Raman scattering characterization of nanopowders and nanowires (rods)

We study nanograin size confinement effects, the effect of the increase of local temperature and strain, and substitutional and nonstoichiometry effects on the Raman spectra in Ce(Ba,Nd,Gd)O₂ nanopowders obtained by self-propagating, room-temperature synthesis, and silicon nanowires obtained by the electrochemical etching process. The local temperature was monitored by measuring the Stokes/anti-Stokes peak ratio. We found large wavenumber shifts, up to 10 (30) cm^-1, and broadenings, up to 40 (20) cm^-1, of the first-order Raman-active modes of Ce(M)O₂ (n-Si), which we attribute to confinement and strain effects (laser heating). The phonon softening and phonon linewidth are calculated using a phenomenological model, which takes into account disorder effects through the breakdown of the k = 0 Raman-scattering selection rule, and also anharmonicity, which is incorporated through the three- and four-phonon anharmonic processes. Very good agreement with experimental data is obtained for the calculated spectra of CeO₂ with nanograin sizes of about 7 nm, taking into account particle size distribution, as well as an increase in the anisotropy constants with respect to those of bulk materials.