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

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N

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-temtoinvestigatetheelectricalpropertiesofCN …… 此处隐藏：18354字，全部文档内容请下载后查看。喜欢就下载吧 ……