## 3 Results and Discussion
### Field-Effect Transistor Device Characterization  
Lastly, the electrical characterization of FETs was also carried out in conjunction with the FLI experiments to demonstrate the area-selective modulation of electrical properties of ML-MoS\({}_{2}\) channel-based FETs via laser-assisted defect engineering. As visualized in Figure 6a, the four FET devices (Dev1-4) were fabricated on a single partially irradiated ML-MoS\({}_{2}\) with the same channel length (\(L\)) and width (\(W\)) as \(\sim\)0.5 \(\upmu\)m and \(\sim\)3.5 \(\upmu\)m, respectively. While the ML-MoS\({}_{2}\) channel of Dev1 could be regarded to be almost pristine, that of Dev4 was irradiated intensively in the middle. Figure 6b shows a set of the drain current vs. gate voltage (\(I_{\text{drain}}\) vs. \(V_{\text{G}}\)) plots measured from all devices in the range of \(V_{\text{G}}\) from \(-\)100 V to 100 V at the drain-to-source voltage, \(V_{\text{DS}}\), of 50 mV in ambient conditions. Following from the \(I_{\text{drain}}-V_{\text{G}}\) data, the devices behaved like the typical n-type channel FET with the large hysteresis, probably resulting from the surface adsorbates and interface contaminants [52]. The relatively low on-off ratio of \(\sim\)10\({}^{3}\) of the Dev1 indicates that native point defects such as S vacancies may exist even in the un-irradiated channel leading to the p-type doping consistently with the PL results and could be improved if the physisorbed O\({}_{2}\) and H\({}_{2}\)O molecules are removed by vacuum annealing [7,25]. The vacuum annealing effects on the FET devices based on the pristine and laser-irradiated channels can be found elsewhere (Figure S4 and Table S1). By the gentle vacuum annealing at 400 K, the hysteresis was observed to shrink significantly and then expand back slowly in the air. The on-off ratio was shown to be enhanced up to \(>\)10\({}^{4}\) (Figure S4). As shown in Figure 6c, the \(I_{\text{drain}}-V_{\text{SD}}\) curves were also measured at \(V_{\text{G}}=0\) V for monitoring the variation in channel conduction by the FLI process as well as evaluating the performance of metal-ML-MoS\({}_{2}\) contacts. It was clearly observed that the channel conduction decreased as the channels were irradiated more strongly, consistently with the trend of \(I_{\text{drain}}-V_{\text{G}}\) behaviors shown in Figure 6b. Further, the linearity of \(I_{\text{drain}}-V_{\text{DS}}\) plots verify that the Ohmic contacts were formed across the metal-ML-MoS\({}_{2}\) interfaces with negligible Schottky barrier during the FET device operation. The locations of the Dev1-4 are expressed by the \(d\), which is defined as the distance from the origin 0 located in the center line of the Dev1 channel along the dashed line parallel to the channel length, as described in Figure 6a. Then, the irradiation effects on device parameters and properties of the ML-MoS\({}_{2}\) channels are discussed based on the analyses of the \(I_{\text{drain}}-\)\(V_{\text{G}}\) data below. The on-state channel conductivity (\(\sigma\)) for the 2D system is defined as \(\sigma=GL/W\) where \(G\) is the conductance at \(V_{\text{G}}=100\) V [53]. As the irradiation spot was approached from Dev1 to Dev4 (with increasing \(d\)), \(\sigma\) was observed to decrease monotonically by more than one order of magnitude from \(\sim\)0.19 \(\upmu\)S (Dev1) to \(\sim\)0.014 S (Dev4), as shown in Figure 6d. In Figure 6e, the subthreshold voltage (\(V_{\text{th}}\)) was determined as the \(V_{\text{G}}\)-axis intercept value of a linear fit to a \(I_{\text{drain}}-V_{\text{G}}\) curve in the linear regime [54]. The shift of \(V_{\text{th}}\) to the direction of positive \(V_{\text{G}}\) can be seen as the influence of irradiation becomes more significant with increasing \(d\). These results can be interpreted as the p-type doping induced by the FLI process.