photoconductivity of carbon nanotube,fujiwara a

7/28/2019 Photoconductivity of Carbon Nanotube,Fujiwara a.

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Encyclopedia ofNanoscience andNanotechnology

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 Iijimain 1991 [1], they have attracted great attention as potentialelectronic materials because of the one-dimensional tubu-lar network structure on a nanometer scale. The varietyof band structures of the CNTs, being either semiconduct-

ing or metallic, depending on the chirality and diameterof the CNT, is also a novel feature. For this reason, it isexpected that CNTs become model samples for an ideal one-dimensional metal and an ideal one-dimensional semicon-ductor. In addition, CNTs are also expected to be one ofthe greatest candidates for nanotechnology materials, suchas nanometer scale wirings and nanometer scale devices[24].

A CNT can be described as a single graphite (graphene)sheet rolled into a cylindrical shape. A concentric tubu-lar structure can be made of two or more nanotubes withdifferent diameters. The former and the latter are calledsingle-wall carbon nanotubes (SWCNTs) and multiwall car-bon nanotubes (MWCNTs), respectively. The thinnest CNTs

are found in a most inner CNT of MWCNTs and SWCNTsgrown in zeolite AlPO4-5 (AFI) single crystals; the diam-eter is about 04 nm [5, 6]. The diameter of thick CNTsis at most a few 10 nanometers. In most cases, CNTs withlarge diameters are MWCNTs; in the case of SWCNTs,a cross-section will not be able to maintain circular struc-ture but it will be distorted. Moreover, in MWCNTs withlarge diameters, structure strongly depends on productionconditions and can be not only concentric, but also the

structure in which a graphene is scrolled upa polyhedralgraphite tube with defects at the vertex and mixtures of them[710].

For the theoretical approach and the interpretation ofexperimental results, SWCNTs are suited, because interac-

tion between layers and the effect of defects must be takeninto consideration in the case of MWCNTs. Most of theexperimental observation had been on MWCNTs in thebeginning of research on CNTs. After the establishment ofa synthesis method for high-quality SWCNTs [1113], exten-sive research on SWCNTs, as well as MWCNTs, is per-formed. Since researches field of photoconductivity of CNTsreviewed in this article have a short history, only the researchfor two kinds of SWCNT samples are reported [1419].Photoconductivity has not been observed in MWCNT sam-ples. Therefore, if there is no notice, there will be a discus-sion about the SWCNT in this article.

This chapter is organized as follows. The physical prop-erties needed for discussion about the photoconductivity of

CNTs are presented in Section 2. In Section 3, the pho-toconductive properties observed in two kinds of SWCNTsare shown. Here, experimental methods are also described,because introduction of this is important because it has beensucceeded only with a few groups in spite of many trials.As related phenomena, two types of photo-induced currentmodulations are presented in Section 4. Section 5 summa-rizes the chapter.

2. PHYSICAL PROPERTIESOF CARBON NANOTUBES

A number of excellent books and review articles have sum-

marized the physical and chemical properties of CNTs [24,20, 21]. In this section, properties related to photoconduc-tivity are briefly described.

2.1. Molecular Structure

A SWCNT can be made by rolling a graphene sheet into acylindrical shape [24, 2023]. Although the tubular struc-ture with any diameter and direction can be made when

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570 Photoconductivity of Carbon Nanotubes

we make it from papers without a pattern on the surface,the network structure of carbon has to be take into accountfor CNTs. In a graphene sheet, carbon atoms form a two-dimensional network of a six-membered ring, namely, ahexagonal (or honeycomb) lattice by connecting their threesp2 hybrid orbitals. There are two sites, A- and B-site, inthe graphene: one carbon is bonded to three carbons of

another site. When graphene is rolled into cylindrical shapeand one carbon is put on any other carbon in the samesite, a CNT can be formed in principle. Relation betweenthese two overlapped carbon atoms on the graphene can bedescribed by chiral vector Ch = na1 +ma2, where n, m areintegers and a1a2 are the unit vectors of the graphene. Thedirection of the chiral vector Ch is perpendicular to the CNTaxis and its length, L Ch = an

2 +m2 + nm1/2 is equalto that of circumference of the CNT, where a is the lengthofa1 and a2. In actuality, because a sp

2 hybrid orbital cannotbe maintained for small diameters and the tubular structureis distorted for large diameters, the diameter of SWCNTs isconsidered to be about 0.42.0 nm.

