Telluriding monolayer MoS and WS via alkali metal scooter.

The conversion of chalcogen atoms to other types in transition metal dichalcogenides has significant advantages for tuning bandgaps and constructing in-plane heterojunctions; however, difficulty arises from the conversion of sulfur or selenium to tellurium atoms owing to the low decomposition temperature of tellurides. Here, we propose the use of sodium for converting monolayer molybdenum disulfide (MoS) to molybdenum ditelluride (MoTe) under Te-rich vapors. Sodium easily anchors tellurium and reduces the exchange barrier energy by scooting the tellurium to replace sulfur. The conversion was initiated at the edges and grain boundaries of MoS, followed by complete conversion in the entire region. By controlling sodium concentration and reaction temperature of monolayer MoS, we tailored various phases such as semiconducting 2H-MoTe, metallic 1T'-MoTe, and 2H-MoS Te alloys. This concept was further extended to WS. A high valley polarization of ~37% in circularly polarized photoluminescence was obtained in the monolayer WS Te alloy at room temperature.