The Sensing Properties, Photoluminescence And Field Emission Performance Of Functional Low-dimensional ZnO Material

ZnO, considered as one of the most important semiconductor functional material, has attracted great attention due to its unique electrical and optical characteristics. ZnO is a very promising candidate in the fields of gas sensors, optoelectronic devices and field emitters. Thus the sensing, photoluminescence (PL) and field emission properties of ZnO have been widely investigated. This thesis is focused on these three properties of ZnO. The main research work and the results as listed as follows:1. Undoped and doped ZnO thin films with the thickness of about 10 nm were deposited on sapphire substrates by pulsed laser deposition (PLD) technique. Some of the undoped films were covered with Pd particles or annealed in air. The H2 sensing properties of all these samples were measured. When P, Mn and Sb elements were doped into the ZnO films, the sensitivities were not improved. And Mg doping could greatly improve the H2 sensitivity of the ZnO film. The sensing mechanism was analyzed to explain the great improvement of sensitivity for Mg doped ZnO film. The ZnO films coated with Pd particles showed good response to H2 at low temperatre. It could detect 10 ppm H2 even at room temperature. Moreover, post-annealing in air could also improve the H2 sensitivity of the ZnO films. Basing on the H2 sensing measurement, we found that the conductivity of the undoped ZnO films converted from p-type at low temperature to n-type at higher temperature. Opposite responses to H2 were observed for ZnO with different conduction type. We explained the reason for such a conversion and the origins of the p-type ZnO were proposed.2. Different structures of one-dimensional ZnO have been fabricated by electrospinning method and the sensing properties of them were tested. It was found that the aligned ZnO nanotubes showed better H2 sensitivity than the ZnO films did. The ethanol and acetone sensing properties of the Tb doped ZnO nanofibers were measured. The results indicated that low concentration (<3%) of Tb doping could improve the sensitivity of the ZnO nanofibers to ethanol and the ZnO nanofibers with the Tb concentration of 2% and 3% showed higher acetone sensitivities among all the Tb doped ZnO nanofibers. Besides, bicomponent SnO2/ZnO nanotubes were fabricated by electrospinning with a side-by-side dual spinneret. The side-by-side SnO2/ZnO nanotubes sample showed good sensing properties to both ethanol and acetone. Its gas sensitivity to ethanol was high at the operation temperature higher than 200℃. And the optimal operation temperature for it to detect acetone was 300℃.3. Al doped ZnO (AZO) nanofibers, Tb doped ZnO (TZO) nanofibers and Eu or Tb doped ZnO/ZrO2 nanaobelts were fabricated by electrospinning technique. The Al doping could simultaneously suppress the green and orange emissions in the PL spectra of the ZnO nanofibers, which derived from oxygen vacancies and excess oxygen in the ZnO nanofibers respectively. This phenomenon could be explained as the separate distribution of these two kinds of the defects in the ZnO nanograins and Al doping could facilitate the neutralization of the two kinds of defects. The PL spectra of the Tb doped ZnO nanofibers showed that using a 325 nm laser as the excitation source, Tb3+ ions could not be well excited and the characteristic peaks of the Tb3+ ions were very weak. That is to say ZnO nanofibers could not be a good matrix for Tb3+ ions. On the other hand, Tb3+ in the ZnO/ZrO2 nanobelts showed strong characteristic peaks of Tb3+ ions, which could be attributed to ZrO2 in the nanobelts. Furthermore, the emissions corresponding to intra-5D0→7Fj(j=0,1,2,3 and 4) transitions of Eu3+ ions in the ZnO/ZrO2 nanobelts could be observed with a high emission intensity. And ZnO oxygen vacancy in the host matrix might act as an effective sensitizer for adjacent Eu ions.4. Homogeneous hierarchical-structure ZnO with ZnO nanoneedles growing on ZnO nanofibers has been fabricated by combining electrospinning and aqueous solution processes. High field emission performance of the hierarchical-structure ZnO, including low turn-on field, high current density at low applied field and high field enhancement factor was obtained. This good FE performance of the hierarchical-structure ZnO is due to the tapered tips of the ZnO nanoneedles, the high aspect ratio of the ZnO nanoneedles and the undulant space distribution of the ZnO nanoneedles.