Ambient hole doping of single-layer WS₂ supported on SiO₂ substrates

Understanding charge transfer (CT) between two-dimensional (2D) crystals and molecules is crucial for the fundamental research and future applications of 2D material systems. Despite recent surge on this research subject, however, the roles of the interface formed by supporting substrates and their surfaces have not been investigated. In particular, ambient water may form a medium on hydrophilic surfaces for electrochemical reactions with oxygen in the air. In this work, we investigated the CT interaction of single-layer WS₂ with the proposed redox couple of O₂/H₂O serving as a hole dopant. Photoluminescence (PL) and Raman spectra, obtained for WS₂/SiO₂ in a gas-controlled optical cell, showed that hole doping is mediated by O₂ and H₂O. Much reduced CT on hexagonal boron nitride substrates suggested a conclusive role of interface between WS₂ and hydrophilic SiO₂ substrate. Wide-field PL imaging was exploited to investigate spatial inhomogeneity and anisotropy. We will further discuss detailed photophysics of charged excitons (trions) and possible roles of defects in the observed CT reactions.