Spectral Diversity in Raman G-band Modes of Metallic Carbon Nanotubes within a SingleChirality

Moonsub Shim,*,† Anshu Gaur,† Khoi T. Nguyen,† Daner Abdula,† and Taner Ozel‡

Departments of Materials Science and Engineering and Physics, UniVersity of Illinois at Urbana-Champaign,Urbana, Illinois 61801

ReceiVed: June 6, 2008

Diversity in the Raman G-band phonon modes within individual metallic carbon nanotubes of the same chiralityis examined. Comparisons between Raman spectra of as-synthesized nanotubes with those obtained underelectrochemical gate potential are made. We show that most of the distribution in line width and peak positionof the G-band modes within a single chirality type can be explained by variations in where the Fermi levellies with respect to the band crossing point (i.e., where the nanotube is at zero charge). Varying degree ofcharge transfer from adsorbed O2 is likely to be the main source of ∼2 eV or larger range of Fermi levelpositions. On average, the Fermi level of individual metallic nanotubes lies on the order of 1 eV below theband crossing point. Both charge transfer and physical disorder are evident upon O2 adsorption. Implicationsof these findings on electron-phonon coupling and charge transfer processes are discussed.

Introduction

Raman spectroscopy has been widely used in characterizingcarbon nanotubes. In addition to structural chiral index (n, m)identification,1 numerous physical and chemical processesincluding electron-phonon and electromechanical coupling,2

charge-transfer doping,3 and chemical reactions4 have beenelucidated. The ability to measure Raman spectra at the singlenanotube level especially when combined with electron transportmeasurements is currently providing unprecedented details.5,6

For example, unusually large softening and broadening of theG-band phonons have been observed in single metallicnanotubes5,6 with Kohn anomalies causing strong electron-phonon coupling.7 However, a large distribution of behaviorsis expected at the single molecule level which can makeinterpretations difficult to make. Hence it is important to quantifythe range of distribution of behaviors and the factors that causesuch distributions.

In metallic nanotubes, often very broad and asymmetricG-band Raman features are observed. It is now well-establishedthat this broad line shape can be used to identify a nanotube asmetallic.8 However, some metallic tubes exhibit narrow line-shapes that are very similar to semiconducting ones and hencethe presence of the broad G-band feature cannot guarantee thatall metallic tubes are identified. The Raman G-band arises fromin-plane C-C stretch and, to date, it has been one of the keyfeatures in spectroscopic characterization of electron-phononinteractions and chemical reactivity of carbon nanotubes.2,4

Understanding the range of diversity and where the differencesin the G-band spectral features come from in individualnanotubes is therefore critical in developing robust chemistriesto selectively functionalize metallic tubes as well as in elucidat-ing the underlying physics of electron-phonon coupling in thisprototypical 1D conductor.

Most comparisons between experimental observations andtheoretical expectations are made with the assumption that the

Fermi levels of as-synthesized nantoubes (and even those thathave been processed for solution suspension) lie at or very closeto the band crossing point halfway between the first pair of vanHove singularities. For metallic tubes, examining the distributionof G-band features may ascertain whether or not such assump-tions hold true. In doing so, two of the key factors to considerare 1) phonon softening due to the Kohn anomaly7 and 2) chargetransfer doping via molecular adsorption especially from ambientO2.9 These two factors are interrelated in that the phononsoftening arising from the strong electron-phonon couplinginduced by the Kohn anomaly disappears upon moving theFermi level away from the band crossing points. In this article,we first show that there is a large range of Raman G-bandphonon linewidths and frequencies even within in a singlechirality. This distribution is then compared to the spectralevolution of nanotubes of the same chirality under electrochemi-cal gate potential indicating that most nanotubes are doped byO2 adsorption with their Fermi levels lying well below the zerocharge point. Implications on electronically selective chemistriesinitiated by charge transfer as well as on comparisons betweenexperimental observations and theoretical predictions especiallythose addressing electron-phonon coupling induced phononsoftening are considered.