2.2. Electronic Structure

It is intelligible when the electronic structure of a SWCNT,as well as molecular structure, is considered on the basisof a graphene sheet [2025]. It is necessary to consider theperiodic boundary conditions along the circumference ofthe CNT to the electronic structure of the graphene. Sincethe graphene sheet has a hexagonal lattice, the first Brillouinzone becomes a right hexagon. In energy-dispersion rela-tions, the (conduction) band and the (valence) bandare degenerate only at the six corners of the hexagonal firstBrillouin zone, K and K points, which the Fermi energypasses. Since the density of states at the Fermi energy iszero, the graphene is a zero-gap semiconductor. Although

all the points in the first Brillouin zone are allowed for thegraphene sheet, in the case of CNTs, allowed points areonly on the line prolonged in the direction of the CNT axisthrough the point (the center of the first Brillouin zone)and the parallel lines that separated 2k/L (k: integer) fromthis 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) pointsand the allowed line through the point is 2 (2n+m/3L.As a result, when 2n+m, that is, nm, is the multiple of 3,CNTs become metallic. In other cases, CNTs become semi-conducting. In the energy dependence of density of statesin CNTs, van Hove singularity (VHS) appears to be reflect-ing one-dimensional nature. Moreover, because (conduc-

tion) and (valence) bands are almost symmetrical, thedensity of state near the Fermi energy shows the symmetri-cal energy dependence in respect to the Fermi energy.

2.3. Optical Absorption

Theoretical prediction shows the optical transition takesplace only between symmetrical bands, under the configura-tion that the polarization vector E is parallel to the nanotubeaxis by taking into account the depolarization effect [26].Corresponding to high transition probability between thesymmetric VHSs, three characteristic absorption bands in

an optical absorption spectrum are observed in the energyrange from an infrared region to a visible region [13, 27].Two absorption bands in the lower energy and one at thehighest energy originate from semiconducting and metallicSWCNTs, respectively.

It is well known that the exciton binding energy becomesinfinite in the limit of an ideal one-dimensional electron-

hole system [2830]. Therefore, the effect of exciton plays animportant role in optical absorption for the one-dimensionalsystem of SWCNTs. This effect is considered to mainlymodify the lowest band of the optical absorption spectrum,which was predicted theoretically [31]. Experimental resultsof optical absorption are consistent with this prediction[13, 27].

2.4. Electronic Transport Properties

Although there are many findings of novel properties andfunctions, such as single electron transport [32, 33], spintransport [34], rectification [35, 36], switching function [37],

tunable electronic structure by magnetic fields [38, 39], sin-gle molecule CNT transistors [4042], and superconductiv-ity [43, 44] in electron transport properties, we focus onthe electron scatteringballistic or diffusive. The ballis-tic conduction in CNTs even at room temperature (RT)was pointed out by observation of quantized conduction inMWCNTs at first [45]. Ballistic conduction of SWCNTs wasproposed by the detailed analysis of the coulomb blockadebehavior of SWCNTs which act as a quantum dot [32]. Fromthe subsequent research for semiconducting SWCNTs, themean-free path is estimated to be about 100 nm which isabout one-tenth of the CNT length, and the result suggestsdiffusive conduction in semiconducting SWCNTs in spite ofballistic conduction in metallic SWCNTs [46]. This is con-

sidered to be due to the electron scattering at defects andthe bending parts of CNTs.

3. PHOTOCONDUCTIVITYOF CARBON NANOTUBES

The research on photoconductivity for two kinds ofSWCNTs has been reported. One is observed in theSWCNTs with a diameter of about 1.4 nm, being closed-packed into bundles and forming a two-dimensional tri-angular lattice [1417]. Another is in the SWCNTs, witha diameter of about 0.4 nm grown in zeolite AlPO4-5(AFI) single crystals, which are one type of the thinnestSWCNTs [18, 19]. In MWCNTs, photoconductivity has notbeen observed thus far.

3.1. Single-Wall Carbon Nanotube Bundles

Photoconductivity of CNTs has been discovered in the filmsample of SWCNT bundles, and the most detailed researchhas been performed for this sample [1417]. In this sec-tion, detailed experimental methods for the observation,photoconductive properties, and possible mechanisms arepresented.


