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QuasiInSituPolymerizationToFabricateCopperNanowire-BasedStretchableConductorandItsApplicationsTaoWang,RanranWang,*YinCheng,andJingSun*TheStateKeyLabofHighPerformanceCeramicsandSuperneMicrostructure,ShanghaiInstituteofCeramics,UniversityofChineseAcademyofSciences,1295DingxiRoad,Shanghai200050,P.
R.
China*SSupportingInformationABSTRACT:Stretchableelectronicshaveprogressedgreatlyandhavefoundtheirwayintovariousapplications,thusresultinginagrowingdemandforhigh-qualitystretchableconductors.
Poly(dimethylsiloxane)(PDMS)isthemostlyfrequentlyexploitedelastomericsubstratefortheconstructionofastretchableandconductiveplatformbecauseofitsvaluablefeatures,suchassuperbstretchability,hightransparency,andreliablebiocompatibility.
However,theweakadhesionbetweenthePDMSsubstrateandtheconductivecomponentshasalwaysbeenanintractableissuewhichunderminesthegoodandstableperformanceoftheresultantdevices.
Weproposedaquasiinsitupolymerizationmethodtoeectivelybuildatightandstableattachmentbetweencoppernanowire(CuNW)andthePDMSsubstrate.
TheCuNWs/PDMSconductorsshowexcellentconductivityandantioxidationstability(R/R0inair),enhancedinterfaceadhesion,andstretchability(80%strain,R/R05),withoutanycomplicatedprecongurationofthePDMSsubstrates.
Forapplicationdemonstration,theCuNWs/PDMSconductorwasdeployedasthestretchableelectricwiringtoilluminatealight-emittingdiode.
Furthermore,astretchablecapacitivestrainsensorwasfabricatedusingtheCuNWs/PDMSaselectrodes.
Thesensorpossessedagaugefactorof0.
82andtheminimumdetectionlimitof1%strain.
KEYWORDS:coppernanowires,PDMS,quasiinsitupolymerization,stretchableelectrode,strainsensor1.
INTRODUCTIONStretchableelectronicswithunconventionalmechanicalper-formancesaregainingnumerousscienticandsocialinterestsinthelast15years.
1,2Intensiveeortsbymaterialscientistsandengineershaveopenedupaspectrumofsophisticatedapplicationelds,includingelastomericcircuits,2,3exible/stretchabledisplays,4exible/stretchableenergy-relateddevi-ces,5,6electroniceyecameras,7smartskins,8muscle-likesoftactuators,9,10implantabledevicesforhumanhealthmonitor-ing,11pressure/strainsensorsforhuman-motiondetection,12,13andsoon.
Stretchableconductorwithbothhighconductivityandstretchabilityisoneofthekeybuildingblocksofstretchableelectronics.
14,15Stretchableconductorsincludetwonecessarymotifs:electricalconductivecomponentsandstretchablesubstrates.
Torealizetheirwideapplicationsforbending,stretching,twisting,andotherdeformationontocomplexandnonplanarsurfaces,exiblenanomaterialscouldbeasuitablealternativeaselectricalconductivecomponentsduetotheirpredominantcompliancecomparedwiththeirbulkcounterparts.
16,17Manyeortshavebeenmadetostudyavarietyofnewnanomaterialsaselectricalconductivecomponentssuchasconductingpolymers,18,19carbonnanotubes,12,13,20graphite,21andmetalnanomaterials.
2,17,2224However,theinstabilityandpoorconductivityofconductivepolymersandtherelativelylowelectricalconductivityofcarbon-basednanomaterialshavebecomethemainlimitationstotheirperformanceandapplications.
Amongthenanomaterials,metallicnanowires(NWs)couldbeverypromisingcandidatesforelectricalconductivecomponentsinstretchableconductors.
Althoughsilvernanowires(AgNWs)havebeenintensivelyinvestigatedformanyyears,thehighpricehinderstheirlarge-scaleapplications.
Veryrecently,coppernanowires(CuNWs)havebecomearesearchfocussincecopperpossessescomparableconductivitywithAg,andisalmost100timescheaperthanAg.
