In this dissertation, we study the complex nature of the surface in cadmium-rich CdS nanocrystals. We found that cadmium carboxylate complexes (CdX2) act as Z-type ligands (Lewis acids) that passivate undercoordinated sulfur sites. To elucidate more information about these states, CdX2 groups were displaced using N,N,N',N'-tetramethylethylenediamine (TMEDA), and monitoring using 1H NMR and photoluminescence spectroscopies. Through these studies, we demonstrate the existence of two distinct binding sites, each with its own binding affinity for CdX2. These sites are under dynamic equilibria, and a two-site binding model was developed to provide a deeper understanding of the thermodynamics of the surface binding in CdS. Thermodynamic data shows that the entropic contribution is a significant difference between each binding site. With a better understanding of the binding thermodynamics of each surface site in hand, we move to determine the role played by each defect site on the photophysical properties of CdS NCs. The absorption profiles of CdS NCs are altered upon the creation of surface defects. There is a direct correlation between the observed red shift of the excitonic peak with the creation of B2 sites on the surface, as well as a correlation between the broadening of the 1Se-2S3/2h peak and the creation of B2 sites. The creation of defects changes the surface polarization, resulting in changes in the absorption profile. Furthermore, a direct correlation between the creation of each vacant site and its impact on the photoluminescence (PL) spectroscopy is discussed. Each site has its effect on PL emission, and B2 vacancies are excellent exciton quenchers and are the primary cause of the energy shift seen in the trap state emission. The trap state emission of CdS NCs was determined to be caused by two distinct trap states. The PL efficiency of each trap state is influenced by the cadmium to sulfur ratio on the surface and correlated to the creation of each surface defect. The excited-state lifetimes of both trap states and of the exciton recombination were probed throughout this study. Our results gave insight into the complex nature of the surface and its impacts on the photophysical NC properties. Overall, we determined that the creation of B2 vacant sites significantly impacts the lifetime of each state. To have greater control over the photophysical properties of CdS NCs, B2 vacant sites needs to be capped with ligands. The impact of these discoveries presented in this thesis will provide greater understanding of CdS NC surface and better control of the surface.
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