one of the focal points of modern research. We are achieving an uncanny ability to design, synthesize, and manipulate structures at the nanoscale. Nanomaterials have received much recent attention because they are expected to be used in various applications based on their excellent and unique optical, electrical, magnetic, catalytic, biological, or mechanical properties. Such properties originate from the finely tuned nanoarchitectures and nanostructures of these materials. However, the fabrication and analysis of nanomaterials remains challenging and, therefore, considerable and continuous efforts have been made to explore novel synthetic and analytical methods for nanoarchitectures and nanostructures by many researchers all over the world. The fascinating world of these nanomaterials and their manifold applications becomes part of our life. Nano-scale materials have brought about many great changes and new research opportunities in physics, chemistry, material science, biology, etc. [1]. Several important events have marked the nanotechnology story. At the beginning of the 1980s, scanning tunneling microscopes (STM) and atomic force microscopes (AFM) were invented providing thus the “experimental techniques, methods and

2

Nanocomposite structures and dispersions

approaches” required for nanostructure measurement and manipulation. Scanning probe microscopy (SPM) has opened up the new world of nanotechnology for observing and manipulating individual atoms and molecules on solid surfaces. Other techniques such as beam-probe techniques, mechanical-probe techniques and particle trapping techniques were introduced to atom manipulation with wider controllability. In a parallel development, expansion of computational capability enabled sophisticated simulations of material behavior at the nanoscale. They stimulated the research with the vision of exciting new discoveries if one could fabricate materials and devices at the atomic/molecular scale. The starting research pointed out that a new class of miniaturized instrumentation would be needed to manipulate and measure the properties of these small “nano” structures. There is also the possibility that the unique properties of nanostructures will result in novel applications and devices. Another reason for the great popularity of this field is that phenomena occurring on this length scale are of interest to physicists, chemists, biologists, electrical and mechanical engineers, and computer scientists. A motivation in nanoscience is also to try to understand how materials behave when sample sizes are close to atomic dimensions. There is also the opportunity to use nanostructures for technology. Making and manipulating matter on the sub-100 nm length scale is a grand challenge for both scientists and engineers. From an engineering standpoint, the sub-100 nm scale is extraordinarily small, and many of the tools that are used routinely to do microfabrication cannot be used for nanofabrication. However, from the chemist’s point of view, this length scale, especially above 10 nm, is extraordinarily large. Chemists are really “Angstrom-technologists”, not nanotechnologists. Even when they work with large molecules, chemists are often manipulating a bond or a localized set of bonds within a larger structure. Although great strides have been made in the area of supramolecular chemistry, the synthetic toolkit required to routinely build structures with control over shape, size, chirality, and function on this length scale does not yet exist. This statement also holds for biochemists and molecular biologists who routinely work with molecules on this length scale. In fact, the investigation of the nanostructure of cells and the development of an understanding of biomolecular interactions on the nanometer scale is a frontier that demands further exploration [2]. At the beginning of any investigation, one is confronted with the selection of the synthesis method, the experimental and simulation techniques to be used, and the choice of materials (metals, ceramics, polymers, organics or carbon-based, composites, etc.). The main challenge is relating the final product properties and production rates to the material properties of the reaction components and precursors

Nanotechnology and nanomaterials

3

and process conditions. The product may be either homogeneous or composite nanostructured particles, with one or multi chemical species, consolidated or aerogels, including coated and doped particles. The nanotechnology has been focused on new concepts and fundamental research to generate nanoparticles at high rates. The work has included contributions on fundamental physics and chemistry for nanoparticle generation with tailored properties via different synthetic methods. The synthetic methods include precipitation from solutions (colloids), gas condensation (aerosols), chemical, plasma, combustion, spray pyrolysis, laser ablation, supercritical fluid expansion, polymerization, modification, chemical reactions, micellar reactions, mechanical attrition, molecular selfassembling, hydrodynamic cavitation, and other processes. Recent scientific literature demonstrates a growing interest in new methods of metal nanoparticles (with more pronounced amorphous core), nanocrystals (with more-developed crystalline core), quantum dots (semiconductor nanocrystal) and metal/polymer (organic) particles synthesis, driven primarily by an every increasing awareness of the unique properties and technological importance of nanostructured materials. The term nanospheres is reserved for the particles generally with different shape, nanoparticles for the spherical structures with less defined crystalline with amorphous or multidomain inorganic cores. The most active nanoparticle research activities in the world include fundamental studies for generation, processing, characterization and modeling; investigations on magnetic nanoparticles; studies on metallic and composite particles; studies on particle colloidal properties of metal, metal/polymer and polymeric particles and selfassembling techniques. There are several important aspects of modern nanoparticle research: (i) the preparation of nanoparticles, (ii) the manipulation and study of individual nanospheres or nanoparticles, (iii) the assembly of two- and threedimensional materials and structures in which the particles are closely packed without allowing the onset of uncontrolled aggregation, (iv) fabrication of nanomaterials, (v) miniaturization of devices, etc. Preparations of nanoparticles have yielded synthesis methods that are widely used to obtain nanoparticle samples for research purposes [3-5]. These preparations have led to detailed examinations of the opto-electronic properties of nanostructures as they deviate from those of the bulk material. For example, the blue shift in the absorption onset as a function of decreasing nanoparticle size can be directly related to quantum confinement of excitons within the nanoparticle [6]. Due to their extremely small size and large specific surface area, nanoparticles usually exhibit unusual physical and chemical properties compared to that of bulk materials [7]. The use of polymer matrix as an environment for in situ nanoparticle growth combines, synergistically,

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the properties of both the host polymer matrix and the discrete nanoparticles formed within it. The nanoparticles of metals and metal oxides embedded in polymer matrices have attracted increasing interest because of the unique properties displayed by materials containing such nanoparticles. These composite materials are expected to have novel magnetic, optical, electrical, catalytical, and mechanical properties [810]. Many techniques have been exploited to prepare metal- or metal oxide-polymer composites [11, 12]. In the last century, we had a number of major changes in the fields of science and technology. Since the invention of transistors half a century ago, electronics has been intimately involved in our daily life, and now has grown to one of key industries. However, its high growth rate up to now will not necessarily be guaranteed in the coming century. For instance, in case of some memory chips, where the memory capacitance has increased with a rate of four times in 3 years, the pace is approaching to physical and technological limits, and the extrapolation of the current technology may suggest the presence of a nonsurmountable wall at 0.01 μm resolution 30 years later in this century The same applies to the materials science. An artificial superlattice film fabricated by depositing a few atoms-thick layers of different elements one over another is an assembly of interfaces, as it were. Hence, the main arena is the world of non-equilibrium, where no text book is available, and neither phase diagrams nor almanacs have authoritative power. Thus, some new concepts or approaches should be fruitful. The last two decades have seen the explosion of miniaturization, based on the development of nanotechnologies, and its use in an increasing number of scientific and technical fields, including biology, chemistry, microelectronics, high density data storage, optics and optoelectronics, sensors, photonics, etc. Nanofabrication and nanoinstrumentation are recently popular research topics in the development of nanotechnology. Nanotechnology is the chance for the realization of that purpose. STM and AFM are two key equipments currently used in the development of nanotechnology. Tremendous amount of research works announced so far were focusing on the applications of STM and AFM on nanofabrication and nanoinstrumentation. Due to the fact that SPM plays a major role in the development of nanotechnology, both STM and AFM were investigated in very details on their functions and working principles. Based on these investigations and obtained knowledge, a comprehensive curriculum in nanotechnology was possible [13]. The “top-down approach miniaturization” is based on a progressive reduction of dimensions. These technologies are mostly based on lithography and pattern transfer, and address dimensions down to 10 nm. It is the basis of today’s application fields,


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and most of the research. The demonstration during the last years of low cost approaches using the “soft-lithography methods” is a key element for the introduction of the technology in all laboratories, and for the application to fields other than microelectronics. The ‘‘bottom-up approach’’ on the contrary relies on the atom per atom, or molecule per molecule building of functionalized elements. Still in its infancy, the research mainly addresses the first mechanical or electrical behaviour of small building blocks. It largely uses the near field methods, and self-assembly properties of atoms and molecules. Characterization at small scale is necessary to control the fabrication and properties of the realized objects. It includes not only observation, in far field or near field, but also in many physical measurements of transport, optical, and magnetic properties [14]. Every substance regardless of composition, when miniaturized to the sub-100 nm length scale, will have new properties. The optical, electrical, mechanical, magnetic, and chemical properties can be systematically manipulated by adjusting the size, composition, and shape of metal and semiconductor nanoparticles or nanospheres on the sub-100 nanometer length scale. Electron transport, manifested in phenomena like Coloumb blockade, as well as the catalytic and thermodynamic properties of structures can be tailored when one can rationally design materials on this length scale. Therefore, analytical tools and synthetic methods that allow one to control composition and architecture on this length scale will yield important advances in almost all fields of science. In the bottom-up approach toward nanomaterials and nanodevice development, two important aspects must be investigated. The first is the synthesis of the nanobuilding block itself and the second is how to assemble these nanobuilding blocks together into predefined structures with desired properties. When one wants to fabricate materials or devices only by the atom and molecule manipulation technique, one has to spend too long a time to finish up. Obviously, actual fabrication of materials and devices should not be pursued by atom-by-atom or molecule-by-atom or moleculeby-molecule processes but by some sort of self-organization. We need a selforganization process including self-ordering, self-assembly, self-limiting phenomena through which a huge number of nanostructures can be fabricated in parallel processing, with atomic accuracies and within a practically acceptable time. While there has been much success with the synthesis of nanobuilding blocks, such as nanoparticles, nanorods, nanotubes [15], the assembling of these materials remains a significant challenge. Among the various approaches, the supramolecular chemistrybased self-assembling technique appears to be most promising in the design and development of a variety of nanomaterial structures, including one-dimensional wires, two-dimensional arrays, three dimensional crystals, and nanocomposites [16].

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To prepare even more complex nanomaterial-based structures and devices, one must first be able to accurately control the chemical structure and functionality of the nanobuilding blocks at the molecular level. Traditional models and theories for material properties and device operations assume that the physical quantities are described by continuous variables and are valid only for length larger than about 100 nanometers. When at least one dimension of a material structure is under this critical length, distinct behavior often emerges that cannot be explained by traditional models and theories. In the semiconductor device field, for instance, quantum effects (tunnel effect, discrete energy levels…) appear when the active layer thickness is smaller than 10 nm. Reducing the dimensions of structures leads to entities, such as carbon nanotubes, quantum wires and dots, thin films, DNA-based structures, and laser emitters, which have unique properties. Nanomaterials and ultrathin functional coatings of nanoparticles will determine the utility of many products in the future: super-hard materials and super-fast computers, dirt repellent surfaces and new cancer treatments, scratchproof coatings and environmentally friendly fuel cells with highly effective catalysts. Highly developed synthetic strategies to such nanoparticles or nanomaterials provide a well-defined geometry, core-shell thickness, and composition, and therefore give controllable properties. The market for products manufactured by nanotechnology is already registering double-digit growth rates and will amount to hundreds of billions of US Dollars by 2004. Conferences on Nanostructured Materials had the ambitious aim of providing a meeting place for scientists from universities, research laboratories, and industry to learn about worldwide activities in nanomaterials research, and to initiate cooperation between the various fields of science and technology. A particular emphasis is placed on examining the synergy between the various scientific disciplines, and the links between the science, the potential applications, and the technical demands of nanoresearch.

Large multinational companies have established specialized groups in their long-term research laboratories where the total precompetitive research expenditure for nanotechnology is estimated to be very important. They deal with the study of mechanism of production of nanospheres, the production and applications of nanostructures and nanospheres and the exploration of novel physical properties of materials in the nanometer scale. The ultrafine particle engineering initiative including synthesis and processing of nanoparticles with controlled properties, with a


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focus on high-yield production processes. Nanoparticles, nanoclusters, layers and tubes are seen as precursors of tailored properties for nanostructures materials and devices. Small businesses have generated an innovative competitive environment in various technological areas including dispersions, filtration, nanoparticle synthesis, functional nanostructures, and various nanoparticle manufacturing processes. For example, USA companies specialized in commercial production of a broad spectrum of nanoparticles are Nanodyne (currently at about 48 t/year and going to 500 t/year) and Nanostructured Materials (currently at about 50 t/year and expanding). Interdisciplinary centers with focus on nanotechnology have been established in the last few years at universities, research centers, private companies creating a continuously growing public research and education infrastructure for this field [17]. The world governments, large computer, chemical and pharmaceutical companies, small and middle size enterprises, as well as state and private foundations provide support for precompetitive nanotechnology. Governments around the world are investing billions of dollars to establish institutes and the infrastructure to carry out state-of-the-art research in this extraordinarily broad and exciting field. It is not an overstatement to say that the nanoscience revolution, in terms of sheer interest and investment, is one of the biggest things to happen to the scientific and engineering communities since the beginning of modern science. “Nano” is also focusing on new issues, such as the industrial applications of nanotechnology products, health and social issues, and business development in different countries. Increased funding opens up the potential of nanotechnology through research collaborations with important industrial sectors, such as information technology and the automotive business. New leading-edge innovations appears: NanoMobil (automotive sector), NanoLux (optics industry), NanoforLife (pharmaceutical and medical technology), NanoFab (electronics), etc.

1.2. Nanoscience and nanotechnology Nanoscience and nanotechnology belong to the broad interdisciplinary area comprising polymer and metal particles, nanoelectrics, supramolecular and colloid chemistry, nanostructured materials, biochemistry and biology. Science is the most powerful means that mankind has to understand the working principles of the material world, as well as to change the world. In the early age of science, most of the scientists were engaged in discovering Nature. As time proceeds, scientists move more and more from discovering to inventing. Technology takes advantage of the

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progress of science to create novel opportunities for practical applications. Since experimental science and technology deal with material objects, it seems fair to say that nanoscience and nanotechnology are science and technology concerning objects of nanometer dimensions, which are atoms (on a scale of tenths of nanometers) and molecules (on a scale of nanometers). Since everything is made of atoms and molecules, nanoscience and nanotechnology could, in principle, be thought to cover all the branches of science and technology. A more satisfactory definition of nanoscience and nanotechnology can be achieved by focusing on the intrinsic properties of the nanoscale objects and on the possibility of organizing them into assemblies in order to perform specific functions. Nanotechnology is really a magic word. It covers any techniques that can manufacture patterns and devices below 1 mm and above a few nanometers. Over the past five years, the scientific and engineering communities have witnessed an explosion of interest and investment in the field of nanoscience and nanotechnology. The field of nanoscience has blossomed over the last twenty years and the need for nanotechnology will only increase as miniaturization becomes more important in areas such as computing, sensors, and biomedical applications. Advances in this field largely depend on the ability to synthesize nanoparticles of various materials, sizes, and shapes, as well as to efficiently assemble them into complex architectures. The synthesis of nanoparticles, however, is a fairly established field as particles of submicron or nanosized dimensions have been synthesized for centuries. The first example of considerable recognition is the Roman Lycurgus Cup, a bronze cup lined with colored glass that dates to the fourth Century AD. Small nanoparticles were often used in later centuries to create stained glass with small, ruby-red Au and lemon-yellow Ag particles. Nanoscience and technology is a field that focuses on: 1) the development of synthetic methods and surface analytical tools for building structures and materials, typically on the sub-100 nanometer scale, 2) the identification of the chemical and physical consequences of miniaturization, and 3) the use of such properties in the development of novel and functional materials and devices. Thus, this field is of greatest interest to handle nanoparticles, nanostructured materials, nanoporous materials, nanopigments, nanotubes, nanoimprinting, quantum dots, and so on and has already led to many innovative applications, particularly in materials science [18, 19]. For basic investigations, an important role is played by manipulation or imaging nanoscale techniques (e.g., AFM and STM). Nanoscience and nanotechnology are still in their infancy. At present, new exciting results [20] and, sometimes, disappointments alternate on the scene, as always


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happens in fields that have not yet reached maturity. Surely, as Feynman said [21], “when we have some control of the arrangement of things on a molecular scale, we will get an enormously greater range of possible properties that substances can have”, and these new properties will lead to a wide variety of applications which we cannot even envisage today. Hopefully, nanoscience and nanotechnology will contribute in finding solutions for several big problems that face a large part of the earth’s population: food, health, energy, and pollution. Nanoscience and nanotechnology have become words that stir up enthusiasm and fear, since they are expected, for good or for bad, to have a strong influence on the nature of mankind. Everybody seems to know what the meaning of these two words is, yet, in fact, even within the scientific community they are not yet well defined and the universally accepted definitions of these two terms will never be attained. This is not surprising, since it is a common experience that, in the field of science, as soon as a definition is established, problems arise. Nanoscience is now an important, central thread in fundamental research, and it will soon become an important part of technology. It focuses on both nano and micro objects. Nanoscience is the science of objects with dimensions on the nanometre scale. This size regime is controlled by quantum mechanical effects, most notable the quantum size effect. This scientific field is in the interaction zone of physics, chemistry, materials science and biology. Nanoscience is truly interdisciplinary in nature, providing potential synergism among the various fields in natural science. For many applications, at present microtechnology is more important than nanotechnology. However, nanoscience becomes a thread woven into many fields of science. Nanotechnology - certainly evolutionary, and perhaps revolutionary - will emerge from it. It is thought that chemistry will play a role; whether this role is supporting or leading will depend in part on how the field develops and what opportunities emerge, and in part on how imaginative chemists and chemical engineers are, or become, in finding their place in it. The similar behavior we can expect for physics or biology. Nanotechnology and biotechnology have both rapidly evolved in recent years, and are considered to be two key technologies for the 21st century. The interplay between these two technologies leads to a very promising and active research field, namely bionanotechnology or nanobiotechnology. It consists of two closely related sides; one focuses on developing nanotechnology with biologically related approaches while the other applies nanotechnology in biomedical studies. Biological systems such as cells and viruses are structured at the nanometer scale and function at the same scale. In that sense, they are natural, proven nanotechnology systems. In developing a human version of nanotechnology, we would like to directly exploit

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existing biological nanostructures, to mimic biological systems and synthesize nonbiological structures, to extract and apply the principles of biological systems. Nanobiotechnology can deal, for example, with protein- and DNA-based nanostructures or devices. Proteins and DNA can selfassemble into various structures with nanometer-scale features. Such biological structures can be used as templates or scaffolds to prepare structures with magnetic and semiconductive nanoparticles, inorganic additives and polymer materials. The resulting structures have interesting physical properties and can be utilized in many technological applications, including nanoelectronic devices, high-density data storage, molecular computations, nanomachines, optical devices, and biosensors. As nanotechnology advances, it provides many new tools for studies of biological systems that would otherwise be impossible. For example, atomic force microscopy allows visualization and manipulation of individual proteins or DNA molecules. Semiconducting nanocrystals are fluorescence labels that can survive for a much longer time than organic fluorescent dyes under strong luminescence. Nanoarrays offer a means to analyze large sets of chemicals or cells in parallel. Up-to-date technology borrows the “nano” from science due to several reasons: Only at this size do the quantum properties of matter become evident so that their advantages can made use of. For photonic components a similar effect occurs when size is reduced to the optical wavelength range. Moreover, in order to save energy, the smallest quantities of matter should be used that can perform a given function, in order to avoid unwanted dissipation and energy waste. This fact is true for electrons, photons, plasmons, and any other elemental excitations that can be used for sensing, detecting, processing, transmitting, actuating, and so on. Finally, size reduction is needed to facilitate integration: If a functioning mechanism is to be useful and cheap (that is to say, reach the widest market) it must be portable and must be capable of being integrated with as many other functions as are required. This includes heterogeneous functions, mixing such properties as optics, electronics, magnetism, and so on. Thus, it is becoming more and more important to find materials with several properties to act as links between devices, based on different phenomena and interactions, which can be processed at the nanoscale. Some such materials have already been created, some are being explored, and some are yet unheard of but will surely soon be discovered. In particular, it is crucial to integrate photons and electrons. The latter have been the main element in processing and communication technologies and the former, having proven their unsurpassable performance in the microworld demand and deserve a place in the nanoworld. This need includes integration of sources, guides, and detectors [22] and aspires to miniaturize them to fit into the small world [23].


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The nanotechnology whose form and importance is yet undefined is “revolutionary nano”; that is, technologies emerging from new nanostructured materials (e.g., nanotubes), or from the electronic properties of quantum dots, or from fundamentally new types of architectures - based on nanodevices - for use in computation and information storage and transmission. Nanosystems that use or mimic biology are also intensely interesting. There is no question that revolutionary nanoscience exists in the laboratories of universities now, and that new forms of nanotechnology will be important; it is just not clear - at the moment - how much of this exciting, revolutionary science will migrate into new technology, and how rapidly this migration will occur. Technology can be defined as the ability of taking advantage of the progress of science to create novel opportunities for practical applications. Technology is the main driving force for the progress of mankind since it provides a wealth of novel materials, devices, and machines capable of improving the quality of life. Taniguchi introduced the term ‘nanotechnology’ in 1974 to describe the manufacturing of products with tolerances less than 1 μm [24]. Feynman introduced the concept of building with molecules, “bottom-up” manufacturing, in contrast with the “topdown” manufacturing, we are familiar with [21]. He suggested that almost any chemically stable structure, that can be specified, can in fact be built. Furthermore, nanotechnology always will remind us of Feynman's statement: “At any rate, it seems that the laws of physics present no barrier to reducing the size of computers until bits are the size of atoms, and quantum behavior holds dominant sway”. The pioneering work of Drexler in molecular nanotechnology is important here; in his works he described nanoscale “assembler” - robots which build structures molecule by molecule and even replicate themselves [25, 26]. To image these tiny structures, special microscopes are needed. Scanning electron microscopes image structures by analysing the scattered electrons on a substrate by computer. Scanning probe microscopes use extremely sharp probes with tips of radius about 10 nm to scan the surface. The scanning tunnelling microscope measures a tunnelling current which occurs when the tip is about 1 nm above the surface and a voltage is applied; the current is held constant by moving the tip vertically while scanning the surface. The restriction that the substrate has to be an electrical conductor, led to the invention of the atomic force microscope. This device also uses a probe, but this one is attached to a flexible (in vertical depiction) cantilever, which is pressed into light contact with the surface while scanning. The vertical movement is followed by detecting the reflections of a laser beam on it; this is one type of AFM, however, several short range very high resolution displacement transducers are used in different types of AFM. Computers construct the final images. These microscopes can also be used to move nanoscale objects (and even single atoms) on a surface, being an important

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device to handle nanotubes. To produce nanoscale shapes or lines, special processes are needed. The most important ones all deal with some kind of energy beam, which reduces the material by ablation (instant vaporisation). These techniques are (in order of increasing power): photolithography, X-ray lithography, electron beam machining, focused ion beam machining, laser beam machining (femtosecond lasers and excimer lasers) [27-29]. Technologies have been developed that use components that are as small as possible, and size reduction of the constituent components plays an important role in the development of these “nanotechnologies”. Nanotechnology is recognized as an emerging enabling technology for the 21st century, in addition to the already established areas of information technology and biotechnology. This is because of the scientific convergence of physics, chemistry, biology, materials and engineering at nanoscale, and of the importance of the control of matter at nanoscale on almost all technologies. Nanotechnology deals with the materials with the size range from 1 nm (molecular scale) to about 50 nm. They appear at the interface between condensed matter and isolated atoms/molecules. The essence of nanotechnology is the ability to work at the molecular level, atom by atom, to create large structures with fundamentally new molecular organization. It is concerned with materials and systems whose structures and components exhibit novel and significantly improved physical, chemical, and biological properties, phenomena, and processes due to their nanoscale size and with development and utilization of structures and devices with organizational features at the intermediate scale between individual molecules and about 100 nm where novel properties occur as compared to bulk materials. It implies the capability to build up tailored nanostructures and devices for given functions by control at the atomic and molecular levels. It is estimated that nanotechnology is at a similar level of development as computer/information technology was in the 1950s. The size range and particularly the new phenomena set apart nanotechnology from MEMS (micro-electrical-mechanical systems, as known in the USA) or MST (microsystems technologies, as known in Europe). Search of effective ways for controlling the morphology of nanophase materials is of principal importance for nanotechnology and for development of advanced nanostructured materials. Nanotechnology is considered to be the technology of the future, it is perhaps today’s most advanced manufacturing technology and has been called “extreme technology”, because it reaches the theoretical limit of accuracy which is the size of a molecule or atom. In manufacturing industry, two interrelated trends are clearly seen: the trend towards miniaturization and the trend towards ultraprecision processing. Both trends are moving in the direction of nanotechnology, because both are tending to dimensions which lie in the range of several nanometres.


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Nanotechnology deals with materials and systems having the following key properties [30]: - they have at least one dimension of about 1–100 nm; - they are designed through processes that exhibit fundamental control over the physical and chemical attributes of molecular-scale structures; - they can be combined to form larger structures. Nanotechnology thus refers to techniques that offer the ability to design, synthesize (or manufacture), and control at the length scale below 50 or 100 nm. The emphasis in this definition of scope is “design and control”, and not only synthesis. Synthesis of materials at nanometer scale has already become routine practice for supported noble metal catalysts after decades of research on the subject. However, there is much room for development to design and control. Nanotechnology has gained substantial popularity recently due to the rapidly developing techniques both to synthesize and characterize materials and devices at the nano-scale, as well as the promises that such technology offers to substantially expand the achievable limits in many different fields including medicine, electronics, chemistry, and engineering. In the literature, there are constantly reports of new discoveries of unusual phenomena due to the small scale and new applications. It is creating a growing sense of excitement because we see an opportunity of unprecedented magnitude looming on the horizon: the ability to arrange and rearrange molecular structures in most of the ways consistent with physical law. If we can arrange atoms with greater precision and flexibility, and at lower cost, then almost all the familiar products in our world will be revolutionized. To name just three, we will: pack more computational power into a cubic centimeter than exists in the world today; make inexpensive structural materials that are as light and strong as diamond; and make surgical tools and instruments of molecular size and precision, able to intervene directly at the fundamental level where most sickness and disease are caused.

Several important points should be noted in the nanotechnology area: 1) New behavior at the nanoscale is not necessarily predictable from that observed at large size scales. 2) The most important changes in behavior are caused not by the order of magnitude size reduction, but by newly observed phenomena intrinsic to or becoming predominant at the nanoscale, such as size confinement, predominance of interfacial phenomena and quantum mechanics. 3) Once it is possible to control feature size, it is also possible to enhance material properties and device functions beyond those that we currently know or even

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consider as feasible. Such new forms of materials and devices herald a revolutionary age for science and technology, provided we can discover and fully utilize the underlying principles. 4) Nanotechnologies are multidisciplinary by nature. Experimental sciences are converging toward the ’nanoworld’: nanosciences, nanotechnology, nanostructures, nanoelectronics. Thus, physics is converging from electrical engineering (m denotes meter), electronic (mm), microelectronic (μm) toward nanoelectronics (nm); biology from cellular biology (μm), molecular biology toward bio-nanostructures and chemistry from atomic chemistry (Å), molecular chemistry (nm) toward nanostructures (Fig. 1).

Fig. 1. Experimental sciences are converging toward the ’nanoworld ’

Current interest in nanotechnology is broad based, and there are several common themes among funding agencies, as well as particularities [17]: 1) A main goal has been realization (synthesis, processing, properties, characterization, modeling, simulation) and use of nanostructured materials, including high-rate production of nanoparticles for potential industrial use. Advanced generation techniques for nanostructures with controlled properties (including gas-, liquid-, solid-, and vacuum based processes, chemistry/bio-self-assembling techniques, and artificially structured materials), methods to simulate structure growth at molecular and mesoscale levels, nanodevice applications, instruments and sensors based on novel concepts and principles, tools for quantum control and


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manipulation, and interdisciplinary research including biology, are important components of the current research activities. 2) Research and development on thermal spray processing and chemistry-based techniques for deposing multilayered nanostructured coatings, processing of nanoscale powders into bulk structures and coatings has been undertaken. 3) Nanofabrication with particular focus on the electronic industry is another major theme. It includes development of technologies seeking improved speed, density power and functionality beyond that achieved by simply scaling transistors, operation at room temperature, use of quantum well electronic devices, and computational nanotechnology addressing physics and chemistry related issues in nanofabrication. 4) Research on nanoscale materials for energy applications has a focus on synthesis and processing of materials with controlled structures, surface passivation and interface properties. The targeted energy-related applications are catalysis, optoelectronics and soft magnets. 5) Miniaturization of spacecraft systems and theoretical modeling addressing the physical and chemical aspects of nanostructures is another area of focus. 6) Biomimetics, smart structures, microdevices for telemedicine, compact power sources, and superlattices are developed in an interdisciplinary environment. 7) Neural communication and chip technologies have been investigated for biochemical applications and sensor development. 8) Metrology activities for thermal, mechanical properties, magnetism, micromagnetic modeling, and thermodynamics of nanostructures have been initiated. Nanoprobes to study nanometer material structures and devices with nanometer length scale accuracy and picosecond time resolution have been developed and others are in development. The basic research in the nanotechnology and obtained results say that ‘the possibilities of nanotechnology are endless. Entirely new classes of incredibly strong, extremely light and environmentally benign materials could be created’ and went on to discuss inexpensive nanostructures for broad applications. For example, the computational molecular-nanotechnology research group examining the ways in which this technology can be used to advance the exploration and human habitation of space. Storing one bit in a few atoms no longer seems outlandish, and molecular switches will someday replace the bulky devices made today using optical lithography. As we move beyond the vision and start asking how we are going to do this and how long it will take, opinions begin to diverge. The remarkable SPM instruments have already demonstrated an ability to move atoms and molecules on a surface in a controlled way, but have so far been confined to two dimensions. Thus, the principles of nanotechnology can be maneuvering things atom by atom.

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In chemistry, the range of sizes from a few nanometers to much less than 100 nanometers has historically been associated with colloids, micelles, polymer molecules, phase-separated regions in block copolymers, and similar structures— typically, very large molecules, or aggregates of many molecules. More recently, structures such as nanotubes, nanorods, and compound semiconductor quantum dots have emerged as particularly interesting classes of nanostructures. In physics and electrical engineering, nanoscience is most often associated with quantum behavior, and the behavior of electrons and photons in nanoscale structures. Biology and biochemistry also have a deep interest in nanostructures as components of the cell; many of the most interesting structures in biology—from DNA and viruses to subcellular organelles and gap junctions - can be considered as nanostructures [31]. Colloids and micelles and crystal nuclei have always been more difficult to prepare and to characterize; developing a “synthetic chemistry” of colloids that is as precise as that used to make molecules is a wonderful challenge for chemistry [32]. Synthesizing or fabricating ordered arrays and patterns of colloids poses a different and equally fascinating set of nanoobjects . The contribution of chemistry to nanoscience one can visualize into several areas as follows [33]: 1) Chemistry is unique in the sophistication of its ability to synthesize new forms of matter. The invention of new kinds of nanostructures will be crucial to the discovery of new phenomena. 2) Chemistry has contributed to the invention and development of materials whose properties depend on nanoscale structure. Chemistry and chemical engineering will, ultimately, be important in producing these materials reproducibly, economically, and in quantity. 3) The molecular mechanisms of functional nanostructures in biology - the lightharvesting apparatus of plants, the ribosome, the structures that package DNA - are areas where chemistry can make unique contributions. 4) Physical and analytical chemistry will help to build the tools that define these nanostructures and further initiate the explosion of nanoscience [34]. 5) Understanding the risks of nanostructures and nanomaterials will require cooperation across disciplines that range from chemistry to physiology, and from molecular medicine to epidemiology [35]. Chemistry is the ultimate nanotechnology. Chemists can make new forms of matter by joining atoms and groups of atoms together with bonds. They carry out this subnanometer-scale activity - chemical synthesis - on megaton scales. Although the initial interest in nanotechnology centered predominantly on nanoelectronics, and on fanciful visions of the futurists, the first new and potentially commercial


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technologies to emerge from revolutionary nanoscience seem, in fact, to be in materials science; and materials are usually the products of chemical processes. For example, nanoballs were the first of the discrete, graphite-like nanostructures. They were followed rapidly by carbon nanotubes - which are long graphite rods. These structures have a range of remarkable properties, including metallic electrical conductivity, semiconductivity with very high carrier mobility, and extraordinary mechanical strength [36]. Nanotubes are, of course, in competition with inexpensive materials such as carbon black and silicon for some of these applications, and cost and safety will determine the winners. Chemistry and chemical engineering play an essential role in developing the catalytic and process chemistry required to make uniform nanotubes at acceptable costs. Nanotechnology is not an independent, isolated circle but rather one that overlaps all of the existing circles and will continue to grow as the field is developed (Fig. 2). This is what distinguishes the field from scientific fads that have focused on a particular class of materials. It is a field fueled by novel tool development that will impact and change almost every conventional scientific and engineering subdiscipline by providing new ways of fabricating or synthesizing structures with well-defined and tailorable properties through control over nanoscale architecture. This give rise to the developing of the new interdisciplinary topics by combination of two or more fields (Fig. 2). The example of such interdisciplinary topic can be taken as nanophotonics. It is neither pure physics, chemistry, or engineering; it is a combination of all three. It is a highly interdisciplinary topic where the level of understanding in each of the three areas has to be very high. This fact makes the topic extremely rich, but this is probably also a big limitation. Just to give one example, you have to be expert in supramolecular chemistry, quantum optics, and optical transmission technology.

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Fig. 2. Nanotechnology encompasses all fields [2].

1.3. Nanoparticles – basic component of nanotechnology Nanoparticles have been empirically synthesized for thousands of years, for example, the generation of carbon black. A fourth century Roman masterpiece, known as the Lycurgus Cup, exhibits the unusual property of dichronism, appearing to be green in the reflected light and red in transmitted light, because of nanometer particles suspended in the glass [37]. Colourful aqueous solutions of gold colloids date again back to Roman times and were known to the medieval alchemists as aurum potabile [38]. The notion that gold sols indeed contain small metallic particles was first expressed in 1857 by Faraday [39], who had conducted a very elegant and simple study of the optical properties of thin films prepared from dried colloidal solutions. He observed reversible colour changes of the films upon mechanical compression. It is noticeable that very few nanoparticle synthesis processes have developed their scientific base decades ago, long before other nanotechnology areas have emerged. One finds in this category the pyrolysis process for carbon black and the flame reaction for pigments, particle polymerization techniques, self-assembling of micelles in colloidal suspensions, and chemistry self-assembling. Several kinds of nanoparticles are routinely produced for commercial use via aerosol and colloid reactors in the world. Research programs on nanoparticles and nanotechnology around the world suggest different strengths have developed in various countries, a fact that would suggest the need of international collaboration [17]. Nanoparticles are seen either as agents of change of various phenomena and processes, or as building blocks of materials and devices with tailored characteristics. Use of nanoparticles aims to take advantage of properties that are caused by the


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confinement effects, larger surface area, interactions at length scales where wave phenomena have comparable features to the structural features, and the possibility of generating new atomic and macromolecular structures. Important applications of nanoparticles are in dispersions and coatings, functional nanostructures, consolidated materials, biological systems and environment. Microparticles or microstructures mostly exhibit physical properties the same as that of bulk form. In the nanometer size regime, new mesoscopic phenomena characteristic of this intermediate state of materials, can appear. Furthermore, the stability of crystal structures can decrease in nanometer sizes, the feromagnetics can be varied when particles reach the nanometer size, the magnetics can become superparamagnetics when materials shrunk to the nanometer scale, the nanometer metal particles can become or loose their catalytic activity, etc. Thus, the electronic and optical properties of metals [40] and semiconductors [41] strongly depend on crystallite size in the nanometer size regime. This can be discussed in terms of organization of atoms or molecules into condensed systems due to which new collective phenomena of materials are developed. Cooperative interaction is responsible for the physical properties of the materials and it varies with the size of agglomerates in the nanometer scale [42]. The production of ultrafine particles is nowadays one of the most important challenges of the new technologies [43]. There are several reasons for this importance: 1) First, technologies today need to reduce as much as possible the size of the components being used. 2) New applications arise due to the great value of the surface/volume ratio associated with ultrafine particles. 3) The properties of ultrafine particles are in some cases very different from those of bulk materials and also from those of isolated atoms/molecules. 4) The method of nanoparticles synthesis often influences the properties of the product, in particularly, synthesis of nanoparticles in confined geometries and structured reaction media can result in anisotropic and size-controlled nanoparticles [44]. Preparation of nanoparticles enables the systematic characterization of the structural, physical, electronic, and optical properties of materials as they evolve from atom form or molecular to bulk via the nanometer size regime. In recent times, new methods of synthesis (inert gas condensation, layer deposition, ultrarapid quenching, mechanical attrition, aerosol, etc) have been used to fabricate magnetic systems with characteristic dimensions on a nanometer scale. Each fabrication technique has its own set of advantages and disadvantages. Among those techniques, the chemical synthesis of nanoparticles is a rapidly growing field with a great potential in making useful materials. However, there are some difficulties to realize it. For example, one

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difficulty is the need to find the proper chemical reactions, the composition of monomer feed and processing conditions for each material or undesirable agglomeration at any stage of the synthesis process, which can change the properties. When the particles are very small the lattice constants of the entire particles are strongly reduced [45]. Furthermore, the large ratio of the surface to interior atoms is connected with a large surface energy and so with the thermodynamic instability of the nanomaterials. In order to prevent the nanomaterials from growth in size, the reduction of the surface energy by the insertion of surface active components into the particle surface is necessary. One of the great challenges in stabilization of nanoparticles is the adsorption and bonding of surface active components into the particle surface. In all cases the chemical stability of the nanoparticles is crucial to avoid degradation processes such as partial oxidation or undesired sintering of particles. The lack of sufficient stability of many nanoparticle preparations has to some extent impeded the development of real world applications of nanomaterials. It has also probably been the reason why gold as a relatively inert metal has played an important role in the pioneering experiments performed mainly by Schmid and co-workers [46], who over the past 20 years have been able to show that single particle studies [47], quantum dot solids [48] and even nano-electronic devices are possible based on ligandstabilised Au clusters. Further important issues associated with nanoparticle (nanocrystal) preparation include the control of particle size and internal structure. Particle synthesis at high production rates has been a major research objective in the last few years. Particle nucleation and growth mechanisms are important scientific challenges. An ultimate goal of nanoparticle and nanocrystal research is to develop the ability to manipulate the size, morphology and arrangement of these ‘superatoms’ in such a fashion that their unique optical, electrical and magnetic properties can be utilized for different applications [49]. While colloid synthesis has the advantage of making bulk quantities of nanomaterials in a manner much simpler and of a smaller size than lithographic techniques, the problem of narrowing the particle size distribution has long plagued colloid scientists [50]. The significance of obtaining a monodisperse colloid is that it will allow us to correlate the physical properties of the entire colloid directly to the physical properties of each single size particle. It is also one of the key requirements in forming superlattice structures using nanocrystals as building blocks [51-53]. Monodisperse colloids, thus, provide ideal systems to study colloidal phase transitions without being affected by the complexity of particle size distribution [54]. Uniform micron size particles have been prepared by LaMer et al. half a century ago using a ‘growth by diffusion method’ [55], the preparation of single size nanocrystal


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colloids, however, are much more difficult due to their fast growth rate and their inherent stability [56]. There has been an increasing interest in semiconductor structures of different sizes and specifically those with dimensions in the order of a few nanometers. Semiconductor nanoparticles promise to play a major role in several new technologies. The intense interest in this area derives from their unique chemical and electronic properties, which gives rise to their potential use in the fields of nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics, as well as other areas. Although synthesis of nanodimensional colloids in biphasic system was known earlier, the problems such as their stability and precise control of reactivity have been tackled only recently using different strategies. Size control is often sought either through the attachment of appropriate protecting agents, such as gelatins, albumines, and other peptides, amphiphiles and macromolecules, such as polyethylene imine or polyvinylimidazole, on the surface of the clusters or to one another without leading to coalescence which results into the loss of their size-induced electronic properties. Another expedient method involves the use of self-assembled monolayer (SAM) formation with alkanethiols and amines for noble metal surfaces leading to the successful synthesis of stable particles. For example, the work reported by Brust et al. [57] involves this type of a method using sodium borohydride reduction of an aqueous tetrachloroaurate in excess diethyl ether. Tetraoctylammonium bromide was used as a phase-transfer catalyst to exchange tetrachloroaurate ions from aqueous to organic phase. Although this is an efficient one-step method for the synthesis of nanoparticles, the transferable metal ion should be in the form of anionic complexes, and hence, this method cannot be extended to common water soluble salts of the metals such as simple salts of silver and copper. It is very important to have general methods for obtaining particles by simple and reproducible techniques. As improved syntheses lead to highly characterized samples with narrow size distributions and regular shapes, the behavior of single nanoparticles or nanocrystals is being examined with increasing rigor and detail. The production of particles in the nanometer range is one of the most important challenges of modern materials science for a variety of reasons: A decade ago quantum-size effects were first recognized in relatively crude 2-4 nm colloidal II-VI semiconductor particles [58]. Optical transitions and spectral characteristics are linked to the particle size of metal and semiconductor colloids [59]. The possibility of a dramatic change in electronic properties by varying the size of metal and semiconductor particles has emerged as an area of important and fruit research activity due to its fundamental and technological relevance. Nanoparticles

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are the core of this technology. These are particles ranging in size from one millionth to 100 millionths of a millimeter - more than 1000 times smaller than the diameter of a human hair. At this order of magnitude, it is not only the chemical composition but also the size and the shape of the particles that determine their properties. Optical, electric, and magnetic properties, but also the hardness, toughness, or melting point of nanomaterials differ substantially from the properties of the macroscopic solids. Synthesis of various particles of with sizes varying from 1 to 100 nm have found promising applications in different fields. When the electrons and holes are confined with the three-dimensional potential well, the continuum of states in the conduction and valence bands is broken into discrete states with a energy spacing, relative to band edge, which is approximately inversely proportional to the square of the particle size [10]. They have a characteristic high surface-to-volume ratio, providing sites for the efficient adsorption of the reacting substrates leading to unusual size dependent chemical reactivity [60]. Atoms and molecules on the air/solid or liquid/solid surfaces have fewer neighbors than those in the sub-surface or solid matrix. The unsatisfied bonds exposed to the surface initiate dangling effect. Thus, the atoms at the surface are under an inwardly directed force and bond distance between the atoms at the surface is smaller than between the atoms in the bulk matrix. The ratio of the surface atoms to interior atoms changes abruptly when the object is strongly decreased. Such dramatic increase in the ratio of the surface to bulk atoms can be correlated to the strong changes in the physical and chemical properties of the nanomaterials. Generaly, nanomaterials include colloidal crystals (nanocrystals), superlatices, nanoparticles, nanorods, nanobelts, nonotubes, nanowires, superlattices, etc. The properties of nanospheres are influenced and modified by reduced dimensions confinement, reduced dimensionality, proximity effects and surface dominating over the bulk [61]. Owing to the fact that in nanospheres the surface/volume ratio can reach very high values, new applications associated with the inner surface have appeared, for example, the development of new catalysts. Optical, magnetic, electric, adsorptive, catalytic, and other characteristics of a given material can also strongly vary with the size and shape of the particles, even though they may have the same composition or molecular structure [62]. In many cases, these properties change in an abrupt manner below a certain particle size, for example, the electrical conductivity or the type of magnetism. Ferromagnetic specimens are build up from many magnetic domains, and there is a critical size for each material below which the particles are single domain. Small, single-domain particles exhibit an exotic magnetic behavior that allows them to reach a limiting magnetism, i.e., the disappearance of the coercivity and remanence at a very high level of magnetization (superparamagnetism). Because of confinement and quantum-size effect, a reduction


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in the dimension of metal domains produces dramatic changes in the behavior of the massive metal properties. The small size also has very important effects on the magnetic behavior of ferromagnetic metals (Table 1). Table 1. Magnetic properties of metals in bulk and small-size categories [63]. Metal Na, K Fe,Co, Ni Gd, Tb Cr Rh

Bulk Paramagnetic Ferromagnetic Ferromagnetic Antiferromagnetic Paramagnetic

Particle cluster Ferromagnetic Superparamagnetic Rotors/superparamagnetic Frustrated paramagnetic Ferromagnetic

As particle sizes become smaller, the ratio of surface atoms to those in the interior increase, leading to the surface properties playing an important role in the properties of the material. Semiconductor nanoparticles also exhibit a change in their electronic properties relative to that of the bulk material; as the size of the solid becomes smaller, the band gap becomes larger. This allows chemists and material scientists the unique opportunity to change the electronic and chemical properties of a material simply by controlling its particle size. A unique property of semiconductor nanoparticles, known as quantum dots (QD) and metal nanoshell, is that they absorb and scatter light of the near infrared, a spectrum region where tissue is essentially transparent. These nanoparticles are often composed of atoms from II–VI or III–V elements in the periodic table. QDs are highly light-absorbable and luminescent with absorbance onset and emission maximum shift to higher energy with decreasing particle size due to quantum confinement effects [64]. These nanoparticles are in the size range of 2–8 nm in diameter. Unlike molecular fluorophores, which typically have very narrow exitation spectra, semiconductor QDs absorb light over a very broad spectral range. This makes it possible to optically excite a broad spectrum of QD colors using a single polyexcitation laser wavelength, which enables one to simultaneously probe several markers. Polymer-mediated nanoparticle assembly provides a versatile and effective method for the creation of structured nanocomposite materials where control over composite morphol-ogy is paramount. In addition to their role in assembling nanoparticles, functionalized polymers can be used to control interparticle distances, the compatibility with broad range of polymers, assembly shape, size, and porosity, and to induce an anisotropic ordering of nanoparticles. The ability to control such structural parameters enables the creation of responsive materials.

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The compatibility or interaction of small particles with cells and tissues is not up to now well understand but there are diseases associated with a few of them: silicosis, asbestosis, “black lung” [65, 66]. Most nanomaterials would be made and used in conditions in which the nanomaterial was relatively shielded from exposure to society (an example would be nanotubes compounded into plastics). Still, we do not know how nanoparticles enter the body, how they are taken up by the cell, how they are distributed in the circulation, or how they affect the health of the organism. If the chemical industry intends to make a serious entry into nanostructured materials, it would be well advised to sponsor arms-length, careful, and entirely dispassionate studies on the effects of existing and new nanoparticles and nanomaterials on the behavior of cells and on the health of living being. The use of nanoparticles in drug delivery is in progress, especially nanoparticulate and nanoporous materials for catalytic and biomaterials applications. This includes stimuli responsive drugdelivery systems, which, for example, release insulin only when glucose concentration is high. A novel synthesis of glucose-responsive nanoparticles for controlled insulin delivery is in progress; the glucose-sensitive polymeric nanoparticles are tailored by respective proteins. The controlled release strategy uses a polymer with acidic degradation products to control the dissolution of a basic inorganic component, resulting in protein release. The potential risks of nanoparticles is well known and broadly discussed. Health issues arise from the altered properties of nanomaterials (such as solubility variations). Questions concerning the inhalation or disposition of nanoparticles (such as polymer or Fe2O3 particles used for drug delivery or surface binding to endogenous proteins) still remain open. A roadmap to safe nanotechnology should include the development and validation of testing methods and increased awareness of the potential environmental and biological hazards of nanoparticles and nanotechnology.

1.4. Nanodevices, nanoelectronics Nanoscale devices and machines are either present in nature [67] or must be synthesized starting from more simple components [68-70]. The idea that atoms could be used to construct nanoscale devices and machines was first raised by Feynman in the previously mentioned address “There is plenty of room at the bottom” [21]. A key sentence of Feynman’s talk is the following: “The principles of physics do not speak against the possibility of maneuvering things atom by atom”. The advent of nanotechnology was depicted in an exciting and visionary way by Eric


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Drexler in mid-1980s [25]. He presented his ideas on nanosystems and molecular manufacturing in a more scientific way claiming the possibility of constructing a general purpose nanodevice. Such a nanorobot should be able to build almost anything, including copies of itself, by atomic-scale precision, “pick-and-place” machine-phase chemistry (mechanosynthesis) [71, 72]. The ideas of simple maneuvering atoms or making molecular mechanosynthesis seem however somewhat against the complexity and subtlety of bond-breaking and bond-making processes [33, 73]. In the nanoscale area, each device is made of a countable number of atoms. At such small dimensions, the physics governing principal device functions eventually transitions from the classical laws, such as Maxwell’s equations [74] and Newton’s laws, into the quantum-mechanical interactions with discrete energy states. As a consequence, principal electronic and/or magnetic and/or mechanical properties of the device might become highly non-linear if the scaling remains the main strategy for miniaturizing the device. The resulting non-linearity can substantially degrade the performance of the device. The deviation from linear scaling laws in the nanoscale is of especial significance for magnetic devices and materials because the characteristic magnetic domain wall width is in the nanoscale range [75]. The domain wall width defines the minimum length of the spatial non-uniformity of the magnetic properties. Therefore, for extending high technologies in the nanoscale, there is a strong need for other fundamental approaches, besides the traditional scaling. The ability to fairly quickly fabricate a nanoscale prototype device for further characterization and optimization becomes a critical factor for continuing the technological progress [76]. A major driving force in the research and development (R&D) of new materials for future information technologies is aimed at the miniaturization of devices down to ultimate limits as determined by basic physics and quantum mechanical principles. Another driving force results from trying to match, in future devices, different performances that are currently achieved separately in biological and technical systems. An often considered example concerns the human brain as compared with the man-made computer. Technologically unmatched performances of the brain concern high information density, low power consumption, high flexibility, excellent association memory, etc. Biologically unmatched performances of the computer concern quantitative information processing, high reproducibility… [77]. With increasing complexity and demands for future information technologies, a trend is to be seen towards the design of “smart” nanostructures which will be interfaced to different substrates. These structures may consist either of chemically synthesized units such as molecules, supramolecules and biologically active (biomimetic) recognition centers, or of natural and hence very complex biomolecular function units with high molecular weight which may be extracted from biological systems.

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The miniaturization of components for the construction of useful nanodevices and nanomachines can be pursued by the top-down and bottom-up approachs. The former approach uses photolithography and related techniques to manipulate progressively smaller pieces of matter in an outstanding way up until this time. It is becoming increasingly apparent, however, that the top-down approach is subject to drastic limitations for dimensions smaller than 100 nm [20]. This size is very small by the standards of everyday experience (about one thousandth of the width of a human hair), but it is very large on the scale of atoms and molecules. Therefore, “there is plenty of room at the bottom” for further miniaturization, as Richard Feynman [21] stated in a famous talk to the American Physical Society in 1959, but the top-down approach does not seem capable of exploiting such an opportunity. An alternative and most promising strategy to exploit science and technology at the nanometer scale is offered by the bottom-up approach, which starts from nano- or subnanoscale objects (namely, atoms or molecules) to build up nanostructures. The bottom-up approach distinguishes two different nanoscale “objects”: 1) Nanoscales are very simple from a chemical viewpoint and do not exhibit any specific intrinsic function (atoms, clusters of atoms, small molecules). 2) Nanoscales have complex chemical composition, exhibit characteristic structures, show peculiar properties, and perform specific functions. All of the artificial molecular devices and machines belong to the second category [78]. Examples of such nanoscale “objects” are the light-driven rotary motors based on the geometrical isomerization of alkene-type compounds containing chiral centers (Figure 3), [79] the prototype of a molecular muscle [80], the light-driven molecular shuttles [81], the artificial molecular elevator [82], the light-driven hybrid systems for producing biocompounds and pumping calcium ions, [68] and the DNA biped walking device [83]. All of the natural molecular devices and machines [67], from the lightharvesting antennae of the photosynthetic systems to the linear and rotary motors present in our bodies, also belong to this category.

Fig. 3. Schematic representation of a molecular motor, based on the photoisomerization of an alkene-type compound containing chiral centers, that exhibits light-induced unidirectional rotation [79].


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The “bottom-up” approach opens virtually unlimited possibilities regarding the design and construction of artificial molecular devices and machines capable of performing specific functions upon stimulation with external energy inputs [84]. Furthermore, such an approach can provide invaluable contributions to give a better understanding of the molecular-level aspects of the extremely complicated devices and machines that are responsible for biological processes. Molecular devices and machines operate via electronic and nuclear rearrangements, that is, through some kind of chemical reaction. Like their macroscopic counterparts, they are characterized by 1) the kind of energy input supplied to make them work, 2) the way in which their operation can be controlled and monitored, 3) the possibility to repeat the operation at will, 4) the timescale needed to complete a cycle of operation, and 5) the function performed [84]. The problem of finding the energy supply to make artificial molecular devices and machines work is of the greatest importance [85]. Since their operation is always based on some kind of chemical reaction, the most obvious way to supply energy to these systems is through the addition of suitable reactants. If an artificial molecular device or machine has to work by inputs of chemical energy, it will need the addition of fresh reactants (“fuel”) at any step of its working cycle, with the concomitant formation of waste products. It is well known for a long time that photochemical and electrochemical energy inputs can cause the occurrence of chemical reactions. In recent years, the outstanding progress made by supramolecular photochemistry and electrochemistry has led to the design and construction of molecular devices and machines powered by light or electrical energy, which work without the formation of waste products [84]. In the late 1970s, a new branch of chemistry called supramolecular chemistry emerged and expanded very rapidly [86, 87]. In the same period, research on molecular electronic devices began to flourish and the idea arose that molecules are much more convenient building blocks than atoms to construct nanoscale devices and machines [88]. The main reasons that provide the basis of this idea are as follows [78]: 1) Molecules are stable species, whereas atoms are difficult to handle; 2) nature starts from molecules, not from atoms, to construct the great number and variety of nanodevices and nanomachines that sustain life; 3) most laboratory chemical processes deal with molecules rather than with atoms;

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4) molecules are objects that already exhibit distinct shapes and exhibit devicerelated properties (e.g., properties that can be manipulated by photochemical and electrochemical inputs); 5) molecules can self-assemble or can be covalently connected to make larger structures. The promising route for the fabrication of nanodevices is the use of metal, polymer and semiconductor nanoparticles as the building blocks [89]. Efforts have been made to assemble nanoparticles into various nanostructures, such as one-, two- and three dimensional nanoparticle arrangements [90]. In addition to the size and composition, the morphology and orientation of the nanoparticles play an important role in modulating the electronic and chemical properties [91]. Million-fold fluorescence enhancement in gold nanorods [92] and distinct quadruple plasmon resonances in silver nanoprisms [91] are some exciting shape-dependent properties that have already been reported. There has been a great deal of interest in the study and application of the unique optical, electronic and catalytic properties of nano- and micrometer-sized particles. Part of the reason lies in the fact that these colloidal particles are useful in a broad range of areas, such as photography [93], catalysis [94], biological labeling [95], photonics [96], optoelectronics [97], surface-enhanced Raman scattering (SERS) detection [98]… In the manufacture of electronic devices such as hybrid integrated circuits and multiplayer ceramic capacitors, the technology of making conductive thick films from metal powders is of considerable importance. A number of examples of devices rely on nanoscale phenomena for their operation. Two dimensional systems such as two-dimensional electron gas can be considered as one dimensional nanotechnologies, quantum wires as two dimensional nanotechnologies and quantum dots as three dimensional nanotechnologies. The extension from one to three nanodimensions is not straightforward but the payoffs can be enormous [99-104]. By using different functional materials, it should be possible to give micro/nanostructures, i.e. micro/nanodevices and micro/ nanomachines, unique properties, which may allow for various applications. The accomplishment of highquality photonic band structures has been confirmed by their bandgap effect. Duan et al. [105] have synthesized Ti4+ ions doped urethane acrylate photopolymerisable resins and investigated their two-photon polymerization, which is applied to threedimensional (3D) micro/nanostructure fabrication. TiO2 nanoparticles were generated in the polymer matrix of micron-sized polymer structures. A 3D diamond photonic crystal structure, which consisted of polymer composite materials of TiO2 nanoparticles, was successfully fabricated by direct laser writing, and its photonic


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bandgap was confirmed. The absorption of electromagnetic radiation by nanocrystallite material is relatively straightforward, but luminescent behavior of such particles is more complicated to understand. In a keynote paper, Chestnoy et al. [106] explained, on the basis of theoretical and experimental studies, the features expected in the luminescence spectra of quantum confined semiconductors and successfully anticipated the results of many subsequent experiments. Progress in nanotechnology demands the capability to fabricate nanostructures in a variety of materials with an accuracy in the nanometre scale and sometimes in the atomic scale. Stringent nanofabrication specifications have to be met in industrially relevant processes due to manufacturability and costs considerations as, for example, in the electronics industry. However, less demanding conditions are needed for developments in optics, sensors and biological applications. In a laboratory environment, at the level of enabling nanofabrication techniques as tools for experiments to understand the underlying science and engineering in the nanometer scale, easily accessible and flexible nanofabrication approaches are required for investigations in, e.g., materials science, organic optoelectronics, nano-optics and life sciences. Alternative techniques to cost-intensive or limited-access fabrication methods with nanometre resolution have been under development for nearly two decades. One clear example is the evolving set of scanning probes techniques, which has become ubiquitous in many research areas. If one considers planar structures, i.e., where nanostructuring is carried out on a surface, as distinct from a threedimensional nanofabrication or multilayer self-assembly, then several emerging nanofabrication techniques can be discussed. Their classification depends on whether the nature of the patterning is chemical or physical, or its modality in time is parallel or sequential, or a hard or a soft mould or stamp is used, etc. This subject is increasing very rapidly and, for example, progress one can observe in micro-contact printing [107], scanning probe-based techniques [108], nanoimprint-based lithography (NIL) technique [109], etc. The single most important fabrication technology of our time is microlithography: the microprocessors and memories that it generates-are the basis for the information technology that has so transformed society in the last half-century. Microelectronic technology has relentlessly followed a Moore’s law; the popular expression of this law is “smaller is cheaper and faster” [110]. Besides this enthusiasm for “smaller” other features-heat dissipation, power distribution, clock synchronization, intrachip communication-have become increasingly important. Still, technical evolution in the semiconductor industry has brought the components of commercial semiconductor devices to sizes close to 100 nm, and miniaturization continues unabated. Understanding the behaviors of matter in < 100 nm structures is, and will continue to be, a part of this evolution, as microelectronics becomes nanoelectronics.

