Fabrication, Characterization And Properties Of ZnO Nanostructures

In recent years, the wide band gap semiconductor has attracted a lot of interests and advanced rapidly, mainly due to their promising applications in short-wave light emitting devices,photodetectors and high power electronic devices. Zinc oxide (ZnO), is one of the most important multifunctional n-type II–VI compound semiconductors, with a direct wide bandgap (3.37 eV), high exciton binding energy (60 meV) at room temperature and superior thermal stability. As a widely used semiconductor material, ZnO have gained itself substantial interest from all over the world due to its unique piezoelectric, pyroelectric, catalytic and photocatalytic properties, and its potential applications in preparing solar cells, gas sensors, UV photodiodes, transparent electrodes and optoelectric devices. ZnO nanostructures with various shapes, such as, nanowires, nanorods, nanowhiskers, nanobelts, nanotubes, nanoflowers have been successfully synthesized by different methods including chemical vapor deposition (CVD), chemical solution deposition method, pulsed laser deposition (PLD), electrochemical deposition, hydrothermal synthesis, spray pyrolysis technique and thermal evaporation.In this paper, various ZnO nanostructures are synthesized by PLD and thermal evaporation. The morphology, structure and compositions of the ZnO nanostructures were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), scanning electronic microscope (SEM), transmission electronic microscope (TEM), high-resolution transmission electronic microscope (HRTEM) and photoluminescence (PL) , respectively. We have put forward the viewpoint of the the morphological differences might be a consequence of the different sizes of the nucleus.The the possible growth mechanism of the ZnO nanostructures is also discussed in brief. All the results are as follows:1. First, the ZnO films and Zn films were deposited on sapphire (001) and Si(111) substrates by pulsed laser deposition(PLD), respectively. This ZnO and Zn films will played a different role in the growth process of the ZnO nanostructures. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the tetrapod-like ZnO nanostructures. The pre-deposited Zn films by PLD on the substrate can promote the formation of the ZnO nuclei and also promote the growth of the ZnO nanostructures effectively.2. Tetrapod-like ZnO nanostructures, multipod ZnO whiskers and flower-like ZnO nanorods were grown on ZnO-coated sapphire (001) substrates and Zn-coated Si(111) substrates by catalyst-free thermal evaporation method. The as synthesized ZnO nanostructures were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), area electron diffraction (SAED) and photoluminescence (PL) , separately.3. Optical properties of the ZnO nanostructures. For the optical property, the measurement of PL spectrum was performed with a Xe lamp as the excitation source (wavelength was 325 nm) at room temperature. There are also have two peaks: a ultraviolet (UV) peak and a green emission peak. The UV peak correspond to the near-band-adge emission, which usually ascribed to the near-band-edge exciton transition. The green emission peak is referred to as a deep-level or trap-state emission. The deep level emission in ZnO may be attributed to the singly ionized oxygen vacancy in the ZnO nanostructures and the results from the recombination of electrons at the conduction band with holes trapped in oxygen-related defects. Thus, the green emission would be a result of the existence of the oxygen vacancies in the ZnO nanostructures. So, we can conclude that there are a number of oxygen vacancies in ZnO nanostructures in our experiment.4. We discussed the growth mechanism of each kind of ZnO nanostructures by analyzing the growth process of the ZnO nanostructures. We studied the influence of the morphology of the ZnO nanostructures. We investigated the influences of different films on the different substrate on the surface morphology of the ZnO nanostructures. The results indicate that the differences morphological of the nanostructures might be a consequence of the different sizes of the nucleus. We also propose the viewpoint that the differences morphological of the nanostructures might be a consequence of the different sizes of the nucleus.