A model of the NbOx ≈ 1 nanocrystals tiling a Nb(1 1 0) surface annealed in UHV

The layer covering a (1 1 0) face of Nb crystal annealed at 1500–2200 K in UHV has been studied by photoemission spectroscopy with synchrotron radiation and scanning tunneling microscopy (STM). This overlayer resulting from segregation of the oxygen dissolved in Nb bulk presents a complex structure with several kinds of Nb oxides. Analysis of photoemission spectra of 3d Nb core levels reveals that: (i) the oxide stoichiometry is mainly ranging from NbOx ≈ 1.2 to NbOx ≈ 0.8; (ii) the oxide thickness corresponds to 1.4 ± 0.3 monolayer of Nb atoms. As observed in STM images, the structure of this oxide overlayer consists of a quasi-periodic arrangement of NbOx ≈ 1 nanocrystals. Each nanocrystal is characterized by a prominent stick formed by 10 ± 1 egg-shaped bumps aligned along a close-packed NbO〈1 1 0〉 direction. These bumps are related to Nb atoms adsorbed as a close-packed row on the outmost (1 1 1) oxygen plane of a NbO nanocrystal, and to chemical bonds involving these Nb atoms and the underlying atoms. The origin of such an atomic structure is discussed.