Electronic and Photonic Properties of Doped Carbon Nanotubes

CONTENTS

1 Introduction 4592 Alkali-MetalHalogen Intercalation 4603 Substitutional Doping with Nonmetals 4664 Transition Metal Doping 4695 Encapsulating with Fullerenes Clusters and Others 4706 Gas Adsorption and Molecule Functionalization 4727 Summary and Outlook 474Acknowledgments 475References and Notes 475

1 INTRODUCTION

Since the discovery of carbon nanotubes by Iijima1 2 therehave been extensive studies on both single-walled carbonnanotubes (SWNTs) and multiwalled carbon nanotubes(MWNTs) because of their unique geometric structures andremarkable mechanical chemical electronic magnetic andtransport properties Their small diameter (on the scale ofnanometers) and the long length (on the order of microme-ters) lead to such large aspect ratios that carbon nanotubescan act as ideal one-dimensional (1D) systems In the massproduction of carbon nanotubes it was found that SWNTsform tube bundles (nanoropes) with close-packed two-dimensional (2D) triangular lattices3 which might offer ahost lattice for intercalation and storage All of these charac-

Electronic and Photonic Properties of DopedCarbon Nanotubes

Jijun Zhaoa and Rui-Hua Xieb dagger

aDepartment of Physics and Astronomy University of North Carolina at Chapel Hill Chapel HillNorth Carolina 27599-3255 USA

bDepartment of Chemistry Queenrsquos University Kingston Ontario K7L 3N6 Canada

The idea of doping carbon nanotubes is attractive since it provides various possibilities for control-ling the physical properties of carbon nanotubes In this review we have summarized recentprogress on the experimental and theoretical studies of carbon nanotubes doped with nonmetalsalkali metals transition metals and clusters The doping effects on the electronic magnetic trans-port and optical properties of carbon nanotubes are reviewed The related applications of carbonnanotubes in nanoelectronics battery eld emission spintronics nonlinear optics and chemicalsensors are discussed

Keywords Doped Carbon Nanotubes Electronic Properties Photonic Properties Magnetic Properties Applications

JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY

teristics make carbon nanotubes the focus of extensive stud-ies in nanoscience and nanotechnology with many potentialapplications The fundamentals of and recent progress in thesynthesis physical and chemical properties and technologi-cal applications of carbon nanotubes can be found in recentbooks4ndash11 and review articles12ndash16

Designing nanotube-based nanoscale materials and de-vices requires the control of physical properties of carbonnanotubes In particular it is desirable to engineer the elec-tronic band structures of the nanotubes The basic idea oftailoring nanotube electronic properties can be classi edinto two directions utilizing the dependence of electronicproperties on the intrinsic tube topology (eg chiralitydiameter and curvature structural defects) or modifyingthe extrinsic variable of carbon nanotubes via substitu-tional doping intercalation etc The latter method referredto as doping carbon nanotube is practically more control-lable and exible

In practice carbon nanotubes can be doped in differentways including intercalation of electron donors like alkali-metal or acceptors like halogen substitutional dopingencapsulating in the interior space coating on the surfacemolecular adsorption covalent sidewall functionalizationetc Figure 1 illustrates the typical approaches of dopingcarbon nanotubes Experimentally doped carbon nanotubeshave been characterized by various techniques such asX-ray diffraction UVIR spectroscopy Raman spectro-scopy nuclear magnetic resonance (NMR) electron spinresonance electron paramagnetic resonance etc17 Plentyof interesting phenomena have been found and some ofthem may lead to technological applications For example

459J Nanosci Nanotech 2003 Vol 3 No 6 copy 2003 by American Scienti c Publishers 1533-4880200303459020$1700+25 doi101166jnn2003241

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Author to whom correspondence should be addressed Present addressInstitute for Shock Physics Washington State University Pullman WA99164 zhaojjmailapsorg

daggerMailing address NIST Mail Stop 8423 100 Bureau DriveGaithersburg MD 20899 rhxienistgov

the electrochemical intercalation of Li in nanotube bundlescan be used as anodes of lithium batteries because of thehigh Li storage capacity The electronic and transportproperties of SWNTs can be engineered by doping eitherelectron donors or electron acceptors which is essentialfor constructing nanoelectronic devices Hybrid structurescreated by lling nanotubes with transition metals exhibitlarge spin polarization and can be used as 1D devices inspintronics The eld emission of carbon nanotubes can beenhanced by substitutional N-doping or alkali-metal inter-calation Larger third-order optical nonlinearities are foundfor N- or B-doped carbon nanotubes constituting good can-didates for optical limiting applications Novel 1D nano-structures can be obtained by encapsulating carbon nano-

tubes with fullerenes clusters and atomic chains The con-ductance of carbon nanotubes is sensitive to gas adsorptionimplying potential applications as chemical sensors

All of these exciting achievements encouraged us towrite this review of the recent progress in understandingthe electronic and photonic properties of doped carbonnanotubes This article is organized as follows In Sections2ndash6 we discuss the physical properties and related appli-cations of carbon nanotubes doped by different methodssuch as alkali-metalhalogen intercalation substitutionaldoping transition metal lling or coating encapsulatingwith fullerenes clusters and atomic chain and moleculeadsorption or functionalization We end this article with abrief summary and outlook in Section 7 The synthesis andcharacterizations of carbon nanotubes are not includedhere but can be found in recent reviews by Duclaux18 andourselves17

2 ALKALI-METALHALOGENINTERCALATION

21 Electronic and Optical Properties

Raman spectroscopy is one of the most ef cient tools forinvestigating the vibration properties of materials in rela-tion to their structural and electronic properties19 In par-ticular the shift of Raman frequency in doped nanotubescan be the evidence of charge transfer between carbonnanotubes and dopants The pioneering work on Ramanspectra of SWNT bundles intercalated by electron donor(K Rb) and electron acceptor (Br2 I2) was done by Raoet al20 21 As shown in Figure 2 the high-frequency tan-gential vibration mode at 1593 cm21 in pristine nanotubesshifts substantially to a lower (for K Rb) or higher (Br2)frequency region implying a charge transfer between thedopants and the nanotubes Little change was found for I2

doping probably because I2 does not intercalate into thenanotube bundles

Zhao and XieProperties of Doped Carbon Nanotubes J Nanosci Nanotech 2003 3 459ndash478

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Dr Jijun Zhao was born in 1973 and educated in Jiangsu China He received his bachelorrsquos degree in physics from NanjingUniversity China in 1992 and his PhD in condensed matter physics from Nanjing University in 1996 From 1997 to 1998 hewas a postdoctoral fellow at the International Centre for Theoretical Physics (ICTP) Italy From 1998 to 2002 he was a researchassociate and then a research assistant professor in the Department of Physics and Astronomy University of North Carolina atChapel Hill NC He is currently a senior research associate at the Institute for Shock Physics Washington State University Hismajor research eld is in computational materials science with a special interest in nanostructures (nanotubes nanowires etc)nanoelectronic devices atomic clusters and cluster-based materials high-pressure physics and molecular crystals He has con-tributed over 70 refereed journal papers and two book chapters in this eld

Dr (John) Rui-Hua Xie is a guest researcher at the National Institute of Standards and Technology (NIST) Gaithersburg MarylandHis research interest and expertise include computational nanoscience (nanoelectronics nanostructured materials nanocrystals andquantum dots) cluster physics molecular physics quantum computing quantum optics quantum theory quantum chemistry andelectronic spectroscopy such as NMR UV-vis and Raman Dr Xie received his bachelorrsquos degree in physics from WuhanUniversity Hubei China in 1991 and his PhD in physics from Nanjing University Jiangsu China in 1996 From 1997 to 1998he was a postdoctoral fellow at the University of Toronto Canada During the period from 1998 to 2000 he moved to Germany asan Alexander von Humboldt fellow working at the Max-Planck-Institut fuumlr Stromungsforschung in the beautiful university town ofGoumlttingen He went to the rst capital of Canada Kingston working at Queenrsquos University before he joined the Quantum ProcessGroup at NIST in 2001 He has contributed over 80 peer-reviewed journal articles and several review chapters in books and ency-

clopedias

Fig 1 Illustration of different ways of doping carbon nanotube (a) Liatom (red balls) intercalation of (1010) nanotube bundle with slice ofelectron charge density (b) Substitutional doping by nitrogen (blue ball)in a (100) tube (c) p-n junction formed by incorporation of C48N12

(right) and C48B12 (left) clusters inside a semiconducting (170) tube(d) Iron atoms lling a (90) tube at a ratio of FeC6 (e) Adsorption of a(100) tube by an organic DDQ molecule with a slice of charge densityfor electron bands crossing the Fermi level (red yellow green blue col-ors on the slice indicate density from higher to lower) (f) Covalent side-wall functionalization of a (66) tube by a COOH group

Successive Raman studies on alkali-metal (Li K Rb Cs)intercalated SWNTs have been done by several groups22ndash27

In in situ Raman studies of K- and Li-doped SWNTswith different concentrations Claye et al demonstrated areversible charge transfer between the dopants and the hostSWNTs which yields a softening in the tangential mode22

Bendiab et al studied the Raman spectra of SWNTs dopedwith Li23 Rb24 25 and Cs24 For Li-doped carbon nano-tube lms with controlled stoichiometries doping-inducedupshift of the tangential mode was evidence for LixC com-pounds (0 x 017) (Ref 23) In a combined in situconductivity and Raman measurement of Rb-dopedSWNTs two different Raman signatures with peaks at1596 and 1555 cm21 were explained by the coexistence oftwo stable doped phases24 Bendiab et al also studied thelow-frequency Raman modes in the Cs- and Rb-dopedSWNTs at saturation concentration25 and observed two low-frequency Raman modes involving both radial motions oftubes and alkali-atom vibrations25 For K-doped SWNTsIwasa et al found a rather stable intermediate phase KC27

before saturation doping of about KC9 (Ref 26) Since theRaman active E2gmode shows an anomalous hardening for

the KC27 phase they postulated that the K ions are insertedbetween two tubes rather than the interstitial site surroundedby three tubes in the nanotube bundle26 Very recently Yeet al intercalated Li atoms into small 04-nm-diameterSWNTs and studied the charge transfer behavior by reso-nant Raman spectra27 With increasing doping concentra-tion the radial breathing mode shifts to higher frequencywhich is attributed to the enhanced stiffness caused by inser-tion of Li atoms

Meanwhile the SWNTs doped by halogen elements(Br I) have also been studied with Raman spectroscopy28ndash30

Kataura et al measured low-frequency resonance Ramanspectra of Br-doped SWNTs using various laser lines to clar-ify the electronic states of the doped SWNTs28 Grigorianet al analyzed the Raman scattering data for I-doped SWNTsamples29 Because of resonant Raman scattering fromcharged (I5)

2 and (I3)2 linear chain complexes in moder-ately doped samples new peaks including a strong one at175 cm21 and a weaker one at 109 cm21 were observed inthe low-frequency region The main effect of polyiodidechain intercalation on the high-frequency Raman tripletobserved in the pristine sample is an up-shift of thesemodes by about 8 cm21 due to the transfer of ordm electronsfrom carbon nanotube to iodine chains More recentlyVenkateswaran et al measured the Raman spectra of iodine-doped SWNT bundles with an elevated pressure up to 7 GPa(Ref 30) In pristine SWNT samples the low-frequencyradial modes show a pressure-dependent shift at about7 cm21GPa whereas that in iodine-doped SWNT exhibits avery small pressure-induced frequency shift A comparisonbetween the pressure dependence of I-doped and pristineSWNTs suggests that polyiodide chains (ie I n