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Focused ion beam (FIB) is a rapid way to fabricate a nanoscale magnetic device for further prototyping [111, 112]. Magnetic recording at areal densities beyond 1 Tbit in-2 is presented as an example of a technology for which the implementation of FIB has played the most critical role for its successful transition into the nanoscale range. The FIB column is very similar to the electron beam (E-beam) column used for electron microscopy (SEM, TEM and others) and E-beam-based lithography, with the main difference being the polarity of the voltages applied to the accelerating and focusing plates and coils in the system. The two main competitors to FIB for defining nanoscale-size patterns are UV-optical and electron-beam lithography [113]. The smallest feature size in the UV-optical case is limited by the UV wavelength and is believed to be of the order of 50 nm. Ideally, both electron-beam and FIB can provide substantially smaller feature sizes, limited only by the quality of the electron/ion columns utilized. E-beam and FIB are capable of feature sizes of 30 and 10 nm, respectively. E-beam-based fabrication has been more extensively explored in the semiconductor industry because of its more traditional approach to defining small features via lithographical masks, thus to some degree reminding one of optical lithography. This is the quality that makes implementation of E-beam fabrication for mass production fairly straightforward [113]. At the same time, the use of the mask patterning makes E-beam a fairly ‘slow’ tool for making individual nanoscale prototype devices necessary for proving a concept. That is exactly when FIB becomes helpful and/or complementary. Although FIB-based fabrication is more different from traditional optical lithography, it has its advantages compared to the E-beam-based fabrication method, especially with respect to magnetic devices and materials [112]. There is a new class of quantum devices being developed which require very small sizes to operate [114]. Although lithography with focused electron-beams and/or ion-beams has been applied to fabricating small features, there is still a need to develop new fabrication techniques for nanometer devices. Scanning tunneling microscopy (STM) developed by Binning et al. has proven to be a powerful technique for the study of atomic resolution in ultra-high vacuum (UHV), air and liquid environments [115]. Soon after the invention of STM, researchers begun to investigate the possibility of utilizing it for the manipulation and modification of solid-state surfaces at an atomic scale [116]. It can generate high electric fields between the tip and substrate, as well as provide an intense and finaly focused source of electrons. This capability makes the STM an ideal tool for nanometer lithography [117]. Two-photon polymerization, which is initiated through the non-linear process of twophoton absorption (TPA) of a photoinitiator, has been gaining greater interest among


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a number of multidisciplinary areas, particularly in the rapidly developing fields of three-dimensional (3D) micro-nanofabrication by using infrared lasers without photomasks. This kind of structure has been used as micro-nanodevices for photonic and electronic applications [118, 119]. However, nanoparticles of noble metals and metallic oxides are interesting species because when size is downscaled to the order of nanometers, materials demonstrate novel optical, electronic, magnetic and mechanical properties, which are the major motivation of the current intense research on nanoparticles. The 3D structures of polymer-metal/metallic oxide nanoparticles composite materials can be expected to play an important role in the field of functional devices for future applications. Interfacing biological molecules and supramolecular assemblies with the synthetic world is critical to many applications in nanotechnology [120]. A particularly exciting class of such hybrid devices utilizes biomolecular motors, which can add active, chemically powered force generation and movement to the functionality of the device. Applications of devices based on biomolecular motors have been explored for nanoscale transport systems (molecular shuttles) [121], surface imaging [122], force measurements [123], single molecule manipulation [124] and lab-on-achip systems [125]. These studies have proven the feasibility of utilizing motor proteins, such as kinesin, and the specific ‘roads’ supporting their movements, such as microtubules, in synthetic environments for a variety of technological purposes. In hybrid devices, the synthetic materials themselves often introduce additional challenges for proteins. Immediate loss of motor protein function upon adsorption has been reported for a number of surfaces, and a significant effort has been devoted to finding surfaces that permit micro- and nanopatterning but also support motor function after adsorption [126, 127]. Now a number of suitable photoresists, which can be used to pattern a glass surface covered with active motor proteins, have been identified. However, not only the material of the surface directly in contact with the motor protein, but also the material properties of surfaces in contact with the buffer solution can affect the lifetime of the proteins. For example, poly(dimethylsiloxane) (PDMS), which is widely used as a biocompatible material [128], has been found to be incompatible with motor protein activity. Different nanotechnologies have developed devices for a continuous drug delivery over an extended period of time. However, most of them suffer from major drawbacks. Degradable polymer implants exhibit an initial ‘burst effect’ prior to sustained release and are typically not as efficient in controlling release rates of small molecules [129]. Implantable devices with percutaneous components such as ambulatory peritoneal dialysis, catheters, intravenous catheters, and orthopaedic implants are often associated with different failure modes. Infection,

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marsupialization, permigration, and avulsion are common occurrences [130]. Osmotic pumps lack the capabilities of electronic integration for achieving higher levels of functionality and are limited with respect to the type of drug they can deliver. Silicon micro- and nanofabrication technology can permit the creation of drug delivery devices that possess a combination of structural, mechanical, and electronic features that may surmount some of these challenges [131]. Ease of reproducibility, tightly controlled dimensions, and ability to manufacture in high volume are other advantages. A nanochannel filter fabricated between two silicon substrates is a potential solution for the bioapplication, proposed in [132, 133]. This device offers good control of channel size and pore distribution, making it possible to control the release rate. The level of integration that can be achieved on a single silicon substrate provides major advantages over other materials used for making drug delivery devices. These sandwich structure nanochannel filters are fabricated using photolithography, selective oxide growth, and removal. They have envisioned nanochannel delivery systems (nDSs) for the delivery of therapeutic molecules. These devices will present progressively increasing degrees of functionality. The device dimensions were optimized for high mechanical strength so that they are suitable for implantation. Nanochannel devices with 60 nm channel height were fabricated in silicon [134]. These nanochannels are in between two directly bonded silicon wafers, and therefore pose very high mechanical strength, compared to nanopores through thin membranes. The nanochannels were defined by selectively growing oxide and then etching that oxide. The glucose flow through a 60 nm channel shows a zero-order release rate for the period investigated. One of the barriers to use in practice is optimizing the size of the nanochannels for a desired drug delivery rate. Different nanochannel sizes deliver different drugs with different rates. A particular drug will require to be delivered at a specified rate, and that will require changing the size of nanochannels. This barrier can be addressed with the integration of electronics onboard. The flow through the nanochannels will then be electrically controlled and changing the voltage externally will change the flow rate. Overall Brunner et al. [135] have shown that a number of polymers (poly(urethane) (PU), poly(methyl-methacrylate) (PMMA), poly(dimethylsiloxane) (PDMS) and ethylene-vinyl alcohol copolymer (EVOH)) can replace glass as packaging material for hybrid nanodevices integrating kinesin motors and microtubules, but that special care has to be taken when intense illumination is needed, for example, for fluorescence imaging. Prolonging the lifetime of biomolecules in their functional states is critical for many applications where biomolecules are integrated into synthetic materials or devices. A simplified molecular shuttle system, which consists


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of fluorescently labelled microtubules propelled by kinesin motor proteins bound to the surface of a flowcell, can serve as a model system to probe the lifetime of a hybrid device. In this system, the functional decay can easily be assayed by utilizing optical microscopy to detect motility and disintegration of microtubules. It was found that the lifetimes of these hybrid systems were mainly limited by the stability of microtubules (MTs), rather than of kinesin. Without illumination, only PU had a substantial negative impact on MT stability, while PMMA, PDMS and EVOH showed stabilities comparable to glass. Under the influence of light, however, the MTs degraded rapidly in the presence of PDMS or PMMA, even in the presence of oxygen scavengers. A similar effect was observed on glass if oxygen scavengers were not added to the medium. Strong bleaching of the fluorophores was again only found on the polymer substrates and photobleaching coincided with an accelerated depolymerization of the MTs. PDMS and PMMA, two widely used materials in micro- and nanofabrication, cause rapid disintegration of microtubules under exposure to light, presumably via release of oxygen into the solution. Many efforts in the field of organic light-emitting diodes (OLEDs) have been made during recent years motivated by their potential for applications in display technology, for instance, to replace liquid-crystal displays (LCDs), which are currently used in computer and television screens. Small-molecule OLEDs (SMOLEDs) [136] as well as polymer-based LEDs (PLEDs) [137] have gained serious industrial interest, and some device displays based on small organic molecules are already on the market. Recently, significant improvements were made possible by making use of new processing technologies. Improved deposition technologies such as injet printing [138] open the way for full-color applications. On the other hand, the ongoing design of new materials leads to higher efficiencies, enhanced brightness, and improved lifetimes of optoelectronic devices. Recently, the additional use of phosphorescent emitters gained much attention because such emitters proved to increase the efficiency of SMOLEDs enormously. The lightemitting electrochemical cells (LECs) provide an alternative to LEDs because of their simple design. Generally, polymeric LECs consist of just one layer, which is a mixture of a conjugated polymer, an ion-conducting polymer, and a salt [139]. Organic light-emitting nanodevices have become very attractive, mainly due to their potential applications in flat-panel displays and lighting. Employing phosphorescent dyes in the electroluminescent light-emitting layer further increases the efficiency [140] of small-molecule-devices since phosphorescent molecules emit from their triplet state. Heavy-metal complexes, particularly those containing Pt(II) and Ir(III), can therefore serve as efficient phosphorescent emitters [141, 142]. In such systems, holes and electrons are injected at opposite sulfaces of a planar multilayer organic-

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thin-film stack. The holes and electrons migrate through the thin films to the interface between two layers, where they recombine to form radiative excited states, or excitons. This electrically generated exciton can be either a singlet or a triplet [143]. Theoretical predictions and experimental measurements agree on singlet-totriplet ratio for these excitons of 1 to 3 [144]. The excited states generated by electron-hole recombination are trapped at the phosphor where strong spin-orbit coupling leads to singlet-triplet state mixing, revealing an efficient phosphorescent emission at room temperature. Both singlet and triplet excited states can be trapped at the phosphorescent emitter, leading to devices with high efficiencies. OLEDs prepared with heavy-metal complexes, such as Ir(III) or Pt(II) complexes, are the most efficient nanodevices reported to date, with theoretical intemal quantum efficiencies of 100 % [145] due to the harvest of both singlet and triplet excitons [146]. In small-molecule devices (thin layers from 5 to 100 nm), each of these layers fulfills a specifie function, such as charge injection, charge transport, or light emission [147]. SMOLEDs utilizing phosphorescent dopants such as Pt(II) or Ir(III) complexes are highly efficient (internal efficiency of 100%), but usually require multilayer architectures. In a first step, glass substrates with a conducting transparent electrode such as indium tin oxide (ITO) are prepared (and often coated by hole-concluding polymers such as poly(3,4-ethylenedioxythiophene)/poly(styrene sullonate) [PEDOT/PSS] [148]. Secondly, a thin organic hole-transporting layer (HTL) composed of carbazole derivatives [148], or triarylamines is applied. Onto this layer, an organic light-emitting host-guest layer of comparable thickness is deposited: this layer contains phosphorescent emitters such as Pt(II porphyrins or Ir(III) complexes. The choice of the host material (hole transporting) is of high importance since the transfer of triplet excitons from the phosphorescent emitter to the host materials has to be prevented. Generally, the triplel energy levels of the host materials need to be higher then those of the employed triplet emitters. For SMOLEDs, the family of carbazoles [149] could be extended to be suitably for red-, green-, and blue-light, and therefore they can be used in full-color displays [150]. Well-known phosphorescent emitter devices are iridium(III) and platinum(II) complexes. When placed in a suitable host material (small molecules of polymeric materials), such emitters find applications in full-color displays. One of the main requirements for OLEDs doped with phosphorescent emitters is that the phosphorescent emitter should exhibit very high phosphorescence quantum efficiencies. Similar interesting properties are also displayed in many other metal complexes, such as osmium(II), rhemium(1), and ruthenium(II). Platinum(II) porphyrinas [141] or iridium(III) complexes containing 2-phenylpylidine,


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benzoquinoline, 2-phenylbenzothiazole [151] or their derivatives are known to exhibit high triplet quantum yields due to the mixing of the singlet and the triplet excited states via spin-orbit coupling, enhancing the triplet state, which, in turn, leads to high phosphorescence efficiencies [152]. A relatively short phosphorescence lifetime significantly improves the performance of a phosphorescent material, particularly with respect to its maximum brightness and efficiency at high currents. [147]. Furthermore, the development of new deposition technologies also allows the use of novel materials such as dendrimers. Dendrimers are a class of materials that have attracted serious interest because of the potential to fabricate high-efficiency devices. [153]. Utilization of ruthenium(II) complexes in light-emitting nanodevices offered an election-to-photon conversion efficiency of 5.5% [154]. The highest phosphorescence quantum yields reported for ruthenium complexes containing substituted 2.2'-bipyridine and 1,10-phenanthroline ligands are around 40% [155]. Advantages of such systems as compared to conventional OLEDs are their high brightness and efficiency at low operating voltages, as well as the fact that they do not need low-work-function cathodes. Within the science of nanomaterials, there has been some focus on novel methods for engineering thermoelectric micro- and nanodevices. In particular, thermoelectric devices were fabricated and evaluated for power generation and cooling performance. Thermoelectrics (TEs) convert heat into electricity (Seebeck effect) and vice versa (Peltier effect). Thermoelectric devices consist of many n-type and ptype thermoelectric elements connected electrically in series and arranged thermally in parallel. The many advantages of TE devices include solid-state operation, zero emission, vast scalability, no maintenance, and a long operating lifetime. Nonetheless, because of their limited energy-conversion efficiencies. thermoelectrics have a rather specific range of applications. Examples of applications include radioisotope thermoelectric generators (RTGs) for power generation and optoelectronic thermal management for cooling purposes. Throughout the microelectronics industry, miniaturization has become affordable, versatile, and readily accessible immediate advantages of miniaturizing thermoelectric devices to the micrometer scale are, as predicted by scaling factors, an increase in specific power (Wcm-2) and improvements in maximum cooling with greater cooling densities. Potentially thousands of thermoelectric microelements could be concentrated within a small area, and they can generate greater voltages at even small temperature differentials. In addition, since the Seebeck effect and the Peltier effect are directly related, optimizing thermoelectric materials for power generation will also optimize them for cooling. Further miniaturization down to the nanoscale affords additional benefits due to quantum-confinement effects. Thermoelectric nanowires less than 10 nm in diameter are predicted to exhibit higher efficiency.

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This enhanced efficiency would result from a higher change-carrier mobility due to a greater density of states and more limited phonon transport. In fact, thermoelectric nanobased devices open a more diverse avenue of applications for increased spot cooling or for use as sensors, such as infrared or micro- and nanocalorimeter sensors [156]. The versatility of physical and chemical properties thus afforded by metal and semiconductor nanoparticles makes them promising as the ultimate miniature devices. In many instances, the ability to exploir nanoparticle properties for device fabrication will require the formation of morphologically controlled or highly ordered arrays of nanoparticles (Fig. 4) [42]. In microelectronics, this is the key challenge in the process of transforming nanoparticles from promising materials into integrated devices [157]. Being able to control the structural arrangement of nanoparticles will make the “bottom-up" a powerful adjund to current top-down technologies (e.g., phololithography and election-beam lithography) in achieving high resolution concomitant with parallel fabrication, and especially in creating complex three-dimensional (3D) structures.

Fig. 4. Polymer-mediated assembly approaches to fabrication of ordered nanocomposites [42, 158].


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Incorporation of a polymer component in nanoparticle - based sensor devices provides greater flexibility than simple nanoparticle assemblies. First, the utilization of flexible polymers to space nanoparticles increases the interparticle space and film porosity, leading to efficient uptake of analyte molecules. Second, the polymer provides multiple interaction sites that can be designed to allow for specific interaction with analyte molecules. Third, the polymer can be designed to enable crosslinking of the nanoparticles, resulting in a mechanically reinforced film and facilitating the controlled creation of a multiple-layer device through the layer-bylayer deposition technique [159]. Taken together, these virtues of nanoparticlepolymer composites all contribute to enhanced selectivity and sensitivity. The specific quantum mechanical properties of many nanodevices require radically novel architectures approaches. Concepts like fault tolerant architecture, parallel processing, neural nets are directly translatable into conventional (Si) hardware, whereas more advanced concepts like non dissipative and quantum computing, DNA-based computing will certainly required the implementation of novel nanodevices. Quantum mechanics (emphasis on tunneling effect); physics (including mechanics, electricity, electromagnetism, optics and photoelectric effect); chemistry (including models of the atom, chemical bonding; aqueous-solution reactions; electrochemistry, photochemistry); stress and strain analysis; vibrations; electronics; circuits analysis; control systems; application of microprocessor; mechatronics (including sensors, actuators, control circuit, piezoelectric actuator); etc. are by the complex way engaged into the nanotechnology area. Biomolecular electronics (BME) is raising increasing interest worldwide, due to the appealing possibility of realizing cheap and easy-to-fabricate devices exploiting the natural self-assembling, self-recognition and self-repairing capability of biological matter. Although very recent, BME has deep roots in the field of organic molecular electronics, whose flag-ships are carbon nanotubes and molecular junctions [160162]. Biomolecules are in general more robust than other organic molecules, thus envisaging a more reliable utilization in electronic devices. Moreover, they are characterized by a number of unique electron transport phenomena, such as charge transfer in proteins, hopping and/or band-like transport (π–π) in self-assembled systems. Finally, both their electronic structure and their ligands can be engineered in a very flexible way, thus allowing a fine tuning of the oxidation potential, and of the selective bonding to different surfaces. Very recently, the attention of BME has been directed toward the identification of molecules combining good conductivity with

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good self-assembling and self-recognition properties. DNA has been one of the most investigated class of biomolecules [163-165], leading to a somewhat controversial description of its electrical properties, and, hence, of its potentiality for electronic applications. Depending on the interconnection mechanism (chemical bonding of the DNA on a metal by a selected sequence of oligonucleotides [163], mechanical contact with a gold interdigitated patterns [164] or single DNA molecule immobilized in a metal contact [165]) the DNA molecules have been found to be conductive, non-conductive or rectifying. From a totally different point of view, some groups are trying to use biological methods to control the formation of semiconductors and metals [166, 167] by investigating the peptide-driven formation of gold crystals, as a prototype mechanism for the formation of natural solids like bones and teeth in the human body. Millions of peptides with specific peptide sequences can be used to distinguish among different crystallographic planes of the most important semiconductors used in technology (GaAs and Si). The peptides could therefore be used to control the positioning and the assembly of materials at nanoscale, which has a tremendous impact on future electronic technologies or nanodevices. High-efficiency photodetectors based on a solid-state self-organized DNA basis, whose figure of merits become appealing even for solar cell applications. The most surprising thing is that most of these groups have discovered the enormous potentiality of biomolecular self-assembling even though they started from different backgrounds and with totally different targets. This gives a clear favor of how general is the cultural revolution we are experiencing. The terms “actuator”, “sensor”, and “transducer” are widely used in the description of measurement device systems [168]. In the broadest sense, a transducer receives energy from one system and transmits this energy to another system, often in a different form. A sensor monitors a system; it responds to physical stimuli, such as heat, light, pressure, or motion, and generates an electronic impulse for detection. An actuator, on the other hand, imposes a state upon a system. Most commonly, this involves converting an input electrical impulse into motion. Actuators and sensors are both transducers intended for different tasks. In accord with these general definitions, an electromechanical transducer converts electrical energy into mechanical energy, and vice versa. An electromechanical system refers to a mechanical element coupled to electronic circuits via electromechanical transducers. For example, the input transducer takes electrical signals from the input circuit and provides mechanical stimuli to the mechanical system; this is generally referred to as actuation. The response of the mechanical element-namely, its motion or displacement-is sensed by the output transducer, which generates electrical signals in the output circuit. These electrical signals in the form of currents and voltages can subsequently be measured. The overall purpose of this conversion of energy back


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and forth between the mechanical and electrical domains may be to accomplish a mechanical task in a controllable manner. For example, the microscopic electromechanical systems that researchers have long been fashioning using the materials and processes of microelectronics. These micromechanical elements beams, cantilevers, gears, and membranes - along with the enabling microelectronic circuits are called microelectromechanical systems (MEMS). MEMS perform a variety of tasks in present day technology, such as opening and closing valves, turning mirrors, and regulating electric current flow. With microelectronics technology now pushing deep into the sub-micrometer size regime, a concerted effort has surfaced to realize even smaller electromechanical systems: nanoelectromechanical systems (NEMS) [169, 170]. Recent demonstrations of NEMS-based nanomechanical electrometry, [171] signal processing, [172] and mass detection [173] have attracted much attention. Elements of nanoelectronic devices, for example, transistors [174] diodes [175] bits of memory [176] and logical gates [177], components of nanoelectromechanical systems, for example, nanorobotic manipulators [178], and nanowheels [179] are envisaged. Novel methods to manipulate carbon nanotubes (CNTs) have been developed to design nanotools [180]. The incorporation of biomaterials into nanoelectronic and nanomechanic systems based on CNTs will undoubtedly result in further impressive advances. Novel nanobiotechnological and biomedical applications of CNT based materials are envisaged, for example, carbon nanofibres could be used as improved neural and orthopedic implants [181], and chemically functionalized CNTs have been suggested as substrates for neuronal growth [182].

1.5. Industrial aspects of nanotechnology Many technologies have been developed to produce nanomaterials, nanospheres and nanostructures. These techniques can be divided into several groups according to the reaction media, the form of products, the way of nanostructures formation, the properties of nanomaterials, etc. as follows: vapor phase nucleation and growth, liquid phase nucleation and growth, solid phase formation, colloidal process, solution-liquid-solid growth, thin film formation, self assembly of small particles, top-down and bottom-up approaches, etc. The so-called bottom–up approach to the fabrication of nanostructures from stable building blocks has become a popular theme in current science and engineering. This construction principle mimics biological systems by exploiting the order-inducing factors that are immanent to the system rather than imposing order top–down from an external source. While the fabrication techniques of current commercial importance such as lithography fall

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practically without exception into the top-down category. The main disadvantage of the top-down is the imperfection of the performed nanostructures. The top-down lithography approach can be accompanied with the significant crystallographic damage to the processes spheres [183]. This includes experimental simplicity down to the atomic size scale and the potential for inexpensive mass fabrication. Bottomup fabrication refers, on the contrary, to the building up the nanomaterials or nanospheres from the bottom to top; atom by atom or molecule-by-molecule. In the colloidal chemistry and polymer chemistry, the polymer nanospheres can be produced by the radical polymerization of unsaturated monomers in the direct or inverse micellar solutions. The reduction of metal salts by radicals and reducing agents in the micellar systems leads to the formation of metal nanoparticles. This method can in the future offer a number of potentially very attractive advantages. Bottom-up promises the best chance to obtain the regular spherical particles with less or mimimum defects. The bottom-up approach to the fabrication of nanostructures from stable building blocks has become a popular theme in current science and engineering. While the fabrication techniques of current commercial importance such as lithography fall practically without exception into the top-down category, bottom-up fabrication may in the future offer a number of potentially very attractive advantages. These include experimental simplicity down to the atomic size scale, the possibility of threedimensional assembly, and the potential for inexpensive mass fabrication. A prerequisite for nanostructure preparation via this self-assembly route is the availability of sufficiently stable building blocks which have to well-characterised and uniform in size and shape. A range of interesting self-assembled structures can be obtained from ligand-stabilized metal nanoparticles, which like-wise show a fascinating wealth of size-related electronic and optical properties. The synthetic methods can be divided into chemical and physical (molecular beam epitaxy, sputter deposition, electron beam lithography, etc.) methods. Chemical methods include a large variety of different chemical techniques with the common property of using reactions in solutions to produce particles of different sizes and materials. In order to control the size and shape of the particles, the synthesis is based on the appropriate control of the parameters that influence nucleation and growth. The use of ligands (stabilizing agents) such as surfactants and polymers (or oligomers) is very common in the specific control of growth and in the prevention of agglomeration of the particles once they have been synthesized [184-188]. Besides of the many advantages there are also disadvantages of the traditional routes to nanomaterials:


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1) The large interface area costs a lot of energy and requires large amounts of stabilizer or embedded surface units. 2) The simple nucleation-and-growth route demands very low in-situ concentrations of the formed colloids, i.e., the mass output is rather low. 3) Concentrating the products or harsh reaction conditions usually leads to the failure of stabilization and the formation of larger aggregates. For these reasons, some modern techniques developed to control the uniformity in size and shape make use of synthesis in mesoscopically confined geometries, such as in vesicles [189], reverse micelles [190, 191], solgel processing [192], zeolites, [193], Langmuir-Blodgett (LB) films [194, 195], microporous glass [196] or organic or inorganic gels. Nanoparticle formation in block-copolymer aggregates can be considered to be a further advancement of these techniques. Research and development are focused on the development of science and technology at the right size - and that size may range from nanometers to millimeters (for the technologies of small things). Associated with these developments, nanostructures offer a new paradigm for materials manufacture by submicron-scale self-organization and self-assembly to create entities from the “bottom up” rather than the “top down” method. However, we are just beginning to understand some of the principles to use to create “by design” nanostructures and how to economically fabricate nanodevices. Each significant advance in understanding the physical/chemical/biological properties and fabrication principles, as well as in development of predictive methods to control them, is likely to lead to major advances in our ability to design, fabricate and assemble the nanostructures and nanodevices into a working system. “Bottom up” approaches, closely linked as they are to the field of molecular electronics are elegant, cheap, and possibly enormously powerful techniques for future mass replication, but their applicability remains limited until total control over the emerging structures in terms of wiring and interconnections can be obtained. It is clear that new architectures are required for such bottom up fabrication approaches [197]. Nanoparticle, crystal and nanolayer manufacturing processes aim to take advantage of four kinds of effects: 1) New physical, chemical or biological properties are caused by size scaling. Smaller particle size determines larger interfacial area, an increased number of molecules on the particle interfaces, quantum electromagnetic interactions, increased surface tension, and size confinement effects (from electronic and optic to confined crystallization and flow structures). The wavelike properties of the electrons inside matter are affected by shape and volume variations on the nanometer scale. Quantum

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effects become significant for organizational structures under 50 nm, and they manifest even at room temperature if their size is under 10 nm. 2) New phenomena are due to size reduction to the point where interaction length scales of physical, chemical and biological phenomena (for instance, the magnetic, laser, photonic, and heat radiation wavelengths) become comparable to the size of the particle, crystal, or respective microstructure grain. Examples are unusual optoelectronic and magnetic properties of nanostructured materials, changes of color of suspensions with particle size, and placing artificial components inside cells. 3) Generation of new atomic, molecular and macromolecular structures of materials by using various routes: chemistry (three-dimensional macromolecular structures, chemical self-assembling), nanofabrication (creating nanostructures on surfaces, manipulation of three-dimensional structures), or biotechnology (evolutionary approach, bio-templating, and three-dimensional molecular folding). 4) Significant increase of the degree of complexity and speed of processes in particulate systems. Time scales change because of smaller distances and the increased spectrum of forces with intrinsically short time scales (electrostatic, magnetic, electrophoresis, radiation pressure, others). Nanoscale phenomena and processes are yet to be understood and the resulting structures to be controlled and manipulated. Critical length and time scales, surface and interface phenomena are essential aspects to be defined. Novel mechanical, optical, electric, magnetic, thermal, chemical and biological properties occur as compared to bulk behavior because of the small structure size and short time scale, but only a small part of these properties have been fully identified and quantified. Several industrial domains have been identified as essential for future applications of nanotechnologies. They include materials as nanostructured materials, nanoelectronics, optoelectronics, magnetics, advances healthcare, therapeutics, diagnostics, environment and energy. We intend to design and fabricate stronger, lighter, harder, self-repairing and safer nanostructured materials. Nanocomposites and nanoparticules reinforced by polymers could for instance be used for automotive applications. It is foreseeable that nanometer structures (nanoelectronics, optoelectronics and magnetics) foster a revolution in information technology hardware rivaling the microelectronics revolution 30 years ago. Advances healthcare, therapeutics and diagnostics via nanotechnology will contribute to significant advances through the development of biosensors and improved imaging technologies. Drugs production and delivery are expected to be drastically change in the next decade.


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A major issue in nanoscale research is how scientific paradigm changes will translate into novel technological processes. Nanoparticle systems, including nanoclusters, nanowires, nanobelts, nanotubes, nanorods, nanostructured particles, and other threedimensional nanostructures in the size range between 1 and about 100 nm are seen as tailored precursors for nanostructures materials and devices. Particle processing (sintering, extrusion, plasma activation, selfassembling, etc.) is the most general method of preparation of nanostructured materials and devices. Research on nanomaterials has been stimulated by the interest for their technological applications, such as catalysts, ceramics, battery materials, color imaging, drug delivery systems, pigments in pains, magnetic tapes, ferrofluids, magnetic refrigerants, giant magnetoresistance, etc [198, 199]. The ability to fabricate nanomaterials and to exploit their special properties is gaining widespread attention. Important areas of relevance for nanoparticles and nanotechnology are advanced materials, electronics, biotechnology, catalysis, pharmaceutics and sensors. These include hard disks in computers, photographic systems, dispersions with novel optoelectronic properties, information recording layers, biodetectors, advanced drug delivery systems, chemical-mechanical polishing, a new generation of lasers, chemical catalysts, nanoparticle reinforced materials, ink jet systems, colorants, and nanosystems on a chip, to name some of the most important. The metal nanoparticles, thus, may serve as efficient catalyst in chemical and photographic processes [200]. Metal clusters and nanoparticles immobilized in polymer films give metallopolymer materials that prove to be useful for technical purposes due to their specific physico-chemical properties [201]. Also, some small metal particles demonstrate a distinct biological activity and may be applied in ecology and medicine, for example, a significant antimicrobial activity of silver nanoparticles allowed the improvement of the water purification in some water filtering apparatus [202]. The demand for smaller materials for use in high density storage media is one of the fundamental motivations for the fabrication of nano-scale magnetic materials. The development of a high density magnetic memory device may be more readily achieved by patterning magnetic nanoparticles into organized assemblies on the surface of a substrate. Since these assemblies usually exhibit unusual electronic, optical, magnetic, and chemical properties significantly different from those of the bulk materials, they have various potential applications such as electronic, optical, and mechanical devices, magnetic recording media, superconductors, highperformance engineering materials, dyes, adhesives, photographic suspensions, drug delivery, and so on [10, 203, 204]. In this connection, catalysis represents one of the single most important applications of nanotechnology. Traditionally, supported catalysts have been produced by wet impregnation using water-soluble metal salts, which results in well-dispersed catalysts with high activity and good stability. The

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particle size of the activated phase is usually in the nanometer range but with a quite broad size distribution and a low degree of control over the particle size. This renders the interpretation of, for example, size-dependent mechanistic phenomena of the catalyst impossible. Another route to prepare nanosized particles is use a water-in-oil (w/o) microemulsion where a metal (platinum) precursor is reduced to metallic platinum in the water pools. With this method of preparation, particles with relatively narrow size distribution down to an average of a few nanometers can be obtained [205]. At least in some systems it has been found that not only the size but also the shape of the particles can be controlled [206]. Novel ideas have been proposed in laser ablation of materials to generate nanoparticles used in nanoelectronics, production of polymer semiconductor composites for development of non-linear optics for waveguides, molecular and nanostructure self-assembly techniques, high-performance catalysts, control of nanoparticles resulted from combustion and plasma processes, and special sensors applied in chemical plants and the environment. Nanoparticle manufacturing is an essential component of nanotechnology because the specific properties are realized at the nanoparticle, nanocrystal or nanolayer level, and assembling of precursor particles and related structures is the most generic route to generate nanostructured materials. Nanoparticles realized at the nanoparticle or nanocrystal/grain level, and the use of precursor nanoparticles as building blocks of tailored properties for nanostructured materials, nanocomponents processes [207]. Nanoparticle manufacturing processes may be separated into the following groups: 1) Processing and conversion of nanoparticles into nanostructured materials (such as advanced ceramics), nanocomponents (such as thin layers), and nanodevices (such as sensors and transistors). Examples of processing methods include sintering, generation of nanostructures on surfaces, evolutionary biotechnology, and molecular selforganization techniques. Research challenges include continuous particle synthesis and processing into functional nanostructures and devices. 2) Utilization of nanoparticles in order to produce or enhance a process or a phenomenon of mechanical, chemical, electrical, magnetic and biological nature. Examples of the more frequently used manufacturing processes are particle contamination control, chemical vapor deposition, use of particles as agents of surface modification, filtration, mass spectroscopy, bioseparation, combustion pollution control, drug delivery and health diagnostics, and use of nanoparticles as catalysts and pigments in chemical plants. 3) Process control and instrumentation aspects. Important problems include off- and on-line measuring techniques for fine particles and their structures. In parallel with the better established characterization methods for particle size, shape and


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composition, new instruments are needed to measure particle interaction forces, their roughness, electric, magnetic and thermal properties. Progress in nanotechnology demands the capability to fabricate nanostructures in a variety of materials with an accuracy in the nanometre scale and sometimes in the atomic scale. Stringent nanofabrication specifications have to be met in industrially relevant processes due to manufacturability and costs considerations as, for example, in the electronics industry. However, less demanding conditions are needed for developments in optics, sensors and biological applications. In a laboratory environment, at the level of enabling nanofabrication techniques as tools for experiments to understand the underlying science and engineering in the nanometer scale, easily accessible and flexible nanofabrication approaches are required for investigations in, e.g., materials science, organic optoelectronics, nano-optics and life sciences. Alternative techniques to cost-intensive or limited-access fabrication methods with nanometre resolution have been under development for nearly two decades. One clear example is the evolving set of scanning probes techniques, which has become ubiquitous in many research areas. If one considers planar structures, i.e., where nanostructuring is carried out on a surface, as distinct from a threedimensional nanofabrication or multilayer self-assembly, then several emerging nanofabrication techniques can be discussed. Their classification depends on whether the nature of the patterning is chemical or physical, or its modality in time is parallel or sequential, or a hard or a soft mould or stamp is used, etc. The literature on the subject is increasing very rapidly, for example, progress in micro-contact printing [208], scanning probe-based techniques [108] and nanoimprint-based lithography (NIL) technique [109], have been published. Recent developments in nanopatterning include dip pen lithography [209] and nanoplotting [210], as well as stenceling [211]. To nanoimprint a surface, three basic components are required. These are: 1) A stamp with suitable feature sizes fabricated by, for example, electron beam lithography and dry etching, if features below 200 nm are needed or, by optical lithography for larger features. 2) The material to be printed, usually a layer of polymer of a few hundred nanometres’ thickness with suitable glass transition temperature Tg and molecular weight, spun of a substrate and 3) equipment for printing with adequate control of temperature, pressure and control of parallelism of the stamp and substrate. NIL has the advantage over conventional nanofabrication methods, of being a flexible, low-cost and biocompatible fabrication technique. There are several variations of NIL including the most popular parallel process using wafer size stamps

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[212], a sequential process called step-and-stamp imprint lithography (SSIL) [213] and roll-to-roll NIL [214] process [215]. There are several key achievements in the development of NIL as a nanofabrication technique and its potential applications reported over the last ca. ten years. These are mentioned in a chronological sequence. (a) The first report of what is known as NIL as a potential nanofabrication technique appeared in 1995 [216]. (b) Feature sizes down to 6 nm by NIL are achieved in 1997 [217] in PMMA. (c) The use of NIL to fabricate polymer-based optical devices is demonstrated in 1998 [218]. (d) Alignment accuracy of 1 nm is demonstrated using commercially available equipment [219]. (e) In 1999, Yu et al. [220] reported on NIL-made metalsemiconductor-metal (MSM) photodetectors with no mobility decrease up to NIL pressures of 600 psi. (f) Broadband waveguide metal polarisers with 190 nm period were reported by Wang et al. [221]. (g) NIL of 150 mm diameter wafers is achieved [212]. (h) A sequential variation of NIL, step-and-stamp imprint lithography is demonstrated using a commercial flip-chip bonder [213]. (j) Low-cost stamp replication using NIL is demonstrated [222]. (k) Mäkela et al. [223] showed that the electrical conductivity of nanoimprinted conducting polymers is not impaired by imprint lithography. (l) A new resistance suitable for NIL, which is also sensitive to electron beam and UV lithography, was reported [224]. (m) The first microfluidic device made by NIL is reported by Studer et al. [225] in 2001. (n) An anti-adhesion treatment for stamps containing sub-100 nm features is reported [226]. The chemical industry faces particularly interesting choices, since taking full advantage of the opportunities of nanotechnology will require it to behave in new ways. Few nanomaterials will be commodities, and few processes for making nanofabricated structures will be carried out in facilities having the scale of those used in the production of commodity chemicals. The value of nanomaterials and nanostructures will come in their function, and in the systems in which they are embedded. Time will tell whether chemical companies will choose to make photonic devices in order to exploit their ability to produce photonic bandgap (PBG) materials, or whether telecommunications companies will choose to make PBG materials in order to exploit the functions that they provide in their devices and systems. Regardless, it seems inevitable that chemical companies active in nanotechnology will find themselves competing with their customers in the areas of high-valued, functional materials, components, and systems. Since there are few new, high-margin markets open to the chemical industry, it may need to move downstream uncomfortable though it may be to do so - in nanotechnology (or other emerging areas) if it is not to stagnate technically and financially. Competition in new markets requires agility, and the ability to move quickly to capture new opportunities is


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always a difficult trick. It will be particularly difficult for an industry that, for some decades, has not been rewarded for embracing new ideas or for accomplishing new tricks, and that, through lack of practice, has become unaccustomed to doing so [33]. Nanotechnology offers the industry several particular opportunities: 1) Production of new tools and equipment for research. 2) Production of new materials of nano- and microstructures. Examples include structural and electrically/magnetically/optically functional polymers, particles, and metal/polymer composites for a range of applications [227] 3) Development of new processes to make new materials for fabrication in the chemical industry. 4) Development of new photoresists and processes with which to fabricate structures with the sub-50 nm dimensions required by nanoelectronics will present opportunities for materials science and chemistry [228] 5) Nanoparticle technology will become important in a wide range of applications— from hydrophobic drugs generated and formulated in nanoparticulate form to improve bioavailability, to electrodes and lumiphores for new kinds of graphic displays. 6) Development of revolutionary nanomaterials or nanoobjects such as nano-CDs, quantum and molecular computers, biocompatible nanoparticles, etc. The chemical industry has used phase-separated copolymers and blends for many years to optimize properties of additive saturated polymeric materials. Nanoscience is beginning to produce new methods of characterizing the structures of the phaseseparated regions (which are often of nanometer dimensions), and thus provide ways of engineering these regions (and the properties of the polymeric materials) in rational ways [229]. Understanding these relationships between the composition of the polymer, and the properties of the materials made from it, will provide a new approach to engineered materials. Nanoscale, phase-separated block copolymers are also finding uses as materials in microelectronics and photonics. For applications in PLEDs, light-emitting polymers have attracted much attention because of their unique properties. They appear able to fulfill functions such as charge injection, charge transport, and emission of light in one active layer in a PLED. Therefore, the complete construction of a PLED can be much simpler than that of a SMOLED. Wet-chemistry fabrication processes are used for applying polymers. They permit coverage of larger areas, which becomes important for applications such as computer displays and television screens [230]. For PLEDs, the organic layers can be deposited by spin-coaling (resulting in monochrome devices)

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or inkjet printing (used for full-color devices). A low-work-function metal cathode is applied by vacuum deposition. Even though in the case of solution-processable polymers fluorescent PLEDs with good efficiencies have been reported, the search for solution-processable materials that allow full-color applications with high brightnesses and increased efficiencies is ongoing. A new class of materials has become prominent in OLEDs, namely, dendrimers [231]. Such light-emitting dendriniers contain surface groups, dendrons, and cores. These materials can be classified as either fully conjugated dendrimers [232] or materials where the fluorophore is attached to dendrons that contain nonconjugated moieties [233]. When comparing conjugated polymers with conjugated dendrimers, a number of potential advantages become apparent. The controlled molecular synthesis of dendrimers would provide greater freedom and a better control of the material properties [234]. Optimization of electronic and processing properties can be tuned and optimized independently. Moreover, the dendrimer generation provides molecular control over the intermolecular interaction.

1.6. Investigative tools Search of effective ways for controlling the morphology of nanophase materials is of principal importance for nanotechnology and for development of advanced nanostructured materials. It is known that the method of nanoparticle synthesis often influences the properties of the product, in particularly, synthesis of nanoparticles in confined geometries and structured reaction media can result in anisotropic and sizecontrolled nanoparticles [235]. The nanostructures are difficult to characterize because they are much smaller than visible light wavelengths and significantly larger than individual molecules. Likewise, simulation at the nanoscale is equally difficult, as the structures are mostly too small for continuum treatments and too large for simulations involving individual atoms and molecules. Investigative tools have played a critical role in the advancement of the entire nanofield. The main research areas and design tools may be grouped as: 1) Modeling and simulation of the connection between structure, properties, functions and processing using atom-based quantum mechanics, molecular dynamics and macromolecular approaches. Simulations aims to incorporate phenomena at scales from quantum (0.1 nm), molecular (1 nm) and nanoscale macromolecular (10 nm) dimensions, to mesoscale molecular assemblies (100 nm), microscale (1000 nm), and macroscale (> 1 μm). A critical aspect is bridging the spatial and temporal scales.


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2) There are a wide range of instruments and techniques, from scanning tunneling mapping of surfaces to atomic force chemistry, nuclear chemistry and near-field visualization for testing and measurements. Scanning probes and optical and laserbased diagnostic techniques are the most widely applied experimental tools. 3) Information technology, including pattern recognition, molecular organization mechanisms, and nanorobotics. Information on surfaces play a key role in selforganization and selfassembling. 4) Techniques such as reaction pathways and process control can be used in order to obtain a predetermined structure or function, and integrate the operation of nanosystems with complex architectures. 5) Unique size dependent properties, phenomena, and processes of nano-particle, droplet, bubble, tube, fiber and layer systems. 6) Fundamental physical (mechanical, thermal, optical, electronic, etc.), chemical and biological characterization of nanoparticles and their interfaces, and development of in-situ and ex-situ instrumentation based on new principles for probing properties and phenomena not well understood at the nanometer scale. 7) Synthesis and processing of nanoparticles and related nanoprecursor structures, including clusters, aerosol and colloid particles, nanotubes, nanolayers, biological structures and self-assembled systems. Approaches may include gas-, liquid-, solid-, and vacuum-based processes, size reduction, chemical and bio-selfassembly. 8) Utilization of nanoparticle systems for enhancing a phenomenon or process, such as chemical reactions, nano-electronics, nano-ionics, magnetic processes, optical processes, heat transfer, bioseparation, bio and chemical reactivity. 9) Utilization of nanoparticles for generating one- to three- dimensional hierarchical structures by assembling, including functional nanostructures in dispersions, structural materials and electronic devices. 10) Utilization of nanoparticles for the formulation and the administration of drugs, including drug and gene delivery systems, transport of molecules in biotechnology, and the use of nanoparticles in the field of the diagnosis. 11) The promise of nanotechnology is being realized through the confluence of advances in scientific discovery that has enabled the atomic and molecular control of material building blocks, and engineering that has provided the means to assemble and utilize these tailored building blocks for new processes and devices in a wide variety of applications. 12) The degree of control over molecular-level organization of amphiphiles and ions that may be exercised at the air-water interface can result in its extensive use in the organization of large inorganic ions [236], and biological macromolecules [237].

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The direct observation of atoms and molecules was initiated more than 20 years ago by the scanning tunnelling microscope [238]. It keeps providing us with fresh insight into structure and dynamics at nanometer scales. But, besides the many varieties of scanning-probe techniques derived from tunnelling microscopy, single-molecule sensitivity can also be reached by purely optical methods, in the far-field. To select a single molecule in a diffraction-limited light spot, one strongly dilutes the active molecules in a nonabsorbing medium, until at most one of them absorbs the exciting laser at any given spot. A detected signal, usually fluorescence, will then necessarily arise from a single molecule. Single-molecule spectroscopy (SMS) that is, the study of single nano-objects (molecules, nanocrystals, metal colloids, etc.), in the focus of an optical microscope, has brought its share of recent surprises. Widespread blinking or flickering, for example, has become a hallmark of single-molecule signals [239] and the routine criterion for reaching the single molecule level. Scanning tunnelling microscopy (STM) relies on measuring the tunnelling current between an atomicsized tip and the sample, which resides on a conductive substrate. The imaging of atomic surfaces is possible by scanning the tip over the sample and registering the interaction for each position. The property that sets STM apart from most other sensitive techniques is its ability to resolve structures and dynamics of surfaces on an atom-by-atom scale. The impact of STM in other fields besides surface science, such as material science and biology, is growing steadily. Since its invention in the early 1980s, scanning probe microscopy (SPM) has been continuously developed to become a versatile and key tool for researchers, particular in the field of materials science and technology. Over the years the basic principle of SPM of measuring a specific interaction between a probe with an ultra-sharp tip and a material’s surface to collect, for example, topographic information with atomic resolution, has generated a complete family of scanning probe microscopy techniques (Scheme 1) [240], such as scanning tunneling microscopy (STM) [241], as its very first member, atomic force microscopy (AFM) [242], and scanning nearfield optical microscopy (SNOM) [243]. The development of new techniques and operating modes to collect more and more information from the nanoworld is continuously in progress.


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Scheme 1. General classification of SPM techniques; indicated are main SPM classes and some of their modifications [240]. Scanning Tunneling Microscopy (STM) Spin-polarized Scanning Tunneling Microscopy (SP-STM) Magnetic Force Scanning Tunneling (MF-STM) Scanning Tunneling Spectroscopy (STS) Inelastic Electron Tunneling Spectroscopy (IETS) Atomic Force Microscopy (AFM) Tapping Mode Atomic Force Microscopy (TM-AFM) Chemical Force Microscopy (CFM) Magnetic Force Microscopy (MFM) Electrical Force Microscopy (EFM) Current Sensing Atomic Force Microscopy (CS-AFM) Atomic Force Acoustic Microscopy (AFAM) Lateral Force Microscopy (LFM) Friction Force Microscopy (FFM) Force Spectroscopy Shear Force Microscopy (SFM) Scanning Near-field Optical Microscopy (SNOM) Scanning Probe Lithography (SPL) Din-Pen Nanolithography (DPN) Mechanical Lithography, Indenting, Ploughing, Scribing Tip-ind uced Oxidation

Beside enabling the organization of matter to be imaged with sub-nanometer resolution, the basic operating principle of SPM provides the power to measure, analyze, and even quantify properties of matter on the nanometer length scale. Using specific probes and measuring conditions, adhesion, elasticity, conductivity, and capacitance data can be obtained, to quote but a few. These data reflect local properties, possibly even of single molecules and atoms and offer new insights into structure-property relations in the nanoworld of matter. Moreover, because of its unique potential to manipulate the organization of atoms, molecules, assemblies, or particles and to structure surfaces in a controlled fashion, SPM has become one of the most powerful tools in the fields of nanoscience and nanotechnology for the preparation and analysis of nanostructures and their functionality.

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The power of scanning probe methods also plays an important role in nanostructured magnetic materials for data storage applications and spintronic devices. Spinpolarized scanning tunneling microscopy and spectroscopy utilizes nonmagnetic probe tips that are coated by a thin (typically less than 10 atomic layers) film of magnetic material, which allows the measurement of both the in-plane and out-ofplane magnetization component of the sample. Thus, the smallest magnetic features, such as domain walls in ferromagnetic iron films in W(110) with a width of 0.6 nm and the atomic-scale antiferromagnetic structure of a manganese monolayer, can be analyzed. In addition, the shape-dependent thermal switching behavior of superparamagnetic nanoislands was explained. The unique resolving power of STM can provide important new information on the atomic-scale realm and on the dynamics of nanostructures. For example, the mobility of defects such as oxygen vacancies on TiO2 surfaces (which become mobile after O2 exposure) can be explored. For the diffusion of O2 molecules on rutile TiO2 (110) surfaces (which plays an important role in understanding (photo)catalytic activity), a charge-transfer-induced diffusion mechanism for the adsorbed O2 molecules was observed. Atomic Force Microscopy (AFM) is a method that quantifies the involved forces arising between a sharp atomic-sized tip and molecules attached to surfaces. Through the scanning of a tip across the surface, AFM can image these forces with submolecular resolution. The ability to perform such experiments under physiological conditions makes it a tool of immense value for the study of biological samples. The forces required revealed the electrostatic, van der Waals, or hydrogenbond forces involved in structural organization. This method introduced a remarkable increase in sensitivity and force resolution. Similar to STM or AFM, scanning-near field microscopy (SNOM) measures the light induced very close to an atomic sized tip. Current SNOM microscopes are either operated in the aperture or scattering mode. The first technique is based on a metalcoated, tapered glass fiber which squeezes the light through an aperture of 50 to 100 nm diameter, while the latter technique exploits the effect of field enhancement when illuminating close to an atomic-sized metal tip.


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1.7. Nano-architectures There is a growing interest in the organization of nanoparticles in two- and threedimensional structures. The main challenge in this area is to develop approaches for the organization of arrays of nanoparticles wherein both the size and separation between the nanoparticles in the arrays can be tailored. Applications based on the collective properties of the organized particles require flexibility in controlling the nanoarchitecture of the materials [62]. Attempts have made to assemble nanoparticles in two-dimensional structures by a variety of methods that include selfassembly of the particles during solvent evaporation [244], immobilization of covalent attachment at the surface of the self-assembled monolayers [245] or surface modified polymers [246], electrophoretic assembly onto suitable substrates [247], electrostatic attachment to Langmuir monolayers at the air-water interface [248] and air-organic solvent interface [249], and by diffusion into ionizable fatty lipid films [250]. The organization of the metal nanoparticles at the air-water interface can be followed by surface pressure-area isotherm measurements while the formation of multilayer films of the nanoparticles by the Langmuir-Blodgett technique can be monitored by quartz crystal microgravimetry, UV-vis spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy. The simple and primary step towards more complex structures is the controlled linkage of particles to each other or to the surface of an already existing structure, which acts as a template. The simplest approach to such systems is to allow the particles to react with bifunctional molecules which can attach to the surface of two particles and link them together. This has been demonstrated for alkane dithiols, which can be used, for example, to precipitate cross-linked networks of gold particles from solution [251] or to assemble spherical ultramicroelectrodes by immersion of dithiol-filled micropipettes in solutions of gold particles [252]. A appealing ‘brick and mortar’ approach to the controlled fabrication of nanoparticle aggregates has been developed by Boal et al. [253] who prepared gold nanoparticles, which contained molecular recognition elements in the ligand shell. These particles aggregated in the presence of specifically designed complementary polymers which acted as a molecular ‘mortar’. The size of the aggregates prepared in this way depended on the temperature in a controllable way. Thiol-stabilised gold nanoparticles have not only been used as building blocks for larger structures comprising hundreds or thousands of particles but are also of interest as individual large molecules, i.e. so-called monolayer protected clusters (MPCs) [254]. They represent nanoscopic metal surfaces and can be regarded as three-dimensional analogues of two-dimensional macroscopic surfaces. This notion

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has been promoted chiefly by the groups of Murray [254, 255] and Lennox [256, 257] who carried out extensive spectroscopic studies including NMR investigations which are not possible with SAMs of thiols on macroscopic surfaces. Murray and coworkers further explored new routes to functionalised MPCs by ligand place exchange reactions [258-260]. Simple alkane thiol ligands can be partially or completely exchanged by more complex functional thiols in order to introduce, for example, electrochemically active [100, 102, 103] or photoluminescent [104] moieties into the ligand shell. This has opened up a new field of preparative chemistry which is still in a very early stage of development. A particularly elegant study by Boal and Rotello [261] describes the evolution of an optimized flavin binding site on an MPC surface containing two different thiols functionalised with pyrene and diaminopyridine moieties, respectively, diluted by a matrix monolayer of octanethiol. The binding of flavin to diaminopyridine by hydrogen bonding is enhanced by the proximity of a pyrene unit, which can provide an additional binding interaction by aromatic stacking. It also confirms that MPCs are quite dynamic systems that do not only readily undergo ligand place exchange reactions but are also capable of remarkable re-organisation processes in their ligand shell. Many technologies including electronics, separations, and coatings will be enhanced by the ability to control the structure of materials on a nanometer-length-scale. Furthermore, the unique properties of nanoscale materials may give rise to entirely new technologies. One approach for constructing mesoscopic structures is to use solution-phase nanocrystals as “building blocks” [10, 262, 263]. Because nanocrystal diameters can range between 2 and 10 nm, these structures would have characteristic dimensions much smaller than those possible using current lithographic technology. One obvious goal for electronic applications is to achieve the capability to position nanocrystals with a high degree of accuracy. A periodic nanocrystal array, for example, requires the precise positioning of nanocrystals with respect to their neighbors. Encouragingly, this architecture is experimentally attainable and it has been found that hydrophobic, sterically stabilized nanocrystals can be organized into close packed arrays simply by evaporating the solvent from a dispersion, provided that the size distribution is sufficiently tight. This general experimental approach to quantum dot superlattice formation has been shown to apply to a variety of materials, such as Au [264], CdSe [265, 266], Ag [267], Ag2S [268] and γ-Fe2O3 [269] nanocrystals. Up to this point, however, superlattice formation remains highly empirical. Because these arrays could provide the possibility of 1) implementing the unique size-dependent physical properties of individual nanocrystals in a device and 2) eliciting collective electronic and optical properties due to “electronic overlap” resulting from the relative positioning of the nanocrystals in the array [270] there is


55

great interest in developing a fundamental understanding of the intrinsic forces that direct superlattice formation. For example, applications of NIL to realise twodimensional photonic crystals have been recently reported [271]. A major motivation for research in the field of assemblies of nano-particles, droplets, bubbles, fibers and tubes remains the challenge to understand how ordered or complex structures form spontaneously by self-assembly, and how such processes can be controlled in order to prepare structures with a pre-determined geometry. For this purpose it is important to build up a broad experimental database, from which a better fundamental understanding of selforganisation processes and eventually predictive power can be developed. A prerequisite for nanostructure preparation via this self-assembly route is the availability of sufficiently stable building blocks which have to be well-characterised and uniform in size and shape [272]. They should ideally also be chemically versatile enough to undergo a range of reactions allowing them to fulfill various structural and/or functional roles within the final system. Examples of such materials include large organic molecules [273], fullerenes [274], carbon nanotubes [274] and inorganic nanoparticles of insulators [275], metals [276], or semiconductors [167]. Impressive progress has been made, particularly in the assembly of semiconductor quantum dot solids. This is due to the possibility of obtaining certain nanoparticles, as highly monodisperse and stable products. These can crystallise from solution into materials, the electronic characteristics of which reflect the quantum confinement properties of the individual building blocks [266]. Similar materials and a range of interesting self-assembled structures can be obtained from ligand-stabilised metal nanoparticles, which likewise show a fascinating wealth of size-related electronic and optical properties. The materials known as Self-assembled monolayers “SAMs” are formed by allowing appropriate surfactants to assemble on surfaces [277]. They provide synthetic routes to nanometer-thick, highly structured films on surfaces that provide biocompatibility, control of corrosion, friction, wetting, and adhesion, and may offer routes to possible nanometer-scale devices for use in “organic microelectronics”. They have also changed the face of surface science as a research enterprise, moving it from the study of metals and metal oxides in high vacuum to the study of organic materials in circumstances more closely approximating the real world. Self-assembly - a strategy best understood and most highly developed in chemistry - is also offering an appealing strategy for fusing “bottom-up” and “top-down” fabrication, and leading to hierarchical structures of the types so widely found in nature [278, 279]. An alternative strategy for the formation of ordered nanoparticle arrays is the selforganization method based on biomolecular templates by direct or synergistic

56


templating techniques. The template-directed synthesis of nanoparticle arrays in mesostructured silica, as well as helical nanostructures in unusual shaped materials such as chiral lipid tubules were developed. Another strategy to utilize the advantages of self-organization processes by means of noncovalent interactions. The self-assembly of grid-type metal ion architectures can generate 1D, 2D and 3D functional metallosupramolecular arrays. Such arrays combine the properties of their constituent metal ions and ligands, and show unique optical, electrochemical, and magnetic behavior. Successful self-organization is based on the interplay of steric, enthalpic, and entropic factors, in terms of both of the ligands and the metal ions. The multitude of different transition metal cations and organic ligand combinations result in a multitude of different grids with a broad variation of properties. The beauty of such grid-type systems is their geometrical simplicity and their small size, which is about 1 000 times smaller than quantum dot arrays, thus opening up plenty of room for complexity at the bottom. The progress has also been attained in the characterization and application of nanostructured materials using block copolymers. Nanostructure fabrication from block copolymers involves polymer design, synthesis, self-assembly, and derivatization. Block copolymers self-assembled into micelle afford a powerful means of manipulating the characteristics of surfaces and interfaces, and therefore, are expected to have novel structures, properties and applications. For example, nanoparticle fabrication using heterobifunctional poly(ethylene glycol) (PEG) and their block copolymer is explored to construct functionalized PEG layers on surfaces, achieving the bio-specific adsorption of a target protein through an appropriate ligand tethered on PEG layers without non-specific adsorption of other proteins. The properties of polymeric micelles formed through the multimolecular assembly of block copolymers are highly useful as novel core–shell typed colloidal carriers for drug and gene targeting. Surface organization of block copolymer micelles with cross-linking core can exhibit non-fouling properties. The surfaces of these aggregates can work as the reservoir for hydrophobic reagents and can be used in diverse fields of medicine and biology to construct high-performance medical devices and drug delivery systems. Furthermore, by controlling metal and semiconductor structure precisely through the concept to construct functionalized PEG layers, one can modify the nanostructures to better suit their integration with biological systems; for example, modifying their surface layer for enhanced aqueous solubility, biocompatibility, and more importantly biorecognition. The use of exquisite recognition properties of biomolecules in organizing non-biological inorganic objects into functional materials led to new applications including ultrasensitive bioassays and multicolor fluorescent labels for high-throughput detection and imaging.


