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DEFECTSINTWO-DIMENSIONAL MATERIALS MaterialsToday DEFECTSIN TWO-DIMENSIONAL MATERIALS Editedby RafikAddou
LuigiColombo
Elsevier
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TypesetbyVTeX
Listofcontributors ix
Abouttheeditors xi
Preface xiii
1.Introduction
2.Physicsandtheoryofdefectsin2D materials:theroleofreduceddimensionality HANNU-PEKKAKOMSAAND
ARKADYV.KRASHENINNIKOV
2.1Introduction7
2.2Classificationofdefects8
2.3Insightsintotheatomicstructuresofdefects fromscanningtunnelingandtransmission electronmicroscopyexperiments10
2.4Productionofdefectsintwo-dimensional materialsunderelectronandion irradiation11
2.5Examplesofdefectsintwo-dimensional materials12
2.6Theoreticalaspectsofthephysicsofdefects inbulkcrystallinesolidsandtwo-dimensional materials18
2.7Calculationsofdefectformationenergiesand electronicstructureusingthesupercell approach21
2.8Electronicstructureof2Dmaterialswith defects26
2.9Pointdefectsandvibrationalpropertiesof2D materialsfromatomisticsimulations32
2.10Conclusionsandoutlook34 References37
3.Defectsintwo-dimensionalelemental materialsbeyondgraphene
PAOLADEPADOVA,BRUNOOLIVIERI,CARLOOTTAVIANI, CLAUDIOQUARESIMA,YIDU, MIECZYSŁAWJAŁOCHOWSKI,ANDMARIUSZKRAWIEC
3.1Introduction43
3.2Borophene45
3.3Silicene49
3.4Germanene52
3.5Stanene55
3.6Plumbene57
3.7Phosphorene59
3.8Arsenene(h-As)andAntimonene (h-Sb)64
3.9Bismuthene70
3.10Seleneneandtellurene72
3.11Gallenene75
3.12Hafnene77
3.13Conclusionsandoutlook78 References79
4.Defectsintransitionmetal dichalcogenides
4.1Introduction89
4.2Pointdefects89
4.3Impurities94
4.4Linedefects104
4.5Controlofdefectsandtheir applications107
4.6Summary109 References110
5.Realizationofelectronicgradegraphene andh-BN
VITALIYBABENKOANDSTEPHANHOFMANN 5.1Challengesoverview:growth,transfer,and integration119
5.2Apparatusandmethodology overview121
5.3Scalablegrowthbychemicalvapor deposition124
RAFIKADDOUANDLUIGICOLOMBO
STEPHENMCDONNELLANDPETRAREINKE
5.4Materialoptimization136
5.5Conclusionsandoutlook149 References149
6.Realizationofelectronic-grade two-dimensionaltransitionmetal dichalcogenidesbythin-filmdeposition techniques
YU-CHUANLIN,RICCARDOTORSI, NICHOLASA.SIMONSON,AZIMKHANKOZHAKHMETOV, ANDJOSHUAA.ROBINSON
6.1Currentchallengesintransitionmetal dichalcogenidesynthesis159
6.2Currentsynthesistechniques161
6.3Controllingnucleationandcrystal growth170
6.4Materialsengineering175
6.5Summary182 References183
7.Materialsengineering–defecthealing& passivation
YULIHUANG,REBEKAHCHUA,AND ANDREWTHYESHENWEE
7.1Introduction195
7.2Defectformationandhealingin2D TMDs196
7.3Defectengineeringbychemicaltreatment andapplications201
7.4Defectcontrolbyexternalsources207
7.5Futureperspectives213 References214
8.Nonequilibriumsynthesisandprocessing approachestotailorheterogeneityin2D materials
DAVIDB.GEOHEGAN,KAIXIAO,ALEXA.PURETZKY, YU-CHUANLIN,YILINGYU,ANDCHENZELIU
8.1Introduction221
8.2Non-equilibriumsynthesis–effectsof chemicalpotentialontheheterogeneityof 2Dmaterials224
8.3Straininducedphenomenain2D materials234
8.4Heterogeneityintroducedbythe self-assemblyofnanoscale‘building blocks’241
8.5Theeffectsofkineticenergyondefectsand doping:hyperthermalimplantationforthe formationofJanusmonolayers247
8.6Summaryandoutlook250 References252
9.Two-dimensionalmaterialsunderion irradiation:fromdefectproductionto structureandpropertyengineering
MAHDIGHORBANI-ASL,SILVANKRETSCHMER,AND ARKADYV.KRASHENINNIKOV
9.1Introduction259
9.2Responseoftwo-dimensionalmaterialstoion irradiation:theoreticalaspects261
9.3Experimentsonionirradiationof two-dimensionalmaterials273
9.4Applications285
9.5Summary,challenges,andoutlook289 References291
10.Tailoringdefectsin2Dmaterialsfor electrocatalysis
LEPINGYANG,YUCHIWAN,ANDRUITAOLV 10.1Introduction303
10.2Defect-tailored2Delectrocatalystsfor hydrogenevolutionreaction(HER)304
10.3Defect-tailored2Delectrocatalystsforoxygen evolutionreaction(OER)311
10.4Defect-tailored2Delectrocatalystsfor nitrogenreductionreaction(NRR)317
10.5Defect-tailored2Delectrocatalystsforcarbon dioxidereductionreaction(CO2 RR)325
10.6Challengesandperspectivesofdefect engineeringfor2Delectrocatalysts330 References332
11.Devicesanddefectsintwo-dimensional materials:outlookandperspectives
AMRITESHRAI,ANUPAMROY,AMITHRAJVALSARAJ, SAYEMACHOWDHURY,DEEPYANTITANEJA, YAGUOWANG,LEONARDFRANKREGISTER,AND SANJAYK.BANERJEE
11.1Introduction339
11.2Defectcharacterizationin2DTMDsusing ultrafastpump-probespectroscopy341
11.3Devicesfabricatedon2DCVD-grown TMDs348
11.4DevicesfabricatedonMBE-grown TMDs355
11.52DvanderWaals(vdW) heterostructures359
11.6Enhancing2Ddeviceperformanceusing defectengineering364
11.7Theoreticalinvestigationofdefectsin2D TMDs378 References387
12.Concludingremarks RAFIKADDOUANDLUIGICOLOMBO
Index 411
Listofcontributors RafikAddou UniversityofTexasatDallas,Richardson,TX,UnitedStates
VitaliyBabenko DepartmentofEngineering,UniversityofCambridge,Cambridge,UnitedKingdom
SanjayK.Banerjee MicroelectronicsResearch CenterandDepartmentofElectricalandComputerEngineering,TheUniversityofTexasat Austin,Austin,TX,UnitedStates
