recent development of two dimensional transition metal dichalcogenides and their applications

Even though 2D TMDs exhibit a breadth of new properties that are distinct from traditional bulk materials or thin films, but also are comparable in performance to the atomic layers produ

Recent development of two-dimensional

1DepartmentofMaterialsScienceandEngineering,MechanicalandEnergyEngineering,UniversityofNorthTexas,Denton,TX76207,UnitedStates

2CenterforIntegratedNanostructurePhysics,InstituteforBasicScience(IBS),Suwon16419,RepublicofKorea

3DepartmentofEnergyScience,SungkyunkwanUniversity(SKKU),Suwon16419,RepublicofKorea

4DepartmentofElectricalandComputerEngineering,TheUniversityofTexasatAustin,Austin,TX78758,UnitedStates

detail.

Introduction

The great successofgraphene hasbeen followed by anequally

impressivesurgeforthedevelopmentofother2Dmaterialsthatcan

formatomicsheetswithextraordinaryproperties.Interestingly,the

2D library grows every year and feature more than 150 exotic

layeredmaterialsthatcanbeeasilysplitintoasubnanometer-thick

materials[1–3].Theseinclude2DTMDs(e.g.molybdenum

disul-fide(MoS2), molybdenumdiselenide (MoSe2), tungstendisulfide

(WS2),andtungstendiselenide(WSe2)),hexagonalboronnitride

(h-BN),borophene(2D boron), silicene(2D silicon),germanene

(2D germanium), and MXenes (2D carbides/nitrides) [4–11]

Figure 1 is a year-wise publication list of 2D materials that

showtheincreasingtrendofstudyingTMDs.Dependingontheir

chemicalcompositionsand structuralconfigurations, atomically

thin 2D materials canbe categorized asmetallic, semi-metallic,

semiconducting, insulating, or superconducting The first gra-phenedescendantsthatsparkedintenseresearchactivityareTMDs, which arealmost as thin,transparent and flexible asgraphene [12,13].Unlikegraphene, many2DTMDsaresemiconductorin natureandpossesshugepotentialtobemadeintoultra-smalland lowpowertransistorsthataremoreefficientthanstate-of-the-art silicon based transistors fighting to cope with ever-shrinking devices [14,15].Besidessharingthesimilaritiesofabandgapin the visible-nearIRrange,high carriermobility,and on/offratio with ubiquitous silicon, TMDs can be deposited onto flexible substratesandsurvivethestressandstraincomplianceofflexible supports[16,17]

2DTMDsexhibituniqueelectricaland opticalpropertiesthat evolvefromthequantumconfinementandsurfaceeffectsthatarise duringthetransitionofanindirectbandgaptoadirectbandgap whenbulkmaterialsarescaleddowntomonolayers.Thistunable bandgapinTMDsisaccompaniedbyastrongphotoluminescence

*Corresponding authors: Choi, W ( wonbong.choi@unt.edu ), Lee, Y.H ( leeyoung@skku.edu )

(PL) and large excitonbinding energy, making thempromising

candidateforavarietyofopto-electronicdevices,includingsolar

cells,photo-detectors,light-emittingdiodes,andphoto-transistors

[18–22].For example, unique propertiesof MoS2 include direct

bandgap(1.8eV), good mobility(700cm2V1s1), high

cur-renton/offratioof107–108,largeopticalabsorption(107m1in

thevisiblerange)andagiantPLarisingfromthedirectbandgap

(1.8eV)inmonolayer;thus,ithasbeenstudiedwidelyfor

elec-tronicsandoptoelectronicsapplications[23]

vanderWaals(vdW)gapsbetweeneachneighboringlayerand

largespecificsurfaceareaduetosheet-likestructuresaredistinct

featuresthatmake2DTMDshighlyattractiveforcapacitiveenergy

storage (e.g.supercapacitors and batteries)and sensing

applica-tions[24,25].Thelargesurface-to-volumeratiobestows2DTMDs

basedsensorswithimprovedsensitivity,selectivityandlowpower

consumption.Unlikedigitalsensors,TMDsbasedsensorsdonot

have physical gates for selectively reacting to the targeted gas

moleculesorbiomolecules[26,27].MoS2-basedFETdeviceshave

potentialapplicationsingas,chemical,andbio-sensors.Another

aspectoftheweaklybonded2DTMDsatomiclayersisthatthey

canbeeasilyisolatedandstackedwithotherTMDstoconstructa

wide range of vdW heterostructures without the limitation of

latticematching[28,29].Stackingtogetherone-atom-thicksheets

of dissimilar TMDs,forexample,vertically stacked

heterostruc-turesallowsfortherealizationofuniquefunctionsandsuperior

propertiesthatcannotbeobtainedotherwise.Byexploitingsuch

novelpropertiesinthesevdWheterostructuresasbandalignment,

tunnelingtransports,andstronginterlayercoupling,severalnew

electronic/opto-electronic devicessuchastunnelingtransistors,

barristers, photodetectors, LEDs and flexible electronicscan be

fabricated[30,31].Figure2depictsthediversedevicesconstructed

fromthe2DTMDsbyusingtheiruniquephysical,chemical,and

opto-electronicproperties[32–37]

