Chinese Journal of Chemical Physics  2017, Vol. 30 Issue (3): 325-332

#### The article information

Yu-lin Chen, Ming-ling Li, Yi-ming Wu, Si-jia Li, Yue Lin, Du Dong-xue, Huai-yi Ding, Nan Pan, Xiao-ping Wang

Two Step Chemical Vapor Deposition of In2Se3/MoSe2 van der Waals Heterostructures

Chinese Journal of Chemical Physics, 2017, 30(3): 325-332

http://dx.doi.org/10.1063/1674-0068/30/cjcp1704063

### Article history

Accepted on: April 19, 2017
Two Step Chemical Vapor Deposition of In2Se3/MoSe2 van der Waals Heterostructures
Yu-lin Chena, Ming-ling Lib, Yi-ming Wub, Si-jia Lia, Yue Lina, Du Dong-xueb, Huai-yi Dinga, Nan Pana,b,c,d, Xiao-ping Wanga,b,c,d
Dated: Received on April 5, 2017; Accepted on April 19, 2017
a. Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China;
b. Department of physics, University of Science and Technology of China, Hefei 230026, China;
c. Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei 230026, China;
d. Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Nan Pan, E-mail:npan@ustc.edu.cn; Xiao-ping Wang, E-mail:xpwang@ustc.edu.cn
Abstract: Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide interest not only for the fundamental research, but also for the application of next generation electronic and optoelectronic devices.Herein, we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In2Se3/MoSe2 heterostructures.Transmission electron microscopy characterization reveals clearly that the In2Se3 has well-aligned lattice orientation with the substrate of monolayer MoSe2.Due to the interaction between the In2Se3 and MoSe2 layers, the heterostructure shows the quenching and red-shift of photoluminescence.Moreover, the current rectification behavior and photovoltaic effect can be observed from the heterostructure, which is attributed to the unique band structure alignment of the heterostructure, and is further confirmed by Kevin probe force microscopy measurement.The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.
Key words: van der Waals heterostructures    Chemical vapor deposition    In2Se3/MoSe2    Kevin probe force microscopy    n+-n junction
Ⅰ. INTRODUCTION

Two-dimensional (2D) layered semiconductors, including transition metal dichalcogenides (TMDs) (MX$_2$, where M=transition metals such as Mo or W, and X=S, Se, or Te) and Ⅲ-Ⅵ group layered chalcogenides (MX or M$_2$X$_3$, where M=Ga or In, and X=S, Se, or Te), have attracted broad attention for applications 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 MoSe$_2$/WSe$_2$ heterostructure owing to type-Ⅱ junction band alignment and ultrafast charge separation [5-7], but also many device applications including photovoltaics, light-emitting diodes, and photodetectors [8-10]. Although stacking different 2D materials by using mechanical transfer techniques is quite convenient and efficient, the stacked orientation cannot be precisely controlled, and the interface is often contaminated [11, 12]. In contrast, direct van der Waals (vdW) epitaxy can realize not only vertical heterostructures with well-defined interlayer orientations and clean interfaces by vertically stacking multiple 2D materials layer-by-layer [5-7, 13-19], but also lateral heterojunctions with seamless connections achieved by successive in-plane epitaxial growth of a second material from the edge of an existing crystal [20-28]. Till now, most of previous studies focused on heterostructures with two different MX$_2$, such as WS$_2$/WSe$_2$ [20], WSe$_2$/MoSe$_2$ [21, 24], WS$_2$/MoS$_2$ [22, 23], and WSe$_2$/MoS$_2$ [25], and many new physical phenomena and functionalities have been demonstrated. It is of great significance to explore 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 growth method for creating 2D vertically-stacked heterostructure consisting of In$_2$Se$_3$ and MoSe$_2$. In$_2$Se$_3$, as a Ⅲ-Ⅵ compound semiconductor, has excellent properties for electronic and optoelectronic devices. Recently, the photodetectors of In$_2$Se$_3$ with ultrahigh responsivity [29] and tunable near-UV behavior [30] have been reported. MoSe$_2$ is an ideal choice not only for its direct band gap ($\sim$1.5 eV) and strong optical absorption due to band nesting [31], but also for the fact that it is unable to form element doping in In$_2$Se$_3$. To our best knowledge, this is the first report of vertically-stacked 2D In$_2$Se$_3$/MoSe$_2$ heterostructures obtained by vdW epitaxial growth. The vertical heterostructures were verified by the characterizations of atomic force microscopy (AFM) and Raman spectroscopy. The investigation of transmission electron microscopy (TEM) reveals the well-aligned lattice orientation between In$_2$Se$_3$ and MoSe$_2$. In addition, owing to the interlayer interaction, the apparent quenching and small red shift of the photoluminescence (PL) can be observed. More importantly, due to the unique band alignment of the two different materials, the fabricated device exhibits both the current rectification behavior and the photovoltaic effect. The surface contact potential difference of the heterostructure is further characterized by using Kevin probe force microscopy (KPFM).

