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 inter-face 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. (c) 2006 Elsevier B.V. All rights reserved.