Synthesis, Characterization, And Optical Properties Of Zinc Oxide Nanostructures

Zinc oxide (ZnO) is an important direct wide band gap (3.37eV) semiconductor with interesting piezoelectric, optoelectronic, and pyroelectric properties. The exciton binding energy of ZnO at room temperature is 60 meV, much lager than those of other semiconductors. Structurally, the alternatively stacking of zinc and oxygen atoms along the c-axis induces two polar surfaces of (0001)-Zn and (0001)-O. In addition, ZnO has three fast growth directions. These unique structural properties have benefited the realization of many kinds of novel ZnO nanostructures, such as nanowires, nanotubes, nanobelts, nanorings, and so on. The various ZnO nanostructures will play important roles in preparing optoelectronic and nanoelectronic devices, since they display many excellent properties different from bulk ZnO.The properties of nanomaterials are associated with the nanostructures, which will be greatly influenced by the technology and method of synthesizing nanomaterials. At present, the development tendency of nanomaterials science and technology focuses on the study of controllable process, including materials'shape, dimension, surface and microstructure, so that their unique properties could find their corresponding applications in the industry. In this thesis, well-ordered ZnO nanostructrues are prepared by the radio frequency magnetron sputtering or thermal vapor deposition process, on the basis of well-ordered alumina membrane. Since the synthesis of nanostructure will be affected by the morphology of membrane, such as order degree, thickness, pore diameter, and so on, we have systemically researched the preparation technology of membrane, synthesized the well-ordered alumina membrane with controllable morphology, and discussed its formation mechanism. Based on the successful preparation of alumina membrane, well-ordered ZnO nanopores array and nanorods have been synthesized by the above-mentioned methods.Scanning electron microscopy (SEM) and transmission electron microscope (TEM) also testify that the prepared single-crystal nanostructures are well-ordered. Photoluminescence (PL) spectra indicate that well-ordered nanostructures have strong ultraviolet emission, which have potential applications in the ultraviolet detectors. We have also studied the distribution of the impurities in membrane, and the influence of the annealing temperature on anion concentration by TEM, infrared absorption spectra, and transmission spectra, to investigate the formation mechanism of the ordered nanostructures. The infrared absorption spectra demonstrate the decrease of COO- stretching modes intensities with increasing annealing temperature, suggesting the annealing process can lead to the decrease of anion concentration due to the decomposition of impurity groups. During the anodizing process, anion in the electrolyte could enter into the alumina membrane owing to the action of electric field and form the inhomogeneous distribution, which results in the formation of localized negative charges on the surface of alumina membrane. Ordered ZnO nanostructures will form due to the effect of localized negative charges during the preparation process. Those nanostructures not only embody the nanounits'collective effect, but also reflect some properties which single nanostructure or nanounit does not possess, thus avoids the formation of complicated crystal boundaries due to out-of-order arrange of nanostructures and prompts their applications.Though ZnO nanostructures with simple morphology have already been successfully synthesized and applied in some nanodevices, higher integrated nanostructures in some advanced systems are still in demand. In this thesis, we have creatively realized the integration of single nanostructures and prepared hierarchical ZnO nanostructures with the controllable morphology, through the two-step oxygen-controlled thermal evaporation process. During the experiments, we have firstly synthesized the Zn nanowires with little oxide on Si substrate, and then prepared the hierarchical ZnO nanostructures successfully by heating Zn source and step-one samples. The integration of ZnO nanorods, nanotubes,and nanowires could be realized by changing the oxygen flow rate, and we have also studied their formation mechanism and change of morphology by TEM,SEM,and a series of designed experiment. The experimental results indicate that ZnO nanorods will grow from the side walls of nanotubes/nanorods through the screw dislocation model at the low oxygen supplement, while high oxygen flow rate will lead to the preferentially growth of nanorods'along [001] orientation from the surface of nanowires.Furthermore, by controlling the evaporation time, we have also realized the modulation of length of nanorods on the side wall of nanotubes. We point out that the oxygen introduced at step one plays an important role on the formation of hierarchical nanostructures. The PL spectra of ZnO nanostructures indicate that the optical properties of hierarchical nanostructures can be modulated by changing the morphology. During the whole experiment, only Zn powder without any doping has been evaporated, which avoids the contamination of impurities on the nanostructure. This simple two-step method also avoids high temperature or some complicated process required in the previous synthesis methods, which will prompt the application of nanostructure in nanodevices. At the same time, this approach can also apply in synthesizing the complex nanostructure of other materials.Finally, the synthesis of ZnO microsphere has been studied. We have investigated the formation mechanism of ZnO hollow microspheres prepared by hydrothermal method, and the influence of ammonia on morphology. In addition, we have also prepared ZnO hollow microspheres with different morphology by evaporating Zn powder, and studied the corresponding formation mechanism through a series of experiments.This work was supported by the National Natural Science Foundation of China under contract Nos. 10734020 and 10674094, National Major Basic Research Project of 2006CB921507, Shanghai municipal projects of 05DJ14003 and 06JC14039, as well as the National Minister of Education Program for ChangJiang Scholars and Innovative Research Team in University of IRT0524.