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    Suspended graphene sensors with improved signal and reduced noise

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    pubs.acs.org/NanoLett
    SuspendedGrapheneSensorswithImprovedSignalandReducedNoise
    ZengguangCheng,QiangLi,ZhongjunLi,QiaoyuZhou,andYingFang*
    NationalCenterforNanoscienceandTechnology,11BeiyitiaoStreet,Zhongguancun,Beijing100190,People’sRepublicofChina
    ABSTRACTWereportenhancedperformanceofsuspendedgraphenefieldeffecttransistors(Gra-FETsassensorsinaqueoussolutions.Etchingofthesiliconoxide(SiO2substrateunderneathgraphenewascarriedoutinsituduringelectricalmeasurementsofdevices,whichenabledsystematiccomparisonoftransportpropertiesforsamedevicesbeforeandaftersuspension.Significantly,thetransconductanceofGra-FETsinthelinearoperatingmodesincreases1.5and2timeswhenthepoweroflow-frequencynoiseconcomitantlydecreases12and6timesforholeandelectroncarriers,respectively,aftersuspensionofgrapheneinsolutionfromtheSiO2substrate.Suspendedgraphenedeviceswerefurtherdemonstratedasdirectandreal-timepHsensors,andcomplementarypHsensingwiththesamenanodeviceworkingaseitherap-typeorn-typetransistorwasexperimentallyrealizedbyoffsettingtheelectrolytegatepotentialinsolution.Ourresultshighlighttheimportancetoquantifyfundamentalparametersthatdefineresolutionofgraphene-basedbioelectronicsanddemonstratethatsuspendednanodevicesrepresentattractiveplatformsforchemicalandbiologicalsensors.
    KEYWORDSGraphene,noise,suspended,pHsensing
    rapheneisattractingtremendousinterestsduetoitsnearlyperfectcrystalqualityandsuperbphysicalproperties,1-5andemergingapplicationsarebeing
    developedbasedonthisnovelbuildingblock.6-9Inparticu-lar,owingtoitsexceptionalcarriermobilityandone-atomicthickness,graphenefieldeffecttransistors(Gra-FETshavebeenproposedtoholdgreatpotentialsforsensitiveandlabel-freedetectionofchemical/biologicalspecies.9Todate,studieshavebeenfocusedexclusivelyongraphenesensorssupportedonsiliconoxide(SiO2substrates,10-13althoughchargetrapsattheinterfaceandintheoxidehavebeenshowntoactasexternalscatteringcentersanddegradetransportpropertiesinsingle-layergraphenewhoseatomsareallexposeddirectlytofluctuationsofextrinsicimpuri-ties.14-18Wehaveundertakenandreporthereinstudiesonperformanceimprovementofgraphenedevicesbysuspend-ingtheminaqueoussolutionthroughanovelinsituetchingtechnique.Ourresultsshowthat,owingtoconcomitantlyincreasedtransconductanceanddecreasednoiselevelbyremovaloftheoxide,thesignal-to-noiseratiosofsuspendedgraphenenanodeviceswereimprovedby14dBinlow-frequencyregime(below1kHzforbothholeandelectroncarrierscomparedwiththosesupportedonSiO2substrates.Asanexample,suspendeddevicesweredemonstratedasreal-timeandsensitivepHsensors,andcomplementarydetectionwitheitherholesorelectronsaschargecarriersinthesamegraphenedevicewasachieved,openingupnewopportunitiesforsuspendedgrapheneasflexiblecandidatesforbioelectronics.
