Raman scattering from low frequency phonons confined in CeO2 nanoparticles

CeO2 nanopowder samples synthesized by self-propagating room temperature method were investigated by Raman spectroscopy. The phonon modes observed in the low frequency region (omega < 70 cm(-1)) can be well described by the elastic continuum model, assuming that nanoparticles are of perfect spherical shape and isotropic. Both rigid boundary and stress-free surface cases were analyzed. The calculated vibrational frequency dependencies on the particle diameter were used to identify the modes: (l=2, n=0), (l=0, n=0) and (l=0, n=1). Using the facts that mode frequencies scale inversely with particle dimension (omega similar to 1/D) and that mode-radiation coupling coefficient for this type of vibrations is 1/omega, the correspondence between the particle size distribution and the Raman intensity was made. Particle diameter values ranging from 7 to 8 run for the rigid boundary condition give the best description of the experimental spectra. This coincides well with the average particle diameter value of 8 nm acquired from the phonon confinement model.