57

Another intriguing route to complex nanostructures is the use of templates onto which the particles can assemble in a pre-determined fashion. An example of this approach is the selective decoration of phase-separated diblock copolymers with thiol protected gold nanoparticles [280]. Such polymers exhibit complex patterns of microphases of the two different components which can have very different affinities for the adsorption of particles. In the case of poly(styrene-block-methyl methacrylate), for example, only the polystyrene phases are decorated with gold particles, which results in the formation of complex gold nanostructures with the shape and size of the polystyrene microphases [280]. Fitzmaurice and co-workers [279] have discovered the use of a particularly interesting template which allowed them to create continuous tubular gold nanostructures in a two-step process. Following the notion that C60 fullerene molecules attach spontaneously to the surface of certain gold particles in organic solution [280] they readily achieved the decoration of bundles of carbon nanotubes by such particles. Although atoms can be arranged in almost infinite permutations, we can currently make only an infinitesimal fraction of what is possible. Very roughly, if we can pack 100 atoms into a cubic nanometer and each atom can be any of the approximately 100 elements, then there are something like 100100 different ways that we can arrange the atoms in just a single cubic nanometer. A cubic micron expands this to 100 100 000 000 000 . The goal that now seems possible is to take a healthy bite out of this enormous range of possibilities and so make most of the things that are possible.

Abbreviations 1D 2D 3D AFAM AFM BME CFM CNTs CS-AFM DPN E-beam EFM EVOH

one-dimensional two-dimensional three-dimensional atomic force acoustic microscopy atomic force microscopes biomolecular electronics chemical force microscopy carbon nanotubes current sensing atomic force microscopy din-pen nanolithography electron beam electrical force microscopy ethylene-vinyl alcohol copolymer

58

FFM FIB HTL IETS ITO LB LCDs LECs LEDs LFM MEMS MFM MF-STM MPCs MSM MST nDSs MTs NEMS NIL OLEDs PBG PDMS PEDOT PStS PEG PLEDs PMMA PU QD R&D RTGs SAM SEM SERS SFM SMOLED SMS SNOM SPL


friction force microscopy focused ion beam hole-transporting layer inelastic electron tunneling spectroscopy indium tin oxide langmuir-blodgett liquid-crystal displays light-emitting electrochemical cells light-emitting diodes lateral force microscopy micro-electrical-mechanical systems magnetic force microscopy magnetic force scanning tunneling monolayer protected clusters metal-semiconductor-metal microsystems technologies, as known in Europe nanochannel delivery systems microtubules nanoelectromechanical systems nanoimprint-based lithography organic light-emitting diodes produce photonic bandgap poly(dimethylsiloxane) poly(3,4-ethylenedioxythiophene) poly(styrene sullonate) poly(ethylene glycol) polymer-based LEDs poly(methyl methacrylate) poly(urethane) quantum dots research and development radioisotope thermoelectric generators self-assembled monolayer scanning electron microscopy surface-enhanced raman scattering shear force microscopy small-molecule OLEDs single-molecule spectroscopy scanning near-field optical microscopy scanning probe lithography


SPM SP-STM SSIL STM STS TEM TEs TM-AFM TPA UHV w/o

59

scanning probe microscopy spin-polarized scanning tunneling microscopy step-and-stamp imprint lithography scanning tunneling microscopes scanning tunneling spectroscopy transmission electron microscopy thermoelectrics tapping mode atomic force microscopy two-photon absorption ultra-high vacuum water-in-oil

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Chapter 2

Preparation of polymer-based nanomaterials 2.1. Introduction 2.2. Solution or bulk polymerization 2.3. Emulsion polymerization 2.4. Microemulsion polymerization 2.4.1. Micelles and microemulsion 2.4.2. Microemulsion polymerization 2.5. Miniemulsion polymerization 2.6. Dispersion polymerization 2.7. Assemblies Abbreviations References 2.1. Introduction Mixtures of oil, water and an emulsifier (surfactant) have attracted much attention in colloid science. Oil and water are essentially not miscible and coexist as a water and oil phase, each saturated with traces of the other component. Emulsifiers are fairly soluble in one or both solvents but form a true molecular solution of emulsifier monomer molecules at low concentrations only. At higher concentrations of emulsifier monomers aggregate into micelles. Three- or four-component mixtures containing water, oil, an emulsifier and coemulsifier can form not only kinetically stable emulsions but also thermodynamically stable microemulsions. An emulsion is a dispersion of one liquid in another where each liquid is immiscible, or poorly miscible in the other [1]. Emulsions exhibit all classical behaviors of metastable colloids: Brownian motion, reversible phase transitions as a result of droplet interactions that may be strongly modified; and irreversible transitions that generally involve their destruction. They are obtained by shearing two immiscible fluids to the fragmentation of one phase into the other. From diluted to highly concentrated, emulsions exhibit very different internal dynamics and mechanical properties. Emulsifiers are usually added to oil/water mixture to enhance the formation of stable monomer emulsions. The molecules of emulsifier adsorb to the surface of oil droplets during homogenization and provide a protective membrane

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that prevents the droplets from flocculating or coalescing. Under certain circumstances, emulsifiers may have a negative impact on emulsion stability because of their ability to form micelles that enhance mass transport processes, such as solubilization and oil diffusion through the aqueous phase [2, 3]. These mass transport processes can cause significant changes in droplet concentration, composition, and size distribution and may therefore adversely influence the bulk physicochemical properties of an emulsion, such as appearance, rheology, and stability. The mass transport among droplets is typically driven by differences in size and composition because they impose differences in chemical potentials for the solutes in each environment. Solubilization involves the movement of oil molecules from emulsion droplets to the surrounding aqueous medium. The rapid increase of the emulsion industry is connected with the environment concerns to regulate or decrease the content of oil in the aqueous phase. Oil-in-water (o/w) emulsions often result as a consequence of cleaning industrial equipment. Because the lifetime of these emulsions may become significant they become good candidates for various commercial applications. All these applications have already led to an important empirical control of these emulsions, from their formation to their destruction. Emulsions are kinetically stable systems that is to say their free energy of formation is greater than zero, and as such will show a tendency to break. The interfacial tension in emulsions is generally of the order of 1 – 10 mN m-1, this in connection with the large interfacial area results in a large positive interfacial energy term. Emulsions are, however, kinetically stable due to the presence of an adsorbed layer at the o/w interface, this barrier may be electrostatic in nature, or steric. The barriers not only prevent emulsion droplets from coming into the direct contact but also serve to stabilize the thin film of liquid between two adjacent droplets. Emulsions may degrade via a number of different mechanisms such as: creaming with or without aggregation and increase in the droplet size, aggregation with or without creaming, increase in the droplet diameter through the oil diffusion and droplet coalescence leading to the production of a separate oil phase. As a result of their thermodynamic instability, emulsions will tend to reduce their total free energy through an increase in droplet size, so reducing their total interfacial area. Creaming and aggregation do not involve the increase in size of the droplets, but are precursors to coalescence since this process requires the droplets to be in close proximity. Ostwald ripening on the other hand does not require the droplets to be close, since the process occurs by transport of dissolved matter through the dispersion medium [4]. Monomer emulsions are supposed to contain the relatively large (1 – 10 μm) monomer droplets and the much smaller monomer-swollen micelles (10 – 20 nm) or even emulsifier micelles (3 – 5 nm), and hence the surface area of the micelles can be orders of magnitude greater than that of the monomer droplets. Consequently, the

Preparation of polymer-based nanomaterials

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probability of interaction between large monomer droplets is very low, and most interactions appear between droplets and micelles or among micelles. This indicates one of possible ways of mass transfer activity. Kinetically stable nano-emulsions (miniemulsions) are much more stable than the coarse emulsions but less stable than microemulsions. Microemulsions are thermodynamically stable, since the interfacial energy term is now very small owing to the very low interfacial tensions (typically 10-1 – 10-2 mNm-1). Moreover, as a result of their very small size, positive entropy of formation of microemulsion droplets, may be orders of magnitude larger than in emulsions. Polymer (latexes) dispersions are generated by the radical polymerization of unsaturated monomers solubilized in the micellar systems. The significant growth in the production of these latexes is due to a number of factors such as: 1) The water-based dispersions, especially paints and coatings, are mostly environmentally-friendly because they avoid of the environmental problems associated with the solvent-based applications. 2) The emulsion polymerization is fast and proceeds smoothly and controllably within the large range of monomers and reactants and their amounts. 3) The prepared polymers are characterized with very large molecular weights which strongly influence the physical properties of final products. 4) The polymer latexes find a broad applications in different fields. 5) The spherical shape of polymer particles and the uniformity of their size distribution are important for their using in many scientific studies. For example, the uniform spherical particles are ideal experimental materials to test the series of colloidal phenomena as stability and coagulation, rheological properties, light scattering, and so on. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. Polymer latexes are heterogeneous systems which consist of two phases, namely, a dispersion medium and a disperse phase. For example, conventional emulsion polymerization systems comprise a continuous aqueous phase and a dispersed oil (polymer) phase. Inverse emulsion systems comprise the continuous oil phase and the dispersed aqueous phase. The dispersion medium is known as the continuous phase or the external phase. As shown, it is aqueous in nature or organic in the case of inverse systems. The disperse phase of a latex is known as the discrete phase, the internal phase and the dispersed monomer or polymer. The polymer latex comprises

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of a large number of separate small particles which are usually spherical in shape. The molecules which are formed by the radical polymerization in the particles are mostly very large polymers of the molecular weigh above 106. The sizes of polymer particles can vary widely according to the reaction approach. The range of diameters for the latex particles prepared by the microemulsion polymerization is ca. 10 – 50 nm. A lower limit of ca. 50 nm and an upper limit of ca. 200 nm are generally obtained by the miniemulsion polymerization. The upper limit of ca. 5000 nm (so called coarse polymer dispersions) can be obtained by the conventional emulsion polymerization. According to the surface charge of polymer particles, lattices are classified into three broad groups: anionic lattices, cationic lattices and non-ionic lattices [5]. The formulation of polymeric nanoparticles (polymer dispersions) dispersed in a non-solvent media are interesting from both academic and industrial aspects. Theses materials are prepared by (macro)emulsion, miniemulsion or microemulsion polymerization in situ or by seeded approach. The emulsion polymerization is broadly used for the production of homopolymers and copolymers by the radical polymerization of unsaturated monomers with similar or different water-solubility. The kinetics and mechanism of hydrophilic unsaturated monomers and especially the copolymerization of a set of monomers with a different water-solubility are very complex. Due to this kinetics, serious disadvantages, such as the lack of homogeneity and restrictions in the accessible composition range, are accompanied. In order to overcome these disadvantages, one has to perform a heterophase polymerization where small, homogeneous, and stable droplets of monomer or polymer precursors are generated, which are then transformed by (as many as possible) polymer reactions to the final polymer latexes, keeping their particular identity during the whole polymerization process.

2.2. Solution/bulk polymerization Water- and oil-soluble polymers are commercially available or can be synthesized by different routes. Among the most important requirements for technological applications of the water-soluble polymers, there are the high solubility in water, and easy and cheap route of synthesis, an adequate molecular weight and molecular weight distribution, chemical stability, high affinity for one or more metal ions, and selectivity for the metal ion of interest. The most usual synthetic procedures


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are addition polymerization, especially radical polymerization, and functionalizing of polymer backbones through polymer-analogous reactions. The macromolecules can be homo- or copolymers, and may contain one or more coordinating and/or charged groups. These groups are placed at the backbone, or at the side chain, directly or through a spacer group and includes the following structures (polymers): -Poly(N-hydroxyethyl)ethyleneimine (PHEI) [6, 7]. -Poly(N-acetyl)ethyleneimine (PAEI) [8]. -Poly[(N-hydroxyethyl)ethyleneimine-co-N-acetyl) ethyleneimine] (PHEI-co-PAEI) [7]. -Poly(ethyleneimine) (PEI) [7, 9]. -Poly(allylamine) (PALA) [10, 11]. -Poly(acrylamide) (PAAm) [12]. -Poly(acrylamide-co-N-maleylglycine) (PAAm-co-MGly) [13]. -Poly(acrylic acid) (PAA) [12, 14]. -Poly(acrylamide-co-acrylic acid) (PAm-co-PAA) [15, 12]. -Poly(N,N-dimethylacrylamide-co-acrylic acid) (PDAm-co-PAA) [12]. -Poly(methacrylic acid) (PMA) [10]. -Poly(α-acetylamino acrylic acid) (PAAA) [16]. -Poly(N-methyl-N’-methacryloylpiperazine) (PAP) [17]. -Poly[(3-(methacryloylamino)propyl)trimethylammonium chloride] (PMPTA) [18, 19]. -Poly(diallyl dimethylammonium chloride) (PDDA) [18, 19]. -Poly(sodium 4-styrenesulfonate) (PStS) [11, 20]. -Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAPS) [10, 21]. -Poly(methacrylic acid-co-2-acrylamido-2-methyl-1- propanesulfonic acid) (PMAco-PAPS) [10, 21]. -Poly(N-methyl-N’-methacryloylpiperazine-co-a-aminoacrylic acid) (PAP-coPAAA) [22]. -Poly(acrylamide-co-N-vinylpyrrolidone) (PAm-co-PVPyrr) [23]. -Poly(N,N-dimethylacrylamide-co-N-vinylpyrrolidone) (PDAm-co-PVPyrr) [24]. -Poly(1-vinylpyrrolidone-co-2-dimethylaminoethylmethacrylate quaternized) (PVPyrr-co -PDAEM) [19]. -Poly(4-vinylpyridine quaternized) (PVPyQ) [25]. -Poly[2-acrylamido-2-methyl-1-propanesulfonic acid-co(3(methacryloylamino)propyl) trimethylammonium chloride] (PAPS-co-PMPTA) [18, 19]. -Poly(vinylphosponic acid) (PVPhA) [18, 19]. -Poly(N-methacryloyl-4-aminosalicylic acid) (PMAAMSA) [26]. -Poly[acrylamide-co-1-(2-hydroxyethyl)aziridine] (PAM-co-PHEA) [27].

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Some mentioned polymers have charged groups, or easily ionizable groups in aqueous solution, while another bear functional groups with the ability to form coordination bonds. The most investigated ligands present in the polychelatogens are amines, carboxylic acids, amides, alcohols, aminoacids, pyridines, thioureas, iminos, etc. Among them, polymers containing amino groups have been extensively studied by ultrafiltration, particularly the functional polyethylenimine [7]. This heterochain polymer contains three different types of amino groups: secondary and tertiary groups in the main chain and secondary and primary amino groups in the side-chain. The most studied polyelectrolytes include those with carboxylic acid, phosphoric acid, sulfonic acid, or ammonium groups in their structure. One versatile route for the synthesis of different (random, block, alternating) copolymers is copolymerization. With a good selection of both comonomers it is possible to improve properties such as water-solubility, metal ion binding capability, and selectivity. A water-soluble polymer solution may be considered as a two-phase system. The polymeric functional groups are kept separated by a distance contained in a range so that they present a local high and nearly constant concentration. The water-soluble polymers undergo interactions with solvent and other high- and lowmolecular weight species present in the aqueous solutions. Due to these interactions, macromolecules in solution exhibit, beyond their chemical structure, different properties such as conformation of the polymer chains, excluded volume, surface activity, or formation of higher-order structures as aggregates, gels, etc. Polyacrylamide and its copolymers with very high molecular weights have gained more and more technical importance for use in many fields. However, for a number of fields of application, the introduction of reactive ionogenic groups into the composition of polyacrylamide (PAAm) macromolecules is promising. Of most interest is the introduction of sulphonic acid groups by copolymerization of acrylamide with salts of styrene sulphonic acid, which improves the flocculating and surface-active properties of the polymers and gives them anti-static, ion-exchange and other valuable properties. Poly-N-isopropylacrylamide (poly-NIPAM) is attracting a great deal of attraction because of its thermosensitivity [28]. PolyNIPAM in aqueous medium has its lower critical temperature (LCST) at 32 oC and poly-NIPAM gel drastically changes its volume at the LCST. The phenomenon is caused by the reversible formation and cleavage of the hydrogen bond with temperature change. Extensive formation of hydrogen bonds between amide group and surrounding water molecules below LCST brings about extensive swelling, and cleavage of the bond above the LCST results in deswelling. The above-mentioned response of poly-NIPAM to a temperature change is believed to change the surface of the polymer from a hydrophilic to a hydrophobic one.


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Copolymerization of acrylamide with multifunctional monomer (crosslinker) leads to the formation of polyacrylamide gels [29]. They have very interesting properties such as the ability to absorb a large amount of water relative to their weight, the sharp collapse transition exhibited in acetone/water mixtures, and biocompatibility of the gels. Numerous potential applications of acrylamide and substituted acrylamide gels have been suggested, including use in electrophoresis, use in immobilization and/or controlled release of biological molecules [30], use in bioseparations [31] and use for moisture retention in soil [32]. Hydrophobically associating polymers consist of a water-soluble polymer (PAAm) containing a small number of hydrophobic groups [33]. In aqueous solution, above a certain polymer concentration, intermolecular hydrophobic interactions lead to the formation of polymolecular associations. As a consequence, these copolymers exhibit thickening properties equivalent to those observed for higher molecular weight homopolymers. The reversible association/ dissociation process gives rise to particular rheological behaviors as a function of shear rate or shear time. Preparation of such materials can be carried out, as for any copolymer synthesis, either by chemical modification of a preformed polymer or by copolymerization of the appropriate monomers or by a combination of both approaches. The very interesting copolymers are the block copolymers characterized by the selfassembling in some solvents which leads to a variety of morphologies. In view of the utilization of these structures, control over size and topology has become an important goal. Particulate structures such as micelles of various shapes as well as ordered, continuous morphologies like lamellae, ordered cylinders, or bicontinuous structures can be prepared. In solvents that selectively dissolve only one of the blocks, AB-diblock copolymers form well-defined micelles with a core consisting of the insoluble block, A, and a shell or corona of the soluble block, B. Spherical and cylindrical micelles as well as more complex, vesicular structures have been described. Most of the block copolymers that have been investigated form micelles either with the more polar or with the more non-polar block pointing outwards, i.e., regular or inverse micelles, depending on the polarity of the solvent. The area of micellization in non-aqueous media has been reviewed by Tuzar and Kratochvil [34] and by Gast [35]. The area of aqueous media was reviewed by Chu [36] and Alexandridis [37]. Two reviews on ionic block copolymers were published by Selb and Gallot [38] and Eisenberg and coworkers [39]. Also the internal structure of block-copolymer micelles, as given by the size of core and corona and the density profile in each domain, has been carefully characterized by static and dynamic light scattering [40] and by small angle neutron scattering

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using contrast variation techniques [41]. The micellar corona has many of the characteristics of a spherical polymer brush. This allows a quantitative understanding of the corona density profile enabling one to improve or adjust the steric stabilization of polymeric or inorganic colloids in a number of applications [41]. Also the interaction potential of block-copolymer micelles has been determined experimentally [42], which allows the rheological properties and ordering phenomena occurring at large micellar concentrations to be predicted. In most cases block copolymers form spherical micelles in dilute solution. In only a few studies was the formation of non-spherical aggregates reported. For example, cylindrical or worm-like micelles were observed for polystyrene-polybutadienepolystyrene (PSt-PB-PSt) triblock copolymers in ethylacetate [43] PSt-PI in N,Ndimethylformamide (DMF) [44] or poly(ethylene oxide) – poly(propylene oxide) – poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers in aqueous solutions [45]. Conditions for the formation of non-spherical micelles currently seem to be clear only for ionic block copolymers. Due to enormous interfacial tension these systems are in a thermodynamic state close to the super-strong segregation limit (SSSL) [46]. Under these conditions, a sequence of shape transitions from spherical cylindrical - lamellar is possible. Such transitions can be induced by increasing the ionic strength of the solution or by increasing the relative length of the core block. This behavior is completely analogous to the phase diagrams of non-ionic alkyl poly(glycol ether)s when the number of ethylene oxide (EO) groups is reduced. 2.3. Emulsion polymerization An emulsifier (surfactant) is a molecule that posses both polar and non-polar moities, i.e., it is amphiphilic. In very dilute water solutions, emulsifiers dissolve and exist as monomers, but when their concentration exceeds a certain minimum, the so-called critical micelle concentration (CMC), they associate spontaneous to form aggregates – micelles (Fig. 1 A). The formation of micelles is controlled by the chemical equilibrium between emulsifier monomers and larger micellar aggregates. At low concentrations the emulsifier dissolves as free monomers but as soon as the emulsifier concentration exceeds the CMC the monomer concentration remains roughly constant and the emulsifier aggregates into micelles.


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Fig. 1. Scheme of emulsifier micelle (unswollen (A) and swollen (B) with oil).

In aqueous solutions, at concentrations not too large with respect to the CMC, say in the range CMC to 10 CMC, ionic emulsifiers form spherical or close to spherical micelles [47]. Micelles are responsible for many of the processes such as: 1) enhancement of the solubility of organic compounds in water (Fig. 1 B), 2) catalysis of many reactions, 3) alteration of reaction pathways, rates and equilibria, 4) reaction loci for the production of polymer products, etc. The outer-core region of the micelle, commonly referred to as the palisade layer, may provide a medium of intermediate polarity that effects the energetics of transition state formation. The primary influence of micelles is to concentrate all reactants in or near the micelles. When ionic surfactants are employed, polar or ionic reactants that are freely soluble in water, may also be concentrated near the micelles by electrostatic or dipole interactions [48]. Nonpolar compounds partitions into the micelle core (Fig. 1) while more polar compounds are formed closer to the micellewater interface. The extent of solubilization, ionic charge of micelle, and the shape of the micelle are also important factors. Micelles are responsible for many of the processes such as enhancement of the solubilization of organic compounds in water (oil-in-water (o/w) emulsion). The formation of o/w micelles is driven by strong hydrophobic interactions of the hydrophobic tail of the emulsifier molecule (o/w micelle). The strong hydrophobic interaction between hydrophobic chains arranges a large number of amphiphiles into the aggregate (micelle). Emulsion polymerization involves dispersion of a relatively water-insoluble monomer (e.g., styrenes, alkyl methacrylates, etc.) in water with the aid of

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emulsifiers, followed by the addition of the water-soluble (e.g., ammonium peroxodisulfate, APS) or oil-soluble (e.g., dibenzoyl peroxide, DBP) initiator. The APS-initiated emulsion polymerization is a two step process (Fig. 2): 1) The first step includes the formation of primary radicals and their transformation to the surface active oligomeric radicals through the addition of monomer units to the growing radical. 2) The second step involves the entry of oligomeric (surface active) radicals into the monomer-swollen micelles (micellar nucleation) or the precipitation of growing radicals (homogeneous nucleation) from the aqueous phase [49-52]: decomposition of initiator (I) in the aqueous phase: I (APS)

⎯→ 2 R.

(1)

water-phase propagation: R. + M ⎯→ RM. ⎯→ RMn. ⎯→ RMz.

(2)

entry of surface active oligomeric radical (RMz.) into the monomer-swollen micelle or polymer particle: RMz. + particle (micelle) ⎯→ RMj. (or active particle)

(3)

Here R. is the charged primary radical derived from peroxodisulfate initiator (I), M monomer in the water phase, RM. and RMn. growing radicals, RMz. the surface active radical with a high degree of hydrophobicity and RMj. the primary particle. The surface active radical enters the polymer particle or monomer swollen micelles, and start the polymerization. Hydrophilic SO4−• radicals derived from peroxodisulfate do not enter the hydrophobic monomer-swollen micelles or polymer particles (Fig. 3). The aqueous phase polymerization generates the surface active oligomeric radicals (RMz.) which can enter the hydrophobic polymer particles. The charged oligomeric radical enters the polymer particle in such a way that the charged group projects to the aqueous phase while the hydrophobic radical end penetrates into the particle surface layer.


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Fig.2. The micellar model for the polymer particle formation (where MD denotes monomer droplet, MP monomer-polymer particle, Mic micelle, and E emulsifier) [49].

This assures negligible repulsion between charged (surface active) oligomeric radicals and charged (hydrophilic) particle surface with the same charge.

Fig.3. Reaction scheme for the radical entry into hydrophobic particles.

Homogeneous nucleation [53, 54] as well as coagulative nucleation [52] are supposed to be operative in the reaction systems which contain the hydrophilic monomer(s), a small amount of emulsifier (below CMC) and interactive particles. In the former, the oligomeric radicals do not enter the polymer particles but propagate

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until they reach a critical degree of polymerization, Pcrit (RMz.), whereupon they become insoluble (precipitate from the aqueous phase) and form primary particles (RMj.) that grow by absorbing monomer and emulsifier: RMn. + M ⎯→ RMz. ⎯→ RMj. (primary particle)

(4)

The unstable primary particles (RMj.) aggregate between themselves and so form larger and more stable particles or flocculate with larger premature particles. In the absence of emulsifier, the flocculation of surface active oligomers governs the particle nucleation, that is, the oligomeric radicals will self-nucleate to form primary particles. The restricted termination of growing radicals within the monomer/ polymer particle leads to a very fast polymerization and the formation of large polymers. The submicron (monomer) polymer particles can be stabilized by the electrostatic repulsion force provided by ionic emulsifier or steric repulsion force provided by nonionic emulsifier to override the van der Waals attraction force between the interactive particles. Ionic emulsifiers are known to form small monomer-swollen micelles (the large surface) and the thin surface layer of polymer particles. Anionic emulsifiers act as a strong particle generator. Nonionic emulsifiers, however, are supposed to be less effective for nucleating and stabilizing the particles while they can act as an auxiliary emulsifier [55]. Nonionic emulsifiers provide latex particle with excellent stability towards high electrolyte concentrations, freeze-thaw cycling and high shear rates [56]. Stabilization of colloidal dispersions can be divided into the two basic mechanisms: electrostatic and steric (Fig. 4) [57]. With the van der Waals-London attractive forces acting continuously between colloidal particles, it is necessary, in order to maintain stability, to introduce a repulsive force (electrostatic and steric) to outweigh the attractive force. The electrostatic stabilization provides the repulsive forces between similarly charged electrical double layers to the interactive particles [58, 59] (Fig. 4). Thus, the electrical double layer imparts the electrostatic stabilization. The steric stabilization becomes important when there are hydrophilic macromolecules or chains adsorbed or bounded to the particle surface [60]. When the layers of two interacting particles overlap the concentration of these macromolecules (chains) increases as well as free energy. The molecules of good solvent enter the overlap layer and then separate the particles. This phenomenon is accompanied with the increased osmotic pressure.


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The excellent ability of nonionics to solubilize and disperse hydrophobic soils such as fats, mineral oils, etc. in water leads to extensive use of this type of emulsifier. Their often superior detergency with respect to solids surfaces is due to a combination of relatively low critical micellar concentration (CMC), allowing emulsification to take place at low emulsifier concentrations, and an ability to adsorb hydrophobically to interfaces and thus, by steric repulsion forces, to disperse hydrophobic liquid or colloid matter. An important group of nonionic emulsifiers is based on ethoxylated alkyl alcohols. Increasing demands for biodegradability and low aquatic toxicity of degradation products of industrial chemicals is expected to make fatty alcohols ethoxylates and nonionic emulsifiers based on natural raw materials an even more important group of chemicals in the future.

Fig.4. Scheme of steric and electrostatic stabilization (where R is the radius of particle core and σ the thickness of particle shell).

An interesting group of nonionic emulsifier is the polymeric type. Its adsorption depends on the chemical nature of the colloidal particles and the hydrophobic group of the emulsifier. Larger amounts of the polymeric amphiphile are expected to be adsorbed when the chemical nature of the hydrophobic group of the amphiphile is similar to the chemical nature of the particles. The stabilizing capacity of the polymeric emulsifier should be dependent on the amount of amphiphile attached to the colloidal particles. These results are anticipated because of the stabilization mode of nonionic polymeric emulsifier. In this case, stability arises from the repulsive forces associated with free-energy changes when polymer segments are mixed together. This free energy of mixing is depending on the concentration of the polymeric segments or the stabilizing moieties. Napper [61] has illustrated that for maximum stabilizing effectiveness in colloidal dispersions the hydrophobic group or anchoring moiety posses a high affinity for the dispersed phase and the hydrophilic

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group or stabilizing moiety must be soluble in the continuous phase. It is widely accepted that the stabilizing effectiveness of emulsifier in the micellar polymerization is related to the number of particles and the rate of polymer or polymer particle formation. The process of particle formation in the emulsion polymerization of a relatively water-insoluble monomer stabilized by anionic emulsifier can be described by the conventional micellar model (Fig. 2) [49]. According to this model, micelles formed by aggregation of emulsifier molecules when the emulsifier concentration is above the CMC, are the principal locus of polymerization. Based on this particle nucleation mechanism, the well-known Smith-Ewart approach predicts that the number of particles nucleated is proportional to both the emulsifier and initiator concentrations. The efficiency of ionic emulsifier depends on several parameters such as the reaction conditions, ionic strength, pH, the type of monomer, initiator, and polymer formed, etc. This dependence can be a major drawback in terms of the stability of the polymer latex. The sterically stabilized polymer particles are more interactive but less stable than the electrostatically stabilized polymer latexes [62]. The behavior of entering radicals is more complex since they are less mobile in the aggregates. In the emulsion polymerization stabilized by mixed anionic/nonionic emulsifiers the competitive adsorption between ionic and nonionic emulsifiers can be the important parameter in the determining the polymer particle size and the rate of polymerization. The addition of small amount of nonionic emulsifier (e.g., Triton X-405, Tr405) to the particles covered with ionic emulsifier (sodium dodecyl sulfate, SDS, close or below CMC) can even be detrimental, since Tr405 was shown to remove some of SDS at low emulsifier contents [63]. Furthermore, the shielding of the charged groups by poly(ethylene oxide) (PEO) chains and the relatively flat conformation of PEO (Tr405) at particle surface can induce the interparticle association. Competitive desorption experiments performed with both emulsifiers (SDS and Tr405) showed that SDS was observed to desorb more readily than Tr405. The particle surface composition was much richer in Tr405 as the total emulsifier concentration decreased. The molar free energy for adsorption for Tr405 is − 36 kJ/mol while for SDS is − 25.6 kJ/mol [64]. At higher concentration of nonionic emulsifier, a more extended conformation has been observed [65]. The increased close packing of extended PEO chains increased the colloidal stability. Nonionic emulsifier exhibits a different behavior in particle nucleation and growth from the anionic counter partner. The emulsion polymerization mostly leads to the formation of polydisperse polymer particles. The monodisperse polymer particles, however, can be formed under certain reaction conditions even during the sterically or electrosterically stabilized emulsion polymerization [66].


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Direct emulsion is formed by emulsification of a water-immiscible monomer (oil) in a continuous aqueous phase using an hydrophilic (oil-in-water) emulsifier. The conventional emulsion polymerization using a water-soluble or oil-souble initiator gives a colloidal suspension of polymer particles in water. The average particle size of conventional latexes is usually 100 – 300 nm in contrast to the original emulsion droplet size of 1000 – 10 000 nm. The emulsion polymerization reaction can be divided into particle nucleation and particle growth intervals. The particles are nucleated by micellar or homogeneous nucleation mechanism and then grow until the supply of monomer is exhausted. According to the micellar model [49], radicals generated in the aqueous phase enter the polymerization. This mechanism was corrected by the additional statement that the primary radicals first initiate the aqueous phase polymerization and then the surface active oligomeric radicals formed enter monomer-swollen micelles and initiate polymerization in the particles. Only a very small fraction of micelles (one from 100 – 1000) captures oligomeric radicals and becomes polymer particles. The unnucleated micelles give up their monomer and emulsifier to nucleated micelles (growing particles). The end of particle nucleation interval (Interval 1) is given by the disappearance of the micelles (Fig. 5). The monomer droplets act as monomer reservoirs, feeding monomer to the growing polymer particles by diffusion through the aqueous phase. Interval 1 is difficult to reproduce while it depends on many parameters such as the type and intensity of agitation, the temperature, the shape of reaction vessel, the rate of radical generation, etc. On the contrary, Interval 2 is mostly well reproducible. The particle nucleation stage can be avoided by using the seeded polymer particles. Seeded emulsion polymerization is used to give the desired concentration of polymer particles as well as their size. Moreover, two-stage or multiple-stage emulsion polymerization can be used to produce core-shell particles and control the extent of grafting between different stages of the polymerization. This seeded process is broadly used in industry to tailor the polymer latex to the specific application. The polymer particles with heterogeneous structure (e.g., core-shell, rusberry-like...) can be formed by the classical batch emulsion copolymerization of unsaturated monomers with different reactivities and hydrophobicities. For example, the emulsion copolymerization of butyl acrylate and vinyl acetate gave polymer particles with a butyl acrylate-rich core and a vinyl acetate – rich shell [67]. The similar structure of polymer particles were reported in the emulsion copolymerization of acrylonitrile with hydrophobic unsaturated monomers such as butyl acrylate [68].

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Fig. 5. Scheme of three Intervals in the emulsion polymerization process.

A compositionally heterogeneous structure of polymer particles can be also obtained by the two - stage emulsion polymerization. In this approach, a monomer polymerizes in the emulsion system containing the seed polymer particles and other components of the reaction system such as initiator and emulsifier. The polymer particles are swollen by the monomer and the addition of initiator starts the polymerization and the accumulation of second polymer either within the particle shell or in the particle core. The degree of swelling, the solubility of polymer in the added monomer and the compatibility of seeded polymer with formed polymer, the ratio of polymer phases and the type of polymer used in the first stage decide the structure of final composite particles. In the limiting case, one can obtain the coreshell structure or a structure of complete separation. The core-shell particles can be obtained under the following reaction conditions: 1) the use of water-soluble initiator, 2) addition of monomer under monomer-starved conditions, 3) sufficiently high seed particle concentration, 4) lipophilicity of the seed particle core in comparison with the hydrophilicity of the second monomer, and 5) incompatibility of the core and shell polymers [51].


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The seeded emulsion polymerization offers the following possible particle morphologies [69]: 1) if polymer A of the seeded particle is insoluble in the monomer B, then polymer B formers surface layers on the seeded particles A, 2) if polymer B is miscible with polymer A and if both have the same hydrophilicity, then core-shell polymer particles are formed. The amount of polymer B in the particle shell will be greater than that of polymer A, 3) if monomer B swells polymer A, but polymer B is immiscible with polymer A, then phase separation in the polymers in the particle takes place, 4) if polymer B is more hydrophilic than polymer A, a core-shell structure can be formed, the shell being mostly formed by polymer B, 5) if polymer A is crosslinked, then polymer core A is surrounded by B-rich shells, and 6) if polymer A is more hydrophilic than polymer B, then polymer B forms separated phases in polymer A (inverse structures). The emulsifier-free emulsion polymerization is used to prepare clear and monodisperse latex particles. The necessary stabilization of the polymer particles is achieved by the accumulation of ionic groups of reactants on the surface of polymer particles. For example, the electrostatic stability of the polymer particles is secured by –OSO3− group from peroxodisulfate initiator. The similar stabilization approach can be observed with the ionic and hydrophilic comonomers in the emulsifier-free emulsion copolymerization of hydrophobic monomer. It was confirmed that the primary particles are formed by a mechanism of homogeneous coagulative nucleation [51]. Reese and Asher [70] have developed emulsifier-free, emulsion polymerization recipes for the synthesis of highly charged, monodisperse latex particles of diameters between 500 and 1100 nm. For example, the poly[styrene-(co2-hydroxyethyl methacrylate)] spherical particles consists of polymer cores whose surfaces are functionalized with sulfate and carboxylic acid groups. The synthesized highly charged particles are spherical and relatively monodisperse up to diameters of ∼ 900 nm (percentage polydispersity < 5), but above 900 nm (percentage polydispersity > 5), the particles become more polydisperse. The particle diameter increases as the reaction mixture ionic strength increases [71]. This dependence on ionic strength occurs because the particles grow via a coagulation process. At low ionic strengths, the particles repel each other early in the reaction and do not collide and fuse into larger particles. At higher ionic strengths, larger particles are formed because increased charge screening allows more particle fusions. The particles repel each other only later in the reaction as they accumulate more surface charge; thus, a larger final diameter occurs.

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2.4. Microemulsion polymerization 2.4.1 Micelles and microemulsion It was observed that the titration of a coarse emulsion by a coemulsifier leads in some cases to the formation of a transparent microemulsion. During the titration of an emulsion with a coemulsifier, the system often undergoes viscosity changes before clearing. Upon addition of coemulsifier the viscosity of the fine - emulsion varies (increases and then decreases) and when the system cleared the viscosity of the microemulsion mostly increased again. The transparence of such system is mostly > 85%. During the addition of a coemulsifier to a coarse emulsion, excess coemulsifier accumulates at the o/w interface. The titration of coarse emulsion by a coemulsifier solution leads in some cases to the formation of a transparent microemulsion structure. The transition from opaque emulsion to transparent solution is spontaneous and well defined. Zero or very low interfacial tension obtained during the redistribution of the coemulsifier plays a major role in the spontaneous formation of microemulsion. As soon as the interface curls and droplets are formed, the interfacial tension increases. Spontaneous microemulsion formation is a function of the selection of primary emulsifier and coemulsifier and the right procedure capable of favouring redistribution between phases. It is not only dependent on simple thermodynamic stability but also on the occurrence of kinetic conditions favourable to the dispersion of the dispersed phase into the o/w system. Depending on the proportion of suitable components and hydrophilic-lypophilic balance (HLB) value of the surfactant used, the formation of microdroplets can be in the form of oil-swollen micelles dispersed in the aqueous phase as for the o/w microemulsion or water-swollen micelles dispersed in oil as the w/o microemulsion (reverse microemulsion). In the intermediate phase region between o/w and w/o microemulsions, there may exist bicontinuous microemulsions whose aqueous and oil domains are interconnected randomly in the form of sponge-like microstructures. In addition to single-phase microemulsions, several phase equilibria are known, namely Winsor systems [72]. Winsor classified the microemulsion systems into three types: 1) the o/w in equilibrium with oil (such as micelles), 2) the w/o in equilibrium with water (such as reverse micelles), and 3) bicontinuous (o+w) in equilibrium with oil and water. These systems are also called Winsor (W) microemulsion types I, II, and III, respectively. Formation of each of these systems depends on the conditions and composition of the phases and each type can be formed from another type by varying one or more of the parameter(s) of the system (salt, coemulsifier, emulsifier structure


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and their concentrations). Thus, depending upon the proportions of components and the influence of certain parameters (ionic strength, temperature, nature of the oil, emulsifier and coemulsifier, etc.), the phase diagram presents single or multiphase domains. In W I systems, o/w microemulsion is in equilibrium with excess oil, while W II systems consist of w/o microemulsion in equilibrium with excess water (Scheme 1). W I and W II microemulsions are of globular form, while the W III middle phase is thought to be bicontinuous with an interface showing a constant mean curvature [73]. All of these systems remain a certain microenvironment; the application of W I and W II type systems in the synthesis of “quantum dot” particles is expected to avoid the limitations that may arise due to solubility constraints. It is known that the flexibility of the surfactant films, presence of additional stabilizing agents, and concentration of the reactants influence the final size of the product particles irrespective of the size of the microdroplets [72].

Scheme 1. O/w and w/o microemulsions and bicontinuous phase.

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The shape of micellar aggregates and the formation of microemulsion can be controlled and understood from the packing parameter of emulsifier molecule in the micellar assembly v/a 1, where v is the emulsifier hydrocarbon volume, a is the polar head area, and l is the fully extended chain length of the emulsifier. When the ratio v/a l is larger than unity, the aggregate curvature will be toward the water. This corresponds to a situation where the oil is penetrating the emulsifier tails and/or the electrostatic repulsion between the charged head group is low. When the ratio is less than unity we have a situation where the electrostatic repulsion is larger and/or the oil is not penetrating the emulsifier tails [74]. Spherical direct micelles are formed when the packing parameter is less than 1/3. The limiting values for packing parameters for cylinders and planar bilayers are 1/2 and 1, respectively. Reverse micellar structures are formed within the right solvent when the packing parameter is greater than 2 (cylinders up to v/a.l ≤ 2 and spherical micelles when v/a.l > 3). When oil is solubilized in hydrophilic micelles, or water in hydrophobic micelles, one can observe the formation of o/w microemulsions for v/a.l < 1; or w/o microemulsions for v/a.l > 1. When v/a.l ≈ 1 lamellar phases or bicontinuous microemulsions are observed [75]. Microemulsion formation involves: 1) a large increase in the interface (e.g., a droplet of radius 120 nm will disperse ca. 1800 microdroplets of radius 10 nm - a 12-fold increase in the interfacial area), and 2) the formation of a mixed emulsifier/coemulsifier film (complex) at the oil/water interface, which is responsible for a very low surface tension (γi). Microemulsions (monomer swollen micellar solution, micellar emulsions, or spontaneous transparent emulsion) are dispersions of oil and water made with emulsifier and coemulsifier molecules. In many respects, they are small - scale versions of emulsions. They are homogeneous on a macroscopic scale but heterogeneous on a molecular scale. They consist of oil and water domains which are separated by emulsifier monolayers. W/o microemulsion solutions are mostly transparent, isotropic liquid media with nanosized water droplets that are dispersed in the continuous oil phase and stabilized by surfactant molecules at the water/oil interface. These surfactant-covered water pools offer a unique microenvironment for the formation of nanoparticles. They not only act as microreactors for processing reactions but also exhibit the process aggregation of particles because the surfactants could adsorb on the particle surface when the particle size approaches to that of the water pool. As a result, the particles obtained in such a medium are generally very fine [76]. Inverse microemulsion droplets, however, are slightly polydisperse due to less strict transformation of


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monomer to assembly form. The microemulsion is thermodynamically stable phase and therefore the polydispersity is an equilibrium property. The microdroplets collide, form transient aggregates, and then revert to isolated droplets. Aggregate lifetimes are typically of the orders of microseconds. The dynamics of the exchange of solute between micelles and the continuous phase is characterized by the rate constant for entry of the solute into the micelle. This process is diffusion controlled, as is the entry of emulsifier molecules into the micelle. Under certain critical conditions, molecules can be transported from one droplet to another without going through the continuous phase. A possible process involves collisions and transient merging of the droplet cores. At low concentration of the dispersed phase, the dispersion is mostly composed of identical spherical isolated droplets. At higher concentrations, the structure of the system depends on the interactions between droplets. If they are repulsive, the collisions are very short and no overlapping between interfaces of colliding droplets occurs. If the interaction are attractive, the duration of collisions increases, and transient clusters of droplets are formed. Interfaces overlapping occurs during collisions, allowing exchanges between touching droplets. These exchanges are achieved by hopping of ions or molecules through the interfaces, or by transient opening of these interfaces with communication between the water cores of the droplets. The electrical conductivity of the w/o microemulsion is an ideal approach to study the percolating events. As the continuous phase of w/o systems is not conducting, electrical conduction needs contact of droplets to allow charge transfer between them. This transfer can be achieved by charge hopping, or transient merging of connected droplets with communication between the water cores [77]. The conditions for proceeding of such events is that the droplet interactions are strongly attractive [78]. When this connectivity is achieved, a steep increase of the conductivity is observed, which has been analyzed as the percolation process, with the percolation threshold φper. Under percolation threshold conditions, water pools of inverse droplets can communicate within the microemulsion system. The transfer of inorganic salts in reverse (w/o) microemulsions has received considerable attention for preparing semiconductor and metal particles [73, 79]. One of the powerful techniques for obtaining the ultrafine particles is based on the use of microemulsions as microreactors in order to control the growth of the particles [80, 81]. For the purpose of the method described for obtaining ultrafine particles, water-in-oil (w/o) microemulsions used are formed by nanodroplets of water dispersed in oil. The size of the microemulsion droplets can be modified in the range 5 – 50 nm by varying the relation of the components of the microemulsion (e.g., changing W = [water]/[stabilizer] in the recipe) or by varying the microemulsion

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itself. Monodispersity of particles and stabilization of particles are very important criteria in controlled synthesis. The volume fractions of oil and water were not that important and that the microemulsion type and stability were determined primarily by the nature of the emulsifier. There are three corner - stones guiding practical microemulsion formulation which address this problem: 1.The Bancroft rule [82], 2.Griffin HLB scale [83] and 3. Shinoda phase inversion temperature [84]. According to Bancroft, the phase in which the emulsifier is predominantly dissolved tends to be the continuous phase, water - soluble emulsifiers tend to stabilize o/w emulsions, while oil - soluble surfactants stabilize w/o emulsions. Griffin suggested an empirical hydrophilic - lipophilic balance (HLB) scale which characterizes the tendency of emulsifiers to form o/w and w/o microemulsions. Emulsifiers with low HLB values (ca. 4) tend to stabilize w/o emulsions, while those with high HLB values (ca 20) stabilize o/w emulsions. The HLB approach does not take into account, however, the effects of temperature and the nature of the oil on emulsion stability. According to Shinoda and Friberg o/w emulsions are stable in the Winsor I region at temperatures ca 20 oC below the phase inversion, w/o emulsions are stable above the phase inversion temperature (PIT) (in the Winsor II region). In the vicinity of the phase inversion temperature (PIT) point (Winsor III region), where oil, water and bicontinuous microemulsion phases coexist in a three phase equilibrium, neither emulsion is stable. If the HLB shifts to hydrophilic, the amount of water swelled between emulsifier aggregates increases rapidly and that of oil decreases, and vice versa. The HLB investigations led to the existence of a w/o microemulsion (3 < HLB < 7) followed by a phase inversion domain (7 < HLB < 9) and by an o/w microemulsion (9 < HLB < 17). Meanwhile, experimental evidence of the existence of bicontinuous (zero and near - zero average mean curvature) structures was found, especially in cases where the microemulsions are in equilibrium with both excess oil and water [85]. Besides, the HLB required to form an o/w or w/o microemulsion depends not only on the emulsifiet type, but also on the oil type. Surface activity of a solute is defined as the ability to reduce the surface tension at an interface without requiring concentrations so large that the distribution between solute and solvent is blurred. From the literature data it appears that the transparent microemulsion systems are prepared under following considerations: 1. an enough emulsifier has to be present to cover the interfacial area, 2. primary emulsion be as finely dispersed as possible,


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3. a large increase in the interfacial area by addition of coemulsifier (1 - 2 orders in magnitude), 4. a formation of a mixed emulsifier/coemulsifier film at the o/w interface, 5. a low value of interfacial tension (γi) is a necessary step in microemulsion formation. Once γi is sufficiently low (< 10-3 dyne/cm), spontaneous dispersion occurred with little or no mechanical work required, 6. the role of coemulsifier is to reduce the rigidity of the interfacial film, allowing the transition from a well - organized phase towards an isotropic microemulsion, 7. the internal interfaces are determined to be flexible and highly disorganized, 8. the flexible interface is absolute requirement for maintaining some microemulsion type systems, and 9. no strict separation into hydrophobic and hydrophilic domains is observed, and 10. no formation of extended aggregates. Microemulsions usually behave like Newtonian fluids; their viscosity is comparable to that of water, even at high droplet concentration, probably because of reversible droplet coalescence. Indeed the microstructure evolves constantly due to constituent exchanges. This is important feature that strongly effects the dynamic properties of microemulsions. Bulk viscosity of a polymer solution depends on the length, weight, size, configuration and structure of polymer. In the case of emulsifier micellar solutions, the linear and non-linear viscoelastic properties of thread like micelles have been predicted to resemble entanglement of ordinary polymers [86]. They explained the increasing viscosity in terms of entanglement of threadlike micelles, which evolve to a network structure. The spherical micelles with no particular structuring have less influence on viscosity but the threadlike micelles forming a random loop structure do have a greater influence. For example, the macroscopic viscosity shows a strong increase above ca. 20% emulsifier (nonylphenol (EO)25 OH) concentration, which should be associated with formation of micellar clusters or other supermicellar structures [87]. The high values of macroscopic viscosities (ca 100 cP) indicate the formation of large and/or interconnected aggregates. The concept of immobilizing reagents or probes onto polymer supports for use in chemistry and biology has received a great deal of attention. Since the activity of supported reagents depends on the accessibility of the active sites and is often limited by diffusion, considerable efforts are made to develop new polymer supports with improved capacity, accessibility and selectivity [88, 89]. In this context, the technique of polymerisation in microemulsion, developed in the early 1980s, offers new opportunities [90]. Indeed, the polymerisation of oil or water-soluble component in oil-in-water or water-in-oil microemulsions allows one to produce stable suspensions of ultrafine particles in the nanosize range (diameter smaller than

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30nm), so called ‘microlatexes’ or ‘nanolatexes’, which exhibit very large specific areas of up to 400–500 m2/g for nanoparticles in the 10–15-nm range [91]. Moreover, the well-defined structure of microemulsions affords a means to synthesize special polymer-based materials with high degrees of chemical functionalisation [92, 93]. 2.4.2. Microemulsion polymerization Microemulsions appear to be excellent media for facilitating chemical reactions. They solubilize a large number of very different compounds, they possess a large internal interface, and they form spontaneously. The studies on chemical reactions in microemulsion media deals mainly with the physical chemistry of the systems themselves. Reactions were studied either as probes for clarifying the physical properties of the microemulsions, or for investigating the influence of an organized reaction medium on the kinetics of the reactions. However, for performing a synthesis the concentrations of the reactants have to be much higher (in the range of one mole per dm3). Increasing concentrations of additives cause increasing problems to control the phase behavior of the microemulsion. This is especially true if high concentrations of electrolytes, amphiphilic polymers, reactive polymers, etc. are added to a microemulsion stabilized by an ionic emulsifier. Microemulsions act as attractive media for polymerization reactions. Polymerization in microemulsions is a new polymerization technique which allows the preparation of ultrafine latex particles within the size range 10 nm < d < 100 nm and with narrow size distribution [91, 94, 95]. However, the formulations of polymerizable microemulsions is subject to severe constraints, due in large part to the high emulsifier level (ca. 10 - 20%) needed for achieving their thermodynamic stability. This fact, together with the requirement of high polymer contents in most applications, raises the problem of keeping specific emulsifier-coemulsifier, monomer-emulsifier and monomer-coemulsifier interactions, which are disrupted in the presence of large amount of polymer tending to destabilize the polymer microemulsion or to produce large-sized polymer particles. While microemulsions can be used as potential media for polymerization in which large molecular - weight polymers with narrower molecular weight distribution (MWD) may be achieved. The microemulsion polymerization system consists of three phases: an aqueous phase (containing initiator, emulsifier, coemulsifier and some amount of monomer), emulsified monomer microdroplets or the monomer swollen micelles and monomer swollen polymer particles. Water is a most important ingredient of the microemulsion polymerization system. It is inert and acts as the


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locus of initiation (the formation of primary and oligomeric radical), the medium of transfer of monomer and emulsifier from monomer microdroplets or the monomer swollen particles micelles to particles and the component of complex emulsifier/coemulsifier/water. An aqueous phase maintains a low viscosity and provides an efficient heat transfer. In addition of the mechanical actions and interfacial energy considerations which will act to reduce the degree of dispersion of an emulsion, there are other considerations which act to limit the stability of emulsions. One such factor is the phenomenon, commonly termed Oswald ripining in which large drops are found to grow at the expense of smaller ones, results from differences in the chemical potential of molecules in small particles relative to those in large ones. Such differences arise from the fact that the pressure (chem. potential) of material inside a drop is inversely proportional to the drop radius. The solubility of the dispersed phase may be so low that diffusion from small to large particles will be exceedingly slow. The decrease in the rate of droplets growth due to flocculation, agglomeration and Oswald ripining can be achieved by using (co)emulsifiers (or hydrophobes) which form a barrier to the passage of dispersed phase molecules into the continuous phase and decrease the water-solubility of solute molecules. For example, an important group of coemulsifier are short-length alcohols and hydrophobes (see later) hexadecane. The most commonly used water - soluble initiator is potassium, ammonium or sodium salt of peroxodisulfate. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc., are also used in microemulsion polymerization. They are, however, less efficient than water - soluble peroxodisulfates. The initiation of microemulsion polymerization is a two - step process: 1) It starts in water by the primary free radicals derived from the water - soluble initiator. 2) The second step occurs in the monomer- swollen micelles by entered oligomeric radicals. Two characteristics of o/w microemulsion polymerization are different from those of conventional emulsion polymerization: 1) No monomer droplets and no inactive micelles exist. 2) The system is optically transparent. The most significant difference between emulsions (opaque) and microemulsions (transparent) lies in the fact that stirring of an crude emulsion or increasing the emulsifier concentration usually improves the stability. This not the case with microemulsions, which appear to be dependent for their formation on specific

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interactions among the constituent molecules. If these interactions are not realised, neither intensive stirring nor increasing the emulsifier concentration will produce a microemulsion. On the other hand, once the conditions are right, spontaneous formation occurs and little mechanical work is required [96]. Microemulsion formation appears to be dependent on specific interactions among the constituent molecules at the o/w interface. Basically, a course emulsion was prepared, and the system was titrated to clarity by adding a coemulsifier (second surface active substance). When the combination of the four components was right, the system cleared spontaneously. The essential features of microemulsion polymerization of unsaturated hydrophobic monomers were reported to be as follows: 1) dependence of the rate of polymerization versus conversion is missing the stationary Interval 2, 2) microemulsion polymerization is slower than emulsion polymerization, 3) a water-soluble initiator is mostly more efficient than oil-soluble one, 4) size and the number of particles increase throughout the course of polymerization, 5) PSD of polymer latexes is very narrow, 6) MWD of resulting linear polymer chains is very broad, 7) Mw is slightly dependent of initiator concentration and conversion, and 8) the radical entry efficiency is very low and the average number of radicals per particle is much below 0.5. A proposed mechanism for the microemulsion polymerization consists of following steps (Fig. 6): the particles are nucleated by capture of radicals from the aqueous phase for both water - soluble and oil - soluble (partly soluble in water) initiators; the microemulsion droplets which did not capture radicals served as reservoirs to supply monomer and emulsifier to the polymer particles. The polymer particles compete with the microemulsion droplets in capturing radicals. However, owing to the much larger surface area provided by microemulsion droplets, the radical flux to the polymer particles was still smaller. The continuous nucleation of polymer particles during polymerization results from the very high number of monomer-swollen micelles or microdroplets. The ratio of monomer to emulsifier or the monomer concentration at the reaction loci decreases with increasing conversion. The result of these two opposing effects is the appearance of maximal rate at ca. 10 - 20% conversion. The light scattering measurements prove the presence of both the microdroplets and mixed micelles (monomer - starved microdroplets). The ratio of microdroplets to mixed micelles


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decreases with increasing conversion. The high emulsifier/water ratio ensures that the dissociation of emulsifier is depressed.

