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Two-Dimensional Materials for Nonlinear Optics: Fundamentals, Preparation Methods, and Applications

Qiang Wang

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Two-DimensionalMaterialsforNonlinearOptics

Two-DimensionalMaterialsfor NonlinearOptics

Fundamentals,PreparationMethods,andApplications

EditedbyQiangWangandHao-LiZhang

Editors

Prof.QiangWang LanzhouUniversity CollegeofChemistryandChemical Engineering No.222TianshuiSouthRoad 730000Lanzhou China

Prof.Hao-LiZhang LanzhouUniversity CollegeofChemistryandChemical Engineering No.222TianshuiSouthRoad 730000Lanzhou China

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Contents

Preface xiii

ListofAbbreviations xv

1Preparationof2DMaterials 1 YueTangandHuaXu

1.1MechanicalExfoliationof2DMaterials 2

1.2Liquid-PhaseExfoliationof2DMaterials 4

1.3ChemicalVaporDepositionGrowthof2DMaterials 6

1.4CVDGrowthofWafer-ScaleSingleCrystal2DMaterials 8

1.5ThicknessControlinCVDGrowthof2DMaterials 10

1.6PhaseControlinCVDGrowthof2DMaterials 12

1.7SummaryandProspect 14 References 15

2AnIntroductiontotheNonlinearOpticalPropertiesof 2DMaterials 21

BolongWangandHao-LiZhang

2.1Introduction 21

2.2NonlinearOpticsof2DMaterials 22

2.2.1SHG,THG,andHHGSetups 24

2.2.2Four-WaveMixing 26

2.2.3Z-ScanTechniques 27

2.2.4NonlinearOpticalImaging 29

2.2.5Pump–ProbeTechniques 32

2.3Applicationof2DNonlinearMaterials 35

2.3.1OpticalLimiting 35

2.3.2Q-SwitchedandMode-LockedLasers 38

2.3.3OpticalSwitchandModulation 40

2.3.4OtherNonlinearOpticalPhenomena 41

2.4Prospect 43

2.4.1PreciseFabricationandFunctionalizationof2DMaterials 43

2.4.2HighResolutionSpatiotemporalCharacterizingTechniques 44

2.4.3NewPhysicsof2DMaterials 44

2.4.4Ever-LastingExpansionofApplications 44 Acknowledgment 45 References 45

3ModulationandEnhancementofOpticalNonlinearityin 2DMaterials 55 XinglinWenandQihuaXiong

3.1Introduction 55

3.2NonlinearOpticsin2DMaterials 57

3.2.1BasicsofNonlinearOptics 57

3.2.2ParametricNonlinearOptics 58

3.2.3NonparametricNonlinearOptics 59

3.3NonlinearityModulationin2DMaterials 60

3.3.1StructuralEngineering 60

3.3.1.1LayerNumber 60

3.3.1.2TwistedAngle 62

3.3.1.3StackingOrder 64

3.3.1.4EdgeStates 65

3.3.2OpticalModulation 66

3.3.2.1ExcitonicEffectsEnhancedNonlinearity 66

3.3.2.2Valley-DependentNonlinearity 69

3.3.2.3OpticalCavity-EnhancedNonlinearity 71

3.3.2.4All-OpticalModulation 73

3.3.3ElectricalModulation 75

3.3.4MagneticModulation 78

3.3.5Perspectives 78

AuthorContributions 78 Notes 79 Acknowledgment 79 References 79

4CharacterizingtheNonlinearOpticalPropertiesof 2DMaterialsbyDouble4fNonlinearImagingSystemwith PhaseObjectandFour-Wave-MixingMicroscopy 87 ZhongguoLiandYinglinSong

4.1Introduction 87

4.2PrincipleofNLOMeasurementTechnique 88

4.2.1NonlinearOpticalInteractionin2DMaterials 88

4.2.2TypicalNLOMeasurementMethodin2DMaterialResearch 89

4.2.2.1Self-PhaseModulation(SPM) 89

4.2.2.2Z-Scan 89

4.2.2.3Third-HarmonicGeneration(THG) 90

4.2.2.4Two-Photon-InducedPhotoluminescence(TPL) 90

4.3CharacterizingNLOResponseof2DMaterialsviaDouble4fNonlinear ImagingSystemwithPhaseObject(NIT-PO)Microscopy 91

4.4CharacterizingNLOResponseof2DMaterialsviaFour-Wave-Mixing Microscopy 95

4.5OutlookandPerspective 97 References 98

5UltrafastCarrierDynamicsinEmerging2DMaterials 103 JiaweiHuang,NingningDong,andJunWang

5.1Introduction 103

5.2UltrafastTime-ResolvedSpectroscopy 104

5.2.1PrincipleofPump–ProbeMeasurement 104

5.2.2ExperimentforUltrafastSpectroscopy 107

5.2.3FundamentalMechanismofRecombinationDynamics 107

5.3UltrafastOpticsinvanderWaals2DMaterials 109

5.3.1Graphene 109

5.3.2TransitionMetalDichalcogenides(TMDC) 109

5.3.3BlackPhosphorus 115

5.4UltrafastOpticsinEmergingQuasi-2DMaterials 118

5.4.12DPerovskite 118

5.4.2Non-Layered2DPlatinumSulfide 120

5.5PerspectivesonUltrafastOpticsfor2DMaterial 124 AuthorContributions 125 Notes 125

