Niobium Oxide Films with Variable Stoichiometry: Structure, Morphology, and Ultrafast Dynamics

Niobium oxide can be stabilized in three distinct stoichiometries, each exhibiting unique physicochemical properties relevant to various technological applications. This study presents a novel procedure for fabricating niobium oxide films and tuning their stoichiometry among the three most stable oxide phases. Starting with a magnetron-sputtered film predominantly composed of Nb₂O₅, its structure and stoichiometry are optimized through thermal treatment in an O₂/N₂ flux. A vacuum reduction treatment transforms the as-grown film into the NbO phase, which can then be reoxidized under controlled oxygen partial pressure to achieve the NbO₂ phase. The films are characterized in terms of surface composition using X-ray photoemission spectroscopy, structure through X-ray diffraction, optical properties via UV-vis spectrophotometry, and morphology using scanning electron microscopy. Additionally, we show that X-ray absorption near-edge spectroscopy at the Nb K-edge, performed with X-ray free-electron laser radiation, can provide insights into the electronic structure and subsurface stoichiometry of the films. The ultrafast mechanisms underlying photoinduced processes in NbO₂ are also discussed.