Fig. 6. The mechanism of microemulsion polymerization [97].

The microemulsion polymerization and copolymerization of amphiphilic monomers and macromonomers can produce the fine polymer latex in the absence of emulsifier [98-100]. The surface active block or graft copolymer stabilizes the latex particles. The chemically bound emulsifier (surface active copolymer) onto the particles surface is known to be much more efficient emulsifier than the emulsifier physically adsorbed onto the particle surface and, therefore, very stable and fine polymer latexes are formed. The similar behavior is expected with the transferred emulsifier radicals. For example, the surface-functionalized nanoparticles in the 12 - 20 nm diameter range can be prepared by a one-step or two-step microemulsion copolymerisation process of styrene (and/or divinylbenzene (DVB)) with the polymerisable macromonomer (Fig. 7) [93, 101]. These nanoparticles exhibit a very high selectivity for cupric ions. In such particles resulting from copolymerisation, the cyclam (macromonomer) residues are distributed between the core and the surface: the ligand accessibility depends on the polymerisation conditions and is closely related to the size [93]. The smaller the particles, the higher the surface-to-volume ratio, the higher the ligand accessibility [93]. Ligand-functionalized nanoparticles may alternatively be obtained by postgrafting. Post-functionalisation of reactive nanoparticles could be a versatile method for binding various functional residues thus giving access to nanoparticles with adjustable functionalities and controlled size from a same microemulsion-

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polymerisation recipe. The postfunctionalisation of classical latexes and polymer gels has been widely used and is well documented [102], there are also some examples of such chemical modifications on nanoparticles prepared by microemulsion polymerization [103].

Fig. 7. Synthetic microemulsion way to prepare cyclam-functionalised nanoparticles [101].

2.5. Miniemulsion polymerization Emulsions are thermodynamically unstable exhibiting flocculation and coalescence unless significant energetic barriers to droplet interactions are present. They degrade toward phase separation via mass transfer, and other mechanisms. When an oil-inwater emulsion is created by the application of shear force to a heterogeneous phase containing surfactants and additives, a distribution of droplet sizes results. Interdroplet mass transfer (Ostwald ripening, [4]) determines the fate of this distribution because of their higher Laplace pressure. If the small droplets are not stabilized against diffusional degradation, they will disappear, increasing the average droplet size. It was shown that this disappearance can be very fast for small droplets [104]. Emulsions are sensitive to coarsening phenomena like coalescence and Ostwald ripening, since their thermodynamically most stable state is the completely demixed one. Besides the molecular diffusion of the dispersed phase, a destabilization of an emulsion can also occur by collision and coalescence processes. Coalescence is often considered as the most important destabilization mechanism leading to coursing of dispersions. However, coalescence can often be prevented by a careful choice of stabilizers and is mainly of interest during processing. On the other hand, Ostwald ripening will continuously occur as soon as curved interfaces are present. The curvature of particles causes higher solubilities of the dispersed phase at


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the particle boundary compared to in the bulk or near to large particles. The concentration gradient in the dispersed phase in the continuous phase causes large particles to grow at the expense of smaller particles. Ostwald ripening involves the movement of oil molecules from small droplets to large droplets (Scheme 2, [105]). It is, thus, the process whereby large droplets grow at the expense of small ones because the solubility of a material within a droplet increases as the interfacial curvature increases [106]. In other words, Ostwald ripening is the process by which larger particles grow at the expense of smaller ones due to the higher solubility of the smaller particles (Gibbs-Thomson or Kelvin effect) and to molecular diffusion through the continuous phase [107]. Ostwald ripening is the process whereby the higher Laplace pressure inside small drops drives the transfer of dispersed oil from small to large drops. The speed of ripening depends primary on the product of the solubility of the dispersed oil in the aqueous continuous phase C∞ and its diffusion coefficient D [10-12]. Oils which are slightly water soluble (so called “mobile” oils) can transfer between droplets at significant rates whereas Ostwald ripening is negligibly slow for oils of sufficiently low aqueous phase solubility, and these oils are termed “immobile”. Oil transport can occur by diffusion of molecularly dissolved oil molecules through the continuous aqueous phase but may also be enhanced by an additional mechanism of transport as solubilized oil within micellar aggregates which are normally present in the continuous phase.

Scheme 2. Schematic representation of the homogenization and monomer droplet degradation [105].

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The physical degradation of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the dispersion medium. Minimizing the interfacial area is mainly achieved by two mechanism: first coagulation possibly followed by coalescence and second Ostwald ripening. However, if properly stabilized against the coagulation/coalescence process, the latter can cause a substantial breakdown of the emulsion. Dissolution in water/oil/emulsifier systems takes place through mechanisms that occur at the molecular level. Kinetic studies aim at determining the limiting step(s) within such mechanisms that dictate mass-transfer rates and transient behavior. Unstable emulsions become more stable with respect to the Ostwald ripening process by the addition of small amounts of a hydrophobic additive, which distributes preferentially in the dispersed phase [108]. This stabilization effect was theoretically described by Webster and Cates [109]. The rate of Ostwald ripening depends on the size, polydispersity and solubility of the dispersed phase in the continuous phase. This means an already ultra-hydrophobic oil dispersed in small droplets of low polydispersity shows low diffusion. But by adding a hydrophobe, the stability can even be increased by additionally building up an osmotic pressure. This was shown for fluorocarbon emulsions based on perfluorodecaline droplets and stabilized with lecithin. By adding a hydrophobic component, e.g. perfluorodimorphinopropane, the droplets' stability was increased [110, 111]. The added material reduces the total vapor pressure as defined by Raoult's law. Hexane and hexadecane (HD) demonstrate a slight negative deviation from ideality, and HD/fluorochemicals a slight positive deviation from ideality [112]. As in the case of the pure oil system, smaller droplets will have a slightly higher vapor pressure (or solubility) than larger ones. To reach the equilibrium state, hexane will leave the small droplets and pass to larger ones. This loss of hexane will cause an increase in the mole fraction of the third component in the small droplets and a decrease in the large droplets. Thus, the small droplets will have a more reduced vapor pressure as compared to the larger ones than originally was the case. The attraction of miniemulsions for application in various fields is due to the following reasons. First, the very small droplet size causes a large reduction in the gravity force and the Brownian diffusion may prevent any creaming or sedimentation. Second, the steric stabilization prevents flocculation or coalescence of the droplets. The small droplet size and the high kinetic stability make miniemulsions suitable for the efficient delivery of active ingredients. Unlike microemulsions, which require a high concentration of emulsifiers for their preparation (usually in the range 10-30 wt%), miniemulsions can be prepared at moderate emulsifier


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concentration (in the range 4-8 wt%). Thus, the droplet size distribution of the nanoemulsions is a complex function of breaking up and coalescence of droplets, the droplet degradation by monomer diffusion and the presence or the absence of emulsifier, coemulsifier (or surface active agent) or hydrophobe. The stability of o/w (nano)emulsion is directly connected with the transport of oil through the aqueous phase. The entry of radicals into the monomer minidroplets leads to the particle nucleation. This means that the droplets become the primary locus of the initiation of the polymer reaction (the reaction system does not contain free monomer-swollen micelles). Polymerization in monomer droplets (nanoreactors) takes place in a highly parallel fashion, i.e., the synthesis is performed in 1018-1020 nano-compartments per liter, which are separated from each other by a continuous aqueous phase. In the first step of the miniemulsion process, small emulsified monomer droplets of 100 - 400 nm in diameter are formed by shearing a system containing the dispersed phase, the continuous phase, a surfactant, initiator and an osmotic pressure agent (Scheme 3). In a second step, these droplets are polymerized without changing their identity. The droplets are small enough to benefit from all the advantages of conventional emulsion polymerization process, e.g., high rates of polymerization, the depressed bimolecular termination of propagating radicals and high molecular weights of the final polymers. Miniemulsion polymerization enables to incorporate water-insoluble materials such as resins, organic pigments, polymers, etc into the polymer matrix. The additive seed allows to control the particle number and particle size during the production process. Furthermore, miniemulsion polymerizations and copolymerizations carried out with acrylic and methacrylic monomers in the presence of unsaturated alkyd resins lead to the production of stable hybrid latex particles containing grafted and crosslinked alkyd resin/acrylic products as coating polymer [114]. In the reaction, the multifunctional resin acts as a hydrophobe as well as the costabilizer of the miniemulsion. The miniemulsion polymerization can be applied for the preparation of the composite particles containing the hydrophobic and/or hydrophilic domains and/or phases. The hydrophobized material, which is to be incorporated, has to be dispersed in the monomer phase. Then, miniemulsification in the water phase has to be carried out. The hydrophilic additives themselves require a hydrophobic surface so that they can be dispersed into the hydrophobic monomer phase. Erdem et al. described the encapsulation of TiO2 particles via miniemulsion in two steps mentioned above. First, TiO2 was dispersed in the monomer using OLOA 370 (poly(butene

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succinimide)) as the stabilizer [115]. This phase was dispersed in an aqueous solution to form stable submicron droplets. The oil/water phase between the droplets and the water phase is not only affected by the surfactants, but also by the component in the monomer.

Scheme 3. The principle of miniemulsion polymerization (where P denotes polymer particle) [113].

Scheme 4. Schematic representation of the sonication process.


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Different sized nanocapsules are formed by a miniemulsion polymerization of variety of monomers in the presence of larger amounts of hydrophobe [117]. Hydrophobe and monomer form a common miniemulsion before polymerization, whereas the polymer is immiscible with the hydrophobe and phase-separates throughout the polymerization to form particles with a morphology consisting of a hollow polymer structure surrounding the hydrophobe. Differences in the hydrophilicity of oil and polymer turned out to be the driving force for the formation of nanocapsules. In the case of poly(methyl methacrylate) (PMMA) and hexadecane (HD), the pronounced differences in hydrophilicity are suitable for direct nanocapsule formation. In the case of styrene as the monomer, the hydrophilicity of the polymer phase has to be adjusted in order to favor the nanocapsule structure, which is done either by the addition of an appropriate comonomer or initiator. The miniemulsion polymerization was used to encapsulate different carbon particles. The surface tension was reported to be of minor importance on the encapsulation process, but both the amount and type of the hydrophobe, as well as the type of monomer turned out to be parameters, which have to be optimized [116]. The fusion/fission process induced by ultrasound is effective for the monomer droplets only, whereas the monomer-coated carbon stays intact (co-miniemulsification). In this way, all monomer droplets are split and hetero-nucleated onto the carbon to form a monomer film; any re-aggregation to monomer droplets would result in liquid units only, which are immediately split again. The thickness of the monomer film depends on the amount of monomer, and the exchange of monomer between different surface layers is − as in miniemulsion polymerization − suppressed by the presence of an osmotically active agent, which cannot exchange between the single dispersed carbon particles. The process can be described best as a polymerization in an adsorbed monomer layer created and stabilized as a miniemulsion (ad-miniemulsion polymerization). There is an optimal range of monomer layer thickness, which can be transferred into a polymer layer under preservation of the morphology. Too low amounts of monomer result in aggregating particles incompletely covered with polymer, whereas too much monomer results in the formation of a second species of pure polymer-containing particles. The insensitivity of the whole encapsulation process against surface tension or the amount of sodium dodecyl sulfate (SDS) in the recipe clearly shows that SDS just keeps the dispersion stable, but is not involved in the layer stability or a possible nucleation process. This again is counterintuitive, but typical of the class of miniemulsion-based processes. In all cases, the surface tension of the final dispersion is above the minimal surface tension of SDS indicating the

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absence of micelles and the incomplete coverage of the polymer-coated carbon particles with surfactant. 2.6. Dispersion polymerization Dispersion polymerization was invented in the 1960s [118], but it was the contributions of Lok and Ober [119] on dispersion polymerization of styrene in alcohol, using water-soluble polymers such as poly(vinylpyrrolidone) (PVP) as the stabilizer, that stimulated the current widespread interest in this methodology. The reaction is easy to carry out, lends itself to scale-up, and yields particles with a very narrow size distribution. It is most suited to the preparation of beads in the diameter range of 1-15 μm. Dispersion polymerization is used to prepare micron-sized monodisperse polymer particles. It is defined as a heterogeneous polymerization by which polymer particles are formed in the presence of a suitable steric stabilizer from an initially homogeneous reaction mixture. The solvent selected as the reaction medium must be a good solvent for both the monomer and the steric stabilizer but a poor solvent or a nonsolvent for the polymer being formed [118,120]. In fact, the dispersion polymerization can be regarded as a special case of precipitation polymerization in which flocculation is prevented and particle size controlled. When conventional surfactants are used in dispersion polymerization, difficulties are encountered which are inherent in their use. Conventional surfactants are held on the particle surface by physical forces; thus, adsorption/desorption equilibria always exist, which may not be desirable. They can interfere with adhesion to a substrate and may be leached out upon contact with solvent. Surfactant migration affects film formation and their lateral motion during particle - particle interactions can cause destabilization of the colloidal dispersion. On the contrary, “reactive surfactants” contain a polymerizable group; thus, they can overcome some of the difficulties encountered with conventional surfactants and can also be incorporated into the surface layer of the polymer particles by copolymerization with other unsaturated comonomers. In this manner, these reactive surfactants are bound to the particle surface and therefore they are prevented from subsequent migration. Polymerization of amphiphilic macromonomer or copolymerization of hydrophobic or nonpolar monomer with hydrophilic or polar macromonomer leads to the formation of surface active polymers or grafted copolymers. Macromonomers are macromolecules with a polymerizable group (see some examples, PSt denotes polystyrene and PMMA poly(methyl methacrylate)):


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PSt-CH2CH=CH2, PSt -CH2CH2OCH=CH2, PSt - CH2CH2OCH - C6H4- CH=CH2 PSt - CH2CH2O- CO- CH=CH2 PMMA-CH2CH=CH2, PMMA -CH2CH2OCH=CH2, PMMA - CH2CH2OCH - C6H4- CH=CH2 PMMA - CH2CH2O- CO- CH=CH2 These compounds afford a powerful means of designing a vast variety of welldefined comb polymers and graft copolymers. Homopolymerization affords a regular comb polymer since the branches are regularly spaced along the backbone [121].

Scheme 5. Basic scheme of polymacromonomer (comb polymer) synthesis [121].

where r and n stand for the DPns of a trunk and a branch, or the number and length of branches, respectively (Scheme 5). The lengths of branches and trunks in a comb polymer can be controlled by the synthesis of macromonomer and the polymerization. An advantage of this approach is the use of a knowm macromonomer as a branch. Thus n and its distribution can be predicted or controlled by the method of preparation. Polymacromonomers can be classified into two types of regular branched forms, i.e., stars and combs (brushes), depending on the degree of polymerization of the backbone and side chains. Polymacromonomers can be treated as star polymers when the number of arms is small. The brush-like conformation develops when the number of branches is larger (Fig. 8). Copolymerization involving two or more kinds of macromonomers appears interesting in providing comb polymers with multiple kinds of branches.

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Polymacromonomers with polymeric branches on the backbone have an extremely high branched density. Therefore, their bulk properties are expected to be significantly different from those of the corresponding linear polymer. The studies by Tsukahara et al. [122] revealed that the glass transition temperature, Tg,PStmacro, of PSt polymacromonomers is predominantly determined by the excess free volume effect of end group per unit MW: MWPstmacro x 10-3/{Tg,Pstmacro = 0.9/56-68, 3.1/84-80, 13.1/98, 14.6/100.

(5)

Fig. 8. Schematic picture of polymacromonomers with various architectures: (a) star-like, (b) rod-like, (c) AnBn star, (d) (AB)n star [121].

Generally, the Tg value was found to increase with the molecular weight (MW) of macromonomer (MWPStmacro) and also that of polymacromonomers. The authors [122] have reported that when PSt polymacromonomers are cast onto glass plates, many cracks are created during the solvent evaporation and the resulting films are too brittle to handle. They ascribed the results to a lack of chain entanglement networks in the polymacromonomer matrix. The SAXS studies demonstrated that the polymacromonomer molecules exist independently of each other [123]. The incompatibility was observed between high MW linear PSt and PSt polymacromonomer with high degree of polymerization [124]. Furthermore, the PSt polymacromonomers form lyotropic main chain liquid crystals in toluene solution


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and in the bulk state [124]. The PEO polymacromonomers from n = 9 to 23 were amorphous material with Tg = - 55 ∼ - 60 oC but those from n = 45 were crystalline with mp = 38 – 44 oC [125]. Copolymerization of a conventional monomer with a macromonomer affords welldefined graft copolymers (Fig. 9). This can be performed in a good solvent for both a monomer and its resulting polymer. The branched structure and heterogeneities in MW and composition of graft copolymers prepared in the solution make the classical techniques used to characterize the graft copolymer inefficient.

Fig. 9. Basic scheme of copolymerization of a macromonomer with a comonomer to a graft copolymer [125].

One of the fascinating application of macromonomers is in the field of dispersion polymerization. The dispersion polymerization in the presence of suitable stabilizers affords mostly monodisperse submicron- and micron-sized microspheres (particles). The macromonomers are graft-copolymerizaed during copolymerization in the continuous phase and so accumulate on the particle surface, so that the resulting particles are effectively sterically stabilized against flocculation. Amphiphilic copolymers synthesized by copolymerization of a hydrophobic conventional monomer with a hydrophilic macromonomer and vice verse present all the typical properties of conventional surfactants. They aggregate between themselves and form a micelle in the aqueous or non-aqueous media. The conformation of a micelle formed by PEO-g-PSt polymer in the aqueous medium consists of a hydrophobic PSt core and a hydrophilic PEO shell (Fig. 10).

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Fig. 10. Schematic picture of organized organization of polyamphiphile (e.g., PSt/PEO) molecules.

Control of surface properties of graft copolymers can be important in technical fields such as coatings, adhesives, films and fibers. For example, a small amount of graft copolymer was efficient to improve the anti-wettability of PMMA films [126]. Some of fluorine- [127], silicon- [128] and PMMA [129] - containing graft copolymers have been used as a promising surface modifier. In blending polymer materials, compatibility between two incompatible polymers is strongly increased by the addition of a small amount of graft copolymer. Poly(St-g-MMA) prepared by macromonomer technique was found to be an effective compatibilizer for PSt and poly(vinyl chloride) (PVC) blends [130]. Furthermore, these species are particularly useful in the field of polymer blends as compatibilizers and/or stabilizers (surfactants). When macromonomer itself is an amphiphilic polymer, then its polymerization in the polar (aqueous) phase occurs usually rapidly as a result of organized aggregation. According to the coagulative nucleation model (derived from the homogeneous nucleation model [54]), the most important point in the dispersion polymerization is the instant at which colloidally stabilized particles form. Indeed, TEM measurements confirmed the agglomeration of small unstable polymer particles into one large (Fig. 11). After this point, coagulation between similar-sized particles no longer occurs, and the number of particles present in the reaction is constant. The dispersion copolymerization of macromonomer is characterized by the following features (Figs. 12 and 13) [100, 131, 132]: 1) The initial reaction mixture of the monomer, macromonomer, initiator and additive is homogeneous, that is, all the reactants are dissolved completely in the solvent.


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Fig. 11. Schematic representation of particle formation through the microspheres (precursors) agglomeration.

2) The decomposition of initiator produces the initial radicals which initiate the polymerization and the formation of oligomeric and polymer radicals, polymers and graft copolymers in the continuous phase. The solubility of these polymers decreases with the polymer molecular weight (MW) and above a certain critical value of MW they precipitate from the continuous phase and form unstable primary particles. 3) These unstable particles coagulate on contact, and the coagulation among them continues until sterically stabilized particles form. 4) This point is referred to as the critical point at which all particles contain sufficient amount of stabilizer groups on the particle surface to provide colloidal stability. Above this point, particles grow by the diffusion of monomer from the continuous phase to the polymer particles and its polymerization and by diffusive capture of oligomers and primary particles (precursors) produced by the polymerization of monomer in the continuous phase. The whole number of polymer particles mostly remains constant and so the final size of polymer particles is a function of amount of polymer formed or the overall concentration of monomer.

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Fig. 12. Shematic model for the particle nucleation and growth of sterically stabilized particles in dispersion polymerization using macromonomer approach [132 ].

In the dispersion copolymerization, the reaction medium play a crucial role for the branches of amphiphilic graft copolymers. They act as a steric stabilizer for the polymer particles. Schematically, the polymer particles generated by the agglomeration of small colloidally unstable primary particles (precursors) develop to a core-shell type as given by Fig. 14. The core is occupied by the insoluble polymer chains and the shell by the soluble, graft-copolymerized macromonomer moieties. The re-organization of a multi-particle aggregate particle to a core-shell structure is initiated by swelling of polymer particles by a conventional monomer. The backbone chains of the graft copolymers, which must be insoluble in the continuous medium but soluble in the conventional monomer phase, serve as the anchors into the core. The amphiphilic block and graft copolymers have the unique molecular structure, which consists of at least two parts with different chemical natures (amphi: of both kinds; philic: having an affinity for) character. In fact parallels can be drawn between typical surfactants and amphiphilic copolymers having both hydrophilic and hydrophobic blocks. Such amphiphilic copolymers find numerous applications as


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emulsifiers, dispersants, foamers, thickeners, rinse aids, immobilizers of metal salts, compatibilizers, etc. This “generalization of amphiphilicity”, i.e., stabilization not

Figure 13. Shematic model in several steps for the particle nucleation and growth of sterically stabilized particles in dispersion polymerization using macromonomer technique [117].

only of the oil/water interface, but of any interface between different materials with different cohesion energies, is enabled by the wide variability of the structure of the polymer through choice of the repeat unit, possible copolymerization, and/or the length and structure of both parts. In addition, various copolymer architectures are possible: random, block, graft, star, multiblock, etc. As a consequence, these polymers can substitute low molecular weight surfactant molecules or extend surfactant applications in many heterophase stabilization problems, such as in emulsion and dispersion polymerization, stabilization of inorganic and metal particles and pigments or the formulation of cosmetics and drugs. Block copolymers with amphiphilic character, having a large solubility difference between hydrophilic and hydrophobic segments, have a tendency to self-assemble into organized agglomerates (micelles) in a selective solvent [133, 134]. In an aqueous solution, micelles with core–shell structure are formed through the segregation of insoluble hydrophobic blocks into the core, which is surrounded by a shell composed of hydrophilic blocks. This self-assembling property of amphiphilic block copolymers provides their high utility in the biomedical field as drug carriers, surface modifiers, and colloidal dispersants [135, 136].

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Fig. 14. Schematic picture of a core-shell polymer particle obtained in dispersion copolymerization using macromonomer technique.

Apart from the wide ranging adaptability of both parts of the polymer, some additional advantages of polymeric amphiphiles are obvious: as high molecular weight stabilizers, additional mechanisms of colloidal stabilization can be invoked, i.e., steric and electrosteric stabilization contributions are added to the standard stabilization by charge repulsion (electrostatic). In addition, the CMC of polymeric surfactants can be controlled so that it is extremely low, so that the dispersion efficiency is kept, even at high dilution. Because of the low mobility of polymetic amphiphiles, their release into the environment is slowed down. For some technological applications, it is interesting to note that the kinetic stability of the aggregated structures is also sensitive to the chemistry and block length: compared to the millisecond exchange of low molecular weight aggregates, the lifetime of blockcopolymer micelles can easily be adjusted to be in the second, minute, or hour region. It is worth mentioning that to solve heterophase stabilization problems biological systems employ practically exclusively polymer-like amphiphiles (proteins, polysaccharides,… ), and the principle of low molecular weight surfactants as emulsifiers is not found in nature. This is a clear hint to all materials scientists working in the area of interfaces: it is the macromolecular architecture of the amphiphilic copolymers and the different length scales, timescales, and levels of


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interaction that it entails which makes the use of these compounds very attractive. Inherent in their various applications is the unique ability of the amphiphilic polymers to self-organize at interfaces and in solution, and thus modify interfacial properties and enhance compatibility or partition. Monodisperse polymeric beads of micrometer-sized diameters have many important applications as separation media, ion-exchange beads, toners, coatings, calibration standards, and in medical diagnostics [137]. For most of these applications, size control and narrow size distribution are of key importance. At present, most particles of this type are prepared by the successive seeded emulsion polymerization method developed by Vanderhoff [138] or by the Ugelstadt [139] activated-swelling suspension polymerization method, although other approaches are also available [140]. These processes for making polymer particles have limitations. They are complex and also difficult to implement on a large scale. Various research groups have found that when they added cross-linking agents, or even hydrophilic monomers to a dispersion polymerization recipe in which all the ingredients were added at the beginning, they obtained poor results [141, 142]. The final particle size was affected significantly. The size distribution became much broader, and sometimes coalescence occurred. Similar results can be abtained when a small amount of a dye-comonomer was added to the reaction (Fig. 15 A and B) [143]. For example, addition of less than 0.5 wt % dye to a traditional one-step dispersion polymerization of styrene in ethanol led to polystyrene (PSt) particles that were spherical and uniform in size, but addition of larger amounts of dyecomonomer (1 wt %) led to polydisperse particles (Figure 15A). To avoid the difficulties described above, the reagents can be added to the reaction after the nucleation stage was complete. The nucleation stage can complete at less than 1% monomer conversion [144]. Comonomers added after this point became incorporated into the particles without disturbing the final particle size and size distribution. In this way dye-labeled or functional group-containing micrometer-sized particles with a very narrow size distribution were prepared. By varying the amount of monomer added in the second stage, the final particle diameter precisely without changing the narrow size distribution can be obtained. Most important of all, this synthetic strategy allowed to prepare crosslinked particles containing up to 3 mol % crosslinking agent. The two-stage dispersion polymerization of styrene was carried out by reactants as the stabilizer (PVP), the co-stabilizer (Triton X-305), initiator [2,2’-azobis(2-methylbutyronitrile), AMBN] and additives. When 1 wt % of dye, based on total monomer, was added to the reaction in the second stage, the particles retained their narrow size distribution (Figure 15 C) and had the same diameter as

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those obtained in the absence of dye comonomer. Similar results were obtained for 3 wt % dye.

A B C Fig. 15. SEM images of copolymer particles prepared by the one-stage method (A) and the two-stage method (C). (A) 1 wt % dyecomonomer; (B) 0.35 mol % DVB; (C) 1 wt % dye [143].

The final particle size increased, and without any additional initiator or PVP, the beads remained colloidally stable. In one experiment, the authors [143] added a total of five aliquots of the same amount of styrene and ethanol. If no new particles were formed (no secondary nucleation), the particle volume (proportional to D3) should increase linearly with the amount of styrene added. The particle size distribution remained very narrow. The particle size could be controlled precisely by adding different amounts of monomer in the second stage. The most important application of the two-stage method is to prepare crosslinked particles. Many research groups have tried to prepare monodisperse crosslinked PSt particles by dispersion polymerization. All of these groups encountered problems. The first experiments to prepare crosslinked micrometer-sized polymer particles by dispersion polymerization were reported by Tseng et al. [145]. In their work, when 0.3 mol % divinylbenzene (DVB) or ethylene glycol dimethacrylate (EGDMA), was added to the reaction mixture, the particle size changed, and the size distribution broadened significantly. Further increase in DVB (or EGDMA) concentration resulted in coagulation of the dispersion. Figure 15 B shows a SEM image of PSt particles prepared by the one-stage method with only 0.35 mol % DVB, similar to the results of Tseng et al. These particles had a very broad size distribution. With a higher DVB content (e.g., 1 mol %), flocculation occurred after only 5 min of polymerization, and the amount of aggregates increased during the polymerization. Rudin et al. [146] carried out the most extensive studies of the effects of DVB on dispersion polymerization. They found that they could obtain a narrow size distribution with tiny amounts of DVB (up to 0.2 mol %). They were able to incorporate up to 1 mol % DVB by linear addition over the major particle growth period, but the resulting particles were heavily dented with a small fraction of


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coagulum. When a crosslinker such as EGDMA is added after the end of the nucleation stage monodisperse particles were obtained. Similar monodisperse particles were also obtained for DVB-crosslinked PSt particles when DVB (1-3 mol %) was added after the end of the nucleation stage. This approach takes into account the different reaction rates for the two vinyl groups of DVB and avoids early consumption of the crosslinker. The homogeneous mixture of monomer/ethanol/water is, for example, the starting point for the so-called “dispersion polymerization process”, which is used to produce, after polymerization very large particles (on the order of 10 to 100 μm) [147]. The size range of the metastable dispersions varied typically between 1 and 4 μm, the particle size increasing with increasing oil concentration in the initial solution (i.e. before mixing into water). Diameters as small as 200 nm have also been obtained using larger ethanol-to-oil ratios [148]. In the Ouzo Domain, the droplet diameter was shown to be a function only of the ratio of excess oil to ethanol (i.e. subtracting the oil which remains dissolved in the final ethanol/water solution from the initial amount of oil) [147]. It did not depend on stirring rate, pH, or ionic strength [149]. The dispersions thus produced are kinetically stable, without the presence of any surfactant. The Ouzo effect produces droplets, which are homogeneously sized and are large enough so that the Ostwald ripening is greatly retarded, but are small enough so that creaming is slow, especially when the density of the oil is close to that of water. Emulsions were made, via the Ouzo effect, using an oil whose density was close to that of water; these emulsions were almost monodisperse and were stable for months [147]. Nonetheless, in most of the studies reported below, surfactants (mostly PEO–PPO block copolymers or Dextran) were added to the aqueous phase prior to emulsification to obtain stable emulsions, which lasted for very long times (typically more than 6 months) [149]. Polymer dispersions in water can be prepared by well-known emulsification techniques, [150] which include solvent evaporation, mechanical means (sonication, microfluidization), and coacervation. The Ouzo effect, which has also been called “coacervation with addition of a nonsolvent”, “solvent displacement process” [151], “spontaneous emulsification” [152] “nanoprecipitation” [153] or “microprecipitation” [154], is clearly a simpler method that could be used to prepare drug-loaded large particles or oil-filled capsules of various commercial polymers. Various polymers containing charged groups in their backbone − for example, carboxylate and quaternary ammonium groups [152], or nonionic hydrophilic groups, such as polyethylene glycol [155] − were successfully emulsified without the need of stabilizers. In all instances, the particles thus formed were smaller than those prepared by means of the solvent evaporation technique (i.e. by dissolving the polymer in dichloromethane prior to emulsification) [156].

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2.7. Self-assemblies Both small molecule and macromolecular amphiphiles can self-assemble into a wide array of soluble organized structures, including micellar, cylindrical micellar, vesicular, and lamellar phases as well as phases that tend to be insoluble, including hexagonal and bicontinuous phases. These organized materials have been used in a wide variety of applications including detergents [157], paints [158], drug delivery agents [159], photonic materials [160] and scaffolds for creating ordered inorganic materials [161]. Because these materials are self-assembled, they undergo phase transitions as a function of percent composition and temperature. Capturing the mesophase structure requires covalent linking. Many phases have been captured via polymerization or crosslinking: micelles [162], w/o micelles [163], cylindrical micelles [164], monolayers [165], hexagonal phases [166], and bicontinuous phases [167]. The extent to which a lyotropic phase is captured depends on the dynamics of the assembly. For example, o/w micelles [168] and w/o composed of small molecules have been polymerized, resulting in molecular weights that are orders of magnitude higher than the weight of the prepolymerized object. Specifically, in the case of w/o micelles, polymerization results in the formation of particles that are roughly 20 times larger than the prepolymerized aggregates [169]. In other words, w/o micelles have not been captured to provide nanometer size domains (Scheme 6).

Scheme 6. Basic scheme of micelle formation by crosslinking approach [170].

Inverse phases such as w/o micelles and inverse hexagonal phases are unique because they place high functional group density toward a core or narrow channel, respectively, and this density can provide unique reactivity [171]. W/o micelles are spherical entities made by dissolving an amphiphile in an oil/water mixture. Under these conditions, the polar headgroup of the amphiphile partitions at the oil-water


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interface, creating a water core that is typically 10-100 Å in diameter [172]. Functional group density around the core and permeability make w/o micelles particularly attractive candidates for microreactors [173]. One limitation of w/o micelles is their capacity to undergo facile phase transitions, thereby limiting the temperature range in which they can be used. Self-organization of amphiphilic (co)polymers has resulted in assemblies such as micelles, vesicles, fibers, helical superstructures, and macroscopic tubes [174, 175]. These nanoscale to macroscale morphologies are of interest in areas ranging from material science to biology [176]. Stimuli-responsive versions of these assemblies are likely to further enhance their scope as “smart” materials. Thermo- or pHsensitive polymer micelles [177] and vesicles [178] have been reported in which the nature of the functionality at the corona changes in response to the stimulus. Some attention has been also paid to realize an environment-dependent switch from a micelle-type assembly with a hydrophilic corona to an inverted micelle-type assembly with a lipophilic corona [179]. Amphiphilic homopolymers containing both hydrophilic (carboxylic acid units) and lipophilic (benzyl moieties) functionalities in each repeat unit have been reported to assembly [180]. These polymers are soluble in both aqueous and organic solvents, where they assemble into micelle-like or inverse micelle-like structures. All these solutions were optically clear. The hydrophilic carboxylic acid unit and the hydrophobic benzyl moiety are placed on the opposite sides of the polymer backbone in solvents of different polarity. The observed solubility characteristics are the result of formation of a micelle-like structure in water, in which the hydrophilic carboxylate groups are exposed to the bulk solvent and the hydrophobic benzyl substituents are tucked in the interior of an assembly (Scheme 7). Similarly, an inverted micelle-like structure would be expected in apolar solvents, in which the functional group placements are reversed. Block copolymers are often the choice for a wide variety of supramolecular assemblies, in which the fundamental driving force involves the mutual immiscibility of the blocks and/or the immiscibility of one of the blocks in the bulk solvent. For example, poly(styrene-co-acrylic acid) block copolymers exhibit several interesting amphiphilic assemblies [181]. These self-assembled structures are the result of the incompatibility between the hydrophobic polystyrene block and the hydrophilic poly(acrylic acid) block. The consequences of incorporating carboxylic acid and benzyl moieties, the key hydrophilic and hydrophobic functionalities in poly(acrylic acid) and polystyrene respectively, within the same monomer of a homopolymer are interesting from an intramolecular phase separation perspective [182].

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Scheme 7. Schematic representation of direct micelle-type (B) and inverse micelle-type (A) assemblies [180].

Block copolymers are well-known examples of sell-assembling systems, in which chemically distinct blocks microphase-separate into the periodic domains. The domains can adopt a variety of morphologies (lamellar, double gyroid, cylindrical, or spherical) and length-scales, depending on the polymer chemistry and molecular weight. [183]. Self-assembled block copolymer domain structures have been used as masks to pattern high-density silica, germanium, and other microelectronic and magnetic materials [184]. Many practical applications such as patterned magnetic recording media require nanostructures with precise positions [185]. Substrates with topographical features or chemical helerogeneities have been used to influence the position and/or orientation of block copolymer domains. Substrates with shallow steps separated by distances of several micrometers have been shown to lead to long-range ordering of a spherical-morphology polystyrene-polyvinylpyridine block copolymer, in which ordered areas up to two hundred domains wide were created perpendicular to the steps [186]. In this system, since the length-scale of the template is vely longe compared to the domain size, the effects of incommensurabilily are negligible, and defects resull primarily from entropic effects. In comparison, at the opposite extreme of template length-scale, lamellar domains in polystyrene-polymethylmethacrylate have been oriented perpendicular to the film plane by using chemically heterogeneous stripes formed lithographically on a substrate, provided that the stripe width is vely similar to the width of the lamellar domains. [187]. Defects in the lamellar domain structure are observed when these is a mismatch between the period of the substrate pattern and that of the block copolymer. In other experiments, thin films of lamellar-morphology block copolymers have been confined within a few times the natural domain spacing by


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two rigid plates. The lamellar periodicity deviates from the bulk value to satisfy the boundary conditions imposed by the confining sulfaces [188]. The domain morphology, periodicity and ordering of self-assembled block copolymers can be influenced by substrate features. Controlling the spontaneous formation of ordered domains in soft materials such as block copolymers [189] may lead to the development of stimuli-responsive materials for applications such as actuators [190] and photonics [191] due to the reversible nature of order formation. However, the stimuli that are typically used to control the morphology of block copolymers are e.g., temperature, pressure, solvent type and concentration... Pioneering work by Abbott and co-workers used the chemical oxidation approach to control the self-assembly of small-molecule amphiphiles containing ferrocene [192]. Rabin and co-workers have shown that the introduction of dissociated charges on one of the blocks of a diblock copolymer leads to stabilization of the disordered phase [193]. They also quantified the increase in χ at the order-disorder transition (ODT), χODT, due to the entropic contribution of the dissociated counterions. The Flory-Huggins parameter,χ, that is used to quantify interactions between polymer chains is assumed to be proportional to the difference in the polarizibility of the blocks [194]. The polarizibility of polyferrocenyldimethylsilane, which is larger than that of either polystyrene or polyisoprene [195], must increase upon oxidation due to the presence of the NO3ions. Eitouni and Balsara [196] have demonstrated that chemical oxidation of poly(styrene-block-ferrocenyldimethylsilane) (SF) and poly(isoprene-blockferrocenyldimethylsilane) (IF) copolymers leads to stabilization of the disordered state. Changing the redox state of 8% of the ferrocene moieties results in a reduction of the order-disorder transition temperature by as much as 40 °C (Fig. 16). This stabilization is suggested to be due to the entropy of a very small fraction of dissociated counterions that are introduced during the oxidation step. An interesting property of ferrocene is the fact that its oxidation state can be altered reversibly by the application of small electric fields. These results suggest that the self-assembly of ferrocenecontaining block copolymers can be controlled by the application of electrochemical potential in a suitable electrolyte. The recent progress in the field of amphiphilic block copolymer assembly in the solution and on the surface, focusing on the biological and biomedical application of poly(ethylene glycol) (PEG) based block copolymers. PEG chains as hydrophilic polymers with a flexible nature can be selected as shell-forming segments, which assemble into dense palisades of tethered chains to achieve unique properties. The

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biocompatibility was guaranteed by the dense PEG shell, which endows the micelle with a stealth character in the blood compartment, achieving a long circulation [197]. PEG chains attached to a surface or forming the corona of a nanosphere exhibit rapid chain motion in an aqueous medium and have a large excluded volume. The steric repulsion resulting from a loss of conformational entropy of the bound PEG chains upon the approach of a foreign substance and the low interfacial free net energy of PEG in water contribute to the extraordinary physiological properties of nanospheres biomedical devices also proved to increase their biocompatibility and to reduce

Fig. 16. Phase diagram of SF (curve 1) and IF (curve 2) polymers as a function of percent oxidation [196].

thrombogenicity [198-200]. Furthermore, surface organization of reactive micelles with crosslinking cores was described, allowing the surface to have extremely high non-fouling character and working as a reservoir for hydrophobic agents. Core segregation from aqueous media is the direct driving force for micellization and proceeds through a combination of intermolecular forces, including hydrophobic interaction [201-205], electrostatic interaction [206, 207], metal complexation [208] and hydrogen bonding [209] of constituent block copopymers. A variety of drugs including genes and proteins, metals, and semiconductors with diverse characteristics can be incorporated into the core-forming segment of the block copolymer so that one can expect a sufficiently strong interaction with core-incoorporated molecules. In


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order to prepare the drug delivery system for site specificity, the outer shell of the polymeric micelle was built in such a way that it was covered with fuctional groups, which react readily with potential pilot molecules or target-specific antibodies. These strategy to construct functionalized PEG layers was further applied to metal and semiconductor nanoparticles, which have attracted much interest in biological assay system due to their unique photochemical and photophysical properties [210-212]. These photonic properties depend on the particle size and composition, which can be varied with the method of preparation, including the use of Langmuir-Blodgett films [213], reverse micelles [214], vesicles [215], and various polymer net-works [216, 217], yet the resulting nanoparticles were not effective in preventing non-specific aggregation in aqueous medium unless their surface was modified with hydrophilic coatings including PEGylation. Accordingly, the surface organization of PEG on these nanoparticles may open the new opportunities to their use in the biological fields. Bilayers of amphiphiles are straightforward mimetics of biomembranes and nanoscopic layered supramolecular structures composed of totally synthesized artificial surfactant molecules such as a double-chain ammonium amphiphile [218] which is spontaneously assembled in water. A large variety of bilayer-forming amphiphiles, which are not directly related to the structure of biolipids, have been synthesized [219]. Both static and dynamic structural characteristics of surfactant bilayers, two-dimensional molecular ordering and thermal phase transition from gel to liquid crystals, are in common with those of biological membranes. The immobilization of aqueous bilayer membranes, keeping their structural characteristics, is essentially required for materialization. Self-standing thin film with multilayered lamella structures can be formed by the simple casting of aqueous bilayer solutions on solid surfaces [220]. Electrostatic complexes of charged amphiphiles and oppositely charged polyelectrolytes can be solved in organic solvent, and their solvent-cast films consist of multilayered lamella structures as well as water-cast films [156]. Composites with polymer compounds improve the mechanical stability and processability of immobilized bilayer films. The Langmuir-Blodgett (LB) technique is one of the most conventional methods of nano-film fabrication. The polyion complex technique [221] was proposed to immobilize water-soluble bilayer forming amphiphiles and counter-charged polymers as polymeric LB films. Water-insoluble polyion complexes are formed at the air/water interface when the charged amphiphiles are directly spread on the surface of the aqueous polyelectrolyte solution. Alternative multilayers of the polymer and the amphiphile with stoichiometric ion pairing can be deposited onto solid substrates by the conventional LB technique [222]. The principle of the

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alternative deposition of the polyion complex LB technique has been expanded into a general and simple method of layer-by-layer self-assembling. This technique requires no special apparatus like a Langmuir trough equipping a dipping item. A large number of substances: polymeric [223, 224], low molecular weight, organic and inorganic [225], etc. can be assembled as layered nanostructures by the sequential stepwise adsorption of counter charged substances on solid substrates. Colloidal nano-particles are no exception to this technique [226]. For photonic and electronic applications, many kinds of particles: metals, semiconductors, magnets, polymers, etc. are chosen for nanoparticle-polyelectrolyte assemblies deposited on flat substrates. Caruso and Moehwald have demonstrated that colloidal particles can also be used as substrates for the nano-scaled layer-by-layer assemblies [227]. The size of the colloidal particles used as microtemplates ranges from tens of nanometers to submillimeters. The selective dissolution of the core template provides hollow capsules of the multilayered assemblies, which can be employed as microreactors [228]. Lvov et al. demonstrated the nano-coating of colloid surface by polyionnanoparticle multilayers. A silica latex of 300 nm in diameter was used as a microtemplate for layer-by-layer assemblies of 75-nm diameter silica spheres [229]. The mesoscopic organization of the nano-scaled supramolecular assemblies is the second stage of the bottom up strategy. Mesoscopic structuring of polymer assemblies have been known as micro- and macro-phase separation of block copolymers and polymer blends, respectively [230]. The main driving force of phase separation in polymer films is the incompatibility of each block of the copolymers or polymers. The surface energy of substrates is another factor affecting the microstructures when the polymer film thickness is in the mesoscopic scale (< 100 nm) [231]. Steiner et al. demonstrated controlled mesoscopic structuring of polymer blends in a cast thin film [232], where the micro-phase separation was induced by a pre-patterned substrate surface modified by the microcontact printing (μCP) technique proposed by Whitesides [233]. The μCP is a pragmatic combination of the top-down and bottom-up strategy of material fabrication. This is an efficient method for pattern transfer using polydimethylsiloxane (PDMS) as an elastomeric stamp, and self-assembled monolayers (SAMs) of thiole derivatives as ink materials, respectively. Patterned modification with SAMs can regulate the local surface energy and then polymer wettability of the substrate for polymer casting. Aksay and co-workers [234] succeeded in the mesoscopic patterning of oriented nanostructured silica thin films polymerized by a surfactant-templated sol-gel technique [235] in combination with a micromolding technique, which is another


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pragmatic combination of micro- or nanofabrication technique with self-assembly proposed by Whitesides. The μCP technique is now commonly used for the surface patterning of biomaterials where the cell/substrate interaction, regulated by chemically and topographically modified surfaces, is one of the dominant factors of cell culturing and functioning. The hybridization of biological systems with conventional hard and/or soft materials, semiconductors and polymers, etc. requires the surface modification techniques to be reliable in both a nanoscopic and mesoscopic scale. Boxer and his coworkers succeeded in the micro-patterning of supported bilayer membranes by using a combination of the μCP technique and vesicle fusion [236]. Artificial networks of hippocampal neurons can be arrayed on semiconductor micro-arrays via thin layered polyamino acid pattern stamped by μCP [237]. Another type of dynamic self-organization, so-called ‘dissipative structure’, is known as a general physical phenomenon which is generated under chemical or physical conditions far from equilibrium [238]. Many spatiotemporal patterns of the dissipative structures are formed in the dissipative processes ranging in size from sub-micrometers to hundreds of kilometers. Several types of regular patterns, e.g. spirals in the Belousov-Zhabotinsky reaction systems, the honeycomb and stripes of Rayleigh/Benard convection, are formed as spatiotemporal patterns in the dissipative processes. To utilize the dissipative structures for self-organization of molecular assemblies, the spatiotemporal patterns have to be frozen as stationary stable structures. Muruyama et al have focussed on the casting process of polymer solutions on solid surfaces because the process is complex enough to form the dissipative structures [239]. Although the formation of dissipative structure is a complicated phenomenon, it can be applied to material fabrication because of its physical generality. The hierarchical superstructures of the molecular assemblies are expected if the mesoscopic dissipative structures are generated from the molecules having a selfassembling nature in nanometer scale. Regularly arranged mesoscopic polymer patterns are spontaneously prepared by simple casting of highly diluted polymer solutions. The driving force of the pattern formation is freezing of dissipative structures, such as Rayleigh-Benard convection and fingering instability [240], dynamically formed in the cast polymer solution [241, 242]. A combination of the fingering instability and solvent front receding provides a regular stripe formation and polymer dewetting in each stripe leads a regular dot pattern. Another finding is that the simple casting of polymer solutions under highly humid conditions can provide micrometer scale honeycomb patterned polymer films [243, 244]. Due to the

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evaporation heat of the solvent, micrometer-size water droplets are condensed onto the solution surface and packed. The systematic investigation of the casting experiments reveals that mesoscopic patterning, based on the dissipative spatiotemporal structuring, is applicable not only for synthetic polymers, but also aqueous biopolymers including DNA [245, 246]. Patterned DNA is applicable to novel functional materials bearing molecular information based on complementary hydrogen bonding between nucleobases, as well as DNA chips. Nano- and microparticles, both inorganic and organic, are also hierarchically organized by this method. Hierarchical structures can be fabricated by combination of hexagonally packed silica nano-particles and the honeycomb patterned polymer substrate (Fig. 17) [238].

Fig. 17. Hierarchical structures fabricated by combination of hexagonally packed silica nanoparticles and the honeycomb patterned polymer substrate [238].

Abbreviations AMBN APS CMC D DBP DMF DPN DVB E

2,2’-azobis(2-methylbutyronitrile), ammonium peroxodisulfate critical micelle concentration diffusion coefficient dibenzoyl peroxide dimethylformamide din-pen nanolythography divinylbenzene emulsifier


EGDMA EO HD HLB I IF l LB LCST M MD Mic MP MW MWD o/w ODT OLOA 370 P PAA PAAA PAAm PAAm-co-MGly PAAm-co-PAA PAAm-co-PHEA PAAm-co-PVP PAEI PALA PAP PAP-co-PAAA PAPS PAPS-co-PMPTA Pcrit PDAAm-co-PAA PDAAm-co-PVP PDDA PDMS PEG

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ethylene glycol dimethacrylate ethylene oxide hexadecane hydrophilic - lipophilic balance initiator poly(isoprene-block-ferrocenyldimethylsilane) fully extended chain length of the emulsifier langmuir-blodgett lower critical temperature monomer monomer droplet micelle monomer-polymer particle molecular weight molecular weight distribution oil-in-water order-disorder transition poly(butane succinimide) polymer poly(acrylic acid) poly(α-acetylamino acrylic acid) polyacrylamide poly(acrylamide-co-N-maleylglycine) poly(acrylamide-co-acrylic acid) poly[acrylamide-co-1-(2-hydroxyethyl)acrylate] poly(acrylamide-co-N-vinylpyrrolidone) poly(N-acetyl)ethyleneimine poly(allylamine) poly(N-methyl-N’-methacryloylpiperazine) poly(N-methyl-N’-methacryloylpiperazine-co-a-aminoacrylic acid) poly(2-acrylamido-2-methyl-1-propanesulfonic acid) poly[2-acrylamido-2-methyl-1-propanesulfonic acid-co- (3(methacryloylamino)propyl) trimethylammonium chloride] critical degree of polymerization poly(N,N-dimethylacrylamide-co-acrylic acid) poly(N,N-dimethylacrylamide-co-N-vinylpyrrolidone) poly(diallyl dimethylammonium chloride) polydimethylsiloxane poly(ethylene glycol)

126

PEI PEO PEO-g-PSt PEO–PPO PEO-PPO-PEO


poly(ethyleneimine) poly(ethylene oxide) poly(ethylene oxide) graft polystyrene poly(ethylene oxide) – poly(propylene oxide) poly(ethylene oxide) – poly(propylene oxide) – poly(ethylene oxide) PHEI poly(N-hydroxyethyl)ethyleneimine PHEI-co-PAEI poly[(N-hydroxyethyl)ethyleneimine-co-N-acetyl) ethyleneimine] PIT phase inversion temperature PMAA poly(methacrylic acid) PMAA-co-PAPS poly(methacrylic acid-co-2-acrylamido-2-methyl-1propanesulfonic acid) PMAAMSAA poly(N-methacryloyl-4-aminosalicylic acid) PMMA poly(methyl methacrylate) PMPTA poly[(3-(methacryloylamino)propyl)trimethylammonium chloride] Poly(St-g-MMA) polystyrene-graft-poly(methyl methacrylate) poly-NIPAM poly-N-isopropylacrylamide PSD particle size distribution PSt polystyrene PSt-PB-PSt polystyrene-polybutadiene-polystyrene PSt-PI polystyrene-polyimine PStS poly(sodium 4-styrenesulfonate) PVC poly(vinyl chloride) PVP poly(vinylpyrrolidone) PVPhA poly(vinylphosponic acid) PVPyQ poly(4-vinylpyridine quaternized) PVPyrr-co -PDAEM poly(1-vinylpyrrolidone-co-2dimethylaminoethylmethacrylate quaternized) R radius R. primary radical RM. and RMn. growing radicals, RMj. active primary particle RMz. surface active radical SAMs self-assembled monolayers SDS sodium dodecyl sulfate SEM scanning electron microscopy SSSL super-strong segregation limit TEM transmission electron microscopy


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glass transition temperature Triton X-405 emulsifier hydrocarbon volume, packing parameter of emulsifier molecule in the micellar assembly, a denotes polar head area W winsor w/o water-in-oil WI, WII, WII and WIV winsor microemulsion types I, II, III, and IV, respectively σ the thickness χ flory-huggins parameter Tg,PSt macro Tr405 v v/a 1

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Chapter 3

Preparation of colloidal metal particles 3.1. Introduction 3.2. Bottom-up approach 3.2.1. Precipitation 3.2.2. Microemulsion 3.2.2.1. Inverse microemulsion 3.2.2.2. Inverse copolymer micelles 3.2.3. Other approaches 3.2.4. Bimetalic particles 3.2.5. Reducing agents and processes 3.2.6. Recipes for magnetic colloidal particles Abbreviations References 3.1. Introduction In recent years, the preparation, characterization, and application of the nanosized metal materials have received increasing attention from many researchers in various fields, e.g., chemistry, physics, material science, biology and the corresponding engineerings [1, 2]. Since the nanoparticles usually exhibit unusual electronic, optical, magnetic, physical and chemical properties significantly different from those of the bulk materials due to their extremely small sizes and large specific surfaces areas, they have various potential application as electronic, optical, and mechanic devices, magnetic recording media, superconductors, high-performance engineering materials, dyes, pigments, adhesives, photographic suspensions, drug delivery, and so on [3-6]. Numerous ways of particle preparation may be divided into two main groups, one including physical and the other-chemical approaches. In the first group, metallic nanoparticles are either assembled from atoms in the process of metal vaporization and subsequent condensation on various supports, or obtained through the treatment of the bigger particles in colloidal dispersions by means of colloidal mills,

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ultrasound, etc. In the second group the main chemical way is the reduction of metal ions in solution in conditions favoring the subsequent formation of small metal clusters or aggregates. The main disadvantages of chemical syntheses in liquid phase are their relatively low stability that requires the use of organic stabilizers and thus complicates the structure and studies of the properties of the whole system. The particle stability can be significantly enhanced for the chemical synthesis in the media saturated with organic stabilizer [7]. With respect to the mode of particle stabilization, chemical methods may be divided into groups: 1) chemical stabilizers as natural or synthetic polymers [8] and 2) stabilizers forming micellar solutions where aggregation process takes place in the aqueous core of reverse micelles and growing particles are surrounded by the surfactant molecules [9]. The particle formation can be governed by the thermodynamic or kinetic processes. In the former case, the synthetic process consists of supersaturation stage, nucleation and subsequent particle growth. In the kinetic process, the formation of nanoparticles is achieved by the limiting the amount of precursors available for the nucleation and the growth. Many approaches have been developed to prepare and stabilize nano scale metal particles, quantum dots or nanocrystals in organic and aqueous media. These include, generally, top-down and bottom-up approaches. Top-down methods broadly employed in semiconductor technology (lithography) approaches are mainly based on milling of metal materials. Both the shape and size accuracy are limited by the resolution of pattering. Furthermore these nanocomposites can have a broad size distribution, varied particle shape and geometry and different defects. In addition, the prepared particles may contain larger or smaller fraction of different impurities. The top-down approach mostly does not allow to produce the desired particle size and shape. The bottom-up approaches built the nanostructures from atoms, molecules or atom clusters using physical and chemical deposition methods. Chemical methods belong to a special category of bottom-up techniques. Nanoparticles are prepared, for example, by chemical reactions from appropriate precursors in the form of a colloid solution as a rule [10]. One of the bottom-up techniques is based on organized aggregation of organic precursors in the presence of stabilizers and costabilizers. The preparation of nanomaterials is one of the most active fields in material science. Number of techniques have been used for the production of nanoparticles: gasevaporation [11], sputtering [12], sol-gel method [13], hydrothermal [14], microemulsion [15, 16], polyols [17], laser pyrolysis [18], sonochemical synthesis [19], chemical coprecipitation [20-22], and so on. Among them, the surfactant assembly mediated synthesis is attracting more attention because it allows for a good

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control of the synthesis process at ambient reaction condition [23]. Surfactant assemblies such as reverse micelles [24, 25], water-in-oil microemulsions [26], Langmuir monolayers [27], self-assembled polyelectrolyte films [28] and various vesicles [29] are often used as micro-reactors to obtain nanoparticles with varying diameters. In other cases, surfactant assemblies such as lyotropic liquid crystals [30], mono-layers [31], vesicles [32] and micelles [33] act as the synthesis templates of the nanomaterials. They can direct the synthesis process through spatial-limitation and/or physical and chemical interaction [34]. In some of these processes (the inverse micelles, photoreduction, ultrasound radiation, etc.), nanosized metal particles can be synthesized with different morphologies (nanoparticulate, nanowire, and nanoprism) and sizes but only with a low concentration of metal colloids (several millimoles per liter or less) in the presence of suitable stabilizers. The stabilizers such as ionic and nonionic surfactants and ionic polymers and random, graft or block copolymers cannot be easily removed from the surfaces of the formed metal colloids, which unavoidably affect the physicochemical properties of the resulting nanoparticles. Furthermore, the nanoparticles ranging in several millimoles per liter or less should result in serious difficulties in separation processes, hindering their practical applications. Among these reported methods, the chemical co-precipitation may be the most promising one because of its simplicity and productivity. Conventional magnetic metal powders, with a microscopic grain size, are usually obtained by mechanical attrition, atomization, condensation of metal vapor, electrolytic decomposition from solution, reduction of metal compounds, centrifugal atomization, decomposition of metal hydrides, etc. Hyperfine metals prepared by chemical method which offers the following principal advantageous [35]: - low cost - simple reaction procedure - possible scaling-up - bulk-quantity of product - high chemical purity metals - monodispersed particles - control of particle size from a very small level Potential applications in electronic, optic, and electrooptical devices and sensors have prompted the development of a large variety of colloid chemical methodologies for the preparation of stable monodisperse nanoparticles [23, 36]. The simplest approach involved the judicious adjustment of the precipitation conditions (type, concentration, order, and rate of additions of the reagents and stabilizers or capping agents, stochiometry, temperature, and solvent) [37]. It is important to recognize that in the absence of stabilizers the particles are attracted to each other by van der Waals

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forces, coagulate, and ultimately precipitate. Stabilization of colloidal particles is, in fact, intimately involved in the preparation procedure. Particles can be stabilized electrostatically or sterically. Coating (capping and derivation) by molecules which form chemical bonds with or chemisorb onto the particles provides an extremely useful method of stabilization [38]. Thiols and dithiols have been demonstrated to be highly suitable capping agents. The capped nanoparticles can be separated from the dispersing solvent, stored as dried powders and redispersed on demand in a suitable solvent to form the same size nanoparticles with the same degree of monodispersity [39]. 3.2. Bottom-up approach 3.2.1. Precipitation The (co-)precipitation (homogeneous and heterogeneous nucleation) of particles is based on the supersaturation of solution by reactantants such as precursors (metal salts), reducing agent, stabilizers, co-stabilizers and various additives. The increased solubility of component in the continuous phase can be reached by the rising in temperature. The supersaturation state can be then reached by the reduction in temperature. Generation of supersaturation through in situ chemical reactions by converting highly soluble chemicals into less soluble chemicals is a good example of this approach. In a typical homogeneous nucleation synthesis consisting of one step process in which precursor(s), stabilizer(s) and other additives are stirred in the oilor water-continuous phase and then treated by the heat [40]. For example, FexOy particles were prepared by simple stirring of oleic acid (OA) in octyl ether, then the solution was heated and the solution of Fe(CO)5 precursor was added [41]. The heterogeneous nucleation of metal particles consists of several-steps process [42]. In a typical heterogeneous nucleation the first step is the formation of the primary (seed) metal particles and then the growth of particles is achieved by the addition of precursor(s), stabilizer(s) and some additives. Nanoparticles through homogeneous nucleation can be generated in three media: liquid, gas and solid. A general scheme for preparing monodisperse metal samples requires a single, temporally short nucleation event followed by slower growth on the existing nuclei. The rapid addition of reagents to the reaction vessel increases the precursor concentration above the nucleation threshold. A short nucleation burst partially relieves the supersaturation. As long as the consumption rate of reactants by the growing particles is faster than the feeding rate of precursors to solution, the second nucleation is strongly suppressed or there are no new nuclei. The size distribution is determined by the time over which the nuclei are formed. If the


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particle growth during the nucleation is small compared with the main growth, the particles become uniform at the end of reaction [43]. The contribution of the second nucleation strongly increases the polydispersity. The polydisperse reaction dispersions can be also accompanied by the degradation of particles. Under such conditions the large particles growth on the expense of the small ones. This growth phase is called Ostwald ripening [44]. In this process the high surface energy of the small particles promotes their dissolution and the dissolved material is then redeposited on the large particles. The decreased fraction of smaller particles can lead to the increase of particle uniformity. Some systems can exhibit a second, distinct, growth stage. The average particle size increases over time, with a compensating decrease in particle number. Higher solution temperatures enhance Ostwald ripening (OR), also leading to larger average particle size [Fig. 1] [45]. In general, particle size increases with increasing reaction time, as more material adds to particle surfaces, and with increasing temperature, as the rate of addition of material to the existing nuclei increases.

