3 D Transition Metal Doped One-dimensional Zno Nanomaterials Magnetic, Optical Mechanism Research

Zinc Oxide (ZnO), a typical wide band gap (Eg=3.37eV) II-VI semiconductormaterial with a good piezoelectric and photoelectric properties, is a promisingapplication in the fields of electrics, optics and magnetics. For ZnO-based dilutemagnetic semiconductor material, its Curie temperature (Tc) is higher than the roomtemperature. In particular, it shows a large solubility of magnetic ions and transparentin the visible light region. These fantastic features make ZnO-based dilute magneticsemiconductor promising in information processing and storage, quantum computingand quantum communication. In this thesis, the first-principles based upon thedensity-functional theory (DFT) are systematically performed to study the geometricand electronic structures, magnetic, optical and electrical properties of3d transitionmetal doped one-dimensional ZnO nanomaterials, and these results may analyse theorigin of ferromagnetism and the mechanism of ferromagnetic coupling of one-dimensional ZnO diluted magnetic semiconductor materials. The effects of transitionmetal doping on the magnetic, optical and electrical properties are investigate, andsome helpful instruction can be provided for the preparation of high-qualityone-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 theadoption of different exchange-correlation potential, the basic structure and propertiesof bulk ZnO are studied using the first-principles calculation based on the densityfunctional theory. The calculated results by using the methods of LDA+U andB3LYP 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 ismuch closer to the experimental data, the above two methods are multiplied in thetime-consuming with respect to the shortest time consuming by the GGA method.2. Study on the structures and properties of ZnO nanowires. The geometricstructures, electronic structures and optical properties are studied by using the densityfunctional theory with respect to one-dimensional ZnO nanowires and nanotubes. It isfound that the band gap and the binding energy are gradually reduced, and the systemis becoming more stable with the increase in ZnO nanowires size. Meanwhile, obvioussize effects and surface effects are observed in the ZnO nanowires. The calculatedcharge density results show that ZnO nanowires look like strong covalent bondscharacter rather than ionic bonds. The strong p-d orbital hybridization appears in ZnONWs and surface charges on the whole of nanowires move outward. The electronicdelocalization is increased, So ionic bonds in ZnO are stronger than covalent bond.For the ZnO nanotubes, the initial polygon structure transforms into a perfectcylindrical tube and the binding energy of different ZnO nanotubes is negative, whichimplies that those ZnO nanotubes can exist in principle. Moreover, the calculatedelectronic structure results show that one-dimensional ZnO nanowires and nanotubesare direct wide gap semiconductor materials, and their band gap values aresignificantly larger than that of bulk ZnO. With the increase of nanotube diameter, thewhole valence band is significantly broadened and moves towards low energy, thedefect 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 asignificant blue shift in the absorption spectrums with the decrease of NWs size, andthe absorption spectrum locate in UV region. This implies that one-dimensional ZnOnanomaterials can be used for the development of UV-electronic devices.3. Study on magnetic, optical and electrical properties of3d transition metaldoped ZnO nanowires. The geometric and electronic structures, together withmagnetic, optical and electrical properties are studied by using the spin-polarized density functional theory with regard to3d 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 isinteresting that Cr, Fe, Co and Ni-doped systems have different magnetic couplingwhen the doped atoms replace the different location, which indicates that Cr, Fe, Coand 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, butthe antiferromagnetic coupling forms half-metal magnetite materials. Meanwhile, thecalculate results of optical properties indicate that there is little change in the opticalproperties, 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 of3d transition metaldoped ZnO nanotubes. The geometric and electronic structures, together withmagnetic, optical and electrical properties of3d transition metals doped ZnOnanotubes are investigated by using the first-principles based on spin-polarizeddensity functional theory. It is found that the V, Cr and Mn-doped systems are moreeasier 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 the3d states oftransition metals split into one triply degeneratet2gand one doubly degenerate egnearthe fermi level, which shows a strong hybridization between TMs-3d and O-2p states.The calculated optical properties show that three obvious absorption peaks areobserved in the UV region, and there is a red shift in the near UV region, meanwhile,the absorption peaks at400nm is a blue shift and the intensity of absorption peaks inMn-, Fe-, Co-, Ni-doped system increase obviously,but the intensity of absorptionpeaks in Cr-doped system decrease. In one word, we can come to a conclusion that3d transition-metal-doped ZnO nanotubes is a good UV magneto-optical electronicmaterials.