25Polydimethylsiloxane(PDMS)isaverycommonstretchablesubstrateandhasbeenbroadlyinvestigated.
15,17Itpossessesnotonlyhighstretchabilityanddurabilitybutalsogoodthermalstability,biocompatibility,andchemicalinertness.
26Additionally,transparencyandcost-eectivefabricationprocesscouldalsobeadvantageousforPDMS.
27,28AlthoughPDMSpossessessomanysuperiorproperties,theweakadhesionbetweenPDMSandelectricalconductivecomponentsisstillasevereproblemconstrainingitsapplicationsinstretchableelectronics.
Thisisattributedtothesuperhydrophobicityofitssurfaceandfastrecoveryfromhydrophilictohydrophobicsurfaceafterbeingtreatedextensivelybyoxygenplasma.
29TheReceived:November17,2015Accepted:February19,2016Published:February19,2016ResearchArticlewww.
acsami.
org2016AmericanChemicalSociety9297DOI:10.
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5b11143ACSAppl.
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Interfaces2016,8,92979304shortcomingsofPDMSimpairtheelectricalpropertiesofstretchableconductors.
Evenworse,theconductivematerialsonPDMScouldbedetachedeasilyfromthesurfaceswhenexternalforceisapplied.
WiththepurposeofenhancingadhesionofthevariousconductivematerialswithPDMS,alotofmeasureshavebeenadopted,suchassurfacefunctionalizationandinsitupolymerization.
W.
C.
Kimetal.
andW.
S.
Kimetal.
modiedthesurfaceofPDMSusingsilaneandpolydopamine,respectively,andenabledthesurfacewithhydrophilicitylastingforalongtime.
ThemethodconsiderablyimprovedtheattachmentofAgNWsontoPDMS.
30,31However,theprocessofchemicalmodicationisrathercomplexandnoteasytocontrol.
Zhuandco-workerreportedthatbyinsitupolymerizationAgNWswereembeddedintoPDMSsubstrates,obtainingAgNWs/PDMSconductorswithhighstretchability.
32Itcanmarkedlyimprovetheadhesion;unfortunately,itisdiculttobetransplantedtotheCuNWs/PDMSsystembecauseCuNWswouldbefullyembeddedandlosetheirconductance.
Tothebestofourknowledge,therearenoreportsdemonstratinggoodattachmentofCuNWswithPDMSyet,whileonlyalittleworkisinvolvedintheweakattachmentofCuNWsonPDMSsubstrateswithoutanytreatment.
17Therefore,searchingafacile,cost-eective,andscalablefabricationmethodtoenhancetheadhesionofCuNWsonstretchablePDMSsubstrateisstillurgent.
ItwasfoundthatthestickinessofPDMSsubstratesvariedwiththepolymerizationtime,whichwouldaecttheattachmentofnanowiressignicantly.
Enlightenedbythisphenomenon,herein,anovel,cost-eective,andeasilyscalablefabricationmethod,quasiinsitupolymerization,composedofthreemainstepsincludingprepolymerization,transferofnanowirenetworks,andpost-treatmentweredeveloped.
Viaregulatingtheprepolymerizationtime,partembedmentofCuNWsinPDMSsubstrateswererealized,whichendowedstrongadhesion,excellentconductivityandstability(R/R0inair),enhancedstretchability(80%strain,R/R05),andbendabilityofCuNWs/PDMScompositeconductors.
Figure1.
(a)SEMtopviewimageofpristineCuNWscoatedonsilicon.
TheinsetshowsahomogeneouslydispersedCuNWink;(b)ThemagniedSEMimageofpristineCuNWs;(c)TheschematicfabricationprocessofCuNWs/PDMS;(d,e)SEMtopviewimagesandmagniedSEMimagesofCuNWsnetworksonPTFEltersbeforeandafterheatannealingat230°C.
TheinsetsshowphotographsofCuNWsnetworksbeforeandafterheatannealing.
ACSAppliedMaterials&InterfacesResearchArticleDOI:10.
