Colloidal quantum dots (QDs) have many advantages of heterogeneous and homogeneous photocatalysts for reactions relevant to energy conversion and organic synthesis. This talk focuses on how the surface chemistry of the particle can be tuned to promote selectivity for certain reaction pathways, enantioselectivity of products, and the formation of colloidally stable assemblies for energy and charge funneling, for reactions such as proton and CO2 reduction, and carbon-carbon coupling. Quantitative characterization of the pKa’s of groups within angstroms of the QD surface, using shifts in optical spectra of the QDs, will also be discussed.
Collective motion and relaxation in glass-forming polymeric liquids are investigated in molecular dynamics (MD) simulations of bulk polymer, thin polymer film and nanocomposite materials. The physical arguments underlying the Adam-Gibbs (AG) model of glass-formation are summarized and we then test whether collective particle exchange motion exists as predicted by AG and whether this model can rationalize the highly variable temperature dependence of the structural relaxation time found in this broad family materials. We observe string-like excitations involving particles undergoing collective exchange motion that we directly identify with with the abstract “cooperatively rearranging regions” of AG (see figure below and Refs. 1 - 3 for discussion). Further, we verify this correspondence by quantitatively describing all our relaxation data based on our extended AG model (string model of glass-formation) and the empirically determed high tempearture Arrhenius activation parameters of transition state theory. Finally, we confirm the predicted inverse relation between the mass of the dynamic strings and the fluid configurational entropy that underlies the AG model. The implications of this model extend far beyond glass-forming polymer liquids; we show the same type of collective motion arises in grain boundaries of crystalline materials, the interfaces of crystals, the interfacial dynamics of nanoparticles, superheated crystals, and in biologically relevant materials such lipid membranes and the internal dynamics of proteins and arises in materials ranging from metallic glasses to polymer materials. Evidently, string-like collective motion is relevant to understanding relaxation in diverse condensed materials.