    *Towhomcorrespondenceshouldbeaddressed.E-mail:fangy@nanoctr.cn.Receivedforreview:02/22/2010PublishedonWeb:00/00/0000
    ©XXXXAmericanChemicalSociety
    G
    Figure1illustratestheoveralldesignofourexperiments.First,Gra-FETswerefabricatedfrommechanicallyexfoliatedsingle-layergraphenesupportedontopofasiliconsubstratewith300nmSiO2.5,19Briefly,source-draincontactstographeneweredefinedbye-beamlithographyandsubse-quentlymetallizationwith5nmCr/70nmAu.Nanodevicesinourstudywereshowntohavesmoothgraphenesurfacewithheightof∼1.0nmbyatomicforcemicroscopy(AFM(seeSupportingInformation,FigureS1a.Typical2-probemobilitiesofa5µmlongGra-FETgatedthrough300nmSiO2inairwerecalculatedas4600and4100cm2/V-secforholeandelectroncarriers(seeSupportingInformation,FigureS1b,respectively,whichareconsistentwithreportedvaluesforpristinegrapheneatroomtemperature.5
    FIGURE1.GrapheneFETsintheelectrolytesolution.(aSchematicrepresentationofourexperimentalsetupwhereasingle-layergrapheneissupportedinsolutionbyCr/Aucontactstobridgeatrenchintheoxide.(bInsituetchingofSiO2underneathgraphene.TheconductanceofgraphenestartstodropgraduallyafterbufferedHFwasaddedtothePDMSchamber.Single-layerdevicesusuallystabilizewithin50to100s,indicatingthecompletesuspensionofgrapheneinsolution.ArrowsindicatethetimewhensolutionwasswitchedinthePDMSchamber.Theinsetshowsascanningelectronmicroscope(SEMimagetakenofasuspendedgraphenedeviceaftersolutionmeasurements.Scalebaris0.5µm.
    A
    DOI:10.1021/nl100633g|NanoLett.XXXX,xxx,000–000

    FIGURE2.TransportcharacterizationofaGra-FETinsolution.BlackcurveisconductancevstheelectrolytegatevoltageofthegraphenedevicesupportedonSiO2substrate,andredcurveisofthesamedeviceaftersuspensionofgrapheneinsolution.Theblackandreddiamondshighlightelectrolytegatepotentialsatwhichthecom-parisonoftransconductanceandnoisespectrawasperformedinthetextandFigure3.
    Apolydimethylsiloxane(PDMSchamberwasthenin-corporatedovertheGra-FETchiptoconfinetheelectrolyticsolutionof100mMpotassiumchlorideand10mMphos-phate(pH7(Figure1a.AnonleakAg/AgClreferenceelec-trodewasplacedinthecenterofthereservoirastheelectrolytegate.Theconductanceofgraphenesensorswasmonitoredthroughthesourceanddraingoldelectrodesusinglock-indetectionbyapplyinga30mVsinusoidalbiasvoltage(Vsdwithafrequencyof109Hz.
    Toquantifyexclusivelyeffectsfromtheunderlyingoxidesubstrate,wedevelopedaninsituetchingtechniquebycarefullykeepingdevicesimmersedinliquidduringswitch-ingofsolutionsandmeasuringofGra-FETs’conductance.Weeliminatedthedryingstepsofsuspendedgraphenedevicesreportedbyformerstudies14,16becausethecycleofdryingandwettingcaneasilygeneratestresstodeformgrapheneandtheirmetalcontacts,andaltertheelectricalpropertiesofdevicesunintentionally(seeSupportingInfor-mation,FigureS2.EtchingofSiO2underneathgraphenewasmonitoredinreal-timebytrackingconductancesignalsthroughdevices(Figure1b,andwefoundthatincubationinbufferedHFresultsinca.100nm/minetchingrateunderneathgraphene.Wenotethatetchingofdevicesinthefollowingdiscussionwascarriedoutfor1min.14,20,21
    TheelectricalmeasurementsofatypicalGra-FET(0.5µmlongand0.6µmwidebeforeandaftersuspensionofgrapheneinsolutionweresummarizedinFigure2andSupportingInformationFigureS3.TheblackcurveinFigure2showstheconductance(dI/dVsdorGofthedeviceplottedagainsttheelectrolytegatevoltage(VgatewithgraphenesupportedonSiO2substrate.Thetransconductanceofholes(dG/dVgateinthenanodevicereachesashighas1mS/VinthelinearoperatingmodebecausetheelectrolytegatewasabletomodulateeffectivelytheFermilevelinsingle-layergraphenethroughthedoublelayerofions.22Notethatthe
