The Impact of Chalcogenide Ligands on the Photoexcited States of Cadmium Chalcogenide Quantum Dots

Quantum dots (QDs) are the foundation of many optoelectronic devices because their optical and electronic properties are synthetically tunable. The inherent connection between synthetically controllable physical parameters, such as size, shape, and surface chemistry, and QD electronic properties provides flexibility in manipulating excited states. The properties of the ligands that passivate the QD surface and provide such synthetic control, however, are quite different from those that are beneficial for use in optoelectronic devices. In these applications, ligands that promote charge transfer are desired. To this end, significant research efforts have focused on post-synthetic ligand exchange to shorter, more conductive ligand species. Surface ligand identity, however, is a physical parameter intimately tied to QD excited state behavior in addition to charge transfer. A particularly interesting group of ligands, due to the extraordinarily thin ligand shell they create around the QD, are the chalcogenides S2-, Se2-, and Te2-. While promising, little is known about how these chalcogenide ligands affect QD photoexcited states. This dissertation focuses on the impact of chalcogenide ligands on the excited state dynamics of cadmium chalcogenide QDs and associated implications for charge transfer. This is accomplished through a combination of theoretical (Chapters 2, 3, and 6) and experimental (Chapters 2, 4, 5 and 6) methods. We establish a theoretical foundation for describing chalcogenide capped QD photoexcited states and measure the dynamics of these excited states using transient absorption spectroscopy. The presented results highlight the drastic effects surface modification can have on QD photoexcited state dynamics and provide insights for more informed design of optoelectronic systems