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Xenes
Xenes
2DSyntheticMaterialsBeyond Graphene
Editedby AlessandroMolle
CarloGrazianetti
EricSalomon,ThierryAngot,LokLewYanVoonandGuyLeLay
1.1Introduction1
1.2Historicalbackground:theSchottkyproblem2
1.3Silicene:theconcept2
1.4Exoticphasesofsiliceneandtheirtopologicalelectronic properties—theory3
1.5Discoveryofone-dimensionalsiliconnanoribbons6
1.6Thebirthoftwo-dimensionalsilicene6
1.7Hydrogenationofthecanonical3 3 3/4 3 4silicenephaseon Ag(111)8
1.8OtherallotropicphasesofsiliceneonAg(111)andmultilayer silicene9
1.9Reactivityandadsorptiononsilicene,siliceneonothersubstrates, andsilicenebywetchemistry11
1.10Exoticvariantsofsilicene:
1.11Properties,applications,andperspectives18 1.12Conclusion19 Note20 References20 2Germanene27
HaroldJ.W.Zandvliet
2.1Introductiontwo-dimensionalDiracmaterials27 2.2Synthesisofgermanene30
2.2.1Introduction30
2.2.2Synthesisofgermanene31
2.3Structuralandelectronicpropertiesofgermanene37
2.4AnomalousquantumHalleffectandquantumspinHalleffect40
2.4.1AnomalousquantumHalleffect40
2.4.2QuantumspinHalleffect41
2.5Bandgapopeningingermanene43
2.5.1Electric-field-inducedbandgapopening43
2.5.2Bandgapopeningbycouplingtoasubstrateandadsorption orintercalationofatomsormolecules44
2.6Bilayergermaneneandtwistedbilayergermanene44
2.7Summary46 References46
3StaneneandPlumbene49
AidiZhao
3.1Introduction:tinandstanene,leadandplumbene49
3.2Thetheoreticalpredictionofstaneneandplumbene49
3.3Theoreticalpredictionforstanenetobeatwo-dimensional topologicalinsulator51
3.3.1StaneneasquantumspinHallinsulatordescribedby Kane Melemodel52
3.3.2DecoratedstaneneasquantumspinHallinsulator describedbyBernevig Hughes Zhangmodel52
3.3.3ModifiedstaneneasquantumanomalousHallinsulator55
3.3.4Theoreticalpredictionforstanenetobeatwo-dimensional Isingsuperconductor56
3.4Thesynthesisandcharacterizationofstanene58
3.4.1Epitaxialgrowthofstanene58
3.4.2Topologicalbandinversioninultraflatstanene59
3.4.3Two-dimensionalIsingsuperconductivityinfewlayer stanene61
3.5Thesynthesisandcharacterizationofplumbene62
3.6Perspectivesontheapplicationsofstaneneandplumbene66 References69
4Borophene73
BaojieFeng,LanChenandKehuiWu
4.1Introduction73
4.2Theoreticalaspectsofborophene75
4.3Synthesisofborophene78
4.3.1BorophenesonAg(111),(110),and(100)78
4.3.2BoropheneonCu(111)82
4.3.3BoropheneonAl(111)84
4.3.4BoropheneonIr(111)86
4.3.5Liquid-phaseexfoliation87
4.3.6Hydrogenatedborophene(borophane)88
4.4Physicalpropertiesofborophene91
4.4.1Mechanicalproperties91
4.4.2Metallicity92
4.4.3DiracconesandDiracnodallines95
4.4.4Superconductivity99
4.4.5Opticalproperties100
4.5Perspective100 References101
5Gallenene107
NicolaGaston
5.1Evidenceoftwo-dimensionalityinbulkgallium108
5.2Evidenceoftwo-dimensionalityingalliumnanostructures109
5.3Experimentaldiscovery111
5.4Electronicproperties111
5.5Thermalstability112
5.6Howuniqueisgallenene?115
5.7Propertiesandapplicationsofgallenene116 Acknowledgments118 References118
6Phosphorene121
YueZheng,JingGao,YuliHuang,TianchaoNiuandWeiChen
6.1Introduction121
6.2Propertiesoftwo-dimensionalblackphosphorus122
6.2.1Crystalandbandstructure122
6.2.2Opticalproperties124
6.2.3Thermoelectricproperties124
6.3Applications126
6.3.1Electronicdevices126
6.3.2Optoelectronicdevices127
6.3.3Bioapplications129
6.4Fabrication130
6.4.1Bulkcrystals130
6.4.2Few-/single-layerBlackphosphorus131
6.4.3Bluephosphorene132
6.5Functionalization135
6.5.1Surfacefunctionalization135
6.5.2Bandgapengineering136
6.5.3Defectsengineering138
6.6Oxidationandsurfaceprotection139
6.6.1Oxidationmechanism139
6.6.2Surfaceprotection141
6.7Conclusionandoutlook143 Acknowledgments144 References144
Contents
7ArseneneandAntimonene149
NikolasAntonatos,EvgeniyaKovalskaandZden ˇ ekSofer
7.1Introduction149
7.2Arsenicandantimonyallotropes150
7.2.1Arsenic150
7.2.2Antimony152
7.3Two-dimensionalarseneneandantimonene154
7.3.1Fundamentals154
7.3.2“Top-down”methods155
7.3.3“Bottom-up”methods157
7.4Dopingofarseneneandantimonene159
7.5Applicationsandfutureprospectsofarseneneandantimonene159
7.6Conclusion165 Acknowledgement166 References166
8Bismuthene173
HanliuZhao,ShiyingGuo,WenZhong,ShengliZhang, LiTaoandHaiboZeng
8.1Introduction173
8.2Structureandpropertiesoftwo-dimensionalbismuth174
