  Monolayer WS2 offers great promise for use in optical devices due to its
direct bandgap and high photoluminescence intensity. While fundamental
investigations can be performed on exfoliated material, large-area and high
quality materials are essential for implementation of technological
applications. In this work, we synthesize monolayer WS2 under various
controlled conditions and characterize the films using photoluminescence, Raman
and x-ray photoelectron spectroscopies. We demonstrate that the introduction of
hydrogen to the argon carrier gas dramatically improves the optical quality and
increases the growth area of WS2, resulting in films exhibiting mm2 coverage.
The addition of hydrogen more effectively reduces the WO3 precursor and
protects against oxidative etching of the synthesized monolayers. The
stoichiometric WS2 monolayers synthesized using Ar+H2 carrier gas exhibit
superior optical characteristics, with photoluminescence emission full width
half maximum values below 40 meV and emission intensities nearly an order of
magnitude higher than films synthesized in a pure Ar environment.
