Preparation And Properties Of Low-dimensional ZnO Nanostructures

ZnO, with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV), is an important semiconducting material. Low-dimensional ZnO nanostructures have recently attracted much attention due to their novel properties and potential applications in manufacturing electronic and optoelectric devices. In this thesis,we prepare ZnO:Y thin films and investigate their electrical and optical properties; synthesize dumbbell-like ZnO microcrystals and ZnO nanotube bundles and study their photoluminescence; fabricate the gas sensors of ZnO nanotube bundles and study their gas-sensing properties. Some important results are obtained.ZnO:Y thin films have been prepared by sol–gel method. The effect of dopant (Y) concentration, heating treatment and annealing in a reducing atmosphere on the structure, morphology, electrical and optical properties of ZnO thin films were investigated. All of the thin films were composed of closed-packed nanoparticles, and exhibited the preferred (002) orientation. When compared with the resistivity values of films without the annealing treatment, the values of films annealed in the reducing atmosphere were decreased by about three orders of magnitude. The lowest resistivity value was 6.75×10-3 ? cm, which was obtained in the 0.5 at.% yttrium-doped ZnO thin film annealed in nitrogen with 5% hydrogen at 500 oC. The average optical transmittance values of the annealed films were more than 80% in the visible range.We present a simple wet chemical approach to the synthesis of dumbbell-like ZnO microcrystals and ZnO nanotube bundles without any additives, templates or substrates at an ambient pressure and low temperature. The crystallinity, morphology, and structure of dumbbell-like ZnO microcrystals and ZnO nanotube bundles were examined, effects of the reactant concentration and the reaction time on the size and shapes of the ZnO products were analyzed, and the formation mechanism of dumbbell-like ZnO microcrystals and ZnO nanotube bundles were discussed from the angle of nucleation and morphology. Furthermore, the room temperature and low temperature PL of the dumbbell-like ZnO microcrystals and ZnO nanotube bundles were also investigated. The results indicated that when dumbbell-like ZnO microcrystals and ZnO nanotube bundles were annealed at 600oC and 300oC in air, respectively, the UV emission intensity was strongest and the green emission intensity was weakest.Based on large-scale ZnO nanotube bundles synthesized by a single wet chemical method, we fabricated the gas sensors of ZnO nanotube bundles. Ethanol, NH3,H2,CO and CH4 sensing characteristics of the fabricated sensors were investigated. The sensitivity to ethanol vapor of the fabricated sensors was highest at wok temperature of 230 oC, and the response and recover time to ethanol vapor was very short. Compared with the sensitivity to ethanol vapor, the sensitivities to NH3,H2,CO and CH4 were very much lower, which indicates that the gas sensors of ZnO nanotube bundles has a very good selectivity.