## 2 Materials and Methods  
Bulk MoS\({}_{2}\) was purchased from Structure Probe, Inc. (SPI Supplies). The bilayer flakes were prepared on Si wafer with 260 nm thermally grown SiO\({}_{2}\) (Silicon Valley Microelectronics, Inc.) via the mechanical exfoliation process. A weight (\(\sim\)3.0 kg cm\({}^{-2}\)) was used for the exfoliation process (Figures 1 and 3-5), pressing it over an exfoliation tape. In the case shown in Figure 6, the pressing weight was not applied. The TFSI molecule was purchased from Sigma-Aldrich, and acetonitrile (Kanto Chemical Co., Inc. Japan) was used as the solvent for the TFSI treatment. The TFSI solution (2 mg mL\({}^{-1}\)) was prepared under an Ar atmosphere in a glove box (UNICO LTd. Japan, UL-1000A), and the TFSI treatment procedure was carried out in an ambient environment. The illustrative image of the process is shown in Figure 2. The UV irradiation process was performed for 5 min each (Figures 2-5). The exfoliated samples on the Si wafer were immersed in the solution at room temperature for 10 min in the normal treatment process (Figure 4). In the case shown in Figure 6, the total UV irradiation times were 15 min (5 min \(\times\) 3) for the initial UV irradiation procedure and 15 min (5 min \(\times\) 3) for the last repeated UV irradiation process to sufficiently modulate the bilayer sample. The lamp house (high-pressure mercury lamp, 170 mW cm\({}^{-2}\)) was placed 2 cm (-4 cm from the lamp) above the Si wafer. PL spectra were measured using a LabRAM HR800 equipped with an EMCCD camera (HORIBA Scientific, Japan). The wavelength was 532 nm for all measurements. The power of the incident beam was \(\sim\)2.7 W cm\({}^{-2}\) for measurements of PL and Raman spectra in Figure 6b and 27 W cm\({}^{-2}\) for other Raman spectra and PL mapping in Figure 1. Atomic force microscope (AFM) images were obtained using the dynamic force mode in SII instruments.  
Figure 3: (**a**) Optical microscope image of the bilayer MoS\({}_{2}\). The labels are the measurement regions (regions of interest (ROIs) 1–4) in Figure 3c–f. (**b**) Illustrative image of the modulation to the bilayer sample via the UV irradiation process. The process induces the bilayer to a monolayer-like state. (**c–f**) PL spectra for the as-exfoliated (original, blue) and the UV-irradiated (orange) samples in (**c**) ROI 1, (**d**) ROI 2, (**e**) ROI 3, and (**f**) ROI 4 in Figure 3a.  
Figure 2: Illustrative image of the process applied to the bilayer MoS\({}_{2}\).  
Figure 4: **(a)** Optical microscope image of the bilayer MoS\({}_{2}\) after the TTSI treatment followed by the UV irradiation. (**b–e**) PL spectra for the TTSI treatment (green) and the second (re-)UV irradiation (red) in (**b**) ROI 1, (**c**) ROI 2, (**d**) ROI 3, and (**e**) ROI 4 in Figure 4a. The values in Figure 4b–e are the magnitude of the PL enhancement in the sample over the as-exfoliated (original) sample.  
Figure 5: **(a,b)** Atomic force microscope (AFM) images for **(a)** the as-exfoliated (original) bilayer MoS\({}_{2}\) exfoliated with the weight and (**b**) the UV-irradiated bilayer MoS\({}_{2}\). (**c**) Raman spectra for the original (blue) and the UV-irradiated (orange) bilayer MoS\({}_{2}\) in ROI 2 in Figure 3. (**d**) Illustrative image of a plausible state of the bilayer MoS\({}_{2}\). The top layer would show a monolayer-like state due to the weak interaction between the top and bottom layers by a sort of misalignment of each layer.