## 3 Results and Discussion  
Figure 1a,b show the detailed synthetic procedures of ML-MoS\({}_{2}\) and ML-WS\({}_{2}\), respectively. Unlike the CVD synthesis method for ML-MoS\({}_{2}\), ML-WS\({}_{2}\) was prepared using the CVT method for easier peeling from the substrate [39]. Then, the formation of the WS\({}_{2}\)/MoS\({}_{2}\) heterostructure (WS\({}_{2}\)/MoS\({}_{2}\) HS) was achieved by transferring ML-WS\({}_{2}\) onto ML-MoS\({}_{2}\) using a wet transfer technique, as illustrated in Figure 2. Typically, the formation of two-dimensional vdW heterostructures is confirmed by integrating the results from optical imaging, AFM imaging, Raman spectra, and PL spectroscopy [25; 40; 41]. Figure 2a is an optical image of the WS\({}_{2}\)/MoS\({}_{2}\) HS on the SiO\({}_{2}\)/Si substrate, displaying twist angles for WS\({}_{2}\) and MoS\({}_{2}\) at approximately 60\({}^{\circ}\). The AFM topography image is shown in Figure 2b, showing that numerous abnormal points with random heights are unevenly distributed throughout the stacking region. The abnormal points are caused by the formation of unavoidable nanobubbles between the two monolayers during the fabrication process, and the gap distances are approximately equal to the diameter of the bubbles. Figure 2c is the height profile along the straight line indicated in Figure 2b. The presence of two steps below 1 nm in the height curve indicates that the heterostructure  
Figure 1: The synthesis procedures of the monolayers. (**a**) Monolayer MoS\({}_{2}\) by CVD. (**b**) Monolayer WS\({}_{2}\) by CVT. is composed of two monolayers pointing to ML-WS2 and ML-MoS2. The height of the second step (0.68 or 0.71 nm) is slightly higher than that of the first (0.63 nm), which is in agreement with the gap of the bilayer. It is also observed that most of the bubbles are less than 2 nm in diameter, but a small number of bubbles can reach diameters of over 10 nm. The nanobubbles enlarge the interlayer distance between the two monolayers, weakening or even eliminating interlayer interactions [42; 43]. Thus, nanobubbles should be avoided in subsequent measurements. Figure 2d presents the Raman spectra of ML-WS2, ML-MoS2, and WS2/MoS2 HS (the selected points are marked with yellow, purple, and blue crosses in Figure 2b, respectively). The characteristic peaks of WS2 located at 424 and 361 cm\({}^{-1}\) are indexed to the out-of-plane A1g mode and in-plane E12g mode of the monolayer WS2 [44]. The out-of-plane A1g mode and in-plane E12g mode of individual MoS2 are located at 410 and 392 cm\({}^{-1}\), respectively. It should be noted that the Raman shifts are relatively higher than those in some previous studies [45; 46; 47; 48; 49], which is induced by the extensive oxidation reactions occurring between atmospheric oxygen and the defects in MoS2 during long exposure to air [50]. The distance between the two Raman peaks is -18 cm\({}^{-1}\), which is consistent with previous studies and confirms the monolayer nature of MoS2 [51]. The Raman spectrum of the WS2/MoS2 region appears to contain all four peaks of the two monolayers, indicating the coexistence of MoS2 and WS2. Upon stacking, the intensities of the peaks corresponding to the WS2 E12g, MoS2 E12g, and MoS2 A1g modes decrease to 1605, 250, and 270, respectively. In contrast, the peak intensity of the WS2 A1g model increases from 254 to 340. Previous studies have shown that the out-of-plane A1g mode of WS2 is extremely sensitive to the number of layers and its intensity increases with increasing layer number [52; 53; 54]. The formation of a bilayer heterostructure can generate interlayer vdW force, enhancing the out-of-plane vibration of WS2. Therefore, the enhancement of the A1g mode of WS2 indicates that there is more than one layer of material in the test area. Combined with the morphology and height images characterized by AFM, these results corroborate the formation of the WS2/MoS2 HS. Figure 2: Characterization of WS2/MoS2 heterostructure. (**a**) Optical image. (**b**) AFM topography image. (**c**) AFM height curve.