Even though 2D TMDs exhibit a breadth of new properties

that are distinct from traditional bulk materials or thin films,

but also are comparable in performance to the atomic layers producedbythestandardexfoliationmethod

Thefieldof2Dmaterialsisanever-expandingresearcharea,and the search for other 2D materials beyond graphene is not just limitedtoTMDs.New2Dmaterialssuchassiliceneand phosphor-enearestrongcontendersintherapidlyemergingrealmsof2D materials [43,44].Several theoretical studiesaddress the funda-mentalpropertiesofthesenew2Dmaterials;however, experimen-talperspectivesarestillintheirinfancyduetostabilityissues Thisreviewhighlightstherecentadvancesinthesynthesisof large-scaleand defect-free2DTMDs.Moreover,wefocusonthe recentprogressinelectronic,opto-electronic,andelectrochemical propertiesofnewlystudiedTMDswithrationaldesignsandnew structuresforpotentialapplications in electronics,sensors, and energystorages.Additionally,wediscussrecentbreakthroughsin thenewestfamiliesof2Dmaterialslikesiliceneandphosphorene Thewiderangeofinterestingpropertiesandpotentialforusein emergent technologies suggest TMDs are likely to remain an importantresearchareaforyearstocome

TMDsarelayeredmaterialsinwhicheachunit(MX2)iscomposed

ofatransitionmetal(M)layersandwichedbetweentwochalcogen (X)atomiclayers.Dependingonthearrangementoftheatoms, thestructuresof2DTMDscanbecategorizedastrigonalprismatic (hexagonal, H), octahedral (tetragonal, T) and their distorted phase (T0) as shown in Fig 3a Typical atomic ratio in layered TMDsexhibitsone transitionmetalto two chalcogenatomsto createMX2exceptseveralcasessuchas2:3quintuplelayers(M2X3) [45]and1:1metalchalcogenides(MX)[46].InH-phasematerial, eachmetalatomputssixbranchesouttotwotetrahedronsin+z andzdirectionswhilethehexagonalsymmetrycanbeseenin the top view (Fig 3a) Therefore, chalcogen–metal–chalcogen arrangementalong z-directionisconsideredassinglelayer,and weakvdWinteractionsbetweeneachlayer(chalcogen–chalcogen) enablemechanicalexfoliationfrombulkTMDstoobtain single layerflake.T-phasehasatrigonalchalcogenlayeronthetopand

180degreerotatedstructure(so-calledtrigonalantiprism)atthe bottominasinglelayerandresultsinhexagonalarrangementof chalcogenatomsinthetopview.Metalatomsaredistortedfurther (ordimerizedinonedirection),calledT0-phase[47,48],resultingin the modification of atomic displacement of chalcogen atoms alongz-direction(d)

Despite the extraordinary mobility of electrons (i.e

15,000cm2V1s1atroomtemperature)ingraphene,thelack

ofa bandgaprestrictsitsuse asanactive elementin FETs[49]

FIGURE1

Year-wisepublicationplotsfor2DTMDsincludingMoS2,MoSe2,black

phosphorus,MXenes,andtotal2DTMDsintheperiodof2005–2016

(searchedbySciFinderScholar(https://scifinder.cas.org),AmericanChemical

Societydatabase(https://www.acs.org/content/acs/en.html),August10th,

2016)

2

nanor-ibbons,AB-stackedbilayergraphene, andchemical dopinghave

metwithmarginalsuccessprovidingthebandgapopeningupto

200meVinmostcases[50–52]

Thisremainsachallengingissueandhasbeenadrivingforcein

developing2DTMDswithafinitebandgap.AslistedinFig.3b,2D

TMDs reveal a wide range of bandgap covering all visible and

infraredrangewiththechoiceofmaterial[53].Most

semiconduct-ing2DTMDsrevealdirectbandgapinmonolayer,whereastheyare

indirectbandgapinbulkformexceptfewcasesofGaSeandReS2

[54,55].For example,monolayer dichalcogenides suchasMoS2

(1.8eV),MoSe2 (1.5eV),(2H)-MoTe2 (1.1eV),WS2 (2.1eV)and

WSe2(1.7eV)showdirectbandgap,whereasbulkphasesexhibit

indirectgapwithsmallerenergies.MostMX2materialshaveboth

metallicphaseandsemiconductingphase[56].Thestablephaseof

MX2materialatroomtemperatureis2Hphase,whereas1Tphase

canbeobtainedbyLi-intercalation[57]orelectronbeam

irradia-tion[58].Thechemicallyexfoliated1TMoS2phaseisknowntobe

107 timesmore conductivethan thesemiconducting 2H phase [59].IncaseofWTe2,1Tor1T0phaseismorestablethan2Hphase

atroomtemperature[60].Both2Hand1T0phaseinMoTe2canbe easily modulated into each other because the cohesive energy differencebetweenbothphasesissimilartoeachother.Besides, thedichalocogenidesoftitanium(Ti),chromium(Cr),nickel(Ni), zinc(Zn),vanadium(V),niobium(Nb),andtantalum(Ta)simply exhibitmetallicbehavior[61]

