Carrier Doping via Adsorption of Atmospheric Molecules in Monolayer MoS ₂

Monolayer molybdenum disulfide (1L-MoS2), owing to its two-dimensional (2D) structure, is highly sensitive to surrounding chemical species, including atmospheric gases; this significantly hinders its practical applications. Charge carrier doping by oxygen and water molecule adsorption on 1L-MoS2 was comprehensively investigated by precisely controlling partial pressures over a wide range and applying a gate voltage using a field-effect transistor (FET) structure in a high-vacuum chamber. The type (electron or hole) and rate of charge carrier doping depended on the relative partial pressures of oxygen and water vapor. Applying a gate voltage during gas exposure modulated both the doping rate and the type of charge carriers by shifting the Fermi energy of 1L-MoS2 relative to the ambient redox potential. These findings suggest that oxygen adsorption doping proceeds via a water-mediated electrochemical mechanism similar to that in graphene. Photoluminescence measurements suggested that sulfur vacancies generated during high-temperature annealing act as adsorption sites for oxygen molecules in 1L-MoS2. A comprehensive phase diagram that predicts the doping behavior under various oxygen–water conditions is presented, offering valuable insights into controlling the electronic properties of 2D materials for applications in electronics, sensors, and catalysts.