Photoconductivity of Carbon Nanotubes

Encyclopedia of Nanoscience and Nanotechnology

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Photoconductivity of Carbon Nanotubes Akihiko Fujiwara Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa, Japan, and Japan Science and Technology Corporation, Kawaguchi, Saitama, Japan

CONTENTS 1. Introduction 2. Physical Properties of Carbon Nanotubes 3. Photoconductivity of Carbon Nanotubes 4. Related Phenomena 5. Summary Glossary References

1. INTRODUCTION Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991 [1], they have attracted great attention as potential electronic materials because of the one-dimensional tubular network structure on a nanometer scale. The variety of band structures of the CNTs, being either semiconducting or metallic, depending on the chirality and diameter of the CNT, is also a novel feature. For this reason, it is expected that CNTs become model samples for an ideal onedimensional metal and an ideal one-dimensional semiconductor. In addition, CNTs are also expected to be one of the greatest candidates for nanotechnology materials, such as nanometer scale wirings and nanometer scale devices [2–4]. A CNT can be described as a single graphite (graphene) sheet rolled into a cylindrical shape. A concentric tubular structure can be made of two or more nanotubes with different diameters. The former and the latter are called single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs), respectively. The thinnest CNTs are found in a most inner CNT of MWCNTs and SWCNTs grown in zeolite AlPO4 -5 (AFI) single crystals; the diameter is about 0
4 nm [5, 6]. The diameter of thick CNTs is at most a few 10 nanometers. In most cases, CNTs with large diameters are MWCNTs; in the case of SWCNTs, a cross-section will not be able to maintain circular structure but it will be distorted. Moreover, in MWCNTs with large diameters, structure strongly depends on production conditions and can be not only concentric, but also the ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

structure in which a graphene is scrolled up—a polyhedral graphite tube with defects at the vertex and mixtures of them [7–10]. For the theoretical approach and the interpretation of experimental results, SWCNTs are suited, because interaction between layers and the effect of defects must be taken into consideration in the case of MWCNTs. Most of the experimental observation had been on MWCNTs in the beginning of research on CNTs. After the establishment of a synthesis method for high-quality SWCNTs [11–13], extensive research on SWCNTs, as well as MWCNTs, is performed. Since researches field of photoconductivity of CNTs reviewed in this article have a short history, only the research for two kinds of SWCNT samples are reported [14–19]. Photoconductivity has not been observed in MWCNT samples. Therefore, if there is no notice, there will be a discussion about the SWCNT in this article. This chapter is organized as follows. The physical properties needed for discussion about the photoconductivity of CNTs are presented in Section 2. In Section 3, the photoconductive properties observed in two kinds of SWCNTs are shown. Here, experimental methods are also described, because introduction of this is important because it has been succeeded only with a few groups in spite of many trials. As related phenomena, two types of photo-induced current modulations are presented in Section 4. Section 5 summarizes the chapter.

2. PHYSICAL PROPERTIES OF CARBON NANOTUBES A number of excellent books and review articles have summarized the physical and chemical properties of CNTs [2–4, 20, 21]. In this section, properties related to photoconductivity are briefly described.

2.1. Molecular Structure A SWCNT can be made by rolling a graphene sheet into a cylindrical shape [2–4, 20–23]. Although the tubular structure with any diameter and direction can be made when Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (569–574)

570 we make it from papers without a pattern on the surface, the network structure of carbon has to be take into account for CNTs. In a graphene sheet, carbon atoms form a twodimensional network of a six-membered ring, namely, a hexagonal (or honeycomb) lattice by connecting their three sp2 hybrid orbitals. There are two sites, A- and B-site, in the graphene: one carbon is bonded to three carbons of another site. When graphene is rolled into cylindrical shape and one carbon is put on any other carbon in the same site, a CNT can be formed in principle. Relation between these two overlapped carbon atoms on the graphene can be described by chiral vector C h = na1 + ma2 , where n, m are integers and a1  a2 are the unit vectors of the graphene. The direction of the chiral vector Ch is perpendicular to the CNT axis and its length, L ≡ Ch  = a n2 + m2 + nm 1/2 is equal to that of circumference of the CNT, where a is the length of a1 and a2 . In actuality, because a sp2 hybrid orbital cannot be maintained for small diameters and the tubular structure is distorted for large diameters, the diameter of SWCNTs is considered to be about 0.4–2.0 nm.

