Polycrystalline cubic CdS nanoparticles (NPs) with a crystallite size (DSch) ~3 nm were synthesized by chemical precipitation method at room temperature. Thermal induced structural and morphological changes have been investigated using x-ray diffraction, high-resolution transmission electron microscope, x-ray fluorescence, Fourier transform infrared and Raman spectroscopy. The influence of these changes on optical absorption and photoluminescence (PL) characteristics have been studied. It was found that increasing annealing temperature (Ta), results in structural phase transitions at 300 and 700 °C, increasing DSch and red shift of the optical band gap (Eg) due to the improvement in crystallinity. The photoluminescence emission spectrum of nonstoichiometric CdS (Cd-rich) nanopowder reveals emission bands at 365, 397, and 434 nm. Furthermore, PL spectrum of colloidal solution exhibits additional green and red emission bands at 535, 570 and 622 nm. To explain the mechanism of PL emission in CdS NPs, trapping and radiative recombination levels have been identified and the corresponding energy band diagrams are suggested. Annealing process results in an overall enhancement in PL intensity due to the improvement in crystallinity associated with the reduction of nonradiative surface state defects. Irradiation of CdS NPs colloidal solution at UV irradiation dose <13 J cm−2 leads to the enhancement of PL quantum efficiency and blue shift of Eg(i.e. photo-brightening) due to the decrease in the particle size deduced from Brus equation (D), This behavior is due to UV irradiation effects suchas photopolymerization, the formation of CdSO4 passivation layers due to photooxidation and the reduction in DBrus by photocorrosion process. At UV irradiation dose <13 J cm−2,PL emission intensity continuously enhances without any change in both Eg and D. This
behavior is discussed in terms of electron filling model. Boltzmann curve fitting successfully describes the dependence of both DBrus and Eg on UV irradiation dose.