Acknowledgment 125 References 125

6TransientTerahertzSpectroscopyfor2DMaterials 131 JingyinXu,KaiZhang,HongLi,andTianwuWang

6.1Introduction 131

6.2GenerationandDetectionofTHzRadiation 132

6.2.1GenerationofTHzRadiationBasedonNonlinearOpticalProcesses 132

6.2.1.1PhotoconductiveAntennas(PCAs) 133

6.2.1.2Electro-optic(EO)Crystals 135

6.2.1.3AirPlasma 136

6.2.1.4THzEmissionof2DSemiconductorMaterials 140

6.2.2TypicalTHzTime-DomainSpectroscopySetup 141

6.2.2.1Time-DomainTHzSpectroscopyUnderPressure 143

6.2.3PhotoinducedChangesoftheDielectricFunctionin2DviaTHz Spectroscopy 147

6.3NanoscaleTHzScanningProbeMicroscopyof2DMaterials 149

6.3.1ScanningNear-FieldTHzMicroscopyforPolaritonin2DMaterials 150

6.3.2THz-STM 151

6.4Perspectives 154

6.4.1TransientTHzSpectroscopy 154

6.4.2High-ResolutionSpatialandTemporalTHzSpectroscopies 155

Acknowledgment 155 References 155

7GrapheneGlassforNonlinearOptics 163 QiXiao,JingyuSun,andHao-LiZhang

7.1LightAbsorptionofGraphene 163

7.2NonlinearOpticalPropertiesofGraphene 164

7.2.1Third-OrderNonlinearOpticalPropertiesofGraphene 164

7.2.2SaturableAbsorptionofGraphene 165

7.2.3High-OrderHarmonicGenerationofGraphene 165

7.3NonlinearOpticalPropertiesofGrapheneGlass 167

7.3.1Graphene-DopedGlass 167

7.3.2GrapheneGlassFabricatedbySpinCoating 167

7.3.3GrapheneGlassFabricatedbyTransferMethod 168

7.3.4DirectlyGrowthGrapheneonGlass 168

7.3.4.1GrapheneGrowthonHigh-Temperature-ResistantGlass 168

7.3.4.2GrapheneGrowthonMoltenGlass 171

7.3.4.3Metal-Catalyst-AssistedGrapheneGrowth 173

7.3.4.4Plasma-EnhancedLow-TemperatureGrapheneGrowth 176

7.4Perspectives 179

Acknowledgment 180 References 180

82DMaterialsforNonlinearOpticalLimiting 185 WenShang,BolongWang,andQiangWang

8.1Introduction 185

8.2NonlinearOpticalLimitingMechanism 186

8.2.1ReverseSaturableAbsorption 186

8.2.2Two-Photon/MultiphotonAbsorption 187

8.2.3FreeCarrierAbsorption 188

8.2.4NonlinearRefraction 188

8.2.5NonlinearScattering 189

8.32DMaterialsforOpticalLimiting 189

8.3.1GrapheneandAnalogs 190

8.3.1.1Graphene 190

8.3.1.2TransitionMetalDichalcogenides 194

8.3.1.3BlackPhosphorus 196

8.3.1.4HexagonalBoronNitride(h-BNS) 197

8.3.2Novel2DMaterials 197

8.3.2.1Antimonene 197

8.3.2.2Tellurene 198

8.3.2.32DPerovskites 200

8.3.2.42DMetal–OrganicFrameworks(2D-MOFs) 201

8.3.3MaterialswithNewStructuresandMechanism 203

8.3.3.1NonlinearPhotonicMetamaterials 203

8.3.3.2PlasmonicEffect-EnhancedNonlinearity 205

8.3.4MultifunctionalOpticalLimitingDevices 206

8.4ConclusionsandProspects 209 Acknowledgments 210 References 210

9TheSaturableAbsorbersBasedon2DMaterials 221 Xin-HaiYan,LeiZhang,andKai-GeZhou

9.1Introduction 221

9.2TheFundamentsinthe2DMaterials-BasedSaturableAbsorbers 222

9.2.1SATheory 222

9.2.2Slow-andFast-SaturableAbsorber 223

9.2.2.1Slow-SaturableAbsorber 224

9.2.2.2Fast-SaturableAbsorber 225

9.3TheFamilyof2DMaterial-BasedSaturableAbsorbers 225

9.3.1Mono-Elemental2DMaterials 226

9.3.1.1GroupIVA-Elemental2DMaterials 226

9.3.1.2GroupVA-Elemental2DMaterials 230

9.3.2Dual-Elemental2DMaterials 234

9.3.2.1TMDs 234

9.3.2.2TopologicalInsulators(TIs) 235

9.3.3Multi-Elemental2DMaterials 237

9.3.3.1MOFs 237

9.3.3.2MXenes 237

9.3.3.32DPerovskites 239

9.4Applications 239

9.4.1FabricationofSADeviceBasedon2DMaterials 239

9.4.1.1ScotchTapeExfoliation 240

9.4.1.2CVD,PVD 240

9.4.1.3LPE 240

9.4.2Q-SwitchedLaser 242

9.4.2.1Q-SwitchedFiberLaser 243

9.4.3Mode-LockedLaser 246

9.5PerspectivesandOutlook 248

Acknowledgment 249 References 249

10Second-HarmonicandThird-HarmonicGenerationsin 2DLayeredMaterials 257 XudongJinandMinZhao

