The Large-Scale Preparation and Optical Properties of MoS2/WS2 Vertical Hetero-Junction.

A variety of hetero-junctions can be constructed to form the basic structural units in the different optoelectronic devices, such as the photo-detectors, solar cells, sensors and light-emitting diodes. In our research, the large-area high-quality MoS2/WS2 vertical hetero-junction are prepared by the two-step atmospheric pressure chemical vapor deposition (APCVD) methods and the dry transfer method, and the corresponding optimal reaction conditions of MoS2/WS2 vertical hetero-junction are obtained. The morphology, composition and optical properties of MoS2/WS2 vertical hetero-junction are systematically characterized by the optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and the field emission scanning electron microscopy. Compared to the mechanical transfer method, the MoS2/WS2 vertical hetero-junction sample obtained by the APCVD and dry transfer methods have lower impurity content, cleaner interfaces and tighter interlayer coupling. Besides, the strong interlayer coupling and effective interlayer charge transfer of MoS2/WS2 vertical hetero-junction are also further studied. The photoluminescence intensity of MoS2/WS2 vertical hetero-junction is significantly reduced compared to the single MoS2 or WS2 material. In general, this research can help to achieve the large-scale preparation of various Van der Waals hetero-junctions, which can lay the foundation for the new application of optoelectronic devices.


Introduction
The electronic chips integration increase with the development of modern semiconductor technology, and the volume of devices is becoming smaller, with the preparation processes becoming more complex [1][2][3]. The size of conventional Si-based CMOS devices has approached the limit, so researchers turn their attention to the two-dimensional transition metal dichalcogenides (2D TMDs) materials with single atomic layers. However, there are some limitations in the application of the single two-dimensional materials. The single h-BN material cannot be used in the devices alone, due to its wide band gap [4]. Additionally, the exposed BP is easily oxidized in the air, which would lead to the degradation of its performance [5]. As we all know, different two-dimensional materials have different energy band structures and band gaps. To further investigate the intrinsic properties of two dimensional materials and expand the application fields, the two dimensional hetero-junction materials can concentrate the advantages of two materials together to achieve the precise adjustment of the band gap, and it can also finely regulate the physical properties of the single materials. The advantage of Van der Waals (VDWs) atomic level hetero-junction has the convenience of modular combination, compared with the conventional hetero-junction. In other words, the atomic layer materials could there is still a huge challenge in using the existing exfoliation or CVD growth processes to achieve the scalable production of TMDs hetero-junctions with well-controlled structures.
The VDWs hetero-junction can be formed by using mechanical transfer technology to stack the different 2D materials, which has limited stacking orientation control. Compared to the mechanical transfer method, the direct growth of MX2 hetero-structure by CVD method not only has inherent scalability, but also has a cleaner interface and tighter interlayer coupling [15]. In this paper, the twostep atmospheric pressure chemical vapor deposition (APCVD) and dry transfer methods are used to epitaxially grow WS2 on MoS2, which can prepare the MoS2/WS2 vertical hetero-junction with its clean interface and strong interlayer coupling. The optimal processes' reaction conditions of MoS2/WS2 vertical hetero-junction can also be obtained. Meanwhile, the morphology, composition and optical properties of MoS2/WS2 vertical hetero-junction are systematically characterized by optical microscopy, Raman spectroscopy, photoluminescence (PL) spectroscopy, atomic force microscopy, and field emission scanning electron microscopy. The results shown in this paper can provide effective guidance for future photonic devices, solar cells, photo-detectors, modulators, and storage devices [16].

The Preparation Experiment of MoS 2 /WS 2 Vertical Heterojunction
Meanwhile, the preparation processes of the MoS 2 /WS 2 vertical hetero-junction by the dry transfer method are also described in previous work [18]. Based on the previous MoS 2 and WS 2 materials prepared by APCVD, the polymethyl methacrylate (PMMA) liquid was spin-coated on the WS 2 at a speed of 4000 r/s, and annealed at 150 • C for 30 min. Additionally, the PMMA film can be immersed in the NaOH solution (Lingping Chemical Glass Instrument Co., Ltd., Guangzhou, China, 0.1 Mol/L). Then, the PMMA film with WS 2 is floated and transferred to another SiO 2 /Si substrate, by using the transfer platform under the condition of CCD monitoring, which would overlap with the MoS 2 . Finally, the PMMA film is dissolved with acetone, and the MoS 2 /WS 2 vertical hetero-junction can be obtained.

