06-AM-Structure-Dependent Electrical Properties of Carbon Na

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DOI:10.1002/adma.200602966

Structure-DependentElectricalPropertiesofCarbonNanotubeFibers**

ByQingwenLi,YuanLi,XiefeiZhang,SatishkumarB.Chikkannanavar,YonghaoZhao,AndreaM.Dangelewicz,LianxiZheng,StephenK.Doorn,QuanxiJia,DeanE.Peterson,PaulN.Arendt,andYuntianZhu*

Spuncarbonnanotube(CNT)fibershavegreatpotentialforconductingandsensingapplicationsowingtotheirunique,tunableelectricalproperties.[1–5]Herewereporttheelectrontransportpropertiesofneat,well-alignedCNTfibersspunfromarraysofmillimeter-longCNTs.Theconductivityofas-spunCNTfibersisaround595.2Scm–1atroomtemperature,anditsvariationwithtemperatureshowsasemiconductivebe-haviorfrom300to75.4K.Theelectrontransportwasfoundtofollowathree-dimensional(3D)hoppingmechanism.[6]Im-portantly,itwasfoundthatchemicaltreatmentsmaysignifi-cantlyaffecttheconductivitiesofas-spunfibers.OxidizingtheCNTfibersinairorHNO3increasedtheconductivities,whilecovalentbondingofAunanoparticlestotheCNTfibersre-markablyimprovedconductivityandchangedconductionbe-havior.Conversely,annealingCNTfibersinAr+6%H2at800°CorundertheCNTarraygrowthconditionsat750°Cledtoadramaticdecreaseinconductivity.

Owingtotheirconjugatedandhighlyanisotropic1Dstruc-tures,carbonnanotubes(CNTs)areafascinatingnewclassofelectronicmaterialsfromboththeoreticalandappliedstand-points.[7]TheexcellentconductivitiesofCNTsandtheirabil-itytocarryveryhighcurrentdensity,alongwiththeirhighthermalconductivity,chemicalstability,andmechanicalstrength,makeCNTsuniquelypromisingforabroadrangeofapplications,includingbuildingblocksfornanoscaleelectron-icdevices,microsensorsforbio-agentsandchemicals,andpowercablesforspaceshuttles.[8–10]TheelectricalresistivityqofindividualCNTshasbeenmeasuredunderballisticconduc-tionstobeaslowas10–6Xcm[11,12]forsingle-walledand

3×10–5Xcm[13,14]formultiwalledCNTs,respectively,indicat-ingthatCNTsmaybebetterconductorsthanmetalssuchascopperatroomtemperature.However,inmostcases,duetothepresenceofvariousdefectsorimpuritiesformedduringtheCNTgrowth,theconductivitiesofindividualCNTsareof-tenmuchlowerthanthoseunderballisticconductionwithnanotubesfreeofdefects.[15,16]

TheelectrontransportinCNTassembliesisdifferentfromthatinindividualnanotubes.Ithasbeenreportedthatsingle-walledcarbonnanotube(SWNT)fibers,eithersynthesizeddi-rectlybyverticalfloatingchemicalvapordeposition(CVD)methods[1,2]orextrudedfromasuper-acidsuspension,[3]ex-hibitroom-temperatureresistivitiesintherangeof1×10–4to7×10–4Xcm,whichisnearly100timeshigherthantheresis-tivitiesofsinglenanotubes.Theresistivitiesofmultiwalledcarbonnanotube(MWNT)fibersaretypicallyoneortwoor-dersofmagnitudehigherthanthatofSWNTfibers.[4,5]Suchlargedifferencesbetweensinglenanotubesandfiberassem-bliesmayarisefromahighimpuritycontent(suchasamor-phouscarbonandcatalyticparticles)inthefibers,whichmayprofoundlyaffectelectrontransportbycausingsignificantscattering,andcontactresistancesbetweennanotubes.