Ⅱ. EXPERIMENTS A. Materials synthesis

In$_2$Se$_3$/MoSe$_2$ heterostructures were grown through a two-step CVD process. The experimental set-up is ahown in FIG. 1(a). In the first step, monolayer MoSe$_2$ nanosheets were synthesized on a clean SiO$_2$/Si substrate in a two-zone furnace. 200 mg selenium (Se) and 10 mg molybdenum oxide (MoO$_3$) powder were used as precursors and put into a quartz boat and an alumina boat, respectively. They were then pushed into a quartz tube with one inch diameter. Se and MoO$_3$ quartz tubes were at the center of the first 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 SiO$_2$/Si substrate was put up-side down with the polished side facing towards MoO$_3$ powder, which located at the bottom of the crucible. The vertical distance between MoO$_3$ powder and SiO$_2$/Si substrate is about 0.5 cm. It took 15 min for zone-1 and zone-2 to heat up from room temperature to 100 and 780 ℃, respectively. Then zone-1 was heated to 270 ℃ within 5 min and kept for 30 min, meanwhile zone-2 was kept at 780 ℃. 40 sccm Ar (started before heating) and 10 sccm H$_2$ (started when zone-1 was 270 ℃) were used as both the carrier gas and reducing atmosphere to promote the reaction. After that, the furnace was cooled down to room temperature rapidly.

 FIG. 1 (a) Schematic of experimental setup for the two step growth of 1L MoSe$_2$ and In$_2$Se$_3$/MoSe$_2$ heterostructures (left), and three kinds of growth process of In$_2$Se$_3$ sheets on MoSe$_2$ (right). (b) Optical images of the obtained triangular 1L MoSe$_2$ on SiO$_2$/Si substrate after the first step growth. Scale bar: 20 $\mu$m. (c-e) Different types of In$_2$Se$_3$/MoSe$_2$ heterostructures after the second step growth, i.e., 1L MoSe$_2$ can be fully covered (c), one-side-uncovered (d) and one-corner-uncovered (e) by In$_2$Se$_3$, respectively. Scale bar: 20 $\mu$m. (f-i) AFM height profiles along the red dash lines from left to right in (b-e) respectively. The MoSe$_2$ with the thickness of $\sim$0.75 nm is monolayer (f) and all of the three In$_2$Se$_3$/MoSe$_2$ heterostructures contain 2 nm thick (bilayer) In$_2$Se$_3$ (g-i).

In the second step, the substrate with 1L MoSe$_2$ on top was used as a new substrate for the second growth of In$_2$Se$_2$, which was also the template for the synthesis of In$_2$Se$_3$/MoSe$_2$ heterostructures. 200 mg Se powder and 10 mg In$_2$O$_3$ powder were used as the Se and In precursors. Heating time and carrier gas were kept the same as the growth conditions for MoSe$_2$, but the temperature of zone-2 was reduced to 660 ℃ and kept for 25 min. All reaction processes are carried out under atmospheric pressure.

B. Characterization

The optical images were taken by optical microscopy (Olympus BX53F). AFM and KPFM were performed using Bruker Demension Icon in the ambient condition. Raman and confocal PL measurements were realized using LabRAM HR 800 under 514 nm excitation. PL mapping measurements was performed with a home-built micro-PL setup. In order to ensure the exciting area to be large enough to cover the whole selected mapping region, 532 nm laser was focused on the back focal plane of objective lens (Olympus M Plan Semi Apochromat, 100$\times$, N.A. 0.9, W.D. 1.0 mm). With a tube lens (300 mm focal length) and long wave pass filter at 550 nm, the luminescence image was enlarged and projected on the slit plane of spectrometer (Princeton Instruments SP2300 with PyLoN:100). For the luminescence image detecting, the width of the slit is set at 2 mm. The zero order interference image reflected by the grating, as the same as the luminescence image, is detected by the CCD of the spectrometer. The spatial resolution is better than 1 $\mu$m. For the spectra measurement, the target $y$ direction cut line of the luminescence image was set at the center of the CCD through tuning the $x$ position of the tube lens. Then the width of the slit was set as 50 $\mu$m. The cut line spectra, the first order interference image 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 luminescence image.

