Correlative imaging of exciton distribution in monolayer of transition metal dichalcogenides

Monolayers of transition metal dichalcogenides (TMD) are direct-gap semiconductor and exhibit strong room-temperature photoluminescence (PL) in the visible and near-infrared range. The nature of the emission is commonly attributed to the formation of exciton states at room temperature. Monolayers of TMD with large surface area can nowadays be grown by chemical vapor deposition (CVD) on arbitrary substrates. However, spatial nonuniformities in the PL emission intensity and wavelength are commonly observed. Here, we combine high-resolution confocal optical microscopy with PL and Raman spectroscopy on CVD-grown WS2 monolayers. We observe an increased and a red-shifted PL emission from the center towards the edge of the individual grains. This red-shifted emission is due to the formation of charged and defect-bound excitons. Our multi-imaging analysis, by correlating PL and Raman emission, allow us to identify presence of local defects, low degree of crystallinity and oxidation.