21 mole-cules) (Ref 29) might reside both in the interstitial channelsand inside the nanotube pores in the SWNT bundles

A Raman study by Maurin et al31 on Li-doped MWNTsshows that lithium species are only trapped at the surfaceof nanotubes probably within the cavities generated bystructural defects or entanglements31 Zhou et al32 havecharacterized iodine-doped MWNTs by means of Ramanscattering Similar to the case of SWNTs MWNTs can beeffectively doped by iodine with charge transfer Iodineatoms form charged polyiodide chains inside tubes of differ-ent inner diameters but cannot intercalate into the graphenewalls of MWNTs

Optical absorption can detect changes in some speci celectronic states of a material Petit et al rst showed thepossibility of tuning the Fermi level of SWNTs by expo-sure to molecules of different redox potentials through theuse of optical absorption spectroscopy33 Later the opticaladsorption spectra of SWNTs doped with halogens and al-kali metals were studied by Kazaoui and co-workers33ndash36

In all of these studies three sets of optical bands at about07 eV 12 eV and 18 eV were observed for the pristinenanotube33ndash36 (see Fig 3) The rst two adsorption bands(07 eV 12 eV) originate from the band-gap transition in

J Nanosci Nanotech 2003 3 459ndash478 Zhao and XieProperties of Doped Carbon Nanotubes

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0

0

0

0

Ram

an I

nten

sity

(ar

b u

nits

)

188

1587

I2

1617

260 Br2

Pristine

Rb

K

186

376 134715261550

15671593

766 9491145

1263

1557(1567)

1554(1564)

12621142

wave numbers500 1000 1500 2000

766 949

Fig 2 Raman scattering spectra for pristine SWNT bundles reactedwith various donor and acceptor reagents From top to bottom I2 Br2pristine SWNT Rb K The asterisks () indicate the positions of peaksassociated with halogen reactant The peak frequencies indicated inparentheses for the K Rb-doped SWNTs are the renormalized phononfrequencies20 Reprinted with permission from Ref 21 A M Rao et alThin Solid Films 331 141 (1998) copy1998 Elsevier Science

semiconducting SWNTs and the third one at 18 eV can beassociated with metallic SWNTs

As shown in Figure 3 doping-induced change on theadsorption spectra of SWNTs depends on the density x ofalkali-metals (K Cs) and halogens (I2 Br2) (Refs 34 and35) At the initial stage of doping (x 0001) only the fea-ture at 07 eV decreases while the others remain Sub-sequent doping up to x 004 causes disappearance of thelow-energy peaks and reduction of the intensity at 12-eVtransition energy At heavy doping level (x 004) all threestructures disappear whereas two new bands at 107 eVand 13 eV transition energies are found for Br015C andCsC respectively The sequent disappearance of the threeadsorption bands for pristine SWNTs was attributed to elec-tron depletion or lling in speci c bands of semiconductingor metallic SWNT34 35 In a consequent experiment by thesame group36 it was established that the semiconductingSWNT can be doped amphoterically The new absorptionpeaks induced by heavy doping were explained by the low-lying valence states in the optical transition

Pichler et al studied alkali-metal intercalated SWNTs byelectron energy-loss spectroscopy (EELS)37 38 The lossfunction at low momentum transfer can be simulated withthe use of a Drude-Lorentz model indicating that all theSWNTs become metallic after intercalation37 As comparedwith the graphite intercalated compound the effective massof charge carrier in the intercalated nanotube is 35 timesgreater37 More recent work by the same group shows thatthere is no hybridization between nanotube ordm states andmetal valence states The states above the tube Fermi levelremain unperturbed by the intercalant38 Based on theirmeasurements of the doping dependence on the opticalexcitation it is possible to tune the Fermi level into conduc-

tion bands upon different electron donor intercalations Theenergy of charge carrier plasmon increases at higher inter-calation level and with the radius of the alkali-metals38

Analysis of NMR spectra of carbon nanotubes providesknowledge of their electronic structures in particular thedensity of states at the Fermi level39 Duclaux et al inves-tigated the modi cations of electronic properties as thepristine MWNT was doped with alkali metals with the useof high-resolution 12C NMR40 The chemical shift of the12C NMR signals for the rst stage of MWNT (KC82 orCsC8) are of the same order as those obtained for graphite-intercalated compounds with an inversion of anisotropyattributed to the dipolar interaction of the 12C nucleus withthe ordm electrons Intercalations of K Rb and Cs in SWNTbundles with stoichiometry near MC8 show similarresults41 In a recent NMR study of Li-intercalated SWNTsthe density of states at the Fermi level increases from0022 states(eV-spin-atom) in the pristine metallic SWNTsto 0031 states(eV-spin-atom) in Li-intercalated puri edSWNTs (LiC57) and to 0043 states(eV-spin-atom) inetched SWNTs with higher Li intercalation density (LiC32)(Ref 42)

So far there have been several theoretical works on theelectronic structures of alkali-metal-doped carbon nano-tubes and bundles43ndash48 The electronic structure of individ-ual K-doped small zigzag SWNTs was rst calculated byMiyamoto et al43 A rigid-band picture of K-doped nano-tubes was proposed in which the Fermi level is shifted intothe conduction band simply because of the charge transferfrom K to nanotube However recent calculations forK-doped zigzag SWNTs found that the effect of K-dopingis not simple charge transfer and the doping effect is sensi-tive to the tube size46 The nearly free electron state of nano-tube hybrids with the 4s orbital of K In the larger (100)and (120) tubes it comes downward and crosses the Fermilevel This state is distributed inside the tube and extendsto the tube direction implying enhanced conductivity

Zhao et al studied the electronic structures of SWNTbundles intercalated with Li44 and other alkali metals (KRb Cs)45 After intercalation complete charge transferfrom Li to nanotube and small structural deformations ofthe nanotube were found44 (see Fig 1a) Band structure cal-culations show that the hybridization between lithium andcarbon transforms the semiconducting nanotube bundlesinto metallic and introduces some new states into nanotubeconduction bands Similarly the electronic states of con-duction bands are signi cantly modi ed by K (also RbCs) intercalation as shown in Figure 4 The most impor-tant nding is that the density of states at the Fermi levelfor metallic and semiconducting nanotube bundles are allsubstantially enhanced and become indistinguishable afterintercalation which is evidenced by NMR experiments45

K-doped SWNT bundles have also been investigated byJo Kim and Lee47 They found an expansion of a 2D lat-tice of up to 8 at K01C with negative binding energies


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CBr x = 0CBr x = 0005CBr x = 0035CBr x = 0040CBr x = 0149

CCs x = 0CCs x = 0004CCs x = 0013CCs x = 0056CCs x = 0070

p-type doping

Abs

orpt

ion

Inte

nsity

[arb

uni

ts]

Energy [eV]

05 10 15 25

n-type dopingstrongly doped SWNTwith Br2 and Cs

107eV130eV

x = 0CBr x = 015CCs x = 015

05 10 15 20

Fig 3 Absorption spectra of doped SWNT for the case of n-type (upperset) and p-type (lower set) doping The doping stoichiometry CDx(C carbon D dopant (ie Cs Br) x their ratio) is given in the legendand the asterisks indicate absorption due to quartz substrate and addi-tional experimental artifacts At high doping concentrations new featuresarise as shown in the inset Reprinted with permission from Ref 35R Jacquemin et al Synth Met 115 283 (2000) copy2000 ElsevierScience

where distortion of tube walls is negligible up to K025Cwithin full relaxation The shift of Fermi level and theamount of charge transfer increase with increasing dopingconcentration and saturate at large concentration47 ForMWNTs Choi et al48 investigated modi cation of theelectronic structure of the aligned MWNTs due to sodiumdoping The change in band structures is largely associatedwith shifts of the Fermi level The changes in the apparentdensity of states in the vicinity of the Fermi level suggesteffects associated with the electron correlation energy

22 Transport Properties and Nanoelectronic Devices

It is well known that SWNTs can be semiconducting ormetallic depending on their chirality Thus carbon nano-tubes are considered as 1D conducting wires for the inves-tigation of mecroscopic transport phenomena and the con-struction of nanoelectronic devices As discussed abovedoping with alkali metal (prototypical electron donor) andhalogen (prototypical electron acceptor) of carbon nano-tubes can directly tune the electronic structures of the pris-tine nanotubes so that their transport properties can be con-trolled In Table I we summarize the measured resistivityof pristine and doped carbon nanotubes from previousworks The nanotube resistivity is usually reduced afterdoping in different ways

Lee et al49 were the rst to study the doping effect on thetransport properties of bulk samples of SWNTs intercalatedwith bromine and potassium They found that doping de-creases the resistivity by a factor of 30 (see Table I) andenlarges the region where the temperature coef cient ofresistance is positive as characteristics of metallic behaviorThese results suggest that doped SWNTs represent a newfamily of synthetic metals Later Ruzicka et al50 reported dctransport and optical conductivity of puri ed and potassium-doped SWNT lms The pristine sample shows a Drude

component in the optical conductivity whereas nonmetallicbehavior is found in dc resistivity measurement because ofthe nonmetallic tube-tube contacts50

Sklovsky et al51 presented in situ four probe dc resis-tance versus pressure of pristine and potassium-dopedSWNT bucky paper up to 90 kbar They found that potas-sium-doped samples show a behavior quite different fromthose of pristine samples (i) by 10 kbar the resistance ofdoped samples drops by 40 (ii) from 10 to 45 kbar theresistance of K-doped SWNTs decreases gradually withpressure contrary to that of pristine nanotubes (iii) at stillhigher pressure resistance increases slightly by 2 from45 kbar to 90 kbar After K-doped SWNTs are exposed toair the resistance is exactly restored to the behavior ofpristine materials indicating that the potassium vapor dop-ing is actually reversible Further work of Lee et al52 in-dicates that K-doping in the nanotube ropes leads to anoverall decrease in the resistance (see Table I) and suppres-sion of the low-temperature divergence According to thedoping-induced change in Vg characteristics chemical dop-ing is a charge transfer process rather than a change in rope-rope contact properties Leersquos work52 also supports the ideathat SWNT materials are inherently p-type because of inad-vertent tube-level doping by exposure to air

The simultaneous measurements of optical absorptionand dc resistance for controlled stoichiometry in p-type(Br2 I2) or n-type (K Cs) doped SWNTs by Kazaouiet al34 demonstrate a decrease in dc resistance accom-plished by the disappearance of absorption bands of pris-tine SNWTs after doping Bendiab et al measured the timedependence of the resistance of SWNTs during a dopingexperiment25 The pristine samples exhibit global semi-conducting character whereas global metallic behavior isfound for the doped samples after a suf ciently long time

Carbon nanotubes are promising building blocks fornanoelectronic devices particularly eld effect transistors(FETs)53 SWNT FETs built from as-grown tubes areunipolar p-type that is there are no electron current ows


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Fig 4 Electronic density of states of pristine (orange dotted line) andK-doped (blue solid line) (170) SWNT bundles (KC17) The valencebands of nanotube are almost unaffected by K intercalations and theconduction bands are signi cantly modi ed by the potassium-carboninteractions

