The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS(2)

We report on preparation dependent properties observed in monolayer WS(2) samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO(2), sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS(2)) exhibit distinctly different optical properties than transferred WS(2) (x-WS(2)). In the case of CVD growth on Si/SiO(2), following transfer to fresh Si/SiO(2) 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-WS(2) is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS(2) for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS(2). 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.