    ©XXXXAmericanChemicalSociety
    B
    conductanceofthedeviceislargeatnegativeelectrolytegate,anditstartstodecreasewithincreasedgatevoltageduetothedepletionofholecarriersinthep-typegrapheneuntilreachesitsminimumattheDiracpoint,thentheconductancestartstoincreaseagainwiththeaccumulationofelectroncarriersinthecurrentlyn-typegraphene.Inter-estingly,theDiracpointofthegraphenedeviceonSiO2settlesat0.1Vwhenthechemicalpotentialoftheneutralsolution(pH7isat0V.WeobservedalargerangeofDiracpointpotentialsinourGra-FETsonSiO2,(0.1(0.2V,whichwasalsoreportedbysimilarstudiesongraphenewithnakedsurfaces.12,13,23ThisvariationofdevicepropertiesinGra-FETsonSiO2isapparentlydisadvantageousinpracticalapplicationswherereproduciblecharacteristicsarecriticalfordevicesas,forexample,reliableclinicaldiagnosticsensors.24
    AfterinsituetchingofSiO2,theresponseoftheGra-FET’sconductancetotheelectrolytegatepotentialwascharacter-izedagainintheneutralelectrolytesolutionasshownbytheredcurveinFigure2.Valuableinformationcanbeim-mediatelydeducedbycomparingtransportpropertiesofthesamedevicebeforeandaftersuspension.(1TheDiracpointofgraphenemovedto∼0Vafteretchingofoxide,whichwasreproducedbyotherdevicesinourstudy,indicatingthattheFermilevelofsuspendedgrapheneiscontrolledsolelybythesolution.(2Theoperatingprincipleofelectricalsensorsisbasedontheirconductancechangesrespondingtolocalpotentialchanges,sotheirsignalsensitivitycanbedescribedasdG/dVgate,thatis,thetransconductanceofdevices.10,24Significantly,thetransconductance,orthesignalsensitivity,ofthegraphenedeviceinthelinearoperatingregionsincreases1.5and2timesforholeandelectroncarriers,respectively,aftersuspension.(3Thehole-electronasymmetrywasfoundforbothelectrolyte-gateandback-gategeometryoftheGra-FETonSiO2(blackcurveinFigure2andSupportingInformationFigureS3b.AsymmetricholeandelectronbrancheswerepreviouslyreportedinbothnonsuspendedandsuspendedGra-FETswithchannellengthbelow1µminvacuum,althoughtheoriginoftheasymmetrywasnotunderstoodthen.14Surpris-ingly,theasymmetryobservedinourshort-channeldevicesdecreasesgreatlyaftersuspensionofgrapheneinelectrolytesolutions(redcurveinFigure2,indicatingeffectsfromdielectricscreeningandreducedscatteringmayberespon-siblefortheimprovedsymmetryinsolution,25,26althoughadetailedstudyisneededinfuturetorevealtheunderlyingmechanism.
    Thelevelofsignalthatcanbeprocessed,ortheresolu-tion,ofasensorisultimatelydeterminedbyitssignal-to-noiseratio.Thusthenoisespectraofgraphenedevicesinelectrolytesolutionswerefurtherexaminedunderourex-perimentalcondition,andwewillconcentrateourdiscussiononlow-frequency(below1kHznoisewhichhasbeenre-portedtobedominantinlimitingperformanceofnanode-vices.15Figure3aandSupportingInformationFigureS4b
    DOI:10.1021/nl100633g|NanoLett.XXXX,xxx,000-–000

    FIGURE3.Noisecharacterizationofgraphenedeviceinelectrolytesolution.(aNormalizedcurrentpowerspectraldensityisshownasafunctionoffrequencyofthegraphenedeviceonSiO2atdifferentelectrolytegatepotentials.ThepowerspectraweremeasuredataDCmodewithabiasvoltageof30mV,andsharppeaks(50Hzanditsharmonicswerecausedbyextrinsicnoisefromthepowersource.(bComparisonofgraphene’snoisepowerspectrainthelinearoperatingmodeswithholesascarriersbefore(black,Vgate-0.10Vandaftersuspension(red,Vgate-0.15V.(cComparisonofgraphene’snoisepowerspectrawithelectronsascarriersbefore(black,Vgate0.20Vandaftersuspension(red,Vgate0.15V.
