Studies On Vapor Deposition Method And Gas Sensitivity Of One Dimensional Wide Band Gap Semiconductor

With the emergence and development of nanotechnology and nanomaterials, it is found that the performance of the material is not only associated with the composition of the material, but also closely related to the microstructure morphology and the aggregation state. In addition, the aggregation state and microstructure are also affected by the preparation methods and preparation technology of materials. As a result, the generation process of material and structure, the mechanism and regulation of the structure are studied to improve material performance through adjusting the parameters so as to improve the development of materials science, which has important research significance and value.Wide bandgap semiconductor materials not only include gallium nitride (GaN), silicon carbide (SiC), diamond (C), aluminum nitride (A1N), zinc oxide (ZnO) and inorganic semiconductor materials, but also contain some organic small molecular materials and polymer semiconductor materials. Wide bandgap semiconductor materials have very important application in gas sensors, photocatalytic, solar cells, and so on, so direct wide band gap semiconductor has become a research hotspot recently. ZnO is a kind of II-VI direct wide bandgap semiconductor material, which showes the potential applications in the gas sensor, piezoelectric, sensor, liquid crystal displays, solar cells, ultraviolet light-emitting devices and catalysis etc. Therefore, the preparation, control and application of ZnO nanostructure have become one of the current research hotspots of nanomaterials. Organic semiconductor materials (especially the conjugated organic small molecules) are now under widely attention and promote the rapid development of molecular electronics, due to light quality, cheap and variety. In addition, its advantage is the structure and performance can be adjusted through molecular design under low temperature. One dimensional organic single-crystal micro-nano structure has many novel properties, which has important application prospects in the field of photoelectric etc.In this thesis, we choose two representative wide bandgap semiconductor materials include Cd doped inorganic ZnO and organic conjugated molecules 3,4,9,10-perlenetetracrboxylic dianhydride (PTCDA) as the research objects. The micro/nano structure was prepared by the modified gas phase methods and the performance was researched. Using the electrophoresis method, the single micro/nano electron devices were assembled, and the gas-sensing properties of materials were studied. We focused on the preparation process of two materials and put forward some key solutions to the practical application. At the same time, we conducted a series of study on the relationship between the microstructure morphology and gas sensing properties of the samples, and realized the effective control of construction materials. We have researched on the relationship between the microstructure morphology, physical properties of sample preparation and gas sensor to provide theoretical and experimental basis for the production and application of semiconductor nanomaterials as follows:Firstly, by a facile vapor-solid route, double helical organic small molecules PTCDA microfibrils were synthesized, which based on spontaneous twisting of supramolecular microtubes. The helices microfibrils, which were obtained by coiling the multilayer microtubes from the inner to the outer molecules layers tend to release their internal rotation stress, so as to present the most stable morphology which was characterized by XRD, FESEM, TEM and UV-Vis. The averaged diameter of the resulting helical structures was about 200 nm and overall length of several micrometers.The changed van der Vaals contact together with the surface free energy among adjacent microtubes molecules layer, were the driving forces to induce the formation of helical structure. The obtained results presented an extremely facile strategy for fabrication of small molecular double helical microfibrils with morphological transformation process. On the basis, we further adopt optical mask technology to assemble the micro fiber single electron device, and the device of this spiral fiber was investigated for the sensitivity to ethanol. The research found that the spiral microfiber has good gas sensitive and good stability to low concentration ethanol (e.g.20 ppm) at room temperature, which is a kind of potential gas sensitive materials with industrial application prospect.Secondly, by a kind of modified chemical vapor deposition (CVD) method, Cd-doped ZnO nanowires (NWs) with different doping concentrations (0.0 wt%,1.0 wt%,2.0 wt%,3.0 wt% and 4.0 wt%) were synthesized. The structures, element valences and properties of the grown NWs have been characterized by XRD, FESEM, SAED, XPS, TEM and BET surface area measurement method. The results showed Cd doped ZnO nanowires had good and orderly structure and high specific surface area when the reaction temperature was 600℃. With further increasing the preparation temperature, nanostructure defects increased obviously, and appeared a reunion phenomenon. The main reason was the higher temperature accelerated the growth rate and the growth direction losed the control. On the basis of these studies, we explored the growth mechanism of the nanowires, and the growth process was in line with the gas-solid (VS:Vapor-Solid) growth mechanism which was discusses in detail. In addition, we used electrophoresis method to assembly the single nanowires devices, and the relationship between the gas sensitive to the reducing gases especially H2S and the Cd doping amount of ZnO nanowires was studied.3wt% Cd doping amount ZnO nanowires showed the best response value which was double of the no doped ZnO. However, the response value for 4wt% Cd doping amount ZnO nanowires decreased. These nanowires devices also had certain of responsiveness to the other reducing gas. In addition, we further investigate the gas sensing mechanism of Cd doped ZnO nanowires, sensitivity to H2S gas. Sensitivity of gas is mainly attributed to the gas molecules in the material surface adsorption and desorption, caused by the surface structure of materials and energy bands changed, barrier height corresponding change, resulting in the carrier transmission speed in the surface of the nanowires is different, the macroscopic resistance of the nanowires changed. In summary, the process is chemical adsorption desorption and electronic sensitization synergy results.