Therefore,twoapproachescanbeusedtoimprovetheelec-tricalconductivityofCNTfibers:1)minimizethecontactre-sistancesbetweennanotubesbyimprovingthealignmentofCNTsandbyincreasingthelengthsofindividualtubes;2)im-provetheconductivityofindividualCNTsbypost-synthesistreatments.ItwastheobjectiveofthestudyreportedheretousethesetwoapproachestoproduceCNTfiberswithhighconductivityandtostudythefundamentalconductionmecha-nismsoftheCNTfibers.

ThinandcleanCNTfibers(typically3lmindiameter)werespunfromarraysofwell-aligned,millimeter-longCNTs,whichweresynthesizedusingethyleneCVDonaFecatalystfilm.[17]BymeasuringtheresistanceofCNTfibersattemper-aturesfrom300Kto75.4K,weinvestigatedtheelectronicpropertiesofas-spunfibersandtheirpossibleconductingmechanisms.ItwasalsofoundthattheconductivityofCNTfiberscouldbetunedthroughmildpost-treatments.

ThespunCNTfiberswerepost-treatedwithfivedifferentprocedures:1)Annealinginairat480°Cforhalfanhourinanattempttocleanofftheamorphouscarbon,whoseoxida-tiontemperatureisoftenaround400°C.[18]2)Oxidizingindilute5MHNO3solutionat40°Ctocauseaweakchemical

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[*]Dr.Y.T.Zhu,Dr.Q.W.Li,Y.Li,Dr.X.F.Zhang,

Dr.S.B.Chikkannanavar,Dr.Y.H.Zhao,A.M.Dangelewicz,Dr.L.X.Zheng,Dr.S.K.Doorn,Dr.Q.X.Jia,Dr.D.E.Peterson,Dr.P.N.Arendt

LosAlamosNationalLaboratoryLosAlamos,NM87545(USA)E-mail:yzhu@lanl.gov

[**]ThisworkwassupportedbytheLaboratoryDirectedResearchand

Development(LDRD)programofficeofLosAlamosNationalLabo-ratory.WethankDr.HonghuiZhouforherhelpinelectricalmea-surements.SupportingInformationisavailableonlinefromWileyInterScienceorfromtheauthors.

©2007WILEY-VCHVerlagGmbH&Co.KGaA,WeinheimAdv.Mater.2007,19,3358–3363

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functionalization.[19,20]3)AnnealinginAr+6%H2atmo-sphereat800°Ctosaturatethedefectivestructures.[21]4)In-troducingmetallicgoldnanoparticlesontoCNTsbydippingfibersfunctionalizedby5MHNO3intoa0.01%HAuCl4eth-anolsolutionfor5min.[22]5)IntroducingcarbonparticlesbyarepeatedtreatmentofthefibersunderCNTarraygrowthconditions.[23]Allofthesetreatmentswerechosentointro-6.0

Ohm-cm *10-3)

5.5

5.0

vity (S/cm * 102)

( ducemildmodificationstotheCNTfiberssothattheirpris-yittictinestructurescouldbemaintainedwhiletheirelectricalvutis4.5

dnpropertiesweremodified.

sieoCFigure1AshowsatypicaluniformCNTfiberspunfromanRarrayofmillimeter-longCNTs.OwingtothethinFecatalyst4.0

usedinitssynthesis,thecatalystcontaminationinourCNTT (K)

arraysismuchlessthaninCNTfiberssynthesizedbythefloatingCVDmethod[1,2]andCNTarraysgrownonthickFeFigure2.Temperaturedependenceoftheresistivity(q)andconductivityfilms.[3,4,24,25]Moreover,CNTsalsoalignverywellinourspun(r)ofaspunCNTfiber.

