Sol-gel routes for metal chalcogenide nanoparticle assembly

This dissertation research is focused on (1) the synthesis of metal chalcogenide nanostructures that are heavier analogs of metal sulfides, (2) the study of the optical properties of chalcogenide gels as a function of dimensionality, (3) optimizing the luminescence properties and (4) discerning the mechanism of gelation.;The successful synthesis of CdSe aerogels by oxidative aggregation of primary nanoparticles followed by supercritical drying is reported. The resultant materials are mesoporous, with an interconnected network of colloidal nanoparticles, and exhibit surface areas up to 224 m2/g and average pore diameters in the range of 16-32 nm. Optical bandgap and photoluminescence studies show that the as-prepared aerogels retain the quantum confined optical properties of the nanoparticle building blocks. Furthermore, the specific optical characteristics of the aerogel can be further modified by surface ligand exchange at the wet gel stage, without destroying the gel network.;The effects of dimensionality on the quantum confinement of chalcogenide nanostructures were studied by synthesizing a series of xerogels (CdS, ZnS, PbS, CdSe) and comparing their optical properties to corresponding aerogels/nanoparticles. Xerogels exhibit optical band-edges that are intermediate between bulk precipitates and the more porous wet gels and aerogels, suggesting the extent of quantum confinement in such colloidal networks is intimately related to the fractal dimensionality. This decrease in bandgap correlates with an increase in densification of the networks as demonstrated by surface areas and porositimetric analyses.;CdSe/ZnS quantum dot networks with high luminosity have been successfully prepared by employing CdSe/ZnS nanoparticles in the sol-gel transformation reactions. In contrast to analogous poorly emissive networks prepared from "naked" CdSe, the extent of quantum confinement in the 3-D connected composite is a function of the core particle size and shows little dependence on the network density.;The mechanism for chalcogenide gelation has been elucidated from chemical and spectroscopic data. Interparticle bonding in CdSe nanostructures is found to be largely due to covalent Se-Se bonds. Hence, chalcogenide gels can be re-dispersed to a nanocrystal sol by adding a variety of reducing agents. A mechanism for the CdSe nanoparticle assembly/disassembly is proposed and the steps necessary for stabilizing CdSe nanocrystals are presented.