Table I Resistivities (R in V cm) of graphite4 for different types ofcarbon nanotubes and doped carbon nanotubes34 49 50 102ndash104 at roomtemperature

Materials R (V cm) Ref

Graphite (basal plane) 5 3 10ndash5 4Carbon nanotube lm 2 3 10ndash2 103Carbon nanotube bundle 65 3 10ndash3 102SWNT thin lm 43 10ndash2 to 4 3 10ndash3 34Bulk SWNT sample 16 3 10ndash2 49Bulk Br2-doped SWNT sample 10ndash3 49Bulk K-doped SWNT sample 3 3 10ndash4 49SWNT lm 5 3 10ndash2 50K-doped SWNT lm 5 3 10ndash3 50Individual MWNTs 53 3 10ndash4 to 19 3 10ndash3 104Individual B-doped MWNTs 74 3 10ndash5 to 77 3 10ndash4 104

In general the nanotube resistivity is reduced after doping by different approaches

even at large positive gate biases This behavior suggests thepresence of a Schottky barrier at the metal-nanotube contactObviously the capability to achieve n-type transistors istechnologically important for the fabrication of nanotube-based complementary logic devices and circuits54 55

In current experiments potassium has been used as then-type dopant54ndash56 58ndash60 Bockrath et al56 reported con-trolled chemical doping of individual semiconducting nano-tube ropes by reversible intercalation and deintercalationof potassium It was found that potassium doping changesthe carriers in the nanotube ropes from holes to electronsThe effective mobility of the electrons (about 20 to 60 cm2

V21 s21) is comparable to that reported for the hole effec-tive mobility in nanotubes53 The controlled n-type dopingrealized in these experiments opens a pathway to makingnanoscale p-n junctions57 58 For example Kong et al59

doped a 04-mm-long semiconducting SWNT into n-typewith potassium vapor Their electrical measurements revealsingle-electron charging at temperatures up to 60 KK-doped SWNT manifests as a single quantum dot or mul-tiple quantum dots in series depending on the range ofapplied gate voltage More recently Kong et al60 realizedan intramolecular p-n-p junction consisting of two p-typesections (doping with molecular oxygen adsorbed fromthe ambient) and a central n section (doping of the SWNTcentral part with potassium) The transport measurementsreveal that nanometer-scale-wide tunneling barriers at thep-n junctions dominate the electrical characteristics of thesystem At low temperatures the system behaves as a singleon-tube quantum dot con ned between two p-n junctions

In addition to potassium doping Martel et al61 Deryckeet al54 and Liu et al55 have shown that p-type to n-type con-version of the carbon nanotube FETs can be made by simplyannealing the device in an inert gas61 or in a vacuum54 55

Recently Derycke et al62 have compared the characteristicsof carbon nanotube FETs produced by both methods andfound fundamental differences in the transformation mecha-nism It was found that the main effect of oxygen adsorptionis not to dope the bulk of the carbon nanotube but to modifythe barriers at the metal-semiconductor contacts Their stud-ies indicate that the oxygen concentration and the level ofdoping of the nanotubes are complementary in controllingthe carbon nanotube FET characteristics

Park and McEuen63 used eld-effect doping to studyboth n- and p-type conduction in a semiconducting carbonnanotube They found that in the n-type region the ends ofthe tube remain p-type because of the doping effect by themetal contacts Thus a p-n junction forms near the contactcreating a small p-type quantum dot between the p-n junc-tion and the contact

23 Li Storage and Battery Applications

Carbon is known as the commercial anode material usedfor Li-ion batteries64 In analogy to the Li intercalation in

graphite65 the crystallites of nanotube bundles might offeran all-carbon host lattice for intercalation and be a can-didate for anode materials for a Li ion battery Table IIsummarizes the maximum Li storage capacity of carbonnanotube-based materials prepared by different methods inprevious experiments31 42 66ndash70 In general MWNTs havea much lower capacity than SWNTs and even lower thangraphite in some cases For SWNTs Li storage capacity upto Li16C6 (600 mAhg) was obtained in the puri ednanotube samples68 signi cantly higher than that in com-mercially used graphite materials LiC6 (372 mAhg) Thismaximum capacity can be further improved by about a fac-tor of 2 via chemical etching42 (see Fig 5) or ball-millingof the nanotube69 with a maximum number as high asLi27C6 (1000 mAhg) In both cases the increase in Licapacity has been attributed to the Li intercalation insidethe nanotube interior space since chemical etching andball-milling are supposed to either open the tube end orcreate defects on the tube sidewall both of which shouldallow Li ions to diffuse into the inside of nanotubes Thusit is important to theoretically investigate the intercalation(insertion) energy and diffusion behavior of Li atoms innanotubes and bundles44 71ndash74

Zhao et al44 calculated the Li intercalation energy in theSWNT bundle at different intercalation sites and as a func-tion of Li density Both the interior of the nanotube and theinterstitial space are susceptible to intercalation They havealso shown that the Li intercalation potential of a SWNTbundle is comparable to that of graphite and independentof Li density up to a saturation density of about Li3C6 (Ref44) in agreement with experiment69 The higher Li capac-ity in nanotube bundles can be related to a carbon densitylower than that in graphite Later on Li insertion into thenanotube interior from the tube end or sidewall has been


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Table II Summary of Li storage capacity of carbon nanotube-basedmaterials from experiments31 42 66ndash70

Reversible Li Preparation storage capacity

Sample method (mAhg) LiC ratio Ref

MWNTs Catalytic decomposition of acetylene 300 Li08C6 31


SWNTs Catalytic decomposition of acetylene 700 Li188C6 67

SWNTs Graphite arc-discharge technique 430 Li116C6 70

SWNTs Laser ablation as prepared 450 Li12C6 68

SWNTs Laser ablation puri ed 600 Li16C6 68SWNTs Laser ablation puri ed

and chemical etched 740 Li2C6 42SWNTs Laser ablation puri ed

and ball-milled 1000 Li27C6 69

For comparison the storage capacity for graphite is 372 mAhg corresponding toLiC6

studied by Kar71 Yang72 Meunier73 and Gurau74 It wasfound that Li ions cannot pass through the sidewall of aperfect nanotube because of the high energy barrier Butthe height of the barrier decreases dramatically as the ringsize of the topological defect on the tube sidewall in-creases72ndash74 Thus Li can enter a nanotube through topo-logical defects like a nine-member ring73 or a ten-memberring74 Similarly insertion of Li ions through the cappedzone of a closed nanotube is also energetically unfavorableunless there are structural defects71 72 On the other handLi insertion into a nanotube from the open end is stronglyexothermic even for a small-radius (60) tube especiallythrough the hydrogen-passivated end71 It was also foundthat Li-Li interaction inside a nanotube is repulsive butstrongly screened71

24 Work Function and Field Emission

Carbon nanotubes were considered to be superior electron eld emitters because of their high eld-emission currents

at low turn-on voltage75 76 Typically the current-voltage(IndashV) characteristics of carbon nanotube eld emission fol-low a Fowler-Nordheim80-type tunneling law76ndash79 In theanalysis within the Fowler-Nordheim model80 the workfunction (WF) of the carbon nanotube is one of the criticalparameters for determining the eld emission properties

Many experimental efforts have been devoted to deter-mining the precise value of the work functions of carbonnanotubes81ndash90 In principle the work function can be esti-mated from the eld-emission spectra based on a Fowler-Nordheim plot81 82 But the WF values obtained are notreliable because of the uncertainty of the local tube geom-etry82 Other experimental techniques such as ultravioletphotoemission spectroscopy (UPS)83ndash89 have been used todetermine the work functions of both SWNTs and MWNTsFrom those experiments the work functions of MWNTsare found to be about 01ndash02 eV lower than that ofgraphite83 84 89 whereas the WFs of SWNT bundles (about48 eV) are slightly higher than the graphite WF85ndash87

Upon intercalation of alkali metal like Cs (Refs 86 and87) or K (Ref 88) the WFs of carbon nanotubes decreasedramatically Figure 6 shows a comparison of the photo-emission spectra of graphite with the pristine and Cs-inter-calated SWNT bundles by Suzuki et al87 It can be seenthat the SWNTs have a slightly larger WF (48 eV) thanthat of graphite (46 eV) With increasing concentration ofCs deposition (from (a) to (c) in Fig 6) the threshold energyof photoemission spectra is shifted to the higher bindingenergy side by 17 (a) 24 (b) and 28 eV (c) resulting inWFs of 31 24 and 20 eV respectively87 In a recentexperiment on SWNTs encapsulated by K (Ref 88) adecrease in WF from 47 eV for pristine SWNTs to 33 eV


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3

2

1

0

2

1

0

2

1

010 215 25 305

LixC6

Cel

l vol

tage

(V

olts

)

Closed SWNTsL = over 10 m

Opened SWNTsAve L = 4 m

(a)

(b)

(c)


Voltage (volts)

Voltage (volts)

Voltage (volts)

APX

PA

PX

PA

PXP

2

0

-2

-4

-6-05 05 15 25

2

0

-2

-4

-6-05 05 15 25

2

0

-2

-4

-6-05 05 15 25

Fig 5 Second-cycle intercalation (discharge) and deintercalation(charge) data collected from the as-puri ed SWNTs (a) and etchedSWNTs with an average bundle length of 4 mm (b) and 03ndash05 mm (c)The data were collected with a two-electrode cell with Li foil and SWNT lm as the two electrodes A 1 M solution of LiClO4 in a 11 volume ratioof ethylene carbonate and dimethyl carbonate was used as the electrolyteLithium intercalation and de-intercalation were carried out with the gal-vanostatic mode at 50 mAg current between 0 and 3 V Reprinted withpermission from Ref 42 H Shimoda et al Physica B 323 133 (2002)copy2002 Elsevier Science

Graphitepristine SWNTsCs-intercalated SWNTs

(c) (b) (a)

20 18 16 14

Binding Energy (eV)

Inte

nsity

(ar

b u

nits

)

Fig 6 Photoemission spectra around the secondary electron thresholdregions of graphite and the pristine and Cs-intercalated SWNT bundles(a) (b) and (c) correspond to different Cs concentrations from lowerto higher Reprinted with permission from Ref 87 S Suzuki et alJ Electron Spectrosc Relat Phenom 114 225 (2001) copy2001 ElsevierScience

for K-doped SWNTs was observed88 which agrees wellwith the theoretical prediction of about a 12-eV drop inWF for KC10 (Ref 45)

Theoretical calculations by Zhao et al show that the workfunctions of SWNTs are insensitive to tube size and chiral-ity45 Upon alkali-metal intercalation the WFs of both metal-lic and semiconducting nanotubes decrease dramaticallywith alkali-metal concentration consistent with Suzukirsquosexperiments86ndash88 Based on the Fowler-Nordheim model80

the reduction of work function upon alkali-metal intercala-tion implies a signi cant enhancement in eld emissionIndeed in the eld emission experiments by Wadhawanet al91 they observed that Cs deposition on SWNT bundlesdecreases the turn-on eld for eld emission by a factor of21 to 28 and increases the eld-emission current by sixorders of magnitude

3 SUBSTITUTIONAL DOPINGWITH NONMETALS

31 Electronic Structures

In analogy to the doping of semiconducting materials sub-stituting carbon atoms with electron donors like nitrogenor acceptors like boron is a possible way of doping carbonnanotubes In a pioneering theoretical work Yi and Bern-holc studied substitutional doping of small semiconductingSWNTs by N and B atoms92 They found that the impuritylevel induced by N is located 027 eV below the bottomof the conduction bands whereas the B-induced level is016 eV above the top of the valence bands

