Study On The Preparation, Structures And Properties Of Co, Ni Doped ZnO Nanorod Arrays And Nanocrystal Thin Films

Zinc oxide is a material with great potential for a variety of practical applications, such as piezoelectric transducers, transparent conductive oxides, sensors, spin functional devices, and UV-light emitters etc.. Its wide bandgap of 3.37 eV at room temperature makes ZnO a promising material for photonic applications in the UV or blue spectral range, while the high exciton-binding energy (60 meV) allows efficient excitonic emission even at room temperature. In addition, ZnO doped with transition metals shows great promise for spintronic applications. There are still a number of unanswered questions concerning the relationship between fabrication conditions and optical properties, the origin of FM ordering in diluted magnetism oxides (DMOs), including TM-doped ZnO, is still a matter of debate. Bottom-up technology of self-assembled nanowires and doping technology remain challenging. The potential application in solar cells and spin field effect transistors (spin-FETs) and all of the above questions make the research in the present paper very valuable. The objective of this dissertation is to hydrothermally synthesize Co, Ni doped ZnO nanorod arrays and prepare Co doped ZnCoO nanocrystal films via sol-gel method, which are studied on the dependence of their properties on the microstructure and preparation condition or the annealing temperatures via the characterization of microstructured, optical and magnetic proerties, and further discuss the origins of ultroviolet emission and room ferromagnetismn and the approaches to enhance room ferromagnetism. The main contents of the research are summarized as follows:1. One-pot hydrothermal method was first used to self-assemble highly c axis oriented Co, Ni doped ZnO nanorod arrays with room ferromagnetism at 70℃for 10h and x ray diffraction(XRD), x ray photoelectron spectra(XPS) and Raman spectra all indicate that Co, Ni instituting Zn indeed were incorporated into ZnO crystal lattice. The effects of growth conditions including growth time, the thickness of ZnO seed layer and PH value (characterized by the amount of ammonia) and annealing temperature on the structures, photoluminescence and magnetic property of Zn0.9Co0.1O nanorod array have been systematically studied. The nanorods with 150nm in average diameter and 4.5um in length grown along [001] direction have been observed, which vertically grown on glass coated with ZnO seed layer by high resolution transmission electron microscope(HRTEM), field emission scan electron microscope(FESEM) and XRD spectrum. The growth of the nanorods proceeds with prolonging growth time from 1h to 10h. ZnO seed layers and PH value were found to play a very important role in the nucleation and growth of ZnCoO nanorods. The less the thickness of the seed layer, the more easy the formation and growth of nanorods, but without ZnO seed layer, it is hard to grow ZnCoO nanorod arrays. PH value can adjust Co doping concentration in the ZnO nanorods, defect related photoluminescence of the nanorods and broaden the PH range to the formation of ZnCoO nanorod arrays. It was also found that the growth factors such as lower PH value, shorter growth time and the less thickness of ZnO seed layer inducing more defects as oxygen vacancy etc. is beneficial to the enhancement of the ferromagnetism of ZnCoO nanorod arrays. It is found from the experiment that annealing at 300℃makes the peak position of ultraviolet emission blushift from 392nm to 387nm and the ratio of bound excitons peak to free excitons peak increase which can be considered to be resulted from the the redistribution of the defects related to bound excitons decreasing the location energy of excitons and bound energy of dopants. At the same time, the intensity of visible light(VL) greatly decreases. Annealing at 400℃can efficiently suppress deep energy level related defect emission. The research results indicate that annealing can not only decrease the amount of defects related to oxygen but also affect the distribution of the defects in the ZnO lattice.2. Co,Ni doped Zn1-xCoxO(x=0.05,0.10,0.15)和Zn1-xNixO (x=0.05,0.10,0.15) nanorod arrays have been synthesized via one pot hydrothermal method and their structural, optical and magnetic properties characterized, both of which exhibite room temperature ferromagnetism. It was also found from experiments that photoluminescence(PL) spectra of ZnCoO consist of broad ultraviolet emission band (UV) and visible light emission band (VL). With an increase of Co content, UV peak obviously widens and redshifts, which can be divided into two peaks at 382nm and 394nm respectively. The redshifting of 382nm peak can be considered to result from the sp-d exchange effect of band electrons and local electrons bound to Co2+ instituting Zn2+, and the redshifting of 394nm results from the enhanced effect of doped Co ions and O ions. With increasing Co dopant, ferromagnetism of Zn1-xCoxO(x=0.05,0.10,0.15) was enhanced. Carrier mediated ferromagnetism mechanism can be used to explain their origins of ferromagnetism. Ni doping having important effects on the shapes of nanorods, with Ni dopant increasing, the cross plane of nanorods changes from unregular shape to tetragon and hexagon. Photoluminescence spectra indicate that free exciton peak at about 380nm redshifts with Ni doping content increasing which is caused by sp-d exchange effect inducing the shrinkage of bandgap of ZnNiO nanorods. Increasing excitation power greatly makes near band edge emission(NBE) reshifting and broadening, which is thought to be the result of higher excitation power activating Zn vacancy defects related recombination and the unstable excitation states of ZnO nanorod arrays with higher Ni doping content. The magnetism of Zn1-xNixO(x=0.05,0.10,0.15) decreases with Ni doping content increasing. It is considered to be caused by more uneven distribution of Ni atoms induced by increasing Ni dopant resulting in antiferromagnetism exchange between nearer atoms appearing, which decreases the contribution to the ferromagnetism.3. Zn0.88Co0.12O films have been prepared via sol-gel method and characterized by x-ray diffraction (XRD), ultraviolet-visible transmittance spectra(UV-Vis), PL and vibrating sample magnetrometer(VSM) measurement indicating that Co2+instituting Zn2+was doped into ZnO crystal lattice. Zn0.88Co0.12O films present room temperature ferromagnetism and ultraviolet emission increases with increasing annealing temperature while visible light emission decreases. The Zn0.88Co0o.12O film annealed at 400-500℃presents the largest saturation magnetization, the analysis of the above phenomena indicates that the room ferromagnetism of Zno.88Coo.12O films results from the coupling effect between Co2+ instituting Zn2+ and carriers. In order to obtain the ZnCoO films with larger room ferromagnetism, it is feasible to adopt proper heat treatment technics to obtain the best combination among defects, carrier concentration and Co ions existing states. 4. The thermal expansion behaviors of ZnO, Zno.9Co0.1O, Zno.88Co0.05Al0.07O nanorod arrays and Zn0.8Mg0.2O thin films at temperatures from 10K and 300K have first been reported and studied via x ray diffractometer with low temperature attachment. The dependences of the crystal constant c on the temperature have been simulated properly by Quartic polynomials. It was found that the maximum of c appears near 50K implying the phase transition occurs, negative thermal expansion appears from the temperature corresponding to the maximum of c to 300K and Co doping increases negative thermal expansion of ZnCoO nanorod arrays near room temperature,.which provides helpful reference for the application of devices made by Co doped ZnO nanorod arrays.