Controllable structure-engineered janus and alloy polymorphic monolayer transition metal dichalcogenides by plasma-assisted selenization process toward high-yield and wafer-scale production

Two-dimensional (2D) materials with an asymmetric structure like Janus transition metal dichalcogenides (TMDCs) exhibit intriguing electronic and optical properties, which can be used for numerous novel electronic and optoelectronic devices application. However, despite tremendous efforts, the synthesis technology of Janus TMDs has not been fully established, especially for high-yield and wafer-scale production. Here, we demonstrate the synthesis of Janus MoSSe by a top-down method for high-yield and wafer-scale production. In particular, we replace the top S atom of monolayer (ML) molybdenum disulfide (MoS2) with Se by controlled vertical plasma-assisted selenization process (PASP). We first synthesized ML Janus MoSSe flakes through PASP. A set of microscopies, spectroscopies and electrical measurements showed that low synthesis temperature (200 °C) leads to the formation of ML Janus MoSSe flake while high temperatures (400 °C and 600 °C) lead to the formation of polymorphic alloy MoSSe flakes. Furthermore, we show that Janus WSSe can likewise be obtained by PASP, indicating the universality of the process. To demonstrate the feasibility of PASP for wafer-scale production of Janus MoSSe, we perform the exact same process but with continuous ML MoS2 on 2-in. sapphire. Our synthesis approach provides a new scalable route to synthesize Janus TMDs toward future electronics.