The Fabrication, Doping And Growth Mechanism Studies Of ZnO Low Dimension Materials By Liquid Chemical Methods

Zinc oxide (ZnO) is one of the most widely researched semiconductors in recent years. As a typical wide bandgap metal oxide semiconductor ofâ…¡-â…¥group, the bandgap of ZnO is 3.4eV, which leads to its excellent shortwave light emitting ability. And the large exciton binding energy of 60meV makes ZnO as the most promising blue-ultraviolet emitting material at room temperature. Besides the photoelectricity capability, ZnO owns many other physical and chemical properties, like gas sensitivity, piezoelectricity, transparent conductivity and so on, which are also favorable in the correlative field. ZnO is a green material that is absolutely nontoxic for human being and it is favorable for the potential application in biomaterials. With the fast development of nanotechnology and thin films technology, it is found that due to the unique polar crystal structure, ZnO has rich nanostructures, which bring many peculiar optical properties and quantum effect. This can extremely improve the devices performance. Thus, the applied researches of ZnO low dimension materials have become more and more extensive and important for the application.So far, the fabrication of high quality thin films and nanostructures has become an important factor for the application of ZnO. The low cost and advantages in nanomaterials synthesis of liquid chemical methods have attracted more attentions. However, the quality improvement of thin films, the controllable growth of special nanostructures and the doping process are always the intractable problems in the liquid chemical methods. The growth mechanism is still ambiguous. They greatly limit the mass production of ZnO low dimension devices.In this paper, the sol-gel, reverse microemulsion and hydrothermal methods were employed as the liquid chemical methods for the fabrication of ZnO low dimension materials, which included ZnO nanocrystalline thin films with different orientation growth, Al-doping and the hollow nanostructures. Combining with the process of experiments, the growth mechanisms were analysed in details. Preferential c-axis oriented ZnO thin films were fabricated on the amorphous glass substrates by sol-gel method. The films are the nanocrystalline thin films, which are composed by many nanograins. The chemical and thermal analysis reveal that the-OH groups on the colloid particle surface play an important role for the dispersancy and stability of the sol. They also make the colloid particles arrange themselves more orderly on glass. The studies of different experimental parameters show that the orientation of ZnO thin film have an evolution from random orientation to c-axis orientation during the sintering. And the c-axis orentation grade can be affected by the preheating route after spin-coating. It is presented that the dipole-dipole interactions between polar ZnO nanograins induce a self-assembly arrangement and lead to the preferential c-aixs oriented growth of ZnO thin films on the amorphous substrates. And the preheating route with high difference in temperature forms a growth model from bottom to top in the film, which should be responsible for the higher c-axis orientation grade.Preferential a-axis oriented ZnO thin films were fabricated by the reverse microemulsion spin-coating method for the first time. FT-IR and TG-DTA analysis of the precursor reveal that CTAB as the surfactant still exists while the temperature is above the nucleation of ZnO, which implies the ZnO nucleation process in CTAB micelles. The SEM and AFM images show that the thin films have the islands growth model, and the increase of aqueous phase can increase the dispersancy of grains on the substrate and decrease the grains size. Due to the very low Zn2+ concentration in the precursor, it is presented that the preferential a-axis orientation attributes to the weakening of dipole-dipole interaction, which results from the large space between grains and the wrapping of CTAB micelles. Then the nonpolar planes along the a-axis of ZnO become more favorable to combine with the nonpolar glass substrate. The PL spectra suggest that the nonpolar thin films on the amorphous substrates are strain-free and have better crystallization than that on the single crystal substrates.The Al:ZnO thin films with the dopant contents of 1-10 at.% on the amorphous quartz and Si(100) substrates were respectively fabricated by sol-gel method. XRD and temperature dependence resistence reveal that the film with Al content of 2 at.% has the highest doping level. The SEM images show that the average grain size of thin film distinctly decreases as the Al contents increase. EDX mapping shows that the Al component undoped into ZnO lattice is rich on the grain boundary. It is presented that the Al atoms may form a state of amorphous Al2O3 and pin the movement of ZnO grain boundary, which leads to the decrease of grain size. The competition between the dopant contents increase and the grain refining makes the Al-doping in ZnO thin film has a maximum.A novel microemulsion hydrothermal method was used to fabricate the ZnO hollow nanostructures. With the templates using the aqueous micelles in the reverse microemulsion, the sol is made to hydrolyse on its surface and hydrothermal treatment. The results reveal that the superfluous sol adding could lead to the aqueous templates precipitation under the wrapping of hydrolysate. With the limit of surfactant micelles and aqueous cores, the production after hydrothermal crystallization is hollow nanospheres. While the sol adding volume is much less, the precursor would keep the state of transparency. The rich NH4+ in the aqueous micelles and the combination between them make the production to form ZnO hexagonal microtubes.