layered materials and their potential applications
Item Type Article
Authors Li, Ming-yang;Chen, Chang-Hsiao;Shi, Yumeng;Li, Lain-Jong Citation Heterostructures based on two-dimensional layered materials
and their potential applications 2015 Materials Today Eprint version Publisher's Version/PDF
DOI 10.1016/j.mattod.2015.11.003
Publisher Elsevier BV
Journal Materials Today
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Heterostructures based on
two-dimensional layered materials and their potential applications
Ming-Yang Li
1,2, Chang-Hsiao Chen
2, Yumeng Shi
1and Lain-Jong Li
1,*
1PhysicalSciencesandEngineeringDivision,KingAbdullahUniversityofScienceandTechnology,Thuwal23955-6900,SaudiArabia
2InstituteofAtomicandMolecularSciences,AcademiaSinica,Taipei10617,Taiwan
The development of two-dimensional (2D) layered materials is driven by fundamental interest and their potential applications. Atomically thin 2D materials provide a wide range of basic building blocks with unique electrical, optical, and thermal properties which do not exist in their bulk counterparts. The van der Waals interlayer interaction enables the possibility to exfoliate and reassemble different 2D materials into arbitrarily and vertically stacked heterostructures. Recently developed vapor phase growth of 2D materials further paves the way of directly synthesizing vertical and lateral
heterojunctions. This review provides insights into the layered 2D heterostructures, with a concise introduction to preparative approaches for 2D materials and heterostructures. These unique 2D heterostructures have abundant implications for many potential applications.
Introduction
The heterostructures formed by alien semiconductors play an importantroleinmodernsemiconductorindustry,andtheyhave been widely used as an essential building block for electronic devices.Aheterojunctioncanbeformedbyinterfacingtwodif- ferentsemiconductors,wheretheelectronicbandstructurenear the interface will be changed according to electrostatics. The semiconductorheterojunctions havebeen appliedin manysol- id-statedevices,suchassolarcells,photodetectors,semiconductor lasers,andlight-emittingdiodes(LEDs).Sincethegreatinfluence ofthe semiconductor junction to our dailylife, Nobel Prize in physicswasawardedto ZhoresI. Alferov,HerbertKroemerand Jack S.Kilby fortheir contribution toward the development of semiconductorheterostructures[1,2]andtoIsamuAkasaki,Hir- oshiAmano,andShujiNakamurafortheinventionofblueLEDs [3,4].
Two-dimensional (2D) materials commonly possess unique opticalbandgapstructures,extremely stronglight–matterinter- actions, and large specific surface area. Graphene, hexagonal boronnitride(h-BN)andsometransitionmetaldichalcogenides (TMDs) have emerged as promising building blocks for novel
nanoelectronics,providingafullrangeofmaterialstypes,including largebandgapinsulators[5,6],semiconductors[7,8],andsemime- tals[9,10].2Dsemiconductorsexhibithighcarriermobility,high on-offcurrentratioandexcellentbendability thatsuitforfuture low-power consumption and flexible electronics [11–15]. These materialsarelaterallycomposedbystrongcovalentbonds,which providegreatin-planestability.Ontheotherhand,theweakvander Waals(vdW)interlayerforceallowsustoisolate2Dmonolayerand restacktheminto arbitrarystackingheterojunctionswithoutthe need to consider the atomiccommensurability asin their bulk counterparts.Hence,anewresearchfieldofheterojunctionsformed by2Dlayeredmaterialshasemerged[16–18].Theverticallystacked 2Dlayerscanbeformedbymechanicalstacking,servingasaquick andconvenientwayofformingheterostructures.Besides,thechem- ical vapor deposition (CVD) method is a growing categoryfor fabricating2Dheterojunctioninthepastfewyears.CVDprocess notonlyprovideslarge-area2Dmaterialformechanicalassembling.
Itisalsopossibletogrowvariousstackingstructuresdirectly.The synthesizedheterojunctioncanprovideamuchcleanerinterfacefor fundamentalresearchand hencebetterdeviceperformance.Fur- thermore, thedevelopmentofthis methodologyhasshownthe possibility to synthesize lateral 2D heterojunction, which goes beyondthelimit ofthemechanicalstackingmethod.2Dlateral
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*Correspondingauthor:.Li,L.-J.([email protected])
1369-7021/ß2015TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).http://dx.doi.org/10.1016/
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heterojunctionhasopeneda newrealminmaterial scienceand device applications.This review focusesonthe preparation and applicationsoftheheterostructuresfromvarious2Dlayeredmate- rials.
Preparation of 2D materials
Thefirstsuccessfulmethodtoobtain2Dmaterialsisthroughthe mechanical exfoliationoftheir bulk crystalsusingscotch tapes (Fig.1a)[19,20].Withthismethod,monolayerorfew-layerflakes canbeobtainedwithoutintroducingtoomanydefects.Although thismethodisnotsuitableforlarge-scaleproduction,themechan- ically exfoliated films normally exhibit highcrystallinity com- pared with the samples obtained via other approaches. To further enhance the production of 2D materials, the chemical exfoliationmethodssuchaselectrochemicalexfoliation[21,22], solvents assistance exfoliation [23] and lithium intercalation approaches (Fig. 1b) [24–28] have been developed. These approachescanprovidemassive 2Dnanoflakesinsolutionsbut the lateral size and thickness of the flakes cannot be precisely controlled. Thequalityofthe 2D flakesusuallydegrades under chemicalprocess,whichremainsasanissueinthisfield.