10.1Introduction 257

10.1.1FundamentalsofSHGandTHG 258

10.1.22DLM-BasedSHG 259

10.1.3Graphene 260

x Contents

10.1.4TransitionMetalDichalcogenides 262

10.1.5Other2DLMs 266

10.1.62DLM-BasedTHG 268

10.1.6.1Graphene 268

10.1.7TransitionMetalDichalcogenides 268

10.1.7.1Other2DLMs 270

10.2ConclusionsandOutlook 274 References 274

112DPerovskitesforNanolasers 281

WeiYuanandChuanjiangQin

11.1Introduction 281

11.2LaserFormationPrincipleandPerformanceParameters 283

11.2.1BasicOpticalPhysicalProcess 283

11.2.2LaserFormationCondition 283

11.2.3PerformanceParameters 284

11.2.4OpticalCavities 285

11.3TheApplicationof2DPerovskitesinNanolasers 286

11.3.1Microplates 287

11.3.2Microrods 287

11.3.3Microcrystals 291

11.3.4Nanowires 291

11.3.5Microrings 293

11.3.6ThinFilms 294

11.3.7ThinFlakes 298

11.4Prospect 299 References 300

122DMaterialsforSpaceUse 303

ShuyanWangandQiangWang

12.1Introduction 303

12.2SpaceRadiationandTwo-DimensionalMaterials 305

12.2.1SpaceRadiationEnvironment 305

12.2.22DMaterialsUnderIrradiation 307

12.2.2.1ElectronIrradiation 307

12.2.2.2IonIrradiation 307

12.2.2.3ProtonIrradiation 308

12.2.2.4 γ-RayIrradiation 308

12.32DMaterialsforSpaceUse 309

12.3.1EmergingNonlinearOpticalApplications 309

12.3.1.1LaserProtectionandModeLocking 310

12.3.1.2OtherNonlinearPhenomena 313

12.3.2SpaceX-rayDetectionandImaging 314

12.3.3PerovskiteSolarCells(PSCs) 316

12.3.42DSingle-PhotonSources 317

12.3.5IntegratedOptoelectronicPlatform 318

12.3.5.1Field-EffectTransistorandPhotodetectors 318

12.3.5.2Sensors 320

12.3.6OtherApplications:Lubricants 320

12.4Perspectives 321

Acknowledgment 322

References 322 Index 333

Preface

Thedevelopmentoftwo-dimensional(2D)materialshasgarneredincreasing interestduetotheiruniquestructureandversatileapplications.Forinstance,inthe optoelectronicandphotonicfields,2Dmaterialshavebeenbroadlyemployedfor opticalmodulationthroughopticallimiting/modelocking,photodetectors,optical communications,miniaturizedall-opticaldevices,andsoon.Theever-expanding 2Dfamilyparticularlyoffersabundantsourcestotheemergingnewphysical insightsandphenomenaintraditionalnonlinearoptics.Althoughpreviously therehavebeenmanyexcellentreviewsontheapplicationsof2Dmaterialsinone specificfield,systematicintroductionintegratingallofthekeyelementsranging frommaterialfabricationstonovelcharacterizingtechniquesandmyriadnonlinear opticalapplicationsisstillrare.

Hence,inthisbook,collectiveeffortsthroughresearchersfrominterdisciplinary fieldsfocuson2Dmaterialsfornonlinearopticalapplicationstofurthertheexciting territorytowardanewhorizon.Preparationmethodsforvarious2Dmaterials includinggrapheneglass,transitionmetaldichalcogenides(TMDs),singleelemental2DmaterialsfromGroup15,andperovskiteshavebeenthoroughlysummarized. Moreover,powerfulnonlinearopticalcharacterizingtechniques,suchasZ-scan measurement,femtosecondtransientabsorptionspectroscopyandmicroscopyare introduced.Theoriginoftheuniqueoptoelectronicpropertiesof2Dmaterialsis elucidatedfromvariousperspectiveslikeexcited-statedynamics.

