Principles of Quantum Dot Photoexcited Charge Transfer

Photoexcited charge transfer is essential to light energy conversion processes in photosynthesis and solar cells. While fundamental theories governing these molecular and bulk charge transfer reactions are well established, the same level of understanding and agreement for nanoscale systems has not been achieved. It is essential to address this knowledge gap, as nanomaterials are becoming attractive candidates for optoelectronics and electrocatalysis. In this dissertation, hole transfer from high photoluminescence quantum yield CdSe-core CdS-shell semiconductor nanocrystal quantum dots to covalently linked molecular ferrocene acceptors is investigated in the context of Marcus Theory of Electron Transfer (Nobel Prize Chemistry 1992). This model quantum dot donor-acceptor system was thoughtfully designed to address the unique properties of QDs that complicate the proper characterization of the rate constants. In this model system, modulating the electronic coupling and thermodynamic driving force for the reaction was done reliably and the hole transfer rate constant per acceptor, kht, was extracted by measuring steady-state and transient photoluminescence to determine the photoluminescence quantum yield in conjunction with quantitative NMR to determine the number of bound acceptors. Our studies varied the electronic coupling by six orders of magnitude by changing the shell thickness and the alkyl chain length. We find that there is a family of universal curves for PLQY as a function of coverage, spanning linear to nonlinear which depends critically on the ratio of the total hole transfer rate to the sum of the native recombination rates in the QD. Thermodynamic driving force was explored by molecular functionalizations on ferrocene that change the oxidation potentials of the acceptor. The rate's dependence on the driving force elucidated that there is no Marcus inverted region, and a new multistate model was posited to explain the result. Parameters that affect the reorganization energy such as the surface ligands were also explored. Our model charge transfer system allowed us to examine the rate of ligand exchange to determine the rate law and the rate order, and explore mechanisms of this exchange reaction. Single particle absorption and photoluminescence via fluorescence and photothermal microscopy were also explored to explore photophysical properties of single QDs, such as how its photoluminescence and absorption degrades over time and how the photoluminescence quantum yield varies in a given batch of this material