TEM, HRTEM images and SAED patterns were collected by a JEOL ARM-200F field emission transmission electron microscope operated at 200 kV. The samples for TEM analyses were transferred onto copper grids coated with 5-nm-thick amorphous carbon film. For sample transfer, polymethylmethacrylate (PMMA, 495A4) was spin-coated on the SiO$_2$/Si substrate with In$_2$Se$_3$/MoSe$_2$ heterostructures at 3000 r/min for 60 s, followed by baking at 120 ℃ for 3 min to remove the solvents. Then the substrate was floated on 1 mol/L NaOH solution and the PMMA layer carrying samples would shed off the SiO$_2$/Si substrate slowly. After that, the PMMA film was transferred to deionized water to remove residual ion and then spread onto the copper grid. Finally, the PMMA mediator was removed by dipping in acetone for 3 h.

C. Device fabrication and measurements

The SiO$_2$/Si substrate with In$_2$Se$_3$/MoSe$_2$ heterostructures was spin coated with PMMA (495A4) resist at 3000 r/min for 60 s followed by a 180 ℃ baking for 90 s. Then the EBL (JEOL, JBX 6300FS) was used to pattern the source and the drain. After development, 5 nm Ti and 50 nm Au contacts were deposited by electron beam evaporation. Lift-off process with acetone finally removed excess metal. The electrical and optoelectronic performance of fabricated device were measured in a Lake Shore CRX-4K probe station ($\sim$10$^{-4}$ Pa) with Keithley 4200S semiconductor parameter analyzer. A 532 nm laser with a power density of $\sim$0.5 mW/cm$^2$ was used as the excitation light.

Ⅲ. Results and discussion

FIG. 1(b) shows the results after the first step growth. As seen, triangular MoSe$_2$ flakes with size about several tens of microns can be found clearly. The thickness of the flake is estimated to be $\sim$0.75 nm by AFM (FIG. 1(f)), indicating that monolayer MoSe$_2$ has been successfully fabricated after the first step CVD process.

After the second CVD process, it is easy to find from FIG. 1(c-e) that the optical contrast of some MoSe$_2$ flakes becomes different, implying that an additional layered material has been grown vertically on the 1L MoSe$_2$. The material can be well identified to be In$_2$Se$_3$ using TEM characterization and Raman spectra (see following paragraphs). Further AFM characterization reveals that the thickness of In$_2$Se$_3$ is about 2 nm, corresponding to bilayer (FIG. 1(g-i)). Therefore, we can conclude that In$_2$Se$_3$(bilayer)/MoSe$_2$(monolayer) heterostructure has been successfully produced by the two-step CVD process. To our best knowledge, this is the first report of direct growth of 2D In$_2$Se$_3$ on top of 1L MoSe$_2$ to form the vertically-stacked In$_2$Se$_3$/MoSe$_2$ heterostructures.

After carefully examining the optical images of produced heterostructures on the whole substrate, we can classify them into three different kinds of morphologies. The representative results are presented in FIG. 1(c-e).The first type is the triangular 1L MoSe$_2$ fullycovered by In$_2$Se$_3$ (FIG. 1(c)), the second is MoSe$_2$ partially covered by In$_2$Se$_3$ flakes with one side uncovered (FIG. 1(d)), and the third is that with a corner of MoSe$_2$ uncovered (FIG. 1(e)). Moreover, we can find that the edge of In$_2$Se$_3$ flake on the top of MoSe$_2$ is rather smooth (FIG. 1(d, e)), hinting that the In$_2$Se$_3$ is unlikely formed from the coalescence of several small flakes but most probably grows up from a single seed. Considering In$_2$Se$_3$ nucleated randomly on the top of MoSe$_2$, we propose three possible growth processes to reach the experimental results, the schematic is shown in the right of FIG. 1(a). When the nucleation is quite near the center of the single crystal MoSe$_2$, the In$_2$Se$_3$ flake tends to fully cover the MoSe$_2$ (type i); when the nucleation is close to one apex of the triangular MoSe$_2$, the In$_2$Se$_3$ flake can partially cover the MoSe$_2$ and leave one side uncovered (type ii); similarly, a corner of the MoSe$_2$ will remain uncovered when the In$_2$Se$_3$ nucleates around one side of MoSe$_2$ (type iii).

