Date of Award

Spring 1-1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Gordana Dukovic

Second Advisor

Joel D. Eaves

Third Advisor

Niels H. Damrauer

Fourth Advisor

David M. Jonas

Fifth Advisor

Garry Rumbles

Abstract

Colloidal semiconductor nanocrystals have many remarkable properties—such as exceptionally tunable excited states and surface chemistry—that have led to an enthusiastic interest in using them for optoelectronic applications such as solar-energy conversion. Such technologies require control over the generation, separation, and extraction of photoexcited electrons and holes. However, the interpretation of experimentally measured excited-state decay curves is challenging because they typically exhibit complicated shapes that are elusive to simple kinetic models. To understand the principles that govern electron and hole relaxation dynamics in these complex systems, models rooted in fundamental physical phenomena are needed. This dissertation describes efforts to understand the dynamics of recombination, charge carrier trapping, trapped holes, and charge transfer in photoexcited Cd-chalcogenide nanocrystals using a combination of ultrafast spectroscopy and kinetic modeling. The first part of this dissertation focuses on studying the spatial dynamics of trapped holes. In CdS and CdSe nanocrystals, photoexcited holes rapidly and efficiently trap to localized states on the surface. We demonstrate evidence that trapped holes are mobile in CdS nanorods, CdSe nanorods, and CdSe/CdS and ZnSe/CdS dot-in-rod heterostructures, and that they likely undergo a diffusive random walk between trap sites on the nanocrystal surface. The second part of the dissertation focuses on modeling charge-transfer kinetics in heterogeneous ensembles of donor–acceptor complexes. In complexes of CdS nanorods and [FeFe] hydrogenase, electron transfer is the key step for photochemical H2 production. By accounting for the distributions in the numbers of electron traps and enzymes adsorbed, we determine the rate constants and quantum efficiencies for electron transfer. The relatively simple analytical models developed here establish a detailed conceptual and quantitative picture of the rich carrier dynamics in these systems, providing important insights for the design of semiconductor nanocrystals for light-driven applications.

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