8.2.1Atomicstructure174
8.2.2Bandstructure174
8.2.3Fundamentalproperties176
8.3Preparationoftwo-dimensionalbismuth178
8.3.1Physicalvapordeposition178
8.3.2Wetchemicalmethods181
8.3.3Exfoliationmethods182
8.4Characterizationoftwo-dimensionalbismuth183
8.4.1Structurecharacterization183
8.4.2Morphologycharacterization184
8.4.3Electricalproperties184
8.5Applicationsoftwo-dimensionalbismuth187
8.5.1Field-effecttransistors187
8.5.2Thermoelectrics188
8.5.3Photodetectors188
8.5.4Batteries190
8.6Summary191 References192
9SeleneneandTellurene197
Pai-YingLiao,Jing-KaiQin,GangQiu,YixiuWang, WenzhuoWuandPeideD.Ye
9.1Introduction197
9.2Selenene199
9.2.1Synthesismethods199
9.2.2Physicalproperties201 9.2.3Applications204 9.3Tellurene206
9.3.1Synthesismethods206
9.3.2Physicalproperties211
9.3.3Electronicapplications214 9.4Conclusions219 References219
10TechnicalevolutionfortheidentificationofXenes: frommicroscopytospectroscopy225 MengtingZhao,HaifengFengandYiDu 10.1Introduction225
10.2Workingprinciplesofscanningtunnelingmicroscopyand angular-resolvedphotoemissionspectroscopy226
10.2.1Scanningtunnelingmicroscopy226
10.2.2Angular-resolvedphotoemissionspectroscopy229
10.3Applicationsofscanningtunnelingmicroscopyand angular-resolvedphotoemissionspectroscopyinXenes230
10.3.1Siliceneandgermanene230
10.3.2Borophene233
10.3.3Stanene236
10.3.4Bismuthene238
10.3.5Phosphorene240
10.3.6Antimonene,arsenene,andtellurene241 10.4EmergingtechniquesusedinXenes243
10.4.1RamanspectroscopystudiesonXenes243
10.4.2Atomicforcemicroscopy246
10.4.3Transmissionelectronmicroscopy246
10.4.4X-rayphotoemissionspectroscopy246
10.4.5Spin-resolvedangular-resolvedphotoemission spectroscopy247
10.4.6Augerelectronspectroscopyandlow-energyelectron diffractionandreflectionhigh-energyelectrondiffraction247 References248
11ChemicalmethodsforXenes255 WarrenL.B.HueyandJoshuaE.Goldberger 11.1Introduction255
11.2Surfacefunctionalizationmetrology256
11.3AnalogyofSi(111)andGe(111)surfacefunctionalization257 11.4ChemicaltreatmentsandreactivityofSi,Ge,SnXenes261 11.5TopotactictransformationsofZintlphasesinthesynthesisof functionalizedXenes262
11.5.1Functionalizedsiliceneanalogsfromtopotactic transformationsofCaSi2
262
11.5.2Functionalizedgermanenefromtopotactic transformationsofCaGe2 264
11.5.3FunctionalizedSnXenesfromthetopotactic transformationsofSn-containingZintlphases267
11.5.4Organic-functionalizedXenesviatopotactic functionalizationofZintlphaseswithhaloalkanes268
11.6LigandsubstitutionreactionsoffunctionalizedSiandGeXenes272
11.6.1SiHtoSiRviahydrosilylation274
11.6.2GeHtoGeRviahydrogermylation275
11.6.3SiHtoSiNRviaamination276
11.6.4SiHtoSiH4PhviaGrignardchemistry276
11.7CovalentmodificationofotherXenes278
11.8Functionalization-inducedchangesinthermalandelectronic propertiesofgroup14Xenes280
11.8.1Functionalization-inducedchangesinelectronicstructure280
11.8.2Functionalization-inducedchangesincarriermobilities286
11.8.3Signaturesofamorphizationandfunctionalization-induced changesinconductivityandthermalstability286
11.8.4Functionalization-inducedchangesinwaterabsorption288
11.9Conclusion289 References289
12TopologicalphysicsofXenes295
YangLi,ZhimingXu,ZetaoZhang,JiahengLiandYongXu
12.1Two-dimensionaltopologicalinsulators295
12.1.1TwotypicalmodelsforquantumspinHallinsulators295
12.1.2TopologicalZ2 invariant298
12.1.3QuantumspinHallinsulatorsinXenes299
12.2QuantumanomalousHalleffectinXene302
12.2.1IntroductiontothequantumanomalousHalleffect302
12.2.2Theoreticalmodelsofquantumanomalous HallstatesinXene303
12.2.3RealizationofthequantumanomalousHall statesinXenes305
12.3TopologicalsuperconductivityandIsingsuperconductivity307
12.3.1Generalintroduction307
12.3.2Type-IIIsingsuperconductivity307
12.3.3Topologicalsuperconductivity309
12.3.4Otherresearchprogresses310
12.4ThermoelectricpropertiesinXenes311
12.5Summaryandoutlook313 References313
13OpticalpropertiesofXenes319 PaolaGori,FriedhelmBechstedtandOliviaPulci Abbreviations319 13.1Introduction319 13.2Theoreticalmethods320
13.2.1Reflectance,transmittance,andabsorbance320
13.2.2Opticalconductivityandthesuperlatticemethod321 13.2.3 Abinitio approaches:fromsingleparticlesto quasiparticles322 13.3Results326
