Addressing the effects of gas adsorption on monolayers beyond charge population analysis: the case of WS2

The optoelectronic properties of two-dimensional (2D) materials can be significantly influenced by charge transfer resulting from surface molecular adsorption. One noteworthy example is observed in WS2 monolayers, where the behavior undergoes an anomalous change when exposed to air, primarily due to the adsorption of oxygen molecules. While the acceptor nature of O2 is widely acknowledged as the underlying cause, the precise electron transfer mechanism remains in need of a comprehensive explanation at the atomistic level. Going beyond conventional charge population analysis, we develop an approach describing the process of molecular adsorption and surface charge transfer that relies on the formalism commonly adopted for charged defects in semiconductors. This method clearly identifies two key factors contributing to electron transfer upon O2 physisorption: the presence of sulphur vacancies and the intrinsic n-type nature of WS2. This approach provides an effective and general scheme to characterize the surface charge transfer in 2D materials exposed to a gas atmosphere.