Functionalized Nanoscale Materials, Devices and Systems (NATO Science for Peace and Security Series B: Physics and Biophysics)

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Functionalized Nanoscale Materials, Devices and Systems

NATO Science for Peace and Security Series This Series presents the results of scientific meetings supported under the NATO Programme: Science for Peace and Security (SPS). The NATO SPS Programme supports meetings in the following Key Priority areas: (1) Defence Against Terrorism; (2) Countering other Threats to Security and (3) NATO, Partner and Mediterranean Dialogue Country Priorities. The types of meeting supported are generally "Advanced Study Institutes" and "Advanced Research Workshops". The NATO SPS Series collects together the results of these meetings. The meetings are coorganized by scientists from NATO countries and scientists from NATO's "Partner" or "Mediterranean Dialogue" countries. The observations and recommendations made at the meetings, as well as the contents of the volumes in the Series, reflect those of participants and contributors only; they should not necessarily be regarded as reflecting NATO views or policy. Advanced Study Institutes (ASI) are high-level tutorial courses intended to convey the latest developments in a subject to an advanced-level audience Advanced Research Workshops (ARW) are expert meetings where an intense but informal exchange of views at the frontiers of a subject aims at identifying directions for future action Following a transformation of the programme in 2006 the Series has been re-named and re-organised. Recent volumes on topics not related to security, which result from meetings supported under the programme earlier, may be found in the NATO Science Series. The Series is published by IOS Press, Amsterdam, and Springer, Dordrecht, in conjunction with the NATO Public Diplomacy Division. Sub-Series A. B. C. D. E.

Chemistry and Biology Physics and Biophysics Environmental Security Information and Communication Security Human and Societal Dynamics

http://www.nato.int/science http://www.springer.com http://www.iospress.nl

Series B: Physics and Biophysics

Springer Springer Springer IOS Press IOS Press

Functionalized Nanoscale Materials, Devices and Systems

edited by

A. Vaseashta Nanomaterials Processing and Characterization Laboratories, Marshall University, Huntington, WV, U.S.A.

I.N. Mihailescu National Institute for Lasers, Plasma and Radiation Physics, “Laser-Surface-Plasma Interactions” Laboratory, Bucharest-Magurele, Romania

Published in cooperation with NATO Public Diplomacy Division

Proceedings of the NATO Advanced Study Institute on Functionalized Nanoscale Materials, Devices and Systems for Chem.-bio Sensors, Photonics, and Energy Generation and Storage Sinaia, Romania 4–15 June 2007

Library of Congress Control Number: 2008931994

ISBN 978-1-4020-8902-2 (PB) ISBN 978-1-4020-8901-5 (HB) ISBN 978-1-4020-8903-9 (e-book)

Published by Springer, P.O. Box 17, 3300 AA Dordrecht, The Netherlands. www.springer.com

Printed on acid-free paper

All Rights Reserved © 2008 Springer Science + Business Media B.V. No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work.

TABLE OF CONTENTS Preface................................................................................................................xi Part I. Invited Contributions Nanoscale Materials, Devices, and Systems for Chem.-Bio Sensors, Photonics, and Energy Generation and Storage A. Vaseashta...................................................................................................3 Nanostructured Thin Optical Sensors for Trace Gas Detection C. Ristoscu, I.N. Mihailescu, D. Caiteanu, C.N. Mihailescu, Th. Mazingue, L. Escoubas, A. Perrone, and H. Du....................................29 X-Ray Photoelectron Spectroscopy and Tribology Studies of Annealed Fullerene-like WS2 Nanoparticles F. Kopnov, R. Tenne, B. Späth, W. Jägermann, H. Cohen, Y. Feldman, A. Zak, A. Moshkovich, and L. Rapoport .................................51 The Development and Application of UV Excimer Lamps in Nanofabrication I.I. Liaw and I.W. Boyd ................................................................................61 Functionalization of Semiconductor Nanoparticles M.-I. Baraton ...............................................................................................77 Flexoelectricity: A Universal Sensoric Mechanism in Biomembranes and in Chem.-Biosensors A.G. Petrov...................................................................................................87 Carbon Nanotubes: From Fundamental Nanoscale Objects Towards Functional Nanocomposites and Applications W. Maser, A.M. Benito, E. Muñoz, and M. Teresa Martínez .....................101 Ultrashort Pulse PLD: A Technique for Nanofilm Fabrication T. Szörényi and Zs. Geretovszky ................................................................121 Laser Ablation and Laser Induced Plasmas for Nanomachining and Material Analysis D. Batani ....................................................................................................145

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Photo-, Dual- and Exoelectron Spectroscopy to Characterize Nanostructures Y. Dekhtyar ................................................................................................169 Laser Interaction with Nano-Spheres: Applications in Sub-Micron Particles Removal and Nanodot Array Fabrication M. Sentis, D. Grojo, Ph. Delaporte, and A. Pereira ..................................185 Clean Fossil Fuels: Advanced Membrane Reactors T. Tran, K. Stoitsas, and J. Schoonman .....................................................199 Nanocrystalline Diamond Films for Advanced Technological Applications C. Popov and W. Kulisch ...........................................................................215 Diamond like Carbon Films: Growth and Characterization S. Tamuleviþius and Š. Meškinis ................................................................225 Fundamentals of Laser-Assisted Fabrication of Inorganic and Organic Films J. Schou ......................................................................................................241 Nanoparticles of Semiconductors in Sol Gel Glasses R. Reisfeld ..................................................................................................257 Electrochemical Sensor Technology Based on Nanomaterials for Biomolecular Recognitions A. Erdem ....................................................................................................273 The Effects of Doping with Elements from the IIA Group on the Thermal and Electronic Properties of Amorphous Selenium G. Belev, D. Tonchev, S.O. Kasap, and H. Mani .......................................279 Nanoscale Materials for Hydrogen and Fuel Cell Systems M. Suha Yazici ...........................................................................................283 Application of Fe-Nanoscale Materials Useful in the Removal of Arsenic from Waters M. Vaclavikova, K. Stefusova, S. Jakabsky, S. Hredzak, and G. Gallios ...291 Nanopatterning Using the Bioforce Nanoenabler K. Arshak, O. Korostynska, and C. Cunniffe .............................................299

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Electrocatalysts and Electrode Design for Bifunctional Oxygen/Air Electrodes V. Nikolova, P. Iliev, K. Petrov, T. Vitanov, E. Zhecheva, R. Stoyanova, I. Valov, and D. Stoychev...........................................................................305 Part II. General Contributions Preparation of Magnetic Chitosan Nanoparticles for Diverse Biomedical Applications D. Kavaz, T. Çirak, E. Öztürk, C. Bayram, and E.B. Denkbaú ..................313 Anomalous Behavior of Carbon Filled Polymer Composites Based Chemical and Biological Sensors K. Arshak, C. Cunniffe, E. Moore, and A. Vaseashta ................................321 Poly(n-isopropylacrylamide) (PNIPAM) Based Nanoparticles for In Vitro Plasmid DNA Delivery N. Ozdemir, A. Tuncel, M. Duman, D. Engin, and E.B. Denkbas..............325 Rapid, Contactless and Non-Destructive Testing of Chemical Composition of Samples O. Ivanov, L. Stoychev, and A. Vaseashta..................................................331 Synthesis and Application of Metal-Containing Silicas K. Katok, V. Tertykh, and V. Yanishpolskii................................................335 Semiconducting Gas Sensors, Remote Sensing Technique and Internet GIS for Air Pollution Monitoring in Residential and Industrial Areas O. Pummakarnchana, V. Phonekeo, and A. Vaseashta..............................339 Self-Assembled System of Semiconductor and Virus like Nanoparticles Yu. Dekhtyar, A. Kachanovska, G. Mezinskis, A. Patmalnieks, P. Pumpens, and R. Renhofa......................................................................347 Thermal Stability and Optical Activity of Erbium Doped Chalcogenide Glasses for Photonics D. Tonchev, K. Koughia, S.O. Kasap, K. Maeda, T. Sakai, J. Ikuta, and Z.G. Ivanova........................................................................................351

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XRD Study of Pulsed Laser Deposited AlN Films with Nanosized Crystallites S. Bakalova, A. Szekeres, A. Cziraki, E. Gyorgy, S. Grigorescu, G. Socol,and I.N. Mihailescu .....................................................................357 Functionalization of Multi-Walled Carbon Nanotubes (MWCNTs) M. Mohl, Z. Kónya, Á. Kukovecz, and I. Kiricsi ........................................365 Sonochemical Synthesis of Inorganic Nanoparticles J. Kis-Csitári, Z. Kónya, and I. Kiricsi ......................................................369 Novel Transparent Molecular Crystals of Carbon G. Kharlamova, N. Kirillova, A. Kharlamov, and A. Skripnichenko .........373 Hydrogen Microsensor Based on Nio Thin Films I. Fasaki, M. Antoniadou, A. Giannoudakos, M. Stamataki, M. Kompitsas, F. Roubani-Kalantzopoulou, I. Hotovy, and V. Rehacek...379 Design and Characterization of Styrene-Based Proton Exchange Membranes D. Ebrasu, I. Petreanu, L. Patularu, I. Stefanescu, and M. Valeanu .........383 Strontium-Substituted Hydroxyapatite Thin Films Grown by Pulsed Laser Deposition C. Capuccini, E. Boanini, A. Bigi, M. Gazzano, F. Sima, E. Axente, and I.N. Mihailescu....................................................................................389 Growing Thin Films of Charge Density Wave System Rb0.3MoO3 by Pulsed Laser Deposition D. Dominko, D. Starešiniü, K. Biljakoviü, K. Salamon, O. Milat, A. Tomeljak, D. Mihailoviü, J. Demšar, G. Socol, C. Ristoscu, I.N. Mihailescu, and J. Marcus..................................................................399 Single Cell Detection with Driven Magnetic Beads B. McNaughton, R.R. Agayan, V.A. Stoica, R. Clarke, and R. Kopelman .403 Antimicrobial Properties of Titanium Nanoparticles B.K. Erdural, A. Yurum, U. Bakir, and G. Karakas...................................409 CsHSO4/Nanooxide Polymer Membranes for Fuel Cell A. Andronie, A. Morozan, C. Nastase, F. Nastase, A. Dumitru, S. Vulpe, I. Stamatin, and A. Vaseashta ....................................................................415

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IV and CV Characteristics of Multifunctional Ilmenite-Hematite 0.67FeTiO3-0.33Fe2O3 C. Lohn, W.J. Geerts, C.B. O’Brien, J. Dou, P. Padmini, R. Schad, and R.K. Pandey, .......................................................................................419 Electrodeposition of BI1-XSBX Nanowires as an Advanced Material for Thermoelectric Applications J.E. Weber, A. Kumar, and W.G. Yelton ....................................................425 A Solid State Nano-Generator: Concept, Design and Theoretical Estimations M. Vopsaroiu, M.G. Cain, V. Kuncser, and J. Blackburn..........................431

Applications of Statistical Physics to Mixing in Microchannels: Entropy and Multifractals M. Kaufman, M. Camesasca, I. Manas-Zloczower, L.A. Dudik, and C. Liu...................................................................................................437 Synthesis and Characterization of Carbon Supported Pd and PtPd Catalysts for DMFCs A. Morozan, A. Dumitru, C. Mirea, I. Stamatin, F. Nastase, A. Andronie, S. Vulpe, C. Nastase, and A. Vaseashta ... .................................................445 Theoretical Study of the Adsorbed Small Molecule on Twisted Nanotubes by Atomic Scale Simulations V. Chihaia, A. Ghita, B.-S. Seong, and S.-H. Suh .. ...................................449 The Defect Structure of Copper Indium Disulfide D. Perniu, A. Duta, and J. Schoonman .....................................................457 ASI Group Photograph....................................................................................465 ASI Group Photograph Legend.......................................................................467 Selected photographs Taken During the ASI..................................................469 List of Participants...........................................................................................475 Author Index....................................................................................................483 Keyword Index................................................................................................487

PREFACE The primary objective of the NATO Advanced Study Institute (ASI) titled “Functionalized Nanoscale Materials, Devices, and Systems for Chem.-Bio Sensors, Photonics, and Energy Generation and Storage” was to present a contemporary and comprehensive overview of the field of nanostructured materials and devices and its applications in chem.-bio sensors, nanophotonics, and energy generation and storage devices. The study has become one of the most promising disciplines in science and technology, as it aims at the fundamental understanding of new physical, chemical, and biological properties of systems and the technological advances arising from their exploration. Such systems are intermediate in size, between the isolated atoms and molecules and bulk material, where the unique transitional characteristics between the two can be understood, controlled, and manipulated. Nanotechnologies refer to the creation and utilization of functional materials, devices, and systems with novel properties and functions that are achieved through the control of matter, atom-by-atom, molecule-by-molecule, or at a micro-molecular level. Advances made over the last few years provide new opportunities for scientific and technological developments in nanostructures and nanosystems with new architectures with improved functionality. The field is very actively and rapidly evolving and covers a wide range of disciplines. Recently, various nanoscale materials, devices, and systems with remarkable properties have been developed, with numerous unique applications in chemical and biological sensors, nanophotonics, nano-biotechnology, and in-vivo analysis of cellular processes at the nanoscale. On a scientifically related note, the potential and risk for inadvertent or deliberate contamination of the environment, food and agricultural products has recently increased due to the global threats of terrorism. As a result, decentralized sensing has emerged as an important issue for several agencies. To detect the contaminants, the current trend is to make laboratory facilities more mobile and conduct clinical trials employing direct reading, portable, labon-chip systems. A nanotechnology-based sensor platform provides platform for direct electrical detection of biological and chemical agents in a label-free, highly multiplexed format over a broad dynamic range during clinical testing. This platform uses functionalized nanotubes, nanoparticles, and nanowires to detect molecular binding with high sensitivity and selectivity. The platform is capable of detecting a broad range of molecules, viz., DNA, RNA, proteins, ions, small molecules, cells, and even pH values. Detection is possible in both liquid and gas phase and is highly multiplexable, enabling the parallel detection of multiple agents. Recent progress in nanostructured materials and its potential applications in chemical and biological sensors are likely to have a significant impact on efficient data collection, processing, and recognition with minimum false positive xi

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counts. Furthermore, nucleic acid layers combined with nanomaterials-based electrochemical or optical transducers produce a new kind of affinity biosensors such as the “DNA Biosensor” or “Genosensor” for small molecular weight molecules. Genosensors are attractive devices for converting the hybridization event into an analytical signal for obtaining sequence-specific information in connection with clinical, environmental, or forensic investigations. The continued development through combined efforts in microelectronics, surface/interface chemistry, molecular biology, and analytical chemistry is expected to lead to the establishment of Genosensor technology as a major component of analytical biochemistry. The design and fabrication of DNA-modified surfaces and materials which are reproducible, stable, and selective to complementary DNA sequences are crucial in the development of emerging analytical tools such as DNA chips or simple diagnostic devices for detecting DNA sequences. These devices have been used extensively not only for the rapid, cost-effective, simple diagnosis of inherited and infectious diseases, but also for the early detection of infectious agents in various environments. In general, the scientific community is confident that nanoscience and nanotechnology will revolutionize research on, and applications in the areas of biology, medicine, and human health. The research will also provide unprecedented means to forewarn and/or protect against the potential and risk for inadvertent or deliberate contamination of the environment and food and agricultural products. Many technologists develop new tools, yet they often have limited understanding of the restrictions that biology places on the proper design of nanotools and nanosystems. Hence, we sought to adopt with this ASI an interdisciplinary approach, bringing together recognized experts in various fields while retaining a level of treatment accessible to those active in specific individual areas of research and development. The NATO ASI titled “Functionalized Nanoscale Materials, Devices, and Systems for Chem.-Bio Sensors, Photonics, and Energy Generation and Storage” was very well planned, organized, and received by the participants. Both directors spent an enormous amount of time in carefully planning the logistics of the event from both international and local organization perspectives. To maximize global participation, the organizing committee focused on a promotional strategy that garnered a tremendous response not only from NATO and partner countries but also from the Asian-Pacific rim. Such an unexpected and substantial response presented a welcome but arduous task for the organizing committee charged with selecting the most deserving participants. Realizing that a productive ASI would result from a large number of participants from multiple scientific disciplines, the organizers accepted a wide range of promising candidates who could be funded through NATO.

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Lectures covering the basic principles and state-of-the-art applications of nanostructured and advanced materials for sensor, optoelectronic, and photovoltaic devices were conducted by 13 experts recognized for advances in nanotechnology. Focused seminar sessions, poster sessions, and interactive feedback sessions stimulated extended interactions between participants and subject matter experts. As a venue for collaborative learning, the interactive lectures and sessions drew enthusiastic response and sharing of information and ideas from all participants. The ASI was held at the Hotel Sinaia, in the eponymous resort town of Romania, which offered a tranquil, congenial setting for the participants. The facility supported formal and informal settings for structured and spontaneous learning and sharing of ideas. The meeting lasted 10 days, with a day free to visit nearby towns, shop, or simply relax. The downtown shops and pubs provided a much-needed break after lectures and in-depth discussions at the ASI. The participants also shared a sightseeing excursion to Bran, Brasov, and Poiana Brasov and a traditional Romanian dinner complete with folk dancing. The unique balance of technical and social interactions materialized in alliances between participants, which have been evidenced by continued correspondence in the months following the ASI. The co-directors interpret the ongoing interaction and positive feedback from participants as an affirmation of a successful ASI. Such a constructive ASI is the outcome of efforts by participants, lecturers, presenters, and co-directors in addition to a host of caring individuals who supported their work. Much appreciation is extended to Mr. Marian Swartz, the Manager of the Hotel Sinaia. We would like to acknowledge editorial assistance from Adina Morozan, Rodica Cristescu, and Silvia Bakalova. Our local organizers, Adina Morozan, Adrian Ghita, Rodica Cristescu, Carmen Ristoscu, Felix Sima, and Ioana Vasiliu handled our everyday logistics with the utmost consideration and efficiency. Our gratitude goes to Dr. F. Pedrazzini, the director of the NATO Scientific Affairs Division for his encouragement, expertise, and financial support. Ms. Annelies Kersbergen with the NATO Publishing Unit of the Springer Academic Publishers has provided us with much appreciated expertise in publishing our proceedings. Similarly, successful proceedings are a result of meticulous preparation of manuscripts by participants to whom thanks are extended. Several funding agencies, such as NSF and TUBITAK, provided travel support for some of the participants and are acknowledged immensely for their generous help. The organizers also acknowledge support from the Romanian Ministry of Education and Research, which provided the conference materials. Thanks are due to the Vaseashta Foundation for poster awards. The co-directors hopefully anticipate that this ASI provides continued success for all participants as they extend collaboration in the pursuit of nanotechnology advancement.

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We would like to close our preface with the following two quotes. “Over the next ten years, the fields of chemistry, physics, material sciences, biology, and computational sciences will converge in a way that will define nanotechnology and impact almost every industry, including computers, semiconductors, pharmaceuticals, defense, health care, communications, transportation, energy, environmental sciences, entertainment, chemicals, and manufacturing. Previously distinct disciplines will also combine: medicine and engineering, law and science, art and physics, etc. This merging will result in developments that are not simply evolutionary; they will be revolutionary.” ............Jack Uldrich and Deb Newberry “The revolutionary promise of molecular nanotechnology (MNT) has become a part of society’s expectations for the future. This technology will provide nanomedicine breakthroughs that could cure cancer and extend lifespace, bring abundance without environmental harm and provide clean sources of energy. These ideas are part of the vision that launched the field of nanotechnology.” ............K. Eric Drexler

Directors Ashok Vaseashta (Washington, DC) Ion N. Mihailescu (Bucharest) April 2008 Organizing Committee Arzum Erdem Ion N. Mihailescu Joop Schoonman Ioan Stamatin Sigitas Tamelevicius Ashok Vaseashta

NANOSCALE MATERIALS, DEVICES, AND SYSTEMS FOR CHEM.-BIO SENSORS, PHOTONICS, AND ENERGY GENERATION AND STORAGE A. VASEASHTA* On detail in Washington DC, USA from Nanomaterials Processing & Characterization Labs, Graduate Program in Physical Sciences, Marshall University, Huntington WV, USA

Abstract – A comprehensive overview of ongoing research efforts and future scientific directions in nanotechnology to develop materials, devices, and systems for potential use in environmental pollution monitoring and mitigation; energy generation and storage; and chemical-biological-radiological-nuclear sensing is presented. Applications of nanomaterials in development of biodegradable, high performance yet light weight and eco-friendly materials are presented to minimize power consumption, green-house gas emissions, and land-fill volume. Societal implications and concerns associated with nanotechnology are addressed by studying fate and transport and development of guidelines for a risk-assessment model. A roadmap of the future of nanomaterials, in-terms of complexity, nexus of disciplines, and emerging green nanotechnologies is presented.

Keywords: Chem.-bio sensors, pollution, satellite, water, energy, storage.

1. Introduction Three most imminent challenges of the 21st century include abundant clean energy, a pollution free environment, and international security and safety. It is imperative that the interdisciplinary scientific community exploit knowledgedriven transformations across scientific fields to develop materials, devices, and systems to address the aforementioned challenges. The scientific study

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To whom correspondence should be addressed: Professor Dr. Ir. A. Vaseashta, email: [email protected] A. Vaseashta and I.N. Mihailescu (eds.), Functionalized Nanoscale Materials, Devices and Systems. © Springer Science + Business Media B.V. 2008

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of nanoscale materials and systems is a promising field. Fundamental understanding and technological advances arise from the potential of nanoscale materials to exhibit unique properties that are attributable to their small size such as surface structure, physical characteristics, and chemical composition.1 These properties drive research that serves as a catalyst for new scientific and technological innovations. Furthermore, the geometrical dimensions span those comparable to the smallest engineered entity, the largest molecules of the living systems, and fundamental physical quantities render their study quite captivating. Materials approaching nanoscale dimensions exhibit atypical characteristics with numerous unique and hitherto unexploited applications. Advances in material synthesis, device fabrication and characterization techniques have provided the means to study, understand, control, or even manipulate the transitional characteristics between isolated atoms and molecules, and bulk materials. The unique characteristics and functionalities of nanomaterials have already been utilized in cosmetic, apparel, and sports industries, while proof-of concept electronic and optical devices have been demonstrated and are largely in the developmental stages. Recently, various nanoscale materials with new architectures and improved functionality have been developed with applications in chemical and biological sensors,2 environmental pollution sensing,3 monitoring,3,4 mitigation and remediation,3 next-generation energy generation and storage devices, nanobiotechnology, nanophotonics, in-vivo analysis of cellular processes and futuristic platforms in health and clinical medicine.5 The potential and risk for inadvertent or deliberate contamination of the environment, food and agricultural products, due to global threats of terrorism make decentralized chemical and biological sensing an important research area for academic institutions and governmental agencies. A nanotechnology based sensor platform enables direct electrical detection of biological and chemical agents in a label-free, highly multiplexed format over a broad dynamic range. Nucleic acid layers combined with nanomaterials-based electrochemical or optical transducers produce affinity biosensors such as the “DNA Biosensor” or “Genosensor” that are attractive devices for converting the hybridization event into an analytical signal for obtaining sequence-specific information in connection with clinical, environmental, or forensic investigations.6,7 A perpetual increase in population and consumption of fossil fuels to meet the current energy demand has led to increased pollution worldwide. Pollution in large cities has reached an alarming level and is widely perceived to be a leading contributor to chronic and deadly health disorders and diseases affecting millions of people each year. In a recent study, the World Health Organization (WHO) reported that over 3 million people suffer from the effects of air-borne pollution. Furthermore, reports from the World Energy Congress

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(WEC) suggest that continued fuel consumption at its current rate will result in pollution creating irreversible environmental damage by 2025. Clinical studies show that inhaling particulate matter (PM) is associated with increased mortality rates that are further magnified for people suffering from diabetes, chronic pulmonary and inflammatory diseases. Of general pollutants that contaminate the urban environment, fine suspended PM, Nitrous Oxide (NOx), sulphur dioxide (SO2), volatile organic compounds (VOCs), and ozone (O3) pose the most widespread and acute risks; thus driving studies and measures, such as cap-and-trade, carbon credit, pollution credit etc., to limit emission of greenhouse gases (GHG). Figure 1 illustrates a pollutions footprint of various sources of energy and a proposed “future distributed-source energy solution”. Accordingly, preliminary results of joint investigations to monitor and mitigate envienvironmental pollution are presented. In addition to air pollution, results from joint investigations into the efficacy of nanostructured materials in the detection and remediation of water pollution are presented.

Figure 1. Sources of energy, pollution level, and distributed model of energy.8

Resistance to new technology and the fact that advances in nanotechnology have progressed faster than development of standards, “voluntary code of conduct” by industry, and implementation of regulations by policymakers, has led to the studies of societal implications and concerns associated with the production and use of nanomaterials. Studies of fate and transport of nanomaterials in air, water, and soil, plume modeling and in-silico risk-assessment models using expert elicitation and statistics based decision analysis are under active investigation and are briefly presented. Roadmap of time vs. complexity,

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nexus of disciplines, and emerging green nanotechnologies is conceptualized to attain a technological singularity in future. 2. Size Effects in Reduced Dimensions Reduced dimensional systems, in which one or more dimensions are reduced such that material begins to display novel quantifiable quantities hold tremendous potential. For solids, typically reduced dimensions amount to reduction of the coordination number; hence the electrons have less opportunity to hop from site to site; thus reducing kinetic energy of electrons or their bandwidth. A higher Coulomb interaction/bandwidth ratio at a site enhances electron correlation and Mott-transition i.e. the tendencies towards the appearance of magnetism. Furthermore, the symmetries of the system are lowered and the appearance of new boundary conditions lead to surface states and interface states. A change of the quantization conditions alters the eigen value spectrum and transport properties of the solid. A high surface area/volume ratio in nanoscale materials alters mechanical and other physical properties. One critical impact is that surface stresses existing in nanomaterials have a different bonding configuration as compared to bulk atoms. As an example, surface elasticity is an effect that occurs due to the lack of bonding neighbors for surface atoms. The effects of the difference between surface and bulk elastic properties become magnified as the surface area/volume ratio increases with decreasing structural dimension. Studies to calculate surface elastic constants using MD simulations, curvature effect using the Cauchy-Born rule, and electronic effects via effective nuclei-nuclei interaction using DFT calculations provide better understanding of surface and interface effects in reduced dimensions. Despite many investigations on surface elastic effects, many questions remain unanswered. Recently, sensing by nanoscale materials has been utilized for chemicalbiological investigations; primarily because of increased surface area, reactivity, and ability of selective functionalization. For instance, nanoscale resonators9 have allowed various research groups to detect mass of molecules by detecting change in resonance frequency with mass.10 Surface effect may play a role in resonant frequency shift if the thickness of the biomolecular layer becomes comparable to the resonator’s thickness; although, the surface stress effect may affect the resonant frequency shift. We have employed the surface plasmon resonance (SPR) on the optical response2,11 as a function of biological molecule interactions resulting from adsorbate-substrate bonding. As the size of nanomaterials approach fundamental physical quantities and biological molecules (shown in Figure 2(l)), many questions remain unanswered to put the broad topic of emergence of size-effect in different materials into perspective. As an


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Figure 2. (l): Dimensional compatibility, (r): roadmap of nanomaterials.

example, when does the surface effect become important? Is there a critical dimension for materials? Do some processes take place only in nanoscale dimensions and do other processes occur even in micro-scale dimensions? What do we know about the interactions of nanomaterials with biological molecules vis-à-vis “spes altera-vitae”? 2.1. NANOMATERIALS-ENVIRONMENT INTERFACE

Size and surface collectively control characteristics of nanoscale materials due to the existence of large boundaries adjoining its surrounding medium and interplay of physicochemical interactions. The surface free-energy is sizedependent and hence increases almost inversely with the decreasing feature sizes of the material. Collective response of a nanomaterial-medium system that is attributable to reduced dimensions, viz. size (area and distribution), surface structure (groups, functionalization), physical (electronic, optical, photoactivation, luminescence), and chemical (crystallinity, purity, solubility) is vital to developing a scientific model that predicts its response as a sensor, bioadverse response pathways for toxicity, adsorption pathways for materials for storage, and interaction with light for optical response. It is an exceedingly complex task due to a large matrix of parameters consisting of nanomaterials (metals, metal oxides, semiconductors, macromolecules and self-assembled), the surrounding environment, and influencing mechanisms of interactions.

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Further complexities arise in nanomaterials production due to variations in, e.g. fabrication with precisely-controlled surface properties, uniform dispersion of nanoparticles in aqueous or organic mediums, linkage of nanoparticles to a polymeric material, and reproducible surface modification of nanoparticles. Hence, as the production and diversity of nanoparticles increases, it will become increasingly important to understand how engineered nanoparticles and biological systems interact in terms of bio-physico-chemical properties of engineered nanoparticles (Figure 2(r)). An extensive investigation is underway in the context of specific functions ranging from bio-nanomaterials interface to toxic potential of industrial pollution. We conducted study of functionalized SAM as biosensors that use biological molecules – usually enzymes, antibodies, or nucleic acids, to recognize simple molecules of interest via hydrogen bonding, charge-charge interactions, and other biochemical interactions to provide molecular information.7 Recent work on affinity biosensors to deliver real-time information about the antibodies to antigens, cell receptors to their glands, and DNA and RNA to nucleic acid with a complimentary sequence,12 provided a multitude of applications. As an example, they can be used to measure blood glucose levels, to detect pollutants and pesticides in the environment, to monitor food-borne pathogens in the food supply, to work as chemical and biological warfare agents, and detect the presence of micro-organisms in foods. A response of a nanomaterials based gas sensor is based on reactions replacement of atoms at the sensing surface of these materials which relies on a change of the resistance of the oxide. Depending on the free electron density in the space charge layer, the depletion region is increased. Since electric properties are influenced by the depletion layer, variation in electrical conductivity indicates sensor response. Similarly, moisture can influence the resistance or conductivity of oxide materials via two pathways: first, the adsorption of monolayer/s of water molecules at the surface; and second, the process of formation of a parallel resistance path by capillary condensation of water via adsorption of the water molecules as protons and hydroxyl groups within pores. The sensitivity and response of nanomaterials of metal-oxide sensors is highly dependent on the roughness of the substrate, which is caused by the increasing surface area and porosity of the film surface modifications in the film surface morphology. The sensitivity (and selectivity) of a can be improved by parameters such as decreasing the crystallite size, the valence control, and using noble metal catalysts. The characteristics which provide beneficial aspects are also believed to be responsible for toxicity of nanomaterials. Consequently, nanoparticle toxicity is studied in context of its ability to induce tissue damage through the generation of oxygen radicals, electron-hole pairs, and oxidant stress by abiotic and cellular


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responses resulting in pro-inflammatory, mitochondrial injury and pro-apoptotic cellular effects in the lung, cardiovascular system and brain.13 It is further believed that nanoparticles absorb cellular proteins which could induce protein unfolding, fibrillation, and thiol cross-linking; leading to neuro-toxicity and reduced enzymatic activity. Nanoparticles which are cationic are also believed to induce toxicity via acidifying endosomes that lead to cellular toxicity and apoptosis in epithelial lung through endosomal rupture through proton sponge mechanism (PSM), mitochondrial targeting, and cytosolic deposition. Nanomaterials composed of redox-active elements are particularly reactive and can possibly provoke potentially damaging chemical transformations. Furthermore, even chemically benign nanoparticles may become activate by light absorption. Hence, fundamental understanding of a nanomaterial-surrounding medium is vital to sustaining technological advances of nanoscale materials as catalyst for new scientific and technological avenues. 2.2. NANOPHOTONICS AND SURFACE PLASMON RESONANCE

Two theoretical models, viz.: the classical electrodynamics for the propagation of light and the solid (or liquid) state theory for the interaction of light with the particle expressed by the complex, frequency dependent dielectric function, DF, of the particle material, which can mathematically be described as,14 according to Mie’s theory: İ(Ȧ) = İ1 + i İ2 = İDrude + Ȥ interband = 1 – (n e2/ İ0 me )/(Ȧ2 + i Ȗ Ȧ) + Ȥ interband

(1)

with n, e, İ0, me, Ȗ are respectively the electron density, elementary charge, field constant, effective mass and relaxation frequency of the conduction electrons of the Drude-Lorentz-Sommerfeld theory, Ȗ = 1/Ĳ with Ĳ the Drude relaxation time and (ne2/İ0me )1/2 = Ȧp the “Drude plasma frequency”. The interband transitions exist in most metals thus causing nanomaterials to demonstrate hybrid surface plasmon polariton (SPP) excitations. They consist of collective electronic excitations in the conduction band and (complex) polarization term for deep bands. Most nanomaterials exhibit optical absorption and scattering spectra with complex multi-peak features, characterized by quantities viz., peak height, spectral peak position ȦMax and band width ī. The dielectric function (DF) includes electronic size and surface/interface effects present in a nanoparticle. Although a detailed description is described elsewhere, it suffices to state here that Mie’s theory is used analogously to Fresnels formulae which was derived for samples with planar geometry. Extension of the theory yield effective

10

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permeability and refractive index to produce negative refractive index materials (NRIM) by plasmonic materials to image sub-O features for producing near field super lens.15 The inverted Mie theory yields the realistic DF including all electronic and optical size and surface/interface effects and resulting evanescent field strongly interacts with adsorbed molecules, thus influencing the resulting spectra. The application of inverse Mie theory is essential for structural, electronic and optical effects for selective detection substances in aqueous/biosurroundings using noble metals as plasmonic nanostructures. Integrating plasmonic elements with existing silicon based technology will likely result in plasmonic nanophotonic arrays with multifunctional capability. 3. Chem.-Bio Sensors and Biomedical Platforms International security threats such as escalation in terrorist activities and asymmetric warfare by adversaries drive needs for novel materials and innovative platforms to detect, interdict, and counter threats due to chem.-biologicalnuclear-radiological-improvised explosive (CBRNE) agents, synthetic DNA, contact-poison, and improvised explosive devices (IEDs) in a time-efficient and reliable manner at the site of event. Additional potential risks include inadvertent or deliberate contamination of the environment, food and agricultural products, or even naturally occurring threats such as avian bird flu. In a possible weapons of mass destruction and terrorism (WMDT) scenario, rapid identification of CBRNE events will allow first responders and emergency personnel to implement critical decisions concerning barricading, evacuating, or efficient decontamination, saving hundreds of lives and preventing responders from becoming victims themselves. Conventional detection methods require time-consuming steps to arrive at meaningful data. As an example, methods such as enzyme linked immuno-sorbent assay (ELISA) and polymerase chain reactions (PCR) have been employed for pathogen detection. ELISA and PCR methods require enrichment, isolation, morphological examination, biochemical, and serological testing to positively identify pathogens. In clinical medicine, decentralized laboratory facilities allow mobile facilities for clinical analysis through direct-reading, portable, lab-on-chip (LOC) systems with wireless communications capabilities to centralized servers, command and control units. Recent progress in nanostructured materials based sensor platforms will significantly impact data collection, processing, and recognition to enable the direct detection of biological and chemical agents in a label-free, parallel, multiplexed, broadly dynamic range. This platform utilizes functionalized nanotubes, nanowires, and nanoparticles to detect a broad range of molecules including DNA, RNA, proteins, ions, cells, pH values, and molecular binding with high sensitivity and selectivity.


11

Biosensors are intrinsically simple and inexpensive systems that use molecules – usually enzymes, antibodies, or nucleic acids – to recognize simple molecules of interest via hydrogen bonding, charge-charge interactions and other biochemical interactions to provide pertinent molecular information (Figure 3(l)). Progress in nanomaterials and advances in fabrication processes provide opportunity to modify, embed in host matrix, or even customize the nanoparticles for use as highly sensitive and selective sensing materials. Nanomaterials based sensors are used in several configurations such as SPR, electrochemical, optical, electrical transduction, and as shock-wave generators for applications ranging from homeland security to studying the environment pollution. Nanostructures will improve our capability to detect CBRNE events with sensitivity and selectivity by several orders of magnitude, protect through filtration, adsorption, mitigation or neutralization of agents, and provide site-specific in-vivo prophylaxis. Joint investigative efforts are focused towards chemical and biological agents, and radiation detection due to a radiological-dispersal device (RDD) or “dirty bomb”.

Figure 3. (l): Electrochemical sensor basics, (r): SPR based sensor.

3.1. NANOMATERIALS BASED CHEMICAL-BIOLOGICAL SENSORS

CNTs are conducting, can act as electrodes, generate electro-chemiluminescence (ECL) in aqueous solutions, and can be derivatized with a functional group that allows immobilization of biomolecules. CNTs have high surface/volume ratios for adsorption, and have surface/weight ratios ~300 m2/g. The uniform chemical functionalization of CNTs is key to the formation of biosensors. Oxidation of nanotubes with HNO3-H2SO4 leads to high concentrations of carboxylic, carbonyl, and hydroxyl groups on the surface, and removal of the tip to expose the tube interior. Carboxyl groups can readily be derivatized by a variety of reactions allowing linking of biomolecules such as proteins, enzymes, DNA, or even metal nanoparticles. The covalent modification of nanotubes facilitates the creation of well-defined probes, which are sensitive to specific intermolecular interactions of many chemical and biological systems. Integration of the

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transducer transducer and probe enables quick, accurate, and reversible measurement of target analytes without the use of reagents. Using sequence-specific attachment, NT-based electronic devices with specific molecular-recognition features of DNA have been reported.16 Covalent modification of single wall CNTs (SWNTs) offers mapping of functional groups at a molecular resolution. Furthermore, chemical processes to link catalysts, such as transition-metal complexes, to the ends of CNTs are useful in creating or modifying the structures at a molecular scale, creating interconnections for electronic devices, and even developing new classes of materials. Covalent functionalization of the sidewalls of SWNTs provides stability and best accessibility; but at the expense of damaging the sidewalls; thereby diminishing the mechanical and electronic properties. However, noncovalent routes to CNTs functionalization offer ease of synthesis and minimum disruption of the tubular structure. During interaction with the polymer coatings, the electrical properties of the nanotubes are altered, enabling detection of the molecules leading to a very sensitive sensing mechanism. In addition to nanotubes, novel materials such as porous silicon17 and porous carbon,18 with porosities of comparable dimensions to those of the biomolecules have been used for biosensor applications. The mesoporous carbon matrix is used for stable immobilization of the biological molecule, and C60 serves as an electron mediator. Both C60 and NTs are good electron mediators when used with a mesoporous carbon matrix or modified metal electrodes. CNT-based transducers, however, show a significant advantage over porous silicon due to the well defined, defect free structures, and also because the NTs promote homogenous electron transfer reactions. Efforts to sort batches of CNTs by length using high-speed centrifuges and functionalization to develop sensors for the food and agriculture industry, genetic analysis, proteomics, drug screening, clinical diagnostics and bio-warfare agent detection are underway. Furthermore, investigations using SAM based SPR and Atomic Force Microscopy (AFM) techniques are in progress to detect several pathogens. The SPR detection technique is rapid, real-time, and requires no labeling, and involves immobilizing antibodies by a coupling matrix on the surface of a thin film of precious metal, such as nanoparticles of gold deposited on the reflecting surface of an optically transparent wave-guide. The precise angle at which SPR occurs depends on several factors (Figure 3(r)). A main response is the refractive index of the metal film, to which target molecules are immobilized using specific capture molecules or receptors along the surface, that cause a change in SPR angle. This can be monitored in real-time by detecting changes in the intensity of the reflected light, producing a sensorgram. The rates of change of the SPR signal can be analyzed to yield apparent rate constants for the association and dissociation phases of the reaction. When the antigens


13

interact with antibodies, the refractive index of the medium surrounding the sensor changes producing a shift in the angle of resonance proportional to the change in the concentration of antigens bound to the surface. Various other sensor platforms, viz.: (a) chemo-mechanical micro-cantilever array to provide a quantitative and label-free platform for high-throughput multiplexed biomolecular analysis for detection of various biomolecules based on binding, (b) microarrays chips fabricated using technique matrix assisted pulsed laser evaporation (MAPLE) and laser induced forward transfer (LIFT) for deposition of biopolymers and a variety of biomolecules to detect dangerous gases, aerosols and micro-organisms, and (c) aligned-CNT probes for biomolecular recognition based on charge transport at the CNT transducer with the accuracy down to molecular level for quantitative and selective detection of a range of metabolites including cholesterol, ascorbic acid and uric acid, in buffer solution as well as in human plasma and blood are under investigation. Several other concepts being considered are resonance based9 shock-wave generators that only could detonate military explosives but will also detect biological and chemical weapons. The shock wave can be used for targeted delivery of drugs, chemotherapy, and cure cells without affecting the whole body. Likewise, nanothermite – a composite of fuel and an oxidizer, which in turn generate combustion waves that can hit velocities ranging from mach 4–7, can safely be used for killing cancer cells. Using electrospinning, we have investigated nanofibers prepared from high performance polymer composites embedded with metal oxides, glasses coated with rare earth metals, and biocompatible compounds for sensing, and even controlling electrical, optical, and chemical and biological response (Figure 4 – both panes). Nucleic acids offer analytical chemists a powerful tool for recognition and monitoring of many important compounds. Recent advances in molecular

Figure 4. Use of electrospinning for chem.-bio sensors, health and medicine.11

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biology are used to study the effects of proteins and drugs on gene expression, viz. gel mobility shift, filter binding, DNA foot-printing and fluorescence-based assays. Most of these methods; however, are indirect and require various labeling strategies. Electrochemical DNA biosensors play an important role for clinical, pharmaceutical, environmental and forensic applications, because they provide rapid, simple and low-cost point-of-care detection of specific nucleic acid sequences. In recent years, there has been a growing interest towards design of electrochemical DNA biosensors that exploit interactions between surface-confined DNA and target drugs/biological molecules for rapid screening.16,19 Binding of small molecules to DNA primarily occurs in three modes: electrostatic interactions with the negative-charged nucleic sugar-phosphate structure, binding interactions with two grooves of DNA double helix, and intercalation between the stacked base pairs of native DNA. Most electrochemical sensors use different chemistries; and employ interactions between the target, the recognition layer and an electrode. We have followed numerous approaches to electrochemical detection including direct electrochemistry of DNA and devices based on DNA-mediated charge transport chemistry. In direct electrochemical DNA sensors, the analysis is based on a guanine signal where a basepairing interaction recruits a target molecule to the sensor, allowing monitoring of drug/biological molecule-DNA interactions, which are related to the differences in the electrochemical signals of DNA binding molecules for DNA barcoding. It is vital to develop sensing strategies to maintain critical dynamics of target capture to generate a sufficient recognition signal. Standard electrochemical techniques, such as differential pulse voltammetry (DPV), potentiometric stripping analysis (PSA), square-wave voltammetry (SWV), etc. are used as genosensors. Since genosensors are compatible with existing micro and nanofabrication technologies, they enable design of low-cost, devices that offer potential for detection and diagnosis of inherited diseases and clinical potential for detecting pathogenic bacteria, tumors, genetic disease, and forensics via credit card-sized sensor arrays. In the context of security, prevention against agroterrorism is by detecting bovine spongiform encephalopathy (BSE). A possible link between BSE and a disease of humans, a variant form of Creutzfeldt-Jakob Disease (vCJD) using nano-technology based platforms is important to provide clues about the disease. 3.2. USE OF NANOMATERIALS IN HEALTH AND MEDICINE

Sensing strategies can be modified towards applications of nanomaterials in targeted drug delivery, diagnostic, and therapeutic actions,5,20 thus creating pathways for use in health and medicine. By incorporating a drug into bio-degradable polymers, a simple and convenient drug delivery system allows time-controlled


15

drug regimen release. We have studied nanofibers for promising medical applications including treatment of primary pulmonary hypertension (PPH), and pulmonary arterial hypertension (PAH) by time-controlled release templates impregnated with anticoagulants and calcium channel blockers, a bioscaffold that mimics extracellular matrix (ECM) topology, polyesters com-bined with phosphatidyl choline for biomimetic applications, intravascular stents from a blend of polyactide and trimethylene carbonate, cellulose based scaffold for cartilage tissue engineering, and esophageal tissue engineering. We studied dystrophin gene immobilized on nanostructured templates as a drug carrier that initiates regeneration and boosted satellite cells mediated repair mechanisms for affected muscles for treating Duchene muscular dystrophy (DMD).21 Likewise, a nanoparticles–cinobufagin-bovine serum albumin based drug delivery mechanism holds promise in treating the hepatocellular carcinoma (HCC) malignancy. Nanotechnology based platforms offer novel opportunities to sense clinical biomarkers by imaging for therapeutic intervention. The use of iron oxide nanoparticles in an MRI system provides 3D image and normal and cancerous hepatocyte cells information.22 Furthermore, nanosized constructs such as dendrimers, liposoms, nanoshells, nanotubes, nanoemulsions, quantum dots (QDs), and even viruses offer use as imaging agents intended as non-invasive probes or targeted disease biomarkers. Gold nanoparticles are used to identify the pathogenic bacteria in a DNA microarray technique,23 and QDs are used to detect the human Y chromosome,24 and for locating cancer markers in cellular imaging.25 One to ten nanometer sized QDs with unique photochemical, photophysical, and physiological properties are proposed for drug delivery or contrast agents in MRI. For medical diagnostics, QDs of varying diameter are embedded into polymeric microbeads to achieve biological assays by multi-color optical coding. Tissue engineering is a relatively new field that seeks to regenerate human tissues through the use of some combination of cells, bioactive molecules such as drugs and mechano growth factors (MGF), and a biomaterial support system or scaffold.26 Hence, biomimetically driven studies are exploring how the topography of a surface can be used to control cell behavior.27 By changing the biomaterial surfaces at nano level, we can observe different types of cell behavior, change in cell adhesion properties – especially for fat-free mass, cell orientation and motility, cytoskeletal condensation, and elastomers for vascular tissue engineering28 to monitor advanced chronic pulmonary diseases (COPD). Recent applications of nanotechnology in dental care (nanodentistry) will permit maintaining near perfect oral health via nanomaterials, biotechnology, and nanorobotics.29 In fact, nanofillers are common nano-dentistry. Additional procedures include renaturalization, permanent hyper-sensitivity cures, and orthodontic realignments. Nanomaterials provide higher mechanical strength, enhanced bioactivity, and restorability for debilitating bone fractures. Nanomaterials-based

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orthopedic applications include nanopar-ticles, nanofibres, nanoscaffolds, nanotubes and nano-composites as orthopedic implants. Developing intelligent biomaterials that closely mimic the molecular composition, mechanical responsiveness, and nanoscale organizations of the natural extracellular matrices (ECM) is vital. Recent reports indicate that adding gold nanoparticles to a modified version of an HIV combating drug, TAK-779 creates a compound that prevents the virus from gaining a cellular foothold. These novel and smart biomaterials, combined with defined biophysical cues and biological factors are essential for functional tissue regeneration. 4. Energy Generation and Storage Driven by population growth and the wave of industrialization in developing countries, energy consumption worldwide is increasing relentlessly. Moving forward, an alternate means to generate and store energy is imperative to ease environmental resource constraints, and to push the economy and society to sustainable development. Reports from the European Union’s (EU) recent research framework program (FP) and the United States’ National Nanotechnology Initiative (NNI) have listed several strategies that identify nanotechnology’s role in power generation and storage. Figure 1 further suggests that future power sources will be distributed allowing reduced dependence on fossil fuels leading to pollution reductions and GHG; that are consistent with Jeremy Rifkin’s model of peer-to-peer, shared, decentralized, and distributed generation using hydrogen energy. Significant nanotechnology contributions will influence research in fuel cells, hydrogen generation and storage, improvement in photovoltaic conversion efficiency, super-strong light-weight materials to reduce power consumption, electrical and optical devices requiring less power yet providing higher lumens, and many others. 4.1. HYDROGEN GENRATION AND STORAGE

Hydrogen produces more energy/volume than any other known sources of fuels. The future of a “hydrogen economy” depends on developing clean, safe and efficient methods for producing and storing hydrogen. Hydrogen can be produced by several methods, such as thermolysis, electrolysis, photocatalysis, hybrid processes, and gasification and CO2 sequestration. For sustainable energy, fuel cells (FC) are an important enabling technology to produce clean and renewable hydrogen energy. There are several different types of fuel cells, e.g. alkali, molten carbonate (MCFC), phosphoric acid (PAFC), proton exchange membrane (PEMFC), and solid oxide fuel (SOFC). We focus on PEM based fuel cells. Technically, water splitting can be achieved with coupled solar cell,


17

i.e. photo-electrolysis systems. A photo-electrochemical (PEC) cell comprises a semi-conducting photo-anode, an aqueous electrolyte, and an inert counterelectrode. The main advantage of the PEC cell is that the evolving hydrogen and oxygen gasses can be collected in separate volumes. The PEC cells are elegant, however are limited in their applicability due to several practical considerations such as hydrogen and oxygen forming in the same volume; limited lifetime of photo catalysts after which the efficiency drops below acceptable levels; and slow charge transfer across the semiconductor/electrolyte interface. Most research efforts on solar water splitting are focussed on systems with photo-catalyst powder suspended in an aqueous solution. In both photoelectrodes and powder-based photo-catalysts, surface reaction kinetics plays a key role. The PECCS cells combine the advantages of a two-electrode system with simultaneous storage of hydrogen in a metal hydride (MH). Once the MH electrode is fully loaded with hydrogen, it can be withdrawn from the cell and placed in a fuel cell system. Upon heating the electrode, the hydrogen is released and can be used to generate power. A Nernst-type sensor measures the concentration of hydrogen formed in the MH. Using a second MH reference electrode with a fixed hydrogen activity, open-circuit voltage across both MHs, EOC is given by the Nernst equation:30

EOC

RTt H F

§ a ( H ) sample · ln ¨ ref ¸ © a( H ) ¹

(2)

R is the gas constant (8.314 JK–1), T is the temperature (in Kelvin), F is Faraday’s constant (96,485 Cmol–1), tH+ is the ionic transport coefficient for protons, and a(H+)sample and a(H+)ref may be substituted with the hydrogen activities in the MH counter- and reference-electrodes, respectively. The current state-of-the-art membranes are fabricated from perfluorinated sulfonic acid (PFSA) polymers, such as Nafion® and Hydrogen uranyl phosphate tetrahydrate, HUO2PO44H2O (HUP) which can be easily synthesized and show high proton conductivity at room temperature.31 These materials have useful, high proton conductivities when fully hydrated limiting their usefulness to temperatures => =>

TiO2 + 2hȞ ĺ 2e– + h+ H2O + 2h+ ĺ ½ O2 + 2H+ 2H+ + 2e– ĺ H2 H2O + 2hȞ ĺ ½ O2 +H2

Recently, hydrogen storage in MHs has been integrated into the photoelectrolysis cell, thus leading to a new type of devices termed as photo-electro chemical conversion and storage (PECCS) cells; in which the cathode is MH allowing in-situ storage of the generated hydrogen. Our efforts are focused on preparation of new materials for PEM fuel cells. Activated carbons (AC), SWNTs and metal–organic frameworks (MOFs) are being investigated as hydrogen adsorption and storage materials.32 The hydrogen storage capacity of nanofibers ranged from |15 > Ȥ > 1 wt % at moderate pressures and temperatures.33 Experiments on SWNTs yielded adsorbed amounts 1), are:

° °n ' ° °° ® °n ' ' ° ° ° °¯

1

2 ·º 2 ª § § · H V ¨ r « ¸¸ ¸¸» 1 1 ¨¨ ¨ «2¨ © 2H 0H rZ ¹ ¸¹»» ¬« © ¼ V 2H 0Z 1

2 ·º 2 ª § « H r ¨1 1 §¨ V ·¸ ¸» ¨ 2H H Z ¸ ¸¸» « 2 ¨¨ © 0 r ¹ ¹» «¬ © ¼

(10)

Equation (10) shows that a variation in the surface conductivity of a material leads1 to a change in its refractive index (the real n’ and imaginary n” parts vary 2 as V ). 2.2. MAIN EXPERIMENTAL SETUPS

In optical sensing, gas detection is achieved by optical interrogation. One has a large choice of laser illuminated interferometers to probe for refractive index variation caused by trace gases. Our option for the studies reported in this paper went to an m-line setup illuminated with a stabilized He-Ne laser source at 633 nm. This technique is suitable and frequently applied in order to measure variations of optical parameters in transparent waveguides.2–4 A typical experimental setup is presented in Figure 1.

NANOSTRUCTURED THIN OPTICAL SENSORS

33

Figure 1. Testing facility for waveguide coupling interrogation schemes.

At the center of the test setup were the prism coupler on its rotation plate and the sensing element, i.e. the thin film, which had been deposited on a transparent substrate pressed on the prism base. Physical interactions between the gas and the sensing material leading to variations in the refractive index of the latter took place at the air-sensitive material interface. Calculations based on equations in Section 2.1. showed5 that the electromagnetic field was enhanced at this interface for the transversal magnetic (TM) polarization state of light and a thin film thickness close to the cut-off thickness of the guided mode (i.e. the thickness below which the mode cannot propagate in the thin film any longer). This configuration ensured the highest sensitivity to refractive index variation under gas exposure. 2.2.1. Coupling by a prism The coupling by a prism of an incident laser beam into a planar waveguide is governed by the incident angle Ts between the beam and the prism base (Figure 2). At a particular incidence angle (called synchronism angle), the resonant coupling of the laser beam into the waveguide can be observed by watching a dark line (known as mode line or m-line) that appears in the reflected beam.

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C. RISTOSCU ET AL.

dark line

laser beam

Ts

prism (nP)

waveguide (nW)

reflected spot in the Fresnel field region

substrate (nS) Figure 2. Prism coupler.