The electronic properties of carbon nanotubes substitu-tionally doped with boron were studied experimentally byCarroll et al93 94 The spatial homogeneity of electronicproperties as characterized by the local density of states(LDOS) of pristine and B-doped MWNTs were investi-gated by scanning tunneling microscopy (STM) and spec-troscopy (STS)94 The undoped carbon nanotubes show asmall band gap (semiconducting or semimetallic behavior)whereas for the B-doped MWNTs the band gap is lledfrom the valence band side with a prominent acceptor-likepeak near the Fermi level The observation of singularpoints in the doped samples is indicative of an exception-ally high structural perfection in the outmost tube cylindersprobed by STS93 Ab initio calculations93 point out that theobserved metallization and strong acceptor states cannot beexplained by isolated B substitutional atoms in the graphitenetwork but can be considered as resulting from nano-domains of BC3 within the metallic nanotube lattice

Carroll et al94 observed distinct variations of the LDOSfor different positions on the body of doped MWNT sam-ples The nonuniformity of the electronic structures is mostlikely related to an inhomogeneous spatial distribution ofthe dopants Closure of carbon nanotubes at the tube endleads to a topology different from that of the sidewall This

effect will result in variations in electronic states at thetube ends re ected by the measured LDOS Figure 7 com-pares the variation of electronic structure at the tube endwith that of the sidewall for both pristine and B-doped car-bon nanotubes94 Because of the existence of pentagonaldefects in the hexagonal lattice the width of the apparentband gap in the pristine case is reduced by the appearanceof states from the valence band-gap edge For the B-dopedcase the prominent peak close to the Fermi level is shiftedto lower energies or even disappears at the tube ends

Electron microscopy and electron diffraction patternsobtained by Blase et al95 have shown that B-doping con-siderably increases the length of carbon tubes and resultsin a remarkable preferred zigzag chirality First-principlessimulations indicate that B atoms in the zigzag geometryact as a surfactant during the growth process and preventthe tube closure whereas this mechanism does not extendto armchair tubes suggesting a doping-induced helicityselection during growth

For a complete analogy to bulk semiconductor dopingtechnology it is necessary to introduce donor states (n-type)to the nanotube similar to the acceptor states (p-type)discussed above Recently Czerw et al96 97 synthesizedN-doped carbon nanotubes by pyrolysis It was found that


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pure carbon tube body

pure carbon tube tip

-05 0 05

Sample Bias (V)

LD

OS

(ar

b u

nits

)

B-doped tube body

B-doped tube tip

-05 0 05

Sample Bias (V)

LD

OS

(arb

uni

ts)

Fig 7 A comparison of the electronic properties approaching the clo-sure structures (tips) in both pure and B-doped nanotubes The top graphcompares tip and body LDOS for a pure carbon tube The bottom graphshows a similar comparison for a B-doped tube In both cases the tubediameter is approximately 10 nm Reprinted with permission from Ref94 D L Carroll et al Carbon 36 753 (1998) copy1998 Elsevier Science

N impurities on the nanotube lattice result in modi cationsof conduction bands including a n-type electron donorstate located approximately 02 eV from the Fermi level96

The local environment of the N impurities within a carbonnetwork mainly consists of N-C structures arranged ina pyridine-like con guration96 Their tight-binding andab initio calculations show that pyridine-like structures areresponsible for the metallic behavior and the prominentfeatures near the Fermi level Moreover Czerw et al96

noted that connections between N- and B-doped carbonnanotubes induce a barrier of about 05 eV

In recent theoretical works the substitution of nonmetalatoms other than boron and nitrogen in carbon nanotubeshas been explored by ab initio methods98ndash100 Zhanget al98 found that oxygen-substitutional doping does notobviously change the binding energy and localized reso-nant states in the substitutional location which wouldaffect the electronic transport and eld-emission proper-ties of nanotubes Mann and Halls99 showed that the bar-rier for inserting an oxygen atom through the center of ahexagonal ring of carbon nanotubes is 137 eV Underthermal conditions the oxygen atom binds to the nanotubewithout a barrier leading to one of two products an epox-ide or an adatom oxygenated nanotube They also foundthat tube curvature effects could lead to an increase of theepoxide binding energy with decreasing tube diameter99

The electronic properties of Si-substitutional doping incarbon nanotubes was investigated by Baierle et al100

Local structural distortion as outward displacement of theSi atom with respect to the tube sidewall was observedThe Si impurity induces a resonant state appearing about07 eV above the Fermi level in the metallic nanotubewhereas doping silicon in the semiconducting tube intro-duces an empty level of about 06 eV above the top of thevalence band These results indicate that the Si substitu-tional impurity will be highly reactive serving as a bindingcenter to other atoms or molecules In their succcessivework chemical functionalization of atoms (F Cl H) andmolecules (CH3 SiH3) at the impurity sites in the Si-dopedSWNTs have been studied101

32 Transport Properties

Wei et al104 investigated the resistivity of individual pureand B-doped MWNTs in the temperature range from 298 to573 K A decrease in the resistivity with increasing temper-ature that is a semiconductor-like behavior was observedfor both B-doped and pure carbon nanotubes As listed inTable I the room-temperature resistivity of B-doped nano-tubes is much lower than that of pure nanotubes and iscomparable to that along the basal plane of graphite More-over the activation energy derived from the resistivity-temperature Arrhenius plots for B-doped MWNTs (55 to70 meV) is smaller than that for the pure nanotubes (190ndash290 meV) Later Liu et al105 examined the transport prop-

erties of B-doped MWNTs They found that the substitu-tional B dopants lower the Fermi level of carbon nanotubesand increase the number of conduction channels withoutintroducing strong carrier scattering In the temperaturerange from 50 K to 300 K the B-doped nanotubes showmetallic behavior with weak electron-phonon couplingand the resistance increases at lower temperature RecentlyHsu and Nakajima106 studied the conductivity of B-dopedMWNT bundles The IndashV relationship is characteristicallylinear at room temperature The contact resistance betweenthe bundle and Au electrodes (about 1ndash2 kV) is signi -cantly lower than G0 5 2e2h 5 129 kV (Ref 108) indi-cating that the contact resistance does not hinder the pas-sage of electrons through B-doped MWNT bundles Theratio of resistancelength for B-doped MWNT bundles isabout 122 kVmm considerably smaller than the ratiofor pure MWNT bundles (10ndash30 kVmm) (Ref 107) Theactivation energy (ie band gap) derived from Arrheniusplots of conductance versus temperature is about 0098 eVto 016 eV smaller than that found for carbon nanotubes(01 eV to 1 eV)109 and comparable to Weirsquos previousresults104

The transport properties of SWNTs with substitutionalimpurity have been studied theoretically110ndash112 For metal-lic SWNTs Choi et al found that a substitutional impurity(boron or nitrogen) induces quasi-bound states of de niteparity and reduces the conductance by one quantum (2e2h)due to resonant backscattering110 Recently Kaun et al111

reported ab initio analysis of IndashV characteristics of carbonnanotubes with nitrogen substitutional doping For zigzagsemiconducting tubes a single nitrogen impurity can in-crease current ow and reduce the current gap for smalltubes Hence they predicted that doping a N impurity pernanotube unit cell would lead to metallic transport behav-ior For armchair metallic tubes they found reduction ofcurrent with substitutional doping because of elastic back-scattering caused by the N impurity which is similar toChoirsquos result110 Furthermore Rochefort and Avouris112

investigated the effects of impurity scattering on the con-ductance of metallic carbon nanotubes as a function of therelative separation of the impurities They found that asingle oxygen impurity reduces the conductance of a (66)nanotube by about 30 Introducing a second oxygenatom leads to oscillations of the conductance versus O-Odistance with a periodicity of half a Fermi wavelength Thestrong electron interference effect is caused by the electronscattering from the oxygen defects

33 Field Emission

Using rst principles methods Zhang et al investigatedthe effect of a substitutional boron or nitrogen atom in thenanotube tip on the eld-emission properties of cappedSWNTs113 They found that the substitutional impurity inthe nanotube tip can signi cantly enhance the LDOS at the


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Fermi level and reduce the tip work function in the case ofnitrogen substitution Accordingly in the low-voltage eldemission nanotubes with substitutional B or N impuritycould provide a much larger emission current than pristinenanotubes The highest occupied molecular orbital (HOMO)lowest unoccupied molecular orbital (LUMO) gap for B- orN-doped carbon nanotubes is also smaller than that of anundoped tube implying that substituting B or N in the tipenhances the local reactivity and thus makes eld emissioneasier However in a later experiment Poa et al114 obtainedlow threshold elds of 16 Vmm and 26 Vmm for eldemission from pristine and boron-doped MWNTs embeddedin polystyrene respectively The higher threshold eld foundfor B-doped MWNTs indicates that the threshold eld forelectron emission is determined by geometry enhancementof the lm surface not by lm resistivity

Wang et al measured eld emission from well-alignedCNx (x up to 9) nanotubes115 and found that the tubesstart emitting electrons at an electric eld of 15 Vmm andthat current densities of 80 mAcm2 are realized at anapplied eld as low as 26 Vmm Doping carbon nanotubeswith N atoms enhances their electron-conducting proper-ties because of the presence of additional lone pairs ofelectrons that act as donors with respect to the delocalizedordm system of the hexagonal framework Hence their worksuggests that the controllable synthesis of well-aligned CNx

nanotubes with high N concentration may open a route toimproving the eld emission properties of nanotubes

34 Third-Order Optical Nonlinearity

Photonic applications such as data processing eyesensorprotection and all-optical switching116 require that thebuilding blocks (for example molecules clusters quantumdots nanocrystals) have large second hyperpolarizabilities reg(also called a third-order optical nonlinear coef cient)However the reg magnitudes of most candidates are usuallysmaller than those needed for photonic devices Thus it isimportant to search the materials with large second hyperpo-larizabilities Previously large nonlinear optical (NLO)responses were achieved in conjugated ordm-electron organicsystems or quantum dots116 117 Recently Xie et al118ndash121

and Jensen et al122 have theoretically shown that carbon nan-otubes are potentially important in photonics owing to their

large reg values The enhancement of the third-order opticalnonlinear coef cients of carbon nanotubes predicted by Xieet al118ndash121 have been con rmed by recent experiment123

Xie has proposed a substitutional doping approach toachieving the large third-order optical nonlinearities of car-bon nanotubes which signi cantly enhances the reg value ofnanotubes by about one order of magnitude with respect toC60 (Ref 124) (see Table III) Because of the distortion ofordm electron distribution in the substituted tubes especiallyaround the dopant atoms the difference between the z andx (or y) components of reg for doped carbon nanotubes ismuch more pronounced than that for the parent ldquopure car-bon nanotubesrdquo124 The study of the dynamic NLOresponses of pure carbon nanotubes118ndash122 indicates thatthe relatively large NLO responses for carbon nanotubes aremainly caused by delocalized ordm electronics as in the con-jugated polymer chains The 3D character of nanotubesleads to severe limitations on their nonlinear optical prop-erties and makes their reg values smaller than those of linearpolymers containing the same number of carbon atomsHowever as shown by Xie118 124 125 the substitutionaldopants (eg B and N atoms) could attract or repel elec-trons and thus introduce a local perturbation of the ordm elec-tron distribution around the dopants leading to the so-called inductive effect On the other hand the dopantions118 124 125 would result in a stronger localization of theoriginal delocalized ordm electrons around them and there-fore may reduce the effective space dimensions of nan-otubes namely the reduction effect Both inductive andreduction effects would make the NLO properties of dopedcarbon nanotubes superior to those of a pure carbon nan-otube In addition it should be mentioned that the localiza-tion effect of the N dopant is stronger than that of the Bimplying a stronger enhancement in N-doped carbon nan-otubes Thus it would be interesting to study the third-order optical nonlinearities of carbon nanotubes with heav-ily N-substitutional doping in future experiments