Experimental Section

Carbon nanotubes were synthesized on Si/SiO2 or fused quartzsubstrates by established chemical vapor deposition methodsusing either ferritin (Sigma-Aldrich) or lithographically patternedFe(NO3)3 ·9H2O/alumina catalyst.10 Simultaneous electrochemi-cal gating and Raman measurements were carried out asdescribed in ref 5 using Au electrodes with Ti wetting layerdeposited on top of SWNTs grown on heavily doped Si/SiO2

substrates. Electrochemical gate potential was applied througha 20 wt % LiClO4 ·3H2O solution in polyethylenimine (PEI,Aldrich).11 Raman spectra were collected with a JY LabRamHR 800 using a 1.96 eV excitation source through a 100× airobjective (laser spot diameter ∼1 µm). Laser power was keptat or below ∼1 mW.

* To whom correspondence should be addressed. E-mail: mshim@uiuc.edu.† Department of Materials Science and Engineering.‡ Department of Physics.

J. Phys. Chem. C 2008, 112, 13017–13023 13017

10.1021/jp8050092 CCC: $40.75 2008 American Chemical SocietyPublished on Web 07/30/2008

Results and Discussion

It has been observed that individual metallic tubes oftenexhibit broad downshifted Raman G-band spectral features (withrespect to semiconducting nanotubes and graphite) with asym-metric Fano line shape.12 It has also been observed that somemetallic tubes exhibit narrow lineshapes that are very similarto semiconducting ones.5 Recent studies combining electro-chemical gating with Raman measurements on individualmetallic nanotubes have now shown that one nanotube canexhibit both broad asymmetric downshifted and narrow up-shifted G-band features depending on where its Fermi levellies.5,6 It is the presence of the Kohn anomaly that leads to thebroad lower frequency G-band mode (often referred to as theG- peak).7 Because of the strong electron-phonon coupling atthe band crossing (Dirac) point, that is, single particle electronicexcitations near the Fermi level coupling to the phonons, theG- peak has been assigned to the axial longitudinal optical (LO)mode.7,13 This is the assignment we follow here. If the metallicnanotubes are charged such that their Fermi levels lie away fromthe band crossing point, line narrowing and frequency upshiftare expected - but only for the LO mode.7 This effect providesa starting point in examining the spectral diversity in individualmetallic nanotubes.

G-Band Spectral Diversity. Figure 1A shows the radialbreathing mode (RBM) and D- and G-band regions of theRaman spectra of several (12, 6) and (14, 2) metallic nanotubes.The assignment of (n, m) chiral vector strictly follows theexperimentally verified RBM frequencies within 2 cm-1 asreported in ref 14. The RBM frequencies are expected to varydue to the variations in the local chemical environment15 andtherefore the assignment of (14, 2) for the nanotubes shown inFigure 1B needs to be justified since (15, 0) nanotube is expectedto have RBM frequency difference of only 1 cm-1. All nano-tubes that we assign to (14, 2) exhibit at least three well-

distinguished peaks in the G-band as shown in Figure 1B. A(15, 0) nanotube is an achiral zigzag tube. Both zigzag andarmchair nanotubes are expected to have only one mode withsubstantial intensities (A1

LO mode for zigzag and A1TO mode for

armchair) when the incident and detected light polarization isparallel to the nanotube orientation.6a,16 The chiral (14, 2)nanotube has a reduced symmetry and multiple peaks observedsuggest that these tubes are not likely to be zigzag tubes.However, we note that the substrate as well as the O2 adsorptioneffects can cause complications and the assignment of (14, 2)is tentative at this stage. Nevertheless, the observed trends inthe G-band linewidths and frequencies do not differ much withinthese tubes.

The most striking deviations within each chirality are thelinewidths and the frequencies of the peaks in the G-band. Thereis also a small but systematic shift in the D-band peak frequency,which is discussed later. To quantify the deviations in theG-band features, we fit all spectra to three Lorentzians and aFano line given by I(ω) ) Io[1 + (ω - ωo)/qΓ]2/{1 + [(ω -ωo)/Γ]2}, where ωo is the spectral position with intensity Io, qis the measure of phonon coupling to a continuum of states,and Γ is the width. Note that in the limit of |1/q|f 0, the Fanoline reduces to a Lorentzian with Γ being the half-width at half-maximum. Therefore, when we refer to linewidths, we use full-width at half-maximum for Lorentzians and 2Γ for the Fanoline. We also note that the actual value of |1/q| obtained fromfitting is very sensitive to small variations in the baseline butthe obtained values are always less than 0.5 with most beingbetween 0.1 and 0.2. Therefore, we limit our discussion to peakposition and line width.

Figure 2A shows examples of how the G-band features arefitted. Three cases shown provide the full range of G-banddiversity observed with the broadest and the most asymmetricat the bottom to the narrowest and the most symmetric at thetop. As seen in the lowermost curve, the intensities of theG-band of those nanotubes that exhibit broad lineshapes extendto frequencies well below the D-band. Therefore, all spectraare fitted including the D-band, which is described fairly wellby a single Lorentzian. The broad line shape of the G-band isassociated with asymmetry and a Fano line is necessary to fitthe spectra (highlighted in red). When the G-band exhibits onlynarrow line features as shown in the uppermost and thelowermost spectra of Figure 1A and B as well as in theuppermost spectrum of Figure 2A, all Lorentzian fit is just asgood as using a Fano line shape. However, for direct compari-son, all spectra are fitted including one Fano line.

The fitting results indicate that the distributions are quite large.Figures 2B shows the average peak positions (filled symbols)and the standard deviation (error bars) for the 2 main featuresof the G-band for three different chirality tubes. The corre-sponding open symbols are the maximum and the minimumfrequencies observed. Figure 2C shows the average linewidths.Also shown are the standard deviation and the maximum andthe minimum linewidths analogous to Figure 2B. The spreadin the peak frequencies are over 20 cm-1 for both the LO andthe TO modes. The spread in the linewidths of LO and TOmodes are also comparable.

Comparison to Electrochemical Gating. In order to explainthese large distributions in the Raman G-band, we now considerthe effects expected due to the Kohn anomaly7 and thepossibility of ambient O2 adsorption induced charge transfer.9

To do so, we compare the spread in the Raman spectra of as-synthesized nanotubes with the effects of electrochemical gating.Figure 3A shows the G- and the D-band regions of a (14, 2)

Figure 1. Raman spectra of (12, 6) and (14, 2) chiral metallicnanotubes showing diversity especially in the G-band features between1500 and 1600 cm-1. Radial breathing modes are shown in the leftpanels with the dashed line indicating the expected peak position.D-band is seen at ∼1320 cm-1. Spectra are offset for clarity.

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nanotube under electrochemical gate potential as indicated. Asexpected and consistent with previous reports, softening andbroadening of the G-band modes are seen around the zero chargepoint near 0 V.

Since we need to correlate these results with phonon modesof as-synthesized nanotubes of the same chirality without anyelectrical contacts, we examine how D-band peak frequencyshifts with the applied gate voltage as a reference point. Whilethe softening and broadening of LO mode may be considered,frequency upshift and narrowing of the line width of this modeappears nearly symmetric away from the band crossing point.This means that we cannot distinguish between electron additionand removal using G-band peak position or its line width. Whilethe D-band is a double resonance process involving a defectscattering,17 it is also associated with the in-plane C-C stretchmuch like the G-band and is therefore also sensitive to thedegree of charging. Figure 3B shows the D-band region enlargedat the same voltages as in 3A. There is small but systematicdownshift in the D-band peak frequency with increasing gatevoltage. Figure 3C shows that this shift is essentially monotonicand nearly linear unlike the G-band. In Figure 3C, we also show

