LOW-FREQUENCY RAMAN-SCATTERING FROM SMALL SILVER PARTICLES EMBEDDED IN SIO2 THIN-FILMS

Intense, low-frequency Raman scattering from localized acoustic vibrations of small, spherical Ag particles embedded in SiO2 thin films has been observed. It was found that the Raman peak shifts to higher frequencies as the particle size decreases. For Ag particles smaller than 4 nm, the size dependence of the peak frequency can be well explained by Lamb's theory, which gives vibrational frequencies of a homogeneous elastic body with a spherical form. The Raman scattering observed is relatively strong and believed to be enhanced by the excitation of the surface plasmons localized in the Ag particles; the enhancement mechanism is analogous to the case of surface-enhanced Raman scattering from molecules adsorbed on rough metal surfaces.