As to the In$_2$Se$_3$/MoSe$_2$ heterostructure fabricated on SiO$_2$ substrate, several issues should be noted. First, monolayer In$_2$Se$_3$ can be hardly observed on 1L MoSe$_2$, probably because it is unstable under our growth condition. Second, the thickness of In$_2$Se$_3$ flakes can be controlled by tuning the vertical distance between the In$_2$O$_3$ source and the substrate as well as the deposition time, therefore different In$_2$Se$_3$(multilayer)/MoSe$_2$(monolayer) heterostructure can be produced (FIG. S1(c) in supplementary materials). Third, the MoSe$_2$/In$_2$Se$_3$ heterostructure, i.e., MoSe$_2$ on the top of In$_2$Se$_3$, cannot be produced on SiO$_2$ substrate. This is because that In$_2$Se$_3$ can hardly nucleate and grow on SiO$_2$ substrate directly, due to the unsaturated dangling bonds on the surface of SiO$_2$ and the large lattice mismatch [4] (FIG. S1(a) in supplementary materials). The result is well consistent with the observation shown in FIG. 1(c-e), in which no individual In$_2$Se$_3$ flake can be found on the SiO$_2$ substrate, except on the monolayer MoSe$_2$. Similar to MoSe$_2$, mica also has no unsaturated dangling bonds on its surface, making it an ideal platform for the deposition of In$_2$Se$_3$ nanofilms (FIG. S1(b) in supplementary materials).

 FIG. S1 (a-b) Optical micrographs of In2Se3 grown on SiO2/Si (a) and mica (b) substrates, under the same synthesis conditions. Only a few of small particles can be formed on the SiO2/Si substrate. On the contrary, triangular In2Se3 nanosheets with different thickness can be grown on the mica substrate, and the shape of In2Se3 flakes becomes more regular with increasing thickness. (c) Thick In2Se3 flakes grown on 1L MoSe2 exhibit brighter optical contrast. Scale bar: 10 μm.

In order to investigate the crystal quality of the heterostructure and the structure relationship between the In$_2$Se$_3$ and the MoSe$_2$, TEM as well as the selected area electron diffraction (SAED) characterizations were carried out on the boundary of In$_2$Se$_3$/MoSe$_2$ heterostructure. The heterostructures were transferred from SiO$_2$/Si substrate to the copper grid with carbon film via traditional PMMA assisted method [32]. FIG. 2(a) shows a low-magnification image focused on the area where 1L MoSe$_2$ is not completely covered by In$_2$Se$_3$ (the right part of the white dash line). FIG. 2(b) is the high-resolution TEM image taken from the white line box in FIG. 2(a). As seen, the left region of MoSe$_2$ uncovered by In$_2$Se$_3$ demonstrates brighter contrast, while the heterostructure on the right region shows darker contrast. Moreover, the crystal lattice of the heterostructre can be clearly observed, indicating its high crystal quality.

 FIG. 2 Structural characterization of the vertically-stacked In$_2$Se$_3$/MoSe$_2$ heterostructure. (a) Low magnification TEM image of the edge area where In$_2$Se$_3$ is grown on 1L MoSe$_2$. Scale bar: 100 nm. (b) The high-resolution TEM image taken from the white box area marked in (a). Scale bar: 10 nm. (c) Fast Fourier transform (FFT) and (d) the The electron diffraction patterns taken from the left (MoSe$_2$) and right (heterostructure) regions in (b), respectively. Scale bar: 5 nm. The two red circles in (c) and (d) stand for the atomic spacing along [10-10] and [11-20] directions of 2H-MoSe$_2$, whereas the green circles in (d) stand for the atomic spacing along [10-10] and [11-20] directions of $\alpha$-In$_2$Se$_3$. (e) Side view of the atomic model of the In$_2$Se$_3$/MoSe$_2$ vertical vdW heterostructure.

