Two Step Chem ical Vapor Deposition of In2Se3/M oSe2van der W aals Heterostructures

Yu-lin ChenM ing-ling LiYi-m ing WuSi-ji LiYue LinDong-xue DuHui-yiDingNn PncdXio-ping Wngcd

a.Hefei National Laboratory for Physical Sciences at the M icroscale,University of Science and Technology ofChina,Hefei230026,China

b.Department ofphysics,University of Science and Technology ofChina,Hefei230026,China

c.Synergetic Innovation Center of Quantum Information&Quantum Physics,University of Science and Technology ofChina,Hefei230026,China

d.Key Laboratory of Strongly-Coupled Quantum Matter Physics,Chinese Academy of Sciences, School ofPhysical Sciences,University of Science and Technology ofChina,Hefei230026,China

Two Step Chem ical Vapor Deposition of In2Se3/M oSe2van der W aals Heterostructures

Yu-lin Chena,M ing-ling Lib,Yi-m ing Wub,Si-jia Lia,Yue Lina,Dong-xue Dub,Huai-yiDinga,Nan Pana,b,c,d?,Xiao-ping Wanga,b,c,d?

a.Hefei National Laboratory for Physical Sciences at the M icroscale,University of Science and Technology ofChina,Hefei230026,China

b.Department ofphysics,University of Science and Technology ofChina,Hefei230026,China

c.Synergetic Innovation Center of Quantum Information&Quantum Physics,University of Science and Technology ofChina,Hefei230026,China

d.Key Laboratory of Strongly-Coupled Quantum Matter Physics,Chinese Academy of Sciences, School ofPhysical Sciences,University of Science and Technology ofChina,Hefei230026,China

Two-dimensional transition metal dichalcogenides heterostructures have stimulated w ide interest not only for the fundamental research,but also for the app lication of next generation electronic and optoelectronic devices.Herein,we report a successful two-step chem ical vapor deposition strategy to construct vertically stacked van derWaalsepitaxial In2Se3/M oSe2heterostructures.Transm ission electron m icroscopy characterization reveals clearly that the In2Se3has well-aligned lattice orientation w ith the substrate ofm onolayer M oSe2.Due to the interaction between the In2Se3and M oSe2layers,the heterostructure shows the quenching and red-shift of photolum inescence.Moreover,the current rectification behavior and photovoltaic effect can be observed from the heterostructure,which is attributed to the unique band structure alignm ent of the heterostructure,and is further confi rm ed by Kevin probe forcem icroscopy m easurem ent.The synthesis approach via van der Waals epitaxy in thiswork can expand theway to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.

van der Waals heterostructures,Chem ical vapor deposition,In2Se3/MoSe2, Kevin probe forcem icroscopy,n+-n junction

I.INTRODUCTION

Two-dimensional(2D)layered sem iconductors,including transition metal dichalcogenides(TMDs) (MX2,where M=transition m etals such as M o or W, and X=S,Se,or Te)and III?VIgroup layered chalcogenides(MX or M2X3,where M=Ga or In,and X=S, Se,or Te),have attracted broad attention for app lications of next-generation electronic and optoelectronic devices[1?4].Particularly,it is of tremendous significance to form novel 2D heterostructures for not only fundamental research,such as long-lived interlayer excitons in M oSe2/WSe2heterostructure ow ing to type-II junction band alignment and ultrafast charge separation[5?7],but also many device applications including photovoltaics,light-em itting diodes,and photodetectors[8?10].A lthough stacking diff erent 2D m aterials by using mechanical transfer techniques is quite convenient and effi cient,the stacked orientation cannotbe precisely controlled,and the interface is often contam inated[11,12].In contrast,direct van der Waals (vdW)epitaxy can realize not only verticalheterostructuresw ith well-defined interlayer orientationsand clean interfaces by vertically stacking multip le 2D materials layer-by-layer[5?7,13?19],but also lateralheterojunctions w ith seam less connections achieved by successive in-p lane epitaxial grow th of a second material from the edge of an existing crystal[20?28].Till now,most of previous studies focused on heterostructures w ith two diff erent MX2,such as WS2/WSe2[20],WSe2/MoSe2[21,24],WS2/MoS2[22,23],and WSe2/MoS2[25],and many new physical phenomena and functionalities have been dem onstrated.It is of great significance to exp lore a variety of novel 2D vdW epitaxy heterostructures constructed beyond TMDs,however,it has been rarely reported[26].

