Download files
Abstract
Development of spin-polarized materials is important for the realization of spintronic devices. Two approaches are taken: (i) investigating magnetism in diluted magnetic semiconductors (DMSs), and (ii) developing novel 2D materials with intrinsic spin currents. The focus of this thesis is to study these two classes of materials using the density functional (DFT) theory.
The first part focuses on ZnO based DMS. The systematic study of C/N doped ZnO with defects shows that ZnO:C exhibits ferromagnetism, but dopant complex C2 tends to inhibit this property. Most importantly, the computational method plays a key role in studying DMS. Due to the popularity of hydrogenated ZnO:Co, this system is tested using the different methods. The DFT+U method shows a very specific configuration of hydrogenated Co complex can generate ferromagnetism. The inadequacies of traditional method are further exposed when the dopant geometry is compared between the hybrid functional method and normal DFT. The hybrid method includes the Fock exchange which significantly improves the description of long-range inter-atomic forces in the geometry relaxation yielding a more accurate description.
The second part focuses on the study of novel 2D materials with. The studies are devoted to increase the band gap of of novel 2D materials. In silicene and germanene, it is shown that the adsorption and substitution of Tl can increase the bandgaps to 0.29 eV at a small doping limit of ~2%. In addition, Tl can also tune the conductivity of silicene from n to p types depending on the doping sites, and preserving the high mobility of the undoped structure. Furthermore, the author also predicts several new wide bandgap quantum spin Hall insulator (QSHI). QSHI is a novel material with an insulating bulk state and spin-polarized metallic edges. By modifying the surfaces through hydrogenation, the orbital interactions of Bi, Pb, Cr, Mo, and W in the low energy level are confined on two dimensional resulting in novel QSHI with giant gap and strongly correlated property.
These results represent important contribution to the study of spintronic materials. First, the understanding between the theoretical method and magnetic properties in semiconductors can help to accurately predict new spintronic materials. Most importantly, the study in orbital engineering of 2D materials with hydrogenation provides new insights into the nature topological materials which have practical applications for the next generation of electronic devices.