Consequently, the application of the method is mainly based on measuring the angles corresponding to the m-lines observed. From the propagation constants of the guided modes determined from these angles, the refractive index n and thickness h of the waveguide can be inferred. In the case of an angle measurement accuracy of 10–3, which is rather easily achievable, we have an accuracy of 10–4 for the refractive index estimation and r2 nm for thickness evaluation. We measured the refractive index and waveguiding properties of thin films for the two polarization states of light. The resonant coupling of the laser beam into the waveguide was strongly affected by perturbations that were due to the interaction of the material and surrounding medium. The effective index N of the guided mode for the resonant coupling is connected to the incident angle Ts of light by Eq. (11):

N

ª º § sin T s · ¸ Ap » n p sin «arcsin¨ ¨ n ¸ «¬ »¼ © p ¹

(11)

where np is the refractive index of the prism, and Ap its characteristic angle. From Eq. (11), we deduce the following expression of 'N:

'N

ª º § sinT s · ¸ Ap » n p cosT s cos«arcsin¨ ¨ n ¸ «¬ »¼ © p ¹ 'T s 2 n 2p sinT s

(12)

2.2.2. Coupling by a diffractive grating Light coupling can also be obtained using a coupling grating etched in the waveguide. The advantage of this system over the prism one consists in the possibility of visualizing in the transmitted beam the actual displacement of the m-line in the presence of gas, as shown in Figure 3.


35

Laser Beam O Ts

/ h

d N: effective index

n

transparent substrate (ns)

e

Figure 3. Light coupling by a diffractive grating engraved in a waveguide.

The angle is measured in both cases in a similar way. The corresponding relation between synchronism angle, Ts, beam wavelength, O, grating period, /, and the effective index for zeroth order, N, is:

N

sin T s

O /

From Eq. (13), we deduced the following expression of 'N:

'N

O· § 'T s 1 ¨ N ¸ /¹ ©

(13)

2

(14) Note that the results of the two methods are comparable in terms of sensitivity. In both of them, device operation is based on the m-line coupling principle, but the use of a grating coupler increases robustness and enables miniaturization (see further Section 4). 2.3. SENSING ELEMENTS

2.3.1. Optical sensors: critical review In the case of electrochemical conductivity sensors (MOS), reversible charge transfer reactions lead to a collective behavior consisting of electrical conductivity changes. Similarly, alterations of the optical properties occur by agent

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adsorption in suitably designed materials, such as metal oxides. These changes can be detected optically with very high accuracy and allow for a wide dynamic range multidimensional parametric space for work. Nanocomposite materials exploration by a two-step production and encapsulation process has also been initiated recently. The functionality of advanced composites would further be enhanced by the inclusion of special nanoparticles. The current trends encourage efforts towards the production of composites based on metals, semiconductors, metal oxides, metal-coated nanoparticles and nanotubes. Non-linear optical mechanisms are expected to offer additional innovative sensing tools that will complement the linear optical response under chemical exposure. Further innovation is foreseen by the use of organic materials incorporated in inorganic matrices to selectively adsorb specific agents. A novel approach consists in applying polymer-based sensitive receptors and including functionalized acidic pendants for nitroaromatic detection. We have to mention, though, that polymers age rapidly and are highly prone to corrosion by various aerosols. Both factors strongly influence stability and reliability in gas detection. At present, the chief concern in optical gas sensing is the reversibility of the physicochemical optical changes that take place in materials, mainly thinfilm structures, upon exposure to the chemical environment. Research efforts are aimed at the selective detection at room temperature with sensitivity in the pp-million range and in some cases (e.g., nitro-aromatics) pp-billion range. The predicted response time of the sensors is on the order of ms (depending on the sensor head), while the anticipated recovery time determined by the adsorption affinity ranges from 10 ms to 10 s. Based on these considerations, we made our choice for highly transparent thin metal oxide films, e.g., ZnO, SnO2, TiO2, WO3, as the simplest, very reliable solution for developing a cheap, easy to operate, potentially miniaturized detection system. 2.3.2. Thin metal oxide sensors: pure, doped and clustered Materials used for waveguides have to meet the basic prerequisite of providing high transmittance in the visible or infrared region. Metal-oxide optical sensors, due to their simple construction, may offer advantages in terms of resistance to severe conditions such as high temperature and corrosive environment, and the capability of being placed in close vicinity of emissions. This type of sensor may also have a short response time so as to enable the possibility of real-time control. However, modern applications of these sensors still face the problems of high cross-sensitivity to gases, i.e. rather low selectivity; high sensitivity to


37

ambient humidity; and long-term drift, related to oxygen bulk diffusion and transformations of the crystal structure of polycrystalline materials.6 A considerable improvement of operating parameters such as gas response selectivity, stability, and rate, can be achieved by optimizing both the bulk and surface structure of the metal oxide films applied. Efforts to optimize sensor output characteristics have so far mainly focused on finding optimal technological methods for doping metal oxides with nanoscale metal catalyst additives. Decreasing crystallites (grains) to nm dimensions has been another major goal. Dopants can boost the catalytic activity of the base oxide, stabilize a particular valence state, favor formation of active phases, stabilize the catalyst against reduction, and/or increase the electron exchange rate. Inserting active additives into base metal oxides can modify their parameters such as charge carrier concentration; chemical and physical properties of the metal oxide matrix; electronic and physicochemical properties of the surface; surface potential and inter-crystallite barriers; phase composition; sizes of crystallites, and so on. Noble metals due to their electronic state and their distribution both on the oxide surface and into the film can strongly promote gas sensor sensitivity and selectivity. Noble metal nanoclusters have been tested as either dopants or surface clusters for boosting gas-sensing efficiency by catalyzing specific reactions with the detected gases. Several catalytic additives/dopants such as Pt,7,8 Pd,7–13 Sb,14 Au,7 and Ru15,16 were chosen for enhancing sensor sensitivity. Different methods have been applied for adding metals to the films. Among them, pulsed laser deposition (PLD) provides some advantages, including reduced contamination due to the use of laser light, controlled composition of deposited structures, and in-situ doping. Moreover, PLD is a versatile, powerful tool for custom designing nanoparticles with the desired size and composition by merely varying the experimental deposition conditions.17–20 This was therefore our choice for obtaining thin metal oxide films doping with noble metal nanoclusters. 3. Optical Gas Sensor with PLD SnO2 Thin Films Doped with Pd Nanoclusters 3.1. MATERIALS AND METHODS

The experiments were performed with a PLD installation schematically depicted in Figure 4. Before each deposition event, the vacuum chamber was evacuated to a residual pressure lower than 10–5 Pa. Meanwhile, the chamber walls were heated to facilitate desorption of water vapors and other contaminants. Vacuum quality was monitored with a quadrupole mass spectrometer.

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The SnO2 (99.99%) target was ablated using an UV XeCl* excimer laser source (Lambda Physik LPX 315i, O = 308 nm, WFWHM 30 ns) operating at a frequency repetition rate of 10 Hz. To deposit one film, 20,000 subsequent laser pulses were applied. Before each deposition event, target surface was cleaned by applying 5,000 laser pulses. During this time, a shutter was interposed between the target and the collector to avoid deposition of ablated material from the first layers of the target, which usually contain contaminants. An AR-coated MgF2 lens with 300 mm focal length was used to focus the laser beam into a 1 mm2 spot on the target surface. The target was rotated at 3 Hz frequency during multipulse laser irradiation to avoid drilling. In-situ doping with noble metal of the SnO2 films was achieved by placing a thin 1 mm diameter Pd wire across the target surface (see inset in Figure 4). The laser beam was incident on the target at an angle of 45º, while incident laser fluence during deposition was set at 10 J/cm2. Films were deposited in a dynamic flux of 10 Pa O2. The Pd concentration in the SnO2 films was controlled by varying laser spot position on the target surface. The experimental conditions were optimized on the basis of previous parametric studies.13

Pd wire (I 1 ) SnO Figure 4. Experimental PLD/RPLD setup; the inset represents the SnO2 target with a thin 1 mm diameter Pd wire.

Ablated material was collected on Si(100) or quartz substrates maintained at room temperature, 350qC, or 500qC, during the deposition. They were placed 60 mm from the target parallel to it. All substrates were cleaned in an ultrasonic bath prior to deposition. Scanning electron microscopy (SEM) measurements were carried out using an electron microscope JEOL 6320F equipped with facilities for conducting energy dispersive X-ray spectroscopy (EDS) analysis. Grazing incidence X-ray


39

diffraction (GIXRD) analysis was carried out using a Siemens D5000 Diffractometer equipped with a Cu KĮ source (O = 0.1541 nm). All measurements were performed at a grazing incidence angle of 3q. Diffractograms were recorded from 10q to 100q at a step of 0.02q. The surface roughness of the deposited films was studied by atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) analysis was carried out using spectrometer system (Kratos Axis Ultra) with a monochromatic AlKD (hQ = 1486.6 eV) photon source. The system was also equipped with a 165 mm hemispherical analyzer for the acquisition of spectra and a concentric spherical mirror analyzer for XP imaging. The m-line technique was used at 633 nm laser wavelength for waveguide coupling interrogation (see also Section 2.2). Light was coupled by a prism to the material in the form of optical resonance (guided mode), and synchronism angle variations following gas introduction were measured. A He-Ne laser source emitting at 633 nm wavelength in transversal electric (TE) mode was used. Transversal magnetic (TM) polarization was obtained using a O/2 plate placed in the exit of the laser beam. The prism used for light coupling was made of TiO2, rutile phase. Its characteristics are given in Table 1. A photo of the prism setup used for light coupling can be seen in Figure 5. TABLE 1. Characteristics of the rutile prism used for light coupling. O (nm) nTE nTM

633 2.8641 2.5821

LASER

SAMPLE HOLDER PRESS

ROTATION STAGE

Figure 5. Photograph of the prism coupler.

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C. RISTOSCU ET AL.

The sample was pressed on the prism by a polyethylene support, the manual translation of which helped control applied pressure. Different translation and rotation systems may improve the positioning of the prism coupler (eight degrees of freedom). The incident angle of the beam on the prism base can be varied in order to attain synchronism conditions. At that stage, a photodiode measured the position of the reflected beam. The system was proven to be capable of detecting hydrocarbons in the range of 100–1,000 ppm.21–25 3.2. SENSING PERFORMANCES AND EXPLOITATION CHARACTERISTICS

3.2.1. Physicochemical characterization Figure 6 shows the morphologies for the three types of Pd:SnO2 samples deposited in 10 Pa O2 at (a) RT, (b) 350qC, and (c) 500qC, respectively. The sample deposited at RT exhibited a rough surface with cracks, while the two others were rather smooth and densely packed. A quasi-ordered nanostructure can be noticed in the sample deposited at 500qC.

a

b

c

Figure 6. SEM microimages of Pd:SnO2 structures deposited in 10 Pa O2 at (a) RT, (b) 350°C, and (c) 500°C.

AFM studies (Figure 7) sustained these observations. The surface of the sample deposited at RT was porous, with large particulates (>100 nm). By contrast, samples deposited at 350qC and 500qC were denser, and their particulate dimensions were down to ~100 and ~80 nm, respectively. An even slight increase in SnO2 grain size can lead to significant changes in the film structure that is responsible for both the catalytic and gas sensing properties of metal oxides.26 Typical GIXRD spectra of the investigated samples are given in Figure 8. The films are polycrystalline without any preferential orientation. Using the diffraction database,27 the visible peaks were clearly assigned to either SnO2 or Si. As can be seen from the spectra, the film deposited at 500qC (Figure 8c) was better crystallized than those obtained at 350qC and RT.


a

b

41

c

Figure 7. AFM microimages of Pd:SnO2 structures deposited in 10 Pa O2 at (a) RT, (b) 350°C, and (c) 500°C.

Note that the films only consisted of SnO2 phase. The most important lines were identified, as e.g. (110) at 26.6q, (101) at 33.9q, (200) at 37.9q, and (211) at 51.7q.27 EDS results confirmed the formation of SnO2 phase including about 2% wt of Pd. 3.2.2. Detection of trace gas (hydrocarbon) All sensors were exposed at room temperature to different concentrations of butane diluted in nitrogen (1,000, 500, 200, or 100 ppm). The applied testing protocol was as follows: (1) the detection cell was evacuated to clear it from impurities (1 min 30 s); (2) the cell was filled with N2 to atmospheric pressure (2 min 30 s); (3) the N2 + butane mixture was let in (3 min); (4) the mixture inflow was stopped, and N2 was introduced to study the return to baseline (3 min); and (5) cycles 1–4 were repeated. The typical optical responses we recorded are given in Figure 9, in which variation due to the presence of butane is evident. An important feature was that the optical response was repeatable for all tested sensors. In vacuum the signal becomes stable at ~400 mV, while the mixture of butane-nitrogen made it stabilize at about 550 mV. This variation was due to butane and could not be attributed to pressure effects. Moreover, the signal always reached the same level regardless of the preceding phase of the protocol, which indicates high reproducibility and reversibility. All films showed similar responses when the concentration of butane decreased. The shape of the signal variation kept unchanged, but the kinetics was slower at lower butane concentrations. The response time was about 1 min for 1,000 ppm, 2 min for 500 ppm, and 3–4 min for 100 ppm. The response time is defined as the time required for stabilizing the signal level from the phase of carrier (nitrogen) gas flow to that of butanenitrogen mixture exposure.

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a

130 120 110 100

SnO2 (220)

30 20

Si (331)

Si (220)

40

Si (400)

50

SnO2 (211)

SnO2 (101)

60

SnO2 (200)

70

Si (311)

80

SnO2 (110)

Lin (Counts)

90

10 0 12

20

30

40

50

60

70

80

90

10

2-Theta - Scale

b)

SnO2 (211) SnO2 (110)

Si (400)

SnO2 (312)

SnO 2

Sn O2 (20 2)

(310 )

SnO2 (220)

SnO2 (321)

SnO2 (301)

SnO2 (200)

100

SnO2 (101)

Lin (Counts)

200

0 10

20

30

40

50

60

70

80

90

10

80

90

2-Theta - Scale

SnO2 (321)

SnO2 (332)

SnO2 (221)

100

SnO2 (220)

SnO2 (101)

200

SnO2 (200)

SnO2 (110)

Lin (Counts)

300

SnO2 (002)

SnO2 (211)

Si (400)

c)

400

0 10

20

30

40

50

60

70

10

2-Theta - Scale

Figure 8. GIXRD spectra of Pd:SnO2 structures deposited in 10 Pa O2 at (a) RT, (b) 350°C, and (c) 500°C, respectively (grazing incidence angle of 3º).

250

N2

N2 + butane

V (mV)

200

N2

N2

a)

50

43

N2 + butane

N2 + butane

150 100

vacuum

vacuum

300

vacuum


1000ppm 200 ppm

0 0

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t (min) 600

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N2 + butane

N2

N2 + butane

vacuum

N2 + butane

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vacuum

V (mV)

400

1000ppm 100

500ppm

b

100ppm

0 0

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t (m in) 600

500

N2

N2 N2 + butane

N2

N2 + butane

vaccum

300

vaccum

vaccum

V(mV)

400 N2 + butane

200

100

1000ppm 500ppm 100ppm

c)

0 0

5

10

15

20

t (min)

Figure 9. Sensitivity of Pd:SnO2 sensor to different concentrations of butane diluted in N2. Quartz substrate temperature during PLD was (a) RT, (b) 350°C, and (c) 500°C.

From Figure 10 we notice that the higher the substrate temperature applied during PLD, the higher the gas sensitivity of the sensors. The (110) and (101) planes of the SnO2 crystal are known to be F (flat) faces. According to the periodic bond chains (PBC) theory,28 surface atoms of F-faces are strongly bound to each other in directions parallel to the face. As a result, these faces have a slight tendency to react with the arriving atoms. It has also been established that (110) and (101) surfaces of SnO2 grow via a layer-bylayer mechanism.

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C. RISTOSCU ET AL. 600

tension (v)

500 400

vacuum

N2

N2 +butane

N2

vacuum

N2

N2 +butane

vacuum

N2 +butane Room temperature 500˚C

300 200 100 0 0

5

10

15

20

t(s)

Figure 10. Effects of Pd:SnO2 film deposition temperature on the sensitivity to 1,000 ppm of butane.

The various crystallographic planes have different distances between the Sn atoms, which can be ranked as follows: d(110) ~ d(100) < d(101) < d(001).29 Sn atoms are centers of oxygen chemisorption. Changing the indicated distance can therefore influence the rate of dissociative oxygen chemisorption, which is in many cases the main process of gas sensing phenomena.19–23 The observed change in SnO2 crystal faceting could actually be one of the reasons behind the considerable modification of gas sensing characteristics in SnO2 films. It was shown in the case of SnO2 sensors made by selective ionic layer deposition (SILD)30 that the small size of the crystallites (grains) was an essential but not sufficient condition for achieving maximum gas sensitivity together with a fast response. When comparing the gas sensing characteristics of metal oxide-based sensors, one has to primarily consider the size of the conglomerates and their porosity, and only after that the size of grains and thickness of the film.30,31 However, we have to note that the above results are only due to part of the metal oxide matrix parameters which can influence gas sensing properties. Along with geometrical parameters, the physicochemical characteristics of the gas sensing matrix have to be taken into account. They include the chemical and phase composition; the concentration of bulk and surface oxygen vacancies; the size and density of both metal catalyst particles and single atoms on the surface of metal oxides; and the type and concentration of uncontrolled impurities. 3.2.3. Gas contamination and multiple operations The potential gas contamination of the sensors as an effect of butane exposure was studied by AFM and XPS. We compared twin samples one of which had been repeatedly exposed to gas action, while another had been kept in controlled, non-polluted atmospheres.


45

AFM analyses (Figure 11) demonstrated that topographies in the cases of unexposed and exposed samples were similar to each other. Nanoparticulates size was in the (45–55 nm) range. The XPS studies indicate that the near surface region of the unexposed sample comprises a mixture of O, Sn, and Pd. The first peak of the Sn 3d doublet occurs at 485.8 eV BE and is assigned to emission from Sn 3d5/2 corelevel of Sn2+ ion in tin oxide. The second component occurs at 494.2 eV and is assigned to emission from the Sn 3d3/2 core level of Sn2+ ions. The 8.4 eV separation of 3/2 and 5/2 components is consistent with this interpretation of SnO/SnO2. The primary component of O 1s occurs at 529.7 eV and is assigned to emission from O2– ions in SnO/SnO2 and PdO. The second component occurs at 530.9 eV and is assigned to emission from chemisorbed OH groups, based on the 2.2 eV separation of the secondary component from the primary one. The third component occurs at 532.9 eV and is assigned to the presence of chemisorbed H2O molecules. The first peak of Pd 3d doublet occurs at 335.7 eV and is assigned to emission from the Pd 3d5/2 core-level of Pd2+ ion in PdO. The second component at 336.8 eV is assigned to emission from the Pd 3d5/2 core-level of Pd4+ ions in PdO2. The XPS spectrum of the “as received” surface of the exposed sample clearly indicates that the near-surface region comprises a mixture of Sn, O, and C. The first component of O1s occurs at 530.6 eV BE and is assigned to emission from O2– ions. The second component occurs at 532.1 eV and, based on the 1.5 eV BE shift, is assigned to emission from adsorbed OH– groups. The Sn 3d5/2 peak occurs at 486.7 eV BE, whilst the Sn 3d3/2 is centered on 495.2 eV BE. The 8.5 eV separation of the two components is consistent with the presence of SnO/SnO2 in the near surface region of the sample. High-resolution C1s spectra were recorded for the unexposed (Figure 12) and exposed (Figure 13) samples, while the main quantitative information was collected in Tables 2 and 3.

a

b

Figure 11. AFM images of Pd:SnO2 samples (a) unexposed and (b) repeatedly exposed to 1,000 ppm butane diluted in nitrogen.

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C. RISTOSCU ET AL. C1s ZnPd25Q

x102

C1s C1s

Intensity(CPS)

80

60

40

20 290

292

288

286

284

282

Binding Energy (eV)

Figure 12. High-resolution C1s XP spectrum obtained from the surface of unexposed sample. C 1s SnPd26 x 10

3

30

25

CPS

20

15

10

5 290

288

286 Binding Energy (eV)

284

282

Figure 13. High-resolution C1s XP spectrum obtained from the “as received” surface of exposed sample.

By comparing data in Tables 2 and 3, we see that the CH radical did not increase in the exposed sample; on the contrary, there was a reduction of this chemical species in the sample exposed to the butane. It should be stressed one would expect the CH radical to be the butane related species.


47

TABLE 2. Summary of quantitative information obtained by analysis of C1s peak as shown in Figure 12. Peak

Position BE (eV)

FWHM (eV)

Raw area (CPS)

RSF

Atomic mass

Atomic conc (%)

Mass conc (%)

C 1s

285.000

1.056

10,004.99

0.278

12.011

60.06

60.06

285.492

1.051

4,625.14

0.278

12.011

27.76

27.76

286.540 287.005

0.890 0.890

639.30 369.07

0.278 0.278

12.011 12.011

3.838 2.215

3.838 2.215

287.946 289.239

1.310 1.191

466.81 553.62

0.278 0.278

12.011 12.011

2.802 3.323

2.802 3.323

(C-C/C-H)

C 1s (C-CO2)

C 1s(C-O) C 1s (C=O)-O

C 1s(C-S) C 1s (shake up)

TABLE 3. Summary of quantitative information obtained by analysis of C1s peak shown in Figure 13. Peak

Position BE (eV)

FWHM (eV)

Raw area (CPS)

RSF

Atomic mass

Atomic conc (%)

Mass conc (%)

C 1s

285.000

0.830

4,350.3

0.278

12.011

45.12

45.12

285.517

0.800

3,052.8

0.278

12.011

31.66

31.66

C 1s(C-O)

285.906

0.728

1,025.1

0.278

12.011

10.63

10.63

C 1s

286.440

0.834

755.6

0.278

12.011

7.84

7.84

289.359

1.175

458.7

0.278

12.011

4.76

4.76

(C-C/C-H)

C 1s (C-CO2)

(C=O)-O

C 1s (shake up)

The latter observation and the results of morphological investigations suggest that the contamination effects due to exposure of the samples to polluted gases are very weak and hence hard to detect, if any. 4. Miniaturized Portable Prototype for Trace Gas Detection The dimensions of the detection system described in Section 2 can be reduced by using an etched coupling grating on the waveguide instead of the prism. We looked for a grating coupler for which the TM0 coupling should take place

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at normal incidence, i.e. Ts = 0 (see Figure 3). The new setup includes the lithographically submicron etched active material that is required to achieve waveguide coupling. The other basic components are: (i) enclosure for gas insertion, (ii) laser diode source and optics, (iii) high resolution CCD to detect the m-line, and (iv) data acquisition and image processing system for detecting variations of the dark line and reference them to the gas analysis. The new detection system redesigned and rescaled is presented in Figure 14. As a remarkable result, we emphasize that its dimensions do not exceed 15 cm length and 6 cm diameter. The system was recently tried in real conditions. Gas traces below 800 ppm, which is the maximum daily exposure to butane authorized under U.S. Federal Regulations, could be reliably detected. The use of the grating as sensing element also made it possible to cut down sharply the detection time. The system is still being worked on to improve its selectivity and sensitivity. Highly innovative schemes based on direct writing of waveguides in different materials have been recently developed, viz: in LiNbO3 in order to enable active electro-optic control of Mach-Zehender Interferometer, and in inorganic-organic hybrid materials deposited by sol-gel method. Multimode interference couplers written with a UV laser allow for the detection of very low refractive index variations.32 5. Conclusions We reviewed the basic physical principles and applied optical schemes to develop a new generation of high-performance optical gas detectors. General physical equations were introduced and the main solutions for optical sensing elements were critically analyzed. We based our decision to select nanostructured metal oxide thin films as sensing elements on their high sensitivity to gas traces and relative robustness and stability for multiple operations. As an application, we analyzed the case of Pd-doped-SnO2 thin films grown on Si (100) and quartz substrates by pulsed laser deposition in 10 Pa O2 at different substrate temperatures (RT, 350, and 500qC). Films deposited on Si were morphologically and structurally investigated. The results showed that 10 Pa O2 background pressure enabled the formation of nanoparticulates by condensation in the gas phase. Twin films deposited on quartz were investigated by the m-line technique to study the variation of optical properties when exposed to test gases (butane). Our results demonstrated that the gas sensing properties were mainly function of the structure and surface morphology of the samples, which in turn depended on substrate temperature. Moreover, it was evident that the higher the substrate temperature during PLD, the higher the gas sensitivity of the obtained SnO2 sensors. Our investigations did not evidence within measurement limits any remanent contamination of the sensing elements.


49

Figure 14. Photo of the miniaturized optical gas sensor prototype.

A miniaturized prototype with a sub-micrometric grating etched in the waveguide film as coupling element was successfully tried for operation in ambient atmosphere. The system is now under development to improve its selectivity and sensitivity to a wide range of detectable gases. ACKNOWLEDGMENTS

The financial support of the EU under the contract NANOPHOS IST-200139112 is acknowledged with thanks. CR also acknowledges the CNR – NATO fellowship Pos. 216.2169, Prot. N. 0015503 and NATO CBP.RIG.982424 contract. The Romanian and French authors acknowledge with thanks the support they received under the 2006–2007 Collaboration Agreement no. 19611 between CNRS and the Romanian Academy.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Born, M., and Wolf, E. (1975) Principles of Optics, Pergamon, Oxford, Chapter II. Agan, S., Ay, F., Kocabas, A., and Aydinli, A. (2003) Applied Physics A 80(2), 341. Shi, J., Cao, A., Zhu, J., and Shen, Q. (2004) Applied Physics Letters 84(17), 3253. Luo, Y., Hall, D., Kou, L., Blum, O., and Hou, H. (1999) Applied Physics Letters 75(20), 3078. Mazingue, T. (2005) Ph.D. thesis, Universite Paul Cezane Aix Marseille II, Marseille, France. Korotcenkov, G. (2005) Sensors and Actuators B 107, 209. Wurzinger, O., and Reinhardt, G. (2004) Sensors and Actuators B 103, 104. Gaidi, M., Chenevier, B., and Labeau, M. (2000) Sensors and Actuators B 62, 43. Chatterjee, K., Chatterjee, A., Banerjee, A., Raut, M., Pal, N., Sen, A., and Maiti, H. (2003) Materials Chemistry and Physics 81, 33.

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10. Korotcenkov, G., Brinzari, V., Boris, Y., Ivanov, M., Schwank, J., and Morante, J. (2003) Thin Solid Films 436, 119. 11. Suda, Y., Kawasaki, H., Namba, J., Iwatsuji, K., Doi, K., and Wada, K. (2003) Surface and Coatings Technology 174–175, 1293. 12. Shatokhin, A., Putilin, F., Safonova, O., Rumyantseva, M., and Gas’kov, A. (2002) Inorganic Materials 38(4), 374. Translated from (2002) Neorganicheskie Materialy 38(4), 462–467. 13. Pererira, A., Cultrera, L., Dima, A., Susu, M., Perrone, A., Du, H., Volkov, A., Cutting, R., and Datta, P. (2006) Thin Solid Films, 497(1–2), 142. 14. Leite, E., Bernardi, M., Longo, E., Varela, J., and Paskocimas, D. (2004) Thin Solid Films 449, 67. 15. Niranjan, R., and Mulla, I. (2003) Materials and Engineering B 103, 103. 16. Rumyantseva, M., Safonova, O., Boulova, M., Ryabova, L., and Gas´kov, A. (2003) Russian Chemical Bulletin International Edition 52(6), 1217 17. Lowndes, D., Geohegan, D., Puretzky, A., Norton, D., and Rouleau, C. (1996) Science 273, 898 18. Chrisey, D., and Hubler G. (Eds.) (1994) Pulsed Laser Deposition of Thin Films, Wiley, New York. 19. Marine, W., Patrone, L., Luk’yanchuk, B., and Sentis, M. (2000) Applied Surface Science 154–155, 345. 20. György, E., Santiso, J., Figueras, A., Giannoudakos, A., Kompitsas, M., and Mihailescu, I. (2005) Journal of Applied Physics 98, 1. 21. Mazingue, T., Escoubas, L., Spalluto, L., Flory, F., Socol, G., Ristoscu, C., Axente, E., Grigorescu, S., Mihailescu, I., and Vainos, N. (2005) Journal of Applied Physics 98(7), 074312 22. Socol, G., Axente, E., Ristoscu, C., Sima, F., Popescu, A., Stefan, N., Mihailescu, I., Escoubas, L., Ferreira, J., Szekeres, A., and Bakalova, S. (2007) Journal of Applied Physics 102, 083103-1-6. 23. Mazingue, T., Escoubas, L., Flory, F., Jacquouton, P., Perrone, A., Kaminska, E., Piotrowska, A., Mihailescu, I., and Atanasov, P. (2006) Applied Optics 45, 1425. 24. Gyorgy, E., Socol, G., Axente, E., Mihailescu, I., Ducu, C., and Ciuca, S. (2005) Applied Surface Science 247, 429–433. 25. György, E., Socol, G., Mihailescu, I., Ducu, C., and Ciuca, S. (2005) Journal of Applied Physics 97, 093527-1_4 26. Golovanov, V., Korotcenkov, G., Brinzari, V., Cornet, A., Morante, J., Arbiol, J., and Rossyniol, E. (2002) CO–water interaction with SnO2 gas sensors: role of orientation effects, in: Proceeding of the 16th International Conference on Transducers, EUROSENSORS-XVI, Prague, Czech Republic, 926–929 (CD). 27. ASTM 21-1250. 28. Hartman P. (Ed.) (1993) Crystal Growth – An Introduction, North-Holland, Amsterdam, The Netherlands. 29. Brinzari, V., Korotcenkov, G., Golovanov, V., Schwank, J., Lantto, V., and Saukko, S. (2002) Thin Solid Films 408(1/2), 51. 30. Korotcenkov, G., Macsanov, V., Tolstoy, V., Brinzari, V., Schwank, J., and Faglia, G. (2003) Sensors and Actuators B 96(3), 602. 31. Korotcenkov, G., Brinzari, V., Cerneavschi, A., Ivanov, M., Golovanov, V., Cornet, A., Morante, J., Cabot, A., and Arbiol, J. (2004) Thin Solid Films 460(1/2), 315. 32. Mazingue, T., Kribich, R., Etienne, P., and Moreau, Y. (2007) Optics Communications 278, 312–316.

X-RAY PHOTOELECTRON SPECTROSCOPY AND TRIBOLOGY STUDIES OF ANNEALED FULLERENE-LIKE WS2 NANOPARTICLES F. KOPNOV1, R. TENNE1*, B. SPÄTH2, W. JÄGERMANN2, H. COHEN3, Y. FELDMAN3, A. ZAK4, A. MOSHKOVICH5, AND L. RAPOPORT5 1 Department of Materials and Interfaces, Weizmann Institute, Rehovot 76100, ISRAEL 2 Fachgebiet Oberflaechenforschung, Fachbereich Materialwissenschaften, Technische Universität Darmstadt, Petersenstrasse 23, 64287 Darmstadt, GERMANY 3 Department of Chemical Research Support, Weizmann Institute, Rehovot 76100, ISRAEL 4 “NanoMaterials” Ltd., Weizmann Science Park, Bldg. 18, 18 Einstein St., P.O. Box 4088, Nes Ziona 74140, ISRAEL 5 Department of Science, Holon Institute of Technology, Golomb St. 52, P.O. B 305, Holon 58102, ISRAEL

Abstract – The temporal chemical changes occurring at the surface of fullerene-like (IF) nanoparticles of WS2 were investigated using X-ray photoelectron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed on the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, ___________ *To whom correspondence should be addressed: R. Tenne, email: [email protected]

A. Vaseashta and I.N. Mihailescu (eds.), Functionalized Nanoscale Materials, Devices and Systems. © Springer Science + Business Media B.V. 2008

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tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.

Keywords: IF-WS2, nanoparticles, XPS (X-ray photoelectron spectroscopy), tribology, annealing.

1. Introduction The extensive research of the structural, physical and chemical properties of layered transitional metal dichalcogenides, MX2 (M = metal, X = chalcogenide) have been going on for the past 5 decades.1 This is due to their unique structural properties, where MX2 layers are bonded by the weak van der Waals forces through relatively large (several Angstrom) van der Walls gap. The van der Waals gap allows accommodation of foreign atoms and molecules leading to the modifications of the physical properties, such as the electrical conductivity and the magnetic susceptibility. Two of these MX2 materials that have been broadly studied are 2H-MoS2 and 2H-WS2 (2H stands for hexagonal unit cell that consists of two layers); they have been shown to be good solid lubricants2 and catalysts.3 Since it is believed that 2H-WS2 can be a promising material for solar cells, much effort has been paid to the synthesis of WS2 thin films and studying their electronic and transport properties by electrical measurements and X-ray photoelectron spectroscopy (XPS).4 It was found that under special conditions layered transition metal dichalcogenides could be produced as closedcage (fullerene-like) nested nanoparticles (IF) and nanotubes (INT).5 Since the first inorganic fullerenes-like structures and nanotubes of MX2 compounds (M = Mo,W; X = Se,S) were synthesized much work has been done in developing new synthetic routs and elucidating their growth mechanisms.6 Moreover, the structural,7 optical,8 electronic,9 and mechanical10 properties of these materials have been extensively studied. In particular, the IF-WS2 phase revealed intriguing tribological11 performance that offers many potential applications where reduced friction and wear are desired. A 1H NMR study12 of the IF-WS2 nanoparticles showed that the nanoparticles entrap water molecules, or OH moieties, which are produced during the conversion of the oxide nanoparticles into the metal sulfide (IF) and perhaps also some residual hydrogen molecules. The water molecules are believed to be present in defects arising due to folding of the IF-WS2 layers. They may also adsorb in the nanopores between the agglomerated IF-WS2 onions; at the nanoparticles surfaces and occupy voids in the central part (core) of the IF nanoparticle as well. A dedicated

ANNEALED FULLERENE-LIKE WS2 NANOPARTICLES

53

vacuum annealing set-up was assembled in order to extract the remnant water molecules produced during the synthesis or adsorbed onto the nanoparticles surface afterwards. Sequential van der Pauw transport measurements13 of the IF-WS2 pellets showed that the vacuum annealed samples demonstrated higher resistivity than the non-annealed ones. However, apart from a preliminary study14 no systematic X-ray photoelectron spectroscopy (XPS) study of the IF-MX2 structures was carried out, so far. Such a study could reveal the electronic properties of the IF material, i.e. the Fermi level position, the position of the tungsten and sulfur bands, which could also help to elucidate the role of the intercalated water molecules within the IF nanostructures. The following work presents XPS studies conducted on the 2H and IF-WS2 before and after vacuum annealing. Two different types of IF powders were studied, i.e. freshly synthesized powder and another powder produced 3 years before the actual measurements. Our measurements showed that the IF material was indeed a p-type semiconductor, but to a lesser degree than bulk 2H-WS2. The main difference between the old and the fresh powders was that the later exhibited a better stoichiometric ratio. These characteristics support our previous results of transport measurements, i.e. the resistivity of annealed IF samples is higher than that of the pristine ones. Complementarily, the XPS-derived binding energies of W and S in pristine (non-annealed) and annealed IF-WS2 demonstrated systematic shifts, as compared to 2H platelets. Moreover, the most salient chemical change under annealing the IF-WS2 was the loss of oxygen, which was indeed indicative of water degassing. 2. Experimental 2.1. SAMPLE PREPARATION AND HANDLING

The synthesis of the IF-WS2 samples was described previously.14 The 2H-WS2 powder came from a commercial source (Alfa Aesar). The XPS measurements were carried out either on pellets or loose powder. 2.2. XPS

The powder from the 3 years old batch was analyzed by X-ray photoelectron spectroscopy (XPS) measurements using an Escalab 250 (by Thermo-VG) setup at a resolution of 0.3 eV. The protocol used for these measurements was as follows: first, survey spectrum was taken with the following parameters: step size 0.5 eV, pass energy 50 eV, dwell time 50 ms. Subsequently, detailed spectra were taken: step size 0.05 eV; pass energy 10 eV, dwell time 50 ms. The X-ray was running with a power of 150 W at 15 kV with a 500 Pm spot size. All the

F. KOPNOV ET AL.

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data presented as figures and as series I (pellets) in Table 2 were done with this setup. All spectra were referred to the Fermi level of a freshly sputtered metallic sample (EF = EB = 0eV). Complementary XPS measurements on the freshly prepared samples were performed (series II in Table 1) with Kratos AXIS-HS analytical system using a monochromatized Al KD (1486.6 eV) source at relatively low power (75 W), with a hybrid magnetic and electro-static lens mode and detection pass energy of 20 eV. Control over sample charging was achieved by an electron flood gun (eFG) with typical acceleration of 2–3 eV. The annealed IF samples were analyzed using this setup 2–5 min after exposure to the ambient (see series II (loose powder) in Table 1). TABLE 1. Summary of the results (series II) of the XPS analysis of IF-WS2 (annealed and nonannealed) powders. The numbers (in bold) represent atomic percentage of the constituent elements. The numbers in parenthesis stand for the energy shifts in mV of the corresponding bands: E (annealed)-E (non-annealed) (the certainty of the measurements is §30 mV according to the shifts under electron flux from the eFG).

IF-WS2 pristine IF-WS2 annealed

W(4f)

S(2p)

28.5

57.9

31.1 62.7 (289 meV) (285 meV)

O(1s) C(1s)

8.1

5.5

1.7

4.5

VB (valence band)

(294 meV)

TABLE 2. Summary of the results (series I) of the XPS analysis of 2H-WS2 and IF-WS2 (annealed and non-annealed). The numbers (in bold) represent atomic percentage of the constituent elements. The numbers in the parenthesis stand for the energy shifts in meV of the corresponding bands: E (annealed)-E (non-annealed).

W(4f)

S(2p)

O(1s)

N(1s)

C(1s)

SOx

2H-WS2

17

38

36

4

5

5.7

IF-WS2 pristine IF-WS2 annealed

18

40

31

5

6

4.7

30 (230 ± 50 meV)

61 (240 ± 50 meV)

4

3

2

0

VB (valence band)

(230 ± 50 meV)


55

2.3. VACUUM ANNEALING

A special setup13 was built for degassing the entrapped gas molecules. The system is evacuated by a turbomolecular pump up to 10–9 Torr (Leybold Turbovac 361). The sample (250–400 mg) is placed in the chamber and heated by a heating element which is controlled by a thermo controller (Eurotherm 2216e). The sample was heated up to 450°C. The usual heating treatment continued for 2 days. The outgoing gases are analyzed by a residual gas analyzer (RGA-Inficon model Transpector 2). 2.4. TRIBOLOGICAL MEASUREMENTS

Friction and wear experiments were performed using a ball-on disk set-up with sliding velocity of 0.4 ms–1 and load of 300 N. A bearing ball with a diameter of 10 mm moved against a steel disk quenched and tempered up to 44–46 HRc. Therefore the harder disk remained intact while the softer ball suffered the wear during the tribological measurements. The surfaces of the ball and the disk were polished (Ra = 0.02 Pm) prior to the experiment. 1 wt % of 2H-WS2 or IF-WS2 powder (pristine and vacuum annealed) were added to a paraffin oil with viscosity of 60 cSt at 20°C. The solid powder was mixed carefully with the oil for 1 h before the test. Few drops of the lubricant were fed to the contact area every minute during the entire duration of the experiment. The friction coefficient and the size of a wear spot on the surface of the ball were measured at a given periods of time after onset of the test. The studied surfaces were analyzed before and after the tribological tests by optical microscopy, electron scanning microscopy (SEM) and Raman spectroscopy. The Raman spectra were measured by a Renishaw micro-Raman microscope 2000 excited with a He/Ne laser (6,328 ǖ). 3. Results and Discussion The most striking observation of the XPS measurements is the shift of the S(2p) and W(4f) bands of the IF samples after annealing. Tables 1 and 2 summarize the results of the XPS analysis. The annealed samples in series II (measured with Kratos AXIS-HS) demonstrated the same band shifts (Table 1) as compared to series I (Table 2) (measured with Escalab 250). The line shifts of the IF powder after annealing are believed to reflect an upwards shift in the Fermi level – EF, i.e. the annealed material becomes less p-doped than the non-annealed one. These results are in line with the previous electrical measurements, where the resistivity of the pristine IF samples was found to be

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56

higher than that of the 2H-WS2 ones, and furthermore the annealed IF samples had higher resistivity than the non-annealed ones. Another interesting observation is that the annealing process of the 3 years old IF powder led to an improved stoichiometric ratio between S and W (Table 2). The vacuum annealing caused also the disappearance of the SOx moieties. However, the magnitude of the sulfur peaks did not alter suggesting that oxygen atoms had evaporated during the annealing. Additionally, the carbon contamination was drastically reduced during annealing, and the C(1s) peak (before annealing) shifted from the binding energy that fits bonded carbon (probably as WC bonding (283.5 eV)) to the value typical for CH species and graphite (284.4 eV) (after annealing) in both sets of powders (I and II). Furthermore, smaller amounts of the contaminant atoms, like C, O, and N, as well as SOx moiety (less than 1% of the sulfur content ) were found in series II (fresh batch) measurements than in series I (3 years old batch). The larger amounts of SOx moieties and other contaminants found in series I may suggest that the surfaces of the pristine IF powder is not chemically homogeneous, or that some oxidation of the nanoparticle surfaces in air must have happened within 3 years. It should be born in mind, however, that transmission electron microscopy (TEM) and x-ray diffraction analyses (XRD) of the 3-years old sample (series I) did not reveal any noticeable differences compared to the fresh sample (series II). Moreover, the annealing process did not have any influence on the outcome of the TEM and XRD analysis. These observations suggest that the IF nanoparticles do not go through structural changes during the annealing process, which affect their outermost surfaces, only. The tribological measurements revealed that the annealed IF powder demonstrated lower friction coefficient than the non-annealed one (Table 3). The results of the XPS analysis can be interpreted along the following scheme: during the synthesis some oxygen from the oxide core or from the entrapped water molecules can react with sulfur atoms of WS2, consequently carbon atoms that enter the reaction as a contaminant bind to the tungsten atoms. These new chemical moieties provide additional acceptor states. Indeed, Table 2 shows that the quantity of SOx and C at the nanoparticles surface TABLE 3. The results of the tribological tests; each figure represents an average of three trials. Material

Friction coefficient

Wear spot (ȝm)

2H-WS2

0.09–0.11

512–544

IF-WS2 (pristine)

0.09–0.1

480–490

IF-WS2 (vacuum annealed)

0.07–0.08

310–312


57

decreased nearly by the same amount after vacuum annealing. Vacuum annealing leads to removal of the WC and SOx species. While some of the reactions are restricted to the nanoparticle surface, others may occur throughout the entire volume of nanoparticles (120 nm in diameter), in the central void of a particle or at the defects arising due to folding of the IF-WS2 layers (Figure 1). The products of such a reaction, i.e. CO2, H2O, SO2 are pumped out during the vacuum annealing. Thus the acceptor states disappear and the material becomes more intrinsic and less conductive. Disappearance of the WC peak indicates that chemically a more complex process occurred during the vacuum annealing in the presented case. One of the possible mechanisms is that WC moiety reacts with the extracted water, and that leads to the formation of a volatile tungsten oxihydrate. Such volatile moieties have been documented in the literature.15 In addition, the XPS measurements showed that the surface of the IF nanoparticles underwent oxidation upon exposure to the ambient for a prolong period of time. Since, there are not any structural differences, according to the TEM and XRD analyses, between the (3 years) old and fresh batch of the IF powder, the oxidation can take place at the defects existing within a nanoparticle or at the outermost layer of the nanoparticle (Figure 1 (inset 3)). Noticeably, when the molecular layers fold sharply, a kink is often formed (see Figure 1) leaving a void, which can be filled with water molecules. These separated layers expose prismatic (hk0) faces, which are more prone to a chemical degradation by an oxidation process. It is a well known fact that the

1

2

3

1

3

2

(a)

(b)

Figure 1. (a) TEM image of an individual IF-WS2 nanoparticle shows clearly the hollow core in the centre of the closed nested WS2 layers. (b) Blown up zones 1, 2 and 3 of the nanoparticle exhibiting defects and voids wherever the folding of the layers induces imperfections in the lattice.

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sulfur terminated (basal) (0001) surface of tungsten disulfide layer in the 2H-WS2 crystal is inert towards oxidation. In contrast to that, the prismatic (hk0) faces of the lattice are chemically reactive due to abundance of dangling bonds. The saturation of these dangling bonds with contaminating atoms leads to the appearance of impurity states that impair the performance of these crystals as solar cells and also adversely affects their tribological performance. 6 4. Conclusions XPS investigation of the pristine and vacuum annealed IF-WS2 nanoparticles was undertaken. Furthermore in the course of the measurements 3-years old and freshly prepared powders were studied. It was found that the freshly synthesized pristine powder had less contamination in the form of SOx, C and O moieties and better stoichiometric ratio than the powder of the 3 years old batch. The annealing improved the stoichiometric ratio in the case of the old batch powder and resulted in a reduced level of contamination for both types of powders. In addition, the annealing process led to a shift of the W(4f), S(2p) atomic bands and the valence band towards higher energies. This shift reflects an upward shift of the Fermi energy such that the annealed material becomes more intrinsic than the non-annealed one. The annealing also resulted in a higher energy shift of the carbon (1s) line. The changes in the XPS spectra of the IF powder and the energy shifts of the corresponding moieties indicate that the annealing process resulted in a more stoichiometric surfaces of the nanoparticles. The chemical reactions that occurred resulted in the disappearance of the acceptor states and thus made the material more intrinsic and less conductive.

References 1. Wilson, J., and Yoffe, A. (1969) Transition metal dichalcogenides discussion and interpretation of observed optical, electrical and structural properties, Adv. Phys. 18(73), 193. 2. Watanabe, S., Noshiro, J., and Miyake, S. (2004) Tribological characteristics of WS2/MoS2 solid lubricating multilayer films, Surf. Coat. Technol. 183(2–3), 347–351. 3. Alonso, G., Del Valle, M., Cruz, J., Licea-Claverie, A., Petranovskii, V., and Fuentes, S. (1998) Preparation of MoS2 and WS2 catalysts by in situ decomposition of ammonium thiosalts, Catal. Lett. 52(1–2), 55–61. 4. Gourmelon, E., Lignier, O., Hadouda, H., Couturier, G., Bernede, J., Tedd, J., Pouzet, J., and Salardenne, J. (1997) MS2 (M=W, Mo) Photosensitive thin films for solar cells, Sol. Energy Mater. Sol. Cells 46(2), 115–121.


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5. Remskar, M. (2004) Inorganic nanotubes, Adv. Mater. 16(17), 1497–1504. 6. Therese, H., Zink, N., Kolb, U., and Tremel, W. (2006) Synthesis of MoO3 nanostructures and their facile conversion to MoS2 fullerenes and nanotubes, Solid State Sci. 8(10), 1133–1137. 7. Hassanien, A., Tokumoto, M., Mrzel, A., Mihailovic, D., and Kataura, H. (2005) Structural and mechanical properties of MoS2-I-x nanotubes and Mo6SxIy nanowires, Physica E 29 (3–4), 684–688. 8. Loh, k., Zhang, h., Chen, W., and Ji, W. (2006) Templated deposition of MoS2 nanotubules using single source precursor and studies of their optical limiting properties, J. Phys. Chem. B 110(3), 1235–1239. 9. Milosevic, I., Vukovic, T., Damnjanovic, M., and Nikolic, B. (2000) Symmetry based properties of the transition metal dichalcogenide nanotubes, Eur. Phys. J. B 17(4), 707–712. 10. Schwarz, U., Komura, S., and Safran, S. (2000) Deformation and tribology of multi-walled hollow nanoparticles, Europhys. Lett. 50(6), 762–768. 11. Hu, J., Bultman, J., and Zabinski, J. (2004) Inorganic fullerene-like nanoparticles produced by arc discharge in water with potential lubricating ability, Tribol. Lett. 17(3), 543–546. 12. Panich, A., Kopnov, F., and Tenne, R. (2006) Nuclear magnetic resonance study of fullerenelike WS2, J. Nanosci. Nanotechnol. 6(6), 1678–1683. 13. Kopnov, F., Yoffe, A., Leitus, G., and Tenne, R. (2006) Transport properties of fullerene-like WS2 nanoparticles, Phys. Stat. Sol. (b) 243(6), 1229–1240. 14. Feldman, Y., Frey, G., Homyonfer, M., Lyakhovitskaya, V., Margulis, L., Cohen, H., Hodes, G., Hutchison, J., and Tenne, R. (1996) Bulk synthesis of inorganic fullerene-like MS2 (M=Mo, W) from the respective trioxides and the reaction mechanism, J. Am. Chem. Soc. 118(23), 5362–5367. 15. Sarin, V. (1975) Morphological changes occurring during reduction of WO3, J. Mater. Sci. 10(4), 593–598.

THE DEVELOPMENT AND APPLICATION OF UV EXCIMER LAMPS IN NANOFABRICATION I.I. LIAW1 AND I.W. BOYD2* Department of Electronic & Electrical Engineering and London Centre for Nanotechnology, 17–19 Gordon Street, London WC1H 0AH, UK 2 Department of Electronic & Electrical Engineering and London Centre for Nanotechnology, 17–19 Gordon Street, London WC1H 0AH, UK 1

Abstract – Photon-induced processes hold many unique advantages for thin film processing and surface modification. These include low thermal budgets, lack of ionisation, chemical selectivity, and high cleanliness levels resulting in new chemical pathways to facilitate processing at reduced geometries. For such applications, a variety of sources are available including a wide range of lasers and incoherent lamp systems. In this presentation, the principles and properties of ultraviolet (UV) and vacuum ultraviolet (VUV) radiation generated by decaying excimer complexes will be described. Excimer lamps based on this principle provide highly efficient, high intensity, narrow-band radiation at various distinct wavelengths, with no self adsorption, which can, through insightful variations of the geometric configurations of these dielectric-barrier discharges, make large-scale applications possible. For deep UV applications, sources emitting at wavelengths as low as 126 nm have been developed. The high photon energy levels generated from these sources have been demonstrated to directly photodissociate nitrogen molecules, enabling direct nitridation of surfaces. By surveying a selection of publications in the field, a range of applications of applications for these novel sources in the field of nano-fabrication is discussed. These include the photo-deposition of low- and high-dielectric constant layers, low-temperature oxidation of Si, SiGe and Ge, photo-etching and micro-structuring of polymer surfaces, photo-induced metallization, cleaning of surfaces and

______ *

To whom correspondence should be addressed: Ian Boyd, email: [email protected]


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UV-curing. From these examples, these relatively low cost lamp systems are demonstrated to be capable of providing low temperature alternatives for largescale materials processing in a wider range of nano-scale applications.

Keywords: Excimer lamps, dielectric barrier discharges, photo-chemistry, ultraviolet, low temperature.

1. Introduction Laboratory UV lasers have widely demonstrated the use of monochromatic photons in the ultraviolet spectral range to selectively initiate photochemical reactions, to dissociate gases or liquids, and on substrate surfaces. For industrial deployment, the need for wavelength selective large throughput systems at low cost is essential. Relatively cheap lamps that possess an extended operating lifetime and are easily fabricated in a variety of geometrical shapes are an intuitive choice. The high energy photons (5–12 eV) generated from these dielectric barrier discharge lamps can readily initiate a range of biological, chemical and physical processes. Initial applications to the generation of ozone for water disinfection, have now extended to photochemical degradation and synthesis, polymerization, and materials deposition at the boundaries of microand nano-science. These sources are also finding possible applications in the microtechnology industry and with the reduction in device geometries towards the nanoscale, their ability to induce specific changes in nanostructures will further highlight their potential towards new generations of nano-fabrication processes requiring low-temperature multi-layered materials. 2. Excimer Radiation from Gas Discharges Dense rare gases at pressures around or above atmospheric pressure have a special property that enables the efficient conversion of electron kinetic energy to electronic excitation energy initially stored in excited atomic and ionic states. In this pressure regime, this excitation is funneled rapidly to a few low-lying atomic and excimer energy levels.1 Excimers (excited dimmers or trimers) are unstable excited molecular complexes, which under normal conditions do not possess a stable ground state. Under the influence of short-pulsed particle bombardment these may disintegrate giving off their binding energy in the form of UV or VUV radiation. As an example, the major reactions for the formation of Xe2* excimers are:

NANOFABRICATION BY UV EXCIMER LAMPS

3

Xe*

Xe*

and

3

P1 , 3 P0 2 Xe o Xe2*

P1 , 3 P2 2 Xe o Xe2*

1

63

¦u Xe

(1)

(2)

1

¦ u Xe

Using Figure 1 to illustrate this further, Xe2* excimer formation only occurs at pressures above 0.1 bar. A three body reaction converts the excited Xe atom, Xe* to the exciplex Xe2* whose radiative decay is faster than that of the Xe* itself, which is responsible for a resonance emission at 147 nm (see Figure 2). The emission bands from the bottom two excimer states dominate and these states decay only by radiation and do not interact with the ground state. The fluorescence radiation of these decaying excimer complexes is normally

Figure 1. Photon energy generated by various excimer sources correlated to common bond dissociation energies (eV).

