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Integration of Carbon Nanotubes to C-MEMS (2)

734IEEETRANSACTIONSONNANOTECHNOLOGY,VOL.9,NO.6,NOVEMBER2010

IntegrationofCarbonNanotubestoC-MEMS

forOn-chipSupercapacitors

WeiChen,MajidBeidaghi,VarunPenmatsa,KevinBechtold,LathaKumari,W.Z.Li,

andChunlei(Peggy)Wang,Member,IEEE

Abstract—Carbonnanotubes(CNTs)/carbonmicroelectrome-chanicalsystems(C-MEMS)compositeswerefabricatedaselectrodematerialsforon-chipsupercapacitors.Byusingpho-tolithographyandpyrolysisprocess,3-DC-MEMSarchitectureswereprepared.TheironcatalystparticleswereconformallycoatedontheC-MEMSbyelectrostaticspraydeposition(ESD)andCNTsweresynthesizedonthesurfacesofC-MEMSbycatalyticCVDmethod.TheCNT/C-MEMScompositesexhibitedhigherspeci ccapacitancethanC-MEMS.Furthermore,thecompositeswithmorehomogeneousCNTsshowedbettercapacitance.Aftertreat-mentofoxygenplasma,thespeci ccapacitanceofthecompositeincreasedduetothecontributionofoxygenfunctionalgroups.IndexTerms—Carbonmicroelectromechanicalsystems(C-MEMS),carbonnanotubes(CNTs),on-chipsupercapacitors.

I.INTRODUCTION

ITHthegrowingdemandforpowersourcesinportableelectronicdevices,miniaturizedenergystoragesystems

haveattractedmoreandmoreinterestoverthepastfewyears[1],[2].Miniaturizedon-chipmicrobatteriesarerecentlyexpectedtohaveenormouspotentialasoneofthepowersupplycandidatesforportabledevices[3].However,conventionalbatteriescanprovidehigherenergydensities,butpowerdensitiesarenotbigenoughforcertainhigh-outputpowerandrapiddischargeapplications.Asalternativeelectricalenergystoragesystems,supercapacitorsshowcharacteristicmerits,suchashigherpowerdensities,fastercharge/dischargecapability,andlongercyclelifetimecomparedtoconventionalbatteries[4],[5].Therefore,webelievethedevelopmentofon-chipsupercapacitorswillbeasimportantasthatofmicrobatteries,especiallyformomentary-loadmicropowerapplication.

Inthepastfewyears,carbonmicroelectromechanicalsys-tems(C-MEMS)techniquehasbeensuccessfullyusedtomi-crofabricatecarbon-basedcurrentcollectorsandelectrodesforminiaturizedLi-ionmicrobatteriesandbiosensors[3],[6],[7].

ManuscriptreceivedDecember31,2009;revisedMarch27,2010;acceptedApril20,2010.DateofpublicationMay3,2010;dateofcurrentversionNovember10,2010.ThisworkwassupportedbytheU.S.DefenseAd-vancedResearchProjectsAgencyYoungFacultyAwardprogramunderProjectHR0011-08-1-0036,NationalScienceFoundationunderMajorResearchInstrumentationProgramCMMI-0821582,andAmericanChemicalSocietyun-derPetroleumResearchFund49301-0N110.TheworkofW.Z.LiwassupportedbytheNationalScienceFoundationunderGrantDMR-0548061.ThereviewofthispaperwasarrangedbyAssociateEditorC.Zhou.

W.Chen,M.Beidaghi,V.Penmatsa,K.Bechtold,andC.(P.)WangarewiththeDepartmentofMechanicalandMaterialsEngineering,FloridaInternationalUniversity,Miami,FL33174USA(e-mail:wangc@ u.edu).

L.KumariandW.Z.LiarewiththeDepartmentofPhysics,FloridaInterna-tionalUniversity,Miami,FL33199USA.

Colorversionsofoneormoreofthe guresinthispaperareavailableonlineathttp://www.wenkuxiazai.com

DigitalObjectIdenti er10.1109/TNANO.2010.2049500

W

TheC-MEMStechniquecombinesconventionalphotolithogra-phyprocessandpyrolysisprocess,providingasimplemeanstofabricatecarbonmicrostructuresfrompatternedphotoresist.Here,forthe rsttime,weproposetofabricatemicroscaleelec-trodestructuresforon-chipsupercapacitorsbasedonC-MEMSmethod.Ontheonehand,relativelylow-energydensityisal-waysafataldrawbackforsupercapacitors[8],especiallyforon-chipsupercapacitors,theenergydensitieswouldfurtherde-creaseduetothelimitationofsmallarealfootprintrequiredforminiaturizeddesigns.AnalternativesolutionistointroducenanoscalestructuretoC-MEMStoincreasethespeci csurfaceareaandoffsetthenegativein uenceofminiaturization.Carbonnanotubes(CNTs)haveattractedextensiveattentionbecauseoftheirnanometersize,hollowstructure,high-accessiblesur-facearea,lowresistance,andhigh-structuralstability[9].Thesepropertiesmakethempotentiallysuitableastheelectrodesforon-chipsupercapacitors.TheintegrationofCNTstoC-MEMSstructureswouldgreatlyincreasesurfacearea,electricalcon-ductivity,andelectrochemicalperformance.However,howtoproduceconformallycoatedCNTsonto3-DC-MEMSisabigchallengeforfurtherdevelopingC-MEMS-basedminiaturizedsupercapacitors.Ontheotherhand,itisreportedthatthecapac-itanceperformanceisgovernednotonlybythespeci csurfaceareasofcarbonmaterials,butalsobythefunctionalgroupsontheirsurfaces[10],whichprovidepseudocapacitanceinvirtueofreversibleredoxreactionofoxygenfunctionalgroups.Modi- cationofcarbonsurfacescanbeachievedbyacidtreatmentorplasmatreatment[11].Forexample,CNTsfunctionalizedbyaninductivelycoupledRFoxygenplasmashowenhancedspeci ccapacitance[12].

Inthispaper,CNTsweredepositedontoC-MEMSbycat-alyticCVDmethod.Wedemonstratedaneffectivemethodforconformallycoatingcatalystparticlesoncomplex3-Dstructuresbyelectrostaticspraydeposition(ESD).Theeffectofdeposi-tiontimeofcatalystonthesynthesisofCNTswasinvestigated.Itwasfoundthatthespeci ccapacitanceofCNT/C-MEMScompositesincreasedupto20timescomparedtothatofbareC-MEMS.Inaddition,wefurtherincreasedthespeci ccapac-itanceofCNT/C-MEMScompositesbyintroducingfunctionalgroupsonthecompositesusingoxygenplasmamethodwithoutdestroyingmicrostructures.

II.EXPERIMENTALSECTION

A.FabricationofC-MEMS

Inordertodepositacarbon- lm-basedcurrentcollector,apositivephotoresist,AZ4620(MicroChemNano),was rstlyspin-coatedonasiliconoxidewaferat500r/minfor12s,and

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