Electron energy loss spectroscopic investigation of Ni metal and NiO before and after surface reduction by Ar + bombardment

Electron energy loss spectra (ELS) have been obtained from Ni metal, NiO and NiO after surface reduction by 2 keV Ar + bombardment. Two predominant loss features in the ELS spectra obtained from Ni metal are assigned as the surface and bulk plasmon excitations, and numerous other features arising from single electron transitions from both the bulk and surface Ni 3d bands to higher-lying conduction bands are also present. The surface and bulk ELS features have been unambiguously identified in this study by varying the primary beam energy. The peak assignments are based on a multitude of reported data on the electronic structure of Ni and NiO obtained using several different techniques, ranging from theoretical calculations to optical measurements and previous experimental ELS data. The ELS spectra obtained from NiO are markedly different than the Ni metal spectra as expected. The fact that the NiO spectral features do not vary appreciably with primary beam energy E p indicates that the surface electronic structure is similar to that of the bulk solid. Although most of the loss features from NiO can be attributed to interband transitions originating from the O 2p and Ni 3d states, the major peak centered at a loss energy of about 22.0 eV may be due to a collective excitation of the valence electrons. As expected, the ELS spectra obtained from the sputtered NiO surface exhibit characteristic features from both metallic Ni and NiO, with the metallic features becoming more prominent with increasing sputtering time. The most distinct change in the ELS spectra following reduction is the growth of the surface plasmon peak at 6.0 eV due to the presence of metallic Ni in the near-surface region. The loss features characteristic of bulk Ni metal are not as pronounced in the spectra obtained from the reduced NiO. These observations indicate that the metallic Ni formed during Ar + bombardment is confined to the very near-surface region, and they also demonstrate that ELS is sensitive to differences in the properties of thin metallic films or clusters and bulk Ni metal. The present results demonstrate that ELS can be used to distinguish between metallic nickel and NiO on the sputtered NiO surface.