Metal Chalcogenide Nanocrystal Assembly: From Synthesis To Applications

This dissertation study is focused on (1) the application of an oxidation-induced sol-gel assembly method to formation of CdTe aerogels; (2) extension of existing aerogel compositions and properties by an alternative synthetic approach: cation-exchange reactions; and (3) probing the functionality of CdSe aerogels as optical sensors of Lewis base analytes. Highly porous CdTe aerogels were successfully synthesized by sol-gel assembly of CdTe nanocrystals with thiolate or nonthiolate (phosphine oxide) ligated surfaces by treatment with tetranitromethane. Both phosphine oxide-capped and the thiolate-capped aerogels exhibit similar structure, morphology and porosimetry properties, suggesting that precursor ligation does not have a no major effect on the gelation process and the resultant aerogel properties. These data clearly shows that thiolate capping is not a necessary step in the gelation process; the exposure of reactive sites and the subsequent surface oxidation reaction to form chalcogenide linkages is the key. Cation-exchange reactions in metal chalcogenide gels were explored for the first time. Ag2Se wet gel monoliths were successfully prepared by an ion-exchange reaction of a monolithic CdSe wet gel and converted to an aerogel by drying under supercritical conditions. The newly prepared Ag2Se aerogels adopt the cubic phase and exhibit a typical pearl-necklace morphology and the high surface area characteristics of aerogels. The ion-exchange synthesis route can also be successfully employed for other gel systems, including PbSe and CuSe. The photoluminescence (PL) response of highly porous CdSe aerogels to triethylamine (TEA) is investigated and compared to prior studies on single crystals and nanoparticle-polymer composites. As-prepared CdSe aerogels show significant and reversible enhancement of luminescence intensity upon exposure to TEA relative to the intensity in pure argon carrier gas. The enhancement in the PL response is concentration-dependant and linear over the range of TEA concentration studied (4.7 103 - 75 103 ppm). The sensing response of previously tested samples exhibits saturation behavior that is modeled using Langmuir adsorption isotherms, yielding adsorption equilibrium constants in the range 300 to 380 atm-1. The response is sensitively affected by the surface characteristics of the aerogel; when the wet gels are treated with pyridine prior to aerogel formation, the response to TEA is diminished, and when as-prepared aerogels are heated in a vacuum, no response is observed. Deactivation is attributed to an increase in surface oxide (SeO2) and decrease in surface Cd2+ Lewis acid sites. Sensing runs of approximately one hour have little impact on the morphology or crystallinity of the aerogels, but do result in partial removal of residual thiolate ligands left over from the gelation process