SIZE CONTROL AND PROPERTIES OF THIOL CAPPED CDS PARTICLES

The growth of CdS particles was radiolytically initiated by the release of HS - ions from a thiol, via its reaction with hydrated electrons, in Cd2+ containing solutions. The size of particles thus obtained could be controlled by the concentration ratio of the solution components Cd2+/H2S/RSH. The higher is the concentration of Cd2+ and RSH at a given H2S concentration the smaller are the particles obtained. A mechanism to rationalize this size control is suggested to involve competition between complexation of thiolate ions with Cd2+ ions at the surface of a core CdS particle and growth of the particles, primarily by ion-cluster addition. The formation of surface thiol-Cd2+ complexes was indicated by conductivity measurements and by energy dispersive X-ray analysis of the particles. The particles thus obtained were found to be strongly fluorescent at sizes larger than ca 16 angstrom. This emission results from recombination of trapped charge carriers. Generation of electrons and holes, by flash photolysis, and injection of electrons into the particles leads to blue shift in the absorption spectra of the particles. This shift of the exciton band to higher energies is shown to result from screening effects on the exciton binding energy rather than from band filling effects.