Inadditiontotheexfoliationmethod,vaporizedprecursorscan reactto formmonolayer2Dmaterialsin a CVDchamberatan elevated temperature andthis methodshows the capabilityfor wafer-scalesynthesis,whichisdesirableforpracticalproduction.
Various2DmaterialshavebeensuccessfullysynthesizedbyCVD approaches including graphene on copper [9,29], and TMDs monolayers (suchasMoS2 [30–32],WS2[33,34],WSe2[14] and MoSe2[35,36])onsapphiresubstrates.HerewetakeMoS2synthe- sisastheexampletodiscusstheprogressofCVDmethods.In2012, Liuandco-workershavereportedatwo-stepthermolysisprocessas shown in Fig.1c [8],wherea three-layeredMoS2 sheetscanbe
achieved.Toimprovethecrystallinity,Leeandco-workersdem- onstrateadirectCVDsynthesisusingMoO3andSasthesources (Fig. 1d) [32]. During the growth, MoO3 is heated into vapor phasesandreactedwithSvaporsfollowedbyatwo-stepreaction, whichallowsthegrowthofmonolayerandsingle-crystallineMoS2
flakesondesiredsubstrates.Furthermore,Linandco-workershave shownthatwafer-scaledepositioncanbeachievedbyusingthe samechemistry[37],wherethefew-layerMoS2wasobtainedafter directsulfurizationofMoO3thin-filmsonsapphiresubstrates.Due toitssimplicityandreproducibility,directsulfurizationorseleni- zation of various metal oxides have been widely adopted to produceTMDslayersuchasMoS2[38],WS2[39,40],MoSe2[35].
Fabrication of 2D heterostructures
Owingto thelayerstructuresof2Dmaterials,the formationof heterostructurecanbeinverticalorlateralfashion.Dependingon thearchitectureoftheheterostructures,thefabricationmethods vary.Here,wesortthefabricationprocessesintotwocategories:
one is the mechanical stacking method and the other is the directlysynthesis.
Heterostructuresbymanualstacking
Thankstothe inventionofexfoliation[19]andtransfer[41,42]
methods of layered materials, the mechanically or chemically exfoliated2Dflakescanbemanuallystackedtoform2Dhetero- structures,wheretheinterlayervdWforceholdstheheterostruc- ture securely. The 2D flakes can be mechanically/chemically exfoliated from bulk materials or isolated from the synthetic (grown) 2D layers on substrates. The stacking order and the interface of the heterojunction are critical for their electrical and opticalproperties. The possibilityto formvarious stacking lattice orientation provides the heterojunction interface with
FIGURE1
Schematicillustrationofmechanicalandchemicalexfoliationprocessesandsyntheticmethodsfor2Dmaterialproduction.(a)Stepsofthemechanical exfoliationprocessfor2Dmaterialisolation[20].ReproducedwithpermissionfromRef[20].Copyright2012,SpringerScience+BusinessMediaB.V.(b)Steps oftheelectrochemicallithiationandintercalationprocessfor2Dnanosheetproduction[27].ReproducedwithpermissionfromRef[27].Copyright2011 WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim(c)Thetwo-stepthermolysisprocessforfew-layerMoS2growth[8].ReproducedwithpermissionfromRef [8].Copyright2012,AmericanChemicalSociety.(d)Theexperimentalset-upforMoS2synthesisthroughthegasphasereactionofMoO3andS[32].
ReproducedwithpermissionfromRef[32].Copyright2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim.
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tunablephysicalpropertiesdependingontheinteractionstrength ofthetwolayeredmaterials.Forexample,thecarriermobilityof grapheneonSiO2isnormallylimitedbythescatteringeffectfrom charge impurities, substratesurface roughness and SiO2 optical phonons.Dean and co-workershave demonstrated a graphene device on h-BN interface layers by using mechanical tacking method (Fig. 2a) [42]. The atomic flat and nearly free charge
trappingofh-BNlayersservesasexcellentsubstrateforgraphene.
Thecarriermobilityofgraphenedevicesfabricatedonh-BNsub- stratesisnearlyanorderhigherthanthedevicesonamorphousSiO2
substrates.Thegraphenelayersonh-BNshowreducedroughness, less dopingandimprovedchemical stability,demonstrating the criticalroleoftheinterfaces.Ontheotherhand,Hsuandco-workers performthesecondharmonicgeneration(SHG)measurementto
FIGURE2
Mechanicalstackingprocessforverticalheterostructures.(a)Schematicillustrationofthetransferandstackingprocessandtheopticalimageofthe graphene/h-BNdevices(scalebars,10mm)[42].ReproducedwithpermissionfromRef[42].Copyright2010,NaturePublishingGroup.(b)Schematic illustrationandopticalmicroscopeimageofthestackedWSe2/MoS2heterojunction[44].ReproducedwithpermissionfromRef[44].Copyright2014, NationalAcademyofSciences.(c)SchematicillustrationandopticalmicroscopeimageoftheCVDMoS2/WSe2stackingheterojunction.Thespatialmapping oftheRamanintensityforWSe2A21gpeakshowstheinterlayercoupledarea[45].ReproducedwithpermissionfromRef[45].Copyright2014,American ChemicalSociety.
