Valence-band electronic structure of Co₃O₄ epitaxy on CoO(100)

Authors

Marjorie Langell, University of Nebraska - LincolnFollow
M.D. Anderson, University of Nebraska - Lincoln
G.A. Carson, University of Nebraska - Lincoln
L. Peng, University of Nebraska - Lincoln
S. Smith, University of Nebraska - Lincoln

Date of this Version

February 1999

Comments

Published by American Physical Society. Phys. Rev. B VOLUME 59, NUMBER 7 (1999). http://prb.aps.org. Copyright © 1999 American Physical Society. Permission to use.

Abstract

Valence-band photoemission studies have been performed on a CoO(100) single crystal as it is slowly oxidized under 1×10^-4 Torr O₂ at 623 K, eventually forming a Co₃O₄ epitaxial film. The most significant changes occur in 3d-related features, with the peak located at the top of the valence band sharpening and shifting to lower binding energies as the spinel oxide forms. Constant initial-state measurements indicate that Co₃O₄ contains admixture from neighboring O 2p in its 3d band, as observed for CoO and other monoxide charge-transfer insulators. Unlike the rocksalt monoxides, which have only a single type of cobalt (Co²⁺) located in octahedral lattice sites, the spinel Co₃O₄ has both octahedral Co³⁺ and tetrahedral Co²⁺ sublattices. The peak at the top of the Co₃O₄ valence band results from the 3d⁶L final state of the octahedral Co³⁺ 3d band. Although some states derived from tetrahedral Co²⁺ may be present at the top of the valence band, the greatest contribution from the tetrahedral Co²⁺ sublattice appears at approximately -3.8 eV, overlapping with O 2p derived features of the spectrum. The Co³⁺ 3d^n-1 satellite is much less intense in Co₃O₄ than in CoO, as is observed for the analogous structure in the cobalt 2p core spectra. An oxygen 2p-derived structure remains fairly constant throughout the oxidation process, with the exception of an intermediate species, which imparts a broad, humplike appearance centered at -5.3 eV to the O 2p region and disappears as oxidation to Co₃O₄4 is completed. The origin of the feature is not clear; however it is most likely due either to an adsorbate or to a defectlike intermediate in the oxidation process.