The Characterization of MoS 2 /WS 2 Vertical Heterojunction
The Raman spectrum and PL spectrum of the MoS 2 /WS 2 vertical hetero-junction are systematically analyzed by the LabRam HR Evolution Raman microscopy, equipped with the 532 nm laser under the 1mW laser power, and the laser spot size is 1µm. The high-resolution images can be obtained by using the 50× objective lens, 1800 lines/mm grating and the 500 nm imaging steps [19]. At the same time, the field emission scanning electron microscope (FESEM) and atomic force microscope (AFM) are also used to characterize the surface morphology and thickness of the MoS 2 /WS 2 vertical hetero-junction.

The Analysis of Characterization Results
In order to obtain better MoS 2 /WS 2 vertical hetero-junction materials, we first need to grow the high-quality large-area single MoS 2 or WS 2 materials by APCVD. Under the optimal growth conditions, the gas concentration and nucleation density of single MoS 2 or WS 2 materials in the tube furnace can achieve the maximum values [20]. The MoS 2 /WS 2 double-layer vertical hetero-junction can be prepared by the two-step APCVD method and the dry transfer processes. At the same time, the MoS 2 /WS 2 vertical hetero-junction samples are systematically characterized by optical microscopy, Raman spectroscopy, PL spectroscopy, AFM and FESEM. In addition, the surface of WS 2 , MoS 2 , and MoS 2 /WS 2 vertical hetero-junction samples are all uniform triangles with a size of several tens of microns after the corresponding growth reaction, and the color contrast between the samples and SiO 2 /Si substrate is relatively obvious, which can be distinguished by the optical microscope, as shown in Figure 2. Raman spectroscopy, PL spectroscopy, AFM and FESEM. In addition, the surface of WS2, MoS2, and MoS2/WS2 vertical hetero-junction samples are all uniform triangles with a size of several tens of microns after the corresponding growth reaction, and the color contrast between the samples and SiO2/Si substrate is relatively obvious, which can be distinguished by the optical microscope, as shown in Figure 2.  The MoS2/WS2 vertical hetero-junction on SiO2/Si substrate can also be observed by the FESEM. It can be found that the size of single WS2 or MoS2 materials are respectively 20 μm and 30 μm, and the contrast between single WS2 or MoS2 materials and SiO2/Si substrate is very uniform.
The reason the overlapping parts are not mixed together to form the WS2-MoS2 lateral heterojunction is that we have adopted the two-step growth process. The first step is that the WoO3 powder forms the WS2 crystals during the high temperature (800 °C) reduction and vulcanization process, and the second step is that the MoO3 forms MoS2 crystals during the low temperature (720 °C) reduction and vulcanization process. Besides, the PMMA-assisted transfer method is used to transfer the monolayer WS2 to the monolayer MoS2 material, and the top WS2 layer has only 0° and 60° orientation relative to the bottom MoS2 layer in the red dotted box, which respectively correspond to The MoS 2 /WS 2 vertical hetero-junction on SiO 2 /Si substrate can also be observed by the FESEM. It can be found that the size of single WS 2 or MoS 2 materials are respectively 20 µm and 30 µm, and the contrast between single WS 2 or MoS 2 materials and SiO 2 /Si substrate is very uniform.
The reason the overlapping parts are not mixed together to form the WS 2 -MoS 2 lateral hetero-junction is that we have adopted the two-step growth process. The first step is that the WoO 3 powder forms the WS 2 crystals during the high temperature (800 • C) reduction and vulcanization process, and the second step is that the MoO 3 forms MoS 2 crystals during the low temperature (720 • C) reduction and vulcanization process. Besides, the PMMA-assisted transfer method is used to transfer the monolayer WS 2 to the monolayer MoS 2 material, and the top WS 2 layer has only 0 • and 60 • orientation relative to the bottom MoS 2 layer in the red dotted box, which respectively correspond to A-A and A-B stacks [21,22], as shown in Figure 3. Therefore, unlike the traditional CVD method, the injection of Mo into Wo 3 to form the lateral MoS 2 -WS 2 mixture is effectively prevented. Figure 4 shows the AFM images of MoS 2 , WS 2 , and MoS 2 /WS 2 vertical hetero-junction. It can be found by observing the surface morphology and thickness that the thickness of the MoS 2 /WS 2 vertical hetero-junction is about 1.7 nm, as shown in red mark region III. In order to further determine the thickness of single MoS 2 and WS 2 materials, the thickness of region II MoS 2 and region I WS 2 are respectively measured as 0.8 nm and 0.9 nm, which indicates the existence of monolayer WS 2 and monolayer MoS 2 , and the lateral dimension is about 20-30 µm.  The MoS2/WS2 vertical hetero-junction on SiO2/Si substrate can also be observed by the FESEM. It can be found that the size of single WS2 or MoS2 materials are respectively 20 μm and 30 μm, and the contrast between single WS2 or MoS2 materials and SiO2/Si substrate is very uniform.
The reason the overlapping parts are not mixed together to form the WS2-MoS2 lateral heterojunction is that we have adopted the two-step growth process. The first step is that the WoO3 powder forms the WS2 crystals during the high temperature (800 °C) reduction and vulcanization process, and the second step is that the MoO3 forms MoS2 crystals during the low temperature (720 °C) reduction and vulcanization process. Besides, the PMMA-assisted transfer method is used to transfer the monolayer WS2 to the monolayer MoS2 material, and the top WS2 layer has only 0° and 60° orientation relative to the bottom MoS2 layer in the red dotted box, which respectively correspond to A-A and A-B stacks [21,22], as shown in Figure 3. Therefore, unlike the traditional CVD method, the injection of Mo into Wo3 to form the lateral MoS2-WS2 mixture is effectively prevented.  Figure 4 shows the AFM images of MoS2, WS2, and MoS2/WS2 vertical hetero-junction. It can be found by observing the surface morphology and thickness that the thickness of the MoS2/WS2 vertical hetero-junction is about 1.7 nm, as shown in red mark region III. In order to further determine the thickness of single MoS2 and WS2 materials, the thickness of region II MoS2 and region I WS2 are respectively measured as 0.8 nm and 0.9 nm, which indicates the existence of monolayer WS2 and monolayer MoS2, and the lateral dimension is about 20-30 μm.