    summarizethecurrentnoisepowerspectraldensitynormal-izedbythemeansquareofcurrent,S(SI/(I2,asafunctionoffrequency,f,ofthesamegraphenedeviceinFigure2beforesuspension.ThenormalizedpowerspectraoverlapinthelinearoperatingmodesandaroundtheDiracpointoftheGra-FET,indicatingthatnoisedensityingraphenegatedbytheelectrolytegateisindependentofthegatepotential,althoughgate-dependentnoisespectrahavebeenreportedingraphenenanobeltsgatedthroughSiO2dielectric.15Fur-ther,thepowerspectraldensityofthegraphenedevicewasinverselyproportionaltothefrequency,representingthe1/fflickernoisecharacteristicubiquitousinnanodevices.ThusthenoisespectraldensityingraphenedevicessupportedonSiO2canbeexpressedas27
    andSIhasaunitofA2/Hz.Thusthepowerofthecurrentnoise,Pnoise,foragivenfrequencyband[f1,f2]inthelow-frequencyregioncanbecalculatedas
    Pnoise
    ∫f
    f2
    1
    SIdf
    ∫f
    f2
    1
    f2AI2
    dfAI2lnff1
    AI2
    SI
    f
    inthelow-frequencyregionfortheelectrolytegategeometryinsolution,whereA∼10-6istheamplitudeofthe1/fnoise,
    ©XXXXAmericanChemicalSociety
    andPnoiseiswithaunitofA2.
    Aftersuspensionofgrapheneinsolution,thenormalizedpowerspectraldensityofthedevicedropped12and6timesforholeandelectroncarriers,respectively,comparedwiththatonSiO2substrate(Figure3b,c.Ithasbeenfoundthattrappedchargesattheinterfaceandintheoxidedegradetransportcharacteristicsofsingle-layergraphene.14,17FormerstudyofgraphenedevicesonSiO2substratewithlow2-probecarriermobilityinvacuum(∼100cm2/V-secwerereportedtoshow∼20timesimprovedperformancebyionscreeningofexternalimpuritychargesinhighionicsolu-tions.25OurGra-FETs,ontheotherhand,havehighmobility(4000to5000cm2/V-seconSiO2intheback-gategeometry
    C
    DOI:10.1021/nl100633g|NanoLett.XXXX,xxx,000-–000

    inair.Thefurtherimprovedperformanceinoursuspendedgrapheneshowsthatdevicedegradationassociatedwithoxidesubstratesisstilldominantinournonsuspendeddevices,despiteanypossibleionicscreeningeffectin100mMsolutions.ItisimportanttonotethatlowionicstrengththatofferslongDebyelengthisdesirablefordetectionofchargedbiomoleculeswithnano-FETs,24,28thustheelimina-tionoftheoxidebysuspensioncanbeincreasinglycriticaltothereductionofnoiseforsingle-layergrapheneinsolu-tionswithlowionicstrength.
    Overall,thesignal-to-noiseratiodefinedas29
    SNR(dB20log
    AsignalPsignal
    10logAnoisePnoise
    wasincreasedby14dBinlow-frequencyregimeforboth
    holeandelectroncarriersingraphenedevicesaftersuspen-sion.Thusourresultsexplicitlyprovethatforsingle-layergrapheneFETbasedsensors,aplatformwithsuspendednanodevicesisadvantageoustoachievelowdetectionlimitinsolution.