SayemaChowdhury MicroelectronicsResearch CenterandDepartmentofElectricalandComputerEngineering,TheUniversityofTexasat Austin,Austin,TX,UnitedStates
RebekahChua DepartmentofPhysics,National UniversityofSingapore,Singapore,Singapore
LuigiColombo UniversityofTexasatDallas, Richardson,TX,UnitedStates
PaolaDePadova IstitutodiStrutturadellaMateriaCNR(ISM-CNR),Rome,Italy
LNF-INFN,Frascati,Rome,Italy
YiDu ISEM-AIIMandInnovationCampus,UniversityofWollongong,Wollongong,NSW,Australia
DavidB.Geohegan FunctionalHybridNanomaterialsGroup,CenterforNanophaseMaterialsSciences,OakRidgeNationalLaboratory,OakRidge, TN,UnitedStates
MahdiGhorbani-Asl Helmholtz-ZentrumDresdenRossendorf,InstituteofIonBeamPhysicsandMaterialsResearch,Dresden,Germany
StephanHofmann DepartmentofEngineering, UniversityofCambridge,Cambridge,United Kingdom
YuLiHuang DepartmentofPhysics,NationalUniversityofSingapore,Singapore,Singapore
MieczysławJałochowski InstituteofPhysics,Maria Curie-SklodowskaUniversity,Lublin,Poland
Hannu-PekkaKomsa DepartmentofApplied Physics,AaltoUniversity,Aalto,Finland
AzimkhanKozhakhmetov ThePennsylvaniaState University,UniversityPark,PA,UnitedStates
ArkadyV.Krasheninnikov Helmholtz-Zentrum Dresden-Rossendorf,InstituteofIonBeamPhysics andMaterialsResearch,Dresden,Germany DepartmentofAppliedPhysics,AaltoUniversity, Aalto,Finland
MariuszKrawiec InstituteofPhysics,MariaCurieSklodowskaUniversity,Lublin,Poland
SilvanKretschmer Helmholtz-ZentrumDresdenRossendorf,InstituteofIonBeamPhysicsandMaterialsResearch,Dresden,Germany
Yu-ChuanLin ThePennsylvaniaStateUniversity, UniversityPark,PA,UnitedStates FunctionalHybridNanomaterialsGroup,Center forNanophaseMaterialsSciences,OakRidgeNationalLaboratory,OakRidge,TN,UnitedStates
ChenzeLiu FunctionalHybridNanomaterials Group,CenterforNanophaseMaterialsSciences, OakRidgeNationalLaboratory,OakRidge,TN, UnitedStates
RuitaoLv StateKeyLaboratoryofNewCeramicsandFineProcessing,SchoolofMaterialsScienceandEngineering,TsinghuaUniversity,Beijing,China
StephenMcDonnell DepartmentofMaterialsScienceandEngineering,UniversityofVirginia, Charlottesville,VA,UnitedStates
BrunoOlivieri CNR-ISAC,Rome,Italy
CarloOttaviani IstitutodiStrutturadellaMateriaCNR(ISM-CNR),Rome,Italy
AlexA.Puretzky FunctionalHybridNanomaterialsGroup,CenterforNanophaseMaterialsSciences,OakRidgeNationalLaboratory,OakRidge, TN,UnitedStates
ClaudioQuaresima IstitutodiStrutturadella Materia-CNR(ISM-CNR),Rome,Italy
AmriteshRai MicroelectronicsResearchCenter andDepartmentofElectricalandComputerEngi-
neering,TheUniversityofTexasatAustin,Austin, TX,UnitedStates
LeonardFrankRegister MicroelectronicsResearch CenterandDepartmentofElectricalandComputerEngineering,TheUniversityofTexasat Austin,Austin,TX,UnitedStates
PetraReinke DepartmentofMaterialsScience andEngineering,UniversityofVirginia,Charlottesville,VA,UnitedStates
JoshuaA.Robinson ThePennsylvaniaStateUniversity,UniversityPark,PA,UnitedStates
AnupamRoy MicroelectronicsResearchCenter andDepartmentofElectricalandComputerEngineering,TheUniversityofTexasatAustin,Austin, TX,UnitedStates
NicholasA.Simonson ThePennsylvaniaState University,UniversityPark,PA,UnitedStates
DeepyantiTaneja MicroelectronicsResearchCenterandDepartmentofElectricalandComputer Engineering,TheUniversityofTexasatAustin, Austin,TX,UnitedStates
AndrewThyeShenWee DepartmentofPhysics, NationalUniversityofSingapore,Singapore,Singapore
RiccardoTorsi ThePennsylvaniaStateUniversity, UniversityPark,PA,UnitedStates
AmithrajValsaraj MicroelectronicsResearchCenterandDepartmentofElectricalandComputer Engineering,TheUniversityofTexasatAustin, Austin,TX,UnitedStates
YuchiWan StateKeyLaboratoryofNewCeramicsandFineProcessing,SchoolofMaterialsScienceandEngineering,TsinghuaUniversity,Beijing,China
YaguoWang DepartmentofMechanicalEngineeringandTexasMaterialsInstitute,TheUniversity ofTexasatAustin,Austin,TX,UnitedStates
KaiXiao FunctionalHybridNanomaterialsGroup, CenterforNanophaseMaterialsSciences,Oak RidgeNationalLaboratory,OakRidge,TN,United States
LepingYang StateKeyLaboratoryofNewCeramicsandFineProcessing,SchoolofMaterialsScienceandEngineering,TsinghuaUniversity,Beijing,China
YilingYu FunctionalHybridNanomaterialsGroup, CenterforNanophaseMaterialsSciences,Oak RidgeNationalLaboratory,OakRidge,TN,United States
Abouttheeditors RafikAddou isaresearchscientistatthe UniversityofTexasatDallas,USA,where heleadseffortsonunderstandingtheinterfaceandsurfacescienceofgraphene, transitionmetaldichalcogenides,andother emerging2Dmaterialsfornano-andoptoelectronics.HeearnedaBScinPhysicsfrom MohamedPremierUniversity,Oujda,Morocco,andMScinMaterialsPhysicsfrom Aix-MarseilleUniversity,France.In2010,he receivedhisPhDdegreeinMaterialsSciencefromEcoledesMines(Nancy,France) inassociationwithEmpaMaterialsScience andTechnologyLaboratory(Thun,Switzerland).BeforejoiningUTD,Dr.Addouwas attheUniversityofSouthFlorida(Tampa FL,USA)asapostdoctoralresearchfellowin Physicswherehestudiedthesurfacephysics ofgraphene.
