Spectroscopic investigations of CdS nanoparticles in sol-gel derived polymeric thin films and bulk silica matrices

Absorption, photoluminescence and Raman scattering spectroscopies have been performed on two kinds of SiO₂ bulk matrices and sol-gel polymeric thin films, containing CdS nanoparticles. Waveguiding in the polymeric CdS films has been demonstrated for wavelengths in the visible region. A simple surface energy diagram, including defect levels, has been used to explain the size-selective luminescence of the nanocrystals. The luminescence spectrum is composed of an excitonic recombination band and a broader red-shifted band, imputed to the transition between the surface trap levels. The particle interaction with the silica xerogels appears to enhance these surface states. Resonance Raman scattering shows that the excitons are coupled to LO-phonon modes, but the spectra exhibit no matrix effect on the frequency vibration of the fundamental LO phonon.