The structures of the monolayer MoSe$_2$ and the In$_2$Se$_3$/MoSe$_2$ heterostructure 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 diffraction patterns were obtained from both single MoSe$_2$ (left region) and In$_2$Se$_3$/MoSe$_2$ heterostructure (right region), consistent with the anticipation of 2H-MoSe$_2$ and $\alpha$-In$_2$Se$_3$. The FFT pattern of MoSe$_2$ in FIG. 2(c) reveals that the probing beam is roughly parallel to $c$ axis of the crystal. The two red circles in the image represent atomic 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 values are in good agreement with the lattice structure of 2H-MoSe$_2$ [26]. The SAED from the heterostructure (FIG. 2(d)) demonstrates two sets of patterns with six-fold symmetry. Compared with FIG. 2(c), we attribute that the spots marked with red circles also come from MoSe$_2$, and the spots marked with green circles are originated from In$_2$Se$_3$. We can further acquire information of the crystal structure of In$_2$Se$_3$ from the set of green circles. The atomic spacing along the [10-10] and [11-20] directions were calculated to be 0.36 and 0.20 nm, respectively, consistent with the lattice structure of $\alpha$-In$_2$Se$_3$ [29]. More importantly, considering the consistency of the crystal orientation of MoSe$_2$ and In$_2$Se$_3$, we consider that 2D $\alpha$-In$_2$Se$_3$ with hexagonal lattice structure has been successfully grown on the top of monolayer MoSe$_2$ by vertical vdW epitaxy. According to the above characterizations, a schematic side view of the atomic model of the In$_2$Se$_3$/MoSe$_2$ vdW heterostructure can be roughly depicted in FIG. 2(e).

The optical properties of the vertically stacked In$_2$Se$_3$/MoSe$_2$ heterostructures were characterized by Raman and photoluminescence (PL) spectroscopy. The excitation is a 514-nm laser with $\sim$1 $\mu$m spot size. FIG. 3(a) shows the optical image of the heterostructures grown on SiO$_2$/Si substrate. Three different positions involving bare 1L MoSe$_2$ (black dot), 2L In$_2$Se$_3$ on 1L MoSe$_2$ (red point) and $n$L In$_2$Se$_3$ on 1L MoSe$_2$ ($n$$>5, blue point) were excited, and the results of Raman and PL spectra are demonstrated in FIG. 3(b) and (c), respectively. As seen in FIG. 3(b), for the heterostructure of In_2Se_3(>5 layers)/MoSe_2, both Raman peaks from In_2Se_3 and MoSe_2 can be observed clearly. Three peaks located at \sim109, \sim177, and \sim206 cm^{-1} (indicated by green dash lines in FIG. 3(b)), can be ascribed to A_1(LO+TO), A_1(TO), and A_1(LO) phonon modes in \alpha-In_2Se_3, respectively [4]. However, because ultrathin In_2Se_3 can be easily damaged even under low-power laser illumination [4], the Raman signal from In_2Se_3 is absent for the 2L In_2Se_3/MoSe_2 heterostructure. The rest four Raman peaks, marked by black dash lines, originate from 2H-MoSe_2 for \sim141 cm^{-1} (weak E_{1\textrm{g}} mode, in-plane), \sim239 cm^{-1} (A_{1\textrm{g}} mode, out-of-plane), \sim289 cm^{-1} (weak E^1_{2\textrm{g}} mode, in-plane), and \sim250 cm^{-1} (defective peak) [26]. With increasing the In_2Se_3 thickness on the top of MoSe_2, the Raman signals of In_2Se_3 increase obviously while the Raman intensities of MoSe_2 slightly decrease (especially for the A_{\textrm{1g}} and E^1_{2\textrm{g}} modes). Moreover, we find that the Raman peak positions of monolayer MoSe_2 are independent of the thickness of In_2Se_3 flake on top.  FIG. 3 Optical properties of In_2Se_3/MoSe_2 heterostructures. (a) Optical image of heterostructures grown on SiO_2/Si. Scale bar: 10 \mum. (b) Raman and (c) PL spectra acquired from the three different points marked in (a), 1L MoSe_2 (black point), 2L In_2Se_3 on 1L MoSe_2 (red point) and nL In_2Se_3 on 1L MoSe_2 (n$$>$5, blue point). (d) Schematic band structure of In$_2$Se$_3$/MoSe$_2$.

