###### Abstract  
A direct band gap nature in semiconducting materials makes them useful for optical devices due to the strong absorption of photons and their luminescence properties. Monolayer transition metal dichalcogenides (TMDCs) have received significant attention as direct band gap semiconductors and a platform for optical applications and physics. However, bilayer or thicker layered samples exhibit an indirect band gap. Here, we propose a method that converts the indirect band gap nature of bilayer MoS\({}_{2}\), one of the representative TMDCs, to a direct band gap nature and enhances the photoluminescence (PL) intensity of bilayer MoS\({}_{2}\) dramatically. The procedure combines UV irradiation with superacid molecular treatment on bilayer MoS\({}_{2}\). UV irradiation induces the conversion of the PL property with an indirect band gap to a direct band gap situation in bilayer MoS\({}_{2}\) when the interaction between the top and bottom layers is weakened by a sort of misalignment between them. Furthermore, the additional post-superacid treatment dramatically enhances the PL intensity of bilayer MoS\({}_{2}\) by a factor of 700\(\times\). However, this procedure is not effective for a conventional bilayer sample, which shows no PL enhancement. From these results, the separated top layer would show a strong PL from the superacid treatment. The monolayer-like top layer is physically separated from the substrate by the intermediate bottom MoS\({}_{2}\) layer, and this situation would be preferable for achieving a strong PL intensity. This finding will be useful for controlling the optoelectronic properties of thick TMDCs and the demonstration of high-performance optoelectronic devices. t 4 March 2021 7 April 2021 15 April coexistence of indirect and direct band gaps, respectively [18]. The PL intensity shown in the bilayer is usually very low compared with ML because of the dominant indirect band gap character. Recently, chemical treatment on ML TMDCs has been reported, and a large enhancement of the PL intensity was demonstrated [19; 20; 21; 22; 23]. In the case of ML MoS\({}_{2}\), treatment by oxidative chemicals such as tetracyanoquinodimethane [19] and quinone [20] derivatives, acids such as sulfuric acid [24], and hydrogen peroxide [25] show a large enhancement of the PL intensity. One of the strongest modulating agents of PL intensity in ML MoS\({}_{2}\) is a superacid molecule, bis(trifluoromethylomethane)sulfonimide (TFSI), for both mechanically exfoliated and chemical-vapor-deposited ML flakes [26; 27; 28; 29; 30]. The PL enhancement mechanism is still under debate, but the existence of TFSI molecules on ML TMDC would be essential for achieving the strong PL intensity. Recently, we reported the consistent substantial PL enhancement of ML MoS\({}_{2}\) via a combination of TFSI treatment and additional UV irradiation [31]. In this work, we found an unusually strong PL enhancement from bilayer MoS\({}_{2}\) by combining the TFSI treatment with UV irradiation (Figures 1 and 2). The PL enhancement is over several hundred and some of them over \(700\times\) from the as-exfoliated bilayer flakes, and the shape of the PL spectrum dramatically changes to a sort of direct band gap. The plausible mechanism would be that the separation into a monolayer-like state would happen under the process. Furthermore, the indirect-to-direct band gap transition was only observed at the specific bilayer sample which would be like twisted bilayers. This work would facilitate controlling the interlayer interaction in TMDCs for modulating electronic structures to achieve atomically thin optoelectronic devices and physics platforms. Figure 1: (**a,b**) Illustrative image of (**a**) the bilayer MoS\({}_{2}\) and (**b**) the bis(trifluoromethylomethane)sulfonimide (TFSI) treatment for the UV-irradiated bilayer MoS\({}_{2}\). (**c,d**) Photoluminescence (PL) mapping images for (**c**) the as-exfoliated bilayer MoS\({}_{2}\) and (**d**) the TFSI-treated sample for the UV-irradiated bilayer MoS\({}_{2}\).