LuigiColombo istheDirectorofStrategicProgramsandAdjunctProfessorinthe DepartmentofMaterialsScience&EngineeringattheUniversityofTexasatDallas,USA. Foralmost40years,heworkedonavarietyofmaterialsresearchanddevelopment programsanddeviceintegrationatTexasInstrumentsinDallas,TX,USA.From2008to 2013,incollaborationwiththeRuoffgroup attheUniversityofTexasatAustin,USA, hediscoveredanddevelopedalargearea graphenefilmgrowthusingacatalyticCVD processonCusubstrates.
Preface Thisbook, DefectsinTwo-DimensionalMaterials,providesa reviewof thefundamentalphysicsandchemistryofdefectsin2D materialsandtheireffectsonthechemical,electronic,opto-electronic,andmechanicalandotherphysicalpropertiesrelevant toapplications.Theprimaryobjectiveisto reviewdefectspresentinavarietyof2D materials,suchastransitionmetaldichalcogenides(TMD),graphene,hexagonalboron nitride(h-BN),andelemental2Dmaterialsbeyondgraphene(e.g.silicene,phosphorene,bismuthene,tellurene,hafnene,etc.). Techniquestocharacterizedefectsin2DM willbeintroducedthroughoutthebook.The discussionsalsohighlighthowunderstandingandcontrollingthesedefectscanprovideapathwaytonovelapplicationsrequiringdefectengineering,enablingscientiststo tunethepropertiesof2Dmaterialstorealize specializedapplications.
Largenumberofreviewpapersandarticlesarebeingpublishedeveryyearon conventionalandnewlydiscovered/synthetized2Dmaterialsfromtheorytoexperimentandapplications.Thisbookmakesit easyforexpertsandnon-expertstokeepup withthelatestreports,discoveries,anddevelopmentsthatinvolvedefectsin2Dmaterials.
Thebookisdesignedtocovergrowth,theoryofdefects,identificationandcharacterizationofdefects,defecthealingandpassivation,defectcreationbyirradiationandannealing,defectengineeringforcatalysisand theroleofdefectsinelectronicdevices.This bookissuitableforbothacademiaandin-
dustryworkinginthedisciplinesofmaterialsscienceandengineeringandmaybeof interesttophysicists,chemists,andelectrical engineersaswellasbeginnerstothefield.
Theeditorswouldliketothankallthe contributorsofthebookfromalloverthe world.Especiallythemainprincipalinvestigatorsforacceptingtoparticipatetothis effort:ArkadyV.Krasheninnikov(HelmholtzZentrumDresden-Rossendorf,Germany),Paola dePadova(ISM-CNR,Italy),StephenMcDonnellandPetraReinke(UniversityofVirginia,USA), VitaliyBabenko andStephan Hofmann(UniversityofCambridge,UK),AndrewT.S.Wee(NationalUniversityofSingapore),JoshuaRobinson(PennStateUniversity,USA),DavidGeohegan(OakRidge NationalLab,USA),RuitaoLv(Tsinghua, China)andSanjayBanerjee(Universityof TexasatAustin,USA).Theeditorsalso thankSilvanKretschmer(Helmholtz-Zentrum Dresden-Rossendorf,Germany)forthebook cover.
Finally,RafikAddouthankshiswife, Sara,daughter,Yasmin,parents,Joudiaand Mimoun,andsisters,Hanan,Wissamand Sanaaforpatienceandunconditionalsupport.LuigiColombowouldliketothankhis patientwife,Sumico,duringtheeditingof thisbook.
RafikAddou ResearchScientist LuigiColombo AdjunctProfessor&DirectorofStrategic Projects August2021
1 Introduction RafikAddouandLuigiColombo
UniversityofTexasatDallas,Richardson,TX,UnitedStates
Two-dimensional(2D)materialshavebeenextensivelystudiedoverseveraldecadesbutespeciallysincetheisolationofgraphenenowover15yearsagobyNobelLaureatesKonstantin NovoselovandAndreK.Geim[1].Thisdiscoveryhasledtoasignificantlevelofeffort acrosstheworldnotonlytounderstandthefundamentalpropertiesofgraphenebutalsoreintroduceandexpandthestudytoothertwo-dimensionalmaterialssuchashexagonalboron nitride(h-BN)andtransitionmetaldichalcogenides(TMDs)[2].Thesearchfornewand“better”2Dmaterialshasalsoledtodiscoveryandgrowthofnewelementaltwo-dimensional materials.Theexpansiontoothertwo-dimensionalmaterialswasdrivenprincipallybythe needtocovertheelectromagneticspectrumwithmaterialsfromthefarinfrared(IR)tothe ultra-violet(UV)tomeetstringentcurrentandfutureapplicationsrequirements.
