| As a direct wide bandgap Ⅱ-Ⅵ compound semiconductor, zinc oxide (ZnO) has a wide band gap (3.37eV), a high exciton binding energy (60meV), outstanding thermal and chemical stability and excellent radiation resistance, which make it a promising candidate in a broad applications in the fields of electronic and optoelectronic devices. In recent years, nano ZnO became a hotspot in the field of condensed matter physics and materials research, for its rich and easy-to-control structures and morphologies, and its fantastic properties. Silicon nanoporous pillar array (Si-NPA) is a silicon micro and nanometer structural composite system, possessing a triple hierarchical structure, i.e. the regular array of micro-sized silicon pillars, the high-density nanopores distributed all over the pillars, and the silicon nanocrystallites composing the pore walls. The unique structure of Si-NPA and its special physical and chemical properties make it an ideal substrate or template for silicon-based materials. Here we prepared ZnO/Si-NPA through chemical vapor deposition (CVD) method, with Si-NPA as the substrate. We studied the regulation and control of the morphology of ZnO/Si-NPA by changing the preparation conditions, the field emission characteristics of ZnO/Si-NPA, the humidity sensing properties of the ZnO/Si-NPA, and the ethanol sensing properties of ZnO hollow spheres. The main research results achieved in this thesis are listed as the followings.1. The preparation and morphology control of ZnO/Si-NPA.Using Si-NPA as the substrate, high-purity zinc powder and oxygen as the zinc source and the oxygen source respectively we prepared ZnO/Si-NPA nanocomposites by CVD method. The morphology of ZnO/Si-NPA can be controled by changing the distance between the zinc source and the substrate, the growth temperature, the oxygen introducing temperature and the rate of oxygen flow:(1) When maintaining the growth temperature (900℃), the oxygen introducing temperature (750℃) and the rate of oxygen flow (2sccm) unchanged, increasing the distance between the source and the substrate, the quantity of ZnO deposited on the Si-NPA substrate reduced gradually. And the ZnO nanorods cluster array gradually changed to be ZnO nanoneedles cluster array.(2) While maintaining the source-substrate distance (2cm), the oxygen introducing temperature (600℃) and the rate of oxygen flow (2sccm) unchanged, increasing the growth temperature, the deposition quantity of ZnO on the Si-NPA substrate gradually increased and their morphology changed. ZnO nanorods were short and radially, selectively distributed on the tip of the silicon pillar when the grown temperature maintained600℃. When the grown temperature reached700℃, The length of ZnO nanorods increased and maintained selectively and radially distributed around the the tip of the silicon pillar. At800℃, the diameter of ZnO nanorods became larger, the density increased and the nanorods evenly distributed on the silicon pillar and the valley between the pillars.(3) Keeping the source-substrate distance (4cm), the growth temperature (700℃) and oxygen introducing temperature (600℃) unchanged, the rate of oxygen flow changed. Under these conditions ZnO nanoneedle radially distributed on tip of the silicon pillar, forming ZnO nanoneedles cluster array. With the rate of oxygen flow increased, the length and density of ZnO nanoneedles reduced the diameter increased.(4) Maintaining the source-substrate distance (4cm), the growth temperature (900℃) and rate of oxygen flow (2sccm) unchanged, the oxygen introducing temperature changed. When the oxygen was introduced at550℃, the clustered ZnO nanorods with a relatively uniform diameter from bottom to top were obtained. When the oxygen was introduced at650℃, ZnO nanorods cluster array was obtained.2. The field emission properties of ZnO/Si-NPAUsing the Si-NPA with the same pillar shape morphology as the substrates, the ZnO/Si-NPA with the nanorods and nanoneedles cluster array were obtained through changing the preparation conditions, and their field emission performances were studied comparatively. And using the Si-NPA with the pillar and crater shape as the substrates, the pillar and crater shape zinc sulfide (ZnS)/Si-NPA were prepared, and the morphology effect of the ZnS/Si-NPA were studied. The results showed that the nanoneedles cluster array had a lower turn on field, a larger field enhanced factor and a higher current emission density compared with the ZnO nanorods cluster array. And the ZnS/Si-NPA pillar array had a lower turn on field, a larger field enhanced factor and a higher current emission density compared with ZnS/Si-NPA crater shape array. And the F-N curves show a distinct characteristic. The electric field distribution around the tip of the unit of the pillar array and crater shape array were simulated by finite element modeling, and the results showed that the tip of the pillar unit presents a stronger field enhancement effect, indicating that the substrate morphology is one of the important factors that effect the cold cathode field emission performance.3. The humidity sensing properties of the ZnO/Si-NPAUsing Si-NPA as the substrate, hydrangea and cauliflower shaped ZnO/Si-NPA were prepared. The corresponding humidity sensors were prepared and their humidity properties were studied. Compared with the hydrangea shaped ZnO/Si-NPA, the cauliflower shaped ZnO/Si-NPA had a shorter response and recovery time and a smaller hysteresis. In addition, the cauliflower shaped ZnO/Si-NPA had good reproducibility and long-term stability. The analysis showed that their different humidity sensing properties were caused by the difference of the morphology, characteristic size, and the structure of the ZnO nanostructure grown on Si-NPA. In addition, SnO2:Sn/Si-NPA nanocomposites were prepared and their humidity sensing performance were studied. The results showed that SnO2:Sn/Si-NPA has a high sensitivity, fast response recovery time, small hysteresis, and good reproducibility and stability. These results showed that Si-NPA is an ideal substrate for preparing a silicon based oxygen semiconductor sensors, and the morphology of the sensing material has great effect on the sensing performance, especially the morphology of the compound semiconductor plays an important role in its humidity sensing property.4. The ethanol sensing properties of the ZnO hollow sphere.During the procedure of preparing the ZnO/Si-NPA by CVD method, a part of the high purity zinc powder was oxidized to form a large quantity of ZnO hollow spheres. By coating the ZnO hollow spheres onto a ceramic tube a resistance gas sensor was prepared and its ethanol sensing properties were studied. The results of the ethanol sensing test showed that the optimum operating temperature of the sensor was350℃. In the whole testing concentration range of10-400ppm, the ZnO hollow spheres sensor showed a high sensitivity (0.25/ppm), a short response and recovery time (5-12s and8-12s), and the response-concentration curve was highly linear. The circular test results for different ethanol concentrations showed that the ZnO hollow sphere sensor had good reproducibility, stability and selectivity (against methanol, formaldehyde and benzene). Based on the morphology feature of the ZnO hollow sphere and the chemical properties of ZnO nanostructure, the sensing mechanism was analyzed. ZnO hollow spheres have a large specific surface area, giving it a high sensitivity, and the hollow spherical structure is conducive to effective transmission of the ethanol gas, giving it a short response and recovery time. ZnO hollow spheres showed good ethanol sensing performance and were ideal materials for the fabrication of ethanol sensor. |
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