    Todemonstratetheviabilityofsuspendedgraphenedeviceasdirectchemicalsensors,weappliedournanode-vicestopHsensing.SolutionsfrompH6topH9weredeliveredtothesurfaceofagraphenedevicesequentially,andforeachsolution,theelectrolytegateresponseofthedevice’sconductancewasmeasuredasshowninFigure4a.WefoundthattheDiracpointofgrapheneshiftspositivelyfrompH6topH9(seeSupportingInformation,FigureS5,whichisconsistentwiththeincreasedchemicalpotentialofsolutionandconfirmsthecapabilityofsuspendedgrapheneforchemicalsensinginsolution.30-32
    Comparedtoformerunipolarsiliconnanowire(SiNWorcarbonnanotube(CNTbasedp-typesensors,33,34suspendedgrapheneshowsnearlysymmetrictransportaroundtheDiracpointinneutralsolution(Figure4a,soitcanworkaseitherap-typeorn-typesensorwhenasmallgatepotential,withoutintroducinganyredoxreactionofmoleculesinsolution,isappliedtoswitchitscarriercharacteristic.Weinvestigatedthetime-dependentresponseoftheGra-FET’sconductancetodifferentpHsolutionswhentheelectrolytegateisbiasedat-0.05and0.05V,respectively.Theconductanceofthenano-sensorwasmonitoredinreal-timeassolutionswithpH6topH9weresequentiallydeliveredtothesurfaceofthesus-pendedgraphene.AsshowninFigure4b,thegrapheneworksasap-typematerialwhen-0.05Vgatepotentialisapplied.Similartoformerp-typedevicesofSiNWsorCNTs,33,35con-ductanceofthegraphenedeviceincreaseswithincreasedpHvalues.Onthecontrary,whenthegatepotentialisswitchedto+0.05V,thesignalflipsitssignandtheconductanceofthegraphenedevicedecreaseswithincreasedpH.Thepolarityflipofsignalsverifiesthatthesensingmechanismingraphenenanodevicesisduetoafield-effect.Theuniquecapabilityto
    ©XXXXAmericanChemicalSociety
    FIGURE4.GrapheneaspHsensor.(aGra-FET’sconductanceasafunctionoftheelectrolytegatevoltageinpH6,7,8,and9solutions.(bReal-timedetectionofsolution’spHbyconductancechangeofthesamegraphenedevicewiththeelectrolytegatepotentialbiasedat-0.05(leftbranchand+0.05V(rightbranch.
    tuneeasilyGra-FETsbetweenp-typeandn-typecharacteristicspresentspowerfulconfirmationonthenatureofsensingsig-nals.WefurthernotethatthepHdetectionofourgraphenedevicesisreversible.Thuscomplementaryandreversiblemeasurementsindifferentoperatingregionsofgraphenede-monstratetheflexibilityofoursuspendedgraphenesensors.Insummary,quantitativestudiesontheimprovedper-formanceofsuspendedgraphenedevicesinsolutionwerereported.Ourresultsshowthat,inlow-frequencyregime,thesignal-to-noiseratioofgraphenedevicesincreased14dBforbothholeandelectroncarriersasaresultofconcomi-tantlyincreasedmobilityanddecreasednoiselevelbysuspensioninsolution.Directandreal-timedetectionofsolution’spHbysuspendedgraphenewasdescribed,andcomplementarysignalsfromthesamegraphenedeviceasap-typeorn-typetransistorwerepresented.Theenhancedelectricalcharacteristicsofsuspendedgraphenedevicesinaqueoussolutionshowthattheyareadvantageousforchemicalandbiologicaldetectionthantheircounterpartssupportedonsubstrates,andourresultsareexpectedto
    D
    DOI:10.1021/nl100633g|NanoLett.XXXX,xxx,000-–000

    bringattentionstosuspendednanodevicesingeneralaspromisingplatformsforbioelectronics.
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    (16Bolotin,K.I.;Sikes,K.J.;Jiang,Z.;Klima,M.;Fudenberg,G.;
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    (17Liu,G.;Stillman,W.;Rumyantsev,S.;Shao,Q.;Shur,M.;Balan-workbySpecialPresidentialFoundationoftheChineseAcademyofSciences,China(08172911ZX,“973”Fund(2009CB930200,andtheNationalNaturalScienceFounda-tionofChina(20973045.
    SupportingInformationAvailable.MethodsandFiguresS1-S5.ThismaterialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
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    EDOI:10.1021/nl100633g|NanoLett.XXXX,xxx,000-–000

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