35 Optical Limiting Property

The laser is a very popular source in the laboratory andindustry However there is the possibility of damage frompulsed lasers or temporary blinding by continuous-wavelasers126 of the thermal camera CCD other optical sen-


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Table III The ratio q 5 gimpuritygpurity of several doped armchair nanotubes C591k310X and doped zigzag nanotubesC591k318X (X 5 B N)

Armchair nanotube Zigzag nanotube

X k 5 0 k 5 1 k 5 2 k 5 9 k 5 18 k 5 1 k 5 2 k 5 5 k 5 10

N 305 307 324 368 412 311 332 360 404B 39 43 49 76 85 47 51 69 77

gimpurityis the calculated static g value of the doped nanotube and gpurityis the static g value of the corresponding pure nanotube and is givenby an empirical formula120for armchair and zigzag nanotubes Adapted from Ref 18

sors and our own eyes Hence it is necessary to developoptical limiters and tunable lters127 for suppressing unde-sired radiation and effectively decrease transmittance athigh intensity or uence126 To design ideal optical lim-iters several critical factors have to be considered (i) rea-sonable linear transmittance at low input uence protectsoptical sensors or eyes against laser pulses of any wave-length and pulse duration (ii) its output energy mustremain at high uences below the optical damage thresh-old of sensors or eyes (iii) the optical damage thresholdmust be as high as possible and the optical activatingthreshold as low as possible Certainly to meet these crite-ria all existing NLO materials need a tightly focused beamto initiate the effect126 For this an adapted optical systemmust be incorporated into the optical limiter Until nowseveral nonlinear effects such as nonlinear absorption128

nonlinear refraction129 and nonlinear scattering130ndash133

have been proved to lead to optical limiting behaviorRecently carbon nanotubes have been experimentally

shown to be good candidates for optical limiting applica-tions134ndash144 For example carbon nanotubes exhibit non-linear scattering134 136 similar to that of carbon black sus-pensions (see recent review by Vivien et al126 for furtherdetails) All of these studies show that NLO transmissionsin carbon nanotubes strongly rely on the width and wave-length of the light pulse as well as the host media The cre-ation of ldquomicrobubblesrdquo in the surrounding solvent due tothe local heating from the dissipation of induced currentshas been argued to be the most reasonable mechanism foroptical limiting of carbon nanotubes since the large aspectratios of carbon nanotubes allow them to behave as effec-tive antennae

An instructive approach that is tailoring the local elec-tronic properties of carbon nanotubes has been proposedto be a good way to understand mechanisms involved inthe limiting behavior of suspensions As discussed abovethe substitutional doping of carbon nanotubes is responsi-ble for stimulating a number of structural and electronicproperties and thus the third-order optical nonlinearities ofcarbon nanotubes118 124 125 Recently Xu et al145 146

have measured the optical limiting properties of B- andorN-doped carbon nanotubes By varying the incident energyand measuring the transmitted energy they observedenhanced optical limiting behaviors of B- or N-doped car-bon nanotubes In comparison with the nonlinear transmit-tance versus incident uence of pure and B- or N-dopedcarbon nanotubes at 532 nm and 1064 nm doped carbonnanotubes are found to have better optical limiting proper-ties (lower threshold values) than pure nanotubes Theirresults on the pure and B-doped carbon nanotubes underidentical input uence (05 Jcm2) indicate that the trans-mittance drops by about 60 and 33 for B-doped andpure carbon nanotubes respectively The optical nonlin-earity within the B-doped sample is stronger than that inthe undoped one Fe catalyst particles were also found in

N-doped carbon nanotubes but were shown to make nocontribution to optical limiting behavior145 146

In another study by Jin et al147 it was found that the opti-cal limiting behavior of carbon nanotubes in poly(vinyli-dene uoride) (PVDF) dimethylformamide (DMF) solutionis size-dependent The tubes of large aspect ratio possessstronger limiting properties However the limiting isobtained by nanotube bundles not by individual tubesUsing electron microscopy Xu et al145 146 have found thatfor each type of doped carbon nanotube the bundles exceed100 mm which is signi cantly longer than the wavelengthof incident light In spite of these studies a full mechanismfor the enhanced optical limiting performance of B- or N-doped carbon nanotubes is still unclear

4 TRANSITION METAL DOPING

Yuan et al150 investigated the electrical transport proper-ties of pure and Au-doped individual MWNTs IndashV mea-surements show that the Au-doped MWNT has a minimumresistance of 20 kV and a maximum of 200 kV which issmaller than those of the undoped carbon nanotubes by afactor of 5 It was also found that the resistance of Au-doped carbon nanotubes decreases with increasing temper-ature conforming to semiconducting behavior Grigorianet al151 studied the Raman spectra and electrical transportproperties of SWNTs doped with transition-metal impuri-ties (eg Cr Mn Co Fe Ni) They found that Raman-scattering spectra for transition-metal-doped SWNTs aresimilar the SWNT radial mode exhibits an unresolveddoublet with peaks at 165 cm21 and 178 cm21 The trans-port properties of SWNTs are strongly in uenced by thepresence of transition-metal impurities which are derivedfrom the catalyst for stimulating nanotube growth Theobserved unusual transport behavior is attributed to theKondo effect that is interaction between the magneticmoment of the transition-metal atom and the spin of con-duction ordm electrons of the nanotube

Carbon nanotubes with high stability and a large aspectratio can be considered ideal templates for fabricating 1Dmetalnanotube hybrid nanostructures Zhang et al148 foundthat titanium atoms can be deposited on the SWNT surfaceand form continuous wires whereas other metals such asgold palladium iron aluminum and lead can only formnoncontinuous and amorphous wires outside the tube wallTo understand the interaction between transition-metaladatoms and carbon nanotubes Yang studied the bindingenergies and electronic structures of metal (Ti Al Au) chainsadsorbed to SWNTs149 The binding energy of a Ti chain onSWNTs is about 20 eVatom signi cantly larger than thoseobtained for Al (052 eVatom) and Au (025 eVatom) indi-cating that titanium is strongly favored energetically overgold and aluminum to form a continuous chain or wire on thecarbon nanotube The coupling between titanium and carbon


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nanotube signi cantly modi es the electronic structuresaround the Fermi level of nanotubes In particular the delo-calized 3d electrons from the titanium generate additionalstates in the band-gap regions of the semiconducting tubestransforming them into metallic materials149

Experimentally it has also proved possible to producedtransition-metal-encapsulated carbon nanotube by lling thetube interior space with a variety of transition metals152ndash158

Thus the magnetic properties of these novel 1D hybridsystems are interesting152 153 158 Lafdi et al152 measuredthe magnetic properties of the pristine and Co nanoparti-cle-doped nanotubes at 10 K with a SQUID magnetometerRana et al153 have investigated magnetic properties ofMWNTs doped by Co particles at 300 K with a vibratingsample magnetometer with an applied eld Z H Z 16 T Inboth studies the undoped carbon nanotubes exhibit dia-magnetic behavior whereas the Co-doped nanotubesexhibit superparamagnetic behavior In particular the coer-cive eld of Co-doped nanotubes increases by a factor ofmore than 5 compared with those of pure Co powder or amixture of Co powder and carbon nanotubes153

Recently the magnetism of 1D hybrid nanostructuresconstituted by SWNTs with Co and other magnetic transitionmetals (Fe Co) inside or outside has been theoretically stud-ied by Yang et al159 It was predicted that such transition-metalnanotube hybrid structures exhibit substantial mag-netic moments that are comparable to the bulk value fortransition metals Figure 8 shows the spin-polarized elec-tron density of state for an Fe- lled (90) tube and itsatomic structure can be found in Figure 1d The large spinpolarization up to about 80 at the Fermi level implies thepossibility of developing 1D devices for spin-polarizedtransport in the emerging eld of spintronics160 with theuse of transition-metal lledcoated carbon nanotubes

Based on an individual MWNT contacted by ferromag-netic electrodes on the two sides spin transport throughcarbon nanotubes was experimentally achieved by Tsuka-goshi et al161 A hysteretic magnetoresistance ratio ranging

from 2 to 10 was reported The presence of the magne-toresistance is attributed to the misalignment of the mag-netic moments of the two electrodes (the spin-valve effect)They used a 9 magnetoresistance ratio and a spin polar-ization of Co at 34 to derive the approximately 14 spinpolarization of the electrons traveling the entire length ofthe nanotube (250 nm) without ipping their spin The spin-scattering length for the nanotube was estimated to be atleast 130 nm Without the spin relaxation the magneto-resistance ratio would have reached a level as high as 21

5 ENCAPSULATING WITH FULLERENESCLUSTERS AND OTHERS

The interior hollow space of a carbon nanotube providesa 1D container for encapsulating a variety of materialsSome materials used to ll nanotube interact only weaklywith the nanotube sidewall and might keep their originalatomic structures One example is peapods made by inser-tion of C60 (Refs 162ndash165) or C70 (Refs 166 and 167)fullerenes as well as endohedral metallofullerenes such asGdC82 (Refs 168 and 169) DyC82 (Ref 171) ScC82

(Ref 170) inside SWNTs Such novel forms of carbon-based materials might lead to new possibilities for electro-chemistry and functionalization of carbon materials

The electronic and transport properties of peapods haveattracted particular attention because of the couplingbetween nanotube and fullerenes The electric resistanceshave been measured for various kinds of peapods Forinstance Pichler et al172 reported doping-induced poly-merization of C60 inside SWNTs with resistivity measure-ments as a probe They found that the resistivity of nano-tube changes from semiconducting to metallic afterdoping For full intercalation a chemical reaction insidethe nanotubes is observed which leads to a one-dimen-sional polymeric C60

26 chain with a metallic character In arecent study of electrical and thermal properties of C60- lled peapods by Vavro et al173 the measured electricalresistivity thermopower and thermal conductivity suggestthat the long C60 chain inside nanotubes provides an addi-tional conductive channel for charge carriers increasesphonon scattering and prevents other gas molecules fromentering nanotube interior sites

For peapods encapsulated with metallofullerenes Chiuet al investigated the electrical transport properties of indi-vidual semiconducting nanotubes doped with DyC82

(Ref 171) It was shown that the DyC82 molecules act aselectron donors and transfer charge to the nanotube andthe amount of charge transfer depends on the temperatureDyC82-doped SWNTs show a transition from p-type ton-type semiconductor when they are cooled from room tem-perature down to 265 K Furthermore metallic behavior isfound at T 215 K whereas single-electron charge phe-nomena become dominant at temperatures below 75 K Inthe latter situation the tubes exhibit irregular Coulomb