Figure 2. Scheme showing the generation of VUV line radiation at low pressure and the generation of VUV excimer radiation at 172 nm at about atmospheric pressure. 2

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restricted to a narrow band of wavelengths (10–15 nm) with a theoretical efficiency between 45–80%3–7 and particle densities around 1025 m-3. The second excimer continua from pure rare gas dimers are shown in Table 1. Using this principle, simple and efficient excimer lamps may be constructed utilizing different rare gases to induce different UV wavelengths. The use of vacuum ultraviolet Xe2* excimer sources has found widespread application due to the availability of high purity silica tubes from which they are constructed which transmit VUV radiation down to 170 nm with little loss. For shorter wavelengths, special window materials such as LiF, MgF2 or CaF2 must be used. At wavelengths 0 on and a negative (inward) initial curvature. This means that at positive static curvature both currents have the same phase, while at negative static curvature the phases are opposite. Since in the first case ' M ' C ! 0 , while in the second case ' M ' C 0 , from both experiments it follows that ' P ! 0 , i.e., f B ! 0 . This positive sign is opposite to the sign of the surface charge established electrophoretically. Repeating the experiment with the UA-modified BLM where recharging of the surface takes place, we see just the opposite behavior, showing that f B 0 in the whole UA concentration range studied. This negative sign is also opposite the positive surface charge due to the strong UO 22 ion adsorption. Another example of a flexoelectric sensor employing patch-clamped membranes of soy lecithin is provided.21 The effect of heavy metal ions Cd2+ and Hg2+ was investigated. While Cd2+ ions resulted in a biphasic effect, firstly increasing and then decreasing the flexoresponse, the effect of Hg2+ (Figure 7) amounted to a monotonic decrease of the flexoresponse in the millimolar range of concentrations.

Figure 7. Flexoelectric sensor effect of heavy metal ions. Hg2+ concentration dependence of the normalized flexocurrents of 4 soy lecithin membranes in patch pipettes excited at 410 Hz. (From Zheliaskova et al.,19 with permission from the Publisher.)

4. Conclusion

In conclusion, possible applications of flexoelectric BLMs as sensors for ion and dipolar species follows from the great sensitivity of flexoresponse to such adsorbed molecules (e.g. Figure 3). First prototypes of such sensors using BLM

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containing lipophilic ions as sensititvity amplifiers18 or patch-clamped bilayers for microscopic usage19 have already been demonstrated. The first observation of the converse flexoeffect in BLMs15,16 permits the potential use of stabilized BLM systems as microtransducers, micro sound wave generators, and microactuators in molecular electronics. Indeed, flexoelectrically induced displacements of a membrane surface that is only nano-meters thick represent (on a molecular scale) a huge, coordinated motion in space of a whole molecular assembly. This effect may, then, find some interesting applications. ACKNOWLEDGEMENTS

This work has been supported by a grant from the National Science Fund of Bulgaria (NP1-03/2004).

References 1. Mueller, P., Rudin, D., Ti Tien, H., and Wescott, W. (1962) Reconstitution of cell membrane structure in vitro and its transformation into an excitable system, Nature 194, 979–980. 2. Coronado, R., and Latorre, R. (1983) Phospholipid bilayers made from monolayers on patchclamp pipettes, Biophys. J. 43, 231–236. 3. Petrov, A. (1999) The Lyotropic State of Matter. Molecular Physics and Living Matter Physics, Gordon & Breach, New York. 4. Passechnik, V., and Sokolov, V. (1973) Permeability change of modified bimolecular phospholipid membranes accompanying periodical expansion, Biofizika 18, 655–660. 5. Ochs, A., and Burton, R. (1974) Electrical response to vibration of a lipid bilayer membrane, Biophys. J. 14, 473–489. 6. Petrov, A. (1975) Flexoelectric model of active transport, in J. Vassileva (ed.), Physical and Chemical Bases of Biological Information Transfer, Plenum, New York, pp. 111–125. 7. Petrov, A., and Derzhanski, A. (1976) On some problems in the theory of elastic and flexoelectric effects in bilayer lipid membranes and biomembranes, J. Phys. Suppl. 37, C3-155–C3-160. 8. Derzhanski, A., Petrov, A., and Pavloff, Y. (1981) Curvature induced conductive and displacement currents through lipid bilayers, J. Phys. Lett. 42, L-119–L-122. 9. Petrov, A., and Sokolov, V. (1986) Curvature-electric effect in black lipid membranes, Eur. Biophys. J. 13, 139–155. 10. Hristova, K., Bivas, I., and Derzhanski, A. (1992) Frequency dependence of the membrane flexoelectric voltage response. Adsorption of multivalent counterions on the surface of curved lipid bilayer, Mol. Cryst. Liq. Cryst. 215, 237–244. 11. Szekely, J., and Morash, B. (1980) The effect of temperature on capacitance changes in an oscillating model membrane, Biochim. Biophys. Acta 599, 73–80. 12. Wobschall, D. (1971) Bilayer membrane elasticity and dynamic response, J. Colloid Interface Sci. 36, 385–396.

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13. Derzhanski, A., Petrov, A., Todorov, A., and Hristova, K. (1990) Flexoelectricity of lipid bilayers, Liq. Cryst. 7, 439–449. 14. Todorov, A., Petrov, A., Brandt, M., and Fendler, J. (1991) Electrical and real-time stroboscopic interferometric measurements of bilayer lipid membrane flexoelectricity, Langmuir 7, 3127–3137. 15. Todorov, A., Petrov, A., and Fendler, J. (1994) Flexoelectricity of charged and dipolar BLM studied by stroboscopic interferometry, Langmuir 10, 2344–2350. 16. Todorov, A. (1993) Experimental investigations of direct and converse flexoelectric effect in bilayer lipid membranes, Ph.D. thesis, Syracuse University, NewYork. 17. Todorov, A., Petrov, A., and Fendler, J. (1994) First observation of the converse flexoelectric effect in bilayer lipid membranes, J. Phys. Chem. 98, 3076–3079. 18. Sun, K. (1997) Toward molecular mechanoelectric sensors: flexoelectric sensitivity of lipid bilayers to structure, location and orientation of bound amphiphilic ions, J. Phys. Chem. 101, 6327. 19. Zheliaskova, A., Naidenova, S., Marinov, Y., Mellor, I., Usherwood, P., and Petrov, A. (2001) Detection of heavy metal ions (Cd2+ and Hg2+) by their influence on flexoelectricity of patch clamped membranes, C.R. Acad. Bulg. Sci. 51(12), 53–56.

CARBON NANOTUBES: FROM FUNDAMENTAL NANOSCALE OBJECTS TOWARDS FUNCTIONAL NANOCOMPOSITES AND APPLICATIONS W. MASER*, A.M. BENITO, E. MUÑOZ, AND M. TERESA MARTÍNEZ Department of Nanotechnology, Instituto de Carboquímica (C.S.I.C.), C/Miguel Luesma Castán 4,E-50018 Zaragoza, SPAIN

Abstract – In this article we give a general introduction into the field of carbon nanotubes. On one side we describe carbon nanotubes as fundamental nanoscale objects and explain their high application potential. On the other side, we focus on carbon nanotubes as non-homogeneous materials as obtained from different sources. Methods for production, characterization, purification and functionalization and dispersions are presented explaining chances, challenges and limitations. Finally, we deal with the broad field of applications for carbon nanotubes with special emphasis onto high performance carbon nanotube composite materials.

Keywords: Carbon nanotubes, production, characterization, purification, functionalization, dispersions, composites, applications.

1. Introduction Carbon nanotubes1 were first described and brought into context by S. Iijima in 1991. It was quickly realized by the scientific community that his findings had wide ranging implications for science and technology. Driven by early theoretical studies, the very first experiments already confirmed the unique structure-property relationship. Since then, these findings boosted the development of a new and highly exciting field of research. Today, more than 15 years after Iijima’s observation, novel and often highly surprising results on carbon nanotubes are still continuously reported in the literature and in international ______ *

To whom correspondence should be addressed: W. Maser, email: [email protected]


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patent databases. The large number of highly promising results, quickly understood as real-world business opportunities, boosted by now the creation of numerous spin-off companies and also contributed to broaden-up core sectors of already well-established companies. With this article we provide the reader with the necessary background knowledge on carbon nanotubes to understand on one hand, their unique properties and opportunities, and on the other hand, the various challenges to be overcome in this broad and fascinating field of research. 2. Carbon Nanotubes: Fundamental Nanoscale Objects 2.1. STRUCTURE

A carbon nanotube (CNT) can be described as a seamlessly rolled-up sheet of graphene, resulting in an open tubular structure composed of carbon atoms arranged in a hexagonal network. The tubular structure is closed by the inclusion of six pentagonal defects into the hexagonal network at each end of the open cylinder to form the corresponding semi-fullerene2 caps. Figure 1 explains the basic structures of carbon nanotubes.

a1 Q

a2

tube axis

armchair A Q C B

zig-zag

A

B

chiral

C Figure 1. The structures of a graphene sheet (A), the family of single-wall carbon nanotubes (B), and a multi-wall carbon nanotube (C).

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103

There are three different ways in which a graphene sheet can be rolled up onto itself thus leading to the three basic forms of CNTs, i.e. the family of single-wall carbon nanotubes (SWNTs): (i) Armchair tubes: These are obtained when rolling-up the sheet in such a way that carbon – carbon bounds are perpendicular to the cylinder axis. Looking at the cross-section of such a tube one observes an “armchair” – like structure of the carbon atoms. (ii) Zig-zag tubes: Here, carbon – carbon bounds are parallel to the tube axis and the cross-section results in a “zig-zag”-like pattern of carbon atoms. (iii) Chiral nanotubes: Here, the hexagons of the original graphene sheet are winding-up around tube axis in a helical way letting form the carbon atoms a helical (chiral) pattern along the length of the tube. In any case, the structure of the nanotubes easily can be described by so-called chiral vector C. It is composed of a two-dimensional pair of integer numbers (n, m) corresponding to the numbers of the hexagonal unit cell vectors a1 and a2, respectively, needed to roll-up the graphene sheet onto itself from on point to another one. Therefore, the (n, m) pair directly reveals the diameter as well as the chiral character for every type of tube, e.g. all armchair tubes are characterized by (n, n) indices. Typically, SWNTs have diameters of about 1 nm and lengths of about 1 Pm to several micrometers, according to experimental observations. Often, due to their small diameters, SWNTs like to arrange themselves into bundles which can adapt diameters even up to 100 nm. Additionally, there exists the family of multi-wall carbon nanotubes (MWNTs): They are composed of individual SWNTs concentrically placed inside each other and separated by an interlayer distance of about 0.34 nm slightly larger than in graphite due to curvature. While length and diameter of the most inner tubes are similar to SWNTs, the outer diameters, depending on the number of individual nanotubes easily can reach values up to 20 or 30 nm. Among the MWNTs, double-wall carbon nanotubes (DWNTs) may play an important intermediate role between SWNTs and MWNTs. 2.2. PROPERTIES

Carbon nanotubes are characterized by a very close and unique structure – property relationship. Composed out of only carbon atoms, carbon nanotubes are very light objects with a very low density of around 1.5 g/cm3. Having all atoms at the surface confers CNTs very high specific surface areas of about 1,400 m2/g (outer surface) or even 2,800 m2/g (if inner surface is considered as well), comparable or even better than highly activated carbons. Furthermore, carbon – carbon bounds are one of the strongest in nature. Therefore, very small individual SWNTs may have Young moduli up to 1,800 GPa (100 times stronger than steel), asymptotically reaching values of graphite for very large diameter

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tubes.3 For SWNTs arranged into bundles, due to sliding effects between individual SWNTs, the Young moduli may lower down to values of about 200 GPa. The maximum tensile strength can reach values of up to 30 GPa. Apart of the high stiffness, individual SWNTs show surprisingly high mechanically flexibility.4 Upon bending to very large angles, SWNTs will not break. Instead, they show a reversible buckling behaviour conferring SWNTs an extremely high toughness. Having micrometer dimensions along their axis and nanometersized diameters, CNTs are nano-scale objects with aspect ratios (ratio length to diameter) as high as 1,000. This is an important property whenever it comes to applications where percolation issues play a critical role. On the other side, depending of the point of view, individual SWNTs might be considered as a large molecule or already as a one-dimensional solid state material. The electronic properties5 easily can be derived applying density functional theory for graphene and zone-folding concepts taking into account additionally periodic boundary conditions for the wave function along the nanometer-sized circumference. These boundary conditions reduce the number of allowed states in the two-dimensional Brillouin zone and introduce one-dimensional van-Hove singularities in the density of states. It can be shown that one third of all SWNTs behave as metals, i.e. all (n, m) tubes with n-m = 3i (I = interger), e.g. all armchair tubes, while two thirds of all SWNTs are semiconductors, i.e. all (n, m) tubes with n-m 3i. It is really remarkable that the structural arrangement of carbon atoms in SWNTs so clearly defines the electronic properties and that these are precisely defined for each for each type of nanotube with a specific (n, m) index. In this sense it has been shown that for semi-conducting SWNTs of a given chirality, the energy gap is inversely proportional to the tube diameter.6–8 Additionally, the one-dimensional electronic character of SWNTs, as expressed by the one-dimensional van-Hove singularities in the density of states, provides these nano-scale objects fundamental electronic, optical and vibrational resonance properties of great importance to spectroscopic characterization, e.g. Raman,9 NIR-absorption,10 photoluminescence,11 and scanning tunnelling spectroscopy,12 as well for applications in the fields of quality control and sensors. Furthermore, experimentally it has been found that SWNTs show ballistic transport13 even at room temperature, have current densities14 as high as 1010 A/cm2 (copper and aluminium show values between 107 and 1010 A/cm2), and behave as excellent electron emitters15,16 with low turn-on fields of 1.5–5 V/ȝm at 1 mA/cm2 and low energy spread of 0.25 eV. Finally, it is worthwhile mentioning that CNTs possess an excellent thermal conductivity17 of up to 3,000 W/mK, superior of diamond, one of the best thermal heat conductors. With this unusual close structure-property relationship and a whole bunch of fascinating combined properties, – light, strong, flexible,

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metal, semiconductor, heat-conductor, electron emitter –, it becomes clear that CNTs bear a great potential for applications in various sectors of technological interest as shown in Figure 2. The fields range from nanoelectronics (CNT based electronic device structures), flexible plastic electronics (photovoltaic and organic light emitting devices), functional composite materials (reinforced structures, tough materials, thermal conductivity, charge dissipation, electromagnetic shielding, intelligent textiles, and coatings, functional adhesives), energy (electrochemical energy storage devices, fuel-cell membranes), nanobio (sensors, drug delivery, cell proliferation, tissue healing), and catalysis (nanocatalyst dispersions). For a more detailed overview on structure and general properties we refer to the book of Dresselhaus.18

Figure 2. An overview of carbon nanotube’s broad application potential.

3. Carbon Nanotubes: Challenging Materials Before profiting from carbon nanotube’s unique properties and using them for various purposes, one has to be aware that carbon nanotubes are not produced as the individual objects described above. Instead, they are obtained as soot materials which are composed of CNTs of all different kinds of characteristics as well as of additional undesired carbonaceous and inorganic byproducts. This is quite challenging since no CNT soot material is like the other and no standard CNT materials exist today. The characteristics of CNT materials largely depends on the production technique being employed: Materials may contain different types of CNTs (SWNTs, MWNTs, DWNTs, …) with different

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Figure 3. Different types of CNT materials: (a) MWNTs from arc, (b) SWNTs from arc/laser, (c) thin MWNT bundles from supported CVD, (d) aligned MWNTs from floating CVD, (e) filled MWNTs from floating CVD, (f) DWNTs from supported CVD, (g) helical tubes from supported CVD.

diameter and chirality distributions, various defect structures, bundle agglomerations, and impurities such as amorphous carbon, graphite particles and attached catalytic nanoparticles. Figure 3 puts in evidence the different CNT materials characteristics. 3.1. PRODUCTION METHODS

To understand better the obtained materials a brief introduction into the most common production methods are given. Basically, one distinguishes between high-temperature and low-temperature processes.19 In high temperature processes, a solid carbon feedstock (e.g. graphite and eventually catalytic particles) is evaporated at temperatures above 3,000ºC by resistive heating or highly concentrated light. In an adequate inert atmosphere carbon species in the gas phase self-assemble to form carbon nanotubes. Electric arc discharge processes20 and laser evaporation methods21,22 are representative for this process. The soot materials obtained are: (i) straight and highly graphitic MWNTs (Øi = 1–3 nm, Øo = 2–35 nm, l = 1 ȝm) with additional polyhedral graphitic nanoparticles but without any catalytic particle (electric arc), (ii) entangled bundles of SWNTs (Øbundle = 20–100 nm, ØSWNT = 1–2 nm, l = 1 ȝm) with amorphous carbon and embedded spherical catalytic nanoparticle, typical Ni, Co, Ni/Y (electric arc, laser evaporation). Low temperature processes are based on the decomposition of a hydrocarbon feedstock at temperatures between 500ºC and 1,000ºC over catalytic particles. Carbon diffuses into the catalytic particle and at lower temperatures precipitates, under the right experimental conditions, in the form of carbon

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nanotubes. Chemical vapor deposition (CVD) techniques are typical methods being used.19,23–29 Depending on the production parameters (type of hydrocarbon gas, flow conditions, temperature, catalysts, catalyst introduction, support materials, additives etc.) usually various different types of MWNTs are obtained in bended form, as a direct consequence of low temperature processes leading to the inclusion of structural defects. More or less homogeneous materials are composed of thick and thin MWNTs, different numbers of layers, more or less bended, organised in bundles, or as individual objects, in various yields, with various amounts of inorganic impurities (between 2 and 40 wt %) from catalyst (even as filling inside CNTs) and catalyst support as well as low or high amounts of amorphous carbon. Sometimes, soot materials with enhanced fractions of SWNTs25 and DWNTs26 are obtained. CNTs also can be grown in aligned or in patterned form.27–29 A variation is the so-called HiPCO method (high pressure disproportionation of carbon monoxide) developed by Rice University30 and commercialized by CNI company which results in SWNT materials (ØSWNT = 2 nm, l = several micrometers) without any amount of amorphous carbon but with considerable amounts of catalytic iron nanoparticles (up to 40 wt % in as produced materials). Producing an almost “monolithic” CNT material composed only of CNTs of well defined diameters, chiralities, number of walls, without any by-products and in large quantities remains the greatest challenge in this field. Today, as a general rule of thumb one can say that today’s low volume production methods (few gram-scale) result in relatively high quality materials in what concerns CNT characteristics while the more recent large scale – low price production methods (tons-scale, 100 Euro/kg) do not aim yet at welldefined materials characteristics but on the availability of CNTs on industrial scale and on applications based on the overall CNT materials characteristics rather than on the individual CNT properties. 3.2. CHARACTERIZATION METHODS

Depending on the further use and application in mind, one type of CNT material might be more suitable than another one. Therefore it is highly recommendable to collect as much information as possible on structure and properties of the used CNT materials. In the following useful standard lab-techniques and their information content respective to bulk CNT materials, as produced, are briefly described. Scanning electron microscopy (SEM): A few milligrams of powder CNT material is deposited on the SEM sample holder. Eventually, depending on the conductivity degree, a very thin conducting metal layer is sputtered on. This technique generally gives highly valuable structural information on a scale of 100 nm to a few millimeters. Although CNTs can not be directly observed, it

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reveals CNT materials morphology, the presence and organization of the different sample products, density, homogeneity, fibre lengths. It also gives hints on the conductivity state (charging and contrast effects). SEM is a rapid, efficient and relatively economic technique for analysing CNT bulk material. Transmission electron microscopy (TEM): CNT materials are suspended in ethanol and ultrasonicated. A drop is deposited on a TEM grid. Solvent is evaporated and the sample can be analysed whenever the deposited material density is transparent for the electron beam. This technique gives structural information from the sub-nanometer to the 100 μm range. Diameter, layer structure, degree of graphitization, defects, bending and inorganic impurities can be observed. Combined with element mapping, selected area electron diffraction and EELS information on type, crystallization and distribution of catalytic nanoparticles as well as CNT chirality can be obtained. TEM gives valuable information on the level of individual CNTs, however, since only a very small (transparent) volume is probed good statistics is needed to get representative results. Induced coupled plasma spectroscopy (ICPS) and elemental analysis: A few tens of milligrams of powder CNT material is dissolved in acids and the precipitate analysed by flame-spectroscopy. It is a completely quantitative technique and reveals the content of metals, as well of other elements such as oxygen, sulphur and halogens in the bulk sample. ICPS allows the calculation of the composition of a sample and also might reveal ideas on the production yield. Thermogravimetric Analysis (TGA): A few tens of milligrams of CNT powders (or pellets) are deposited in a suspended crucible whose weight-loss is measured as a function of temperature increase. TGA gives information on the oxidation temperatures and residual mass. It may reveal CNT materials composition (presence of different carbon materials) and the amount of catalyst material in the sample. TGA is a valuable qualitative and comparative technique. Very easily different types of CNT materials can be distinguished from each other and information on further treatment temperatures can be obtained. If used for qualitative analysis, a strict protocol has to be employed (oxidation temperatures and metal rest content strongly depend on various experimental parameters. Slow temperature ramps between 3–5ºC/min, low gas flows are recommended). Furthermore, various error sources have to be taken into account (buoyancy effects, sample weight, sample morphology, “fluffy”sample or pellet effect, spontaneous combustion). Powder X-ray diffraction (XRD. About 100 mg of sample material is deposited on a flat-bed sample holder. XRD reveals the sample content, i.e. the type of carbon CNT, Camorph, Graphite, the degree of graphitization, of bundling

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and alignment, as well as the presence of catalysts or other elements. Easily different CNT samples can be distinguished from each other. Raman spectroscopy: a few milligrams of CNT powder material is placed on a sample holder. Four characteristic features can be obtained: low frequency CNT radial breathing modes (RBM) around 100–200 cm–1 (frequencies are inversely proportional to SWNT diameters), tangential shear mode (G-line) at about 1,580–1,590 cm–1, defect mode (D-line) at about 1,350–1,450 cm–1 and double resonant G’ mode at around 2,700–2,800 cm–1. Therefore, Raman gives information on the carbon structures and the quality of the CNT material. It easily allows to distinguish between different types of CNTs (SWNTs, MWNTs, DWNTs) and reveals the presence of carbonaceous defects and of graphite. Diameters of SWNTs and their distribution as well as the degree of bundling and alignment can be calculated. The conducting character as well as doping and charge transfer effects may be revealed. Due to the one-dimensional electronic structure and the presence of van-Hove singularities, resonance effects upon tuning the laser excitation frequency are observed. Resonance Raman spectroscopy is used to reveal the (n, m) structure of individual SWNTs. Raman spectroscopy is one of the most powerful standard characterization technique for CNTs with a high information content on the sample characteristics and on individual objects.9 The homogeneity degree of the samples has to be probed by mapping different sample areas. Only limitation might be eventual sample degradation effects when using elevated powers which, on the other hand, reveal information about CNT’s thermal stability. Four-point conductivity: Pellets or films are contacted using pressure contacts (most easy approach). This technique establishes the conducting behaviour of bulk CNT networks. It is a good comparative approach to distinguish between MWNTs, SWNTs or modified CNTs. However, absolute values for conductivity strongly depend on the exact calculation of the geometry factor as well as the protocol for pellet pressing and contacting the sample. Nitrogen adsorption isotherms (BET): Bulk technique using a few tens of milligrams of CNT powder (or pellets or films) deposited in the corresponding sample holder. BET gives information on the gas-adsorption process (physisorption or chemisorption), gas kinetics, CNT pore size and specific pore-size area. Different types of CNTs easily can be distinguished. The above mentioned set of techniques to characterize bulk CNT materials reveals sufficient information for further materials processing and also may give first hints on the suitability of CNT materials for certain kinds of applications. Most of them are use in a qualitative and comparative way. It still remains critically distinguishing between different carbon components and quantifying their respective content in the CNT material. An approach to

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solve this problem may by the use of NIR spectroscopy11 carried out in diluted CNT suspensions. 3.3. PURIFICATION AND TREATMENT

As discussed above, CNT materials are not pure and thus there is great interest in improving the samples’ quality for many purposes ranging from the development of proper characterization standards to further use and applications. Improvements refer to: (i) removal of metal particles, (ii) removal of carbonaceous non-CNT contributions, (iii) separation of bundle structures, (iv) removal defective CNTs (v) separation according to length, diameter and chirality. Main difficulty here is that CNTs (and the whole CNT materials) are chemically quite inert and non-soluble. Therefore, purification and treatment methods, in a first step, focus onto the removal of metals and Camorph, and the improvement of CNT structure by oxidation approaches, as shown in Figure 4.

Figure 4. A typical purification scheme for CNT materials.

Typically two types of treatments can be distinguished: On one hand, chemical oxidation in liquid phase using acids (e.g. HNO3,31,32 HCl,31,33 HCl/HNO3,31 HF/HNO3,34 H2SO4/HNO3), under reflux and, on the other hand, thermal treatments, such as air-oxidation processes at temperatures between 300–450ºC. Chemical oxidation focuses onto the removal of metal nanoparticles in the CNT materials. The efficiency strongly depends on the concentration and concentration ratio of the used acids, the reaction time, and the amount of CNTs. Thermal treatments are directed towards the removal of amorphous carbon as well as the creation of pore-structures (activation process). The efficiency depends on the temperature applied (determined beforehand by TGA), the gas-flow, the amount of CNT materials and the macroscopic sample distribution, i.e. the exposed materials surface to the heat and gas-flow. If necessary, both processes can complement each other, even in several repetitive cycles.

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Typically, a final annealing step is employed in order to heal structural CNT defects, and improve the bundle recrystallization in case of SWNTs. There exists a wide range of recipes which strongly depend on the CNT materials used and on the further use of the material. The following general observations can be made32: Chemical treatments lead to a significant reduction of metal particles up to 95 wt %. This is accompanied by significant material losses (20–60 wt %) including non-metal material losses (5–10 wt%) due to attacked carbon materials. Additionally, basic functional groups, especially carboxylic groups are attached to defect sites in attacked CNTs and also intercalation effects may be observed. The obtained material is highly compacted with drastically reduced specific surface areas of about 2 m2/g and nearly untreatable. Airoxidation does not affect the metal content, but removes defective carbon material, resulting in further material losses and in a relative increase of the metal content. On the other hand, it also results in highly activated materials with drastically increased specific surface areas as high as 750 m2/g. It is to mention that also this process creates defects at the tips and sidewall of CNTs to which carboxylic groups may be attached. The final annealing step contributes to heal defect structures, to recover the bundle organization, to remove created carboxylic groups and intercalated molecules. As one can see, purifycation is a multi-step approach which can be efficiently controlled using the above mentioned bulk techniques in order to judge the materials quality after each step. However, due to serious material losses, one has to find a compromise between resulting CNT quality, process yields, and final materials needs. 3.4. FUNCTIONALIZATION AND DISPERSIONS

Due to the high chemical inertness of CNTs and their insoluble character, chemistry on carbon nanotubes came later into the game and only created increased interest when it became clear that simple pre-oxidation steps as taken out in purification treatments lead to the creation of structural defects in CNTs (tip-opening and side-wall) to which various oxygen containing groups (mainly carboxyl groups) are attached as a function of the strength of the oxidation process (acid strength, and oxidation time) as illustrated in Figure 5. The oxidatively introduced carboxyl groups represent useful sites for further modifications, as they enable the covalent coupling of molecules through the creation of amide and ester bonds. By this method the nanotubes can be provided with a wide range of functional moieties such as dendrimers, nucleic acids, enzymes, metal complexes, nanoparticles etc. Furthermore, the presence of (modified) carboxyl groups leads to a reduction of van der Waals interactions between the CNTs, which strongly facilitates the separation of nanotube bundles

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Figure 5. A nanotube after oxidative treatment with defect sites at tip (open) and at side-walls to which oxidative functional groups (mainly carboxyl groups) are attached.

into individual tubes. Additionally, the attachment of suitable groups renders CNTs soluble in aqueous or organic solvents, opening the possibility of further modifications through subsequent solution-based chemistry. In addition to this “defect” functionalization, direct side-wall addition reactions through thermally activated reactions (preferentially on more reactive small diameter tubes) are documented in the literature. The functionalization degree is low and may vary between 3% and 15%. Fluorination35 and cyclo-addition36 are one of the most prominent side-wall reaction which further can be used for additional substitution reactions with alcohols, amines, Grignard reagents, alkyl lithium compounds and other specific linker molecules.37 Beside covalent functionalization (defect and side-wall addition), noncovalent functionalization is becoming of increased importance. It is based on the highly delocalized ʌ-electron system of CNTs to which many long-chain molecules and polymers have a strong affinity and get non-covalently linked to the CNT surface via van der Waals forces or ʌ-ʌ interactions. Surfactants such as triton X-100 or sodium dodecyl sulfate (SDS) lead to the formation of micelles and help to separate and disperse CNTs.38 Long-chain molecules/ polymers, such as polyelectrolytes,39 peptides,40 lipids,41 DNA,42 ʌ-conjugated polymers43–45 may even fully wrap around the CNTs by undergoing corresponding conformational changes to adapt to the underlying CNT network. These noncovalent approaches bear several advantages: (i) CNT structures are not disturbed and thus the original CNT properties are maintained, (ii) they render CNTs soluble in various solvents and thus is the base for the development of stable homogeneous CNT dispersions and for spectroscopic characterization, (iii) they facilitate selective SWNT separation according to diameter, chirality, length, and conductivity via adequately designed molecules, (iv) they add

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further functionality to CNTs, (v) they improve interface interactions with matrix materials for the development of high-performance composite materials. Both, covalent functionalization and, even more promising, non-covalent supramolecular functionalization are the base for the development of stable and homogeneous dispersions. This is a very important step towards CNT homogenization, separation, improved solution characterization (NIR,10 photoluminescence,11 dynamic light scattering46 and optical spectroscopy), solution processing of CNTs as a whole material (the deposition and incorporation techniques, transformation technology (buckypaper and fibre fabrication)) and achieving compatibility and synergetic interactions with many molecules and polymer systems. Therefore, stable and homogeneous dispersions are considered as a key-stone to transfer carbon nanotube’s high application potential into realworld applications. 4. Carbon Nanotube Applications With their bunch of fascinating properties carbon nanotubes are of great interest to technological applications in several fields, as already indicated in Section 2.2. In the following, we will not group applications into different sectors, but instead will classify the various applications with respect to the appearance of CNTs in a certain form. This means we distinguish between applications based on individual CNTs, on assemblies of CNTs and on CNT-hybrid materials, such as CNT based composites. This type of classification will allow to better understand the chances and challenges common to CNTs in a certain form of appearance. Hereby, special emphasis will be put onto the field high performance CNT composite materials. 4.1. INDIVIDUAL CARBON NANOTUBES

An individual carbon nanotube with a well defined structure and thus, electronic properties is a highly promising object for the development of nanoelectronic devices. A metal type SWNT can be imagined as a metal wire of great use as vertical interconnector in classical semi-conducting devices. On the other hand, a semi-conducting SWNT can be seen as important logic element in field effect transistor device (FET). It has been demonstrated by several groups around the world that semi-conducting SWNTs can be incorporated successfully as active element into classical silicon-based FET devices. Here they have proven (superior) operational functionality as logic gate,47,48 as gas or bio sensing devices,49,50 or as nanoelectromechanical device.51 Key issues towards comercialization of these products mainly relate to time-costly assembly technologies, which directly is related to the lack of pure SWNT materials composed of only

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one type of SWNTs. Today, three assembling approaches are followed: (i) selection of an individual SWNT of a specific electronic characteristics out of a bulk SWNT material (usually requires coupled AFM, Raman, SEM technology steps), placing the selected SWNT onto a pre-fabricated electronic structure using a nanomanipulator (AFM-based technology), contacting (usually lithographic steps). (ii) dropping a diluted SWNT dispersion onto a prefabricated electronic structure, checking if SWNTs got deposited onto (between) the right contact structures, probing the electronic characteristics of these SWNTs (requires combined AFM (SPM) and Raman techniques), contacting (lithography steps). The limitations for both techniques refer to exhaustive selection and characterization processes for having the right SWNT at the right place. Alternatively, a third approach (iii) is to grow individual SWNTs in CVD processes directly in prefabricated channels of semi-conducting device structures into which catalyst seed particles are incorporated.52 Since no control of CNT characteristics during growth can be achieved yet, these device structures show only functionality at random places. Finally, it remains to mention the dream for purely nanotube-based electronic devices achieved when connecting SWNTs of different specific characteristics. While theoretically easily possible, again, limitations refer to efficient SWNT selection and assembly technologies. Further progress strongly relates to improved SWNT production technology as well as to efficient chemical separation and reliable assembly techniques. 4.2. ASSEMBLIES OF CARBON NANOTUBES

There are several electronic applications which are based on a more statistical approach using as grown assemblies of CNTs. Most prominent example are flat panel devices making use of the electron emission effect of CNTs. As mentioned in Section 3.2, CNTs can be grown in a pixel-like pattern on a conducting substrate.27–29 The micrometer-sized pixel areas contain millions of parallel aligned MWNTs. Placing a top-electrode and applying an electric field, statistically sufficient electron emission can be obtained (although some individual CNTs may fail) to fully satisfy conditions for building commercial flatscreen devices,53 and other devices based on field emission, such as X-Ray cathodes,16 and SEM/TEM tips.54 Aligned grown assemblies of CNTs also have been used for the development of various kinds of filter membranes55 for hydrocarbon separation and waste-water treatment. As a next step, CNT dispersions can be used to obtain statistical networks of CNTs. This can result in micrometer thick CNT networks (buckypapers) or even in ultra thin and even transparent networks of either conducting or semiconducting CNTs which can be coated onto flexible transparent substrates

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resulting in transparent (vis-IR) and conducting CNT film coatings.56 These further can be assembled into flexible and transparent thin film transistors57 maintaining their electrical performance even upon strong bending. CNT networks (properly functionalized) also serve as transducing material for gas58,59 and bio60 sensing applications as well as biological scaffold materials for cell proliferation.61 CNT dispersions also are used to produce CNT fibre materials by spinning technologies as base for the development of intelligent textiles.62,63 Incorporated into textiles they operate as electrochemical element using the proper sweat of the human body. They may act as supercapacitors (storage of electrochemical energy), detect molecules (sensors), change color (electrochromic device), actuate (close and open pores) and absorb shock-energy. 4.3. CARBON NANOTUBE BASED COMPOSITE MATERIALS

The development of high performance nanocomposite materials is one of the most promising fields for CNT applications impacting on a broad range of technological sectors. The basic idea is to incorporate nanotubes into a matrix material and transfer their unique properties to the host system resulting in a material with enhanced functional, structural and processing properties. Essentially two types of matrix materials are considered: Polymeric and ceramic materials. Since only few works have been reported in the field of ceramics, in the following we will focus only on the class of polymer-CNT composites. Here, most research has been performed in the fields of thermoplastics and thermosets (both of great technological relevance in daily applications) as well in the area of electroactive polymers (of increasing importance for the development of plastic electronics). Independent of the matrix system, there are two key issues of paramount importance to obtain highly functional CNT-based composite materials: (i) Homogeneous CNT dispersion in the host system contributing to achieve maximum stable separation of CNTs and to avoid CNT agglomerations thus resulting in a homogeneous composite material. (ii) Efficient transfer of CNT properties to the host matrix. The development of nanocomposites clearly is an issue at the interface where the formation of a proper interface layer acting as link between CNT and the rest of the host material determines the degree of CNT property transfer and thus the final characteristics of the whole composite material. The interface layer simply may lead to a reorganized polymer structure in the vicinity of the CNTs,64,65 enough to observe already some first significant changes in the overall materials characteristics, or, in the best cases, even allow highly favorable CNT property transfer leading to completely new materials characteristics.45 The development of a favorable interface layer impacting on the whole materials characteristics

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directly is linked to a high degree of CNT dispersion. Both, CNT dispersion and load-transfer are highly complex issues strongly depending on the type of CNT material and the host matrix system, both defining the way towards successful compounding. Latest at this point, it should become clear why a thorough knowledge on all aspects around CNTs, as mentioned in the former chapters is critical towards the development of functional CNT-based nanocomposites. Incorporation of nanotubes into plastics can potentially provide light structural materials with dramatically increased modulus and strength. Thermoplastics, such as polypropylene,66 polystyrene,67 as well as thermosets,68 such as epoxies are widely used for this purpose. While for low CNT loadings, typically in the region between 0.1 and 2 wt %, moderate improvements, due to the development of a proper interface layer, are observed (elastic moduli improvements in the range of a factor or 3–4, increase of thermal conductivity by a factor of 1%), at higher loading rates, agglomeration takes place and the developed materials get highly inhomogeneous. This indicates that far more work is needed to achieve mechanical properties closer to the ones of the proper CNTs. On the other hand, due to CNTs high aspect ratio and conducting (electrical and thermal) properties, conducting filler networks can be obtained already at very low percolation values. Ultra-low percolation thresholds in the range 0.001 and 0.01 wt % of CNT filler ratio (a factor of 1,000 loser than for spherical carbon black particles) have been observed to reach conductivity values around 0.1–1 S/cm.69 Therefore, the formation of percolated CNT networks in different kinds of polymer matrices for the development of lightweight materials able to dissipate electrostatic charge, to shield electromagnetic radiation, and to dissipate heat, is of increased technological interest for commercial applications. Finally, it has been shown that CNTs may interact in a very favorable way with electroactive polymer matrix systems, such as polyaniline, polythiophene, polypyrrole and respective derivatives. Their highly conjugated backbone structure with a highly delocalized ʌ-electron system is highly compatible with the CNTs extended ʌ-electron system leading to composite materials with drastically improved characteristics of special interest to optoelectronic applications. Composites of CNT/poly(p-phenylenevinylene) (CNT/PPV),70–72 CNT/poly(3,4-ethylenedioxythiophene) (CNT-PEDOT)73 have proven their functionality in light-emitting diodes and photovoltaic cells, CNT/polyaniline (CNT(PANI) have been used as printable conductors for thermal-imaging techniques.74 Especially when in-situ polymerization processes are carried out, i.e. polymerization in the presence of CNTs, as shown for CNT/PANI,45,75 highly synergetic effects can be obtained ranging from increased conductivity, thermal stability, deprotonation stability, optical activity, combined with improved processing characteristics. All these are highly promising results towards the development of future plastic electronic devices including improved flexible

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organic light emitting devices (OLEDs), photovoltaic cells, circuits, and sensing devices. Additionally, these electroactive polymer/CNT composites are of great interest to the development of energy storage devices, such as supercapacitors.76 5. Conclusions and Outlook In this article we have explained the unique properties of carbon nanotubes and shown that these are fundamental nanoscale objects of interdisciplinary, intersectorial interest and of great educative value. They are impacting on broad fields of science and technology. We have shown that carbon nanotubes as a material is highly complex and bears several challenging key issues ranging from proper characterization and purification to functionalization and the development of homogeneous stable dispersions and highly functional CNT-composite materials. While carbon nanotubes certainly will proof advantageous for certain applications, other ones probably will not develop at all. In any case, more than 15 years-time of research in this field has taught us that carbon nantoubes are highly enabling materials and good for many further surprises. ACKNOWLEDGEMENTS

The authors gratefully acknowledge funding from Regional Government of Aragon (DGA) under its “Group of Excellence” programme (DGA-T66). W.M. gratefully acknowledges funding from Spanish Ministry of Education and Science (MEC) and European Regional Development Fund (ERDF) under project NANOPOLICOND (MAT 2006-13167-C02-02). E.M. and A.M.B gratefully acknowledge support by MEC and ERDF under project SENAGAS (TEC200405098-C02-02). A.M.B. and M.T.M. gratefully acknowledge funding from DGA under Priority Research Line Projects (PIPO21/2005 and PIPO15/2005).

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ULTRASHORT PULSE PLD: A TECHNIQUE FOR NANOFILM FABRICATION T. SZÖRÉNYI1* AND Zs. GERETOVSZKY2 1 Research Group on Laser Physics, Hungarian Academy of Sciences, University of Szeged, PO Box 406, H-6701 Szeged, HUNGARY 2 Department of Optics and Quantum Electronics, University of Szeged, PO Box 406, H-6701 Szeged, HUNGARY

Abstract – In the present contribution the peculiarities of laser ablation are discussed with special emphasis on the differences in the mechanisms of nanoparticle formation when ablating materials with pulses of nanosecond vs. femtosecond duration. In the case of ablation using nanosecond pulses the dominating species leaving the target surface are principally atoms and ions. Cluster formation and growth mainly take place, via nucleation and condensation, from the plasma plume within the surroundings. The principal control parameter is the ambient pressure. When the major goal is not the production of colloids (either in form of an aerosol or a sol) but layer growth instead, nanostructured films can be made at pressures higher than a few pascals. On the other hand, ablation with ultrashort pulses produces a plasma plume of biphasic character: its leading edge, consisting of ionic and atomic components is followed by a spatially and temporally well separated cloud of nanoparticles. In this case nanoparticle formation is a direct consequence of the interaction of the ultrashort laser pulse with the target material. This process even works in high vacuum, which provides an additional proof for that here, contrary to nanosecond-ablation, those are the laser parameters that control the characteristics of the nanoparticles produced.

Keywords: Ablation, laser processing, clusters, nanoparticles, thin films

______ *

To whom correspondence should be addressed: T. Szörényi, email: [email protected]


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1. Pulsed Laser Ablation: General Characteristics In laser ablation the energy of the laser pulse is absorbed within a small volume of the surface layer of the target material and converted into heat almost instantaneously. The fast and confined heating builds up extreme pressures, induces phase transitions and leads to material ejection from the heated volume. While the technique is conceptually simple, the strongly nonlinear and far-fromequilibrium nature of the involved processes make it rather complex. Though being clearly an over-simplification, ablation is often envisaged as a sequence of steps, starting with the interaction of laser radiation with the target, followed by the absorption of energy and localised heating within a thin surface layer, phase transitions, and subsequent (very much forward directed) material ejection in form of a plume, which either expands into vacuum or gas, or may even interact with a liquid environment. Depending on the duration of the laser pulse and the nature of the environment, the properties of the components of the ablation plume may further change, due to collisions within the plume and/or between the constituents of both the plume and the ambient, in certain cases even through laser-plume interactions.1,2 Historically, production of nanoparticles by laser ablation dates back to the mid-1980s, when the first papers on fullerenes were published.3 For about 2 decades nanosecond lasers remained the only tools of nanoparticle production. In the third Millennium ultrashort pulse lasers moved to the forefront of laser ablation research, as well. Nevertheless, it became evident only very recently, that nanosecond and femtosecond ablation proceed along fundamentally different routes.4 In this contribution the peculiarities of nanoparticle production by laser ablation will be reviewed by following the life story of species ejected from the target while focusing on the differences between nanosecond vs. femtosecond processing. Special emphasis will be given to that particular setup in which the species are collected on a substrate, i.e. when we grow a layer. First a brief overview of the techniques available for in- and ex-situ analysis of the properties of the ablated species and their interaction within and outside the plume will be given. Then the characteristics of nanoparticle formation will be revealed by analyzing the results of selected case studies. 2. Analytical Toolbox Techniques – The Life Story of the Ablated Species The range of applicable analytical techniques is determined by the ambient the ablated species expand into. In vacuum or in a low pressure gas ambient in-situ plasma spectroscopies offer the most direct way to identify and characterize the just born species and their interaction with the surroundings during plume

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expansion. Gated intensified CCD photography is routinely used to record the time evolution of the spatial distribution of the plume. Differentiation between various plume components is possible by working in carefully chosen spectral domains. Once the spatial and temporal evolutions of the plume have been identified via imaging, optical emission spectroscopy can be used to identify and quantify species in their excited states in different locations within the plume. On the other hand, laser-induced fluorescence or optical absorption spectroscopy can probe the “dark” part of the plume, i.e. measure unexited atoms and molecules, along with small clusters. Scattering can also be used to reveal the presence of NPs. Rayleigh scattering imaging is effective for particles larger than just 2–10 nm, so it can be used to study nanoparticle growth. Blackbody radiation can also be imaged, or spectroscopically analysed to reveal the presence and temperature of NPs. Time-of-flight techniques are also routinely applied for spaceand time-resolved analysis of the dynamics of plasma species. In vacuum or relatively low pressures (up to several hundred pascals) the plume components can be collected for (in- and ex-situ) analyses and/or layers can be grown by immersing a substrate into the expanding plume. By following the process of film growth in-situ or by the ex-situ characterization of film properties, information on the characteristics of the ablation products can be derived. It is worth noting, that this technique offers a relatively simple, yet extremely versatile method of thin film production, also known as Pulsed Laser Deposition, PLD. Due to the popularity of PLD, gaining insight into the behaviour of plasma species by means of indirect film-based techniques has a well studied basis. Contrary to the plethora of techniques available in low pressure environments the analytical toolbox is rather empty in denser media. The main reason of that stems from the reduced mean free path at high pressures or in liquid ambients. In certain cases characterization of detonation waves, cavitation bubble formation by shadowgraphy does work, but in the general case the range of analytical techniques reduces to ex-situ characterization of the ablation products. Colloidal systems formed in liquids or in gas atmospheres at pressures above several hundred pascals can be analyzed using the standard techniques of nanoscience/nanotechnology. While stabilizing a nano-dispersed phase in an aerosol is rather problematic, gases offer the cleanest environment and therefore allow for the fabrication of nanoparticles of the highest purity. On the other hand, ablation into a liquid has the advantage of providing an easy means to hinder agglomeration and preserving the original size distribution of the individual nanoparticles via protecting their surface.

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3. Ablation with Nanosecond Pulses 3.1. SINGLE-WALL CARBON NANOTUBE SYNTHESIS BY LASER VAPORIZATION

Synthesis of carbon nanotubes by laser ablation is worth mentioning not only because it was the first method used to produce fullerenes in the gas phase,3 but also because this technique remained one of the best methods to grow high-quality, high-purity single wall carbon nanotubes, SWNT. A single laser pulse (of typically nanosecond duration) is sufficient to produce a vapor cloud of carbon and catalyst atoms, from which, inside a hot oven and in inert gas atmosphere, the species self-assemble to form SWNTs. The yield is high and the process is relatively insensitive to the diameter of the metal catalyst nanoparticle.5–7 SWNT can also be grown at process temperatures well below 1,000–1,100ºC, even at RT, where high-repetition-rate8 or long-pulse9 lasers provide the sufficient heating of the target and the plasma species. A necessary prerequisite of exploiting the full potential of carbon nanotube growth by laser evaporation is the understanding of the process of SWNT formation. The potential of spectroscopies in following the time evolution of the plasma processes has convincingly been demonstrated by the spatial and temporal mapping of SWNT synthesis in a series of papers from Dave Geohegan’s group at ORNL.10–12 From the results of these studies the timeline of the process has been constructed which depicts, in Figure 1, the birth of laser generated SWNTs. The plume initially consists of atomic and molecular species. Condensation of carbon starts within 0.2 ms after ablation. The small C clusters begin to aggregate and form swirling vortex rings appearing as smoke in the gas flow. As time passes the plasma temperature continuously decreases and the excited metal atoms relax into their ground states. In the next millisecond, also the ground state metal atoms condense, and so by t | 2 ms the plume becomes almost entirely composed of clusters, trapped within swirling smoke rings. This mixture of carbon clusters and metal catalyst nanoparticles forms the feedstock for nanotube formation. The growth is however a relatively long process. After 20 ms, only SWNTs of about 200 nm length are found. Growth up to 10 μm lengths occurs over much longer times – from 100 ms to s – indicating growth rates of 0.5–5 Pm/s. While the conclusion of general interest of these time-resolved growth studies is that SWNTs grow through the conversion of condensed-phase carbon clusters and metal catalyst nanoparticles, in the context of our analysis which aims to compare nanosecond ablation to femtosecond one, the main message is that the clusters, acting as the feedstock of SWNT growth, are formed from atomic species of the plasma by condensation in an inert ambient.


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Figure 1. The stages of SWNT formation from laser generated plasma as a function of time and plume temperature.11

3.2. THE ROLE OF PRESSURE IN DETERMINING GAS PHASE PROCESSES; FORMATION OF CARBON NANOCLUSTER FILMS

When the plume expands into a gas, the fate of its species is determined by the frequency and the nature of collisions with the surrounding atoms/molecules. Up to approximately 1 Pa the behaviour of the plume is essentially vacuumlike. With increasing background pressure the frequency of collisions increases, which reduces the initially high kinetic energy of the species. At sufficiently high pressures the expansion eventually stops, and the species become thermalized.12,13 Fast imaging techniques provide insights into the details of these events.14–16 In Figure 2 the effect of confinement is exemplified by comparing the position and shape of carbon plasma plumes in vacuum and in 20.5 Pa Ar. This latter pressure lies already within the pressure domain where the ablated atoms and ions have slowed down so much that cluster formation becomes possible. Now, the size of the clusters is jointly controlled by the laser parameters and the actual pressure. This pressure control of the kinetic energy of plasma species materializes in profound changes in the morphology of films built by collecting the ablated material on substrates.17–19 As an example, Figure 3 shows the evolution of the surface morphology of films deposited by ablating a graphite target with 6 J cm2 pulses of a KrF excimer laser (15 ns, 248 nm) in argon at different background pressures.18 At 0.67 Pa hard, scratch resistant films of smooth morphology are produced. As the Ar pressure increases to 5.32 Pa the films become much

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Figure 2. Wavelength filtered ICCD images of a carbon plume expanding into vacuum (a)–(c), and into 20.5 Pa Ar (d)–(f). Twenty nanoseconds gate with a delay after the laser pulse of 120 ns for images in the first column, 250 ns for the second column and 400 ns for the third column. The length of each frame is 30 mm.16

Figure 3. SEM images of carbon films deposited at Ar pressures of (a) 0.67, (b) 5.32, (c) 13.3 and (d) 45.2 Pa. Note the different scale bars.18

rougher due to nanoscale clustering. The cluster size derived from the SEM picture is approximately 15 ± 3 nm. At 13.3 Pa clustering becomes more pronounced, and leads to clusters of §45 nm in size. These layers can be easily scratched off, indicating much reduced hardness. By 45.2 Pa the film morphology changes from nanoscale clusters to more filamentary growth together with a further reduction in film density. The appearance of the material resembles very much that of the low density diamond-like carbon nanofoam produced by high repetition rate Nd:YAG laser in argon atmospheres of pressures exceeding §13 Pa.20 When decorated by surface-enhanced Raman-active components,


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e.g. metal nanoparticles, such large surface area carbon nanoscaffolds may serve as efficient, low-cost sensor platforms.21 The evolution of film morphology with pressure is very similar in different materials systems, though with different threshold values depending on the material ablated and the nature of the ambient gas used.22 Since the changes in growth mechanism materialize in the variation of all film properties, from the proper analysis of these, information on the underlying mechanisms can also be derived. The comparative analysis of the growth characteristics of carbon and carbon nitride films fabricated by ablating identical graphite targets with an ArF excimer laser (Ȝ = 193 nm) in the same configuration in Ar and N2 atmospheres, respectively, is another example for such a film-based approach.23 The pressure dependence of the apparent growth rates, defined as measured film thickness per number of pulses, is shown in Figure 4. Up to approximately 0.5 Pa the nature of the atmosphere has no effect on the growth rate. It is still in line with the expectations, that in Ar the growth rate drops between 0.5 and 5 Pa. The unexpected increase between 5 and 100 Pa, however, pinpoints a change in the growth mechanism. When ablating in N2, the domain where the increase of pressure has apparently no influence on the growth rate extends up to approximately 50 Pa, suggesting contribution of more than one process to film growth. In the changes in the apparent growth rate both the variation of the number of the constituting atoms and the changes in film microstructure manifest themselves. The two contributions can be separated by recording the change in the number of constituting atoms deposited over unit film area per pulse, within the same pressure domain (Figure 5). A close-up of the change in carbon growth rate in Ar atmosphere reveals that the decrease in the apparent growth rate between 0.5 and 5 Pa, shown in Figure 4, is a consequence of the decrease in the number of carbon atoms reaching the substrate due to collisions with background gas particles. However, the threefold decrease in the number of film building atoms from 0.6 to 0.2 × 1015 atoms cm–2 pulse–1 is only accompanied by an approximately twofold decrease in the apparent growth rate, suggesting a concomitant decrease in the compactness of the films at higher pressures. In the 5–100 Pa domain the carbon growth rate keeps decreasing, while the apparent growth rate increases, revealing that the decrease in film density accelerates. In nitrogen atmospheres, up to approximately 5 Pa, the incorporation of nitrogen nearly compensates for the loss in carbon deposition, resulting in a practically constant apparent growth rate. When exceeding 5 Pa this trend discontinues. The number of carbon atoms incorporated into the films keeps decreasing while that of nitrogen atoms starts to decrease, as well. While the


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dependence of carbon and nitrogen deposition rates on N2 pressure accounts for the practically constant N/C ~ 0.35 ratios measured in the 5–50 Pa domain,24 it can not any further account for the unchanged apparent growth rate (cf. Figure 4).

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Figure 4. The apparent growth rate of carbon (Ƒ) and carbon nitride (Ŷ) films as a function of pressure, fabricated by ablating identical graphite targets in Ar and N2 atmospheres, respectively.

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Figure 5. The deposition rate of carbon (Ƒ and Ŷ) and nitrogen (ż) atoms in carbon (Ƒ) and carbon nitride (Ŷ and ż) films, as a function of pressure.

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Figure 6. The density of carbon (Ƒ) and carbon nitride (Ŷ and ż) films as a function of pressure.

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The comparative analysis reveals that the pressure dependence of the absolute number of film constituting atoms (Figure 5) alone can not explain the variations in the apparent growth rates (Figure 4). The mass density vs. ambient pressure plots, derived by dividing the sum of the mass of all constituting atoms over unit area by the measured thickness, shown in Figure 6, display similar dependences for both carbon and carbon nitride, and indicate profound structural changes, indeed. In particular, these results confirm that the macroscopic density of carbon nitride films starts to decrease when the composition exceeds N/C ~ 0.2.25–28 The sudden decrease in mass density above 5 Pa explains why practically no change has been recorded in the apparent deposition rate in the 5–50 Pa domain (Figure 4) in spite of the continuous decrease in both C and N arrival rates (Figure 5). Atomic force microscope17,29,30 and TEM31 studies (Figure 7) verified, that the decrease in mass density of both DLC and carbon nitride films was a direct consequence of increased surface roughness and porosity.

