Growth, stability, optical and photoluminescent properties of aqueous colloidal ZnS nanoparticles in relation to surfactant molecular structure.

The interaction between organic molecules and the surface of nanoparticles (NPs) strongly affects the size, properties and applications of surface-modified metal sulfide semiconductor nanocrystals. From this viewpoint, we compared the influence of cationic surfactants with various chain lengths and anionic surfactants with different head groups, as surface modifiers during synthesis of ZnS NPs in aqueous medium. The surfactant adsorbs on the surface of the particles as micelle-like aggregates. These aggregates can form even at the concentration lower than critical micelle concentration (cmc) due to interaction between the polar groups and the NPs. The nature of interaction depends specifically on the surfactant polar group. The ability of surfactant to form the micelle-like aggregates on the surface of the NPs correlates with their cmc. This leads to the fact that the surfactant with longer tail stabilizes the NPs better since its cmc is lower. The adsorption of the surfactant on the NPs also stabilizes them by the change of their charge which is in accordance with the correlation of zeta potential with the particles stability. The energetics of surface states generating interesting photoluminescence (PL) properties in ZnS NPs has been governed by the nature of surfactant molecules. In general, the size, structure, and stability of the ZnS NPs can be controlled by the choice of suitable surfactant.