## 3 Results and Discussion
(**b**) PL spectra with probe contact. of the WS\({}_{2}\)/MoS\({}_{2}\) HS not only experiences a substantial decrease in intensity but also undergoes a significant change in shape. The peak of MoS\({}_{2}\) A-excitons is highlighted, enabling the detailed study of the excitonic properties in a vdW heterostructure using PL spectroscopy. Moreover, an AFM equipped with a Si probe can serve as not only an electrode for electron transport but also a means to exert out-of-plane controllable pressure on 2D materials [57]. Previous studies have shown that 2D materials exhibit good reversibility under a pressure of 12 GPa and a strain of 0.08%, and applying pressure to 2D material using an AFM probe does not cause damage [58; 59]. To investigate the pressure-related exciton characteristics, the PL spectra of ML-WS\({}_{2}\), ML-MoS\({}_{2}\), and WS\({}_{2}\)/MoS\({}_{2}\) HS are measured under different applied forces in the range of 0-120 nN at 20 nN intervals, as shown in Figure 4a-c, respectively. The emission peak intensities of all three regions exhibit a decreasing trend with increasing pressure, which can be related to the out-of-plane pressure-induced transition from the direct to indirect bandgaps of the monolayers, leading to a reduction in quantum yield and a significant weakening of PL intensity [15; 60; 61]. The spectral behavior in the heterostructure region is more complex than that in the monolayer region, as the PL properties of the heterostructure are influenced not only by the electronic structure of the monolayers but also by interlayer coupling [62]. To visually demonstrate the relationship between the PL spectra and pressure in the heterojunction region, the 2D pressure-dependent wavelength-intensity image is shown in Figure 4d. Three peaks of the heterostructure exhibit a strong pressure dependency. As the pressure increases, the intensity of the three peaks declines with varying degrees, where the peak related to WS\({}_{2}\) trions exhibits the most noticeable decrease. To gain insights into the influence of pressure on the exciton characteristics of the vdW heterostructure, the pressure-dependent PL curves are fitted with the Lorentzian function. The evolution of the intensities and positions of the fitted exciton peaks of ML-WS\({}_{2}\), ML-MoS\({}_{2}\), and WS\({}_{2}\)/MoS\({}_{2}\) HS with increasing pressure are shown in Figure 5. Figure 5a reveals that with increasing pressure, the intensity of A excitons and trions in ML-WS\({}_{2}\) decreases at the same rate but the exciton energy remains almost constant (Figure 5b). For ML-MoS\({}_{2}\)  
Figure 4: (**a–c**) PL spectra of (**a**) ML-WS2, (**b**) ML-MoS2, and (**c**) WS2/MoS2 HS under out-of-plane pressure ranging from 0 to 120 nN. (**d**) Two-dimensional mapping of the HS PL spectra. the intensity of A excitons decreases much faster than that of B excitons (Figure 5c), and the variation in exciton energy is consistent with that of ML-WS\({}_{2}\) (Figure 5d). The decline in PL intensity in the monolayer TMDCs is associated with the transfer of electrons from the K valley to its neighboring valleys during conduction. ML-WS\({}_{2}\) and ML-MoS\({}_{2}\) are both semiconductors with direct bandgaps. With increasing pressure, the conduction band exhibits an upward displacement at the K valley and a downward displacement at the \(\Gamma\) valley, enhancing the K\(-\)K bandgap and reducing the K\(-\)\(\Gamma\) bandgap [61]. Consequently, the quantum yield diminishes, resulting in a reduction in PL intensity. Once the pressure surpasses a certain threshold, the optical bandgap undergoes a transition, shifting from direct to indirect. In the vdW heterostructure, the WS\({}_{2}\) A-excitons and the MoS\({}_{2}\) A-excitons exhibit similar decreasing trends in peak intensity under increasing pressure (Figure 5e). However, the WS\({}_{2}\) trions experience a rapid decline with a reduction rate approximately twice that of the WS\({}_{2}\) A-excitons.