Figure 7. TEM micrographs taken on films deposited with 10 J cm–2 pulses in 5 Pa N2 (left) and with 7.5 J cm–2 pulses in 50 Pa N2 (right). Each picture is 1 × 1 ȝm2 in size.31

The above results confirm that in the particular case of carbon and carbon nitride film growth the critical pressure is at around 5 Pa, indeed. Cluster growth starts at pressures above 5 Pa due to gas-phase collisions of carbon atoms/ions. The thermalized species build a layer of loose, porous network when reaching a room temperature substrate. The corollary of these and any other studies alike is that the primary products of nanosecond-ablation are atoms and ions. Cluster formation is the consequence of the interaction of the expanding plume and the ambient gas species. In inert atmospheres, the contribution of atomic vs. cluster growth to film formation is controlled by pure mechanics, i.e. by the number of collisions.


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Increasing pressure promotes cluster formation. When ablating in reactive atmospheres, a chemical effect should also be added to the kinetics: the collisions may result in formation of species composed of components of both the plasma and the atmosphere. 3.3. A UNIQUE APPROACH: BACKWARD DEPOSITION OF LASER GENERATED NANOCLUSTERS

As the mean free path of plasma species at pressures higher than a few pascals falls below a centimetre, in the absence of a forced gas flow the nanoclusters formed remain in the vicinity of the source, i.e. close to the ablation spot. A genuine approach is the collection of backward-propagating clusters on substrates placed in the target plane near to the ablated area.32,33 By tuning the laser parameters and the nature and pressure of the surrounding gas, nanoparticles of a great variety of sizes can be produced. The size distribution of nanoclusters deposited by ablating a Si wafer in He (Figure 8) illustrates well the potential of this geometry.

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Figure 8. Size distribution of Si nanoclusters deposited in He atmosphere at 532 Pa at various energy densities of the ablating ArF laser pulses.32

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3.4. NANOPARTICLE PRODUCTION BY ABLATING UNDER LIQUIDS

Nanoparticles intended for chemical- and bio-applications in liquid environments should (i) be directly formed in the particular liquid most appropriate for the planned application, (ii) possess a relatively narrow size distribution, (iii) be free of surface impurities, and (iv) have reactive chemical groups on their


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surfaces to simplify further attachment of (bio)molecules, if necessary.34 Backed by 100 years of experience colloid chemistry can produce a wide spectrum of nanoparticles, partly meeting these requirements. The peculiarity of ablation is that performed in purified and filtered deionized water a great variety of ultrapure nanomaterials can be produced, and after being in the solution, surface engineering of the nanoparticles becomes possible. Since systematic studies on the physical aspects of ablation in liquid environment have only been carried out for about 7 years, the full potential and the limitations of this technique is far from being explored. Technical problems like the formation of relatively long-living (300–500 ȝs) cavitation bubbles, absorption of laser light by and reflection on the nanoparticles in the liquid, self-focusing (time dependent) inhomogeneities in the refractive index limit the possibilities of following the events in-situ and make the analysis and description of the complicated processes involved rather hard. The state-of-the-art is the following: When ablating with nanosecond pulses in chemically inert liquids (e.g. in pure water), the size of the nanoparticles produced is relatively large, since coagulation and aggregation can hardly be overcome. In practice strongly dispersed particles of 10–300 nm mean diameter have been obtained with limited control by the laser parameters.34 When ablating in chemically active solutions chemistry helps to stabilize the primary nanoparticles, which leads to smaller mean diameters and narrower size distributions, as compared to the inert liquid case. Significant progress has been achieved by the use of aqueous solutions of surfactants. The surfactant covers the just born NPs and prevents them from agglomeration. Sodium dodecyl sulfate (SDS) proved to be one of the most efficient surfactants to stabilize gold and silver nanoparticles at around 5 and 12 nm mean sizes (Figure 9), respectively, in best cases.35–37 3.5. NANOPARTICLES BY NANOSECOND-ABLATION – SUMMARY

When ablating with pulses of nanosecond duration the material leaves the target in form of atoms, ions, and to a much less extent as small clusters. To generate nanoparticles, an environment of relatively high pressure is a prerequisite in order to create an efficient confinement of the plasma and increase its density. The formation and the evolution of the properties of the nanoparticles are essentially determined by the interaction of ejected (atomic) species and the environment. When ablating in gases, the key parameter is the pressure. Here the material can also be collected on substrates, leading to the formation of nanostructured films. Ablation in a liquid ambient results in the formation of colloidal nanoparticle solutions, but small sizes are available only when further chemical treatments are applied.

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Figure 9. Electron micrograph and the derived size distribution of silver nanoparticles produced by ablation with the second harmonic (532 nm) of a Nd:YAG laser in 0.05 M SDS aqueous solution.35

4. Ablation with Ultrashort Pulses 4.1. INTRODUCTION

In the past decade, femtosecond lasers emerged as unique tools allowing processing of practically any material with unprecedented precision. The advantages of ultrashort pulse processing over conventional laser machining with pulses of nanosecond duration originate in the 4–6 orders of magnitude shorter timescale of energy deposition. The energy injected into the material within picoseconds is absorbed instantaneously by the electrons of a very thin surface layer (skin depth), while the transfer of energy to the lattice takes longer time, typically tens of picoseconds. Although the details of the interaction are different for metals and dielectrics, the net results are the same: clean removal of a small, but well defined volume of material with minimal thermal load to the surrounding material. Historically, motivated by the interest of industrial applications which required high-precision patterning, most efforts have been devoted to the optimization of material removal with much less attention to the properties of the material removed.38 As a consequence, the potentials of femtosecond ablation in nanoparticle production remained hidden until very recently. Ultrashort pulse lasers produce a hot, highly ionized plasma plume that expands in a well-defined fashion and serves as an excellent source for thin film deposition. The plume does not contain micron-sized particulates and droplets,


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which is often the case when ablating with nanosecond lasers, enabling thereby the production of particulate-free films. The high intensity pulses produce plumes with large concentrations of ionic species with high kinetic energies (>100 eV) which has been considered beneficial e.g. in the production of high quality diamond-like carbon films.39 However, from the data available to date, one may conclude that ablation with high intensity femtosecond pulses does not necessarily lead to improvement in film properties Instead of striving to achieve maximum intensities, ablation with pulses of lower intensities, conveniently attainable with commercial femtosecond systems, seems to be a more pragmatic approach.40 Moreover, recent results revealed that plasmas generated in low pressure environments by ultrashort pulses having intensities not far above ablation threshold were ideal sources of nanoparticles.41–46 Following the same outline we used to present the peculiarities of nanosecond ablation, in this second part we will highlight those features of the ultrashort ablation process which differ most from that of the nanosecond case. Our approach will again address both the characteristics of the plume, and the properties of the ablation products. This will be achieved by summarizing the results of selected case studies, rather than trying to give a full coverage of all aspects of ablation with ultrashort pulse lasers. 4.2. ABLATION IN VACUUM: PLUME PROPERTIES

In a series of papers published in the last few years S. Amoruso and coworkers convincingly demonstrated that – contrary to the nanosecond case where the material leaves the target surface in a single package – in the plasma generated by ultrashort pulses of intensities within the 1011–1013 W cm–2 domain three populations could be identified, each of which is characterized by different expansion dynamics.41–43 Snapshots of a silicon plume and especially the intensity profiles derived by integrating the emission along directions parallel to the target surface, and shown on the right of the images (Figure 10), clearly prove the presence of three populations in the expanding plasma. From the distances the two fast components traveled in 30 ns a decent estimate can be given to their expansion velocities, while for the determination of the velocity of the third, slow component, which remains very close to the target surface on this timescale, delays of the order of microseconds are necessary (Figure 11). The expansion velocities of the three populations derived from these images are ~107, ~106 and ~5 × 104 cms–1, respectively. Spectral analysis has shown that the fastest population consists of ions, while the second component of the plasma contains both neutral and ionized species. At time delays below 0.5 ȝs the spectra, shown here for the particular

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Figure 10. The early stage of expansion of a silicon plume produced by ablation with 0.9 ps @1,055 nm pulses of 8.4 × 1011 W cm–2 intensity into vacuum. Gate time: 10 ns.41

Figure 11. Evolution of the slow component of a silicon plume produced by ablation with 0.9 ps @1,055 nm pulses of 8.4 × 1011 W cm-2 intensity into vacuum. Gate times : 1 ȝs at 5.5 ȝs and 5 ȝs at 22.5 ȝs delays, respectively.41

case of Au (Figure 12), are dominated by emissions from atoms. At longer delays, however, structureless, broad black-body spectra appear, indicating the arrival of hot nanoparticles. Detailed analysis of the spectra yields initial temperatures in the order of few thousands Kelvin, continuously decreasing with time due to radiative cooling.42,44,45 The comparison of results obtained on ablating Si, Ni, Fe, Au, Ag, Ti and TiC with pulses of different durations (80–900 fs) and intensities (1011–2 × 1013 W cm–2) suggests that the behaviour of the plasma, outlined above, represent a common, general feature of femtosecond-ablation, at least within the parameter window examined.41–46


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Figure 12. Time evolution of the emission spectra of a gold plume recorded 1 mm above the target surface. Ablation with 120 fs pulses of 5 × 1012 W cm–2 intensity of a Ti:sapphire laser @780 in vacuum.42

4.3. NANOPARTICLE FABRICATION IN VACUUM

Similar to the behaviour of the plasma, the size and the size distribution of the nanoparticles produced from different materials are akin, as well. AFM analysis of nanoparticle films of Ag, Au, Ni, Si and Fe of less than a monolayer coverage yielded mean nanoparticle radii ranging from 8 to 25 nm with standard deviations between 5 and 20 nm, as illustrated for the case of Ag in Figure 13.

Figure 13. Left: AFM image of nanoparticles deposited on mica by ablating a silver target with 120 fs pulses of 5.0 × 1012 W cm–2 intensity in high vacuum. Right: size histogram.42

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Size control of nanoparticles is possible by tuning the laser parameters. An increase in intensity within the 1012–1013 W cm–2 range leads to a slight increase in the mean radii.42 The relatively broad size distribution, characteristic of ablation in the visible, can be narrowed by using UV wavelengths.47 The increase of pulse duration up until the electron-phonon relaxation time, Ĳe-ph of the material to be ablated has apparently no significant effect on the ablation products. There was very little difference in the morphology of Ag (Ĳe-ph ~20 ps) and Ni (Ĳe-ph ~7 ps) films deposited in high vacuum using 10 ps and 200 fs pulses.48 In both cases the films comprised of almost spherical nanoparticles of 50–300 nm in diameter, with apparently more aggregation when ablating with 10 ps pulses. Nevertheless, more experimental work is required to completely characterise the ablation and deposition processes at different pulse durations. In the light of potential future applications it is important to note, that synthesis of nanocrystals of different materials is also possible with this technique. An example: The analysis of X-ray diffraction patterns of Ni and Fe nanofilms indicated that the films consisted of crystallites with sizes of few tens of nanometers, whereas AFM imaging gave exactly the same nanoparticle sizes, suggesting that each and every particle has the same single-crystalline domain, i.e. the films are composed of randomly oriented, but identical nanocrystals.49 The analysis of the magnetic behaviour of Ni nanoparticle films led to the conclusion that the film behaved as a system of isolated magnetic particles, i.e. the individual nanocrystals of 40 ± 19 nm size approached fairly well the ideal, single-domain behaviour.50 4.4. PLUME EXPANSION IN A REACTIVE GAS. A POSSIBLE ROUTE TO COATED NANOPARTICLE FABRICATION

The differences in the mechanisms and the products of ablation performed with pulses of nanosecond vs. femtosecond duration have, to the first glimpse, surprising consequences when performing the experiments in reactive atomspheres. The synthesis of carbon nitride is a good example. The prediction of the unique properties of the ȕ-C3N4 phase by Liu and Cohen in the late 1980s51 initiated tremendous efforts to synthesize this “magic” compound. By ablating carbonaceous targets in nitrogen containing atmospheres carbon nitrides, CNx with × 0.7, i.e. well below its nominal 57 atm % N content could only be obtained.52,53 There were speculations that when producing more energetic carbon species the efficiency of CN formation could be promoted. The experiments performed using femtosecond lasers had puzzling results: Films with maximum nitrogen contents less than 15 atm %, i.e. much lower than those fabricated using nanosecond pulses, could be produced.54,55


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Recalling that compound formation materializes as a result of collisions of the carbon plasma with the N2 molecules, it is plausible that the highest nitrogen content can be reached when single C atoms/ions meet N2 molecules or (excited) nitrogen atoms. The interaction of C clusters with nitrogen results in a compound with smaller N content. Meeting of carbon nanoparticles and nitrogen species is the worst case scenario in terms of carbon nitride chemistry. The differences in the plasmas produced by nanosecond and femtosecond pulses therefore explain the apparently surprising result. While the ablation of carbonaceous targets with femtosecond pulses can not produce species rich in N, it may well be that their nitrogen content originates from the outer surface of the nanoparticles. If that is the case, ablation by femtosecond pulses in reactive atmospheres could be a dedicated technique for the production of core-shell nanoparticles. 4.5. SOLID NANOPARTICLE FILM FROM A LIQUID TARGET

Very recently Szörényi and coworkers fabricated Si-doped amorphous carbon nanofilms by ablating a commercial silicone oil with extremely clean 700 fs @248 nm pulses of 4–5 × 1011 W cm–2 intensity in high vacuum.56 AFM and SEM unambiguously proved that the surface of the films, deposited between RT and 250ºC, consisted of nanoparticles (Figure 14). The formation of nanoparticles suggests that the ablation mechanism of a liquid target might be very similar to that of a solid one. The results of a parallel TEM study confirmed that on >100 nm-scale the films were morphologically inhomogeneous, indeed (left panel of Figure 15).

Figure 14. a-C:Si nanoparticle films deposited onto silicon substrate at RT (left) and 200ºC (right) by ablating silicone oil with 700 fs @248 nm pulses of 12.2 and 14.7 mJ total energies, respectively, focused onto 4.1 mm2 areas.

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Figure 15. Top-view HRTEM images of a free standing a-C:Si film. The blurred contrast is due to defocus and noise filtering.

Even at the highest magnification used (right panel of Figure 15) no ordering could be detected, revealing that at the nanoscale the material is homogeneous and isotropic, possessing a perfectly amorphous structure. 4.6. ABLATION IN VACUUM. EXPERIMENT VERSUS THEORY

The results analysed above suggest that ablation with ultrashort laser pulses is characterized by a number of general features, independently of the nature of the target material. In particular, ablation at intensities not much higher than the plasma formation threshold, more specifically within the 3 × 1011 – 2 × 1013 W cm–2 domain, inevitably leads to the generation of nanoparticles of the target material. This characteristic seems to be true not only for elemental, but also for multicomponent (e.g. binary or even more complex) materials. These general features are consistent with the physical picture of the process, as outlined by the predictions of recent theoretical analyses.57,58 At laser intensities in the range of 1012–1013 W cm–2, corresponding to initial temperatures of a few electron volts, the (nearly) adiabatic cooling drives the material into a metastable phase, and results in the production of a relatively large fraction of nanoparticles through phase decomposition processes. On the other hand, at larger intensities (>1014 W cm–2) the material can never reach the metastable phase, resulting in an almost fully atomized plume. Thus the most promising laser intensity range in the context of nanoparticle production is theoretically predicted to be 1012–1013 W cm–2. Though further comparative studies are inevitably necessary for judging how general this statement could


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be, the intensity values predicted by the theoretical approaches turn out to be in very good quantitative agreement with the limited experimental works’ that led to an efficient generation of nanoparticles. 4.7. NANOPARTICLE PRODUCTION IN LIQUID ENVIRONMENT

As it was shown in the case of nanosecond ablation, ablation in liquids offers a clean and efficient way for colloidal nanoparticle production. Now, the limited number of studies and the immature state of femtosecond ablation in liquid ambient do not allow us to draw solid and general conclusions.34,59 One can state though, that with femtosecond pulses – as compared to the nanosecond case – nanoparticles with smaller mean sizes and lower dispersion can be produced, but at much lower production yield.34,59–61 It sounds feasible that in liquid environment two ablation mechanisms compete, depending on the intensity.59,61 The first mechanism manifests itself at relatively low intensities (e.g. at I < 3.6 × 1015 W/cm–2 in the case of ablation of a gold target in pure deionized water) and produces almost monodisperse gold colloids of very small (~3–10 nm) sizes, as shown in the inset of Figure 16. The second process becomes active at high intensities and produces particles of much larger size and broader size distribution.

Figure 16. Size distribution of gold nanoparticles prepared by ablating a gold target under deionized water with 110 fs pulses of 5.5 × 1014 W cm–2 intensity.61 Inset shows a respective TEM micrograph.

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Similarly to the nanosecond case, in chemically active liquids enhanced stabilization can be established and leads to more efficient control of nanoparticle size.62,63 The first results are quite impressive and support the assumption that ablation with ultrashort pulses in both low pressure and liquid ambients could be a competitive approach to nanoparticle production. 5. Conclusions-Outlook Laser ablation is a general and practical route for the fabrication of nanoparticles and nanoparticle films of both elemental and multicomponent materials in vacuum, gas or liquid environments. From the point of view of the ablation products the principal difference between ablation with traditional, nanosecond and ultrashort, femtosecond lasers is that while in the former case a condensed ambient is indispensable for nanoparticle production (either as a liquid or in the form of a gas atmosphere at pressures above approximately 1 Pa), in the latter one this can even be accomplished under vacuum conditions. In the case of nanosecond ablation the dominating species leaving the target surface are atoms and ions, and cluster formation and growth take place in the plume. When ablating with femtosecond pulses of intensities not far above the threshold of plasma formation, the ablated material leaves the target in form of nanoparticles, i.e. ablation with femtosecond pulses directly produces nanoparticles and therefore independent from the nature and the pressure of the ambient. The properties of nanoparticles can be further tuned by choosing an appropriate environment, making in-situ engineering possible, and thereby opening up the way to produce more exotic (e.g. coated or functionalized) nanoparticles.

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ULTRASHORT PULSE PLD 60. 61. 62. 63.

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LASER ABLATION AND LASER INDUCED PLASMAS FOR NANOMACHINING AND MATERIAL ANALYSIS D. BATANI* Università di Milano Bicocca, ITALY

Abstract – The goal of this paper is to introduce the basics of laser ablation. After introducing some general concepts for the description of lasers, and their usefulness in laser micromachining, and of the materials with which they interact, we describe in more details the femtosecond and nanosecond pulse duration regimes.

Keywords: Laser ablation, spatial coherence, ablation threshold flux, plasma frequency, Spitzer’s law, hydrodynamics, collisional absorption, crater formation

1. Introduction Soon after the invention of lasers, these systems begun to be used for applications in machining. Initially CO2 lasers were successfully used for efficient laser cutting of metals (and other materials) in various shapes. CO2 lasers are still probably the most used system at the industrial level. Laser cutting is nowadays so used that it deserved a voice in Wikipedia. More recently lasers have been used in applications like micro-hole drilling, creation of surface structures, introduction of calibrated leaks in pharmaceutical packaging (so to have a prolonged and controlled release of medicaments), selective material removal (e.g. polymer jacket from optical fibers), engraving for code marking or decoration, 2.5D micro milling (i.e. creation of structures in depth in materials), 3D engraving of glass and other transparent materials, laser cleaning in art conservation, guaranteeing high precision and high reproducibility in all cases.

______ *

To whom correspondence should be addressed: D. Batani, email: [email protected]


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Advantages of using lasers include economic advantages (reduction of work times; increase in production quality) but above all technological advantages: laser machining is precise, clean and silent; the beam can be focused on extremely small areas; the area in proximity to the edge may have very low heat alteration; the laser cut has the capacity of operating on complex profiles and with very small rays of curvature. Also, the laser is a non-contact instrument that guarantees absence of mechanical pressure on the piece (unlike water and traditional cutting systems); working capability independent of hardness of the material; capability of cutting coated or surface treated materials; no contact contamination of materials; no wear on the laser (which affects precision) Laser cutting also has a high degree of automation and flexibility able to offer ease of integration with other automated systems; and capability of adapting immediately to changes in production requirements. Finally, in many cases, laser cutting can produce finished pieces that do not require further processing. All these applications of lasers are based on ablation i.e. the capability to remove matter from a substrate by irradiating it with sufficiently high intense radiation. Focusing a high intensity laser on a solid target brings many effects: (1) ablation (removal) of material (hole drilling, micromachining, surface modifications), (2) emission of radiation (this can be used for plasma and material diagnostics but also for developing pulsed radiation sources, in the XUV and soft X-ray range), and (3) redeposition of the ablated material on a substrate (thin film production, PLD, pulsed laser deposition). In this paper, I will present only the first point and describe the basics of laser ablation in the femtosecond and nanosecond pulse duration regime. For PLD please see the contribution by J. Schou in this book. 2. The Laser System Why are lasers good for micromachining? This depends on characteristics of laser vs. “normal” light. Usual light is characterized by: low directionality, low mono-chromaticity, low coherence, low power (thermal source). Laser light is instead characterized by: high directionality, high mono-chromaticity, high degree of spatial coherence, high power (being non-thermal sources, lasers are not constrained by Kirchoff’s law, i.e. the emitted power may exceed the black body limit at a given wavelength). As it is well known, the peculiarities of laser light comes from two “ingredients”, i.e. (i) the different emission mechanism (stimulated vs. spontaneous emission), and (ii) the role of the optical cavity. Lasers appear as an ideal tool for concentrating energy in space and in time thereby realizing huge irradiation intensities with little energy, and higher intensities imply larger effects on materials. The intensity on target is given by

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I(W/cm2) = E(J) / (S(cm2) W(s))

(1)

where E is the laser pulse energy, S the focal spot area and W the pulse duration. Focusability is related to spatial coherence and hence to beam quality: Lasers are spatially coherent therefore the beam can be focused to a very small spot. This implies a precise cut but also saving of energy because intensity is inversely proportional to focal spot area. Due to diffraction, focal spot diameter decreases with the wavelength O. With perfect beams and aberration-free focusing optics, we get d = 2.44 O f / D = 2.44 O/F#

(2)

where F# = D/f (the constant 2.44 holds is flat-top laser beam profile in the near field, otherwise a different number appears, always of the order of unity). Provided their optical quality is good, short wavelengths offer the possibility of improved resolution and also of allowing higher intensities on target I v (F#/O)2

(3)

The other way of increasing laser intensity is by reducing the pulse duration W. With this respect after the introduction of Q-switching and mode-locking in the 1970s, which allowed pulses in the picosecond-regime to be produced, the recent breakthrough is the well known introduction of the Chirped Pulse Amplification (CPA) technique1 in the 1990s, which has allowed the generation of laser intensities as high as 1021 W/cm2. Such values are of course not directly interesting for laser-ablation and here we will limit ourselves to much lower irradiation values. In any case CPA has made possible to extend laser ablation to the femtosecond-pulse duration regime. TABLE 1. Laser parameters, in red the useful range for laser ablation.

Wavelength Repetition frequency Average power Energy per pulse Pulse duration Peak power Intensity (irradiance)

Visible, near IR, near UV CW, 100 MHz, MHz, kHz , Hz, single shot MW, W, kW pJ, nJ, mJ, J, kJ μs, ns, ps, fs MW, GW, TW mW/cm2, 109, 1016, 1021 W/cm2

O Q P E = P/Q W Po = E/W I = P/S

Several parameters are useful for a discussion of laser performance in ablation, and in all cases the variation extends over several orders of magnitude: see Table 1 (in red the useful range for laser ablation). As for the choice of the laser-type (i.e. the active medium and the pumping tool) the systems, which are more largely used in today’s laser ablation and laser micromachining, are gas

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lasers (especially excimers due to their high efficiency and short wavelength), and solid-state lasers (Nd:glass, Nd:YAG and Nd:YLF and Ti:Sa for the femtosecond regime). For such brief discussion, it appears that the “ideal” laser for ablation and micromachining should have the following characteristics: high beam quality; short wavelength; short pulse duration; high energy per pulse (and high absorption); high repetition frequency; and finally high conversion efficiency (conversion from plug-in to laser energy which implies reduced costs). 3. The Material After describing the laser, let’s now present the other principal actor of laser ablation, i.e. the material. First of all, we must notice how ablation is the removal of matter from the target (i.e. of atoms). But atoms are heavy and inertial so that laser light preferentially interacts with the electrons in the material, while the atoms are heated later due to energy exchange processes. Therefore first we need to characterize the electrons in the materials (Figure 1) and their interaction with light, and then the energy transfer to ions.

Figure 1. Electronic structure in the material.

Let’s first consider the case of metals (we will briefly speak about the case of insulators in next section). As it is well known, electrons are packed in the conduction band and only electrons at the top of the band can effectively exchange energy (the others being constrained by Pauli’s exclusion principle). Electrons in the conduction band are characterized by the value of the Fermi Energy. In practical units EF (eV) § 3 10–7 (ne(cm–3))2/3

(4)

where ne is the density of free electrons in the conduction band. Usually EF is a few electron volts, for instance for aluminum it is 11.63 eV, for copper 7 eV.

LASER ABLATION

149

Another important value is the extraction work ), i.e. the energy difference between the most energetic electrons in the conduction band and the continuum. As for the atoms in the material, they are bonded to the lattice with energies which are characteristics of the materials, but typically Ebond § 1 eV to a few electron volts. This defines an ablation (evaporation) heat: :§ (ni Ebond) = U NA Ebond/A

(5)

where ni is the atom (ion) density in the material, U the material density, NA§ 6 1023 the Avogadro number, A the atomic weight of the material. For instance in the case of Al, U/A§0.1 and assuming Ebond§1eV =1.6 10–19 J we get :§9,000 J/cm3. The corresponding value per unit mass is :U§ 3,300 J/g or, in general :U§ NA Ebond/A. Let’s also notice that the density of ions an free electrons in the material are related by ne§Z* ni, where Z* is the ionization degree or the average number of electrons in the conduction band per ion. Finally we will need to describe thermal conduction in the material and its own time scale Wth. Since Wth is finite and, as we will see, of the order of a few 10 ps, we can already infer that a fast deposition of laser energy (short pulse W, femto and picoseconds) will not allow spreading and penetration of energy to large volumes. In this regime therefore high intensity ablation (I > 1013 W/cm2) will be characterized by direct evaporation of the matter and negligible thermal effects to the surrounding material, resulting in high precision. On the contrary, for very long pulses (microseconds or sub-microseconds) ablation will take place at low laser intensities and evaporation will be accompanied by fusion of material resulting in low precision. To this we must add the fact that in general short pulse lasers are also characterized by a better beam quality, as compared to longer pulse ones. 4. Laser Ablation in the Femtosecond Regime Although of course laser ablation in the femtosecond regime is recent, follow-ing the introduction of femtosecond lasers, we start by describing it because it is a clearer process. We will also see how the description also applies to the few picosecond regime. Laser ablation is a multi step process. First, energy is deposited in a region of thickness G (which, as we’ll see soon, can be identified with the skin depth) being absorbed by the free electrons in the material (electrons in the conduction band). Second, energy is transported by electrons to a thickness l (electron thermal conduction, diffusion process). Third, we have the interaction with the atomic lattice, heating of the atoms, breaking of the chemical bonds, and finally ablation of the material and the formation of a plasma plume. In order for this third step to take place, after thermalisation, ion energy must exceed the bond-

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ing energy: this condition can be expressed as CiTi > U: where classically Ci = 3/2ni with number of atoms per unit volume (ionic density). The steps are qualitatively illustrated in Figure 2. In the following we will illustrate the basic physical mechanisms determining the depths G and l. Before that, however, let’s notice one important point. Let’s assume that energy is deposited with an exponential profile in the material with a typical scale-length l, i.e. the energy flux (energy per unit surface) is F (J/cm2) = Fo exp (-x/ l)

Figure 2. Schematic of the ablation process. Energy is absorbed to a thickness G and then diffuses to a larger penetration depth l.

Then the energy deposited between xo and xo + dx over a surface S is

dE S

dF dx dx x x 0

S Fo exp(xo / l )

dx l

(6)

For ablation to take place, this deposited energy this must exceed the ablation heat U: in the volume dV = S dx

dE dV

§x · Fo exp¨ ¸t U: l ©l ¹

(7)

The distance x = L at which the two quantities are equal defines the depth of ablated material. By solving for L we get

§F · § F · L l ln¨ o ¸ l ln¨ o ¸ ©U:l ¹ ©Fth ¹

(8)

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151

where the quantity Fth (J/cm2) = U:l is the threshold flux for ablation. This must be corrected for the absorption coefficient of laser light A = 1-R, so that Fth = U:l/A. Let’s notice two important consequences of the logarithmic dependence of L over Fo: (1) Below Fth there is no ablation, but L = l when the logarithm is 1, i.e. when Fo = e Fth § 3 Fth. (2) If you are working with a high repetition frequency laser and above the threshold flux, increasing the inward energy flux Fo (therefore the laser inten-sity I) is not advantageous. Indeed with N shots at a given laser intensity I, the total ablation depth is §NL, while if we increase the intensity N times, the ablation depth only increases as ln(N) (Moreover increasing the intensity may increase unwanted “collateral” effects). Let’s finally observe that this model is 1D. However neglecting 2D dynamics and flows and assuming a radial Gaussian profile of the energy source we can also use the above law in order to derive an equation for the radial size of the ablated region:

F(r) Fo exp(r2 / 2wo2 ) t Fth §F · 2wo2 ln¨ o ¸ ©Fth ¹

r2

From which

(9) (10)

4.1. THE OPTICAL PENETRATION DEPTH G

The initial distance G over which the laser energy is absorbed is determined by the penetration of light in the material, i.e. for a metal, metal, the skin depth. The free electrons in the conduction band form a plasma with electronic density ne = Z* ni and the dispersion relation of the electromagnetic wave (laser) in the plasma, in the collisionless approximation is

Z 2 Z 2p c 2 k 2 4Snee 2 me

Zp

(11) (12)

where Zp is the plasma frequency, which defines the plasma refractive index

n

1

Z 2p Z

2

n 1 e nc

(13)

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where nc is the critical density at which the (angular) frequency of laser light equals the plasma frequency. In practical units Zp and nc are given by

Z p (Hz) 5.64 10 4 ne (cm3 ) nc (cm3 )

1.1 10 21 ( O(Pm))2

(14) (15)

Usually Zp !!Z and then the refraction index n becomes imaginary. Therefore the wave is evanescent in the material, penetrating only over a skin depth G § c/Zp For instance for Al, Z* § 3 and ni § 6 1022 cm–3. Then Zp § 2.4 1016 Hz and G § 12 nm. In reality, in a real case, the plasma will be collisional (even strongly collisional) and the plasma frequency may not be much higher than the laser frequency. Therefore determining the skin depth may become a complex problem. Finally, from G, we can calculate the average energy in the initial plasma contained in the volume ʌwo2G, which is

Ee

A EL 2 (Swo G ) niZ *

(16)

Now, as said before, electrons in the conduction band are characterized by the value of the Fermi Energy. As a guiding rule, we can assume that if Ee EF then the characteristics of the material are not affected. We call this “cold solid approximation”. On the contrary if Ee > EF, which happens at high laser intensities, then we will need to take into account modifications in the target characteristics and, in particular, to include plasma effects. Finally the case Ee > G we get the expression which we have written before, which yields the value of the flux ablation threshold Fth

CiTeq

§ z · Fo exp¨ ¸ l © l ¹

(20)

§ z · exp¨ ¸ G © G ¹

(21)

If instead case G >> l then we get

CiTeq

Fo

Penetration of energy is still exponential but it is now governed by optical propagation, yielding a value of ablation threshold Fth = U:G.

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4.3. CALCULATION OF THE ATOMIC PARAMETERS

Let’s consider as an example the case of Cu irradiated by laser light at 532 nm (2nd harmonic of Nd:YAG). Cu has a density of atoms ni = 8.5 1022 cm-3 and Z* = 1, therefore ne = ni. This corresponds to Zp= 1.6 1016 Hz, and G = 1.8 10-6 cm = 18 nm. Also such electronic density gives EF = 7 eV and vF = 1.57 108 cm/s. This is the velocity of the electrons at the top of the conduction bands, those with energy EF and the only ones that can exchange energy. Therefore it seems appropriate to substitute v = vF in the expression for the thermal diffusivity D. The calculation of the electronic relaxation time We requires some specifications. Such time is related to the dc electrical conductivity V by the well-known expression

V

nee 2W e me

Z 2pW e 4S

(22)

therefore it is a function of material density and temperature. Figure 3 reports the behavior of V for solid density Al for temperatures between room temperature and 10 keV. Experimental measurements have been made by Milchberg et al.3 by measuring the reflectivity of femtosecond-heated Al and calculating resistivity. These are compared to the classical Drude model and to a simplified quantum calculation (Eidmann-Huller model4). All curves gives the same behavior: first conductivity decreases and then increases again when the material temperature becomes so large that the plasma regime is reached. In this case conductivity scales with temperature according to the Spitzer’s model, i.e. Vv Te3/2. Let’s notice that the simple classical Drude model, although quantitatively wrong, gives the correct qualitative behavior. All curves show a minimum value of conductivity, called “saturated” conductivity. It is possible to show that this minimum corresponds to We= a/vF, where a is the inter-ionic distance in the material (Ioffe-Regel’s limit). In other terms, in this limit, the electrons moving at velocity vF make a collision with every atom they meet on their path. In the case of copper, we get a = ni-1/3 = 2.3 10-8 cm = 2.3 Å and We = a/ vF = 2.2 10-16 s = 0.22 fs. It is easy to see that such is an incredibly fast time, much shorter than even shorter laser pulses used in ablation. The ion relaxation time is much longer because the energy exchange between very light and very heavy particles is proportional to the mass ratio, i.e. is given by:

Wi which yields § 26 ps in our case.

miW e me

(23)

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155

109

Electrical conducibilty MKS (:m)

-1

experimental data from Milchberg et al. Quantum model by Eidmann and Huller Semiclassical Model (Drude + Spitzer)

108

Vcold Al = 3.7 107 :-1m -1

107

106

T (eV)

105 0.1

1

10

100

1000

4

10

Figure 3. Behavior of electrical conductivity vs. temperature for solid state Al.

Using these values we get for the thermal diffusivity D = (1/3) vF2We = 1.8 cm /s. Now, assuming as an example W= 40 ps (at the very upper limit of the validity range for the present model of laser ablation in the femtosecond-regime) we get l = (DW § 85 nm. Finally we can calculate the threshold flux for ablation as Fth = U:l/A = 1.2 J/cm2 where we have taken into account that A = 0.3 at 532 nm. Of course this is the value calculated for a polished and clean Cu surface, which will certainly be very different from a real rough and oxidized surface; however this is not too important: A appears inside a logarithmic terms so small differences in A will not change the value of Fth too much. Coming back to the cold solid approximation, we therefore see that in such approximation (Ee§EF) no characteristics of the material is strongly affected: the electronic relaxation time remains the same, there is no ionization above the normal value of Z* therefore the electronic density doesn’t change, the values of G, A, EF, vF don’t change. On the other end, as already said, if Ee EF. This results in strong heating of the material and plasma effects: (1) ionization of

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material above the normal value Z: the electronic density changes inducing changes in G, A, EF, vF; (2) high temperature of the initial plasma. Formulas for heat transfer derived from solid state physics are no longer applicable. In particular there is an increase in the velocity of electrons (vF is replaced by electronic thermal velocity vth) and there is an increase in We for which we must now use Spitzer’s law giving We in a plasma

We

10 6 Te3 / 2 s 2.91 ne ln /

(24)

The plasma ionization degree should be calculated in a self consistent way. Figure 7 shows the average ionization degree Z* for gold at density U = 0.1 g/cm3 vs. temperature in various approximation or using different formulas. 80 70 60

Z*

50 40 30 20 10 0 10

1000

100

104

T (eV)

Figure 7. Average ionization degree Z* for gold at density U = 0.1 g/cm3. Squares: coronal limit; circles: Thomas Fermi model; solid lines: analytical formula for coronal equilibrium (CE); dashed line: analytical formula by Colombatnt and Tonon6 (valid in Collisional Radiative Equilibrium, CRE).

In principle, we should also calculate the energy lost to ionization by summing over the different ionization energies for each electron in Cu. (shown in Figure 8). However it is possible to show that usually the total energy lost to ionization Eion is negligible with respect to the thermal energy. The following table shows a comparison of the ablation parameters calculated in the cold solid approximation and in plasma model. The pulse duration is 400 ps. In the plasma case, values are calculated for an intensity I § 1014 W/cm2 giving a temperature Te § 500 eV.

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159

104

1000

100

10

1 0

5

10

15

20

25

30

Figure 8. Ionization energy of each electrons for Cu ions. TABLE 2. Comparison of ablation parameters calculated in the cold solid approximation and taking the plasma effects into account.

Z* Free electron density ne Electron velocity ve Electron relaxation time We Penetration depth l Ablation threshold flux Fth = U:l/A

Cold solid approximation 1 8.5 u 1022 cm–3 vF = 1.6 108 cm/s 2.2 u 10–16 s 0.1 μm 1.2 J/cm2

Plasma model 18 – 19 15 u 1023 cm–3 vth = 1.5 109 cm/s We (Spitzer) = 10-15 s 2 μm 70 J/cm2

The transition between the cold solid approximation and the plasma case is qualitatively illustrated in Figure 9. The plasma case onset corresponds to a higher ablation threshold flux, but gives larger ablation depths L. L L

§ F · o ¸ lplasma ln¨ ¸ ¨F © th plasma ¹

L

§ F · lcold ln¨ o ¸ ©Fth cold ¹

Figure 9. The transition between the cold solid approximation and the plasma case The plasma case onset corresponds to a higher ablation threshold flux, but gives larger ablation depths L.

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As an illustration of ablation measurements in the plasma case, let’s consider the following measurements performed at Milano Bicocca.7 A Nd:YAG (Quanta System) laser was used with maximum energy 60 mJ per pulse, 10 Hz repetition frequency, working in the green (532 nm) with focal spot radius wo=25 μm and pulse duration W = 40 ps giving a maximum intensity on target I = 1014 W/cm2. Figure 10 shows the depth and diameter of the ablation region on Cu targets in air and vacuum, as a function of energy flux. Figure 11 instead compares results obtained in vacuum for Copper and Lead. In both cases, results were obtained by analyzing the irradiated samples with optical microscopy. First, data show that the presence of air decreases the ablation depth of about 40%: as in the case of the previous experimental results obtained at LOA (Figure 5) this is due to the re-deposition of the ablated material (while the threshold flux is not affected). From available data, we know that for Cu U = 8.96 g/cm3 and : = 4723 J/g, while for Pb U = 11.35 g/cm3 and : = 859 J/g. Then interpolation of data yields the following penetration depths and ablation threshold: Cu, l | 5 Pm and Fth | 70 J/cm2, Pb, l | 4 Pm and Fth | 7 J/cm2. The ablation rate for lead is larger than for copper but this is due to the different in the evaporation rate while the heat penetration rate is practically the same (indeed this is mainly a function of plasma conditions, including electron temperature, which are mainly fixed by the char-acteristics of the laser pulse). Also, we see that in this experiments, the penetra-tion lengths are of the order of a couple of micrometer, as indeed it should happen in the plasma case, and not of several 10 nm, as in the cold solid approximation.

Diameter Depth (Pm)

30

(Pm)

160 140

VACUUM

25 120

VACUUM 20

100 80

15

60 10 40 5

AIR

AIR

20 0

0 0

1000

2000

3000

Flux (J/cm2)

4000

5000

0

1000

2000

3000

4000

5000

Flux (J/cm2)

Figure 10. Depth and diameter of the ablated region on Cu vs. energy flux on target. Data have been interpolated with the formulas introduced in Section 4.

Alternatively, the ablation depth per shot can be measured by counting the number of shots, which is needed to drill a clear hole in a foil with a given

LASER ABLATION

Diameter

Depth (Pm)

161

( Pm)

40

200

LEAD

35 150

LEAD

30 25

100

20 15

COPPER 50

10

COPPER

5 0

0 0

1000

2000

3000

4000

Flux (J/cm2)

5000

0

1000

2000

3000

4000

5000

Flux (J/cm2)

Figure 11. Depth and diameter of the ablated region for Cu and Pb vs. laser flux in vacuum.

Figure 12. AFM images of surface modifications induced on the irradiated Al target.

thickness. For instance, in the same experiments, we measured that 53 shots at the maximum energy flux are needed to drill a hole in a Cu foil 1-mm thick. This implies an ablation depth per shot of 19 μm, in fair agreement with the measurements reported in Figure 11. In the same conditions, the measured hole diameter was 120 μm. Finally, let’s notice that removal of material is of course not the only consequence of laser ablation. Indeed the characteristics of the material surface are also modified, an aspect that is also often used in application8,9 (e.g. surface hardening, surface cleaning, surface polishing). Let’s see for instance the analysis of the irradiated Al surface obtained with the same laser system. The Atomic Force Microscopy (AFM) pictures in Figure 12 (40 u 40 μm and 80 u 80 μm respectively) show two samples irradiated at Milano Bicocca (10 laser shots on an Al surface). The laser induced a change in the surface: the smooth surface was not irradiated, the part with “bubbles” was irradiated). The second picture shows a granular and irregular surface following laser irradiation.

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5. Laser Ablation in the Nanosecond Regime The situation for laser ablation in the nanosecond regime is very different: there is indeed one very important point, which is implicit in what we said until now. During the duration of the laser pulse W, matter has no time to move, there-fore the ionic density ni remains constant. Indeed the sound velocity is

cs

JZ * Te mi

9.8 10 5

JZ * Te (eV ) A

cm / s

(25)

Even if Te = 100 eV, we get cs = 7 106 cm/s, and then if W = 100 fs, then the typical extension of the plasma is L= cs W = 0.07 μm. This shows that there is no appreciable movement or expansion of the target material. The situation in the nanosecond regime is very different being completely dominated by hydrodynamics. Because matter has time to move, we have plasma expansion in vacuum. A low density plasma corona is formed in front of the target surface. The laser interacts with the plasma and it is absorbed in the plasma corona at the critical surface. Energy is then transported inwards by electronic thermal conduction (and/or radiation and/or suprathermal electrons). Matter is constantly removed from the solid at the ablation surface (where U = Uo of the cold solid). The plasma expands in vacuum and plasma expansion is fed by laser ablation. Beyond the ablation surface a shock is formed as a consequence of momentum conservation: the material is compressed by such ablation (shock) pressure. Shock front and ablation front moves inward in the material. The situation is evidenced in Figure 13. Ablation region

Plasma Expansion

ne ns

Overcritical region

LASER

Te ^Kev nc corona

xs xa

xc

Figure 13. Typical density and temperature profiles established during nanosecond-laser interaction.

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163

It is possible to accurately describe the 1D hydrodynamics though analytical formulas obtaining scaling laws for the relevant quantities, the ablation pressure and the mass ablation rate.10,11 In practical units: 2/ 3 § 2 · § ·2 / 3§ A ·1 / 3 I (W / cm ) 1 ¸ ¨ P( Mbar ) 12.3¨ ¨ ¸ ¨ ¸ ©O (Pm) ¸ 14 ¹ ©2Z ¹ © 10 ¹ 1/ 3

§ · I (W / cm 2 ) ¸ m (kg / s cm 2 ) 150¨ ¨ ¸ 14 ¹ © 10

4/3

§ 1 · ¸ ¨ ©O (Pm) ¹

(26) 2/ 3

§ A · ¨ ¸ ©2Z ¹

= 32 kg/cm2s. For instance irradiation at 1012 W/cm2 with O = 1 μm yields m For a laser pulse duration W = 1 ns, this gives a mass removed for unit surface m W § 3 10-5 g/cm2. Finally for U = 2.7 g/cm3 (Al), this gives and ablated depth W/U = 0.12 μm. At 1014 W/cm2 it becomes 0.6 μm. d= m The ablation rate is a large number but the ablated mass and the ablated depth become very small because of the small focal spot and short pulse duration. However such ablated depth can be difficult to be measured because in this regime often the shock is also a very effective factor producing further ablation of material, as we will see in the following. Let’s also notice that ablation pressure can very easily reach the several 10 Mbar range. Indeed this kind of experiments are also used to compress matter to high pressures and study physical characteristics and equation of state of compressed matter.12 5.1. HYDRODYNAMICAL CODES

A number of 1D and 2D hydro codes are well adapted to describing the hydrodynamics and the ablation of nanosecond-laser irradiated targets, e.g. the code MULTI developed by R. Ramis et al. at Universidad Politecnica, Madrid.13,14 In Figure 14, we see the interaction of a 6 μm foil Al target with laser radiation with W = 3 ns at 800 nm and 1012 W/cm2. After shock breaks out on target rear side (at about 0.75 ns after the beginning of the laser pulse), the whole target is accelerated by the laser beam. At the same time a relaxation wave is generated and reaches the front side again. Since the laser pulse is still on at this time, then a second shock is generated and reaches the rear side just before 2.5 ns. The ablated mass corresponds to the rear side reaching low densities (green area). Of course if the target is massive, then the shock will never breakout on

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D. BATANI

Figure 14. Interaction of a 6 μm Al foil with a laser with W = 3 ns at 800 nm and 1012 W/cm2.

rear side and only front side dynamics will be important. In this case relaxation will take place only at the end of the laser pulse, although some residual superficial compression may be maintained in the material (and actually it may be useful for surface hardening of the material). 5.2. THE QUESTION OF LASER ABSORPTION

The question of laser absorption is very important for an efficient laser-target coupling and then for an efficient ablation, as we have noticed in Section 2. While the absorption of femtosecond-lasers is still poorly understood, the situation is clearer for nanosecond-laser pulses In the corona laser energy is absorbed collisionally (i.e. by inverse bremsstrahlung). The absorption coefficient then reads,

D (cm1 ) 3.1 107 Z * ne2Z 2 ln / Te3/ 2 n 1

(27)

On this basis, very often we find in text books that IR lasers are absorbed more because of longer wavelengths (smaller Z). In reality this view is only partial and true only for very low intensities (or rather short pulses) for which an extended plasma corona does not develop. But as soon we get an extended corona (which is the main feature of nanosecond-interactions) this is not true. We must remember the plasma refraction index n becomes 0 as the critical density is approached, and also that the critical density is inversely proportional to the square of wavelength. Therefore, at shorter wavelengths, the laser can penetrate deeper in the plasma and reach higher intensities where absorption is higher.

LASER ABLATION

165

Experimental measurements by Fabre et al.15 performed already in the 1980s, clearly show such behavior (see Figure 15) A= absorption % 2.1010 2.1011 100

2.1012

intensity (W / cm2) 2.1013 2.1014 2.1015

0.53Mm 2ns

90 80 70 60

1.06Mm100 ps

1.06Mm 2.5ns

50 40 30 20 E(J) for 100 ps pulse 1/25E(J) for 2.5 ns pulse

10 0 10 4

10 3

10 2

10 1

1

10

Figure 15. Fraction of laser light absorbed in a plasma as a function of laser intensity on target, laser wavelength and pulse duration.

5.3. SHOCK EFFECTS AND CRATER FORMATION

At high laser irradiances in the nanosecond-regime, the shock wave becomes the dominant aspect of the interaction and leads to the formation of a crater.16 This is shown for instance in the SEM images in Figure 16. In general this regime is not interesting for micromachining, unless precision is not a requirement. However it is still very interesting from the point of view of physics.

Figure 16. SEM images showing the surface morphology of caters created on Al flat targets: (left) diameter D = 30 μm, flux § 2,200 J/cm2; (right) D = 100 μm, flux § 4,000 J/cm cm2.

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D. BATANI

Shock dynamics is governed by Rankine-Hugoniot relations which represent the conservation laws for mass, momentum, and energy (per unit mass) across the shock front. For strong shocks, these read: Po UoUD

P

Uo D U ( D U ) 'E

E Eo

(28)

1 2 U 2

Here P, U, E, D and U are respectively the shock pressure, material density, internal energy per unit mass, shock and fluid velocity (i.e. the velocity of matter behind the shock front), and the suffix “o” refers to the unperturbed material ahead of the shock front. The condition for ablation requires the energy increase per unit mass to be larger than ablation threshold, i.e. 'E = U2/2 > :. Hydrodynamics is governed by scaling laws i.e. it is universal or scaleinvariant. Crater formation is governed by self-similar laws (i.e. power laws). The same laws concerning crater depth and crater diameter hold from micronsize laser crater to huge craters created by nuclear explosion and meteorite impacts (according to the theory formulated by Raizer in the approximation of a pointlike, in space and in time, energy release17). It is possible to derive the scaling law for crater size in the following way. The crater size Rcrat is determined by the condition 'E(Rcrat) = :. In the case of crater formation we have a spherical shock, which is expanding in the material starting from the laser focal spot, and thereby reduces its pressure according to:

§Ro ·2 Po ¨ ¸ © R ¹

P

(29)

In a material the shock and fluid velocity are usually related by the relation D = cs + SU where cs is the sound velocity in the material. For strong shocks gives D§ SU from which P = Po + UoDU §UoDU §UoSU2. Therefore

'E

U2 2

P 2UoS

2 Po §Ro · ¨ ¸ 2 U o S © R ¹

§Ro ·2 k ¨ ¸ © R ¹

and the crater size is

Rcrat

Ro Po / 2 Uo S:

Ro k / : Ro

Rcrat / k / :

We now integrate to get the total energy spent in creating the crater

(30)

LASER ABLATION

Eabs

³

Rcrat 0

167

§Ro ·2 k ¨ ¸ 2SR 2 dR 2SkRo2 Rcrat © R ¹

(31)

and by substituting the previous relation between Rcrat and Ro we finally get

Eabs

2SkRo2 Rcrat

2Sk Rcrat / k / :

Rcrat 2

3 2SkRcrat / : (32)

or in other words a cubic root dependence 1/ 3

Rcrat v Eabs

(33)

Under such conditions, the crater may (or indeed will) be much larger than the focal spot (i.e. the laser acts as a point source). 6. Conclusions We have revised the physical basics of laser ablation in the femtosecond and nanosecond pulse duration regimes. Laser ablation is a complex physical phenomenon which involves optical penetration of laser in the material and the transport of energy inside the material, a problem at the border line of solid state and plasma physics, which may imply quantum mechanical calculations. In the nanosecond-laser interaction regime, ablation is dominated by hydrodynamics: ablation feeds plasma expansion continuously and the laser interacts within the corona, producing an increased absorption at shorter wavelengths. Finally, shock dynamics and the formation of a laser crater is often a key phenomenon associated to high-intensity nanosecond-laser interactions.

References 1. 2. 3. 4. 5.

Perry, M., and Mourou, G. (1994), Science, 264, 917–924. Nolte, S. et al. (1996), J. Opt. Soc. Am. B, 14(10), 2716. Milchberg, H. et al. (1988), Phys. Rev. Lett., 61(20), 2364. Eidmann, K., Meyer-ter-Vehn, J., Schlegel, T., and Hüller, S. (2000), Phys. Rev. E, 62, 1202. Di Bernardo, A., Batani, D., Desai, T., Courtois, C., Cros, B., and Matthieussent, G. (2003), Laser Part. Beams, 21, 59–64. 6. Tonon, G., and Colombant, D. (1973), J. Appl. Phys., 44, 3524–3537.

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7. Desai, T., Batani, D., Rossetti, S., and Lucchini, G. (2005), Radiat. Eff. Defects Solids, 160(10–12), 595–600. 8. Trtica, M., Gakovic, B., Maravi, D., Batani, D., Desai, T., and Redaelli, R. (2006), Mater. Sci. Forum, 518, 167. 9. Trtica, M., Gakovic, B., Batani, D., Desai, T., Panjan, P., and Radak, B. (2006), Appl. Surf. Sci., 253, 2551–2556. 10. Lindl, J. (1995), Phys. Plasmas, 2, 3933. 11. Batani, D., Stabile, H., Ravasio, A., Desai, T., Lucchini, G., Desai, T., Ullschmied, J., Krousky, E., Juha, L., Skala, J., Kralikova, B., Pfeifer, M., Präg, A., Nishimura, H., Ochi, Y., et al. (2003), Phys. Rev. E, 68, 067403. 12. Batani, D., Stabile, H., Tomasini, M., Lucchini, G., Ravasio, A., Koenig, M., BenuzziMounaix, A., Nishimura, H., Ochi, Y., Präg, A., Hall, T., Milani, P., Barborini, E., Piseri, P., et al. (2004), Phys. Rev. Lett., 92, 065503. 13. Ramis, R., Schmalz, R., Meyer-ter-Vehn, J. (1988), Comput. Phys. Commun., 49, 475. 14. Ramis, R., and Meyer-ter-Vehn, J. (1992), MULTI2D-A computer code for two-dimensional radiation hydrodynamics, MPQ-174. 15. Garban-Labaune, C., Fabre, E., Max, C., Fabbro, R., Amiranoff, F., Virmont, J., Weinfeld, M., and Michard, A. (1982), Phys. Rev. Lett., 48, 1018–1021. 16. Bussoli, M., Batani, D., Desai, T., Milani, M., Trtica, M., Gakovic, B., and Krousky, E. (2007), Laser Part. Beams, 25, 121–125. 17. Zeldovich. Y., and Raizer, Y. (1967), Physics of Shock Waves and High Temperature Hydrodynamic Phenomena, Vol. 1, Academic, New York.

PHOTO-, DUAL- AND EXOELECTRON SPECTROSCOPY TO CHARACTERIZE NANOSTRUCTURES

Y. DEKHTYAR* Biomedical Engineering and nanotechnologies Institute, Riga Technical University, Kalku 1, Riga LV-1658, LATVIA

Abstract – Physical basics of prethreshold photo-, dual- and exo-electron spectroscopy to characterize nanostructures, as well as relevant measurement technique are described. Application cases are considered.

