Phonons of clean and metal-modified oxide films: an infrared and HREELS study

Vibrational properties of thin dielectric films may be investigated by infrared spectroscopy and high-resolution electron-energy-loss spectroscopy (HREELS). Infrared absorption by dielectric films occurs near the frequencies of longitudinal optical bulk phonons (Berreman effect); electron energy losses due to Fuchs–Kliewer surface phonons are observed at similar energies. We report a comparative study of the phonon features of an ultra-thin, well-ordered aluminum oxide film on NiAl(1 1 0) by infrared reflection-absorption spectroscopy (IRAS) and HREELS. At least fourteen vibrational modes are identified. While the spectra are similar to those expected from macroscopic dielectric theory, a detailed band assignment will require microscopic lattice dynamical calculations. Metal particles are grown on the alumina film by vapor deposition of palladium, rhodium, iridium, vanadium, and aluminum. Their sizes, as determined by scanning tunneling microscopy (STM), range from few atoms up to several thousand atoms. Infrared spectroscopy serves to study the impact of the admetals on the oxide phonons. Detailed data on the damping, frequency shift, and broadening of the most intense oxide phonon as a function of metal exposure are presented, spanning an exposure range from below 0.01 to several monolayers. Pd induces a red-shift of the phonon mode, while Rh, Ir, V, and Al initially cause a blue-shift. At larger exposures, most metals induce a shift to lower frequencies. Adsorbed CO and ethene give rise to further changes of the phonon spectrum. Possible reasons for the observed behavior are discussed.