Low-Temperature Large-Scale Synthesis and Electrical Testing of Ultralong Copper Nanowires.pdf

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pubs.acs.org/Langmuir©2010AmericanChemicalSocietyLow-TemperatureLarge-ScaleSynthesisandElectricalTesting†ofUltralongCopperNanowiresMelindaMohl,PeterPusztai,AkosKukovecz,andZoltanKonya*DepartmentofAppliedandEnvironmentalChemistry,UniversityofSzeged,RerrichB.ter1.,6720Szeged,HungaryJarmoKukkolaandKrisztianKordasMicroelectronicsandMaterialsPhysicsLaboratories,UniversityofOulu,PL4500FIN-90014Oulu,FinlandRobertVajtaiandPulickelM.AjayanDepartmentofMechanicalEngineeringandMaterialsScience,RiceUniversity,Houston,Texas77251ReceivedApril7,2010.RevisedManuscriptReceivedMay21,2010Coppernanowires(NWs)withuniformdiametersandlengthsrangingfromseveralhundredsofnanometerstoseveralmicrometershavebeenpreparedwithhighyieldbyasimplehydrothermalprocedure.TheX-raydiffraction(XRD)andenergydispersiveX-rayspectroscopy(EDS)analysisdataindicatethatthecoppernanowiresarefreeofanycontamination,whiletheelectrondiffraction(ED)analysishasrevealedthenanowirestobesinglecrystals.Thenanowiregrowthmechanismhasalsobeendiscussed.Hexadecylamineisthesurfacestabilizingagentinourmethod,whileglucosefacilitatesformationofsingle-crystallineseedsonwhichthecoppernanowiresgrow.Theelectricalpropertiesoftheas-synthesizedcopperNWshavealsobeeninvestigated.7Introductionvapordeposition,etc.),thecontrollednanowiresynthesisisgene-rallyperformedbygrowingcrystalsinthepresenceofphysicalCopperisprobablythemostfrequentlyusedmetallicnano-templatesorcappingagentsselectivelyadheringtocrystallinewire.Lately,one-dimensional(1D)nanocopperreceivedout-facetssuchassurfactantsand/ororganicpolymers.Applicationsstandingattentionbecauseofitshighelectricalconductance.requirethenanowirestohaveuniformsizedistributionandtobeThispropertymakesitespeciallysoughtafterbythemicroelec-1freeofdefects.Therefore,afacilesynthesisapproachcapableoftronicsindustry.Beingusedinabroadrangeofapplicationsproducinglargequantitiesofnanowireswithtunableaspectratiostartingfromelectriccircuitstoelectricpowerleads,1Dnano-1isingreatdemand.copperofferstrulyuniqueelectricalandthermalproperties.Thereareanumberofmethodsforpreparingcoppernano-Typicalfieldsofintensiveuseincludedevicesconstructedfrom8structures:chemicalvapordeposition,electrochemicaldeposi-nanosizeelectronic,optoelectronic,andmagneticbuildingele-9,1011-13tion,templateormembraneprocesses,reversemicellarments.Somorjaiandco-workershavepioneeredresearchonthe14,15systems,andsoon.Probablythemostwidelyusedmethodforheterogeneouscatalyticapplicationpotentialofcoppernano-2,3thepreparationofCunanocrystalsisthetemplate-directedelec-structuresofcontrolledmorphologyandoxidationstate.Their16,17trochemicalmethod.Itiseasytoperform,andthemorpho-mostimportantresultsarerelatedtouncoveringthedetailsof4logyandcrystallinityofnanowires(NWs)arerelativelyeasilysurfacechemistryoncopperandtothebridgingthegapbetween185adjustable.However,themethodconsistsofseveralsteps,anditmodelcatalystsandrealheterogeneouscatalyticsystems.isgenerallyonlyapplicableforproducingverysmallamountsofToday,itisgenerallyagreedthattheoriginalintrinsicproper-nanowires.tiesofmetalnanostructuresaremostlydeterminedbytheirsize,6shape,composition,structure,andcrystallinity.Intensivere-(7)Liu,Z.;Bando,Y.Adv.Mater.2003,15,303–305.searchhasbeenongoingfornanostructuresofwell-definedshape(8)Choi,H.;Park,S.J.Am.Chem.Soc.2004,126,6248–6249.andstructure,sincesizeandshapeinfluencethephysicalandchemi-(9)ToimilMolares,M.E.;Hohberger,E.M.;Schaeflein,Ch.;Blick,R.;€Neumann,H.R.;Trautmann,C.Appl.Phys.Lett.2003,82,2139–2141.calpropertiessignificantly.Withafewexceptions(e.g.,vacuum(10)ToimilMolares,M.E.;Buschmann,V.;Dobrev,D.;Neumann,R.;Scholz,R.;Schuchert,I.U.;Vetter,J.Adv.Mater.2001,13,62–65.