In the present work,we report a two-step CVD grow th method for creating 2D vertically-stacked heterostructure consisting of In2Se3and M oSe2.In2Se3, as a III?VI com pound sem iconductor,has excellent properties for electronic and optoelectronic devices.Recently,the photodetectors of In2Se3w ith ultrahigh responsivity[29]and tunable near-UV behavior[30]havebeen reported.MoSe2is an ideal choice not only for its directband gap(～1.5 eV)and strong opticalabsorption due to band nesting[31],but also for the fact that it is unable to form elem ent doping in In2Se3.To our best know ledge,this is the fi rst report of vertically-stacked 2D In2Se3/MoSe2heterostructures obtained by vdW epitaxial grow th.The vertical heterostructures were verified by the characterizations of atom ic force m icroscopy(AFM)and Raman spectroscopy.The investigation of transm ission electronm icroscopy(TEM)reveals thewell-aligned lattice orientation between In2Se3and MoSe2.In addition,ow ing to the interlayer interaction,the apparent quenching and small red shift of the photolum inescence(PL)can be observed.M ore im portantly,due to the unique band alignment of the two differentmaterials,the fabricated device exhibits both the current rectification behavior and the photovoltaic eff ect.The surface contact potential diff erence of the heterostructure is further characterized by using Kevin probe forcem icroscopy(KPFM).

II.EXPERIM ENTS

A.M aterials synthesis

In2Se3/M oSe2heterostructures were grown through a two-step CVD process.The experimental set-up is ahown in FIG.1(a).In the fi rst step,monolayer MoSe2nanosheets were synthesized on a clean SiO2/Si substrate in a two-zone furnace.200mg selenium(Se)and 10 mg m olybdenum oxide(M oO3)powder were used as precursors and put into a quartz boat and an alum ina boat,respectively.They were then pushed into a quartz tube w ith one inch diam eter.Se and M oO3quartz tubeswere at the center of the fi rst heating zone (zone-1)and the second heating zone(zone-2)respectively.These two heating zones can be controlled separately and the distance between two sources is 23 cm. The SiO2/Si substrate was put up-side down w ith the polished side facing towards MoO3powder,which located at the bottom of the crucible.The vertical distance between M oO3powder and SiO2/Si substrate is about 0.5 cm.It took 15 m in for zone-1 and zone-2 to heat up from room temperature to 100 and 780?C, respectively.Then zone-1 was heated to 270?C w ithin 5 m in and kept for 30 m in,meanwhile zone-2 was kept at 780?C.40 sccm Ar(started before heating)and 10 sccm H2(started when zone-1was270?C)wereused asboth the carrier gas and reducing atm osphere to prom ote the reaction.A fter that,the furnace was cooled down to room tem perature rapidly.

In the second step,the substrate w ith 1L MoSe2on top was used as a new substrate for the second grow th of In2Se2,which wasalso the tem plate for the synthesis of In2Se3/MoSe2heterostructures.200 mg Se powder and 10 mg In2O3powder were used as the Se and In precursors.Heating time and carrier gaswere kept the same as the grow th conditions for M oSe2,but the temperature of zone-2 was reduced to 660?C and kept for 25 m in.A ll reaction processes are carried out under atmospheric pressure.

B.Characterization

The optical imageswere taken by opticalm icroscopy (O lym pus BX 53F).AFM and KPFM were perform ed using Bruker Dem ension Icon in the ambient condition. Raman and confocal PL measurements were realized using LabRAM HR 800 under 514 nm excitation.PL mapping measurements was perform ed w ith a hom ebuilt m icro-PL setup.In order to ensure the exciting area to be largeenough to cover thewholeselectedmapping region,532 nm laserwas focused on the back focal p lane of ob jective lens(O lym pus M Plan Sem i Apochromat,100×,N.A.0.9,W.D.1.0 mm).W ith a tube lens(300 mm focal length)and long wave pass fi lter at 550 nm,the lum inescence image was enlarged and projected on the slit p lane of spectrometer(Princeton Instruments SP2300 w ith PyLoN:100).For the lum inescence image detecting,the w idth of the slit is set at 2 mm.The zero order interference im age reflected by the grating,as the sam e as the lum inescence image,is detected by the CCD of the spectrometer.The spatial resolution is better than 1μm.For the spectra measurem ent,the targetydirection cut line of the lum inescence im age was set at the center of the CCD through tuning thexposition of the tube lens.Then the w idth of the slit was set as 50μm.The cut line spectra,the fi rst order interference im age reflected by the grating,are detected by the CCD.The spectrum resolution is better than 0.5 nm,and spatial resolution equals to that of the lum inescence image.

TEM,HRTEM images and SAED patternswere collected by a JEOL ARM-200F field em ission transm ission electron m icroscope operated at 200 kV.The samp les for TEM analyses were transferred onto copper grids coated w ith 5-nm-thick amorphous carbon fi lm. For samp le transfer,polymethylmethacrylate(PMMA, 495A4)was spin-coated on the SiO2/Si substrate w ith In2Se3/M oSe2heterostructures at 3000 r/m in for 60 s, followed by baking at 120?C for 3 m in to remove the solvents.Then the substrate was floated on 1 mol/L NaOH solution and the PMMA layer carrying sam p les would shed off the SiO2/Sisubstrate slow ly.A fter that, the PMMA fi lm was transferred to deionized water to remove residual ion and then spread onto the copper grid.Finally,the PMMA mediator was rem oved by dipping in acetone for 3 h.