Keywords: Prethreshold photo-, dual- and exo-electron spectroscopy, nanostructures.

1. Introduction Advances in nanotechnologies need non-destructive characterization methods. To reach this, electron emission measurements, when energy to escape an electron from a tested object is not enough to destroy its molecular/atomic couples, are employed as such energies have values ~0.1–1 eV for non-destructive characterization. In such a case the excited electron has a mean free path (L) ~10–100 nm,1 correspondingly within the analysed emitter. These values of L fit sizes of nanostructures. This is a key point to apply electron emission for nanomaterials characterization. The paper is targeted to consider a prethreshold photo-, dual- and exo-electron emissions and their capabilities in respect with nanotechnologies. 2. Photoelectron Emission Analysis 2.1. PHYSICAL BASICS

A current (I ) of the prethreshold single photon photoelectron emission (PE) is described by the classical formula

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To whom correspondence should be addressed: Y. Dekhtyar, e-mail, [email protected]


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Y. DEKHTYAR

170

conts(hQ M ) m

I

(1)

where hQ – energy of exciting photon, Melectron work function, m-power index. For the prethreshold emission mode (hQ t M the ҏvalues of M are equal to several electron volts,2 and therefore a mean pass of the photoelectron within the emitter is around 10–100 nm.1 To emit the electron it is excited from an initial (i) to a final (f) states that correspond to the energies Ei and Ef . The electron that leaves the specimen to the vacuum has the energy

E f Evac

W

(2)

Evac – energy of the vacuum level. Because of the energy conservation law

hQ

E f Ei

following main photoelectron spectroscopy methods could be employed. When hQ = const,

W

Ei hQ Evac ,

dI dW

dI dEi

(3)

the approach on initial states spectroscopy is provided. When Ef = const, hQ = var

dI d (hQ )

dI dE f

(4)

the spectroscopy of the final states is available. If Ei = const, however Ef = var and hQ = var with the condition 'Ef = 'hQ (' – means increment) (1) comes again to (4). When electron emission is delivered from the local states (Ei = const) and hQ = var, W

I ~ ³ N i ( E ) N f ( E , hQ )dE ,3

(5)

0

N i , N f – density of the initial and final electron states, correspondingly. Using

this way

dI ~ N i (W ) N f (W , hQ ) . dW

(6)

PHOTO-DUAL- AND EXO-ELECTRON EMISSION

171

However,

dI dW

dI . dhQ

(7)

Because of (6) estimation of the N i , N f combination could be estimated. The electron work function in a case of emission from metals and semimetals that do not have an energy gap (Eg) is equal to an electron affinity (F). For the materials with Eg (semiconductors, dielectrics)

M Eg + F . The index m (formula (1)) characterises transition channel to excite the electron (see please the Table 1 below): TABLE 1. Index m in for different electron transitions. Material

Transition

Crystal

Direct Nondirect From the local level

Amorphous

Value of m

Source

1 2.5 1.5 >3

5 5 6 7

2.2. INSTRUMENTATION

To get the advantages (estimation of m, M and I(M) of the prethreshold PE measurements the photon energy should be as possible as close to the electron work function: hQ | t M. To supply the single photon electron emission mode and to avoid heating of a tested object, the flux of the photons should be rather weak. This stipulates small values of I. Because of this very sensitive electron detectors must be in use. Typically the secondary electron multipliers that have a noise 0.1–1 electron/second are applied. The value of F could be effected by ions/dipoles sorption/desorption on/from an emitter surface.4 This asks for stable and “passive“ electron emission measurements environment (preferably vacuum; 10–2…–7 Pa8). Typically the values of M are not distributed uniformly over a surface of the emitter. As a result a contact potential difference is induced between the surface arrears and electrical fields are induced. It decreases/increases I from the places with lower/higher M, correspondingly.4 In fact, when characterisation of M distribution is an aim of measurements, their results could become false. To avoid this, an external compensating electrical field ~102…3 V/cm directed from the surface to the detector should be provided.4

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To measure energy of the escaped electron (for the approach by the formula (3)) an energy analyser should be in use. The approaches by (4)–(7) are available, when a monochromator of the photons is provided. 2.3. APPLICATION CASES

2.3.1.

Employing of the electron work function

2.3.1.1. Mechanical tensions of crystalline silicon surface layer Influence of any mechanical load on a negative increment of the silicon energy gap9 was in use to identify location of mechanical tensions induced within a surface layer because of its local doping with boron.10 A thickness of the doped layer was equal to 100 nm. A distribution of related to the tensions I over the surface is demonstrated in Figure 1. A crown at the boundary of the image corresponds to increased local deformation. Doping (B) 1000 Å

Si (B) |V|

Energy of electron

x Vacuum Cond. band

Cond. band

M2 Valence band

Valence band

Cond. band

M1 Valence band

Figure 1. PE Image of the mechanical tensions distribution over the Si surface.10

2.3.1.2. Thickness of nanofilm An insulator film Si3N4 was deposited with a different thickness (5–100 nm) on a Si substrate. Because of simultaneous electrons emission from the film and the substrate, both of them having differentM, their contribution to the value of I depended on a thickness of the film. Figure 21 provides the PE image of the non-uniform nanofilm. Figure 3 demonstrates a response of I during plasma chemical etching of a multi layer structure. Measurements were supplied in situ using a PE spectrometer incorporated into an etching machine. 2.3.1.3. Size depended electrical potential of nanoparticles It is known that mechanical surface tensions depend on a size of the spheri-cal like particles. This could influence of density of the surface charge, particularly in a case of ion crystals. As the result value of M should be affected.


173

Figure 2. PE current distribution over the nanofilm having different thickness.

Figure 3. PE current in dependence on multi layer system etching (in situ).

Figure 4 demonstrates a correlation of M on a size (X) of the hydroxyapatite nanoparticles.11 2.3.2.

Density of electron states

2.3.2.1. Electron states of the nanoparticles Computational simulation (CS) evidenced that the result in Figure 4 was stipulated because of different electron state density induced by protons.11 To verify these approaches (6) and (7) were employed.11 The achieved results demonstrated that increasing of the particle size (X) shifts the detected local

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0,71

log M>M@ = eV

0,705 0,7 0,695 0,69 0,685 0,68 0,675 0,67 1

1,5

2

2,5

3

3,5

log X, [X]= nm Figure 4. Correlation of M on a size of the nanoparticles.11

dI/dhQ , arb. units

Shift of the local state

Tail of states

1,2 1 0,8 0,6 0,4 0,2 0

X=100 -1000 nm X=20-60 nm

-0,2 -0,4 -0,6 4,9

5,4

5,9

hQ , eV

Figure 5. Distribution of particle size.11

dI / d (hQ ) ~ N i N f

on dependence on the hydroxyapatite nano-

centre in accord with CS the and provides a tailed distribution of the electron states that is similar with disordered systems (Figure 5). The latter was evidenced because of the index m (1) increasing on 50%.11 2.3.2.2. Electron states of the bone The approach (3) was applied jointly with fluorescence and dual emission (see please below) measurements to explore electron states of the bovine bone.12 The result presented in Figure 6 evidenced that the bone has a semiconductor like electron density states distribution.12


E eV

vacuum level

175

Density of electron states, arb. units

-1.0 -2.0 -3.0 -4.0 -5.0 -6.0 Figure 6. Electron density of the states in bone.12

3.

Dual Electron Emission Analysis

3.1. BASIC PHYSICS

When the tested object has and energy gap, electrons and hols may be generated because of radiation by photons having an energy hQad t Eg. The excited charge carries are mobile and therefore provide an opportunity to compensate a surface charge that is resulted by the Fermi level pinning at the surface13 and adsorbed ions/dipoles.14 If the photons are switched alongside with PE measurements of I, the latter demonstrates an increment ('I). This is a dual emission mode (DE). The values of 'I may be calibrated in terms of the surface charge density, behaviour of 'I on time reflecting surface electrical potential relaxation induced by photon flux switching on/off.12 3.2. INSTRUMENTATION

The technique on DE measurements is similar that is in use to detect PE, additional light source and monochromator to select suitable hQad should incurporated. To exclude influence of hQad on direct escaping of the photoelectron, the DE is restricted by the condition hQad < hQ, where hQ is the photon energy from (1). On the other hand to minimize an effect of hQ on DE, a ratio of light fluxes must be taken into account:

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Fad /F >> 1, where Fad and F are the fluxes of light beams that supply hQad and hQ, correspondingly; Fad is not enough for multi photon processes. To select a contribution of hQad , it should be supplied in a pulse mode. 3.3. APPLICATION CASES

3.3.1.

Detection of the energy gap

'I, arb. units

Figure 7 provides an example on Eg detection of the CdTe semiconductor. The value of 'I becomes from zero to a positive value, when hQad = Eg .

1 0.8 0.6 0.4 0.2 0

Eg

1

1.1

1.2

1.3

1.4

1.5

1.6

hQad, eV Figure 7. 'I dependence on hQad in respect to evaluate the energy gap of CdTe surface layer.

3.3.2.

Light induced time depended excitation and relaxation

The flux Fad was switched (+hQad) to radiate CdTe crystal and the value of 'I was increased in time (Figure 8 ). When Fad was switched off (-hQad), 'I had been relaxed. 4. Exoelectron Emission Analysis 4.1. BASICS OF PHYSICS

Electron emission, when its current is induced15 or modulated16 because of relaxation processes within a surface layer of the solid is named as exoelectron


+hQ Qad

177

-hQ Qad

1

'I, arb. units

0.95 0.9 0.85 0.8 0.75

time, minutes

0.7 0

2

4

6

8

10

Figure 8. Behavior of 'I because of Fad , in a case of CdTe.

emission (EE). To reach EE two steps should be applied. First, a tested object in advance typically has to be provided with imperfections, their concentration is not in a thermodynamically equilibrium state. After that heat is delivered to the object to induce relaxation of imperfections. As a result EE is supplied. This is a mode of thermostimulated EE (TSE). On the other hand the object tested with PE could be heated alongside to supply EE. In such a case photothermostimulated EE (PTSE) is achieved. EE current (IEE) depends on temperature (T) of the tested specimen, behaviour of IEE having a maximum (Figure 9). The latter is a subject of research and practical applications of EE for analysis. The temperatures to reach EE are not enough to provide significantly detectable thermoelectron emission current. TSE is typically delivered by following mechanisms: (a) Thermoionisation of local states trapping electrons and belonged to the imperfections15; such a mechanism is available from materials with Eg; the ionisation potential should have a little value. (b) Auger transitions of electrons,15 from local electron traps of imper-fections; this channel is available when Eg > F. (c) Field emission from the electron traps by imperfections because of heat induced electrical polarisation/depolarisation of the emitting surface layer.17

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IEE

Ti

Tmax

Tf

T

~ (+20 … 600) o C Figure 9. Typical spectrum of EE. (Ti, ,Tf , Tmax – initial, final temperatures of the spectrum and temperature of its maximum, correspondingly).

PTSE is provided as a single photon prethreshold PE, when heat delivers: (a) Modulation of the density of the electron local states induced by imperfections and emitting electrons16 (b) Shift of the Fermi level because of (a)16 The above mechanisms fit materials having the energy gap. EE from metals is supplied perhaps from their surface oxides having properties of insulators or semiconductors.15 For the ionisation mechanism Tmax corresponds to the ionisation potential of the electron trap.18 There are experimental evidences15 that

I EE (T ) ~

dC dT

(8)

where C – concentration of relaxing imperfections. In fact the total emitted charge (Q) is directly proportional to C at Ti.15,16 An activation energy (Er) of relaxation for the simplest first order annealing reaction may be estimated from (8)15,16: Er = - kT ln [IEE(T)/N(T))]

(9)

where the k-Boltzmann constant, Tf

N (T )

³I

T

EE

(T )dT

(10)


179

In a case of semiconductors the EE spectrum is contributed by annealing of point type complexes of defects supplying single imperfections. Such the reaction is represented by the up-going branch of the spectrum (Ti < T < Tmax ). 16 For the first order reaction its activation energy (Eact) is available from the equation (9)16. However T f should be replaced with Tmax .16 The down-going branch of the EE spectrum (Tmax < T < Tf ) corresponds to migration of the generated single imperfection to the surface of the specimen. An activation energy (Em ) of this process in a case of uniformly distributed single defects within a specimen may be estimated from the equation:16

Em

kT ln

I EE (T max) I EE (T )

d[

I EE (Tmax ) ] I EE (T ) dT

(11)

4.2. INSTRUMENTATION

The above conditions considered to detect PE and DE (when DE is applied jointly with EE) are related to EE too. However, the temperature of the specimen should not exceed the condition, when a significant thermoelectron emission current is provided. 4.3. APPLICATION CASES

4.3.1.

Electron traps at the interface

A thin film of Si3N4 (10 nm) was deposited on a SiO2 substrate. The speci-men was irradiated with weak electrons. The latter were supplied with differ-enced energy to fill in the traps in Si3N4 , Si3N4/SiO2 , consequently. TSE was detected after each step of radiation. The TSE current demonstrated significantly different behaviour, when electron radiation reached the Si3N4/SiO2 interface (Figure 10). This is in favour to apply EE to detect interfaces of the nanostructures. 4.3.2.

Concentration of implanted atoms

A crystalline Si was irradiated by As+ (50 keV) ions. Measured total emitted charge of PTSE was proportional to the delivered fluence of ions (Figure 11).16 The value of Q was sensitive to 10–6 atomic % of As atoms concentration.

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Si3N4

8 IEE , arb. units

Si3N4/SiO2

4

0 0

300

600

o

T, C Figure 10. TSE of the multi layer system Si3N4/SiO2.19

lg Q, arb. units

2 1.8 1.6 1.4 1.2 1 10

11

12 13 14 15 -2 lg (Flunece), cm

16

Figure 11. Emitted charge of PTSE in dependence on As+ ions fluence supplied to Si.16


4.3.3.

181

Activation energy of point type defects annealing and migration

4.3.3.1. Annealing of complex of single defects Tetra vacancies (V4) were generated in a monocrystalline Si because of P+ ions (100 keV) radiation. An activation energy (Er) of complexes annealing was acquired from PTSE measurements (9).16 At a high values of delivered fluence (>5.1014 cm–2) Er decreased because of interactions between V4 (Figure 12).

0.8

Er , eV

0.6 0.4 0.2 0 13.5

14

14.5

15

lg (Fluence), cm-2 Figure 12. Behavior Er of Si tetra vacancies in dependence on P+ ions fluence.16

4.3.3.2. Migration of single defects Single vacancies were generated in a monocrystallne Si owing to dissociation of the vacancy contending defect complexes. The latter were induced due to electron radiation (5 MeV). A activation energy (11) of the liberated vacancy decreased at the fluence level >4.1015 cm–2 (Figure 13), that was stipulated by interference of imperfections.16 From the experimental data, as shown in Sections 4.3.3.1 and 4.3.3.2, it is demonstrated that the photothermostimulated emission instrument can be used to estimate: (a) A thermodynamically non equilibrium concentration of defects (b) A threshold of the concentration of imperfection that provides their interaction (c) Annealing and migration activation energies of imperfections.

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lgEm, [Em]=eV

14.8

15

15.2 15.4 15.6 15.8 16

16.2

0 -0.5 -1 -1.5 -2 lg (Fluence), cm-2

Figure 13. Em of Si single defects in dependence on electron fluence.16

5. Conclusion The review demonstrates that PE, DE and EE are capable to supply sensitive, accurate, and non-contact technique for the needs of nanotechnologies. The techniques are directed to measure the electron work function (to characterise surface potential), distribution of the electron states density (PE); energy gap and charge relaxation (DE); concentration of the point type imperfections, their annealing and migration (EE). ACKNOWLEDGMENTS

The author graciously acknowledges figures and contributions provided by the Author’s colleagues Mr. V. Noskov, Riga Technical University, Latvia, Dr. A. Balodis, Riga Technical University, LV, and Mr. V. Noskov, Riga Technical University, LV.


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References 1. Froitzheim H. (1977), Electron Spectroscopy for Surface Analysis. Ed. H. Ibach, Springer, New York, 315 p. 2. Fomenko V.S. (1981), Emission Properties of Materials. Naukova Dumka, Kiev, 338 p. (in Russian). 3. Kinding N.B., Spicer W.E. (1963), Band structure of cadmium sulphide photoemission studies, Phys. Rev. 138: A561–A576. 4. Dobrecov L.N., Gomoyunova M.V. (1966), Emission Electronics. Nauka, Moscow 564 p. (In Russian). 5. Kane E.O. (1962), Theory of photoelectric emission from semiconductors, Phys. Rev. 127(1): 131–141. 6. Pihtin A.N. (1983), Physical Basics of Quantum Electronics and Optoelectronics. Vsishaya shkola, Moscow, 304 p. (in Russian). 7. Dekhtyar Yu. D., Vinyarskaya Yu. A. (1994), Exoelectron analysis of amorphous silicon, J. Appl. Phys. 75(8): 4201–4207. 8. Dekhtyar Yu.D., Sagalovich G.L. (1988), Exoelectron emission of monocrystalline silicon and its practical application, Proc. USSR Academy of Sci., Physical Ser. 52: 1611–1613. (in Russian). 9. Polyakova A.L. (1979), Deformation of Semiconductors and Semiconductor Devises. Energiya, Moscow, 168 p. (in Russian). 10. Sagalovich G.L., Balodis A.Ya., Dekhtyar Yu.D. (1984), Emission test of mechanical tensions in surface layers of silicon, Electron. Tech., Ser. 8. 5(110): 25–27. 11. Bystrov V., Bystrova N., Dekhtyar Yu., Filippov S., Karlov A., Katashev A., Meissner C., Paramonova E., Patmalnieks A., Polyaka N., Sapronova A. (2006), Size depended electrical properties of hydroxyapatite nanoparticles, in: IFMBE proceedings. V. 14. CD version. Springer, Seoul, pp. 3149–3150. 12. Arvin H., Bogucharska T., Dekhtyar Yu., Hill R.M., A. Katashev, Pavlenko A., Pavlenko I., Zakaria M. (2000), Electronic transitions and structural changes in bone, Latvian J. Phys. Tech. Sci. 6(S): 50–55. 13. Nesterenko B.A., Cnitko O.V. (1983), Physical Properties of Atomic Clean Surface of Semiconductors. Naukova Dumka, Kiev, 264 p. (in Russian). 14. Volkenstein F.F. (1987), Electron Processes on Semiconductors Surface During Chemosorption. Nauka, Moscow, 430 p. (in Russian). 15. Kortov V.S., Shifrin V.P. Gaprindashvili A.I. (1975), Exoelectron spectroscopy of semiconductors and insulators, Microelectronics 8: 28–49 (in Russian). 16. Dekhtyar Yu.D. (1993), Exoelectron Spectroscopy of Point Type Defects in Semiconductors. Riga Technical University, Latvia, Riga, 59 p. (in Russian). 17. Rosenman G.I., Rez I.S., Chepelev Yu.L., Angert N.B. (1981), Exoemission of defected surface of lithium tantalite, J. Tech. Phys. 51(2): 404–408 (in Russian). 18. Nassenshtein G. (1962), Electron emission from the solid state surface after mechanical treatment, in: Exoelectron Emission, Inostrannaya literature, Moscow, pp. 72–95 (in Russian). 19. Rosenman, M., Naich, M., Molotskii, Dekhyar Yu, Noskov V. (2002), Exoelectron emission spectroscopy of silicon nitride thin films, Appl. Phys. Let. 80(15): 2743–2745.

LASER INTERACTION WITH NANO-SPHERES: APPLICATIONS IN SUB-MICRON PARTICLES REMOVAL AND NANODOT ARRAY FABRICATION M. SENTIS1*, D. GROJO1, PH. DELAPORTE1, AND A. PEREIRA2 1 Lasers Plasmas and Photonic Processing Laboratory, CNRS – Universities of Aix-Marseille, Campus de Luminy – Case 917, 13288 Marseille Cedex 9, FRANCE 2 INRS-EMT, 1650 blvd. Lionel-Boulet, Varennes (Quebec) J3X1S2, CANADA

Abstract – Laser-assisted nanoparticle removal processes like selective particle ablation, mechanical ejection, local substrate ablation, explosive evaporation are considered by experimental and theoretical approaches. Experiments are based on the estimation of particle removal efficiencies and thresholds depending on material and size of particles. Afterwards, a novel and efficient photonicbased method to synthesize porous alumina membranes (PAMs) and subsequent metal nano-dot arrays on various substrates, based on near laser field enhancement by nano spheres, is reported.

Keywords: Laser nano-particle interaction, near-field enhancement, nanodot arrays.

1. Laser-Assisted Nanoparticle Removal Processes 1.1. INTRODUCTION

Due to the emerging nanotechnologies, new cleaning solutions providing the ability to remove nanometer defects from material surfaces are now required to ambition a reliable fabrication of nanoscale devices. In this context, the dry

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To whom correspondence should be addressed: M. Sentis, email: [email protected]


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laser cleaning (DLC) technique might be consider as a potential approach. In this method, the particles are ejected from the surfaces by the illumination of the polluted materials with nanosecond laser pulses. Some studies have already demonstrated that this technique is efficient for specific applications1 and allows eliminating particles with size below 100 nm.2,3 However, there is still a lack of knowledge of its underlying mechanisms. This may be explained by the complexity of the processes which can be combined in the studied situations. Among them, the adhesion and the laser-matter interactions involved in the study of particles at the nanometer scale exhibit singular characteristics. The literature on DLC, for more than 15 years now, shows that the removal of particles results from a combination of at least three mechanisms: (i) the mechanical force (inertia or elastic) exerted on the particles while they move with the rapid thermal expansion of materials,4,5 (ii) the local substrate ablation due to the enhancement of the laser light which can be observed underneath small particles,6,7 (iii) the explosive evaporation of the adsorbed humidity from the air at the interface between the particles and the surface of the substrate.8,9 On the basis of this latter contribution which has been evidenced recently, we may consequently wonder how dry the so-called dry laser cleaning technique is. Indeed, as we shall see later on, the humidity can play a major role on the physical processes occurring in the experiments. For this reason, we extensively investigated this aspect. The interpretation of the results of these experiments contributes to a better understanding of the humidity-based adhesion and laser-assisted removal processes involved in experiments with submicrometer particles. 1.2. EXPERIMENTAL DETAILS

Laser-induced particle ejection experiments were carried out with an ArF (Olas = 193 nm) excimer laser delivering pulses of 15 ns duration and 300 mJ energy. The samples were irradiated by single laser pulses in residual pressure conditions to avoid particle redeposition. The image relay of a metallic mask (using a lens) permits a near-uniform irradiation (S = 2 u 1.5 mm2) of the target materials. Samples were silicon (Si) (100) substrates on which 250 nm radius SiO2 spheres were spin-coated. A density of #105particles/cm2 was obtained with more than 90% of isolated particles. The samples were placed in a vacuum chamber allowing to reach a residual pressure of 10–4 Pa. Inside the chamber, the stainless steel sample holder was back-side equipped with a halogen lamp to adjust the temperature of the irradiated target up to 750K. Thus, the substrates can be subjected to different degassing step between each of the experiments and the water meniscus inevitably formed in a humid medium at the interface between the particle and surface can be gradually eliminated. In our experiments, the water meniscus may be formed due to the humid particle deposition process

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or condensed from ambient (i.e. air) humidity before the sample was entered into the chamber. Software analysis of optical micrographs (dark field) of the laser cleaned substrates was used to determine the Particle Removal Efficiency (PRE ) as a function of the experimental conditions (laser energy, degassing step). The values of PRE correspond to the statistic fraction of particles which was removed by the irradiation i.e. PRE = 1–n/n0 where n and n0 are respectively the densities of particles on the surface before and after the laser irradiation. Fast imaging of the ejected particle clouds was also performed with a fast intensified CCD (ICCD) and a forward scattering-based probing technique. The temporal and spatial resolutions of these measurements were ~1 μs and 5 μm, respectively. A complete description of the experimental apparatus can be found in Ref.10 On the basis of this diagnostic, time-of-flight analyses of the ejected particles allow to determine the detachment velocity of the particles. The measurement of the initial velocity, which can be considered as an experimental signature of the different ejection mechanisms,11 is used to distinguish between them and to identify their respective validity domains. 1.3. RESULTS AND DISCUSSIONS

1.3.1. Time-of-flight (T.O.F.) analyses Detailed information on the particle cloud ejection dynamics is obtained from the distribution of the scattering signal from a CW probe beam as a function of the distance from target I(z) for different delays t between the laser pulse and the ICCD observation gate of 1 μs duration. The distance between the particles (#30 μm) being large compared to the probe wavelength (Oprobe = 532 nm) the measured signal is directly proportional to the particle density. The I(z) curves which were captured for a time delay t = 8 μs and for three different laser fluences above the particle removal threshold fluence (Fth # 130 mJ cm–2), are shown on Figure 1. Each profile was fitted with a Gaussian function G F ,t ( z ) . Consequently, according to the consistency of the fits, the complete velocity distribution f F (v ) of the ejected particles for a given laser fluence F is also well described by a Gaussian function which can be deduce from the profiles that are captured at a time delay t. This distribution is given by the expression: f F ( v ) v tG F ,t ( vt ) . The profiles shows that very different behaviors are observed as a function the laser energy deposited onto the polluted substrates. In particular, Figure 1b shows that under appropriate conditions, the ejected particle cloud is split in two distinct components with different velocities. According to the position of the detected species zmax at about 200 and 650 μm for each of these components, the corresponding propagation velocities of the particles are respectively 25 and

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81 m s–1. This observation means that, although we study calibrated systems, the particle detachment results from a competition between at least two different mechanisms in this given situation. For lower laser fluences (Figure1a), only the slow component is observed. For larger laser fluences (Figure1c) only the fast one remains. These observations evidenced the existence of two regimes where only one removal mechanism is responsible for the particle ejection.

Figure 1. Spatial repartition of the ejected particles (given by the forward scattering signal) with respect to the distance from target for a time delay t of 8 μs after the irradiation of the target with the nanosecond laser pulse. The profiles (and their Gaussian fitting curves) are shown for three different laser fluences: (a) Flas = 270 mJ cm–2, (b) Flas = 410 mJ cm–2 and (c) Flas = 550 mJ cm–2.

To analyze the different regimes in more details, Figure 2 shows the determined velocities of the particles as a function of laser fluence. We note that the analyses were possible for laser fluences very close to the removal threshold fluence Fth. Above Fth, particles are ejected with a characteristic velocity which gradually increases from 7.6 to 36.8 m sí1. In the range 300–500 mJ cmí2, while the scattered intensity from the particle cloud decreases a faster component appears. This transitory behavior evidenced that some particles are now removed on the basis of a physical process of different nature to that observed for the low laser energy domain. For larger fluences, only the fast component remains and particles acquire considerably more speed when increasing Flas. Propagation velocity of the particle cloud linearly increases from 66 to 231 m sí1 when fluence is varied from 415 to 680 mJ cmí2. Thus, analyzing the characteristic velocities of the ejected particles, we determined two laser energy domains where two different detachment processes dominates but that still have to be identified.


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Figure 2. Velocities of the ejected particles measured by fast imaging of the scattering clouds as a function of the laser fluence. The determined velocities (above the particle removal threshold fluence) are compared with a calculation of the surface expansion dynamics for similar conditions.

Among the possible mechanisms, we first examined the likelihood of a mechanical ejection resulting from the rapid thermal expansion of the materials. For submicron particles irradiated with nanosecond laser pulses, the elasticity of particles can be neglected and the contaminants move as a whole, following the thermally-induced displacement of the surface of the substrate.10 Figure 2 presents the results of a calculation of the maximal surface velocity which is reached in the vicinity of the particles. The model consists of a numerical resolution of the 3D thermo-elasticity problem, taking into account the focusing power of the SiO2 spheres. This aspect is described using the Lorentz-Mie theory. Indeed, we already showed that, the surface of the substrate is locally illuminated by a bright spot with a Gaussian-like shape and a full width at half maximum of 160 nm12 underneath each of the spheres. The subsequent hot spots enhance the expansion dynamics of the materials (in comparison with a uniform irradiation with clean surface). However, as shown on Figure 2, the expansion velocity of the surface remains two orders of magnitude below the experimentally measured particle ejection velocities. This demonstrates that the dominant removal mechanism in these experiments is not the rapid thermal expansion of materials.

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1.3.2. Self-limited local ablation of the surface After the irradiation experiments, the cleaned substrates were carefully analyzed by means of SEM. For Flas corresponding to the presence of the fast component (i.e. Flas > 300 mJ cm–2), we successfully observed ablated craters in places where originally located the particles. The hole sizes were in the order of 135 nm in diameter for laser fluences less than 500 mJ cmí2. This size is relatively consistent with the laser energy distribution given by the Lorentz-Mie theory solution.12 It demonstrates that, in this fluence domain, the ejection of the particles results from the local ablation of the substrate which is induced by the optical near-field enhancement underneath the particles. This approach is not relevant in a context of extreme cleaning but could be used as a technique of nanomanufacturing. 1.3.3. Humidity dependence of the laser-induced removal of particles As stated previously, the nature of the interactions between, the laser light, the submicrometer particles and the surface may be strongly modified by the water meniscus at the interface between the particles and surface. The influence of this aspect on the particle removal with moderated laser fluences is analyzed in this section. To better understand the role of the humidity, we repeated PRE measurements as a function of laser energy after successive periods during them the polluted substrates were conserved under vacuum and at relatively high temperature. Consequently, the water meniscus under the particle was progressively reduced with respect to time in our experimental investigations. Figure3 presents the evolution of the particle removal threshold when the humidity is progressively reduced. The last value which corresponds to the driest substrate (as dry as we could get) is close to the local substrate ablation threshold reported in the previous paragraph. It means that the removal of the water meniscus leads to the disappearance of the low fluence regime mechanism which was discriminated in the TOF analyses. Thus, for dry substrates, the only way to eject particles is the substrate ablation. From this result, we propose that the explosive evaporation of the water meniscus which is induced by the laser irradiation8,9 is the particle removal mechanism in this regime. The measured ejection velocities (see Figure 2) strengthen us in our proposition. Indeed, the velocities found in the literature relating the ejection (ablation) of thin films of condensed water on metal surfaces by nanosecond laser pulse irradiations are also in the range 10–40 m s–1 for similar conditions.13 In this approach, the cleaning is still based on the momentum transfer from laser-ablated species (water) but does not require the consumption of the substrate.


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Figure 3. Dependence of the particle removal threshold fluence as function of time during an experiment where the humidity trapped on the substrate is gradually reduced.

It would be over-simplified to claim that the presence of humidity transforms only the laser-matter interaction processes in our experiments. Indeed, the nature of the adhesive interaction between the particle and the surface is also strongly modified by the water meniscus which is formed between them. In particular, this aspect needs to be taken into account to explain the nonmonotonous variation of the particle removal threshold shown in Figure 3. The observation of a minimum as a function of time evidences that it exists an amount of water which optimizes the removal process. This behavior is confirmed by the ejected particle velocity measurements performed before and after that the samples was left during 24 h in a 10–2 Pa atmosphere (corresponding to the first and second points in Figure 3). Figure 4 shows that this gentle degassing step leads to a 20% decrease of the particle removal threshold Fth and a significant increase of the ejection velocity for laser fluences close to Fth It reveals that, when the humidity is slightly decreased the particle ejection becomes easier (in terms of laser energy); hence it corresponds to a better compromise between the adhesion and the cleaning force. In principle, the adhesion of submicrometer objects to surfaces is dominated by van der Waals forces. A typical value of these forces for a 250 nm silica particle is #100 nN.14 However, the presence of a water meniscus may provide an additional contribution to the adhesion by the capillary force. The capillary force can be estimated by using the standard approximation Fc = 4SJR where

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Figure 4. Measurement of the velocities of the ejected particles as function of the laser fluence. After that the humidity was slightly reduced by a degassing step of 24 h under a pressure P = 10–2 Pa, a decrease of the particle removal threshold fluence (# 20%) and a significant increase of the kinetic energy of the ejected particles are observed.

J is the surface tension of water (7.28 u 10–4N cm–1).15,16 For R = 250 nm, this (#200 nN) is not negligible and can easily dominate the van der Waals force. Surprisingly, on the basis of this often used approximation, the adhesion force is independent on the relative humidity RH, and consequently, on the amount of water at the interface between the particle and the surface. However, recent numerical simulations16,17 and capillary force measurements (based on Atomic Force Microscopy)18 demonstrated that while this approximation is viable for particles above 1 μm, its validity is limited for particles as small as the particles we studied. In particular, O. Pakarinen et al.16 developed a model to numerically calculate the exact (non-circular) meniscus profile from the Kelvin equation and, the subsequent capillary force in real environments. They showed that for the geometry “sphere on flat surface”, the standard approximation remains acceptable for RH # 1. However, the adhesion vanishes gradually with the humidity (with slopes increasing as the size of the particles decreases). Consequently, the non-monotonous variation of the particle removal threshold given in Figure 3 is explained by the competition between two humidity-based forces: the capillary adhesion force and the cleaning explosive evaporation (of the water meniscus) force. In the experiments, at the very beginning, when the humidity is gently reduced, the adhesion force is severely decreased but there is still enough water trapped under the particles to eject them efficiently


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by the laser-produced explosive evaporation of this humidity. Thus, the removal threshold is reduced as a result of an optimization of the environment. After this time, while the sample is degassed further, the removal threshold increases progressively as a result of the reduction of the quantity of water possibly ablated and the subsequent dragging force exerted on the particles. This is observed until the quantity of water becomes sufficiently low to make impossible the ejection of the particles by momentum transfer from the evaporated water. Then, the only way to remove the particles is based on the local ablation of the substrate. 1.4. CONCLUSION OF LASER NANOPARTICLE REMOVAL

We demonstrated that the removal of submicrometer particles in nanosecond laser experiments results from a competition between two mechanisms based on the momentum transfer from laser ablated species to the particles. The local ablation of the substrate resulting from the near-field enhancement underneath the particles provides the cleaning force for large fluences. At low laser fluence, we observed that the high efficiency of the direct illumination laser cleaning technique is damage-free. However, the experimental results reveal a large dependence on the amount of humidity inevitably trapped at the interface between the particles and surface. The humidity plays a major role in the cleaning force as well as in the adhesion force exerted on the particles. This humidity dependence may explain the lack of constancy of the results of the numerous experimental studies on DLC performed in non-controlled atmospheres for more than 15 years now. The explosive evaporation of the trapped water is actually the cleaning mechanism in our studied systems. Consequently, the so-called DLC is a humidity-based process. Thus, it is rather similar to the Steam Laser Cleaning (SLC) where a liquid film is deposited on the surface before the laser irradiation. In comparison with SLC, the advantage of our approach is that the water is located only where the water is needed (at the interface between the particle and the surface). However, to optimize the technique, the relative humidity must be controlled and adjusted to lead to the optimum quantity of water trapped under the particles. 2. Preparation of Metal Nanodot Arrays by Near-Field Enhancement In this paragraph we exploit the local ablation of substrates resulting from the near-field enhancement underneath the particles to synthesize Porous Alumina Membranes (PAM) and subsequent metal nanodot arrays on various substrates.

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2.1. LASER PROCESS

The fabrication process is outlined in Figure5. A monolayer of self-assembled spheres is formed onto a thin alumina (Al2O3) film, which was previously coated on a silicon substrate by means of Pulsed Laser Deposition (Figure 5a). Then, pores are optically drilled in the Al2O3 film by particle-assisted near field enhancement. This is accomplished through illumination of the spheres with a single nanosecond laser pulse at the wavelength Olas = 193 nm. This leads to the local removal of the 20 nm thick Al2O3 film under each sphere. Since the spheres are arranged in a hexagonal array at the surface of the substrate, the aluminum oxide film is decorated with an ordered arrangement of holes. (Figure 5b). Using this Laser Fabricated PAM (LF-PAM) as a mask for the deposition of metal (Figure 5c), a series of ordered metal nanodots are formed at the surface of the substrate upon dissolution of the alumina layer (Figure 5d). As we shall see later on, we demonstrated the controllability of this schematic approach by forming an ordered array of gold nanodots.

Figure 5. Scheme of the nanodot array fabrication method.

Numerous studies devoted to nanostructuring by particle-assisted near-field enhancement deal with 1-μm PolyStyrene (PS) spheres. However, Piparia et al.19 demonstrated recently that, in the near UV region (10 at. %) we observe two crystallization peaks; a possible indication of phase separation. We have also observed crystalline inclusions on the X-ray diffraction pattern, and significantly reduced optical activity. Thus (Ge30(SeS)1-x(Te)x)94Ga6 glasses with 5 at. % Te or higher amounts of Te are less stable. The crystallization and glass transition vs. Ga content behavior in (GeSe2)1-y (Ga2Se3)y, y = 0 0.3, glasses exhibit less stable regions as the Ga content becomes large. The structure of GeSeGa glasses were studied by DSC, Raman scattering, and XPS.4 The Ga content beyond which the (GeSe2)1-y(Ga2Se3)y glasses become less stable, where Tc Tg shows a sharp fall, is about 12 at. % Ga. We have also observed a good correlation between thermal and structural properties and optical properties (PL intensity) as shown in Figure 2.

PL Intensity (a.u.)

0

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6

8

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G a C o n t e n t ( a t .% )

Figure 2. Photoluminescence intensity dependence of 1 at. % Er3+ doped (GeSe2)1x(Ga2Se3)x chalcogenide glasses at 1,538 nm on Ga (at. %) content. The dashed line is a guide to the eye.

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PL intensity increases with the Ga content as more Er3+ become activated until the Ga content reaches about 10–12 at. %. Beyond the latter value there is a sharp fall in the PL intensity, and the structure is less stable. Er3+-doped GeSGa, GeSeGa and Ge(SeTe)Ga glasses exhibit the expected characteristic PL emission spectra, as shown, as an example, in Figure 3, for 1.8, 2.1 and 2.4 mol % Er2S3 doped (GeS2)75(Ga2S3)25 glasses.

PL intensity (arb. un.)

1 .0

E r2S

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0 .8 0 .6

m ol % 2 .4 2 .1 1 .8

0 .4 0 .2 0 .0 1450

1500

1550

1600

1650

W a v e le n g th ( n m ) Figure 3. Examples of photoluminescence spectra from Er3+-doped (GeS2)75(Ga2S3)25 glasses.

The PL line-shape is virtually independent of the doping level, with minor variations which can be attributed to self-absorption effects, as discussed elsewhere.2 Most of the erbium doped GeSeGa and GeSGa-based glass compositions we have fabricated were optically active and have shown excellent photoluminescence properties, provided the Ga content in the structure was sufficient. In particular stoichiometric compositions have shown better thermal and optical properties. We have observed a good correlation between the maximum active Er3+ dopants that can be dissolved homogenously and the Ga content in GeSGa and GeSeGa glasses. In the case of GeSeGa glasses the Ga to Er3+ concentration ratio should be typically more than ~6; the glass containing 6 at. % Ga can dissolve up to about 1 at. % Er3+. We have also measured the PL lifetime, and found values in the milliseconds range for both Er2S3 doped stoichiometric GeSGa and GeSeGa compositions; such long lifetimes are obviously highly desirable for optical amplifier applications, along with high a concentration of dissolved and activated Er3+. Table 1 summarizes some of the important experimental observations.

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TABLE 1. Summary of various properties of GeSGa, GeSeGa, and Ge(SeTe)Ga glasses doped with Er2S3. Glass system

Properties

(GeS2)1y(Ga2S3)y Stoichiometric y = 0.2, 0.25

Can be doped with up to 1.5 at. % Er3+. Tg |420 qC (y = 0.25). Two crystallization peaks when non-stoichiometric glasses but one Tg. Characteristic PL emission band, which increases with Er up to 1.5–2 at. % Er3+; and good PL lifetime 1.5–3.5 ms. Tg initially decreases from 410qC to 370qC from 0 to 2 at. % Er3+, and then constant between 2–12 at. % Ga. Beyond 12 at. % Ga, stability decreases rapidly with Ga. Er3+ dissolution and activation depends on the [Ga]/[Er3+] ratio, which should be more than 6. Characteristic PL emission band and good PL lifetime 1–2 ms, up to 2 at. % Er3+. Non-stoichiometric glasses with two Tc and one Tg. Tg and Tc remain constant up to 1 at. %Te, and then both decrease linearly with Te. Thermal stability decreases as Te content increases. Non-stoichiometric Te content samples have two crystallization peaks. Low optically activity when X-ray detected crystalline state.

(GeSe2)1y(Ga2Se3)y Stoichiometric y = 0–0.3

(Ge30 (Se1-xTex)70)94Ga6 doped with 1 at. % Er

4. Discussion Based on DSC, Raman scattering, and XPS measurements, Maeda and Lucovsky4 have recently discussed the structure of stoichiometric (GeSe2)1y(Ga2Se3)y glasses as the Ga content is increased. Most important compositional range is the stoichiometric range that is able to dissolve substantial amounts of Er3+. The reason for these host glasses being able to dissolve Er3+ readily can be easily understood by noting the stoichiometric composition behaves as a pseudobinary alloy (GeSe2)1-y (Ga2Se3)y, in which the two-fold coordinated Se atoms can bridge pairs of Ge-atoms, pairs of Ga-atoms, and Ga and Ge atoms as well. The structure has neutral GeSe4/2 groups, negatively charged GaSe4 (or equivalently Se3GaSe1) groups, and Ga3+ ions. In the absence of Er3+, three Se3Ga Se1 groups are required to neutralize each six-fold coordinated Ga3+ ion. The bonding coordination of Er in oxides, sulfides and selenides is 6 (six),5 the same as the coordination of Ga in the alloys. In addition, the formal chemical valence of Er is 3, the same as Ga. The case of Er incorporation into the (GeSe2)1-y (Ga2Se3)y alloys, and the difficulty of incorporating trivalent Er into GeSe2, derives from the differences in formal chemical valence between Er and Ge, 3 as opposed to 4, as well as the differences in bonding coordination, Er = 6 as opposed to Ge = 4, and the differences in the electronegativities of Er and Ge, about 1.2 for Er, and 1.8 for Ge. The incorporation of Er into the alloys

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may be best understood by treating the Er doped system as a pseudoternary alloy: (GeSe2)1-y-G(Ga2Se3)y (Er2Se3)G, where G 10 at. % Te) were optically active and so were (GeS2)1-y(Ga2S3)y glasses. 5. Conclusions Both stoichiometric GeSGa and GeSeGa glasses can be very good hosts for Er3+ dopants. We can dissolve up to 2 at. % Er3+ in GeSGa and in GeSeGa glasses. Most of GeSGa and Ge(SeTe)Ga based chalcogenide glasses we have studied have two crystallization peaks but only a single heat capacity step change type of glass transition. GeSGa glasses are nonetheless optically active when doped with Er2S3, and exhibit typical PL spectra and good lifetimes in the milliseconds range. Replacement of Se with Te in GeSeGa glasses leads to less stable glasses in which both Tg and Tc decrease with the Te content beyond 1 at. % Te addition. The Ga content in these glasses is critical to dissolving and activating the Er3+. At high Ga concentrations, beyond about 12 at. % Ga, the GeSeGa glasses become less stable.

References 1. Zakery, A., and Elliott, S. (2003) J. Non-Cryst. Solids, 330, 1 and references therein. 2. Kasap, S., Koughia, K., Munzar, M., Tonchev, D., Saitou, D., and Aoki, T. (2007) J. Non-Cryst. Solids, 353, 1364 and references therein. 3. Maeda, K., Sakai, T., Tonchev, D., Munzar, M., Ikari, T., and Kasap, S. (2005) Mater. Sci. Eng. B, 122, 20. 4. Maeda, K., Sakai, T., Sakai, K., Ikari, T., Munzar, M., Tonchev, D., Kasap, S., and Lucovsky, G. (2007) J. Mater. Sci. Mater. Electron, 18, 367–370. 5. Cotton, F., and Wilkenson, G. (1972) Advanced Inorganic Chemistry, third ed. Interscience, New York.

XRD STUDY OF PULSED LASER DEPOSITED AlN FILMS WITH NANOSIZED CRYSTALLITES S. BAKALOVA1*, A. SZEKERES1, A. CZIRAKI2, E. GYORGY3, S. GRIGORESCU3, G. SOCOL3, AND I.N. MIHAILESCU3 1 Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, Tzarigradsko Ch 72, Sofia, BULGARIA 2 Eotvos Lorand University, Faculty of Solid State Physics, 1 Pazmany Peter str., 1117 Budapest, HUNGARY 3 Laser-Surface-Plasma Interactions Laboratory, Lasers Department, National Institute for Lasers, Plasma and Radiation Physics, PO Box MG-54, 77125 Bucharest-Magurele, ROMANIA

Abstract – The structure of pulsed laser deposited AlN films was investigated by X-ray diffractometry. The AlN films were deposited on (111) single-crystalline Si wafers in ambient nitrogen at a pressure of 0.1 Pa via ablation of an AlN target using KrF* excimer laser radiation (248 nm wavelength, t >= 7 ns) with 3.7 J/cm2 incident fluence. The obtained films had a polycrystalline structure with cubic phase nanocrystallites. The size of the crystallites, as estimated from the Bragg peaks, was about 55 nm slightly depending on the post-deposition cooling rate.

Keywords: Aluminium nitride, pulsed laser deposition, XRD, crystalline structure, nanocrystallites, XPS analysis.

1. Introduction Aluminium nitride (AlN), a wide-bandgap semiconductor material, possesses great potential for versatile applications ranging from electronics, acoustic wave devices, and photonic devices to antiwear coatings. Obtaining AlN films with

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To whom correspondence should be addressed: S. Bakalova, email: [email protected]


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definite structure and crystalline quality is still a difficult task for most deposition techniques. Some comprehensive research is thus necessary to find out how exactly the crystallographic structure and degree of crystallinity of AlN films depend on the film preparation method and conditions. AlN generally crystallizes in either the hexagonal wurtzite (h-AlN) structure or the cubic zinc-blende (c-AlN) polytype. The wutrzite structure shown in Figure 1 differs from the cubic one mainly by the relative position of the third neighbours and beyond. The cubic phase has been reported to be theoretically metastable.1 Theoretical investigations of the thermal conductivity of AlN have shown that the energy difference between two kinds of phase structure of AlN is so small that both hexagonal and cubic phases are able to coexist in a realistic 2 material. Still, relevant data on the formation of each of the crystalline phases of AlN have been scant so far.

AI AI

AI N 107.7˚ AI N

N N

110.5˚

Figure 1. Locations of the Al and N atoms in the wurtzite structure.

AlN films deposited by reactive radio frequency and coherent magnetron sputtering have been reported to exhibit c-axis oriented hexagonal structure.3,4 A highly oriented (0001) polycrystalline h-AlN structure has been obtained by metal organic chemical vapour deposition.5 Films of AlN hexagonal polytype have been deposited by molecular-beam epitaxy.6 Pulsed laser deposition (PLD) method has resulted in AlN material in either cubic or hexagonal phase, depending on the preparation conditions. By varying deposition temperature and ambient pressure, different crystalline structures have been obtained. Vispute et al. reported the synthesis of h-AlN with (0001) orientation,7 while Wen-Tai Lin et al. obtained cubic AlN films.8 Both groups used silicon wafers as substrate. The metastable c-AlN was found in films synthesized by nitrogen-ion-assisted PLD9 and by solid state reaction.10

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Our recent investigations are focused on the preparation of AlN films by pulsed laser deposition and their properties. In this study, PLD AlN thin films were analyzed by X-ray diffractometry (XRD), and the influence of the postdeposition cooling rate on the crystallographic structure and crystallite size was discussed. 2. Materials and Methods 2.1. SAMPLE PREPARATION

AlN films were synthesized by laser ablation from bulk AlN target by a KrF* excimer laser operating at 248 nm with a pulse duration of 7 ns, energy fluence of 3.7 J/cm2 and repetition rate of 2 Hz. To avoid piercing, the target was both rotated (0.3 Hz) and translated (along two orthogonal directions). Depositions were performed in low-pressure nitrogen at a dynamic pressure of 0.1 Pa. Single crystalline substrates of (111)Si were heated in the vacuum chamber (5 × 10-4 Pa) up to 800°C prior to deposition in order to remove the native oxide11 and were kept at this temperature during deposition providing good conditions for the crystalline growth of AlN.7 After applying 8,000–15,000 laser pulses, the samples were cooled down to room temperature with an average cooling rate of either ~25°C/min (fast cooling) or ~5°C/min (slow cooling). Details on the processing, including pulse numbers, cooling rates, and also the film thickness are presented in Table 1. TABLE 1. Number of laser pulses applied, post-deposition cooling rates and the corresponding films thickness. Sample series A B C

Laser pulses

Cooling rate (°C/min)

Thickness (nm)

8,000 15,000 20,000

25 25 5

560 1,200 1,400

2.2. DIAGNOSTICS

The crystalline phases in the AlN films were determined with a large-angle (24 = 0–90°) X-ray Philips X’Pert diffractometer working with Cu radiation (Ȝ = 0.154056 nm). The XRD patterns were identified using the ASTM database.12 Assuming a spherical shape of the crystallites, their average grain size was deduced from the Bragg peaks with Sherrer’s formula.13

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Chemical analysis using X-ray Photoelectron Spectroscopy (XPS) technique was applied to get information about the chemical bonding. 3. Results and Discussion For crystalline AlN there are two types of lattice structures: hexagonal and cubic ones. The deposited films crystallize in these two phases but in different proportions. Typical X-ray diffraction patterns of the PLD films are shown in Figure 2. These spectra are representative of polycrystalline structure with predominant cubic phase of AlN crystallites, the corresponding lattice parameter being a = 0.4045 nm (ASTM 46-120012). The hexagonal phase is also observed in the case of fast cooling and greater thickness – series B (see Figure 2b), but it is quite possible, that hexagonal crystallites are also presented in the thinner films of series A. However, due to the smaller thickness, the Bragg peaks corresponding to these crystalline phases are very weak for the thinner samples and the signal-to-noise ratio is greater (Figure 2a). For deposition of the films followed by slow post-deposition cooling (series C), the XRD patterns show the presence of solely cubic phase (Figure 2c) and a (111) preferential orientation of the cubic crystallites. Bragg peaks related to the hexagonal crystalline phase are not detected in this XRD spectrum. The shape of the Bragg peaks reveals the presence of residual non-uniform strains and/or crystallites with sizes in the nanometres range. Concerning the internal strains we do not have enough number of peaks to estimate the stress contribution to the peak broadening. However, we observe asymmetry of the XRD peaks, which is an indication of an inhomogeneous film structure. This is clearly shown in Figure 2b, where the splitted C(002) peak is magnifies and inserted into the figure. The related inhomogeneities may have been caused by lattice parameter or composition variations in the films. We deconvoluted the C(002) peaks using a Philips Pro’Fit commercial program in order to determine the lattice parameter difference and the corresponding crystallite size. Precise calculations of the crystallite size could not be made due to the instrumental sources of error and the contribution of the residual stress to the peak broadening. However, rough estimation can be made from the most intensive peaks by applying the Sherrer formula and assuming spherical shape of the crystallite grains. The crystallite size values, as inferred from the FWHM of the strongest C(111) XRD peaks, are given in Table 2. For the different sample series we obtained crystallite size of 48.9–55.4 nm with an accuracy of ±5 nm.


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24(degree) Figure 2. XRD patterns of AlN films deposited with different number of laser pulses and at different post-deposition cooling rates.

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TABLE 2. Size of crystallites grown along C(111) preferential orientations. Sample series

A (fast cooling) B (fast cooling) C (slow cooling)

Peak center (24) 38.36 38.36 38.39

Lattice parameter (nm r 0.0005 nm) 0.4060 0.4064 0.4057

FWHM (24) 0.11 0.17 0.15

Crystallite size (nm r 5 nm) 55.4 48.9 54.2

For series B samples, as we mentioned above, the C(002) peak was splitted and, therefore, the crystallite size was calculated after deconvolution of this peak into two ones. The estimated diameter of the crystallite grains were 58.7 and 82.6 nm, respectively. The corresponding lattice parameters varied from 0.4066 to 0.4053 nm, most probably due to non-uniform strains in the structure. As mentioned in the Introduction, during PLD AlN films crystallize in either cubic or hexagonal phase depending on experimental conditions.7,8 The XRD analysis of samples revealed that under given deposition conditions we obtain AlN films with dominantly cubic structure. This is similar to results of Wen-Tai Lin et al.8 performing experiments at a Si substrate temperature of 630°C and N2 pressure of 20 Pa. In contrast other pulsed laser depositions at 700°C and in NH3 ambient7 resulted in hexagonal AlN films structure. The obtained AlN films were characterized with XPS in order to determine the nature of the chemical bonds. We measured the core level spectra of Al 2p and N 1s (Figure 3). The corresponding binding energies (BE) are presented in Table 3. For the slow cooled films we registered a maximum at 73.89 eV in the XPS spectrum. This energy position is reported to correspond to Al (2p3/2) spin-orbital split component of the transition in Al chemically bonded to N in AlN.14 The same observation was made in the case of fast cooling, where the Al peak appearing at 74.3 eV can be attributed to Al in AlN.15 However, we cannot exclude the possibility that at the surface we have also some oxygen and OH traces, because at this position of 73.9 eV Wagner et al.16 have attributed this maximum to Al (2p3/2) bonded with OH radical in Al(OH)3 configuration and Taylor17 has related the 74.3 eV maximum to Al in Al(OH)3 bond configuration. The maxima in the N 1s core level spectra (see Figure3a and b) appeared at 396.61 and 396.66 eV for fast and slow cooling rates, respectively, corresponded to N bonds in either AlN14 or NO/Al bond configurations reported in the literature.18 Taking into account that by the XRD method we did not detect any peaks corresponding to oxygen compounds in the films, we can conclude that even if there is any oxygen related material it is limited to the surface region only.


