The oxide/metal interface induced by surface segregation of oxygen during the annealing of a Nb
single crystal in UHV has been studied by photoemission spectroscopy with synchrotron radiation.
With 260 and 350 eV photons, four well-resolved peaks A, B, C, D are found in spectra within the
200–210 eV range of binding energy. One couple of peaks ~A and C! is associated with 3d5/2 and
3d3/2 core levels of Nb atoms in the metal while the other one ~B and D!, shifted by 1.4 eV when
compared to A and C, corresponds to 3d levels of oxidized Nb atoms. The metal peak A at 202.3
eV is formed by three 3d5/2 components: a peak due to a metallic state ~202.1 eV! and two
components shifted by 0.2 and 0.5 eV, which are attributed to Nb6O and Nb4O compounds due to
interstitial atoms of oxygen, respectively. The estimated concentration of the interstitial oxygen
atoms in the nanometer-thick metal skin underlying the NbO/Nb interface corresponds to a large
enrichment when compared to the one in the Nb bulk. |