fibers,astheycanonlybespunfromsuper-alignedCNTar-rays.[24,25]OurspunCNTfibershavegoodCNTalignmentandhighpurity[17]andthereforemayserveasagoodmodelsys-465.3Scm–1at300K,whichisabout22%lowerthanthefi-temtoinvestigatetheelectricalpropertiesofCNTassemblies.berfromanarrayof1.0mmCNTs.ThisindicatesthatCNTTheinsetinFigure1ArevealsthattheCNTsaremultiwalledfibersspunfromarraysoflongerCNTswillhavelowercon-(2–7walls)withanaveragediameterof7nmandthattheytactresistance.ThisislikelyduetothefactthatafibermadearemoredefectivethanSWNTsandgraphite.[26]Thisisevi-oflongerCNTswillhavefewerendconnectionsandlargerdentfromtheRamanspectrumshowninFigure1B,wherecontactareabetweenneighboringtubes.ForagivenspecifictheintensityoftheDpeakishigherthanthatoftheGpeak.contactresistance,thelargerthecontactarea,thesmallertheFigure2showsthetemperaturedependenceoftheresistiv-totalcontactresistance.Asaresult,ourCNTfiberismoreityqandconductivityrofaCNTfiberbetween75.4andconductivethanotherreportedCNTfibers,itsconductivity300K.Theresistivitydecreasesmonotonicallyandsmoothlybeingatleast49.5%higherthanthosereportedpre-from2.19×10–3Xcmat75.4Kto1.68×10–3Xcmat300K.viously.[3,4]

Conversely,theconductivityincreaseswithincreasingtemper-Thetemperaturedependenceofconductivitycanhelpwithature,from456.6Scm–1at75.4Kto595.2Scm–1at300K,in-understandingtheconductionmechanismofaCNTfiber.Un-dicatingasemiconductingbehavior,butwithmuchsmallerlikethatofasingletube,theresistivityofaCNTfiberisde-temperaturedependencethancommercialgraphiteandcar-rivedfromtwocomponents:theresistanceofindividualCNTsbonfibers.[27]Underthesamesynthesisandspinningcondi-andthecontactresistancebetweenCNTs.Asmentionedtionsexceptforthegrowthtime,theCNTfibersspunfromanabove,theresistivitiesofindividualtubesareoftentwoordersarrayof0.3mmlongCNTsshowedaconductivityof

ofmagnitudelowerthantheirassemblies,[3,4]whichsuggests

30000

B

1300

25000

1593

y

ti

s20000

netn15000

I10000

5000

Raman Shift (cm-1

)

Figure1.A)Scanningelectronmicroscopy(SEM)imageofaspunCNTfiber;thetypicaldiameterofthefiberusedinthestudyis3lm.Inset:Trans-missionelectronmicroscopy(TEM)imageshowingCNTsaremultiwalledwithanaveragediameterof7nm.B)Ramanspectrumofthespunfiber,re-vealingahighlydefectivestructureformedintheCNTs.

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thatthecontactresistancesattheinterfacialconnectionsofCNTsplayasignificantroleintheconductionbehaviorofaCNTfiber.Ingeneral,twomainmechanismscanbeusedtoexplaintheconductionbehaviorofsemiconductiveCNTs:variablerangehopping(VRH)[28]andtunnelingconduction(TC).[29]Theycanbedescribedwiththefollowingtwoequa-tions,respectively:rT1/2=exp(–B/T1/4)r=r0exp(–A/T1/2)

(1)(2)

whereristheelectricalconductivity,r0,A,andBarecon-stants,andTisabsolutetemperature.AsshowninFigure3,plottingourdataintheformln(rT1/2)versusT–1/4,basedonEquation1,givesamuchmorelinearplotthanlnrversusT–1/2,basedonEquation2,whichindicatesthattheconduc-tioninourspunfibersispredominantlycontrolledbythehop-pingmechanism.TheslightdeviationfromthestraightlineinFigure3Aathightemperatureswasprobablycausedbyelec-trontunnelingbetweensomeCNTs.