N 1s

20000 pulses slow cooling

Intensity (arb. unit)

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Al 2p

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15000 puses fast cooling

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80 78 76 74 72 70 68 66

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Binding energy (eV)

Figure 3. The XPS spectra of N 1s and Al 2p core levels in the slow cooled AlN samples (a) and fast cooled ones (b). TABLE 3. The BE of the peaks of Al 2p3/2 and N 1s in the case of slow and fast cooling and the related chemical bonding. Core level

Cooling rate

Al 2p3/2

Slow – 5°C/min

73.89

Fast – 25°C/min

74.34

Slow – 5°C/min Fast – 25°C/min

396.61 396.66

N 1s

BE (eV)

Bonding and related BE (eV) AlN Al(OH)3 AlN Al(OH)3 NO/Al AlN

– – – – – –

73.9 73.9 74.4 74.3 396.8 397.3

4. Conclusions The analysis of the XRD patterns of AlN films produced by low nitrogen pressure pulsed laser deposition at a nitrogen pressure of 0.1 Pa has revealed a cubic film structure with preferential (111) orientation of the crystallites. The average crystallite size was about 50 nm. Post-deposition cooling rates only affected the structural ordering. More specifically, fast quenching resulted in more disordered film structure. The chemical binding energies of the film surface were determined by XPS analysis and they were assigned to Al and N core levels, related to AlN, Al(OH)3 and NO/Al compounds.

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ACKNOWLEDGEMENTS

The authors acknowledge with thanks the support of this work under the 2004– 2007 Collaboration Agreement between the Bulgarian Academy of Sciences and Romanian Academy of Sciences.

References 1. Paisley, M., and Davis, R. (1993), J. Cryst. Growth. 127, 136–142. 2. Kitagawa, H., Shibutani, Y., and Ogata, S. (1995), Model. Simul. Mater. Sci. Eng. 3, 521–531. 3. Ho, C., Shing, T., and Li, P. (2004), Tamkang J. Sci. Eng. 7, 1–4. 4. Chu, K., Chao, C., Lee, F., and Huang, H. (2001), J. Electron. Mater. 30(1), 1–5. 5. Boo, J., Lee, S., Kim, Y., Park, J., Yu, K., and Kim, Y. (1999), Phys. Stat. Sol. (a) 176(1), 711–717. 6. Kipshidze, D., Schenk, H., Fissel, A., Kaiser, U., Schulze, J., Richter, W., Weihnacht, M., Kunze, R., and Kräusslich, J. (1999), Semiconductors 33(11), 1241–1246. 7. Vispute, R., Narayan, J., Wu, H., and Jagannadham, K. (1995), J. Appl. Phys. 77(9), 4724–4728. 8. Lin, W., Meng, L., Chen, G., and Liu, H. (1995), Appl. Phys. Lett. 66(16), 2066–2068. 9. Ren, Z., Lu, Y., Ni, H., Liew, T., Cheong, B., Chow, S., Ng, M., and Wang, J. (2000), J. Appl. Phys. 88(12), 7346–7350. 10. Petrov, I., Mojab, E., Powell, R., Greene, J., Hultman, L., and Sundgren, J. (1992), Appl. Phys. Lett. 60(20), 2491–2493. 11. Miyata, N., Shigeno, M., Arimoto, Y., and Ito, T. (1993), J. Appl. Phys. 74(8), 5275–5276. 12. Index to the Powder Diffraction File (2000), Published by Joint Committee on Powder Diffractions Standards. 13. Klug, H., and Alexander, L. (1962), X-Ray Diffraction Procedures, Wiley, New York, p. 491. 14. Taylor, J., and Rabalais, J. (1981), J. Chem. Phys. 75(4), 1735–1745. 15. McGuire, G., Schweitzer, G., and Carlson, T. (1973), Inorg. Chem. 12(10), 2450–2453. 16. Wagner, C., Passoja, E., Hillery, H., Kinisky, T., Six, H., Jansen, W., and Taylor, J. (1982), J. Vac. Sci. Technol. 21, 933. 17. Taylor, J. (1982), J. Vac. Sci. Technol. 20, 751. 18. Pashutski, A., and Folman, M. (1989), Surf. Sci. 216(3), 395–408.

FUNCTIONALIZATION OF MULTI-WALLED CARBON NANOTUBES (MWCNTS) M. MOHL, Z. KÓNYA*, Á. KUKOVECZ, AND I. KIRICSI Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1., 6720 Szeged, HUNGARY

Abstract – The surface of the CNTs can be chemically modified to impart a specific desired property. In general functionalization has usually been done by oxidation using HNO3, KMnO4, etc. The defects and the ends of the CNTs are thus functionalized by carboxyl groups. The current focus of the research is to develop a convenient and fast method for the functionalization of carbon nanotubes. We explore a covalent chemical strategy for functionalization of MWCNTs with amine-containing groups. The chemical interaction of the amine-containing group and the oxidized CNTs surface is confirmed by FT-IR and XPS.

Keywords: MWCNT, functionalization, amine-containing groups.

1. Introduction Carbon nanotubes are extremely promising materials for applications in materials science and medicinal chemistry. CNT consist of graphitic sheets, which have been rolled up into a cylindrical shape. Carbon nanotubes can be considered as attractive candidates in nanotechnological applications such as sensors or molecular tanks. However, the lack of solubility and the difficult manipulation in any solvents have imposed great limitation to the use of CNTs.1 2. Experimental The MWCNTs were prepared by a catalytic vapor decomposition process at 650ºC using acetylene as carbon source and 2.5% Co,Fe/MgO as catalyst.

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MWCNTs were first refluxed first in concentrated HNO3 for 24 h and second in H2O2 for 12 h. Afterwards the MWCNTs were sonicated in deionized water and KMnO4 solution was added drop wise. After 10 min vigorous stirring, citric acid solution was added to quench the KMnO4. It was filtrated, washed with deionized water and dried (A).2 To convert the carboxylic groups into – NH2 functional groups CNTs were first reacted with SOCl2 and after that with 1,8-diaminooctane (B). 3-aminopropyl-triethoxysilane (APTES) functionalized CNTs were prepared as follows. The as grown CNTs were first oxidized as described above, and then they were reacted with APTES dissolved in acetone under ultrasonic treatment (C).3 Rh containing CNTs were prepared as follows. The as grown CNTs were first oxidized as described above and after that they were treated with aqueous solution of Rh(III) in an ultrasonic bath for 2 h. Afterwards aqueous hydrogen peroxide solution was added drop wise under vigorous stirring and refluxed for 4 h at 80°C. The precipitate was filtered and washed by deionized water to neutral (D). 3. Result and Discussion The resultant composites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron dispersion X-ray (EDX) analysis. The Figure 1 shows the C 1s and O 1s core levels of the oxidized CNTs (A) and the amine-functionalized CNTs (B). The spectra corresponding to the oxidized sample show a large peak at 284.4 eV (C = C bonds), a smaller peak at 291.0 eV (O-COO or –COO bonds), another large peak at 531.6 eV (C=O bonds) and a smaller at 533.1 eV (O-C bonds). In the aminated samples (Figure 1B) the following bonds were assigned: 284.3 eV (C = C), 535.8 eV and 532.1 eV (C = O or/and O-C bonds) and 401.1 eV (-NH2 bonds).4 In Figure 2A and B, the cylindrical structure of CNTs are clearly revealed and show that the sample does not contain any bundles or aggregation of APTES. On TEM image (Figure 2B), RhO2 particles can not be seen on the surface of the CNTs, probably caused by their very small size. Figure 3 shows the FT-IR spectra of oxidized (A), aminated (B) and APTESfunctionalized (C) samples. Acidic treatment resulted in the appearance of a new peak at 1,715 cm-1 (Figure 3B) that corresponds to the ›C=O stretching indicating the introduction of carboxylic groups. The peak around 1,640 cm-1 (Figure 3) -NH bending vibration of amine group. The large peak at 3,436 cm-1 can be assigned to the vibrational modes of the –OH groups. The appearance of new peaks: 1,117 cm-1 and 1,043 cm-1 corroborated to Si-O-Si stretching and

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Si-O-C stretching can be due to the chemical interactions between silane and CNT. The XRD analysis result shown in Figure 3 demonstrates that the crystal structure of CNTs remains unchanged after supporting the rhodium oxide nanoparticles.

Figure 1. XPS plots of the oxidized CNT (A) and amine-functionalized sample (B).

Figure 2. TEM images of APTES functionalized nanotubes (left and middle frames) (C) and RhO2/CNT sample (right frame) (D).

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Figure 3. FT-IR spectra of (A) oxidized, (B) aminated, and (C) APTES functionalized CNT and XRD diffraction patterns of RhO2 /CNT.

Figure 4. EDX analysis of APTES functionalized sample (left frame) (C), RhO2 /CNT sample (right frame) (D).

The EDX investigation of APTES-functionalized CNTs shows a high concentration of Si (Figure 4A), while on Figure 4B the presence of Rh can be seen. 4. Conclusion The functionalization methods described in this article have proved to be successful for MWCNTs. All the functionalized CNTs can prove to be suitable for gas sensor applications.

References 1. Tasis, D., Tagmatarchis, N., Bianco, A., and Prato, M. (2006) Chem. Rev. 106, 1105–1136 2. Kordas, K., Mustonen, T., Toth, G., Jantunen, H., Lajunen, M., Soldano, C., Talapatra, S., Kar, S., Vajtai, R., and Ajayan, P. (2006) Small 2, 1021–1025 3. Shanmugharaj, A., Bae, J., Lee, K., Noh, W., Lee, S., and Ryu, S. (2006) Comp. Sci. Technol. 67, 1813–1822 4. Okpalugo, T., Papakonstantinou, P., Murphy, H., McLaughlin, J., and Brown, N. (2005) Carbon 43, 153–161.

SONOCHEMICAL SYNTHESIS OF INORGANIC NANOPARTICLES J. KIS-CSITÁRI1,2, Z. KÓNYA2* AND I. KIRICSI1,2 Bay Zoltán Foundation for Applied Research, Pf.:46, 3515 Miskolc-Egyetemváros, HUNGARY 2 Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, 6720 Szeged, HUNGARY 1

Abstract – Sonochemistry has applications across almost the whole breadth of chemistry. A number of theories were developed in order to explain how a 20 kHz sonic radiation can break chemical bonds. They all agree that main event in sonochemistry is cavitation, in other words the creation, growth and collapse of bubbles (so called hot spots) formed in the liquid. These hot spots have temperatures of roughly 5,000°C, pressures of about 500 atmospheres, and heating and cooling rates greater than 109 K/s. The enormous local temperature, pressure and the extraordinary heating/cooling rates generated by cavitational collapse provides an unusual mechanism for generating high energy chemistry. A novel sonochemical method for the continuous preparation of metal- and metal-oxide nanocrystalline materials has been developed. The products were characterized by transmission electron microscopy. We observed that the choice of the source metal salt, the reactant and the optimal usage of high-power ultrasound are both important in the formation of the nanostructures.

Keywords: Sonochemistry, cavitation, nanoparticles.

1. Introduction The chemical effects of ultrasound do not derive from a direct coupling of the acoustic field with chemical species on a molecular level. Instead, sonochemistry and sonoluminescence derive principally from acoustic cavitation: the formation, growth and implosive collapse of bubbles in liquids irradiated with highintensity ultrasound. Bubble collapse during cavitation serves as an effective means of concentrating the diffuse energy of sound: compression of a gas

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generates heat. When the compression of bubbles occurs during cavitation, heating is more rapid than thermal transport, creating a short-lived localized hot spot. The chemical effects of ultrasound can be divided into three general types: neat liquids; heterogeneous liquid-liquid; and heterogeneous liquid-solid systems. Over the past few years, the synthesis of inorganic materials has developed as one of the most important applications of sonochemistry.1 2. Experimental 2.1. SYNTHESIS OF MAGNETITE NANOPARTICLES

The synthesis of magnetite nanoparticles was carried out as follows: distilled water solution of FeCl2 and FeCl3 was sonicated using a high-intensity ultrasounic horn (Hielscher Ultrasound Homogenizater UIP1000, 20 mm,Ti horn, 20 kHz). During the sonication oleic acid was added. At the end of the synthesis a black solution of iron oxide was obtained. It was filtrated, washed with distilled water several times and the particles separeted by centrifugation. 2.2. SYNTHESIS OF SILVER NANOPARTICLES

The synthesis of polyvinylpyrrolidon (PVP)-stabilized silver nanoparticles was carried out as follows: distilled water solution of PVP was sonicated using high intensity ultrasonic horn (Hielscher Ultrasound Homogenizater UIP1000, 20 mm,Ti horn, 20 kHz). Solution of AgNO3 and NaBH4 were added and sonicated the mixture for 10 min. At the end of the synthesis a grey solution of silver was obtained. It was filtrared and washed with distilled water several times. 2.3. SYNTHESIS OF COPPER NANOPARTICLES

The synthesis of polyvinylpyrrolidon (PVP)-stabilized copper nanoparticles was carried out as follows: distilled water solution of PVP was sonicated using high intensity ultrasonic horn (Hielscher Ultrasound Homogenizater UIP1000, 20 mm,Ti horn, 20 kHz). Solution of CuSO4 and NaH2PO2 were added and sonicated the mixture for 10 min. A brown solution of copper nanoparticles was obtained which filtrated and washed with distilled water.

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2.4. SYNTHESIS OF COBALT-OXIDE

The cobalt-oxide was prepared as followed: distilled water solution of (NH4)2SO4 was sonicated using high intensity ultrasounic horn (Hielscher Ultrasound Homogenizater UIP1000, 20 mm,Ti horn, 20 kHz). CoSO4 and Zn were added and sonicated the mixture for 10 min. A green-grey solution of cobalt-oxide was obtained. It was filtrated and washed with distilled water. 2.5. CHARACTERIZATION

For the characterization of the nanoparticles a Philips CM 10 transmission electron microscopy was used. 3. Results and Discussion It is easier said than done to prepare nanoparticles using sonochemistry. Easy, because the sonotrode gives enough energy to force the reaction between the reactants, however, to control the reaction by itself is not an easy job. In order to synthesize nanoparticles with narrow size distribution, we have to control several reaction parameters, like reaction temperature, ultrasonic energy, ultraonic amplitude, reaction time, etc. Usually the formation of the nanoparticles can be divided into two steps; the first is the reaction between the reactants, while the second is the actual growng of the nanoparticles.2 We have found that the most important question is the rate of the reaction. If we can separate the two main steps, we can easily and precisely control the size and the shape of the particles.3 In the sonochemistry owing to its very high energy this approximation is a hard task since it is almost impossible to separate the two main steps. We have found that one of the most important questions is to properly choose the reactant; if the basic reaction is too fast, the sonochemical effect is hardly controllable. However, if we can find a reaction, which is running only if we use ultrasound, but then the rate of the reaction is very fast, and we can introduce a proper strucure directing agent, the reaction and the growing of the nanoparticles can be quasi separated. Figure 1 shows different nanoparticles synthesized by sonochemistry driven reactions. The size distribution of the nanoparticles is quite good. According to the TEM measurements, the average particles sizes are 8.4, 16.3, 28 and 137 nm for magnetite, silver, copper and cobalt-oxide, respectively. It can be conluded that for magnetite and silver, we can precisely control the reaction, while for copper and cobalt-oxide, we still have to work on the synthesis parameters.

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Figure 1. TEM images of magnetite (a), silver (b), copper (c) and cobalt-oxide (d) nanoparticles prepared by sonochemical method.

4. Conclusions We reported in this short communication the one step sonochemical synthesis of magnetite, silver, copper and cobalt-oxide nanoparticles. We have found that controlling the reaction precisely a quite narrow size distribution of the nanoarticles can be achieved. Although still much work remains to be done to find the best synthesis paraeters in order to optimize the reactions, it is a simple and efficient way to produce nanoparticles, which may find applications in many fields.

References 1. Suslick, K., Didenko, Y., Fang, M., Hyeon, T., Kolberg, K., McNamara, W. III, Mdleleni, M., and Wong, M. (1999), Philos. Trans. Roy. Soc. Lond A 357, 335–353. 2. Puntes, V., Krishnan, K., and Alivisatos, A. (2001), Science 291, 2115–2117. 3. Konya, Z., Puntes, V., Kiricsi, I., Zhu, J., Alivisatos, A., and Somorjai, G. (2002), Nano Lett., 2, 907–910.

NOVEL TRANSPARENT MOLECULAR CRYSTALS OF CARBON G. KHARLAMOVA1*, N. KIRILLOVA2, A. KHARLAMOV2, AND A. SKRIPNICHENKO2 1 Kiev National Taras Shevchenko University, 14 Glushkov str., room. 89, 03187 Kiev, UKRAINE 2 Frantsevich Institute for Problems of Materials Science, NAS of Ukraine, 3, Krjijanovskogo str., 03680 Kiev, UKRAINE

Abstract – Novel molecular transparent thread-like crystals of carbon at evaporating powdery carbon and transformation of molecules of aromatic hydrocarbons are obtained.

Keywords: Carbon threads, SiC nanostructures, exothermal nanosynthesis, sublimation.

1. Introduction The great variety of compounds of carbon, in particular with hydrogen, is caused, first of all, ability of its atom to form chains ɋ-ɋ – bonds. Directions of these bonds, as believed earlier, are strictly fixed. In diamond and molecule of methane atoms are coupled by means of only sp3 – bonds, and in graphite and in a molecule of benzene are coupled by means of only sp2 – bonds. However opening of new allotropic state of carbon, in particular fullerenes and carbon nanotubes, convincingly have shown, that the corners between ɋ-ɋ – bonds can be a little bit distinct from standard. In molecule of phenyl ethylene C8H8 and spheroidal molecule of hydrocarbon – dodecahedrane C20H20 the corners are not standard. Two of three allotropic state of carbon are diamond and fullerene are transparent and both have cubic crystal structure.

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2. Experimental Investigations As diamond, and fullerene, as against graphite, are non-conductors. Isotropic crystals of diamond are colorless whereas isotropic crystals of fullerene ɋ60 are reddish. Here we represent original experimental results concerning obtaining and certification of the new forms of transparent carbon. The basic attention will be given to thread-like crystals of carbon, which in transmission polarized light are, as a rule, bright colored (Figure 1). The similar painted threads of length more 40 mm and diameter up to 50 mkm are formed in conditions two radically different processes. However here one of them we shall consider in more details.

Figure 1. Optical microscopy in transparent polarized light images of painted trans-parent threads formed simultaneously at synthesis of silicon carbide.

3. Characterization 3.1. OPTICAL CHARACTERIZATION

The transparent painted threads are revealed by means of optical micro-scopy in a powdery product of synthesis of silicon carbide, where as initial reagents the powders of silicon and carbon are used. The study of these threads in transmission polarized light has shown that they are anisotropic crystals with a parameter of two-refraction approximately equal 1,575. (We shall note that the crystals of quartz are also anisotropic with a parameter of two-refraction close to 1,575).

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Figure 2. X-ray spectral analysis of transparent colored microthreads.

The element composition of the revealed transparent threads was estimated by means of X-ray spectrometer (Camebax), which typical spectra are presented on Figure 2. From spectra follows that the transparent threads basically consist of carbon, instead of silicon and oxygen. Some ends of microthreads of carbon have extremely unusual structure (Figure 1). The research of a transparent micro thread by means of scanning electronic microscopy also has shown that all carbon micro thread consists of great number of threads considerably smaller diameter and, hence, the micro thread is a rope. The chemical analysis of a painted micro thread by means of electron dispersive spectroscopy (EDS) has shown that this rope consists of more thin transparent threads from carbon. Painted transparent microthreads of carbon are formed parallel with growth of thread-like and tubular structures of silicon carbide. As we believe, the painted microthreads of carbon and nanothreads of silicon carbide grow with participating of atoms of evaporating carbon (Figure 3). It is possible, that new thread-like crystals of carbon grow immediately from intermediate thermostable molecules of carbon.

Figure 3. The scheme of growth process of carbon and SiC threads.

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The basic part of atoms of carbon goes on formation nanostructures of silicon carbide therefore microthreads of carbon is formed in very small amounts. The growth of anisotropic particles of carbon and silicon carbide, as well as many other substances,1–5 for example boron carbide, silicon and carbon nanostructure,6,7 synthesized by us, is fulfilled according to the mechanism of low temperature exothermic nanosynthesis. Exothermic nanosynthesis is self-accelerating growth of anisotropic particles of silicon carbide from atoms of evaporating substances, in particular of carbon and silicon. In vicinities of nanocenter the sublimation of initial powdery reagents essentially raises because of considerable growth of temperature at realizing of highly exothermic reaction of formation of silicon carbide. Let’s note that it was accepted earlier to consider that the interaction between solid reagents is accomplished according to diffusion the mechanism. The stage of diffusion of atoms of silicon or carbon through a layer of a formed product of reaction (silicon carbide) is considered limiting in this process. The painted transparent microthreads of carbon were obtained following a detailed study of process of thermal transformation of molecules aromatic hydrocarbons, in particular benzene and toluene. As it is visible (Figure 4), these microthreads, at least, on color and morphology are very similar to the previous microthreads, which grow at evaporation of carbon. Crystal-optical investigations have shown that these thread-like crystals are also anisotropic with a parameter of two-refraction close 1,575. The study of chemical structure of these microthreads by means of the X-ray analyzer has shown that they consist also basically of carbon. Pulverized threads of carbon are investigated by means of X-ray diffractional analysis (Figure 5) and infrared (IR) spectroscopy. .

Figure 4. Optical microscopy in transparent polarized light images of painted trans-parent threads formed at thermal transformation of aromatic hydrocarbons.

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Figure 5. X-ray diffraction patterns of coloured transparent threads of carbon.

4. Conclusions Threads have original hexagonal crystalline structure with the following parameters: a = 0.498 nm and c = 0.826 nm. In IR spectrum the group of lines from 1,157 up to 1,724 sm-1 is visible which are usually responsible for skeletal fluctuations of ɋ-ɋ – bonds in aromatic hydrocarbons. (In IR spectrum fullerene ɋ60 in this range there are three lines: 1,182; 1,428 and 1,539 sm-1). The lines in the range less 900 sm-1 can be referred to deformational fluctuations of aromatic ɋ-ɇ – bonds.

References 1. Kharlamov, A. I., Kirillova, N. V., Karachevtseva, L. A. and Kharlamova, A. A. (2003) Theoretical and Experimental Chemistry. 39 (6) pp.374–379. 2. Kharlamov, A. I. and Kirillova, N. V. (2002) Theoretical and Experimental Chemistry. 38 (1) pp. 59–63. 3. Kharlamov, A. I., Kirillova, N. V. and Kaverina, S. V. (2003) Theoretical and Experimental Chemistry. 39 (3), pp. 141–146. 4. Kholmanov, I., Kharlamov, A. I. and Milani, P., et al. (2002) Journal of Nanoscience and Nanotechnology. 2 (5), pp. 453–456. 5. Kharlamov, A. I., Kirillova, N. V. and Kaverina, S. V. (2002) Theoretical and Experimental Chemistry. 38 (4), pp. 232–236. 6. Kharlamov, A. I., Kirillova, N. V. and Ushkalov, L. N. (2006) Theoretical and Experimental Chemistry 42 (2), pp. 90–95. 7. Kharlamov, A. I., Loythenko, S. V., Ʉirillova, N. V., Kaverina, S. V. and Fomenko, V. V.

(2004) Report of Academia of Science of Ukraine (1), pp. 95–100 (Russian).

HYDROGEN MICROSENSOR BASED ON NIO THIN FILMS I. FASAKI1,2, M. ANTONIADOU1,2, A. GIANNOUDAKOS1,2, M. STAMATAKI1, M. KOMPITSAS1*, F. ROUBANIKALANTZOPOULOU2, I. HOTOVY3, AND V. REHACEK3 1 National Hellenic Research Foundation, Theoretical and Physical Chemistry Institute, Vasileos Konstantinou Ave. 48, 11635 Athens, GREECE 2 National Technical University of Athens, School of Chemical Eng. 9 Iroon Polytechniou St., 15780 Zografou, Athens, GREECE 3 Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, SLOVAK REPUBLIC

Abstract – A multitude of industries use H2 either as part of their process or as a fuel. All these applications motivate nowadays the development of hydrogen sensor devices which enable its safe and controlled use. Since H2 is explosive above the lower explosion limit at 40,000 ppm, devices which permit the detection of its presence and measure its concentration become indispensable. In this work, we present a microsensor based on NiO thin films produced with dc reactive magnetron sputtering on GaAs, with an incorporated Pt heater, all on a DO-8 package ready for use. The microsensor was tested to H2 concentrations 5,000 and 10,000 ppm at different working temperatures. The change of the electrical resistance of NiO thin films was the signal for hydrogen sensing. The response of the sensor was not proportional to concentration of the gas neither to the working temperature.

Keywords: NiO, H2, gas sensors 1. Introduction Hydrogen as an industrial gas is being used by a multitude of industries. Some of the major industries today are chemical industries (refining crude oil, plastics, reducing environment in float glass industry, etc.), food industry (hydrogenation

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of oils and fats), semiconductor industry (as processing gas in thin film de-position and in annealing atmosphere), transportation (as fuel in fuel cells, rockets for space vehicles) and use H2 either as part of their process or as a fuel.1 All these applications necessitate the development of hydrogen sensor devices which enable its safe and controlled use. Since hydrogen is explosive above the lower explosion limit (LEL – 40,000 ppm) devices which permit the detection of its presence and its concentration become indispensable.2 Nickel oxide (NiO) is frequently considered as a model for p-type semiconductors. It is a wide band-gap (Eg | 4 eV) transition metal oxide, with a cubic rock-salt structure and antiferromagnetic properties below its Néel temperature, 523 K.3 Due to their excellent chemical stability NiO films have a wide range of applications as catalysts,4 electrochromic display devices5 and fuel cells.6 Moreover, recent works have shown that thin NiO films are attractive sensing material in gas and humidity detection devices.7,8 NiO thin films have been deposited by different techniques, including chemical self-assembly,9 sol-gel,10 RF,11 DC sputtering12 and recently pulsed laser deposition (PLD).6,13,14 The preparation method is fundamental in determining the microstructure and consequently the functional properties of the synthesized materials. In this work we demonstrate the response of NiO thin films to hydrogen. 2. Experimental The different layers of the microsensor were deposited by dc reactive magnetron sputtering on a GaAs substrate (Figure 1). By using a suitable mask and photolithographic process, platinum integrated heater having a shape of meander was realized. A layer of polyimide was deposited on Pt heater for electrical isolation. At the top NiO thin films were deposited. The microsensor was placed on a DO-8 package ready for use. The sample was mounted inside a gas test chamber which was evacuated at 10–2 mbar. The chamber was filled with dry air and then heated at different temperatures. The microsensor was tested at 5,000 ppm (working temperatures 210ºC, 240ºC and 280ºC) and 10,000 ppm (working temperatures 185ºC and 205ºC) of H2. The concentration of hydrogen was calculated using the partial pressures of the sensing gas and air in the chamber. The change of the electrical resistance of NiO thin films was the signal for hydrogen sensing. 3. Results and Discussion The response of the microsensor at 5,000 ppm (working temperature 240ºC) and at 10,000 ppm (205ºC) of H2 is seen at Figure 2. The response of the sensor

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The increase of the electrical resistance was expected, considering that NiO is a p-type semiconductor and hydrogen is a reducing gas. As it is known, the NiO p-type conductivity is due to the non-stoichiometry of the prepared samples, in which vacancies occur in cation sites, i.e. the NiO films showed a metal deficiency.15 Atmospheric oxygen is expected to be present on the surface of NiO as O2,adsí and Oadsí negative charged chemical species. The high coverage with adsorbed oxygen species causes an increase in the concentration of the holes of the NiO film and an increase in its conductivity. The presence of H2 causes a decrease of the electrical conductivity, because H2 reacts with adsorbed oxygen and forms water vapor, injecting electrons in the NiO p-type semiconducting film2:

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2H2,gas + O2,adsí ļ 2H2Ovap + eí H2,gas + Oadsí ļ H2Ovap + eí 4. Conclusions A NiO microsensor has been developed by DC Magnetron Sputtering. The sensor’s response to H2 down to 5,000 ppm has been recorded. The resistance increased as expected for a p-type semiconductor in a reducing gas atmosphere.

References 1. Fuel Cell Standards Committee (2001) “Basic Consideration for Safety of Hydrogen Systems”, Technical Report ISO TC 197 N166, International Standards Organization. 2. Hotovy, I., Huran, J., Siciliano, P., Capone, S., Spiess, L., and Rehacek, V. (2004) Sens Actuators B, 103, 300. 3. Seehra, M., and Giebultowicz, T. (1988) Phys. Rev. B 38, 11898. 4. Blaumer, M., and Freund, H. (1999) Prog. Surf. Sci. 61, 127. 5. Jiao, Z., Wu, M., Qin, Z., and Xu, H (2003) Nanotechnology 14, 458. 6. Chen, X., Wu, N., Smith, L., and Ignatiev, A. (2004) Appl. Phys. Lett. 84, 2700. 7. Shi, J., Zhu, Y., Zhang, X., and Baeyens, W. (2004) Trends Anal. Chem. 23, 1. 8. Ando, M., Sato, Y., Tamura, S., and Kobayashi, T. (1999) Solid State Ionics 121, 307. 9. Wang, Y., Ma, C., Sun, X., and Li, H. (2004) Micropor. Mesopor. Mater. 71, 99. 10. Jiao, Z., Wu, M., Qin, Z., and Xu, H. (2003) Nanotechnology 14, 458. 11. Souza Cruz, T., Hleinke, M., and Gorenstein, A. (2002) Appl. Phys. Lett. 81, 4922. 12. Lee, M., Seo, S., Seo, D., Jeong, E., and Yoo, I. (2004) Integr. Ferroelectr. 68, 19. 13. Zbroniec, L., Sasaki, T., and Koshizaki, N. (2005) J. Ceram. Process. Res. 6, 134. 14. Sasi, B., and Gopchandran, K. (2007) Nanotechnology 18, 115613. 15. Lee, M., Seo, S., Seo, D., Jeong, E., and Yoo, I. (2004) Integr. Ferroelectr. 68, 19–25.

DESIGN AND CHARACTERIZATION OF STYRENE-BASED PROTON EXCHANGE MEMBRANES D. EBRASU1*, I. PETREANU1, L. PATULARU1, I. STEFANESCU1, AND M. VALEANU2 1 National Research Institute of Cryogenics and Isotopic Technologies, Uzinei Street no. 4, 240050, Rm. Valcea, ROMANIA 2 National Institute of Materials Physics, P.O. Box MG-7, 077125, ROMANIA

Abstract – This paper deals with preparation of PEM, based on commercial block copolymer of the styrene-butadiene. The copolymer was structurally changed by sulfonation followed by cross linking, in order to design a Proton Exchange Membrane for Fuel Cells. The membranes were structural tested by FTIR Spectroscopy and Scanning Electron Microscopy. Ionic Exchange Capacity (IEC) and thermal behavior by Differential Scanning Calorimetry (DSC) were measured too.

Keywords: Membrane, polymer, fuel cells

1. Introduction The commercial breakthrough of fuel cells is hindered by the high price of fuel cell components. PEM is the most expensive part of the fuel cell. Lower prices will be achieved by developing new materials and improving performance. Perfluorinated copolymers are the current state-of-the-art proton exchange membranes (PEM).1 The typical operation temperature of PEMFCs is in the range of 60–90°C.2 This operating temperature range is currently limited by the perfluorinated proton exchange membrane. Tri-block copolymers produced ______ *

To whom correspondence should be addressed: D. Ebrasu, email: [email protected]


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by DAIS-Analytic Corporation which may be based on sulfonated styrene-coethyleneco-butylene have been described in the literature.3 These sulfonated Kraton®-type block copolymers are post-sulfonated. The stability of these aliphatic hydrocarbon copolymers is, in general, inferior to the current state-ofthe-art perfluorinated copolymers. For this reason, the DAIS membranes are being promoted for the low temperature (Ca10-xSrxHA (x = 0–1)@ were prepared by direct synthesis in aqueous medium at 90°C. Sr2+ insertion led to a decrease of crystallinity degree, which accounted for the simultaneous reduction of the crystal dimensions. For PLD experiments, we used an UV excimer (KrF*) laser source (248 nm, ~7.4 ns) operating at a repetition rate of 2 Hz. The fluence during target irradiation was set at 2.4 J/cm2, and substrate temperature kept at 400°C. The depositions were performed from HA at different degrees of Sr2+ substitution for Ca2+ (x = 0; 0.1; 0.5; 1). All structures were post-treated in a H2O enriched atmosphere for 6 h. The results of structural and morphological characterizations carried out on the obtained structures indicated that the coatings, which adhered well to the substrates, were made of crystalline HA and contained strontium with a (Ca + Sr)/P molar ratio close to the stoichiometric value of HA.

______ *

To whom correspondence should be addressed: C. Capuccini, email: [email protected]


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Keywords: Isomorphous substitution, strontium, hydroxyapatite, PLD

1. Introduction Apatites are widely spread in nature; in particular, the inorganic phase of the hard tissues of vertebrates can be assimilated to the synthetic hydroxyapatite Ca10(PO4)6(OH)2, CaHA. The similarity with biological apatites accounts for the high biocompatibility of synthetic CaHA. This bioactive ceramic material does not possess acceptable mechanical properties, as it is brittle in bulk. However, it does demonstrate significant potential for use as a coating on metallic orthopaedic and dental prostheses.1 At present, titanium and its alloys due to their excellent mechanical and biomedical properties are the most widely used materials for the production of metal implants.2 The presence of a CaHA coating improves osteointegration and creates a barrier to the release of metallic elements from the implant. Pulsed laser deposition has proved to be a competetive technique for growing thin calcium phosphate structures on metallic substrates. PLD utilizes a short, generally UV pulsed laser beam that is focused onto a rotating target placed inside a reaction chamber, where a controlled atmosphere can be maintained. The species that are expulsed by each subse-quent laser pulse form the coating as they reach the substrate, which can also be heated to a fixed temperature. The stoichiometry and crystallinity of the deposited material can be selected by a proper choice of the ablation and deposition parameters.3–7 The high stability and flexibility of the hydroxyapatite structure justify the wide variety of possible ionic substitutions.6 Among the bivalent cations that can replace calcium in CaHA, strontium has attracted a remarkable interest for its potential biological role. Strontium is present in the mineral phase of the bone, especially in the regions of high metabolic turnover,8 and its beneficial effect in the treatment of osteoporosis is well known.9 In vitro, strontium promotes the proliferation of osteoblasts and decreases the number and activity of osteoclasts10,11; in addition, strontium administration reduces bone resorption and stimulates bone formation.12–14 Strontium can replace calcium in the HA structure over the whole range of composition. The solid solutions that have been obtai-ned by hydrothermal methods or by treatment at high temperatures, display a linear variation with composition in the lattice parameters, whereas different data have been reported on the preferential substitution site of Sr for Ca in CaHA.15–17 To better clarify the interaction with HA structure of Sr, we previously synthesized and characterized Sr-Ca-HA solid solutions across the whole range of concentrations and proved that Sr when in low concentration showed an unexpected preference for site (1) of the HA structure.6 In this paper, we investigate the possibility to obtain thin CaHA films at different degrees of Sr

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substitution for calcium, up to 10%. The films were grown on pure Ti substrates by pulsed laser deposition, which provides several advantages over other techniques for preparing HA thin films. Specifically, PLD allows precise control over HA growth parameters at low deposition temperatures. An appropriate selection of the deposition and post-deposition parameters can result in HA thin film microstructures and nanostructures having unique biological properties. One of the most important and successful biomedical applications of calcium phosphates is in coatings of endosseous implants.18 The deposition of calcium phosphate coatings on titanium seems to enhance the bioactivity of the surface, which improves fixation between hard tissue and the metal implant and stimulates bone apposition.19,20 The use of PLD has lately been extended to coating Ti substrates with calcium phosphate.20–23 Compared with other physical techniques, the use of laser light in PLD has the additional advantage of causing much lower pollution of the deposited material.3,4,24 The presence of Sr2+ ions is all the more interesting as awareness of their biological role has recently increased following the development of strontium ranelate, a drug that has been shown to reduce the incidence of fractures in osteoporotic patients.25–27 2. Synthesis and General Characterization (Ca–Sr) hydroxyapatites (Ca–Sr–HA) with Sr/(Ca + Sr) molar ratios in the range from 0 to 0.1 were synthesized in N2 atmosphere using 50 mL solutions with different Sr/(Ca + Sr) ratios that were prepared by dissolving the appropriate amounts of Ca(NO3)2·4H2O and Sr(NO3)2 in CO2-free deionized water and adjusting the pH to 10 with NH4OH. The total concentration of [Ca2+] + [Sr2+] was 1.08 M. The solution was heated to 90°C, and 50 mL of 0.65 M (NH4)2HPO4 solution, pH 10 adjusted with NH4OH, was added dropwise under stirring. The precipitate was maintained in contact with the reaction solution for 5 h at 90 °C under stirring, then centrifuged at 10,000 rpm for 10 min and washed repeatedly with distilled water. The product was dried at 37°C overnight. X-ray diffraction analyses were carried out using a PANalytical X’Pert PRO powder diffractometer equipped with a monochromator in the diffracted beam. Cu KĮ radiation was used (40 mA, 40 kV). The 2ș range was 10–60 at a scanning speed of 0.75/min. To evaluate the coherence lengths of the crystals and perform the line profile analysis, further X-ray powder data were collected over the 2ș range in the step scanning mode with a fixed counting time of 10s per 0.030/step. Calcium and strontium contents were determined using a Perkin Elmer AAnalyst 400 atomic absorption spectrophotometer (Ȝ(Ca) = 422.7 nm; Ȝ(Sr) = 460.7 nm). The samples were diluted to an appropriate volume with 10% lanthanum in 50% HCl, in order to suppress interferences. Phosphorus

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content was determined spectrophotometrically in molybdovanadophosphoric acid using a Varian Cary50Bio instrument (Ȝ = 400 nm).28 For TEM investigations, a Philips CM 100 transmission electron microscope operating at 80 kV was used. A small amount of powder was dispersed in ethanol and submitted to ultrasonication. Furthermore, a drop of the calcium phosphate suspension was transferred onto holey carbon foils supported on conventional copper microgrids. 3. Thin Films Generation and General Characterization Disk shaped targets (13 mm in diameter and 1 mm in thickness) were manufactured by pressing the Sr-doped CaHA powders at 3 MPa and sintering at 380°C for 6 h The films were pulsed laser deposited on etched Ti substrates. In our experiments, we used an UV KrF* excimer laser source (Ȝ = 248 nm, Ĳ ~ 7.4 ns). The reaction chamber was evacuated down to a residual pressure of 10í4 Pa prior to every deposition. Films were deposited in 50 Pa water vapor flux on substrates heated to 400°C. The substrates were placed parallel to the targets 4 cm away from them. Fluence was set at 2.4 J cmí2 and we applied 25,000 subsequent laser pulses for each deposition. The as-deposited samples were submitted to annealing treatments in water vapor and ambient pressure for 6 h at temperatures identical to those applied during deposition. The average thickness of the obtained structures, measured by profilometry, was ~1ȝm. Grazing incidence XRD measurements were performed on the coatings with an X’Pert Philips Diffractometer using CuKĮ radiation and a grazing angle of 0.3–1.0°. The 2ș angles ranged from 10° to 40° with a 0.005°/s scanning speed. Morphological investigations of the synthesized products were conducted using a Philips XL-20 Scanning Electron Microscope. The samples were sputter coated with gold before examination. EDX analyses were also performed on uncoated specimens. 4. Results and Discussion The X-ray diffraction patterns of the solid products synthesized with different Sr/(Ca + Sr) molar ratios are shown in Figure 1. As all of the patterns show, they are made of hydroxyapatite as a unique crystalline phase. The patterns of the samples corresponding to CaHA (Sr0%) display well-defined sharp peaks in agreement with a high degree of their crystallinity. On the other


393

Figure 1. Powder X-ray diffraction patterns of (a) Sr0; (b) Sr5 and (c) Sr10. The strontium content in the solid phase is given in Table 2.

hand, the patterns of the samples containing both Ca and Sr generally exhibit broader diffraction peaks. This is in agreement with a reduced degree of crystallinity of the mixed Ca–Sr–HA, which suggests an increasing difficulty for CaHA to host a larger strontium amount (Ca 2+ ionic radius = 0.100 nm; Sr2+ ionic radius = 0.118 nm). The line broadening of the 0 0 2 and 3 1 0 reflections was used to evaluate the length of the coherent domains (Ĳh k l) both along the c-axis and perpendicular to it. The values Ĳh k l were calculated from the widths at half maximum intensity (ȕ1/2) using the Scherrer equation:

Ĳ

hkl

KO , E 1/ 2 cos T

(1)

where Ȝ is the wavelength, ș the diffraction angle and K a constant depending on crystal habit (chosen as 0.9). The silicon standard peak 111 was used to evaluate the instrumental broadening. The values of Ĳ002 shift from 469(±5) to 385(±5) Å, those of Ĳ310 decrease from 224(±6) to 153(±6) Å on increasing strontium content up to 10% (Table 1).

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TABLE 1. Lengths of the coherent domains along the 002 and 310 directions. Samples

Ĳ0 0 2 (Å)

Ĳ3 1 0 (Å)

Sr0 Sr5 Sr10

469 (5) 396 (1) 385 (5)

224 (6) 217 (4) 153 (6)

The relative amount of strontium in the solid products, ȤSr, as evaluated through atomic absorption spectrometry is presented in Table 2 as a function of the strontium content in solution. The values of ȤSr increase on increasing Sr/(Ca + Sr) in the starting solution, in agreement with the quantitative incorporation of strontium in the solid phase. The ratio between the two cations in the solid phase is slightly smaller than that in the synthesis solution. The isomorphous substitution does not significantly affect the stoichiometry of HA, as can be deduced from the (Ca + Sr)/P molar ratio, which supports a mean value of 1.68 ± 0.03, very close to the stoichiometric value of 1.67, independently of the Ca and Sr contents. The TEM investigation results indicate that the morphology of the apatite crystals is little affected by the chemical composition. Ca–HA is constituted of plate-shaped crystals, with mean dimensions up to about 200 × 40 nm2 (Figure 2a). By low Sr2+ contents (0 ȤSr2 +d 0.1), the Ca–Sr–HA nanocrystals display more perturbed shapes and ill-defined edges (Figure 2b), in agreement with the lower degree of crystallinity that is revealed by the broadening of the X-ray diffraction peaks. TABLE 2. Relative amount of strontium in the solid products, evaluated through atomic absorption spectrometry reported as a function of strontium content in solution. Sample

Sr0 Sr5 Sr10

Sr/(Ca + Sr) molar ratio in solution 0 0.05 0.10

Sr/(Ca + Sr) = ȤSr molar ratio in the solid product 0 0.03 0.07

The X-ray diffraction patterns of the PLD thin films are shown in Figure 3. As those recorded from the powders, all these patterns indicate that the films are made of hydroxyapatite as a unique crystalline phase and display welldefined sharp peaks in agreement with a high degree of crystallinity achieved by the Pulsed Laser Deposition technique.


(a)

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(b)

Figure 2. TEM micrographs of (a) Sr0; (b) Sr10. Scale bars = 200 nm.

Figure 3. Grazing incidence X-ray diffraction patterns of thin films deposited from (a) Sr0; (b) Sr5; (c) Sr10 samples. Asterisks indicate the reflections due to Ti.

The SEM investigation results (Figure 4) show that the apatite thin films exhibit a granular surface, with mean grain dimensions smaller than 1 Pm. The presence of Sr2+ does not affect the morphology of the coating, which is close to that previously obtained from apatites of different composition.29 The results of EDX indicate [(Sr/Sr + Ca)100] values of 7% and 10% for the thin films deposited from Sr5% and Sr10% powders, respectively. The data are clearly in excess with respect to the results of the chemical analysis carried out on the original powders. Although the EDX method is less reliable than atomic absorption spectroscopy (AAS), it confirms that PLD succeeds in preserving the same composition in the thin films as in the initial powder.

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(a)

(b)

Figure 4. SEM micrographs of thin films deposited from (a) Sr0; (b) Sr10 samples. Scale bars = 5 Pm.

5. Conclusions The main contributions reported in this paper deal with (i) the chemical synthesis of Ca-Sr-HA powders and (ii) their use in nanostructured depositions onto Ti substrates by PLD. We therefore summarized our main results accordingly. I. We synthesized in N2 atmosphere (Ca–Sr) hydroxyapatite (Ca–Sr–HA) powders with Sr/(Ca + Sr) molar ratios in the range from 0 to 0.1. (1) The XRD patterns of the samples corresponding to CaHA (Sr0%) display well-defined sharp peaks in agreement with a high degree of crystallinity. The samples containing both Ca and Sr generally exhibit broader diffraction peaks, indicating a reduced degree of crystallinity of the mixed Ca–Sr–HA. (2) The TEM investigations indicate that the morphology of the apatite crystals is little affected by the chemical composition. Ca–HA consists of plate-shaped crystals with mean dimensions up to about 200 × 40 nm2. The Ca–Sr–HA nanocrystals with low Sr2+ contents display more perturbed shapes and ill-defined edges, in line with the lower degree of crystallinity the broadened X-ray diffraction peaks reveal. II. Nanostructured Ca-Sr-HA coatings were successfully grown on Ti substrates by PLD. (1) The XRD recorded patterns show that the films are made of hydroxyapatite as a unique crystalline phase and display well-defined sharp peaks. (2) The Sr:HA films exhibit a granular surface, with mean grain dimensions smaller than 1 ȝm, as seen from the SEM micrographs. The presence of Sr2+ does not affect the morphology of the coating, which remains close to that previously obtained from apatites of different composition. The EDX analyses reveal [(Sr/Sr + Ca) %100] values of 7% and 10% for the thin films deposited from Sr5% and Sr10% powders, respectively.


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We conclude that PLD is fully appropriate for obtaining stoichiometric, well crystallized Ca-Sr-HA nanostructures compatible with further use in biomimetic Ti implants applications. ACKNOWLEDGEMENTS

We acknowledge with thanks the support provided to this research under theme 36 “New biomimetic calcium phosphate coatings for metallic implants” in the framework of the 2006–2008 Agreement on Scientific and Technological Co-operation between Italy and Romania.

References 1. Calvert P., De Rossi D., Petty M.C., Editors (2007) Material Science and Engineering C 27, 484. 2. Lacefield W. (1999) Advance in Dental Research 12, 21. 3. Chrisey D. and Hubler G., Editors (1994) Pulsed laser deposition of thin films, Wiley, New York. 4. Mihailescu I. and Gyorgy E. (1999) Pulsed laser deposition: an overview. In: Asakura T., Editor, Trends in optics and photonics. Springer series in optical science, Springer, Berlin, 201. 5. Bauerle D., Editor (1996) Laser processing and chemistry, Springer, Berlin. 6. Bigi A., Boanini E., Capuccini C., and Gazzano M. (2007) Inorganica Chimica Acta, 360, 1009. 7. Nelea V., Mihailescu I., and Jelinek M. (2007) Pulsed laser deposition of thin films: ApplicationsLED growth of functional materials. In: Robert Eason J., Editor, J. Wiley & sons Inc., Hoboken, New Jersey, pp. 421–456. 8. Blake G., Zivanovic M., and McEwan A. (1986) European Journal of Nuclear Medicine 12, 447. 9. Shorr E. and Carter A. (1952) Bulletin of the Hospital for Joint Diseases Orthopaedic Institute 13, 59. 10. Canalis E., Hott M., Deloffre P., Tsouderos Y., and Marie P. (1996) Bone 18, 517. 11. Chang W., Tu C., Chen T., Komuwes L., Oda Y., Pratt S., Miller S., and Shoback D., Endocrinology 40, 5883. 12. Grynpas M., Hamilton E., Cheung R., Tsouderos Y., Deloffre P., Hott M., and Marie P., Bone 18, 253 13. Marie P., Ammann P., Boivin G., and Rey C. (2001) Calcified Tissue International 69, 121. 14. Dahl S., Allain P., Marie P., Mauras Y., Boivin G., Ammann P., Tsouderos Y., Delmas P., and Christiansen C. (2001) Bone 28, 446. 15. Zhu K., Yanagisawa K., Shimanouchi R., Onda A., and Kajiyoshi K. (2006) Journal of the European Ceramic Society 26, 509. 16. Kikuchi M., Yamazaki A., Otsuka R., Akao M., and Aoki H. (1994) Journal of Solid State Chemistry 113, 373. 17. Bigi A., Bracci B., Cuisinier F., Elkaim R., Fini M., Mayer I., Mihailescu I.N., Socol G., Sturba L., and Torricelli P. (2005) Biomaterials 26, 2381.

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18. Sun L., Berndt C., Gross K., and Kucuk A. (2001) Journal of Biomedical Materials Research Applied Biomaterials 58, 570. 19. Ducheyne P. and Qiu Q. (1999) Biomaterials 20, 2287. 20. Torrisi L. and Setola R. (1993) Thin Solid Films 227, 32. 21. Fernández-Pradas J., Sardin G., Clèries L., Serra P., Ferrater C., and Morenza J. (1998) Thin Solid Films 317, 393. 22. Arias J., Garcia-Sanz F., Mayor M., Chiussi S., Pou J., Leon B., and Perez-Amor M. (1998) Biomaterials 19, 883. 23. Antonov, E., Bagratashvili V., Popov V., Sobol E., Davies M., Tendler S., Roberts C., and Howdle, S. (1997) Biomaterials 18, 1043. 24. Bauerle D. (1996) Laser processing and chemistry, Springer, Berlin. 25. Meunier P., Lorenc R., and Smith I., et al. (2002) Osteoporosis International 13 (Suppl. 3), 66. 26. Meunier P., Roux C., Seeman E., Ortolani S., Badurski J., et al. (2004) The New England Journal of Medicine 350, 459. 27. Reginster J., Sawicki A., Devogelaer J., Padrino J., et al. (2002) Osteoporosis International 13 (Suppl. 3). 28. Quinlan K. and De Sesa M. (1955) Analytical Chemistry 27, 1626. 29. Gyorgy E., Torricelli P., Socol G., Iliescu M., Mayer L., Mihailescu I., Bigi A., and Werckman J. (2004) Journal of Biomedical Materials Research 71A, 353.

GROWING THIN FILMS OF CHARGE DENSITY WAVE SYSTEM Rb0.3MoO3 BY PULSED LASER DEPOSITION D. DOMINKO1*, D. STAREŠINIû1, K. BILJAKOVIû1, K. SALAMON1, O. MILAT1, A. TOMELJAK2, D. MIHAILOVIû2, J. DEMŠAR2, G. SOCOL3, C. RISTOSCU3, I.N. MIHAILESCU3, AND J. MARCUS4 1 Institute of Physics, HR-10001, Zagreb, P.O. Box 304, CROATIA 2 J. Stefan" Institute, Jamova 39, SI-1000, Ljubljana, SLOVENIA 3 National Institute for Lasers, Plasma and Radiation Physics, PO Box MG-54, Bucharest-Magurele, ROMANIA 4 Institut Neel, CNRS, BP 166, F-38042, Grenoble, FRANCE

Abstract – We prepared high-quality epitaxial thin films of charge density wave system Rb0.3MoO3 on MgO substrate. In continuation to the femtosecond spectroscopy performed in,1 new studies of femtosecond time-resolved Terahertz conductivity dynamics, necessary to directly probe the relaxation processes of photo-excited carriers, need high-quality thin films. Film morphology was characterized by EDS, AFM, and STM, while optical and electrical properties were studied using FTIR and UV-Vis spectrometers, and terahertz conductivity measurements, respectively. Certain degree of crystallinity has been observed in some films by x-ray diffraction.

Keywords: Thin films, blue bronze

1. Introduction Blue bronzes K0.3MoO3 and Rb0.3MoO3, the two most known CDW compounds, undergo a Peierls transition into 3D CDW ordered state at 183 K. Amplitudons and phasons, excitations of the amplitude and phase of the CDW complex order parameter were widely investigated,1 as they are at the origin of many fascinating CDW properties. Collective charge transport by moving CDW is probably

______ *

To whom the correspondence should be addressed: D. Dominko, email: [email protected]


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the most known one. CDW sliding (and pining) was studied in bulk and only recently in film compounds.2 CDW properties on mesoscopic scales, as well as possible applications of the CDW effects, especially in thin films, are potentially attractive. 2. Motivation Femtosecond spectroscopy investigation on blue bronze gave first information on the collective (amplitudon and phason) and single particle excitations. With optical pump – terahertz probe in transmission geometry we could measure the dynamics of the photoexcited phason directly (since it is IR active and lies in this frequency range).

Figure 1. Arrhenius plot of temperature dependence of resistivity for various sample thickness.3

Finite size effects were investigated on 1D2,3 and 3D CDW systems only. TP and width of transition increase for wires thinner than transverse correlation length (observed on TaS3 only), Figure 1 As similar effects were observed in doped samples (doping induced/enhanced CDW), interplay doping/dimensionality can be an additional information. 3. Deposition UV excimer laser system has been used, with Ȝ = 248 nm and FWHM p 7 ns. Deposition rate is estimated to be §0.5 nm/pulse. Conditions that influence film quality are: (1) preferable commensurability between substrate surface lattice and lattice of film deposit, (2) higher substrate temperature results larger grains and higher corrugation, (3) oxygen pressure reduces oxygen loss during deposition,4 (4) higher deposition rate result in smaller grains. 5 Thin films of various thicknesses were grown by pulsed laser deposition applying 2,000 to

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6,000 subsequent laser pulses with an excimer laser system COMPexPro 205 (Lambda Physik). The depositions were performed in oxygen pressure 0.1–0.3 Torr on substrates heated to 400°C. The incident laser fluence was 2.5 J/cm2 (Table 1). It has been shown5 that using SrTiO3 [510] substrate, chains are aligned in one direction. With our deposition conditions we failed so far in obtaining well defined chain direction. TABLE 1 Deposition conditions.

