Nonlinear optical properties in CdS/silica nanocomposites

Porous CdO/silica gel glasses were prepared through a sol-gel process. The nano-sized CdS was obtained by a gas reaction. For CdO/SiO2, the average pore radii were examined by mercury porosimetry. X-ray diffraction (XRD) was used to study the structural properties of the embedded CdS particles. Their crystallite sizes were estimated using Scherrer's equation. The band gap energy of embedded CdS was evaluated by optical absorption measurement. A shift in the band gap energy, with respect to the bulk materials, was observed in CdS/SiO2 nanocomposites. The amount of this shift increased with decreasing crystallite size. Based on the absorption spectra, the estimated effective mass of exciton was 0.315m0. The photoluminescence (PL) spectrum also showed a blue-shift characteristic near the energy band gap, but the shifted values were smaller than those of the absorption edge. Furthermore, a strong PL emission peak, observed in the range between 500 and 700 nm, also demonstrated a blue shift characteristic. The mechanism of this PL is regarded to be a re-combination process from surface states to the valence band, which resulted from the dangling bonds of the porous silica matrix, in the interband of the energy band gap of nanometre CdS particles. The 3rd nonlinear optical susceptibility was found to be 4.5×10-11 esu (CW laser) and 2.3×10-11 esu (pulse laser), respectively.