(11)Gerein,N.J.;Haber,J.A.J.Phys.Chem.B2005,109,17372–17385.PartoftheMolecularSurfaceChemistryandItsApplicationsspecial(12)Liu,Z.;Yang,Y.;Liang,J.;Hu,Z.;Li,S.;Peng,S.;Qian,Y.J.Phys.Chem.issue.B2003,107,12658–12661.*Towhomcorrespondenceshouldbeaddressed.E-mail:konya@(13)Yen,M.;Chiu,C.;Hsia,C.;Chen,F.;Kai,J.;Lee,C.;Chiu,H.Adv.Mater.chem.u-szeged.hu.Telephone:þ36-62-544-620.Fax:þ36-62-544-619.2003,15,235–237.(1)Azarian,A.;Irajizad,A.;Dolati,A.Condens.Matter2007,19,446007.(14)Filankembo,A.;Pileni,M.P.J.Phys.Chem.B2000,104,5865–5868.(2)Jernigan,G.G.;Somorjai,G.A.J.Catal.1994,147,567–577.(15)Pileni,M.Nat.Mater.2003,2,145–150.(3)Zhang,Y.W.;Huang,W.Y.;Habas,S.E.;Kuhn,J.N.;Grass,M.E.;(16)Dobrev,D.;Vetter,J.;Angert,N.;Neumann,R.Appl.Phys.A:Mater.Sci.Yamada,Y.;Yang,P.;Somorjai,G.A.J.Phys.Chem.C2008,112,12092–12095.Process.1999,69,233–237.(4)Fu,S.S.;Somorjai,G.A.J.Phys.Chem.1992,96,4542–4549.(17)Gao,T.;Meng,G.W.;Zhang,J.;Wang,Y.W.;Liang,C.H.;Fan,J.C.;(5)Somorjai,G.A.;York,R.L.;Butcher,D.;Park,J.Y.Phys.Chem.Chem.Zhang,L.D.Appl.Phys.A:Mater.Sci.Process.2001,73,251–254.Phys.2007,9,3500–3513.(18)Tian,M.;Wang,J.;Kurtz,J.;Mallouk,T.E.;Chan,M.H.W.NanoLett.(6)Somorjai,G.A.;Tao,F.;Park,J.Y.Top.Catal.2008,47,1–14.2003,3,919–923.16496DOI:10.1021/la101385ePublishedonWeb07/02/2010Langmuir2010,26(21),16496–16502 Mohletal.ArticleIn1996,Setluretal.describedanintriguingnovelmethodofn-hexane(C6H14,Sigma-Aldrich,96.5%),andethanolabs.producinglargequantitiesofcarbonnanotubesandencapsulated(C2H6O,MolarChemical,99,99%)wereusedaswithoutfurthercoppernanowiresinahydrogenarc.19Atthesametimeanewpurification.methodhasbeenpresentedbyFilankemboandPileniwherein1.SynthesisProcess.Inthecourseofatypicalprepara-copperNWsareproducedinamicelletemplatesystem((AOT)2-tionprocedure,copperchloride(0.17g,12.5mM)andglucose(0.391g,∼2mmol)weredissolvedin80mLofdistilledwaterinanisooctane-watersystem)andtheshapeofcoppernanocrystalcanErlenmeyerflask.Thenhexadecylamine(1.44g)wasslowlyaddedbeadjusted/controlledbytheadditionofdifferentsaltswhilethe14tothewatersolutionfollowedbyvigorousmixingfor5hbysametemplateremainsunchanged.magneticstirreruntilalightblueemulsionwasobtained.ThePresumably,hydrothermalmethodsseemtobeoneoftheemulsionwasplacedinaTeflon-linedstainlesssteelautoclaveofsimplestwaysfortheproductionofmetalnanowiresinalarge200mLcapacity.Theautoclavewasrotatedat393Kfor2hunderscale.Liuetal.,whowerethefirstamongthosewhoappliedtheautogenouspressureandthenallowedtocooltoroomtempera-2-Cu(II)-glycerolcomplexinthepresenceofphosphite(HPO3)ture.Theresultingreddishbrown(foxycolor)solutionwasandSDBS,successfullypreparedsome10μmlongcoppernano-centrifuged(2000rpm)andthenwashedwithdeionizedwater,wiresat393Kbyahydrothermalmethod.Inthecourseofthen-hexane,andethanolsequentially.Theprocesswasrepeatedsynthesis,SDBSactsasacappingagent.12Afewyearslater,severaltimestoremovetheexcesssurfactant,andareddishfluffysolidwasobtained.Theproductwaskeptundern-hexanetoavoidZhengetal.preparedporouscoppernanowireswithrectangulartheoxidationofthecopperNWs.crosssectionsinaone-stephydrothermalsynthesisusingascorbic2.Characterization.Themorphologyandtheprogressoftheacidasreducingagentandpvpascappingagentatrelativelylow20,2122synthesiswereassessedbytransmissionelectronmicroscopytemperature.WhileWangetal.preparedultralongsub-(TEM;PhilipsCM10).Samplesweresonicatedinethanolbefore23micrometernanowiresbyapplyingascorbicacid,Shietal.usingbeingdroppedonacoppermountedholeycarbonfilmanddried.octadecylaminesimilarlysucceededinobtainingcoppernano-Thestatisticalcharacterizationofthediametersizedistributionwireswithalargeaspectratio.Inbothcases,theaddedreducingofeachsamplewasperformedbasedontheTEMimages.Theagentactedasthecappingagentaswell.AtthesametimeDengmorphologyandelementalcompositionofthepreparedcopper24nanowireswerecharacterizedbyscanningelectronmicroscopyetal.elaboratedasimplehydrothermalmethodtoproduceinonestepcoppercore/carbonsheathnanocables.Thesecopper@(SEM;HitachiS-4700)andenergydispersiveX-rayspectroscopycarbonnanocablesareformedinthepresenceofcetyltrimethyl-(EDS),respectively.Samplesweredroppedonthesurfaceofapieceofsiliconwaferwhichwasattachedtothealuminumsampleammoniumbromide(CTAB)byahydrothermalreduction/holder.ThepowderX-raydiffraction(XRD)patternswerecarbonizationstep.Inthehydrothermalprocess,CTABplaystheobtainedfrompowdersamplesmountedonglassslidesinakeyroleasastructure-directingagent.RigakuMiniflexIIXRDinstrumentoperatingwithCuKRContrarytoseveralpublications,whichdetailpossibleelec-radiation(λ=1.5406A˚).tricalapplications,availablefortheproductionofcoppernano-3.ElectricalMeasurements.Fortheelectricalmeasure-wiresbyahydrothermalway,thedeterminationofelectricalments,coppernanowiresweredispersedinethanol,sonicated,propertiesofcoppernanostructuresisratherfarfromcompleteinanddrop-castedonaSi/SiO2chipequippedwithPt/Tielectrodestherelevantliterature.Sofar,determinationofcurrent-voltage(15μmspacing).Allthecurrent-temperatureandcurrent-(I-V)characteristicsofcoppernanowirespreparedbyelectro-voltagemeasurementswereperformedinaLinkamTHMS600deposition9,25havebeenperformed.heatingandfreezingstageapplying6K/minheatingandcoolingInthispaper,wedescribeaneffectiveprotocolforpreparingrates.AKeithley2636Asourcemeterwasusedinallresistance-temperature,current-voltage,andresistance-gatevoltagemea-single-crystallineultralongcoppernanowiresinalargescale.Insurements.Thesampleswereinvestigatedinthe150-625Kthenewhydrothermalmethod,glucosereducescopperinthetemperaturerangeunderdifferentgasatmospheres(N2,syntheticpresenceofhexadecylamine(HDA).Inordertoexploretheroleair,H2/Ar,at1barpressure).ThechangeofelectricpropertiesofglucoseandHDAinthesynthesis,differentamountswereduringatransitionofmetalliccoppertoCu(I)2OandCu(II)Oadded.Surprisinglyenough,varyingtheamountofglucosehadasemiconductorwasalsoassessed.dramaticeffectontheshapeofthecoppernanowires.Further-more,theeffectsofreactiontimeontheproductswerestudiedResultsandDiscussionaswell.InordertoexploittheelectricalpropertiesofcopperThesynthesisresultedinareddishbrownsuspensioncontain-nanowires,weaimedtosynthesizecoppernanowireswithuni-ingbothnanowiresandnanoparticles.Aftercentrifugation,aformdiameterandsingle-crystallinenature.Current-voltage,reddishbrownsolidprecipitatewasobtained.Fortheseparationresistance-gatevoltage,andresistance-temperaturecharacteri-26ofnanowiresfromnanoparticles,arotationspeedof2000rpmsticsindifferentgasenvironmentshavebeenstudied.wascriticalduringcentrifugation.Theyieldbyweightafterdryingwasabout50%,indicatingasignificantmateriallossthatpre-ExperimentalSectionsumablyhappenedinthewashingphase.Chemicals.Reagent-gradechemicalscopper(II)chloride1.Phase,Morphology,andStructureCharacterization(CuCl232H2OReanal)puriss,D-glucose(C6H12O63H2O,Re-oftheProducts.AcharacteristicTEMimageofasinglenano-anal),hexadecylamine(C16H35N,Sigma-Aldrich,Tech90%,),wire(Figure1)clearlyrevealsthenanowiremorphology.Electrondiffractionanalysisgaveevidenceofthesinglecrystallinenature(19)Setlur,A.A.;Lauerhaas,J.M.;Dai,J.Y.;Chang,R.P.H.Appl.Phys.Lett.oftheas-synthesizednanomaterial.Themorphologyanddimen-1996,69,345.sionoftheas-preparedproductswerealsoexaminedbySEM.Itis(20)Zhang,X.;Zhang,D.;Ni,X.;Zheng,H.SolidStateCommun.2006,139,shownaSEMimage(Figure1,left)thatthefinalproductis412.