Fuchs-Kliewer HREEL Phonon Spectrum of Single Crystal NiO(100) and of Ni(100)/NiO(111) and Ni(100)/NiO(100) Thin Films

The high resolution electron energy loss spectrum (HREELS) of an ionic material is generally dominated by inelastic scattering of the electron with Fuchs-Kliewer phonon modes of the lattice surface. The phonon motion, in which the cations and anions vibrate out of phase to each other, yields a substantial dipole moment and thus an intense phonon spectrum along the specular scattering direction. In NiO, the high symmetry of its fcc rocksalt structure produces a single fundamental Fuchs-Kliewer phonon mode. However, the large cross section for phonon excitation results in multiple scattering events and thus a series of losses at integral multiples of the fundamental phonon energy. We report Fuchs-Kliewer phonon spectra for single crystal NiO(100) and for thin films of nickel oxide grown on Ni(100) single crystal substrates. The NiO(100) orientation is nonpolar, with interpenetrating square arrays of cations and anions, and is the thermodynamically most stable nickel oxide surface. The entire phonon spectrum of the thin film is considerably less than that of the NiO(100) single crystal substrate due to the damping effect of the underlying Ni(100) metal. NiO(100) is produced for Ni(100) substrate temperatures <=300 K under comparable oxygen partial pressures. The NiO(100) orientation is metastable and grows to a limiting thickness of three layers. With a single loss energy of 69.6 meV, the Ni(100)/NiO(1111) Fuchs-Kliewer phonon loss is equal to that of NiO(100) but the multiple losses are more poorly developed despite the comparable thicknesses of the two thin films. The poorer definition of the phonon spectrum is believed to result from a greater amount of disorder in the metastable NiO(111) thin film.