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.