Fig. 1. Scheme depicting the stages of nucleation and growth for the preparation of monodisperse particles in the framework of the La Mer model (Conc PRE denoted the precursor concentration) [46].

The variation in the reaction temperature initiates both the supersaturation and the chemical reaction as well. The sufficient high temperature of the solution initiates the

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decomposition of the reagents, and so forming a supersaturation of species in solution. Upon nucleation the concentration of these species in solution drops below the critical concentration for nucleation, and further material can only add to the existing nuclei. An alternative synthetic approach involves mixing the reagents in a vessel at a temperature low enough to preclude any appreciable reaction. A controlled ramp of the solution temperature accelerates the chemical reaction and produces the requisite supersaturation, which is then relieved by a nucleation of primary particles. As long as the temperature is adjusted to keep the rate at which the reagents react less than or equal to the rate at which material adds to the existing nuclei, the supersaturated state is never revisited and no new nuclei form. In either approach, the size distribution of the particle sample is limited primarily by the short time interval in which the nuclei form and begin to grow. The systematic adjustment of the reaction conditions - time, temperature, and concentration and chemistry of reagents and surfactants - can be used to control particle size and thus prepare a size series of particle samples. The chemical reduction initially leads to the formation of primary metal atoms or small (primary) particles. These particles subsequently aggregate to form larger particles - which then further grow by nucleation and growth processes [Fig. 2]:

Fig. 2. Scheme depicting the stage of growth by nucleation and aggregation.

When the particle reaches the desired size, further growth is arrested by cooling the solution. The particle dispersions are stable if the interaction between the capping groups and the solvent is favorable, providing an energetic barrier to counteract the van der Waals and magnetic (for magnetic materials) attractions between particles. The particles are then isolated from their growth solution. Introducing a nonsolvent that is miscible with the first solvent but has an unfavorable interaction with the capping groups (hence “nonsolvent”) reduces the barrier to aggregation and destabilizes the particle dispersion, causing their flocculation. Centrifuging the resulting turbid suspension allows the solvent to be decanted and powders of the


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particles to be isolated. These powders consist of the desired particles and their intimate organic capping layer and can be re-dispersed in a variety of solvents. The high-temperature precipitation method is widely used for preparation of nanosized metal colloid in organic liquid phase. Types and concentration of precursor and dispersing agent are important to obtain colloidal solution in which particle size is controlled. Generally, when precursor concentration is increased or the concentration of dispersing agent is decreased at the given condition, the prepared colloid particle size is increased. However, the size control of colloidal particle by concentration of precursor or dispersing agent is difficult, because the change of precursor concentration is interrelated with the change in dispersing agent concentration. Furthermore, some components of reactant might act as impurities of final colloid. Therefore, it is not always feasible to control the colloidal particle size only by the concentration of dispersing agent or precursor. A high-temperature solution-phase synthesis provides a method of preparing such uniform nanomaterial samples for a variety of metals [47] and semiconductors [45]. Each particle or nanocrystal in a sample consists of an inorganic (crystalline) core surrounded by an organic monolayer (Fig. 3). Structural and chemical probes of the inorganic (or metal) core and the organic monolayer are necessary to develop structural models of particle samples. Only with careful characterization can the sizedependent magnetic, optical, and electronic properties characteristic of the particles in the sample be uncovered. These nanomaterial samples may then be used as the building blocks for close-packed particle solids. The organic monolayer coordinating each particle surface enables uniform particle samples, under controlled conditions, to self-assemble into particle superlattices [48]. The hybrid organic–inorganic materials combine the unique properties characteristic of the “individual” particle with new collective properties arising from interactions between neighboring particles in the superlattice [49]. Controlling the size and composition of the particles and the length and chemical functionality of the organic monolayer allows the properties of the individual particle building blocks and the collective properties of the nanomaterial superlattices to be engineered. Higher temperature reduction of metal salts in the presence of stabilizing agents can be employed to produce transition metal (e.g., Co, Ni...) particles that do not crystallize well at room temperature [50]. In this approach metal salts are dissolved in high-boiling inert solvents (e.g. octylether, phenylether) along with a combination of long-chain alkylphosphines and long-chain carboxylic acid (e.g., oleic acid, OA). Metal particles nucleate and grow until the reagent is consumed. The requisite supersaturation and subsequent nucleation can be triggered by rapid addition of

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metal-precursors into a vigorously stirred flask containing a hot coordinating solvent. The solvents usually used are mixtures of long-chain alkylphosphines, alkylphosphine oxides, alkyl amines, etc. where alkyl can be butyl or octyl [45]. In the synthesis of II-VI nanocrystals (MeE where Me = Zn, Cd, Hg; E = S, Se, Te), metal alkyls (e.g. dimethylcadmium, diethylzinc, dibenzylmercury) are generally selected as the group II sources. Group VI sources are often organophosphine chalcogenides or bistrimethylsilylchalcogenides. Injection of reagents into hot solvents (alkylphosphites, alkylphosphates, pyridines, alkylamines, furans, etc.) all produce nanocrystals [51].

Fig. 3. Scheme of inorganic (crystalline) core - organic shell structure of metal particle.

Higher temperature reduction of metal salts in the presence of stabilizing agents can be employed to produce transition metal (e.g., Co, Ni...) particles that do not crystallize well at room temperature [50]. In this approach metal salts are dissolved in high-boiling inert solvents (e.g. octylether, phenylether) along with a combination of long-chain alkylphosphines and long-chain carboxylic acid (e.g., oleic acid, OA). Metal particles nucleate and grow until the reagent is consumed. The requisite supersaturation and subsequent nucleation can be triggered by rapid addition of metal-precursors into a vigorously stirred flask containing a hot coordinating solvent. The solvents usually used are mixtures of long-chain alkylphosphines, alkylphosphine oxides, alkyl amines, etc. where alkyl can be butyl or octyl [45]. In the synthesis of II-VI nanocrystals (MeE where Me = Zn, Cd, Hg; E = S, Se, Te), metal alkyls (e.g. dimethylcadmium, diethylzinc, dibenzylmercury) are generally selected as the group II sources. Group VI sources are often organophosphine chalcogenides or bistrimethylsilylchalcogenides. Injection of reagents into hot


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solvents (alkylphosphites, alkylphosphates, pyridines, alkylamines, furans, etc.) all produce nanocrystals [51]. There are generally two approaches to obtain nanocrystal colloids with uniform size distribution. The first approach is the size selective precipitation method, which is a separation technique relying on the size dependent solubility of nanocrystals in a solvent mixture. Several groups have successfully used this technique to separate different size particles from a polydisperse hydrosol [52] and organic phase colloids [53]. The second method is a result of continuing efforts following the original idea of LaMer et al. [54] and Sugimoto [55], in which a uniform particle size is obtained by a fast nucleation occurring at the early stage of the reaction followed by a diffusion-controlled growth process that is carefully monitored during the rest of the synthesis [56]. Size-selective precipitation which narrows the particle size distribution involves the slow titration of a nonsolvent into the dispersion to bring about its partial flocculation [57]. Since the largest particles experience the greatest attractive forces, they aggregate first. If the dispersion is allowed to flocculate only partially, filtering or centrifuging the suspension isolates a precipitate enriched in the larger particles and leaves the smaller nanoparticles dispersed in the supernatant, which is then decanted. Additional nonsolvent may be added to the supernatant to isolate a second fraction of smaller particles. The precipitates isolated can in turn be redispersed in a solvent and subjected recursively to this gentle destabilization/ redispersion procedure to further narrow the sample size distribution. Narrower initial size distributions allow the desired value to be attained with fewer stages of size-selective precipitation and thus provide higher yields. The monolayer of organic capping groups bound to the nanoparticle surface can be exchanged with other competing capping groups. Repeated exposure of the nanoparticles to an excess of the competing capping groups, followed by precipitation and redispersion in fresh solvent, isolates capexchanged nanoparticles [58]. This process allows the length and chemical functionality of the organic capping layer to be precisely adjusted. Nanoparticle samples with narrow size distributions can be deposited from solvents to assemble into particle superlattices. The solvent used to deposit the particle superlattices is selected for its polarity and for its boiling point. The solvent polarity is chosen so that the interaction between nanospheres will become mildly attractive as the solvent evaporates and the dispersion becomes more concentrated. The boiling point of the solvent is selected to permit the nanospheres enough time to find equilibrium lattice sites before the solvent evaporates on the growing nanospheres superlattice (Scheme 1).

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Scheme 1. Schematic representation of the synthetic procedures to synthesize particle samples by high-temperature solution-phase routes and deposition [59].

Stabilizing agents must be present during growth to prevent aggregation and precipitation of the particles. The particle stability can be significantly enhanced for the chemical synthesis in the media saturated with organic stabilizer [7]. When the stabilizing molecules are attached to the particle surface as a monolayer through covalent, dative, or ionic bonds, they increase the stability of dispersion (stabilizing molecules acts as capping groups [39]. The capping of the reagents on the particle surface is analogous to the binding of ligands into the metal coordination sphere in the coordination chemistry. This method allows to bind the function group(s) and so separate tail from the head (function). Thus, the tail and head groups are independently tailored through well-established chemical substitutions. The particle surface can be also modified through the ligand exchange by repeated exposures of the particles to an excess of a competing capping agent [60]. This recursive approach can cap particles with a wide range of chemical functionalities. The cap exchange process is used to adjust the dimensions of the organic layer surrounding the particles and thus the minimum inter-particle spacing in particle assemblies [48]. The average particle size is varied by the ratio of capping groups to metal salt [61]. Tailoring the ratio of the concentration of reagents to that of surfactants provides control over particle size, since high stabilizer-to-reagent concentrations favor the formation of more small nuclei initially and thus a smaller particle size [47]. The


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chemistry of the surface agent can also be chosen to control particle size. During particle growth, the surfactants in solution adsorb reversibly to the surfaces of the particles, providing a dynamic organic shell (capping layer) that stabilizes the particles in solution and mediates their growth. Surfactants that bind more tightly to the particle surface or larger molecules providing greater steric hindrance (bulkier surfactants) slow the rate of materials addition to the particle, resulting in smaller average particle size. For example, bulkier alkyl segments of surfactants provide larger steric hindrance than more compact alkylsurfactants, slowing particle growth. An effective strategy involves using a pair of surface agents of which one binds tightly to the particle surface, hindering growth, and the other is less tightly bound, permitting rapid growth. For example, judicious adjustment of the ratio of carboxylic acid (tightly bound) and alkylphosphine (weakly bound) stabilizers allows the growth rate and therefore the size of the particles to be controlled. Alternatively, the size of particles may also be increased by supplying additional reagent feedstock to a solution of growing particles. As long as the rate of feedstock addition does not exceed the rate of material addition to the particles, the particles continue to grow without creation of new nuclei. This controlled addition of reagents can be optimized to narrow or “focus” the particle size distribution as material adds to all particles at nearly equal rates and produces an initial variation in particle size that is small compared to the larger, final size of the particles [56].

3.2.2. Microemulsion 3.2.2.1. Inverse microemulsion Reverse micelle synthesis utilizes the natural phenomenon involving the formation of spheroidal aggregates in a solution when a surfactant is introduced to a polar organic solvent, formed either in the presence or in the absence of water [62]. Micelle formation allows for a unique encapsulated volume of controllable size through which reactions and subsequent development of metal and metallic compounds can be produced. Aggregates containing water to surfactant molar ratios of less than 15 are called reverse micelles and have hydrodynamic diameters in the range of 4-10 nm [63], whereas water to surfactant molar ratios of greater than 15 constitute microemulsions, which have a hydrodynamic diameter range between 5 and 100 nm. Within the micelles, reactants for chemical processes can be contained. In developing nanocrystalline materials, solutions of separately prepared reverse micelles containing encapsulate reactants A and B are mixed together.

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The fabrication of nanoparticles within reverse microemulsions [15, 64-66] has been shown to be a convenient route to monodisperse particles of controllable size. This method exploits two useful properties of reverse microemulsions: the capacity to dissolve reactants in the water core and the constant exchange of the aqueous phase between micelles. Within the microemulsion, micelles are in constant Brownian motion and thus collide frequently. A small fraction of these collisions result in micelle fusion which give rise to short lived dimmers [67]. The dimmers subsequently separate to form new micelles containing a mixture of the solutions enclosed in the two originals micelles. Thus, by mixing microemulsions containing different reactants, it is possible to perform chemical reactions inside the reverse micelle water pool, using it as a nanoreactor [68]. If this reaction results in a solid compound, nanoparticles are created and their growth is limited by the micelle size. By using sonication or stirring to further enhance collisions between the microdroplets or micelles containing the different reactants, fusion between two micelles can increase, which produces a transient dimer that exchanges the water cores of the collided micelles. The dimer breaks down again into two reverse micelles with the contents from one micelle transferred into the other [62]. The mixing of the two reactants produces a precipitation reaction, from which nanoparticles can be obtained through centrifugal extraction of the solution. This method has been studied for some years and used for metal [64, 65], semiconductor [66] and oxide [69] nanoparticle synthesis. A schematic picture of this process is represented in Scheme 2. It can be seen that after mixing both microemulsions containing the reactants, it takes place the interchange of the reactants (here referred as Metal salt and Reducing agent) during the collisions of the water droplets. This interchange of reactants is very fast [70], so that for the most commonly used microemulsions it occurs just during the mixing process. The reaction then takes place inside the droplets (nucleation and growth) which control the final size of the particles. Interchange of nuclei or particles between “nucleated” droplets is hindered because it would require the formation of a big hole during droplet collisions and this in turn would require a great change in the curvature of the surfactant film around the droplets which is not favorable energetically. Microemulsions should be chosen so that the curvature radius is similar to the natural radius, otherwise the surfactant film can be opened during collisions which form “transient droplet dimmers”, leading to interchange of particles/nuclei, and therefore not appropriate for the growth control [71]. Once the particles attain the final size, the surfactant molecules are attached to the surface of particles, thus stabilizing and protecting them against further growth.


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Scheme 2. Proposed mechanism for the formation of metal particles by the microemulsion approach [64].

The dynamic exchange of reactants such as metallic salts and reducing agents between droplets via the continuous oil phase is strongly depressed due to the restricted solubility of inorganic salts in the oil phase. This is reason why the attractive interactions (percolation) between droplets play a dominant role in the particle nucleation and growth in the w/o microemulsion reaction medium. For example, a bimolecular chemical reaction between two hydrophilic reactants metallic salt (A ≡ CoCl2, CuCl2, FeCl3, FeCl2, etc.) and inorganic reducing agent (B ≡ sodium borohydride, NaBH4, NH4OH, etc.) forms a product C ( ≡ a metal particle) with all reacting species confined within the dispersed aqueous phase of a w/o microemulsion system (Scheme 3).

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Scheme 3. Proposed percolation and reaction mechanism [64].

The reactant A located in the water pool of one microdroplet somehow finds the reactant B located in the water pool of other microdroplet. Thus, this can occur (1) by desorption of a reactant molecule out of the water pool, migration through the hydrocarbon phase and re-entry into a pool containing other reactant, or (2) by direct transfer between pools during the time of the collision between two droplets. The possibility of this process occurring would be enhanced in the case of the more energetic collisions and if collisions were strongly interactive. Eicke et al. [72] have already shown, using a hydrophilic fluorescer/quencher system, that inter-droplet communication is very rapid, and occurs via a transitory “dimmery” species, formed as a result of droplet collision. Generally the chemical reactions of reactants (metallic salt and reducing agent) within the microdroplet is very fast and therefore the ratedetermining step in the overall reaction will be the initial communication step of the microdroplets with different reactants. The rate of communication has been defined by a second-order communication-controlled rate constant kcom, expressed in terms of the droplet concentration in the continuous hydrocarbon medium. This rate constant is then analogous to a diffusion-controlled rate constant in a homogeneous medium, and represents the fastest possible rate constant for the system. The subsequent reaction or the reaction yield then simply serves as an indicator how fast reactants A and B have communicated. Fisher et al. [73] estimated kcom to be ca. 106 – 107 dm3 mol-1 s-1 for the w/o AOT/water/heptane microemulsion (where AOT denotes bis(2-ethylhexyl)sulfosuccinate). This means that approximately 1 in 103 of


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collisions between droplets leads to exchange, since a diffusion rate constant kD is ca. 1010 dm3 mol-1 s-1 in a solvent with viscosity of n-heptane. The process of microdroplet exchange, which leads to further growth, continues until the particles reach a terminal size determined by the system and the stabilization of the particles by the surfactant [63]. The size of the metallic particles produced is a function of the reaction time, water content in the micelle, the concentration of reactant solutions contained within the micelle, and the solvent type [63]. When we use anionic emulsifier such as AOT or sodium dodecyl sulfate (SDS) then the interface is negatively charged. The metal cations of their salts will be preferentially located close to this interface, whereas the hydrophilic co-reactants prefer to be located in a region away from the interface. This can lead to the separation of reactants within the larger microdroplet. This separation factor can depress or inhibit the rate of reaction. Microemulsions with a small water pool (W < 5 where W denotes the ratio ([water] / [surfactant])) no effective separation within a pool can occur since the pools are very small and hence no retardation due to this effect is possible. The reverse is true for the larger pools (maximal separation with W ca. 20). Also, for very large pools, the effect surface/volume ratio is reduced, and partitioning due to this effect should be again less pronounced. For the metal salt – reducing agent reaction in the aqueous pool, the rate-determining step can be loss of ligand (water, alcohol, aminoalcohol, etc.) from the metal ion. When ligand molecules are tightly bound to the metal ion, a sharp reduction in the reaction rate can be observed [74]. Generally addition of salt strongly influences the degree of dissociation of emulsifier, the solubility of emulsifier in the aqueous phase and the micelle aggregation number. For example the addition of NaCl can even induce a transition from an o/w structure to a bicontinuous structure and a w/o structure [75]. For the relatively low W values, where the droplets are monodisperse and stable, the size of the droplet (which already contains a high concentration of Na+ ions) does not change significantly on further addition of salt. However, at higher W values, microemulsions can be destabilized on addition of salt, so that the size is clearly affected in this case. The supply of metal salt must be regulated and when the small particles are needed then the particle growth must be stopped at a small size by cutting off the supply of reagent. For this reason, very low concentrations (10-3 – 10-4 mol/dm3) are used and a stabilizing agent must be added to preserve monodispersity. An amount of simple metal salts dissolved in water pools stabilized by emulsifiers is, thus, severely limited (mmol kg-1 order concentration in organic solvent) and the yield of the metal nanoparticles is very low for the amount of organic solvent used. The systems serving to control particle growth are themselves nanostructured (micelles, microemulsions, interlamellar space of clay minerals, etc.), just like the particles to be synthesized. The procedures most widely used are described in

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Scheme 3. The average size of the nanoparticles synthesized by the microemulsion method depends on the size of the microemulsion droplet, which is mainly determined by the W ratio [76]. Further progress in this direction was made when the radiation-chemical technique was applied in a reverse micellar system [77]. Here it was possible to obtain some metal nanoparticles living for months and even year. W/o microemulsions have been used for many years as microreactors for the synthesis of ultrafine metallic particles [78, 79]. Since the pioneer works of Boutonnet et al. [80], who studied the production of colloidal Pt, Pd, Rh, and Ir particles by hydrogen or hydrazine (N2H4) reduction in w/o microemulsions, many studies have been made on the synthesis of this type of material. A reverse micelle (microemulsion) method, as a kind of soft technique, is a suitable way for obtaining the uniform and size controllable nanoparticles. The droplet dimension was modulated by various parameters, in particular W [81]. Some studies indicated that with the assistant of cosurfactant, the size of nanoparticles prepared in quaternary reverse micelle system is more controllable [82]. For example, compared with the anionic (AOT) ternary reverse micelle system, the droplet dimension of the quaternary cationic (cetyltrimethyl-ammonium bromide, CTAB) reverse micelles can be elaborately adjusted by changing W with the additional modulation of cosurfactant at the interface of water and oil. The microstructure and dynamic exchange process are dominated by the influence of cosurfactant on the curvature radius and interface rigidity of the droplets in the quaternary reverse micelle [82]. As suggested by several researchers, inverse microemulsions consist of water droplets ranging from 5 to 15 nm [83]. Primary particles can be firstly formed within these water droplets, followed growth stages due to the dynamic nature and collision process among the water domains [84-86]. Eventually, each water domain in inverse microemulsions may contain few particles in the later reaction stage. Oxidation of the out-layer of metal particles, of course, will diminish the crystallinity and purity of metal. This can be depressed by the modification of microemulsion process by the formation of polymer shell or matrix. Such organic or polymer matrix can actually be regarded as a networked material with metal sites. Nanosized composite particles of metal and polymer can be easily isolated and this structure can therefore avoid oxidation and particle growth during the annealing stage. One of disadvantages of the microemulsion method lies in its expensiveness due to the large amounts of surfactant (as much as 20-30%) added to the system. Another drawback is that the surfactant ensuring colloidal stability is adsorbed on the surface of the nanoparticles, thereby decreasing their usability. The disadvantages may be circumvented by the application of micellar synthesis, in the course of which the


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desired reaction takes place in the interior of the micelle. When long-chain surfactants are used, micelles with diameters of 2-10 nm are formed which incorporate substantially less surfactant than with the classic method [87, 88]. Another problem of using w/o microemulsions for nanoparticle synthesis is the separation and removal of some (highly-boiling point) solvents from products. The most obvious way to circumvent the above-mentioned problems is to decrease the amount of surfactant or event to avoid the use of surfactants at all. In the former case the covalently bound hydrocarbon tails of surfactants to the particle surface increase the stability of dispersion and the stable dispersion can be obtained even at a relatively small amount of stabilizer. In the latter case particle synthesis may be controlled by two-dimensional structures, for example, the layers of clay minerals [89]. These themselves are nanostructured systems and their lyophilicity can be modified by the incorporation of alkyl chains. The monodispersity of nanoparticles grown in the interlamellar space is attained by stopping the supply of reagent; otherwise, the increasing voluminous particles of various sizes will push the lamellae apart [87]. Problem concerning of removal of some (highly-boiling point) solvents from products can be solved by synthesis of particles in the solvents with low-boiling temperature, such as CO2 solution. Silver and copper nanoparticles were synthesized in AOT reverse micelles in compressed propane and supercritical- CO2 solutions [90]. A water-in-supercritical-CO2 microemulsion with silver nitrate dissolved in the aqueous core was used to form metal particles [91]. However, the solubility of reducing agent (NaBH3CN, NaBH(OAc)3,..) in supercritical carbon dioxide (SF CO2) is small, on the order of 10-4 M at 40 oC and 200 atm [92]. Therefore, an adition of ethanol, for example, increases the content of the reducing agent in the SF CO2 system. The introduction of a reducing agent to the fluid phase caused reduction of Ag+ to elemental Ag. The reducing agent used was sodium triacetoxyborohydride, NaBH(OAc)3. The following two reducing agents, sodium cyanoborohydride (NaBH3CN) and N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD), were found very effective for synthesis Ag and Cu nanoparticles in the water-in-supercritical CO2 microemulsion [90]. Both of them are more soluble than NaBH(OAc)3 in supercritical carbon dioxide (SF CO2). Under the specified conditions, NaBH3CN can effectively cause reduction of Ag+ and Cu2+ in the microemulsion system, leading to the formation of nanometer-sized metal particles. On the contrary, TMPD is very soluble in SF CO2, with a solubility estimated to be > 0.5 M at 40 oC and 200 atm in CO2. Therefore, no ethanol or other modifier is needed when TMPD is used as a reducing agent for the SF CO2 experiments. Supercritical carbon dioxide (CO2,scrit) offers several advantages over conventional organic solvents including (1) being one of the most environmentally friendly and low-cost solvents available,

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(2) rapid separation of dissolved solute from the solvent by reduction of pressure, (3) providing high diffusivity and thus accelerated reaction rate, (4) tunable solvent strength through manipulation of the density and thus providing some control of the solubility of solutes. Ji et al. [91] have used the RESS (rapid expansion of supercritical solution) method [93] to collect the silver nanoparticles synthesized in the water-in-CO2 microemulsion using NaBH(OAc)3 as the reducing agent. TEM pictures of the collected silver nanoparticles showed an average size of approximately 5-25 nm, larger than the calculated size based on the spectroscopic information. The type and the amount of reducing agent is a further parameter which strongly influences the particle formation in the microemulsion. For example, the reducing agent ((CH3)4NOH) was used to synthesize the nanosized NiZn-ferrite particles by the inverse microemulsion approach [94]. The reverse micellar CTAB/1 hexanol/water microemulsions with identical weight ratios of their three basic constitutive components (CTAB/1-hexanol/wate) were prepared; one comprising an acidic stoichiometric aqueous solution of sulphate salts of divalent precursor cations (Ni, Fe and Zn) and the second comprising a solution of (CH3)4NOH, which served as the precipitating agent. If the amount of precipitating agent was less than the stoichiometric amount needed to precipitate the precursor cations in the form of divalent hydroxides, the resulting powder was primarily made up of goethite (αFeOOH). In other words, if the pH value of the precipitation was below 8, goethite was formed, whereas if the pH of the precipitation larger was higher than 8, a spinel phase with better crystallinity was obtained. The particles synthesized between precipitating pH values of 8 and 10 were found to have average particle sizes at the order of 2–3 nm, whereas powders synthesized at pH values higher than 10 had average particle sizes of ∼ 4 nm. This size of the produced particles is consistent with the estimated diameters of the reverse micelles’ ‘‘water pools’’ in herein used microemulsion system for the given water content [95], used in these experiments. The average particle sizes deduced from the specific-surface area measurements were slightly higher than the values of the same quantity derived from the X-ray diffraction lines-broadening analysis. This is probably a result of the agglomeration of nanosized particles, which occurs not during the chemical synthesizing procedure, but during the isolation of the particles from their parent microemulsion. A reverse microemulsion method is applied for the preparation of composite nanoparticles [96]. By controlling the amount of surfactant and water, fabrication of particles in water-in-oil microemulsions (reverse micelles) affords great control over the size and shape of the particles [15]. This procedure takes advantage of two selforganizing processes. First, the reverse micelles are used to synthesize metallic


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nanoparticles in the water pools of the reverse micelles. Then we take advantage of the constricted environment of the reverse micelle and use it to form a layer of metal on the core magnet. In a second self-organizing process, since metal (for example gold) and sulfur form spontaneous bonds, the gold coating will direct the ferromagnetic nanoparticles into an ordered array on the surface of a thiol functionalized substrate such as silicon or glass to produce a thin film. TEM images of self-assembled gold coated iron nanoparticles onto thiolated TEM grids was reported to show dramatic differences when compared to images of non selfassembled particles [97]. Once the iron nanoparticles have formed inside the micelle, the addition of Au (III) precursor initiated the formation of particle shell. Because gold and iron grow with complementary crystal structures, the metallic gold forms a coating on the outer surface of the iron particles. The gold shells on the iron particles provide functionality and thin films of the gold particles have been made by selfassembly reactions between the gold surface of the particle and thiol functionalized substrates. An X-Ray diffraction pattern obtained on a powder sample of gold coated iron proved the position of two peaks. The 2.5 nm thick gold coating effectively prevents any oxidation of the metallic iron core. Core/shell structured Fe/Au nanoparticles were synthesized by a reverse micelle method [98]. The Au shell was expected to protect the Fe core and to provide for further organic functionalization. These nanoparticles had a size distribution of 5-15 nm diameter and an average size of about 10 nm. The X-ray diffraction pattern showed peaks assigned to Au and Fe, but no diffraction was associated with oxide. The blocking temperature was reported to be 42 K. Other short reports have reported the preparation of core/shell nanoparticles by the microemulsion [99, 100]. The Xray absorption spectroscopy (XAS) showed that the Fe core was extensively oxidized during the preparation and precipitation processes. The oxide was most similar to that of γ-Fe2O3 [101]. It was proposed that the Fe nanoparticle may not be centered in the micelle, resulting in an asymmetric Au shell. An alternate explanation was that there may be grain boundaries in the Au shell that allow for diffusion of oxygen and oxidation of the metallic core. In the report by Kinoshita et al. [102], the same synthetic method was followed, and the sample was characterized by the same methods, along with X-ray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS). The XANES spectra were consistent with the core magnetic phase being primarily Fe3O4. Other studies have suggested that the Fe/Au nanoparticles may not be prepared via the reduction route using the reverse micelle method [103]. The key issues here are the chemical states of the core materials and whether the oxide forms during or after the synthesis process.

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Cho et al. have synthesized Fe-core/Au-shell nanoparticles by a reverse micelle method and investigated their growth mechanisms and oxidation-resistant characteristics [104]. The core/ shell heterostructure and the presence of the Fe and Au phases have been clearly confirmed. The Au shell appears to grow by nucleating at selected sites on the Fe core surface before coalescing. The rough surface could compromise the oxidation resistance of the Au shell. Indeed, the magnetic moments of such nanoparticles, in the loose powder form, decrease over time due to oxidation. The oxidized product does not show crystalline Fe oxides in the powder diffraction pattern. In the pressed pellet form, electrical transport measurements show that the particles are fairly stable, as the resistance and magnetoresistance of the pellet do not change appreciably over time. These results provide direction for new synthesis routes to achieve truly airtight Au shells over Fe cores. The Fe-Ni core-shell particles have also been prepared by the microemulsion approach [105]. First, the Fe nanoparticles were prepared in the nonylphenol poly(ethoxylate) ether/ cyclohexane/water/FeCl3/ NaBH4 imicroemulsions. Then, the Fe particles were passivated by NiCl2 in the same microemulsion type. The particles consist of metallic cores, having an average diameter of 6.1 nm, surrounded by an oxide shell, averaging 2.7 nm in thickness, for a total average particle diameter of 11.5 nm. Elemental analysis determined by XAS and ICP-OES determined the nickel and boron concentrations to be < 4 at. %. The nickel is present to aid in the formation of the passivating layer. The small amount of boron present most likely located on the surface of the growing particle results in disorder in the iron core. The nanoparticles presented here consist of a metallic iron glass surrounded by a disordered oxide shell. This shell protects the metallic core from oxidation for at least 6 weeks. This core-shell structure maintains the favorable magnetic properties of metallic iron while protecting the nanoparticle from oxidation. Using reverse micelles as reaction vessels, it is possible to synthesize iron nanoparticles that are coated with a native oxide shell [106]. Using the aqueous cores of reverse micelles allows for rapid homogeneous nucleation, while the micellar diffusion maintains slow particle growth [107]. The micellar factors play an important role in determining the particle size. Carrying out the reaction in a sequential fashion allows for the product of the first step to act as a nucleation site for the second passivating shell formation [108]. A reverse-micelle technique is considered an efficient route to produce high quality, monodisperse magnetic and superparamagnetic nanoparticles. It has been shown that the basic magnetic properties such as coercivity (Hc), saturation magnetization (Ms), Curie temperature (Tc) as well as lattice constants can be tuned by varying the cation stoichiometry [109]. In this method, cation occupancy, elemental composition,


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morphology and final particle size can be carefully controlled [110]. This control allows for scalable synthesis of tailored magnetic nanoparticles that exhibit increased Ms and lower Hc at room temperature compared to that of the bulk phase material. Nanoparticles have a large surface area and it has been shown that the overall physical properties are determined by the surface environment and bonding to the surface cations. The magnetic properties have been shown to be affected considerably by the type of ligands used for the surface passivation of nanoparticles [111]. Sahoo et al. have presented a detailed analysis of the surface chemistry, particle isolation, dispersion and interaction for various surfactant systems on ferrite particles [112]. Metal nanoparticles were reported to be prepared by the reverse micelles and microemulsions composed of bis(N-octylethylenediamine) metal(II) complexes. These systems have characteristic features that the metal ions are highly condensed in the mesoscopic water pools and a variety of the morphologies of the aggregation system are dependent on the water content and the complex concentrations [113, 114]. Size-tunable silica nanotubes were prepared using a reverse-microemulsion-mediated sol-gel (RMSG) technique [115]. The main advantages of the RMSG approach are that it is easily adaptable to large-scale fabrication, and the diameter of the silica nanotubes is tunable through the use of diffelent apolar solvents. The overall procedure for synthesizing silica nanotubes is illustrated in Figure 4. Silica nanotubes were fablicated by a sol-gel synthesis in a reverse niicroemulsion system. Cylindlical reverse micelles (sofl templates) were fomed from surfactant and metal salt in an apolar solvent: the sol-gel process occurred at the water-oil intelface of the cylindrical reverse micelles, producing silica nanotubes. Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), which can solubilize a relatively large amount of water [116], was selected as the sulfactant for the reverse microemulsion system. An aqueous FeCI3 solution was added into an AOT/ apolar solvent mixture at room temperature. Subsequently, cylindrical reverse micelles, soft templates for the fabrication of nanotubular structures, were generated [117]. AOT micelles are generally a few nanometers in size in the absence of water. However, the addition of water dramatically increases the average aggregation number of the reverse niicelles and thus the hydrodynamic radius of the aqueous micellar core increases [118].

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Fig. 4. Schematic representation of the fabrication of silica nanotubes using the RMSG approach consisting of several steps: 1) formation of reverse AOT micelles, 2) cylindrical reverse AOT micelles, 3) adsorption and hydrolysis of TEOS on the micelle surface, 4) condensation of TEOS, and 5) formation of final silica nanotubes [115 ].

Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), which can solubilize a relatively large amount of water [116], was selected as the sulfactant for the reverse microemulsion system. An aqueous FeCI3 solution was added into an AOT/ apolar solvent mixture at room temperature. Subsequently, cylindrical reverse micelles, soft templates for the fabrication of nanotubular structures, were generated [117]. AOT micelles are generally a few nanometers in size in the absence of water. However, the addition of water dramatically increases the average aggregation number of the reverse niicelles and thus the hydrodynamic radius of the aqueous micellar core increases [118]. Furthermore, the electrical double layer of the micelle is compressed by the enhanced ionic strength, and the repulsion between the anionic head groups of neighboring AOT molecules decreases in the presence of metal cations [119]. Therefore, more AOT molecules participate in micelle formation, and the shape transition originates from the driving force that tends to expose the maximum number of free water molecules to the anionic head groups of the AOT molecules Under these


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experimental conditions, ferric chloride, a metal salt, plays an important role in the formation of the cylindrical reverse micelles because it increases the ionic strength of the solvent and also decreases the second critical micelle concentration (CMC II) of AOT [120]. When tetraethyl orthosilicate (TEOS, the silica precursor) was introduced into the FeCl3/AOT/apolar solvent mixture, it was hydrolyzed at the water-oil interface of the cylindrical reverse micelles. Then, an appropriate amount of sodium hydroxide solution led the condensation reaction of the hydrolyzed TEOS without deforming the cylindrical assembly. The as-prepared silica nanotubes maintained their tubular structure after heat-treatment. The diameters of nanotubes prepared in heptane are 150 nm and their lengths are more than 2 um. A wall thickness is 27 nm. Whereas the diameter and wall thickness of the silica nanotubes prepared in hexane are about 120 nm and 13 nm, respectively, those of the silica nanotubes prepared in isooctane are 180 nm and 34 nm, respectively. Judging from these results, the diameter of the silica nanotubes increases on increasing the hydrocarbon spacer of the apolar solvent. In other words, the average diameter of the silica nanotubes increased in the older hexane >n

(11) (12) (13) (14) (15)

The particle size is strictly related to the reaction time. In order to obtained very small particles, a limited reaction time is usually required. Much larger metal particles are obtained using longer reaction times. Owing to the presence of a mechanical barrier around the particles made by the polyol molecule that bond to the electrophilic metal surface, clusters do not coalescence by sintering.


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Polyol process can be effectively used, for example, for the synthesis of larger amounts of single-crystal nanoparticles of Pt [236]. The key strategy of this approach is the introduction of a trace amount of Fe2+ (or Fe3+) species to greatly reduce the level of supersaturation of Pt atoms and thus the growth rate by slowing down the reduction reaction. Figure 8 summarizes all major steps and changes involved in a typical synthesis. In this polyol process, ethylene glycol (EG) serves as both a reducing agent and a solvent [237]. In the first step, Pt(II) species were formed when H2PtCl6 or K2PtCl6 was reduced by EG at 110 °C in the presence of poly(vinyl pyrrolidone). At room temperature, the Pt(II) species were stable and no particles were formed when the solution was stored in a vial for one month. When the reaction continued in air at 110 °C, the Pt(II) species were reduced slowly to generate Pt nanoparticles with a diameter of ~ 5 nm. If a small amount of FeCl3 or FeCl2 was added after the reaction had proceeded, Pt nanoparticles were produced at a relatively slower rate and tended to assemble into spherical agglomerates and larger structures. This agglomeration might be attributed to the destruction of the stabilization layer around each Pt nanoparticle. Interestingly, the Pt(II) species were reduced at an extremely slow rate by the end of the reaction, and the resultant Pt atoms started to nucleate and grow into uniform nanowires on the surface of each agglomerate. Note that PVP had to be present in the synthesis; otherwise, no Pt nanospheres or nanowires could be formed.

Fig. 8. Schematic illustration detailing all major steps and changes involved in the formation of single-crystal Pt nanospheres and nanowires through an iron mediated polyol process [236].

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Once the nucleation of Pt(0) particles had started, further reduction of the Pt(II) species was accelerated via an autocatalytic process. Although the presence of oxygen could slow this process, the reduction was still too fast to induce anisotropic growth. In contrast, reduction of the Pt(II) species was largely diminished when Fe2+ or Fe3+ was added to the reaction mixture. Since Fe2+ could be readily converted to Fe3+ by oxygen under the conditions used to perform the synthesis, the function of both Fe2+ and Fe3+ species seemed to be similar: to oxidize Pt(0) to Pt(II) (by Fe3+) and thus greatly reduce the supersaturation of Pt atoms. The resultant Fe2+ could be recycled to Fe3+ by oxygen, so that only a small amount of Fe2+ or Fe3+ was needed in this synthesis [236].

3.2.4. Bimetalic particles The advantages in tuning many physical and chemical properties using a bimetallic combination has triggered special interest in the synthesis and stabilization of bimetallic particles over monometallic particles. Here, bimetal refers to particles containing two different kinds of metals, which has either a core-shell or an alloy structure and the kind of structure is decided by the method of preparation. Bimetals can be prepared by either physical or chemical routes. Physical routes mainly consist of vapor deposition of one metal on top of the other, whereas chemical bonds involve simultaneous reduction of two metal ions or reduction of one after another in presence of a suitable stabilizer [238]. Additionally, bimetals generate properties that are different from monometallic components. After preparation of the desired colloid, the microdomains can be reloaded with precursor materials, which can subsequently be reacted to obtain intermetallic nanocolloids, sometimes in the form of onion-type clusters. Currently, there exist a large variety of technological applications of composite particles consisting of a nucleus and shell with different type and compositions. In any case, the aim is to take advantage of the properties of both the nucleus (e.g., size, shape, magnetic susceptibility) and the shell (e.g., surface electrical characteristics, adsorption properties) materials. The solid nucleus (hematite), for example, can be prepared as colloid particles of defined geometry, monodisperse in size and shape, covering a wide range of both particle diameters (from about 10 nm to several microns) and geometries (spheres, spheroids, cubes, needles,..) [239]. Like in many other fields of colloid science, special interest exists on such particles when they can prepared with controlled shape, stability and size [240, 241]. In the case of particular combination, the nucleus is a well-known solid that can be prepared as colloidal


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particles of defined geometry, monodisperse in size and shape, and covering a wide range of both particle diameters and geometries [242]. They can be thought of as supports for other materials that cannot be prepared with such a high versatility. An increasing elegant method of yielding of bimetallic core-shell particles is the microemulsion approach. By controlling the amount and type of metal precursor, surfactant and water, the core-shell particles can be prepared in water-in microemulsions (reverse micelles) [97]. First, the reverse micelles are used to synthesize metallic nanoparticles in the water pools of the reverse micelles. For example, metallic iron nanoparticles are synthesized in reverse micelles of cetyltrimethylammonium bromide using hydrazine as a reducing agent. Ones the iron nanoparticles have formed inside the micelle (the core), an aqueous solution of second precursor (HAuCl4) is added to the seed iron/CTAB stabilized particles. An X-Ray diffraction pattern obtained on a powder sample of gold coated iron proved the position of two peaks. Furthermore, the 2.5 nm thick gold coating effectively can prevent any oxidation of the metallic iron core. Antonietti et al. reported that the size and morphology of bimetal colloids in block micelles were strongly dependent on the type of reducing agents [243, 244]. The use of a strong reducing agent favors simultaneous nucleation at many sites, producing many smaller colloids per micelle. The Au/Pd bimetallic colloids with different metal ratios in micelles or microdroplets can be prepared [244]. It was found that the catalytic activity of Au/Pd bimetallic colloids in the hydrogenation of cyclohexane is higher than that of Pd monometallic colloids and depends on the Au/Pd molar ratio. The phenomenon was explained by the core-shell structure of the metal colloids: Pd atoms are located on the surface of the cluster particles with Au cores. The preparation of core-shell (to overcoat the premature particle) structured metal particles can proceed under a few modest constraints: 1) The particle seed must withstand the conditions under which the second phase is deposited, 2) the surface energies of the two phases must be sufficiently similar so that the barrier for the heterogeneous nucleation of the second phase is lower than that for homogeneous nucleation, and 3) the seed particles and the overcoat material must not readily inter-diffuse under the deposition conditions. First, the seed particles are prepared and isolated and then redispersed in a fresh solution of solvent and stabilizers. The stock solution is then heated while precursors for the inorganic shell are gradually added to allow the material to heterogeneously nucleate on the seed particles. If the rate of precursor addition does not exceed the

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rate of deposition on the seeds, the precursor concentration never reaches the threshold for homogeneous nucleation of a second inorganic phase [245]. The precipitation method first devised by Aiken and Matijevic [241] has been used for synthesizing spherical bimetallic colloidal particles formed by a hematite (α Fe2O3) nucleus and an yttrium oxide (Y2O3) shell [246]. The spherical hematite nuclei were obtained by homogeneous precipitation in a solution containing FeCl3 and HCl. The spherical and reasonable monodisperse particles with the average diameter up to 60 nm were obtained. Then the produced hematite particles were covered by a layer of yttrium. The different electrical surface characteristics of yttrium and iron oxides, as well as the diameters of both types of spherical particles, dominate the overall process of particle aggregation. The aggregation kinetics of the suspensions can be followed by measuring their optical absorbance as a function of time (Fig. 9). The quantity plotted is the absorbance increment, that is, the difference between the absorbance, A, at any time and the initial absorbance, A0. The initial increase in absorbance seems an indication of the fact that a doublet of particles has a larger extinction cross section, Cext, than two individual units sufficiently far apart. This situation is most likely to occur during the initial stages of aggregation; hence, they can be useful in explaining the results of Fig. 9 for short aggregation times.

Fig. 9. Optical absorbance, A, referred to its initial value, A0, as a function of time, for hematite suspension (1) and hematite/Y2O3 core/shell particles (2) of the pH values ca. 7-8, ionic strength 10-3 M NaCl [241].


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In both systems, the dependence of the initial slopes of absorbance vs. pH was described by a maximum at ca. pH 7 for hematite and pH 8 for the mixed particles. This behavior can be understood quantitatively in terms of the values of the pH of both kinds of particles: pH = 7.5 for hematite and pH = 8.5 for the mixed colloids. The absence of electrostatic double-layer repulsions between the particles for those pH values brings about a rapid aggregation, and hence a fast increase in absorbance, as a consequence of the above-mentioned increase in scattering cross sections of doublets as compared with pairs of individual particles. The increase in absorbance (particle aggregation) is much more pronounced in hematite than in mixed particles. The same applies to the effect of NaCl concentration on the initial absorbance slopes. The data yield an increase of the slopes with ionic strength in practically monotonous fashion for the two types of materials. The screening of electrical double-layer repulsions by increasing amounts of ionic species in the medium can account for the faster aggregation. The faster increase in turbidity was found for both hematite and core/shell particles as larger concentrations of NaCl are added to the supporting solution. The essential role played by pH on the charge generation of the two oxides and the shift of one pH unit between the isoelectric points of hematite and yttria manifests in two features: 1) the stability decreases on approaching the isoelectric point from either the acid or basic side and 2) the maximum instability is found for hematite at pH 7 and for hematite/yttria at pH 8, that is, close to the isoelectric points of α-Fe2O3 and Y2O3, respectively. The role of added electrolyte is simply to yield the suspensions of either type more unstable. The colloidal stability of dispersions of hematite/yttria core/shell particles is another essential feature of the colloidal behaviour of particles [246]. The optical absorbance data are used as a probe of their stability and that of suspensions of the pure hematite nuclei and the bimetallic particles. The results can be considered as an independent proof of the efficiency of α-Fe2O3 coating by yttrium oxide, mainly considering that the differences between their isoelectric points (i.e.p. or pH of zero zeta potential) should manifest themselves in clearly distinguishable aggregation-pH trends. The different electrical surface characteristics of yttrium and iron oxides, as well as the diameters of both types of spherical particles, dominate the overall process of particle aggregation. The further studies have been devoted to the summarization of the lightscattering properties [247], surface characteristics [248], and magnetism [249] of the hematite/yttria core-shell nanomaterials.

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The formation multicomponent metal particles can proceed by thermolysis (the precipitation at high temperature). Co-Pc/Fe nanocomposite particles, for example, were prepared by one-step precipitation at higher temperature with precursor complexes [250]. Cobalt (II)- phthalocyanine (Co-Pc) and liquid Fe(CO)5 are employed as the precursors. The complex formation with cobalt-phthalocyanine happened owing to the highly reactive surface of Fe nanoparticles enabled the formation of Co-Pc/Fe nanocomposite particles. The Co-Pc/Fe nanocomposite particles have an almost regularly spherical shape and a relatively smooth surface. The density of nanocomposite particles was determined to be 3.66 g/cm, which is much lower than that of pure iron powders, about 7.8 g/cm3. The value of 3.66 g/cm3 is also much lower than 5.46 g/cm, which is based on the assumption that the density of the nanocomposite particles obeys a linear law of mixture, and the density values of pure iron and organic Co-Pc are 7.8 and 1.52 g/cm3, respectively. The lower density of the nanocomposite particles was attributed to their special structure and organic Co-Pc in the nanocomposites. A HRTEM micrograph confirmed the shellcore structure. The α-Fe nanoparticles derived from the thermal decomposition of the liquid Fe(CO)5 were primarily encapsulated into the interior of composite particles. The surface of the nanocomposite particles is completely covered by Co-Pc layers due to existence of white ring, which represents the organic coating Co-Pc because it has a lower conductivity than metal iron. The average thickness of Co-Pc layers on the surface of nanocomposite particles is certified to be about 100 nm. Both the Fe cluster nanoparticles and aggregates of the cluster nanoparticles were completely covered by Co-Fe during the formation of the Co-Pc/Fe nanocomposite particles. From the 15 nm nanoparticle size obtained from the X-ray diffraction it was estimated that a micrometer-sized magnetic nanocomposite particle consists of hundreds of thousands of α-Fe nanoparticles on the inside dispersed in organic CoPc. A separation of about 5 nm between α-Fe nanoparticles, indicating the presence of a thinner layer, possibly a monolayer of Co-Pc on α-Fe nanoparticles, which implicates an almost molecular level dispersion of the organic component. This preserves a barrier that prevents Fe nanoparticles from growing into bigger particles. TG and DTA analyses indicated the superior antioxygenation behavior of Co-Pc/Fe nanocomposites which was attributed to the currant-bun morphology and covering organic layers of cobalt-phthalocyanine. Fig. 11 indicated that the weight change of Co-Pc/Fe nanocomposite particles started from 150 oC and ended at 545 oC. First, the weight goes down due to loss of the organic component, and later weight goes up due to oxidation of iron. The nanocomposite particles will completely oxidize into Fe2O3 once the Co-Pc layers are lost at a temperature of above 314 oC.


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Fig. 11. TG () and DTA () curves of Co-Pc/Fe nanocomposite particles [250].

The chemical vapor condensation (CVC) method was successfully used to prepare ferromagnetic nanoparticles with a core-shell structure by the pyrolysis of iron pentacarbonyl ([Fe(CO)5]) [176, 177]. The colloidal particles and their properties were observed to depend on the reaction conditions. The synthesis of samples (a) (particle diameter d = 5.4 nm) and (c) (d = 12.3 nm) were carried out under the He atmosphere while the synthesis of the sample (b) (d = 10.3 nm) was carried out under the presence of a little quantity of oxygen (Ar + 1 vol % O2). The decomposition temperature of Fe precursor was 1000 oC for samples (b) and (c) and 400 oC for the sample (a). The XRD patterns showed no visible peaks of Fe for the sample (a) but they are present in the samples (b) and (c). However, the high-resolution TEM showed the particles (a) with a crystal Fe core and glasslike oxide shell, although no diffraction peaks occur. The fitting Mosbauer spectra of the CVC-prepared Fe nanoparticles showed that the shells consist of only Fe3O4 and part of them exhibit superparamagnetism. HRTEM data of samples of (b) and (c) show that Fe3O4 layes epitaxially grow according to the crystallographic direction of a crystal Fe core. In the case of the sample (a), the shell of Fe particles is completely composed of amorphous matter. The presence of a distribution of hyperfine fields is representative of a disordered, amorphouslike structure [251]. Mosbauer spectroscopy investigations performed on Fe-based nanocrystalline alloys show [252] that the hyperfine magnetic field experienced by the resonant iron atom reduces by decreasing the number of Fe nearest neighbors. As revealed by the fitted result of Mossbauer spectra, a distribution of the hyperfine field of about 24% occurs in sample (b). It is assumed that the oxygen atoms are dissolved in the crystal Fe core to

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influence interatomic spacing or coordination, and give rise to a distribution of exchange interaction. A further example of (co)precipitation approach is the preparation of silverpalladium (Ag-Pd) nanobimetallic colloidal particles [253]. The reduction of Ag+ and Pd2+ ions can be monitored by measuring the UV-visible absorbance of sol (sodium borohydride as a reducing agent). The absorption study is somewhat simple while AgNO3 does not absorb in the UV-visible region, whereas PdCl2 gives two absorption peaks at 460 nm and 325 nm [6]. The absence of these two peaks confirms the complete reduction of PdCl2. The color of bimetallic sols ranged from less intensive brown to intensive brown depending on the metal content. Organic modified aminosilicate (N’-[3-(Trimethoxysilyl)propyl] diethylene triamine, TPDT) is used as a supporting matrix as well as a stabilizing agent, to obtain very uniform, well-distributed bimetallic particles. The monometallic dispersion of Pd is found to be very stable for several months while that of Ag is not. This may be due to the low affinity of Ag towards the –NH2 group [254]. However, bimetallic Ag-Pd particles show a good stability over a long period. These nanoparticles are found to be stable for several months in both the solid and the liquid phases. The core-shell nature of the bimetallic particles formed during simultaneous reduction of metal ions is mainly decided by the kinetics of reduction that in turn depends on the stability constant of the individual metal complexes. The stability constant of Pd-amine complex is much higher than that of the corresponding Ag-amine complex. The reported log K1 of the Pd-dien complex is 34 while that of Ag-dien complex is 6.1 [255]. Based on these values, it may be expected that the reduction process would result in the formation of Ag first and then Pd resulting in a core-shell type of structure with Ag as the core and Pd as the shell. The mean particle size (d/nm) slightly increases with increasing concentration of metal ions with respect to silane (silane:AgNO3:PdCl2): d(nm)/(silane: AgNO3: PdCl2): 2/(100:2.5:2.5), 2.5/(100:2.5:3.75), 3/(100:2.5:7.5 )

(16)

The particle size is nearly spherical in all the compositions and the size distribution is very narrow and unimodal. The absence of bimodal size distribution is also an indirect evidence of the formation of bimetal. A bimodal type of size distribution is generally expected due to the difference in growth rates for two metal colloids [256]. Pd and Ag are known to form a series of solid solutions over the whole range of compositions [257]. The complete reduction of the Ag-dien complex to the corresponding metal takes place in ca. 3 min while the Pd-dien complex takes 10 min for complete reduction under the same experimental conditions. This may give rise to


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the presence of different amounts of silver and palladium particles at any given time leading to the formation of an alloy since the two metals are miscible at any composition. Also, a compositional difference is expected between the bulk and the surface. Indeed, this has been observed in the atomic ratios calculated from the XPS intensities. XPS of the as-prepared Ag-Pd bimetallic sample does not show either Ag or Pd peaks. However the corresponding Ag and Pd peaks start to appear after 10 min of etching and the intensities of the Ag and Pd peaks increase with further etching of the sample. Both Ag and Pd peaks are observed after the first etching itself. The calculated concentration ratio of Ag/Pd is 3.2 showing that there is a clear silver enrichment on the surface. Further etching reduces the ratio gradually and at an etching time of 30 min and above the ratio stays to 1. This confirms the fact that the particle consists of both silver and palladium at any stage. This rules out the possibility of a well-formed core-shell structure as observed for the Pt-Pd system using silane as the stabilizing agent [258]. The surface composition of Ag-Pd nanostructures is very different from that of the bulk. For example, Noordermeer et al. [259] reported that the surface of a Pd0.67Ag0.33 alloy has a composition of Pd0.1Ag0.9. This enrichment of silver on the surface is explained based on the difference in surface energy of Ag and Pd. The Ag(3d) and Pd(3d) peaks in Ag-Pd bimetallic cluster could be resolved into a set of respective spin-orbit doublet. Accordingly, Ag(3d5/2 and 3d3/2) doublet peaks are observed at 368.0 and 374.9 eV and they were attributed to metallic Ag. The binding energy values for Pd are observed at 338.0 and 343.1 eV that correspond to Pd(3d5/2 and 3d3/2) peaks. Venezia et al. [260] synthesized pumice-supported Ag-Pd bimetallic particles and characterized them by XRD and X-ray photoelectron spectroscopy (XPS). Huang et al. [261] synthesized Ag-Pd bimetallic alloy particles by a chemical reduction method, characterized them by elemental analysis and XRD, and further used these particles for electronic component fabrication. Rao and co-workers synthesized FCC structured Ag-Pd and Cu-Pd nanoscale alloys in bulk quantities and characterized them with various techniques such as transmission electron microscopy and XRD [262]. It was also reported that that Pt-Pd nanobimetallic particles having a core-shell structure can be prepared in a single-step process using silanes as stabilizers [258]. Several other palladium-containing bimetallic nanoparticles including core/shell structures have been synthesized and reported (Fig. 12 a) [263]. In some of these core/shell nanoparticles, palladium is in the core and the other metal such as copper or nickel covers the surface of palladium (Fig. 12 b). For example, Miyake and co workers synthesized core/shell Pd/Ni nanoparticles from the consecutive reduction of H2PdCl4 and nickel acetate through the polyol method [264]. The synthesis of Pd-Ni bimetallic nanoparticles with a Ni-rich core/Pd-rich shell structure (Fig. 12 c) was

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performed from the thermal decomposition of palladium and nickel precursors [265]. This synthetic procedure employs the thermal decomposition of metal-surfactant complexes [266]. The metal-surfactant complexes are prepared by the reaction of reactants (e.g. trioctylphosphine, TOP), metal salts such as Pd(acac)2 and Ni(acac)2 and oleylamine) at various temperatures. The elemental analysis of the nanoparticles prepared at different temperatures revealed that a large fraction of PdTOP complex was not decomposed below 235 oC. When the injection temperature was increased gradually from 205 to 235 oC, the mole fraction of Pd increased from 7 to 41% , demonstrating that Ni-TOP complex decomposes at a lower temperature than Pd-TOP complex. The particle size of pure nickel nanoparticles was larger than that of pure palladium nanoparticles synthesized under the same reaction conditions.

