Reliable Synthesis of Large-Area Monolayer WS₂ Single Crystals, Films, and Heterostructures with Extraordinary Photoluminescence Induced by Water Intercalation

Two-dimensional transition metal dichalcogenides (TMDs) hold great potential for future low-energy optoelectronics owing to their unique electronic, optical, and mechanical properties. Chemical vapor deposition (CVD) is the technique widely used for the synthesis of large-area TMDs. However, due to high sensitivity to the growth environment, reliable synthesis of monolayer TMDs via CVD remains challenging. Here a controllable CVD process is developed for large-area synthesis of monolayer WS₂ crystals, films, and in-plane graphene–WS₂ heterostructures by cleaning the reaction tube with hydrochloric acid, sulfuric acid, and aqua regia. The concise cleaning process can remove the residual contaminates attached to the CVD reaction tube and crucibles, reducing the nucleation density but enhancing the diffusion length of WS₂ species. The photoluminescence (PL) mappings of a WS₂ single crystal and film reveal that the extraordinary PL around the edges of a triangular single crystal is induced by ambient water intercalation at the WS₂–sapphire interface. The extraordinary PL can be controlled by the choice of substrates with different wettabilities.