Raman and optical studies of the oxides of palladium and platinum.

Abstract

Raman and optical studies are used to investigate oxides of Pd and Pt. Polarized measurements performed on oriented single crystals of PdO grown by a vapor transport technique are used to identify the allowed Raman modes, $B\sb{1g}$ and $E\sb{g}$, as the lines at 651 and 445 cm$\sp{-1}$. Intensity versus excitation wavelength profiles for these lines show a strong resonance behavior, which leads to the appearance of numerous additional features in the spectrum. By fitting these profiles to an exciton model of resonant Raman scattering we obtain $\hbar\omega\sb{ex}=2.423 \pm 0.011$ and $\hbar\Gamma=0.071\pm0.018$ eV for the exciton energy and half-width. Dielectric response functions parallel and perpendicular to the c axis are obtained by analysis of ellipsometric data. Strong anisotropy is observed between $\varepsilon\sb{\Vert}(\omega)$ and $\varepsilon\sb{\perp}(\omega);$ however both exhibit a prominent peak near 2.5 eV that correlates well with the resonance Raman data. Utilizing results from lattice dynamics calculations and experiments on isotopically substituted Pd$\sp{18}$O, we are able to assign most of the additional features in the Raman spectrum to second-order scattering from overtones and combinations or to forbidden LO phonon scattering. The overtone of the $B\sb{1g}$ mode appears as a sharp line in the spectrum, very similar to the case for the single Raman-active mode in diamond. A combination of Raman scattering and ellipsometry is used to study the oxidation kinetics of Pd at temperatures of 300-500$\sp\circ$C. Thin film oxides of platinum are prepared by reactive sputtering and characterized primarily by x-ray diffraction and Raman scattering. Different phases of the PtO system are obtained by adjusting sputtering parameters such as gas composition, deposition rate, and substrate temperature. The Raman spectra from polycrystalline PtO show two broad peaks at 657 and 438 cm$\sp{-1}$, close to those observed in isostructural PdO. The absence of a resonance Raman effect in PtO is shown to be consistent with the measured optical properties. Infrared reflectivity spectra yield two of the three ir-active phonons, and along with scanning tunneling microscopy measurements suggest that PtO is a semiconductor. Thermoelectric measurements identify the predominant carriers as p-type, and ir data indicate a carrier concentration of $\sim1.6\times10\sp{20}$ cm$\sp{-3}$.