Photoluminescence Enhancement in WS2 Nanosheets Passivated with Oxygen Ions: Implications for Selective Area Doping

The dominant defect types in chemical-vapor-deposited (CVD) tungsten disulfide (WS2) monolayers (ML) were regulated through mask-assisted scattered-oxygen-ion (O+) implantation. A shadow mask allowed for two distinctive implantation regions: directly bombarded and mask-shaded. Upon direct implantation, photoluminescence (PL) was universally suppressed, whereas in the mask-shaded region, PL was enhanced by up to 500% at low doses before suppression at doses of >3 × 1013 ions/cm2. We verified that the introduction of scattered O+ ions and low-density structural atomic defects are the two prerequisites for PL enhancement by replacing O+ ions with C+ ions and eliminating the involvement of physisorbed gases or laser treatment. Density functional theory calculations were carried out, suggesting a possible mechanism for the vacancy-induced dangling bonds in WS2. Sulfur- or tungsten-related vacancies create in-gap deep trap states, hindering electron–hole recombination. Scattered-oxygen treatment passivates these sulfur vacancies by effectively eliminating these in-gap nonradiative pathways. In addition, it further increases the transition probability by creating more tungsten-dominated states near the conduction band edge through charge transfer. This work demonstrates a facile and successful single-step method to passivate sulfur vacancies with scattered oxygen ions. It has the potential to heal the PL quenching that originated from the intrinsic high defect density in a CVD-grown WS2 ML.