DTIC ADA560657: Revealing New Electronic Behaviours in the Raman Spectra of Chirality-Enriched Carbon Nanotube Ensembles

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Phys. StatusSolidi B247, Nos.11–12, 2768–2773(2010) / DOI10.1002/pssb.201000350 a s i b
c u d
si at oli
y
t s
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p www.pss-b.com
basic solid state physics
Revealing new electronic behaviours
in the Raman spectra of chirality-enriched
carbon nanotube ensembles
Juan G. Duque1, Hang Chen2, Anna K. Swan2, Erik H. Ha´roz3, Junichiro Kono3, Xiaomin Tu4, Ming Zheng4,
and Stephen K. Doorn*,1
1Center forIntegrated Nanotechnologies, Los AlamosNational Laboratory,MS-K771,Los Alamos, NM87545, USA
2Department of Electrical andComputer Engineering, Boston University, Boston,MA 02215,USA
3Department of Electrical andComputer Engineering, Rice University,PO Box1892, MS-366,Houston, TX77005, USA
4PolymerDivision,NationalInstituteofStandardsandTechnology,100BureauDr.,Stop8540,Gaithersburg,MD20899-8540,USA
Received 21May2010, accepted14July 2010
Publishedonline 13September2010
Keywords carbonnanotube, Gband,Raman, resonance profile, separations
*Correspondingauthor:e-mail [email protected],Phone:[phone], Fax:[phone]
We present Raman spectroscopy of single-walled carbon intensitiesoftheresonancecouplingtoincidentandscattered
nanotubes (SWNTs) that are enriched in metallic species by photons.Theexperimental data maybefit usingafour-level
density gradient ultracentrifugation (DGU) and enriched in molecular model for Raman scattering and the strong
singlesemiconductingchiralitiesthroughDNA-basedsepara- asymmetrycanbeunderstoodasaconsequenceofthepresence
tions.Radialbreathingmode(RBM)spectrademonstratethat ofnon-Condoneffects.Theresultrequiresareassessmentofthe
DGUsamplesarehighlyenrichedinarmchairchiralities.The assumption that the incident and scattered resonances are
enrichment allows acquisition of pure G-band spectra of the equivalent.Theconsequencesofsuchnon-Condoneffectson
armchair SWNTs and reveals that the LO mode is absent in other SWNT electronic and optical processes will be an
these structures. Raman excitation profiles for the G-band in importanttopicforfuturestudy.
nearlypure(10,2)samplesrevealsastrongasymmetryinthe
(cid:1)2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
1 Introduction Resonance Raman spectroscopy of Anequallyrichsetofbehavioursprovidingcomplemen-
single-walledcarbonnanotubes(SWNTs)hasbeenapower- taryinformationisavailablethroughG-band((cid:1)1590cm(cid:2)1)
ful tool for sample characterization and probing of a broad studies, which can provide insight into doping processes
rangeofvibrationalandelectronicbehaviours[1].Thespecial [12],metallicityofSWNTsthatcompriseasample[13],and
importanceheldbytheradialbreathingmode(RBM)inthis electron–phonon coupling behaviours linked to transport
regard is in large part due to the strong dependence of its properties [14] as just a few examples. While G-band
frequencyonnanotubediameter.Itsrelativelylowfrequency intensities and frequencies have allowed study of certain
((cid:1)100–300cm(cid:2)1)alsogivesitanarrowresonancewindow, ensemble behaviours, such as monitoring of different
thus reducing the overlap of the resonance responses of chemical and redox processes, many fundamental beha-
different(n,m)speciesoverbroaddiameterranges.Thesetwo viours and their chirality dependences are masked at this
characteristicsallowspectroscopicisolationofrelativelypure level. Thus, much of the information tied to the G-band
behavioursofsingle(n,m)speciesinmixed-chiralitysamples. responsehasonlybeenaccessibleatthesingle-tubelevel.In
Asaresult,theRBMhasplayedacriticalroleindeveloping contrasttotheRBM,thisresultsfromtheweakdependence
spectroscopicapproachestoidentifying(n,m)structuresand ofG-bandfrequencyonSWNTstructure,pairedwithabroad
mapping the chirality dependence of electronic transitions resonance window resulting from its relatively high
[2–4]. It has also provided a window for understanding the frequency. The two characteristics together make it nearly
nature of the optical excitations [5–7] and exciton–phonon impossibletochooseanexcitationwavelengththatsuitably
coupling[8–11]. isolatesthepurespectralbehaviourofasinglechirality.