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demonstrate the effect of the twisting angle on stacking TMD bilayers [43]. The SHG from the twisted bilayers is a coherent superpositionofthesecondharmonicfieldsfromtheindividual stackinglayersandthephasedifferenceisonlydeterminedbythe stacking angle.Fortheinterlayercoupling,Fangandco-workers haveinvestigatedtheopticaltransitionpropertiesonMoS2/WSe2
stackingheterojunction(Fig.2b)[44].Thebandalignmentandthe interlayer coupling at the interface result in a Stokes shift of 100meV,wherethedirectabsorptionandindirectemissionwere observedontheheterojunction.Chiuandco-workershavefabri- cated vertical MoS2/WSe2 heterojunction by manually stacking CVD-grownMoS2 andWSe2 monolayersand demonstratedthat the interlayer coupling can be enhanced by thermal treatment (Fig.2c)[45].Throughthestudyoninterlayercouplingwithoptical transitionandthebandalignmentwithscanningtunnelingspec- troscopy (STS)andX-rayphotoemission spectroscopy(XPS),the bandalignmentandtheexcitonbindingenergyoftheheterojunc- tioncanbedetermined.Themechanicalstackingmethodprovides asimplestwaytoconstructarbitrary2Dheterostructures.However, thequalityoftheinterfacecanbeeasilyaffectedbythetrappingof solventsorchemicalsusedfortransfer,whichstillremainsasan issuetobesolved.
Directsynthesis of2Dheterostructures:
Althoughtheexfoliationtechniquecanproducehighquality2D crystals for fundamental study, it still remains challenging to controlthelocation,layernumberandinterfaceoftheproduced heterojunction, which hinders the practical fabrication. Very recently,theCVDtechniqueshowsgreatpromisefortheproduc- tionoflargedomain2DbuildingblocksforTMDheterostructures.
Italsoallowsthe directsynthesisofvarious2Dheterojunctions with vertically stacked or laterally stitched interface. Here, we introducethe verticaland lateral heterojunctions fabricatedby CVDmethodseparately.
Vertically stacked 2D heterojunctions
WiththeCVDmethods,theverticallystackedheterojunctioncan beobtainedbygrowingone2Dmaterialonanother.Thedirect growth in principle results in clean heterojunction interfaces.
Since thereexistsonlyvdWinteractionbetween2D layers,the mechanismislikelythroughthevdWepitaxy,wherethelattice mismatchisthekeychallengetoovercome.Theearlyexperiment starts fromgrapheneand h-BN whichsharesimilar lattice con- stant.Yangandco-workershavereportedaplasma-assisteddepo- sitionmethodforthegrowthofsingledomaingrapheneonh-BN substrate(Fig.3a)[46].Grapheneregistersontheh-BNlatticewith apreferredorientationandthesizeofgrapheneisonlyrestricted bytheareaofunderlyingh-BN.Furthermore,Shiandco-workers have obtained a vertically stackedMoS2/graphenestructure via thermaldecompositionofammoniumthiomolybdateprecursors ongraphenesurfaces(Fig.3b)[47].Notethatalthoughthelattice constant of MoS2 is about 28% larger than that of graphene, grapheneisstillagoodgrowthplatformforMoS2.Itislikelythat the verticalepitaxialgrowthcantoleratethe crystalorientation differenceandthegrowthofMoS2ongraphenemayinvolvestrain toaccommodate thelatticemismatch.Linandco-workershave furtherdemonstratedthedirectgrowthofMoS2,WSe2,andh-BN on graphene through CVD methods, where the underlying
graphenewasgrownepitaxiallyonSiC(Fig.3c)[48].Ithasbeen revealed that the morphology of the graphene layer strongly affectswiththegrowthandthepropertiesoftopheterostructures, wherestrain,wrinkling, and defectson thesurface ofgraphene providesthe nucleationcentersforupperlayermaterialgrowth.
AnotherresearchinterestisfocusedonTMDheterojunctionsfor- mation.Gong and co-workers havereported a one-step vapor phasegrowthmethodforthedirectformationofverticalWS2/ MoS2 heterostructures[49].TheWS2can growontop ofMoS2
withthe2Hstackingstructure.Ontheotherhand,Linandco- workerspresentthedirectsynthesisofMoS2/WSe2/grapheneand WSe2/MoS2/graphene heterostructuresex situbyoxide powder vaporization and metal-organic chemical vapor deposition (MOCVD) methods as shown in Fig. 3d [50]. The electrical transport of the heterostructure exhibits a sharp negative
FIGURE3
Growthof2Dverticalheterostructures.(a)Schematicillustrationofthe plasma-assisteddepositionmethodforgraphenegrownonh-BNandthe Morie´ patternoftheheterostructure.Thewhitearrowsindicatethe graphenelayer(scalebars100nm)[46].Reproducedwithpermissionfrom Ref[46].Copyright2013,NaturePublishingGroup.(b)TheTEMimageand theschematicillustrationoftheMoS2grownongraphene[47].Reproduced withpermissionfromRef[47].Copyright2012,AmericanChemicalSociety.
(c)Cross-sectionalHRTEMofMoS2/graphenestackedheterojunctionona SiCsubstratestepedge[48].ReproducedwithpermissionfromRef[48]
Copyright2014,AmericanChemicalSociety.(d)Schematicillustrationofthe MoS2/WSe2/grapheneandWSe2/MoS2/grapheneverticalheterostructures formedbyoxidepowdervaporizationandMOCVDapproaches[50].
ReproducedwithpermissionfromRef[50].Copyright2015,Nature PublishingGroup.
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differential resistance at room temperature resulting from the specialresonanttunneling ofcharge carriersat interface.This observationhasnotbeenfoundin mechanicallystackedjunc- tions.Thesedirectsynthesesareinprincipleapplicabletoother
materials whichopensnewwaysforthefabricationofvarious heterojunctions. However, the thermodynamicallydominated processmightrestrictthecontrollabilityonstackingorder,twist- ingangleordomainsize.
