Controllable Water Vapor Assisted Chemical Vapor Transport Synthesis of WS₂–MoS₂ Heterostructure

The vapor phase synthesis of two-dimensional transition-metal dichalcogenides (MX2) and their heterostructures is often poorly reproducible and sensitive to uncontrolled environmental humidity. It was recently realized that water vapor can play important roles in the growth of MX2 by reacting with MX2 at high temperature to form volatile metal oxyhydroxide (MOx(OH)y) and hydrogen chalcogenides (H2X) that dramatically change the growth processes. Here we report the controllable synthesis of WS2, MoS2, and their heterostructures using water-assisted chemical vapor transport (CVT). The water vapor can be tunably delivered by thermal dehydration of calcium sulfate dihydrate (CaSO4·2H2O) solid precursor, which not only provides a much lower vapor pressure baseline and a wider tunable range than liquid water but also can be readily integrated into a chemical vapor deposition process. This allows controlled growth of monolayer, multilayer, and spiral nanoplates of WS2, as well as the lateral epitaxial growth on the edge of MX2, and more reproducible growth of large area WS2–MoS2 heterostructures. Raman and photoluminescence spectral mappings confirm the various types of WS2–MoS2 heterostructures. These results reveal insights into the growth mechanisms of MX2 and provide a general approach to the controllable growth of other metal chalcogenides.