Optical properties of citrate-stabilized CdS nanoparticles

Citrate-stabilized CdS nanoparticles of size 4 nm are obtained by varying the sulfide:citrate ion concentration in a simple aqueous synthesis method. The optical absorption and photoluminescence properties of the nanoparticles are studied. The size of the crystallites is found to be less affected by sulfide:citrate ratio. At lower concentrations of S²⁻, trap state emission is favoured and at higher concentrations excitonic transition is predominant as shown by optical absorption and photoluminescence spectra. Effective surface capping and optimum concentration of S²⁻ leads to the quenching of surface-defect-related emission. Increase in citrate ion concentration is found to increase the intensity of photoluminescence band arising from trap state emission revealing the role of sulfide:citrate ratio on surface modification of CdS nanocrystals. The nanoparticles are hexagonal as shown by the X-ray diffraction and selected area electron diffraction pattern.