the 2D peak (sometimes called the G′, D′, or D*) position. Boththe D- and the 2D-bands show the same trend with the appliedgate. Similar gate dependence of 2D (as well as the G) bandfrequencies have been observed in graphene.18 The 2D-band isalso a double resonance process but it involves two phononsrather than a phonon and a defect. Hence the 2D-band may beused even in the absence of physical disorder. In this regardand the fact that the shift in 2D-band is twice as large as theD-band, 2D-band may be a better reference point to comparecharging effects. However, the 2D-band intensity can be quitelow in some nanotubes and we have a more complete D-bandspectral data set for the current samples. Therefore, we resortto using D-band frequency for comparing electrochemical gatingresults with the spectral distribution in the as-synthesizednanotubes. We note that the D-band frequency is expected tobe dependent on the incident excitation energy.17 Since allmeasurements are carried out with the same laser line here, thisdependence does not affect our results.

In Figure 3D, the shifts in the positions of the three mainpeaks of the G-band for this (14, 2) nanotube are plotted asfunction of the D-band peak position. The maximum softeningof all three modes is seen between 1318 and 1320 cm-1. Thisregion corresponds to applied gate potential range of 0-0.3 V.The gate potential dependence of the peak positions are similarto those shown in ref 5. Since the gate dependence of D-band(as well as the 2D-band) peak position is nearly linear, directcorrelation with D-band frequency and Fermi level shift cannow be made.

The results of Figure 3D (filled symbols) are compared tothe G-band frequencies of as-synthesized nanotubes of the samechirality without any electrical contacts (open symbols) in Figure4A. The blue half-filled symbols are for the same nanotube thatthe electrochemical gating measurements were carried out onprior to the application of polymer electrolyte. The red opensymbols labeled 1, 2, and 3 correspond to one nanotube underdifferent gas ambient whose spectra are shown in Figure 4B.The effects of the ambient gases are discussed in the nextsubsection. Figure 4C shows the same comparison betweenelectrochemical gating and the G-band spectral distribution ofas-synthesized nanotubes all having (12, 6) chiral index. Allvariations in the G-band peak frequencies overlap closely orextrapolate well with electrochemical gating measurementsindicating that the major factor leading to the diversity in theG-band spectra even within a single chirality is the distributionof where the Fermi level lies.

In both (12, 6) and (14, 2) nanotubes, the minimum frequencyfor the G-band LO mode corresponds to the band crossing point(zero charge point) where the phonon softening effect due tothe Kohn anomaly is the strongest. Notice that, with theexception of four nanotubes, all as-synthesized nanotubes ofboth chiralities lie at higher D-band frequencies than where thezero charge point lies. Referring back to Figure 3C and D, thesehigh D-band frequencies correspond to negative voltages wherethe gate potential induces positive charge on the nanotube. Thatis, most as-synthesized metallic nanotubes are positivelycharged. As-synthesized metallic nanotubes of another chirality,(13, 4), shown in Figure 4D exhibit monotonic increase (i.e.,no minimum) in the G-band frequencies with D-band peakposition suggesting that all of these tubes are charged positive.

The Fermi level lying below the band crossing point (beingpositively charged) may not be too surprising given that as-synthesized semiconducting tubes are known to exhibit p-typebehavior.9 What is striking is that a large number of as-synthesized nanotubes’ Fermi levels lie well below the most

Figure 2. (A) Demonstration of curve fitting for the D- and G-bandspectral regions of three metallic tubes exhibiting large variations inthe G-band features. Gray lines are the data. Components of the fit (asdescribed in the text) are also shown. Fano line highlighted in red isconsidered as the main LO mode and the blue highlighted Lorentzianis considered as the TO mode here. (B) The distribution of LO andTO mode frequency for the indicated chiral index. Filled symbols arethe average values with the error bars indicating the standard deviation.Open symbols are the minimum and the maximum values observedfor the as-synthesized metallic tubes. (C) Same as in (B) but forlinewidths. For the Lorentzian TO mode, fwhm is shown. For the Fanoline, 2 Γ which is analogous to fwhm (see text) is shown.

