Stoichiometry modulation of molybdenum oxide thin films by direct sputtering for energy applications

Stoichiometry of molybdenum oxide, MoOx, thin films was controlled by simply modulating the argon working pressure during the non-reactive sputtering deposition. Rutherford Backscattering Spectrometry revealed that the O/Mo ratio increased between 2.6 and 3.0 as the argon pressure increased. X-Ray Photoemission Spectroscopy (XPS) analyses pointed out that the sub-stoichiometry led to the formation of oxygen vacancies, closely related to the presence of Mo5+ reduced oxidation state, while in fully stoichiometric films only Mo6+ oxidation state appeared. The strong correlation between composition and optical properties was demonstrated through optical absorption measurements. Sub-stoichiometric films exhibited a narrower bandgap and significant infrared absorption at 1.55 eV, attributed to increased disorder and a higher number of defects, such as oxygen vacancies. Finally, by merging the XPS and optical absorption data, we demonstrated that the band structure of MoOx was effectively controlled by simply modulating the Argon deposition pressure. This tunability offers significant potential for tailoring the optical and electronic properties of MoOx for applications in energy storage, photovoltaic and photonic devices.