SincemostoftheMX2arefreefromdanglingbonds,andsome

ofthemexhibithighmobility,dependingonthechoiceof appro-priatesubstrateandmetalcontactsaswellasmobilitysuppression throughgrainboundaries,etc.Forexample,MoS2givesamobility

of700cm2V1s1onSiO2/Sisubstratewithscandium(Sc)contact and33–151cm2V1s1onBN/Sisubstrate(encapsulated)atroom temperature[62,63]

Besides excellentelectrical transport,TMDs aremechanically flexible and strongsimilar to graphene An exceptionally high Young’s modulus(E) of 0.330.07TPa has been reported in

FIGURE2

Electronic,opto-electronicandenergydevicesbasedon2Dtransitionmetaldichalcogenides(TMDs).(ReprintedwithpermissionfromRef.[32].2015,Nature PublishingGroup;Ref.[33].2014,NaturePublishingGroup;Ref.[34].2015,RoyalSocietyofChemistry;Ref.[35].2012,JohnWileyandSons;Ref.[36].2015, NaturePublishingGroup;andRef.[37].2014,AmericanChemicalSociety.)

3

suspendedfew-layerMoS2 nanosheets[64].Bertolazzietal.[65]

reportedhighin-planestiffnessandEofsingle-layerMoS2,thatis,

18060Nm1and270100GPa,respectively.TheYoung’s

modulus of monolayer MoS2 outperforms the stainless steel

(204GPa)andgrapheneoxide(207GPa)[66],whichareattributed

totheabsenceofstackingfaults,highcrystallinityanddefect-free

natureoftheatomicallythinTMDs

Morerecentprogressinsinglecomponent2Dlayersisexpanded

to the group III and V elements Atomically thin boron layer

‘borophene’ synthesis was recently carried out in an ultrahigh

vacuum system withthe evaporationofpure boronelement at

hightemperature (450–7008C) onsilver (Ag)(111) Anisotropic

metallicity is confirmed by scanning tunneling spectroscopy,

whilebulkboronallotropesaresemiconductors[67].Bulkblack

phosphorous(BP)wassynthesizedbychemicalvaportransportof

redphosphorinthepresenceoftransportagent[68]or

pressuri-zation(>1.2GPa,2008C)[69,70].BPfilmisastrongcandidatefor

applications to electronic devices due to their high mobility

(1000cm2V1S1) with ambipolarity [71–73] Layered metal

carbides/nitrides,MXenes[74],arelocatedatthebottominthe

schematicwithgraphenealsoshowmetallicbehavior

The structure and properties such as charge density wave

(CDW),magnetism(ferromagneticandanti-ferromagnetic),and

superconductivityof2DTMDsaresummarizedinFig.4.Thedetail

descriptionofallthesepropertiesineachmaterialisbeyondthe

scopeofthisreview

In addition to TMDs, borophene, silicene, germanene and

stanene are predicted as exotic2D materials that could show

many intriguing properties However, these materials are

quite unstablein air[75]andtherefore needencapsulation or

hydrogen termination to generate SiH or GeH in silicene or germanene.Boropheneischaracterizedasametal.Siliceneopens

abandgapslightly by1.9meVandgermaneneby33meVand staneneby 101meV, which is an opposite trendwith atomic number[76].Recentworkrevealstheperformanceofsiliceneas

a field effect transistor (FET), which is promising for future

FIGURE3

(a)Typicalstructuresoflayeredtransitionmetaldichalcogenides.Cleavable2H,1Tand1T0structuresinlayeredTMDareshown.(b)Bandgapof2Dlayered materialsvaryingfromzerobandgapofgraphene(whitecolor)towidebandgapofhBN.Thecolorinthecolumnispresentingthecorresponding wavelengthofbandgap,forexample,thebandgapforMoS2(1.8eV)isredcolorandWS2(2.0eV)isorangecolor.Indirectmaterialsarerepresentedatleft (SnS2,ZrS2,HfS3,ZrS3,ZrSe3,HfS2,HfSe3,HfSe2,ZrSe2andZrTe2)anddirectbandgapmaterialsarerepresentedatrightsideofthecolumn(h-BN,WS2,MoS2, WSe2,MoSe2,2H-MoTe2,TiS3andTiSe3)

FIGURE4 Tableforvarious2DTMDsandother2Dmaterialsexhibitingvarious physicalpropertiessuchasmagnetism(ferromagnetic (F)/anti-ferromagnetic(AF)),superconductivity(s)andchargedensitywave(CDW) andcrystalstructures(2H,1T)

