## 3 Results and Discussion
### Electrical Characterization  
We further conducted electrical measurement of the device. The sublinear I\({}_{\text{DS}}\) (drain current) \(-\)V\({}_{\text{DS}}\) (drain voltage) characteristics of a p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction transistor in Figure 3a indicate the ohmic contacts at source and drain terminals. The I\({}_{\text{DS}}\)\(-\)V\({}_{\text{C}}\) characteristic of the p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction transistor in Figure 3b shows an on/off of \(\sim\)10\({}^{7}\), which is comparable to that of the best MoS\({}_{2}\) transistors [2; 6; 8; 17; 28; 34]. The gate insulator capacitance, \(C_{g}\), for the 90 nm SiO\({}_{2}\) and 5 nm WSe\({}_{2}\), is calculated to be 3.77 \(\times\) 10\({}^{-8}\) F\(\cdot\)cm\({}^{-2}\), using the following formula:  
\[\frac{1}{C_{g}}=\frac{1}{C_{SiO_{2}}}+\frac{1}{C_{WSe_{2}}} \tag{1}\]  
where \(C_{SiO_{2}}\) = 3.84 \(\times\) 10\({}^{-8}\) F\(\cdot\)cm\({}^{-2}\) is capacitance of 90 nm SiO\({}_{2}\), \(C_{WSe_{2}}\) = 2.07 \(\times\) 10\({}^{-6}\) F\(\cdot\)cm\({}^{-2}\) is capacitance of 5 nm WSe\({}_{2}\) (\(C_{OX}\) = \(\varepsilon_{0}\varepsilon_{OX}\)/\(t_{OX}\); \(\varepsilon_{SiO_{2}}\) = 3.9; \(\varepsilon_{WSe_{2}}\) = 11.7; \(t_{SiO_{2}}\) = 90 nm; \(t_{WSe_{2}}\) = 5 nm; where \(\varepsilon_{0}\) is the vacuum permittivity, \(\varepsilon_{OX}\) is relative permittivity, \(t_{OX}\) is gate  
Figure 1: Phototransistor based on laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) van der Waals heterojunction: (**a**) three-dimensional (3D) schematic diagram of the phototransistor; (**b**) two-dimensional (2D) schematic diagram of the phototransistor’s operation principle; (**c**) optical microscopy image of the phototransistor; (**d**) atomic force microscopy image (AFM) of the phototransistor.  
insulator thickness). The field-effect mobility (\(\mu\)) of the phototransistor is 32.74 cm\({}^{2}\)/V\(\cdot\)s according to the following formula:  
\[\mu=\frac{L\cdot g_{m-max}}{W\cdot C_{g}\cdot V_{DS}} \tag{2}\]  
where \(L\) = 8.95 \(\upmu\)m is channel length of the device, \(W\) = 24.07 \(\upmu\)m is channel width of the device and \(g_{m-max}\) = 1.66 \(\times\) 10\({}^{-5}\) S is maximum transconductance according to the dark transfer curve, \(V_{\rm{DS}}\) = 5 V. We further examined the transfer characteristics of a few-layer WSe\({}_{2}\) FET on the 90 nm SiO\({}_{2}\)/Si substrate, and the I\({}_{\rm{DS}}-\)V\({}_{\rm{G}}\) transfer characteristics before and after laser-scanned process (using 633 nm laser, with laser power of 4 mW, exposure time of 0.75 s and laser spot diameter of 0.61 \(\upmu\)m) are shown in Supporting Information Figure S7. The I\({}_{\rm{DS}}-\)V\({}_{\rm{G}}\) transfer characteristics are transferred from ambipolar to unipolar p-type apparently, which indicates that WSe\({}_{2}\) is effectively p-type doped through the laser-scanned process. This can help to promote a strong built-in electric field between the laser-scanned WSe\({}_{2}\) and MoS\({}_{2}\).  
Figure 3: Electrical characteristics of laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) field-effect transistor (FET). (**a**) Output characteristics of laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) FET. (**b**) Transfer characteristics of laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) FET. I\({}_{\rm{DS}}\) (drain current), \(V_{\rm{DS}}\) (drain voltage), \(V_{\rm{G}}\) (gate voltage).  
Figure 2: Raman spectra characterization of the laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction. (**a**) Raman spectra of WSe\({}_{2}\), MoS\({}_{2}\) and laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction. (**b**) photoluminescence (PL) spectra of WSe\({}_{2}\), MoS\({}_{2}\) and laser-scanned WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction.