EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part I: Plasmon and Near-Edges Fine Structure

Abstract

A comprehensive electron energy-loss spectroscopy study of niobium (Nb) and stable Nb-oxide phases (NbO, NbO₂, Nb₂O₅) was carried out. In this work (Part I), the plasmons and energy-loss near-edge structures (ELNES) of all relevant Nb edges (Nb-N_2,3, Nb-M_4,5, Nb-M_2,3, Nb-M₁, and Nb-L_2,3) up to energy losses of about 2600 eV and the O-K edge are analyzed with respect to achieving characteristic fingerprints of Nb in different formal oxidation states (0 for metallic Nb, +2 for NbO, +4 for NbO₂, and +5 for Nb₂O₅). Chemical shifts of the Nb-N_2,3, Nb-M_4,5, Nb-M_2,3, and Nb-L_2,3 edges are extracted from the spectra that amount to about 4 eV as the oxidation state increases from 0 for Nb to +5 for Nb₂O₅. Four different microscopes, including a 200 keV ZEISS Libra with monochromator, were used. The corresponding wide range of experimental parameters with respect to the primary electron energy, convergence, and collection semi-angles as well as energy resolution allows an assessment of the influence of the experimental setup on the ELNES of the different edges. Finally, the intensity of the Nb-L_2,3 white-line edges is correlated with niobium 4d-state occupancy in the different reference materials.