Specifically,Chapter1providesacomprehensiveperspectiveonthefundamentals andpreparationmethodsof2Dmaterialswithnonlinearopticalproperties.Chapter 2isanoverallintroductiontothenonlinearopticalpropertiesof2Dmaterials, andChapter3elaboratesonthemodulationandenhancementofopticalnonlinearityin2Dmaterials.Furthermore,Chapters4–6discussthenonlinearoptical characterizationtechniquesfor2Dmaterialsandtheirapplicationsinthespecific field,includingnovelnonlinearopticalimagingsystems,ultrafasttime-resolved spectroscopyforinvestigatingcarrierdynamicsinemerging2Dmaterials,transient terahertzspectroscopy,etc.Chapter7expoundsongrapheneglassfornonlinear opticswhileChapters8–12emphasizetheapplicationsof2Dmaterialsforoptical limiting,saturableabsorber,SHGandTHG,nanolasers,andspaceuse,respectively. Thetopicscoveredinthisbookareofvitalimportanceinunderstandingthe uniqueoptoelectronicpropertiesandapplicationsof2Dmaterialsandthephysical

xiv Preface

insightsbehindthem.Wehopethebookwillbeahandytoolkitforprofessionals, researchers,andgraduatestudentsworkingorinterestedinthefieldofnonlinear opticsandmaterials.

Theeditorsthankallthecontributorsofthisbookfortheirinsightfulchapters.We aregratefultotheproductioneditorsatWiley,Ms.LifenYangandKatherineWong, fortheirdiligentworkandexpertiseinmanagingtheprocess.