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3.1.1. Experimental Technique

The samples of SWCNT bundles were synthesized by evap-oration of composite rods of nickel (Ni), yttrium (Y), andgraphite in helium atmosphere by arc discharge [12, 13] orablating a graphite target containing Ni and cobalt (Co) cat-alysts at 1250 C in an argon atmosphere by using a pulsedNd:YAG laser [11]. Observations by transmission electron

microscopy (TEM) revealed that soot is mainly composed ofSWCNTs and also amorphous carbons and metal particles.The diameter of the SWCNTs is determined to be about14 02 nm by the Raman frequency of a breathing modeand TEM observation. The typical length of SWCNT bun-dles estimated by scanning electron microscopy (SEM) is afew micrometers.

To prepare film samples, soot-containing SWCNTs weredispersed in methyl alcohol by an ultrasonic vibrator andsuspension of SWCNTs was dropped on a glass substrate.The typical film sample size is about 100 m 100 m andthe thickness of the film is between 300 and 500 nm. Thesamples were annealed in vacuum at 106 Torr and 673 Kfor 2 hours to remove the absorbed gases and methyl alco-

hol from samples. A pair of gold electrodes separated by a10-m gap was evaporated in vacuum onto the surface ofthe film samples and connected to a DC regulated powersupply (100 mV). In order to reduce the number of junc-tions between SWCNTs in the current pass, the narrow gapof 10 m was chosen because the resistance of the junctionsdominates the total resistance of the sample and obscuresthe intrinsic transport properties of SWCNTs.

The samples were mounted in a continuous-flow cryostatand cooled by flowing the vapor of liquid He and liquid N 2in the temperature range from 10 K to RT and from 100 Kto RT, respectively. As a light source, an optical paramet-ric oscillator (OPO), excited by a pulsed Nd:YAG laser, wasused. 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 afew tens nJ/pulse to 1500 nJ/pulse. The temporal profiles ofthe laser pulse and the photocurrent were monitored witha digitizing oscilloscope. In order to avoid spurious ringingin the fast pulse detection, we were obliged to use the inputimpedance of the oscilloscope (50 ) as the reference resis-tor, despite the obvious disadvantage of lower sensitivity.The resistance of samples in the dark is ca. 100 at RT and800 at around 10 K.

3.1.2. Observed Behaviors

The temporal evolution of photocurrent shows a Gaussian-like peak with a 5 ns width corresponding to the pulse

duration of the laser. Photocurrent increases with increas-ing incident light intensity. When photocurrent is less than10 A, the photocurrent intensity responds linearly to inci-dent light intensity. On the other hand, it shows a satura-tion behavior above 10 A; this saturation is often observedunder intense light intensity and might be due to the lackof replenishment of carriers [47]. In photoconductivity exci-tation spectra estimated from the slope of the linear part inlight intensity dependence of photocurrent, two clear peaksin photoconductivity excitation spectra at RT are observedaround 0.7 and 1.2 eV. These energies are very close tothe energy difference of first and second symmetrical pairs

of VHSs of semiconducting SWCNTs with a diameter of1.4 nm. In addition, these spectra are very similar to theoptical absorption spectra of SWCNTs prepared by the samemethod [13, 27].

The photoconductive response at 13.2 K is much higherthan that at RT, whereas the optical absorption is hardlyenhanced even at a low temperature [48]. Moreover, the

enhancement strongly depends on the photon energy; theintensities of the peak in photoconductivity excitation spec-tra at 0.7 and 1.2 eV were enhanced by about two and oneorders of magnitude, respectively. The observed photocon-ductive response monotonically increases with a decrease intemperature between 10 K and RT, and shows the saturationaround 10 K.

3.1.3. Mechanism

From the correspondence between the optical absorp-tion spectra of semiconducting carbon nanotubes and pho-toconductivity spectra, it is clear that the photocurrentoriginates from photoexcitation of electrons in semiconduct-

ing SWCNTs. Temperature dependence of photoconductiv-ity T can be represented by T= nTeT =nT e2/m {lT/vT}, where nT , e, T , m,lT, and vT are carrier numbers increased by light irra-diation, carrier charge, mobility of charge carrier, effec-tive mass of charge carrier, mean-free path, and thermalvelocity [47]. Therefore, the temperature dependence ofT should be attributed to that ofnT and/or T lT/vT. T3/2 dependence of T is expected in con-ventional semiconductors with the regime of the diffusivetransport due to electronphonon interactions, because lT and vT, respectively, follow T1 and T1/2. On the otherhand, by assuming the ballistic conduction, lT is expectedto be limited to the nanotube length and to become inde-pendent of temperature. In this case, T is expected tofollow T1/2. Therefore, T is expected to follow T3/2

or T1/2 for the interband transition, because nT hardlydepends on temperature. In this way, photoconductivityincreases with a decrease in temperature for the interbandtransition, although temperature dependence of photocon-ductivity changes owing to the type of transportballistic ordiffusive.

