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Investigation On The Synthesis And Properties Of Undoped /doped Zno Nanomaterials

Posted on:2011-05-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:D D WangFull Text:PDF
GTID:1101360305990371Subject:Condensed matter physics
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ZnO nanomaterials have great potential applications in the photoelectronic devices and spintronic device owing to the native properties and shape characters. Therefore, it is very important to do the research on the fabrication and properties of undoped and doped ZnO nanomaterials. The main points for this thesis are not only successfully realize the synthesis of undoped/doped ZnO nanomaterials, and also investigate the structure and optical properties in detail by the methods of field emission scan electron microscope (SEM), transmission electron microscope (TEM), resonant Raman, photoluminescence (PL), time resolved PL spectrum, spectrofluorimeter and superconducting quantum interference device (SQUID).In the thesis, firstly, vertical aligned ZnO nanowires were successfully prepared on the substrate with Au layer by the classical vapor transport method. Results show that the thickness of the catalyst has big effect on the size of the nanowires, the thicker catalyst, and the bigger diameter of the nanowires. At the same time, we investigate the optical properties of vertical undoped ZnO nanowires. UV emission displayed an evident blueshift with increasing excitation power and the corresponding energy shift might be as large as 10 meV. This anomalous phenomenon correlates to the band bending level caused by the surface built-in electric field due to the existence of substantial oxygen vacancies. By increasing the excitation power, the enhanced neutralization effect near the surface will reduce the built-in electric field and lead to a reduction of band bending which triggers the blueshift of the UV emission. Secondly, we choose the rare earth element as the dopant, and investigate the structure and properties of rare earth doped ZnO. We mainly focus on two rare earth elements doping as follow:1. Eu doped ZnO. We synthesize the Eu doped ZnO by two methods; one includes the synthesis of the source powders by a sol-gel method and the growth of nanowires by vapor transport method; another one is the growth of nanosheets based microflower. Then we investigate the properties of Eu doped ZnO. Results show that the Eu ion has doped into the matrix of ZnO. Under the UV excitated, there is a sharp red emission related to the intra-4f transitions of Eu3+ ions. Our systematic studies on photoluminescence (PL) excitation, temperature-dependent photoluminescence and time-resolved photoluminescence suggest that intrinsic defects, in particular singly ionized oxygen vacancies, can serve as the media for the energy transfer from the ZnO host to the Eu3+ ions. For the nanosheets based microflower, there are both the oxygen vacancy and oxygen interstice defects, and we also observe the red peak due to energy transfer from the ZnO host to the Eu3+ ions.2. Nd doped ZnO. We synthesize the Nd doped ZnO nanowires by two steps method including synthesis of source powders the solid state reaction method and the growth of nanowires by vapor transport method. XRD, TEM, XPS and EDS results confirm that the Nd has doped into the matrix of ZnO. The Nd doped ZnO nanowires show room temperature ferromagnetic properties. They have large coercivity and shape anisotropy. The magnetic moment also show anisotropic properties. The ferromagnetic properties are due to the bound magnetic polaron mechanism. The oxygen vacancies polarize the magnetic dopants and form the polarons, and the interaction of the adjacent polarons made the Nd doped ZnO nanowires give the ferromagnetic properties. The higher coercivity is due to the magnetic anisotropy and the pining effect of the defect in Nd doped ZnO nanowires. The anisotropic property of magnetization is due to the orbital contribution of the Nd magnetic moment.This thesis not only provides the optimized growth conditions for undoped/doped ZnO nanomaterials, but also obtains some beneficial results in aspects of their optical and magnetic properties, which builds theoretical and experimental foundation for much better and broader applications of undoped/doped ZnO nanostructures.
Keywords/Search Tags:ZnO, rare earth, vapor transport method, hydrothermal method, sol-gel method, solid state method, optical properties, magnetic properties
PDF Full Text Request
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