Lanzhou,People’sRepublicofChina QiangWang Hao-LiZhang

ListofAbbreviations

Number

2DTwodimensional

2DLMs2Dlayeredmaterials

2DCSTwo-dimensionalcoherentspectroscopy

1DOnedimensional

0DZerodimensional

2PATwo-photonabsorption

3PAThree-photonabsorption

3PMThree-photonmicroscopy

ABCDAir-biasedcoherentdetection

AC-STEMAberration-correctedscanningtransmissionelectionmicroscopy

AIEAggregation-inducedemission

AFMAtomicforcemicroscope

ASEAmplifiedspontaneousemission

BiBismuthene

BCBirefringencecrystals

BPBlackphosphorus

BSBeamsplitter

BPDsBalancedphotodiodes

BBOBetabariumborate

BNBoronnitride

BLGBilayergraphene

BGRBandgaprenormalization

xvi ListofAbbreviations

CARSAnti-StokesRamanscattering

CMOSComplementarymetal-oxidesemiconductor

CVDChemicalvapordeposition

CSSChineseSpaceStation

COFsCovalentorganicframeworks

CBConductionbands

CWContinuouswave

DACDiamondanvilcell

DFGDifference-frequencygeneration

DLCDiamond-likecarbon

D-ADonor–acceptor

DBRDistributedBraggreflector

DSMDiracsemimetal

DFWMDegenerateFWM

DTDifferentialtransmission

DFBDistributedfeedback

DWPDouble-wavelengthwaveplate

ESAExcited-stateabsorption

EDFErbium-dopedfiber

EBLElectronbeamlithography

ENZEpsilonnearzero

EDElectricdipole

EQElectricquadrupole

EOElectro-optic

EFISHElectric-field-inducedsecondharmonics

FWMFour-wavemixing

FWRFour-waverectification

fsFemtosecond

FCAFreecarrierabsorption

FETField-effecttransistor

FTOFluorine-dopedtinoxide

FWFundamentwavelength

F-PFabry–Pérotcavity

FWHMFullwidthathalfmaxima

GeGermanium

GOGrapheneoxide

GDGraphdiyne

GEOsGeostationaryorbits

GSAMGraphenesaturableabsorbermirror

HGHarmonicgeneration

HHGHigh-harmonicgeneration

h-BNHexagonalboronnitride

HEOsHighlyellipticalorbits

HAADFHigh-angleannulardarkfield

HTHightemperature

HPHalf-waveplate

HEGHydrogen-inducedexfoliatedgraphene h-MOHexagonalmetaloxides

ISSInternationalSpaceStation

IPAIsopropylammonium

LEEDLow-energyelectrondiffraction

LDLaserdiode

LEOsLow-Earthorbits

LiDARLightdetectionandranging

LDOSLocaldensityofstates

LPELiquid-phaseexfoliation

LAPCAsLarge-aperturephotoconductiveantennas

ListofAbbreviations xvii

xviii ListofAbbreviations

MMagnetization

MPLMultiphotonlithography

MPEAChiral β-methylphenethylammonium

MPAMultiphotonabsorption

MRRsMicro-ringresonators

MZIMach–Zehnderinterferometer

MEOsMedium-Earthorbits

MOFsMetal–organicframeworks

MOCVDMetal–organicchemicalvapordeposition

MRAMetamaterialresonantabsorber

MDMagneticdipole

MIMultiphotonionization

MSHGMagnetization-inducedSHG

MAEFMultiphoton-absorption-excitedfluorescence

MXenesTransitionmetalcarbidesandnitrides

NLONonlinearoptics

NLANonlinearabsorption

NLRNonlinearrefraction

NLSNonlinearscattering

NWsNanowires

NRsNanorods

NDsNanodots

NSsNanosheets

NIT-PONonlinearimagingsystemwithphaseobject

NMPN-methylpyrrolidone

OPAOpticalparametricamplifier

OMOpticalmicroscope

OLOpticallimiting

OCOutputcoupler

OAOpenaperture

OROpticalrectification

PMMAPolymethylmethacrylate

PDMSPolydimethylsiloxane

PEEMPhotoemissionelectronmicroscopy

PtPlatinum

PCPolarizationcontroller

PDPhotodiode

PCAsPhotoconductiveantennas

POPhaseobject

PI-ISOPolarization-independentisolator

PSEPolymersolidelectrolyte

PVDPhysicalvapordeposition

PVTPhysicalvaportransport

PBPhotoinducedbleaching

PMParabolicmirrors

PMTPhotomultipliertube

PIAPhotoinducedabsorption

PtSPlatinumsulfide

PMTPhotomultipliertube

QWsQuantumwells

QDsQuantumdots

RSAReversesaturableabsorption

rGOReducedgrapheneoxide

RIEReactiveionetching

SHGSecond-harmonicgeneration

SRSStimulatedRamanscattering

SASaturableabsorption

SAESonication-assistedexfoliation

SEEsSingle-eventeffects

SEUSingle-eventupset

SEFISingle-eventfunctionalinterruption

SETSingle-eventtransient

SELSingle-eventlatch-up

SNSPDSuperconductingnanowiresingle-photondetectors

SDIspectral-domaininterferometry

SOISilicononinsulator

SEMScanningelectronmicroscopy

SFMSum-frequencymixing

ListofAbbreviations xix

xx ListofAbbreviations

SPMSelf-phasemodulation

SNOMScanningnear-fieldopticalmicroscopy

STESelf-trappedexciton

SVEASlowlyvaryingenvelopeapproximation

SESAMSemiconductorsaturableabsorbermirror

STMScanningtunnelingmicroscopy

STSScanningtunnelingspectroscopy

SECTSlowevaporationatconstanttemperature

SNRSignal-to-noiseratio

TTPATwo-photonabsorption

THGThird-harmonicgeneration

TPFTwo-photonfluorescence

TPFTiltedpulsefront

THzTerahertz

TFISHTHz-field-inducedsecondharmonic

TPLTwo-photonluminescence

TPEFTwo-photonexcitationfluorescence

TAMTransientabsorptionmicroscopy

TASTransientabsorptionspectroscopy

TACThermallyassistedconversion

TCRTransition-Cherenkovradiation

TMDsTransitionmetalchalcogenides

TMDCsTransitionmetaldichalcogenides

TITunnelionization

TIDTotalionizingdose

tr-SNOMTime-resolvedscanningnear-fieldopticalmicroscope

TRTSTime-resolvedTHztime-domainspectroscopy V

vdWvanderWaals

VBValencebands

ListofAbbreviations xxi

WTungsten

WPWollastonprism

WDMWavelengthdivisionmultiplexer

WGMWhispering-gallerymode

XUVExtremeultraviolet

Preparationof2DMaterials

YueTangandHuaXu

ShaanxiNormalUniversity,KeyLaboratoryofAppliedSurfaceandColloidChemistryofMinistryof Education,ShaanxiKeyLaboratoryforAdvancedEnergyDevices,SchoolofMaterialsScienceand Engineering,Xi’an710119,P.R.China

Two-dimensional(2D)materials,anemergingnewclassofnanomaterialswith richstructuresandremarkableproperties,wouldbringmanytransformative technologiesandapplications[1].Sincethediscoveryofgrapheneforthefirsttime in2004,the2Dmaterialfamilyhasexpandeddramaticallytoincludeinsulators (hexagonalboronnitride[h-BN]),semiconductors(mosttransitionmetaldichalcogenides[TMDCs],blackphosphorus[BP],andtellurium[Te]),semi-metals(some TMDCsandgraphene),metals(transitionmetalcarbidesandnitrides[MXenes]), superconductors(NbSe2 ),andtopologicalinsulators(Bi2 Se3 andBi2 Te3 )[2,3].The atomicthicknessanddanglingfreesurfaceof2Dmaterials,togetherwiththeir superioroptical,electrical,magnetism,thermal,andmechanicalproperties,endow themwithgreatpromiseforapplicationsinopticalcommunication,electronics, optoelectronics,spintronics,memory,thermoelectric,andenergyconversationand storagedevices[4,5].

AshighlightedbythefamousnanomaterialscientistZhongfanLiu,“preparation determinesthefuture”isaninexorablelawforallmaterials.Inthepastdecade,a seriesofpreparationtechnologieshavebeendevelopedtofabricate2Dmaterialsfor satisfyingtherequirementsoftheirfundamentalstudiesandvariousapplications.In viewofthelayeredstructureof2Dmaterials,theprimarypreparationtechnologies canbedividedintotwomajortypes:top-downandbottom-upapproaches.Inthis chapter,wewillintroducetherecentlydevelopedpreparationtechnologiesfor2D materials,includingtwotop-downapproaches(mechanicalexfoliationandliquid exfoliation)andonebottom-upapproach(vaporphasegrowth).Here,wegivemore spacetointroducesinglecrystalgrowth,thicknesscontrol,andphasecontrolinthe vaporphasegrowthof2Dmaterials.

