## 3 Results and Discussion  
### Optical Characterization  
A 3D (three-dimensional) schematic diagram, 2D schematic diagram of the operation principle and an optical image of the phototransistor based on laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) van der Waals heterojunction are shown in Figure 1a-c, respectively. The MoS\({}_{2}\) and WSe\({}_{2}\) nanoflakes were exfoliated onto PDMS and SiO\({}_{2}\) substrates, respectively. The WSe\({}_{2}\) surrounded by a white dotted-line triangle in Figure 1c was fully scanned using a 633 nm wavelength laser. After the laser-scanned process, MoS\({}_{2}\) nanoflakes were transferred onto the surface of laser-scanned WSe\({}_{2}\). Source and drain electrodes were patterned via lithography on MoS\({}_{2}\) and then 5 nm Ni and 50 nm Au were deposited by the electron-beam evaporator, followed by the lift-off process. Figure 1d presents the height profiles measured using AFM (Supporting Information, Figure S4). The thicknesses of pristine MoS\({}_{2}\) and WSe\({}_{2}\) and the heterojunction were 5 nm. This shows a consistent thickness. It has been shown that the ambipolar transfer characteristic of the pristine WSe\({}_{2}\) field-effect transistor (FET) was converted into unipolar p-type behavior after laser-scanning treatment according to our former works [5; 27]. The laser-oxidized product WO\({}_{\rm x}\) is responsible for the p-type doping of WSe\({}_{2}\)[5; 27]. The laser-scanned method can also be used for p-doping MoTe\({}_{2}\)[26; 31]. In order to establish the proper power and exposure time for p-doping, multilayer WSe\({}_{2}\) samples with laser treatment of different power and exposure time were measured (Supporting Information Figure S2 and Table S1). As shown in Figure S2c, the PL peak intensity of WSe\({}_{2}\) decreased with exposure time, suggesting effective oxidation originating from serious damage to the structure. As shown in Figure S2e, the PL peak intensity of WSe\({}_{2}\) increased with the 633 nm laser-scanned power, indicating a photo-induced enhancement in the electron concentration. After elaborative comparison, the 633 nm laser (laser power of 4 mW, exposure time of 0.75 s) was chosen for controllably p-doping the WSe\({}_{2}\). Figure 2a shows the Raman spectra of multilayer WSe\({}_{2}\), MoS\({}_{2}\) and the overlapping region between laser-scanned WSe\({}_{2}\) and MoS\({}_{2}\). The corresponding fitting curves are shown in Supporting Information Figure S5; one strong peak (248 cm\({}^{-1}\)) and four weak peaks (306, 357, 371 and 392 cm\({}^{-1}\)) can be observed from WSe\({}_{2}\)'s Raman spectra, and two strong peaks (381 and 406 cm\({}^{-1}\)) can be observed from MoS\({}_{2}\)'s Raman spectra. The Raman spectra of the overlapping area show three strong peaks at 248 cm\({}^{-1}\), 381 and 406 cm\({}^{-1}\), which means that the laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) heterostructure is successfully conducted [6]. However, the Raman peak intensities of the MoS\({}_{2}\) are reduced for the laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction. These intensity reductions can be attributed to ultrafast charge transfers due to strong interfacial coupling in these structures between laser-scanned WSe\({}_{2}\) and MoS\({}_{2}\)[32; 33]. Furthermore, we conducted the PL spectra of pristine WSe\({}_{2}\), MoS\({}_{2}\) and laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction, as shown in Figure 2b. Two prominent emission peaks of MoS\({}_{2}\) are observed at 1.82 and 1.95 eV, corresponding to emissions originating from the indirect transition and A exciton, respectively. PL of WSe\({}_{2}\) also exhibited an indirect transition and A exciton at 1.44 and 1.61 eV, respectively. In the laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction, the PL peak intensity of MoS\({}_{2}\) dropped, which indicates charge  
transfer between p-doped WSe\({}_{2}\) and MoS\({}_{2}\)[32]. In order to reveal the effect of doping, we fabricated a new WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction with undoped and doped regions.