Inmoredetail,therelationshipbetweenconductivityandtemperatureinMott’svariablerangehoppingmodelcanalsobeexpressedasr∝exp(–A/T[1/(d+1)]),whereAisaconstantanddisthedimensionality.[6]TheplotoflnrversusT–1/4(ford=3),T–1/3(ford=2),andT–1/2(ford=1)Table1.EffectsofchemicaltreatmentsonthestructuresandelectricalpropertiesofCNTfibers.havelinearfittingcoefficientsof

ConductivityDpeakpositionGpeakpositionIG/IDSampleConductivity0.997,0.995,and0.992,respectively.

–1–1–1–1

[cm][cm]at75.4K[Scm]at300K[Scm]Thissuggeststhattheelectrontrans-Originalfiber456.6595.2130015930.67portinCNTfibersisconsistentwith

Annealedin642.7818.3130015980.54a3Dhoppingmechanism.Thisbe-airat480°C

haviorismostlikelyduetothede-AnnealedinAr+6%H2at29.370.0130015980.48

fectstructuresofCNTs,inwhich800°Celectronscannotbeconfinedinthe692.5969.0130015940.41Oxidizedin1DchannelsofCNTs,andinstead5NHNO3

Coatedwith907.41152.7130515980.52hopfromonelocalizedsitetoan-Aunanoparticlesother,orpossiblyfromoneCNTto

Re-growthat750°C55.371.1130015830.62

another.Theenergydifferencebe-

tweentheinitialandfinalstatesisbridgedbyanelectron–phononscatteringprocess.[30]

Thetwo-terminalcurrent–voltage(I–V)characteristicsofaspunCNTfiber(showninFig.S2,SupportingInformation)indicatealinearnatureoftheI–Vcurve,whichrevealsgoodohmiccontactsbetweentheCNTfiberandtheelectrodes.TheresistanceRasderivedfromthelinearI–Vcurveis2.2×103X,basedonR=qL/S,wherethelengthLis1mmandtheareaSis7.1×10–12m2foraCNTfiberwithdiameterof3lm.Thecorrespondingresistivityiscalculatedtobe1.56×10–3Xcm,whichisconsistentwiththefour-probemea-surement.

TheconductivitiesoftheCNTfiberschangedafterdiffer-entpost-synthesistreatments.AsshowninTable1,oxidiz-ingtheCNTfiberinbothairandHNO3ledtoanincreaseinconductivity.TreatingtheCNTfiberunderhightempera-tureinforminggas(Ar+6%H2)ledtoadramaticde-creaseinitsconductivity.Also,annealingtheCNTfiberun-derregrowthconditions(containinghydrogenaswell),dramaticallydecreaseditsconductivityalthoughsomenewCNTswereformedonthefiber(showninFig.4A).Amongthefivetreatments,covalentlycoatinggoldnanoparticlesontothefiberledtothegreatestincreaseinconductivity.AsseenfromFigure4B,althoughsuchcoatingwassparse,

ln (α Τ1 /2)

lnσ

T-1/4

T-1/2

Figure3.FittingoftheconductivitydataofourCNTfiberswithtwodifferentconductionmechanisms.A)Plotofln(rT1/2)versusT–1/4,basedonthevariablerangehopping(VRH)mechanism.B)PlotoflnrversusT–1/2,basedonthetunnelingconduction(TC)mechanism.

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Thechangesoftheelectricalproper-tiesofCNTfiberscanberelatedtotheirstructuralmodifications,[34]whichwereelucidatedusingRamanspec-troscopy.Thecontentandtypesofde-fectsintheCNTstreatedusingdiffer-entrouteswereevaluatedbasedonthelocationsoftheDandGpeaks,andtheratioIG/ID.[35]InatypicalRa-manspectrumofCNTs,theDpeakislocatedbetween1330and1360cm–1andisassignedtodisorderedcarbon(defectsandamorphouscarbon),whiletheGpeakisaround1580cm–1andcorrespondstothestretchingmodein

thegrapheneplane.Asshownin

Table1,exceptfortheregrowthroute,

)