2.2. Electronic Structure It is intelligible when the electronic structure of a SWCNT, as well as molecular structure, is considered on the basis of a graphene sheet [20–25]. It is necessary to consider the periodic boundary conditions along the circumference of the CNT to the electronic structure of the graphene. Since the graphene sheet has a hexagonal lattice, the first Brillouin zone becomes a right hexagon. In energy-dispersion relations, the ∗ (conduction) band and the (valence) band are degenerate only at the six corners of the hexagonal first Brillouin zone, K and K  points, which the Fermi energy passes. Since the density of states at the Fermi energy is zero, the graphene is a zero-gap semiconductor. Although all the points in the first Brillouin zone are allowed for the graphene sheet, in the case of CNTs, allowed points are only on the line prolonged in the direction of the CNT axis through the  point (the center of the first Brillouin zone) and the parallel lines that separated 2k /L (k: integer) from this line. Therefore, when these straight lines pass through K (K  ) points, since energy dispersion will pass Fermi energy, CNTs become metallic. The distance between K (K  ) points and the allowed line through the  point is 2 (2n + m /3L. As a result, when 2n + m, that is, n − m, is the multiple of 3, CNTs become metallic. In other cases, CNTs become semiconducting. In the energy dependence of density of states in CNTs, van Hove singularity (VHS) appears to be reflecting one-dimensional nature. Moreover, because ∗ (conduction) and (valence) bands are almost symmetrical, the density of state near the Fermi energy shows the symmetrical energy dependence in respect to the Fermi energy.

Photoconductivity of Carbon Nanotubes

an optical absorption spectrum are observed in the energy range from an infrared region to a visible region [13, 27]. Two absorption bands in the lower energy and one at the highest energy originate from semiconducting and metallic SWCNTs, respectively. It is well known that the exciton binding energy becomes infinite in the limit of an ideal one-dimensional electronhole system [28–30]. Therefore, the effect of exciton plays an important role in optical absorption for the one-dimensional system of SWCNTs. This effect is considered to mainly modify the lowest band of the optical absorption spectrum, which was predicted theoretically [31]. Experimental results of optical absorption are consistent with this prediction [13, 27].

2.4. Electronic Transport Properties Although there are many findings of novel properties and functions, such as single electron transport [32, 33], spin transport [34], rectification [35, 36], switching function [37], tunable electronic structure by magnetic fields [38, 39], single molecule CNT transistors [40–42], and superconductivity [43, 44] in electron transport properties, we focus on the electron scattering—ballistic or diffusive. The ballistic conduction in CNTs even at room temperature (RT) was pointed out by observation of quantized conduction in MWCNTs at first [45]. Ballistic conduction of SWCNTs was proposed by the detailed analysis of the coulomb blockade behavior of SWCNTs which act as a quantum dot [32]. From the subsequent research for semiconducting SWCNTs, the mean-free path is estimated to be about 100 nm which is about one-tenth of the CNT length, and the result suggests diffusive conduction in semiconducting SWCNTs in spite of ballistic conduction in metallic SWCNTs [46]. This is considered to be due to the electron scattering at defects and the bending parts of CNTs.

3. PHOTOCONDUCTIVITY OF CARBON NANOTUBES The research on photoconductivity for two kinds of SWCNTs has been reported. One is observed in the SWCNTs with a diameter of about 1.4 nm, being closedpacked into bundles and forming a two-dimensional triangular lattice [14–17]. Another is in the SWCNTs, with a diameter of about 0.4 nm grown in zeolite AlPO4 -5 (AFI) single crystals, which are one type of the thinnest SWCNTs [18, 19]. In MWCNTs, photoconductivity has not been observed thus far.

2.3. Optical Absorption

3.1. Single-Wall Carbon Nanotube Bundles

Theoretical prediction shows the optical transition takes place only between symmetrical bands, under the configuration that the polarization vector E is parallel to the nanotube axis by taking into account the depolarization effect [26]. Corresponding to high transition probability between the symmetric VHSs, three characteristic absorption bands in

Photoconductivity of CNTs has been discovered in the film sample of SWCNT bundles, and the most detailed research has been performed for this sample [14–17]. In this section, detailed experimental methods for the observation, photoconductive properties, and possible mechanisms are presented.

