The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2

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.