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Under mildly oxidizing ultrahigh vacuum conditions, it is possible to form on top of CoO(100) single crystal substrates, thin films that have higher oxygen content but that preserve the overall symmetry of the CoO(100) low-energy electron diffraction pattern. X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy (HREELS) data indicate that the epitaxial film grown on CoO(100) at 625 K and 5×10−7 Torr is Co3O4-like in both oxygen content and XP/HREEL spectroscopic characteristics. Both materials are closest packed in lattice oxygen, with the mismatch of bulk O2−–O2− distances of approximately 5%. However, the Co3O4 is only able to grow to a thickness of approximately 5 Å before the oxidation process halts. It is proposed that the orientation of Co3O4 that forms most readily on the CoO(100) surface does not present a thermodynamically stable orientation of the bulk Co3O4 substrate but is that which grows under the constraint of the best CoO(100)/Co3O4 epitaxial arrangement. While the mismatch in lattice parameters may in part be to blame for the limitation of higher oxide thickness, thicker oxide films have been grown under conditions with significantly larger mismatch.

Single crystal MnO͑100͒ substrates can be selectively oxidized to produce Mn 2 O 3 -and Mn 3 O 4 -like surfaces under mild oxidation/reduction conditions readily accessed under ultrahigh vacuum ͑UHV͒. MnO͑100͒ yields a characteristic Mn 2p x-ray photoelectron spectroscopy ͑XPS͒ satellite structure and appropriate O/Mn concentrations from O 1s/Mn 2p XPS intensity ratios. Its high resolution electron energy loss ͑HREEL͒ spectrum shows a series of Fuchs-Kliewer multiple phonon excitations with a single loss energy of 70.9 meV, characteristic of the cubic manganese monoxide structure. However, the HREEL spectral ͑HREELS͒ background is high and the phonons are not as well resolved as those typically observed on comparable metal monoxides. Annealing the MnO͑100͒ substrate at 625 K and 5ϫ10 Ϫ7 Torr O 2 slowly forms Mn 2 O 3 , as indicated by O 1s and Mn 2p XPS, and does so without significantly altering the symmetry of the MnO͑100͒ low energy electron diffraction pattern. The MnO͑100͒-Mn 2 O 3 surface can be selectively reduced to Mn 3 O 4 -like composition by heating under UHV to 775 K and to MnO͑100͒ at 1000 K. HREEL spectra for the UHV annealed surfaces are well-resolved, and for the MnO͑100͒-Mn 3 O 4 substrate a second fundamental phonon loss is observed at 55.6 meV as a result of the lower symmetry of the Mn 3 O 4 spinel structure. The UHV-annealed MnO͑100͒ surface appears to be more highly ordered since its HREELS phonon loss peaks are better resolved. It is also somewhat reduced, however, resulting in a less intense phonon spectrum with a fundamental loss energy of only 65.1 meV.

Despite the relevance to a variety of materials applications, the electronic and bonding properties of spinel transition metal oxides are not well established. We report here the slow oxidation of CoO(100) to Co3O4, studied by photoemission (UPS and XPS), low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS) with the aim of elucidating the valence band electronic structure of the Co3O4 spinel. The original Mott insulator picture of the parent CoO substrate has been revised in recent times, after careful analyses and extensive debate, to the more detailed charge-transfer insulator model which includes some admixture of oxygen 2p levels in the 3d-derived valence band. No equivalent band structure analysis has been performed on the spinel oxides, perhaps in part because of the greater complexity of the 56-atom unit cell with two different cation lattice sites and oxidation states. In this study, we determine the valence band structure of the spinel oxide and address the question of whether Co3O4 can be modeled as a charge-transfer insulator in analogy with its closely related rocksalt substrate.