Theoretical Insight into the Manipulation of the Optical and Magnetic Properties of Transition-Metal-doped II-VI Semiconductor Quantum Dots

Abstract

The ability to tune the electronic, magnetic, and optical properties of II-VI semiconductor quantum dots (QDs) makes these materials ideal candidates in the fabrication of new solar energy, spin electronic, and phosphorescent devices. This dissertation makes use of electronic structure theory to provide insight into the physical underpinnings of magnetic exchange and photoexcitation in Mn<super>2+</super>- and Co<super>2+</super>-doped CdSe and ZnO QDs. Specifically, new methods for controlling these physical effects via the incorporation of various dopants or changes in the QD's size and shape are presented. Several examples of these control techniques are discussed in this dissertation. Ferromagnetic alignment of multiple unpaired Mn<super>2+</super> 3<italic>d</italic> electrons is predicted when the p-type defect N<super>2-</super> is present in ZnO QDs. Control over the magnetic exchange interactions between charge carriers and TM<super>2+</super> dopants that give rise to ferromagnetism is achieved by distorting the QD's shape along one or two dimensions. Aliovalent doping with Al<super>3+</super> produces QDs that are spectroscopically identical to photochemically charged QDs, yet exhibit different reactivities. A unique temperature dependence of the luminescence and photoconductivity in large Co<super>2+</super>-doped ZnO QDs is predicted upon excitation of its mid-gap excited state. Lastly, a new method for the optimization of transition state molecular geometries is shown to exhibit fast optimization and to be advantageous for difficult optimizations where the reaction path is flat, ideal for optimizing large QD structures.