13.3.1FreestandingXenes327 13.3.2HydrogenatedXenes:theXanes336 13.3.3Beyondthefreestandingcase:substrateeffects339 13.4Summaryandconclusions345 Acknowledgments346 References346
14Two-dimensionalmagnetisminXenes353 AndreyM.Tokmachev,DmitryV.Averyanov,IvanS.Sokolov, AlexanderN.Taldenkov,OlegE.Parfenov,IgorA.Karateevand VyacheslavG.Storchak
14.1Introduction353 14.2TheoreticalpredictionsofmagnetisminXenes354 14.3IntrinsicmagnetisminXenemultilayerthree-dimensional compounds357
14.4Two-dimensionalferromagnetisminsilicenematerials361 14.5Two-dimensionalferromagnetismingermanenematerials363 14.6Electrontransportintwo-dimensionalmagneticXenecompounds364 14.7Extensionoftwo-dimensionalferromagnetismtographene366 14.8Conclusion368 Acknowledgments369 References369
15Xeneheterostructures377 CarloGrazianettiandAlessandroMolle 15.1Introduction377 15.2Xenehomostructures381 15.3Xeneheterostructures385 15.4Challenges,bottlenecks,potentialandperspectives396 Acknowledgement399 References399
16IntegrationpathsforXenes405 GabrieleFaraone,Md.HasibulAlam,XiaoXu,ZhaoyingDang, LiTao,DejiAkinwandeandDeepyantiTaneja
16.1Introduction405
16.1.1Majorplayersfromadeviceperspective405
16.1.2GrowthtechniquesandfunctionalizationofXenes406
16.1.3ApplicationofXenedevices407
16.1.4Challengesindevicefabrication408
16.2Reviewofexistingelectronicdevices409
16.2.1Group14devices409
16.2.2Group15(pnictogens)devices418
16.2.3Group16(chalcogens)devices420
16.3CurrentandfutureapplicationsofXenes421
16.3.1Photonicsdevices421
16.3.2Biomedicaldevices422
16.3.3Topology-basedelectronicdevices423
16.3.4Memorydevices425
16.3.5Xene-basedjunctions426
16.4PerspectivesonintegrationofXeneswithsilicon428
16.5Concludingremarks429 References430 Furtherreading438 Index439
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Introduction
CarloGrazianettiandAlessandroMolle
Two-dimensionalmaterialsbeyondgraphene:therise oftheXenes
Thediscoveriesabouttheone-dimensional(1D)andthetwo-dimensional(2D)allotropesofcarboncanbeprobablyconsideredasthebeginningoftheeraofthelowdimensionalmaterials.Althoughtheconceptoflow-dimensionalmatterwasalready presentinsolid-statephysics,forexample,afterthediscoveryofscanningtunneling microscopy(STM)thatenabledthesurveyofmatterattheatomicscale,thesynthesisofthe1Dcarbonnanotubesandtheisolationofasingle-layer2Dgraphenein 1991and2004,respectively,pavedthewaytoaneweraincondensedmatterphysics [1,2].Inparticular,withintheframeworkof2Dmaterials,graphenejuststarted arevolution.Thenumberofscientificmanuscriptsandbooksthetopicsistoday incalculable.Ofcourse,theexistenceofgraphenewaswell-knownevenbeforeits isolationbecausemanygraphenelayersputtogetheraregraphite.Thereforeitis veryintriguingtoconsiderthatsomeonethoughtwhethersiliconandgermanium mayalsohavelow-dimensionalallotropes,beingsiliconandgermaniumplaced directlybelowcarboninthecolumnIVAoftheperiodictable.Althoughthisquestionsoundsacademic,andactuallyitwas,itsimportanceisrelatedtotheubiquitous useofsiliconandgermaniuminelectronicssincethedawnoftheelectronicsera (considerthattheinterfacebetweensiliconanditsoxideisprobablythemoststudiedinterfaceever).Therefore,inthisscenario,theeffortstofindthe2Dhoneycomb latticesmadeofsiliconandgermanium,thatis,siliceneandgermanene,were mainlydrivenbytheurgentrequestofultrascaledsemiconductorsforextendingthe so-calledMoorelawtrend [3,4].TakedaandShiraishiwonderedifthesiliconand germaniumatomsarrangedintographene-likelatticeswerestableandsurprisingly theanswerisaffirmativealthoughinapeculiarway,namelywhatwemayterma pseudo-planarlattice [5].Inshort,thisexcitingtheoreticalfindingsuggeststhat freestandingsiliceneandgermanenecanbestabilizedbypuckeringthegraphene latticewithsomesilicon(orgermanium)atomsoutofthebasalplane(see Fig.1).
Thesiliceneandgermanenecrystalsthereforeincludeanadditionallattice parameter(tothein-planelatticevectors)termedbuckling,namelytheverticaldistancebetweenthetopandbottomsatoms,andtheirrelatedcrystalsarereferredto asbuckled(whilegrapheneisplanarornon-buckled).Evenifthetheoryprovesthe
Figure1 Sideandtopviewsoftheplanarhexagonalringofgraphene(left)andbuckled hexagonalringtypicaloffreestandingXeneslikesilicene(right).Althoughthebucklingisa keyparameterforthepostgrapheneXenes,so-calledultraflatconfigurations(e.g.,for stanene)canbeobservedevenforatomsheavierthancarbon.
