Controlling the Electron-Transfer Kinetics of Quantum-Dot Assemblies
© 2017 American Chemical Society. Electron transfer theory is used to explore the size-dependence of electron transfer (ET) between dithiol-bridged quantum dots (QDs) and make predictions that can be tested experimentally. Electronic couplings, electronic densities of states, and reaction-free energies are all found to be size-dependent and to influence the ET rates. As the acceptor QD radius grows at fixed edge-to-edge donor-acceptor distance, the reaction-free energy becomes more negative. As a result, both electron and hole transfer rates show a peak as a function of acceptor radius for donor radii ranging from 9.5 to 21.5 Å however, this rate maximum as a function of radius is weaker than that observed in molecules, since the increasing acceptor density of states partially compensates both the Marcus inverted effect and the decreased electronic coupling with increasing radius. The electronic coupling decreases as the donor radius grows because the wave function probability density per surface atom decreases and the acceptor density of states at the donor's band edge energy decreases. The through-solvent and through-bond electronic couplings have different dependencies on QD radii, with a switch in the dominance of the coupling mechanisms as the QD radius changes. For large QDs, the through-solvent coupling dominates, so the chemistry of the through-bond linkage does not strongly influence the coupling. Finally, we discuss how the electron and hole transfer rates can be matched by varying the QD radii, thus providing an approach to optimize the performance of solar cells based on type II QD assemblies.
Liu, R; Bloom, BP; Waldeck, DH; Zhang, P; Beratan, DN
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