Oxygen vacancy induced electronic transport modulation in defective transition metal oxide MoO3−x

Amorphous molybdenum trioxide (MoO3) thin films represent a promising yet underexplored class of disordered transition metal oxides, where polaronic charge transport, defect chemistry, and morphology interplay to dictate electrical performance. This thesis investigates the formation, dynamics, and stability of small polarons in amorphous MoO3 films, elucidating their coupling with oxygen vacancies, mixed Mo oxidation states (Mo6+/Mo5+/Mo4+), and metal diffusion under varying electrical and environmental conditions. Through systematic electrical measurements, including temperature-dependent conductivity, clear signatures of polaron hopping emerge, characterized by thermally activated transport with activation energies tied to local lattice distortions and defect densities. Complementing these, structural and chemical analyses via scanning electron microscopy (SEM) reveal nanoscale morphology evolution, such as porosity and roughness, influencing percolation pathways; Raman spectroscopy maps vibrational modes indicative of Mo–O bond disorder and phase segregation; and X-ray photoelectron spectroscopy (XPS) quantifies oxygen vacancy concentrations and valence state distributions, linking reductions in Mo6+ to polaron stabilization and enhanced conductivity. Metal diffusion, probed through depth-profiled XPS and correlated with ion intercalation, highlights how structural flexibility in the amorphous network facilitates reversible cation insertion while modulating polaron mobility. By integrating these studies, the work establishes a unified framework for polaron physics in amorphous MoO3, demonstrating how processing-induced defects and morphology govern charge transport mechanisms—from insulating to conductive regimes—and enable tailored electrochemical responses. These insights provide fundamental design principles for optimizing amorphous TMOs in energy storage, electrochromics, and catalysis, advancing their deployment in next-generation thin-film devices.