existenceofsiliceneandgermanene,theirrealizationdoesnotcomewithout practicalconsequences.Thefirstdifferencewithgrapheneistheabsenceofasolid three-dimensional(3D)graphite-likeparentcrystalofsilicon(germanium)madeof silicene(germanene)layersstackedbyvanderWaals(vdW)forces.Afterall,this conditioniscommontomostofthechemicalelementssurroundingcarboninthe periodictable.Hence,thesubsequentquestioniswhetherthesecrystalscanbesynthesizedinalternativewaystomechanicalexfoliation(themethodoriginallyused byGeimandNovoselovtoderivegraphenesheets).Theseconddistinguishingfeatureofsilicene(andgermanene)withrespecttographeneisthechemicalbond. Unlikecarbon,whichiswell-knowntohybridize sp 2 ingraphite(and sp 3 indiamond),siliconandgermaniumprefertohybridizetheirthree p orbitalswith s orbitalstomakeonly sp 3 tetrahedralconfiguration.Typically,ionicradiilargerthan carbonpromote sp 3 hybridization,whereas sp 2 hybridizationisenergeticallymore favorableincarbon.Siliceneandgermanenethusshowamixed sp 2 sp 3 hybridizationthat,consideringtheirlatticesasbigmolecules,canberelatedtoapseudoJahn-Tellerdistortionoriginatingtheircharacteristicbuckling [6].Despitethese difficultiesandafteralong-debatedsurvey,thecompellingexperimentalevidence ofsilicenesynthesiswaseventuallyreportedin2012 [7].Soonafterthegroundbreakingsilicenediscovery,inparticular,themethodologyusedtoaccomplishits growth,aplethoraof2Dartificiallatticesflourishedincludinggermanene,stanene, plumbene,borophene,gallenene,phosphorene,antimonene,arsenene,bismuthene, selenene,andtellurene.Indeed,quitesurprisingly,theXenesincludeelementsnot onlybelowcarbonbutevenintheneighborcolumnsoftheperiodictable(see Fig.2).
Interestinglyafterthegrapheneboom,elementsoftheperiodictablecloseto carbonhavebeenmade2Dflatinlessthan10yearsandanewpanoramaof2D materialsshowedup.Althoughthechallengeofachievingnewmonoelemental2D
Figure2 TheXenesperiodictableoftheelements.TheelementsfromcolumnIIIA VIA areschematicallydividedintothreegroupsdependingontheir(theoreticallyexpected) bandstructure:metal/semimetal(blue),semiconductorwithagap(yellow),andtopological insulator(green).Althoughgraphene,silicene,andgermaneneareQSHIs,theirspin orbit coupling-inducedbandgapsarenegligiblewhencomparedtothoseofheavierelementslike tinorbismuth.Thisclassificationshouldbeintendedasflexibleasthekeyfeatureofthe Xenesisthepossibilitytotunetheirproperties.
materialsisofparamountimportancefornewhiddenphysicstocomeout,additionaleffortsintheXenesresearcharedevotedtoexpandingtheclassof2Dmaterialswithawealthofnewelectronicproperties.Theweaknessrelatedtotheir metastablenature(e.g.,makingthempronetoenvironmentaldegradationinsome cases)canbeturnedintoapositiveopportunityopeningaflexibleandversatile playgroundwhereengineeringtheirpropertieswithanatomiccontrol.Plentyof roomformaterialsscientistswhendealingwithmetallicXeneslikeborophene (withDiracfermions)andgallenene,orDiracsemimetalfromsilicenetostanene withtheincreasingatomicnumber(andthenthespin orbitcouplingeffects)that convertsahoneycomblatticeintoaquantumspinHallinsulator(QSHI),or—last butnotleast—semiconductorslikephosphoreneandtellurenechallengingthetransitionmetaldichalcogenides(TMDs)forapplicationsneedinganopticaland/or indirectgaptowardelectronicand/orphotonicfunctionalities.Hence,manyelectronicflavorsarenowavailablewithintheXenes,andprobably,manymoreareyet tocome.Here,wewillbrieflyintroduceXeneswiththeirmainelectronicproperties eveniftodaytermslike“metal,”“semimetal,”or“semiconductor”areonlyahasty descriptionoftheirelectronicbehavior.Indeed,theimportanceoftopologyinthe descriptionofsolidmatteriswell-recognized,andtherecentfindingsoftopology ofmatterareimprovingthesedefinitionswithmoreprecision.Forinstance,the
semimetaldefinitionbelongstoa“bufferzone”betweensemiconductorsand metals,andischaracterizedbyasmalloverlapbetweenconductionandvalence bands,buttheamountofthisoverlapisroughlydefinedbecausethemomentumis nottypicallyincludedinthegeneralclassification.Inthenextsections,wewill illustratebrieflythemainpropertiesofeachindividualXenethatwillbethesubject ofadedicatedchapterinthefirstpartofthepresentbook.Afterward,theXenesare scrutinizedintermsoftheirpotentialforapplicationstonanotechnologies.
AbriefoverviewoftheXenesfromtheperiodictable
Afterthedebutofsilicenein2012,furtherstepsonmaterialsresearchhaveraised thebaroveranincreasingnumberofelementsrecastingasXenecrystals.Currently availableXenesarelistedbelowfollowingthecolumnarsequenceoftheperiodic tableoftheelements.
Boropheneandgallenene
Ontheleftsideofthecarboncolumn,boropheneandgallenenerepresentthelightestXenes.Theirbehaviorismetallic(withDiracfermionsinborophene)andmakes themsuitableforplasmonics,flexibleandtransparentelectronics,electrodes,displays,hydrogenstorage,andbatteryapplications.Boropheneandgalleneneare reportedinChapters4and5,respectively.