Fig. 12. Model structures of Pd-Ni bimetallic nanoparticles [263].

The strategy of synthesis of Ni/Pd core/shell nanoparticles was as follows: the first (e.g., Ni-TOP) complex is decomposed at the relatively lower temperature, where the second (e.g., Pd-TOP) complex is rarely decomposed. After aging at lower (ca. 205 o C) temperature to decompose Ni-TOP complex completely, the temperature was slowly increased to decompose Pd-TOP complex, generating the Pd shell on the top of Ni core. The particle size and Pd mole fraction after aging at 205 oC for 30 min were 3 nm and 9 mol %, respectively. After aging at 235 oC for 30 min, the particle size was increased to 4 nm and the Pd content was simultaneously increased to 47 mol %. Field-emission Auger electron spectroscopy (FE-AES) demonstrated that bimetallic Pd-Ni nanoparticles had a Ni-rich core and a Pd-rich shell, similar to the structure shown in Figure 1c. A high resolution TEM image of the nanoparticles revealed the highly crystalline nature of the nanoparticles. The X-ray diffraction pattern of the nanoparticles revealed an FCC Pd crystal structure.


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Yue and Cohen prepared (ZnCd)S2 colloids by multiple loading experiments [164]. The carboxylic acid coordination sites are regenerated and can be re-used to make onion-type binary clusters. This technique allows for the possibility of cluster-size control and the synthesis of core-shell clusters through multiple metal-loading and reduction cycles. Recharging leads to ZnCdS colloids. In a similar fashion, bimetallic Au/Pd nanocolloids have been synthesized as catalysts for hydrogenation of dienes [244]. Eisenberg and coworkers used this method to increase the size of CdS colloids [267]. A one step precipitation approach was used to prepare the bicomponent particles Cd and Se in the presence of polyvinyl alcohol (PVA) [268]. The multicomponent metal nanoparticles can be formed via the alloying approach. Kinetic considerations concerning of alloying of small particles indicate that, for the two metals to mix, the diffusion coefficient needs to be many orders of magnitude larger than that for the bulk materials [269]. Shimizu et al. reproduced Mori and Yasuda’s results of rapid alloying using two-dimensional molecular dynamics simulation and concluded that the surface melting plays an important role [270]. They have used relatively small NPs (~ 100 atoms). Studies on Sn alloying at Cu surfaces reveal a process driven by surface free energy that leads to migration of tin atoms at the NPs surface [271]. Because Tm of the smaller particles is much lower than the corresponding bulk, much faster interdiffusion of the atoms is expected in NPs. Preservation of twinning boundaries during the alloying of copper in gold NPs led Mori et al. to conclude that the particles remain solid during the alloying process [272]. A number of studies on mechanical alloying have suggested that interfacial imperfections enhance the diffusion by many orders of magnitude [273]. It is also clear that often-used supports play an important role in the crystallization and morphology of NPs [274]. The atoms at the boundary between the two metals in small NPs, which were initially constructed as core-shell structures, were reported to spontaneously interdiffuse [275]. The rate of alloying of the two components is size dependent but cannot be explained merely by size-dependent depression of the melting point that leads to enhanced diffusion. It is hypothesized that vacancy defects at the boundary between the two metals cause the observed enhancement. Molecular dynamics estimations demonstrated that vacancies at the boundary dramatically enhance the rate of mixing. The presence of vacancies at the bimetallic interface can explain the observed termination of the alloying process. The alloying may be viewed as a competition between percolation of the defects to the outer surface and migration of the metal atoms into the vacancies. Once a vacancy percolates to the outer interface, its penetration back into the lattice is expected to be extremely slow. Preferred directionality in migration toward the surface is expected because of the larger volume fraction and smaller curvature of the outer layers. Furthermore, a higher energy barrier is required to generate a vacancy in the bulk relative to the

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surface. Both effects will ensure prohibitively slow vacancy penetration back into the bulk. Combinations of Au and Ag have been extensively studied, often because of the great interest in their plasmon band [276]. In characterizing Au-Ag particles using TEM, Hodak et al. have shown clear phase boundaries between the Au core and the Ag shell for particle sizes ≥ 20 nm [277]. For particles of smaller sizes, the phase boundaries are not observed. Reports using laser-induced melting of mono- and bimetallic nanoparticles outlined the events that lead to melting and intermetallic mixing of the two metals [278]. Furthermore, light-induced melting has recently been used to control the shape and morphology of metallic NPs and thereby manipulate their absorption characteristics [279]. On the other hand, the depression of Tm in NPs was used to thermally control their shape [280]. Among the most important for the broad applications of nanomaterials are the atomically ordered intermetallic compounds of the transition metals, which remain impossible to synthesize as solution-dispersible nanocrystals. While most reports of bimetallic nanocrystals involve atomically disordered alloys [281], a few have succeeded at transforming disordered alloys into ordered intermetallic compounds. For example, Sun and co-workers accessed the ordered face-centered tetragonal (fct) form of FePt by annealing a spin-cast film of atomically disordered FePt nanocrystals at high temperatures [282]. As synthesized, the atomically disordered FePt nanocrystals are superparamagnetic, but transform to ferromagnetic FePt nanocrystals with high coercivity and high magnetic anisotropy after annealing above 550 oC. [42]. Likewise, Teng and Yang recently transformed Pt-Fe2O3 coreshell nanoparticles into atomically ordered fct FePt nanocrystals by high-temperature (550 oC) reduction and annealing of a surface-confined monolayer film [283]. These methods prove the feasibility of solid-state transformations in nanocrystals and attest to the enhanced properties that can be achieved through such reactions. However, there remain several critical limitations. The high annealing temperatures remove the stabilizing ligands, preventing redispersion in solution for high-volume applications. The high annealing temperatures also tend to significantly increase the crystallite size through sintering. As a result, only surface-confined nanocrystals have been amenable to solid-state transformations [284] since multilayer films and bulk powders significantly coalesce at high temperatures. To prevent the detrimental effects of sintering on the morphology of internally structured nanocrystals and nanocrystalline films, new lower-temperature routes to atomically ordered nanocrystals are necessary. To that end a new multistep approach to the synthesis of intermetallic nanocrystals that relies on the low-temperature


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annealing of bimetallic nanoparticle precursors is needed. A new multistep approach was developed to synthesize atomically ordered intermetallic nanocrystals, using AuCu and AuCu3 as model systems [285]. Bimetallic nanoparticle aggregates are used as precursors to atomically ordered nanocrystals, both to precisely define the stoichiometry of the final product and to ensure that atomic-scale diffusion distances lower the reaction temperatures to prevent sintering. In a typical synthesis, PVPstabilized Au-Cu nanoparticle aggregates synthesized by borohydride reduction are collected by centrifugation and annealed in powder form. At temperatures below 175 oC, diffusion of Cu into Au occurs, and the atomically disordered solid solution CuxAu1-x exists. For AuCu, nucleation occurs by 200 oC, and atomically ordered AuCu exists between 200 and 400 oC. For AuCu3, an AuCu intermediate nucleates at 200 oC, and further diffusion of Cu into the AuCu intermediate at 300 oC nucleates AuCu3. Atomically ordered AuCu and AuCu3 nanocrystals can be redispersed as discrete colloids in solution after annealing between 200 and 300 oC.

3.2.5. Reducing agents and processes With respect to the nature of reducing agent, chemical methods may be subdivided into classical ones, using the well-known chemical reducing substances (hydrazine, borohydrates: LiAlH4, NaBH4, LiBEt3H, NaBH(OAc)3, NaBH3CN,… hydrogen, alkylsilanes, formaldehyde, alcohols, (CH3)4NOH, polymers, etc.) [7] or some special reducing medium and radiation-chemical approaches including radicals where the reduction process is initiated by solvated electrons generated by the ionizing radiation [286]. Chemical reduction methods such as alcohol reduction [287], hydrogen reduction [288], sodium borohydride reduction [289] etc. have been the most common ways of synthesizing colloidal metal nanoparticles. Other reduction methods such as electrochemical [290], photochemical [291], and sonochemical [292] reduction methods have also been used to a smaller extent. The introduction of a reducing agent to the fluid phase causes the reduction of metal precursor. The reduction can proceed under different reaction conditions, such as the solubility of a reducing agent in the continuous (fluid) phase, pH of the reaction mixture, temperature, etc. Reduction of metal salts requires adjustment of the reactivity of the reducing agent to the redox potential of the metal. The redox potential of a given metal salt can depend on the coordinated ligand. For example, complexes of soft-base ligands with many transition metals (Ag+, Pt2+, Au3+, Cd2+, Pd2+) have low redox potentials because of partial charge transfer from the polarizable ligands to the positively charged metal. It is often surprisingly difficult to

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reduce noble-metal salts in polymeric ligand matrices. For example, AuCl4− is a strong oxidizing agent with a standard potential of +1.00 V in contrast to Au(CN)2−, which has a standard potential of −0.6 V. It is thus not possible to reduce Au(CN)2− with hydrogen even under high pressure, usually a standard method for the preparation of noble-metal colloids. By mixing microemulsions containing precursor and reducing agent, it is possible to perform reduction inside the reverse micelle water pool, using it as a nanoreactor [68]. This is the most simple and common way to prepare metal particles at room temperature by using a classical water soluble reducing agent (borohydrates, hydrazine, ascorbic acid. To the group of reducing agent belongs also (CH3)4NOH whose amount was shown to play a crucial role in the process of synthesizing the nanosized NiZn-ferrite particles by the inverse microemulsion approach [94]. The reverse micellar CTAB/1-hexanol/water microemulsions with identical weight ratios of their three basic constitutive components were prepared and mixed. If the amount of precipitating agent was less than the stoichiometric amount needed to precipitate the precursor cations in the form of divalent hydroxides, the resulting powder was primarily made up of goethite (α-FeOOH). If the pH value of the precipitation was below 8, goethite was formed, whereas if the pH of the precipitation larger was higher than 8, a spinel phase with better crystallinity was obtained. The powders synthesized between precipitating pH values of 8 and 10 were found to have average particle sizes at the order of 2–3 nm, whereas powders synthesized at pH values higher than 10 had average particle sizes of ∼ 4 nm. This size of the produced particles is consistent with the estimated diameters of the reverse micelles “water pools” in herein used microemulsion system for the given water content [95], used in these experiments. When the reaction proceeds under a very low temperature the solubility of common reducing agents is strongly depressed. This is the case with NaBH (OAc)3 as a reducing agent in the supercritical CO2 microemulsion - its low solubility in the fluid phase. The involving of NaBH(OAc)3 into reduction reactions was accomplished by dissolving the reduction agent in ethanol followed by injection of the ethanol solution into the supercritical CO2 (SF CO2). Then, the silver and copper nanoparticles could be synthesized in sodium bis(2-ethylhexyl)sulfosuccinate reverse micelles in compressed propane and supercritical ethane solutions [90]. A water-in supercritical-CO2 microemulsion with silver or copper nitrate dissolved in the aqueous core in the presence of reducing agent and solvent was used to form metal particles [91]. Furthermore, the other reducing agents, such as sodium cyanoborohydride (NaBH3CN) and N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) were found very effective for synthesis Ag and Cu nanoparticles in the


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water-in-supercritical CO2 microemulsion [90]. The reducing efficiency of reducing agents depend on their stability under the different reaction conditions (temperature, pH, type of solvents, etc.). For example, NaBH3CN and TMPD are more soluble than NaBH(OAc)3 in supercritical carbon dioxide (SF CO2). NaBH3CN is stable in aqueous solutions at least in the pH range 2-8 [293]. The pH of water in equilibrium with CO2 was measured to be around 2.9 and 3.5 [294, 295]. NaBH3CN is also stable in the aqueous core of the water-in-supercritical-CO2 microemulsion. The oxidation potentials of these reducing agents in supercritical CO2 are unknown. In organic solvents such as diglyme, the redox potential of NaBH4 was reported (E = - 0.6 V vs NHE) [296]. The oxidation potential of NaBH3CN is probably higher than that of NaBH4. The standard redox potential of TMPD in N,N-dimethyl formamide for the reaction TMPD → TMPD.+ + e− was reported to be E = 0.45 V (vs NHE) [297]. The reducing agent system can control the architecture of final metal particles. The chemical reduction in aqueous solutions of the common silver compound, silver nitrate, are broadly used to prepare metal particles [298]. Hydrazine hydrate and formaldehyde were widely used to prepare ultrafine metallic powder as reducing agent. The reducing power of hydrazine hydrate or formaldehyde was reported to be too strong to obtain uniform size and monodispersed silver powder. Ascorbic acid having middle reduction power was chosen as an ideal reducing agent for this task. In this process, the following chemical reaction occurred: 2Ag+ + C6H8O6 ⎯→ 2Ag0 + C6H6O6 + 2H+

(17)

If the reaction rate is too fast, rapid formation of a large amount of metal nuclei will result in the too small particles to separate easily. If the reaction rate is too slow, it is very difficult to prevent the powder from coming together. In the course of reaction, addition of dispersed AgNO3 droplet into the dispersed ascorbic acid is important that a confined amount of silver ion in a droplet of AgNO3 may govern the size of silver powders. In the absence of dispersion agent, it is possible that monodispersed particles tend to grow once silver ion remained in the solution. According to Eqs. (18) and (19), the potential varies with pH value, and it shows the increase of reduction ability of ascorbic acid [298]. In addition, the existing state of silver ion varies with the pH value in aqueous solution and so the reduction process. C6H6O6 + 2H+ + 2e = C6H8 E0 = 0.08V (pH = 6.4) E = E0 + 0.059 pH

(18) (19)

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In some cases the particle morphology can be varied by the selection of the reducing agents. The silver nanorods can be obtained by the variation of reducing agent efficiency and the seed-mediated growth method, the latter involves at least two steps [299, 300]: 1) the preparation of small size spherical silver nanoparticles, and 2) growth of the prepared spherical particle in surfactant (CTAB) media. In the first step, silver nanoparticles with diameters ca. 6 nm were formed by chemical reduction of AgNO3 with a strong reducing agent such as NaBH4 in the presence of sodium citrate dihydrate to stabilize the nanoparticles: 2NaBH4 + 2AgNO3 + 6H2O ⎯→ 2Ag + 2NaNO3 + 2H3BH4 + 7H2

(20)

Prepared spherical silver nanoparticles are then added to a solution containing more silver salt, a weak reducing agent (ascorbic acid) and a surfactant (CTAB). Ascorbic acid is a weak reducing agent and cannot reduce the silver salt in the present of the micelle without the presence of seeds [301]: C6H8O6 + 2AgNO3 ⎯→ 2Ag0 + C6H6O6 + 2HNO3

(21)

CTAB forms a bilayer structure around silver nanorods (formed in the previous step) during the growth of nanorods [302]. El-Sayed and coworkers provided two reasonable explanations for the formation of faceted and rod-shaped particles; first, the growth rates vary at different planes of the particles and second, particle growth completes with the capping action of stabilizers [303]. The difference in standart potentials of two metals or solid template can induce or promote the reduction process. This is the case with HAuCl4 and silver nanorod template. Gold nanoshells have been synthesized by reacting aqueous HAuCl4 solutions with solid templates (silver nanorods), because the standard reduction potential of AuCl4−/Au pair (0.99V, vs. SHE) is higher than that of Ag+/Ag pair (0.80V, vs. SHE), silver nanostructures suspended in solution can be oxidized by HAuCl4 according to the following replacement reaction 3Ag(s) + AuCl4− (aq) ⎯→ Au(s) + 3Ag+(aq) + 4Cl−(aq)

(22)

This shell seems to have an incomplete structure in the initial stages because both HAuCl4 and AgCl can continuously diffuse across this layer until the silver template has been completely consumed [304, 305]. The nanosized silver particles have been prepared from an aqueous solution of silver nitrate employing only formaldehyde and trisodium citrate as a reductant and a stabilizer, respectively, which can be easily


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removed from the resulting nanoparticles by washing. The chemical states of Ag+ in the initial reaction solution have a profound influence on the size and size distribution of the resulting silver nanoparticles. In the polyol process methoxy polyethylene glycol (MPEG, CH3O(CH2CH2O)n–H) acts as a reducing agent. It generates free radicals by ultraviolet irradiation which reduce the metal ions. MPEG also act as the template of the metal particles. Thus, photolysis of water in presence of a ultraviolet source (hν) leads to the formation of radicals which then take part in the further steps [306]: H2O(hν) ⎯→ H• + OH• (23) The scavenger of the H• and OH• radicals is CH2CH2–OH, and this yields −CH2CH•−OH. −CH2CH2OH (OH• ) ⎯→ −CH2CH•−OH + H2O −CH2CH•−OH (H• ) ⎯→ −CH2CH•−OH + H2

(24) (25)

−CH2CH•−OH has a very powerful reducing property and the standard electrode potential (E0) value is: E0 (−CH2CHO/−CH2CH•−OH) = −1.9V (26) which is able to reduce silver ions as follows: Ag+ + −CH2CH•−OH ⎯→ Ag0 + CH2CHO

(27)

The polyol process [35] represents a convenient chemical route for the synthesis of bulk quantities of metal clusters. The method is based on the alcoholic reduction of a metal ion at high temperature. In the synthesis of metal clusters, for example, cobalt (II) hydroxide (Co(OH)2) is quantitatively reduced to the zero-valence state by di(ethylene glycol) (O(CH2 – CH2 – OH)), at the refluxing temperature (i.e., 245 oC). Owing to the low tendency of the metal particles to coalescence during the growth step, polymeric protective agents (e.g., poly(vinylpyrrolidone)) are not required to ensure the steric stabilization of metal clusters. The metal precursor is slightly soluble in the glycole and therefore a heterogeneous phase reaction is involved. A preparative method for silver particles by taking the advantage of the orientation of polymer methoxy polyethylene glycol (MPEG) in an ultraviolet environment has been reported by Mallick et all. (see above) [306]. MPEG generates free radicals which serve as the reducing agent for the silver ions in the presence of ultraviolet irradiation and also act as the template of the silver particles involved in the

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formation of the silver chain. For the synthesis of metal nanoparticles, the first step involves the reduction of the metal ions in solution. The atoms produced act as nucleation centers and catalyze the reduction of the remaining metal ions present in the bulk matrix. This stage of the process has an autocatalytic nature. The reduction potential of the metalion/metalatom and the metalion/metalcluster systems becomes more negative than that of the corresponding bulk metal. The reduction potential of the Agion/Agbulk (aqueous) system is + 0.79V versus normal hydrogen electrode (NHE), but for the Agion/Agatom (aqueous) system it is -1.8V versus NHE [307]. In the ethylene glycol process, the alcohol served both as solvent and reducing agent for metal ions. However due to slow reaction rate, the synthesis is often carried out at higher temperatures, e.g. 90–160 oC to shorten the reaction time. Carotenuto et al. [308] used sonication at room temperature instead and also obtained silver nanoparticles after 24 h of reaction. On the other hand, if the reaction was performed in aqueous solution, a reducing agent, such as formaldehyde or hydrazine is needed to produce silver colloids. To enhance the reducing power of formaldehyde, additional alkaline agent such NaOH or Na2CO3 is often added in appropriate amounts. When NaOH was used to promote the reduction reaction, the precursor solution would quickly turn into dark color upon NaOH addition and then gradually change into yellowish green color as reaction was completed. Presumably, the dark color suggested that silver ion was reduced into extremely fine silver particles [309]. When sodium carbonate was used to promote the reduction of Ag salts, the reaction path was significantly changed [309]. First, one would observe a color change to dark gray upon the addition of sodium carbonate, suggesting the formation of white Ag2CO3 precipitate. In an alkaline solution, the reduction of silver ion by formaldehyde can be represented by the following reaction: 2Ag+ + HCHO + OH− ⎯→ 2Ag0 + HCOO− + 2H2O

(28)

Yet when Na2CO3 was used, the silver ion would first combine with carbonate ion to form Ag2CO3. As the carbonate ion was hydrolyzed to produce some hydroxyl ions, it started to reduce silver ion into silver metal. This reaction decreases the carbonate ion concentration and Ag2CO3 precipitate would then dissolve back to release more carbonate ions. As a result, the suspension would gradually become dark color, similar to when NaOH was added. And finally, when the reaction was complete, the solution would change to a color corresponding to the size of silver colloids. Poly(alkyl metharylate)s belong to the group of weak reducing agents. Yin and Horiuchi have applied the reducing power of some block copolymers to produce and assemble palladium (Pd) nanoparticles in two-dimensional (2D) periodic arrays in


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nanoscales [310]. They have also used block copolymer (BC) thin films as templates. A symmetric diblock copolymer of poly(methyl methacrylate)-block-poly(2 hydroxyethyl methacrylate) (PMMA-b-PHEMA) was coated on a Si-wafer by dipcoating from three different solvents having different solubility against each component. The monolayer films thus-prepared were exposed to the vapor of palladium (II) bis(acetylacetonato), and Pd nanoparticles were selectively produced in the PHEMA phase due to its stronger reducing power than the PMMA phase. A new reducing agent (one of the natural plant pigments, quercetin, Qr) for the synthesis of Ag nanoparticles in reverse micelles was reported to generate highly stable and rather monodisperse particles [311]. It was reported that apart from its strong interaction with performed nanoparticles, Qr reduces silver ions from aqueous salt solutions, presumably through the formation of an intermediate complex where electron density is shifted towards the silver ion. Barnickel et al. [312] have reported that the terminal hydroxyl groups of the poly(ethylene oxide) (C12E5) surfactant, which are oriented toward the inner part of the droplets, are capable of reducing Ag+ ions upon radiation. Under daylight irradiation Ag colloids were generated in C12E5/cyclohexane/water microemulsion systems. The precipitation process yields highly concentrated and stable dispersions of monodispersed silver nanoparticles in a simple manner, by reduction of concentrated aqueous solutions of silver nitrate with ascorbic acid in aqueous medium [313]. Ascorbic acid has ability to reduce Ag+ salts and so to precipitate metallic silver in acidic solution according to 2Ag+ + C6H8O6 ⇔ 2Ag0 + C6H6O6 + 2H+

(29)

Most of other common reducing agents, such as hydrazine and formaldehyde, are effective only in solutions of neutral or basic pH and, therefore, require a base to complete the reaction. The reduction of Ag+ can proceed by the radicals derived from initiator by its decomposition at elevated temperatures. The Cu2+ reduction might be initiated by the formation of a small amount of radicals at elevated temperatures; this radical can then react with Cu2+ to produce Cu+, followed by disproportionation to Cu2+ and Cu0 [314].

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3.2.6. Recipes for magnetic colloidal nanoparticles General Magnetic (oxide) colloidal nanoparticles or nanocrystals of the elements in the fourth row of the periodic table (Fe, Co, and Ni) are important for the understanding of magnetic properties in a nanometer regime [315] and several technical applications [316], ranging from magnetic resonance imaging, drug delivery, battery materials, catalysts, biosensing, to nanoelectronic materials, etc. Realization of these goals relies on the availability of size- and shape-controlled nanocrystals. Colloidal magnetic oxide nanocrystals are traditionally synthesized through the precipitation of nanocrystals from basic aqueous solutions with a broad size distribution [317]. The nanocrystal particles can be generated in heterogeneous media of microemulsions where aqueous micelles of variable sizes act as microreactors [318]. When the precipitating agent (OH−) is slowly generated in aqueous solution by hydrolysis of a molecular precursor (urea, urotropin, etc.) the particle size is easier to control [319]. The greatest advantage of these techniques is that the surface of the produced nanocrystals remains active and available for postsynthesis chemical modification. Other methods use the nucleophilic property of water in hydrolysis of metal precursors. Reactions usually are slower than ion metathesis reactions, and therefore crystallization is easier to control. Metal and metal oxide particles are synthesized by a variety of methods (see above) based on relatively simple chemical reactions yielding products with desired composition, purity, and crystal structure. Some methods are rooted in the ion metathesis reactions in solution. These straightforward reactions provide good yield and purity of the products, but they occur instantly on mixing and leave little possibility to control the course of the crystallization. Different techniques were developed to address this problem. According to one of them, ion metathesis reactions are performed under strictly controlled conditions of mixing such as addition of the dilute reactant solutions at a restricted rate with vigorous stirring and maintaining the proper temperature [320]. The nanocrystalline metal oxides can be prepared by the hydrolysis of chelate metal alkoxide complexes at elevated temperature in solutions of the parent chelating alcohols [321]. This method allows the preparation of nonaggregated nanocrystals with variable size and composition and high crystallinity. The surface of the isolated nanoparticles is coated with a labile layer of the solvent and remains chemically active and available for further derivatization. Nanocrystals in this state are capable of forming stable aqueous colloids without using capping ligands or surfactants. The


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developed method represents a highly economical and facile “green” process that can be used for scaled preparations. Hydrolyzable salts of metal ions are used for synthesis of corresponding oxides in colloidal form by their forced hydrolysis under hydrothermal conditions [322] or in high-boiling solvents (polyols) [323]. Hydrolysis in nonaqueous solutions has been applied also to metal alkoxides [324] and diketonates [325], offering a convenient route to the uncapped nanoparticles. Synthesis of oxide nanocrystals has been directed to nonaqueous approaches [326-328] mostly inspired by the success of the synthesis of high quality semiconductor nanocrystals in nonaqueous media [329]. The quality of the nanocrystals yielded by these nonaqueous solution methods is generally better than that of the nanocrystals synthesized in aqueous solutions. The thermal decomposition technique is usually used in conjunction with tactical targeting control over composition of the surface of growing nanocrystals. This technique offers a convenient way to manipulate the kinetics of crystallization and therefore the nanocrystal dispersity. The thermal decomposition of oxygen-rich molecular precursors or metal carbonyls in the presence of oxygen or oxygen donors in solutions of high-boiling nonpolar solvents leads to nanocrystals [330]. The synthesis reactions are performed in the presence of complexing agents that reversibly bind to the coordinatively unsaturated metal atoms at the crystal surface. These complexing agents (capping ligands) contain one or more substituents that provide steric separation between nanocrystals and adjust their affinity to the medium, stabilizing their colloids. Capping ligands [331] and coordinating polymers [332] are sometimes used also to passivate the nanocrystal surface in combination with hydrolytic and ion metathesis methods. Metal carboxylate salts, including their fatty acid salts, are the most common compounds for most metals and these compounds are compatible with nonaqueous media. Colloidal nanocrystals are the most developed ones in terms of synthetic chemistry due to the success of the organometallic approaches [333] and the alternative (or greener) approaches [334]. The key to this success, as revealed by the mechanism studies, is to maintain a balance between the nucleation and growth stages [335]. This balance can be better achieved by the noncoordinating solvent approaches introduced recently [335]. This is so because the reactivity of precursors in noncoordinating solvents can be fine-tuned by varying the bonding strength of the ligands to the monomers, concentration, chain length, and/or configuration of the ligands for the monomers [336]. Synthetic approaches using metal fatty acid salts were reproducible, significantly better than the ones using metal oxide powders dissolved by fatty acids directly in the reaction flasks as the starting material [337].

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To tune the activity of the metal fatty acid salts, a certain amount of the corresponding free fatty acids was employed as the ligands for both monomers and nanocrystals. The noncoordinating solvents were either octadecene (ODE), n eicosane, tetracosane, or a mixture of ODE and tetracosane. Activation reagents, either primary amines or alcohols, were used in cases for accelerating the reaction rate and lowering the reaction temperature. Water-soluble nanoparticles (particles soluble in polar solvents) Magnetite (Fe3O4) nano-particles (recipes): A stoichiometric ratio 1:2 ferrous sulfate hepta-hydrate (FeSO4 x 7H2O) and ferric chloride hexa-hydrate (FeCl3 x 6H2O) is dissolved in deionized water under vigorous stirring to prepare total concentration of 0.20 M ferrite solution as an iron source [338]. Concentrated ammonia was then dissolved in an aqueous solution to form 3.5 M ammonium hydroxide (NH4OH) as a base source. Dextran was dissolved in hot deionized water to form a coating solution. A 50 ml ferrite solution was mixed with an equal volume of dextran aqueous, then appropriate amount of urea was added into the mixed solution. The mixture was heated gently up to 80 – 100 oC in order to decompose the urea. Thus, the pH can be changed homogeneously all over the mixture. After that, during rigorous stirring, the mixture was titrated to have a pH of around 10 - 11 by adding drops of 3.5 M ammonium hydroxide at room temperature. The solution became black due to the formation of Fe3O4 particles. The black mixture was then heated at 60 – 70 oC in a water bath for 30 min to coat the Fe3O4 particles with dextran. Aggregates were then removed by centrifugation in a lows peed centrifuge at 4000 rpm for 5 min. The excess unbound dextran was separated by gel filtration chromatography on Sephacryl-300, using 0.10 M sodium citrate at pH 5.0 as an eluent. Nanoscale iron oxide (Fe2O3) nanoparticles were also prepared by the coprecipitation of ferric and ferrous ions in ammonium hydroxide solution [339-343]. A 10.8 g portion of FeCl3.6H2O and 4.0 g FeCl2.4H2O were dissolved in 50 mL of water. The resulting solution was poured with vigorous stirring into 500 mL of a 1.0 M NH4OH solution. The resulting black precipitate was collected with a magnet. A 500 mL portion of 1 M tetramethylammonium hydroxide (TMAOH) solution was added to the precipitate, and the mixture was sonicated for 1 h. After that, 6.3 g of oleic acid and 1.0 g of sodium dodecyl benzene sulfonate (SDBS) were added to modify the magnetic colloid surface properties [344].


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Massart’s method used for the preparation of the Fe3O4 nanoparticles consists of the following steps [345, 346]. A mixed aqueous solution of ferric chloride (50 mL, 1M) and ferrous chloride (10 mL, 2M, in 2M HCl) were added slowly into ammonia solution (500 mL, 0.7 M) under vigorous stirring for 30 min at room temperature in a nitrogen atmosphere. The resulting black precipitates were collected by a permanent magnet. An aqueous solution of 1 M tetramethylammonium hydroxide and 3 mg of adipic acid were added to the precipitates. The solution reacted overnight at room temperature. Then the precipitates were separated from the solution by a permanent magnet and redispersed in 20 mM of α,α,α-tris(hydroxymethyl)methylamine (Tris) containing 20 mM NaCl. The concentration of the colloidal solution was about 25 mM. Caruntu et al. have prepared magnetite nanoparticles in diethylene glycol, N-methyl diethanolamine or in mixture of diethylene glycol and N-methyl diethanolamine (1:1, w/w) [321]. A 2 mmol amount of FeCl2‚4H2O and 4 mmol of FeCl3‚6H2O were dissolved in 80 g of diethylene glycol (DEG) in a Schlenk flask under protection with argon. Separately, 16 mmol of NaOH was dissolved in 40 g of diethylene glycol. The solution of NaOH was added to the solution of metal chlorides with stirring at room temperature, causing an immediate color change from yellow-brown to deep green brown. After 3 h, the temperature of solution was raised during 1.5 h to 210 °C and then kept constant for 2 h in the temperature range 210-220 °C. The solid product was isolated by cooling the reaction mixture to room temperature and centrifuging. A black solid was obtained and washed with ethanol twice and with a mixture of ethanol and ethyl acetate (1:1, v/v) three times to remove the excess of diethylene glycol and was dried in a flow of nitrogen. The typical yield was 95- 96%. When reactivity tests or the preparation of colloids were planned, the solids were used without drying. The size of the nanoparticles is controlled by changing the complexing strength of the reaction medium [321]: 5.7 nm in diethylene glycol, 16.8 nm in N-methyl diethanolamine, and 12.7 nm in a 1:1 mixture of both solvents. The nanocrystalline powdered Fe3O4 products were isolated with a high yield. The surface of the obtained nanocrystals is passivated by molecules of adsorbed donating solvent that provide stability against agglomeration, provide solubility in polar protic solvents (water and methanol), and allow reactions at the nanocrystal surface. Sun et al. [347] have synthesized the hydrophilic Fe3O4 nanoparticles by the modification of hydrophobic ones. Under ambient conditions, a hexane dispersion of hydrophobic Fe3O4 nanoparticles (about 20 mg in 0.2 mL) was added to a suspension of tetramethylammonium 11-aminoundecanoate (about 20 mg in 2 mL) in dichloromethane. The mixture was shaken for about 20 min, during which time the particles precipitated and separated using a magnet. The solvent and nonmagnetic

200


suspension were decanted, and the precipitate was washed once with dichloromethane and separated again using a magnet to remove excess surfactants before drying under N2. The product was then dispersed in deionized water or 1 mM phosphate buffer at neutral pH. Fe3O4 nanoparticles are commonly produced via coprecipitation of ferrous (Fe2+) and ferric (Fe3+) ions by a base, usually NaOH, or NH3‚ in an aqueous solution [348], or they may be made by thermal decomposition of alkaline solution of Fe3+ chelate in the presence of hydrazine [349] and by sonochemical decomposition of hydrolyzed Fe(II) salt followed by thermal treatment [350]. The disadvantage of these aqueous solution syntheses is that the pH value of the reaction mixture has to be adjusted in both the synthesis and purification steps, and the process toward smaller (120 oC) and longer times than in HDAm. The particles (~5 nm) in dichloromethane show an absorption shoulder at 450 nm and a narrow band emission at 480 nm (λexc = 370 nm). However, using TOP or TBP (tributyl phosphine) as solvents (excluding TOPO altogether) results in the formation of CdS particles from Cd xanthates even at room temperature upon standing overnight, in sheer contrast to the “TOP/TOPO” route using Cd salt which proceeds at 250-350 oC. These findings indicate the crucial role of Lewis bases in lowering the reaction temperature (involving, most probably, a β-

234


elimination mechanism as in the Chugaev reaction) [55]. Also in differential scanning calorimetry (DSC), solid Cd HDX exhibits a major endothermic transition at 131 oC, with two minor ones at 91 and 108 oC, in HDAm a broad transition is observed already at ~70 oC, while in TOPO no transition appears up to 200 oC. Oleic acid belongs to the traditional ligand that can yield much more stable active metal nanocrystals than other saturated fatty acids [56]. Such stability is provided by the random packing of the hydrocarbon chain of the ligand monolayer. In the case of stearic acid, a saturated fatty acid, the hydrocarbon chains are packed in a crystalline manner, at least partially. Such crystalline packing of the hydrocarbon chains of the ligands on a nanocrystal creates gaps between each crystalline domain, provided the spherical nature of the nanocrystal surfaces. These gaps act as the diffusion channels for oxygen molecules. Consequently, this makes the nanocrystals significantly less stable if they are coated with ligands with a saturated hydrocarbon chain. This phenomenon is similar to the enhanced stability of nanocrystals by coating them with hype-branched ligands, organic dendron ligands [45]. Aldana et al. have followed the effect of pH on the formation of the nanocrystalligand coordinating bond [57]. The precipitation pH of semiconductor nanocrystals coated with deprotonated hydrophilic thiol ligands did not depend on the concentration of the nanocrystals, the concentration of the free thiol ligands, or the nature of the anion of the strong acids used for the titrations [HCl, H2SO4, or trifluoroacetic acid (TFA)]. Within the pH range tested, the nanocrystal precipitates were found to be recoverable, without any noticeable change in their absorption spectra, by adjusting the pH of the solution to the pKa of the thiol ligands. According to the work of Brus [58], a chemical explanation of quantum confinement of semiconductor nanocrystals is the enhanced bonding strength between the internal atoms of a semiconductor nanocrystal as particle size decreases. The stability of the nanocrystal-ligand complexes increases as the size of the nanocrystals decreases. The precipitation equilibrium using CdSe nanocrystals as the example was suggested to follow the next mechanism [57]: (CdSe)n − Lm + mH+

' ((CdSe)n)m+ + mHL,

Keq

(2)

where ((CdSe)n)m+ and (CdSe)n − Lm correspond to bare nanocrystals (precipitates) and nanocrystals coated with ligands, respectively. The positive charges of the bare nanocrystals in the precipitates should be balanced by the negative charges of the counterions of the strong acid used for titration. Keq is the equilibrium constant of equilibrium 2. Equilibrium 2 can be expressed as the two equilibria listed below.

Modification and passivation of colloidal particles

((CdSe)n)

m+

HL ' H+ + L− , + mL− ' (CdSe)n − Lm,

Ka (Ks)m

235

(3) (4)

Equilibrium 4 is a general equation for the formation of a complex and (Ks)m is the formation constant for (CdSe)n − Lm. It is assume that the equilibrium constant Ks, the formation constant of a single nanocrystal-ligand coordinating bond, in equilibrium 4 is the same for all cadmium sulfur bonds between a surface cadmium atom on the surface of a given sized nanocrystal and its deprotonated thiol ligands. Another way to state this assumption is to view Ks as an average value of the formation constant of all nanocrystal-ligand coordinating bonds on a given nanocrystal. There are two pieces of evidence to back up this assumption. One, the precipitation of nanocrystals occurred quite abruptly as evidenced by the titration curves. Two, NMR data revealed that approximately all ligands detached from the surface of the nanocrystals at the precipitation point. With this assumption and the relationship among the above three equilibriums, equilibrium 4 plus m times of equilibrium 3 being equal to negative equilibrium 2, Ks can be expressed as follows and

(Ks)m = 1/[(Ka)m Keq]

(5)

Ks = 1/[(Keq)1/m Ka]

(6)

Equilibrium 2 tells us that Keq = {[HL]m [((CdSe)n)m+]}/{[(CdSe)n − Lm]([H+]eq)m}

(7)

Combining eqs 6 and 7, we obtain eq 8. Ks = {[(CdSe)n − Lm]1/m[H+]eq}/{[HL][((CdSe)n)m+]1/m Ka}

(8)

The fact that the equilibrium precipitation pH of the nanocrystals did not depend on the concentrations of the nanocrystals and the free ligands implies that eq 8 can be simplified as eq 9. Ks = [H+]eq/Ka

(9)

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Equation 9 enables the calculation of the average reaction Gibbs free energy of the formation of a nanocrystal-ligand coordinating bond (ΔrG°) using the following equation. ΔrG° = − RT ln Ks = − RT ln([H+ ]eq/Ka) (10) Equation 10 reveals that the Gibbs energy change solely depends on the equilibrium dissociation pH determined by titration experiments. The pH at the interface and the bulk solution after reaching diffusion equilibrium should be the same. However, Aldana et al. did observe that when some net charges exist on the outer surface of the ligand monolayer, the equilibrium pH was different from that in the bulk solution because of the screening and trapping effects [57]. The equilibrium precipitation pH of the nanocrystals was found to be size-dependent approximately in their strong quantum confinement size regime. Consequently, a similar size dependence of the ΔrG° defined using eq 10 is obtained (Figure 1).

Figure 1. Size-dependent reaction Gibbs energy (ΔrG°) for the formation of the nanocrystalligand coordinating bond for ligand-coated nanocrystals, 1) CdS, 2) CdSe, 3) CdTe [57].

Figure 1 demonstrates that the Gibbs free energy change for the formation of nanocrystal-ligand coordinating bonding (ΔrG°) decreases as the size of the nanocrystals increases for a given type of semiconductor. ΔrG° of the nanocrystals


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with the same size increases as the bulk band gap of the semiconductor increases. The values in Figure 1 are reasonably consistent with the free energy of the Cd-S bond for the complexes of cadmium ions with thiol ligands reported [59]. The value was found to be about 49 kJ/mol, which is close to the values reported for thiol and Au bulk crystals, 20-40 kJ/mol [60]. Evidently, the results shown in Figure 1 are reasonably similar to these values. Brus [58] pointed out a very interesting fact regarding quantum confinement of semiconductor nanocrystals. The chemical indication of quantum confinement is that, within the quantum confinement size regime, the bonding strength between interior atoms in a nanocrystal increases as the size of the nanocrystals decreases. As nanocrystal size decreases, the potential energy curve of the ground state becomes deep and steep, increasing the “hardness” of the material. The results (similar to those shown in Figure 1) imply that the bond enhancement inside nanocrystals in the Brus picture may have some connection to the nanocrystal-ligand coordinating bonds on the surface of inorganic nanocrystals. However, it should be pointed out that the ΔrG° is associated with the formation of the coordinating bonds between negatively charged thiolate ligands and positively charged nanocrystal core. Traditional titrations approach (adding a strong acid into solutions) can be used for the quantitative determination of the formation constant and the related free energy change of nanocrystal-ligand complexes. However, slow diffusion of hydrogen ions through the ligand monolayer on a nanocrystal technically prevents ones from using common titration techniques. The precipitation pH of nanocrystals with different chemical compositions and/or with different ligands can occur [42]. Among new types of surface ligands, recently developed oligomeric phosphines [48] and thiol dendrimers [61] should be mentioned. A new family of oligomeric phosphine ligands [48] electronically passivate particles as well as the initial growth ligands. They yet form thin and secure organic shells that allow tunable compatibility in diverse environments and flexibility for further chemistry. Alkyl phosphines passivate CdSe and CdSe/ZnS (core/shell) QDs effectively as a major component of the growth ligands. However, monomeric alkyl phosphines are labile as are many other monodentate ligands. A considerable concentration of free monodentate ligands is required in solution to keep the particles well passivated. When particle growth solutions are diluted or embedded in an environment in which no excess ligands are present, quatum yields (QYs) tend to decrease and the particles aggregate. As polydentate ligands, however, oligomeric phosphines (OPs) bind more effectively to particle surfaces. For example, they can securely anchor targets. To avoid aggregation of the particles, the outermost part of the organic shell must be compatible with the bulk environment. Furthermore, OPs can be easily modified with

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many functional groups, allowing homogeneous incorporations into various matrices. The organic shell is designed to form sublayers around the particle (crystal) [48]: 1) inner phosphine layer, 2) a linking layer, and 3) an outer functionalized layer. The inner phosphine layer passivates the particle surface; the linking layer protects it; while the outer functionalized layer delivers desirable chemical properties, including miscibility, the ability to copolymerize with other polymer matrices, cross-linking on the surface of the dots, and further chemical modifications such as conjugations to biomolecules. The oligomeric phosphines passivated CdSe/ZnS (core/shell) QDs sample loses ~ 80% of its initial QY after ligand-exchange by the thiol ligand mercaptoundecanoic acid (MUA), while carboxylic acid (CarbA) maintains the initial QY (consistently in the 20-40% range) through the ligand-exchange process and for weeks following. Carboxylic acid (CarbA) efficiently passivates QDs in various buffer solutions of pH ranging from 5.5 to 12 and in high salt concentration solutions. To passivate QDs even more efficiently, the chemical flexibility of CarbA is varied with further cross-linking around QDs. The cross-linked ligand passivates the QDs efficiently, with a loss of less than 20% of the initial QY after 24 h in serum at 37 °C. To demonstrate the versatility of OPs, streptavidin is conjugated to CarbA. Oligomeric phosphines with methacrylate (OPMA) can enable homogeneous incorporation (i.e., copolymerization) of QDs into many polymer matrices without the need for additional free ligands such as TOP in the matrix. The polymerizable ligands OPMA can become incorporated into host polymers and offer synthetic routes to micrometer- and submicrometer-sized polymer-QD composites [48]. The formation of ligands monolayer achieved by the cross-linking of all surface ligands of each nanocrystal, increases the colloidal stability of nanoparticles [9]. To reach the desired global cross-linking, for example, a dendron with thiol as the anchoring group and several carbon-carbon double bond as the terminal groups can be employed as the surface ligands. The relatively good chemical stability of the dendron ligands coated nanocrystals (dendron-nanocrystals) [45] enabled the nanocrystal/ligands complexes to readily survive the cross-linking reaction and related purification procedures. The multiple double bonds of each dendron ligand made it possible to obtain global cross-linking of all the dendron ligands on the surface of each nanocrystal through RCM [62] to form a dendron box around each nanocrystal (box-nanocrystal). The formation of the dendron boxes was confirmed by NMR and MS. High quality CdSe/CdS core/shell box-nanocrystals demonstrated superior stability against HCl etching, chemical oxidation with H2O2, photochemical


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oxidation, and thermal sintering. The resulting stable semiconductor nanocrystals should be of great interest for many fundamental studies, such as measurement of the sized dependent melting point of nanocrystals and studies on the size-specified reactivity of nanocrystals. Furthermore, those luminescent and stable core/shell nanocrystals should greatly benefit several types of technical applications, such as light emitting diodes (LEDs) [63], solid-state lasers [64], and biological labeling [65] and so forth. The empty dendron boxes formed by the dissolution of the inorganic nanocrystals in concentrated HCl represent a new class of nanometer sized polymer capsules, which are nearly monodisperse, soluble, stable, and with a very thin peripheral. The chemical nature and physical dimension of the ligands for nanocrystals have been widely noticed to act as an important factor during the formation of nanocrystals. For example, selective binding on the surface of nanocrystals, or the ligand template effect, has been widely suggested to control the shape of nanocrystals [66, 67]. However, any ligands added to a synthetic system are not only the ligands for the resulting nanocrystals but also the ligands for the monomers. It is possible that, in the growth stage, the ligands may influence the monomers (precursors) and nanocrystals simultaneously. The ligand effects, thus, can be classified as two types, effects on monomers and effects on nanocrystals [68, 69]. The effects on the monomers should dominate ligand effects on the nucleation stage because nucleation is the process which creates the “seeds” for the growth/formation of nanocrystals. The bonding strength between ligands and metal ions significantly affects the nucleation process. When phosphonic acid was used, cadmium precursors were much less reactive in comparison to the cases with fatty acids. As a result, a small amount of monomers were consumed in the nucleation stage in general and formed a small amount of nuclei. This observation is consistent with other evidence. For example, if phosphonic acids were used as the ligands instead of fatty acids, nucleation of CdS and ZnSe nanocrystals was not possible [68, 70]. Phosphonic acids were so strong that the nucleation as well as the growth of the CdTe nanocrystals showed no response by varying the chain length of the phosphines that were used as the ligands for the Te monomers. In contrast, the nucleation process was dramatically affected by the chain length of the Te ligands, phosphines, when fatty acids were employed as the ligands for the cations. For the examples, the number of nuclei was significantly less in the case of trioctylphosphine (TOP) than that when tributylphosphine (TBP) was used. The ligand effects on monomers observed in the nucleation stage was suggested to be divided into three different classes [70]:

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1) A strong coordination bond between monomers and ligands will decrease the reactivity of the monomers. 2) ligands with longer hydrocarbon chains will suppress the reactivity of the monomers. 3) as reported previously, the higher the ligand concentration is, the lower the reactivity of the monomers will be [68, 69]. The last two effects are most likely due to steric factors and the first one is caused by the stability of the monomers. If the monomers are very stable, the steric effects will not play a role as demonstrated by the octadecylphosphonic acid (ODPA) -related reactions. Furthermore, the nature, structure, and configuration of the ligands are important factors for determining the reactivity of the monomers. The activity coefficient may cover both the steric and stability effects and the effect of temperature, as well. The shape differences can be discussed within the context of this approach [71]. Elongated shapes require a high chemical potential environment to grow [72]. This is equivalent to high monomer activity, which is a combination of a high remaining monomer concentration after nucleation and a high activity coefficient. However, to maintain a high remaining monomer concentration in solution for a sufficient period of reaction time, the concentration of nuclei must be low. If the nuclei concentration is too high, the high monomer concentration will quickly be depleted to an undesirable low level due to the rapid consumption of monomers by the highly populated nuclei. Consequently, growth of elongated structures will no longer be supported. For the formation of elongated shapes, the nucleation stage prefers a low activity coefficient and the growth stage requires a high activity coefficient. These two contradictory factors make a medium level activity coefficient a good choice for a simple reaction system. When phosphonic acids were used as ligands, the activity coefficient of cadmium monomers was too low to promote the growth of elongated shapes in noncoordinating solvents. If fatty acids and TBP were used, the activity coefficient of the monomers was too high and too many nuclei were formed; as a result, elongated shapes could not be formed. With fatty acids and TOP, elongated shapes could be formed at a relatively high initial monomer concentration due to a reasonable activity coefficient of the monomers [70]. Although phosphonic acids prohibited the formation of elongated CdTe nanocrystals in 1-octadecene (ODE), it did promote the formation of elongated CdTe nanocrystals in a coordinating solvent, TDPA mixed with TOPO [71]. This can be simply explained by the decreased activity coefficient of the cadmium monomers by the excessive existence of a weak ligand, TOPO. The results observed in the formation of elongated CdSe nanocrystals in coordinating solvents suggested that the cadmium monomers at high temperatures should have both phosphonic acid and TOPO as their ligands. Because these ligands


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are all somewhat labile at high temperatures, both types of ligands would have a chance to bond to cadmium atoms. CdTe, CdSe, and CdS can all be in the form of either zinc blende or wurtzite structures. However, high quality nanocrystals for both CdSe and CdS systems have been limited to the wurtzite form. The formation of dot-shaped wurtzite CdSe and CdTe was attributed a sudden phase transition when the size of the particles reaches a critical value [72]. This transition has also been observed in the case of CdS formed in aqueous solutions, from “magic-sized clusters” with zinc blende bonding geometry to largely sized CdS wurtzite nanocrystals [73]. When the reaction temperature is lower, the decomposition of both Cd-phosphonate and Cd-carboxylate becomes more difficult. As a result, lower temperatures benefit the formation of zinc blende crystals for the phosphonic acid-related reactions and zinc blende stacking faults in the wurtzite nanocrystals for the fatty acid-related reactions. There is an alternative interpretation of this interesting phenomenon. Phosphonic acids bond strongly to cadmium atoms on the surface of a nanocrystal in comparison to fatty acids. The phase transition of the nanocrystal could be hindered by these strong ligand-nanocrystal interactions. However, this alternative explanation is not consistent with the results observed in the reactions performed in coordinating solvents. When CdTe nanocrystals were synthesized in a coordinating solvent [71], TOPO mixed with tetradecylphosphonic acid (TDPA), the nanocrystals were wurtzite, although NMR studies confirmed that TDPA were the only ligands on the surface of the nanocrystals. In the context of ligand effects on monomers, the formation of wurtzite structure in the coordinating solvents was explained by the enhanced activity coefficient of cadmium monomers due to the excessive existence of a weak ligand (TOPO). The stability of zinc blende nanocrystals at high temperatures increases as the atomic mass of anions of the cadmium chalcogenides increases [66]. This is the reason why the crystal structures of high-quality CdTe nanocrystals is to be realized. The high-temperature stability of the zinc blende structure of CdTe nanocrystals can also explain why it is much easier to form tetrapods with a very high yield. An aging of of less stable, phosphine-protected gold nanoparticles was reported to lead to a rapid increase in particle size [74]. On the contratry, thiol-protected particles showed very slow change in particle size. Subtle changes in nanoparticle behavior during aging was attributed to changes in the particle morphology [74]. Rapid mass loss is observed above 200 °C, consistent with desorption of the ligands from the metal surface (freshly made phosphine-protected nanoparticles) [75]. The

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decomposition of the aged nanoparticles, however, occurs at a slightly higher temperature than the freshly prepared sample. This is further confirmed by the differential scanning calorimetry. The enhanced thermal stability of the aged nanoparticles is consistent with the proposed surface reorganization during aging. If such reorganization involved just 3-5 defect sites on the surface, one might expect to see no noticeable changes in the nanoparticle thermal properties; results thus suggest that reorganization probably affects a substantial proportion of the surface atoms. Recent studies revealed that cadmium based semiconductor nanocrystals did not affect the biological functions if they were completely coated with organic ligands [76]. After the ligands were detached, the nanocrystals became extremely toxic [77]. Several studies centered on tailored ligands to cover quantum dots include the use of synthetic polymers [78], oligopeptides [79], and oligonucleotides [80]. 4.3. Ligand exchange The ligand exchange reaction is an extremely versatile tool for the preparation of functionalized metal nanoparticles [81]. This method is fast and simple to use; it allows one to introduce functional groups that are incompatible with other methods for nanoparticle synthesis. The importance of ligand exchange reaction made it a subject of several mechanistic studies [82]. This method is fast and simple to use; it allows one to introduce functional groups on the particle surface. For example, the thiols replace can be used to modify the particle surface, vary the solubility of particles and increase the colloidal stability of metal particles. There remain significant challenges associated with the use of ligand-exchange techniques to give the tailored coverages, as surface oxidation, changes in nanocrystal size and size distribution, and diminished photoluminescence often accompany ligand-exchange chemistries. Nevertheless, ligand exchange is standard practice for the introduction of new surface functionality to quantum dots, as the high-temperature nanocrystal growth methods are not compatible with most organic functional groups. Ligand exchanges of organics-stabilized nanoparticles with free ligands, the use of different ligands can lead to changes in core size, incomplete exchange, or no exchange at all. [83]. The exchange rate depends on the concentration and type of ligands and reaction conditiond. For example, ligand exchange between the small gold - triphenylphosphine (TPP or PPh3) precursor particle and functionalized thiols can be achieved by combining the phosphine-stabilized nanoparticle with an excess


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of the thiol (approximately 90-200 molar equivalents with respect to the nanoparticle) in an appropriate solvent. Organic-soluble exchange products are prepared in a monophasic system with a variety of alkyl- and arylthiols. The reaction time strongly depends on the thiol ligand used and generally increases with increasing chain length, ranging from 30 min for exchanges with propanethiol up to 18 h for octadecanethiol (ODT). For aromatic thiols, reactions times are usually longer to achieve complete exchange. This trend in reactivity is consistent with the differences in reactivity for thiol-for-thiol ligand exchange reactions and has mainly been attributed to steric effects [81] (see scheme 1).

Scheme 1. Ligands used in the ligand exchange reaction, where R denotes -(CH2)nCH3 n = 2 - 17 -p-Ph-OH, -p-Ph-CH3, -p-Ph-CH3, -p-Ph-Ph, -(CH2)3Si(OCH3)3 -(CH2)nCOOH n = 1- 6, 11 -(CH2CH2O)nCH2CH2OH n = 1, 2 -(CH2)2PO(OH)2, -(CH2)2NHMe+2Cl-, -(CH2)2NMe+3Cl-(CH2CH2O)nCH2CH2NMe+3Cln = 1, 2 [84].

