## 3 Results and Discussion
Therefore, the sequence of the TFSI treatment and the UV irradiation does not matter for achieving the PL enhancement; rather, the combination of both processes is the essence of obtaining the strong PL in the bilayer MoS\({}_{2}\) flakes. Figure 5 shows atomic force microscope (AFM) images for the as-exfoliated (original) and UV-irradiated bilayer MoS\({}_{2}\). The morphologies of bilayer MoS\({}_{2}\) maintain the structure via the UV irradiation process. Raman spectra for the sample maintain typical E\({}_{\text{2g}}\) and A\({}_{\text{1g}}\) modes for lateral and vertical vibration signals of MoS\({}_{2}\). However, the peak separation of the two signals is less than 20 cm\({}^{-1}\). In previous work, it was found that the peak separation of E\({}_{\text{2g}}\) and A\({}_{\text{1g}}\) modes is strongly dependent on the interlayer stacking of the bilayer, and the peak separation reduces if the bilayer is twisted [33; 34; 35]. We hypothesize that the bilayer sample might twist under the exfoliation process, because we pressed the exfoliation tape onto a substrate with a weight (\(\sim\)3.0 kg cm\({}^{-2}\)) when exfoliating the flake. MoS\({}_{2}\) is well known as a lubricant, and sliding between layers would happen with low friction [36]. This twisting would require more study in order to completely understand the situation. Importantly, the Raman signal is maintained after the UV irradiation process over the bilayer flake (Figure 5c and Figure S2), even though the character of the PL spectra converted from an indirect to a direct band gap nature, as shown in Figure 3.  
The unusual event shown in the indirect-to-direct band gap conversion for the bilayer MoS\({}_{2}\) would be explained by the interlayer separation between the top and bottom layers (Figure 5d). Previously, a systematic study of twisted bilayer MoS\({}_{2}\) showed different PL and Raman signals [33; 34; 35]. By twisting the top and bottom layers, the peak separation of E\({}_{\text{2g}}\) and A\({}_{\text{1g}}\) reduces, as observed in our case. In addition, the peak position of the PL signal corresponding to the indirect band gap upshifts about 0.1 eV. Furthermore, the interlayer separation is also dependent on the interlayer stacking of the bilayer. If the bilayer is twisted, weak interaction between the top and bottom layers is expected. This scenario requires more study, but it would be a plausible explanation for the strong PL intensity from the bilayer MoS\({}_{2}\). Another probable scenario to explain our observations would be etching the top layer and thinning it to a ML of MoS\({}_{2}\) via UV irradiation, because a laser-thinning process has been reported to reduce the thickness of MoS\({}_{2}\)[37; 38; 39]. However, the morphology and color of the sample are maintained in the process, as shown in Figure 5. It also would be hard to imagine the uniform etching of the whole region of the flake; therefore, we think the structure is maintained in the process. To further confirm the process with a conventional, non-twisted bilayer MoS\({}_{2}\). We prepared bilayer MoS\({}_{2}\) without using the weight simply by contacting bulk MoS\({}_{2}\) with a Si/SiO\({}_{2}\) wafer and exfoliating a bilayer flake (Figure 6a). The Raman spectrum showed E\({}_{\text{2g}}\) and A\({}_{\text{1g}}\) signals with a peak separation of 21.6 cm\({}^{-1}\), corresponding to a typical (non-twisted) bilayer MoS\({}_{2}\) (Figure 6b,c) [19; 40; 41]. The PL measurement for the bilayer MoS\({}_{2}\) showed an indirect band gap around 1.5 eV and a direct band gap nature around 1.86 eV, respectively. The indirect band gap obtained without the weight showed a downshift from the bilayer signal exfoliated via pressing with the weight. These PL and Raman tendencies indicate a non-twisted bilayer MoS\({}_{2}\) via the exfoliation process without pressing with the weight. Then, the non-twisted bilayer flake was applied with the whole process of UV irradiation, TFSI treatment, and supplemental UV irradiation.