  We report on preparation dependent properties observed in monolayer WS2
samples synthesized via chemical vapor deposition (CVD) on a variety of common
substrates (Si/SiO2, sapphire, fused silica) as well as samples that were
transferred from the growth substrate onto a new substrate. The as-grown CVD
materials (as-WS2) exhibit distinctly different optical properties than
transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following
transfer to fresh Si/SiO2 there is a ~50 meV shift of the ground state exciton
to higher emission energy in both photoluminescence emission and optical
reflection. This shift is indicative of a reduction in tensile strain by
~0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between
neutral and charged exciton by exposure to moderate laser power, while such
optical control is absent in as-WS2 for all growth substrates investigated.
Finally, we observe dramatically different laser power-dependent behavior for
as-grown and transferred WS2. These results demonstrate a strong sensitivity to
sample preparation that is important for both a fundamental understanding of
these novel materials as well as reliable reproduction of device properties.
