Research On Fabircation And Sensing Propeties Of Gas Sensors Based On Carbon Nanotube Films

With the increased need for high-performance gas sensor in industrial production, home security, environmental monitoring and medicine and so on, there are an uprising of research and development activities of gas sensors. Utilizing the outstanding physical and chemical properties of carbon nanotubes (CNTs) in gas sensors, a new approach will be provided to improve the performance of the devices, which may have advantages such as high sensitivity, lower working temperature, lower power consumption and so on. In this dissertation, in order to realize a high sensitivity gas sensor based on a CNT film, the fabrication process of depositing CNT films onto microelectrodes, and functional modification methods for enhancing the gas sensitivity of CNT films are investigated. The sensitivity of the sensors to several gases is tested accordingly.A conductivity model of the CNT film is established based on the CNT resistances and the CNT junction resistances. Combining with CNT film structure; through the tunneling theory, the theoretical formula of the CNT junction resistance is obtained. The sensitivity of the CNT film is explained using the conductivity model. The adsorption of gas molecules will affect the CNT resistances and CNT junction resistances.A design of field effect transistor (FET) structure based on a CNT film as both sensitive material and signal conversion is proposed. For depositing CNT films onto microelectrodes, dielectrophoresis (DEP) and layer-by-layer self assembly (LbL) methods are utilized respectively. The density of assembled CNTs mainly depends on the magnitude of the applied ac electric field and the concentration of CNT solution on DEP. A feedback control circuit is utilized to control the resistances of assembled CNT films. Polymers and metals are employed to functional modify CNT films. Three kinks of polymers, including Poly (dimethylammonium chloride) (PDDA), styrene-butadiene-styrene copolymer (SBS) and mixture of polyethyleneimine and Starch (PEI/Starch) are used to modify the CNT surface, respectively, and four kinds of metals, including Au, Pt, Ti, Ni are deposited onto CNT films respectively by magnetron sputtering for decoration. The stability of the CNT film resistance is effectively improved by annealing treatment. Experimental results show that the CNT films have a negative temperature coefficient of resistance, don't possess the electrical properties of FET, and obtain higher sensitivity at low frequency during impendence test.Gas sensing properties of different CNT films are investigated for CO2, humidity and ethanol gas. The resistance of carboxylated multi-walled carbon nanotube (MWNT) films assembled by dielectrophoresis increases linearly with humidity increasing, the sensitivity is about 0.5%/%RH. CNT/PDDA films have higher sensitivity to humidity, the resistance increases exponentially with humidity. The thickness of CNT/PDDA films is thinner, the response is faster, and there is an optimum thickness to make the film having a greatest sensitivity for ethanol gas. The gas sensing performance of MWNT/SBS films was not improved. The response of the MWNT films modified by PEI/Starch for CO2 is significantly increased. The response of the MWNT films decorated by metals significantly enhance the sensitivity for ethanol gas, in particular, the film modified by Au can achieve the detection of 200ppm ethanol gas concentration, and the film modified by Pt has a better sensitivity for benzene.