4

topo-logicallayer

Considerableeffortshave beendevotedtothesynthesis of

con-trollable, large-scale, and uniform atomic layers of diverse 2D

TMDsusingvarioustop-downandbottom-upapproaches,

includ-ingmechanical exfoliation, chemicalexfoliation, and chemical

vapordeposition(CVD).Mostofthereporteddataandtheoryon

thefundamentalphysicsanddeviceson2D TMDshavelargely

reliedontheexfoliationmethodduetoitshighquality.However,

thecriticallimitationsoftheflakesizeandfilmuniformityhave

draggeditsdevelopmentbeyondthefundamentalstudies.Onthe

contrary, the CVD process has been studied for scalable and

reliable production of large area 2D TMDs Nevertheless, CVD

grownTMDsshowpoorqualityascomparedtotheirexfoliated

counterparts

Veryrecently,attemptshavebeenmadetoobtainhighquality

TMDswith thickness controllabilityand wafer-scaleuniformity

using atomic layer deposition (ALD), metal-organic-CVD

(MOCVD),anddirectdepositionmethods(sputtering,pulsedlaser

deposition(PLD), e-beam) The 2D materials forming chemical

reactionsgenerallyuseeitherthermalenergyfromaheated

sub-strateornon-thermalenergysuchasmicrowaveorphotonenergy

intothereactionprocessand the 2Dmaterialsforming process

depends on lattice parameter of substrates, temperatures, and

atomic gas flux [78,79] Here, we will focus our discussion on

the2DTMDsgrowthbyCVD,MOCVD,andALDmethodswith

theirprosandcons

TheCVDisoneofthemosteffectivemethodstoachievelargearea

growth of atomically thin 2D TMDs for the successful device

applications.ThesimplestformofCVDtogrow2DTMDsisthe

co-evaporationofmetaloxidesandchalcogenprecursorsthatlead

tovaporphasereactionfollowedbytheformationofastable2D

TMDovera suitablesubstrate The growth mechanismofCVD

methoddiffersineachsynthesisprocessasthematerialsforming

processalsodependson(1)propertiesofsubstrate,(2)temperature

and(3)atomicgasfluxasbrieflydiscussedinthefollowingsection

(1) Properties of substrate: the atomic layer of 2D materials is

influenced by nanoscale surface morphology and terminating

atomic planes of substrates as well as lattice mismatching It

wasreportedthatthe surfaceenergyofsubstrateaffectsthe

nu-cleationandgrowthof2DTMDs[80].(2)Temperature:thereaction

process islimited by the growth temperature Normally,if the

growthtemperature ishigh,that is,thesurface diffusionisfast

enough,arandomlydepositedadatomwillmovetothe

energeti-callymostfavorableplacesandresultsina3Dislandgrowth.On

theotherhand,ifthesubstratetemperatureistoolow,an

amor-phousorpolycrystallinefilmwillformsinceadatomswillnothave

enough kineticenergy to diffuseand find the lowest potential

energysite [81] (3) Atomic gasflux: atomic gasflux is another

importantparametertoachievehighquality2Dmaterialsgrowth

Onlyasufficient highvaporpressure enablesmixing ofatomic

gases and transport the atomic species to the substrate The

stability of vaporizedatomsis required toprevent unnecessary

reactionduringvaporizedatom transport to the substrate The

vaporizedatomsaretransportedbyacarriergastothesubstrate and theflowrateofvaporedatomisgovernedbytheClausius– Clapeyronequation:d(lnP)/dT=DH/kT,whereDHistheenthalpy

ofevaporation,Pisthe partialpressure oftheevaporatedatom [82].Leeetal.[83]reportedlarge-scaleMoS2layersby chemical vaporreactionofmolybdenumtrioxide(MoO3)andsulfurpowder

atelevatedtemperatures(6508C).MoO3isinitiallyreducedintoa suboxideMoO3x,whichreactswithvaporizedsulfurfurtherto forma2DlayeredMoS2film

Thissimpleprocessiscapableofproducinglarge-scaleMS2; how-ever,itoftenresultsintheformationofrandomlydistributedflakes ratherthanacontinuousfilm.TheinhibitionofthegrowthofMoS2 wasattributedtothepresenceofinterfacialoxidelayerasa signifi-cantobstacle.Inasimilarapproach,Najmaeietal.[84]synthesized MoS2atomiclayersonSi/SiO2substratesbyusingthevapor-phase reactionofMoO3 andS powdersand reportedthe formationof MoS2monolayertriangularflakesonthesubstratesratherthanthe formationofacontinuousMoS2layer.Theaveragemobilityand maximum current on/off ratio of the MoS2 flakes showed 4.3cm2V1s1 and 106

, respectively Wang et al [85] found

aninterestingshapeevolutioninCVDgrownMoS2domainsfrom triangular to hexagonal geometriesdepending upon the spatial locationofthesiliconsubstrateasshowninFig.5a.Yuetal.[86] developedanewmethodthatpreciselycontrolthenumberofMoS2 layersoveralargeareabyusingMoCl5andsulfurasprecursors.But, themobilityofchargecarriersinMoS2-FETwasfoundtobevery low(0.003–0.03cm2V1s1)

Anotherfacilemethodforgrowinglargeareaandcontinuous TMDsisusingthe‘two-stepmethod,depositingtransitionmetal thin film (e.g Mo, W, Nb, etc.) on substrate (usually Si/SiO2) followed by thermalreactionwith chalcogen (S,Se, Te) vapor Thefollowingreactionoccurstoformastable2DTMDduringthe CVD processathightemperatures (300–7008C)and inert atmo-sphere