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Revealing new electronic behaviours in the Raman spectra of
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Phys.StatusSolidiB247,Nos.11–12(2010) 2769
Enabling a broader set of G-band studies at the 2.2 Raman spectroscopy The wide range of exci-
ensemble level is important for addressing a number of tation wavelengths used in this study (460–850nm) was
issues that are difficult or laborious to probe at the single provided from Ti:Sapphire, dye (using Kiton Red and
tube level, such as establishing the generality of specific Rhodamine6G),andfrequency-doubledTi:Sapphirelasers.
behavioursacrossawiderangeofstructures,orprocesses Ramanspectraareobtainedinabackscatteringconfiguration
requiring measurement of specific resonance behaviours. with typically 25mW of incident power. Scattered light is
These difficulties may be overcome by working with collected,dispersedinatriple-monochromator,anddetected
ensemblesamplesthathavebeenhighlyenrichedinsingle on red or green sensitive CCDs with 2–5min integration
chiralities or specific electronic types. Tremendous pro- times.
gress has been made recently in the ability of separations
chemistry to provide just these kinds of samples. Ion- 3 Resultsanddiscussion
exchangechromatographyofSWNTswrappedinspecific 3.1 Kohn anomaly in the armchair limit G-band
DNA sequences is providing a variety of nearly pure spectraforsemiconductingandmetallicSWNTsareshown
semiconducting species [15]. Density gradient-based inFig.1.Thenarrowbandsinthesemiconductingspectrum
separations [16, 17] allow separation by metallicity and (weak G(cid:2) lower frequency transverse optical (TO) mode,
diameter, and are providing alternative routes to single- and strong Gþ high frequency LO mode) transform in
chirality samples. Access to these types of samples has metallicSWNTstoabroadandmoreintenseG(cid:2)relativeto
allowed new photophysical behaviours that were pre- thestill-narrowGþ.Ithasbeenshownboththeoretically[14]
viously inaccessible to be revealed. Examples include and experimentally [22–24] that the occurrence of a Kohn
numerousdynamicsstudiesonpure(6,5)samples[18]and anomaly in metallic nanotubes results in significant soft-
probingofdarkexcitonicstates[19,20]. eningandbroadeningoftheLOmode,sothattheG(cid:2)andGþ
Wedemonstratehereexamplesofhowsamplesenriched assignments are reversed from the semiconducting case.
in specific types of SWNTs have allowed us to probe Spectraobtainedonchirality-identifiedsingletubesindicate
behaviours linked to low-energy electron excitations the LO broadening is an intrinsic feature of all metallic
coupled to transport processes and the fundamental nature SWNTs,butitswidthandintensityrelativetotheGþfeature
of the Raman scattering process at resonance with optical displays a significant chirality dependence [22].
excitations,boththroughobservationofG-bandspectra.We Interestingly, the LO mode was found to be absent at the
show that by tuning specific parameters in density-based singletubelevelforthe(15,15)armchairchirality[22],and
separations, samples highly enriched in armchair metallic morerecentlyinthe(10,10)structure[25],inagreementwith
SWNTsareproduced.G-bandresponsesuggeststhataloss
of the longitudinal optical (LO) mode is general to all
armchairs. Single-chirality semiconducting samples allow
a)
ustoobtainforthefirsttimefullresonanceexcitationprofiles LO
fortheG-band.Theobservedprofilesoverturnthelong-held
3000
assumption that intensities are equivalent for response
resonant with the incident photon in comparison to the
responseofthescatteredphotonresonance.