C.Device fabrication and m easurem ents

The SiO2/Si substrate w ith In2Se3/M oSe2heterostructureswas spin coated w ith PMMA(495A 4)resist at 3000 r/m in for 60 s followed by a 180?C baking for 90 s.Then the EBL(JEOL,JBX 6300FS)was used to pattern the source and the drain.A fter development,5 nm Ti and 50 nm Au contacts were deposited by electron beam evaporation.Lift-off process w ith acetone finally removed excessmetal.The electricaland optoelectronic performance of fabricated device weremeasured in a Lake Shore CRX-4K probe station (～10?4Pa)w ith Keithley 4200Ssem iconductor parameter analyzer.A 532 nm laser w ith a power density of～0.5mW/cm2was used as the excitation light.

III.RESULTS AND DISCUSSION

FIG.1(b)shows the resultsafter the fi rst step grow th. As seen,triangular MoSe2flakes w ith size about several tens ofm icrons can be found clearly.The thickness of the flake is estimated to be～0.75 nm by AFM (FIG.1(f)),indicating thatm onolayer M oSe2has been successfully fabricated after the fi rst step CVD process.

A fter the second CVD process,it is easy to find from FIG.1(c?e)that the optical contrast of some MoSe2flakes becomes different,im p lying that an additional layered m aterial has been grown vertically on the 1L MoSe2.Thematerial can bewell identified to be In2Se3using TEM characterization and Raman spectra(see follow ing paragraphs).Further AFM characterization reveals that the thickness of In2Se3is about 2 nm,corresponding to bilayer(FIG.1(g?i)).Therefore,we can conclude that In2Se3(bilayer)/MoSe2(monolayer)heterostructure has been successfully produced by the twostep CVD process.To our best know ledge,this is the fi rst report of direct grow th of 2D In2Se3on top of 1L MoSe2to form thevertically-stacked In2Se3/MoSe2heterostructures.

A fter carefully exam ining the optical im ages of produced heterostructures on the whole substrate,we can classify them into three different kindsofmorphologies. The representative resu ltsare presented in FIG.1(c?e).The fi rst type is the triangular 1L M oSe2fullycovered by In2Se3(FIG.1(c)),the second is MoSe2partially covered by In2Se3flakesw ith one side uncovered(FIG. 1(d)),and the third is that w ith a corner of M oSe2uncovered(FIG.1(e)).M oreover,we can find that the edge of In2Se3flake on the top of MoSe2is rather sm ooth(FIG.1(d,e)),hinting that the In2Se3isunlikely formed from the coalescence of several small flakes but most probably grows up from a single seed.Considering In2Se3nucleated random ly on the top ofMoSe2,we propose three possible grow th processes to reach the experimental results,the schematic is shown in the right of FIG.1(a).W hen thenucleation isquitenear the center of the single crystal M oSe2,the In2Se3flake tends to fully cover the M oSe2(type i);when the nucleation is close to one apex of the triangular MoSe2,the In2Se3flake can partially cover the MoSe2and leave one side uncovered(type ii);sim ilarly,a corner of theM oSe2w ill remain uncovered when the In2Se3nucleatesaround one side of MoSe2(type iii).

As to the In2Se3/M oSe2heterostructure fabricated on SiO2substrate,several issues should be noted. First,monolayer In2Se3can be hardly observed on 1L M oSe2,probably because it is unstable under our grow th condition.Second,the thickness of In2Se3flakes can be controlled by tuning the vertical distance between the In2O3source and the substrate as well as the deposition time,therefore diff erent In2Se3(multilayer)/M oSe2(m onolayer)heterostructure can be produced(FIG.S1(c)in supplementary materials).Third,the MoSe2/In2Se3heterostructure,i.e., M oSe2on the top of In2Se3,cannot be produced on SiO2substrate.This is because that In2Se3can hard ly nucleateand grow on SiO2substrate directly,due to the unsaturated dangling bonds on the surface of SiO2and the large latticem ism atch[4](FIG.S1(a)in supp lem entary materials).The result is well consistent w ith the observation shown in FIG.1(c?e),in which no individual In2Se3flake can be found on the SiO2substrate,except on themonolayer M oSe2.Sim ilar to M oSe2,m ica also has no unsaturated dangling bonds on its surface, making it an idealplatform for the deposition of In2Se3nanofi lm s(FIG.S1(b)in supp lem entary m aterials).