Graphenewithitszerobandgapcanbeusefulformanyapplicationsbuthighperformance electronicdevicestypicallyneedabandgaptoachievelowdeviceoffcurrents.Elemental2D materials[3]withnarrowbandgapscouldbeusedforsomeelectronicapplicationsrequiringnarrowbandgaps, < 1 eV,butitistheTMDswiththeirtunablebandgapsandpotential forheterostructurefabrication[4]thatareattractiveformanyelectronicandoptoelectronic applicationsincludingtransistorstohelpscalecomplementarymetaloxidesemiconductor (CMOS)devices.Finally,hexagonalboronnitride(h-BN),sometimesreferredtoaswhite graphite,aninsulatorwhichisthe2Dmaterialwiththehighestbandgap,about6eV,isa highlysoughtmaterialbecauseofitsexceptionallowinterfacetrapdensityandabilityto screenchargesfromsubstrates[5].
Fromamaterialsscienceperspective,thesuccessoftheelectronicsandoptoelectronics industryislargelyduetotheavailabilityoflargelowdefectdensitysinglecrystalsofSi, III–V,andII–VIcompounds.Further,theelectronicpropertiesofthesematerialshavebeen controlledbyintrinsicandextrinsicdopingatlevelsbelow 1014 cm 3 .Theabilitytocontrol impurities,dopingandalloyingofthesematerialshasenabledtheelectronicsindustryover thelastsevendecadestoharnesselectronictransportandelectro-opticaleffectswithunparalleledsuccess.Whiledefectchemistryandcontrolareverymatureinthesecubicsystems, thescientificcommunityisnowfacedwithasimilarchallengefor2Dmaterials.Thisbook reviewsthestatusofdefectsanddefectchemistryin2Dmaterialsaswellaschallengesand opportunitiesforthefabricationofnewdevicesandapplications.
Duringtheearlydaysof2Dmaterialsdevelopmentthefocuswasonusingsmallvery high-qualityexfoliatedgraphenefromgraphiticcarbonsourcestodevelopbasicunderstandingofthefundamentalproperties.However,asmanufacturinggrowthprocessesweredevelopedtoaddressthelackoflargeareafilmsofanyofthe2Dmaterialsandevaluatethemfor variousapplications,anotherchallengecametolight,defects.Aswillbedescribedindetail inthisbook,eventhough2Dmaterialsareatomicallythin,thereisarichnessofdefectsasin thecaseof3Dcrystals.
Thesuccessoftheelectronicsindustryisbasedonthesimultaneouscreationofthehighest crystallinequalitymaterialsinconjunctionwithprecisecontrolofdefects.Controlofdefects insemiconductorshasenabledtheindustrytofabricatemanydifferenttypesofelectronicdevicesfromsimpletransistorstoCMOSdevicestohigh-electronmobilitytransistors(HEMT) usingIII–Vcompoundheterostructures,andhigh-performanceinfrareddetectorsusingII–VI compounds.Thefabricationofthesedeviceswasenabledbythegrowthofthehighestqualitybulkcrystalsand/orthinfilmsand/ortheirheterostructuresandcontrolofinterfacesand pointdefectstomanipulatetransportofelectronsandholes,andbandstructure.Thebasic understandingofdefectsanddefectchemistryinthebulk,andinterfacesofsemiconducting heterostructuresandsemiconductorswithdielectricshasledtothefabricationofnotonly high-performancedevicesbutalsoreliabledevicessupportingafewtrillion-dollarelectronicsindustry.
Thesuccessof2Dmaterialsforelectronicapplicationswillalsolargelydependonour abilitytonotonlygrowthehighestqualitymaterialsbutalsocontroldefectsandsurfaces toasimilardegreeasintraditionalelectronicmaterialsinproductiontoday.Underideal conditions,defectsin2Dmaterialsarelikethosein3Dsemiconductormaterialswithone majordistinction,2Dmaterialshavetwosurfacesand“nobulk”.Inotherwords,surfaces playacriticalroleinthedefectformation,types,characteristics,andmethodstocontrolthem. Thehopeformanyapplicationsistotakeadvantageofthelowsurfacestatedensitydueto thesp2bondingnatureofthe2Dmaterials,andtheatomicallythinnatureforelectrostatic controlofscaledtransistors.
Thebookstartswithachapterby H.-P.KomsaandA.V.Krasheninnikov onanoverviewof recentadvancesandcurrentunderstandingofthephysicsofdefectsin2DMs.Inthischapter theauthorsdiscussthechangesinthetheoreticaldescriptionofnativeandextrinsicdefects, andthereduceddimensionalityofsomedefects:forexample,agrainboundaryisalinedefect in2D,andedgedislocationisapointdefect.Interstitialatomsdonotexistinthemajorityof 2Dmaterials(e.g.graphene,h-BN,MoS2 ),asitisenergeticallymorefavorablefortheatomto takeanadatompositionratherthanbeembeddedintotheatomicnetwork.Theauthorsalso discusstheplausibilitythatvariousspeciescanbeadsorbedatthereactivedanglingbondsof theatomsnexttovacancies(e.g.,hydrogenornitrogenatoms)in2DMoS2 andotherTMDs. These“defects”cangiverisetoshallowoccupiedstatesclosetoconductionbandminimum (CBM)oremptystatesnexttovalencebandminimum(VBM).Theauthorsfurtherdiscuss howthegeometryof2Dmaterialsrequiresmakingchangesinthetheoreticaldescription ofchargedpointandlinedefects,asthescreening/electrostaticsisstronglyanisotropicand inhomogeneous,andtheresultsmaydependontheshapeofthesimulationcellandelectrostaticcorrectionschemeused.Theauthorsfurtherstatethattheenvironmentshouldalsobe carefullyaccountedfor,bothinmodifyingthescreeningandbychoosingthevaluesofatom chemicalpotentialsmatchingtheexperimentalsituation.
