.3 D Transition Metal-doped One-dimensional Zno Nanomaterials Magneto-optical Mechanism

Zinc Oxide (ZnO), a typical wide band gap (E_g =3.37eV) II-VI semiconductor material with a good piezoelectric and photoelectric properties, is a promising application in the fields of electrics, optics and magnetics. For ZnO-based dilute magnetic semiconductor material, its Curie temperature (T_c) is higher than the room temperature. In particular, it shows a large solubility of magnetic ions and transparent in the visible light region. These fantastic features make ZnO-based dilute magnetic semiconductor promising in information processing and storage, quantum computing and quantum communication. In this thesis, the first-principles based upon the density-functional theory (DFT) are systematically performed to study the geometric and electronic structures, magnetic, optical and electrical properties of 3d transition metal doped one-dimensional ZnO nanomaterials, and these results may analyse the origin of ferromagnetism and the mechanism of ferromagnetic coupling of one-dimensional ZnO diluted magnetic semiconductor materials. The effects of transition metal doping on the magnetic, optical and electrical properties are investigate, and some helpful instruction can be provided for the preparation of high-quality one-dimensional ZnO magnetic nanomaterials with high Curie temperature.The main contents and results are listed below:1. Theoretical study on electronic structure of bulk ZnO materials. With the adoption of different exchange-correlation potential, the basic structure and properties of bulk ZnO are studied using the first-principles calculation based on the density functional theory. The calculated results by using the methods of LDA + U and B3LYP are better than those calculated by using the individual method of GGA and LDA. Although the calculated band gap by using the LDA + U and B3LYP methods is much closer to the experimental data, the above two methods are multiplied in the time-consuming with respect to the shortest time consuming by the GGA method.2. Study on the structures and properties of ZnO nanowires. The geometric structures, electronic structures and optical properties are studied by using the density functional theory with respect to one-dimensional ZnO nanowires and nanotubes. It is found that the band gap and the binding energy are gradually reduced, and the system is becoming more stable with the increase in ZnO nanowires size. Meanwhile, obvious size effects and surface effects are observed in the ZnO nanowires. The calculated charge density results show that ZnO nanowires look like strong covalent bonds character rather than ionic bonds. The strong p-d orbital hybridization appears in ZnO NWs and surface charges on the whole of nanowires move outward. The electronic delocalization is increased, So ionic bonds in ZnO are stronger than covalent bond. For the ZnO nanotubes, the initial polygon structure transforms into a perfect cylindrical tube and the binding energy of different ZnO nanotubes is negative, which implies that those ZnO nanotubes can exist in principle. Moreover, the calculated electronic structure results show that one-dimensional ZnO nanowires and nanotubes are direct wide gap semiconductor materials, and their band gap values are significantly larger than that of bulk ZnO. With the increase of nanotube diameter, the whole valence band is significantly broadened and moves towards low energy, the defect levels appear in the valence band top due to surface effects. At the same time, the calculated optical results for ZnO nanowires and nanotubes show that there is a significant blue shift in the absorption spectrums with the decrease of NWs size, and the absorption spectrum locate in UV region. This implies that one-dimensional ZnO nanomaterials can be used for the development of UV-electronic devices.3. Study on magnetic, optical and electrical properties of 3d transition metal doped ZnO nanowires. The geometric and electronic structures, together with magnetic, optical and electrical properties are studied by using the spin-polarized density functional theory with regard to 3d transition metal doped ZnO nanowires. The calculated results indicate that V-doped system is only ferromagnetic coupling, and the Mn-doped system is only antiferromagnetic coupling. However, it is interesting that Cr, Fe, Co and Ni-doped systems have different magnetic coupling when the doped atoms replace the different location, which indicates that Cr, Fe, Co and Ni-doped ZnO nanowires possess more rich magnetism phenomenon. Especially, the ferromagnetic coupling forms magnetite semiconductor materials for Co-doped, which exhibits excellent magneto-optical properties from theoretical prediction, but the antiferromagnetic coupling forms half-metal magnetite materials. Meanwhile, the calculate results of optical properties indicate that there is little change in the optical properties, and significant blue shift and red shift are respectively observed in the Mn, Fe, Co, Ni-doped systems and Cr-doped system.4. Study on magnetic, optical and electrical properties of 3d transition metal doped ZnO nanotubes. The geometric and electronic structures, together with magnetic, optical and electrical properties of 3d transition metals doped ZnO nanotubes are investigated by using the first-principles based on spin-polarized density functional theory. It is found that the V, Cr and Mn-doped systems are more easier to form the ferromagnetic coupling and possess a strong magnetic properties, while the Fe and Co-doped system are more easier to form antiferromagnetic coupling. but the Ni-doped system is instable for the magnetic properties. Furthermore, overlook from the electronic structure calculation, it is clear that the 3d states of transition metals split into one triply degenerate t_2g and one doubly degenerate e_g near the fermi level, which shows a strong hybridization between TMs-3d and O-2p states. The calculated optical properties show that three obvious absorption peaks are observed in the UV region, and there is a red shift in the near UV region, meanwhile, the absorption peaks at 400nm is a blue shift and the intensity of absorption peaks in Mn-, Fe-, Co-, Ni-doped system increase obviously,but the intensity of absorption peaks in Cr-doped system decrease. In one word, we can come to a conclusion that 3d transition-metal-doped ZnO nanotubes is a good UV magneto- optical electronic materials.