Surface-enhanced Raman scattering of SnO$_{2}$ bulk material and   colloidal solutions
  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. 
Enza Fazio, Fortunato Neri, Salvatore Savasta, Salvatore Spadaro,   Sebastiano Trusso
cond-mat.mtrl-sci