FIGURE4
In-planeepitaxialgrowthof2Dlateralheterostructures.(a)Schematicillustrationforformationofgraphene/h-BNlateralheterojunctionsusingCVDgrowth method[51].ReproducedwithpermissionfromRef[51].Copyright2012,NaturePublishingGroup.(b)Schematicillustrationoftheone-potsynthesissetup andopticalimageoftheWS2/MoS2lateralheterojunction[49].ReproducedwithpermissionfromRef[49].Copyright2014,NaturePublishingGroup.(c) Schematicillustrationofthetwo-stepgrowthofthemonolayerWSe2/MoS2lateralheterojunction,andtheopticalimageofthejunction.Therightpicture showsthecorrespondingHigh-resolutionSTEMimagestakenattheinterface[55].ReproducedwithpermissionfromRef[55].Copyright2015American AssociationfortheAdvancementofScience.
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TABLE1
2Dheterostructuresandapplications.
Heterojunctiontype Layerstructure Application Deviceperformance Ref
Semi-metal/
semiconductor
Graphene/MoS2 DNAbiosensor DetectionDNAconcentration1attomole Loanetal.[60].
Graphene/WS2 Solarcells Powerconversionefficiency3.3% Shanmugametal.[67]
Graphene/WS2/graphene/
h-BN
VerticalFETs ON/OFFratio>1106 Georgiouetal.[75]
Graphene/MoS2 VerticalFETs Currentdensity5000Acm 2 ON/OFFratio>103
Yuetal.[76]
Graphene/MoS2 VerticalFETs Currentdensity104Acm 2 ON/OFFratio105
Moriyaetal.[77]
Graphene/h-BN/MoS2/ graphene
FETs Electronmobility33cm2/Vs.
ON/OFFcurrentratio>106
Royetal.[68]
Graphene/MoS2/graphene Photodetector Externalquantumefficiency55%
Internalquantumefficiency85%
Photoresponsivity0.22A/W
Yuetal.[80]
Graphene/WS2/graphene Photodetector Externalquantumefficiency30%
Photoresponsivity0.1A/W
Britenalletal.[79]
Graphene/MoS2 Photodetector Photogain>108
Internalquantumefficiency15%
Photoresponsivity>107A/W
Zhangetal.[89]
Graphene/MoS2 Photodetector Gain5–101010
Quantumefficiency32%
Photoresponsivity11010A/Wat 130K,5108A/Watroomtemperature
Royetal.[90]
Graphene/h-BN/MoS2(WS2)/
h-BN/graphene
Electroluminescence Extrinsicquantumefficiency10% Withersetal.[88]
Semi-metal/insulator Graphene/h-BN FETs Mobility60,000cm2/Vs Deanetal.[42]
h-BN/Gra./h-BN/Gra./h-BN;
h-BN/Gra./MoS2/Gra./h-BN
VerticalFET B/G/B/G/B:ON/OFFratio50 B/G/M/G/B:ON/OFFratio1104
Britnelletal.[74]
Graphene/h-BN/graphene Thermoelectrical power
Seebeckcoefficient 99.3mV/K ZT=1.0510 6
Chenetal.[92]
Semiconductor/
semiconductor
WSe2/MoS2 Solarcells Powerconversionefficiency0.2%
Externalquantumefficiency1.5%
Furchietal.[63]
p-WSe2/n-WSe2 CMOS Fulllogicswingvoltagegainupto38 Yuetal.[76]
p-MoS2/n-MoS2 Solarcells Powerconversionefficiency2.8% Wietal.[65]
MoS2/p-Si Solarcells Powerconversionefficiency5.23% Tsaietal.[66]
WSe2/MoS2
Gra./WSe2/MoS2/Gra.
Solarcells Photoresponsivityis2mAW 1 Photoresponsivityis10mAW 1 Externalquantumefficiency2.4%, 12%and34%formonolayer, bilayerandmulti-layerTMDs
Leeetal.[64]
WSe2/MoS2 p–ndiode
Photodetector Electroluminescence
Currentrectificationfactor1.2 Externalquantumefficiency12%
Chengetal.[81]
Blackphosphorus/MoS2 p–ndiode Photodetector
Currentrectificationfactor105 Externalquantumefficiency0.3%
Photoresponsivity418mA/W
Dengetal.[82]
MoS2/WS2 Chargetransfer Ultrafastholetransfertime<50fs. Hongetal.[93]
WS2/MoS2 Solarcells Open-loopvoltage0.12V
Close-loopcurrent5.7pA
Gongetal.[49]
WSe2/WS2 Solarcells Open-loopvoltage0.47V
Close-loopcurrent1.2nA
Duanetal.[53]
WSe2/MoS2 Solarcells Open-loopvoltage0.22V
Close-loopcurrent7.7pA
Lietal.[55]
WSe2/MoSe2 Solarcells Powerconversionefficiency0.12% Gongetal.[56]
MoS2/p-Ge Band-to-band
tunnelingFET
Subthresholdswingminimumof 3.9mV/decade,average31.1mV/decade
Sarkaretal.[78]
MoS2/p-Si Solarcells
Electroluminescence
Externalquantumefficiency4% Lopez-Sanchezetal.[83]
Semiconductor/
insulator
MoS2/h-BN/graphene FETs Mobility>45cm2/Vs ON/OFFratio104–106
Leeetal.[94]
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Laterally stitched 2D heterojunctions
Sincethe2Dmaterialsarecovalentlybonded,thelateralhetero- junctioncanonlybeformedbydirectgrowth.Levendorfandco- workershavedemonstratedthelateralstitchedgraphene/h-BN heterojunction by growing h-BN on patterned graphene withCVD method, which results in a continuous 2D hetero- junctionfilm(Fig.4a)[51].Notethatthegeneralissueforlateral heterojunctionisthestrainrelease betweentwomaterials. Lu andco-workershaveusedinsituscanningtunnelingmicroscopyto studytheinterfaceofgraphene/h-BNlateraljunctiononaRu(0001) surfacetounderstandhowthelatticestraincanbereleasedalong
thelateraldirection[52].Thedefect-free,pseudomorphicgrowthof h-BN from graphene edge occurs only within a short distance (<1.29nm)perpendiculartotheinterfaceandthezero-dimensional dislocationsoccurtoreleasetheelasticstrainenergyandtostabilize thestructure.ForthelateralTMDheterojunction,theearlydem- onstrationhasbeenachievedbyone-potsynthesis.Forexample,the MoS2/MoSe2andWS2/WSe2lateralheterojunctionsareobtainedby physicalvaportransportmethodwithsequentialsourceswitching [53], MoSe2/WSe2 by physical vapor transport method with mixedsources[54]andWS2/MoS2byone-spotCVDgrowthwith multi-sources(Fig.4b)[49].Atomicallyresolvedtunnelingelectron
FIGURE5
Heterostructuresforbiosensor.Schematicillustrationofthegraphene/MoS2stackingheterostructuresensorandmeasurementsetup[60].Bottomimages showtheMoS2photoluminescencepeakmappingswhenhybridizedwiththecomplementarytargetDNA(up)andthemismatchedDNA(down)atdifferent concentrations.ReproducedwithpermissionfromRef[60].Copyright2014,WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim.