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negative PEI electrolyte gate potential of -1 V. Note that forelectrolyte gating, PEI adsorption on carbon nanotubes is knownto induce electron transfer.19 That is, positively charged metallicnanotubes are brought back near the zero charge point uponPEI adsorption and hence the maximum phonon softening occursnear zero gate bias. Given the nearly ideal gate efficiencies ofpolymer electrolyte gate,11 this implies that the average metallictube has Fermi level more than 1 eV below the zero chargepoint. Furthermore, the large distribution in the G-band featuresindicates that the distribution in the Fermi level position maybe larger than 2 eV even within a single chirality.

Effects of O2 Adsorption. In individual semiconductingnanotubes, the role of ambient O2 adsorption on the observedelectrical and optical properties has been and continues to bedebated.9,19 Both direct doping effects and nanotube/metalcontact Schottky barrier modulations have been argued in thecase of single nanotube transistors.9,20 We have previouslyshown that both effects are important.9a In this study, mostnanotubes examined do not have any metal contacts. In fact,the very last sample preparation step for most tubes shown hereis the CVD growth. Therefore, contact effects and otherfabrication/processing induced effects are not applicable here

Figure 3. Spectral evolution of one (14, 2) nanotube under polymer electrolyte gate potential (Vg). (A) D- and G-band regions under indicated gatepotential. (B) Enlarged D-band region in (A). Spectra are offset for clarity. (C) Gate dependence of D and 2D band frequencies (ωD and ω2D) bothshowing nearly linear dependence with Fermi level shift. (D) Correlation of the frequencies (ωG) of the main three peaks of the G-band with ωD.LO and TO modes are filled squares and filled triangles, respectively.

Figure 4. (A) G-band frequency distribution of (14, 2) nanotubes. Electrolyte gating on one nanotube (filled symbols) with the distribution inas-synthesized nanotubes (open symbols) are compared. Blue half-filled symbols are for the same nanotube that the electrolyte gating was carriedout on but prior to polymer electrolyte applications. Open red symbols labeled 1, 2, and 3 correspond to the spectra shown in (B). (B) Ramanspectra of one (14, 2) nanotube in O2 ambient (1), in Ar after annealing at 450 °C in Ar for 15 min. (2), and after re-exposure to O2 (3). All threespectra are collected at room temperature and offset for clarity. (C) Same as (A) but for (12, 6) nanotubes without the O2/Ar/O2 cycle. (D) Sameas (C) but (13, 4) nanotubes without comparison to electrochemical gating.

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and we focus on possible doping effects to explain most metallicnanotubes having Fermi level significantly below the zero chargepoint.

If O2 adsorption leads to significant charge transfer insemiconducting tubes, metallic tubes are more prone to thiseffect. Any charge transfer process involving carriers mustovercome at least ∼0.1 to ∼1 eV barrier depending on the bandgap of the semiconducting nanotubes whereas the finite densityof states at the Fermi level readily provides electrons and emptystates in metallic tubes. This difference has been the basis fordeveloping several electronically selective reactions.4b,d Theavailable electrons in metallic tubes should therefore facilitatecharge transfer interaction with O2. Indeed, we have previouslyshown that such effect can be observed through the Ramanspectra of metallic tubes.5 Here, we explore this effect further.

Figure 4B shows the D- and G-band regions of a (14, 2) tubeunder O2 ambient (bottom curve labeled “1”), after Ar annealingand maintained under Ar (middle curve labeled “2”), and afterre-exposure to O2 (uppermost curve labeled “3”). Initialrelatively narrow and upshifted G-band features become broad,downshifted, and asymmetric after Ar annealing. Upon re-exposure to O2, the G-band recovers back to the line shape thatis nearly identical to before annealing. These observations areconsistent with our previous report.5 There is also a small butsignificant shift in the D-band peak position which allows usto map these changes onto the G-band/D-band correlation shownin Figure 4A. The red open squares labeled “1”, “2”, and “3”in Figure 4A correspond to these changes and follow the changesdue to charging via electrochemical gate potential nearlyidentically. The downshift in the D-band upon Ar annealing(removal or reducing the O2 induced effects) is equivalent to achange of ∼200 to 300 mV in gate potential in the direction ofremoving excess positive charges.