If exciton absorption is dominant in the semiconduct-ing SWCNTs, as pointed out by the theoretical and exper-imental approach on optical absorption [13, 27, 31], freecarriers contributing to the photoconductivity nT arecreated through thermal dissociation of excitons. In this

case, nT will decrease with a decrease in tempera-ture, and then, T will decrease, which is contrary tothe experiment result. Experimental results naively supportthe theory that the photocarriers originate from the usualinterband transition. Very recently, photoconductivity wasobserved in SWCNTs with the diameter of 0.4 nm [18, 19] asdescribed in the next subsection. Since these samples have astronger one-dimensional structure, binding energy of exci-ton is expected to be much larger than our samples. It isexpected that the comparison of the photoconductive prop-erties between these samples gives valuable information tosolve this contradiction.


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3.2. Single-Wall Carbon Nanotubesin Zeolite Single Crystals

Another example of observation of photoconductivity isshown in this section [18, 19]. Novel features of this typeof sample are the smallest diameter and the almost perfectalignment of SWCNTs in the zeolite single crystals. For thesereasons, this sample is very suitable for estimation of theeffect of the one dimensionality and the optical anisotropy.

3.2.1. Experimental Technique

The samples of SWCNTs in one-dimensional channels ofzeolite AFI single crystals were synthesized by thermal treat-ment of paralyzed carbon encapsulated in the AFI crys-tal at 500800 C [6, 49]. Single-wall carbon nonotubesare grown in one-dimensional channels along the c-axis ofAFI. Observations by TEM revealed that after dissolving theAFI framework residual materials are SWCNTs and raft-like graphite; the diameter of the SWCNTs is determined tobe about 042 02 nm. Three possible structures, nm=5 0 3 3 4 2, are proposed for this type of nanotube.

Typical dimensions of an AFI single crystal containing SWC-NTs are 75160 m in a cross-section diameter of a hexag-onal face and ca. 300 m in length along the c-axis of AFI.

The gold wires of 100150 mm in diameter were attachedto both hexagonal faces of the AFI single crystal. Bias volt-age was applied between these two wires by a DC regulatedpower supply up to 1.5 V. Current flows along the c-axis ofAFI, namely, the CNT axis. Linearly polarized light froma CW-Ar+-ion laser or a CW-Ti/sapphire laser pumped byan Ar+-ion laser was focused onto the central part of onesurface of the AFI crystal. The direction of incident light isperpendicular to the CNT axis. The size of the illuminatedspot was set about 100 m in diameter. The resistance ofsamples in the dark is around 100 M at RT.

3.2.2. Observed Behaviors

Conductance increases during the photo irradiation withthe period of about 10 s. The response time is less than50 ms. Photocurrent is proportional to the incident lightintensity at low intensities. Although neither photon energydependence (photoconductivity excitation spectra) nor tem-perature dependence have been investigated in detail, theinformation about optical anisotropy have been presentedin this sample. By using this sample, it is confirmed thatthree absorption bands characteristic of CNTs are observedwhen a polarization vector is parallel to the c-axis of theAFI crystal (E c), that is, the CNT axis, whereas opti-

cal absorption has hardly been detected for the case thatthe polarization vector is perpendicular to the nanotube axis(E c) [19, 50, 51], which is consistent with the theoreticalprediction as described in Section 23 [26]. Correspondingto this, photoconductive response strongly depends on theangle between the polarization vector and the CNT axis;photocurrent for E c is about twice as large as that forE c.

Intrinsic resistance of the SWCNT sample in the darkincreases to more than twice the values by irradiating anintense light of about 10 mW. During the process of anincrease in the resistance, the increases in photoconductive

response and in optical anisotropy are also observed. Thisresult suggests that the intense light irradiation results not inthe collapse of CNTs but in the increase in an effective semi-conductor SWCNT, namely, the rearrangement of the nano-tube structure within zeolitefor example, the connectionof a divided semiconductor nanotubes and the structure con-version from the metallic SWCNTs to semiconducting ones.