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FIGURE6
Heterostructuresforphotovoltaicdevices.(a)SchematicillustrationandopticalmicroscopeimageoftheMoS2/WSe2hetero-diode[63].(b)Schematic illustrationofthep-MoS2/n-MoS2photovoltaicdeviceandthecurrent-voltagecharacteristicsunderAM1.5Gsolarsimulatorillumination[65].Reproduced withpermissionfromRefs[63,65].Copyright2014,AmericanChemicalSociety.(c)TheopticalimageofthelateralWSe2/WS2heterojunctionp–ndiodeand itsI–Vcharacterizationunderdifferentgatevoltage(left)andunderlaserillumination(right).Scalebar,2mm[53].ReproducedwithpermissionfromRef [53].Copyright2014,NaturePublishingGroup.
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FIGURE7
Heterostructuresforfield-effecttransistors.(a)Schematicillustrationofthefabricationprocessandtherelatedopticalimagesoftheall-2DFETs[68].
ReproducedwithpermissionfromRef[68].Copyright2014,AmericanChemicalSociety.(b)SchematicillustrationandopticalimageoftheWSe2CMOS structure.TherightfigureshowsitsvoltagetransfercharacteristicsofthelogicinverterwithVddsupplyfrom2to6V[73].Reproducedwithpermission fromRef[73].Copyright2015,AmericanChemicalSociety.(c)Schematicillustrationofthegraphenefield-effecttunnelingtransistorandthe
correspondingbandstructureunderoperation[74].ReproducedwithpermissionfromRef[74].Copyright2015AmericanAssociationforthe AdvancementofScience(d)Opticalimageofthegraphene/MoS2/metalverticaltransistordevice[76].ReproducedwithpermissionfromRef[76].
Copyright2013,NaturePublishingGroup.
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FIGURE8
Heterostructureforphotodetector.(a)Externalquantumefficiency(EQE)ofthegraphene/WS2/grapheneheterostructureatdifferentwavelengthsand powers.Rightimageshowsthephotocurrentmapsofh-BN/Gr/MoS2/Grdeviceswithout(up)andwith(down)spatteringofgoldnanoparticlesfor plasmonicenhancement[79].ReproducedwithpermissionfromRef[79].Copyright2013,AmericanAssociationfortheAdvancementofScience.
(b)Schematicillustrationofthesideviewofthegraphene/MoS2/graphenestackingheterostructuredevice,andtheEQEatdifferentlaserpowers[80].
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microscope (TEM) imagessuggest that the atominter-diffusion (alloy structure) exhibits at the interfaces of these lateral junctionssincetheTMDalloysarethethermodynamicallystable forms.Meanwhile,duetothelimitationofthesyntheticmethod, theTMDmaterialsinheterostructuresarewithonecommonele- menteitherthemetal orthechalcogen.Liand co-workershave recently demonstrated a two-step CVD approach for epitaxial growthofWSe2/MoS2lateraljunction,whereWSe2isfirstlysynthe- sizedthroughvanderWaalsepitaxyonsubstratefollowedbythe edgeepitaxyofMoS2alongtheWgrowthfront(Fig.4c)[55].The criticalpointforthetwo-stepCVDapproachistorevealthereaction condition to avoid the alloy formation during the second-step growth.TheTEMimagerevealsthattheatomicallysharpinterface isobtainedatthe WSe2-MoS2interface.Similarly,Gongand co- workersalsoreportedaWSe2/MoSe2heterojunctionbyusingtwo- step CVD method [56]. During the second CVD process, WSe2 bilayerwill growthfirstly from monolayerMoSe2 edge forming lateraljunctionandthencovertheMoSe2surfacedependingon thesynthesistime.The two-stepgrowthoffersthe possibilityto constructheterojunctions withvaried compositionsand desired pattern for scientific research and applications. Besides the novelty of structure, these lateral heterojunction also exhibits intrinsic p–n junction behaviors such as rectifying properties andphotovoltaiceffects,promisingforfuturemonolayerelectron- ics[49,53,55,56].
