Disentangling the effects of doping, strain and disorder in monolayer WS₂ by optical spectroscopy

Monolayers of transition metal dichalcogenides (TMdC) are promising candidates for realization of a new generation of optoelectronic devices. The optical properties of these two-dimensional materials, however, vary from flake to flake, or even across individual flakes, and change over time, all of which makes control of the optoelectronic properties challenging. There are many different perturbations that can alter the optical properties, including charge doping, defects, strain, oxidation, adsorbed molecules and water intercalation. Identifying which perturbations are present is usually not straightforward and requires multiple measurements using multiple experimental modalities, which presents barriers when attempting to optimise preparation of these materials. Here, we apply high-resolution photoluminescence and differential reflectance hyperspectral imaging in situ to CVD-grown WS₂ monolayers. By combining these two optical measurements and using a statistical correlation analysis we are able to disentangle three contributions modulating optoelectronic properties of these materials: electron doping, strain and disorder arising from adsorbates, substrate fluctuations and/or defects. In separating these contributions, we also observe that the B-exciton energy is less sensitive to variations in doping density than A-excitons. Our approach is not limited to monolayers of WS₂ and is also applicable to other 2D materials with optical transitions, and the general approach could be extended to other characterization modalities. (paper)