The reaction proceeds to completion only when an excess of the incoming thiol is used. Smaller amounts of thiol usually result in incomplete exchange. On the other hand, if too large an excess of thiol is used (more than 300 molar equivalents in most cases), the phosphine-stabilized nanoparticles rapidly decompose rather than undergoing ligand exchange [85]. Decomposition is also observed when the exchange reaction is carried out at elevated temperatures presumably due to the limited thermal stability of the phosphine-stabilized precursor particles in solution [86]. On the base of the experimental data, Hutchinson et al. proposed a three-stage mechanism for the ligand exchange reaction between particles and thiols [84]:

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1) In the initial stage, part of the phosphine ligand shell is rapidly replaced in the form of AuCl(PPh3) until no more particle-bound chlorides are available. 2) This initial phase is followed by removal of the remaining phosphine ligands either as free PPh3 in solution (pathway I) or through direct transfer of PPh3 to closely associated AuCl(PPh3) (pathway II). 3) During the final stage, the completed thiol ligand shell is reorganized into a more crystalline state. Murray group suggests a new role of reaction conditions in mediating the exchange reaction [82]. It was demonstrated that triphenylphosphine (TPP)-stabilized gold nanoparticles [86] undergo ligand exchange reactions with a few ω-functionalized thiols to produce functionalized nanoparticles that preserve the core dimensions of the precursor particles but exhibit highly increased stability against heat, aggregation, and decomposition [87]. The development of a convenient synthesis of 1.5-nm AunTPP has generated considerable interest in the ligand exchange reactions of these nanoparticles. Disulfides, which are usually inactive in the exchange reaction, undergo partial exchange with Au nanoparticles protected by phosphines or short-chain thiols [88, 89]. This reaction can be conveniently monitored by EPR spectroscopy, provided the disulfide is functionalized with a spin label, e.g., disulfide II (Scheme 2).

Scheme 2. Place-exchange reaction of Au nanoparticles with disulfide II [89]

The kinetics of the reaction can be followed by the disappearance of one or two peaks in the EPR spectra. These peaks originate from the spin-spin interactions between the adjacent spin labels in unbound disulfide II; as the S-S bond is broken


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during the exchange reaction, these peaks gradually disappear from the spectra [88]. Aging of the Au nanoparticles in solution affects the rate of the ligand exchange reaction. A freshly prepared solution of gold nanoparticles reacted with disulfide at a much faster rate than the solution which had been left to age for several hours. The poor reproducibility of the exchange kinetics was thus due to different times which the Au nanoparticles had been aged for during their synthesis and sample preparation. Nanoparticles with the same aging history showed good reproducibility of kinetics of the ligand exchange. Freshly prepared solutions react up to 10 times faster than the solutions aged for 1 week. The core of the Au nanoparticles having ca. 500 gold atoms is coated with ca. 150 ligands. Exchange reaction with excess disulfide, however, would only replace a maximum of ca. 3-5 ligands per particle. As reactions were carried out at 1:2 ligand to nanoparticle stoichiometry, nearly complete exchange was observed at longer times. Aging, however, appears to have no impact on the maximum number of exchangeable ligands. Increase in particle size (from 2.6 nm to > 3 nm) and partial aggregation was only observed at much longer aging times ( > 2 weeks). This is consistent with the observation that while particle size increases substantially with increased reaction time during nanoparticle synthesis, purified particles are stable in core size for weeks [90]. The feature of the spectra is a surface plasmon peak (a shoulder at ca. 520 nm). Although all spectra have similar line shape, peak-fitting analysis revealed that λmax for the plasmon peak contribution undergoes a substantial red-shift with increased aging time. The intensity of the plasmon peak was, however, nearly constant. This further confirms that the particle size is not affected by aging [91]. The shift in the position of the plasmon band is therefore most likely caused by small changes in particle morphology upon aging. In a typical nanocrystals’ synthesis, ligands preventing their aggregation are present in the reaction medium. Depending on the intended application, these “original” ligands must be replaced by new ones, which introduce the desired solubility and/or functionality. For this purpose, bifunctional ligands X-Y-Z are frequently used, in which X is a chemical function with a high affinity for the nanocrystal surface, Y is a spacer of alkyl or aryl type, and Z is the group transferring the desired property to the nanocrystal. The main advantages of this approach are the well-defined surface chemistry of the nanocrystals after functionalization and the possibility to maintain essentially their original size. In most literature examples, the function X stands for one or two thiol groups [92]. This preference can be partially attributed to the large commercial availability of thiols with numerous spacers Y and functional groups Z. Although convenient in use, thiol-based ligands show however some weaker points. For example, photodegradation studies reveal that thiols coordinated on some nanocrystals readily undergo photooxidation accompanied by the formation of

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disulfides, which leads to the precipitation of the crystals [42]. This photochemical instability can be ascribed to the relatively weak interaction between the nanocrystal surface and the thiol ligand, which is not of a covalent nature. Furthermore, quasiquantitative substitution ( >95%) of the original surface ligands by thiols requires usually extended exchange reaction times and can be achieved only in relatively drastic reaction conditions which, in turn, might affect other properties of the nanocrystals. These shortcomings can be overcome by a new type of bifunctional ligands, which contain a carbodithioate (-C(S)S-) function serving as the anchor group for the nanocrystal surface [43]. Several features make them attractive candidates for nanocrystal functionalization. Their high affinity for metal atoms, which arises from the bidentate chelating binding of the carbodithioate group, facilitates the replacement of the original surface ligands. Quasi-quantitative exchange is achieved in a room-temperature reaction within several hours. In addition, nanocrystals capped with the carbodithioate ligands exhibit a significantly enhanced stability against photodegradation with respect to the corresponding thiol ligands. High quality CdSe nanoparticles were obtained by passivation with trioctylphosphine oxide (TOPO) ligands which gives a hydrophobic and chemically inert ligand shell [71]. Conversion of TOPO-covered quantum dots to water-dispersible materials requires ligand exchange to give coverage, for example, with thioalkanoic acids [42], or related compounds with multivalent coordination sites. The thiols replace TOPO from the CdSe nanocrystal surface [93] and the deprotonated negatively charged carboxylic functions provide the solubility in water [94]. Because of the synthesis, the particles are initially covered with trioctylphosphine oxide and are soluble in chloroform. These particles can be transferred from the chloroform into the water phase via attachment of deprotonated mercapto acetic acid (MeAA) at pH = 11 [65]. The positively charged sites of the added polymer are attracted by the negatively charged nanocrystal surface ligands, while the aliphatic chains stick out into the solvent to provide the solubility change [95]. This reaction worked only when the amount of charged groups within the polymer was adjusted to about 30-50%. No transfer reaction could be observed for lower amounts of polar chains, probably because of uncompensated negative charges on the nanocrystal surface, while polymers with a higher ratio of charged groups are not soluble in chloroform. A CdSe-nanocrystal is 3 nm in diameter, covered by 2 monolayers of ZnS and by about 9 polymer chains (estimated from TGA). For comparison, one can assume that every MeAA molecule covers a surface area of 13 Å2 [96] and hence each nanocrystal is covered with 425 MeAA molecules. Because a


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polymer chain with 30% positively charged end groups could compensate approximately 90 negative charges, only about 5 polymers could be attached to one nanocrystal, according to the considerations. However, most likely the polymers are not wrapped smoothly around the surface, and hence the coverage is somewhat higher as was determined by the TGA experiments. While pure MeAA covered NCs precipitate after some time [42], the nanocrystal/polymer composites did not precipitate even when the oxygen-saturated solution was exposed to UV light for more than a day. This is most likely due to the fact that the possible ligand dimerization happens at the inorganic particle surface while the stabilizing polymer stays attached to the nanocrystal due to the multiple bonding and strong ionic interaction. Ligand-exchange with various thiol ligands, including dithiols and thiol dendrimers [45, 65] provides more sophisticated architectures which have been built on these thiol ligands covalently, electrostatically, or via sol-gel chemistry [97]. Ligandexchange with thiols usually diminishes the quantum yield (QY) of the as-grown quantum dots (QDs) photoluminescence (PL). Encapsulating QDs and their initial ligands with macromolecules such as polymers or lipids can preserve QY, but generally adds a large volume to the QDs, resulting in a final size that may be bulkier than desired [76]. For example, the increased size can diminish imaging sensitivity by decreasing the number of QDs that can be attached to a target. For in vivo imaging, bulky QDs may have limited accessibility to target systems. For many applications, thin functionalizable organic shells are ideal. For example, thin shells are desired for potential sensors that function by energy transfer to and from QDs [98]. The exchange of sulface-capping organic ligands on as-synthesized nanocrystals in favor of shorter insulating ligands appears to play a significant role in achieving high electroluminescence quantum efficiency devices. PbS-nanocrystal particles capped with alkylamines were investigated [99]. The procedure was repeated to ensure uniform ligand exchange, and the final precipitate was dispensed into the desired organic solvent for polymer-composite fabrication. Whereas photoluminescence intensities for unexchanged and exchanged nanocrystal samples were comparable, the ~ 2 nm long oleate-capped nanocrystal devices exhibited a much lower electroluminescence intensity in the infrared than those capped with 1 nm long alkylamine ligands. It was noted that for electroluminescence, excitations generated within the polymer matrix must be transferred to the nanocrystals, and that the rate of energy transfer is presumably sensitive to both the choice of capping ligand and the proximity between exciton donor and acceptor [100]. This observation prompted subsequent investigations using photoluminescence excitation spectra of the

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influence of ligand chemistry and ligand length on energy transfer from a polymer matrix to nanocrystals [101]. PbS nanocrystals capped with as-synthesized oleale (~ 2 nm), octylamine (~ 1 nm), dodecylamine (~ 1.5 nm), and octadecylamine (~ 2 nm) ligands were combined with the host matrix and formied into films. Composites based on as-synthesized PbS nanocrystals capped by oleale ligands provided transfer efficiencies- the number of excitons transferred to the nanocrystals divided by total number of excitons generated in the polymer-of 20%. Replacing these ligands with the shortest ligands gave a threefold improvement in excitation-transfer efficiency (ETrE): ETrE (%)/ ligand: 24/oleate (C18), 31/octadecylamine (C18), 55/dodecylamine (C12), 61/octylamine (C8) [101] The exchange process can be applide to to prepare multishells particles such as spherical Se-Ag2Se core-shell colloids. Such particles with tunable shell thickness have been successfully synthesized by taking advantage of the high reactivity of αSe toward silver (Scheme 3) [102]. To prevent possible agglomeration during the exchange process Poly(vinyl pyrrolidone) (PVP) was added as a stabilizer. For the conversion from Ag2Se to CdSe, a small amount of tributylphosphine (TBP) was introduced into the colloidal suspension in the presence of excess Cd2+. Phosphines have been commonly used as ligands in metal complex chemistry [103]. In particular, TBP can bind to Ag+ cations on the surfaces of Ag2Se shells to form intermediate complexes, facilitating the replacement of Ag+ by Cd2+ at elevated temperatures. In comparison, the conversion of CdSe to Ag2Se could proceed spontaneously in the presence of excess Ag+ at room temperature without the use of any catalyst or ligand.

Scheme 3. Schematic illustration showing how Se/Ag2Se core-shell spherical colloids were transformed into Se-CdSe methanol colloids with the assistance of tributylphosphine (TBP) at 50 oC [102].


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Cation-exchange reactions can be employed to diversify the chemical composition of semiconductor colloids further. Se-Ag2Se core-shell colloids could be transformed into Se-CdSe core-shell colloids through a cation-exchange reaction [102]. Ionexchange reactions have been extensively studied in the general areas of catalysis and thin film technology [104]. A number of cations such as Ag+, Sb3+, Bi3+, and Cu+ have been used to replace Cd2+ in thin films made of CdSe and CdS particles [105]. An anion-exchange reaction has recently been used by Konenkamp and coworkers to transform columnar ZnO into tubular ZnS by exposure to H2S gas [106]. The resultant ZnS tubes could be further converted into Ag2S, Bi2S3, and Cu2S with preservation of the tubular shape through a cation-exchange reaction in an aqueous solution that contained the appropriate salt precursor [105]. Alivisatos and coworkers successfully demonstrated the use of a cation- exchange reaction between Ag+ and Cd2+ to convert CdSe nanocrystals into Ag2Se and vice versa [107]. Although the replacement of Cd2+ by Ag+ is spontaneous because of the high mobility of Ag+ and their large solubility product difference [108], the replacement of Ag+ by Cd2+ needs a small amount of tributylphosphine (TBP) and temperature elevation to facilitate the exchange process. Although high quality CdS nanocrystals with strong band gap emission have become available recently [109], the intrinsic toxicity of cadmium places ZnSe and ZnS nanocrystals in an advantageous position. The wide band gaps of ZnSe and ZnS also make them an ideal choice as an inorganic passivation shell for a variety of semiconductor core/shell nanocrystals [8] in order to improve the stability and emission properties of the semiconductor core nanocrystals with a relatively narrow band gap. These wide band gap semiconductor nanocrystals are also attractive hosts for the formation of doped nanocrystals [110]. For these reasons, synthesis of high quality ZnSe and ZnS nanocrystals is still an attractive subject [111], although synthetic chemistry of cadmium chalcogenide nanocrystals has been well developed in the recent years [70]. A solid-phase place exchange reaction was used to synthesize gold nanoparticles with monofunctional group attached to the surface (Fig. 2, Scheme 4) [112]. This approach is based on a “catch and release” mechanism. Bifunctional thiol ligands with a carboxylic end group were first immobilized on a solid support such as a polymer resin with a controlled density. The density was low enough that neighboring thiol ligands were far apart from each other. When the modified polymer support was incubated in a butanethiol-protected gold nanoparticle solution, a one-toone place exchange reaction took place between the polymer-bound thiol ligands and the nanoparticles. After cleaving off from the solid support, nanoparticles with a single carboxylic group were obtained as the major product. Jacobson et al.

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published an almost identical approach toward the synthesis of gold nanoparticles with a single amino acid moiety [113]. These nanoparticles with a single functional group attached can be treated as giant “molecules” and linked together into very sophisticated structures through traditional chemical reactions, just like the total synthesis of complicated natural product from small molecular units.

Fig. 2 The synthesis of gold nanoparticles with a single acid moiety [113].

Scheme 4. Solid-phase synthesis of gold nanoparticles with a single carboxylic group and its coupling reaction with alkyldiamine [114]


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The first example of this concept is based on triphenylphosphine-protected gold nanoclusters reported by Hainfeld et al. [115]. The functional ligand is attached to the nanoparticle with other nonfunctionalized ligands by controlling the stoichiometric ratio of these two different ligands during nanoparticle synthesis. Furthermore, triphenylphosphine-protected gold nanoparticles have poor environmental and thermal stability, which limits the number of applications for which they may be used. In a second example, Alivisatos et al. reported the separation of gold nanoparticle-DNA conjugates with discrete numbers of DNA molecules by electrophoresis [116]. However, this approach is not a general synthetic method to prepare gold nanoparticles with different organic functional groups in large quantities. It has been argued and hypothesized that by strict stoichiometric control of incoming ligand ratio versus the nanoparticle-bound ligands, one may be able to attach a single functional group to the nanoparticle surface using the typical solution-phase place exchange reaction [81]. It was found from solution-phase place exchange reaction that an even distribution of gold nanoparticles with one, two, three, and other discrete numbers of functional groups was obtained. The efficiency of solid-phase place exchange reaction is clearly much higher than the solution-phase place exchange reaction. However, there are still a significant number of obstacles that must be overcome before the solid-phase synthesis approach can become a versatile synthetic strategy for the controlled chemical functionalization of nanoparticle materials. Due to the large size, high molecular weight, and many other unique properties associated with nanoparticles compared to small organic and inorganic compounds, extensive modification of the solid-phase reaction conditions needs to be made to accommodate these properties. 4.4. Particle growth techniques General An important question is the role of capping agents. As the nuclei grow, van der Waals interactions can cause rapid coalescence of nuclei and unrestrained particle growth. Ligands such as oleate and trioctylphosphine (TOP) are chemically bonded to both the precursors and to the particles that form. Such bonds resist van der Waals forces. Capping agents may be added during synthesis to adsorb and limit particleparticle aggregation, though such molecules may, in principle, hinder monomer deposition too. Furthermore, it is possible that complexation of capping agents to the monomers and precursors may impede nucleation, which will then lead to larger, rather than smaller, particle sizes.

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The nucleation of CdSe in hot octadecene provides an ideal model system to verify kinetic models of particle growth in solution [117]. An aim in this model is to estimated and measure the number of nuclei formed during the nanocrystal synthesis and to estimate the initial size of these nuclei prior to growth. The simple kinetic model for the particle growth is based on the following equation ( d[M]t /dt) = − kA(t)[M]t N(t)

(11)

where [M]t is the concentration of available M at time t, A(t) is the surface area of each particle at time t, N(t) is the number of particles at time t, and k is an interfacial rate constant with units (length/time) that reflects the ratedetermining steps during deposition. The results presented by [117] indicate that the oleic acid is not only an efficacious capping agent for CdSe QDs in octadecene but it markedly influences the primary nucleation steps in two distinct ways. First, the number of nuclei is reduced almost linearly as oleic acid is added. Nucleation is more difficult in the presence of oleic acid due to its complexation to Cd. However, the results also show that the initial nuclei in the presence of oleic acid are smaller than in its absence. This is counter-intuitive, since the lower degree of supersaturation in the presence of the ligand should lead to larger nuclei. Hence, the complexing agent reduces the rate of nucleation by reducing the active monomer concentration, but it also rapidly caps the nuclei as they form. These two effects compete with each other. If there is too much capping agent, nucleation can be completely hindered, ultimately leading to indiscriminate growth of a small population of nuclei. However, because there are fewer nuclei formed in the presence of the ligand, ones find the somewhat unexpected result that larger clusters form as we increase the concentration of oleic acid. It should be pointed out that at low ligand concentrations, the main effect is the capping action. Under these conditions, the ligand may act to stabilize small nanocrystals and prevent growth through coalescence or ripening, as one normally expects. Epitaxial approach The stability and processibility of plain core nanocrystals are both far from ideal, and for many reasons this instability and poor processibility may be intrinsic [118]. Therefore, core/shell nanocrystals are the desired structures when either the nanocrystals must undergo complicated chemical treatments, such as for biomedical applications [65] or the nanocrystals require constant excitation as for LEDs [63] and lasers [64]. The quality of core/shell nanocrystals [119] has not yet reached that of


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plain core nanocrystals in terms of size and size distribution control. Two critical issues for maintaining the size distribution of nanocrystals during the epitaxial growth of shell materials include: (1) the elimination of the homogeneous nucleation of the shell materials; and (2) homogeneous monolayer growth of shell precursors onto all core nanocrystals in solution, yielding shell layers with nearly the same thickness around each core nanocrystal. In principle, the reactivity of the precursors should be weak enough to prevent independent nucleation, but sufficiently strong to promote the epitaxial growth around the existing core nanocrystals. Therefore, the relatively stable, inexpensive, and relatively safe alternative precursors [120] should be more suited for the growth of high-quality core/shell nanocrystals than the traditional organometallic precursors, such as dimethyl cadmium, dimethyl zinc, and trismethylsilane sulfide. Less active precursors are much easier to control as has been demonstrated in the case of the development of the alternative synthetic schemes for high-quality plain core nanocrystals [69]. A brief report on the synthesis of CdSe/ZnSe core/shell nanocrystals using zinc stearate as the zinc precursor and Se-trioctylphosphine as the Se precursor in a coordinating solvent by Reiss et al. was published [8]. An impressively high PL efficiency was demonstrated. However, the size distribution of the core/shell nanocrystals was significantly worse than that of the original core nanocrystals, although Reiss et al. were very careful in choosing the growth conditions (dropwise addition of the shell growth solution). Thus, precursors alone are unlikely to be sufficient for the improvement of the size distribution of the core/shell nanocrystals. The epitaxial growth of a colloidal compound semiconductor onto another is a modified version of successive ionic layer adsorption and reaction (SILAR) [121]. Results revealed this new SILAR technique can provide excellent control of the shell thickness with submonolayer accuracy for several semiconductor pairs [122]. This growth of multiple particle systems on a single nanocrystal, enabling sequential epitaxial growth of different semiconductor nanocrystals onto given core nanocrystals. For many purposes, epitaxially grown core/shell semiconductor nanocrystals are of critical importance. If the shell possesses a higher band gap than the core material and the band offsets of the core/shell structures are type-I (the core possesses a lower conduction band and a higher valence band in comparison to those of the shell), a photogenerated electron and hole inside the nanocrystal will be mostly confined in the core. Consequently, such core/shell nanocrystals typically show bright photoluminescence (PL) [8] and electroluminescence [123] and are more stable against photooxidation [9]. For example, the photoluminescence quantum

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yield (QY) of core/shell nanocrystals can be as high as 50-80% [124] although 1040% the photoluminescence quantum yield is more routinely achieved [5]. The results reveal that core/shell nanocrystals may also be more chemically and thermally stable than the corresponding plain core nanocrystals [61]. Advancement in the synthesis of semiconductor nanocrystals has made it possible to obtain highly luminescent plain core nanocrystals [125]. A very interesting concept is atomic-layer-epitaxy (ALE). In ALE, the molecular beams of the anionic and cationic species are switched on and off in an alternating pattern. In this way, only one-half of a monolayer will grow in each period with either the cationic or the anionic beam on. Because the cationic and anionic species do not coexist with each other in the growth chamber, local nucleation on the substrate or in the gas phase is avoided. Consequently, the thin film grows in a wellcontrolled manner. This concept was extended to the deposition of thin films onto solid substrates using solution bath, successive ion layer adsorption and reaction (SILAR) [126]. The impressive results recently demonstrated by Park et al. [127] revealed that SILAR is a very successful technique for the growth of high-quality thin films. The stability of CdSe nanocrystal cores was reported to enhance by the epitaxial growth of a thin layer of CdS on their surface [9]. It was confirmed that such core/shell structures can improve the photochemical stability of CdSe nanocrystals by the confinement of the photogenerated charges inside the core material [9]. The stability enhancement of the ligands monolayer was achieved by the cross-linking of all surface ligands of each nanocrystal through the standard ring-closing metathesis (RCM). A number of different methods have been used for the heteroepitaxial growth of semiconductor (e.g., SiGe) particles, but the two most commonly employed techniques are molecular beam epitaxy (MBE) utilizing solid Si and Ge and ultrahigh vacuum chemical vapor deposition (UHV-CVD) or gas-source molecular beam epitaxy (GSMBE) utilizing SiH4 and GeH4 or Si2H6 and Ge2H6. There are two ultimate, but also diverse, objectives in the growth of Si1-xGex/Si. The first is the achievement of defect-free Si1-xGex layers, which may take the form of strained layer superlattices, while the second is the growth of self-assembled coherent Si1-xGex islands or quantum dots [128]. The control of composition x of Si1-xGex nanocomposites at the atomic level is achieved via the design of single-source precursors containing precise atomic arrangements with direct Si-Ge bonds. Uniform composition reflecting the stoichiometry of the precursor is observed in all cases without any segregation of either Ge or Si. The growth of Si1-xGex films proceeds via


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the Stranski-Krastanov growth mode. Morphological control of the size and shape of the islands is achieved by simple adjustments of the precursor flux rate and the growth temperature [129]. Vapor-liquid-solid and vapor-solid approaches Depending on the presence or absence of metal catalysts in the synthesis processes, two growth mechanisms, i.e., vapor-liquid-solid (VLS) and vapor-solid (VS) mechanisms, have been adopted to account for the growth of the one-dimensional oxide nanostructures. In the VLS process [130], a liquid alloy droplet composed of a metal catalyst component (such as gold, iron, etc.) and a nanoparticle (e.g., nanowire, nanobell...) component is first formed under the reaction conditions. The liquid droplet serves as a preferential site for the adsorption of gas-phase reactant and, when supersaturated, as the nucleation site for crystallization. Nanowire growth, for example, begins after the liquid becomes supersaturated in reactant materials and continues for as long as the catalyst alloy remains in a liquid state and the reactant is available. During growth, the catalyst droplet directs the nanowire's growth direction and defines the diameter of the nanowire [131]. Unlike the well-developed VLS process, the detail of the VS process, for example, how atoms or other building blocks can be rationally assembled into one-dimensional nanostructures with wirelike on bell-like morphologies, is still not fully understood. This process is believed to be dominated by the direct vaporization of the solid at a higher temperature zone, with deposition occurring at a lower temperature region. This process appears to be most relevant to the growth of oxide nanobells. Solution- liquid- solid approach For materials with a zinc blende cubic lattice, the solution - liquid - solid (SLS) mechanism [132] has been widely used to synthesize wirelike crystallites. The SLS method is analogous to the vapor-liquid-solid (VLS) method [133], which generates single-crystal wires in relatively large quantities. Very recently, quantum rods and wires of InAs [134] and InP [135] with controllable diameters and excellent crystallinity were synthesized by the SLS method. However, the fabrication of III-V semiconductor quantum rods (QRs) remains difficult regarding the control of size, length, and size uniformity. Another difficulty with QRs synthesized by the SLS method is residual metallic catalyst spherules (Au, In) present at the rod tip. The presence of these metal catalyst particles interferes with measurements of the optical and electronic properties of the QRs.

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Other approaches An alternative approach is to modify the nanoparticle growth reaction in situ to favor geometric and morphological changes in the final nanoparticle. It has been demonstrated that it is possible to fabricate branched CdSe nanocrystals of tunable size by controlling the growth kinetics during different phases of the reaction to favor growth along specific crystal facets of the material [136]. To create anisotropic metal nanoparticles, a similar in situ growth control can be achieved by the inclusion of surfactants in the growth solution. Metal nanoparticles including nanorods [137], nanocubes [138], and nanotubes [139] are some examples of particles that have been prepared using surfactant-based methods. The size and shape of the surfactant micelles provide control over particle morphology during nanoparticle growth [140]. The selective adsorption of surfactant molecules and their respective counterions on certain crystallographic facets during particle growth is also believed to affect nanoparticle shape [141]. Studies that have attempted to characterize the mechanisms controlling the size and shape of metal nanoparticles produced through a surfactantmediated approach have found the process to be highly dependent on the various chemical species in solution and the thermodynamic stability of the nanoparticle crystalline domains [142]. Virtually all prior work in this area has focused on the role of the surfactant in nanoparticle synthesis. The structure and geometry of metal nanoparticles can also be modified after synthesis. Specifically, metal nanoparticles have been converted from one geometry to another using pulsed laser irradiation [143] and photoinduced and thermalinduced conversion processes [144]. The nanoparticle reshaping is dependent upon the presence of emulsifier (e.g., cetyltrimethylammonium bromide, CTAB), and that the final morphology depends on the reactant environment of the nanostructure and also on the morphology of the starting nanostructure [145]. Specifically, Aguirre et al. have observed the room temperature reshaping of the gold shell to form highly asymmetric gold rod-like or beanlike structures attached to silica and gold nanoparticles. They also have observed that the silica nanoscale structures present reshape and ultimately dissolve over time. As has been suggested by many studies on CTAB-mediated nanoparticle growth, it is possible that the stabilization and formation of different nanocrystalline shapes is due to the presence of other molecular species during nanoparticle synthesis, possibly byproducts of the reduction reaction. The formation of nonspherical particles can be synthesized by the growth of the spherical particles by variation of reaction conditions. For example, rodlike CdSe/CdS core-shell particles have grown from the spherical CdSe nanocrystals (Fig. 3 a, b) [146]. At the first stage of the synthesis, CdSe core nanocrystals with narrow size distributions (Figure 3 a) were synthesized in the coordinating solvent mixture of hexadecylamine (HDAm), tri-n-octylphosphine oxide (TOPO), and tri-n-


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octylphosphine (TOP) using dimethylcadmium and tri-n-phosphine selenide (TOPSe) as cadmium and selenium precursors, correspondingly CdS shells were grown on the CdSe cores by slowly adding a solution of cadmium and sulfur precursors to a diluted dispersion of CdSe cores in the HDAm-TOPO-TOP mixture.

Fig. 3. Overview TEM images of (a) spherical CdSe nanocrystals used as seeds for the growth of (b, c) CdSe/CdS nanorods [146].

The relatively low temperature used for growing the CdS shell prevented the alloying of the core and shell materials [9]. If the molar ratio of cadmium and sulfur precursors was in the range from 1:1 to 1:1.6, then spherical CdSe/CdS nanocrystals with a well-controlled shell thickness were formed. In the presence of an excess of the sulfur precursor (Cd/S = 1:3-1:5), an asymmetric growth of the CdS shell leading to rodlike nanocrystals with an aspect ratio of up to 5:1 took place [146]. Using CdSe cores of different sizes, the diameter of the nanorods can be varied from 3 to 6 nm. Their length can be easily tuned by changing the amount of shell precursors added (Figure 3 b). The nanorods grown at relatively low reaction temperatures (120-130 oC) had the same diameter as the initial cores because of purely asymmetric shell growth, whereas at higher temperatures (140-180 oC) the growth along the c axis was accompanied by an increase in particle diameter. In contrast, nanocrystals prepared at 280 oC have a spherical shape due to the isotropic growth of the CdS shell. The transition from the solely anisotropic growth of the CdS shell at low temperatures (120 oC) to isotropic growth at 280 oC can be explained if we consider both thermodynamic and kinetic factors affecting the shell growth. The difference

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in interfacial strain energy for the CdS shell growing on the different facets of the CdSe core results in favorable growth of the CdS shell along the (001) axis. However, at high temperature this energetic difference may approach the value of kT, resulting in a transition from anisotropic to isotropic growth of the CdS shell. The proposed kinetic model is based on the higher reactivity of the {001} facet of CdSe seeds. The observed regimes of temperature-dependent shell growth can be determined by a transition from reaction kinetics-limited (anisotropic) growth at low temperatures to diffusion-limited (isotropic) shell growth at higher temperatures. Such a transition is expected to occur as the reaction rate increases more strongly with temperature than the rate of diffusion. 4.5. Digestive ripening process For the studies of various properties of nanostructures are required nanoparticles with a very narrow distribution. The similar claims are necessary for the design, fabrication and the particle assemblies. The narrowing of particle distribution can be done by the digestive ripening process. The mechanism for this remarkable process is complex and not entirely clear. However, a few useful facts are known. First of all, nanoparticles are the necessary starting material; that is, normal metal powder is not susceptible to digestive ripening, showing again that nanosized particles are intrinsically more chemically reactive than bulk samples [147]. The narrow size distribution of the particles can be achieved by the simple and remarkable procedure of “digestive ripening” [148]. This procedure is based on the reflux of a polydisperse nanoscale colloid for a certain amount of time, resulting in a dramatic improvement of the size distribution of the particles. The ripening process probably involves the dissolution of surface atoms or clusters of atoms by the ligand molecules, which are in excess in the solution [27]. This phenomenon is best known for gold nanoparticles [149], but other metals can behave in a similar way with the appropriate ligands [56, 150]. So a dissolving and reprecipitation process is likely, and reactive sites (corners, edges) would be the first atoms susceptible. The narrow size distribution of the particles achieved by digestive ripening can be discussed in terms of the close-packed particle shell performed by the thiol headgroups and the alkyl chains of thiols for the certain size or curvature of metal particles. Unlike Ostwald ripening, where small particles dissolve preferentially in favor of larger particles [151], the digestive ripening occurs through a process in which large particles break apart and small particles increase in size until a stable and uniform size is reached for the entire colloid.


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A digestive ripening process was successfully used to get mainly noble metal monodisperse nanoparticles [152]. The authors [152] demonstrated a simple and straightforward approach to obtain narrow size distribution gold nanoparticles from a very polydisperse colloid by ligating the nanoparticles with dodecanethiol followed by a digestive ripening process. Temperature induced size segregation can be used to further select the desired particle size. Stoeva et al. have applied the digestive ripening process for the formation of a monodisperse colloid from the polydisperse Au-toluene-thiol colloid and discussed the mechanism of particle variations [27]. The procedure involves heating under reflux of a Au-toluene-thiol colloid. A polydisperse colloid containing particles with sizes ranging from 1 to 40 nm is transformed into an almost monodisperse colloid with particle sizes of about 4-4.5 nm. The UV/vis absorption spectrum of the colloid after cooling to room temperature (Figure 4) shows an appearance of a definite plasmon absorption maximum at 513 nm, which is in agreement with the size and monodispersity of the obtained particles. The UV/vis absorption spectrum (Figure 4) of colloid 2 is in agreement with the sizes of the particles observed in TEM. It is characterized by a broad plasmon absorption band with no definite maximum [153].

Figure 4. UV/vis absorption spectra of as-prepared colloid 2 (1) and the digestive ripened colloid (2) [27].

Heating of Au-toluene-thiol colloid under reflux results in a dramatic narrowing of the particle size distribution and increased organization of nanoparticles [26]. TEM studies of a hot colloidal solution showed formation of spherically shaped particles with sizes of about 4 nm. They have an increased tendency to organize into 2D layers. Some of the particles from the hot colloid organized in nice 3D structures. The amazing result of the increased narrowing of particle size distribution is that the particles predominantly organize on the TEM grid in large 3D structures in only

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about 15 min after the digestive ripening process is finished. A small number of areas of 2D arrangement are also observed. Even larger 3D structures are observed after 1 day and after ~ 2 months. A reflux digestion caused a dramatic improvement of particle size distribution of metal colloids prepared in the didodecyldimethylammonium bromide (DDAB)/water/toluene inverse micelle system [152]. The prepared gold particles have a wide size distribution, from tiny particles as small as 1 nm to very large particles with the size of 80 nm. The large size distribution was apparently caused by the inhomogeneous growth of the nanoparticles due to the low DDAB concentration. After stirring of prepared colloid with dodecanethiol, the color of the colloid turned from dark red to slightly purple. Dodecanethiol has a strong affinity to the gold surface [154] which results in a change of the interaction strength between the particles. The purple color was caused by the aggregation of the gold particles [155]. The ligand modification and reflux digestion caused a dramatic improvement of particle size distribution. The average particle size was 6.2 nm, with a much narrower size distribution (σ = 0.3 nm). The same size particles existed throughout the entire grid. A longer time digestion only improve the size distribution slightly. The amount of dodecanethiol also affected the final size of the system. If an extra dodecanethiol was added to colloid, a somewhat larger particle size was observed after digestion, with average size of 8.7 nm. It is also reported that significant amount of 6.2 nm particles also exist in the size distribution. Therefore a large amount of extra dodecanethiol molecule is needed for this complete digestion to occur. The amount of water existent in the reaction precursor also influenced the final particle size after digestion. As the amount of water in the DDAB/water/toluene system was increased by several times, the prepared sample is also very polydisperse with a significant amount of very smaller particles with diameter 3–4 nm. One possible explanation is the increased water concentration allows more nucleation sites in the system. By sharing materials between these nucleation sites, the overall particle size after digestion decreases. Long time digestion, however, did not seem to further narrow the particle size distribution. Among the many contributions that Wilhelm Ostwald made to contemporary physical chemistry, [156, 157] “Ostwald ripening” is perhaps the phenomenon most well known to today’s material chemists. This ripening process involves “the growth of larger particles (crystals) from those of smaller size which have a higher solubility than the larger ones” [158, 159], and it has been commonly observed in general crystal growth for more than a century. In their preparative solution, for example, a group of free standing crystallites with unequal sizes in nonequilibrium form will further differentiate and redistribute themselves through the above solid–solution–


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solid process to achieve a more uniform size distribution. As illustrated in Figure 5, to lower the total energy of a system, smaller crystallites would eventually dissolve into solution and regrow on the larger ones during Ostwald ripening (or secondary recrystallization, aging, etc.).

Fig. 5. General process of Ostwald ripening [160]

The intrinsic polydispersity of particles in colloidal solution usually results in the Ostwald ripening growth mechanism, where smaller particles dissolve, providing the monomers for building the large ones [161]. Peng et al. [162] showed experimentally that the growth of nanocrystals in colloidal solutions occurs either with a “focusing” of size distribution if the monomer concentration in the solution is high and all the nanocrystals grow with narrowing of the size distribution or through Ostwald ripening, when the larger particles grow at the expense of smaller particles, resulting in varying of the size distribution. Peng also showed that this behavior is in accord with the model of Sugimoto [163]. “Focusing” and “defocusing” of size distribution do not originate from different particle growth mechanisms and can be described in the framework of the same kinetic model of evolution of an ensemble of nanoparticles in a colloidal solution [164]. The term “monomer” is used to describe any molecular species, excluding nanocrystals, containing Cd, In, As, Se, Te, etc. which are necessary for the formation and growth of nanocrystals [71]. During Ostwald ripening, the particles in an ensemble have different growth rates [164], which has to affect their properties to some extent. Thus, larger particles have positive growth rates while smaller particles show negative ones. In the design and fabrication of traditional (spherical) and nontraditional (core–shell, rod, tube,..) nanostructures, which has attracted much attention in nanomaterials research in recent years [165-167], the century-old ripening phenomenon may offer us newer alternatives if we revisit its type of matter relocation. For three-dimensional colloidal aggregates, regardless of crystalline or amorphous form, it is comprehensible that some interior space would be eventually generated within the

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solids, as larger particles are essentially immobile while the smaller ones are undergoing mass transport through dissolving and regrowing. A number of scenarios can be further analyzed. For an unorganized solid aggregate, where the large and small crystallites are likely to be randomly mixed, the resultant voids will then be created in a similar fashion, that is, the voids will also be randomly distributed. However, for an organized solid aggregate, where there is a crystallite-size distribution or particle-density gradient across the solid, the design and architecture of the void space then become possible upon Ostwald ripening. In this regard, it was reported that the central space within a spherical colloidal aggregate can be created by Ostwald ripening (Figure 6) [168].

Fig. 6. Various schemes of Ostwald ripening for spherical colloidal aggregates: 1) core hollowing process; 2) symmetric Ostwald ripening for formation of a homogeneous core– shell structure; 3) asymmetric Ostwald ripening in formation of a semi-hollow core–shell structure; 4) a combination of 1 and 2 [160].

Two new conceptual schemes, which is termed “symmetric Ostwald ripening” and “asymmetric Ostwald ripening”, are illustrated in Figure 6 (steps 2 and 3, respectively). Unlike scheme 1, which results in only simple hollow spheres, these two schemes allow us to fabricate even more complex inorganic materials with core– shell and basketlike configurations of the same chemical constituent ( homogeneous core–shell structures) [169-171] to differentiate them from heterogeneous core–shell structures, which are made from more than one type of material [167]. In the fabrication of hollow heterogeneous core–shell structures, the different chemicophysical properties of the various components can be utilized to generate the required interior space. For example, by selective solution dissolution, organic combustion, and redox reactions, certain components within precursor core–shell structures can be removed, which leaves interior spaces behind [169]. Unlike the heterogeneous type, homogeneous core–shell structures are in general more difficult


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to fabricate because they do not possess chemical inhomogeneity in their material precursors. 4.6. Deposition The nanoscale coating of colloid particles with materials of different compositions has been an active area of research in nanoscience and nanotechnology [2]. Deposition of metal nanoparticles on different colloid particles to form core–shell particles has been one of the most effective tools for achieving such composite nanostructures [172]. In particular, a number of studies on such composite structures were concentrated on the fabrication of metal coated latex particles, because of their potential applications in the fields of surface-enhanced Raman scattering (SERS), catalysis, biochemistry, and so forth [173]. Conventionally, silver shells on polymer latex were prepared via wet-chemistry methods, which involve the activation of a latex surface by ‘seeds’ of a different metal, followed by the deposition of the desired metal [174], or the modification of the latex with groups capable of interacting with the metal precursor ions on the latex surface via complex or ion pairs, and subsequent reduction [175]. The work [176] describes several in situ chemical reduction methods for depositing silver layers on polystyrene (PSt) latex spheres. Compact silver or gold nanoshells on PSt spheres have been fabricated through electrostatic attraction between ligand stabilized metal nanoparticles and polyelectrolyte modified PSt spheres [177]. Recently, the Fitzmaurice group [178] reported the templated assembly of gold nanoparticles on the surface of patterned latex nanospheres. Other methods involve thermal evaporation techniques or sputtering onto latex particles [179]. Although silver coating on PSt latex has been reported by several investigators, formation of silver nanoshells built up with silver nanoparticles of different sizes and morphologies is still a significant challenge. The deposition of silver nanoparticles instead of smooth silver layers is generally desirable, considering the application of such core–shell particles as appropriate substrates for organic molecules in SERS studies [176]. It has been reported that negatively charged sulfate stabilized polystyrene latex can serve as core particles and be coated with a smooth inorganic layer by the hydrolysis of metal ions, and poly(vinylpyrrolidone) (PVP) was added to the system to prevent the aggregation of core and core–shell particles [180]. In the preparation of silica core–gold shell nanoparticles by chemical methods, one model was suggested wherein nanoparticles are initially nucleated but ultimately coalesce into a continuous coating [181].

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Poly-(styrene–acrylic acid) (PStAA) latex spheres ca. 600 nm in diameter were used as core materials [182]. To deposit silver nanoparticles onto PStAA spheres, a colloid solution containing PStAA latex, AgNO3, urotropine, and PVP was aged.

Figure 7. (a) A TEM image of PStAA latex particles, (b) a TEM image of PStAA/silver particles with one cycle of coating, (c) TEM images of PStAA/silver particles with three cycles of coating, [Ag+] = 0.002 mol dm-3 [182].

Figure 7 (b) shows that it is merely silver particles 10–20 nm in diameter that are immobilized on the surface of PStAA particles in the first cycle of coating. Furthermore, these silver particles grow into larger particles and PStAA spheres seem to be coated fairly well after three cycles of coating (figure 7 (c)). This reveals that silver nanoshells are actually constructed from discrete nanoparticles of uniform size and diameters of ~ 50nm on average. When the concentration of AgNO3 increases to 0.004 mol l-1, silver deposits on PStAA spheres remain discrete nanoparticles with a slightly larger size of about 60 nm. Larger silver particles and some rodlike particles with diameters in the range of 80–100 nm are observed when the concentration of AgNO3 is 0.008 mol l-1, and these particles are closely packed on the surfaces of PStAA particles and starting island formation. Both SEM and TEM images indicate that although the silver coating on the surface of PStAA latex spheres appears to be fairly uniform, these metallic deposits form not a continuous coating or shell but a coating of closely packed silver particles of different sizes and morphologies. Silver nanoshells composed of closely packed silver nanoparticles on PStAA latex spheres have been fabricated through a simple chemical reduction of AgNO3 with formaldehyde generated from the decomposition of urotropine. The shell features could be controlled easily by choosing different reaction conditions.


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When the concentration of AgNO3 increases from 0.002 to 0.008 mol l-1, silver nanoparticles immobilized on PStAA exhibit different morphologies and show uniform size or regular shapes. Although the coating mechanism and the coalescence mechanism cannot be ruled out, the formation process of the discrete silver nanoparticle coating seems to be different from those for the continuous coatings. With a lot of carboxyl groups on its surface, the PStAA latex can be easily adsorbed by Ag+ ions. It is known that urotropine can slightly decompose to formaldehyde and ammonia upon ageing in aqueous solution. The adsorbed Ag+ ions can be reduced in situ by the freshly produced formaldehyde to form silver nuclei, and these silver nuclei are immobilized on the surfaces of PStAA latex spheres. Then, the nucleation stage is followed, especially in the next cycle of coating, by diffusion of silver ions and reducing agent to the surface of the PStAA latex spheres, feeding the in situ reducing reaction, and silver nuclei grow into silver nanoparticles. Deposition of small clusters using a STM tip can be used [183]. It could also position metal clusters at desired locations on a flat substrate. The technique can be scaled up using the millipede approach to deposit a large number of clusters simultaneously [184]. Potentially, it can be coupled with dip-pen technology for a continuous supply of the metal precursor [185]. At present, however, these techniques have not yet achieved atomic control. In other words, there is no technique yet to generate supported metal clusters of uniform composition, shape, and size. Electrodeposition is appropriate to fabricate magnetic nanostructures with a high aspect ratio [186], and at a relatively low cost. Even semiconducting substrates may have a sufficiently large conductivity to allow electrodeposition. Thus it is possible to grow defined magnetic nanostructures directly on semiconductors without the usual disadvantages such as shading and heating of physical vapor deposition processes. It is an established fact that the properties of these materials change with reduction in size in the nanometer range [187, 188]. The electro-deposition approach was used to prepare metal particles [189]. The method is inexpensive, allows high aspect ratio of the deposit and can be applied on a large surface area. The cobalt dots produced by continuous electrodeposition mainly have a multi domain structure in the MFM picture. The MFM picture for the Co dots array grown in the pulsed mode shows that the dots are single domain magnets with a magnetization oriented perpendicular to the surface. This result is again confirmed by the FMR measurements. The spectrum exhibits a very sharp resonance line at 122 mT, indicating that nearly all the dots are in the same state,

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with their easy magnetization direction aligned along the dots axes. The alignment is mainly due to the influence of the shape anisotropy due to the large aspect ratio. As a matter of fact, confined electrochemical growth within holes leads to a polycrystalline hexagonal close packed (hcp) structure [190]. The elementary atomic processes governing the growth of nanostructures on surfaces under ultrahigh vacuum (UHV) conditions are summarized in Fig. 8. They include diffusion processes on terraces, at edges, between layers and across steps, as well as nucleation and coalescence. Each of these processes i is temperature dependent and can be described by a simple Arrhenius-type kinetic equation (12): ki = k0,i exp (- Ea,i/kBT)

(12)

where ki , k0,i , and Ea,i are the rate constant, the pre-exponential, and the activation barrier for process i; kB is the Boltzmann constant, and T is the absolute temperature. The parameters, which are under the control of the experimentalist, are the type and size of the cluster, the type of surface, the surface temperature, the cluster flux, the total coverage or number density of clusters on the surface, and the impact energy of the cluster.

Figure 8. Basic surface processes for the deposition of atoms from the gas phase [191].

The structures generated by deposition may range over several orders of magnitude in size, from just a few angstroms up to the micrometer range [191]. Alloy formation in nanoislands might be useful as a possible source for the generation of chemical power for nanoengines [192]. Fractal particles possess a very high surface to volume ratio, which makes them very interesting candidates for catalytic applications.


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Intriguing issues concerning microscopic crystallization phenomena can be addressed by the growth of nanoparticles. Nanoislands containing a crystalline or amorphous arrangement of clusters can amplify the properties of the single clusters by cooperative effects. Furthermore, they represent new elemental modifications, which seem to be stable only in a very limited size range. Electrochemical atomic layer epitaxy (ECALE), developed by Stickney [193], has been used to produce a wide variety of well-ordered semiconductor deposits. The principle of ECALE is that a deposit is formed one atomic layer at a time, using surface limited reactions through underpotential deposition (upd) [194], in a repeating cycle. Separate solutions and potentials are used to deposit atomic layers of each element electrochemically in a cycle in this method. The problems with this technique are that the electrode is required to be rinsed after each upd deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems for ECALE were developed to overcome these problems [195]. Another particularly promising method is successive ionic layer adsorption and reaction (SILAR) which is based on the sequential immersing of the substrates in solutions of each element, with rinsing between with water [196]. PbS has been prepared by cycling the potential between anodic and cathodic potentials using the disodium salt of ethylenediaminetetraacetic acid (EDTA) as a comlexing agent for Pb [197]. A new electrochemical process was reported to be based on the co-deposition of Pb and S precursors from the same solution containing EDTA, Pb2+, and S2- at a constant potential, which is determined from the upd potentials of Pb and S [198]. Preliminary results indicate that this onestep process could also be used for the electrochemical deposition of II-VI compound semiconductors in the form of single crystalline thin film. Additionally, this method allows easy control of the thickness of the material by simply using different deposition times and has some advantages over the present electrochemical deposition methods to overcome the deposition problems. Oznuluer et al. have shown that it is possible to grow highly crystalline deposits of PbS by a novel electrochemical method, which is based on a simple idea, in which Pb is complexed to prohibit the precipitation in the solution. Then, both of the Pb and S precursors are deposited from the same solution at a constant potential at which both upd waves occur. By interrupting a bulk electrodeposition process in the initial stages of nucleation and growth, an electrode surface can be covered by well-separated nanocrystals. This makes electrodeposition a low-cost and easy-to-use approach for the formation of semiconductor nanocrystals in which quantum confinement leads to an atomic-like energy spectrum [199]. Electrodeposition of quantum-confined structures could be of

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use, for instance, to decorate Si spheres in order to modify the optical properties of Si in optoelectronic devices. Lead selenide is a semiconductor that shows strong confinement effects due to the low effective mass of both electrons and holes [200]. PbSe can be made by electrodeposition from solutions containing lead acetate and selenous acid [201]. For example, the authors deposited PbSe layers from an aqueous solution of lead acetate and selenous acid on flame-annealed gold [202]. To prevent precipitation of PbSeO3 and Pb(OH)2, Pb2+ ions were complexed with EDTA and the solution was made acidic by adding acetic acid. Electrodeposition of PbSe on gold occurs by a two-dimensional nucleation and growth mechanism. This results in strongly flattened structures. The nanocrystals have a staircase-like electron density of states, typical for a quantum well, and the dielectric function of the PbSe nanocrystals shows a marked dependence on the nanocrystal height. Deposition–precipitation has a potential advantage over coprecipitation in that all of the active component remains on the surface of the support and none is buried within it. In fact, this method has been used to prepare a number of supported gold catalysts [203], although NaOH was used as a precipitation agent. The benefit of using the thermal decomposition of urea is that hydroxyl ions are generated slowly and uniformly throughout the liquid phase and their concentration is always low because they are consumed almost as soon as they are formed, thus preventing the precipitation away from the support surface. Two groups successfully used this method for the preparation of Au/TiO2 spheres [204, 205]. Khoudiakov et al. have reported the synthesis of nanoscale Au/Fe2O3 particles by a deposition–precipitation technique using the thermal decomposition of urea [206]. A dispersion of metallic Au nanoparticles having typical diameters of 3-7 nm is deposited. They were supported on highly crystalline α-Fe2O3 grains with an average size of about 20 nm.

4.7. Recipes for nanocomposite particles General Nanocomposite materials [207] provide the possibility for enhanced functionality and multifunctional properties in contrast with their more-limited single-component counterparts. One example of a nanocomposite material is the inorganic core-shell structure. In the case where semiconductors comprise the core and shell, the coreshell motif has permitted enhanced photoluminescence [208], improved stability against photochemical oxidation [209], enhanced processibility [210], and


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engineered band structures [211]. Where metals have been combined in core-shell structures, noble metals have been grown on magnetic metal cores [212] and the reverse [213], for example, causing changes in magnetic, optical, and chemical properties compared to those of the individual components [213]. While examples of enhancement or modification of properties resulting from the core-shell structures are becoming more common, instances of truly multifunctional behavior remain rare. For example, iron oxide nanoparticles overcoated with a dye-impregnated silica shell were shown to retain the magnetic properties of the core, while exhibiting the luminescent optical properties of the organic dye [214]. Core-shell particles The core-shell approach was described by Sun et al. follows [215]. In a typical heterogeneous nucleation synthesis of 7 nm diameter particles, platinum seed clusters were formed by the polyol reduction of 4 mg of platinum acetyl acetonate, Pt(acac)2 by 150 mg of 1,2-hexadecanediol in a dioctyl ether solution containing 0.5 g of surfactant [216]. The initial surfactant was a 1:1 molar combination of oleic acid (OA) and oleylamine (OAm). The solution was heated while stirring to 100 °C, and 0.2 mL of Fe(CO)5 was added. The yellow solution darkened as it was heated past 160 °C, becoming black by 200 °C. The color change indicated the growth of Fe on the Pt seed clusters. Heating continued until reflux at approximately 287 °C, and then the heating mantle was removed, and the solution was cooled to room temperature. An additional stage of growth followed. After the solution was cooled, the volume of octyl ether was doubled, and the solution was reheated. An additional 1.2 mL of Fe(CO)5 was added at 100 °C, and heating continued until 260 °C, when the sample was again immediately cooled and removed to an argon atmosphere glovebox for washing. The initial heating stage lasted less than 25 min with no refluxing, and the second stage lasted no more than 10 min. The final sample showed ferrofluidic response to a permanent magnet. To make smaller particles, less iron pentacarbonyl was used. To increase the particle size, larger amounts of iron carbonyl were used and the heating times were longer during both stages. Larger particles were also prepared using reduced amounts of the Pt seeding agent, but this method was less reliable. Pt-Fe2O3 core-shell nanoparticles were made in octyl ether (2.5 mL) with a reactant mixture of platinum acetylacetonate (100 mg, 0.25 mmol), iron carbonyl (Fe(CO)5, 0.4 mmol), oleic acid (0.6 mmol), oleylamine (0.6 mmol), and 1,2-hexadecanediol (0.2 g, 0.75 mmol) [217, 218]. After the reaction, the core-shell nanoparticles went through several size selection cycles using ethanol and hexane [215]. These coreshell particles were then deposited as a monolayer on a substrate using a LangmuirBlodgett system at a surface pressure of 45 mN m-1. The Langmuir-Blodgett (LB)

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technique was also used to deposit γ-Fe2O3 nanoparticles [219]. The solid-state conversions to the final FePt products were conducted at either 550 or 650 °C for 9 h under a flow of Ar(95%)/H2(5%) gas in a tube furnace. The iron oxide shell was reduced to iron and formed the FePt alloy via solid-state reactions [220]. A procedure similar to that for making FePt nanoparticles was used to convert Pt-Fe2O3 core-shell nanoparticles into fct FePt films. In the latter case, a multilayered assembly, instead of the LB monolayer of Pt-Fe2O3 core-shell nanoparticles made by drop casting, was used. The synthetic procedure which employs the thermal decomposition of metalsurfactant complexes of different stability was used for the preparation of bimetallic core-shell structures [221, 222]. The metal-trioctylphosphine (Me-TOP) complex solution, prepared from the reaction of 1:1 mixture of Pd(acac)2 (0.328 mmol) and Ni(acac)2 (0.328 mmol) in 2 mL of trioctylphosphine, was injected to 7 mL of oleylamine at various temperatures (82 – 96 oC). The resulting solution was aged at the indicated temperature for 30 min, producing colloidal nanoparticles. The elemental analysis using energy dispersive X-ray spectroscopy (EDX) in TEM and ICP-AES of the nanoparticles revealed that a large fraction of Pd-TOP complex was not decomposed below 235 oC. When the injection temperature was increased gradually from 205 to 235 oC, the mole fraction of Pd increased from 7 to 41%, demonstrating that Ni-TOP complex decomposes at a lower temperature than PdTOP complex. The particle size of pure nickel nanoparticles was larger than that of pure palladium nanoparticles synthesized under the same reaction conditions.

Figure 9. Model structures of Pd-Ni bimetallic nanoparticles [222].