FIG. 3(c) shows the PL spectra from monolayer MoSe$_2$ and In$_2$Se$_3$/MoSe$_2$ heterostructure. As seen, the maximal PL intensity is obtained in monolayer MoSe$_2$ (black line spectrum in FIG. 3(c)), thanks to its direct band gap [33]. The peak position of PL is approximately at $\sim$820 nm. For the In$_2$Se$_3$(2L)/MoSe$_2$ (1L) heterostructure (red line spectrum in FIG. 3(c)), the PL spectrum shows not only the decrease of intensity but also the red shift of peak position to $\sim$825 nm. The behaviors can be further confirmed by the PL mappings of 1L MoSe$_2$ and In$_2$Se$_3$(2L)/MoSe$_2$(1L) heterostructure (FIG. S2 in supplementary materials). Because the bilayer In$_2$Se$_3$ owns a too large indirect band gap ($\sim$2.8 eV) to absorb the excitation laser [30], we attribute the predominant reason for the decrease of PL intensity of the heterostructure to the type-Ⅱ band alignment between 2L In$_2$Se$_3$ and 1L MoSe$_2$, 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 photoluminescence [5].

 FIG. S2 (a-c) PL mappings of intrinsic 1L MoSe2. (a) Optical image of a monolayer MoSe2 sheet. Scale bar: 5 μm. (b) PL intensity mapping (integrated from 700 to 900 nm) collected from the region in (a). The lower PL intensity at the edge is analogously caused by defects, which are more intensive around the edges. These defects can quench the intrinsic PL or act as non-radiative recombination sites.1 (c) Normalized intensity mapping of PL emission along the black dash line marked in (b). The peak of PL emission shows no shift. (d-f) PL mappings of heterostructure with 1L MoSe2 partially covered by 2L In2Se3. (d) Optical image of the In2Se3/MoSe2 heterostructure. Scale bar: 10 μm. (e) PL intensity mapping (integrated from 700 to 900 nm) collected from the rectangle region marked by red dotted line in (d). The dash circle shows the edge of In2Se3. The PL of MoSe2 is obviously quenched by the covered In2Se3. (f) Normalized intensity mapping of PL along the black dash line marked in (e). As compared to the peak position ~820 nm for intrinsic monolayer MoSe2, a distinct peak red-shift to ~825 nm is observed in the region where the MoSe2 is coved by In2Se3.

1. X. Wang, Y. Gong, G. Shi, W. L. Chow, K. Keyshar, G. Ye, R. Vajtai, J. Lou, Z. Liu and E. Ringe, ACS nano, 2014, 8, 5125-5131.

It is found that the full width at half maximum (FWHM) of PL spectra shown in FIG. 3(c) are rather large, which might be caused by the co-existed neutral exciton and trion emissions. In this context, the spectra can be well fitted by two Gaussian functions with neutral exciton (X$^0$$\approx818 nm, 1.52 eV) and negative trion (X^-$$\approx$834 nm, 1.49 eV). The results are shown in FIG. S3 (supplementary materials) with the red and green lines, respectively. The evaluated trion binding energy $\sim$30 meV agrees well with previous report [34]. The integrated intensity ratios of negative trion to neutral exciton is found to increase from $\sim$1.0 for monolayer MoSe$_2$ (FIG. S3(a) in supplementary materials) to $\sim$1.3 for In$_2$Se$_3$(2L)/MoSe$_2$(1L) heterostructure (FIG. S3(b) in supplementary materials). The dominant negative trion emission in the heterostructure implies that the electrons tend to transfer from In$_2$Se$_3$ to MoSe$_2$, leading to more negative charge doping in MoSe$_2$ [35]. The result can also account for the PL red shift of the heterostructure in FIG. 3(c).

 FIG. S3 PL spectra of the monolayer MoSe2 (a) and In2Se3(2L)/MoSe2 heterostructure (b). Both of PL can be fitted by the neutral exciton peak (X0~818 nm, 1.52 eV) and the negative trion peak (X−~834 nm, 1.49 eV). However, the ratios of the integrated intensity of negative trion to that of neutral exciton are calculated to be ~1.0 in (a) and ~1.3 in (b). The results indicate that the negative trion is dominant in the PL for heterostructure, leading to the red-shift of PL peak as compared to that of MoSe2.