Sample PO2 (Torr) Pulse No.

1 2 3 4 5 6

0.3 0.3 0.1 0.1 0.2 0.2

2,000 3,000 2,000 6,000 2,000 6,000

Other information

Substrate MgO (100) T (°C) 400 3 FluenceO2 (J/cm ) 2.5 Dsubstrate-target (cm) 5

4. Film Characterization 4.1. CROSS POLARISED MICROSCOPY

Figure 2 clearly shows homogeneous structure for sample 1, while samples 4 and 5 exhibit nonhomogeneous structure. In addition, leaf-like texture has been found in samples 3 and 4. Similar structures have been observed for films deposited in the same oxygen pressure, indicating its crucial roll for obtaining films with higher degree of crystallinity.

200X

200x200x

400

Figure 2. SEM images of samples 1, 4 and 5.

4.2. AFM, SEM, EDXS AND RBS

Due to high corrugation of the rest of the films only sample 5 could have been scanned by AFM. Walls were observed in surface structure. SEM results from previous works5 have shown clear substrate dependence of film quality. So far

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SEM on our samples did not show similar chainlike features. Only Rb and Mo concentration ratios could be deduced from EDXS and RBS, due to oxygen contained in MgO substrate. We reached desired stoichiometry for Rb0.3MoO3. 4.3. X-RAY AND THZ CONDUCTIVITY

X-ray has been performed on samples 1, 2, 3 and 5. Figure 3 shows that sample 1 (as is the case for sample 2 as well) has the right peak as expected for blue bronze, at 11Û. GISAX showed oscillations for samples 3 and 5, which revealed smoother surface and consequently amorphous structure. Thus samples 1 and 2 are most likely to have crystalline structure of blue bronze. With GISAX film thickness of sample 5 has been estimated to be 90 nm. Two orders of magnitude lower conductivity at 1 THz is obtained from film 4, compared to bulk 4.

Figure 3. X-ray for sample 1 (upper) compared with the theory (lower). Different colors represent different angle regions.

5. Conclusions We deposited blue bronze films on MgO substrate with right stoichiometry. This substrate has been used because it is transparent to IR wavelengths needed for femtosecond spectroscopy investigation. Certain degree of crystallinity has been obtained from film deposition with higher oxygen pressure.

References 1. 2. 3. 4. 5.

Demšar, J. et al. (1999) Physical Review Letters 83, 800. Mantel, O. et al. (1999) Journal of Applied Physics 86, 4440. Zaitsev-Zotov, S. (2003) Microelectronic Engineering 69, 549. Travaglini, P. (1984) Physical Review B 30, 1971. Van Der Zant H. et al. (1996) Applied Physics Letters 68, 3823.

SINGLE CELL DETECTION WITH DRIVEN MAGNETIC BEADS B.H. MCNAUGHTON1, R.R. AGAYAN1,2, V.A. STOICA1, R. CLARKE1, AND R. KOPELMAN1,2* 1 University of Michigan, Department of Physics, Ann Arbor, MI 48109 2 University of Michigan, Department of Chemistry, 930 N. University, Ann Arbor, MI 48109-1055

Abstract – Shifts in the nonlinear rotational frequency of magnetic beads (microspheres) offer a new and dynamic approach for the detection of single cells. We present the first demonstration of this capability by measuring the changes in the nonlinear rotational frequency of magnetic beads driven by an external magnetic field. The presence of an Escherichia coli bacterium on the surface of a 2.0 Pm magnetic bead affects the drag of the system, thus changing the nonlinear rotation rate. Measurement of this rotational frequency is straightforward utilizing standard microscopy techniques.

Keywords: Nonlinear rotation, nonlinear dynamics, magnetic microspheres, magnetic beads, single cell detection

1. Introduction The ability to detect and measure single biological agents is of fundamental importance for rapid and accurate medical diagnostics. Recent investigations have focused on the development of micro and nanoscale oscillating systems as novel detection schemes that are both ultra-sensitive and rapid. Detection methods utilizing this technology offer a powerful and diverse group of extremely sensitive tools that have already demonstrated single biological agent detection.1–4 Microand nanoscale oscillators can be classified into several general categories, some ______ *

To whom correspondence should be addressed: R. Kopelman, email: [email protected]


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of which include resonant nanomechanical (NEM) cantilevers,3,4 rotationalbased oscillators bound to a substrate via carbon nanotubes,5–7 and fluid-based magnetically actuated systems.8–10 A key distinction of fluid-based magnetically actuated systems is that they exhibit a nonlinear behavior that enables a new sensing scheme – see Figure 1a, a scheme where sensitivity is unaffected by viscous losses11 (unlike with cantilevers,12–14 which work best in air or vacuum). In 1990, the seminal experimental and theoretical work of Helgesen et al. detailed the rotational dynamics of a pair of magnetic holes (non magnetic microspheres in a ferrofluid), in which nonlinear behavior was observed at sufficiently high external magnetic field rotation rates.15 Many groups followed this work, with various nonlinear rotation studies of small scale systems.10,16,17 For example, Shelton and coworkers used angular momentum from polarized light to torque a glass nanorod.17 They theorized and experimentally verified that the average rotation rate of the glass rod had a nonlinear dependence on the rotation rate of the polarized light and showed that the nonlinear rotation rate is dependent on the optical torque and fluidic drag of the system. The nonlinear rotation rate of magnetic beads has the same dependence on drag as a torqued glass nanorod. Indeed, Biswal and Gast showed that chains of paramagnetic beads are governed by similar rotational dynamics.16 Recently, Cebers and Ozols performed a rigorous theoretical analysis on single particle systems, but did not focus on applications of such systems. Other types of rotational systems that could be used for nonlinear rotation experiments include magnetic nanorods (or “nanowires”)18,19 and substrate-based rotational actuators (if the system experiences drag).5 While many systems have been shown to exhibit nonlinear rotational dynamics, few studies considered applications, such as single cell detection. To fill this research gap, we have studied the nonlinear rotation of magnetic microparticles and explored a number of applications.11,20,21 Indeed, the rotational dynamics of magnetic particles offer potential use in the detection of biological agents. We report on such an application, demonstrating single cell sensitivity. 2. Methods The orientation and rotation rate of magnetic particles can be measured because of a physical or optical asymmetry that the particle has, i.e. a “nanocap” on one side of a particle – shown in Figure 1b. A nanocap can be made by depositing a thin layer of a light attenuating metal, such as aluminum. Nonlinear rotation occurs at high frequencies when the phase-lag between an external rotating magnetic field and the dipole vector of an aligning magnetic particle becomes larger than ʌ/2. After this point, the magnetic particle cannot overcome the viscous

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drag (to remain phase-locked with the external field’s rotational frequency) and thus “slips,” rotating asynchronously (nonlinearly) with the driving field, e.g. the average rotational frequency of the magnetic particle has a lower value than that of the driving field.11,20–22 This type of asynchrony also appears in the flashing of fireflies and Josephson junction voltage dynamics.23

Ab2Ab1

Objective Lens

Xenon Light Field: 5-20 Oe Rot:0-10 Hz Visc: 122 cSt

2.0 micron

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ic Ce

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m Average Nonlinear Rotation Rate

(b)

ii) Post-detection Slow Rotation

i) Pre-detection Fast Rotation Single Bacterium

(a)

Filter Cube

CCD interfaced to Image Analysis Software 270˚

Figure 1. (a) Schematic of the nonlinear rotation rate changes that a magnetic bead undergoes when bound to a bacterium. The magnetic bead is functionalized with a secondary antibody (Ab2) and a primary antibody (Ab1). The bottom series shows fluorescence microscopy images of a rotating 2.0 Pm magnetic bead with a single Escherichia coli bacterium attached. The dotted circle indicates the location of the magnetic bead. (b) Schematic of the microscopy setup used to perform single cell sensitivity measurements (reflection microscopy is shown, but fluorescent microscopy was also utilized).

The rotational frequency was measured using microscopy and image analysis techniques reported elsewhere.20,21 Measurements were performed in two homemade ~100 Pm thick fluidic cells: one contained magnetic bead solution with no bacteria present and the other contained magnetic beads with bacteria bound to their surfaces. Samples were mixed with glycerol before being placed in the fluidic cells to a glycerol-water mass fraction of 0.5. The approximation of the average nonlinear rotational frequency was determined by performing a discrete Fourier transform of the beads’ intensity fluctuations – see Figure 2b. This measurement was performed for 20 single magnetic bead without bacteria in one fluidic cell and for 20 single magnetic beads with one E. Coli bound to each surface – see Figure 2c. To obtain nonlinear rotation, the particles were rotated in an external field with a magnitude of approximately 10 Oe at a driving frequency of 2.5 Hz. The beads were ferromagnetic and obtained from Spherotech, Inc.


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3. Results and Discussion One of the physical properties that the nonlinear rotation rate depends on is fluidic drag. When a bacterium attaches to a nonlinear rotating magnetic particle, the particle’s volume and shape are drastically changed. This produces more drag and, therefore, the nonlinear rotation rate slows considerably. This technique has been used to measure a change of drag caused by the attachment of a 1.0 ȝm particle to a 1.9 ȝm nonlinear rotating magnetic bead.21 Here, the binding of a single bacterium, to a 2.0 ȝm sphere, is shown to be detectable; in fact, the nonlinear rotation rate slowed down, on average, by a factor of ~3.8 – see Figure 2. The technique is also dynamic in the sense that a change in drag causes a direct change in the nonlinear rotation rate; thus the growth of an attached bacterium would cause further changes in drag. While an entire range of frequencies can be scanned to determine the point of criticality (when the motion changes from linear to nonlinear, which is given by : C ), for a magnetic particle with an attached bacterium, as was done in Figure 2a, it is much faster and more straightforward to simply measure the value of the nonlinear rotation frequency, T , at a given external driving frequency of : . From the rotation rate of the magnetic bead and the rate of the external driving field, the critical frequency can be calculated, as

:C

T

1 2

> 2: T @

1 2

mB / NKV ,

(1)

where m is the strength of the magnetic moment of the bead, B is the external magnetic field amplitude, ț is the shape factor, Ș is the dynamic viscosity of the surrounding fluid and V is the bead volume. Figure 2b shows the results from such an approach, where a Fourier transform of the modulated signal was performed for a typical magnetic bead with a single bacterium attached to its surface, and for one without (the antibody used was anti-E. coli IgG with specificity for all “O” and “K” serotypes of E. Coli obtained from Cortex Biochem). The peaks indicate the average rotation rate, which directly corresponds to the critical frequency and, therefore, to the fluidic drag, as described in Eq. (1). Figure 2c shows the curves for the rotation rates of 20 particles with single attached bacteria and for 20 particles without bacteria. Individual bacterium attachment was confirmed by imaging the bacteria with fluorescent microscopy, where DsRed fluorescent protein vectors were used for transformation of the E. coli. The presence of the bacteria on the surface of the magnetic beads caused a measurable change in the average rotation frequency, namely the average frequency of the magnetic beads at a 0.72 Hz to T2 0.19 Hz , driving frequency of 4.0 Hz changed from T1

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Figure 2. Nonlinear rotation dynamics of magnetic beads with and without bacterium attached. (a) The rotational response of a single magnetic particle with attached bacterium at various external driving frequencies, where the squares are data and the line is a theoretical fit. (b) The fast Fourier transform of the intensity fluctuations of a typical particle, with a bacterium attached (solid curve) and for one without (dashed curve). (c) The average nonlinear rotation frequency of 20 particles in a fluidic cell with bacteria present (solid curve) and a fluidic cell without bacteria (dashed curve).

a factor of ~3.8. This change in rotation frequency is similar in value to our previous measurements on a 1.0 Pm particle that was attached to a single ~1.9 Pm ferromagnetic bead.21 Once a bacterium is attached to a magnetic bead, this technique could also be used to monitor a single bacterium’s growth. Monitoring changes in nonlinear rotation rate could lead to the study of single bacterium growth dynamics and thus to rapid antibiotic susceptibility measurements. The ability to use the change in nonlinear rotation of magnetic particles to detect bacteria has been demonstrated. In summary, the nonlinear rotation frequency of 2.0 Pm magnetic beads changed on average by a factor of 3.8. These data show that a dynamic micro-oscillator is sensitive enough to detect a single bacterium in a fluidic environment. 4. Conclusions The ability to use the change in nonlinear rotation of magnetic particles to detect bacteria has been demonstrated. The nonlinear rotation frequency of 2.0 ȝm magnetic beads changed on average by a factor of 3.8. For the first time, we have shown that a dynamic micro-oscillator is sensitive enough to detect a single bacterium in a fluidic environment. ACKNOWLEDGMENTS

The authors would like to thank Fierke, Hernick, and Hurst for help with the bacteria growth and transformations, and NSF for funding (DMR # 0455330).

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References 1. Craighead, H. (2000) Science 290(5496), 1532. 2. Ekinci, K., and Roukes, M. (2005) Review of Scientific Instruments 76(6), 61101–61101. 3. Ilic, B., Czaplewski, D., and Zalalutdinov, M., et al. (2001) Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 19, 2825. 4. Ilic, B., Yang, Y., and Craighead, H. (2004) Virus detection using nanoelectromechanical devices, Applied Physics Letters 85(13), 2604–2606. 5. Fennimore, A., Yuzvinsky, T., and Han, W., et al. (2003) Nature 424, 408–410. 6. Papadakis, S., Hall, A., and Williams, P., et al. (2004) Physical Review Letters 93(14), 146101. 7. Williams, P., Papadakis, S., and Patel, A., et al. (2002) Physical Review Letters 89(25), 255502. 8. Anker, J., and Kopelman, R. (2003) Applied Physics Letters 82(7), 1102–1104. 9. Fan, D., Zhu, F., and Cammarata, R., et al. (2005) Physical Review Letters 94, 247208. 10. Korneva, G., Ye, H., Gogotsi, Y., et al. (2005) Nano Letters 5(5), 879–884. 11. McNaughton, B., Agayan, R., and Kopelman, R. (2006) Arxiv preprint cond-mat/0610144. 12. Bhiladvala, R., and Wang, Z. (2004) Physical Review E 69(3), 36307. 13. Paul, M., and Cross, M. (2004) Physical Review Letters 92(23), 235501. 14. Vignola, J., Judge, J., Jarzynski, J., et al. (2006) Applied Physics Letters 88(4), 41921–41921. 15. Helgesen, G., Pieranski, P., and Skjeltorp, A. (1990) Physical Review Letters 64(12), 1425–1428. 16. Biswal, S., and Gast, A. (2004) Analytical Chemistry 76(21), 6448–6455. 17. Shelton, W., Bonin, K., and Walker, T. (2005) Physical Review E 71(3), 36204. 18. Lapointe, C., Cappallo, N., Reich, D., et al. (2005) Journal of Applied Physics 97(10), 10. 19. Tok, J., Chuang, F., and Kao, M., et al. (2006) Angewandte Chemie International Editon England. 20. McNaughton, B., Kehbein, K., Anker, J., et al. (2006) Journal of Physical Chemistry B 110, 18958–18964. 21. McNaughton, B., Agayan, R., Wang, J., et al. (2007) Sensors and Actuators B 121, 330–340. 22. Cebers, A., and Ozols, M. (2006) Physical Review E 73(2), 21505. 23. Strogatz, S. (1994) Nonlinear dynamics and chaos. (Addison-Wesley, Reading, MA).

ANTIMICROBIAL PROPERTIES OF TITANIUM NANOPARTICLES B.K. ERDURAL, A. YURUM, U. BAKIR, AND G. KARAKAS* Department of Chemical Engineering, Middle East Technical University, 06431 Ankara, TURKEY

Abstract – In the present study, nanostructured titania particles were synthesized using hydrothermal processing and their photocatalytic antimicrobial activities were characterized. Sol-gel synthesized TiO2 samples were treated with a two step hydrothermal treatment. The first stage treatment was the alkaline treatment with 10 M of NaOH for 48 h at 130ºC, followed with the second step which applied with distilled water for 48 h at 200°C. Scanning Electron Microscope (SEM) images showed that alkaline treatment yields lamellar structure particles from the sol-gel synthesized anatase. Further treatment of nanoplates with distilled water results in crystal growth and the formation of nano structured thorn like particles. The photocatalytic antimicrobial activities of samples were determined against Escherichia coli under solar irradiation for 4 h. It was observed that the samples treated under alkaline conditions have higher antimicrobial activity than the untreated samples.

Keywords: Titania, nanostructured, hydrothermal, photocatalytic, antimicrobial

1. Introduction Microbial contamination and growth on the surfaces are potential risks for human health. Various applications are utilized for disinfections of surfaces such as, detergents, alcohols and chlorine components. However these agents are ineffective for long term disinfection and are not environmentally benign. In addition to these methods, UV irradiation is also used for disinfection. This method is an effective, but a temporary and hazardous one requiring lower

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To whom correspondence should be addressed: G. Karakas, email: [email protected]


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wavelength UV-C light source. However, photocatalysis is an alternative to direct UV disinfection. Large band semiconductor metal oxides, e.g. TiO2, SnO2 and ZnO are potential alternatives because of their higher wavelength UV absorption (UVA) which exists in natural sunlight and artificial illumination.1–6 TiO2 photocatalysts with anatase structure generate strong oxidizing power under UV irradiation which corresponds to wavelengths less than 385 nm as a result of charge separation. The hole (h+) and electron (e–) pairs formed react with H2O and O2 over the surface and hydroxyl radicals (OH-) and super oxide ions (ƔO2-) are generated. The hydroxyl radicals are highly toxic towards microorganisms.7 The photocatalytic efficiency of many synthetically produced TiO2 samples depends on many structural parameters such as band gap, surface area, particle size and crystallinity. During the last decade, much effort has been devoted to the increase of the photocatalytic efficiency of TiO2 materials.8 The most important reason of low photocatalytic efficiency of TiO2 is the competition of hole/electron charge recombination reaction with charge separation and free radical production reactions. In present study, the effects of alkaline and neutral hydrothermal treatments were examined on the sol-gel synthesized TiO2 particles. 2.

Experimental

2.1. SUBSTRATE PREPARATION

In the synthesis, titanium tetraisopropoxide (TTIP, Aldrich, extra pure grade), ethanol (C2H5OH, 99.5%) and 35% HClaq. were used. TTIP was first dissolved in ethanol and hydrolyzed with dropwise addition of ethanol–aques hydrochloric acid solution in thermostated bath at 0ºC, under continuous stirring. The resulting sol was dried in the oven at 60ºC overnight, and after mashing the aggregates in mortar, the samples were calcined in air at 600ºC for 2 h. Afterwards the alkaline post treatment step was carried out hydrothermally by using 1.5 g of sol-gel synthesized samples with 100 ml of 10 N NaOH in autoclave sealed at 130ºC for 48 h. After cooling the autoclave, the suspension was filtered, washed repeatedly with 0.1 N HCl solution followed by washing with distilled water until a neutral filtrate (pH = 7) was obtained. After drying at 60ºC for 2 h, sample stored in a desiccator for further analyses and second stage post treatment. The second stage post treatment was applied to 1 g of sample with 100 ml distilled water and the mixture was autoclaved at 200ºC for 48 h. The treated samples were filtered and dried in air at 80ºC for 4 h.

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2.2. CHARACTERIZATION

The photochemical antimicrobial activity of samples was measured against Escherichia coli. For this purpose microbial cell concentrations were determined by viable count procedure on agar plates after serial dilutions of the culture in 0.1% peptone water. Total reaction mixture volume was 20 ml. The reaction mixture contained 3.5 g/l TiO2 samples and E. coli about 103 cells/ml in 0.1% peptone water. The mixture was agitated with a magnetic stirrer at 250 rpm at ambient temperature while irradiated with artificial irradiation source which was applied vertically. Osram Ultra-Vitalux (Product number: 03313) 300 W bulb with similar spectral distribution to solar spectrum between 280 and 780 nm was used as artificial irradiation source. The light intensity over the test bench was adjusted to achieve 10 mW/cm2. Microbial inactivation was followed for 4 h with the removal of the samples of reaction mixture at various time intervals. 200 Pl of samples was directly spread onto agar plates and incubated at 35ºC for 24 h to determine the survivors by counting the colonyforming units (CFUs). Two sets of control experiments were carried out. “Light control” experiments were tested without TiO2 samples were performed with artificial light and dark control experiments were performed with TiO2 without artificial light. 3. Results and Discussion The effects of alkaline and neutral hydrothermal treatments on the photocatalytic antimicrobial performance of sol-gel synthesized TiO2 (SGS) samples were evaluated. The antimicrobial performances of the SGS TiO2 and its derivatives which was hydrothermally treated under alkaline conditions for 48 h are presented in Figure 1. A limited antimicrobial activity was observed for SGS sample. At the end of 4 h, only 70% of the microorganisms could be inactivated by using SGS. However, when SGS sample was treated with NaOH for 48 h (S48) nearly complete inactivation were achieved within 3 h. Our detailed analysis on the effect of hydrothermal treatment9 under alkaline conditions revealed that sol-gel synthesized titania particles are re-structured to trititanate nanoplates which have poor crystallinity. Therefore, complete inactivation of alkali treated samples could be explained by the formation of trititanate structure. Second stage of hydrothermal post treatment was applied to the S48 sample for 48 h (S48h48) with distilled water at 200ºC to test the effect of hydrothermal treatment under neutral conditions on the crystallinity and the photocatalytic activity. In our previous study we have reported that the increase of

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Figure 1. Effect of alkaline hydrothermal treatment on antimicrobial efficiency of sol-gel synthesized samples under irradiation (10 mW/cm2) (x, SGS; o, S48; , dark; , under irradiation). (C/C0 = (number of survival microorganisms)/(number of initial microorganisms)).

crystallinity of anatase phase increase with hydrothermal post treatment period.9 Unexpectedly, the photocatalytic antimicrobial performance of S48h48 sample resulted in poorer activity than S48 sample in spite of S48h48 has better anatase crystallinity.9 When the activity of S48 samples were compared with S48h48 sample, it is clear that the initial inactivation rates are almost the same. However the initial activity of S48h48 sample has not been able to sustain after 20 min of irradiation. Surprisingly the activity was resumed after about 3 h and similar inactivation rate was observed in this period (Figure 2). Ineffective period in microbial inactivation might be attributed to the density of active surface sites and active species.9 The concentration of •OHads species depends on the consumption rate by microbial inactivation and production rate by charge separation (e- and h+). Over the ineffective period, the microbial inactivation rate might be decreased with the lower concentration of active species. With increasing neutral hydrothermal treatment time, more water may start to be adsorbed over the surface. On the other hand, when anatase is irradiated with UV, adsorbed molecular water species dissociate8 and the concentration of oxidative species (such as •OHads and H2O2) directly influences the photocatalytic microbial inactivation. To support this suggestion, S48h48 was agitated in peptone water for 4 h under irradiation and in dark separately. After 4 h period, E. coli was inoculated to the aqueous mixtures and antimicrobial activity of S48h48 was tested for 100 min.

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0

ln(C/C0)

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-3

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-5 0

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Figure 2. Comparison of effects of alkaline hydrothermal treatment for 48 h (S48) (o), neutral hydrothermal treatment for 48 h (S48h48) (x), treatment with water and UV for 4 h (ź) on antimicrobial activity, dark; (), under irradiation ().

As shown in Figure 2, 80% microbial inactivation was achieved within 100 min over the sample which was treated with UV for 4 h. However, keeping the substrate in water in dark have no antimicrobial activity which indicates the surface sites are saturated with water species and UV radiation is essential for •OHads formation.9 4. Conclusion The photocatalytic antimicrobial activity of sol-gel synthesized TiO2 anatase particles can be enhanced with the hydrothermal treatment under alkaline conditions. Further hydrothermal post treatment under neutral conditions improves the crystallinity but not photocatalytic activity. However treatment under UV irradiation facilitates the photocatalytic activity by converting surface water species to active •OHads.

References 1. Matsunaqa, T., Tomoda, R., Nakajima, T., and Wake, H. (1985), FEMS Microbiol. Lett., 29(1–2), 211–214. 2. Saito, T., Iwas, T., Horis, J., and Morioka, T. (1992), J. Photochem. Photobiol. B, 14, 369–379. 3. Benedix, R., Dehn, F., Quaas, J., and Orgass, M. (2000), Lacer, 5, 157. 4. Hong, J., and Otaki, M. (2003), Biosci. J. Bioeng., 96(3), 298–303.

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5. Erkan, A., Bakir, U., and Karakas, G. (2006), J. Photochem. Photobiol., 184(3), 313–321. 6. Huang, Z., Maness, P., Blake, D., and Wolfrum, E. (2000), J. Photochem. Photobiol. A: Chem., 130, 163–172. 7. Seven, O., Dindar, B., Aydemir, S., Metin, D., Ozinel, M., and Icli, S. (2004), J. Photochem. Photobiol. A., 165, 103–107. 8. Diebold, U. (2003), Surf. Sci. Rep., 48, 53–229. 9. Erdural, B., Yurum, A., Bakir, U., and Karakas, G. (2008), J. Nanosci. Nanotechnol. 8, 878–886.

CsHSO4/NANOOXIDE POLYMER MEMBRANES FOR FUEL CELL A. ANDRONIE1, A. MOROZAN1, C. NASTASE1, F. NASTASE1, A. DUMITRU1, S. VULPE1, A. VASEASHTA2, AND I. STAMATIN1* 1 3Nano-SAE Research Centre, University of Bucharest, PO Box MG-38, Bucharest-Magurele, ROMANIA 2 On detail from Nanomaterials Processing & Characterization Laboratories, Marshall University, Huntington, WV, USA

Abstract – Composite solid acid/nanooxide polymer membranes with good electrical behaviour can be obtained by thermocentrifugal field processing. The supporting polymer matrix is designed to embed solid acids or strong solid acids/ nanooxides making it appropriate for proton conducting composite membranes. Nanocomposites CsHSO4-YSZ/PAN membranes were investigated by Raman, FT-IR and SEM techniques. The dependence of the electrical conductivity on temperature is evaluated.

Keywords: Nanocomposite membrane, solid acid, superprotonic transition

1. Introduction Due to their high efficiency and low emissions, fuel cells have merged as attractive alternatives to combustion engine. With good electroactivity and waste heat treatment solid acid electrolytes have potential for implementing in novel fuel cells.1,2 SAFCs utilize an anhydrous, nonpolymeric proton-conducting electrolyte that can operate at slightly elevated temperatures.3 Their brittleness, the narrow temperature range of the superprotonic phase and chemical instability in hydrogen atmospheres (are reduced to form H2S) are the limiting factors for electrochemical application of these protonic electrolytes.4 For fuel cell

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To whom correspondence should be addressed: I. Stamatin, email: [email protected]


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applications, CsHSO4 offers the advantages of anhydrous proton transport and high temperature stability.1 In the present study, we developed composite CsHSO4/YSZ embedded in a stable polymer matrix of polyacrylonitrile. The effect of doping the ionic salt with nanooxide particle is the ionic conductivity enhancement due to the interface interaction between the two species.3,5,6 2. Experimental Crystals of CsHSO4 samples were synthesized at room temperature from an aqueous solution of Cs2SO4 (Sigma–Aldrich) and H2SO4, Cs:SO4 = 1:2. The precipitated product with ethanol was then dried at 60°C for 3 h. The same procedure was followed for CsHSO4/YSZ synthesis, adding from the beginning a certain amount of YSZ nanopowder (ZrO2 contains 3% Y2O3, 100–120 m2/g, d < 100 nm, Sigma–Aldrich) (2.5, 5, and 7.5% (w/w) of YSZ to CsHSO4). The samples were indexed as follows: CsHSO4, CsHSO4/YSZ_2.5, CsHSO4/YSZ_5 CsHSO4/YSZ_7.5. Dimethylformamide (DMF)–10 wt % polyacrylonitrile (PAN) solutions (DMF-PAN) were prepared and then mixed with CsHSO4/YSZ by ultrasonic processing until homogeneous mixtures were formed. Thin membranes (about 100 Pm) as continuous and pore-free films were formed from each mixture (CsHSO4 or CsHSO4/YSZ: PAN = 1:4) in thermocentrifugal field up to 2000C. The corresponding sample indexing is: CsHSO4-PAN, CsHSO4/YSZ_2.5-PAN, CsHSO4/YSZ_5-PAN, CsHSO4/YSZ_7.5-PAN. 3. Results and Discusions 3.1. SEM IMAGING

SEM (FEI-Quanta 400) reveals a sponge-like morphology of CsHSO4/YSZ composite at different scale as shown in Figure 1.

Figure 1. SEM imaging of CsHSO4/YSZ_2.5 at different scales.

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3.2. FT-IR AND RAMAN MEASUREMENTS

The Raman (Raman NRS–3100 JASCO) and FT-IR (Jasco FT/IR – 6200 Spectrometer) spectra of CsHSO4/YSZ compounds are shown in Figure 2. The Raman peaks around 1,000 cm–1 observed in all the spectra are basically attributable to stretching vibration of sulfate ions. Stretching modes between 800 and 1,300 cm–1 and bending modes between 400 and 600 cm–1 were observed in the cases of Cs2SO4, but also of CsHSO4 and its YSZ composites. The FT-IR spectra of Cs2SO4, CsHSO4 and its YSZ composites show stretching modes due to sulfate ions between 800 and 1,300 cm–1.

Figure 2. (Left) Raman spectra of: (a) Cs2SO4, (b) CsHSO4, (c) CsHSO4/YSZ_2.5, (d) CsHSO4/ YSZ_5, (e) CsHSO4/YSZ_7.5; (right): FT-IR spectra of: (a) Cs2SO4, (b) CsHSO4, (c) CsHSO4/ YSZ_2.5, (d) CsHSO4 /YSZ_5, (e) CsHSO4/YSZ_7.5.

3.3. ELECTRICAL MEASUREMENTS

The electrical conductivity measurements were performed using Keithley 2400, and a Faraday box with a temperature controller.7 The conductivity of CsHSO4 and CsHSO4/YSZ composite polymer membranes was measured between 353 K and 463 K, as shown in Figure 3. A reduction of the jump in conductivity at the phase-transition temperature has been obtained. Increasing the amount of YSZ leads to a reduction of the conductivity at low temperature.

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Figure 3. Graphical representation of the conductivity for membranes: CsHSO4-PAN, CsHSO4/ YSZ_2.5-PAN, CsHSO4/YSZ_5-PAN, CsHSO4/YSZ_7.5-PAN.

4. Conclusions Composite solid acid/nano-oxide polymer membranes can be easily obtained by thermo-centrifugal field processing. The introduction of YSZ increases the low-temperature proton conductivity of the material and reduces the jump in conductivity at the phase-transition temperature. Further studies are required for obtaining a proton conducting composite membrane appropriate for commercial fuel cells. ACKNOWLEDGEMENTS

The authors acknowledge research support Grant CEEX-MENER # 704/2006.

References 1. Haile, S., Boysen, D., Chisholm, C., and Merle, R. (2001), Nature, 410, 910–913. 2. Uda, T., and Haile, S. (2005), Electrochemical and Solid-State Letters, 8(5), A245–A246. 3. Ponomareva, V., and Shutova, E. (2005), Solid State Ionics, 176, 2905–2908. 4. Baranov, A., Grebenev, V., Khodan, A., Dolbinina, V., and Efremova, E. (2005), Solid State Ionics, 176, 2871–2874. 5. Ponomareva, V., and Lavrova, G. (1998), Solid State Ionics, 106, 137–141. 6. Colomban, P. (1992), Protonic Conductors (Cambridge University Press, Cambridge). 7. Nastase, C., Mihaiescu, D., Nastase, F., Moldovan, A., and Stamatin, I. (2004), Synthetic Metals, 147, 133–138.

IV AND CV CHARACTERISTICS OF MULTIFUNCTIONAL ILMENITE-HEMATITE 0.67FeTiO3-0.33Fe2O3 C. LOHN1, W.J. GEERTS1*, C.B. O’BRIEN1, J. DOU2, P. PADMINI2, R.K. PANDEY2,3, AND R. SCHAD2 1 Texas State University-San Marcos, Department of Physics, 601 University Drive, San Marcos, TX 78666, USA 2 The University of Alabama, Department of Physics, Tuscaloosa, AL 35487, USA 3 The University of Alabama, Department of Electrical and Computer Engineering, Tuscaloosa, AL 35487, USA

Abstract – We investigated the IV and CV properties of an [(FeTiO3)0.67 (Fe2O3) 0.33] epitaxial thin film. The four point probe (4pp) measurements revealed that the material has a linear IV relation and has a resistivity of approximately 0.56 : cm. In contrast, the two point (2pp) measurements are highly non-linear suggesting the existence of Schottky barriers. The CV data suggest that the material under the contacts is depleted. From the corrected CV data, the carrier concentration is found to be of the order of 1023/cm3.

Keywords: Ilmenite-hematite, multi-functional materials, CV, IV

1. Introduction In conventional electronics, devices manipulate charge in order to store and transfer data. The emerging technology of spintronics uses the phenomenon of electron spin to encode data. Spintronic devices, such as the spin MOSFET first proposed by Datta and Das,1 combine charge transport with spin-dependent effects that arise from the interaction of the charge and properties of the magnetic materials. A major obstacle of spintronics development has been in finding materials with both ferromagnetic and semiconductor properties above room

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To whom correspondence should be addressed: W.J. Geerts, email: [email protected]


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temperature. The ideal ferromagnetic semiconductor should have the Curie point well above the device operating temperature, have high mobility, exhibit a spinsplit band structure and be controllably doped to produce p-type or n-type material. The prediction of Curie temperatures much above room temperature in ferromagnetic semiconductors2 has led to the approach of doping nonmagnetic semiconductor material with magnetic ions. Alternatively, one can investigate materials that are ferro- or ferrimagnetic above room temperature and have semiconductor properties. Individually, ilmenite (FeTiO3) and hematite (Į–Fe2O3) are antiferromagnetic insulators, but compositions of ilmenite-hematite (IH) [(FeTiO3)(1-x)-(Fe2O3)x] are ferrimagnetic over a wide composition range. 3,4 IH systems with a Curie point above room temperature have been demonstrated.5 Furthermore, adjusting the composition can produce p-or n-type material.6 Although the magnetic properties of IH systems have been well documented, data on its dielectric constant, its mobility, and the type of contact it makes to metals are scarce in literature. IH is a multifunctional material; its unique magnetic and dielectric properties make the interpretation of results obtained with standard semiconductor characterization techniques far from trivial. In this paper we attempt to characterize some of the electric properties of the IH system that will be important for a potential device. The focus is on the characterization of metal semiconductor contacts by IV and CV analysis. 2. Experimental Details 2.1. SAMPLE PREPARATION

The IH film was deposited on a single crystal sapphire substrate (10 × 10 mm) by pulsed laser deposition (PLD). The deposition was carried out in an argon atmosphere of 10–3 Torr and at a substrate temperature of 750°C.7 The epitaxial [(FeTiO3)0.67(Fe2O3)0.33] (IH-33) thin film has a thickness of 92 nm as determined by XRR. An Al2O3 layer (142 nm) with a width of approximately 4 mm was deposited at the center of the sample by PLD, leaving on both sides a strip of approximately 3 mm of exposed IH. Six silver contacts were made on the perimeter of the exposed IH thin film using silver epoxy (three on each surface area). A 7th contact was made on top of the oxide in the center of the sample by sputtering Pt through a mask of approximately 2 × 2 mm. Since the oxide contained pinholes or cracks, this center contact should be considered to be a metal-semiconductor contact of which we do not know the effective surface area. The Ag contacts were annealed at 120°C for 20 min to reduce their resistance. The area, the perimeter, and the position of each contact were determined by using a digital top view image of the sample loaded into the Canvas software

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program (error > Creverse the measurement data will mainly show the behavior of the smaller of the two, i.e. Creverse. Also the conductance appears to depend on the bias voltage and is maximum for zero bias voltage. This is in qualitative agreement with the non-linear IV characteristics of the contacts. We used the minimum and maximum capacitance in Figure 2b to calculate the maximum and minimum depletion depth.

Figure 2. (a) Impedance analyzer raw data; (b) CV data corrected for series resistance.

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Assuming the dielectric constant to be 30 and the surface area to be equal to the contact area, we found dmax = 20 Pm and dmin = 5 Pm. This suggests that the whole area under the contacts might be depleted and that the effective contact area is proportional to the contact’s perimeter times the film thickness. We plotted the square of the reciprocal capacitance per area as a function of the bias voltage and determined the carrier concentration in the IH film from the slope8 to be of the order of 1023/cm3. This number is in agreement with former results of Dou7 and suggests that the crude assumptions we made in this paper bare some truth. Calculations using a device simulator to get a better idea on the size and shape of the depletion area as well as measurements on specially designed test structures that will give a more accurate value of the capacitance are planned for the near future. ACKNOWLEDGEMENTS

We would like to thank the US ONR (# N00014-03-1-0358) and the US DoE (#DE-FG02-03ER46039) for their sponsorships. One of the authors, Lohn acknowledges NATO, NSF and Texas State University for travel grants. References 1. Datta, S., and Das, B. (1990) Electric analog of the electro-optic modulator, Appl. Phys. Lett. 56(7), 665–667. 2. Dietl, T., Ohno, H., Matsukura, F., Cibert, J., and Ferrand, D. (2000) Zener model description of ferromagnetism in zinc-blende magnetic semiconductors, Science 287(5455), 1019–1022. 3. Ishikawa,Y., and Akimoto, S. (1957) Magnetic properties of the FeTiO3-Fe2O3 solid solution series, J. Phys. Soc. Jpn. 12(10), 1083–1098. 4. Ishikawa, Y. (1958) Electrical properties of FeTiO3-Fe2O3 solid solution series, J. Phys. Soc. Jpn. 13(1), 37–42. 5. Hojo, H., Fujita, K., Tanaka, K., and Hirao, K. (2006) Epitaxial growth of room-temperature ferromagnetic semiconductor thin films based on the ilmenite-hematite solution, Appl. Phys. Lett. 89, 082509. 6. Zhou, F., Kotru, S., and Pandey, R. (2002) Pulsed laser-deposited ilmenite–hematite films for application in high temperature electronics, Thin Solid Films 408, 33–36. 7. Dou, J., Navarrete, L., Kale, P., Padmini, P., Pandey, R., Guo, H., Gupta, A., and Schad, R. (2007) Preparation and characterization of epitaxial ilmenite-hematite films, J. Appl. Phys. 101, 053908. 8. D. K. Schroder (1998) Semiconductor Material and Device Characterization (Wiley, New York).

ELECTRODEPOSITION OF BI1-XSBX NANOWIRES AS AN ADVANCED MATERIAL FOR THERMOELECTRIC APPLICATIONS J.E. WEBER1,2, W.G. YELTON3, AND A. KUMAR1,2* Department of Mechanical Engineering, Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, FL USA 2 Department of Mechanical Engineering, Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, FL USA 3 Photonics Microsystems Technologies, Sandia National Laboratories, Albuquerque, NM USA 1

Abstract – This paper focuses on the electrodeposition of high density nanowire arrays in porous anodic aluminum oxide (AAO) templates. A two step anodization technique was used to develop a template for Bi1-xSbx nanowire growth directly on Si substrates. Uniform pores with virtually no grain boundaries were achieved. Fundamental electrochemistry experiments on Bi3+, Sb3+, and both cations in dimethyl sulfoxide (DMSO) were carried out to characterize the ideal chronopotential pulse and to determine the thermodynamic, diffusion and mass transport issues when plating Bi1-xSbx into nano dimension pore structures. Such chronopotentiometry resulted in uniform nanowire growth within porous channels of the AAO template.

Keywords: Nanowire, electrodeposition, thermoelectric application

______ *

To whom correspondence should be addressed: A. Kumar, email: [email protected]


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1. Introduction The measure of thermoelectric efficiency is based on a dimensionless ZT figure of merit:

ZT

S 2V

N

T

(1)

where S = Seeback coefficient, ı = electrical conductivity, and ț = thermal conductivity. Currently, bulk Bi1-xSbx with 12% Sb will yield a ZT = 0.88 at 80 K. However, Dresselhaus and coworkers1 have predicted a ZT § 1.25–1.5 at a wire diameter ~35–45 nm and Sb concentration ~ 10–15%. Once synthesized, the Bi1-xSbx nanowire arrays would serve as the P-type semiconducting leg in a thermoelectric device. The nanowire composition is controlled by controlling the plating bath composition and by manipulating the deposition potential. To get a large ZT, it is necessary to have large Seeback coefficients, small electrical conductivity, and small thermal conductivity. The fabrication of well aligned nanowire arrays is a vital project for the realization of novel microelectronic and nano electrical mechanical systems (NEMS). Anodic aluminum oxide (AAO) porous membranes produced on silicon substrates are suitable for a diverse field of applications, including template scaffolds for nanowire growth.2 In this work, aluminum films were anodized to create a porous template in which Bi1-xSbx nanowires were fabricated via electrodeposition. 2. Methods Smooth Nd-doped Al films were grown on a Si substrate on top of a stress-free Pt layer, which bonds to the Si substrate via a W adhesion layer. A two-step anodization process was used to obtain the porous templates. The aluminum films were anodized in 3% oxalic acid chilled at 2°C. The initial anodization was at 50 V for less than 1 min. This quick anodization step provided the rough template. Immediately following the first anodization, the films were etched in a 5% H3PO4, CrO3 bath at room temperature. The time of this etch varied from 10 min to 30 s in order to achieve an optimal etch of the oxide layer. The second anodization step began at 50 V, 2°C with a systematic potential reduction in order to more efficiently etch the barrier oxide layer. Electrochemical experiments were performed using a Radiometer Analytical Voltalab 40 electrochemical analyzer. All electrochemical measurements were executed in a standard three electrode system at room temperature in a solution of dimethyl sulfoxide (DMSO) due to the high solubility of Sb salt in DMSO.

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3. Results Cyclic voltammetry was used to characterize the kinetic limitations of plating Bi3+ Sb3+ into nanoscale pores. It is necessary to know where reduction takes place and what the rate of reduction is (current density, or speed). Figure 1 shows that Bi has a large reduction potential (3 mA/cm2 @ –500 mV) when compared to Sb (–0.5 mA/cm2 @ –500 mV). Electrochemical impedance spectroscopy (EIS) was used to determine the ideal pulse parameters for nanowire growth. This technique was utilized to help explore the electron transfer rate from the electrode to the ion. The Nyquist plot (Figure 2) was used to determine the diffusion coefficient (2.4 × 107 cm/s2) of Bi3+ Sb3+. The time constant was also extrapolated to be 159 ms and the

Figure 1. CV’s of Bi3+, Sb3+, and both cations in DMSO at room temp. Potential vs. Ref: Hg:HgCl (sat KCl).

Figures 2 and 3. Nyquist plot of 50 mM Bi3+ and 50 mM Sb3+ in DMSO at room temperature. Chronopotentiometric pulse used for electrodeposition of Bi1-xSbx in a porous alumina template.

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activation current was 419.2 ȝA. To summarize, a pulse of at least 419.2 ȝA for the duration of 159 ms was needed in order to begin the nanowire deposition process. The Bi1-xSbx nanowires were deposited via chronopotentiometry. The application pulse was at –1.5 mA for 250 ms. A small etching pulse of 0.25 mA for 2 s was also used as shown in Figure 3. 4. Discussion Chronopotentiometry was utilized to pulse the deposition potential in order to avoid a large Nernst diffusion layer and create more crystalline nanowires. The short on-pulse helped emulate nanoscale features during the growth process. It was found that by including a small etch pulse a better array of nanowires were produced. The reason for this was twofold. First, the 2 s etch allowed the concentration of ions at the interphase to return to the concentration of the bulk solution. This step helped maintain the correct Sb concentration in the nanowires. The 0.25 mA on-pulse also aided in the etch of any nanowires that have grown up and out of the porous channels. This additional step kept the surface smooth and the current uniformly distributed in the pores during deposition. Figure 4 illustrates that a smaller deposition pulse will create a more uniform growth rate and smoother template surface. To conclude, fundamental electrochemical studies were carried out on Bi3+, Sb3+, and both cations in order to determine any mass transport issues that may arise during nanowire plating. EIS was used to determine the ideal electrodeposition parameters and the Bi1-xSbx were successfully fabricated in an anodic aluminum oxide (AAO) template.

Figure 4. (a) Top-view SEM of Bi1-xSbx nanowires grown out of AAO template at low resolution and at right (b) same sample at high resolution.

SYNTHESIS OF BI1-XSBX NANOWIRE ARRAYS

References 1. 2.

Rabin, O., Lin, Y., and Dresselhaus, M. (2001) Appl. Phys. Lett. 79, 81–83. Liu, C. et al. (2003) Adv. Mater. 15, 838–841.

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A SOLID STATE NANO-GENERATOR: CONCEPT, DESIGN AND THEORETICAL ESTIMATIONS M. VOPSAROIU1*, M.G. CAIN1, V. KUNCSER2, AND J. BLACKBURN1 1 National Physical Laboratory, Hampton Road, Module G9/A05Teddington TW11 0LW, UK 2 National Institute for Materials Physics, 77125, Bucharest – Magurele, ROMANIA

Abstract – Nano-technology is a very attractive area of research and innovation because it allows the current trends in miniaturization to continue. The transition from micro scale to nano scale devices has already taken place in many applications such as electronics, magnetic recording and nano-biophysics. However, as we scale down the size of the structures and devices, it becomes obvious that the classical behavior will break down at the nano-scale and an interesting superposition of classical and quantum effects will emerge. Therefore, the validity of classical physics is questioned and many aspects of physics are now being revisited from the point of view of nano-technologies. In line with the new developments in miniaturization and nano-technologies, we propose in this letter a simple mechanism that applies the Faraday effect at the nano-scale in order to create a possible solid-state energy nano-generator device. The proposed nanogenerator functionality is based on what we shall call the Super-Paramagnetic Electromotive Force (SPEF) effect. This has the potential to produce a very small voltage on short time scales by converting directly thermal energy at room temperature to electromotive energy without the need for external work or mechanical motion.

Keywords: Thermal energy, superparamagnetism, nano-generator, Faraday effect

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To whom correspondence should be addressed: M. Vopsaroiu, email: [email protected]


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1. Introduction and Concept Description The Faraday effect states that an electromotive voltage can be induced in an N turn coil, by a time fluctuating magnetic flux. Mathematically the electromotive voltage generated (e) is proportional to the rate of change of the magnetic flux through the circuit:

e

N

dI dt

N

d BA cos T dt

(1)

where I is the magnetic flux, B is the magnetic field, A is the area of the coil and T the angle between the magnetic field vector and the normal to the coil plane. According to Eq. (1) the only requirement to induce a voltage in the coil is the time variation of either B, A, T or any combination between them. This is typically achieved by mechanical periodic motion through the rotation of the coil in a constant magnetic field or the magnetic field around a fixed coil. Hence, mechanical work is converted into electrical energy via the Faraday effect (i.e. electromagnetic induction). The main idea of this letter is to reduce the size of the system down to the nano-meter range and then to apply the same principle. However, when we scale the system down to the nano-meter range, the main advantage is that the requirement for mechanical motion is no longer necessary and a time variation of the magnetic flux can be engineered by other means. Let us imagine that the magnet is scaled down to the size of a single domain ferromagnetic nano-particle having an ellipsoidal shape. A magnetic single-domain state can be achieved in a nanoparticle by reducing its volume below a critical size where the magnetic exchange interactions dominate the magnetostatic interactions. The combined shape and magneto-crystalline anisotropies will generate an easy axis parallel to the long axis of the ellipsoid forcing the net magnetic moment (m) of the particle parallel to this easy axis. It is assumed here that the magneto-crystalline is dominant over the shape anisotropy. The nano-particle system is in effect a small magnetic dipole, which can generate at a distance r a magnetic field B given in absolute value by Eq. (2).

B

P0 m 4Sr 3

1 3 cos D 2

1 2

(2)

where P0 is the magnetic permeability of vacuum and m is the magnetic dipole moment and D is angle between the easy axis and the r direction. We now assume a single nano-coil surrounding the magnetic nano-particle (Figure 1). Just as in the classical Faraday effect, a time dependent magnetic flux through the coil is

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Figure 1. Schematic diagram of the system nano-coil/superparamagnetic nano-particle. The long axis of the particle can be either in the plane of the coil or perpendicular to it. Thermal jumps between the two equilibrium states +m and –m along the easy axis will generate a time dependent magnetic flux through the nano-coil.

required to produce an electromotive force. In order to generate the time dependent magnetic flux, we can make use of the super-paramagnetic state of the nanoparticle. In the super-paramagnetic state the nano-particle possesses an intrinsic magnetic moment that randomly fluctuates between the two 0° and 180° equilibrium states due to the thermal activation (see Figure 2). This is achieved in nanoparticles for which the magneto-crystalline energy term (KuV) is dominated by the thermal Boltzman energy (KBT) at a temperature T z 0. Because the anisotropy term depends on the volume of the particle, by reducing its size, a super-paramagnetic state is achieved at room temperature. Also the magnetocrystalline energy term (KuV) can be further reduced by choosing ferromagnetic materials with anisotropy energy constant Ku as small as possible. Therefore, from the magnetic point of view, we deal with a magnetic dipole, which fluctuates along the easy axis direction. Recalling relation (2), a thermally induced time fluctuating magnetic dipole m(t) would generate at a distance r a time dependent magnetic field B(t), which in turn leads to a time dependent magnetic flux, I (t ) , through the nano-coil. The Faraday effect is induced, except that in this case there is no mechanical work or motion involved and the time dependent magnetic flux is achieved by converting the thermal energy of the environment to thermal fluctuations of the magnetization via the super-paramagnetic effect. Hence, the surrounding room temperature thermal energy is converted directly into electromotive energy and we propose to name this effect: The Super-Paramagnetic Electromotive Force (SPEF) effect.

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Energy

KBT

m

+m

0

60

F

120

180

Figure 2. Energy (E = KuVsin2I) of a magnetic nano-particle in zero applied field as a function of the angle I between its magnetization and the easy axis. There are two energy minima (0°, 180°) with the energy barrier KuV between them. When the thermal Boltzman energy KBT is large enough to overcome the barrier, thermal jumps between the two energy minima occur randomly.

2. Theoretical Estimations and Discussions Let us assume the following relation describing the stochastic time fluctuating magnetic moment: m(t) = m cos(Zt) + f(t)

(3)

where m is the absolute value of the magnetic moment and f(t) is a stochastique noise parameter superimposed on to a harmonic oscillation, Z = 2S/W and W is the super-paramagnetic relaxation time given by the relation:

W

1

# 2 Q 0 e

K uV K BT

(4)

where Q0 # 109 s–1 and represents the attempt frequency to overcome the energy barrier. Using the relation m = MV, where M is the magnetization of the material and V is the volume of the particle, combined with relations (2) and (3), the magnitude of the magnetic field can now be expressed as:

B(t )

1 P 0 MV cos(Zt ) 2 2 1 3 cos D 4Sr 3

(5)

where for simplicity we have ignored the stochastic noise parameter. The approximate induced electromotive voltage through a nano-coil of radius L when the particle is located at the centre of the coil with the long axis normal to the plane of the coil (i.e. D = 90°) is then:

SUPER-PARAMAGNETIC ELECTROMOTIVE FORCE (SPEF)

e

dI dt

P 0 MV 2L

Z sin(Zt )

emax sin(Zt )

435

(6)

In order to estimate the maximum induced voltage (emax) we now consider a soft NiFe nano-particle. The following parameters are used: M = 1 T = 107/4S A/m, Ku = 102 J/m3, nano-coil radius L = 1.5 × 10–7 m. The calculations have been made for a relaxation time of W = 10-8 s and the thermal energy at room temperature is KBT = 4.14 × 10–21 J. Using relation (4), the volume of the NiFe particle has been estimated as: V # 1.24 × 10–22 m3. Using these values and relation (6), the estimated induced maximum voltage by a single nano-particle is and emax = 259 nV. Assuming a single Cu nano-coil of radius L = 150 nm, cross section area of A = 10–15 m2 and resistivity U = 2.24 × 10–8 : m, we can estimate the electrical resistance of the nano-coil as: R = U2SL/A = 21 :. Hence, the maximum current flowing through the Cu nano-coil will be Imax = 12.3 nA. However, in order to minimize the power loss through electron collisions in the coil, an interesting and challenging exercise would be to use carbon nano-tubes instead of Cu. These have been intensively studied and they are known as ballistic 1D conductors1 with mean free path extending to micrometers. By using a high conductivity carbon nano-tube coil the Joule heat loss in the coil could be minimized and the efficiency of the nano-generator could be maximized. The system nano-coil/super-paramagnetic nano-particle will behave in effect as a nano AC voltage generator by converting the surrounding room temperature thermal energy into electrical energy. The average frequency of the induced voltage from a particle is given by its super-paramagnetic relaxation time. Although the induced voltage/current appear to be very small, it is important to mention that our estimations are for a single nano-particle inside a single Cu nano-coil. Besides using carbon nano-tubes instead of Cu, other options to increase the power output are to optimize the particle material, the shape/size of the particle and coil, the use of an N turns nano-coil instead of a single nanocoil and a large number of nano-particles. Of course, one has to consider the technical challenges in fabricating such a device (for example we have ignored the auto-inductance effects, coil impedance, thermal Johnson noise, stochastic nature of the fluctuations, etc.) but the purpose of the paper is to communicate this idea rather than to give specific solutions to the practical realization. Most important fact is that the SPEF effect used with the nano-generator has the potential to produce a very small amount of electromotive work from the thermal energy of the environment. Since the electromotive work is produced directly from the heat of the environment, this appears to violate the second law of thermodynamics. However, new theoretical and experimental scientific developments have shown that the second law is applicable to large macroscopic systems while its violation is possible in small systems on short time scales. This has been

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widely reported in the literature: Second law of thermodynamics “broken” [NewScientist, 19 July 2002; Second law broken, Nature July 23, 2002] and the justification is based on the Fluctuation Theorem developed by Evans et al.,2,3 which has been proven both theoretically4 and experimentally.5,6 In support of the idea presented in this letter we would like to quote a paragraph from reference3 that implies exactly the possibility of converting heat into work as in the SPEF effect: “If the work performed during the duty cycle of any machine is comparable to the thermal energy per degree of freedom in the system (i.e. KBT), the Fluctuation Theorems say that there is a significant probability that the machine will actually run backwards. In violation of the Second Law, heat energy from the surroundings can be converted into useful work to provide sufficient energy for the machine to run in reverse” p. 1580. 3. Conclusion The Super-paramagnetic Electromotive Force (SPEF) effect has been proposed in this paper. This works by converting the thermal energy of the environment into a time variation magnetic flux via the super-paramagnetic effect, which in turn is used to create electromotive work in a nano-coil. Although the proposed energy nano-generator is very promising, the practical realization and optimization of such a device remains at this stage a challenge for the nano-fabrication research groups around the world. If successfully fabricated, the nano-generator has possible applications as in-situ power source for nano/molecular devices and for fundamental studies of relaxation phenomena in nano-magnetic systems.

