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

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Stoichiometry of molybdenum oxide, MoO x , 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 Mo 5+ reduced oxidation state, while in fully stoichiometric films only Mo 6+ 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 MoO x was effectively controlled by simply modulating the Argon deposition pressure. This tunability offers significant potential for tailoring the optical and electronic properties of MoO x for applications in energy storage, photovoltaic and photonic devices. • Control of molybdenum oxide composition by direct sputtering of a stoichiometric target. • Demonstration of strong correlations between stoichiometry and optical properties. • O/Mo ratio varied between 2.6 and 3.0 only by tuning Ar sputtering pressure. • Mo 6+ oxidation state predominant in all the samples, while Mo 5+ contribution decreasing as Ar pressure increased. • The increased IR absorption contribution correlates with a higher density of oxygen vacancies, confirming XPS measurements.