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

• Main Authors: Yuki Yamada, Takeshi Yoshimura, Atsushi Ashida, Norifumi Fujimura, Daisuke Kiriya
• Format: Article
• Language: English
• Published: MDPI AG 2021-04-01
• Series: Applied Sciences
• Subjects: transition metal dichalcogenide (TMDC); photoluminescence; superacid; molecular treatment; bilayer; molybdenum disulfide (MoS₂)
• Online Access: https://www.mdpi.com/2076-3417/11/8/3530

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.