4. RELATED PHENOMENA

In this section, two related phenomena, photo-induced cur-rent modulation, are presented.

4.1. Conductance Modulation Dueto the Molecular Photodesorption

Conductance of an individual semiconducting SWCNTdecreases by 10% upon ultraviolet (UV) illumination in air,NO2, and NH3 atmosphere, contrary to the photoconductiveresponse [52]. The conductance recovers after the light isturned off. This reaction occurs in reverse by repeating irra-diation of light. This is caused by photodesorption of gaseswhich acts as an electron donor or accepter. Through pho-todesorption of molecules, SWCNTs becomes intrinsic semi-conductors without any carrier doping and its conductancedecreases. Therefore, when UV illumination is performedin a high vacuum, the conductance decreases drastically by afew orders of magnitude, and exhibit no appreciable recov-ery when the light was switched off.

4.2. Photo-Induced Tunneling Currentin 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 semi-conducting and metallic SWCNTs, only when the photonenergy exceeds energy difference of the first symmetricalpairs of VHS in the density of state. It is observed at botha 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 pho-toconductive response and electronic structure modulationthrough light irradiation in nanometer scale spatial resolu-tion, it is necessary to clarify the extrinsic effects, such asthe effect of thermal expansion of the CNT sample and thetip of STM to tunnel current, which is extremely sensitive tothe distance between the sample and the tip.

5. SUMMARY

In this chapter, we have reviewed current states of photo-conductivity of carbon nanotubes. Photoconductive proper-ties for the bundles of SWCNTs with a diameter of 1.4 nmand the SWCNTs with a diameter of 0.4 nm in Zeolitesingle crystals are presented. Two types of photo-inducedcurrent modulation are also presented. The mechanism ofphotoconductivity is still unclear because of its short his-tory. However, from the viewpoint both of nanoscale sci-ence and practical application for nano-scale devices, the


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Photoconductivity of Carbon Nanotubes 573

understanding of this phenomenon is very important. It isexpected that the origin of photocarriers, the conductioncharacteristic of a semiconductor nanotubeballistic or dif-fusive will be clarified from the detailed experiments on thetemperature dependence of photoconductivity. In additionto this, the effect of the one dimensionality and the contribu-tion of the exciton for the carrier generation will be clarified

by comparing the photoconductive properties between thesetwo types of SWCNTs with different diameters. This field isnow ongoing and will develop further.

GLOSSARY

Carbon nanotube (CNT) A tubular molecule made of car-bon with nanometer diameter.

Exciton A mobile, electrically neutral, excited condition ofholes and electrons in a crystal. One example is a weaklybound electron-hole pair.

Multiwall carbon nanotube (MWCNT) A type of carbonnanotube with the structure that a concentric tubular struc-

ture can be made of two or more nanotubes with differentdiameters.

Optical parametric oscillator (OPO) A laser-pumped crys-tal with nonlinear optical properties that generates coher-ent light whose output can be tuned continuously over widerange of wavelengths.

Photoconductivity An electrical conductivity increaseexhibited by some nonmetallic materials, resulting from thefree carriers generated when photon energy is absorbed inelectronic transitions.

Photocurrent A current produced by photoelectric or pho-tovoltaic effects.

Scanning electron microscope (SEM) A type of electronmicroscope that uses a beam of electrons to scan the samplesurface, ejecting secondary electrons that form the pictureof the sample.

Scanning tunneling microscope (STM) A high-solutionmicroscope that can detect and measure the positions andheights of individual atoms on the surface of the sample.

Single-wall carbon nanotube (SWCNT) A type of car-bon 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 transmita beam of electrons through the sample; the electrons arethen focused on a fluorescent screen to form an enlargedimage or a diffraction image.

van Hove singularity (VHS) A singularity observed inenergy spectrum of density of states for electrons andphonons, showing divergences of the slope.

ACKNOWLEDGMENTS

The authors wish to thank Dr. H. Suematsu at JapanSynchrotron Radiation Research Institute and ProfessorK. Miyano, Professor N. Nagasawa, and Mr. N. Ogawa at theUniversity of Tokyo for the continuous discussions and/orcollaboration on this problem.

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