(21)Zhang,X.;Zhang,D.;Ni,X.;Zheng,H.Chem.Lett.2006,35,1142.composedofalargequantityofCunanowireshavingauniform(22)Wang,W.;Li,G.;Zhang,Z.J.Cryst.Growth2007,299,158–164.diameterof64(8nmandlengthofafewmicrometers,thus(23)Shi,Y.;Li,H.;Chen,L.;Huang,X.Sci.Technol.Adv.Mater.2005,6,761–765.(24)Deng,B.;Xu,A.;Chen,G.;Song,R.;Chen,L.J.Phys.Chem.B2006,110,11711.(26)Sun,Y.;Yin,Y.;Mayers,B.T.;Herricks,T.;Xia,Y.Chem.Mater.2002,(25)Cao,H.;Wang,L.;Qiu,Y.;Zhang,L.Nanotechnology2006,17,1736–1739.14,4736–4745.Langmuir2010,26(21),16496–16502DOI:10.1021/la101385e16497 ArticleMohletal.Figure1.CopperNWproductsofa24hsynthesisinvestigatedbySEM(left)andTEM(right).Figure2.Leftpanel:PowderXRDpatternsofcoppernanowiresaftersynthesis(bottomcurve)andcoppernanowiresexposedtoair(topcurve).Rightpanel:dspacingversus1/MbasedonXRDmeasurements.featuringanaspectratioabove50.Consequently,themethodCuOappeared.PeaksrelatedtopotentialcontaminatingspeciesmentionedaboveisapplicablefortheproductionofuniformCuorCuOwereabsentfromtheXRDprofile,indicatingthatpureNWsinbulkquantities.Asshownintheimages,theCuNWsmetalliccopperproductswereobtained.However,thecolorofproducedinourlabaremostlystraight;nevertheless,someofcoppernanowiresturnedfromfoxytodrabduringprolongedthemdohaveangularandbendingsectionsasshownintheTEMstorageinair.Thisphenomenoncouldbeavoidedbystoringtheimage(Figure1,right).Wetentativelyassumethattheangula-CuNWsunderethanol.XRDanalysisofthediscoloredproductsritiesarebroughtaboutbythesimultaneousgrowthofdifferentverifiedthepresenceofoxidizednanowiresasshowninFigure2,facetsoftheseednanoparticles.EventhoughtheEDpatternsleft:aminorpeakappearedat36.5°whichcanbeattributedtothegaveevidenceforthecoppernanowirestobesinglecrystallinein{111}crystalplaneofCu2Oascribedtotheoxidizedsurfacelayernature(insetinFigure1),thestraightsectionsofNWsshowofthenanowires.ItislikelythattheoxygendissolvedinthewaterangularitiesandotherformsasshowninFigure1.Thus,basedonusedintheHDAremovalstepinitiatestheformationofathinTEMimages,itisassumedthatstraightandangularNWsareCu2Olayeronthesurfaceofthenanocrystal.Insummary,thecomposedofmicrometerlongsinglecrystalsorassembliesofXRDpeakpositionswereingoodagreementwiththevaluessuchsinglecrystals,respectively.Moredetailswillbegivenbelow.reportedintheliteratureandconfirmedthecrystallinenatureofThechemicalcompositionofnanowireshasbeeninvestigatedbycoppernanowires.EDS.AtypicalEDSspectrumshowspresenceofAl(1.48keV),Si2.GrowthMechanismoftheCuNanowires.Toinvesti-(1.74keV),andCu(8.07and8.92keV)lines.Thepresenceofthegatetheinfluenceoftheamountofthesurfactant(HDA)andtheCuKRlineintheEDSspectrum(Figure1)confirmsthereducingagent(glucose)onthemorphologyofthefinalproduct,formationofCunanowires,whereastheSiandAllinesoriginatecoppernanowireswithdifferentmolarratioswerepreparedbyfromthesiliconsubstrateandaluminumsampleholder,respec-varyingtheamountofHDAorglucosewhilekeepingthetively.amountsofwaterandCuCl2*2H2Ofixed.Figure2depictstheXRDpatternofcoppernanowirefilmsa.AmountofGlucose.Atthemolarratioofglucose/copperdrop-castedonaglasssubstrate.Coppernanowireswereidenti-1:1,mostlynanowireswereformed.Copperparticlesofotherfiedonthebasisofthethreeclearlydistinguishablediffractionmorphologieswerepracticallyabsentfromthisproduct(Figure3,peaksat2θ=43.3°,50.4°,and74.2°correspondingtothe{111},sampleB).Uponincreasingtheratioofglucosebyafactorof{200},and{220}crystalplanesofface-centeredcubic(fcc)1.5-2,moreandmorenanoparticlesofdiverseshapesandsizescopper,respectively.ThelatticeconstantofthiscubicphaseappearedintheSEMimages(Figure3,sampleCandD).Atthewasfoundtobea0=3.619A˚whichagreeswellwiththelitera-ratioofglucos/copper4:1,theproductconsistedalmostexclu-12turevalueofa0=3.615A˚.IntheX-raydiffractograms,neithersivelyof50-300nmnanoparticlesandhardlyanynanowiresanypeakprovingpresenceofcontaminatingspeciesnorthatofwereformed.Supposedly,athighglucosecontents,onlyafew16498DOI:10.1021/la101385eLangmuir2010,26(21),16496–16502 Mohletal.ArticleFigure3.SEMimagesofcoppernanowiressynthesizedwiththefollowingglucose/coppersaltratios:(A)0.2:1,(B)1:1,(C)3:1,and(D)4:1.seedsareactiveinwiregrowth.Decreasingtheamountofglucoseyetturnedintoreddishbrownwithincreasingsynthesistimeto1/10yieldedexclusivelynanowires;however,abluesuspension(SupportingInformationFigureSI-2).TEMimagesofsampleswasformedwithalittleamountofreddish-brownprecipitate.Itissynthesizedfor1,3,and6hareshowninFigure4.Inthecaseofassumedthatinthiscaseonlyasmallamountofcopperwas1hsynthesis,thesamplesweremostlycomposedof2.1(0.7nmreducedduringsynthesis.TheSEMimage(Figure3,sampleA)coppernanoparticlesandhardlyanynanowiresappeared.Atthisclearlyrevealstheresultingcrystaldefectsofthesewires.earlystage,theHDAisobservableasplanesintheTEMimages.b.InfluenceoftheRatioofHDAtoCuCl23H2O.TheWhereasinthecaseof2and3hsamples,somecoppernanowires23,28morphologyandtheaspectratioofthenanowireswerefoundshowupandthestructureofHDAappearstobecoiledrathertobeacomplexfunctionofthemolarratioofHDAtocopperthanplanar.SEMimages(SupportingInformationFigureSI-3)chlorideasevidencedbySEM.(FigureSI-1intheSupportingconfirmedthecoiledstructureofHDAwrappedaroundtheInformationdepictscoppernanowirespreparedinthepresencecoppernanowires.TheouterdiameterofthewholestructureofvariousamountsofHDAwhilekeepingallothersynthesiswas214(39nm,andthewrappednanowiresmeasured31(parametersconstant.SamplesA,B,C,andDcorrespondto7nmindiameter.Consequently,thegrowthofcoppernanowires50%,75%,125%,and150%oftheoriginalHDAamount,res-issupposedtobeinitiatedbytheformationofthecopperpectively.)ItisassumedthatincreasingtheamountofHDAnanoparticles,thatis,single-crystallineseedsappearingatthemightcausetoohighsurfactantcoverageontheinitialNWseedsearlystageofgrowth.HDAplaysanessentialroleinthewhereasaninsufficientHDAamountisunabletoeffectivelynucleationprocess.Theimageofasamplecontainingeitherno26passivatetherequiredfaces.BothcasesresultinaratherHDA(B)ornoglucose(C)isshowninSupportingInformationisotropicgrowth.FigureSI-2.Thecolorofthemixtureindicatesthatnonanowiresc.Time-DependentAnalysis.Sincethegrowthmechanismareformedintheabsenceofglucose.Wesuggestthatglucose,asaofcoppernanowiresisnotyetfullyunderstood,hydrothermalreducingagent,playsasignificantroleinthesynthesis.Itissynthesesofvariousdurationswereperformedinordertogainresponsibleforbringingabouttheseeds,thatis,thenanoparticlesinsightintothegrowthkineticsofcoppernanowires.Thepro-fromwhichthenanowiresgrowatlaterstages.IntheabsenceofductswereinvestigatedbyTEM,SEM,andXRD.ProductsHDA,alightyellowsuspensionisformedthatyieldsawhiteobtainedafter1-72hhydrothermaltreatmentat393Kexhibitedprecipitateafter1day.Nocoppernanowireswereidentifiedinalmostidenticalmorphologies,indicatingthatsynthesistimeisthissample.Inthecomplex-surfactant-assistedhydrothermal12notacriticalfactorundertheseexperimentalconditions.methodpresentedbyLiuetal.,theroleofHDAislikethatd.SuggestedGrowthMechanism.Thekeyparametersde-ofSDBS.Firstofall,ithinderstheagglomerationofcopperterminingthefinalshapesofnanocrystalsareknowntobethenanoparticlesattheearlysynthesisstages.Moreover,thesinglecrystallographicstructureoftheseedsinthenucleationprocesscrystallinenatureofCuNWsisalsoattributedtothepreferential27andinthefollowingstagesofgrowth.ThecoloroftheproductadsorptionofHDAonselectedcrystallographicfacetsofcopperCuNWsuspensionswasbluishgreenafter1-2hsynthesistime(28)Li,Y.;Li,X.;Deng,Z.;Zhou,B.;Fan,S.;Wang,J.;Sun,XAngew.Chem.,(27)Wiley,B.;Herricks,T.;Sun,Y.;Xia,Y.NanoLett.2004,4,1733–1739.Int.Ed.2002,41,333–335.Langmuir2010,26(21),16496–16502DOI:10.1021/la101385e16499 ArticleMohletal.Figure4.TEMimagesofsamplesatdifferentstagesofhydrothermaltreatment(A)1h,(B)2h,(CandD)3h,(E)6h,and(F)24h.nanoparticles.EventhoughEDresultsindicatedthatthenano-meltingandcrystallizingduringtemperaturecycles.Sampleswiresaresinglecrystals,theymaycontainangularandbendingheatedinairatmospherewereoxidizedasshownbythefieldsectionsaswell.ThisapparentcontradictioncanberesolvedbyemissionSEM(FESEM)andEDSmeasurements.Inthecourserealizingthatearlyinthesynthesis(3h,Figure4D)thereactionoftemperaturecycles,theresistanceincreasedanddecreasedmixturecontainswirecompositesofmicrometersizethataresignificantlyinsynchronywiththefirstfewoxidation-reductionformedbytheadhesionofseveralseeds,thatis,nanoparticles.stages.Insomesynthesisproducts,spottedwirescouldbeobservedTheFESEMimagesofselectedsamplesA-Dareshownin(SupportingInformationFigureSI-4).FurtherinvestigationisSupportingInformationFigureSI-5.SampleAistheoriginalneededtogiveapreciseandreliableaccountofthegrowthmecha-CuNWbeforetemperaturetreatment,sampleBreducedinannismofsuchnanowires.Twoformationmechanismsappeartoatmosphereof5%H2inargon,andsampleCoxidizedinairandbeplausible.Eitherthenanowireandnanoparticlegrowtharesubjectedtothefollowingheatcycleswitharateof6K/min:competitiveprocessesleadingtocomplexNWsurfacesdueto300Kf550Kf350Kf550Kf300K.SampleDwasproximityinducedaggregationandrecrystallizationreactions,orexposedtoseveralheatcyclesupto625Kinairandin1000ppmthespottedNWistheresultofdirectgrowthcomplicatedbyH2inargon.Whileminorclusterformationisobservableevenonlocallyvaryingfacetgrowthspeedsduetoconcentrationfluctua-sampleA,onsamplesB(treatedat550KinH2)andC(550K,air)tionsalongthewireaxis.theclusterformationismuchmoreintensive.Byfar,thebiggest3.ElectricalMeasurements.a.Oxidation/ReductionclusterswerefoundinthecaseofsampleD,whichwasrepeatedlyStudyoftheCopperNWs.Thenanowireswereexposedtoheatedto625Kinbothhydrogenandairatmospheres.differenttemperaturetreatmentsunderalteringgas(N2,air,H2/TheCu/OatomicratioscalculatedfromEDSmeasurementsAr)environment.Evenatarelativelylowtemperatureof550K,areshowninTable1.ItisclearlyseenthatsampleAcontainsonlyclusterswereformedonthesurfaceofthenanowires.Supposedly,asmallamountofoxygen,supposedlyduetoathinsurfaceCu2OtheyarebroughtaboutbyHDAresiduesorothercontaminantslayer.ThelowoxygencontentsofsampleBmayalsoberelatedto16500DOI:10.1021/la101385eLangmuir2010,26(21),16496–16502 