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Interfaces2016,8,929793049298Stretchablelight-emittingdiode(LED)circuitandstretchablesensorswithCuNWs/PDMSconductorswerefabricatedtodemonstratethefeasibilityofthisapproach.
Theresultingcapacitivesensorwhichcandetectstrainaslowas1%andhasquitehighsensitivity(gaugefactor0.
82,closetothetheoreticalvalueof1)surpassesthepreviousresults.
Itisbelievedthatthequasiinsitupolymerizationmethodcanbeextendedtoothermaterialsandndmoreapplicationsinexibleandstretchabledevices.
2.
EXPERIMENTALSECTIONSynthesisofCuNWs.
CuNWsweresynthesizedbythemethodweproposedinpreviousstudies.
33,34Hexadecylamineandcetyl-trimethylammoniumbromideweremixedandheatedtoformaliquid-crystallinemediumastubularstructuretoguidethegrowthofCuNWs.
Thentheprecursor,copperacetylacetonate[Cu(acac)2],wasaddedintothemediumandthereactionsystemwasincubatedforacertainperiodoftime.
Thereafter,acertainamountofPtnanoparticleswithdiameterof5nmformedbyreducingPtCl2withethyleneglycolwereintroducedtocatalyzethereductionofCu2+.
After12hofreactionat180°C,CuNWsweresynthesized.
Afterbeingrinsedwithtolueneseveraltimes,thenanowireswerekeptintoluenewithaconcentrationof0.
28mg/mL.
PreparationofCuNWs/PDMSCompositeFilms.
CuNWssuspensionsof5mLwereltratedtoformlmsonpolytetrauoro-ethylene(PTFE)ltermembranes.
ThentheCuNWs/PTFElmswereheat-treatedatvarioustemperatures(190,210,230,250,and290°C)inaquartztubefurnacefor30minunderaconstantowofFigure2.
(a)SheetresistancechangeoftheCuNWs/PDMSastheprepolymerizationtimeisprolonged.
Thetopviewandcross-sectionalSEMimagesof(b)CuNWs/PDMS-3,(c)NWs/PDMS-5,(d)NWs/PDMS-10,(e)NWs/PDMS-15,(f)CuNWs/PDMS-20,and(g)CuNWs/PDMS-30.
(h)Adhesivestrengthtests(3Mscotchtapetest)ofCuNWs/PDMS-3,CuNWs/PDMS-5,CuNWs/PDMS-10,CuNWs/PDMS-15,CuNWs/PDMS-20,andCuNWs/PDMS-30.
ACSAppliedMaterials&InterfacesResearchArticleDOI:10.
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5b11143ACSAppl.
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Interfaces2016,8,929793049299hydrogentoremoveorganiccomponentsandoxidesoverCuNWstogainconductivenetworks,andtheeectofannealingtemperatureonconductivityofCuNWslmswasinvestigated.
LiquidPDMS(Sylgard184,DowCorning)wascastontoglassslidesbymixingthe"base"andthe"curingagent"witharatioof10:1,followedbycuringat80°Cfordierenttimes(3,5,10,15,20,and30min)untilslightlystickyPDMSsubstrateswereobtained.
Theeectofprepolymerizationtimewasalsostudied.
TheannealedCuNWs/PTFElmswereplacedcarefullyontothePDMSsubstrateswithCuNWsfacedown,andthentheywereputintoavacuumovenfor2htopolymerizecompletely.
CuNWs/PDMScompositelmsweregainedafterpeelingoPTFEandtheywerecutintostripswiththewidthof10mmforelectricalandmechanicalpropertiestests.
FabricationofCapacitiveStrainSensor.
ACuNWs/PDMSlmwiththeCuNWssurfacefacingdownwasplacedonaglassslide.
ThePDMSliquidwasspreadoverthesurfaceoftheCuNWs/PDMSlm.
Beforecuring,anotherCuNWs/PDMSwiththeCuNWssurfacefacingupwaslaminatedontothewetPDMSandorientedtomakesurethatbothpatternsarefullyaligned.
Thenthewholepiecewasdegassedandcuredthermallyinavacuumovenat80°Cfor2h.