Silicene,germanene,stanene,andplumbene
Theelementsbelowcarbonattractedmaininterestafterthesilicenesynthesis,and subsequentlygermanene,stanene,andfinallyplumbeneappearedonthestagein thischronologicalorder.TheyaretheXenesthatmorecloselyresemblegraphene althoughtheirstructureisdifferent.Theincreasingatomicmassmakesthem QSHIs,andtheirmagnetic,optical,andtopologicalpropertiesarestillbeingstudied thussuggestingmanyintriguingapplicationsforthemspanningfromlow-power andtopologicalelectronics,photonicsandoptoelectronics,spintronics,thermoelectricity,andquantumcomputing.ColumnIVAXenesaredescribedindetailin Chapters1 3.
Phosphorene,arsenene,antimonene,andbismuthene
Ontherightsideofthecarboncolumn,thepnictogensreducedtothe2Dlevel, namelyphosphorene,arsenene,antimonene,andbismuthene,representthe“semiconductive”alternativetoboropheneandgallenene.Phosphorenecanbeobtained byexfoliationofblackphosphorussimilarlytographeneandMoS2,andrecently, manyeffortsaimatitsepitaxialsynthesis.Evenarseneneandantimoneneshowa thickness-dependentbandstructure,andwithphosphorenetheymightbeusefulfor
electronics,optoelectronics,ionstorage,andgassensors.Conversely,bismuthene showsexcitingtopologicalpropertiesandmorecloselyresemblesstaneneasalarge gapQSHIthuslookingpromisingforelectronicandspintronicapplications(operatingatroomtemperature).Chapters6 8willsummarizethepropertiesofcolumn VAXenes.
Seleneneandtellurene
Furtherontherightsideofthecarboncolumn,withinthechalcogensgroup,seleneneandtellurenearealsotwosemiconductingXenesofpotentialinterestfora broadrangeofapplicationsincludingelectronics,optoelectronics,thermoelectricity, sensors,anddetectors.Intheirbulkform,theyarestructuredas1Dspiralchains thataremutuallykepttogetherbyvdWforces.Seleneneandtellurenearediscussed inChapter9.
Throughoutthisoverview,theroleoftheXenesinmoreextensiveunderstanding thephysicsof2Dsystemswillbeelucidated,andemphasiswillalsobegiventothe morerecentpathstointegratetheXenesinnanotechnologyaimingattheexploitation oftheirpeculiarpropertiesineverydaylifedevicesorapplications.
Xenestowardnanotechnologies
Followingupgrapheneinthe2Dmaterialsscenario,Xenesinitiallyattractedattentionascandidatesforchannelmaterialsintransistorswheregrapheneitselfirremediablyfailsforitslackofasizeablegap,namelyamandatoryconditionforgate modulationoftheFermilevel.Inthisview,siliceneappearednotonlyasthefrontrunneramidtheXenesbutalsoasthebridgetothecontinuousscalingtrendin digitalnanoelectronics [8].Thelatteraspectreliesonthetechnologyevolutionof high-performancefield-effecttransistors(FETs)inthecomplementarymetal-oxide semiconductortechnologyinordinarilyusedhigh-performancemicrochips.While thesiliconbodyinaFETstructurehasgotprogressivelyshrunkdowntoanultrathinbodywiththicknessbelow3nmintheso-calledFinFETarchitecturesand beyond,thephysicallimitofasilicon-madetransistorchannelisnexttobereached withdetrimentalimplicationsinthecarrierconduction(e.g.,mobilitydegradation withdecreasingthickness).Inthisrespect,theclassoftheXenesappearedasone ofthepossibleadvancedmaterialsfrontierswheretodrivetheFETtechnology towardanultimatelyscaledfully2Dprototype.Benefitsinthissensewerenotonly restrictedtotheatomicthicknessbuttheyalsocomprisemobilitypreservationasan intrinsicfeatureoftheXenestructure(nolongerexplainableasbarethickness reductionofthe3Dbulk).Nonetheless,Xenes,asmuchasmostoftheotherplayers inthe2Dmaterialsplayground,arestillfarawayfromalab-to-fabtransitionin termsofintegrabilityandreliability.Takethecaseofsilicene.Ifreducingsilicon atomstoahexagonalmonolayerwasasensationalachievementintermsofnanomaterialsynthesis,inpracticesilicenecanbeepitaxiallyproducedonlyviaasubstrate
selectivescheme,anditultimatelyturnsouttobehighlyunstableinenvironmental conditionsthereinneedingan adhoc stabilization.Theseaspectslimitbyfarthe optionsforsilicenedeploymentinatechnologyframework.Chapter16takesthis matterintoaccountbyreportingonthelatestadvancesintheXenesprocessing towardatechnologytransferplatforminnanoelectronics.Similartographene,two approachescanbepursuedtothisend:the“directgrowth”onasubstrateandthe “growth&transfer” [9].TheformerapproachimpliesXenestobegrownonasubstratethatisreadilyexploitableinatargetapplication.Thenumberofconfigurations inthisrespectisquitelimitedbythefactthatXenesarebasicallygrownonmetal substrateswhichinturnarenotreadilycompliantwithatechnologyflow.AnexceptionforthisfashionisthecaseofXenesonsapphire,wherehoweverthestructureof theXenesisnottriviallyobservablewithusualsurfacesciencetechniques [10,11]. SomepromisingadvancesinthisframeworkmaycomefromXenes-liketellurene whosegrowthwasprovenonamorphousSiO2 substratesbycondensationalthoughit isnotcleartodatewhatisthereachableinferiorlimitofthegrownfilmthickness [12].The“growth&transfer”approachmostlyreferstoXenesasepitaxialfilmona commensuratetemplate(seethecaseofsiliceneonsilver)andthesubsequentdisassemblyfromthenativesubstratetoendupinatransferablemembrane [13].This approachissolidlybasedonthestructuralvalidationofXenesbyatomically resolvedprobesliketheSTM(seeChapter10asareviewofthetechnicalmethodologiesfortheXenesidentification),butitsuffersfromthesevereinstabilityofmost oftheXenesinvokingstabilizationschemeslikeencapsulationwithaprotective layer.Inmanycases,Al2O3 wasproventohaveastabilizingeffectontheXenes underanenvironmentalconditionwithnoconsequencesontheunderlyingXene structure [14].AnalternativeapproachtomanipulatingXenesreliesonthechemical synthesisfromtopotacticde-intercalationfromareferenceZintlphaseasdetailedin Chapter11.ThisisthecaseofsiliceneandgermaneneextractedfromlayeredCaSi2 andCaGe2 compounds,respectively,andeventuallyresultinginH-functionalized Xenes(silicaneandgermanane)readilyintegrableinopticaldevices [15].
