Fabrication and Characterization of Zinc Oxide (Zno) Nanostructures
Author: Yu-Hang Leung, 梁宇恆
Publisher: Open Dissertation Press
Release Date: January 27, 2017
This dissertation, "Fabrication and Characterization of Zinc Oxide (ZnO) Nanostructures" by Yu-hang, Leung, 梁宇恆, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author.
Abstract of thesis entitled FABRICATION AND CHARACTERIZATION OF ZINC OXIDE (ZnO) NANOSTRUCTURES Submitted by Leung Yu Hang For the degree of Master of Philosophy at The University of Hong Kong in January 2006
Since the discovery of carbon nanotubes by Iijima in 1991, increasing research effort has been devoted to the synthesis and characterization of nanostructured materials. Materials at this scale exhibit novel properties which cannot be found in their bulk form. In silicon nanowires, for example, the bandgap of Si nanowires increases from 1.1 eV to 3.7 eV when the nanowire diameter decreases from 7 nm to 1.3 nm. In recent years, nanostructures of various semiconductors like Si, gallium arsenide, gallium nitride, and zinc oxide have been demonstrated. Among them, ZnO has been recognized as a promising material for a variety of applications in the field of photonics, optoelectronics, gas sensing, field emission, and piezoelectrics. To realize the incorporation of ZnO nanostructures into real life applications, lots of work need to be done, including synthesis of the nanostructures, and understanding of their properties. In this study, a number of ZnO nanostructures (tetrapods, nanorods, nanoribbons/combs, multipods) were fabricated by thermal evaporation and hydrothermal synthesis methods. The morphologies and, structural and optical properties of the resultant nanostructures were then characterized by various techniques, such as scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, X-ray diffractometry, photoluminescence and electron paramagnetic resonance. It was observed that fabrication conditions could significantly affect the properties of the nanostructures. All nanostructures exhibited one UV peak and one broad peak in the visible spectrum. The major difference is that the nanostructures fabricated by thermal evaporation exhibited green PL emission while the nanorods synthesized by hydrothermal method gave yellow emission. Green emission was observed even without the presence of the EPR signal at g 1.96 in some samples. This result contradicts the commonly cited oxygen vacancy hypothesis of green emission of ZnO. Obviously, that hypothesis does not apply to all ZnO samples. The nanorod sample with yellow emission also exhibited strong EPR signal similar to the green emitting sample. A possible explanation for the obtained results is that there are two different deep levels responsible for the green and yellow emission respectively. Stimulated UV emission of three different nanostructures (tetrapods, nanoribbons/combs, and nanorods) was studied by time-resolved photoluminescence (TRPL). It was observed that different nanostructures exhibited different lasing thresholds, emission delay times and decay times. The difference in the lasing behaviors of the nanostructures is likely due to the difference in morphology, mode of cavity, or even native defects present in the material.