Mohletal.ArticleFigure5.I-Vcurvesfordifferentgatevoltagesinroomatmo-Figure6.Resistance-temperaturesweepinnitrogenatmosphere.sphereofthefreshlypreparedsamplesbeforeanytemperatureorTemperaturewasswept300Kf500Kf150Kf500Kf150Kgastreatments.Insetshowssource-drainresistanceversusgatef300K.Note,becauseofthelowcurrentsthroughthissample,voltageat50mVSDbias.resistancevaluescouldnotbeacquiredreliablybelow230K.Table1.Cu/OAtomicRatiosCalculatedfromEDSMeasurementsCuOsampleA8.41sampleB9.71sampleC3.31sampleD1.21asurfaceoxidelayer.Ontheotherhand,insampleC,veryhighoxygencontentisdetected,mostprobablycausedbytheoxida-tionofcopperintheairatmosphere.LetusnoteherethatonsampleCthesizeoftheclustersisbiggerandtheirsurfaceisrougherthanthatofsampleB.ThiscorrelateswellwithoxideformationonthesurfacethatexpandsandmergesnanowiresFigure7.Resistance-temperaturecurvesmeasuredinnitrogentogether.SampleDwasexposedtoseveralheatcyclesinbothatmosphere.Temperaturewasswept300Kf150Kf500Kf150Kf500Kf300K.Inset:Arrheniusplotforconductanceinoxidativeandreductiveatmosphere.Thissamplehasclearlythethetemperaturerange150-500K.biggestclustersofallwhichisexplainedbythehighertemperatureof625Kandthelongerdurationoftheheattreatmentsmakingofconductance,twoenergybarrierscouldbedistinguishedclusterformationandgrowthpossible.Theatomicratiowas(0.16and0.29eVforthelowandthehightemperatureregimes,foundtobeCu/O=6:5forthissample.Theoxygencontentwasrespectively)whichmightberelatedtoSchottkycontactseitherveryhighinspiteofthereductiveatmosphere(1000ppmH2)attheinterfacesofsemiconductingmetaloxidenanowiresandCuappliedinthelasttreatment.Consequently,theintensiveclusternanowiresoratthePtelectrodes;however,wecannotexcludetheformationappearstopreventnanowirereduction.possibilitythattheexponentialtemperaturedependenceofcarrierInconclusion,itisclearthatCunanowiresformclustersevenconcentration,typicalforsemiconductingmaterials,alsoplaysaat550K.Twopossibleexplanationscanbesuggestedfortherole.Earlier,Singhetal.haveobservedtheI-Vcharacteristicsofformationofclusters:(i)TheremightbesomecontaminationleftCu-Senanowireheterostructurestoexhibitaresonanttunnelingonthesurfaceofthenanowireswhichgetsmeltedorfusedduring29diode(RTD)behavior.theheatingtreatments.Inviewofthecircumstancesofthepre-Inthenextstep,thereductionofnanowirestometalliccopperparationofthenanowires,HDAresiduesonthesurfaceorglu-wasattempted,whilemeasuringthechangeofresistancewhilecosemightbringaboutthisphenomenon.Thisseemstobethethetemperaturewasvaried(Figure8a)in1000ppmhydrogenmostlikelyexplanationforclusterformation.(ii)Inprinciple,theatmosphere.Reductionofthecopperoxidetometalliccopperwell-knownphenomenonofmeltingpointloweringinnano-happensaround570K.Theresistanceofthereducednanowirestructurescouldalsoberesponsibleforclusterformation.How-network(Figure8b)wasfoundtobequitelow,exhibitinglinearever,bulkcoppermeltsat1358K,atemperaturesignificantlyI-Vcharacteristicswithnogatetunabletransport,andshowedhigherthantheoneoccurringinourexperiments(625K).alineartemperaturedependencewithapositivetemperatureTherefore,thisargumentdoesnotseemtoberelevantenough.coefficient(Figure8c),bothtypicalformetals.b.ResultsoftheElectricalMeasurements.TheI-Vchar-Uponoxidationofthereducedwiresinsyntheticairatmo-acteristicsshowninFigure5arelinearandindependentofthesphere,theresistanceconsiderablyincreasesaround450Kgatevoltageasisexpectedforametallicconductor.Thesamples(Figure9a).Notethattheoxidationtemperatureishigherthanwereverysensitiveforoxidationinair.InFigure6,thesamplethatinFigure6andtheresistanceresulthereissignificantlylowerseemstobeoxidizedat360K,whichiscausedbyalowamountofthanthatafterthefirstoxidation.Figure9bpresentstheresultsoflaboratoryairmixedintotheinertatmosphereofthemeasure-attemptedreductionsofthesample(curves1-3)