This ‘two-stepmethod’hasdemonstratedwafer scale fabrica-tion(2in.)andsuccessfulthicknessmodulationofMoS2layers (multilayertomonolayer)onSiO2/Sisubstrates(Fig.5b)[87].After metaldeposition(W,Mo)with controlledthickness,the metal-coatedsubstrateand sulfurpowderwereplaced insidethe CVD furnace, and the reactionenvironment was kept inert under a constantflowof200sccmArat6008Cfor90min.However,in monolayerMoS2grown,pointdefectsanddoublelayers’domains werepresentasconfirmedbyhigh-resolutiontransmission elec-tronmicroscopy(HRTEM)andRamananalysis.Electrical measure-ments on MoS2 FETs revealed a semiconductor behavior with much higherfieldeffectmobility(12.24cm2V1s1) and cur-renton/offratio(106)ascomparedtopreviouslyreported CVD-grownMoS2-FETsandamorphoussilicon(a-Si)ororganicthinfilm transistors Zhanet al [88] usede-beam evaporation and CVD methodstogrowlargeareaMoS2filmsandfoundap-type con-duction but with very poor mobilities in the range of 0.004– 0.04cm2V1s1.ThepresenceofMo-containingseedshasbeen

5

ofhighlycrystallinemonolayer MoS2[89].Thepatternsor seed

moleculesonthesubstratescanprovidethecontrollednucleation

of2DTMDsinpredefinedlocations.Thegrowthsoflarge

crystal-lineislandofMoS2 witha sizeof100mmwerereported.Device

measurements exhibited carrier mobility and on/off ratio that

exceeded10cm2V1s1and106,respectively.However,the

met-al-sulfurization methodoffers controllable thickness with large

scaleproduction,butitisstilllimitedtotheproductionofsmall

grainsizewithdefects.Inadditiontometalfilmapproach,direct

sulfurization/selenization of various metal oxide and chloride

precursorssuchas(NH4)2MoS4,MoO3,WO3andMoO2havebeen

widely employedtogrow TMDs.Eliasetal [90]controlled the

thicknessoftheWO3coatingforthelarge-areagrowthof

single-layerand few-layerWS2sheets.(NH4)2MoS4 and similarsulfide

precursorshavebeenused,butwithuncontrolledlayerthicknesses

[91].Duringselenizationofoxideandchloridecoatings,hydrogen

gasisusuallyintroducedtoassistthereactionand toithelpin

tailoringthecrystallineshapeofTMDs[92]

TheMOCVDissimilartoaconventionalCVDexceptthat

metal-organic or organic compoundprecursor are used asthe source

materials [93,94] In MOCVD reaction, the desired atoms are

combined with complex organic molecules and flown over a

substratewherethemolecules aredecomposedbyheatand the

target atomsaredepositedonthe substrateatomby atom The

qualityoffilmscanbeengineeredbyvaryingthecompositionof

atomsatatomicscale,whichresultsinthedesiredthinfilmwith

highcrystallinity.Figure6istherepresentativeschematicofthe

MOCVDmethodshowingvariousstepsinvolvedduringthe

syn-thesis of 2D materials As shown in Fig 6, a series of surface

reactionsoccurduringMOCVDprocessincludingadsorptionof

precursormoleculesfollowedbysurfacekinetics(i.e.surface

dif-fusion), nucleationand growthofdesiredmaterialwith the

de-sorptionofthevolatileproductmolecules

MOCVDhasbeenusedtogrow2DTMDsonlyvery recently

The advantagesof MOCVD in 2D TMDs growth are: (i) itcan

achievelarge-scaleanduniformgrowthof2DTMDs,(ii)itprovides

aprecisecontroloverbothmetaland chalcogenprecursorsand therebycontrolsthecompositionandmorphologyof2DTMDs.In thisregard,Kangetal.[95]synthesizedwafer-scale(4-in.) mono-layerand fewlayersMoS2 andWS2 filmson SiO2 substratesby usingmolybdenumhexacarbonyl(Mo(CO)6),tungsten hexacar-bonyl(W(CO)6),ethylenedisulfide((C2H5)2S),andH2gas-phase precursorswithArgascarrier Theteamshowslarge-scaleMoS2 andWS2filmson4-in.fusedsilicasubstrates(Fig.7a),andabout

8000MoS2FETdevicesfabricatedbyastandardphotolithography process(Fig.7b).TheMoS2-FETsshowedhighelectronmobilityof

30cm2V1s1atroomtemperatureand114cm2V1s1at90K (Fig.7c).Figure7disthetimeevolutionofthemonolayercoverage overtheentiresubstrateasafunctionofcriticaltime(t0) Recently,Eichfeldetal.[96]claimedthefirstreportonthe large-area growth of mono and few-layer WSe2 via MOCVD using W(CO)6 and dimethylselenium ((CH3)2Se) precursors They showedthatthetemperature,pressure,Se:Wratio,andsubstrate choicethathavesignificantimpactonthemorphologyofWSe2 films.ItisclearfromFig.8athatWSe2hasdistinctmorphologyon differentsubstratesincludingepitaxialgraphene,CVDgraphene, sapphireand BN.WSe2 grew with large nucleation density on

FIGURE6 DepositionprocessonthesubstrateandsurfaceprocessesinMOCVDwhile growingactivelayersonthesubstrate.Thegaseousprecursorsare thermallydecomposedandadsorbedonthesubstratefollowedbysurface diffusionkineticstoformhighqualitythinfilms

FIGURE5

(a)SchematicdiagramofCVDprocessandAFMimagesshowingshapeevolutionofMoS2crystalsfromtriangulartohexagonaldependingonthespatial locationofsiliconsubstrate.(ReprintedwithpermissionfromRef.[85].2014,AmericanChemicalSociety.)(b)Largeareagrowthof2–3layersofMoS2using

MoseedlayersulfurizedinaCVDfurnace.(ReprintedwithpermissionfromRef.[87].2015,AmericanInstituteofPhysics.)

