Towards understanding electronic structure of cobalt and manganese doped zinc oxide quantum dots with Density Functional Theory Methods

The chapters in this dissertation describe investigation of the electronic structure, excitations and magnetic exchange interactions in transition metal doped zinc oxide (TM2+:ZnO) nanocrystals. A computational scheme has been developed to model and predict those characteristics with the density functional theory (DFT) methods. A set of the spherical-like wurtzite ZnO nanocrystals have been built and capped with pseudo-hydrogen atoms to remove non-physical surface-states from semiconductor bandgap. The tests on the performance of the various DFT functionals showed that inclusion of the non-local Hartree-Fock exchange is important to correctly describe the relative positions of the transition metal dopant and semiconductor host energy levels.;DFT results obtained with our scheme provide explanation to the experimentally observed charge-controlled magnetization in Mn2+-doped ZnO colloids. Injected electrons activate new ferromagnetic Mn2+-Mn2+ interactions by formation of the bound magnetic polaron. These interactions are strong enough to overcome antiferromagnetic coupling between nearest-neighbor dopants, making the full magnetic moments of all dopants observable. Analysis shows that this large effect occurs in spite of small pairwise electron-Mn 2+ exchange energies, because of competing electron-mediated ferromagnetic interactions involving distant Mn2+ ions in the same nanocrystal.;The effects of TM2+-concentration on absorption spectra and magnetic exchange parameters have been addressed. The excitonic transition maximum shifts to higher energy and decreases in intensity with increasing Mn2+ concentration. Increased Mn2+ concentration leads to broadening and increase in the intensity of the sub-bandgap charge-transfer electronic absorption band. The charge-transfer band broadening is a result of excited-state splitting arising from double-exchange-like magnetic interactions involving Mn2+ ions and the photogenerated hole. Delocalization of the photogenerated hole via the double-exchange-like mechanism leads to stabilization of the ferromagnetic configuration in the charge-transfer excited state. The strength of this ferromagnetic double-exchange interaction depends strongly on the inter-Mn2+ distance within the quantum dot. Analysis of the MLCBCT excited state wavefunctions reveals that the more stable excited state originates from the out-of-phase interaction between localized Mn d-orbitals, which is mediated by the exchange interactions between Mn spins and ZnO valence band. The charge-transfer band intensity grows linearly with the number of dopants due to the increase of excited state density and non-linearly through the delocalization of charge-transfer excited states. In Co2+ doped QDs, the double-exchange stabilization is also observed and for the dt2 → CB ML CBCT transitions and has a value comparable to the Mn2+-doped QDs. The lowest excitonic state is stabilized by strong Zener-type sp-d exchange between the TM2+ and ZnO conduction and valence band spins.;At last, the excited state relaxation of the MLCBCT and d-d transitions has been analyzed with the linear response TD-DFT approach for the Co2+-doped ZnO nanocrystals. Substantial vibronic excited state stabilization leads to the crossing between the ligand field and CT excited states potential energy surfaces. This explains the experimentally observed photocurrent arising from the localized ligand field transitions.;Overall, presented in this document is the detailed investigation of electronic structure, magnetic exchange interactions and absorption spectra of TM2+-doped ZnO QDs with the quantum chemistry methods. The methodology described below is based on the easily available quantum chemistry computational package (Gaussian); it is carefully validated, and can he used for prediction of these extremely interesting and important phenomena in other II-IV DMS quantum dots. (Abstract shortened by UMI.)...