As to the In$_2$Se$_3$($>$5L)/MoSe$_2$(1L) heterostructure, PL emission can hardly be found as shown in FIG. 3(c) with blue line. We proposed two causes for the result. One is the exciton quenching effect, which is similar to that observed in the In$_2$Se$_3$(2L)/MoSe$_2$(1L) heterostructure. The other is that the optical band gap of In$_2$Se$_3$ decreases rapidly with the thickness ($\sim$2.2 eV in 5.5 nm thick In$_2$Se$_3$ and 1.45 eV in bulk In$_2$Se$_3$) and can even transform into a direct band gap [30], resulting in the strong absorption to the excitation light and weakening the emission.

To investigate the electrical properties of the In$_2$Se$_3$/MoSe$_2$ heterostructures, the source-drain contacts consisting of Ti/Au (5/50 nm) were deposited on 2L In$_2$Se$_3$/MoSe$_2$ and 1L MoSe$_2$ regions, respectively. The final device is shown 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 MoSe$_2$ and In$_2$Se$_3$ are inclined to be intrinsic n-doping during the growth [33, 36]. The $I_{\textrm{ds}}-V_{\textrm{ds}}$ curves under different 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 $\sim$100 when $V_\textrm{g}$$< 40 V. We attribute the behavior to the n^+-n heterojunction formed between In_2Se_3 and MoSe_2, because both materials are n-type semiconductor. The interpretation can be further verified by the fact that the rectification behavior is degraded as V_\textrm{g} becoming large. For instance, the rectification ratio is only \sim5 when V_\textrm{g}$$\approx$60 V, as shown in FIG. 4(b).

 FIG. 4 Electrical characterization of the In$_2$Se$_3$/MoSe$_2$ heterostructure. (a) Optical image of a device. The inset is the schematic structure of the device. Scale bar: 10 $\mu$m. (b) $I_{\textrm{ds}}-V_{\textrm{ds}}$ curves of the device at different back-gate voltages under dark (from bottom to top, in steps of 20 V). The inset is the transfer characteristic curve with $V_{\textrm{ds}}$ fixed at 20 V. (c) Surface contact potential difference measured along the red arrow line depicted in (a). (d) Band alignment of monolayer MoSe$_2$ and bilayer In$_2$Se$_3$ based on KPFM characterization.

In order to investigate the band alignment of the heterostructure and identify the proposed n$^+$-n heterojunction, Kelvin probe force microscope (KPFM) measurement was performed along the red arrow line depicted in FIG. 4(a). As observed from FIG. 4(c), the surface contact potential difference (CPD) across the edge of monolayer MoSe$_2$ and heterostructure can reach $\sim$200 meV and the width of depletion region is about 4 $\mu$m. Because KPFM measures the CPD between the AFM tip and the surface of sample, i.e., CPD$_{\textrm{sample}}$=$\phi_{\textrm{tip}}$-$\phi_{\textrm{sample}}$, we can get the Fermi level difference between In$_2$Se$_3$ and MoSe$_2$ by:

 $\Delta E_\textrm{F}\hspace{-0.15cm}=\hspace{-0.15cm}E_{\textrm{F}(\textrm{In}_2\textrm{Se}_3)}-E_{\textrm{F}(\textrm{MoSe}_2)}\nonumber\\ \;\;\;\;\;\;\;=\hspace{-0.15cm}\phi_{\textrm{MoSe}_2}-\phi_{\textrm{In}_2\textrm{Se}_3}\nonumber\\ \;\;\;\;\;\;\;=\hspace{-0.15cm}\textrm{CPD}_{\textrm{In}_2\textrm{Se}_3}-\textrm{CPD}_{\textrm{MoSe}_2}$ (1)

where $\phi_{\textrm{MoSe}_2}$ and $\phi_{\textrm{In}_2\textrm{Se}_3}$ are the work functions of MoSe$_2$ and In$_2$Se$_3$, respectively [37]. Therefore, the value of $\Delta E_\textrm{F}$ between In$_2$Se$_3$ and MoSe$_2$ can be estimated at $\sim$200 meV and the band alignment of 2L In$_2$Se$_3$ and monolayer MoSe$_2$ is schematically demonstrated in FIG. 4(d). As seen, compared to n-type MoSe$_2$, In$_2$Se$_3$ is heavily n-doped semiconductor. Consequently, the In$_2$Se$_3$ and MoSe$_2$ can form n$^+$-n heterojunction, which is the predominant cause for the rectification behavior in FIG. 4(b).

