Strong Photoluminescence Enhancement from Bilayer Molybdenum Disulfide via the Combination of UV Irradiation and Superacid Molecular Treatment

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 MoS2, one of the representative TMDCs, to a direct band gap nature and enhances the photoluminescence (PL) intensity of bilayer MoS2 dramatically. The procedure combines UV irradiation with superacid molecular treatment on bilayer MoS2. UV irradiation induces the conversion of the PL property with an indirect band gap to a direct band gap situation in bilayer MoS2 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 MoS2 by a factor of 700×. 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 MoS2 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.