Introduction and Analysis of the Spectrum Characteristics
Raman spectrum and PL spectrum have become the effective way to detect and identify the optical properties and layer number of MoS2, WS2, and MoS2/WS2 vertical hetero-junction grown by the APCVD and dry transfer methods. The corresponding test characterizations require constant temperature and humidity, which are performed in an ultra-clean room environment.

The Spectrum Characterization of Monolayer WS2
It can be found, by observing Figure 5a, that there are two characteristic peaks in the Raman spectrum of the WS2 sample, and the peak positions of the E 1 2g and A1g characteristic peaks are respectively located at 349 cm −1 and 413 cm −1 , which are fitted by the Gaussian function. The peak shift between two characteristic peaks decreases with the layer number decreases, which can be explained by the increase of the Van der Waals force. The Raman spectrums on the triangular WS2 sample remain consistent at different points, which can indicate that the sample is uniform. In Figure

Introduction and Analysis of the Spectrum Characteristics
Raman spectrum and PL spectrum have become the effective way to detect and identify the optical properties and layer number of MoS 2 , WS 2 , and MoS 2 /WS 2 vertical hetero-junction grown by the APCVD and dry transfer methods. The corresponding test characterizations require constant temperature and humidity, which are performed in an ultra-clean room environment.

The Spectrum Characterization of Monolayer WS 2
It can be found, by observing Figure 5a, that there are two characteristic peaks in the Raman spectrum of the WS 2 sample, and the peak positions of the E 1 2g and A 1g characteristic peaks are respectively located at 349 cm −1 and 413 cm −1 , which are fitted by the Gaussian function. The peak shift between two characteristic peaks decreases with the layer number decreases, which can be explained by the increase of the Van der Waals force. The Raman spectrums on the triangular WS 2 sample remain consistent at different points, which can indicate that the sample is uniform. In Figure 5b, the strongest PL spectrum peak position of monolayer WS 2 is located at 636 nm. The band gap of monolayer WS 2 is calculated at 1.96 eV from the conversion relationship between the wavelength and electron volt, which is the same as the reported results. Besides, the PL spectrum also has an exciton peak of 2.04 eV. This is because the WS 2 sample has energy band splitting [23]. Figure 5c shows the Raman spectrum of monolayer WS 2 under the different laser powers. The Raman spectrum shape and relative position of the WS 2 sample move, to a certain extent, when the laser power increases. The Raman intensity of the E 1 2g characteristic peak increases with laser power increase, and the E 1 2g characteristic peak shape changes, especially when the laser power exceeds 10%. Meanwhile, the A 1g characteristic peak intensity remains constant, but the position has a certain deviation. The PL spectrum intensity of the WS 2 sample strongest peak increases with a laser power increase, the strongest peak position is red-shifted, and the shape changes to some extent, as shown in Figure 5d. This phenomenon can be explained from two aspects here; on the one hand, higher power would generate more heat on the WS 2 sample, which has an effect on the power PL spectrum of WS 2 ; on the other hand, the SiO 2 /Si substrate selected in the WS 2 growth experiment is an n-type doped semiconductor material [24].
Molecules 2020, 25, x FOR PEER REVIEW 6 of 11 generate more heat on the WS2 sample, which has an effect on the power PL spectrum of WS2; on the other hand, the SiO2/Si substrate selected in the WS2 growth experiment is an n-type doped semiconductor material [24].