TheCuNWs/PDMSsubstratewascutintostripstoproducecapacitivesensorsfortesting.
Structural,Electrical,andMechanicalCharacterization.
ThemicromorphologyandEDXanalysisofCuNWswereconductedwithaeld-emissionscanningelectronmicroscope(HitachiS-4800).
ThesheetresistanceofsampleswasmeasuredwithMitsubishiChemicalLaiesta-EPMCP-T360.
Thevolumeresistancewasmeasuredusingatwo-probemethodwithaFLUKE-15Bdigitalmultimeter.
TheresistanceversusstrainofCuNWs/PDMSwasanalyzedwiththecombinationofPrincetonAppliedResearchParastat2273andahigh-precisionelectronicuniversaltestingmachine(CMT6103,MTSSystems(China)Co.
,Ltd.
).
ThebendabilityoftheCuNWs/PDMSwasmeasuredbyamovablestage(ZXT_050-300_MA06(China),ShanghaiZhengxinOptio-electricalTechnologyCo.
Ltd.
).
Fouriertransforminfrared(FTIR)spectroscopydatawerecollectedwithaFTIRspectrometer(BrukerTensor27),whichwasequippedwithanattenuatedtotalreectanceaccessoryandsettledinagloveboxlledwithAratmosphere.
RamanmeasurementswerecarriedouttostudytheoxidationstateofCuNWslmsbyusinga532nmlaserunderambientconditionswithaRamanmicroscope(ThermoScienticRamanDXR),andthelaserspotandpowersizewere0.
7μmand8mW.
ThermogravimetryanalysiswascarriedoutwithNETZSCHSTA449Cinthetemperaturerangeof40500°Cataheatingrateof10°C/mininnitrogen.
ThecapacitanceofcapacitivestrainsensorwascharacterizedwithYD2616Dcapacitancetestingapparatus(YangziElectronicCo.
Ltd.
(China))at1kHzfrequencywitha0.
1Vacsignal.
3.
RESULTSANDDISCUSSIONFigure1adisplaysthetop-viewSEMimageofpristineCuNWsonasiliconsubstrate,whereadensenetworkoflongCuNWsisformed.
Fromtheinsetwecanndthat,unlikeotherbundledCuNWsreported,35homogeneousstablenanowiredispersionwithoutanyvisibleaggregatescanbeeasilyobtainedviamildsonication.
Throughcalculationofrandomlyselected50nanowiresfromSEMimage(Figure1b),theaveragediameteroftheCuNWsis60nm.
Thelengthsofthenanowiresvaryfrom10sto100sofmicrometers.
Figure1cillustratestheschematicfabricationprocessofCuNWs/PDMSstretchableconductors.
Initially,CuNWswerecollectedonPTFEltersthroughvacuumltration.
Afterannealingtreatment,liquidPDMSwascastontoglassslides,followingbycuringat80°CforaperiodoftimeuntilslightlystickyPDMSsubstrateswereobtained.
ThentheCuNWs/PTFElmswerecarefullytransferredontothesubstratesandtheywerefullypostpolymerizedat80°Cfor2hinavacuumoven.
Finally,beingpeeledofromthelters,theCuNWs/PDMScompositelmswereobtained.
TheroughsurfaceofCuNWsshownfromthemagniedSEMimage(Figure1d,inset)implicatedtheexistenceoforganicresiduesandoxides,whichwasfurtherconrmedbyFTIR,energy-dispersiveX-rayspectroscopy,andRamananalysis(FigureS1,SupportingInformation).
Theorganicresidueswillincreasethecontactresistancebetweennanowiresgreatlyandneedtoberemoved.
Therefore,CuNWs/PTFElmswereannealedatacertaintemperature(190290°C)inaquartztubefurnacefor30minunderaconstantowofhydrogen.
Figure1eshowsthedigitalphotographandSEMimagesofCuNWlmannealedat230°C.
ThesurfacesofCuNWsbecamemuchcleanerafterannealing.
Moreover,thepeaksascribedtoNH2(3391cm1),CH2(2917,1467,and719cm1),andCH3(2847cm1)disappearedintheFTIRspectrumafterannealing,verifyingtheremovaloforganicresidues.