2Dheterostructuresand theirapplications
The recent advances in 2D heterostructures indicate a broad rangeofapplicationssuchasfield effect/tunnelingtransistors, biosensors,light emittingdiodes,light detectors,photovoltaic and energy storage devices. The 2D heterostructures and applications reported in the recent fiveyears are summarized inTable1andseveralapplicationsarediscussedinthefollowing paragraphs.
Biosensor
Thelargeaspectratioof2Dmaterialsandtheirultrasensitivityto environmentalchangesmakethemsuitableforpossibleapplica- tionsin bio-sensing[10,57].Previousworkshave demonstrated theDNAdifferentiationabilityinMoS2 throughitsopticaland electro-chemicalcharacterization[58,59].However,MoS2isalso sensitivewiththepresenceofmoistureandoxygen,whichlimits theirapplicationsinaqueoussolutions.Inorder toincreasethe stabilityandsensitivity,Loanandco-workershavedemonstrateda graphene/MoS2heterostructureforlabel-freeselectivedetectionof DNAhybridizationthroughthephotoluminescence(PL)intensity changingofMoS2(Fig.5)[60].Thetopgraphenelayernotonly avoidsthe contactof MoS2with moisture and oxygenbut also hoststheDNAowingtoitsbiocompatibility[60,61].Thedetection ability of the graphene/MoS2 sensor can down to 1atto mole (10 18M)withfewminutesoffastresponsetime,demonstrating the potential ofultrasensitive DNA detectionusing 2D hetero- structures.
Solar cells (photovoltaic)
Asolarcell comprises aninterface (junction) oftwo materials withoppositecarriertypes,whichallowsefficientcarrierssepa- rationofthephotogeneratedelectron–holepairsattheinterface to induce thephotovoltaic(PV)effect.Although theinterface canbeeasilyconstructedbyformingaSchottkyorp–njunction in 2D heterojunctions, the key question is still whether it is possibletoachieveenoughsolarenergyconversionwithinsuch athinlayer.Bernardiandco-workershavestudiedthePVeffects of2Dheterojunctiontheoretically,includingtheSchottkybar- rier between MoS2 and graphene and the MoS2/WS2 stacking heterostructure[62].Thesimulationshowsthateventheatom- ically thinactivelayer canproducepowerconversionefficien- cies (PCE)upto 1%ora powerdensity ofupto2.5MW/kg.
Comparedwiththeexistingultrathinsolarcells,the2Dhetero- structuresolarcellcouldachieve1–3ordersofmagnitudehigher efficiencies.
Furchiandco-workershaverevealedthePVeffectsonastacking heterojunction diode made of n-type MoS2 and p-type WSe2
monolayers(Fig.6a)[63].Byadjustingtheelectricaldopinglevel throughabackgatetuning,the2Dhetero-diodeshowsthePCE about 0.2% and the external quantum efficiency (EQE) about 1.5%.However,thelowin-planemobilityoftheTMDlayerresults in a short carrier diffusion time comparable to the interlayer electron–hole recombination lifetime, which leads to the de- creasedphotoresponsivity.Inordertoovercomethecarriertrans- port issue in this kind of stacking device, Lee and co-workers demonstratea MoS2/WSe2 stackedjunctionwithgrapheneelec- trodesdirectlyonthetopandthebottomoftheheterostructure [64].The carrierextractionrate isenhanced throughthe direct charge transferto graphenesemi-metal layerand the photore- sponsehasincreasedaboutfactorof5.AnothersimilarTMDp–n heterojunctionisformedby usingp-MoS2/n-MoS2junction.Wi andco-workersdemonstratethePVdevicewithaverticallystack- ingindiumtinoxide(ITO)/n-MoS2/plasma-dopedMoS2/Auhet- erostructure (Fig. 6b) [65].This stackingdevice exhibits a high short-loop photocurrentdensity of 20.9mA/cm2 (underAM1.5 illumination),PCEof2.8%andhighEQEvaluesfrom37to78%
forthewavelengthsbetween300and700nm.Tsaiandco-workers furtherutilizeCVDMoS2monolayerswithp-Sitoformalargescale type IIstacking p–n heterojunction,where the built-in electric fieldneartheinterfacecanfacilitatephoto-generatedcarriersepa- ration[66].Thislargescaleheterojunctiondevicecanachievea PCEof 5.23%,andthe successful combinationof MoS2with Si implies the possibility for integrating 2D materials into the Si manufacturing process.Anotherstackingphotovoltaicdeviceis throughtheformationofsimpleSchottkybarrierwithmetal(semi- metal)/semiconductor junction to establishthe built-in electric fieldattheinterface.Shanmugamandco-workershavereported the graphene/WS2 Schottkybarriersolarcell by usingWS2 asa photoactivelayer,whichshowsagoodphotonabsorptionability inthe visiblespectralrangeanda photoelectricconversioneffi- ciencyabout3.3%[67].
ReproducedwithpermissionfromRef[80].Copyright2013,NaturePublishingGroup.(c)Schematicillustrationandopticalimageofthelarge-area graphene/MoS2stackingphotodetector,andthephotogainatdifferentlaserpowers[89].ReproducedwithpermissionfromRef[89].Copyright2014, NaturePublishingGroup.