The Spectrum Characterization of Monolayer MoS2
In Figure 6a, the Raman spectrum of monolayer MoS2 has two characteristic peaks, wherein the E 1 2g characteristic peak is at 379 cm −1 , the position of A1g characteristic peak is 398 cm −1 , and the ratio of A1g to E 1 2g characteristic peaks is approximately 1.051. Besides, the Raman spectrum intensity and position of the MoS2 sample at different points are consistent, which shows that the monolayer MoS2 sample is highly uniform. It can be seen from Figure 6b that the strongest PL spectrum peak position of monolayer MoS2 on SiO2/Si substrate is at 681.2 nm, and the electron volt is 1.82 eV, which is the same as the direct band gap width of monolayer MoS2. Besides, the PL spectrum of monolayer MoS2 also has an exciton peak at 1.95 eV. The reason is that the Mo atom has 3d orbital electron interaction [25]. As shown in Figure 6c, the Raman spectrum intensity of MoS2 sample increases when the laser power increases; the positions of E 1 2g and A1g characteristic peaks are blue-shifted simultaneously, which is caused by the n-type doped semiconductor characteristics of SiO2/Si substrate [26]. Figure

The Spectrum Characterization of Monolayer MoS 2
In Figure 6a, the Raman spectrum of monolayer MoS 2 has two characteristic peaks, wherein the E 1 2g characteristic peak is at 379 cm −1 , the position of A 1g characteristic peak is 398 cm −1 , and the ratio of A 1g to E 1 2g characteristic peaks is approximately 1.051. Besides, the Raman spectrum intensity and position of the MoS 2 sample at different points are consistent, which shows that the monolayer MoS 2 Molecules 2020, 25, 1857 7 of 11 sample is highly uniform. It can be seen from Figure 6b that the strongest PL spectrum peak position of monolayer MoS 2 on SiO 2 /Si substrate is at 681.2 nm, and the electron volt is 1.82 eV, which is the same as the direct band gap width of monolayer MoS 2 . Besides, the PL spectrum of monolayer MoS 2 also has an exciton peak at 1.95 eV. The reason is that the Mo atom has 3d orbital electron interaction [25]. As shown in Figure 6c, the Raman spectrum intensity of MoS 2 sample increases when the laser power increases; the positions of E 1 2g and A 1g characteristic peaks are blue-shifted simultaneously, which is caused by the n-type doped semiconductor characteristics of SiO 2 /Si substrate [26]. Figure 6d shows the PL spectrum of monolayer MoS 2 under different laser powers. As the laser power increases, the strongest photoluminescence spectrum peak intensity of monolayer MoS 2 increases, and the shape and position of characteristic peaks do not change. It can be judged that MoS 2 on SiO 2 /Si substrate is monolayer, by characterizing and analyzing the Raman spectrum and photoluminescence spectrum of MoS 2 .
Molecules 2020, 25, x FOR PEER REVIEW 7 of 11 substrate is monolayer, by characterizing and analyzing the Raman spectrum and photoluminescence spectrum of MoS2.