2000
TO
2 Experimental y
t 1000
2.1 Chirality enrichment Isolation of armchair si
n
SWNT species is obtained through density gradient ultra- e
nt 1500 1550 1600 1650
centrifugation(DGU)usingapreviously-reportedapproach d I b)
[21].Briefly,SWNTsaresuspendedin1%aqueoussodium n
deoxycholate (DOC), with DGU performed in a 3-com- Ba TO
ponentmixtureofsodiumdodecylsulphate,sodiumcholate G- 4000
LO
and DOC. After isolation of the fraction of interest, the
sample was dialysed into 1% DOC to provide stable
suspensionandeliminationoftheiodixanoldensitygradient
2000
matrix. Enrichment in single-chirality semiconducting
species is obtained from ion-exchange chromatography of
suspensionsofDNA-wrappedSWNTs[15].Forthespecific
example shown below of the (10,2) species, wrapping in 0
1500 1550 1600 1650
(TATT) TAT provides the required selectivity. After
2 Raman Shi(cid:1) (cm-1)
chromatographic isolation of the (10,2)-enriched fraction,
the sample is dialysed into DOC (1wt.%) for Raman Figure 1 Representative G-band spectra for (a) semiconducting
analysis. and(b)metalliccarbonnanotubes.
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2770 J.G.Duqueetal.:NewelectronicbehavioursintheRamanspectraofchirality-enrichedCnanotube
the expectation that it is symmetry forbidden for armchair 0.6 a)
structures.Incontrast,recenttheorysuggestsarmchairspecies
will still display significant LO mode intensity [26]. It is 0.5
therefore of interest to determine if its absence is a general
0.4
featureofallarmchairchiralities.Thechallengeforaddres- e
c
sing this question at the ensemble level is to obtain spectra n
fromsamplesthatarehighlyenrichedinarmchairspecies. rba 0.3
o
Absorption spectra of as-produced SWNT suspensions s
b 0.2
in 1% aqueous DOC, in comparison to DGU-enriched A
fractions, are shown in Fig. 2a. The spectrum of the DGU 0.1
fractionclearlyshowseliminationoftheabsorptionfeatures
originatingintheE ,E andhigher-ordersemiconducting 0.0
11 22
transitions,suggestingasignificantdegreeofenrichmentin 400 600 800 1000 1200
metallicspecies.Correspondingphotoluminescencespectra Wavelength (nm)
b)
(notshown)fromthe same samplesshow completelossof 80000 (10,10)
(8,3)
emission from the metallic-enriched sample, indicating
significanteliminationofthesemiconductingspecies.
y 60000
While the absorption and photoluminescence spectra sit
n
demonstrate a high degree of metallic enrichment ((cid:1)98% e
[21]), the details of the final chirality distribution in this M Int 40000 (13,4) (7,5)
fraction canonlybeprovided throughitsextensiveRaman
B
characterization.RamanspectraoftheRBMregionforthe R 20000 (7,6)
as-produced and metallic-enriched fractions are shown in
Figs.2bandc.Thespectrawereobtainedwithexcitationsat 0
655and500nm,respectively;neartheE resonancepeaks
11
for the (10,10) and (7,7) species. The spectrum at 655nm 150 200 250 300 350
excitation for the as-produced sample is dominated by RBM Frequency (cm-1)
semiconductingspeciesofthe(2nþm)¼19and20families 25000 c)
(Fig. 2b top, black trace). RBM features for the metallic (7,7) (8,5)
(2nþm)¼30familyarealsopresent,butrelativelyweakin 20000
comparison.At500nmexcitation(Fig.2ctop,blacktrace), y
semiconducting transitions are avoided, but RBM features nsit 15000
e (9,3)
from the (8,5) and (9,3) species overlap with the (7,7) nt
response. M I 10000
A striking difference is observed for the RBM spectra B
R
takenofthemetallic-enrichedfraction.Allsemiconducting 5000
(8,2)
features have been eliminated. Of special note is that, at
655nmexcitation,the(10,10)RBM,previouslytheweakest 0
feature in the as-produced spectrum, is now the dominant
150 200 250 300 350
feature (Fig. 2b, bottom, red trace). Likewise, we observe RBM Frequency (cm-1)
with 500nm excitation (Fig. 2c bottom, red trace) that the
(7,7) RBM becomes the dominant feature, with only weak Figure 2 (onlinecolourat:www.pss-b.com)(a)Absorptionspec-
contributionfromthe(8,5)alsoappearing. traofas-producedHiPconanotubesuspension(upper,blacktrace)
Thisbehaviourisfoundtobegeneralforallthemetallic andmetallic-enrichedfraction(lower,redtrace).(b)RBMspectraof
2nþm families. From the relatively large 2nþm¼33 as-producednanotubesuspension(upper,blacktrace)andmetallic-
family to the smallest diameters (2nþm¼18), the RBM enriched(lower,redtrace)fractionat655nmexcitationand(c)at
feature of the armchair goes from being the weakest RBM 500nmexcitation.