In order to investigate the crystal quality of the heterostructure and the structure relationship between the In2Se3and the M oSe2,TEM as well as the selected area electron diff raction(SAED)characterizationswere carried out on theboundary of In2Se3/MoSe2heterostructure.The heterostructureswere transferred from SiO2/Si substrate to the copper grid w ith carbon fi lm via traditional PMMA assisted method[32]. FIG.2(a)shows a low-magnification image focused on the area where 1L M oSe2is not com p letely covered by In2Se3(the right part of thewhite dash line).FIG.2(b) is the high-resolution TEM image taken from thewhite line box in FIG.2(a).As seen,the left region of M oSe2uncovered by In2Se3dem onstrates brighter contrast, while the heterostructure on the right region shows darker contrast.Moreover,the crystal latticeof theheterostructre can be clearly observed,indicating its high crystal quality.

FIG.2 Structural characterization of the vertically-stacked In2Se3/MoSe2heterostructure.(a)Low magnification TEM im age of the edge area where In2Se3is grown on 1L M oSe2. Scale bar:100 nm.(b)The high-resolution TEM image taken from the white box area m arked in(a).Scale bar: 10 nm.(c)Fast Fourier transform(FFT)and(d)the The electron diff raction patterns taken from the left(M oSe2)and right(heterostructure)regions in(b),respectively.Scale bar:5 nm.The two red circles in(c)and(d)stand for the atom ic spacing along[10-10]and[11-20]directions of 2HMoSe2,whereas the green circles in(d)stand for the atom ic spacing along[10-10]and[11-20]directions ofα-In2Se3.(e) Side view of the atom icm odel of the In2Se3/M oSe2vertical vdW heterostructure.

The structures of the monolayer MoSe2and the In2Se3/MoSe2heterostructure are further analyzed by fast Fourier transform(FFT)and SAED.The results taken from the left/right region of FIG.2(b)are presented in FIG.2(c)and(d),respectively.Obviously, typical six-fold diff raction patternswere obtained from both single M oSe2(left region)and In2Se3/M oSe2heterostructure(right region),consistent w ith the anticipation of 2H-MoSe2andα-In2Se3.The FFT pattern of M oSe2in FIG.2(c)reveals that the probing beam is roughly parallel tocaxis of the crystal.The two red circles in the image represent atom ic spacing along[10-10]and[11-20]directions,and the corresponding lattice constants were calculated to be 0.28 and 0.16 nm,respectively.These valuesare in good agreem entw ith the lattice structure of 2H-MoSe2[26].The SAED from the heterostructure(FIG.2(d))demonstrates two sets of patternsw ith six-fold symm etry.Com pared w ith FIG. 2(c),weattribute that the spotsmarked w ith red circles also come from MoSe2,and thespotsmarked w ith green circles are originated from In2Se3.We can further acquire information of the crystalstructure of In2Se3from the set of green circles.The atom ic spacing along the [10-10]and[11-20]directionswere calculated to be 0.36 and 0.20 nm,respectively,consistent w ith the lattice structure ofα-In2Se3[29].M ore im portantly,considering the consistency of the crystal orientation of MoSe2and In2Se3,we consider that 2Dα-In2Se3w ith hexagonal lattice structure has been successfully grown on the top ofm onolayer M oSe2by vertical vdW epitaxy. According to the above characterizations,a schematic sideview of theatom icmodelof the In2Se3/MoSe2vdW heterostructure can be roughly depicted in FIG.2(e).

FIG.3 Optical properties of In2Se3/M oSe2heterostructures.(a)Optical im age of heterostructures grown on SiO2/Si.Scale bar:10μm.(b)Raman and(c)PL spectra acquired from the three diff erent points marked in(a),1L MoSe2(b lack point),2L In2Se3on 1L M oSe2(red point)andnL In2Se3on 1L M oSe2(n>5,b lue point).(d)Schem atic band structure of In2Se3/M oSe2.

The optical properties of the vertically stacked In2Se3/MoSe2heterostructures were characterized by Raman and photolum inescence(PL)spectroscopy.The excitation is a 514-nm laser w ith～1μm spot size. FIG.3(a)shows the optical image of the heterostructures grown on SiO2/Si substrate.Three different positions involving bare 1L MoSe2(black dot),2L In2Se3on 1L M oSe2(red point)andnL In2Se3on 1L M oSe2(n>5,blue point)were excited,and the results of Raman and PL spectra are demonstrated in FIG.3(b) and(c),respectively.As seen in FIG.3(b),for the heterostructure of In2Se3(>5 layers)/M oSe2,both Raman peaks from In2Se3and MoSe2can be observed clearly. Three peaks located at～109,～177,and～206 cm?1(indicated by green dash lines in FIG.3(b)),can be ascribed to A1(LO+TO),A1(TO),and A1(LO)phonon modes inα-In2Se3,respectively[4].However,because ultrathin In2Se3can be easily damaged even under lowpower laser illum ination[4],the Raman signal from In2Se3is absent for the 2L In2Se3/M oSe2heterostructure.The rest four Ram an peaks,marked by b lack dash lines,originate from 2H-MoSe2for～141 cm?1(weak E1gmode,in-p lane),～239 cm?1(A1gmode,outof-p lane),～289 cm?1(weak E12gmode,in-p lane),and～250 cm?1(defective peak)[26].W ith increasing the In2Se3thickness on the top of M oSe2,the Raman signals of In2Se3increase obviously while the Raman intensities of MoSe2slightly decrease(especially for the A1gand E12gm odes).M oreover,we find that the Raman peak positions ofmonolayer MoSe2are independent of the thickness of In2Se3flake on top.

