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

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.