Inaddition,theoxidelayerwasalsoremovedbyannealing,asconrmedbyRamananalysis.
Sincetheannealingtemperaturehasaneectontheintegrityandconductivityofthenanowirelms,itwasinvestigatedindetail.
AsdisclosedinFigureS2,CuNWslmannealedat230°Cpossessesthebestconductiveperformance,whichisattributedtotheeectiveremovaloforganicresiduesandoxidelayers,aswellastheweldingofnanowires.
Higherannealingtemperatureswillleadtobreakageofnanowires,whichwilldecreasetheconductanceofthelm.
Therefore,CuNWslmswereannealedat230°Cinthefollowingdiscussionifnototherwisestated.
Duringthequasiinsitupolymerizationprocess,prepolyme-rizationtimeisacrucialfactorsinceitdeterminesthestickinessandhardnessofthePDMSsubstrates,whichwillaectthetransferofCuNWsgreatly.
Therefore,compositelmsfabricatedwithdierentprepolymerizationtimeswerecollected,tested,andcharacterizedbySEM.
AsshowninFigure2a,thesheetresistanceofthecompositelmsdecreasescontinuouslywiththeprolongingofprepolymerizationtimeandlevelsoafter10min.
ThereasonliesinthedierentembeddingdepthsofCuNWsintothePDMSsubstrates,asdisclosedinFigure2bg.
Astheprepolymerizationtimeisprolongedfrom3to30min,theembeddingdepthdecreases.
Prepolymerizationtimeof3and5minleadstostickierandsoftersurfacesofPDMSsubstrate,whichwillbringindeeperembeddingofCuNWnetworksduringthetransferprocess,andviceversa.
Whentheprepolymerizationtimeisprolongedto30min,CuNWsarescarcelyburied,andmostofthemcoverontopofthePDMSsubstrate(Figure2g).
Deeperembeddingofthenanowirenetworkresultsinlargersheetresistanceofthecompositelmssincewrappingbypolymerwouldincreasethecontactresistanceoftheunweldedjunctions,whichexplainsthevariationtrendofthesheetresistance.
Moreover,theembeddingstatusofCuNWsinuencestheirattachmenttoPDMSsubstratespronouncedly.
AsseenfromFigure2h,CuNWs/PDMSsamplesprepolymerizedfor3,5,and10min(markedasCuNWs/PDMS-3,CuNWs/PDMS-5,andCuNWs/PDMS-10,respectively)exhibitsstrongadhesiontothesubstrates,andnonanowirescanbepeeledothesubstratesby3Mscotchtapes.
Incontrast,forCuNWs/PDMS-15,CuNWs/PDMS-20,andCuNWs/PDMS-30,theCuNWswillbepartlyremovedbyscotchtapeandtheintegrityofCuNWsnetworkswillbebroken.
ThestretchabilityofCuNWs/PDMScompositeconductorswerecharacterizedbyatensilesystematthespeedof10%/min.
R/R0wasdenedasaratiooftheinstantaneousresistanceataspecictensilestraintotheinitialresistanceatzerostrain.
Figure3ademonstratesthechangeofR/R0asafunctionofACSAppliedMaterials&InterfacesResearchArticleDOI:10.
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Interfaces2016,8,929793049300strainofCuNWs/PDMSlmswithdierentprepolymeriza-tiontimes.
Theresistancesofthecompositelmsriseslowlyandthenrapidlywithstretch,whichissimilartothetendencyofresistancechangeasreportedbefore.
36CuNWs/PDMS-10revealsthebesttensilestraintolerance,andtheresistanceincreasesby4timesatthetensilestrainof80%,whichoutperformsmanynanomaterial-basedstretchableconductorswiththenonstructurallyengineeredelastomericsubstratestodate.
StretchableconductorsbasedonAgNWs/polyacrylateandCuNWs/polyurethaneshowedincreaseontheresistanceby10and15timesatthestrainof80%,respectively.
36,37Thersttencyclesofthesixsampleswithstrainupto20%wereinvestigatedandshowninFigure3b.
