Size-dependent properties of ${\mathrm{CeO}}_{2-y}$ nanoparticles as studied by Raman scattering

The combined effects of strain and phonon confinement are seen to explain why the Raman peak near $464{\mathrm{cm}}^{-1}$ in ${\mathrm{CeO}}_{2-y}$ nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric (on the low-energy side) as the particle size gets smaller. The increasing lattice constant measured for decreasing particle size explains this Raman shift well. The linewidth change is fairly well explained by the inhomogenous strain broadening associated with the small dispersion in particle size and by phonon confinement. The spectra are also likely to be directly affected by the presence of oxygen vacancies. Comparison of the temperature dependence of the Raman lineshape in the nanoparticles and the bulk shows that phonon coupling is no faster in the nanoparticles, so size-dependent phonon coupling does not contribute to the large nanoparticle peak red shifts and broadening at room temperature. Irreversible thermally induced changes are observed in the Raman peak position of the nanoparticles.