2 1Preparationof2DMaterials

(a)

(b)

Figure1.1 Mechanicalexfoliationpreparationof2Dmaterials.(a)Schematicdiagramof mechanicalcleavageprocessandopticalmicrographofoneofthegrapheneﬂakesonthe SiO2 /Sisubstratewithdifferentthicknesses.Source:ReproducedwithpermissionfromYuan Huangetal.[7]/AmericanChemicalSociety.(b)SchematicdiagramoftheAu-assisted mechanicalexfoliationprocessandcorrespondingopticalmicroscope(OM)imagesof obtainedvdW-layered2Dmaterials.Source:YuanHuangetal.[8]/SpringerNature/CCBY 4.0.

1.1MechanicalExfoliationof2DMaterials

In2004,GeimandNovosolovfirstlypreparedmonolayergraphenebyexfoliating bulkgraphiteusingScotchtape[6].Theschematicexfoliationmethodandthe obtained1-4-layergrapheneareshowninFigure1.1a[7].Sincethen,mechanical cleavage,commonlyreferredtoastheScotchtapemethodandinvolvingno

1.1MechanicalExfoliationof2DMaterials 3 chemicalreactions,isconsideredtobethesimplestandbestapproachtoobtain large-area,high-quality2Dmaterialsthatretaintheirpristinestructuresand properties.Until2010,Heinz’sgroupfirstlyextendedthemechanicalcleavage methodtopreparemonolayerMoS2 anddiscoveredtheindirecttodirectbandgap transitionwiththethicknesschangingfrombulktomonolayer[9].Then,A.Kis’s groupfabricatedthefieldeffecttransistorbasedontheexfoliatedmonolayerMoS2 andachievedroom-temperaturecurrenton/offratiosof108 andultralowstandby powerdissipation[10].Later,mechanicalcleavagetechniqueswerewidelyused toproducedozensof2Dtransitionmetaldichalcogenides(TMDCs:WS2 ,WSe2 , MoSe2 ,ReS2 ,etc.)[11].However,withtheincreasingrequirementsonthickness anddomainsizeof2Dmaterialsfordevicefabricationandpropertyexploration, cleavagetechnologypursuesefficientpreparationof2Dmaterialswithlargeareas andhighquality.Hence,manyoptimizedmechanicalcleavagetechniqueshave beendevelopedinrecentyears.Forexample,Huang etal.developedauniversal Au-assistedmechanicalcleavagetechnique,asshowninFigure1.1b[8].TheoreticalcalculationindicatesthatAuandmany2Dmaterialscanformquasi-covalent bonds,whicharelargerthanvanderWaals(vdW)interactionsbutsmallerthan covalentbonds.Intheexperiment,athinAulayerwasfirstlydepositedontoa substratecoveredwithathinTiorCradhesionlayer,andafreshlycleavedbulk crystalontapewasbroughtintocontactwiththeAulayer.Thentheadhesivetape wasplacedontheoutwardsideofthecrystal,andgentlepressurewasappliedto establishagoodcrystal/Aucontact.Finally,peeloffthetapetoremovethemajor portionofthecrystal,leavingmonolayerorfew-layerflakesontheAusurface. Usingthisapproach,theyobtainedlarge-areamonolayerflakesontheAusurface, includingMoS2 ,FeSe,PtSe2 ,PtTe2 ,PdTe2 ,andCrSiTe3 .Usingtheaboveexfoliation methods,aseriesofnew2D-layeredmaterialshavebeensuccessfullyprepared, suchasFe3 GeTe2 ,MnBi2 Te4 ,CrOCl,NbSe2 ,andNbOCl2 ,[12–16]whichexhibit superiorferromagnetic,superconductionandnonlinearopticproperties.Inaword, mechanicalcleavagetechnologyhasbeenwidelyutilizedtoprepare2D-layered materialsforstudyingtheirfundamentalproperties.

However,themethodsmentionedabovearerestrictedto2D-layeredmaterials inwhichtheinterlayerinteractionsaredominatedbyweakvdWforce,andthus thesemethodsarenotapplicabletomaterialsinwhichinterlayerinteractions aredominatedbynon-vdWforce.Inviewofthis,Zhangetal.developedanew mechanicalcleavagestrategy[17].Thepolishedmetallicsurfaceisoxidizedunder acontrolledenvironmenttoenablethegrowthofhexagonalmetaloxides(h-MO). Thesehigh-crystallineh-MOwithlayeredstructures,withoutionicdopantsor vacancies,canbeeasilyexfoliatedbystampingthemontothetargetsubstrates. Thiscleavagestrategywasfirstlyappliedtopreparethethree-dimensional(3D) transitionmetal-basedh-MO(TiO2 ,Fe2 O3 ,andNi2 O3 ),anditcouldbereadily extendedtoprepareavarietyofothermetaloxidesforexploringtheirnovel2D quantumproperties.Mostrecently,FengxiaGeng’sgroupreportedacalendaringpretreatmentmechanicallyexfoliateapproachtoprepare2Dmaterialsfrom non-vdWstructures[18].Onthebasisofthetraditionalscotchtapemethod,an externalmechanicalforcewasappliedtothenon-vdWmaterials.Thisapproach involveslaterallyslidinginthecloselypackedneighboringlayerstotransformthe

