Influence of Cu doping and post-heat treatment on the microstructure, optical properties and photoluminescence features of sol-gel derived nanostructured CdS thin films

Cu doped CdS nanocrystalline thin films were prepared by the sol-gel dip-coating method incorporated in polyethyleneglycol matrix. After heat treatment in air at 250 and 350 °C, the prepared thin films were investigated by studying their structural, morphological, compositional, optical (linear and nonlinear) and photoluminescence properties. X-ray diffraction (XRD) results suggest that the samples are polycrystalline in nature with hexagonal crystal structure and the average grain size is in the range of 15-19 nm. The surface morphology of the films was examined by scanning electron microscopy (SEM). The results showed that the films consist of nanocrystalline grains included in the clusters with uniform coverage of the substrate surface. To examine the chemical composition of the films, the X-ray photoelectron spectra (XPS) of the composite films were measured. It was found that the transmittance and the band gap decreased totally as the Cu concentration/annealing temperature increased. The refractive index of the films was studied and the related dispersion was discussed in terms of the Wemple-Didomenico single oscillator model. The third-order nonlinear polarizability of the films was estimated through a semi-empirical relation based on single oscillator model and the results showed that the prepared films are proper to use as optical switch. For photoluminescence (PL) study, 200 and 365 nm excitation wavelengths were examined and compared together. The intensities of emission peaks for the prepared thin films varied with Cu doping/annealing temperature and the behaviors of different band peaks were discussed with giving reasons.