Adsorption of Oxygen Molecules on Individual Carbo

1AdsorptionofOxygenMoleculesonIndividualCarbonSingle-walledNanotubesA.Tchernatinsky†,B.Nagabhirava‡,S.Desai†,G.Sumanasekera†,B.Alphenaar‡,C.S.Jayanthi†,andS.Y.Wu†*†DepartmentofPhysics,UniversityofLouisville‡DepartmentofElectricalandComputerEngineering,UniversityofLouisvilleLouisville,KY40292*Correspondingauthor.E-mail:[e-mail]adsorptionofoxygenmoleculesonindividualsemiconductiongsingle-walledcarbonnanotubesatambientconditionsrevealsthattheadsorptionisphysisorption,thattheresistancewithoutO2increasesby~twoordersofmagnitudeascomparedtothatwithO2,andthatthesensitiveresponseisduetothepinningoftheFermilevelnearthetopofthevalencebandofthetuberesultingfromimpuritystatesofO2appearingabovethevalenceband.2Interestingasadsorptionbycarbonnanotubesatambientconditionshasbeenspurredbythedemonstrationsofthepotentialofsingle-walledcarbonnanotube(SWCNT)-basedgassensors1-3,specifically,theestablishmentoftheinterdependencebetweengasadsorptionandtransportpropertiesofCNTs.Inrecentyears,experimentalstudiesontheadsorptionofoxygenmoleculesbySWNT-bundlesormatsincludedthemeasurementsofelectricalresistanceandthermoelectricpower2,3,theeffectofadsorptionofO2onthebarrierofmetal-semiconductorcontact4,5,andthekineticsofO2adsorptionanddesorption6.Thepictureemergedfromthesestudiesrelevanttogassensingindicatesthattheelectricalresistancechangesbyabout15%betweengassinganddegassing2,thattheholedopingofsemiconductingSWNT(s-SWNT)inairisbytheadsorptionofO2inthebulkofs-SWNTs5ratherthanatthecontact4,andthattheadsorptionofO2hasthecharacteristicsofphysisorption6.TheoreticalinvestigationsoftheadsorptionofO2onSWNTshavealsobeencarriedout,usingspin-unpolarizedaswellasspin-polarizeddensityfunctionaltheory(DFT)methods7-12.StudiesoftheadsorptionofO2onsmall-diameter(8,0)SWNTbasedonthespin-unpolarizedDFTwithinthelocaldensityapproximation(LDA)predictedaweakhybridizationbetweenstatesofO2andthoseofthes-SWNTwithanestimatedchargetransferof~0.1e7,9,leadingtoabindingofO2atadistancelessthan3Åfromthes-SWNT.Theholedopingofthes-SWNTwasattributedtothepinningoftheFermilevelatthetopofthevalencebandduetotheadsorptionofO2.WiththeO2moleculehavingatripletgroundstate,themorerealisticcalculationsbasedonthespin-polarizedgradient-correctedDFT8,10,12,ontheotherhand,yieldedaveryweakbondingat~4Åwithnosignificantchargetransfer,indicatingthatanO2moleculeinthemorestabletripletstateisonlyphysisorbedonas-SWNT.ForthetripletstateofO2adsorbedonthe(8,0)SWNT,twodegeneratedppπ*3bandswerefoundtosplitintofourbands,withthetwoun-occupiedppπ*(↓)bandsrising~0.35eVabovethetopofthevalencebandattheΓpoint12,castingsomedoubtabouttheholedopingpicturededucedfromtheun-polarizedcalculation.InordertoobtainacoherentandconsistentpictureoftheadsorptionofO2byindividuals-SWNTs,wehaveconductedacarefulexperimentalandtheoreticalinvestigationoftheadsorptionofO2moleculebyindividualSWNTstoshedlightonthenatureofadsorptionanditseffectonthetransportpropertiesofSWNTs.Experimentally,contactsweremadetoafewverydispersedSWNTsusinge-beamlithography.Theexperimentwasfirstconductedunderambientconditionsinair(roomtemperatureandatmospherepressure).Theresistancewasmonitoredduringeachexposuretoairandsubsequentpumping(10-6torr).AresistancechangeofmorethanoneorderofmagnitudewasobservedforthefirsttimeasaresultoftheadsorptionofO2byindividuals-SWNTs,indramaticcontrasttoamere15%changeobservedforSWNTbundlesormats.Furthermore,theonsetofthechangeinresistancewasinminutes.Theseobservationsclearlydemonstratedthefeasibilityofconstructings-SWNT-basedchemicalsensors.Tobemoreconsistentwiththeexperimentalresult,wehavecarriedoutastudyontheadsorptionofanO2moleculebyalargerSWNTthantheoneconsideredinpreviousstudies,the(14,0)SWNT,thatisclosertotherangeofdiametersintheexperiment,usingthespin-polarizedDFTmethod.Ourcalculation(spin-polarizedGGA)yieldedashallowpotentialwellofdepthoftheorderof~0.05eVat~3.6ÅfromthesurfaceoftheSWNT,consistentwiththepictureofphysisorption.WehavedeterminedthepinningoftheFermienergyduetotheimpuritylevelassociatedwithO2.Ourestimateoftheresistancebasedontheresultofthe(14,0)tubewiththeadsorptionoftheO2moleculeisinexcellentagreementwiththeobservedinitialresistanceinair,4indicatingthemetallizationofthes-SWNTbyholedopingassociatedwiththephysisorbedO2.Wehavealsopredictedachangeintheresistanceabouttwoordersofmagnitudebetweengassinganddegassing.IndividualSWNTsweresynthesizedusingCVDwithFecatalystandCH4onaSiO2/Sisubstratewithprepatternedgridmarks.Silicon(100)withathinoxidelayer(0.4µm)wasselectedforthegrowthprocess.Gridpattern(Aualignmentmarks)wasfabricatedontheSiO2/Sisubstrateusinge-beamlithographyandetchedusingBOE.Thealignmentmarkswereetchedsothattheycanbeseeninatomicforcemicroscopy(AFM)imaging.ThepreparationofcatalystsolutionfollowstheproceduregiveninRef.13.FenanoparticlesweredispersedonthesubstratefromFe(NO3)3propanolsolution.Afterwashingwithhexane,thesubstratewasloadedintotheCVDreactorandheatedto9000CinflowingAr/H2(100sccmof10%H2inAr).Afterreductionat9000Cfor10minutes,Methane,thecarbonfeed,wasintroduc