  Surface enhanced Raman scattering (SERS) effects on tin dioxide in the form
of bulk material, nanostructured thin films and colloidal solutions were
investigated. Raman spectra are characterized by the three Raman scattering
peaks at 478, 633, and 776 \invcm, assigned to the E$_g$, A$_{1g}$ and B$_{2g}$
modes, typical of rutile SnO$_2$. In presence of the silver nanoparticles, in
addition to the enhancement intensity of some of the fundamental tin dioxide
rutile Raman features, the appearance of a new Raman scattering peak at about
600 cm$^{-1}$ is observed. This spectral features is observed, in presence of
silver nanoparticles, also in other SnO$_2$ based system such as pulsed laser
deposited thin films, with different stoichiometry, and in water colloidal
solutions. The observed SERS effects are explained in terms of electric-field
gradient mechanism that are generated near a metal surface. In particular, the
appearance of a peak near 600 \invcm is attributed to the Raman activation of
the infrared E$_u$ transverse optical (TO) mode.
