September 24, 2008
FRONTIERS IN MATERIALS SCIENCE
VINSE COLLOQUIUM SERIES
Dr. Sanat K. Kumar
Department of Chemical Engineering
"Modeling the Growth of Surfactant Decorated Nanocrystals"
Abstract. Nanoscale materials have qualitatively different properties compared to their macroscopic counterparts. For example, their photoluminescence changes dramatically with crystal size due to quantum confinement effects. To obtain materials with desired properties it is necessary that we possess the ability to synthesize monodisperse crystals of desired size. While there has been considerable success in synthesizing monodisperse nanocrystals mediated by surfactants, a theoretical understanding of such syntheses remains elusive.
We have developed a mean-field theory which can calculate the equilibrium sizes of surfactant-decorated nanocystals. This theory forms the basis from which we study the kinetics of nanocrystal growth. We have focused on two systems: gold and zinc sulfide. The time-evolution of the synthesis of thiol-capped ZnS nanocrystals using a novel “digestive ripening” procedure leads to monodisperse 1.8nm diameter crystals. Theoretically, we show that these results reflect that the binding of the thioglycerol to the zinc is weaker than the zinc binding energy in the nanocrystal. The calculated nanocrystal free energy density using this fact shows two minima, at ~1.8nm and for infinite crystals: we suggest that the experiments correspond to driving the system to the metastable nanoscale minimum. Apparently, tuning ligand-crystal interaction provides a means of exquisite size control. Giving credence to this suggestion we show that different crystal-ligand binding energies can allow us to get crystals of any size (going from the nanoscale to macroscale crystals). The case of gold-dodecane thiol is particularly interesting since experiments show that we can get small monodisperse crystals in the presence of excess surfactant, while only large macroscopic crystals results for systems lean in surfactant. The molecular bases for these results will also be discussed.