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Fig 8 Spin-polarized density of states for the Fe- lled (90) SWNT ata ratio of FeC6 (see Figure 1d for its atomic structure) Large spin-polar-ization (89) is found near the Fermi level

0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)

blockade oscillations and can be considered as a series ofquantum dots171 Hirahara et al studied the electronicstructures of (GdC82)nSWNTs from their EELS anddc electric resistance168 Chemical state analysis of Gdatoms based on EELS shows evidence for charge transferfrom Gd to either fullerene or the nanotube The slopes ofthe temperature dependence of resistance for (GdC82)n

SWNTs or (C60)nSWNTs are much steeper than thosefor empty SWNTs implying that the electron scatteringis due to the electrostatic potential from inside the ful-lerenes168 Later the FET behavior of (GdC82)nSWNTsand (C60)nSWNTs was investigated by Shimada et al169

C60 peapods exhibit unipolar p-type behavior whereasGdC82 peapods show ambipolar behavior with both p-and n-type characteristics by tuning gate voltage Hencetunable electronic properties of peapods can be achievedby choosing the different types of encapsulated fullerenemolecules

The electronic structures of peapods have been calcu-lated by Okada and co-workers with the use of rst-principles methods174 175 C60(1010) SWNT peapod wasshown to be metallic174 with two types of carriers onepropagating along the nanotube and the other on the interiorC60 chain In their recent work175 it was found that the elec-tronic states of peapods depend on the space between encap-sulated fullerenes and the outer nanotube and re ect the typeof fullerenes The multicarrier effect in peapods is caused bythe deep energy position of the lowest unoccupied state offullerene as well as hybridization between fullerene ordm statesand the nearly free-electron states of nanotube Rochefortalso studied the electronic and transport properties of metal-lic and semiconducting SWNTs encapsulated with C60 as afunction of tube diameter176 Weak charge transfer ( 01e)from tube sidewall to C60 was found corresponding to aweak orbital mixing between C60 and the nanotube Thecharge transfer and orbital mixing increase slightly as thetube diameter becomes smaller within the exothermic pea-pod limit whereas the change can be dramatic in the case ofendothermic peapods

Recently Xie et al177 demonstrated that C602mBm andC602nNn molecules could be engineered as acceptordonor pairs desired for molecular electronics by properlycontrolling the number m and n of the substitutionaldopants in C60 These acceptordonor pairs can be promis-ing components for making nanotube-based p(n)-typetransistors p-n junctions and so on For example placingan acceptor C48B12 into a (1710) tube induces a 1067echarge on the SWNT and results in a p-type tube-basedtransistor incorporating donor C48N12 into a (170) tubeleads to a 2039e charge on the SWNT and results in an-type tube-based transistor A prototype of p-n junctionusing C48N12 and C48B12 molecules encapsulated in a(170) SWNT is shown in Figure 1c

Similar to carbon fullerenes other magic-numberedclusters with spherical geometry and high stability might

also be incorporated inside the nanotube The insertion ofMet-Car clusters M8C12 (M 5 Sc Ti V) inside SWNThas also been explored theoretically with extended Huumlckeltheory178 They found that the most active states (responsi-ble for the cluster-cluster and cluster-nanotube interactions)are the d states of transition-metal atoms The electronicproperties of the hybrid structures might be effectivelycontrolled by targeted modi cation of the chemical com-position of met-cars Sun et al studied the heteropeapodwith WSi12 clusters encapsulated in SWNT and foundenhanced density of states at the Fermi level via WSi12

doping179 Recently Zhao and Xie investigated the insertionof Na6Pb clusters into SWNTs of different diameters180

Their ab initio results demonstrate that Na6Pb clusters canbe incorporated into carbon nanotubes of diameters $ 1 nmwith an insertion energy up to 277 eV per cluster Forcomparison it was shown that only nanotubes wider than13 nm can accept C60 (Ref 174) and the insertion energyfor a C60 cluster inside (1010) SWNT is 173 eV (Ref175) (see Table IV for a comparison of the insertionenergy for different clusters inside SWNTs) Band struc-ture calculations for Na6Pb(88) SWNT further showthat the hybridization between nanotube and incorporatedclusters increases the number of conduction channels ofthe armchair metallic SWNT from two to three ThusNa6Pb and other stable clusters can be used as nanoscaleblocks for insertion into carbon nanotube which modifythe electronic properties of nanotubes for example byenhancing the tube conductivity

Very recently carbon atom chain was inserted intoMWNTs forming a novel carbon nanowire (CNW)181

HRTEM observations indicate that a CNW consists of aMWNT with a long 1D linear carbon chain inserted into itsinnermost tube about 07 nm in diameter Raman scatter-ing and HRTEM studies show the formation of long linearcarbon chain containing more than 100 atoms inside thenanotube First-principles calculations nd an increase inthe density of states at the Fermi level of SWNTs with theinsertion of carbon chain which implies that the insertionof carbon chain may improve the conductivity of metallicnanotube and even transform a semiconducting nanotubeinto a metallic one181 In a theoretical work by McIntoshet al the energies and electronic properties of SWNTencapsulated with a polyacetylene were studied182 Theweak coupling between polyacetylene and nanotube leads


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Table IV Insertion energy per cluster (in eV) for different kinds ofclusters inside SWNTs175 179 180

C70 C60 WSi12 Na6PbClusters (Ref 175) (Ref 175) (Ref 179) (Ref 180)

Tube chirality (1111) (1010) (99) (88)Tube diameter (nm) 149 136 122 108Insertion energy (eV) 171 173 018 277

Only the nanotubes with the lowest insertion energy are presented

to a peak of DOS at the Fermi level which may raise thesuperconducting transition temperature in this system

Carbon nanotubes have also been encapsulated withmany other materials Here we will not discuss those sys-tems because of limited space For example 1D ionic KClcrystals grown within SWNT have been obtained183 fur-ther details on this direction can be found in a recent re-view by Greenrsquos group184 It is also interesting to note thata novel nanothermometer can be made by lling MWNTswith gallium185

6 GAS ADSORPTION AND MOLECULEFUNCTIONALIZATION

61 Electronic and Transport Properties

Molecule adsorption and covalent sidewall functionaliza-tion on a carbon nanotube constitute another type ofdoping carbon nanotube that is different from all of theapproaches discussed above The molecules can attach tothe carbon nanotube either by weakly van der Waalsndashlikeinteraction or a covalent bond formed between moleculeand nanotubes The tubendashmolecule interaction may havesubstantial in uence on the carbon nanotubes and lead toapplications like chemical sensors

Experimentally the electronic and transport propertiesof carbon nanotubes were found to be sensitive to gasadsorption39 186 187 189 190 For instance Kong et alfound that the electrical resistance of an individual semi-conducting SWNT dramatically decreases (increases)upon NO2 (NH3) gas adsorption186 This effect can be uti-lized as the basis for nanotube molecular sensors whichexhibit fast response and high sensitivity In a parallelstudy Collins et al found that exposure to oxygen gas dra-matically affects the conductivity thermopower and localdensity of states of individual semiconductor SWNTswhereas Ar He and N2 have no noticeable doping effect187

These electronic parameters can be reversibly ldquotunedrdquo by asmall amount of gas concentration whereas oxygen adsorp-tion generally converts semiconducting tubes into apparentconductors187 In addition to the potential sensor applica-tion as proposed by Kong186 their results also indicate thatthe air exposure effect on the measured properties of as-prepared nanotubes should be carefully examined Manysupposedly intrinsic properties measured on tube samplesmight be severely compromised by extrinsic air exposureeffects187 A NMR experiment by Wursquos group has pro-vided further evidence for the increase in density of state atthe Fermi level of SWNTs after exposure to oxygen39 188

whereas most other gases like He H2 and CO2 do not havesuch an effect188 The effects of gas adsorption and colli-sions on the thermopower and resistivity of tangled SWNTbundles have been studied by Sumanasekera et al189 Itwas found that the resistance of metallic nanotube bundles

decreases as gas molecules (most importantly oxygen) areremoved from the sample The transport properties ofSWNT bundles were found to be quite sensitive to eveninert gas because of the gas collisions with the nanotubewall More recently the same group observed the ordm elec-tron coupling between aromatic molecules (such as ben-zene C6H6) and carbon nanotube190 The four-probe resis-tance and thermoelectric power of nanotube samples areconsiderably modi ed by the adsorption of aromatic C6H6whereas the effect of the nonaromatic C6H12 molecule issmall In analogy Liu et al observed a dramatic decreasein the resistance in SWNT bundles upon doping of 23-dichloro-56-dicyano-14-benzoquinone (DDQ C8N2O2Cl2)molecules191 The effect of DDQ adsorption is much strongerthan that due to O2

The effect of gas adsorption on carbon nanotubes hasbeen studied theoretically by rst-principles methods192ndash202

Most calculations focus on the adsorption of O2 (Refs192 194ndash199) NO2 and NH3 (Refs 192 193) on smallSWNTs Zhao et al have systematically investigated thebinding energies and electronic properties of SWNTs uponadsorption of a variety of inorganic gaseous molecules(NO2 O2 NH3 N2 CO2 CH4 H2O H2 Ar)201 andorganic molecules (benzene C6H6 cyclohexane C6H12DDQ C8N2O2Cl2)202 The major theoretical results forbinding energy and charge transfer of the gas adsorptionon SWNTs from those rst-principles calculations aresummarized in Table V It can be seen that most gas mole-


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Table V First-principles calculations for the equilibrium tube-moleculedistance d adsorption energy Ea and charge transfer Q of variousmolecules on SWNTs

Molecule Tube d (Aring) Ea (eV) Q (e) Method Ref

NO2 (100) 26 03 mdash LDA 192O2 (100) 27 01 2009 LDA 192NO2 (100) 23 042 2011 LDA 193NH3 (100) 29 018 004 LDA 193CO2 (90) mdash 0109 mdash HF-MP2 200O2 (90) 323 0107 mdash HF-MP2 196O2 (80) 27 025 2010 LDA 194O2 (55) 246 0306 20142 LDA 194NO2 (55) 216 0427 20071 LDA 201NH3 (55) 299 0162 0033 LDA 201H2O (55) 268 0128 0033 LDA 201CH4 (55) 333 0122 0022 LDA 201CO2 (55) 354 0109 0014 LDA 201N2 (55) 323 0123 0011 LDA 201H2 (55) 319 0084 0016 LDA 201Ar (55) 358 0082 0011 LDA 201O2 (80) 343 0038 2001 GGA 195O2 (100) 28 0097 2009 GGA 202DDQ (100) 32 0317 20212 GGA 202C6H6 (100) 37 0103 0012 GGA 202C6H12 (100) 39 0118 0039 GGA 202

Charge transfer Q is de ned as the total Mulliken charge number on the moleculesthat is positive Q means charge transfer from molecule to tube For comparison theexperimental adsorption energies for O2and CO2molecules on SWNT bundles areabout 0192 eV (Ref 203) and 0024 (Ref 200) eV respectively

cules adsorb weakly on SWNTs and are charge donors tothe nanotubes201 whereas the electronic properties ofSWNTs are sensitive to the adsorption of certain moleculessuch as NO2 O2 and DDQ as charge acceptors The chargetransfer and gas-induced charge uctuations could signi -cantly affect the electronic and transport properties ofSWNTs For example Jhi et al194 showed weak hybrid-ization between carbon and oxygen for the valence-bandedge states which leads to conducting states near the bandgap Similar results have been found for NO2 adsorptionon semiconducting SWNTs by Zhao et al201 For the ad-sorption of organic molecules (noncovalent functionaliza-tion)202 as shown in Figure 1e there is strong coupling ofordm electrons between tubes and aromatic molecules for theconduction bands across the Fermi level consistent withexperiments190 The hybridization between the DDQ mol-ecular level and nanotube valence bands transforms thesemiconducting tube into a conductor202