1Preparationof2DMaterials

structurefromastabletoametastablephase,weakeningtheinterlayerbinding. Usingthisapproach,avarietyof2Dmaterialshavebeenprepared,includingmetals (Bi,Sb),semiconductors(SnO,V2 O5 ,Bi2 O2 Se),andsuperconductingcompounds (KV3 Sb5 ).Thismethodformechanicallyexfoliatingnon-vdWmaterialsincreases theavailabilityof2Dmaterialsfortheexplorationoftheirphysicalcharacteristics andpotentialapplications.Tosumup,usingthemechanicalcleavagemethod canpreparemostofthetarget2Dmaterialswithhighcrystalquality,butthe samplesacquiredviathisapproachalsopossessseveralproblems,suchasirregular morphology,uncontrollablethickness,smalldomainsize,andlowyield.Therefore, thepresentmechanicalcleavagetechnologyfor2Dmaterialpreparationstillfaces greatchallenges,whichlimitsitsbasicresearchtopracticalapplication.Itisworth tofurtheroptimizethepresentcleavagemethodsandexplorenovelcleavage technologiestoachieve2Dmaterialswithcontrollablelayersandsizes.

1.2Liquid-PhaseExfoliationof2DMaterials

Asdemonstratedabove,thepreparationof2Dmaterialsviamechanicalexfoliation islimitedbytheinevitablelowyieldanduncontrollablethickness,domainsize,and morphology.Toresolvetheseissues,liquid-phaseexfoliation(LPE)wasdeveloped toprepare2Dmaterials,whichhastheadvantagesofhighefficiency,largescale, andbettercontrollability[19–23].TheLPEof2Dmaterialscanbedividedintotwo primaryapproaches:directexfoliationandchemicalexfoliation[24].Directexfoliationmethodsincludesonication-assistedexfoliation[SAE]andshearexfoliation. Chemicalexfoliationmethodsincludechemicalintercalationandelectrochemical intercalation[25,26].

FortheSAEmethod,layeredbulkcrystalwasfirstlydispersedinasolvent,followedbymaterialexfoliationviaultrasonicenergyandremovalofthenon-exfoliated materialviacentrifugation[27].Thedispersionisthencentrifugedandpurifiedto achieve2Dnanosheetswithuniformdomainsizeandthickness.Moreover,ashear exfoliationmethodhasbeenwidelyusedtodispersethepretreatedlayeredcrystals, whichenablestheproductionofhigh-quality2DnanosheetswhileavoidingSAE’s fragileissues.However,theexfoliationisonlysuitablefor2Dmaterialswithweak interlayerforce,whileitisdifficulttoeffectivelyexfoliate2Dmaterialswithstrong interlayercoupling.

Huangetal.developedanefficientandstablethree-stagesonication-assisted liquid-phaseexfoliation(TS-LPE)methodformasspreparationofhigh-structuralintegrityfew-andsingle-layerFe3 GeTe2 nanoflakes[28].Thethreestagesinclude slicingcrystals,weakeninginterlayervdWforces,andusingultrasoniccavitation (Figure1.2a).Atthefirststage,ballmillingisusedtoreducethethicknessandsize ofFe3 GeTe2 crystals.Atthesecondstage,theshearofthesolventviastirringin N -methylpyrrolidone(NMP)andtheshockwavescausedbythecollisioneventsin thecontainerleadtoexpandinglayerspacingandweakeninginterlayervdWforces tofacilitatethesubsequentsheddingofthenanoflakes.Thethirdstageisultrasonic cavitationtofacilitatetheexfoliationofnanoflakes.Finally,Fe3 GeTe2 nanoflakes

Figure1.2 Liquid-phaseexfoliationof2Dmaterials.(a)SchematicdiagramoftheTS-LPE method.Source:ReproducedwithpermissionfromMaetal.[28]/Americanchemicalsociety. (b)SchematicillustrationsoftheelectrochemicalLi+ intercalation-basedexfoliation process.Source:ReproducedwithpermissionfromYangetal.[29]/SpringerNature.

wereextractedbycentrifugation.ThehighestyieldofFe3 GeTe2 nanoflakesis 22.3wt%,withsinglelayersaccountingfor6%,andamaximumof103 μmis available.The200mglevelofoutputhasovercomethelimitationsofmechanical exfoliationandmolecularbeamepitaxyineconomicallyamplificatedproduction.

