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This publication describes how to distinguish under-oxidized and over-oxidized phases of the metal-insulator transition material NbO2 by employing a combination of the Nb 3d and O 1s core-level and valence-band spectra. Niobium oxides (NbOx) are grown under various conditions by molecular beam epitaxy on several different substrates, mostly (111)-oriented strontium titanate (SrTiO3), in order to obtain phase-pure NbO2. The films are characterized by in situ reflection high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy to determine crystallinity, lattice constants, and epitaxial relationships. Niobium metal oxidation states and film stoichiometry are determined with in situ X-ray photoelectron spectroscopy. Asymmetric Nb 3d core-level spectral components are observed in phase-pure NbO2, with a binding energy for Nb 3d5/2 of 206 eV. The effect of substrate type, growth temperature, and oxygen pressure on the film composition is also described.
Hadamek et al. (2016) studied this question.