In Situ Assembly Of Bridged Metal Oxide Nanowires Sensing Devices And Sensitivity Enhancement Via Pulsed Temperature Modulation

With the advent of the Internet of Things,people's demand for gas sensors is increasing.In numerous gas sensors,metal oxide semiconductor(MOS)sensor with the advantages of small size,low power consumption and ease of integration has received the widespread attention.Although nanostructured MOS gas sensors can be integrated into intelligent terminals,the deficiency of sensor sensitivity,detection limit,power consumption index,and integration bottleneck are the main factors restricting the application of the Internet of Things.This work improved the gas sensing performance of MOS nanostructures prepared based on the device structure design,Pt sensitization,and pulse temperature modulation(PTM),which provides a way of thinking of preparing in-situ integration for high sensitivity,low power sensor.The specific research results and innovation points are as follows:1.In-situ assembly of regularly bridged CuO micro-hemisphere nanowire arrays(RB-MNAs)for high-performance sensing applications.Herein,RB-MNAs have been rationally designed on(indium-tin-oxide,ITO electrode)substrates,via thermal oxidation of ordered Cu micro-hemisphere arrays obtained by solid state dewetting of patterned Ag/Cu/Ag film.Both the position and spacing of CuO micro-hemisphere nanowires could be well controlled by as-used shadow mask and the thickness of Cu film,which allows to homogeneously manipulate the bridging of adjacent nanowires grown from neighboring CuO hemispheres,and thus benefits for achieving highly sensitive(trimethylamine,TMA)sensor.The electrical response of 3.62 toward 100 ppm TMA is comparable to the state-of-the-art CuO based sensors.The combination of in-situ assembly of RB-MNAs device arrays and conventional photolithography technology provides the potential for large-scale applications of oxide(CuO)nanowire devices.The advantage of the "bridging" structure has significantly improved the gas sensitivity,but it is still inferior to the traditional sensitization methods(such as noble metal sensitization).The next part of the work will use noble metal sensitization to improve gas sensing performance.2.Unveiling the low-temperature growth of WO2.72 nanowires by oxidation of metastable ?-W and using noble metal sensitization to improve gas sensitivity.With the unique geometrical structure,versatile polymorphs and sub-stoichiometric compositions featuring innate tunnels and oxygen vacancies,quasi-one-dimensional(q-1D)tungsten oxide(WOx)nanowires have been explored as multifunctional materials with diverse applications.Though the existing vapor phase transport or thermal oxidization methods allows to in-situ patterning of WOx nanowires,the relatively high growth temperature(?500-1300 ?)hinders their emerging applications.In this work,to gain a close insight into the driving force that determines the 1D anisotropic growth of WOx nanowires during thermal oxidation,the temperature and oxygen partial pressure dependent growth have been systematically investigated for both W film and powders.WO2 72 nanowires could be steadily obtained under appropriate temperature and oxygen pressure range for both cases,while the growth temperature for W film(metastable ? phase dominant)could be much lower than that of W powder(alpha phase).The structural analysis indicates the metastable ?-W is susceptible to oxidation in comparison with ?-W,and thus enables us to tailor oxidation induced chemical compression for nanowire growth.The growth temperature of WO2.72 nanowires could be reduced to 400?,which paves the way for the in-situ patterning of WO2.72 nanowires on the indium-tin-oxide(ITO)glass substrates and flexible substrates.At the same time,conventional Pt chemical sensitization can significantly improve the gas sensitivity of WO2.72 nanowires.However,the traditional chemical sensitization method has complicated steps and limited sensitization effect.Therefore,in the next part of the work,a simpler physical sensitization method will be proposed to greatly improve the sensitivity to VOCs.3.A generic approach to boost the sensitivity of metal oxide sensor by decoupling the surface charge exchange and resistance reading process is proposed.In this work,the alternative to conventional strategies of designing sensitive surfaces via morphology/defect/heterojunction control(then operating at an optimized isothermal temperature with a maximal response),a facile enhancement approach by decoupling surface charge exchange and resistance reading process(possessing different temperature-dependent behaviors)through pulsed temperature modulation(PTM)is reported.Substantially magnifying electrical responses of a generic metal oxide(e.g.WO3)MEMS sensor toward diverse analyte molecules is demonstrated.Under the optimal PTM conditions,the response toward 10 ppm NO2 can be boosted from(isothermal)99.7 to 842.7,the response toward 100 ppm acetone is increased from(isothermal)2.7 to 425,respectively,which are comparable to or even better than most of the state-of-the-art WO3 based sensors.In comparison to conventional(isothermal)operation,PTM allows to sequentially manipulate the physi/chemisorption of analyte molecules,generation of surface reactive oxygen species(ROS),and sensor resistance reading,and thus provides an additional opportunity in boosting the response of oxide sensors.4.It is proposed that PTM combined with noble metal sensitization can greatly increase the gas sensitivity to low concentration VOCs gas molecules and reduce the limit of detection.The weak response of conventional MOS sensors to trace(ppb-level)VOCs molecules limits their application in indoor air quality monitoring.To expand the detection limit of the MOS sensor,Pt single atom sensitized WO3 sensing material was designed by combining PTM physical sensitization with traditional chemical sensitization,and a PTM test was carried out.Under the optimal PTM temperature(?300?),the response to 1 ppm TMA can be increased from(isothermal)1.9 to 6541.5 and the detection limit is 10 ppb,the response to 1 ppm p-xylene can be increased from(isothermal)10.3 to 1001.1 and the detection limit is 10 ppb,which are better than the reported WO3 based sensors.