Influence of the Cd/S Molar Ratio on the Optical and Structural Properties of Nanocrystalline CdS Thin Films

Nanocrystalline CdS thin films have been deposited using precursors with different thiourea concentration onto glass substrates by sol-gel spin coating method. The crystalline nature of the films has been observed to be strongly dependent on thiourea concentration and annealing temperature. The CdS films are found to be nanocrystalline in nature with hexagonal structure. The grain size is found to be in the range of 7.6 to 11.5 nm depending on the thiourea concentration and annealing temperature. The high resolution transmission electron microscopy (HRTEM) results of the CdS films prepared using cadmium to thiourea molar ratio of 0.3:0.3 indicate the formation of nanocrystalline CdS with grain size of 5 nm. Fourier transform infrared (FTIR) analysis shows the absorption bands corresponding to Cd and S. The optical study carried out to determine the band gap of the nanostructured CdS thin films shows a strong blue shift. The band gap energy has been observed to lie in the range of 3.97 to 3.62 eV following closely the quantum confinement dependence of energy on crystallite radius. The dependence of band gap of the CdS films on the annealing temperature and thiourea concentration has also been studied. The photoluminescence (PL) spectra display two main emission peaks corresponding to the blue and green emissions of CdS.