62 Chemical Sensors

The sensitivity of the electronic and transport properties ofcarbon nanotubes upon gas adsorption implies the possi-bility of developing chemical sensors from carbon nano-tubes Inspired by the pioneer work of Dairsquos group186

there has been increasing interest in nanotube-based chem-ical sensors204ndash212 MWNTs lling the cavity of a micro-electrode were shown to be a promising candidate for anitrite sensor204 Chopra et al developed a nanotube-tubebased resonant-circuit sensor for ammonia detection205

In their experiments the sensor consists of a circulardisk electromagnetic resonant circuit coated with eitherSWNTs or MWNTs both of which are highly sensitive toadsorbed gas molecules Upon exposure to ammonia adramatic downshift is found in the electrical resonant fre-quency of the sensor On the other hand gas sensors forsub-ppm NO2 gas detection are realized by Valentini et alby deposition of carbon nanotubes on Si3N4Si sub-strates206 The sensor is highly sensitive to NO2 gas at con-centrations as low as 10 ppb (parts per billion) and exhibitsfast response time and good selectivity Recently Dairsquosgroup developed a strategy to fabricate large microarraysof SWNT sensor devices with 100 yield208 They usedpolymer functionalization to enhance the sensitivity andselectivity The n-type nanotube devices made by polyethyl-eneimine coating can detect NO2 at less than 1 ppb con-centration but are insensitive to NH3 whereas coatingna on on nanotubes blocks NO2 and switches to selectivesensing of NH3 These advances demonstrate the prospectof developing highly sensitive nanotube-based sensors forspeci c molecular detections

In addition to NO2 and NH3 alcohol vapor sensors areobtained from semiconducting SWNTs in FET geome-try207 Signi cant changes in FET current are found when

the nanotube-based device is exposed to various kinds ofalcoholic vapors and these responses are reversible andreproducible over many cycles Furthermore Dekkerrsquos grouphas demonstrated the use of individual semiconductingSWNTs as single-molecule biosensors209 The nanotubecoated with redox enzyme glucose oxidase on its sidewallis found to act as a pH sensor with a large and reversiblechange in conductance upon changes in pH

Instead of detecting change in electrical conductivityby gas adsorption Chopra and co-workers have devel-oped a carbon nanotube sensor with a conducting circulardisk coated by SWNTs which detects the change in thedielectric constant due to the presense of gases212 Anoticeable shift in resonant frequency was found for bothpolar (NH3 and CO) and nonpolar (He Ar N2 and O2)gases The sensor is selective for a number of gases sincedifferent resonant frequency shifts were observed for dif-ferent gases It can detect low concentrations (100 ppm)of gases with a small response time as compared with con-temorary sensors

Based on the ab initio calculations Peng and Cho213

proposed that nanotubes substitutionally doped with impu-rity atoms (such as boron or nitrogen) can serve as sensorsfor detecting CO and H2O molecules which only weaklyinteract with perfect nanotube201 The sensitivity of thesesensor devices can be controlled by the doping level of theimpurity atoms

63 Effect of Gas Adsorption on Field Emission

In the eld emission of carbon nanotubes it was found thatthe in uence of residual gases in vacuum chamber is a crit-ical factor for the long-term stability of emission cur-rent91 214 215 216ndash220 Dean and Chalamala performed gasexposure experiments with H2 H2O Ar and O2 and re-corded the eld-emission current as a function of time214

Exposures to H2 and Ar show no signi cant effect on thenanotube emission characteristics whereas exposure toH2O leads to rst an increase in current and then a subse-quent small reduction in current A dramatic decrease incurrent was found in the O2 environment The damage isirreversible after long exposure (40 h) Lim et al havestudied the eld-emission properties of nanotube eldemission arrays exposed to various gases (O2 N2 H2)

215

They found that the changes at high eld are stronglyrelated to the electronegativity of the individual speciesand nature of the adsorption Oxygen gas dominates the eld-emission properties upon adsorption and degrades thesurface morphologies because of a possible oxidative etch-ing whereas hydrogen gas has much less effect SimilarlyWadhawan et al studied the effects of N2 and O2 gaseson the eld-emission properties of Cs-deposited SWNTsand found that the emission current is stable in N2 butdecreases during exposure to O2 (Ref 91) They further


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compared the effects of O2 Ar and H2 gases on the eld-emission properties of single-walled and multiwalled nano-tubes216 They found that H2 and Ar did not have a signi -cant effect on the eld-emission properties of either SWNTsor MWNTs On the other hand exposure to O2 temporarilyincreases the turn-on eld of SWNTs (MWNTs) by 22(43) and reduces the eld-emission current by two (three)orders of magnitude for SWNTs (MWNTs) For SWNTsthe eld-emission properties completely recover after about40 h whereas only partial recovery is observed in the caseof MWNTs216

Hata et al studied the eld-emission microscopy (FEM)of MWNT in an atmosphere of various gases (H2 CO N2O2)217 218 They found that a MWNT with a clean surfacepresents FEM patters consisting of six bright pentagonalrings and the adsorbed gas molecules are recognized asbright spots in the FEM pattern These adsorbates prefer toreside on the pentagonal sites with strong electric eld andlead to stepwise increase in the emission current The de-sorption on a MWNT emitter can be realized via heat treat-ment at about 1300 K Recovery of the FEM pattern isfound after desorption of H2 and N2 whereas the MWNTtip structure is damaged or even destroyed after desorptionof CO and O2 (Ref 218) The effect of carbon-containingresidual gases (CO CO2 CH4 C2H4) on the eld-emissioncurrent of MWNTs has recently been investigated bySheng and co-workers219 They found that exposures toCO and CO2 at 1025 Pa reduce the current from 22 to44 and the reduction can be fully recovered by continu-ous emission under a high vacuum of 1026 Pa In contrastexposure to CH4 and C2H4 increases the current with poorstability and the change cannot be recovered219

The effects of gas adsorption on carbon nanotube eldemission have also been theoretically studied by Park et alfor atomic and molecular oxygen220 and by Maiti et al forH2 and H2O (Ref 221) Emission currents can be enhancedby oxygen adsorption particularly for the molecular ad-sorption cases220 For atomic adsorption the enhancementof the local electric eld leads to an increase in emissioncurrent and the new electronic states induced by an O2

molecule can explain the large current in the case of oxy-gen molecular adsorption They suggested that the mainreason for the current degradation in experiments might bestructural change involving oxidative etching220 Maiti et alshowed that the interactions between both polar H2O mole-cules and nonpolar H2 molecules and nanotube are weak ina zero electric eld221 However under eld-emission con-ditions the binding energy between polar H2O moleculesand metallic nanotube tip increases substantially The watermolecular adsorption lowers the ionization potential andmakes the HOMO level in the nanotube more unstableBoth effects are enhanced with an increasing number ofH2O molecules up to a saturation density In contrast non-polar H2 molecules weakly interact with nanotubes evenunder an electric eld221

64 Covalent Sidewall Functionalization

Different from adsorption or noncovalent functionaliza-tion the molecule can attach to the sidewall of a carbonnanotube via a tube-molecule covalent bond Such cova-lent functionalization might lead to new opportunities innanotube-based materials and devices222ndash224 Experimentson nanotube covalent functionalization start with the u-orination of SWNTs225 and the substitution reaction of uo-rinated SWNTs in solutions226 On the other hand directfunctionalization to the sidewall of SWNTs by various chem-ical groups such as atomic hydrogen227 aryl groups228

nitrenes carbenes and radicals229 COOH and NH2 (Ref230) N-alkylidene amino groups231 alkyl groups232 andaniline233 have been reported

The covalent bond formed between functional groupsand a carbon nanotube sidewall is expected to disturb theperfect tube ordm bonds via the local sp3 rehybridizationThus the electronic and optical properties of carbon nano-tube should be modi ed by the functionalization It wasfound that the band-to-band transition features of ordm elec-trons in the UV-visible spectra of pristine SWNTs dis-appear upon covalent functionalization226 228 231ndash233 Theresistance of functionalized nanotubes changes dramati-cally from that of the pristine sample225ndash227 Chiu et alobserved Raman shifting in functionalized SWNTs indi-cating charge transfer between functional groups (eg-COOH -NH2) and SWNTs230

In recent theoretical calculations by Zhao et al it wasshown that covalent functionalization on the nanotubesidewall will introduce a sp3 defect and induce an impuritystate around the Fermi level which may signi cantly mod-ify the electronic and transport properties of carbon nano-tube234 The impurity state was found to be extended overa large distance (1 nm) even though the structural defor-mation is con ned to the vicinity of the functional site Theeffect of covalent sidewall functionalization is differentfrom those of substitutional doping alkali-metal intercala-tion and topological defects on the tube sidewall but issimilar to that of the vacancy defect Thus covalent side-wall functionalization might provide an effective pathwayfor band structure engineering nanoelectronic devicesand sensor applications Because of the limited space wewill not discuss the other details of covalent functionaliza-tion of carbon nanotube which can be found in recentreview articles16 222ndash224

7 SUMMARY AND OUTLOOK

Doping of carbon nanotubes has clearly led to new oppor-tunities in nanotube-based materials and devices As wediscussed here the electronic and photonic properties ofcarbon nanotube are signi cantly modi ed upon doping inmany cases Thus doped carbon nanotubes become a fas-cinating class of nanoscale materials and devices with a


474

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variety of novel properties and applications reviewed inthis article The unique geometric and electronic propertiesmake the carbon nanotube bundles promising batterymaterials with high lithium capacity The enhanced eld-emission properties of the doped carbon nanotubes makethem viable for future development in planar displays Thelarge third-order optical nonlinearities of doped carbonnanotubes have potential applications as photonic devicesincluding all-optical switching and data processing As anideal optical limiter doped carbon nanotubes have reason-able linear transmittance at low input uence protectingoptical sensors or eyes against laser pulses of any wave-length and pulse duration

The remarkable electronic and transport properties ofdoped carbon nanotubes make them important buildingblocks in nanoelectronics which may speed up the devel-opment of molecular circuits and related devices In partic-ular the controlled pn doping and reversible adsorptiondesorption of gas molecules that are strongly coupled withnanotube electronic states are key issues in obtainingnanotube-based devices with desirable and tunable con-ductance Moreover nanotubes can serve as a 1D templateto build up metalnanotube and clusternanotube hybridnanostructures which may lead to novel applications innanoelectronics and spintronics

Certainly a lot of work on doped carbon nanotubes forexample the optical and magnetic properties of p- andn-type doped carbon nanotubes or peapods and covalentsidewall functionalization of nanotubes needs more care-ful and systematic studies The potential applications ofdoped carbon nanotubes also require us to obtain in a con-trolled manner highly reproducible preparations of dopedSWNTs or MWNTs possessing the desired characteristicsThis also applies to the other chemical manipulations withthese nanoscale materials So far most theoretical studieshave focused on doped SWNTs It is important and chal-lenging to explore the electronic structures and other phy-sical properties of doped MWNTs and nd the differencein doping behavior between SWNT and MWNT withthe use of accurate rst-principles methods and elaborateexperiments