References 1. Poncharal, P., Berger, C., Yi, Y., Wang, Z., and de Heer, W. (2002), J. Phys. Chem. B 106, 12104. 2. Evans, D., Cohen, E., and Morris, G. (1993), Phys. Rev. Lett. 71, 2401. 3. Evans, D., and Searles, D. (2002), Adv. Phys. 51, 1529–1585. 4. Evans, D., and Searles, D. (1994), Phys. Rev. E 50, 1645–1648. 5. Wang, G., Sevick, E., Mittag, E., Searles, D., and Evans D. (2002), Phys. Rev. Lett. 89, 050601. 6. Carberry, D., Reid, J., Waang, G., Sevick, E., Searles, D., and Evans, D. (2004), Phys. Rev. Lett. 92, 140601.

APPLICATIONS OF STATISTICAL PHYSICS TO MIXING IN MICROCHANNELS: ENTROPY AND MULTIFRACTALS M. KAUFMAN1*, M. CAMESASCA2, I. MANAS-ZLOCZOWER2, L.A. DUDIK3, AND C. LIU3 1 Department of Physics, Cleveland State University, Cleveland, OH.44115, USA 2 Department of Macromolecular Science, Case Western Reserve University, Cleveland OH.44106, USA 3 Electronics Design Center, Case Western Reserve University, Cleveland OH.44106, USA

Abstract – We apply rigorous measures of mixing based on entropy in conjunction with fractals to the field of microfluidics. First we determine the entropy and multifractal dimensions of images of mixing a fluorescent and a non-fluorescent fluid in a microchannel. We find the microstructures to be self-similar (fractals). Second we propose a new approach for patterning the walls of microchannels using the Weierstrass function. We have evidence from numerical simulations that by properly adjusting the dimension of the Weierstrass function one can get microfluidic devices that exhibit better mixing than the current ones.

Keywords: Microfluidics, entropy, fractals, mixing 1. Introduction Microfluidic systems operate in a pressure driven flow regime with no moving parts to drag the fluids. Since the flows are laminar and diffusion is in general small, mixing can be achieved in such devices by patterning the channel walls.1 To design microchannels that are efficient mixers it is important to develop rigorous assessment and quantification tools of mixing. To this end we proposed2 to use Shannon and Renyi entropies. To further our understanding of mixing, we also characterize the geometric structures generated by the flow by using

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To whom correspondence should be addressed: M. Kaufman, email: [email protected]


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multifractal dimensions. We apply the entropic and multifractal tools on the published1 images of mixing of a fluorescent and a non-fluorescent fluid in a microchannel. Stroock et al.1 have achieved mixing in microchannels by patterning the bottom wall of the duct in a device called the Staggered Herringbone Mixer (SHM). We investigate3 a non-periodic patterning design based on the Weierstrass fractal function.4 We present numerical simulations of three channels designed accordingly and a fourth were the patterning is periodic. We quantify the degree of mixing by using the mixing entropy. We also report here on our work to develop an experimental prototype of a fractal microchannel mixer. 2. Renyi-Shannon Entropy and Multifractal Dimensions The Shannon5 entropy S measures mixing as it is uniquely determined from the Khinchin axioms6: it depends on the probability distribution p only; the lowest entropy corresponds to one of the p’s being 1, i.e. no mixing; the largest value for the entropy is achieved when all p’s are equal to each other, i.e. perfect mixing; S is additive over partitions of the outcomes. For an experiment with N outcomes the Shannon entropy is: N

S

¦ p j ln p j

(1)

j 1

If the last Khinchin axiom is relaxed to consider only statistically independent partitions, Rényi7 found that the information entropy is replaced by a one-variable function:

S (E )

§ N · 1 ln ¨ ¦ p j E ¸ 1 E © j 1 ¹

(2)

For ȕ = 1 the Renyi entropy equals the Shannon entropy. Multifractals8,9 are self-similar complex structures characterized by a spectrum of dimensions d(E). The multifractal dimensions are obtained4 from the linear dependence of the Renyi entropy S(E) on ln(N). The slope equals d(E)/D, where D = 2 is the embedding dimension. In Section 3 we apply this formalism to images of binary fluids mixing in a microchannel.1 In view of the additivity axiom, the Shannon entropy for a multicomponent system can be expressed as:

S

S (locations) Slocations ( species)

(3)

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439

where: N

Slocations ( species)

¦ ª¬ p S ( species) º¼ j

j

(4)

j 1

C

S j ( species)

¦ ª¬ pc / j ln pc / j º¼

(5)

c 1

N

S (locations )

¦ ª¬ p j ln p j º¼

(6)

j 1

where: pj is probability of a particle to be in bin #j and pc/j is probability of a particle to be of species c conditional on being in bin #j. In section 4 we analyze the mixing of two incompressible fluids. Since the density at any location does not change in time, the entropy S(location) is a time constant. Thus we concentrate on Slocation(species) which measures the local quality of mixing of the two fluids averaged over space. Since 0 Slocation(species) ln(C), we normalize the species entropy conditional on locations Slocation(species)/ln(C) to get an index of mixing varying between 0 for perfect segregation and 1 for perfect mixing. 3. Fractal Analysis of Images We analyze the six structures generated in the staggered herringbone mixer and reported by Stroock et al.1 in terms of their multifractal dimensions. The six images are converted to grayscale. Each pixel j has a gray scale value xj varying between 0, black, and 255, white. x measures the local concentration of the fluorescent (white) fluid. Renyi entropies are calculated for each image by substituting in Eq. (2) the probabilities computed using the reading from each pixel:

pj

xj

¦x

(7)

i

The multifractal dimensions are obtained from a multi-scale analysis of the pictures. For each picture we considered 20 scales of observation. The slope of the linear dependence of entropy on the logarithm of the number of bins yields the dimension. The high values of the Pearson correlation coefficient, in all cases considered, point to the fact that the structures are self-similar (multifractals).

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In Figure 1 we show the Renyi entropies for ȕ = 0, 1, and 2 for the six sections presented in Stroock et al.,1 measuring the spatial distribution of the fluorescent (white) fluid. The qualitative conclusion that mixing progresses from Sections 1 to 6 is confirmed quantitatively by the monotonic increase in entropy. In Figure 2 we show two sections from Stroock et al.1 and the corresponding multifractal dimensions function d(ȕ). The dimensions of the later section are higher, again consistent with the higher degree of mixing in the later section.

Figure 1. Renyi entropies versus section; symbol × for ȕ = 0; symbol + for ȕ = 1 (Shannon entropy); symbol square for ȕ = 2.

Figure 2. Sections #4 and #6 of Stroock et al. and the corresponding multifractal dimensions functions d(ȕ).


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4. Microchannels with Fractal Grooves 4.1. WEIERSTRASS FUNCTION AND SURFACE PATTERNING

We recently proposed3 a new geometric design to pattern the microchannel wall. On one hand we preserve Stroock et al.1 asymmetric V shaped grooves, as they generate two stagnation points in the cross sections, and thus fluid rotation in clockwise and counter-clockwise direction at the same time. On the other hand, unlike Stroock et al., we locate the grooves in a non-periodic pattern along the channel axis. To this end we use the Weierstrass function to generate the coordinates of the “V” grooves tips. The Weierstrass function4 is:

W ( x)

f

¦

cos 2n x

n 0

2

n (2 D )

(8)

Its fractal dimension is D is easily controlled parametrically. We evaluate the function at M points x j j 2MS for j = 1, 2 …M. The coordinates of the grooves vertices are (xj, W(xj)). We also apply a linear transformation on each coordinate separately to insure that the M vertices occupy the length, 5,000 ȝm, and the width, 200 ȝm, of the channel. To generate asymmetric V-shaped ridges these vertices are connected to the lateral walls. As shown in Figure 3, to better approximate a more complex function, of higher dimension D, one needs a larger sampling rate M. Both the dimension D and the sampling rate M are important parameters. In our exploratory work we used a fixed M = 50 for three dimensions D = 1.25, 1.50, 1.75, labeled respectively: “W1.25”, “W1.50”, “W1.75”. In Figure 4 we show the W1.25 channel. All the channels have a length of 5,000 ȝm, width of 200 ȝm and height of 150 ȝm. Each ridge has a height of 50 ȝm. We have also studied numerically the SHM channel of Stroock et al.1 The flow field for the four different geometries is obtained by solving numerically the continuity equation for incompressible fluids and the NavierStokes equation of motion with no-slip boundary conditions:

G Gº G ª wv G U « v v » p P2 v ¬ wt ¼ G v 0

(9) (10)

In Eqs. (9) and (10) v is the velocity vector, ȡ the density, t the time, ȝ the viscosity and p the pressure. The fluid is Newtonian with density of 103 kg/m3

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M. KAUFMAN ET AL.

Figure 3. Weierstrass function plots for the cases of D = 1.25, D = 1.50, D = 1.75. In each case we show M = 50 vertices (circles).

Figure 4. The “W1.25” channel.

and viscosity of 10–3 Pa s. We integrate numerically the velocity field, to get the trajectories of 26,000 massless non-interacting particles of two species (13,000 blue and 13,000 red), and compute the mixing entropy defined in Eq. (4). In Figure 5 the entropy is calculated using 1,200 bins. The mixing efficiency as measured by the mixing entropy follows the following order: W1.25 > W1.50 > mSHM > W1.75.


443

Figure 5. Normalized mixing entropy versus distance along channel. The W1.25 and W1.50 exhibit higher mixing entropies than the SHM.

4.2. MANUFACTURE THE MICROCHANNEL WITH WEIERSTRASS PATTERNING

A physical model of the channels is needed in order to validate the computer model. A method to produce a three dimensional representation of the grooves in a microchannel that also had fluid inlet and outlets was developed. Since the dimensions of the microchannel are small (width of the channel is approximately 1 mm with the raised grooves having height and width of 50 μm), it was necessary to use a photolithographic technique along with other fabrication methods in order to create the device. The final device created is shown below in Figure 6. It consists of a Pyrex substrate with the 50 micrometer high ridges, a 200 μm high patterned plastic sheet with adhesive on both sides, a piece of acrylic with holes and tubing connectors. The ridges need to be at the bottom of a channel in

Figure 6. Fabricated microchannel.

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M. KAUFMAN ET AL.

which fluid will flow. Creating a straight walled channel can be difficult using conventional microfabrication techniques. In order to create a testing device that would allow for different thicknesses of channels we adapted a technique used in the production of commercial multi-layered polymer sensors such as the glucose sensors used by diabetics. The technique involves patterning different layers of plastic and then laminating them together to form a monolithic device. 5. Conclusions Mixing can generate complex structures. A fractal analysis of images complements the entropic mixing characterization. Future calibration work is needed to study the influence of the type of image file (.bmp, .jpeg, .tif) on the estimated fractal and entropic measures. Microchannels with patterned surfaces using a fractal function have been designed and characterized for their mixing efficiency. We found numerically that two of the mixers designed using the fractal approach in patterning are better mixers than the SHM of Stroock et al.1 One would expect the higher fractal dimension (more complex) structure to yield higher mixing. The results of Figure 5 show the opposite trend. We believe that the sampling rate M needs to be higher for the higher D structure. This problem needs further study.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Stroock, A. et al. G. (2002) Science 295, 647–651. Camesasca, M. et al. (2005) J. Micromech. Microeng. 15, 2038–2044. Camesasca, M. et al. (2006) J. Micromech. Microeng. 16, 2298–2311. Schroeder, M. (1991) Fractals, Chaos, Power Laws (New York, W. H. Freeman). Weaver, W., and Shannon, C. (1963) The Mathematical Theory of Communications (Urbana, IL, Illinois University Press). Khinchin, A. (1957) Mathematical Foundations of Information Theory (New York, Dover). Rényi, A. (1960) Theory of Probability (Amsterdam, North-Holland). Mandelbrot, A. (1982) The Fractal Geometry of Nature (New York, W. H. Freeman). Grassberger, P., and Procaccia, I. (1984) Physica D 13, 34–54.

SYNTHESIS AND CHARACTERIZATION OF CARBON SUPPORTED Pd AND PtPd CATALYSTS FOR DMFCs A. MOROZAN1*, A. DUMITRU1, C. MIREA1, I. STAMATIN1, F. NASTASE1, A. ANDRONIE1, S. VULPE1, C. NASTASE1, AND A. VASEASHTA2 1 3Nano-SAE Research Centre, University of Bucharest, P.O. Box MG-38, 077125, Bucharest-Magurele, ROMANIA 2 On details in Washington DC from Nanomaterials Processing and Characterization Labs. Marshall University West Virginia, USA

Abstract – Carbon supported Pd and PtPd catalysts were prepared by chemical reduction of metal precursors with sodium borohydride. Multi-wall carbon nanotubes (MWNTs) were used as carbon support. The composition of prepared catalysts was given by EDX technique. Cyclic voltammetry (CV) measurements were used to evaluate the electrocatalytic activities for methanol electrooxidation after their immobilization onto carbon paper.

Keywords: Catalyst, methanol oxidation, MWNTs.

1. Introduction Direct methanol fuel cell (DMFC) is one of the most advanced low-temperature fuel cells owing to the choice for many portable/micro power applications. However, there are some limitations such as the operational temperature, sluggish methanol oxidation kinetics, methanol crossover through the membrane electrolyte and the requirement for a greater loading (density) of catalyst to electrodes.1,2 Pt catalyst for hydrogen oxidation is not suitable for methanol due to CO formed as a reaction intermediate product, irreversibly absorbed to the Pt surface, rapidly lowering its activity.3 The electrocatalytic system Pt-Ru for methanol oxidation reduces the CO-poisoning by Ru, which removes COabs as CO2 (gas).4

______ *

To whom correspondence should be addressed: A. Morozan, email: [email protected]


445

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A. MOROZAN ET AL.

It is well established that bimetallic systems, with Pt as one of the components, give substantial tolerance compared to Pt alone, to the presence of CO in the fuel stream. Another noble metal with catalytic activity is Pd, which has the remarkable ability to store and release substantial amounts of hydrogen.5 For practical electrodes of a DMFC, carbon nanotubes (CNTs) are considered to be the most appropriate potential supports, especially for heterogeneous catalysts to be employed in liquid-phase reactions. Comparing with the most widely used Vulcan XC-72R carbon support, CNTs have significantly higher electronic conductivities and extremely high specific surface area.6 Moreover, higher activity of the oxygen reduction reaction and better performance of the DMFC compared to the catalysts supported on commercial carbons, was achieved.7,8 2. Experimental MWNTs from Shenzhen Nanotech PortoCo., Ltd., China (dIn @ >V @ xxx i

''' In

(1c)

x ' Cu Cu Vix l Cu ix VCu

K F,Cu

>Cu @ >V @ x i

' Cu

(2a) (2b)

>Cu @ >V @

(2c)

SSx Vix l Si'' VSxx

(3a)

x i

' Cu

DEFECT STRUCTURE OF COPPER INDIUM DISULFIDE

>S @ >V @ >S @ >V @ xx S

'' i

K aF , S

xx S

'' i

459

(3b) (3c)

Here, it is further assumed that the thermal generation of Frenkel disorder in the copper sublattice is energetically favorable. CIS is a ternary material and can be formed from the binary metal sulfides Cu2S and In2S3 and its formation is given by reaction (4a) with reaction equilibrium constant (4b). Cu2S + In2S3 ĺ 2CuInS2

K CIS

1/ 2 a Cu a 1In/22S3 2S

(4a) (4b)

CuInS2 is the ideal composition, which is just a special case. In general, it is expected that CIS will have a range of compositions by changing the molecularity and/or stoichiometry,3 which results in extrinsic defect formation mechanisms, presented as follows. The deviation from stoichiometry, that is the sulfur incorporation in or loss from the CIS lattice, can be expressed by lattice reaction (5a) and Eq. (5b),

1 S 2 ( g ) VSxx l SSx 2h x 2

>h @ >V @ p

(5a)

x 2

KS

xx S

(5b)

1/ 2 S2

For explaining the deviations from molecularity, the equations for Cu2S and In2S3 incorporation, i.e., the Cu-rich/In-rich material (equations in groups (6) and (7)) can be written as follows: CuInS 2 x Cu 2 S 2 o 2Cu Cu 2VIn''' S Sx 3VSxx

K Cu2 S

>V @ >V @ ''' 2 In

xx 3 S

(6b)

a Cu2 S

CuInS 2 In 2 S 3 2 o 2VCu' 2 In Inx 3S Sx V Sxx

K In 2S3

(6a)

>V @ >V @ 2 ' Cu

a In 2S3

xx S

(7a) (7b)

D. PERNIU, A. DUTA, AND J. SCHOONMAN

460

Ternary compounds, such as CIS, may exhibit both a deviation from stoichiometry and from molecularity and extrinsic ionic/electronic point defects are generated in the case of such deviations. The groups of equations labeled (8) and (9) express the lattice reactions, the laws of mass action, and the electro-neutrality equation for the Cu-rich material and In-rich material,

Cu 2 S

(8a)

3 CuInS 2 x S 2 ( g ) 2 o 2Cu Cu 2V In''' 4 S Sx 6h x 2

>V @ >h @ ''' 2 In

K Cu2 S , S

(8b)

x 6

aCu2 S p S32/ 2

> @ >h @

(8c)

x

3 VIn'''

1 ' In 2S 3 S 2 ( g ) CuInS 2 o 2VCu 2In Inx 4SSx 2h x 2

(9a)

>V @ >h @ 2 ' Cu

K In 2S3 , S

a In 2S3 p

>V @ >h @

x 2

(9b)

1/ 2 S2

(9c)

x

' Cu

The electronic equilibrium and equilibrium constant are as follows (10a and b):

null l e ' h x

(10a)

>e @ >h @

(10b)

Ki

x

'

3. The Brouwer Diagram The concept of the 3D Brouwer diagram is presented in Figure 1. The diagram represents the variations of a particular defect concentration as function of an external parameter. In the case of a ternary material, like CIS, the deviation from molecularity is expressed by the variation in activity of the metal sulfide and the deviation from stoichiometry by the variation in the sulfur partial pressure. In order to construct the 3D-Brouwer diagram the formal electroneutrality condition is the starting point. For the specific case of CIS, the electroneutrality condition is given by the following Eq. (11):

>V @ 3>V @ >e @ >Cu @ 3>In @ >h @ 2>V @ ' Cu

''' In

'

x i

xxx i

x

xx S

(11)


Ig [defect]

Deviation from stoichiom etry

Deviation from molecularity

Ig acu2s

461

Ig ps2

Ig aIn2S3

Figure 1. The 3D Brouwer diagram concept for the CIS material.

The diagram comprises different regimes, which are discriminated by a Brouwer approximation per specific regime. The choice of the Brouwer approximation, which is the reduced electroneutrality condition for the specific regime, is made considering the most probable defect formation reaction. In the following, the Brouwer regimes are characterized by the reduced electroneutrality conditions and the concomitant defect concentrations. Based on the Brouwer approximations and considering the equations expressing the laws of mass action for the lattice reactions, the defect concentrations were calculated for each regime in the diagram. 3.1. THE INTRINSIC REGIME

The preferred reaction is the Frenkel disorder in the copper cation sublattice, with the electroneutrality condition (2c). Considering this assumption, the concentrations of the point defects can be calculated (Eqs. (12a), (13a), and (14a)):

>V @ >Cu @ K >V @ >In @ K >e @ >h @ K ' Cu

x i

1/ 2 F ,Cu

(12a)

''' In

x xx i

1/ 2 F , In

(13a)

'

x

1/ 2 i

(14a)

2 with K F1 /,Cu ²² K F1 /, In2 ²² K i1 / 2 .

3.2. THE EXTRINSIC REGIME

If an excess of one of the binary metal sulfides is present in CIS, the material exhibits a deviation from molecularity. A deviation always leads to ionic point defects. Lattice reactions (8a) and (9a) describe the extrinsic disorder taking


462

into account both a deviation from molecularity and stoichiometry. In the 3D Brouwer diagram, the deviation from molecularity is presented in the front part of the diagram. Here, the defect concentrations are plotted versus the metal sulfide activity, taking the partial sulfide pressure constant. The deviation from stoichiometry is represented in the laterals and the defect concentrations are plotted versus the sulfur partial pressure, the activity of the metal sulfide being constant. The diagram contains two parts: one describing the Cu-rich materials, and the other the In-rich materials. In the Cu-rich region, reaction (8a) is predominant, and Eq. (8c) stands for the Brouwer approximation. In the In-rich region, the predominant reaction is (9a) and the Brouwer approximation is presented in Eq. (9c). The defect concentrations are calculated from the equations expressing the laws of mass action as a function of the sulfur partial pressure. The expressions for the defect concentrations are given in Eqs. (12b–18b) for the Cu-rich material and in Eqs. (12c–18c) for the In-rich region. For the Cu-rich material,

>Cu @ 3 K K K >V @ 3 K K >In @ 3 K K >V @ 3 K >V @ 3 K K >h @ 3 K >e @ 3 K K x i

1 CIS

1/ 4

F , Cu

1 / 4

' Cu

1/ 2 In 2 S 3 , S

CIS

xxx i

1 / 2 In 2 S 3 , S

3/ 4

F , In

3 / 4

''' In

xx S

x

1/ 4

(14b)

2

(15b)

2

1/ 4 1 / 8 aCu S pS 2

(16b)

2

1/ 8 3 / 16 aCu S pS 2

(17b)

2

1 / 8 Cu 2 S , S

i

1 / 8 3 / 16 aCu S pS 2

1/ 8 3 / 16 aCu S pS 2

1/ 4 Cu 2 S , S

1 / 4

'

(13b)

2

1 / 8 Cu 2 S , S

1/ 8 Cu 2 S , S

(12b)

2

1 / 8 5 / 8 1 / 16 K Cu aCu S pS 2 2S,S

1/ 8 Cu 2 S , S

1 S

1/ 2

1/ 8 5/8 1 / 16 K Cu aCu S pS 2 2S ,S

1 / 8 3 / 16 aCu S pS 2

(18b)

2

For the In-rich material,

>Cu @ K K K >V @ K K a >In @ K K K K >V @ K K K x i

' Cu

xx x i

1/ 4 In2 S 3 , S

F , In

''' In

1 / 4 In2 S 3 , S

F ,Cu

3 / 2 CIS

3/ 2 CIS

1 / 2 CIS

1/ 2 CIS

1 / 2 Cu 2 S , S

1/ 2 Cu 2 S , S

1/ 4 1 / 8 aCu S pS 2

1 / 4 Cu 2 S

p1S/28

3/ 4 In2 S 3 , S

3 / 4 In2 S 3 , S

(12c)

2

5 / 4 aCu pS23 / 8 2S

5/ 4 aCu pS32/ 8 2S

(13c) (14c) (15c)


>V @ K K >h @ K >e @ K K xx S

1 S

x

1/ 2 In2 S 3 , S

1/ 4 In2 S 3 , S

i

(16c)

1 / 2 1 / 4 K CIS aCu S pS 2 2

(17c)

1/ 2 1/ 4 1/ 8 K CIS aCu S pS 2

1 / 4 In2 S 3 , S

'

463

2

(18c)

1 / 2 1/ 4 1 / 8 K CIS aCu S pS 2 2

The defect concentrations are plotted versus the variable parameter considered for each regime. The slopes are the result of the derivations of the defect concentrations versus the metal sulfide activity or sulfur partial pressure. Figure 2a and b presents the two views of the 3D-Brouwer diagram. It is to be mentioned that on the diagram faces the extreme conditions are represented, while in the interior of the diagram both the deviation from molecularity and the deviation from stoichiometry can be viewed.

lg [defect]

lg [defect]

Cui

lg pS2

a

Vcu

VIn Vcu Ini Vs h e lg acu2s

Cui

Ini Vs

VIn h

lg pS2 e

lg aIn2S3

lg acu2s

lg aIn2S3

b

Figure 2. (a) The 3D-Brouwer diagram showing the Cu-rich regime, (b) the 3DBrouwer diagram showing the In-rich regime.

4. Conclusions The defect chemistry of the chalcopyrite-structured material CuInS2 is governed mainly by extrinsic defects as a result of deviations from molecularity and stoichiometry. Keeping the partial pressure of sulfur constant, expressions have been derived for the concentrations of the point defects. In a similar approach, expressions for the defect concentrations have been derived under the condition that the activity of one of the binary sulfides is kept constant. Here, it is assumed that the intrinsic disorder is described by a Frenkel mechanism in the copper-ion sublattice. The expressions for the defect concentrations have been used to obtain the slopes in a three-dimensional 3D-Brouwer diagram. This study

464


describes for the first time a 3D-Brouwer diagram for this chalcopyrite material. It is anticipated, that using the present approach, the 3D-Brouwer diagrams for other members of the visible-light absorbing chalcopyrites can be constructed. The predominant defect structures will be used to explain electrical conductivity and optical experiments on CuInS2 with a deviation from molecularity and for a deviation from stoichiometry.

References 1. Kröger, F. (1964) The Chemistry of Imperfect Crystals, North-Holland, Amsterdam. 2. Maier, J. (2004) Physical Chemistry of Ionic Materials. Ions and Electrons in Solids, Wiley, Chichester. 3. Binsma, J. (1983) J. Phys. Chem. Solids 44, 237–244. 4. Ueng, H., and Hwang, H. (1990) J. Phys. Chem. Solids 51, 11–18. 5. Márquez, R., and Ricón, C. (1999) Mater. Lett. 40, 66–70. 6. Nanu, M., Schoonman, J., and Goossens, A. (2004) Thin Solid Films, 451–452, 193–197. 7. Perniu, D., Vouwzee, S., Duta, A., and Schoonman, J. (2007) J. Optoelectron. Adv. Mater. 9, 1568–1571.

ASI GROUP PHOTOGRAPH

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ASI GROUP PHOTOGRAPH LEGEND

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

60

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60.

Adrian Ghita Cyril Popov Jan Boyd Emilia Pecheva Colm Cunniffe Olga Korostynska Dana Perniu Carmen Ristoscu Nicoleta Preda Damir Dominko Daniela Ebrasu Dan Tonchev Wolfgang Maser Brandon McNaughton Judit Kis-Csitari Marian Vopsaroiu Konstantin Petrov Adina Morozan Arezki Benfdila Melinda Mohl Jorgen Schou Ganna Kharlamova Ornprapa Pummakarnchana Vesselin Paunov Anna Kachanovska George Gallios Sigitas Tamulevicius Mr. Reisfeld Renata Reisfeld Uwe Kriebig Rodica Cristescu Felix Sima Ion N. Mihailescu Marina Turcan Ashok Vaseashta Doga Kavaz Alex G. Petrov Miroslava Vaclavikova Kseniia Katok Christopher S. Lohn Elena Lupan Andreea Purice Alexander Kukhta Tamer Cirak Pavel D’yachkov Oleksiy Kharlamov Yuriy Gnatyuk Adriana Andronie Jessica Weber Marie I. Baraton Marijus Brikas Temenuga Hineva Perica Paunovic Joop Schoonman Petia Petrova Aysegul Gokalp Silvia Bakalova Lyubomir Stoychev Nalan Ozdemir Ioanna Fasaki

467

SELECTED PHOTOGRAPHS TAKEN DURING THE ASI

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SELECTED PHOTOGRAPHS


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LIST OF PARTICIPANTS Dr. ALVELO, Jose I. Technical Program Support Northrop Gruman Information Technology Defense Group 8211 Terminal Road, Suite 1000 Lorton, VA 22079, USA [email protected] ANDRONIE, Adriana-Elena 3Nano-SAE Research Centre University of Bucharest PO Box MG-38, 077125 Bucharest-Magurele, ROMANIA [email protected] AXENTE, Emanuel Lasers Plasmas and Photonic Processing Laboratory - LP3 CNRS – Université Aix-Marseille Campus de Luminy – Case 917 13288 Marseille Cedex 9, FRANCE [email protected] Dr. AYVAZYAN, Gagik Semiconductor R&D Center Engineering Academy of Armenia 41 Arshakunyats, Technopark 0026 Yerevan, ARMENIA [email protected] BAKALOVA, Silvia Institute of Solid State Physics Bulgarian Academy of Sciences Tzarigradsko Chaussee Blvd. 72 1784 Sofia, BULGARIA [email protected]

Prof. BARATON, Marie-Isabelle SPCTS – UMR CNRS 6638 University of Limoges 87060 Limoges, FRANCE [email protected] Prof. BATANI, Dimitri Dipartimento di Fisica“G.Occhialini” Università di Milano Bicocca Piazza della Scienza 3 20126 Milano, ITALY [email protected] Dr. BENFDILA, Arezki LMPDS, Faculty of Electrical Engineering and Computer Science The University M. Mammeri BP 17 RP, 15000 Tizi-Ouzou, ALGERIA [email protected] BRIKAS, Marijus Institute of Physics Savanoriu Ave. 231 LT-02300 Vilnius, LITHUANIA [email protected] CIRAK, Tamer Hacettepe University Department of Chemistry Biopolymeric Systems Research Gr Beytepe, Ankara TURKEY [email protected] CAPUCCINI, Chiara Biomimetics and Materials Department of Chemistry Bologna, ITALY [email protected] 475

476

LIST OF PARTICIPANTS

Dr. CRISTESCU, Rodica National Institute for Lasers, Plasma and Radiation Physics Lasers Department, Laser-SurfacePlasma Interaction Laboratory Atomistilor 409, P.O. Box MG-36 Bucharest Magurele RO-077125, ROMANIA [email protected] D’YACHKOV, N. Pavel Institute of General and Inorganic Chemistry Russian Academy of Sciences Leninskii pr. 31 119991 Moscow, RUSSIA [email protected] DAMIR, Dominko Institute of Physics, HR-10001, P.O. Box 304 Zagreb, CROATIA [email protected] Prof. DEKHTYAR, Yuri Riga Technical University Biomedical Engineering and Nanotechnologies Institute 1 Kalku str, Riga, LV1658, LATVIA [email protected] ERDURAL, Beril Korkmaz Department of Chemical Engineering Middle East Technical University 06431 Ankara, TURKEY [email protected] FASAKI, Ioanna National Hellenic Research Foundation

Theoretical and Physical Chemistry Institute Vasileos Konstantinou Ave. 48 11635 Athens, GREECE [email protected] Prof. FONTCUBERTA, Josep Institut de Ciència de Materials de Barcelona-CSIC Campus UAB, Bellaterra 08193 Catalunya, SPAIN [email protected] Dr. FORSEA, Anna Maria Dermatology Department Elias University Hospital Carol Davila University of Medicine & Pharmacy Bucharest, ROMANIA [email protected] Dr. GALLIOS, George Aristotle University of Thessaloniki School of Chemistry Laboratory of General & Inorganic Chemical Technology (116) GR-540 06 Thessaloniki, GREECE [email protected] GHITA, Adrian Institute of Physical Chemistry “Ilie Murgulescu” 202 Splaiul Indepentei Bucharest, ROMANIA [email protected] GNATYUK, Yuriy Surface Photonics Laboratory O. Chuiko Institute of Surface Chemistry of NAS of Ukraine 03164 Kyiv, UKRAINE [email protected]


GOKALP, Aysegul Nanomaterials Processing & Characterization Laboratories Marshall University, One John Marshall Drive Huntington, WV 25575-2570, USA [email protected] Dr. GURSAN ERDEM, Arzum Ege University Faculty of Pharmacy Analytical Chemistry Department 35100 Bornova, Izmir, TURKEY [email protected] HINEVA, Temenuga Laboratory of Thin Films Technology University of Chemical Technology and Metallurgy Department of Physics 8 Kl. Ohridsky blvd 1756 Sofia, BULGARIA [email protected] Dr. HORVÁTH, J. Zsolt Hungarian Academy of Sciences Research Institute for Technical Physics and Materials Science Budapest 114, P.O. Box 49, H1525, HUNGARY [email protected] KACHANOVSKA, Anna Biomedical Engineering and Nano Technologies Institute Riga Technical University Riga, LATVIA [email protected] Dr. KAUFFMAN, Miron Physics Department

477

Cleveland State University Cleveland, OH 44115, USA [email protected] KATOK, Kseniia Institute of Surface Chemistry National Academy of Sciences of Ukraine 17 General Naumov Str., 03164 Kyiv, Ukraine [email protected] KAVAZ, Do÷a Hacettepe University Chemistry Department Biochemistry Division Beytepe, Ankara, TURKEY [email protected] KHARLAMOV, Oleksiy I.N. Frantsevich Institute for Problems of Materials Science Kiev, UKRAINE [email protected] KHARLAMOVA, Ganna Kiev National Taras Shevchenko University Kiev, UKRAINE [email protected] KIS-CSITÁRI, Judit Bay Zoltán Foundation for Applied Research Iglói út 2, 3519 Miskolc-Tapolca HUNGARY [email protected] Prof. KREIBIG, Uwe I. Physical Institute IA RWTH Aachen GERMANY [email protected]

478


Dr. KUKHTA, V. Alexander Institute of Molecular and Atomic Physics National Academy of Sciences Nezalezhnastsi Ave. 70 220072 Minsk, BELARUS [email protected] LOHN, S. Christopher Department of Physics Texas State University at San Marcos TX 78666, USA [email protected] LUPAN, Elena Center of Optoelectronics Institute of Applied Physics Academy of Sciences of Moldova Chisinau MD-2028, REPUBLIC of MOLDOVA [email protected] Prof. MASER, Wolfgang Department of Nanotechnology Instituto de Carboquímica (CSIC) C/Miguel Luesma Castán 4 E-50018 Zaragoza, SPAIN [email protected] McNAUGHTON, H. Brandon Applied Physics Program University of Michigan Ann Arbor, MI, USA [email protected] Prof. MIHAILESCU, Ion N. Director, National Institute for Lasers, Plasma and Radiation Physics Lasers Department, “Laser-SurfacePlasma Interactions” Laboratory

PO Box MG-54, RO-77125, Bucharest-Magurele, ROMANIA [email protected] MOHL, Melinda Department of Applied and Environmental Chemistry University of Szeged, Rerrich B. tér 16720 Szeged, HUNGARY [email protected] Dr. MOROZAN, Adina 3Nano-SAE Research Centre University of Bucharest PO Box MG-38, 077125 Bucharest-Magurele, ROMANIA [email protected] NAIDENOVA, Tsvetelina Institute of Electronics Bulgarian Academy of Sciences 72 Tzarigradsko Chaussee blvd 1784 Sofia, BULGARIA [email protected] ÖZDEMIR, Nalan Erciyes University Chemistry Department Kayseri, TURKEY [email protected] Dr. PAPAKONSTANTINOU, Pagona Nanotechnology Research Institute (NRI) School of Electrical and Mechanical Engineering University of Ulster at Jordanstown Newtownabbey, County Antrim BT37 OQB Northern Ireland, UK [email protected]


Dr. PAUNOV, Vesselin N. Surfactant & Colloid Group Department of Chemistry University of Hull, UK [email protected] PAUNOVIû, Perica Faculty of Technology and Metallurgy University Sts. Cyril and Methodius Skopje, R., MACEDONIA [email protected] Dr. PECHEVA, Emilia Institute of Solid State Physics Bulgarian Academy of Sciences blvd. Tzarigradsko Chaussee. 72 1784 Sofia, BULGARIA [email protected] Dr. PERNIU, Dana Transilvania University of Brasov Center: Product Design for Sustainable Development Brasov, ROMANIA [email protected] Acad. Prof. PETROV, Alexander G. Institute of Solid State Physics Bulgarian Academy of Sciences 72 Tzarigradsko chaussee Sofia 1784, BULGARIA [email protected] Dr. PETROV, Konstantin Institute for Electrochemistry and Energy Systems Bulgarian Academy of Sciences 1113 Sofia, BULGARIA [email protected]

479

PETROVA, Petia Central Laboratory of Photoprocesses “Acad. J. Malinowski” Bulgarian Academy of Sciences “Acad. G. Bonchev”str., bl. 109, 1113 Sofia, BULGARIA petiakl@mail,bg Dr. PIVIN, Jean-Claude CSNSM, 91405 Orsay Campus, FRANCE [email protected] Dr. POPOV, Cyril Institute for Nanostructure Technologies and Analytics (INA) University of Kassel, GERMANY [email protected] Dr. PREDA, Nicoleta Roxana Optics and Spectroscopy Lab. National Institute of Materials Phys PO Box MG-7, BucharestMagurele, ROMANIA [email protected] Dr. PUMMAKARNCHANA, Ornprapa Department of Environmental Science School of Science Silpakorn University Nakornoathom, 73000 THAILAND. [email protected]

480


Prof. REISFELD, Renata Department of Inorganic Chemistry The Hebrew University Jerusalem, ISRAEL [email protected]

Lasers Department “Laser-Surface-Plasma Interactions” Laboratory PO Box MG-36, RO-77125, Bucharest-Magurele, ROMANIA [email protected]

Dr. RISTOSCU, Carmen National Institute for Lasers, Plasma & Radiation Physics Lasers Department, “Laser-SurfacePlasma Interactions” Laboratory PO Box MG-36, RO-77125, Bucharest-Magurele, ROMANIA [email protected]

Dr. STAMATIN, Ioan 3Nano-SAE Research Centre University of Bucharest P.O. Box MG-38 077125 Bucharest-Magurele, ROMANIA [email protected]

Prof. SCHOONMAN, Joop Department of DelftChemTech Delft University of Technology Delft, THE NETHERLANDS [email protected]

STOENESCU, Daniela National Institute for Cryogenics and Isotopic Technologies ICIT Rm. Valcea, ROMANIA [email protected]

Prof. SCHOU, Jørgen Department of Optics and Plasma Research Risø National Laboratory Technical University of Denmark DK-4000 Roskilde, DENMARK [email protected]

STOYCHEV, Lyubomir Geordi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Science 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, BULGARIA [email protected]

Prof. SENTIS, Marc Lasers Plasmas and Photonic Processing Laboratory CNRS – Universities of AixMarseille Campus de Luminy – Case 917 13288 Marseille Cedex 9, FRANCE [email protected]

Prof. SZÖRÉNYI, Tamas Research Group on Laser Physics University of Szeged PO Box 406, H-6701 Szeged, HUNGARY [email protected]

SIMA, Felix National Institute for Lasers, Plasma and Radiation Physics

Prof. TAMULEVICIUS, Sigitas Institute of Physical Electronics Kaunas University of Technology Savanoriu 271 Kaunas LT50131, LITHUANIA [email protected]


Prof. TENNE, Reshef Department of Materials and Interfaces Weizmann Institute Rehovot 76100, ISRAEL [email protected] Dr. TONCHEV, Dan Department of Electrical Engineering University of Saskatchewan Saskatoon, CANADA [email protected] TOURLEIGH, Yegor V. Bioengineerings Department Lomonosov Moscow State University Moscow, RUSSIA [email protected] ğURCAN, Marina Center of Optoelectronics Institute of Applied Physics Academy of Sciences of Moldova 1 Academiei Street Chisinau MD-2028, REPUBLIC Of MOLDOVA [email protected] Dr. VACLAVIKOVA, Miroslava Institute of Geotechnics Slovak Academy of Sciences Watsonova 45

481

SK-043 53 Kosice, SLOVAKIA [email protected] Prof. Dr. Ir. VASEASHTA, Ashok Director, Nanomaterials Processing & Characterization Labs. Department of Physics & Grad. Program in Physical Sciences Marshall University One John Marshall Drive Huntington, WV 25575-2570, USA [email protected] Dr. VOPSAROIU, Marian National Physical Laboratory Teddington TW11 0LW, UK [email protected] WEBER, Jessica E. Department of Mechanical Eng. Nanomaterials & Nanomanufacturing Research Ctr. University of South Florida, Tampa, FL, USA [email protected] Dr. YAZICI, M. Suha Unido International Centre for Hydrogen Energy Technologies Sabri Ulker SK 38/4 CevizlibagZeytinburnu 34015 Istanbul, TURKEY [email protected]

AUTHORS INDEX Agayan 403 Andronie 415, 445 Antoniadou 379 Arshak 299, 321 Axente 389 Bakalova 357 Bakir 409 Baraton 77 Batani 145 Bayram 313 Belev 279 Benito 101 Bigi 389 Biljakoviü 399 Blackburn 431 Boanini 389 Boyd 61 Cain 431 Caiteanu 27 Camesasca 437 Capuccini 389 Chihaia 449 ÇÕrak 313 Clarke 403 Cohen 51 Cunniffe 101, 321 Cziraki 357 Dekhtyar 169, 347 Delaporte 185 Demšar 399 Denkbas 313, 325 Dominko 399 Dou 419 Du 27 Dudik 437 Duman 325 Dumitru 415, 445 Duta 457 Ebrasu 383

Engin 325 Erdem 273 Erdural 409 Escoubas 27 Fasaki 379 Feldman 51 Gallios 291 Gazzano 389 Geerts 419 Geretovszky 121 Ghita 449 Giannoudakos 379 Grigorescu 357 Grojo 185 Gyorgy 357 Hotovy 379 Hredzak 291 Ikuta 351 Iliev 305 Ivanov 331 Ivanova 351 Jägermann 51 Jakabsky 291 Kachanovska 347 Karakas 409 Kasap 279, 351 Katok 335 Kaufman 437 Kavaz 313 Kharlamov 373 Kharlamova 373 Kiricsi 365, 369 Kirillova 373 Kis-Csitári 369 Kompitsas 379 Kónya 365, 369 Kopelman 403 Kopnov 51 Korostynska 299

483

484

AUTHORS INDEX

Koughia 351 Kukovecz 365 Kulisch 215 Kumar 425 Kuncser 431 Liaw 61 Liu 437 Lohn 419 Maeda 351 Manas-Zloczower 437 Mani 279 Marcus 399 Martínez 101 Maser 101 Mazingue 27 McNaughton 403 Meškinis 225 Mezinskis 347 Mihailescu, I. 27, 357, 389, 399 Mihailescu, C. 27 Mihailoviü 399 Milat 399 Mirea 445 Mohl 365 Moore 321 Morozan 415, 445 Moshkovich 51 Muñoz 101 Nastase, C. 415, 445 Nastase, F. 415, 445 Nikolova 305 O’Brien 419 Ozdemir 325 Öztürk 313 Padmini 419 Pandey 419 Patmalnieks 347 Patularu 383 Pereira 185 Perniu 457 Perrone 27 Petreanu 383

Petrov, A. 87 Petrov, K. 305 Phonekeo 339 Popov 215 Pummakarnchana 339 Pumpens 347 Rapoport 51 Rehacek 379 Reisfeld 257 Renhofa 345 Ristoscu 27, 399 Roubani-Kalantzopoulou 379 Sakai 351 Salamon 399 Schad 419 Schoonman 199, 457 Schou 241 Sentis 185 Seong 449 Sima 389 Skripnichenko 373 Socol 399 Späth 51, Stamataki 379 Stamatin 415, 445 Starešiniü 399 Stefanescu 383 Stefusova 291 Stoica 403 Stoitsas 199 Stoyanova 305 Stoychev, D. 305 Stoychev, L. 331 Suh 449 Szekeres 357 Szörényi 121 Tamuleviþius 225 Tenne 51 Tertykh 335 Tomeljak 399 Tonchev 279, 349 Tran 199

AUTHORS INDEX

Tuncel 325 Vaclavikova 291 Valeanu 383 Valov 305 Vaseashta 3, 321, 331, 339, 415, 445 Vitanov 305 Vopsaroiu 431

Vulpe 415, 445 Weber 425 Yanishpolskii 335 Yazici 283 Yelton 425 Yurum 409 Zak 51 Zhecheva 305

485

KEYWORD INDEX 3D-Brouwer diagram 457 A Ablation 121 ablation threshold flux 145 adhesion 347 adsorption 447 akaganeite 291 aluminium nitride 357 amine-containing groups 365 annealing 51 antimicrobial 409 applications 101 arsenic 291 B bifunctional air/oxygen electrode 305 BioForce NanoeNablerTM 299 biomedical applications 215 biomembrane 87 biosensors 87, 215 biotechnology 313 blue bronze 399 C carbon membrane reactors 199 carbon nanotube 445 carbon nanotubes 101 carbon threads 373 catalyst 445 cavitation 369 ceramic membranes 199 chalogenide glass 351 characterization 101 chemical composition test 331 chemical gas sensors 77 chitosan 313 clusters 121

CH4 449 collisional absorption 145 composites 101 conducting polymer composites 321 contact angle with water 225 copper indium disulfide 457 crater formation 145 crystalline structure 357 crystallization temperature 351 CV 419 D diamond like carbon films 225 dielectric barrier discharges 61 dispersions 101 DNA 273 doped a-Se 279 DSC 351 dual-electron spectroscopy 169 E electrical properties 279 electrocatalysts 305 electrochemical sensors 273 electrodeposition 425 energy 283 entropy 433 ESR 313 excimer lamps 61 exo-electron spectroscopy 169 exothermal nanosynthesis 373 F Faraday effect 431 field-matter interaction 331 flexoelectricity 87 fractals 437 FTIR spectroscopy 77

487

488

KEYWORD INDEX

fuel cells 283, 383 functionalization 101, 365 G gas diffusion electrodes 308 gas sensors 379 gas-surface interactions 77 gene delivery 325 glass transition 279 gold 335 greenhouse effect 199 H hydrodynamics 145 hydrogen 283 hydrogen oxidation 335 hydrogen production 199 hydrothermal 409 hydroxyapatite 389 H2 379, 449 I IF-WS2 51 ilmenite-hematite 419 inorganic-organic hybrid matrices 257 internet GIS 339 intrinsic defect 457 in situ reduction 335 ion beam synthesis 225 isomorphous substitution 390 IV 419 L laser 145 laser ablation 145 laser nano-particle interaction 185 laser processing 121 low temperature 61 M magnetic beads 403 magnetic microspheres 403

magnetic nanoparticles 313 magnetite 291 matrix-assisted pulsed laser evaporation (MAPLE) 241 mechanosensitivity 87 membrane 383 metal oxide films 27 methanol oxidation 445 microdevices 299 microfluidics 437 mixing 437 m-line technique 27 modeling 291 modified surface 335 multi-functional materials 419 MWCNT 365 MWNTs 445 N nanocomposite membrane 415 nanocrystalline diamond films 215 nanocrystallites 357 nanodot arrays 185 nano-generator 431 nanomaterials 273, 283 nanoparticles 51,121, 291, 369 nanopatterning 299 nanostructured coatings 27 nanostructured 409 nanostructures 169 nanowire 425 near-field enhancement 185 NiO 379 non-destructive testing 331 nonlinear dynamics 403 nonlinear rotation 403 O optical and electrical properties 257 optical gas sensor 27 optical properties 225

KEYWORD INDEX

P photocatalytic 409 photo-chemistry 61 photoluminescence 351 Plasma frequency 145 PLD 241, 389 point-of-care diagnostics 299 polymer 383 polymeric nanoparticles 325 photo-electron spectroscopy 169 production 101 pulsed laser deposition (PLD) 241, 357 purification 101 R raman scattering spectroscopy 225 rare earth doping 351 real time air pollution monitoring 339 remote sensing 339 S semiconductor metal oxides 77 semiconductor nanoparticles 347 semiconductor quantum dots and rods 257 sensor systems 321 sensors 321, 339 sensors arrays 299 SiC nanostructures 373 silica 335 silicon hydride groups 335 single cell detection 403 SiOx containing DLC 225 sol gel glasses 257 solid acid 415 sonochemistry 369

489

sorption 291 spatial coherence 145 Spitzer’s law 145 strontium 389 sublimation 373 superparamagnetism 431 superprotonic transition 415 surface chemical species 77 surface chemistry 77 T thermal energy 431 thermal properties 279 thermodynamic diagrams 291 thermoelectric application 425 thin films 121, 399 titania 409 tribology 51 tripolyphosphate 313 U ultraviolet 61 V VSM 313 W water treatment 291 X XPS (X-ray photoelectron spectroscopy) 51 XPS analysis 357 XPS study 225 XRD 357 Z zirconia- silica- polyurethane 257

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Nuclear Power and Energy Security (NATO Science for Peace and Security Series B: Physics and Biophysics)

Imaging for Detection and Identification (NATO Science for Peace and Security Series B: Physics and Biophysics)

Optical Waveguide Sensing and Imaging (NATO Science for Peace and Security Series, B: Physics and Biophysics)

Countering Nuclear and Radiological Terrorism (NATO Science for Peace and Security Series B: Physics and Biophysics)

Extreme Photonics & Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Complexity and Security (Nato Science for Peace and Series) (Nato Science for Peace and Series)

Biophysics and the Challenges of Emerging Threats (NATO Science for Peace and Security Series B: Physics and Biophysics)

Nanostructured Materials for Advanced Technological Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Nanostructured Materials for Advanced Technological Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Properties and Applications of Thermoelectric Materials: The Search for New Materials for Thermoelectric Devices (NATO Science for Peace and Security Series B: Physics and Biophysics)

Nuclear Science and Safety in Europe (NATO Science for Peace and Security Series B: Physics and Biophysics)

Advanced Materials and Technologies for Micro Nano-Devices, Sensors and Actuators (NATO Science for Peace and Security Series B: Physics and Biophysics)

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Mid-Infrared Coherent Sources and Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Metamaterials and Plasmonics: Fundamentals, Modelling, Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Molecular Imaging: Computer Reconstruction and Practice (NATO Science for Peace and Security Series B: Physics and Biophysics)

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Security and Reliability of Damaged Structures and Defective Materials (NATO Science for Peace and Security Series C: Environmental Security)

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Hamiltonian Dynamical Systems and Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Hamiltonian Dynamical Systems and Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Hamiltonian Dynamical Systems and Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Nuclear Power and Energy Security (NATO Science for Peace and Security Series B: Physics and Biophysics)

Imaging for Detection and Identification (NATO Science for Peace and Security Series B: Physics and Biophysics)

Optical Waveguide Sensing and Imaging (NATO Science for Peace and Security Series, B: Physics and Biophysics)

Countering Nuclear and Radiological Terrorism (NATO Science for Peace and Security Series B: Physics and Biophysics)

Extreme Photonics & Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Complexity and Security (Nato Science for Peace and Series) (Nato Science for Peace and Series)

Biophysics and the Challenges of Emerging Threats (NATO Science for Peace and Security Series B: Physics and Biophysics)

Nanostructured Materials for Advanced Technological Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Nanostructured Materials for Advanced Technological Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Properties and Applications of Thermoelectric Materials: The Search for New Materials for Thermoelectric Devices (NATO Science for Peace and Security Series B: Physics and Biophysics)

Nuclear Science and Safety in Europe (NATO Science for Peace and Security Series B: Physics and Biophysics)

Advanced Materials and Technologies for Micro Nano-Devices, Sensors and Actuators (NATO Science for Peace and Security Series B: Physics and Biophysics)

Detection and Disposal of Improvised Explosives (NATO Science for Peace and Security Series B: Physics and Biophysics)

Mid-Infrared Coherent Sources and Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Metamaterials and Plasmonics: Fundamentals, Modelling, Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Molecular Imaging: Computer Reconstruction and Practice (NATO Science for Peace and Security Series B: Physics and Biophysics)

Stand-off Detection of Suicide Bombers and Mobile Subjects (NATO Science for Peace and Security Series B: Physics and Biophysics)

Mid-Infrared Coherent Sources and Applications (NATO Science for Peace and Security Series B: Physics and Biophysics)

Detection and Disposal of Improvised Explosives (NATO Security through Science Series B: Physics and Biophysics)

Evolution from Cellular to Social Scales (NATO Science for Peace and Security Series B: Physics and Biophysics)

Smart Materials for Energy Communications and Security NATO Science for Peace and Security Seri

Mini-Micro Fuel Cells: Fundamentals and Applications (NATO Science for Peace and Security Series C: Environmental Security) (NATO Science for Peace and Security Series C: Environmental Security)

Building and Using Datasets on Armed Conflicts (Nato Science for Peace and Security) (Nato Science for Peace and Security)

Stem Cells and Their Potential for Clinical Application (NATO Science for Peace and Security Series NATO Science for Peace and Security Series A: Chemistry and Biology)

Environmental Security and Public Safety (NATO Science for Peace and Security Series C: Environmental Security)

Energy and Environmental Challenges to Security (NATO Science for Peace and Security Series C: Environmental Security)

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