Systematic Study of Oxygen Vacancy Tunable Transport Properties of Few-Layer MoO3-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 (MoO3-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 MoO3-x down to three layers thick, the most 2D-like MoO3-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 MoO3-x into the 2D family, as well as highlight the promise of MoO3-x as a functional, tunable electronic material.