## 4 Conclusions  
In this study, we developed an ultrasensitive phototransistor based on a laser-induced p-doped WSe\({}_{2}\)/MoS\({}_{2}\) van der Waals heterojunction. A simple, efficient and selective p-doping approach was used for WSe\({}_{2}\). The interface between p-doped WSe\({}_{2}\) and MoS\({}_{2}\) can separate the photogenerated electron-hole pairs and inject electrons into the MoS\({}_{2}\) channel layer under laser illumination due to the strong built-in electric field. The p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction phototransistor shows a high photoresponsivity of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Device Structure** & \(R\) (\(\mathbf{A}\cdot\mathbf{W}^{-1}\)) & \(D^{\star}\) **(Jones)** & \
\begin{tabular}{c} **Rise/Decay** \\ **Time** \\ \end{tabular} &
\begin{tabular}{c} **Spot Size** \\ **(Diameter)** \\ \end{tabular} & **Reference** \\ \hline Laser-induced p-type doped WSe\({}_{2}\)/MoS\({}_{2}\) (5 nm) & \(1.28\times 10^{5}\) & \(7.17\times 10^{13}\) & - & 1038 nm & This work \\ \hline Out-of-plane WSe\({}_{2}\)/MoS\({}_{2}\) (9 nm) & 2700 & \(5\times 10^{11}\) & \(\sim\)10 ms & 872 nm & [6] \\ \hline Out-of-plane MoS\({}_{2}\) p-n homojunction (7 nm) & \(7\times 10^{4}\) & \(3.5\times 10^{14}\) & \(\sim\)10 ms & 2 mm & [2] \\ \hline Monolayer MoS\({}_{2}\) (0.7 nm) & 0.0075 & - & 50/50 ms & 10 μm & [28] \\ \hline Multilayer MoS\({}_{2}\) (10 nm) & 0.18 & \(3\times 10^{10}\) & - & - & [8] \\ \hline Laser-induced p-type doped WSe\({}_{2}\) & 0.8 & - & 139/39 ms & 1038 nm & [5] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performance comparison between phototransistors based on 2D-material semiconductors. Figure 6: Time-dependent photoresponse. (**a**) Time-dependent photoresponse of the phototransistor achieved using an optical chopper with a frequency of 1/30 Hz, for V\({}_{\rm G}=-10\) to 0 V, V\({}_{\rm DS}=5\) V and incident power of 164 nW. (**b**) Time-dependent photoresponse of the device achieved using an optical chopper with a frequency of 1/30 Hz, for V\({}_{\rm G}=0\) to 10 V, V\({}_{\rm DS}=5\) V and incident power of 164 nW. (**c**) Response time as a function of V\({}_{\rm G}\). \(\sim\)1.28 \(\times\) 10\({}^{5}\) A\(\cdot\)W\({}^{-1}\) and a high specific detectivity of \(\sim\)7.17 \(\times\) 10\({}^{13}\) [PERSON]. Moreover, our phototransistor can operate in two modes. In the first mode, the photocurrent of our phototransistor decreases as the time increases under positive gate voltage, which means that our device can work in bright backgrounds. In the second mode, the photocurrent of our phototransistor increases as the time increases under negative gate voltage, indicating that our device can work in dark backgrounds. The capability of two work modes in our device indicates its potential application in bioinspired in-sensor visual adaptations [42]. By introducing trap states into the interface between p-doped WSe\({}_{2}\) and MoS\({}_{2}\), our device can be used for optoelectronic memory, benefiting from the large memory window. Together, these open a new avenue for future photoelectronic devices based on emerging 2D van der Waals heterojunctions with optimum device engineering.