Thethirdchapterby PaoladePadovaetal.on“Defectsintwo-dimensionalelementalmaterialsbeyondgraphene”presentsanextensivereviewofelemental2Dmaterialsbeyond graphenesuchasBorophene,Silicene,Germanene,Stanene,Plumbene,Phosphorene,Arsenene,AntimoneneBismuthene,SeleneneandTellurene.Here,theauthorsreviewdifferent typesofstructuralpointdefects,includingStone-Wales,singlevacancy,andbi-vacancies,as wellasgrainboundaries(lines)andadatomsasdescribedbasedontheoreticalandexperimentalpublishedreports.Thediscussionextendstohighlightingthatsomeoftheseemerging 2Dmaterialsshownon-trivialtopologicalbehaviorduetothepresenceofdefectsthatcould beofgreatinterestfortheirpotentialuseinseveralelectronicapplications.
Thefourthchapterisby StephenMcDonnellandPetraReinke on“Defectsintransitionmetal dichalcogenides”providesthereaderwithanoverviewoftheroleofdefectsontheproperties ofTMDmaterials.Theauthorslistthevarioustypesofdefectsuniqueto2Dmaterialsand provideexamplesfromtheexperimentaldatabaseandpointoutthatthecommunityisstill alongwayfromsynthesizingtrulyhigh-purityfilmscomparabletosilicon.Inthelongterm, achievinghighpuritymaterialwillsignificantlysimplifypropertyengineeringofTMDs,but inthenearterm,wemustcontinuetothoroughlycharacterizethematerialsandcontinue toconsiderbothrandom/uncontrolledandengineereddefectswheninterpretingtheirfunctionalproperties.
Tocomplementthechaptersondefects,twochaptershavebeendedicatedtocrystal growthofgraphene,h-BNandTMDs.WhileChapter 5 writtenby VitaliyBabenkoandStephan Hofmann reviewsthegrowthofelectronicgradegrapheneandh-BN,Chapter 6 writtenby YuChuanLinetal. reviewsthegrowthofelectronicgradeTMDthinfilms.Overthelastdecade orsoalargeefforthasbeendedicatedtothedevelopmentoflarge-scalecrystalgrowthof 2Dmaterials.Bottom-upthinfilmapproachesarebeingdevelopedfor2Dmaterialstomeet industrialrequirementsandextensiveeffortsarealsobeingdedicatedtotheunderstanding andcontrolofdefectsinsyntheticmaterialsasopposedtonaturallyoccurring2DmaterialslikegraphiteandsomeMoS2 .HexagonalboronnitrideandmanyotherTMDshavenot beenfoundinnature.Theseprocessesaredevelopedtoreplacetheexfoliatedprocessfor graphene,h-BNandTMDsextensivelyusedtogeneratesamplesforfundamentalstudies. Today,waferscaleprocessessuchascatalyticchemicalvapordepositionofgrapheneand h-BNhavebeendevelopedandnumerousvapordepositionprocessesarebeingdeveloped forTMDs.Theauthorsofthesetwochaptersreviewanddescribeindepthvariousprocesses usedtogrowlargearea2Dmaterialsfilmswithcontrolleddefectdensitiestomeetthemany deviceperformancerequirements.
ThepreviouschaptersongrowtharefollowedbyChapter 7 by DavidB.Geoheganetal. on“Nonequilibriumsynthesisandprocessingapproachestotailorheterogeneityin2Dmaterials”wheredefectsarestudiedwithrespecttonucleationandgrowthwherestochastic variationsinchemicalpotential,temperature,fluxofdifferentspeciespushthesynthesis environmentoutofequilibrium.Cooperativeeffects,suchasstrainaccumulationduetocoalescencewithothercrystallinedomainsduringgrowth,canalsoinducebothlocalizedand long-rangeheterogeneities.Inthecaseof2DTMDmaterialssucheffectsaremanifestedas changesinoptoelectronicproperties.Theauthorsgoontodescribeasynergisticapproachto revealthesyntheticoriginsofheterogeneityin2DTMDmaterialsthatinvolvesacombination of:(1)temporally-andspatially-resolvedinsitudiagnosticsofgrowthenvironment,using primarilyopticalspectroscopicandelectronmicroscopytechniques,(2)acorrelationbetween
spectroscopicmapsofoptoelectronicpropertiesandatomisticcharacterizationofheterogeneity,usingprimarilyZ-contrastscanningtransmissionelectronmicroscopy,and(3)correlated theoryofelectronicandvibrationalproperties,leadingtocomputationalmodelingsimulationsofsynthesisdynamics.Theauthorsthendiscussrecentprogressontheroleofkinetic energyondefectgenerationinatomicallythin2DTMDcrystalsfortheformationofJanus TMDmonolayers.
InChapter 8 “Materialsengineering–defecthealing&passivation”theauthors YuLi Huangetal.reviewstrategiesondefecthealingandpassivationasappliedtoTMDmaterials. Theydiscusstheformationofintrinsicdefectsandself-healingwithsmallmolecules(e.g., O2 )aswellasviachemicaltreatments,e.g.,decorationwithorganiclayers,whichresultin improveddevice.Thermalannealing,electronbeamirradiation,plasmatreatment,andencapsulation,arealsodiscussed.Theyconcludethechapterwithanoutlookonthechallenges andopportunitiesfordefectminimizationin2Dmaterials,withafocusonTMDs.
MahdiGhorbani-Asletal. inChapter 9 presentareviewon “Two-dimensionalmaterialsunder ionirradiation:fromdefectproductiontostructureandpropertyengineering” wheretheydiscuss theeffectsofionirradiationof2Dmaterialsandtheroleofreduceddimensionality.They goontocomparetheimpactofionson3Dand2Dmaterials.Theprimaryobjectiveofthis chapteristoreviewanddiscusstheeffectofionbeamson2Dmaterialsandhowtheycanbe usedtoengineertheirstructureandproperties.Itshouldbenotedthattheworkreviewedin thischapterismostlyongrapheneandMoS2 giventhelimiteddataonother2Dmaterials.
Chapter 10 by LepingYangetal.isdedicatedto“Tailoringdefectsin2Dmaterialsforelectrocatalysis”wheretheauthorssummarizethestate-of-the-artadvancesindefectengineeringof 2Dmaterialsforelectrocatalyticapplicationsinvolvinghydrogenevolutionreaction(HER), oxygenevolutionreaction(OER),carbondioxidereductionreaction(CO2 RR),andnitrogen reductionreaction(NRR).Thechallengesthatthecommunityfacesinthisveryfertilefield aretuningsurfaceelectronicstatestooptimizethereactionenergybarriers,theincorporation ofsufficientactivesitestoimprovethesurfaceelectrochemicalreactivity,andenhancingthe electricalconductivitytoaccelerateelectrontransportamongthereactioninterfaces.
Thelastchapterofthebookbeforethesummaryby A.Raietal. on“Devicesanddefects intwo-dimensionalmaterials:outlookandperspectives”presentsacomprehensivereview on2Dmaterials-baseddevicesandeffectsofdefects,interfaces,interfacialdefects,anddielectricsonelectronicdevicecharacteristicsandtransport.Theyreportondevicesfabricated onCVDandMBEgrown2Dfilmsandusingheterostructuresformedusingavarietyof 2Dmaterials.TheypresentaplethoraofdataontheeffectofmanydielectricsonthedevicepropertiesofTMDsbaseddevices.TheyalsodiscussthefabricationofVanderWaals heterostructureswhereanyatomicallythin2Dmaterialcanbestacked,rotated(twisted)to achievethedesiredbandstructurethusleadingtounprecedentedadvancementinelectronics.ThechapterconcludeswithareviewonthetheoreticalinvestigationofdefectsinTMDs.
References
[1]ScientificBackgroundontheNobelPrizeinPhysics2010,ClassforPhysicsoftheRoyalSwedishAcademyof Sciences. https://www.nobelprize.org/uploads/2018/06/advanced-physicsprize2010.pdf,2010.
[2]K.S.Novoselov,D.Jiang,F.Schedin,T.J.Booth,V.V.Khotkevich,S.V.Morozov,A.K.Geim,Two-dimensional atomiccrystals,ProceedingsoftheNationalAcademyofSciencesoftheUnitedStatesofAmerica102(30)(2005) 10451–10453.
[3]N.R.Glavin,R.Rao,V.Varshney,E.Bianco,A.Apte,A.Roy,E.Ringe,P.M.Ajayan,Emergingapplicationsof elemental2Dmaterials,AdvancedMaterials32(7)(2020).
[4]F.N.Xia,H.Wang,D.Xiao,M.Dubey,A.Ramasubramaniam,Two-dimensionalmaterialnanophotonics,Nature Photonics8(12)(2014)899–907.
[5]C.R.Dean,A.F.Young,I.Meric,C.Lee,L.Wang,S.Sorgenfrei,K.Watanabe,T.Taniguchi,P.Kim,K.L.Shepard, J.Hone,Boronnitridesubstratesforhigh-qualitygrapheneelectronics,NatureNanotechnology5(10)(2010) 722–726.
Physicsandtheoryofdefectsin2D materials:theroleofreduced dimensionality Hannu-PekkaKomsaa andArkadyV.Krasheninnikovb,a
a DepartmentofAppliedPhysics,AaltoUniversity,Aalto,Finland b Helmholtz-Zentrum Dresden-Rossendorf,InstituteofIonBeamPhysicsandMaterialsResearch,Dresden,Germany
2.1Introduction Defectsincrystallinesolidsareunavoidable.Indeed,thesecondlawofthermodynamics statesthatacertainamountofdisordershouldbepresentinanymaterialatfinitetemperatures,providedthatthesystemisinthermodynamicequilibrium.Moreover,theconcentrationofdefectsinsyntheticmaterialscanbewellabovetheequilibriumvaluedepending uponthepreparationprocess.Typicallymaterialsgrownundernearequilibriumconditions attemperaturesclosetooratthemeltingpointtendtohavelowerdefectdensitiesthanmaterialgrownundernon-equilibriumconditions.Defectscanalsoappearduetoexternalfactors, suchaspressureorirradiationwithenergeticparticles.Defectsstronglyinfluencethebehaviorofcrystallinesolids,somuchsothatbothscientistsandengineershavedevotedtheir careerstonotonlyreducethem,butcontrolthemwithgreataccuracy[1].Specifically,defects cancompletelygoverntheelectronic,optical,thermal,andmechanicalpropertiesofthesolid. Althoughtheword“defect”hasnegativeconnotation,imperfectionsincrystalsdonot alwayshavedetrimentaleffectsonmaterialsproperties[2],withthemostprominentexamplebeingthedopingofsemiconductorsbycontrollableintroductionofimpuritiesusingion implantationorcolorcentersinwide-gapsemiconductors[3].Otherexamplesincludethe pinningofmagneticvorticesondefectsintype-IIsuperconductors[4],orthepossibilityto controlmechanicalstrengthandductilityofmetalsbyintroducingdislocations,grainboundariesandimpurities[5].Toemphasizetheversatileandimportantroleofdefects,itistemptingtoquotefromtheAshcroft-Mermin’stextbook[6]:“Likehumandefects,thoseofcrystals comeinaseeminglyendlessvariety,manydrearyanddepressing,andafewfascinating.”
2.Physicsandtheoryofdefectsin2Dmaterials:theroleofreduceddimensionality
Asforirradiation-induceddefects,theycaneasilybeproducedinirradiation-hostileenvironments,suchasopenspaceorfission/fusionreactors.Moreover,thetreatmentsofsolids withbeamsofenergeticionsandelectronshavebeenshowntobeveryusefulfortailoring propertiesofmaterialsaftersynthesis.Theseaspectshavestimulatedhugeinterestnotonly inthephysicsofdefects,butalsointheirproductionmechanismsunderirradiation,includingknock-onorballisticdamage,electronicexcitationsand,inlow-dimensionalmaterials, irradiation-inducedchemicaletching.
