## 3 Results and Discussion
### Cryogenic Photoluminescence Characterization  
In order to elucidate the defect-driven effects more clearly, a set of low-temperature micro-PL experiments were also carried out on another area-selectively irradiated ML-MoS2 flake displayed in the OM image of Figure 5a. As shown in Figure 5b, the map of \(I_{\text{Total}}\) of the ambient PL acquired on the ML-MoS2 flake after the FLI process shows  
Figure 4: N-AES analyses on the ML-MoS2 irradiated area selectively using a focused laser beam: (**a**) A set of representative n-AES spectra in the derivative form of \(E(dN(E)/dE)\) vs. \(E\) where \(E\) and \(N\) are the kinetic energy and the number of detected Auger electrons, respectively. These spectra were measured successively by moving the focused e-beam along the white dashed line in Figure 3c from the origin 0 to the spot of \(d=\) \(\sim\)6.41 μm across the irradiated region. (**b**) The variations in \(I_{\text{A+A}^{-}}\) and the peak-to-peak intensity of S LMM Auger signal, \(I_{\text{PP-S}}\). (**c**) The data plot of \(I_{\text{A+A}^{-}}\) vs. \(I_{\text{PP-S}}\) normalized to the value at \(d=0\). To match the two sets of data set for the same \(d\) values, \(I_{\text{PP-S}}\) corresponding to the \(d\) values for the \(I_{\text{A+A}^{-}}\) data are given by linear interpolation. Note that the red-dashed curve is drawn only for guidance. (**d**) The changes in normalized peak-to-peak intensities, \(I_{\text{PP}}\)s, of S LMM (150.0 eV) and Mo MNN (186.5 eV) Auger signals with \(d\). For a comparison, (**a,b**) include the data measured from the un-irradiated region of pristine ML-MoS2. the local enhancement of PL, similar to the previous results. The five positions a-e in Figure 5b were measured as the ML-MoS2 sample was loaded in the cryostat. In Figure 5c, the two pairs of PL spectra were measured in positions a (almost pristine region) and d (close to the irradiation spot) at RT in air and vacuum after gentle annealing at -400 K, respectively. In a vacuum, both the two positions showed very low PL intensity with similar PL characteristics since the A radiation was suppressed in the absence of the interplay between S vacancies and O2 and H2O molecules in the air, as also observed in the previous studies [7; 26; 32; 37]. However, once the ML-MoS2 was exposed to air, both two positions showed the augmentation of PL intensity but by different amplitudes. The variation in PL modulation in terms of \(I_{\text{A+A-}}\) was observed to be much large in position d than in position a by 2.5 times. The reason why the un-irradiated region also showed the modulation of PL as environment conditions would be the existence of thermodynamically native point defects such as S vacancies [7; 25]. These results imply that the defect engineering by the FLI process may be available for enhancing the air gas sensing capability of ML-MoS2 as well as tuning its electronic state and optical properties. Figure 5: Low-temperature micro-PL characterization of the area-selectively irradiated ML-MoS2: (**a**) An OM image of the ML-MoS2 flake, including the red dashed circle, which indicates the spot for the selective FLI process on the ML region. (**b**) The map of \(I_{\text{Total}}\) acquired after the FLI process. (**c**) Two pairs of PL spectra measured in the positions of a (\(d=0\)) and d (\(d=3\)\(\upmu\)m) at room temperature (RT) in air and vacuum (Vac). (**d**) A series of PL spectra measured at 77 K in vacuum in the five different positions a-e along the white dashed line in (**b**) across the irradiated region. The positions b, c, and e are located at \(d=1,~{}1.5\), and \(4.5\)\(\upmu\)m, respectively. For comparison, the PL spectra measured in position d at RT in (**c**) are also drawn together. (**e**) The variations in \(I_{\text{A+A-}}\) at RT in air and at 77 K in vacuum and the integrated intensity of defect-activated peak (D), \(I_{\text{D}}\), with \(d\). For a comparison, (**e**) includes the ambient \(I_{\text{A+A-}}\) measured from the pristine ML-MoS2 before the FLI process at RT.