FIG.3(c)shows the PL spectra from m onolayer M oSe2and In2Se3/M oSe2heterostructure.As seen,the maximal PL intensity is obtained in monolayer MoSe2(black line spectrum in FIG.3(c)),thanks to its direct band gap[33].The peak position of PL is approximately at～820 nm.For the In2Se3(2L)/M oSe2(1L) heterostructure(red line spectrum in FIG.3(c)),the PL spectrum shows not only the decrease of intensitybut also the red shift of peak position to～825 nm.The behaviors can be further confi rmed by the PLmappings of 1L M oSe2and In2Se3(2L)/M oSe2(1L)heterostructure(FIG.S2 in supp lementarym aterials).Because the bilayer In2Se3owns a too large indirect band gap(～2.8 eV)to absorb the excitation laser[30],we attribute the predom inant reason for the decrease of PL intensity of the heterostructure to the type-II band alignment between 2L In2Se3and 1LMoSe2,as shown in FIG.3(d). This zigzag band structure is prone to quench the excitons and promote the charge separation,leading to the distinct decrease of photolum inescence[5].

It is found that the full w idth at half m aximum (FWHM)of PL spectra shown in FIG.3(c)are rather large,which m ight be caused by the co-existed neutral exciton and trion em issions.In this context,the spectra can be well fi tted by two Gaussian functions w ith neutral exciton(X0≈818 nm,1.52 eV)and negative trion(X?≈834 nm,1.49 eV).The resultsare shown in FIG.S3(supp lem entary m aterials)w ith the red and green lines,respectively.The evaluated trion binding energy～30meV agreeswellw ith previous report[34]. The integrated intensity ratios of negative trion to neutralexciton is found to increase from～1.0 form onolayer MoSe2(FIG.S3(a)in supplementarymaterials)to～1.3 for In2Se3(2L)/MoSe2(1L)heterostructure(FIG.S3(b) in supp lem entary materials).The dom inant negative trion em ission in the heterostructure im p lies that the electrons tend to transfer from In2Se3to MoSe2,leading tomore negative charge doping in MoSe2[35].The result can also account for the PL red shift of the heterostructure in FIG.3(c).

As to the In2Se3(>5L)/M oSe2(1L)heterostructure, PL em ission can hardly be found as shown in FIG.3(c) w ith blue line.We proposed two causes for the result.One is the exciton quenching eff ect,which is similar to that observed in the In2Se3(2L)/M oSe2(1L)heterostructure.The other is that the optical band gap of In2Se3decreases rapidly w ith the thickness(～2.2 eV in 5.5 nm thick In2Se3and 1.45 eV in bulk In2Se3)and can even transform into a direct band gap[30],resulting in the strong absorption to the excitation light and weakening the em ission.

To investigate the electrical properties of the In2Se3/M oSe2heterostructures,the source-drain contacts consisting of Ti/Au(5/50 nm)were deposited on 2L In2Se3/MoSe2and 1L MoSe2regions,respectively. The finaldevice isshown in FIG.4(a)and the schematic structure of the device is shown in the inset.The transfer characteristic of the device shows apparent n-type feature(inset of FIG.4(b)).This is due to that both MoSe2and In2Se3are inclined to be intrinsic n-doping during the grow th[33,36].TheIds-Vdscurves under diff erent back-gate voltages ranging from 0 V to 60 V clearly show rectification behavior(FIG.4(b)), and the rectification ratio can reach as high as～100 whenVg<40 V.We attribute the behavior to the n+-n heterojunction formed between In2Se3and MoSe2,because both materials are n-type sem iconductor.The interpretation can be further verified by the fact that the rectification behavior is degraded asVgbecom ing large.For instance,the rectification ratio is only～5 whenVg≈60 V,as shown in FIG.4(b).