Finally, we present the primary results for the photoresponse of the devices under light illumination. A 532 nm laser with a power density of $\sim$0.5 mW/cm$^2$ was used as the excitation light. The $I_{\textrm{ds}}-V_{\textrm{ds}}$ curves of the device with and without light are demonstrated in FIG. S4 in supplementary materials. It can be found that the current increases dramatically with light on. The feature is more striking for the reverse biased device. Specifically, when the device under reverse bias, the current switching ratio of the device with light on to that with light off can be as high as $\sim$200. However, it decreases to $\sim$4 for the forward biased device. This phenomenon can be well understood with the band structure of the junction shown in FIG. S4(b) and FIG. S4(c) in supplementary materials. Under the reverse bias (FIG. S4(b) in supplementary materials), in addition to weak dark current, the large band offset can promote the separation of excitons in MoSe$_2$, leading to a high switching ratio; on the contrary, the small band offset under the forward bias voltage (FIG. S4(c)) enables large dark current and therefore lowers the switching ratio. This is similar to the operating principle of p-n photodiode, which further convince the n$^+$-n band alignment across the In$_2$Se$_3$/MoSe$_2$ junction. Additionally, we also observe photovoltaic effect in such a n$^+$-n heterojunction device. As shown in the inset of FIG. S4 in supplementary materials, the device shows an open-circuit voltage $V_{\textrm{OC}}$$\approx0.5 V and short-circuit current I_{\textrm{SC}}$$\approx$0.8 pA under light illumination.

 FIG. S4 Optoelectronic properties of the In2Se3/MoSe2 heterostructure shown in Fig. 4a. (a) Ⅰds-Ⅴds curves of the heterojunction in the dark (black line) and under 532 nm laser illumination (red line). The heterojunction also demonstrates the photovoltaic effect, as shown in the inset. (b-c) Schematic band profiles of In2Se3/MoSe2 heterojunction under reverse-bias voltage (b) and under forward-bias voltage (c).
Ⅳ. Conclusion

In summary, we fabricated the vertical heterostructures with bilayer In$_2$Se$_3$ on the top of monolayer MoSe$_2$ through a two-step CVD process. The vdW epitaxial character and the possible growth schematic are revealed by the structural characterization. It is found that the quenching behavior and red-shift of photoluminescence of the heterostructure can be modulated by the interaction between MoSe$_2$ and In$_2$Se$_3$. Due to the uniquely aligned band structure, such In$_2$Se$_3$/MoSe$_2$ heterostructures can exhibit distinct current rectification behavior and photoelectric response. More importantly, the reported approach in this work can be extended to other novel 2D heterostructures involving different layered compounds, enabling to enrich the variety of 2D vdW heterostructures for basic research and applications for next generation optoelectronic devices.

Supplementary materials: More optical micrographs about the growth of In$_2$Se$_3$. PL mappings of intrinsic 1L MoSe$_2$ and heterostructure with 1L MoSe$_2$ partially covered by 2L In$_2$Se$_3$. Fitted PL spectra of the monolayer MoSe$_2$ and In$_2$Se$_3$(2L)/MoSe$_2$ heterostructure. Optoelectronic properties of the In$_2$Se$_3$/MoSe$_2$ heterostructure.

Ⅴ. Acknowledgments

This work was supported by the Ministry of Science and Technology of China (No.2016YFA0200602), 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 Fundamental Research Funds for the Central Universities (WK2030020027, WK2060190027).

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a. 中国科学技术大学, 合肥微尺度物质科学国家实验室, 合肥 230026;
b. 中国科学技术大学物理系, 合肥 230026;
c. 中国科学技术大学, 量子信息与量子科技前沿协同创新中心, 合肥 230026;
d. 中国科学技术大学, 中国科学院强耦合量子材料物理重点实验室, 合肥 230026