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3.1.1. Experimental Technique The samples of SWCNT bundles were synthesized by evaporation of composite rods of nickel (Ni), yttrium (Y), and graphite in helium atmosphere by arc discharge [12, 13] or ablating a graphite target containing Ni and cobalt (Co) catalysts at 1250  C in an argon atmosphere by using a pulsed Nd:YAG laser [11]. Observations by transmission electron microscopy (TEM) revealed that soot is mainly composed of SWCNTs and also amorphous carbons and metal particles. The diameter of the SWCNTs is determined to be about 1
4 ± 0
2 nm by the Raman frequency of a breathing mode and TEM observation. The typical length of SWCNT bundles estimated by scanning electron microscopy (SEM) is a few micrometers. To prepare film samples, soot-containing SWCNTs were dispersed in methyl alcohol by an ultrasonic vibrator and suspension of SWCNTs was dropped on a glass substrate. The typical film sample size is about 100 m × 100 m and the thickness of the film is between 300 and 500 nm. The samples were annealed in vacuum at 10−6 Torr and 673 K for 2 hours to remove the absorbed gases and methyl alcohol from samples. A pair of gold electrodes separated by a 10-m gap was evaporated in vacuum onto the surface of the film samples and connected to a DC regulated power supply (100 mV). In order to reduce the number of junctions between SWCNTs in the current pass, the narrow gap of 10 m was chosen because the resistance of the junctions dominates the total resistance of the sample and obscures the intrinsic transport properties of SWCNTs. The samples were mounted in a continuous-flow cryostat and cooled by flowing the vapor of liquid He and liquid N2 in the temperature range from 10 K to RT and from 100 K to RT, respectively. As a light source, an optical parametric oscillator (OPO), excited by a pulsed Nd:YAG laser, was used. The photon energy was in the range of 0.5 to 2.8 eV and the pulse duration was 5 ns. The light intensity is from a few tens nJ/pulse to 1500 nJ/pulse. The temporal profiles of the laser pulse and the photocurrent were monitored with a digitizing oscilloscope. In order to avoid spurious ringing in the fast pulse detection, we were obliged to use the input impedance of the oscilloscope (50 ) as the reference resistor, despite the obvious disadvantage of lower sensitivity. The resistance of samples in the dark is ca. 100  at RT and 800  at around 10 K.

3.1.2. Observed Behaviors The temporal evolution of photocurrent shows a Gaussianlike peak with a 5 ns width corresponding to the pulse duration of the laser. Photocurrent increases with increasing incident light intensity. When photocurrent is less than 10 A, the photocurrent intensity responds linearly to incident light intensity. On the other hand, it shows a saturation behavior above 10 A; this saturation is often observed under intense light intensity and might be due to the lack of replenishment of carriers [47]. In photoconductivity excitation spectra estimated from the slope of the linear part in light intensity dependence of photocurrent, two clear peaks in photoconductivity excitation spectra at RT are observed around 0.7 and 1.2 eV. These energies are very close to the energy difference of first and second symmetrical pairs

of VHSs of semiconducting SWCNTs with a diameter of 1.4 nm. In addition, these spectra are very similar to the optical absorption spectra of SWCNTs prepared by the same method [13, 27]. The photoconductive response at 13.2 K is much higher than that at RT, whereas the optical absorption is hardly enhanced even at a low temperature [48]. Moreover, the enhancement strongly depends on the photon energy; the intensities of the peak in photoconductivity excitation spectra at 0.7 and 1.2 eV were enhanced by about two and one orders of magnitude, respectively. The observed photoconductive response monotonically increases with a decrease in temperature between 10 K and RT, and shows the saturation around 10 K.

3.1.3. Mechanism From the correspondence between the optical absorption spectra of semiconducting carbon nanotubes and photoconductivity spectra, it is clear that the photocurrent originates from photoexcitation of electrons in semiconducting SWCNTs. Temperature dependence of photoconductivity  T can be represented by  T = n T × e T = n T e2 /m∗ {l T /v T }, where n T , e,  T , m∗ , l T , and v T are carrier numbers increased by light irradiation, carrier charge, mobility of charge carrier, effective mass of charge carrier, mean-free path, and thermal velocity [47]. Therefore, the temperature dependence of  T should be attributed to that of n T and/or  T ∝ l T /v T . T −3/2 dependence of  T is expected in conventional semiconductors with the regime of the diffusive transport due to electron–phonon interactions, because l T