In order to investigate the band alignment of the heterostructure and identify the proposed n+-n hetero junction,Kelvin probe force m icroscope(KPFM) measurement was performed along the red arrow line depicted in FIG.4(a).As observed from FIG.4(c), the surface contact potential diff erence(CPD)across the edge ofm onolayer M oSe2and heterostructure can reach～200 meV and the w idth of dep letion region is about 4μm.Because KPFM measures the CPD between the AFM tip and the surface of sam p le,i.e., CPDsample=?tip??sample,we can get the Ferm i level diff erence between In2Se3and MoSe2by:

where?MoSe2and?In2Se3are the work functions of MoSe2and In2Se3,respectively[37].Therefore,the value of?EFbetween In2Se3and MoSe2can be estim ated at～200 m eV and the band alignment of 2L In2Se3and monolayer MoSe2is schematically demonstrated in FIG.4(d).As seen,compared to n-type M oSe2,In2Se3is heavily n-doped sem iconductor.Consequently,the In2Se3and M oSe2can form n+-n heterojunction,which is the predom inant cause for the rectification behavior in FIG.4(b).

Finally,we present the primary results for the photoresponse of the devices under light illum ination.A 532 nm laser w ith a power density of～0.5 mW/cm2was used as the excitation light.TheIds-Vdscurves of the device w ith and w ithout light are dem onstrated in FIG.S4 in supp lementary m aterials.It can be found that the current increases dramatically w ith light on. The feature ismore striking for the reverse biased device.Specifically,when the device under reverse bias, the current sw itching ratio of the devicew ith light on to that w ith light off can be as high as～200.However,it decreases to～4 for the forward biased device.Thisphenomenon can be well understood w ith the band structure of the junction shown in FIG.S4(b)and FIG.S4(c) in supp lementary materials.Under the reverse bias (FIG.S4(b)in supp lementary materials),in addition to weak dark current,the large band off set can promote the separation of excitons in M oSe2,leading to a high sw itching ratio;on thecontrary,thesmallband offset under the forward bias voltage(FIG.S4(c))enables large dark current and therefore lowers the sw itching ratio.This is sim ilar to the operating princip le of pn photodiode,which further convince the n+-n band alignm ent across the In2Se3/M oSe2junction.Additionally,we also observe photovoltaic eff ect in such a n+-n heterojunction device.As shown in the inset ofFIG.S4 in supplementary materials,the device shows an open-circuit voltageVOC≈0.5 V and short-circuit currentISC≈0.8 pA under light illum ination.

FIG.4 Electrical characterization of the In2Se3/MoSe2heterostructure.(a)Optical image of a device.The inset is the schem atic structure of the device.Scale bar:10μm.(b)Ids-Vdscurves of the device at diff erent back-gate voltages under dark(from bottom to top,in steps of 20 V).The inset is the transfer characteristic curvew ithVdsfixed at 20 V.(c)Surface contact potential diff erencem easured along the red arrow line depicted in(a).(d)Band alignm ent ofm onolayer M oSe2and bilayer In2Se3based on KPFM characterization.

IV.CONCLUSION

In summary,we fabricated the vertical heterostructuresw ith bilayer In2Se3on the top ofmonolayerMoSe2through a two-step CVD process.The vdW epitaxial character and the possible grow th schematic are revealed by the structural characterization.It is found that the quenching behavior and red-shift of photolum inescence of the heterostructure can bem odulated by the interaction between M oSe2and In2Se3.Due to the uniquely aligned band structure,such In2Se3/MoSe2heterostructures can exhibit distinct current rectification behavior and photoelectric response.M ore importantly,the reported approach in this work can be extended to other novel 2D heterostructures involving diff erent layered com pounds,enabling to enrich the variety of 2D vdW heterostructures for basic research and applications for next generation optoelectronic devices.

Sup p lem en tary m aterials:M ore optical m icrographs about the grow th of In2Se3.PL mappings of intrinsic 1L MoSe2and heterostructurew ith 1L MoSe2partially covered by 2L In2Se3.Fitted PL spectra of the monolayer M oSe2and In2Se3(2L)/M oSe2heterostructure.Optoelectronic properties of the In2Se3/M oSe2heterostructure.

V.ACKNOW LEDGM ENTS

Thiswork was supported by the M inistry of Science and Technology of China(No.2016YFA 0200602), the National Natural Science Foundation of China(No.21421063,No.11374274,No.11404314, No.11474260,No.11504364),the Chinese Academy of Sciences(XDB01020200),and the Fundam ental Research Funds for the Central Universities (WK 2030020027,WK 2060190027).

[1]S.Z.Butler,S.M.Hollen,L.Y.Cao,Y.Cui,J.A. Gupta,H.R.Guti′errez,T.F.Heinz,S.S.Hong,J. X.Huang,A.F.Ismach,E.Johnston-Halperin,M. Kuno,V.V.Plashnitsa,R.D.Robinson,R.S.Ruoff, S.Salahuddin,J.Shan,L.Shi,M.G.Spencer,M.Terrones,W.W ind l,and J.E.Goldberger,ACS Nano 7, 2898(2013).

[2]B.Radisav ljevic,A.Radenovic,J.Brivio,V.Giacom etti,and A.K is,Nat.Nanotechnol.6,147(2011).

[3]X.F.Li,M.W.Lin,A.A.Puretzky,J.C.Id robo, C.Ma,M.F.Chi,M.Yoon,C.M.Rouleau,I.I. K ravchenko,D.B.Geohegan,and K.X iao,Sci.Rep. 4,5497(2014).

[4]W.Feng,W.Zheng,F.Gao,X.S.Chen,G.B.Liu,T. Hasan,W.W.Cao,and P.A.Hu,Chem.M ater.28, 4278(2016).

[5]F.Ceballos,M.Z.Bellus,H.Y.Chiu,and H.Zhao, ACSNano 8,12717(2014).

[6]P.Rivera,J.R.Schaibley,A.M.Jones,J.S.Ross,S. W u,G.A ivazian,P.K lem ent,K.Sey ler,G.C lark,N. J.Ghim ire,J.Yan,D.G.M andrus,W.Yao,and X. Xu,Nat.Commun.6,6242(2015).

[7]P.Rivera,K.L.Seyler,H.Y.Yu,J.R.Schaibley,J.Q. Yan,D.G.M andrus,W.Yao,and X.D.Xu,Science 351,688(2016).

[8]A.Pospischil,M.M.Furchi,and T.M ueller,Nat.Nanotechnol.9,257(2014).

[9]J.S.Ross,P.K lement,A.M.Jones,N.J.Ghim ire,J. Q.Yan,D.G.M andrus,T.Taniguchi,K.Watanabe, K.K itamura,W.Yao,D.H.Cobden,and X.D.Xu, Nat.Nanotechnol.9,268(2014).

[10]K.N.Zhang,T.N.Zhang,G.H.Cheng,T.X.Li,S. X.Wang,W.Wei,X.H.Zhou,W.W.Yu,Y.Sun,P. W ang,D.Zhang,C.G.Zeng,X.J.Wang,W.D.Hu, H.J.Fan,G.Z.Shen,X.Chen,X.F.Duan,K.Chang, and N.Dai,ACS Nano 10,3852(2016).

[11]S.J.Haigh,A.Gholinia,R.Jalil,S.Romani,L.Britnell,D.C.Elias,K.S.Novoselov,L.A.Ponom arenko, A.K.Geim,and R.Gorbachev,Nat.M ater.11,764 (2012).

[12]W.Yang,G.R.Chen,Z.W.Shi,C.C.Liu,L.C. Zhang,G.B.Xie,M.Cheng,D.M.Wang,R.Yang,D. X.Shi,K.W atanabe,T.Taniguchi,Y.G.Yao,Y.B. Zhang,and G.Y.Zhang,Nat.M ater.12,792(2013).

[13]A.K.Geim and I.V.Grigorieva,Nature 499,419 (2013).

[14]R.Cheng,D.H.Li,H.L.Zhou,C.Wang,A.X.Yin, S.Jiang,Y.Liu,Y.Chen,Y.Huang,and X.F.Duan, Nano Lett.14,5590(2014).

[15]M.H.Chiu,C.D.Zhang,H.W.Shiu,C.P.Chuu,C. H.Chen,C.Y.S.Chang,C.H.Chen,M.Y.Chou,C. K.Shih,and L.J.Li,Nat.Commun.6,7666(2015).

[16]H.Fang,C.Battaglia,C.Carraro,S.Nem sak,B.Ozdol, J.S.Kang,H.A.Bechtel,S.B.Desai,F.K ronast,A. A.Unal,G.Conti,C.Conlon,G.K.Palsson,M.C. M artin,A.M.M inor,C.S.Fad ley,E.Yab lonovitch,R. Maboudian,and A.Javey,Proc.Natl.Acad.Sci.USA. 111,6198(2014).

[17]C.H.Lee,G.H.Lee,A.M.Van Der Zande,W.C. Chen,Y.L.Li,M.Y.Han,X.Cui,G.A refe,C.Nuckolls,T.F.Heinz,J.Guo,J.Hone,and P.K im,Nat. Nanotechnol.9,676(2014).

[18]S.Tongay,W.Fan,J.Kang,J.Park,U.Koldem ir,J. Suh,D.S.Narang,K.Liu,J.Ji,J.B.Li,R.Sinclair, and J.Q.Wu,Nano Lett.14,3185(2014).

[19]F.W ithers,O.Del Pozo-Zamudio,A.M ishchenko,A. P.Rooney,A.Gholinia,K.Watanabe,T.Taniguchi, S.J.Haigh,A.K.Geim,A.I.Tartakovskii,and K.S. Novoselov,Nat.Mater.14,301(2015).

[20]X.D.Duan,C.Wang,J.C.Shaw,R.Cheng,Y.Chen, H.L.Li,X.P.Wu,Y.Tang,Q.L.Zhang,A.L.Pan, J.H.Jiang,R.Q.Yu,Y.Huang,and X.F.Duan,Nat. Nanotechnol.9,1024(2014).

