  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 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, multilayers, 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.