Asexpected,CuNWs/PDMS-10showsthebestperformancecomparedwiththeothersamplesandthecurvesofthechangeofR/R0asafunctionofstretchingcyclesrevealsxedperiodicity.
However,insteadofrecoveringtheinitialvalueafterreleasing,theresistanceascendsgraduallywithcycling.
ThisphenomenonismainlyattributedtotheslidebetweenCuNWswhenstretched,which,unfortunately,canonlyslidebackbyacertaindegreeafterreleasingduetothefrictionforcebetweenCuNWsandPDMSmatrix.
TheslideofCuNWsmayleadtoasmalleroverallcontactarea,thusresultingintheincreaseofresistanceshowninFigure3c.
32Exceptforthereasonmentionedabove,thereisresidualstraininCuNWs/PDMSlayeruponunloadingaswell,duetotherearrangementofCuNWsduringtherststretching,whichissimilartonetworksofCNTscompressedbyoutsideload.
38Figure3ddisclosesthatthevariationcurveofR/R0turnsatwithcycling,whichoriginatesfromtheformationofabuckledwavyshapeofCuNWs/PDMSlayer.
32Suchwavy,sinusoidalshapeisbenecialtothestretch-releaseperformanceanddurabilityofstretchableconductors,whichhasbeenconrmedinmanypreviousreports.
39,40ThebendablepropertiesoftheCuNWs/PDMScompositelmswerealsoinvestigated(showninFigure3e).
Forconvenience,thedistancebetweenthetwoendsofthebendedlmwastakenastheassessmentofthedegreeofbending.
Films15mmlongwerebentfromattoabendingdegreeof3mmgraduallyandthenreturnedtoat.
ThevariationcurvesofR/R0revealthatthesesixcompositelmsmaintaintheconductivestabilityintherstcycle,especiallyCuNWs/PDMS-10,CuNWs/PDMS-15,andCuNWs/PDMS-30.
Furthermore,acyclicbendingtestatabendingdegreeof6mmindicatesthattheconductivityofCuNWs/PDMS-10,CuNWs/PDMS-15,CuNWs/PDMS-20,andCuNWs/PDMS-30remainsperfectlystable,evenaftercyclingfor1000timeswithoutconspicuousdegradation.
TheresistanceofCuNWs/PDMS-3,however,increasesapparentlywithcycling.
AschemeofthestructurechangeofCuNWs/PDMScompositelmswithstretchingandreleasewasproposedtoillustratethereasonwhyCuNWs/PDMS-10hasthebeststretchingandbendingperformance.
AsshowninFigure3f,thecompositelmismainlycomposedofthreelayers,includingthebottomPDMSlayer,CuNW-PDMScompositelayer,andtheunembeddedCuNWlayer.
ThethicknessofeachlayerisdependentontheprepolymerizationtimeasshowninFigure2.
Whenbeingstretched,nanowiresinthecompositelayerwillslidealongthestretchingdirectionduetothefrictionforcebetweenthemandthePDMSsubstrate.
Thiswillleadtothedamageofsomeconductingpaths,thereforeresultingintheincreaseofthesheetresistance.
Incontrast,thecombiningbetweenPDMSwithCuNWs,whicharenotwrappedbypolymersintheunembeddedlayer,isratherweak.
Whenbeingstretched,slidingwilloccuramongthenanowiresaswellasbetweenthenanowireandthePDMSsubstrate.
Therefore,theunembeddedCuNWnetworkdeformslesscomparedwithCuNW-PDMScompositelayer,andstaysmoreconductiveafterFigure3.
(a)R/R0versustensilestrainupto200%ofCuNWs/PDMSwithdierentprepolymerizationtimes.
(b)R/R0changeofCuNWs/PDMS-3,CuNWs/PDMS-5,CuNWs/PDMS-10,CuNWs/PDMS-15,CuNWs/PDMS-20,andCuNWs/PDMS-30withtensilestrainupto20%inthersttencycles.
(c)R/R0variationofCuNWs/PDMS-10withtensilestrainupto20%intherstcycle.