6

graphene, while the largest domain size of about 5–8mm was

observedon sapphire.Figure 8 showsthe influenceof thegas

flowrateonthedomainsizeandshapeofWSe2grownonepitaxial

graphene.Besides flowrate, the domain size increaseswith

in-creasingtotalpressureandtemperature.A200%incrementinthe

grainsize(700nmto1.5mm)occurredastemperaturerosefrom

800 to 9008C The results weresimilar on sapphire substrates

Besidespressure andtemperature,the Se:Wratioand totalflow

throughthesystemgreatlyimpactthedomainsizeofWSe2films

AnincreaseinSe:Wratiofrom100to2000allowsanincreasein

thedomainsizefrom1to5mm.Anincreaseintotalgasflowfrom

100 to 250sccm enhances domain size up to 8mm The I-V

characteristicsconfirmthepresenceofatunnelbarriertovertical

transportcreatedbytheWSe2andtherebyevidencesthata

pris-tinevanderWaalsgapexistsinWSe2/grapheneheterostructures

Furthermore,thismethodisusedtogrow MoS2/WSe2/graphene

andWSe2/MoS2/grapheneheterostructures(Fig.8c)[97]

Interest-ingly, they discovered that directly grown heterostructures by

MOCVDexhibitresonanttunnelingofchargecareersthatleads

tolargenegativedifferentialresistance(NDR)atroomtemperature

as shown in Fig 8d The MOCVD method is versatile, highly

scalable, and provides significant stoichiometric control over

thefilms,buttheuse oftoxicprecursors,slowfilmgrowthrate

andhighproductioncostsetitbackforthewidespreaduse

ALDisa gasphasechemicalprocess todeposit atomicallythin

filmsofvariousmaterialslayerbylayerbyusingprecursorstoreact

themwithsubstrate.AlthoughALDhasbeenusedwidelyforoxide materials,severalbinarysulfidematerialshavebeenstudied suc-cessfully by several research groups These include TiS2, WS2, MoS2,tin(II)sulfide(SnS),andlithiumsulfide(Li2S).However,

inthescopeofthecurrentarticle,wesummarizeonlyimportant resultsofthe2DTMDsbyALDmethod.Tanetal.[98]provided precisecontrolovertheMoS2filmthicknesspreparedbythe self-limiting reactions of molybdenum pentachloride (MoCl5) and hydrogen sulfide (H2S) on sapphire substrate However, high-temperature(8008C)annealingwasperformedtogrowlargesize (2mm)triangular MoS2crystals Songetal.[99]demonstrated thewaferscalegrowthofWS2usingtheALDgrowthoftungsten oxide(WO3)withsubsequentconversionviaH2Sannealing Fig-ure 9a shows the ALD growth steps for the synthesis of WS2 nanosheets.The numberofMoS2layerscanbecontrolled effec-tivelybytuningthenumberofALDcyclesforMoO3growth.The camera image of large-area (approximately 13cm in length) mono-, bi-, and tetralayer WS2 nanosheets onSiO2 substrateis showninFig.9b

Monolayer WS2 FETs fabricated in the top-gate geometry showed n-type conduction with an electron mobility of

3.9cm2V1s1 (Fig 9c) Furthermore, the high conformal growthabilityofALDhelpedthemtorealizethicknesscontrolled growthof1DWS2 nanotubes(WNTs)bysulfurizingWO3layers deposited onSi nanowires(NWs) Followingthe firstreporton ALD growth of2D materials,Jinet al [100]presentedanother chemical route to deposit MoS2 on SiO2/Si substrate using Mo(CO)6 anddimethyldisulfide (CH3SSCH3,DMDS)as Moand

FIGURE7

LargescaleMOCVDgrowthofcontinuous(a)MoS2monolayersonfusedsilicausingall-gasphaseprecursors.(b)Thescalablegrowthenablesmass

productionof8000ofFETdevices.(c)Fieldeffectmobility(mFE)measuredfromfiveFETdeviceswithdifferentlengthscales.Aconsistentmobilityof

30cm2V1s1wasobserved.(d)OpticalimagesofMoS2filmsatdifferentgrowthtimes,wheret0istheoptimalgrowthtimeforfullmonolayercoverage (scalebar:10mm).(ReprintedwithpermissionfromRef.[95].2015,NaturePublishingGroup.)