InChapter13,Xenesarealsodeemedintermsoftheiropticalpropertiesasthey maybearpotentialasphotonicmaterials,anotherfieldwheregrapheneandTMDs havebeenlongstudied [16 18].Inthisrespect,epitaxialsilicenewasobservedto retainasurfaceplasmonfeatureaccordingtoelectronenergylossspectroscopythat distinctlydiffersfromthemainplasmonicsignalfromthemetalsubstrate [19] Thisisapromisingscenarioforsilicenetobeexploitedasanopticallyactivematerialinaplasmonresonancegratingeitherasastand-alonelayerorinterplaywith thenativemetalsubstrate.WhileinthelattercaseXeneswouldbecoupledwith theirsubstrate,intheformercasegraphene-likeXeneswithlinearlydispersed energybandsareexpectedtodisplayaTHzplasmonicresponsethatmakesthem particularlyinterestingfortheemergingfieldofTHztechnologies [20].
MakingXenesfullyexploitablefortechnologytransferwouldpavethewaytoa richergroundforapplicationsthatmayreadilybenefitfromthecharacteristicsof eachspecificXene.Nanoelectronicsisonlyonepossibleoutcomeoutofthisscenario, butXenesmaymeettherequirementsofothernanotechnologiestoo.InChapter14, therealizationofsilicenebyintercalationofrareearthelementsintobulksilicon(e.g.,
GdSi2)isshowntodiscloseanunprecedentedscenarioofaXene-basedmagnetism encompassingathickness-dependentantiferromagnetic ferromagnetictransitionand colossalmagnetoresistanceatthe2Dlevel.InChapter12,specificXenesareshown tobearanontrivialtopologyasaresultoftheirintrinsicallyhighspin orbitinteraction.TheyaredescribedasQSHIsaslongastheyareexpectedtodisplayaconductancequantizationwithnoexternalmagneticfieldappliedowingtotheirnontrivial topologicaltexture.InaXenenanoribbonlayout,theQSHstateisqualifiedbytopologicallyprotected1Dedgesinavirtuallyinsulating2Dbodythusreflectingadimensionalreductionofwhatisknowntotakeplaceinaconventional3Dtopological insulator(thisisthereasonwhytheQSHmaterialsareoftendenotedas2Dtopologicalinsulators).AlthoughthetopologicalphysicsofXeneshasreceivedscarceexperimentalconfirmationssofar,topologicalpropertiesmaypavethewaytooutstanding implicationsintermsofquantumelectronics,spintronics,andthermoelectrics [21]. Indeed,theXenesarepreparatorytoconceptualizeatopologicaltransistorwherethe binarystatecanbeswitchedonandoffbychangingthephysicalstatefromatopologicaltoatrivialinsulatorandviceversa [22].Thenontrivialtopologyoftheheavy Xenes-likestaneneinitsmultilayered α-phaseisalsoshowntoeffectivelycouple withantiferromagneticlayertogiverisetoanewfashionofspintronicdevicesbased ontheso-calledspin-orbitronicsparadigm [23].Moreover,topologicalfeaturesmay boosttheinherentpotentialofXenesasthermoelectricmaterialsaslongastheycould leadtoextremelyhighedgeconductivityagainstarelativelylowthermalconductivity throughthebodytherebyincreasingthethermoelectriczTfactorwellbeyondthestate oftheart [24 26]
Xene-relatedenergytechnologiesarenotbarelylimitedtothermoelectricityas longasXenesinthemultilayerform(whenallowed)mayaddressrequirementsfor postgraphiteelectrodesinLi(orbeyondLi)ionbatteries [27,28].Anincreasing effortiscurrentlyintheplaceofdevelopinghigh-powerandenergydensityLi-ion batteriestotackletheever-increasingenergydemandsofconsumerelectronicgoods likeportableelectronicdevicesorelectricvehicles(tonameafew),andprospects towardtheInternet-of-Thingsconceptposeevenmorestringentchallenges.Inparticular,multilayersilicenecanfalldownintotheongoingquestforsilicon-based anodes [29].Inthisrespect,silicenemayalleviatecurrentdrawbacksofconventionalsiliconliketheintercalation-inducedstructuralexpansionorthechemical reactivitywiththealkalimetalspecies,bothofthemdramaticallydegradingthe batterycellsperformance.