In addition to the G-band broadening and D-band downshift,there is also a decrease in the D-band to G-band integratedintensity ratio. However, this particular nanotube shows verylittle D-band intensity initially and the changes are not obvious.To examine the possible role of O2 actually inducing physicaldisorder, we have carried out the same Ar annealing/O2 exposurecycle on another metallic tube that exhibits larger initial disorder.As seen in Figure 5A, there is a large reduction in the D-bandintensity upon Ar annealing and a recovery when re-exposedto O2. In Figure 5B, we plot the G-band LO mode line width,which is sensitive to charging of metallic tubes, with theintegrated D/G ratio. The integrated D/G ratio (ID/IG) iscalculated including the asymmetry of the Fano line followingref 20. These results indicate that O2 adsorption causes bothcharge transfer and physical disorder in metallic nanotubes.

To further examine O2 adsorption induced disorder and chargetransfer, we plot the G-band LO mode line width of as-synthesized metallic tubes vs their ID/IG ratios in Figure 5C. Ifthe ambient O2 adsorbs and bonds to metallic tubes (whichwould lead to an increase in the actual physical disorder), thepolar bond formed could lead to a reduction in electron densityin the nanotube. Such effects should result in larger degree ofcharging in nanotubes (which would in turn cause narrowerG-band line width and upshift in the D-band frequency) withincreasing ID/IG ratio. This correlation appears to be presentwhen different degree of O2 adsorption is examined on the samenanotube as shown in Figure 5B. However, when different as-synthesized individual nanotubes are compared, there is noobvious correlation between G-band line width (or D-band peakposition) with ID/IG ratio as shown in Figure 5C. This apparentdifference is likely to arise from variations in the local chemical

environment which may provide additional disorder and/orcharge transfer effect different from what we can observe byAr annealing and O2 exposure. Further work is needed and isunderway to sort out these issues.

Implications on Diameter Trends, Phonon Softening, andCharge Transfer Initiated Chemistries. The diversity in theRaman D- and G-band modes of individual metallic tubes andthe observation that Fermi levels of as-synthesized metallic tubesare on average well below the zero charge point have severalimportant implications. One immediate implication is on thediameter dependence of these in-plane phonon modes. Thecharge dependent D-band frequency combined with the fact thatindividual metallic tubes on average have their Fermi levelslying well below the zero charge point can explain the apparentdifferent diameter dependence of D-band frequencies of semi-conducting and metallic tubes. Pimenta et al. have shown thatthe diameter dependence of D-band frequencies of metallic tubesare about 10 cm-1 above the expected values based ondependence observed in semiconducting tubes and thereforefollow a different trend.22 The average D-band frequency foreach chirality of as-synthesized metallic tubes in Figure 4 isabout 5-10 cm-1 higher than the expected D-band frequencyat zero charge point. Then, it may be the charging effects dueto ambient O2 exposure shifting the D-band frequencies higher

Figure 5. (A) Raman spectra of one metallic nanotube in O2 ambient(bottom), in Ar after annealing (middle), and after re-exposure to O2

(top). The annealing conditions are the same as in Figure 4B. LO modeline width correlation to integrated D/G intensity ratio (ID/IG) for thespectra of the nanotube in A is shown in (B) and for the as-synthesizednanotubes is shown in (C). No correlation is seen in as-synthesizednanotubes.

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rather than metallic tubes following a different diameterdependence than semiconducting tubes.