spectral feature in the as-produced sample to the strongest
featureintheenrichedspectra.TheRBMdatademonstrate at 500nm, the broad G(cid:2) feature expected for metallic
the interesting result that not only does the DGU process nanotubesisclearlypresentat1525cm(cid:2)1intheas-produced
enrichthefinalsampleinmetallicnanotubes,butalsothatthe sample (Fig. 3a top, black trace). However, the G-band
enrichmentisselectivetowardsthearmchairspecies. spectrumofthearmchair-enrichedsample(Fig.3abottom,
The ability to spectrally isolate the armchair species redtrace)showsasinglesharpGþbandat1590cm(cid:2)1.The
allows us to obtain a pure G-band spectrum for these RBM spectrum of Fig. 2c clearly shows that at 500nm
chiralities. In Fig. 3 are shown the corresponding G-band excitation resonance is with solely the (7,7) chirality. The
spectraforexcitationsat500and655nm,obtainedfromthe resultofFig.3athusindicatesthatthebroadLOfeaturehas
as-producedandmetallic-enrichedsamples.Withexcitation beeneliminatedinthe(7,7)structure.
(cid:1)2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.pss-b.com
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Phys.StatusSolidiB247,Nos.11–12(2010) 2771
a) symmetryargumentsthattheLOmodeshouldbeabsentin
25000
armchair structures is confirmed. This result has important
implicationsfor understanding andengineeringofelectron
20000
scatteringbehaviourintransportapplications.Becauseofits
500 nm, (7,7)
strong electron–phonon coupling, the LO mode has been
15000
shown to be a significant source of electron scattering in
transportprocesses[27].Asaresult,largenon-equilibrium
10000
populations of G-band excited vibrational states are
observed in devices under source-drain bias [28]. The LO
5000
mode’s absence in armchair structures suggests that this
y
sit 0 source of scattering may be eliminated, implying that
n armchairsmaybeattractivecomponentsforinterconnects.