7

9008Ctocrystallizethemintoa2H-MoS2phase.Theabovestudies

revealclearlythatthegrowthtemperaturewasquitehigh(800–

10008C)andfilmsresultedincrystallitesizeinsub-10nmrange

Hence,thedevelopmentofabetterALDapproachtogrowhigh

quality,large-scalewithprecisecontrolofatomiclayerthickness

inMX2isimperative.Inthisdirection,Delabieetal.[101]

dem-onstratedthelowtemperature(300–4508C)growthofWS2atomic

layersenabledbySiandH2plasmareducingagentsforCVDand

ALD,respectively,inthepresenceofWF6andH2Sprecursors.No

template layeror postdeposition annealingtreatmentwas

per-formedontheselayers[102].ALDmethodishighlyscalablewith

precisethicknesscontrollabilityusuallyatlowsubstrate

tempera-tures;itsexpensivenatureanduseofhighlysensitiveprecursors

arebigconcerns

Applications 2DTMD materialsareconsidered attractivefordiverse applica-tions including electronics, photonics, sensing, and energy devices.Theseapplicationsareinspiredbytheuniqueproperties

oflayeredmaterialssuchasthinatomicprofilethatrepresentsthe idealconditionsformaximumelectrostaticefficiency,mechanical strength,tunableelectronicstructure,opticaltransparency,and sensorsensitivity [103].Ofparticular interestforapplications is flexiblenanotechnology,whichisconsideredforpotentially ubiq-uitouselectronicsandenergydevicesthat canbenefitfromthe rangeofoutstandingpropertiesaffordedby2Dmaterials.Flexible technologycomprisesa widearrayofscalablelarge-areadevices includingthinfilmtransistors(TFTs),displays,sensors, transdu-cers, solar cellsand energy storage onmechanically compliant substrates

FIGURE8

(a)AFMsurfacemorphologyoftheWSe2filmgrownondifferentsubstrates;epitaxialgraphene,CVDgraphene,sapphire,andboronnitride.(b)FESEM imagesofWSe2showinganevolutionofdomainsizeasafunctionofflowrate.(ReprintedwithpermissionfromRef.[96].2015,AmericanChemicalSociety.) (c)SchematicandAFMimageofdifferentheterostructures(d)I–Vcurvesfordifferentcombinationofdichalcogenide-grapheneheterostructuresindicating resonanttunnelingandNDR.(ReprintedwithpermissionfromRef.[97].2013,AmericanChemicalSociety.)

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Electrical and optoelectronics applications

Thescalinglimitsofconventionalsilicon-basedtechnologyover

thelastdecadessuggestthatatomicallythinsemiconductorssuch

as TMDs might be applicable for future generation large-scale electronics[104],providedmanufacturingand integration chal-lengescanberesolved.Indeed,thefirstconsumerproduct featur-inggraphenetouchpaneldisplaysinsmart-phoneswasreleasedin Chinain2014,afteronlytenyearsofglobalgrapheneresearch,a relativelyshorttime inthe innovation cycle Moreover,in this timeframe,severalarticleshavereviewedbreakthroughsand per-ceivedapplicationsof2Dmaterials[105,106].Here,wewilldiscuss progressinflexibleelectronics,particularly2DTFTs,whichisthe coredevicerequiredformanyflexibletechnologydeviceconcepts, much like the conventionalones (FET is the centraldevicefor virtuallyallusesofsemiconductortechnology)

Afterseveral yearsofactiveresearchanddevelopment, high-performance2DTFTsbasedonsynthesizedMoS2havenowbeen achieved.TheseTFTsoperatingatroomtemperaturefeaturethe characteristic high on/off current ratio and current saturation expectedfromhigh-qualityTMDs(Fig.10a,b).Inparticular, elec-tronmobility50cm2V1s1andcurrentdensity250mA/mm havebeen observed,whichisvery encouragingfor high-perfor-mance TFTs Importantly, cut-off frequencies exceeding 5GHz havebeenrealizedonflexibleplasticsubstratesatachannellength

of0.5mm(Fig.10c).Atfirst,thiswasrathersurprisinggiventhe relativelylowmobilityofMoS2;however,atthehighfieldsneeded formaximumhighfrequencyoperation,transportisdetermined

by thesaturation velocity(vsat)that turnsouttobesufficiently reasonable(2106cm/s)toachieveGHzspeedsatsub-micron channellengths[107].Inaddition,flexiblemonolayerMoS2TFTs

FIGURE9

(a)SchematicoftheALDprocessforthesynthesisoflargeareaand

thicknesscontrolledWS2filmsand(b)demonstrationofthelargearea

(approx.13cm)mono-,bi-,andtetralayerWS2fabricatedonSi/SiO2

substrates.Thegrowthareaisaboutthesizeofacellularphonedisplay

screen.(c)TransfercharacteristicsofsinglelayerWS2FETexhibitinga

n-typeconductionbehavior.(ReprintedwithpermissionfromRef.[99].2013,

AmericanChemicalSociety.)

FIGURE10

RepresentativeCVD-grownMoS2FET(L=1mm,W=2.6mm)on280nmSiO2/Si.(a)Electricaltransfercharacteristics.Theinsertshowsthelow-fieldmobility

54cm2V1s1,whichisatthehigh-endformonolayerMoS2.(b)ID–VDcharacteristicsfeaturinglinear-saturationprofileexpectedforwell-behaved

semiconductingFETs.(c)Extractedcut-offfrequencyforflexibleMoS2transistorswithintrinsicfT5.6GHz(L=0.5mm).Insetisanillustrationofthedevice structure.(d)Multi-cyclebendingtestsshowingthattheflexibleMoS2affordstrongelectricalstabilityafter10,000cyclesofbendingat1%tensilestrain

(e)SchematicofflexibleMoS2RFtransistorusedasanAMdemodulatorwithinawirelessAMreceiversystem(AMradioband0.54–1.6MHz).Insetshows

theAMreceiveroutputspectrum.(ReprintedwithpermissionfromRef.[107].2015,JohnWileyandSons.)