Spectrum Characterization of the MoS 2 /WS 2 Heterojunction
The Raman spectrum of the MoS 2 /WS 2 hetero-junction is tested with the 532 nm laser excitation wavelength, to evaluate vibration characteristics and thickness. There are E 1 2g and A 1g Raman active modes in the Raman spectrum of MoS 2 /WS 2 hetero-junction, wherein the E 1 2g is the in-plane optical mode that corresponds to the vibrational motion of Mo and S atoms in the x and y planes, while the A 1g is an out-of-plane vibration mode that corresponds to the vibrational motion of two S atoms along the z unit cell axis [27]. It can be found that four Raman characteristic peaks, WS 2 -E 1 2g , MoS 2 -E 1 2g , MoS 2 -A 1g , and WS 2 -A 1g , appeared in the MoS 2 /WS 2 hetero-junction region, and the corresponding peak positions are respectively located at 350 cm −1 , 379 cm −1 , 398 cm −1 and 413 cm −1 . In the four Raman peaks of MoS 2 /WS 2 hetero-junction, two peaks are the same as the Raman peaks of MoS 2 film, and the other two peaks correspond to the Raman peaks of WS 2 film. The four Raman peaks positions of MoS 2 /WS 2 hetero-junction did not show red or blue shift, which indicates that WS 2 and MoS 2 did not affect the long-distance Coulomb interaction between the effective charges [28]. The photoluminescence spectrum can be used to identify the band gaps of WS 2 and MoS 2 . Figure 7b shows the PL spectrum of WS 2 /MoS 2 hetero-junction under the 532 nm wavelength. The PL peak positions of WS 2 and MoS 2 are respectively located at 635 nm (1.96 eV) and 682 nm (1.82 eV), which is due to the direct exciton transition energy of bottom layer MoS 2 and top layer WS 2 . It can be seen by comparing and analyzing the PL spectrum of MoS 2 /WS 2 vertical hetero-junction and single MoS 2 or WS 2 thin films that the PL peak position of MoS 2 /WS 2 hetero-junction is the same as that of the single MoS 2 or WS 2 films, but the peak intensity of the MoS 2 /WS 2 hetero-junction is much lower than that of the single WS 2 films, which can be caused by interlayer exciton relaxation. At the interface between MoS 2 and WS 2 composites, electrons from the conduction band of WS 2 are transferred to the conduction band of MoS 2 , and the holes from the valence band of MoS 2 are moved to the valence band of WS 2 , which would cause the separation of photo-generated electrons and holes [29]. A slight PL position shift and an additional weak peak of 1.82 eV can be observed in the epitaxial MoS 2 /WS 2 hetero-junction, which can be attributed to the recombination of spatially separated carriers. The two main factors of the PL spectrum quenching are the energy and charge transfer. In Figure 7c, the four characteristic peaks intensity of MoS 2 /WS 2 hetero-junction gradually increases with the laser power increase, wherein the E 1 2g and A 1g characteristic peak positions of bottom layer MoS 2 are blue-shifted, and the A 1g characteristic peak position of top layer WS 2 also shifted blue. Different laser powers on MoS 2 /WS 2 hetero-junction would generate different heat, which can affect the Raman spectrum of MoS 2 /WS 2 hetero-junction. The top layer WS 2 may prevent the effective collection of Raman scattered signals from the bottom layer MoS 2 . Meanwhile, the Raman signal from WS 2 is also weakened when it passes the bottom layer of MoS 2 , indicating that the bottom layer MoS 2 can still affect the Raman intensity of the top layer WS 2 . Figure 7d shows the photoluminescence spectrum of the MoS 2 /WS 2 hetero-junction under different laser powers. The strongest photoluminescence peak intensity of the MoS 2 /WS 2 hetero-junction increases with laser power increase; the strongest photoluminescence spectrum position red-shifted, and the strongest photoluminescence spectrum shape of MoS 2 /WS 2 hetero-junction also changes.
Molecules 2020, 25, x FOR PEER REVIEW 7 of 11 substrate is monolayer, by characterizing and analyzing the Raman spectrum and photoluminescence spectrum of MoS2.  The Raman spectrum of the MoS2/WS2 hetero-junction is tested with the 532 nm laser excitation wavelength, to evaluate vibration characteristics and thickness. There are E 1 2g and A1g Raman active modes in the Raman spectrum of MoS2/WS2 hetero-junction, wherein the E 1 2g is the in-plane optical mode that corresponds to the vibrational motion of Mo and S atoms in the x and y planes, while the A1g is an out-of-plane vibration mode that corresponds to the vibrational motion of two S atoms along the z unit cell axis [27]. It can be found that four Raman characteristic peaks, WS2-E 1 2g, MoS2-E 1 2g, MoS2-A1g, and WS2-A1g, appeared in the MoS2/WS2 hetero-junction region, and the corresponding peak positions are respectively located at 350 cm −1 , 379 cm −1 , 398 cm −1 and 413 cm −1 . In the four Raman peaks of MoS2/WS2 hetero-junction, two peaks are the same as the Raman peaks of MoS2 film, and the other two peaks correspond to the Raman peaks of WS2 film. The four Raman peaks positions of MoS2/WS2 hetero-junction did not show red or blue shift, which indicates that WS2 and MoS2 did not affect the long-distance Coulomb interaction between the effective charges [28]. The photoluminescence spectrum can be used to identify the band gaps of WS2 and MoS2. Figure 7b shows the PL spectrum of WS2/MoS2 hetero-junction under the 532 nm wavelength. The PL peak positions of WS2 and MoS2 are respectively located at 635 nm (1.96 eV) and 682 nm (1.82 eV), which is due to the direct exciton transition energy of bottom layer MoS2 and top layer WS2. It can be seen by comparing and analyzing the PL spectrum of MoS2/WS2 vertical hetero-junction and single MoS2 or WS2 thin films that the PL peak position of MoS2/WS2 hetero-junction is the same as that of the single MoS2 or WS2 films, but the peak intensity of the MoS2/WS2 hetero-junction is much lower than that of the single WS2 films, which can be caused by interlayer exciton relaxation. At the interface between MoS2 and WS2 composites, electrons from the conduction band of WS2 are transferred to the conduction band of MoS2, and the holes from the valence band of MoS2 are moved to the valence band of WS2, which would cause the separation of photo-generated electrons and holes [29]. A slight PL position shift and an additional weak peak of 1.82 eV can be observed in the epitaxial MoS2/WS2 hetero-junction, which can be attributed to the recombination of spatially separated carriers. The two main factors of the PL spectrum quenching are the energy and charge transfer. In Figure 7c, the four characteristic peaks intensity of MoS2/WS2 hetero-junction gradually increases with the laser power increase, wherein the E 1 2g and A1g characteristic peak positions of bottom layer MoS2 are blue-shifted, and the A1g characteristic peak position of top layer WS2 also shifted blue. Different laser powers on MoS2/WS2 hetero-junction would generate different heat, which can affect the Raman spectrum of MoS2/WS2 hetero-junction. The top layer WS2 may prevent the effective collection of Raman scattered signals from the bottom layer MoS2. Meanwhile, the Raman signal from WS2 is also weakened when it passes the bottom layer of MoS2, indicating that the bottom layer MoS2 can still affect the Raman intensity of the top layer WS2. Figure 7d shows the photoluminescence spectrum of the MoS2/WS2 hetero-junction under different laser powers. The strongest photoluminescence peak intensity of the MoS2/WS2 hetero-junction increases with laser power increase; the strongest photoluminescence