e
nt 1400 1450 1500 1550 1600 1650 1700
d I 700000 3.2 Non-Condon effects in the G-band
n b)
a resonance window Thecommonmodelsusedinunder-
B
G- 600000 standing Raman scattering from SWNTs originate in the
500000 Kramers–Heisenberg formalism for the polarizability. The
655 nm, (10,10) inherentproblemofaccountingforthesumoverstatesinthe
400000 electronicoverlaptermshasbeensolvedthroughincorpor-
atingexpressionsfordensityofstates[29]andintroducing
300000
many-bodyeffectstoaccountfortheexcitonicnatureofthe
200000 opticalexcitations.Theresultisacommonlyusedexpression
that describes the bandshape of the Raman excitation
100000
profile[30]:
0
(cid:3) (cid:3)2
1400 1450 1500 1550 1600 1650 1700 (cid:3) M (cid:3)
G-Band Frequency (cm-1) I/(cid:3)(cid:3)(cid:3)ðiðEii(cid:2)ElaserÞþGÞ(cid:1)iðEii(cid:2)Elaser(cid:2)EphononÞþG(cid:2)(cid:3)(cid:3)(cid:3) : (1)
Figure 3 (onlinecolourat:www.pss-b.com)(a)G-bandspectraof
as-producednanotubesuspension(upper,blacktrace)andmetallic- In Eq. (1), M includes the matrix elements for exciton–
enriched(lower,redtrace)fractionat500nmexcitationand(b)at phononandexciton–photoncoupling,E istheenergyofthe
ii
655nmexcitation. resonantelectronictransition,E istheexcitationenergy,
laser
E istheenergyofthemodebeingprobedintheRaman
phonon
measurementandGisaphenomenological dampingfactor
Asimilarresultisfoundfor655nmexcitation,whichas thatdefinesthetransitionwidth.Theresultingprofilegives
indicated in Fig. 2b should be selective for the (10,10) twopeakscorrespondingtoresonanceswiththeincidentand
chirality in the armchair-enriched sample. The G-band scattered photons. The underlying assumption in Eq. (1) is
spectrum obtained at this wavelength in the as-produced that the two resonances have equivalent intensities.
sample(Fig.3btop,blacktrace)reflectsthelargeresponse Thisformulahasbeenprovensuccessfulintheanalysis
expectedfromthesemiconductingspeciesthatarepresentin ofexcitationprofilesoftheRBMandhasalsobeenassumed
theunseparatedsample.Uponremovalofthesemiconduct- to be appropriate for other modes as well. However, the
ingspecieswiththearmchairenrichment,however,weare validityofthismodelhasnotbeentestedunderconditionsin
leftwithaG-bandspectrumthatisprimarilyasinglesharp which the two resonances can be sufficiently resolved to
Gþ feature. The broad residual G(cid:2) response likely occurs confirm the assumption of equal contributions. Sufficient
fromthesmalldegreeofnon-armchairmetallicsthatremain separationoftheresonancesrequiresprobingofarelatively
from the 2nþm¼30 family after separation. These are highfrequencymodeliketheG-band.Toavoidoverlapping
apparent as the shoulders at higher frequency from the spectral contributions to the G-band profile from multiple
(10,10) peak in the armchair-enriched RBM spectrum chiralities, however, requires measurement of samples
(Fig.2b,redtrace)thatcorrespondtothe(11,8)and(12,6) highly enriched in a single chirality, such as the single-
structures. Loss of the LO mode for the larger diameter chiralitysemiconductingfractionsmadeavailablefromion-
(10,10) is therefore also indicated, in agreement with the exchangechromatographyofDNA-wrappedSWNTs[15].
resultsofRef.[25]. As an example, we show in Fig. 4a the absorption
Obtainingthisresultattheensemblelevelissuggestive spectrumforafractionthathasbeenhighly-enrichedinthe
that the loss of intensity in the LO mode in armchair (10,2)chirality.Thespectrumisdominatedbythefeatures
chiralities is a general result for all armchairs. While this correspondingtotheE (at1061nm)andE transitions(at
11 22
mustbefurtherdemonstratedbyobtainingsimilarlypureG- 740nm).Minorfeaturesat907and656nmcorrespondtothe
bandspectraforotherarmchairspecies,theexpectationfrom respectivephononsidebandsofthetwotransitions,withonly
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2772 J.G.Duqueetal.:NewelectronicbehavioursintheRamanspectraofchirality-enrichedCnanotube
a)
0.15
)
u.
e a.
rbanc 0.10 sity (
o n
s e
Ab nt
0.05 d I
n
a
B
-
G
0.00
500 600 700 800 900 1000 1100
1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Wavelength (nm)
400000 b) Excitation Energy (eV)
(10,2) Figure 5 (online colour at: www.pss-b.com) Plot of (10,2) Gþ
intensityasafunctionofexcitationenergy(bluecircles)withafit
y 300000 tothedatausingthefour-levelmodelofEq.(2)(redsolidline)andan
nsit overlayofthecorrespondingexpectationfortheexcitonmodelof
nte Eq.(1)(blackdashedline).