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that of CVD graphene, and on/offratios typically >10–10 at

roomtemperature[108]

Inessence,BPcanbeviewedasa2Dcrystalthatoffers

optoelec-tronic properties in between zero-bandgap high-mobility

gra-phene and large-bandgap low-mobility TMDs A primary

applicationofBPisforhigh-performancehigh-mobilityflexible

nano-optoelectronics.Inthisregard,thefirstflexibleBPTFTswas

reportedin2015[109],andofferedambipolartransportwithhole

andelectronmobilities(Fig.11b)higherthantheestablished

thin-filmmaterialsbasedonmetaloxides,organicsemiconductorsand

amorphousSi.Thisisallthemoreinterestingsincethefabricated

flexible BP TFTs were not optimized in terms of contacts and

interfaces,thereby suggesting significantroom forperformance

improvement Output characteristics of flexible BP TFTs show

strongcurrent saturation(Fig.11c) The currentsaturation and

2DTMDsaregainingsignificantattentionaselectrodematerials forenergystorages,suchassupercapacitorsandLi-ionbatteries, duetotheir atomically layeredstructure,highsurface areaand excellentelectrochemicalproperties.Suchlayeredstructures pro-vide more sites for ions in energy storage while maintaining structurestability duringchargeand dischargecycles.Thehigh surface area of 2D materials (e.g the surface area of graphene shows2630m2/g,whichisthehighestamongcarbonmaterials) whencombinedwithsurfacefunctionalityandelectrical conduc-tivity,makethemasanidealelectrodeforenergystorages[113– 115]

Asupercapacitoris a high-capacityelectrochemicalcapacitor with capacitance values one order higher than Li-ion batteries consistingoftwosymmetricelectrodesseparatedbyamembrane, andanelectrolyteionicallyconnectingbothelectrodes.Whenthe electrodesarepolarizedbyanappliedvoltage,ionsinthe electro-lyteformelectricdoublelayersofoppositepolaritytothe electro-de’s polarity In certain electrode materials, some ions may permeatethedoublelayerandbecomespecificallyadsorbedions andcontributewithpseudocapacitancetothetotalcapacitanceof the supercapacitor MoS2 exhibits large electrical double layer capacitance (EDLC) due to its stacked-sheet-like structure and largepseudo-capacitanceowingtodifferentMooxidationstates (+2to+6)andhasbecomeapromisingsupercapacitorelectrode materialinthe 2Dmaterials fraternity[116–118].However, po-tentialproblemsassociatedwithitswidespreadusearesmallflake size,productionwithlowyieldanduncontrollablethicknessand defectsbytheexfoliationandhydrothermalmethods[119,120] Tuning the surface morphology of MoS2 nanosheets is an important parameter for their superior electrochemical perfor-mance.Touretal.[121]fabricatededgeoriented/verticallyaligned MoS2 nanosheets that opened more van der Waals gaps and offeredreactivedanglingbondssites totheelectrolyteionsand therebymanifestlargecapacitiveproperties(Fig.12a).The sponge-like vertically aligned MoS2 flexible supercapacitor electrodes demonstrateda higharealcapacitanceupto12.5mFcm2.The limited electrical conductivity of the most common 2H-MoS2 phasemakesitlessattractivematerialforsupercapacitorelectrode

asreportedatinstances[122,123].ThiscompelsRutgersuniversity researcherstodevelopametallicMoS2phase(1T)whichhas107 timeshigherconductancethansemiconductingphase(Fig.12b) ThechemicallyexfoliatedMoS2nanosheets-derived supercapaci-tor electrodes demonstrated excellent capacitive performance, withcapacitance values rangingfrom 400to 700Fcm3in a variety of aqueous electrolytes [124] Choudhary et al [34] reported the direct fabrication of a large-scale and unique

FIGURE11

(a)Fieldeffectmobility(mFE)(opencircles)Ion/offratio(filledbluetriangles)

ofBPfilmswithvaryingthickness.(ReprintedwithpermissionfromRef

[108].2014,NaturePublishingGroup.)(b)Transfercharacteristicsof

encapsulatedBPambipolarTFTonpolyimide,showinglowfieldhole

mobilityof310cm2V1s1,andelectronmobilityof89cm2V1s1.The

on/offratio>103.Vd=10mV,andflakethicknessis15nmandW/

L=10.6mm/2.7mm.(c)Outputcurvesofthesamedevicedisplaying

currentsaturation.Gatebias,Vg=0to2.5Vfrombottomtotop

(ReprintedwithpermissionfromRef.[109].2015,AmericanChemical

Society.)(d)Representativeflexiblehigh-frequencynanosystemapplications

of2Dmaterialsbasedonexperimentallyachievedsaturationvelocities

Contemporaryorganicandmetal-oxide(e.g.IGZO)filmsaremostlysuitable

forlowerfrequencyfunctionssuchasdisplayTFTs.(Reprintedwith

permissionfromRef.[110].2016,AmericanChemicalSociety.)

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