Photoinduced Charge Transfer from Quantum Dots on the Timescale of Chemistry

Abstract

Measuring and modulating charge-transfer processes at quantum dot interfaces are crucial steps in developing quantum dots as photocatalysts. In Chapter 1, a viewpoint for conceptualizing photoinduced charge transfer as a bimolecular, multi-step process is presented. The conventionally accepted mechanism that charges directly transfer to an acceptor following exciton dissociation is outlined and challenged. Then, existing methodology for evaluating photoinduced charge transfer is introduced. In Chapter 2, cyclic voltammetry under illumination is demonstrated to measure the rate of photoinduced charge transfer from CdS quantum dots by directly probing the changing oxidation states of a library of molecular charge acceptors, including both hole and electron acceptors. Observed rates for photoinduced charge transfer on the order of 0.1 s-1 are measured, which are distinct from the picosecond dynamics measured by conventional transient optical spectroscopy methods. Surprisingly, we found that charge transfer takes ca. 30 min to reach a maximum observed rate and charge transfer lasts for ca. 30 minutes after illumination ends, ~12 orders of magnitude longer than would be expected if charge transfer was directly from exciton dissociation. This timescale challenges the conventionally accepted mechanism of charge transfer. In Chapter 3, we investigated this new pathway for charge storage and transfer. Altogether, our results confirm that excited electrons are stored at ligated surface Cd, these sites are competent charge donors, and this storage is charge balanced by X-type ligand desorption. We found that charge storage occurs in every QD system studied, including CdS, CdSe, and InP capped with carboxylate and phosphonate ligands.