Jillian Dempsey (University of North Carolina)

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Location: B01 McCourtney Hall

Abstract:  As-synthesized colloidal semiconductor nanocrystals are capped by surface-coordinating ligands that act to passivate surface sites and suspend the colloidal particles in solution. Displacement of these ligands from the nanocrystal surface can be promoted by reaction with other ligands, chelating agents, or addition of charge carriers. By monitoring the ligand displacement reactions using UV-Vis absorption, 1H NMR, and FTIR spectroscopies, along with inductively-coupled plasma mass spectrometry, insight to the nanocrystal electronic structure and native surface composition can be gleaned. For instance, upon thermochemical doping of CdSe with chemical reductants, we find that a portion of the native anionic ligands are liberated from the nanocrystal surfaces and observe that added electrons reduce surface-based Cd and Se ions before filling the conduction band. These data reveal a new mechanism for charge balance in thermochemical doping reactions wherein anionic oleate ligands are liberated from the surface as free carriers are trapped in a subpopulation of cadmium-based localized surface states. Separately, the addition of chelating diamine ligands to PbS strips native Z-type lead oleate ligands. Quantifying the binding isotherms of these displacement reactions allows us to map the relative subpopulations of Z-type ligands and thereby elucidating the native surface topology. These indirect imaging studies support computational predictions that PbS nanocrystals have a size dependent shape, transitioning from an octahedron to cuboctahedron. In a third example, we show that PbS nanocrystal react with thiol-terminated ligands through three mechanisms. When small amounts of UDT are added to PbS NCs, Z-type Pb(oleate)2 ligands are displaced along with binding of L-type thiol ligands to the NC surface. When higher equivalents of thiol are added, L-type binding of thiol ligands not associated with Z-type displacement occurs along with an X-type exchange between native oleates and thiolates.

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