High resolution electron energy loss spectroscopy of MnO(100) and oxidized MnO(100)

Abstract

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.

Surface composition of MnxCo1−xO solid solutions by X-ray photoelectron and Auger spectroscopies

“…In comparison with literature values, typical 2p 3/2 binding energies of Mn 3+ -containing oxides such as Mn 3 O 4 and Mn 2 O 3 have been reported over the range of 641.2-641.9 eV [13,18,24,26,29] whereas Mn 4+ -containing materials are reported to be between 642.0 and 642.7 eV [18,26,29,30]. High-quality thin fi lms of epitaxially grown MnO 2 tend to have the highest binding energies and very little satellite structure [26] whereas both naturally occurring and synthetic mineral structures are represented by the lower part of the binding energy range.…”

“…The pure MnO trace yields 2p 3/2 and 2p 1/2 binding energies of 640.8 and 658.7 eV, respectively, and weak satellite peaks at 647.0 eV for the 2p 3/2 transition and at 658.7 eV for the 2p 1/2 transition. The binding energies and spectral structure are characteristic of MnO [13,23], which has only very weak, broad satellites in comparison to those observed for the CoO sample. Some enrichment in manganese is apparent for spectra from samples with x bulk = 0.2-0.4, although more dramatic is the broadening and shift in 2p peak energies that occur for the Mn 2p spectra in this bulk concentration range.…”

“…Singlecrystal substrates of MnO(100) and CoO(100) were used as reference samples for the x = 0 and x = 1 extremes of the compositional range. Their preparation and surface characterization are described more fully in [11][12][13].…”

“…The samples tend to be semiconducting due to the tendency of the compound to form slightly nonstoichiometric compositions and no problems were encountered with charging in either of the electron spectroscopies used. Nevertheless, the XP binding energies were calibrated relative to the monoxide O 1s value of 529.4 eV, in agreement with values from MnO and CoO [11][12][13] to correct for contact potential differences between the sample and the spectrometer. No decomposition or other changes were observed in the samples during the course of the data acquisition.…”

“…However, both manganese and cobalt have other readily accessible oxidation states and oxide crystal structures available to them in the solid state [10], raising the question of how similar bulk and surface compositions might be for these solid solutions. For example, Co 3 O 4 forms epitaxially on CoO(100) [11,12] and both Mn 3 O 4 and Mn 2 O 3 form on MnO(100) [13]under very mild, UHV-accessible oxidizing conditions. In addition to phase separation, other mixed-metal manganese-cobalt oxide compositions are possible, as well.…”