Differential growth and photoluminescence of ZnS nanocrystals with variation of surfactant molecules

Growth of colloidal ZnS nanoparticles in aqueous solution has been extensively investigated in presence of two water-soluble thiols, cysteine and mercaptoethanol. In addition to controlling the particle size by duration of heating, an effective control over growth rate (dr/dt) was also obtained just by altering the functional groups of thiol-surfactants, even without changing heating rate or temperature. The growth rate was found to vary linearly with the average radius of growing nanoparticles. Higher growth rate for cysteine-capped ZnS NPs was observed which could be attributed to greater specific surface energy at the solvent–NP interface as compared to mercaptoethanol capped ones. Photoluminescence of these nanoparticles was found to be influenced by the concentration and nature of the surfactant molecules. Maximum PLQY of 2.3% and 2.0% was obtained for mercaptpethanol and cysteine capped NPs, respectively at an optimum Zn2+:surfactant molar ratio, 1:1.5. For given particle size, mercaptoethanol-capped ZnS NPs show greater PLQY as compared to cysteine-capped ones. The lower PLQY of cysteine-capped ZnS NPs is attributed to the sulfur vacancy defects, which was not found in case of mercaptoethanol-capped ZnS nanoparticles.