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RESEARCH:Review
Besides the vertically stacked heterojunction, Gong and co- workers havereported the lateral p–njunction with WS2/MoS2 heterostructure,whichshowsaclearPVeffect[49].Anopen-loop voltage of 0.12V and close-loop current of 5.7pA is measured without further external electrical field tuning. Duan and co- workersforms a lateralheterojunctionp–n junctionwith WSe2
and WS2 (Fig. 6c)[53].The lateraldiode showsanopen-circuit voltageof0.47Vandshort-circuitcurrentof1.2nA.TheEQE ofthephoton-to-electronconversionisdeterminedas9.9%,and internalquantumefficiencyof43%.Liandco-workersfurther demonstrateacontrolled-edgeepitaxylateralWSe2-MoS2mono- layerheterojunction. The intrinsicp–n junctionexhibits open- circuitvoltageof0.22Vandclose-circuitcurrentof7.7pAunder whitelightillustration,andaPCEatleast0.2%[55].Thesmaller photoactiveareaandthemonolayeredthicknessinthesein-plane solarcellsmightrestricttheirsolarabsorption.ThelowPCEissue mayberesolvedbyintroducingmetallicnanoparticlesorphotonic structuresonit.
Field effect transistors
Although graphene shows potentialin realizing2D highspeed bipolarfieldeffecttransistor(FET)devicesduetoitshighcarrier mobility and ambipolar effect, the zero band gap and Klein tunneling nature limit itsapplication forFET due to low ON/
OFFcurrentratios.Oneapproachforrealizinghighperformance 2D FETs is to use TMD as the channel material. The proper bandgap and reasonable mobility of the TMD materials make them morepromisingthangraphene.Royandco-workershave demonstratedann-typeMoS2FETwithallcomponentsformedby 2Dmaterials,includingMoS2astheactivechannelmaterial,h-BN asthedielectriclayerfortopelectrodeisolation,andgrapheneas the semi-metal contacts (Fig. 7a) [68]. The high mobility and tunable Fermi-level properties of graphene allow it to froman OhmiccontactwithTMDmaterials.Theh-BNlayerscanactasan isolatinglayerwithitslargebandgap,andithasatomicallysmooth surfaces withoutchargetrapping, makingitagreat interfaceor substratefor2Dmaterials[42].TheMoS2n-typeFETexhibitsan ON/OFF ratio larger than 106, and an electron mobility of 33cm2/Vs.Likewise,ap-typeFEThasbeendemonstratedusing WSe2astheactivechannel[13].SinceWSe2isaslightlyp-doped material, itis easyto tune the transport carriertypes by extra dopingorbyselectionofthecontactmetaltoserveforn-typeorp- type FETs [13,69,70]. Furthermore, the complementary metal oxide semiconductor (CMOS) has been reported by using two differentchannelmaterials[71]ormaterialswithdifferentdoping levels[70,72,73].Yuandco-workershavedemonstratedthehigh performanceCMOSbyusingWSe2asthemainactivechanneland tunethecarrierchannelthroughchemicaldoping(Fig.7b)[73].
ThisWSe2CMOSexhibitsfulllogicswing,idealvoltagetransfer characteristic,highnoisemarginandlargevoltagegainupto38in inverteroperation,promisingforlowpowerelectronicdevices.
Verticaltransportthrough2Dheterojunctionsisanothertype of2DFET.Britnellandco-workershavereportedatunnelingFET withtheverticallystackedheterostructurecomposedofgraphene andthinh-BNorMoS2(Fig.7c)[74].Thebasicprincipleisbasedon thequantumtunneling,wherethegrapheneelectrodesaresepa- ratedbythininsolationbarriers(h-BNorMoS2)andthetunneling is controlled by tuning the density of states in graphene and
adjoinedbarrierbyanexternalvoltage.ThetunnelingFETexhibits anON/OFFratioof50and104byusingBNandMoS2asbarrier, respectively.Thetransittimeisexpectedtobemuchfasterthan planarFETbytunnelingmechanism,whichispotentialforhigh- speedoperation.Sharingthesameidea,Georgiouandco-workers have reported a graphene vertical FET by using WS2 layers as barrier.ThesmallbandgapofWS2notonlyallowsfortunneling transportbutalsoforthermionictransportbyelectricallytuning the Fermi-levelof graphene.Meanwhile, itexhibits a highON currentandbettercurrentmodulationwithanON/OFFcurrent ratioup to 106 [75].This device can further operate stably on transparentand flexiblesubstrateswith similarperformance on SiO2 substrates.AnothertypeofverticalFETissimplywiththe structureofgraphene/TMD/metal,wherepropermetallayerpro- videsanOhmiccontactwithTMDtofacilitycarriertransportand thecurrentmodulationiscompletedbyelectricaltuningthrough Schottkybarrierbetweengrapheneand TMDlayers.Yuand co- workershave reporteda verticalFETwith a graphene/few-layer MoS2/metalheterostructure,whichexhibitsanON/OFFratiolarg- erthan103andhighcurrentdensityof5000A/cm2(Fig.7d)[76].
Moriyaandco-workersfurtherimprovedthecurrentmodulation to larger than105 and current density up to 104A/cm2 with a similarstructurebutbetterinterfacefabrication[77].Theadvan- tagesofthistypeofverticalFETsarethelargecurrentdensityand thesmalldevicescale,providinghighpotentialforfuturehigh- densityintegrationcircuits.Besidestheabovecommercialtunnel- ingFETs,Sarkarandco-workershaverecentlyrevealedaband-to- bandtunnelFETwiththe verticalstackingheterojunctionofp- type Ge and n-type MoS2, where its subthreshold swing (SS) 31.1mV/decadesissignificantlylowerthanthethermioniclimi- tation[78].Theirresultsopenavenueforfuturehigh-performance andlowpowerelectronics.
