Photoinduced doping and photoluminescence signature in an exfoliated WS₂ monolayer semiconductor

Following graphene, atomically thin two-dimensional transition metal dichalcogenides (2D-TMDs) are quickly emerging as a new multidisciplinary frontier across condensed matter physics, materials science and inorganic chemistry. Compared with graphene, the optical and optoelectronic properties of 2D-TMD materials are more attractive largely due to the nature of their direct band gap. In this article, we show an interesting demonstration of the photoinduced doping effect in a mechanically-exfoliated high-quality tungsten disulfide (WS₂) monolayer semiconductor. By utilizing a focused laser beam and increasing its intensity, we successfully observed a photoinduced doping effect, indicated by the gradual tuning of dominant light emission from a single narrow emission band peaking at 2.017 eV (Peak 1) to a broad asymmetric emission band (Peak 2) eventually located at around 1.955 eV at room temperature. Moreover, the peak position of Peak 2 shows a distinct red shift dependence on the excitation intensity, predicted by the band gap renormalization theory due to the heavy doping. Justified from their spectral features and excitation intensity dependence, the narrow emission band is ascribed to the fundamental band edge free exciton transition, whereas the broad asymmetric one is ascribed to the localized state ensemble induced by photo doping.