and v T , respectively, follow T −1 and T 1/2 . On the other hand, by assuming the ballistic conduction, l T is expected to be limited to the nanotube length and to become independent of temperature. In this case,  T is expected to follow T −1/2 . Therefore,  T is expected to follow T −3/2 or T −1/2 for the interband transition, because n T hardly depends on temperature. In this way, photoconductivity increases with a decrease in temperature for the interband transition, although temperature dependence of photoconductivity changes owing to the type of transport—ballistic or diffusive. If exciton absorption is dominant in the semiconducting SWCNTs, as pointed out by the theoretical and experimental approach on optical absorption [13, 27, 31], free carriers contributing to the photoconductivity n T are created through thermal dissociation of excitons. In this case, n T will decrease with a decrease in temperature, and then,  T will decrease, which is contrary to the experiment result. Experimental results naively support the theory that the photocarriers originate from the usual interband transition. Very recently, photoconductivity was observed in SWCNTs with the diameter of 0.4 nm [18, 19] as described in the next subsection. Since these samples have a stronger one-dimensional structure, binding energy of exciton is expected to be much larger than our samples. It is expected that the comparison of the photoconductive properties between these samples gives valuable information to solve this contradiction.

572 3.2. Single-Wall Carbon Nanotubes in Zeolite Single Crystals Another example of observation of photoconductivity is shown in this section [18, 19]. Novel features of this type of sample are the smallest diameter and the almost perfect alignment of SWCNTs in the zeolite single crystals. For these reasons, this sample is very suitable for estimation of the effect of the one dimensionality and the optical anisotropy.

3.2.1. Experimental Technique The samples of SWCNTs in one-dimensional channels of zeolite AFI single crystals were synthesized by thermal treatment of paralyzed carbon encapsulated in the AFI crystal at 500–800  C [6, 49]. Single-wall carbon nonotubes are grown in one-dimensional channels along the c-axis of AFI. Observations by TEM revealed that after dissolving the AFI framework residual materials are SWCNTs and raftlike graphite; the diameter of the SWCNTs is determined to be about 0
42 ± 0
2 nm. Three possible structures, n m = 5 0  3 3  4 2 , are proposed for this type of nanotube. Typical dimensions of an AFI single crystal containing SWCNTs are 75–160 m in a cross-section diameter of a hexagonal face and ca. 300 m in length along the c-axis of AFI. The gold wires of 100–150 mm in diameter were attached to both hexagonal faces of the AFI single crystal. Bias voltage was applied between these two wires by a DC regulated power supply up to 1.5 V. Current flows along the c-axis of AFI, namely, the CNT axis. Linearly polarized light from a CW-Ar+ -ion laser or a CW-Ti/sapphire laser pumped by an Ar+ -ion laser was focused onto the central part of one surface of the AFI crystal. The direction of incident light is perpendicular to the CNT axis. The size of the illuminated spot was set about 100 m in diameter. The resistance of samples in the dark is around 100 M at RT.

3.2.2. Observed Behaviors Conductance increases during the photo irradiation with the period of about 10 s. The response time is less than 50 ms. Photocurrent is proportional to the incident light intensity at low intensities. Although neither photon energy dependence (photoconductivity excitation spectra) nor temperature dependence have been investigated in detail, the information about optical anisotropy have been presented in this sample. By using this sample, it is confirmed that three absorption bands characteristic of CNTs are observed when a polarization vector is parallel to the c-axis of the AFI crystal (E c), that is, the CNT axis, whereas optical absorption has hardly been detected for the case that the polarization vector is perpendicular to the nanotube axis (E ⊥ c) [19, 50, 51], which is consistent with the theoretical prediction as described in Section 2–3 [26]. Corresponding to this, photoconductive response strongly depends on the angle between the polarization vector and the CNT axis; photocurrent for E c is about twice as large as that for E ⊥ c. Intrinsic resistance of the SWCNT sample in the dark increases to more than twice the values by irradiating an intense light of about 10 mW. During the process of an increase in the resistance, the increases in photoconductive

Photoconductivity of Carbon Nanotubes

response and in optical anisotropy are also observed. This result suggests that the intense light irradiation results not in the collapse of CNTs but in the increase in an effective semiconductor SWCNT, namely, the rearrangement of the nanotube structure within zeolite—for example, the connection of a divided semiconductor nanotubes and the structure conversion from the metallic SWCNTs to semiconducting ones.

