Synthesis, Optical And Magnetic Properties Of Zn_1-xCu_xO Micro/Nanostructures

The physical and chemical properties of micro/nano materials are afunction of their composition, morphology, and size. At present,controllable synthesis, characterization and application of micro/nanomaterials is of great interest to physicists, chemists, and materialscientists. On the other hand, doping is an efficient method to improve theelectrical and optical properties of semiconductors. In this dissertation,we have chosen the Cu-doped ZnO （Zn1-xCuxO） system to investigate.Compared with the other transition metal, Cu is not only an effectiveferromagnetism dopant, but also has its own advantages. Such as, thesubstitution of Zn by Cu is a p-type doping, and the size mismatchbetween Cu and Zn is very small, resulting in the low formation energy.In addition, Z_1-xCu_xO is free of ferromagnetic impurities, making iteasier to investigate the origin of ferromagnetism in diluted magneticsemiconductor.So far, considerable effort has been devoted to the fabrication ofZ_1-xCu_xO materials with room temperature ferromagnetism （RTFM）through various methods. However, it remains controversial whether the observed RTFM originates from the extrinsic Cu introducing or theintrinsic structural defects. Most of the investigations have been focusedon bulk materials or thin films, whereas only a few reports onone-dimensional （1D） nanostructures （e.g., nanowires, nanonails, andnanoneedles）.In recent years, increasing attention has been paid to design andcontrollable preparation of hollow spherical structures andthree-dimensional （3D） hierarchical ZnO architectures because of theirunique geometrically shape and widely applications in many fields.Compared with the reported1D Cu-doped ZnO nanostructures, sphericalstructures could apply to catalysis, sensors, drug delivery, energy-storagemedia, and so forth because of their geometrically hollow shape and largesurface area.3D hierarchical ZnO architectures, which derived from1Dnanostructures as building blocks，show considerable promise for thedevelopment of multiple functional nanodevices （e.g., gas sensor andphoto-catalytic hydrogen generation）. For these reasons, it is veryimportant and necessary to fabricate the Z_1-xCu_xO hollow sphericalstructures and3D hierarchical micro/nano structures. Thus far, there havebeen no reports of such Z_1-xCu_xO hollow spherical and hierarchicalstructures.We concentrate on exploring new method to prepare the Zn1-xCuxOhollow spherical and hierarchical micro/nano structures, studying the influence of Cu doing on the morphologies and structures of theas-synthesized samples. The optical and magnetic properties are also beeninvestigated after Cu introducing.Experimentally, the processes of prepare the Z_1-xCu_xO hollowspherical and hierarchical micro/nano structures often require highertemperature, multi-step process, or introduce impurities by foreigncatalysts or the templates, which is harmful to the application in device.Therefore, it is still a challenge to find a simple and controllable methodto fabricate hollow spherical and hierarchical micro/nano structures.In this dissertation, we firstly report a simple chemical vapordeposition method to prepare the Z_1-xCu_xO hollow spherical structures.Through a series of controlled experiments by changing the growthconditions, we study on the changes of morphology and structures afterCu doping, and propose the growth mechanism of the as-preparedspherical structures. The visible luminescence is greatly enhanced afterCu introduced. We attribute the enhanced RTFM with Cu contents in ourZ_1-xCu_xO hollow spherical structures to the increased intrinsic defectstriggered by the Cu doping.Followed that, we have realized the fabrication from1D nanorod to3D hierarchical Z_1-xCu_xO micro-cross structures by a simplecatalyst-free vapor-phase method. The key point is that the highertemperature is helpful to form a central core and epitaxial growth of the hierarchical structure. The introduction of abundant Cu in the core causethe usual hexagonal ZnO structures growing into four-fold hierarchicalcross-like structures. Interestingly, we have observed the distinctinhomogeneous cathodeluminescence （CL） in a single Zn1-xCuxOmicrocross structure, which is attributed to the different concentrations ofCu.On the other hand, high concentration of excitons can be generated atroom temperature because of the large exciton binding energy （60meV）in ZnO, which will benefit to produce laser in a low excitation energy. Wealso investigate the laser emission properties of pure ZnO nanostrucures.Different hierarchical ZnO nanostructures, such as nanotube surroundednanowires, and nanotube assembled micro spherical shell structures havebeen prepared by chemical vapor deposition method. We can observe therandom action in the as-prepared ZnO hierarchical nanostructures.Moreover, we have prepared the micro-tower arrays, which can be servedas laser micro cavity. The observed fixed laser mode at room temperaturein the ultraviolet region at room temperature in micro-tower structurescan be attributed to the Whisper Gallery Mode lasing assisted byFabry-Pérot resonance.