m

alltreatmentsledtoanobviousde-ccreaseofIG/ID,suggestingthatCNTs/Sbecomemoredefectiveafterthese( yttreatments.iviInterestingly,annealingCNTfiberstcuinairat480°CdidnotreducetheIDdnpeak,butresultedinanIG/IDratiooCevenlowerthanthatoftheas-spun

fiber.Asamorphouscarbonisreport-edtostartburningat300°C,[18]the

Temperature (K)increaseoftheIDpeakinsteadofa

decreaseafterheattreatmentindicates

Figure4.A)SEMimageoftheCNTfibertreatedunderregrowthconditions,somenewnanotubes

thatthecontributionofthedefectivewerefoundgrowingonthefiber.B)SEMimageoftheCNTfibercoatedwithAunanoparticles.Inset:

sp2carbontotheDpeakisdominantTEMimageshowingthatgoldnanoparticleswereformedsparselyonthenanotubes.C)Compari-sonoftheconductivitytemperaturedependencebetweenthepureCNTfiberandAu-coatedfiber.inthepresentcase.Burningthe

fibersampleinairataround480°C

enabledthedefectivecarbonstobefunctionalizedintocarboxylicgroups

Au-coatedfibershowedthebestconductivityof(COOH),[19,36]

leadingtoahigherDpeakandashiftinits

1150Scm–1

,whichismuchhigherthanthoseofcommercialGpeak.

carbon(285Scm–1)[31]andcarbonfiber(560Scm–1).[27]

ItRefluxingCNTsindiluteHNO3hasbeenreportedtoyieldappearsthatsimpleandmoderatechemicaltreatmentsofetchedcarbonaceousparticlesandtofunctionalizedefectsitestheas-spunCNTfibersresultinremarkableimprovementofCNTsintocarboxylicgroups.[19]Ourexperimentsindicateinelectricalconductivity.thatoxidizingtheCNTsamplein5MHNO3causesmorese-ExceptfortheAu-coatedfiber,alltreatedCNTfibersveredamagetoCNTsthanoxidationinair.Thisalsoshowedshowedaconductivityincreasewithtemperatureovertheen-thelowestIG/IDofthefivetreatments,indicatingthatmore

tiretemperaturerangefrom75Kto300K.Incontrast,asdefectsiteswereformedonnanotubesundertheseconditionsshowninFigure4C,theAu-coatedfiberexhibitedauniquewhereCNTswerefunctionalizedwithC Ogroups.conductivity–temperaturerelationship.Attemperaturesbe-AsshowninFigure4B,dippinganacid-treatedfiberintoa

low250K,theconductivityincreasedwithtemperature,indi-0.01%H4AuCl3solutionspontaneously,butnotdensely,de-catingasemiconductivenature.Attemperatureshigherthanpositsgoldnanoparticlesontothesidewallsofnanotubes250K,theconductivebehaviorchangedtometallic,butthenwithouttheassistanceofanyotherreducingreagent.Thisswitchedbacktosemiconductivewhentemperatureswerekindofreactioncanonlyoccuronfunctionalizednano-above280K.Suchconductivityfluctuationswerealsoob-tubes.[37]TheresultinFigure4Balsoindicatesthatthefunc-servedinDNA-linkedAunanoparticleaggregates[32]andAutionalizationefficiencyin5MHNO3islow,andgoldnanopar-composites,[33]

implyingthatAunanoparticlesplayanimpor-ticleswithanaveragesizearound10nmweresparsely

tantroleintheconductingbehaviorofthefiberathighertem-distributedonthenanotubes.

peraturesbuttheconductanceatlowertemperaturesisdomi-Annealingtheas-spunCNTfiberinhydrogenatmosphere

natedbyCNTs.(Ar+6%H2)atahightemperaturealsoresultedinobvious

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damagetothenanotubestructures.AnincreaseoftheDpeakandaredshiftoftheGpeaksuggestthatthepresenceofhy-drogenathightemperaturesmayhelptoreducedefectivesp2bonds(C C)intosaturatedsp3bonds(C–H).[38]TreatingtheCNTfiberunderregrowthconditionsincludingahydrogenat-mosphereleadstotheformationofsomesmallCNTsonthefiber,butwithadropinIG/IDaswell.