The strategy of synthesis of Ni-Pd core-shell nanoparticles was as follows: Ni-TOP complex is decomposed at the relatively low temperature of 205 oC, where Pd-TOP complex is rarely decomposed. After aging at 205 oC for 30 min to decompose NiTOP complex completely, the temperature was slowly increased to 235 oC to decompose Pd-TOP complex, generating the Pd shell on the top of Ni core. The


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particle size and Pd mole fraction after aging at 205 oC for 30 min were 3 nm and 9 mol %, respectively. After aging at 235 oC for 30 min, the particle size was increased to 4 nm and the Pd content was simultaneously increased to 47 mol %. Fieldemission Auger electron spectroscopy (FE-AES) demonstrated that bimetallic Pd-Ni nanoparticles had a Ni-rich core and a Pd-rich shell, similar to the structure shown in Figure 9. By mixing oleic acid and Fe(CO)5 with benzyl ether solution of Pt(acac)2 first and heating the mixture at 130 oC for about 5 min before oleylamine was added, a portion of highly faceted FePt nanoparticles was obtained [223]. If the mixture contained excess of Fe(CO)5, by refluxing in a shorter period of time and air oxidation, core/shell-structured FePt-Fe3O4 nanoparticles were separated [224]. The 7-nm/1.2nm FePt-Fe3O4 nanoparticles were prepared with Fe(CO)5/Pt(acac)2) [225]. Further experiments indicate that various FePt-Fe3O4 nanoparticles with tunable core and shell can be readily prepared in this one-step synthesis. The synthetic results seem to support the following particle formation mechanism. The Pt-rich nuclei are formed from the reduction of Pt(acac)2 either at temperature > 200 oC or by Fe atoms from the decomposed Fe(CO)5, or by both. More Fe atoms will then coat over the existing Pt-rich nuclei, forming larger clusters. Exposing these clusters to air leads to formation of Pt-rich FePt-Fe3O4 as evidenced by XRD analysis. Heating the clusters to refluxing at 300 oC leads to atomic diffusion and formation of fcc-structured FePt nanoparticles. In the presence of excess of Fe(CO)5, the extra Fe will continue to coat over, leading to core-shell-structured FePt-Fe that is further oxidized to FePt-Fe3O4. Lai et al. have presented the one-step synthesis of core-shell magnetic nanoparticles [226]. The nanoparticles were prepared by adding 0.21 mL of Fe(CO)5 and 39.98 mg of Cr(CO)6 to 100 mL of degassed mesitylene (preheated to 70 °C). The molar ratio surfactant : carbonyls : solvent was 1:100:40000, and 9:1 for Fe(CO)5 : Cr(CO)6. After stirring for 5 min, 226 mg of the polymer surfactant, Pluronic F127, was added. The temperature was then raised to reflux (~165 °C) for 24 h under reducing environment (5% H2; 95% Ar). After cooling to room temperature, the resulting precipitate was collected by centrifugation, washed with toluene, and dried under vacuum. MFe2O4 nanoparticles Monodisperse CoFe2O4 nanocrystals are synthesized by using a combination of a nonhydrolytic process and seed-mediated growth [227]. The general strategy is using coordination compounds of iron(III) and cobalt(II) acetylacetonate, Fe(acac)3 and Co(acac)2, as precursors in a nonhydrolytic process to synthesize CoFe2O4 spherical nanocrystals with a mean diameter of ca. 5 nm. Such nanocrystals then serve as seeds

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to grow larger spherical or cubic nanocrystals in the seed-mediated growth process. In a typical synthesis, a mixture of 2 mmol of Co(acac)2, 40 mL of phenyl ether, 20 mmol of 1,2-hexadecanediol, 10 mL of oleic acid, and 10 mL of oleylamine was heated to 140 °C followed by a droplet addition of 4 mmol Fe(acac)3 in 20 mL of a phenyl ether solution. The temperature of the nonhydrolytic reaction was then increased quickly to 260 °C, and the mixture was kept at reflux for 30 min before being cooled down to room temperature. After addition of ethanol and centrifuging, spherical CoFe2O4 nanocrystals with a diameter of 5 nm were obtained. Such 5 nm nanocrystals were used as seeds to grow 8 or 9 nm spherical nanocrystals in the solution of Co(acac)2 and Fe(acac)3 precursors. For instance, to produce 8 nm spherical nanocrystals, 100 mg of seeds was used in a particle growth solution consisting of 1 mmol of Co(acac)2, 2 mmol of Fe(acac)3, 10 mmol of 1-octadecanol, 5 mL of oleic acid, and 5 mL of oleylamine. Then, the solution temperature was raised to 260 °C at a rate of 10-15°/min and kept at reflux at 260 °C for 30 min. The nanocrystals precipitated out after ethanol was added. The nanoparticulate samples were suspended in hexane for TEM studies. Using the seed-mediated growth, bigger nanoparticles of CoFe2O4 up to 20 nm were prepared. MnFe2O4 nanoparticles with a mean size of ~ 9 nm were stirred overnight in 1.0 M aqueous initiator solution, 3-chloropropionic acid. The pH of the solution was kept at 4 by adding HCl. The particles were collected with a magnet and washed several times to remove excess initiator. Air-dried nanoparticles (0.22 g) were added into 8 mL of nitrogen-purged styrene solution. CuCl (0.3 mmol) and 4,4’-dinonyl-2,2’dipyridyl (1.1 mmol) were dissolved in 4 mL of xylene, and then the solution was added to the styrene/nanoparticle mixture. The final solution was stirred and kept at 130 °C for 24 h. The particles were collected with a magnet and repeatedly washed with toluene [228]. Using the seed-mediated growth, bigger nanoparticles of MnFe2O4 up to 18 nm have been made [224, 229]. Sun et al. demonstrated the formation of nearly monodisperse CoFe2O4, and MnFe2O4 nanocrystals using metal acetylacetonates as the precursor in the presence of 1,2-hexadecanediol, oleylamine, and oleic acid in phenol ether. [224, 229]. The sizes of the resulting nanocrystals in these three high-temperature and nonaqueous solution approaches were varied between a few nanometers and about 20 nm. Alloys FePt, CoPt, and FePd alloys with high magnetocrystallic anisotropy and chemical stability inspired much interest because of their potential applications in high-density data storage and highperformance permanent magnets. Because the magnetic stability of an individual particle is related to the anisotropy constant and the particle


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volume, these alloys become promising candidates to fabricate ultrahigh-density magnetic storage devices. Chemically prepared FePt nanoparticles have generated great interest recently because of their ease of synthesis, chemical stability, and potential applications in high-density data storage1 and high performance permanent magnets [230]. The surfactants surrounding each FePt nanoparticle can be replaced by other bifunctional stabilizers, rendering the particles water soluble and suitable for site-specific biomolecule attachment and magnetic field-assisted bioseparation [231]. Nanoparticles of iron-platinum alloys have been under intensive study in recent years because of their magnetic properties [232]. There are three major different types of ordered FePt alloys, namely, Fe3Pt, FePt, and FePt3 [233]. The face-centered tetragonal (fct) (also known as L1o phase) FePt alloy is particularly desirable because of its high magnetic anisotropy, high coercivity, small domain wall width (2.8-3.3 nm), small minimal stable grain sizes (2.9-3.5 nm), and chemical stability. These properties make fct FePt nanoparticles excellent candidates and widely studied systems for applications in ultrahigh density magnetic storage media and for making advanced magnetic materials [234]. For applications, there are several requirements for FePt nanoparticles (NPs). First of all, although a smaller size is better, the diameter of NPs should be larger than the superparamagnetic limit, ~3.3 nm, if one wants FePt NPs to be ferromagnetic [235]. Second, the size distribution of NPs should be narrow enough for the application. Third, undesirable sintering between each nanoparticle, that takes place when NPs are annealed in order to transform the crystalline structure from the chemically disordered fcc to the chemically ordered fct (L10 phase), should be avoided. Finally, the FexPt100-x NPs should be within the composition of 40 ≤ x ≤ 60 to be transformed into the L10 structure according to the phase diagram [236]. Until now, a number of efforts have been devoted to prepare monodisperse magnetic nanocrystals via a wet chemical method with binary or multiple surfactants, such as trioctylphosphine oxide (TOPO)-oleic acid [56], adamantanelcarboxylic acid (ACA)hexadecylamine (HDAm) [237], HDAm - TOPO- TOP [238]. Currently, monodisperse magnetic nanoparticles of FePt alloy are synthesized through the simultaneous reduction of metal salts and thermal decomposition of organometallic compounds in the presence of mixed surfactants [215]. The stabilizing agent is important for the formation of monodisperse FePt nanoparticles in the disordered face-centered cubic (fcc) phase [232]. The assembled nanoparticles can be converted into fct FePt alloys after annealing at enhanced temperatures. It is important to note that the monodisperse FePt nanoparticles made using wet

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chemistry methods typically have diameters in the size range of 4 to 6 nm [232]. For many practical applications, magnetic nanoparticles larger than 6 nm are preferred because coercivity Hc and remanence to saturation magnetization ratio Mr/Ms of the nanoparticles are closely related to the volume or size of magnetic nanoparticles [239]. These two properties have maximum values when the nanoparticles reach the critical sizes, which are around a few tens of nanometers in diameter depending on the chemical composition and crystalline structure of the particles. For instance, 2040 nm nanoparticles have the highest coercivities for the CoNi alloy system [240]. Nanoparticles gradually become superparamagnetic due to random anisotropy when they are smaller than the critical size. Domain structure exists in particles larger than the critical size and is responsible for the decrease of coercivity and remanent magnetization. Unfortunately, the Ostwald ripening, which has been successfully applied to production of semiconductor quantum dots [241] and silver nanowires [139] of various sizes does not seem to work on FePt nanoparticles. FePt nanoparticles The FePt particles are commonly synthesized via decomposition of iron pentacarbonyl, Fe(CO)5, and reduction of platinum acetylacetonate, Pt(acac)2, [236] co-reduction of iron salt and Pt(acac)2, [242] or seed-mediated growth where smaller FePt nanoparticles are used as seeds and more FePt is coated over the seeds. The decomposition and reduction or co-reduction methods can yield FePt nanoparticles with controlled composition but fail to produce particles larger than 4 nm, while the seed-mediated growth method gives larger FePt particles without accurate control on Fe/Pt ratio at different sizes. As both structural and magnetic properties of FePt nanoparticles depend not only on the size, but also on the composition of the particles [243]. In the one-step synthesis of FePt nanoparticles, platinum acetylacetonate (Pt(acac)2) and iron pentacarbonyl (Fe(CO)5) and Fe(CO)5 was mixed at excess of stabilizers at 100 °C, then the mixture was heated to more than 200 °C, and kept it at that temperature for 1h, before it was heated to reflux [215, 223]. It was found that with benzyl ether as solvent and oleic acid and oleylamine as stabilizers, one-pot reaction of Fe(CO)5 and Pt(acac)2 could give nanosized FePt particles (3 - 4 nm). Size, composition, and shape of the particles were controlled by varying the synthetic parameters such as molar ratio of stabilizers to metal precursor, addition sequence of the stabilizers and metal precursors, heating rate, heating temperature, and heating duration. Monodisperse FePt nanocrystals were prepared by hydrolysis of pentacarbonyl iron and reduction of metal complexes in the presence of oleic acid and oleylamine [215].


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The popular synthetic method for the preparation of FePt NPs is the procedure with Fe(CO)5 as a precursor [215]. The drawbacks are the high toxicity and high flammability of reactants and the atomic composition distribution of FePt NPs (mean diameter 3 nm) is extremely broad and the fraction of FexPt100-x NPs which are within a composition of 40 ≤ x ≤ 60 was reported to be less than 30% [244]. When iron(III) acetylacetonate [245] or iron(II) chloride [232] is chosen as a precursor, the reducing agent, such as a sodium or boron compound, has to be use. A purification process is needed to remove such elements, otherwise these elements are included in the FePt NPs as impurities [232]. In addition, in many cases, only NPs smaller than the superparamagnetic limit (~3.3 nm) have been produced [246]. Although one can synthesize L10 FePt NPs directly using a polyol reduction method at high temperatures, the resulting NPs often tend to aggregate [247] and the quality of FePt NPs was inferior to that of those obtained by using Fe(CO)5. Chen et al. succeeded in making 6-nm and 9-nm FePt NPs with a good average composition by controlling the heating rate and Ar gas flow rate precisely [223]. Pt content in FePt NP tends to increase with increasing NP size. Thus, a unique synthetic approach has been proposed to obtain large FePt NPs with an equiatomic composition by several groups. For example, Teng and Yang reported that they succeeded in synthesizing large (average diameter 17 nm) Fe50Pt50 NPs by making Pt-Fe2O3 core-shell NPs followed by annealing in reductive atmosphere [218]. The synthesis of Pt-Fe2O3 core-shell NPs was sequential. First, the Pt core was synthesized using Pt(acac)2, and then the Fe2O3 shell was coated on the Pt core by a second injection of Fe(CO)5 into the reaction system. In addition, this synthetic method required precise temperature regulation. To precisely control the size and composition distribution among FePt NPs in these synthetic methods, it was important to know the thermal decomposition and/or the reduction rates of iron and platinum precursors as well as the nucleation and growth rates of FePt NPs. Saita and Maenosono have reported the synthesis of FePt NPs using iron(III) ethoxide [Fe(OEt)3] and platinum(II) acetylacetonate [Pt(acac)2] as precursors without any reducing agent. Fe(OEt)3 is a brown powder at room temperature (nonvolatile) and is a highly reactive late transition metal alkoxide [248]. In addition, decomposition products of Fe(OEt)3 are less toxic. For these reasons, the use of Fe(OEt)3 as a precursor is suitable for industrialization. An amount of 1 mmol of Fe(OEt)3, 0.5 mmol of Pt(acac)2, 17 mL of octyl ether, 1.6 mL of oleic acid, and 1.7 mL of oleylamine were placed in a three-necked flask at ambient air. The color of the resulting mixture was brown. The molar ratio of Fe to Pt in the mother reaction solution was adjusted to 2. The flask was then evacuated to remove oxygen and volatile components from the reaction solution. After evacuation, the flask was

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purged three times by high-purity Ar. Subsequently, the temperature was raised to 297 °C under an Ar atmosphere. At around 150 °C, white smoke billowed from the solution and the reactant turned black gradually. After 30 min of reaction at 297 °C, the flask was cooled to 50°C and ethanol bubbled by N2 was added into the flask. By centrifuging this mixture, a black powder was separated from the matrix. Then the powder was redispersed in hexane. Precipitation and redispersion processes were repeated several times to remove impurities completely. By this synthetic procedure, FePt NPs capped with oleic acid were obtained. Equiatomic FePt NPs of 4.5-nm average diameter with good composition distribution were obtained by this synthetic method. Chen et al. have presented a one-step synthesis of FePt nanoparticles with controlled composition and size tunable up to 9 nm in diameter [223]. The size of the particles is tuned by controlling the molar ratio of stabilizers to Pt(acac)2 and heating conditions. A ratio of at least 8 is essential to make FePt nanoparticles larger than 6 nm. In the synthesis, Pt(acac)2 with Fe(CO)5 were mixed and excess of stabilizers at 100 oC, then heated the mixture to more than 200 oC, and kept it at that temperature for 1h, before it was heated to reflux. Heating rate of ~ 15 oC/ min and temperature of 240 oC would finally yield 6-nm FePt, while the rate of ~5 oC/min and heating temperature of 225 oC led to 9-nm FePt. The composition of the particles is controlled by varying the molar ratio of Fe(CO)5 and Pt(acac)2. Under current reaction conditions, Fe(CO)5/Pt(acac)2 ratio of 2 gave 6-nm Fe53Pt47 and 9-nm Fe44Pt56 nanoparticles. In both cases, the particles have narrow size distribution with standard deviation < 10%. The particles have commonly known chemically disordered fcc structure [215] and the diffraction peaks are narrower when the size of the particles is larger. Estimation using Scherrer’s formula [249] on (111) peak gives the average particle size that is consistent with that observed from TEM images, indicating that the as-synthesized FePt nanoparticles are of good crystallinity. The similar approach, that is, hydrolysis of pentacarbonyl iron and reduction of metal complexes in the presence of oleic acid was used to prepare FexCoyPtz [250] nanocrystals. Using the Pt-Fe2O3 core-shell nanoparticle precursors, one can make FePt magnetic nanoparticles not only with the preferred fct phase but also in a size range that has not been achievable using wet chemistry synthetic approaches [218]. Pt-Fe2O3 coreshell nanoparticles were made in octyl ether (2.5 mL) with a reactant mixture of platinum acetylacetonate (100 mg), iron carbonyl (Fe(CO)5, 55 μL), oleic acid (80 μL), oleylamine (80 μL), and 1,2-hexadecanediol (0.2 g) [217]. After the reaction, the core-shell nanoparticles went through several size selection cycles using ethanol and hexane [215]. The solid-state conversions to the final FePt products were


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conducted at either 550 or 650 °C for 9 h under a flow of Ar(95%)/H2(5%) gas in a tube furnace. The iron oxide shell was reduced to iron and formed the FePt alloy via solid-state reactions [220]. A procedure similar to that for making FePt nanoparticles was used to convert Pt-Fe2O3 core-shell nanoparticles into fct FePt films. In the latter case, a multilayered assembly, instead of the LB monolayer of Pt-Fe2O3 core-shell nanoparticles made by drop casting, was used. FePd nanoparticles To prepare highly monodisperse FePd nanoparticles, Hou et al. used a combination of ACA and alkylphosphine to stabilize FePd nanocrystals [251]. The chemical synthetic route is based on the reduction of paladium acetylacetonate (Pd(acac)2) with hexadecanediol and thermal decomposition of Fe(CO)5. Note that the combination of ACA and tributylphosphone (TBP) or oleic acid plays a key role for preparing monodisperse FePd nanoparticles. The size and composition of FePd nanoparticles can be readily controlled by adjusting the combination of various stabilizers and the reaction condition. For example, with 4:1 molar ratio of Fe(CO)5 to Pd(acac)2, Fe28Pd72 particles were produced by using oleic acid-TBP as stabilizers, while Fe50Pd50 particles were produced by using ACA/TBP as the stabilizers. The reaction process was followed by the standard Schlenkline technique. In a typical synthesis, under Ar atmosphere protection conditions, 0.08 g of Pd(acac)2, and 0.54 g of ACA were dissolved in 1 mL of TBP to form Pd-TBP-ACA mixture at room temperature. Subsequently, 0.14 mL of Fe(CO)5 (1 mmol) was injected into the mixture of Pd-TBP-ACA. After stirring for 30 min, orange stock solution of the FePd-TBP-ACA complex was obtained. In a separate flask, the reducing solution consisting of 390 mg of hexadecanediol and 15 g of TBPO was heated to 120-180 °C and bubbled with Ar for 60 min. Sequentially, the Fe-Pd-TBP-ACA stock solution was quickly injected into the reducing solution, and the reaction temperature was slowly increased and maintained at 280-300 °C for 30-60 min. The color slowly changed from orange to black, indicating the formation of FePd nuclei. After the designated time, the heater was removed and the solution was cooled to roomtemperature naturally. The post-prepared process was performed in air. The black precipitates were isolated from the solution by adding nonpolar solvent such as ethanol or acetone and centrifuging. If necessary, a cycle of the washing process with hexane and acetone was performed. CoPt3 nanoparticles CoPt3 nanocrystals were synthesized [252] in a high-boiling coordinating solvent mixture (hexadecylamine-diphenyl ether). In a typical synthesis, relevant amounts of 1,2 hexadecandiol (0.13 g), Pt(acac)2 (0.0328 g) and 1-adamantanecarboxylic acid (ACA, 0.25 g) were dissolved in a mixture of 4 g HDAm and 2 mL of diphenyl ether

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and heated to 65 oC in a three-neck flask until a clear solution was formed [253]. Then, the solution was heated to a certain temperature in the range from 140 oC to 220 oC and the cobalt stock solution was swiftly injected into the hot reaction mixture under vigorous stirring. After injection the color changes from pale yellowish to black indicating the formation of CoPt3 nanocrystals. Further heating usually continued for 1 h at the injection temperature and was followed by annealing at refluxing temperature (~ 275-285 oC) for 1 h. The cobalt stock solution has to be freshly prepared before the synthesis by dissolving 0.043 g of Co2(CO)8 in 0.6-0.7 mL of 1,2-dichlorobenzene at ~ 35 oC under airless conditions. The molar ratio of 1,2-hexadecandiol to Pt(acac)2 was always kept as 6 to 1. After cooling the reaction mixture to 50oC all subsequent steps were performed in air. The crude solution of CoPt3 nanoparticles was mixed with ~ 5 mL of chloroform. Subsequently, 20 mL of propanol-2 were added, resulting in a black precipitate which was isolated by centrifugation. The precipitate was redissolved in toluene (~ 5 mL) and filtered. To wash out the excess of stabilizers, the nanocrystals were precipitated again by addition of ~ 20 mL of 2-propanol and centrifuged. The resulting black precipitate containing CoPt3 nanocrystals can be redissolved in various nonpolar solvents (toluene, hexane, chloroform, etc.). A small amount (~0.1 mg) of HDAm can be added to the solution of the nanocrystals to improve their solubility and the stability of the colloidal solution. CoPt3 nanocrystals can be formed via a modified “polyol” process [215, 252, 254] in a high-boiling coordinating solvent. During particle formation, Pt(acac)2 was reduced by a long-chain 1,2-diol and cobalt carbonyl was thermally decomposed. The synthesis was carried out using standard Schlenk line technique under dry argon. Cobalt stock solution was freshly prepared before synthesis by dissolving 0.043 g of Co2(CO)8 in 0.4 mL of 1,2-dichlorobenzene at room temperature under airless conditions. In a typical synthesis, 0.033 g of Pt(acac)2, 0.13 g of 1,2- hexadecandiol, and 0.084 g of 1-adamantanecarboxylic acid (ACA) were dissolved in a mixture of coordinating solvents and heated to 65 oC in a three-neck flask until a clear solution was formed. Three types of coordinating mixtures were used: (i) diphenyl ether (2.0 mL) and HDAm (4.0 g), (ii) diphenyl ether (6.0 mL) and n-tetradecylphosphonic acid (TDPA) (0.04 g), and (iii) HDAm (0.04 g) and 1-hexadecanol (6.0 g). To produce CoPt3 nanocrystals, the reaction mixture was heated either to 100 or to 170 o C and cobalt stock solution was injected into the flask under vigorous stirring. CoPt3 nanocrystals of 2.6 nm diameter were prepared by injection of a mixture of cobalt stock solution, 1,2-hexadecandiol, and 1.5 mL of diphenyl ether into an HDAmdiphenyl ehter coordinating mixture at 220 oC. Different temperatures and duration of further heating were tested to control the size and the shape of CoPt3 nanocrystals. The yield of the reaction was ~ 0.040 g of properly washed CoPt3 nanocrystals for all


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coordinating mixtures and temperature regimes. This method allows the preparation of CoPt3 particles with sizes ranging from ~ 1.5 to 5.5 nm, depending on reaction conditions and the compositions of coordinating mixture. To prepare larger CoPt3 nanocrystals (up to ~10 nm), additional injections of cobalt and platinum precursors into the reaction mixture were required. A solution of precursors prepared by mixing of relevant amounts of Co2(CO)8 (0.0225 g), Pt(acac)2 (0.0164 g), 1,2hexadecanediol (0.065 g), 1,2-dichlorobenzene (0.4 mL), and diphenyl ether (3.0 mL) was used. This solution of precursors was dropwise introduced into the reaction mixture at 155 oC. After injection, the temperature was slowly (~2 grad/min) increased up to 230 oC and heating was continued at this temperature for 40 min. All subsequent injections were carried out at the same conditions. After cooling the reaction mixture to 50 oC, all subsequent steps were performed in air. The crude solution of CoPt3 nanoparticles was mixed with 5 mL of chloroform. Subsequently, 20 mL of ethanol were added, resulting in a black precipitate which was isolated by centrifugation. The almost colorless supernatant was discarded. The precipitate was redissolved in chloroform (~5 mL) and filtered. To wash out the excess of stabilizers, the nanocrystals were precipitated again by addition of ~ 20 mL of ethanol and centrifuged. The resulting black precipitate containing CoPt3 nanocrystals can be redissolved in various nonpolar solvents (toluene, hexane, and chloroform, etc.). The color of the solution was changed from black-brown to brown in accordance with the concentration of CoPt3 nanocrystals. A small amount (~ 0.1 mg) of HDAm can be added to nanocrystals prepared in TDPA-diphenyl ether coordinating mixture to improve their solubility. When the synthesis described above yielded monodisperse samples, no further size selection was applied. If a distribution of nanocrystals size was observed, the conventional solvent/nonsolvent size selective precipitation technique [255] was applied. Hexane and ethanol were used as solvent and nonsolvent, respectively. The FeCo composite colloids were synthesized by using the sonochemical radiation approach [237, 256, 257]. In one case, stable magnetic fluid consisting of amorphous iron oxide nanoparticles was synthesized by ultrasound irradiation of a decalin solution of Fe(CO)5 at 30 oC for three hours in the presence of oleic acid as a solutions of Co[CH3(CH2)3CH(C2H5)CO2]2 or Co[CH3(CH2)3CH(C2H5)CO2]2 and Fe(CO)5 in decalin, which were sonochemically irradiated at –60 oC for 4 hours. In this method the role of a stabilizing agent is played by the organic ligand [CH3(CH2)3CH(C2H5)CO2]−.

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Metal/inorganic particles Sun et al. [258] have improved the way to coat magnetite nanoparticles with silica. First, stable ferrofluids were obtained by modifying the magnetite surface with citric acid. Then the ferrofluids were transferred into ethanol. After this, water glass treatment was carried out by directly adding a critical volume of silicate solution into the ethanol. Finally the silica coating was formed by Stober process [259]. Magnetite nanoparticles were synthesized according to a procedure proposed by Qu et al. [260]. The concentration of Fe3O4 was adjusted to 2 mg/ml and the pH was adjusted to 3.0 by HCl (0.1 mol/dm3). Citric acid (5% molar ratio of Fe) was added to the suspension under magnetic stirring. After 4 h the solution was washed with water by magnetic decantation 3 times and adjusted to the original volume. Then 2 ml of the magnetite solution was ultrasonically dispersed in absolute ethanol, and 120 ml sodium-silicate solution (Na2O . SiO2, 0.22 wt%SiO2) was added to the ethanol under vigorous mechanical stirring. After four hours, 4ml water, 1.4 ml NH3 . H2O (27wt% NH3) and various volume of tetraethylorthosilicate (TEOS) (5, 20, 50, 250 ml, respectively) were consecutively added to the reaction mixture. The hydrolysis of TEOS was carried out under mechanical stirring for 12 h at ambient temperature. The final product was obtained by magnetic separation and washed with water by magnetic decantation 4 times. Tartaj et al. have reported the synthesis of monodisperse air-stable superparamagnetic α-Fe nanocrystals encapsulated in nanospherical silica particles of 50 nm in diameter [261]. The development of strategies to produce magnetic metallic particles encapsulated in inorganic diamagnetic matrixes is not only important in biotechnology but also in recording applications. For example, an unusual magnetic behavior (beating the superparamagnetic limit) in metallic magnetic nanocrystals embedded in inorganic matrixes has been recently obserbed [262]. In this approach a microstructure with a core containing superparamagnetic αFe nanocrystals dispersed in a silica matrix was prepared. A shell containing silica is ultimately responsible for screening the magnetic dipolar attraction. The absence of dipolar chains suggests that this type of microstructure is adequate for screening dipole-dipole interactions between nanocomposites [263]. The experimental method selected to build the microstructure was based on the subtle interplay controlling the formation of nanospherical silica particles by the ammonia base-catalyzed hydrolysis of tetraethoxysilane (TEOS) in water-in-oil (W/O) microemulsions [264]. In this system, TEOS molecules, easily dissolved in the external oil phase, interact with


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water within the micellar aggregates to produce hydrolyzed species (Si-OH groups) that remain bound to the micelles due to their amphiphilic character. A sol-gel technique mixing ethanolic tetraethoxysilane (TEOS, Aldrich 98%; ethanol, EtOH, Carlo Erba 96%) solutions with hydro alcoholic solutions of Co(CH3COO)2‚4H2O, and Fe(CH3COO)2 was used to prepare (FexCo1-x)y (SiO2)1-y nanospheres [265]. The hydrolysis-condensation reactions were promoted by adding bidistillate water and acetic acid. The pH was kept in the range 3.5-4. Metal solution concentrations were selected to obtain (FexCo1-x)y (SiO2)1-y samples with the following characteristics: (a) a fixed Fe/Co ratio (x = 0.5) and a selected overall metal content (y = 0.02, 0.05, 0.1, 0.2); (b) a fixed overall metal content (y = 0.10) and selected Fe/Co ratios (x = 0.5, 0.55, 0.70). In a typical preparation the starting mixture was stirred for 1 h to obtain a clear sol, which was then poured into a flat Teflon beaker and allowed to gel in air at room temperature. Fresh alcogel was powdered and calcined in air at 623 K for 1 h to eliminate residual water and organics. Xerogel cooled to room temperature (RT) was then put in a quartz reactor, heated with a ramp rate of 10 K/min, and treated at 1073 K for 2 h under a hydrogen flux. After cooling at RT, the H2 flux was replaced by a slow commercial argon flux (impure by some ppm of oxygen) to passivate the particle surface and to prevent their ignition.

Abbreviations 2D 3D ACA ALE BC CarbA Cd HDX CTAB DDAB DDAm DSC ECALE EDTA EDX

two-dimensional three-dimensional adamantanelcarboxylic acid atomic-layer-epitaxy block copolymer carboxylic acid cadmium xantate cetyltrimethylammonium bromide didodecyldimethylammonium bromide dodecylamin differential scanning calorimetry electrochemical atomic layer epitaxy ethylenediaminetetraacetic acid energy dispersive X-ray spectroscopy

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ETrE excitation-transfer efficiency fcc face-centered cubic fct face-centered tetragonal FE-AES Field-emission Auger electron spectroscopy GSMBE gas-source molecular beam epitaxy hcp hexagonal close packed HDAm hexadecylamine ICP-AES inductively coupled plasma – Auger electron spectroscopy LB Langmuir-Blodgett LcA lauric acid LEDs light emitting diodes MBE molecular beam epitaxy MeAA mercapto acetic acid Me-TOP metal-trioctylphosphine MFM magnetic force microscopy MUA mercaptoundecanoic acid NPs nanoparticles OA oleic acid OAm oleylamine OctA octyl acid OctAm octyl amin ODE 1-octadecene ODPA octadecylphosphonic acid ODT octadecanethiol OPs oligomeric phosphines OPMA oligomeric phosphine methacrylate PL photoluminescence PMMA-block-PHEMA poly(methyl methacrylate) – block – polyhydroxyethylmethacrylate PSt polystyrene PStAA poly(styrene–acrylic acid) PVP poly(vinyl pyrrolidone) QDs quantum dots QRs quantum rods QY quantum yield RCM ring-closing metathesis RSH alkyl thiol RT room temperature SAMs self-assembled monolayers SERS surface-enhanced Raman spectroscopy


SILAR SLS SP STM TBP TDPA TEM TEOS TFA TGA TOP TOPO TOPSe TPP or PPh3 UHV UHV-CVD VLS VS W/O XRD

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successive ionic layer adsorption and reaction solution - liquid - solid surface plasmon scanning tunneling microscopes tributylphosphine tetradecylphosphonic acid transmission electron microscopy tetraethylorthosilicate trifluoroacetic acid thermogravimetric analysis trioctylphosphine tri-n-octylphosphine oxide tri-n-phosphine selenide triphenylphosphine ultrahigh vacuum ultrahigh vacuum chemical vapor deposition vapor-liquid-solid vapor-solid water-in-oil x-ray diffraction

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293

Index A

B

ablation, 12 aerogels, 3 aerosols, 3 AFM, 1, 4, 8, 11, 50, 51, 52, 57, 59 aggregates, 16, 41, 53, 56, 75, 77, 83, 89, 90, 91, 92, 98, 111, 113, 115, 136, 145, 160, 172, 180, 187, 258, 259 aggregation, 3, 71, 83, 89, 102, 107, 120, 136, 140, 144, 149, 155, 156, 158, 161, 168, 171, 178, 179, 224, 228, 234, 241, 242, 248, 257, 260 albumines, 21 alkanethiols, 21 alkoxides, 170, 195 alkyl amines, 142 alkylalcohol, 225 alkylamine, 200, 225, 244 alkylphosphates, 142, 143 alkylphosphine oxides, 142 alkylphosphites, 142, 143 alkylsilanes, 187 alkylthiol, 225 amphiphiles, 21, 49, 78, 111, 115, 118, 120 amphiphilic, 77, 93, 96, 103, 107, 109, 110, 111, 116, 118, 159, 160, 227, 278 amphiphilic copolymers, 109, 111 amphiphilic polymers, 93, 112 antiferromagnetic, 52 antiwear properties, 163 ascorbic acid, 188, 189, 190, 193 atomic diffusion, 268 atomic force microscope, 11 atomic-layer-epitaxy, 251, 278 atomization, 137

Bancroft rule, 91 band-gap semiconductor, 223 battery materials, 43, 194 beam-probe techniques, 2 beanlike structures, 253 bicontinuous microemulsions, 87, 89 bicontinuous structures, 76, 159 bifunctional ligands, 228, 242 bilayer solutions, 120 bilayer structure, 190 bimetallic colloids, 177 biomaterials, 24, 39, 122 biomembranes, 120 biomolecular electronics, 57 bionanotechnology, 9 biorecognition, 56 biosensing, 194 biosensors, 10, 42 biotechnology, 1, 9, 12, 42, 43, 44, 49, 277 bis(2-ethylhexyl)sulfosuccinate, 148, 188 block copolymer micelles, 56, 161, 163 block copolymers, 16, 47, 56, 76, 77, 110, 114, 116, 117, 118, 121, 137, 159, 160, 172, 192, 207 borohydrates, 187, 188 Brownian motion, 70, 146 bulkier surfactants, 145 butyl acrylate, 84

C capping agents, 137, 248 capping layer, 141, 143, 145 carbazole derivatives, 34 carbon black, 17, 18 carbon particles, 102 catheters, 31 cavitation phenomena, 165

294 cetyltrimethyl-ammonium bromide, 150, 207 chemically bound emulsifier, 96 clusters, 20, 21, 26, 43, 49, 90, 92, 136, 162, 167, 170, 174, 176, 185, 191, 222, 225, 238, 249, 255, 262, 263, 264, 266, 268, 286 coacervation, 114 coagulation, 72, 86, 99, 107, 108, 113 coagulative nucleation, 80, 86, 107 coalescence, 21, 71, 92, 97, 99, 100, 112, 164, 174, 191, 248, 249, 262, 263 coarse emulsion, 87 coatings, 6, 15, 19, 54, 72, 107, 112, 120, 222, 262 cobalt nanocrystals, 204 cobalt octacarbonyl, 203 coemulsifier, 70, 87, 89, 92, 93, 94, 95, 100 collisions, 90, 146, 148 colloidal, 72, 81, 82, 83, 84, 103, 108, 110, 111, 121 colloidal dispersions, 82, 135 colloidal mills, 135 colloidal nanocrystals, 224, 229 colloidal particles, 28, 81, 82, 121, 135, 138, 176, 178, 181, 182, 228 colloidal solution, 141, 197, 256, 258, 275 colloidal solutions, 18, 258 colloidal stability, 150, 179, 223, 235, 239 colloids, 3, 16, 18, 20, 21, 41, 70, 77, 137, 143, 160, 161, 162, 177, 179, 185, 187, 192, 193, 194, 195, 197, 245, 246, 276 comb polymers, 104 composite materials, 4, 28, 31, 167, 222 composite particles, 3, 85, 100, 150, 166, 176, 180 condensation of metal vapor, 137 conventional emulsion polymerization, 72, 84, 94, 100 core/shell nanoparticles, 153, 183 core-shell, 6, 84, 85, 86, 109, 111, 154, 164, 176, 177, 179, 181, 182, 183, 185, 186, 224, 245, 246, 253, 265, 266, 267, 268, 272, 273 core-shell particles, 84, 85, 177, 266

Index creaming, 71, 99, 114 critical concentration for nucleation, 140 critical micelle concentration, 77, 123, 157, 207 crosslinked particles, 112, 113 crosslinker, 76, 114 cylindrical micelles, 76, 115, 159

D dendrimers, 35, 48, 234, 244 dendron ligands, 235 digestive ripening, 255, 256, 257 dichronism, 18 diketones, 203 dip-coating, 193, 227 dip-pen technology, 262 discrete phase, 73 dispersion, 54, 70, 71, 72, 78, 87, 90, 92, 94, 99, 102, 103, 106, 107, 109, 110, 111, 112, 113, 114, 140, 143, 144, 151, 155, 170, 180, 182, 189, 197, 200, 201, 203, 206, 208, 228, 254, 265 dissipative structure, 122 divinylbenzene, 96, 113, 123 DNA, 6, 10, 16, 26, 37, 38, 123, 248 double gyroid, 117 drug delivery, 24, 31, 32, 43, 44, 56, 115, 120, 135, 194 drug delivery systems, 43, 56

E election-to-photon conversion, 35 electro-deposition approach, 262 electrochemical atomic layer epitaxy, 278 electroluminescence quantum, 244 electrolytic decomposition, 137 electron beam lithography, 40, 45 electron microscopy, 30, 58, 125, 161 electrooptical devices, 137 electrooxidation, 162 electrostatic double-layer repulsions, 179 electrostatic repulsion, 81, 89

Index elementary atomic processes, 263 emulsification, 82, 84, 114 emulsifier, 70, 71, 72, 77, 78, 80, 81, 82, 83, 84, 85, 86, 87, 89, 90, 91, 92, 93, 94, 95, 96, 99, 100, 123, 124, 126, 149, 253 emulsion, 70, 71, 72, 73, 78, 79, 83, 84, 85, 86, 87, 89, 91, 94, 95, 97, 99, 100, 110, 112 emulsion polymerization, 72, 73, 79, 83, 84, 85, 86, 95, 112 encapsulation, 100, 102, 162, 167 energy dispersive, 267, 278 epitaxial growth, 250, 251 epitaxial-type shell, 223 ethanolic tetraethoxysilane, 278 ethylene-propylene copolymers, 159 ethylene-vinyl alcohol copolymer, 32, 57

F ferric chloride hexa-hydrate, 196 ferrocene, 118 ferrofluids, 43, 277 ferromagnetic, 23, 52, 153, 164, 181, 186, 270 ferromagnetic nanoparticles, 153, 164, 181 ferrous sulfate hepta-hydrate, 196 flocculation, 81, 94, 97, 99, 103, 106, 113, 140, 143 Flory-Huggins, 118 Focused ion beam, 30 fullerenes, 55 functionalized nanoparticles, 96, 241 furans, 142, 143 fusion, 102, 122, 146

G gel, 75, 120, 121, 136, 155, 170, 196, 208, 216, 244, 278 gelatins, 21 gels, 41, 75, 76, 97

295

Gibbs energy, 233 goethite, 152, 188 gold, 18, 20, 28, 38, 53, 57, 153, 161, 162, 167, 170, 177, 185, 224, 225, 226, 238, 239, 241, 242, 246, 247, 248, 252, 253, 255, 256, 257, 260, 265 gold nanoparticles, 53, 57, 161, 226, 238, 241, 242, 246, 247, 248, 253, 255, 256, 260 gold nanoshells, 260 Griffin HLB scale, 91

H heavy-metal complexes, 34 hematite, 176, 178, 179 heterogeneous nucleation, 138, 177, 266 heterogeneous phase reaction, 174, 191 heterogeneous reagents, 161 heterogeneous systems, 72 hexadecane, 94, 99, 102, 124, 226 hollow capsules, 121 hollow polymer structure, 102 homogeneous monolayer growth, 250 homogeneous nucleation, 79, 84, 107, 138, 154, 164, 177, 250 honeycomb patterned polymer, 122, 123 hybrid latex, 100 hybrid organic-inorganic materials, 141 hydrazine, 150, 161, 162, 177, 187, 188, 189, 192, 193, 198, 207 hydrodynamic cavitation, 3 hydrophilic, 73, 76, 80, 81, 82, 84, 86, 87, 89, 91, 92, 100, 103, 106, 109, 110, 112, 114, 116, 118, 120, 124, 147, 148, 149, 159, 160, 172, 197, 207, 231 hydrophilic-lypophilic balance (HLB), 87 hydrophobic, 47, 54, 56, 76, 78, 79, 80, 82, 84, 86, 89, 92, 95, 99, 100, 103, 106, 109, 110, 116, 119, 167, 197, 223, 243 hydrothermal, 136, 195 hype-branched ligands, 231

296

I independent nucleation, 250 inorganic cores, 3 interface effects, 227 interfacial area, 41, 71, 89, 91, 92, 99 interfacial phenomena, 13, 127 interfacial tension, 71, 77, 87, 92, 160 interchange of particles, 146 intermetallic nanocrystals, 186 inverse micelles, 76, 137, 159 ionic strength, 77, 83, 86, 88, 114, 156, 160, 178, 179 ionizable groups, 75 iron nanoparticles, 153, 154, 177, 203 iron pentacarbonyl, 164, 181, 202, 203, 266, 271 isotropic microemulsion, 92

K Kelvin effect, 98

L lamellae, 76, 151, 159 lamellar pattern, 227 Langmuir monolayers, 53, 137 Langmuir-Blodgett films, 120 Langmuir-Blodgett technique, 53, 224 laser ablation, 3, 44 laser pyrolysis, 136 latexes, 72, 73, 83, 84, 95, 96, 97 layer-by-layer assemblies, 121 layers, 4, 7, 34, 43, 44, 47, 52, 56, 81, 86, 102, 120, 137, 151, 180, 185, 223, 224, 250, 251, 256, 260, 263, 264, 265 ligand design, 227, 228 ligand exchange reaction, 239, 240, 242 ligand modification, 257 ligands, 35, 37, 40, 54, 56, 75, 144, 155, 162, 168, 171, 186, 187, 194, 195, 203, 222, 224, 225, 228, 229, 231, 232, 234, 235, 236, 237, 238, 239, 241, 242, 243, 244, 245, 246, 248, 251, 255

Index light emitting diodes, 228, 236, 279 light scattering, 72, 77, 95, 159 light-emitting dendriniers, 48 locus of polymerization, 83 loop structure, 92 lyotropic liquid crystals, 137

M maghemite, 198, 199, 202 magnetic colloidal nanoparticles, 194 magnetic fluid, 276 magnetic materials, 43, 52, 117, 140, 270 magnetic nanoparticles, 3, 43, 155, 268, 270, 273 magnetic resonance imaging, 194, 229 magnetism, 10, 15, 22, 179 magnetite, 162, 197, 198, 199, 200, 277 magnetocrystallic anisotropy, 269 magnetoresistance, 43, 154 Massart’s method, 197 materials science, 4, 8, 9, 17, 21, 29, 45, 47, 50 mechanical attrition, 3, 19, 137 mercaptoundecanoic acid, 235, 279 metal alkyls, 142 metal clusters, 136, 167, 174, 191, 262 metal coated latex particles, 260 metal colloids, 50, 137, 170, 177, 182, 188, 225, 257 metal nanoparticles, 3, 40, 43, 53, 55, 149, 161, 168, 173, 174, 185, 187, 192, 203, 223, 226, 239, 253, 260 metal nanoshell, 23 metal oxide-polymer composites, 4 metal oxides, 4, 55, 170, 194 metal particles, 7, 19, 43, 90, 110, 135, 136, 137, 138, 147, 150, 151, 159, 165, 166, 167, 168, 174, 177, 179, 188, 189, 191, 222, 223, 225, 229, 239, 255, 262 metallopolymer materials, 43 metathesis methods, 195 methoxy polyethylene glycol, 191, 207 micellar aggregates, 77, 161, 278 micellar assembly, 89

Index micellar concentration, 82 micellar nucleation, 79 micellar reactions, 3 micellar solutions, 40, 92, 136, 163 micellar systems, 40, 72, 157 micelles, 16, 18, 41, 56, 70, 71, 72, 76, 77, 78, 79, 81, 83, 84, 87, 89, 90, 92, 93, 94, 95, 100, 103, 110, 111, 115, 116, 119, 135, 136, 137, 145, 146, 149, 150, 151, 152, 154, 155, 156, 157, 159, 160, 161, 162, 163, 173, 177, 188, 193, 194, 227, 253, 278 microemulsion, 44, 70, 72, 73, 87, 89, 90, 91, 92, 93, 94, 95, 96, 97, 126, 135, 136, 145, 146, 147, 148, 150, 151, 152, 153, 154, 155, 156, 158, 177, 188, 193, 208 microemulsion polymerization, 73, 93, 94, 95, 96, 97 microemulsion-microwave synthesis, 158 microprecipitation, 114 microreactors, 89, 90, 116, 121, 150, 194 mimetics, 120 miniemulsion, 73, 100, 101, 102 molecular beam epitaxy, 40, 251, 279 molecular recognition, 53 molecular selfassembling, 3 molecular weight distribution, 74, 93, 124 monodentate ligands, 234 monodisperse isotropic nanoparticles, 229 monodisperse latex particles, 86 monodisperse nanoparticles, 137, 198, 256 monodisperse particles, 114, 139, 146, 178, 193, 230 monodispersed silver nanoparticles, 193 monodispersity, 138, 149, 151, 167, 168, 256 monolayer films, 193, 227 monolayer protected clusters, 53, 58 monomer reservoirs, 84 monometallic particles, 176 multilayer films, 53, 186 multishells particles, 245

297

N N,N-dimethylformamide, 77, 160, 207 nanoarchitectures, 1 nanobell, 252 nanobiotechnology, 9 nanobuilding block, 5 nanocapsules, 102 nanoclusters, 7, 43, 161, 167, 248 nanocolloids, 161, 162, 176, 185 nanocomposite particles, 179, 180, 181, 222, 265 nanocomposites, 5, 36, 136, 180, 224, 251, 277 nanocrystal colloids, 21, 143 nanocrystalline films, 186 nanocrystalline materials, 145, 171 nanocrystals, 3, 10, 20, 21, 22, 50, 54, 136, 142, 143, 158, 186, 194, 195, 197, 200, 202, 204, 206, 223, 224, 227, 228, 229, 231, 232, 233, 234, 235, 236, 237, 238, 239, 242, 243, 244, 246, 249, 250, 251, 253, 254, 258, 264, 268, 269, 270, 271, 273, 274, 275, 277, 286 nanocubes, 253 nanoelectronic devices, 10, 39 nanoelectronic materials, 194 nano-emulsions, 72, 100 nanofabrication, 2, 4, 15, 29, 31, 32, 33, 42, 45, 46, 122 nano-film fabrication, 120 nanochannel, 32, 58 nanoimprint-based lithography, 29, 45, 58 nanoimprinting, 8 nanoinstrumentation, 4 nanolayer, 41, 44 nanolithography, 57, 159 nanomachines, 10, 26, 27, 28 nanomaterials, 1, 3, 5, 6, 16, 20, 22, 24, 35, 39, 40, 43, 46, 47, 70, 136, 165, 167, 174, 179, 186, 258 nanoparticle synthesis, 7, 18, 48, 146, 151, 239, 242, 248, 253

298 nanoparticles, 3, 5, 6, 8, 10, 14, 18, 19, 20, 21, 22, 23, 24, 28, 31, 36, 37, 43, 44, 47, 48, 49, 53, 55, 73, 89, 93, 96, 97, 120, 135, 136, 138, 143, 146, 150, 151, 152, 153, 154, 155, 157, 158, 159, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 177, 180, 181, 182, 183, 184, 186, 188, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 201, 202, 203, 204, 206, 224, 226, 227, 228, 229, 230, 235, 238, 239, 240, 241, 242, 243, 246, 248, 253, 255, 256, 257, 258, 260, 261, 262, 264, 265, 266, 267, 268, 269, 270, 271, 273, 274, 275, 276, 277, 279 nanopigments, 8 nanoporous materials, 8, 24 nanoprecipitation, 114 nanoreactor, 146, 188 nanorods, 5, 16, 22, 28, 43, 165, 167, 190, 253, 254 nanoscience, 2, 7, 8, 9, 11, 16, 17, 51, 260 nanosized metal materials, 135 nanospheres, 3, 5, 6, 22, 39, 119, 143, 175, 222, 260, 278 nanostructure, 2, 28, 40, 44, 55, 167, 253 nanostructured materials, 3, 7, 8, 11, 14, 24, 42, 43, 44, 56, 167 nanostructured particles, 3, 43 nanostructures, 1, 2, 3, 5, 6, 10, 12, 14, 15, 16, 17, 19, 25, 26, 28, 29, 39, 40, 41, 42, 43, 44, 45, 46, 48, 49, 51, 52, 56, 57, 117, 121, 136, 165, 183, 190, 223, 252, 255, 258, 260, 262, 263 nanosystems, 25, 43, 49 nanotechnology, 1, 3, 4, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 18, 24, 29, 31, 37, 39, 42, 43, 44, 45, 46, 48, 49, 51, 60, 260 nanotubes, 5, 6, 8, 11, 12, 16, 17, 24, 37, 39, 43, 49, 55, 57, 155, 156, 157, 253 nanowire, 137, 252 nanowires, 22, 35, 43, 165, 175, 271 Newtonian fluids, 92 noble metal, 13, 21, 168, 222, 256

Index noble metal crystallites, 222 nonylphenol poly(ethoxylate) ether, 154

O octadecylphosphonic acid, 237, 279 oil-filled capsules, 114 oligomeric radicals, 79, 80, 81, 84, 94 oligomers, 40, 81, 108 oligonucleotides, 38, 239 oligopeptides, 239 one-step precipitation, 179 optoelectronics, 4, 15, 21, 28, 29, 42, 45 organic dendron ligands, 231 organic light-emitting diodes, 33, 58 organophosphine chalcogenides, 142 organophosphine stabilizers, 206 osmotic pressure, 81, 99, 100 osmotically active agent, 102 Ostwald ripening, 71, 97, 99, 114, 139, 161, 207, 255, 257, 258, 259, 271 Ouzo effect, 114

P paints, 72, 115, 223 palladium (II) bis(acetylacetonato, 193, 227 palladium (II) bis(acetylacetonato), 193, 227 paramagnetic, 23 particle nucleation, 81, 83, 84, 100, 109, 110, 147, 230 particle trapping techniques, 2 passivated nanosized metal, 222 passivation, 15, 155, 222, 223, 225, 228, 229, 243, 246 passivation of colloidal particles, 222 Peltier effect, 35 peptides, 21, 38 perfluorodimorphinopropane, 99 phosphines, 203, 234, 235, 236, 241, 279 phosphorescent emitter, 34 photochemical synthesis, 168 photolithography, 12, 26, 32

Index photoluminescence, 223, 229, 239, 244, 250, 265, 279 photoluminescent, 54 photonic materials, 115 photonics, 4, 28, 47, 118 photopolymerisable resins, 28 photovoltaic devices, 228 phthalocyanine, 180 plasma activation, 43 plasmon excitation, 168 platinum acetylacetonate, 266, 271, 273 poly(2-vinylpyridine), 162, 172, 207, 208 poly(3,4-ethylenedioxythiophene)/ poly(styrene sullonate, 34 poly(3,4-ethylenedioxythiophene)/ poly(styrene sullonate), 34 poly(4-vinylpyridine quaternized), 125 poly(acrylamide-co-acrylic acid), 124 poly(acrylamide-co-N-maleylglycine), 124 poly(acrylamide-co-N-vinylpyrrolidone), 124 poly(acrylic acid), 116, 124, 173 poly(alkyl methacrylates), 159, 208 poly(allylamine), 124 poly(butene succinimide), 101 poly(dimethylsiloxane), 31, 32, 58 poly(ethylene glycol), 56, 58, 118, 124 poly(ethylene oxide), 77, 83, 125, 193, 207, 208 poly(ethyleneimine), 125 poly(methacrylic acid), 125 poly(methyl methacrylate), 58, 102, 103, 125, 193, 279 poly(methyl-methacrylate), 32 poly(N-acetyl)ethyleneimine, 124 poly(N-hydroxyethyl)ethyleneimine, 125 poly(N-methyl-N’-methacryloylpiperazine), 124 poly(N-vinylpyrrolidone), 168, 208 poly(sodium 4-styrenesulfonate), 125 poly(styrene-block-ferrocenyldimethylsilane), 118 poly(urethane), 32, 58 poly(vinyl alcohol), 173 poly(vinyl chloride), 107, 125 poly(vinylphosponic acid), 125 poly(vinylpyrrolidone), 103, 125, 174, 191

299

poly[(3-(methacryloylamino)propyl) trimethylammonium chloride], 125 polyacrylamide, 75, 76, 124 polydisperse colloid, 256 polydisperse nanoscale colloid, 255 polydisperse reaction, 139 polyelectrolytes, 75, 120, 172 polyethylenimine, 75 polyferrocenyldimethylsilane, 118 polychelatogens, 75 polyion complex technique, 120 polyisoprene, 118, 159, 160, 208 polymacromonomers, 105 polymer brush, 77, 159 polymer dispersions, 73 polymer latex, 73, 83, 84, 96, 260 polymer materials, 10, 107 polymer-analogous reactions, 74 polymer-based LEDs, 33, 58 polymeric nanoparticles, 24 polymerisable macromonomer, 96 polymolecular associations, 76 poly-N-isopropylacrylamide, 125 polyols, 136, 195 polystyrene, 57, 77, 103, 112, 116, 117, 118, 125, 160, 162, 163, 166, 208, 224, 260, 279 polystyrene-block-polyisobutylene, 163 polyvinyl ether, 159, 208 polyvinylimidazole, 21 polyvinylpyridine, 117, 160 pyridines, 75, 142, 143

Q quantum dots, 3, 8, 11, 16, 23, 28, 58, 136, 239, 243, 244, 251, 271, 279, 286 quantum wires, 6, 28

R radical polymerization, 40, 72, 73, 74, 219 radioisotope thermoelectric generators, 35, 58 reaction loci, 78, 95

300 reactive polymers, 93 reactive surfactants, 103 reverse micelles, 120, 145, 146, 152, 154, 155, 157, 177 reverse microemulsion, 87, 158 reverse-microemulsion-mediated sol-gel (RMSG) technique, 155

S scaffolds, 10, 115 scanning tunneling microscopy, 50, 52, 59 Seebeck effect, 35 self-assembled monolayers, 53, 121, 125, 224, 226, 279 self-assembled polyelectrolyte films, 137 self-assembling, 3, 5, 14, 18, 37, 38, 42, 76, 110, 121, 122 self-assembly, 5, 29, 40, 41, 44, 45, 53, 55, 56, 118, 122, 153, 157, 159 self-assembly reactions, 153 self-organization, 5, 41, 55, 122 semiconductive colloids, 222 semiconductor nanocrystal, 3, 231 semiconductor nanoparticles, 23, 157 semiconductors, 19, 29, 38, 55, 119, 121, 122, 141, 160, 262, 264, 265 semimetal particles, 222 sensors, 1, 4, 8, 14, 29, 36, 37, 38, 43, 44, 45, 137, 226, 244 shape-controlled nanocrystals, 194 shell materials, 250, 254 shell precursors, 250, 254 Shinoda phase inversion temperature, 91 Scherrer’s formula, 273 Schlenk line technique, 275 silica nanotubes, 155, 156, 157 silver nanoparticles, 43, 152, 168, 169, 191, 261 silver nanoshells, 260, 261 sintering, 20, 43, 44, 174, 186, 236, 270 smart structures, 15 sodium borohydride, 21, 147, 182, 187 sodium cyanoborohydride, 151, 188, 207

Index sodium dodecyl benzene sulfonate, 168, 196, 208 sodium dodecyl sulfate, 83, 102, 125, 149, 208 sodium triacetoxyborohydride, 151 soft-lithography methods, 5 solely anisotropic growth, 254 solid-phase place exchange reaction, 246, 248 solution phase synthesis, 171 solvated metal atom dispersion technique, 170, 208 sonication, 101, 114, 146, 192, 199 sonochemical radiation approach, 276 sonochemical synthesis, 136, 166 spin coating, 224 SPM, 2, 4, 15, 50, 51, 59 sponge-like microstructures, 87 spray pyrolysis, 3 sputter deposition, 40 standard ring-closing metathesis, 251 star polymers, 104 starved microdroplets, 95 steric hindrance, 145, 206 steric repulsion, 81, 82, 119 STM, 1, 4, 8, 30, 50, 51, 52, 58, 59, 262, 280 Stober process, 277 strained layer superlattices, 251 Stranski-Krastanov growth, 252 sulfonated poly- (ether-ether)ketone, 173 supercritical fluid, 3 superlattice, 4, 20, 54, 55, 141, 143 superparamagnetism, 22, 181 supersaturation, 136, 138, 139, 141, 142, 175, 176, 249 super-strong segregation limit, 77, 125, 160, 208 supramolecular chemistry, 2, 5, 17, 27 supramolecular structures, 120 surface tension, 41, 89, 91, 102 surface-enhanced Raman scattering, 28, 260 surfactant, 70, 77, 87, 88, 89, 100, 103, 110, 114, 120, 121, 128, 136, 145, 146, 149, 150, 152, 155, 157, 158, 159, 163, 167, 177, 184, 190, 193, 203, 224, 227, 253, 266, 267, 268

Index

T

V

templates, 10, 55, 57, 137, 155, 156, 172, 190, 193, 227 tetrachloroaurate, 21 thin films, 6, 18, 34, 117, 121, 153, 159, 193, 227, 246, 251 thiol-modified supports, 224 thiols, 54, 224, 228, 229, 239, 240, 241, 242, 243, 244, 255 transient droplet dimmers, 146 transmission electron microscopy, 53, 59, 183, 208, 280 trialkoxysilane, 223 triblock copolymers in ethylacetate, 77, 160 tributylphosphine, 205, 206, 236, 245, 246, 280 trioctylphosphine, 184, 204, 206, 209, 236, 243, 248, 250, 267, 270, 279, 280 triphenylphosphine, 239, 241, 248, 280 Triton, 83, 112, 126 tubular gold, 57 tunnel effect, 6 two-photon absorption, 30, 59 two-photon polymerization, 28

van der Waals interactions, 248 vesicles, 41, 116, 120, 137 vinyl acetate, 84

U ultrahigh vacuum chemical vapor deposition, 251, 280

W water-cast films, 120 water-in-oil microemulsions, 92, 137, 152 water-soluble polymer, 75, 76 Winsor systems, 87 wormlike micelles, 163

X X-ray lithography, 12 X-ray photoelectron spectroscopy, 183, 209

Y yttrium, 178, 179

Z zeolites, 41, 158

301


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