Research On Fabrication, Structure And Properties Of ZnO Nanomaterials

In the last decades, wide band gap semiconductors have become a hot research direction in electronics, materials, physics, chemistry and biomedical areas due to their potential applications in short-wave light emitting devices, microelectronic devices, biosensors, photodetectors and photocatalysts. Among them, zinc oxide(Zn O) is a promising material with extensive applications in fabricating nanoelectronic, sensor and luminescent devices owing to its wide direct bandgap(3.37 e V) and large exciton binding energy(60 me V). Moreover, Zn O nanomaterials exhibit good optical, electrical and magnetic properties which are quite different from bulk Zn O materials. Numerous researches have been performed to investigate the preparations and growth mechanisms of Zn O nanomaterials, and many Zn O nanomaterials with different various shapes have been fabricated.In this thesis, by adjusting the buffer layer type, source material ratio and temperature, we have fabricated a series of Zn O nanomaterials with different morphologies using the chemical vapor deposition(CVD) method. The structure, components, and optical properties of the samples are characterized by x-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), high-resolution transmission electron microscopy(HRTEM), selected area electron diffraction(SAED), energy dispersive spectroscopy(EDS), photoluminescence(PL) spectroscopy, cathodeluminescence(CL) spectroscopy, x-ray photoelectron spectroscopy(XPS) and Raman scattering. Based on the experiment results, the growth mechanisms of different Zn O nanomaterials are also discussed. The main research results are summarized as follows.1. Tetrapod-like Zn O nanomaterialsCaltrop-like, awl-like and antenna-like Zn O nanomaterials are prepared by using the CVD method on Cr2O3, Ni and Au coated Si(100) substrates, respectively. XRD, TEM and SEM results demonstrate that all the three types of Zn O nanomaterials are hexagonal wurtzite structure. The sharp and intense diffraction peaks indicate that the as-prepared samples are of good crystalline quality. Room-temperature PL and CL measurements indicate that the Zn O nanomaterials possess good optical properties. Moreover, light spots in the CL image of Zn O nanocaltrops and nanoawls, as well as the blue-shift of ultraviolet peak and red-shift of the green peak in the CL spectra, are observed and explained. No catalyst particles are observed on the tips of the three tetrapod nanomaterials, indicating that the main growth mechanism follows the vapor-solid(VS) mode. Based on the octa-twin model and other growth models, the growth mechanisms of the tetrapods are proposed and discussed. It is believed that the wet ablitiy and Young modulus between substrate and buffer layer, the reactant vapor density and the substrate temperature result in the different morphologies.2. Stalactite-like Zn O nanorodsStalactite-like ZnO nanorod arrays are synthesized on the Ni-coated Si(100) substrate by using the CVD method. The XRD pattern demonstrates that the wurtzite Zn O nanorods are of good crystalline quality. A weak diffraction peak appearing at about 43.0° may come from the γ-Ni5Zn21 phase with high catalyzing ability. Scanning transmission electron microscopy(STEM) and TEM results show that each nanorod is about 100-200 nm in diameter at the tip and 3-5 μm in length. The bright spots are observed at the tips of nanorods in the CL image, indicating their strong green emission features. Irregular catalyst particles are observed at the nanorod tips, suggesting that the growth is dominated by the vapor-solid-solid(VSS) mode. A possible growth mechanism is then discussed in the framework of VSS theory. The catalysis of Ni-Zn alloy and the buffer layer of zinc sub-oxide may play important roles during the growth process of nanorods.3. Urchin-like Zn O nanomaterialsUrchin-like ZnO nanomaterials are synthesized on Si(100) substrates by using the CVD method. A structural characterization reveals that the Zn O urchin is also of single crystalline wurtzite structure. Each Zn O urchin is composed of a nucleus and tens of nanofeet which are preferentially grown along the [0001] direction. The CL measurements reveal an ultraviolet emission at 380 nm and three green emissions at 521 nm, 520 nm and 512 nm, respectively. With decreasing radius of the nanofoot, the surface state density increases, both the Zn concentration and the number of oxygen vacancies decrease, the surface curvature and the number of free excitons increase, leading to a green emission blue-shift from 521 nm to 512 nm. In the framework of classic nucleation theory, the growth mechanism of the urchin-like Zn O nanomaterials, the relations between the nucleation rate, nanofeet density and the critical nuclei size, and the major factors influencing the formation of different morphologies of Zn O nanomaterials are discussed.