Nano-structuredmaterialsalsohavedefectsandimpurities.Thereduceddimensionality ofthesesystems,however,makesthephysicsofdefectsquitedifferentfromthatinbulk systems.Thisisparticularlyrelevanttotwo-dimensionalmaterials(2DMs),whichsincethe isolation[7]ofasinglesheetofgraphenebyA.GeimandK.Novoselov,havebeenatthe forefrontofresearchduetoauniquecombinationoftheirelectronic,optical,andmechanical propertiesandpotentialapplicationsinnanoelectronics,photonics,catalysis,aswellasenergystorageandconversion,andmore,seeRefs.[8–11]foranoverview.Thefamilyofthe experimentallysynthesized2DMsincludesnowaboutahundredmembers,withthemost prominentonesamongthem(inadditiontographene)beinghexagonalboronnitride(hBN)[12]andtransitionmetaldichalcogenides(TMDs)[13].
In2DMs,evenwhentheformationenergyofdefectsishighasinthecaseofgraphene,so thattheequilibriumconcentrationofdefectsisnegligibleoverawiderangeoftemperatures, imperfectionscanappearduetotheinteractionwiththeenvironment[14–16],as2DMshave averyhighsurface-to-volumeratio,Fig. 2.1(a,b).Moreover,anyspeciesonthesurfaceof 2DMscanhavestrongeffectonthematerialproperties,whiletheprocessesonthesurfaces ofbulksystemsarenormallycompletelyignoredinthecontextofpoint(andfrequentlyline) defects.
Inthischapter,wefirstgiveabriefoverviewofthetypesofdefectsinbulksystemsand 2DMswiththemainfocusonthedifferencesoriginatingfromthereduceddimensionalityof thelatter.Webrieflydiscusshowthegeometryof2DMsalsomakesitpossibletodeliberately introducedefectswithnearlyatomicspatialresolutionusingfocusedionandelectronbeams, whichalsohelpsinimagingthedefectsusingscanningprobeandtransmissionelectronmicroscopy.Wealsodiscussthechangeswhichshouldbemadeinthetheoreticaldescriptionof nativeandextrinsicdefects.Finallywebringforththechallengesinthisfieldandtheissues whichstilllackbasicunderstanding.Westressthatourmaingoalisnottogiveacomplete overviewofallpossibledefectsinall2DMs,buttoanalyzegeneraltrendsandillustratethem byafewexamples.
2.2Classificationofdefects Priortodiscussingdefectsin2DMs,itisinstructivetogiveageneraldefinitionofdefects incrystallinematerialsandtheirclassification.Thesimplestnotionofadefectinacrystalline solidistheideaofastructuralimperfection(e.g.,missingatomorpresenceofanimpurity), thatisadeviationfromtheperfectorder(periodicity).Amorerigorousdefinitioncanbe formulatedasfollows:“Astructuraldefectisaconfigurationinwhichanatom(orgroupof atoms)doesnotsatisfythestructurerulespertainingtotheidealreferencesystemormaterial”.
FIGURE2.1 (a)Schematicillustrationofavacancyformationinabulkcrystallinesolid.Theeffectsoftheenvironmentarenormallyneglected.Thegreyarearepresentsthesupercellusedindefectsimulations.(b)Formationof avacancyina2DM.Unlessvacuumisperfect,thereactivedanglingbondsatthevacancycanpickupatomsfrom theenvironment,makingtheequilibriumvacancyconcentrationdependentonthechemicalenvironment.Notethat inideal2DMs,likegrapheneorh-BN,adatomsplaytheroleoftheinterstitialsasinbulkcrystals.(c)Impactofan energeticionontoabulktarget.Theionkineticenergyistransferredballisticallytothetargetatoms,whichresults inatomdisplacement,alongwithtemperatureandpressurerise,andultimatelytodefectformation.Theatomsare coloredaccordingtotheirkineticenergyfromblue(zeroenergy)tohigh(red)energies.(d)Impactofanionontoa 2DM.Whilealltheionenergyisdepositedintothebulksystems,onlyafractionofionenergyistransferredtothe 2Dtarget.Besides,aconsiderableamountofthedepositedenergyistakenawaybysputteredatoms.
Defectscanbeclassifiedaccordingtotheir dimensionality.Correspondingly,in bulk systems,onecandifferentiatebetween:
•Pointdefectsor0Ddefects,e.g.,vacancies;
•Lineardefectsor1Ddefects,e.g.,dislocations;
•Planardefectsor2Ddefects,e.g.,grainboundaries;
•Volumedefectsor3Ddefects,e.g.,voids.
Wenoteherethatedgedislocationscouldalsobeconsidered2Ddefectsduetotheaddedor removedhalf-planeora3Ddefectduetofinitethicknessofthehalf-plane.Thegivenexamplesfrombulksolidsmaynotbeappropriateinthecaseof2DMs.Theedgedislocationsin 2DMsare0Ddefects,sincethedislocationlinecannotexistperpendiculartothelayer,andthe sameistruefordisclinations.Screwdislocationscannotbepresentinthecaseofmonolayers either,butcanconnectlayersinmultilayerstructures[17–19].Similarly,grainboundariesin 2DMsbecome1Ddefects.Voidsin2Dsystemsareholesformedby,e.g.,agglomerationof vacancies.
2.Physicsandtheoryofdefectsin2Dmaterials:theroleofreduceddimensionality
Pointdefectscanbefurthercharacterizedaccordingtothe chemicalcontent:
•Intrinsicdefects,e.g.,vacancies,self-interstitials;
•Extrinsicdefects,e.g.,impurityatoms;
•Antisitedefectsincompoundsolids;
•Isotopes.
Notethatimpurityatomscanbeinbothsubstitutionalandinterstitialpositions,thatis, theycanoccupythepositionsofhostatoms,orbebetweenthem.Asforisotopes,allthe atomsinthiscasearechemicallyequivalent,butdifferentmassesoftheisotopeatomsaffectvibrationalandindirectlyelectronicproperties,givingrise,e.g.,totheisotopeeffectin superconductors.