[21]Y.J.Gong,S.D.Lei,G.L.Ye,B.Li,Y.M.He,K. Keyshar,X.Zhang,Q.Z.W ang,J.Lou,Z.Liu,R. Vajtai,W.Zhou,and P.M.A jayan,Nano Lett.15, 6135(2015).

[22]Y.J.Gong,J.H.Lin,X.L.Wang,G.Shi,S.D.Lei, Z.Lin,X.L.Zou,G.L.Ye,R.Vajtai,B.I.Yakobson,H.Terrones,M.Terrones,B.K.Tay,J.Lou,S.T. Pantelides,Z.Liu,W.Zhou,and P.M.A jayan,Nat. M ater.13,1135(2014).

[23]H.Heo,J.H.Sung,G.Jin,J.H.Ahn,K.K im,M.J. Lee,S.Cha,H.Choi,and M.H.Jo,Adv.M ater.27, 3803(2015).

[24]C.M.Huang,S.F.Wu,A.M.Sanchez,J.J.P.Peters,R.Bean land,J.S.Ross,P.Rivera,W.Yao,D.H. Cobden,and X.D.Xu,Nat.M ater.13,1096(2014).

[25]M.Y.Li,Y.M.Shi,C.C.Cheng,L.S.Lu,Y.C.Lin, H.L.Tang,M.L.Tsai,C.W.Chu,K.H.W ei,J.H. He,W.H.Chang,K.Suenaga,and L.J.Li,Science 349,524(2015).

[26]X.F.Li,M.W.Lin,J.H.Lin,B.Huang,A.A.Puretzky,C.M a,K.W ang,W.Zhou,S.T.Pantelides,M.F. Chi,K.K ravchenko,J.Fow lkes,C.M.Rouleau,D.B. Geohegan,and K.Xiao,Sci.Adv.2,e1501882(2016).

[27]L.Liu,J.Park,D.A.Siegel,K.F.M cCarty,K.W. Clark,W.Deng,L.Basile,J.C.Id robo,A.P.Li,and G.Gu,Science 343,163(2014).

[28]Y.F.Yu,S.Hu,L.Q.Su,L.J.Huang,Y.Liu,Z.H. Jin,A.A.Purezky,D.B.Geohegan,K.W.K im,Y. Zhang,and L.Y.Cao,Nano Lett.15,486(2015).

[29]R.B.Jacobs-Ged rim,M.Shanmugam,N.Jain,C.A. Durcan,M.T.M urphy,T.M.M urray,R.J.M atyi,R. L.Moorell,and B.Yu,ACSNano 8,514(2014).

[30]J.Quereda,R.Biele,G.Rubio-Bollinger,N.Agra¨?t, R.D’Agosta,and A.Castellanos-Gom ez,Adv.Opt. Mater.4,1939(2016).

[31]A.Carvalho,R.M.Ribeiro,and A.H.C.Neto,Phys. Rev.B 88,115205(2013).

[32]J.Xia,X.Huang,L.Z.Liu,M.Wang,L.W ang,B. Huang,D.D.Zhu,J.J.Li,C.Z.Gu,and X.M.M eng, Nanoscale 6,8949(2014).

[33]Y.J.Gong,G.L.Ye,S.D.Lei,G.Shi,Y.M.He,J.H. Lin,X.Zhang,R.Vajtai,S.T.Pantelides,W.Zhou, B.Li,and P.M.A jayan,Adv.Funct.M ater.22,2009 (2016).

[34]A.Singh,G.M oody,K.Tran,M.E.Scott,V.Overbeck, G.Bergh¨auser,J.Schaibley,E.J.Seifert,D.Pleskot, N.M.Gabor,J.Q.Yan,D.G.Mand rus,M.Richter, E.M alic,X.D.Xu,and X.Q.Li,Phys.Rev.B 93, 041401(2016).

[35]J.S.Ross,S.F.W u,H.Y.Yu,N.J.Ghim ire,A. M.Jones,G.A ivazian,J.Q.Yan,D.G.M andrus,D. Xiao,W.Yao,and X.D.Xu,arXiv p reprint arXiv: 1211.0072,(2012).

[36]M.Lin,D.Wu,Y.Zhou,W.Huang,W.Jiang,W.S. Zheng,S.L.Zhao,C.H.Jin,Y.F.Guo,H.L.Peng, and Z.F.Liu,J.Am.Chem.Soc.135,13274(2013).

[37]K.Chen,X.W an,J.X.Wen,W.G.X ie,Z.W.Kang, X.L.Zeng,H.J.Chen,and J.B.Xu,ACS Nano 9, 9868(2015).

ceived on April 5,2017;Accepted on April 19,2017)

?Authors to whom correspondence shou ld be add ressed.E-m ail: npan@ustc.edu.cn,xpwang@ustc.edu.cn