The shift in the average Fermi level below the zero chargepoint also needs to be considered in comparing theoretical andexperimental G-band LO phonon frequencies. Lazzeri et al. haveshown that by assigning the lower frequency G-band peak toLO mode softened by the Kohn anomaly, the observed diameterdependence trend of G+/G- splitting can be explained well.7b

However, there is a consistent offset where the experimentallyobtained values of the LO mode frequencies are ∼20 to 30 cm-1

higher than those calculated by density functional theory (seeFigure 3 of ref 7b). This discrepancy has been considered toarise from neglecting dynamic effects.7c However, we note thatthe ∼20 to 30 cm-1 offset should be expected due to the averageFermi level offset (corresponding to ∼ 5 to 10 cm-1 upshift inthe D-band position) from O2 adsorption induced charging whichleads to on average ∼15 to 20 cm-1 upshift in the LO frequencycompared to that at zero charge. Furthermore, nonadiabaticdensity functional calculations predict that charge injectionshifting the Fermi level away from the zero charge point shouldremove the LO mode softening eventually to the point where itcrosses over to higher frequencies than the transverse optical(TO) mode.7d Since the polymer electrolyte gate that we usehere has nearly ideal efficiencies,11 the gate voltage range shouldcorrespond closely to the actual Fermi level shift. The crossingof LO and TO is predicted to be between 0.5 and 0.6 eV awayfrom the zero charge point in ref 7d. Our gate voltage rangespans -1 to +1 V and we observe no evidence of LO/TOcrossing in Figures 3D and 4A and C. This is most likely dueto the higher frequency TO mode exhibiting nearly as largesoftening as the LO mode especially in the (14, 2) nanotubeshown in Figure 3. However, the line broadening, while alsosignificant, is not as large in the TO mode as in the LOmode suggesting that TO mode softening near the zero chargepoint may be a different effect. Nevertheless, all of theseobservations indicate that further work is necessary in under-standing electron-phonon coupling and the phonon softeningeffect in metallic nanotubes.

Finally, we point out that the large variation of ∼2 eV orlarger in the Fermi level position of metallic tubes has importantimplications on charge transfer processes in general. Chemicalreactions initiated by charge transfer are being exploited indeveloping electronically selective chemistries to sort outmetallic tubes from semiconducting ones.4 Reactions such asthose with aryl diazonium compounds4a,d that can be applieddirectly to nanotubes on substrates are particularly appealingin electronics applications since already established fabricationtechniques can be used. However, large variations on wherethe Fermi level lies, at least in metallic tubes on substrates,would lead to large variations in the initial charge transfer stepand therefore reduce selectivity.

Conclusions

We have shown that there is a large distribution of linewidthsand peak positions in the Raman G-band modes of metallicnanotubes even within a single chirality. This distribution arisesmainly from the variations in the degree of charging/dopingcaused by O2 adsorption either directly on nanotubes orindirectly via the substrate. This large variation means that theFermi level position in metallic tubes with respect to the bandcrossing point can vary by as much as 2 eV or more. On average,we have found that individual metallic tubes have their Fermilevels on the order of 1 eV below the zero charge point (i.e.,positively charged). There are several important implications

of this offset in the Fermi level along with the chargingdependent D- and G-band modes. In particular, the averageobserved G- peak frequencies (as well as G+ and D-bandfrequencies) of as-synthesized metallic tubes are likely to beupshifted by about 10-20 cm-1 due to the O2 adsorptioninduced charging. This upshift is concurrent with narrowerlinewidths (∼20 to 40 cm-1 narrower than at zero charge)especially for the G- peak. Therefore, comparisons betweencalculations on G-band (as well as D-band) linewidths andpositions should take this offset into account. The largevariations in the Fermi level positions of metallic tubes are alsoimportant to consider in general for any studies in charge transferprocesses involving metallic nanotubes.

Acknowledgment. This material is based upon work sup-ported by NSF (Grant Nos. DMR-0348585 and CCF-0506660).K.T.N. acknowledges support from the Vietnam EducationFoundation.

References and Notes

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Spectral Diversity in Raman G-band Modes J. Phys. Chem. C, Vol. 112, No. 33, 2008 13023