Investigation Of Microstructural Gas Sensitive Thin Film And Integrated Sensors

With the boom of the Internet of Things(IoT)and trend of automation,the research and development of sensors,as the front-end core devices and cornerstone of the IoT system,has garnered enormous attention.Among numerous types of sensors,gas sensors(including humidity sensors)targeting specific gas molecules in the ambient air play a vital role in various fields of application like air quality monitoring,smart home services and unmanned storage.To meet these needs,gas sensors are advancing towards high performance,low power consumption,miniaturization,multi-function,and integration.However,the performance of existing gas sensors is still restricted by such imperfections as limited sensitivity,high power consumption,bulky size,single feature and lack of integration,resulting in insufficient properties for their later use.To address this issue,this thesis gives prominence to the fabrication and investigation of microstructural gas sensitive materials with integrated sensors.The sensing mechanism analysis is presented through exploring several aspects in particular,i.e.the fabrication methods of microstructural sensitive films,the influence of sensing film structure on its sensing properties,the design and fabrication of microsensor structure,and the construction and application of sensor system.The main body of this thesis includes the following five sections.1.Protonation process is employed on polyethyleneimine(PEI)to overcome flocculation when it is mixed with graphene oxide(GO)aqueous dispersion.A stable mixture material of protonated PEI(P-PEI)and GO is successfully obtained and decorated on quartz crystal microbalance(QCM)via dip coating method.The protonation is confirmed through the spectroscopy.The morphology and oscillation frequency shift show that the as-prepared film with the ripple-like surface owns higher specific area and mass-loading amount,which provides more surface adsorption sites.The humidity sensing properties measurement results show that the mixture composite material increases the sensitivity of sensor and overcomes the oscillation failure of pristine PEI at high humidity level.The sensor achieves a sensitivity of 19.1 Hz/%RH,as well as good performance in repeatability,selectivity and long-term stability.The hysteresis of composite sensor is reduced compared with pristine sensor.2.Different deposition techniques are employed in preparing the GO/PEI layered composite films on QCM devices.PEI and GO are deposited in sequence onto the surface of QCM devices via spray method.The stable bonding effect produced by amide reaction and ionic effect is confirmed through spectroscopy analysis.Based on the morphology study of composite film,a wrinkled surface is observed and GO sheets are inserted into PEI layer due to the redissolution effect in micro-area.The adsorption area is increased by the constructed 3D microstructure.Humidity sensing properties measurement results suggest that the layered GO/PEI achieves good sensing properties in sensitivity(27.3 Hz/%RH),repeatability,selectivity,long-term stability and hysteresis.To analyze the influence of film structure on the humidity sensing performance of layered films,GO and PEI are alternately deposited on the surface of QCM benefit from the chemical bonding between GO and PEI,as well as the intrinsic stickiness of PEI.Results reveal that the QCM sensors through layer-by-layers methods shows longer response and recovery time with larger humidity hysteresis in comparison with sensors through spray method.By comparing the structure of films that fabricated through spray method and self-assembly method,it could be found that the embedded structure of GO sheets benefits the adsorption and delivery of water molecules,indicating that the film microstructure has obvious influence on the humidity sensing properties of QCM humidity sensors.The humidity sensing mechanism is hence established by comparing the different microstructure of layered films prepared via different methods and combining the measurement results of layered films,PEI film and GO film.3.Standard semiconductor technologies are employed in the design and preparation of gas sensitive field effect transistors(FET)with different structures.The sensing function of gas sensitive field effect transistors is verified.Firstly,ultra-thin body(UTB)FETs are fabricated from a silicon on insulator(SOI)wafer.The humidity sensor with sensitivity of 8.6%/%RH is obtained through channel decoration by using humidity sensitive material via spin-coating.The sensing mechanism is analyzed with help from the bottom gate.Then,gate-free FETs are fabricated in bulk silicon structure in order to make it easier for operation and fabrication and reduce the cost.Phthalocyanine copper(CuPc)is deposited on the channel of FET for the fabrication of gas sensors.The as-prepared sensor demonstrates a good sensitivity and selectivity towards nitrogen dioxide with the capability in the detection of nitrogen dioxide as low as 50 ppb.The feasibility of the strategy to fabricate microstructure gas sensors based on the FET structure is proven by the experiments.4.Ion implantation is employed during the fabrication of sensitive FETs to shift the threshold voltage of transistor.Gate-free FET based ammonia sensors,hydrogen sulfide sensors and humidity sensors are fabricated via photolithography and thermal evaporation.The prepared ammonia sensor and hydrogen sulfide sensor achieve outstanding sensing properties,i.e.large detection range with low detection limit from 10 ppb to 5 ppm,with the responses toward 1 ppm of target gas being 13.3 and 724.5 for ammonia sensor and hydrogen sulfide sensor,respectively.The sensitivity of humidity sensor is 33.9%/%RH.All three sensors have good repeatability and selectivity,and the cross-sensitivity of three sensors and hysteresis are negligible.Given the measurement results,the sensing mechanism of gas sensitive FET is established based on the adsorption theory and energy band theory.5.A gas sensing system is constructed by combining sensor array,test chamber and reading circuit.The gas sensor array consists of multiple FET-based gas sensors.The freshness of foods like egg and pork is monitored by the concentration readout of ammonia,hydrogen sulfide,and humidity via above system.The food spoilage time of both foods at different temperature conditions are tested.Based on these results,the integrated microstructural gas sensing system is proven to be a functional example.