Tunable Electronic and Optical Properties of Hydrogenated MoO₃ Films by Metal–Acidic Ionic Liquid Treatment

A MoO₃ film with the main phase of orthorhombic α-phase was deposited on a glass substrate by pulsed laser deposition. Metal–acid ionic liquid treatment caused the film to change from transparent to blue, while also reducing its bandgap and resistance values. These properties can be controlled by using different metals with lower work functions than MoO₃, different acids, and varying treatment times. In this study, the film treated with Al-acid ionic liquid for 15 min showed the lowest bandgap of 2.61 eV and the smallest resistance of 9.48 × 10³ Ω. This treatment resulted in a modulation of 0.59 eV in the bandgap and a more than 5 orders of magnitude increase in MoO₃ conductivity. Both variable range hopping and activated behavior may be present throughout the entire temperature range in this study. The treating process causes an expansion of the crystal lattice, and the reduction of the Mo element. The manipulation of microstructures and properties may be attributed to the hydrogenation of the MoO₃ thin films, driven by the difference in work function between the metal and oxide. This study expands the use of acids from inorganic acids to acidic ionic liquids, and enriches the method of metal–acid treatment. The concept of controllable hydrogen-doping through metal–acidic ionic liquid treatment has significant implications for the design of multifunctional materials in various applications.