Conclusions
The 2D TMDs materials have different energy band structures and band gaps, so that the physical properties of 2D TMDs materials can be finely adjusted by stacking with each other. The large-area, high-quality monolayer MoS 2 and WS 2 materials are first prepared by APCVD method in this paper. Then, the MoS 2 /WS 2 vertical hetero-junction is prepared by the dry transfer method, which can provide a well-defined interface between WS 2 and MoS 2 in the vertical dimension. The interface environment is the key factor affecting the performance of VDW hetero-structure modulation devices. The optimal reaction conditions of MoS 2 /WS 2 vertical hetero-junction by the APCVD and dry transfer methods are obtained through experiments. The composite materials are divided into the MoS 2 , WS 2 and MoS 2 /WS 2 hetero-junction regions, according to the morphology evolution of the MoS 2 /WS 2 vertical hetero-junction on the SiO 2 /Si substrate. Compared to the single MoS 2 or WS 2 materials, the PL spectrum characterization results show that the PL intensity of the MoS 2 /WS 2 hetero-junction is reduced by half, which is due to the effective photoelectron-hole separation phenomenon which occurred during the recombination process. Meanwhile, the strong interlayer coupling and effective interlayer charge transfer of the MoS 2 /WS 2 hetero-junction are systematically studied to strengthen the basic understanding of interlayer coupling, which can provide an unstructured and convenient method to explore the interface environment of the VDW hetero-structures. At the same time, the characterization results can help the mass production of various VDW hetero-junctions in future optoelectronic devices.