Electron spin dynamics in colloidal n-type zinc oxide quantum dots

In this dissertation, electron spin dynamics of n-type colloidal zinc oxide quantum dots (ZnO QDs) have been studied by time resolved Faraday rotation (TRFR) and electron paramagnetic resonance (EPR) spectroscopies. TRFR was used to study the ensemble electron spin dephasing dynamics of ZnO QDs. A biexponential decay was observed where the short (∼100 ps) component was attributed to excitonic recombination and the long (∼1 ns) component was attributed to a metastable state formed by charge carrier separation due to hole trapping on the surface. A g* value of ∼1.96 was observed, which is consistent with what is expected based on k·p band theory and also with what is observed by EPR of bulk and epitaxial ZnO samples as well as photochemically reduced n-type colloidal ZnO QDs. A size dependence of this g* value is observed by EPR of the n-type colloidal ZnO QDs, supporting the hypothesis that the photochemically injected electrons are conduction band-like. A detailed mechanistic EPR study revealed the important factors for electron spin relaxation. Colloidal ZnO QDs with varying nuclear spin concentrations have been chemically synthesized to investigate the effects of electron-nuclear hyperfine interactions. A strong dependence of the spin-spin relaxation time (T2) on nuclear spin concentration was observed and a T2 of approximately 85 ns was measured. Other important factors for electron spin relaxation in ZnO QDs include electron-electron interactions, QD diameter, and temperature.