Systematic Study of Oxygen Vacancy Tunable Transport Properties of Few-Layer MoO_{3− x} Enabled by Vapor-Based Synthesis

Bulk and nanoscale molybdenum trioxide (MoO₃) has shown impressive technologically relevant properties, but deeper investigation into 2D MoO₃ has been prevented by the lack of reliable vapor-based synthesis and doping techniques. Herein, the successful synthesis of high-quality, few-layer MoO₃ down to bilayer thickness via physical vapor deposition is reported. The electronic structure of MoO₃ can be strongly modified by introducing oxygen substoichiometry (MoO_{3− x}), which introduces gap states and increases conductivity. A dose-controlled electron irradiation technique to introduce oxygen vacancies into the few-layer MoO₃ structure is presented, thereby adding n-type doping. By combining in situ transport with core-loss and monochromated low-loss scanning transmission electron microscopy–electron energy-loss spectroscopy studies, a detailed structure–property relationship is developed between Mo-oxidation state and resistance. Transport properties are reported for MoO_{3− x} down to three layers thick, the most 2D-like MoO_{3− x} transport hitherto reported. Combining these results with density functional theory calculations, a radiolysis-based mechanism for the irradiation-induced oxygen vacancy introduction is developed, including insights into favorable configurations of oxygen defects. These systematic studies represent an important step forward in bringing few-layer MoO₃ and MoO_{3− x} into the 2D family, as well as highlight the promise of MoO_{3− x} as a functional, tunable electronic material.