  Two-dimensional (2D) 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 we develop a controllable CVD process
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.