Finally,theemergingtopicregardingthecreationof adhoc assembledhomo andheterostructuresmadeofXeneswillbeexploredinChapter15.Onceagain inspirationpartiallycomesfromtheoutstandingfindingsrelatedtobilayerandtrilayergraphenewheretheadjacentlayersaremanuallytwistedatsome“magic” anglestomodifytheelectronicbandstructureandthereforerangeoverdifferent phasesofmatter [30,31].Althoughsuchastraightforwardsurveycanbehardly extendedtoXenes,becauseoftheirmetastablecharacter,thechanceofgrowing newmaterialsbyassemblingdifferentXenesispossiblebymakinguseofepitaxial methodsthatareconventionallyadoptedintheXenesynthesis [32].Thispathpaves thewaytoanunexploredfieldofmaterialsengineeringattheatomiclevelwhere
Figure3 TheXeneswheelsummarizingthefourmainareaswheretheXenescontributionis expectedtoprovidesubstantialimprovementinnanotechnology.
Xenesofdifferentkindscanbeaccommodatedtogetherinordertodesignand boostpropertiesondemand(considerforinstancethepossiblefabricationofatomicallythinelementaljunctionortheisolationoftopologicalXeneoutofthesubstrateproximity).FuturechallengesontheXeneheterostructuresshouldthenface thestabilityaswellasthewholeelectronicbehavioroftheso-createdsystem.
Thenanotechnologyareasdiscussedsofar,wheretheroleoftheXenesis expectedtopushforwardourknowledge,aresketchedin Fig.3,eventhoughwe willbenotsurprisedifnewareaswillcomeupshortly.
Conclusions
ThequicklyexpandingXenesfamilyismatureenoughforatwofoldpurpose.On theonehand,arichphysicsisworthoffurtherstudydevotedtounravelandcontrol propertiesarisingfromchemicalelements“forced”intoallotropicphasesnotexistinginnature.Ontheotherhand,thesepropertiesmaymeetsomekeyrequirements
fromcurrenttechnologicalandsocietalchallengeslikeminiaturizationandenergysavingissuesinelectronicdevices.ExpandingtheknowledgeoftheXenesasa materialscategoryrepresentsthestartingsteptoassesstheXenepotentialinthis direction.Withthispurposeinmind,therecentfindingsontheXenesandtheir propertiesarepresentedanddiscussedinthesubsequentchapters.
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Listofcontributors
DejiAkinwande MicroelectronicsandEngineeringResearchCenter,Department ofElectricalandComputerEngineering,TheUniversityofTexasatAustin,Austin, TX,UnitedStates
Md.HasibulAlam MicroelectronicsandEngineeringResearchCenter, DepartmentofElectricalandComputerEngineering,TheUniversityofTexasat Austin,Austin,TX,UnitedStates
ThierryAngot Aix-MarseilleUniversity,CNRS,PIIM,Marseille,France
NikolasAntonatos DepartmentofInorganicChemistry,UniversityofChemistry andTechnology,Prague,CzechRepublic
DmitryV.Averyanov NationalResearchCenter“KurchatovInstitute”,Moscow, Russia
FriedhelmBechstedt IFTO,FriedrichSchillerUniversity,Jena,Germany
LanChen InstituteofPhysics,ChineseAcademyofSciences,Beijing,P.R.China; SongshanLakeMaterialsLaboratory,Dongguan,P.R.China
WeiChen DepartmentofPhysics,NationalUniversityofSingapore,Singapore; DepartmentofChemistry,NationalUniversityofSingapore,Singapore;National UniversityofSingapore(Suzhou)ResearchInstitute,Jiangsu,P.R.China
ZhaoyingDang SchoolofMaterialsScienceandEngineering,Southeast University,Nanjing,P.R.China;Centerfor2DMaterials,SoutheastUniversity, Nanjing,P.R.China
YiDu SchoolofPhysicsandBUAA-UOWJointResearchCentre,Beihang University,Beijing,P.R.China;InstituteforSuperconductingandElectronic Materials,UniversityofWollongong,Wollongong,NSW,Australia
GabrieleFaraone LNESSandDipartimentodiScienzadeiMateriali,Universita ` degliStudidiMilanoBicocca,Milano,Italy
BaojieFeng InstituteofPhysics,ChineseAcademyofSciences,Beijing,P.R.China
HaifengFeng SchoolofPhysicsandBUAA-UOWJointResearchCentre,Beihang University,Beijing,P.R.China
JingGao DepartmentofPhysics,NationalUniversityofSingapore,Singapore
NicolaGaston TheMacDiarmidInstituteforAdvancedMaterialsand Nanotechnology,DepartmentofPhysics,TheUniversityofAuckland,New Zealand
JoshuaE.Goldberger DepartmentofChemistryandBiochemistry,TheOhio StateUniversity,Columbus,OH,UnitedStates
PaolaGori DepartmentofIndustrial,ElectronicandMechanicalEngineering, Rome,Italy
CarloGrazianetti ConsiglioNazionaledelleRicerche-Istitutoperla MicroelettronicaeMicrosistemi(CNR-IMM),UnitofAgrateBrianza,Agrate Brianza,Italy
ShiyingGuo KeyLaboratoryofAdvancedDisplayMaterialsandDevices, MinistryofIndustryandInformationTechnology,SchoolofMaterialScienceand Engineering,NanjingUniversityofScienceandTechnology,Nanjing,P.R.China