in1000ppmmentchamber.Seemingly,thesamplereachesanoxidizedstateathydrogen.Itappearsthatthesamplecannotbereducedbackabout360Kastheresistancesuddenlyincreasesbyagreatextent.tothemetalliccopperstatewiththeseheatandgastreatments.ItwasnotpossibletochangeitbacktoitsoriginalstateviafurtherTheirreversibilityintheprocessismostprobablycausedbytemperaturechanges.Figure7showsexponentialtemperaturetheformationofnanowireclustersshowninFESEMimagesdependenceoftheresistancefortheoxidizednanowiresmeasuredinnitrogenatmosphere.FromtheArrheniusfit(insetinFigure7)(29)Singh,R.;Kumar,R.;Chakarvarti,S.K.Phys.E2008,40,591–593.Langmuir2010,26(21),16496–16502DOI:10.1021/la101385e16501 ArticleMohletal.Figure8.(a)Changeoftheresistanceofthenanowirenetworkwhenthenanowiresarereducedin1000ppmH2inargon.(b)Source-draincurrent-voltagecurvesfordifferentgatevoltagesinlaboratoryairatmosphereafterreduction.(c)TemperaturedependenceofresistanceforthereducednanowiresnetworkmeasuredinN2atmosphere.Figure9.(a)Changeofresistanceduringoxidationofthenanowirenetworkinsyntheticair.Temperaturewassweptinthesequence:300Kf625Kf350Kf625Kf300K.(b)Resistanceplotasafunctionoftemperaturewhileattemptingtoreducetheoxidizednanowiresin1000ppmH2.(c)Current-voltagecurveinlaboratoryairatmosphere;measurementswerecarriedoutaftertheoneshadbeenfinishedinpanels(a)and(b).(SupportingInformationFigureSI-5).Asaresultofclusterfor-reducedandreoxidizedagainafterthefirstoxidation-reductionmation,thecoppercannotbeexposedtothegasinitsenviron-cycle.Insubsequentcycles,theagglomerationofnanopar-mentandtherebyitcannotbereducedproperly.ItispossiblethatticleswasobservedandeffectiveredoxstepscouldnotbeHDAresiduescoverthesurfaceofthenanowireshinderingperformed.Thisphenomenonhindersconsiderablythepoten-furtherredoxcycles.Themeasuredpoorsampleconductivitytialapplicabilityofcoppernanowiresasgassensors.OnthewithnonlinearI-Vbehaviorjustifiesthatthereductionattemptotherhand,oncethewiresareprotectedfromthesurroundingfailedandthenetworkiscomposedofsomeoxidephasesoftheenvironment,theymayfindapplicationsinelectricalintercon-nanowires(Figure9c).nectsaswellasinthermalcompositesduetotheirexcellentthermalpropertiesinheritedfromthebulkcopperphase.FurtherConclusionsresearchisnecessarytofindamoreeffectivepurificationofnanowires.Coppernanowireswithalargeaspectratiohavebeensuccess-fullyobtainedbyasimplehydrothermalmethod.Intheprocess,Acknowledgment.ThisworkwassupportedbytheAcademyglucoseactsasareducingagentandhexadecylaminefunctionsasofFinland(Projects1120853and1128626)andTEKES(Projectsatemplatemolecule.Ourresultsindicatethatbothglucoseand52478,52433,and52467).ThefinancialsupportoftheHungarianHDAplayasignificantroleinthereactionandhaveastrongScientificResearchFund(OTKA)throughProjectsNNF-78920influenceoncoppernanowireformation.Theelectricalpropertiesand73676isacknowledged.ofcoppernanowireshavealsobeeninvestigatedandwerefoundtobesimilartothosereportedforelectrodepositedCunanowires;SupportingInformationAvailable:AdditionalSEM,FES-however,duetoclusteringandmultiplecontactsbetweentheEM,andTEMimages;visualcomparisonofselectedreac-wiresinourfilms,theoverallconductionprocessismorecomplex30tionmixturesinvolvedinthehydrothermalsynthesisofthanthatforindividualnanowires.Thesamplecouldnotbecoppernanowires;plotofHDAcoverageoncoppernano-wires.ThismaterialisavailablefreeofchargeviatheInternet(30)ToimilMolares,M.E.;Hohberger,E.M.;Schaeflein,Ch.;Blick,R.H.;€Neumann,R.;Trautmann,C.Appl.Phys.Lett.2003,82,2139.athttp://pubs.acs.org.16502DOI:10.1021/la101385eLangmuir2010,26(21),16496–16502