(d)StretchingandreleasingpropertyofCuNWs/PDMS-10in50cycles(theinsetwasa10cyclescurveofthecompositeaftercycling20times).
(e)R/R0ofCuNWs/PDMSversusthebendingdegreefromatto3mm,andbacktoat.
Thephotographsarethecompositelmsbentatvariousbendingdegrees,fromatto9mm,6mm,andbacktoat.
TheinsertwasthecyclicbendingtestofCuNWs/PDMSatabendingdegreeof6mm.
(f)SchemeofthestructurechangeofCuNWs/PDMScompositelmswithstretchingandrelease.
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ThiscanexplainthepoorstretchingperformanceofdeeplyembeddedsamplesincludingCuNWs/PDMS-3andCuNWs/PDMS-5.
However,ifthepolymerizationtimeisprolongedtoover20min,thenanowirenetworksarebarelyembedded,andtheirattachmenttothesubstratebecomesveryweakaswell,whichresultsineasierdetachmentofCuNWsfromPDMSsubstratesduringthestretchingprocess.
Onthebasisofthereasonsmentionedabove,theCuNWs/PDMS-10showsthebeststretchableperformance.
Whenbeingreleased,nanowirescouldnotslidebacktotheinitialstate,andmanyconductingpathsweredestroyedduetothehinderingofthesurroundingpolymermoleculesinthecompositelayers,whichexplainstheinferiorrecoveryperformanceofCuNWs/PDMS-3andCuNWs/PDMS-5.
Exceptfortheseproperties,theantioxidationabilitywasalsoinvestigated.
AsshowninFigureS3,thechangeofR/R0islessthan1.
4afterthelmswerestoredinairfor50days,testifyingtotheexcellentantioxidantpropertyofthecompositelms.
TodemonstratethefeasibilityofCuNWs/PDMSconductors,apieceofCuNWs/PDMS-10withthelengthof2cmandinitialresistanceof2Ωwasusedasstretchableelectricwiringtoilluminatealight-emittingdiode(LED)lamp.
TheLEDlampwasoperatedatavoltageof4Vwhenitwasstretchedfrom0%to120%asshowninFigure4.
Asseenfrominsets,noconspicuousdegradationofbrightnessisobservedduringstretchingfrom0%to120%.
Althoughthecurrentdecreasedastheelectriccircuitwasstretchedtoalargerstrain,owingtothepartialdamageoftheintegrityoftheCuNWsnetwork,theLEDcanstillmaintainitsnormalworking.
Thecurrentvoltagecharacteristicswerepartlyrecoveredwhenthestrainwasreleased.
ThesetestingresultssuccessfullydisplaythepossibleapplicationsoftheCuNWs/PDMSlmsinstretchableelectronics.
Becauseofitshighconductivity,stretchability,andbendability,CuNWconductorhaspotentialaselectrodesinstrainsensorapplication.
Herein,acapacitivesensorwasfabricatedbylaminatingtwoCuNWs/PDMS-10electrodeswiththeconductingsurfacefacingoutward,whichissimilartoaparallelplatecapacitor(Figure5a).
Theinitialcapacitanceofthesensorisgivenbyεε=CAd00r00,whereε0istheelectricconstant,εristhedielectricconstant,andA0andd0aretheareaoverlapandseparationbetweenthetwoelectrodes(thespacebetweentwoelectrodesisabout1mm),respectively.
Whenitwasuniaxiallystretched,thechangeincapacitanceΔCwiththeappliedstrainεisgivenbyΔC=aεC0,whereaisthegaugefactor.
32,41Figure5bplotsthechangeofcapacitanceasafunctionofstrain,whichshowsgreatlinearitywithstandarddeviationR2of0.
998underalargestrainrange(upto60%).
(ΔC/C0)/ε,isdenedasacapacitivegaugefactorwhichistheslopeofthelineartasshowninFigure5bandistakenasaperformanceindex.
42Thegaugefactorofourstrainsensoris0.
82andtheminimumdetectablestrainisabout1%.
Theperformanceoutperformsmanystrainsensorswiththesameworktheoryandparallel-platestructure.