Photodetector
Thezerobandgapandhighmobilitypropertiesofgraphenecan providebroadbandabsorptionand fastresponsetimein photo- electronics,butthezerogapnatureandweakopticalabsorption limitthephotoresponsivity.ThemonolayerTMDmaterialshavea directbandgapandthusahighabsorptioncoefficient.However, the low mobility of TMDs will reduce the response time and extractionrate.Therefore,thedevelopmentinrelatedheterostruc- turesistryingtocombinebothadvantagestoenhancethephoto- responses.Britnellandco-workershavedemonstratedanefficient photodetector with graphene/TMDs/graphene stacking hetero- structures(Fig.8a)[79].TheTMDslayersserveasalightabsorber andthegrapheneelectrodescanprovidegoodcarrierseparation bytuningitsFermilevelthroughdoping.Thecombinedhetero- structure(graphene/WS2/graphene)forPVdevicesshowsphoto- responsivityabove0.1A/W,correspondingtotheEQEupto30%.
Furthermore, Yu and co-workers provide an extra gate voltage controlingraphene/MoS2/grapheneheterostructuretoadjustthe schottkybarrierbetweengrapheneandMoS2,wheretheexternal electricalfieldcanenhancethephoto-carriergeneration,separation andtransportprocessesinthisverticalstackingdevice(Fig.8b)[80].
IthasbeendemonstratedthattheEQEcanachieve55%and85%of internalquantumefficiency(IQE)byproperphotocurrentmodula- tion.Leeandco-workershaveextendedtherangeofthesevertical stackingheterostructurethroughinducingp–njunctionintothe 12
RESEARCH:Review
photovoltaicdevice,wheretheMoS2/WSe2p–njunctionissand- wichedbygrapheneelectrodes[64].Thep–njunctioncanfacili- tatephoto-excitedelectron–holeseparationandtheEQEhasbeen showntobedependentonthethicknessofp–njunction.Cheng andco-workershavefabricatedaverticalp–njunctionbymono- layerCVDWSe2andmultilayerexfoliatedMoS2[81].Thedevice showsphotoresponses onentireoverlappingareawith amaxi- mumEQEupto12%.Similarly,Dengandco-workersreportap–n diode based on a p-type black phosphorus/n-type monolayer MoS2stackedp–nheterojunction[82].Theblackphosphorusis anintrinsicp-typesemiconductorwith ahighcarriermobility value(10,000cm2/Vs)andasmalldirectbandgapabout0.3eV inmonolayer,suitableforbroadbandphotodetectors.Thisblack phosphorus/MoS2heterojunctionexhibitsthephoto-responsiv- ityof418mA/WandEQEof0.3%under633nmlaserillumina- tion, potentially useful for broadband photodetectors. It is noteworthythattheelectroluminescencepeaks havebeenob- servedatvariousjunctionsincludingverticallystackedp–njunc- tion[81,83],electricalfieldinducedjunction[84–87],anddouble heterojunction[88],suggestingthepossibilityofusingTMDfor LEDdevices.
Besides above mentioned heterostructures, the simple gra- phene/TMDheterojunctioncanprovidesensitivephotodetection byproperlytuningtheelectricalpotentialattheinterface.Zhang and co-workers have constructed a large area graphene/MoS2
heterojunctionwithmonolayerCVDgrapheneandMoS2,where theelectrodesarecontactedonlyontopgraphenelayerandthe MoS2 is covered by graphene (Fig. 8c) [89].The photo-excited electronsfromMoS2willtransporttographeneandtheholesare trapped inMoS2 basedonthe tunablebuild-in potentialatthe interface.At thesametime,the highmobilityingraphene will facilitatetheelectronextraction,buttheholeswillbetrappedin MoS2resultinginamultiplerecirculationofelectronsingraphene.
Therecirculationphenomenonleadstoaphotogainhigherthan 108 anda photoresponsivitylarger than107A/W.Onthe other hand,Royand co-workersshowsthat the graphene/multi-layer MoS2heterojunctionnotonlyprovidesahighphotoresponsivity near5108A/Wbutalsoexhibitspersistent photoconductivity through electric field tuning, which can serve as gate-tunable photodetectorsandopticalswitches[90].
Thermoelectric devices
Onthebasisofthelayeredstackingstructure,theheatdissipa- tionin2Dherterostructuresislimitedbythethermaltransport inverticaldirection[91].Thetemperaturegradientcanbeeasily built in few-layer 2D materials or vertical heterostructures, potentialforapplicationsinthermoelectrics.Chenandco-work- ershavereportedaSeebeckstudyonagraphene/h-BN/graphene heterostructure[92].Through applyingatemperaturegradient ontopandbottomgraphenelayersandmeasuringthethermo- electricvoltage,theheterostructureshowsaSeebeckcoefficient about 99mV/K.Togetherwiththeelectricalandthermalcon- ductance,thepowerfactor(S2G)of1.5110 15W/K2andthe thermoelectricfigure ofmeritZT=1.0510 6 canbederived.
Although the thermoelectric energy conversion efficiency is still low in this device, the research area is at an early stage andit is anticipated that progresses can bemade in thenear future.
Conclusion
Wesummarizetherecentprogressesonthefabricationandappli- cationsof2Dverticalandlateralheterostructuresinthisreview.
The variousheterostructuresbasedon2Dmaterialsopenanew researchfieldandthebandstructureengineeringbyheterostruc- tureformationprovidesthepossibilitytodevelopnoveldevices with desired properties and potential energy applications. It is anticipated that the further progresses in 2D material growth, heterostructure formation and device fabrication shall lead to practicalapplicationsinthefuture.
Acknowledgements
L.J.L.thankssupportfromKAUSTSaudiArabia,AcademiaSinica TaiwanandAOARD-134137USA.
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