Phosphorene-An-Unexplored-2D-Semiconductor-with-a-

LIUETAL.VOL.8’NO.4’4033–4041’2014:AnUnexplored2DSemiconductorwithaHighHoleMobilityHanLiu,†,‡AdamT.Neal,†,‡ZhenZhu,§ZheLuo,‡,^XianfanXu,‡,^DavidToma´nek,§andPeideD.Ye†,‡,*†SchoolofElectricalandComputerEngineeringand‡BirckNanotechnologyCenter,PurdueUniversity,WestLafayette,Indiana47907,UnitedStates,§PhysicsandAstronomyDepartment,MichiganStateUniversity,EastLansing,Michigan48824,UnitedStates,and^SchoolofMechanicalEngineering,PurdueUniversity,WestLafayette,Indiana47907,UnitedStatesPrecedingthecurrentinterestinlayeredmaterialsforelectronicapplications,researchinthe1960sfoundthatblackphosphoruscombineshighcarriermobilitywithafundamentalbandgap.Weintroduceitscounterpart,whichwecallphosphorene,asa2Dp-typematerial.SameasgrapheneandMoS2,wefindsingle-layerphosphorenetobeflexibleandcapableofmechanicalexfoliation.Thesefindingsarein-linewiththecurrentinterestinlayeredsolidscleavedto2Dcrystals,representedbygrapheneandtransitionmetaldichalcogenides(TMDs)suchasMoS2,whichexhibitsuperiorme-chanical,electrical,andopticalpropertiesovertheirbulkcounterpartsandopenthewaytonewdeviceconceptsinthepost-siliconera.1
4Animportantadvantageoftheseatomicallythin2Dsemiconductorsistheirsuperiorresistancetoshortchanneleffectsatthescalinglimit.5MasslessDiracfermionsendowgraphenewithsuperiorcarriermobility,butitssemimetallicnatureseriouslylimitsitsdeviceapplications.6,7SemiconductingTMDs,suchasMoS2,donotsufferfromavanishinggap8,9andhavebeenappliedsuccessfullyinflexiblen-typetransistors4thatpavethewaytowardulti-matelyscaledlow-powerelectronics.RecentstudiesonMoS2transistorshaverevealedgooddeviceperformancewithahighdraincurrentofuptoseveralhundredmA/mm,asubthresholdswingdownto74mV/dec,andanIon/Ioffratioofover108.3,10
12DuetothepresenceofSvacanciesinthefilmandthepartialFermilevelpinningneartheconductionband,11,13,14MoS2transistorsshown-typeFETcharacteristics.InpreviouslydemonstratedMoS2logiccircuitsbasedonn-typetransistorsonly,thestaticpowercon-sumptionislikelytoolargeforlow-powerintegratedsystems.15,16Thisfactalonecallsfornewp-typesemiconductorsthatwouldallowtherealizationofCMOSlogicina2Ddevice.Inthisstudy,weintroducephos-phorene,anamewecoinedforasingle-layerorfew-layerofblackphosphorus,asnovel2Dp-typehigh-mobilitysemiconductorsforCMOSapplications.Westudytheopticalandelectronicpropertiesandtransportbehavior*[e-mail]2,2014andacceptedMarch17,2014.Publishedonline10.1021/nn501226zABSTRACTWeintroducethe2Dcounterpartoflayeredblackphosphorus,whichwecallphosphorene,asanunexploredp-typesemiconductingmaterial.SameasgrapheneandMoS2,single-layerphosphoreneisflexibleandcanbemechanicallyexfoliated.Wefindphosphorenetobestableand,unlikegraphene,tohaveaninherent,direct,andappreciablebandgap.Ourabinitiocalculationsindicatethatthebandgapisdirect,dependsonthenumberoflayersandthein-layerstrain,andissignificantlylargerthanthebulkvalueof0.31
0.36eV.Theobservedphotoluminescencepeakofsingle-layerphosphoreneinthevisibleopticalrangeconfirmsthatthebandgapislargerthanthatofthebulksystem.Ourtransportstudiesindicateaholemobilitythatreflectsthestructuralanisotropyofphosphoreneandcomplementsn-typeMoS2.Atroomtemperature,ourfew-layerphosphorenefield-effecttransistorswith1.0μmchannellengthdisplayahighon-currentof194mA/mm,ahighholefield-effectmobilityof286cm2/V3s,andanon/offratioofupto104.Wedemonstratethepossibilityofphosphoreneintegrationbyconstructinga2DCMOSinverterconsistingofphosphorenePMOSandMoS2NMOStransistors.KEYWORDS:phosphorene.anisotropictransport.transistor.inverterARTICLELIUETAL.VOL.8’NO.4’4033–4041’2014firstCMOSinverterusingfew-layerphosphoreneasthep-channelandMoS2asthen-channel.Blackphosphorus,thebulkcounterpartofphos-phorene,isthemoststablephosphorusallotropeatroomtemperature17,18thatwasfirstsynthesizedfromwhitephosphorusunderhighpressureandhightemperaturein1914.19Similartographite,itslayeredstructureisheldtogetherbyweakinterlayerforceswithsignificantvanderWaalscharacter.20
22Previousstudieshaveshownthismaterialtodisplayasequenceofstructuralphasetransformations,superconductivityathighpressureswithTcabove10K,andtemperature-dependentresistivityandmagnetoresistivity.17,22
27Two-dimensionalphosphoreneis,besidesgraphene,theonlystableelemental2Dmaterialthatcanbemechanicallyexfoliated.RESULTSANDDISCUSSIONWehavedeterminedtheequilibriumgeometry,bonding,andelectronicstructureofblackphosphorus,few-layerandsingle-layerphosphoreneusingabinitiodensityfunctionaltheory(DFT)calculationswiththePBE28andHSE0629functionalsasimplementedintheSIESTA30andVASP31codes.AsseenintheoptimizedstructuredepictedinFigure1a
c,phosphorenelayersshareahoneycomblatticestructurewithgraphenewiththenotabledifferenceofnonplanarityintheshapeofstructuralridges.Thebulklatticeparametersa1=3.36Å,a2=4.53Å,anda3=11.17Å,whichhavebeenoptimizedbyDFT-PBEcalculations,areingoodagreementwiththeexperiment.Therelativelylargevalueofa3iscausedbythenonplanarlayerstructureandtheprese