Temperature-Dependent Studies on the Photophysics and Ligand Shell Structure of CdSe/CdS Quantum Dots

Colloidal quantum dots (QDs) are a class of materials that have received significant interest due to their disinctive optical properties when compared to other materials. Their tunable, sharp, and bright emissions, improved photostability, and high molar extinction coefficients make them of interest to a number of fields. In addition, QD structures can be synthesized with different compositions and geometries in order to maximize luminescence efficiency, or rates of charge separation. The presence of a shell of surface ligands provides another mechanism by which the properties of QDs can be tuned, including their photoluminescence and coupling with other QDs or molecules. Effective utilization of QDs in a number of applications is reliant upon understanding how to efficiently radiatively recombine or separate photoexcited carriers, while minimizing undesirable loss mechanisms. To better control photoexcited charges in QD systems, it is necessary to understand the mechanisms that underpin each charge recombination process. Because these processes are often complex and interconnected, knowing the temperature-dependence can be used to determine the relevant energetics governing excited state processes, which often yields mechanistic insights. In addition, developing an understanding the structure of the QD surface allows for further control of QD properties. In Chapter 1, I will introduce different excited state relaxation pathways in colloidal QDs. The roles of the core/shell interface, binding of surface ligands, interactions involving the ligand tail group, and redox-active ligands in modulating QD properties will be discussed. In addition, I will briefly describe why performing temperature-dependent measurements is of interest for each of these parameters. Chapter 2 shows how the presence of the core/shell interface modulates the radiative rate. This leads to a decrease in the radiative rate in CdSe/CdS QDs as temperature is increased. How this relationship is impacted by both core size and shell thickness is described. In addition, different factors that can impact the radiative rate, including thermal fluctuations, and electronic excitations are evaluated computationally to show which factors most change the radiative rate. In Chapter 3, I show how the temperature-dependent structure of the ligand shell is modified by the ligand composition. Introduction of either short chain ligands or a ligand with a cis double bond shifts an order/disorder transition within the ligand shell to lower temperatures. This order/disorder transition is observed experimentally using both temperature-dependent photoluminescence and infrared spectroscopies. The observed behavior is then explained by invoking a simple lattice model, which shows how domains of one ligand can seed disorder into another. In Chapter 4, I will describe temperature-dependent studies of hole transfer from photoexcited QDs to molecular acceptors. The observed temperature-dependence of the hole transfer rate found does not match with several Marcus theory models of the charge transfer process. Using a trap-mediated hole transfer model, the temperature-dependence can be explained. This observation indicates the importance of reversible surface trapping in changing QD properties. Finally, Chapter 5 will discuss the findings found in this dissertation and provide an outlook for future experiments