Pointdefectscanalsobeclassifiedaccordingtothelocalnumberofatomsinacertain volume(areain2DMs):Whenlocallytherearefeweratomsthaninthepristinelattice,one cantalkaboutvacancy-typedefects,andwhentheoppositeholds,aboutinterstitials.Atthe sametime,thenumberofatomscanbethesameasintheperfectlattice,buttheycanbe arrangedinadifferentway,dueto,e.g.bondrotations(notethatsuchconfigurationshavea higherenergy).Examplesofsuchdefects,alsoreferredtoasWignerdefects[20],arebound vacancy-interstitialpairsingraphite[20]andsilicon[21],orStone-Walesdefects[22]inthe sp2 -hybridizedcarbonsystems.Thelattercanrelativelyeasilybeannealed,as,forexample, contrarytovacanciesandinterstitials,theydonotrequiremigrationofthedefects,andthey maystoreaconsiderableamountofenergy.
Onecanalsoclassifydefectsaccordingtotheir origin:forinstancenativedefects(vacanciesandinterstitials)maynaturallyexistinthespecimenatfinitetemperatures(itisassumed thatthesystemisinequilibrium)ortheycanbeinducedbyhigh-temperatureannealingina specificatmosphereorirradiation–theso-called“irradiation-induceddefects”.Thiswording isfrequentlyusedtoemphasizethattheconcentrationofdefectsiswellabovetheequilibriumvalue,whileitisfundamentallyimpossibletosayifaparticularvacancy(oranyother defect)wasproducedduetoathermalfluctuationortheimpactofanenergeticparticle.In thecaseofhigh-temperatureannealing,suchasvacanciesinmulti-componentII–VIandIII–V systemsannealedunderdifferentpressuresofoneofthecomponents,thedefectconcentrationmaybeclosetoequilibriumattheelevatedtemperature,butwellabovetheequilibrium concentrationatroomtemperature.
2.3Insightsintotheatomicstructuresofdefectsfromscanningtunnelingand transmissionelectronmicroscopyexperiments
Variousexperimentaltechniquescanbeusedtoprobedefectsin2DMs,seeChapter 3 for details.Defectsandimpuritiescanbedetectedusingopticalspectroscopy[23]orbyX-ray absorptionspectroscopyandrelatedmethods(XAFS,XANES)basedonprobingthedefectinducedelectronicstatesbyexcitingcoreelectronsintheatomsnexttothedefect[24].Also Ramanspectroscopycanbeusedtoidentifythepresenceofdefectsinthesampleandto assessdefectconcentration[25,26].
High-resolutiontransmissionelectronmicroscopy(HR-TEM)andscanningtransmission electronmicroscopy(STEM)haveproventobeveryefficienttoolstogetinsightsintothe
atomicstructureofdefects.Infact,thequickprogressintheinvestigationofdefectsin2DMs canbepartlycreditedtotherecentimpressivedevelopments[27–29]inaberration-corrected HR-TEM(asevidentfromthe2020KavliPrizeinNanoscienceawardedexactlyforthese developments)andtheverynatureofany2Dsystem.TheTEMstudiesofbulkmaterials assumefabricationofathin(yetpreservingthemorphologyofthebulksystem)sample,followedbythereconstructionofitsatomicstructurefromtheTEMimage,whereonespotcan correspondtoacolumnofatoms.TEMcharacterizationof2DMsdoesnotrequirethis,asthe structureisalreadyatomicallythin,andeveryatomcandirectlybe“seen”.Moreover,byfocusingelectronbeamonspecificareas(essentiallyonsingleatom),andusingelectronenergy lossspectroscopy(EELS),itispossiblenowtoidentifyimpurityatomsinsubstitutionalpositions[30]andgettheinformationonthelocalelectronicstructure/bondingconfigurations [29,31],magneticstates[32],andevenphonons[33].
Thebombardmentofthesamplebytheenergeticelectrons(inthe30-200keVrange)may leadtosampledamage.Ononehand,fastorexcessivedamagemayhindertheexperiments, butontheotherhand,theabilitytocreatedefectsondemandisidealwhenonewantsto studydefects.
Nowadays,eventhefullthree-dimensionalstructure,yieldinginformationaboutrippling andbendingarounddefectscanbeobtainedusingSTEMwithtilting(tomography)and computer-aidedreconstructiontechniquestakingadvantageof(S)TEMimagesimulations [34,35].Recently,ithasalsobecomepossibletoextractchargedensityandelectricfieldmaps arounddefectsusingelectronholographyandelectronptychography[36,37].
Theuniquegeometryof2DMs(surfaceonly),enablesonetodirectly“see”thedefects usingscanningprobemicroscopy(scanningtunnelingmicroscopy,STM,andatomicforce microscopy,AFM,whichcanbothbereferredtoasscanningprobemicroscopy,SPM)techniques.ByscanningtheSTMtipbias,denotedscanningtunnelingspectroscopy(STS),itis alsopossibletoobtainenergy-resolvedinformationontheelectronicstatesclosetothetip and,forinstance,findtheenergiesofdefectstateswithinthebandgap.Theobtainedspectraoftencloselyresemblethelocaldensityofstates,whichcanbereadilycomparedtothose obtainedfromelectronicstructurecalculations.SinceSTM/STSyieldsspatially-resolvedand energy-resolvedinformationabouttheelectrondensity,itthuscomplementsthemoredirect structuralinformationobtainedfrom(S)TEM.STMisalsolargelynon-destructivemethod, andthusenablesindepthstudiesofelectrondynamicsin/aroundthedefects,e.g.,defect chargingorTomonaga-Luttingerliquid[38,39].Ontheotherhand,theinterpretationofSTM imagesisoftenmorecomplicated,asitprobestunnelingcurrenttolocalelectronicstatesina certainenergyrange,nottheatoms,andmayrequirecomparisonwiththecalculatedimages.
2.4Productionofdefectsintwo-dimensionalmaterialsunderelectronandion irradiation
Thereduceddimensionalityof2DMsalsoaffectthebehaviorofthesesystemsunderimpactsofenergeticparticles,suchaselectronsandions.Herewebrieflytouchuponthemain aspectsoftheirradiationresponseof2DMs,whilethedetailedanalysisoftheexperimental