Forexample,Lipomietal.
reportedstrainsensorbasedonCNTlmwiththegagefactorof0.
41,43whileHuetal.
andYaoandZhufabricatedAgNWstrainsensorswiththegaugefactorof0.
5and0.
7,respectively.
41,44Figure5crecordsthevariationofcapacitanceversustimeovertenstretchingandreleasingcycleswiththe20%stretchingstrain.
Ineachstretchingcycle,thecapacitanceincreasesquicklyto1.
16timesoftheoriginalvalueandcanreversebacktotheinitialvalueafterreleasing,exhibitingastableresponse.
Furthermore,thedurabilityofthesensorwastestedover1000cyclesofstretchingandreleasing.
AsseenfromFigure5d,thecapacitancekeepsconstantduringcycling,manifestingtheexcellentdurabilityoftheCuNWs/PDMScapacitivesensor.
4.
CONCLUSIONSInsummary,wehaveproposedaquasiinsitupolymerizationmethodtofabricateCuNWs/PDMSstretchableconductors.
Viaregulatingtheprepolymerizationtime,partialembedmentofCuNWsinPDMSsubstrateswererealized,whichendowedtheconductorswithstrongadhesion,excellentconductivityandstability(R/R0inair),enhancedstretchability(80%strain,R/R05),andbendabilityofCuNWs/PDMSconductors.
ThecompositeconductorhasbeenusedasthestretchableelectricwiringtoilluminateanLED,whichdidnotshowobviousdegradationofbrightnessduringstretchingfrom0%to120%.
Stretchablecapacitivesensorwithdetectionlimitaslowas1%strain,andsensitivityashighas0.
82hasalsobeenconstructedwithCuNWs/PDMSconductorsaselectrodes.
ItisbelievedthatthequasiinsitupolymerizationmethodcanbeFigure4.
CurrentvoltagemeasurementofLEDintegratedbyCuNWs/PDMS-10atvarioustensilestrainsover120%.
InsetsshowthedigitalimagesoftheintegratedLEDcircuitsoperatedat0%,40%,80%,and120%strain.
(Note:Thecurvefor100%tensilestraincoincideswiththeoneofrecovery.
)Figure5.
(a)SchematicallyshowingacapacitivestrainsensorwithCuNWs/PDMS-10astheelectrodes(top)beforeand(bottom)afterbeingstretched.
(b)ChangeofcapacitanceΔC/C0versusstrain.
(c)Changeincapacitanceversustimeovertenstretching/releasingcycleswith20%stretchingstrain(stretching/releasingspeedis50%/min).
(d)DurabilitytestofCuNWs/PDMSstrainsensorover1000timesstretchingandreleasinguptotensilestrainof20%.
ACSAppliedMaterials&InterfacesResearchArticleDOI:10.
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ASSOCIATEDCONTENT*SSupportingInformationTheSupportingInformationisavailablefreeofchargeontheACSPublicationswebsiteatDOI:10.
1021/acsami.
5b11143.
TheFTIRspectraofCuNWsonsiliconwaferbeforeandafterheattreatment,SEMandEDXofCuNWnetworkwithoutheattreatment,RamanspectraforCuNWsonPTFEbeforeandafterheattreatment,theeectofdierentheattreatmenttemperaturesonCuNWs/PTFElms,andlong-termstabilityofCuNWs/PDMSunderairatmosphere(PDF)AUTHORINFORMATIONCorrespondingAuthors*Telephone:+86-21-52412717.
Fax:+86-21-52413122.
E-mail:wangranran@mail.
sic.
ac.
cn.
*E-mail:jingsun@mail.
sic.
ac.
cn.
NotesTheauthorsdeclarenocompetingnancialinterest.
ACKNOWLEDGMENTSThisworkwasnanciallysupportedbytheNationalBasicResearchProgramofChina(2012CB932303),theNationalNaturalScienceFoundationofChina(GrantNo.
61301036),ShanghaiMunicipalNaturalScienceFoundation(GrantNo.
13ZR1463600),andTheInnovationProjectofShanghaiInstituteofCeramics.
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