Zengetal.developedanelectrochemicallithium-ionintercalation-basedexfoliationmethodthatcanbeeasilyconductedatroomtemperaturewithin26hours (Figure1.2b)[29].Thisexfoliationmethodisrelativelysimpleandstraightforward. Itoffersahigherdegreeofcontrolundermildconditions,andthusitwaswidelyutilizedtopreparevarious2Dmaterialsonalargescale.Theobtained2Dnanosheets possesshighcrystalquality,uniformthickness,andarelativelylargedomainsize. However,duringthelithium-ionintercalationprocess,theinsertionofeachLi+ ion involvestheinjectionofoneelectronintothehostcrystal.Thus,theintercalationof alargenumberofLi+ ions(oneperformulaunitinLiMoS2 )wouldleadtomassive electroninjectionintotheMoS2 crystal,whichwouldinduceanundesiredphase transitionfromthesemiconducting2Hphasetothemetallic1Tphase[30–32].Theoreticalstudiessuggestthatthisphasetransitionoccursonlywhentheelectroninjectionexceedsacertainthreshold(0.29electronsperMoS2 formulaunit)[33,34].A possiblewayofreducingelectroninjectionintothehost2DcrystalandthuspreventingtheundesiredphasetransitionistoreplacethesmallLi+ ions(diameter d ∼2Å) withlargercations,suchasquaternaryammoniumions[32].Duanetal.developedanelectrochemicalexfoliationmethodviaintercalatinglarge-sizetetramethyl ammoniumbromide(THAB; d ∼20Å)moleculesintotheMoS2 crystal[35].Driven bythenegativeelectrochemicalpotential,THA+ cationswereinsertedintotheMoS2 layer,causingasubstantialvolumeexpansion.Then,theexpandedMoS2 crystal

wasimmediatelysonicatedintheorganicsolventtodispersethinMoS2 nanosheets withinseveralminutes.Thisapproachcanachievehigh-qualityMoS2 nanosheets onalargescalewitharelativelyuniformthicknessthathavebeenappliedtoconstructinglarge-areathin-filmelectronicdevices,andthismethodhasbeenextended toprepareother2DTMDs[20,36].

Tosumup,theLPEmethodcanpreparelarge-scale2Dmaterialswithmonolayerandfew-layer,butitstillneedstoimprovethecontrollabilityofthicknessand morphology.Inaddition,high-powerultrasonicenergy,organicsolvents,orchemicalintercalatorsinevitablydegeneratetheintrinsicstructureandpropertiesof2D materials,whichhinderstheirhigh-performanceopticalandelectronicapplications. Hence,the2DmaterialspreparedbyLPEmethodsarefavorabletosomeparticularfields,suchassupercapacitors,lithium-ionbatteries,catalysis,printerink,and compositematerials.

1.3ChemicalVaporDepositionGrowthof2DMaterials

Comparingwiththeabovetop-downmethods,bottom-upsynthesisbychemical vapordeposition(CVD)provestobeaneffectivemethodtogrowlarge-scale2D materialswithcontrollabledomainsize,morphology,thickness,andelectronicgradequality,whichshowsgreatpotentialforfabricatinghigh-performanceoptical, electronic,andoptoelectronicdevicesforcommercialapplications.TheCVD methodusedtosynthesize2Dmaterialsreferstothegas-vaporgrowthofmaterial onasubstratewithinthetubefurnacesystem(Figure1.3a)[37,41].Tobespecific, twoormoreprecursorsareintroducedintothehigh-temperaturereactionzone inquartztubebycarriergasforchemicalreaction,nucleation,growth,andfinal formationof2Dflakesorcontinuousfilmsonthesubstrate.

Inthepastdecades,CVDhasbeenwidelyusedinthepreparationofvarious2D materials,suchaselementalgrapheneandBP,binaryh-BN,TMDs(MoS2 ,WSe2 , MoTe2 ,VS2 ,TiS2 ,ReSe2 ,etc.),ternarychalcogenides,andtheiralloysandheterostructures[42–47].TaketheCVDgrowthofMoS2 asanexample,VanderZande etal.synthesizedmonolayerMoS2 nanosheetswithuniformtrianglemorphology byusingCVDgrowthwithsulfur(S)powderandmolybdenumtrioxide(MoO3 )as precursors[48].TheobtainedMoS2 possesseshigh-quality,uniformmonolayerand largegrainsizeupto120 μm,whichenablescomparableelectricalandphotoelectric propertieswiththoseofmechanicallyexfoliatedMoS2

Duringtheprocessof2Dmaterialgrowth,manyfactorsaffectthequalityand propertiesoftheproduct,suchasprecursor,substrate,growthtemperature,carriergasflow,andadditives[49–51].Thus,greateffortshavebeenmadetooptimizetheCVDgrowthmethodtoachievethepreparationoflarge-area,highquality,homogeneous-thickness2Dmaterials[49].Forinstance,toachieveastableand continuoussupplyofMosource,electrochemicallyoxidizedarchedMofoilwasutilizedastheprecursorinsteadofthetraditionalpowder,andthegrowthofMoS2 withcontrollablethicknesswasrealized(Figure1.3b)[38].Inaddition,researchers haveconfirmedthattheadditionofsynergisticadditivescanmodulatethegrowth chemistryof2Dmaterials,facilitatethegrowthrate,andadjustthemorphologyand