ThedependenceofconductivityofCNTfibersontheirstructurescanbeattributedtothevariationincarrierdensityandp-bondingsystemofCNTscausedbydifferentchemicaltreatments.Bothas-spunandtreatedCNTfibersshowacon-ductivityincreaseatlowtemperatures,suggestingthatthecar-rierdensityincreasedwithtemperature.WhenCNTfibersarefunctionalizedwithcarboxylicgroups,wherethep-conjugatedsystemofCNTscanbemaintained,theCNTsaredopedbytheacceptordopantgroups.Asaresult,theconductivityofthefibersisenhancedduetoanincreaseincarrierdensity.[34]Incontrast,thecovalentcoatingofmetallicAunanoparticlesontoCNTsappearstofurtherincreasecarrierdensity,leadingtoanobviousincreaseinconductivity.Conversely,theforma-tionofsp3bondsinCNTsduringannealinginhydrogen-con-tainingatmosphereathightemperaturesmayinterrupttheplanarp-conjugatedstructureofCNTs,causingaseveredis-ruptionoftheelectrontransportinCNTsandthereforeasharpdecreaseoftheconductivity.[38,39]

Insummary,CNTfibersspunfromarraysofmillimeter-longCNTsarecomposedofcleanandwell-alignedCNTs.Theirconductivebehaviorindicatesthattheyaresemicon-ductinginthetemperaturerangefrom75Kto300K.Furthermore,theyshowbetterconductivitythancommercialcarbon,carbonfiber,andpreviouslyreportedCNTfibers.Theconductionoftheas-spunCNTfiberfitstheT–1/4hoppinglawverywell,indicatingthatelectronsconductalongtheCNTfi-berbya3Dhoppingmechanism.Thecarrierdensityseemstoplayanimportantroleintheirconductionbehavior.

TheelectricalpropertiesofCNTfibersarelargelydepen-dentonstructuralchangescausedbydifferentchemicaltreat-ments.IntroductionofacceptordopantsintoCNTconjugatedsystemsbyoxidizingCNTsinairorHNO3helpstoincreasetheconductivityofCNTfibers,whileannealingCNTsinahy-drogen-containingatmospheremaysignificantlylowertheirconductivityduetotheformationofsp3carbonbonds.Cova-lentcoatingofAunanoparticlesontotheCNTsnotonlysig-nificantlyenhancesconductivity,butalsochangesthecon-ductingmechanism.Theseresultsprovideinsightintotheelectrontransportbehaviorinthep-conjugatedCNTsystem,andusefulstrategiestomanipulatetheelectronicpropertiesofCNTsforpotentialapplicationsinelectronics,sensing,andconductingwires.

anddrawingat5cmmin–1.Thediametersofthespunfibersweremeasuredusingscanningelectronmicroscopy(SEM).

Electricalmeasurementswereconductedusingthefour-probemethodattemperaturesfrom75.4Kto300Kwithasamplinginter-valof0.02K.Theconstantdirectcurrentpassingthroughthefiberwassetat10lA.Inordertobuildupagoodcontactbetweenthethinfiberandtheelectrode,aprepatternedglasssubstratewithfourPtelectrodestripswasmadeusingsputteringthroughashadowmask.ThefourPtstripswere300nmthick,1mmwide,5mmlong,andsep-aratedby1mm.Thefibersweretransferredontotheprepatternedsubstrates.AsillustratedinFigureS1,athinlayerofsilverpastecov-eredthefiberateachPtelectrode.

Received:December27,2006

Revised:May25,2007

Publishedonline:September20,2007

Experimental

Arraysofmillimeter-longCNTsweresynthesizedat750°Cwith100sccmethyleneand100sccmforminggasfor15min.TheCNTfi-berswerespunfromCNTarrayswithaspindlerotatingat2500rpm

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