Acknowledgments We thank Dr H S Nalwa ProfV H Smith Jr Prof J P Lu and Prof C K Yangfor valuable comments and stimulating discussions JZacknowledges support from the University Research Coun-cil of the University of North Carolina at Chapel HillOf ce of Naval Research Grant N00014-98-1-0597 andNASA Ames Research Center

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167 H Kataura Y Maniwa M Abe A Fujiwara T KodamaK Kikuchi H Imahori Y Misaki S Suzuki and Y Achiba ApplPhys A 74 349 (2002)

168 K Hirahara K Suenaga S bandow H Kato T OkazakiH Shinohara and S Iijima Phys Rev Lett 85 5384 (2000)

169 T Shimada T Okazaki R Taniguchi T Sugai H ShinoharaK Suenaga Y Ohno S Mizuno S Kishimoto and T MizutaniAppl Phys Lett 81 4067 (2002)

170 K Suenaga T Okazaki C R Wang S Bandow H Shinohara andS Iijima Phys Rev Lett 90 055506 (2003)

171 P W Chiu G Gu G T Kim G Philipp S Roth S F Yang andS Yang Appl Phys Lett 79 3845 (2001)

172 T Pichler H Kuzmany H Kataura and Y Achiba Phys RevLett 87 267401 (2001)

173 J Vavro M C Liaguno B C Satishkumar D E Luzzi and J EFischer Appl Phys Lett 80 1450 (2002)

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(2003)176 A Rochefort Phys Rev B 67 115401 (2003)177 R H Xie G W Bryant J J Zhao V H Smith Jr A D Carlo

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Ivanovskii Chem Phys Lett 351 35 (2002)179 Q Sun Q Wang Y Kawazoe and P Jena Phys Rev B 66

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185 Y Gao and Y Bando Nature 415 599 (2002) Y Gao Y Bandoand D Golberg Appl Phys Lett 81 4133 (2002)

186 J Kong N R Franklin C Zhou M G Chapline S Peng K Choand H Dai Science 287 622 (2000)

187 P G Collins K Bradley M Ishigami and A Zettl Science 2871801 (2000)

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Received 26 July 2003 RevisedAccepted 23 September 2003

the electrochemical intercalation of Li in nanotube bundlescan be used as anodes of lithium batteries because of thehigh Li storage capacity The electronic and transportproperties of SWNTs can be engineered by doping eitherelectron donors or electron acceptors which is essentialfor constructing nanoelectronic devices Hybrid structurescreated by lling nanotubes with transition metals exhibitlarge spin polarization and can be used as 1D devices inspintronics The eld emission of carbon nanotubes can beenhanced by substitutional N-doping or alkali-metal inter-calation Larger third-order optical nonlinearities are foundfor N- or B-doped carbon nanotubes constituting good can-didates for optical limiting applications Novel 1D nano-structures can be obtained by encapsulating carbon nano-

tubes with fullerenes clusters and atomic chains The con-ductance of carbon nanotubes is sensitive to gas adsorptionimplying potential applications as chemical sensors

All of these exciting achievements encouraged us towrite this review of the recent progress in understandingthe electronic and photonic properties of doped carbonnanotubes This article is organized as follows In Sections2ndash6 we discuss the physical properties and related appli-cations of carbon nanotubes doped by different methodssuch as alkali-metalhalogen intercalation substitutionaldoping transition metal lling or coating encapsulatingwith fullerenes clusters and atomic chain and moleculeadsorption or functionalization We end this article with abrief summary and outlook in Section 7 The synthesis andcharacterizations of carbon nanotubes are not includedhere but can be found in recent reviews by Duclaux18 andourselves17

2 ALKALI-METALHALOGENINTERCALATION

21 Electronic and Optical Properties

Raman spectroscopy is one of the most ef cient tools forinvestigating the vibration properties of materials in rela-tion to their structural and electronic properties19 In par-ticular the shift of Raman frequency in doped nanotubescan be the evidence of charge transfer between carbonnanotubes and dopants The pioneering work on Ramanspectra of SWNT bundles intercalated by electron donor(K Rb) and electron acceptor (Br2 I2) was done by Raoet al20 21 As shown in Figure 2 the high-frequency tan-gential vibration mode at 1593 cm21 in pristine nanotubesshifts substantially to a lower (for K Rb) or higher (Br2)frequency region implying a charge transfer between thedopants and the nanotubes Little change was found for I2

doping probably because I2 does not intercalate into thenanotube bundles


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Dr Jijun Zhao was born in 1973 and educated in Jiangsu China He received his bachelorrsquos degree in physics from NanjingUniversity China in 1992 and his PhD in condensed matter physics from Nanjing University in 1996 From 1997 to 1998 hewas a postdoctoral fellow at the International Centre for Theoretical Physics (ICTP) Italy From 1998 to 2002 he was a researchassociate and then a research assistant professor in the Department of Physics and Astronomy University of North Carolina atChapel Hill NC He is currently a senior research associate at the Institute for Shock Physics Washington State University Hismajor research eld is in computational materials science with a special interest in nanostructures (nanotubes nanowires etc)nanoelectronic devices atomic clusters and cluster-based materials high-pressure physics and molecular crystals He has con-tributed over 70 refereed journal papers and two book chapters in this eld

Dr (John) Rui-Hua Xie is a guest researcher at the National Institute of Standards and Technology (NIST) Gaithersburg MarylandHis research interest and expertise include computational nanoscience (nanoelectronics nanostructured materials nanocrystals andquantum dots) cluster physics molecular physics quantum computing quantum optics quantum theory quantum chemistry andelectronic spectroscopy such as NMR UV-vis and Raman Dr Xie received his bachelorrsquos degree in physics from WuhanUniversity Hubei China in 1991 and his PhD in physics from Nanjing University Jiangsu China in 1996 From 1997 to 1998he was a postdoctoral fellow at the University of Toronto Canada During the period from 1998 to 2000 he moved to Germany asan Alexander von Humboldt fellow working at the Max-Planck-Institut fuumlr Stromungsforschung in the beautiful university town ofGoumlttingen He went to the rst capital of Canada Kingston working at Queenrsquos University before he joined the Quantum ProcessGroup at NIST in 2001 He has contributed over 80 peer-reviewed journal articles and several review chapters in books and ency-

clopedias

Fig 1 Illustration of different ways of doping carbon nanotube (a) Liatom (red balls) intercalation of (1010) nanotube bundle with slice ofelectron charge density (b) Substitutional doping by nitrogen (blue ball)in a (100) tube (c) p-n junction formed by incorporation of C48N12

(right) and C48B12 (left) clusters inside a semiconducting (170) tube(d) Iron atoms lling a (90) tube at a ratio of FeC6 (e) Adsorption of a(100) tube by an organic DDQ molecule with a slice of charge densityfor electron bands crossing the Fermi level (red yellow green blue col-ors on the slice indicate density from higher to lower) (f) Covalent side-wall functionalization of a (66) tube by a COOH group














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186

376 134715261550

15671593

766 9491145

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1554(1564)

12621142

wave numbers500 1000 1500 2000

766 949











462

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p-type doping

Abs

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463

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467

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474

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766 949











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62 Chemical Sensors












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62 Chemical Sensors












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62 Chemical Sensors












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-2

-4

-6-05 05 15 25



(c) (b) (a)

20 18 16 14

Binding Energy (eV)

Inte

nsity

(ar

b u

nits

)














466

RE

VIE

W



-05 0 05

Sample Bias (V)

LD

OS

(ar

b u

nits

)

B-doped tube body

B-doped tube tip

-05 0 05

Sample Bias (V)

LD

OS

(arb

uni

ts)














33 Field Emission



467

RE

VIE

W












468

RE

VIE

W



X k 5 0 k 5 1 k 5 2 k 5 9 k 5 18 k 5 1 k 5 2 k 5 5 k 5 10

N 305 307 324 368 412 311 332 360 404B 39 43 49 76 85 47 51 69 77














469

RE

VIE

W















470

RE

VIE

W


0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)













471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W









465

RE

VIE

W

3

2

1

0

2

1

0

2

1

010 215 25 305

LixC6

Cel

l vol

tage

(V

olts

)



(a)

(b)

(c)


Voltage (volts)

Voltage (volts)

Voltage (volts)

APX

PA

PX

PA

PXP

2

0

-2

-4

-6-05 05 15 25

2

0

-2

-4

-6-05 05 15 25

2

0

-2

-4

-6-05 05 15 25



(c) (b) (a)

20 18 16 14

Binding Energy (eV)

Inte

nsity

(ar

b u

nits

)














466

RE

VIE

W



-05 0 05

Sample Bias (V)

LD

OS

(ar

b u

nits

)

B-doped tube body

B-doped tube tip

-05 0 05

Sample Bias (V)

LD

OS

(arb

uni

ts)














33 Field Emission



467

RE

VIE

W












468

RE

VIE

W



X k 5 0 k 5 1 k 5 2 k 5 9 k 5 18 k 5 1 k 5 2 k 5 5 k 5 10

N 305 307 324 368 412 311 332 360 404B 39 43 49 76 85 47 51 69 77














469

RE

VIE

W















470

RE

VIE

W


0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)













471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W














466

RE

VIE

W



-05 0 05

Sample Bias (V)

LD

OS

(ar

b u

nits

)

B-doped tube body

B-doped tube tip

-05 0 05

Sample Bias (V)

LD

OS

(arb

uni

ts)














33 Field Emission



467

RE

VIE

W












468

RE

VIE

W



X k 5 0 k 5 1 k 5 2 k 5 9 k 5 18 k 5 1 k 5 2 k 5 5 k 5 10

N 305 307 324 368 412 311 332 360 404B 39 43 49 76 85 47 51 69 77














469

RE

VIE

W















470

RE

VIE

W


0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)













471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W














33 Field Emission



467

RE

VIE

W












468

RE

VIE

W



X k 5 0 k 5 1 k 5 2 k 5 9 k 5 18 k 5 1 k 5 2 k 5 5 k 5 10

N 305 307 324 368 412 311 332 360 404B 39 43 49 76 85 47 51 69 77














469

RE

VIE

W















470

RE

VIE

W


0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)













471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W













468

RE

VIE

W



X k 5 0 k 5 1 k 5 2 k 5 9 k 5 18 k 5 1 k 5 2 k 5 5 k 5 10

N 305 307 324 368 412 311 332 360 404B 39 43 49 76 85 47 51 69 77














469

RE

VIE

W















470

RE

VIE

W


0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)













471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W














469

RE

VIE

W















470

RE

VIE

W


0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)













471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W
















470

RE

VIE

W


0

20

-4 -2 0 2 4

20

0

Majority spin EF

Minority spin

Den

sity

of

stat

es (

arb

uni

t)

Energy (eV)













471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W














471

RE

VIE

W

















472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W














472

RE

VIE

W






62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W



62 Chemical Sensors












215



473

RE

VIE

W













474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W














474

RE

VIE

W





































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W






































475

RE

VIE

W






























Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W































Carbon 37 61 (1999)































476

RE

VIE

W






























70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W































70 105 (2000)
































477

RE

VIE

W




























086403 (2003)



























478

RE

VIE

W





























086403 (2003)



























478

RE

VIE

W


Documents

Electronic and Photonic Properties of Doped Carbon Nanotubes