Low-frequency Raman scattering from small silver particles embedded inSiO2thin films

Intense, low-frequency Raman scattering from localized acoustic vibrations of small, spherical Ag particles embedded in ${\mathrm{SiO}}_2$ 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.