4. RELATED PHENOMENA In this section, two related phenomena, photo-induced current modulation, are presented.

4.1. Conductance Modulation Due to the Molecular Photodesorption Conductance of an individual semiconducting SWCNT decreases by 10% upon ultraviolet (UV) illumination in air, NO2 , and NH3 atmosphere, contrary to the photoconductive response [52]. The conductance recovers after the light is turned off. This reaction occurs in reverse by repeating irradiation of light. This is caused by photodesorption of gases which acts as an electron donor or accepter. Through photodesorption of molecules, SWCNTs becomes intrinsic semiconductors without any carrier doping and its conductance decreases. Therefore, when UV illumination is performed in a high vacuum, the conductance decreases drastically by a few orders of magnitude, and exhibit no appreciable recovery when the light was switched off.

4.2. Photo-Induced Tunneling Current in STM Measurements It is expected that the local photocurrent can be measured by the scanning tunneling microscopy (STM) method [53]. A photo-induced tunneling current is observed for semiconducting and metallic SWCNTs, only when the photon energy exceeds energy difference of the first symmetrical pairs of VHS in the density of state. It is observed at both a positive and negative bias voltage of STM measurements, and increases linearly to the light intensity and is reversible. Although this behavior is preferred to observations of photoconductive response and electronic structure modulation through light irradiation in nanometer scale spatial resolution, it is necessary to clarify the extrinsic effects, such as the effect of thermal expansion of the CNT sample and the tip of STM to tunnel current, which is extremely sensitive to the distance between the sample and the tip.

5. SUMMARY In this chapter, we have reviewed current states of photoconductivity of carbon nanotubes. Photoconductive properties for the bundles of SWCNTs with a diameter of 1.4 nm and the SWCNTs with a diameter of 0.4 nm in Zeolite single crystals are presented. Two types of photo-induced current modulation are also presented. The mechanism of photoconductivity is still unclear because of its short history. However, from the viewpoint both of nanoscale science and practical application for nano-scale devices, the

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understanding of this phenomenon is very important. It is expected that the origin of photocarriers, the conduction characteristic of a semiconductor nanotube—ballistic or diffusive will be clarified from the detailed experiments on the temperature dependence of photoconductivity. In addition to this, the effect of the one dimensionality and the contribution of the exciton for the carrier generation will be clarified by comparing the photoconductive properties between these two types of SWCNTs with different diameters. This field is now ongoing and will develop further.

GLOSSARY Carbon nanotube (CNT) A tubular molecule made of carbon with nanometer diameter. Exciton A mobile, electrically neutral, excited condition of holes and electrons in a crystal. One example is a weakly bound electron-hole pair. Multiwall carbon nanotube (MWCNT) A type of carbon nanotube with the structure that a concentric tubular structure can be made of two or more nanotubes with different diameters. Optical parametric oscillator (OPO) A laser-pumped crystal with nonlinear optical properties that generates coherent light whose output can be tuned continuously over wide range of wavelengths. Photoconductivity An electrical conductivity increase exhibited by some nonmetallic materials, resulting from the free carriers generated when photon energy is absorbed in electronic transitions. Photocurrent A current produced by photoelectric or photovoltaic effects. Scanning electron microscope (SEM) A type of electron microscope that uses a beam of electrons to scan the sample surface, ejecting secondary electrons that form the picture of the sample. Scanning tunneling microscope (STM) A high-solution microscope that can detect and measure the positions and heights of individual atoms on the surface of the sample. Single-wall carbon nanotube (SWCNT) A type of carbon nanotube with the structure that a single graphite (graphene) sheet rolled into a cylindrical shape. Transmission electronic microscope (TEM) A type of electron microscope that uses magnetic lenses to transmit a beam of electrons through the sample; the electrons are then focused on a fluorescent screen to form an enlarged image or a diffraction image. van Hove singularity (VHS) A singularity observed in energy spectrum of density of states for electrons and phonons, showing divergences of the slope.

ACKNOWLEDGMENTS The authors wish to thank Dr. H. Suematsu at Japan Synchrotron Radiation Research Institute and Professor K. Miyano, Professor N. Nagasawa, and Mr. N. Ogawa at the University of Tokyo for the continuous discussions and/or collaboration on this problem.

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