Electron spin dynamics in quantum dots, and the roles of charge transfer excited states in diluted magnetic semiconductors

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Electron spin dynamics in quantum dots, and the roles of charge transfer excited states in diluted magnetic semiconductors

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Title: Electron spin dynamics in quantum dots, and the roles of charge transfer excited states in diluted magnetic semiconductors
Author: Liu, William K
Abstract: In this dissertation, photoelectrochemical measurements have been used in combination with absorption and magnetic circular dichroism spectroscopic measurements to investigate the electronic structural properties of nanocrystalline Co2+:ZnO and Mn2+:ZnO diluted magnetic semiconductors that give rise to macroscopic charge separation when these materials are excited with photons throughout the visible energy range. From analysis of the spectroscopic results, sub-bandgap charge transfer transitions have been identified and shown to be responsible for the photoinduced charge separation in these materials. In a broader context, these charge transfer excited states are shown to be relevant to the understanding of ferromagnetism of TM2+:ZnO, where TM2+ denotes 3d transition metal cations. The assignment of the charge transfer transition (ligand-to-metal versus metal-to-ligand) can reveal the polarity of the carriers that mediate the ferromagnetism ( n-type versus p-type, respectively). To investigate the exchange interaction between the carrier and magnetic dopant cation of TM2+:ZnO diluted magnetic semiconductors, electron paramagnetic resonance measurements were performed on colloidal Co2+:ZnO and Mn2+:ZnO nanocrystals possessing additional quantum-confined conduction band electrons. Additionally, the electron-nuclear hyperfine interaction between nuclear spins of 67Zn cations and additional quantum-confined conduction band electrons, as reflected in the spin dephasing time, in colloidal ZnO quantum dots is investigated by electron paramagnetic resonance spectroscopy.
Description: Thesis (Ph. D.)--University of Washington, 2007.
URI: http://hdl.handle.net/1773/8588

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