Powdered samples of the type Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb, are synthesized over the range 0≤x≤0.5 starting from nitrate solutions of the rare earths. X-ray diffraction and Raman scattering are used to analyze the samples. These compounds, at least in the low doping regime and for strictly trivalent dopants, form solid solutions that maintain the fluorite structure of CeO2 with a change in lattice constant that is approximately proportional to the dopant ionic radius. The single allowed Raman mode, which occurs at 465 cm−1 in pure CeO2, is observed to shift to lower frequency with increasing doping level for all the rare earths. However, after correcting for the Grüneisen shift from the lattice expansion, the frequency shift is actually positive for all the strictly trivalent ions. In addition, the Raman line broadens and becomes asymmetric with a low frequency tail, and a new broad feature appears in the spectrum at ∼570 cm−1. These changes in the Raman spectrum are attributed to O vacancies, which are introduced into the lattice whenever a trivalent RE is substituted for Ce4+. This conclusion is supported by a simple model calculation of the effects of O vacancies on the Raman spectrum. The model uses a Green’s function technique with the vacancies treated as point defects with zero mass.

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Polarized Raman measurements are performed on oriented single crystals of PdO, grown by a vaportransport technique. The allowed Raman modes, 8&g and Eg, are assigned to lines at 651 and 445 cm respectively. Intensityversusexcitation-wavelength profiles for these lines show a strong resonance behavior, the presence of 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 %co,"=2. 423+0.011 and AI =0.071+0.018 eV for the exciton energy and half-width. Both the position and width match well a prominent peak in Im(e) appearing in the PdO optical properties reported by Nilsson and Shivaraman [J. Phys. C 12, 1423Phys. C 12, (1979]. Phonon-dispersion curves and the density of states are calculated with a simple valence-force model incorporating the important bond-stretching and -bending motions. With these calculations and with results from experiments on isotopically substituted Pd' 0, 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 8&g mode appears as a sharp line in the spectrum, very similar to the case for the single Raman-active mode in diamond.

Thin-film oxides of platinum have been prepared by reactive sputtering and characterized primarily by x-ray diffraction (XRD) and Raman scattering. Different phases of the Pt-O system were obtained by adjusting sputtering parameters such as gas composition, deposition rate, and substrate temperature. Optimum conditions for producing crystalline α-PtO2 and PtO were identified. Raman spectra of α-PtO2 films show two sharp lines at 514 and 560 cm−1, in agreement with those from commercially available powders. XRD and high-resolution scanning electron microscopy (HRSEM) analyses indicate that these samples have a poorly crystallized structure with lack of order along the c axis. The Raman spectra from PtO show two broad peaks at 438 and 657 cm−1, close to the lines seen in PdO, which has the same structure as PtO. Estimates from XRD line broadening and HRSEM photographs indicate mean crystallite sizes on the order of 300 Å. 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 show that the predominant carriers are p-type, and ir reflectivity data indicate a carrier concentration of ∼4×1018 cm−3.

We used in situ spectroscopic ellipsometry to measure the photocatalytic activity of titania films on fused silica and glass substrates. Amorphous and anatase TiO2 films with a variety of microstructures were prepared by reactive sputtering and pyrolytic deposition. The titania films were coated with thin, spin-cast films of stearic acid [CH3(CH2)16COOH] to represent an organic contaminant. Photooxidation rates were determined from ellipsometric measurements of the reduction in stearic acid film thickness during exposure to UV irradiation at 313 or 365 nm. The photooxidation rate was found to be proportional to I α, where I is the irradiance. The exponent α correlated with the TiO2 crystallinity, having values of approximately 0.7 and 0.8 for amorphous and anatase films, respectively. The largest photooxidation rate was observed for the pyrolytically deposited anatase sample on which X-ray reflectometry and spectroscopic ellipsometry measurements detected the presence of a low-density TiO2 surface layer. To assess the performance of these films in practical applications, the specimens were exposed to wavelength and irradiance conditions that simulated a solar UV spectrum. The most photocatalytically active sample had a stearic acid film removal rate of 22 nm/h, which would be suitable for self-cleaning window applications.