YuliHuang DepartmentofPhysics,NationalUniversityofSingapore,Singapore
WarrenL.B.Huey DepartmentofChemistryandBiochemistry,TheOhioState University,Columbus,OH,UnitedStates
IgorA.Karateev NationalResearchCenter“KurchatovInstitute”,Moscow, Russia
EvgeniyaKovalska DepartmentofInorganicChemistry,UniversityofChemistry andTechnology,Prague,CzechRepublic
GuyLeLay Aix-MarseilleUniversity,CNRS,PIIM,Marseille,France
LokLewYanVoon UniversityofWestGeorgia,Carrollton,GA,UnitedStates
JiahengLi StateKeyLaboratoryofLowDimensionalQuantumPhysics, DepartmentofPhysics,TsinghuaUniversity,Beijing,P.R.China;FrontierScience CenterforQuantumInformation,Beijing,P.R.China
YangLi StateKeyLaboratoryofLowDimensionalQuantumPhysics,Department ofPhysics,TsinghuaUniversity,Beijing,P.R.China;FrontierScienceCenterfor QuantumInformation,Beijing,P.R.China
Pai-YingLiao SchoolofElectricalandComputerEngineering,PurdueUniversity, WestLafayette,IN,UnitedStates
AlessandroMolle ConsiglioNazionaledelleRicerche-Istitutoperla MicroelettronicaeMicrosistemi(CNR-IMM),UnitofAgrateBrianza,Agrate Brianza,Italy
TianchaoNiu BeihangHangzhouInnovationInstituteYuhang,Hangzhou,P.R.China
OlegE.Parfenov NationalResearchCenter“KurchatovInstitute”,Moscow,Russia
OliviaPulci DepartmentofPhysics,andINFN,UniversityofRomeTorVergata, ViadellaRicercaScientifica1,Rome,Italy
Jing-KaiQin SchoolofElectricalandComputerEngineering,PurdueUniversity, WestLafayette,IN,UnitedStates;SchoolofMaterialsScienceandEngineering, HarbinInstituteofTechnology(Shenzhen),Shenzhen,P.R.China
GangQiu SchoolofElectricalandComputerEngineering,PurdueUniversity, WestLafayette,IN,UnitedStates
EricSalomon Aix-MarseilleUniversity,CNRS,PIIM,Marseille,France
Zdene ˇ kSofer DepartmentofInorganicChemistry,UniversityofChemistryand Technology,Prague,CzechRepublic
IvanS.Sokolov NationalResearchCenter“KurchatovInstitute”,Moscow,Russia
VyacheslavG.Storchak NationalResearchCenter“KurchatovInstitute”, Moscow,Russia
AlexanderN.Taldenkov NationalResearchCenter“KurchatovInstitute”, Moscow,Russia
DeepyantiTaneja MicroelectronicsandEngineeringResearchCenter,Department ofElectricalandComputerEngineering,TheUniversityofTexasatAustin,Austin, TX,UnitedStates
LiTao SchoolofMaterialsScienceandEngineering,SoutheastUniversity, Nanjing,P.R.China;Centerfor2DMaterials,SoutheastUniversity,Nanjing, P.R.China;SchoolofMaterialsScienceandEngineering,JiangsuKeyLaboratory ofAdvancedMetallicMaterials,SoutheastUniversity,Nanjing,P.R.China
AndreyM.Tokmachev NationalResearchCenter“KurchatovInstitute”,Moscow, Russia
YixiuWang SchoolofIndustrialEngineering,PurdueUniversity,WestLafayette, IN,UnitedStates
KehuiWu InstituteofPhysics,ChineseAcademyofSciences,Beijing,P.R.China; SongshanLakeMaterialsLaboratory,Dongguan,P.R.China
WenzhuoWu SchoolofIndustrialEngineering,PurdueUniversity,West Lafayette,IN,UnitedStates
XiaoXu SchoolofMaterialsScienceandEngineering,SoutheastUniversity, Nanjing,P.R.China;Centerfor2DMaterials,SoutheastUniversity,Nanjing, P.R.China
YongXu StateKeyLaboratoryofLowDimensionalQuantumPhysics, DepartmentofPhysics,TsinghuaUniversity,Beijing,P.R.China;FrontierScience CenterforQuantumInformation,Beijing,P.R.China;RIKENCenterforEmergent MatterScience(CEMS),Saitama,Japan
ZhimingXu StateKeyLaboratoryofLowDimensionalQuantumPhysics, DepartmentofPhysics,TsinghuaUniversity,Beijing,P.R.China;FrontierScience CenterforQuantumInformation,Beijing,P.R.China
PeideD.Ye SchoolofElectricalandComputerEngineering,PurdueUniversity, WestLafayette,IN,UnitedStates
HaroldJ.W.Zandvliet PhysicsofInterfacesandNanomaterials,MESA+ Institute forNanotechnology,UniversityofTwente,Enschede,TheNetherlands
HaiboZeng KeyLaboratoryofAdvancedDisplayMaterialsandDevices,Ministry ofIndustryandInformationTechnology,SchoolofMaterialScienceand Engineering,NanjingUniversityofScienceandTechnology,Nanjing,P.R.China
ShengliZhang KeyLaboratoryofAdvancedDisplayMaterialsandDevices, MinistryofIndustryandInformationTechnology,SchoolofMaterialScienceand Engineering,NanjingUniversityofScienceandTechnology,Nanjing,P.R.China
ZetaoZhang StateKeyLaboratoryofLowDimensionalQuantumPhysics, DepartmentofPhysics,TsinghuaUniversity,Beijing,P.R.China;FrontierScience CenterforQuantumInformation,Beijing,P.R.China
AidiZhao SchoolofPhysicalScienceandTechnology,ShanghaiTechUniversity, Shanghai,P.R.China
HanliuZhao SchoolofMaterialsScienceandEngineering,JiangsuKeyLaboratory ofAdvancedMetallicMaterials,SoutheastUniversity,Nanjing,P.R.China
