II−VI colloidal semiconductor nanocrystals (NCs),
such as CdSe NCs, are often plagued by efficient nonradiative
recombination processes that severely limit their use in energy-conversion schemes. While these processes are now well-known to
occur at the surface, a full understanding of the exact nature of
surface defects and of their role in deactivating the excited states of
NCs has yet to be established, which is partly due to challenges
associated with the direct probing of the complex and dynamic surface of colloidal NCs. In this dissertation, we report a detailed study of the
surface of cadmium-rich zinc-blende CdSe NCs. The surfaces of
these cadmium-richspecies are characterized by the presence of
cadmium carboxylate complexes (CdX2) that act as Lewis acid (Z-
type) ligands that passivate under-coordinated selenide surface
species. The systematic displacement of CdX2 from the surface by N,N,N′,N′-tetramethylethylene-1,2-diamine (TMEDA) has been studied using a combination of 1H NMR and photoluminescence spectroscopies. We demonstrate the existence of two independent surface sites that differ strikingly in the binding affinity for CdX2 and that are under dynamic equilibrium with each other. A model involving coupled dual equilibria allows a full characterization of the thermodynamics of surface binding (free energy, as well as enthalpic and entropic terms), showing that entropic contributions are responsible for the difference between the two surface sites. Importantly, we demonstrate that cadmium vacancies only lead to important photoluminescence quenching when created on one of the two sites, allowing a complete picture of the surface composition to be drawn where each site is assigned to specific NC facet locale, with CdX2 binding affinity and nonradiative recombinationefficiencies that differ by up to two orders of magnitude.To understand the effect of steric hindrance and types of functional groups in different ligands on X-type ligand exchanges, using NMR, PL and UV-Vis absorption spectroscopy, we studied X-type exchanges on CdSe NCs capped with native carboxylates, with oleic acid, oleyl thiol, benzoic acid and benzenethiol ligands. We discussed the results and occurrence of undesired pathways including displacement of Z-type ligands, and suggested ligand exchange strategies that most likely lead to 100% X-type exchange.The structural complexity of surface of CdS NCs is also discussed in this dissertation. We demonstrate presence of two different sulfur surface defects on CdS NCs with ligand binding equilibrium constants that are two orders of magnitude apart and 20-60% smaller than those of selenium on similar size CdSe NCs. We also correlated the different surface defects to the PL quenching efficiency of CdS NCs.
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