M I 200000 (12,1)
B
R
by invoking a four-level model for resonance Raman
100000 X 100
scatteringdescribedinEq.(2)[31]:
(cid:3) (cid:3)2
0 (cid:3) m m (cid:3)
175 200 225 250 275 300 325 I/(cid:3)(cid:3)(cid:3)ðiðEii(cid:2)Ela1serÞþGÞþ(cid:1)i(cid:1)Eii(cid:2)Elaser(cid:2)E2phonon(cid:2)þG(cid:2)(cid:3)(cid:3)(cid:3) :
RBM Frequency (cm-1)
(2)
Figure 4 (onlinecolourat:www.pss-b.com)(a)Absorptionspec-
trum of (10,2) enriched sample. (b) RBM spectra of the (10,2) InEq.(2),m andm nowrepresentthematrixelementsfor
1 2
enriched sample with excitation on resonance at 730nm (black
the incidentand scattered resonant processes,respectively.
trace)andoff-resonanceat800nm(redtrace).
Thefour-levelmodelisappropriateinthecaseofresonance
with discrete electronic and vibrational levels such as in
molecular systems. Such a description works well for
minor additional features being observed from impurity SWNTsbothasaresultofthediscretenatureoftheexcitonic
species. Likewise, the RBM spectrum obtained with transitionsandthepresenceofthevanHovesingularitiesin
excitation near resonance with the E transition (730nm, the one-dimensional (1D) density of states. Furthermore,
22
Fig. 4b black trace) shows a single Lorentzian feature at momentumconservationforphotonandphononabsorption
265cm(cid:2)1, indicating the extremely high enrichment in andemissionlimitsopticalactivitytothezonecentre.
(10,2). Minor presence of other semiconducting species is The four-level fit accurately reproduces the E value
22
evident in RBM spectra taken at resonance with other obtained from the absorption spectrum (1.67eV).
semiconductingfamilies.But,asseeninthespectrumtaken Furthermore,thefitresultsfromidenticalvaluesofgamma
at resonance with the 2nþm¼25 E transitions (800nm, (26meV)beingincorporatedforbothresonances,indicating
22
Fig. 4b red trace), intensities for these species are <1% of thatthedifferenceintheirintensitiesdoesnotoriginatefrom
thatfoundforthe(10,2)structure. variabilityinthebroadeningmechanismsforeachterm.We
Thehighdegreeofenrichmentinthissampleallowsthe findtheratioofthematrixelements(m /m )tobe(cid:2)0.75.Itis
2 1
acquisition of G-band spectra corresponding to the pure noteworthythatabestfitrequiresthetwomatrixelementsto
behaviour of the (10,2) chirality. A plot of the (10,2) Gþ haveoppositesign.
intensity as excitation energy is varied is shown in Fig. 5. The origins of this strong asymmetry between the two
Remarkably, we find a clear asymmetry in the excitation resonancescanbeunderstoodastheresultofviolationofthe
profile.Thescatteredresonancepeakat1.86eVisseentobe Condon approximation, which assumes no nuclear motion
significantlyweakerthantheincidentresonanceat1.67eV. duringanelectronicexcitation.Suchnon-Condoneffectsare
Overlaidontheexperimentaldata(blackdashedline)isthe generalized here as the introduction of a strong nuclear
expectationfortheshapeoftheresonancewindowbasedon coordinate dependence inthe transition moment dipole for
the exciton description of Eq. (1), which clearly fails. In the accompanying absorption and emission processes.
contrast,wefindthattheresonancelineshapecanbewell-fit Behaviour similar to that in Fig. 5 is commonly observed
(cid:1)2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.pss-b.com
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Phys.StatusSolidiB247,Nos.11–12(2010) 2773
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of a LANL-LDRD Directors Postdoctoral Fellowship. AKS Y.Sfeir,L.E.Brus,J.Hone,andT.F.Heinz,Phys.Rev.Lett.
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