Gas Sensor Based On Highly Oriented Vertical Silicon Nanowires Junction Array

With the rapid development of Internet technology and artificial intelligence,we are about to entering the Internet of Tings(IoT)era.The goal of the IoT is to realize the interconnection of all things to make our environment fully intelligent.The information transmitted by the IoT will come from ubiquitous intelligent terminals.The core devices that collect environmental data from intelligent terminals are sensors.Due to the wider range of application conditions and the trend towards miniaturization of electronic devices,the fabrication of suitable sensing materials and structures is particularly important.Gas sensors are one of the most important sensors which have important applications such as public safety,industrial control,environmental protection and personal health.Numerous new types of nanomaterials and nanostructures have been used in gas sensing devices,including nanoparticles,nanowires,nanotubes,and diverse types of two-dimensional nanomaterials.Silicon nanowires are considered as a promising nanomaterial for their high specific surface area,good compatibility and low power consumption.The preparation methods of silicon nanowires include chemical vapor deposition,laser ablation,reactive ion etching,solution method,metal assisted chemical etching,etc.In these methods,metal assisted chemical etching(MACE)gets widespread attention due to directly formed array,cheapness,convenience,size adjustable and other advantages.In this paper,we use MACE method to fabricate silicon nanowire arrays and establish a reliable gas sensing platform to achieve high sensitivity of gas detection and gas recognition.The main contents of this thesis are as follows:The first chapter introduces the research of electrical gas sensor based on nanomaterials,including chemiresistor,field effect transistor,field ionization,chemical capacitor and heterodyne oscillation,etc.Finally,the advantages and disadvantages of various principle sensors are discussed.Then the fabrication methods of silicon nanowires are introduced,and emphatically discusses the application of MACE method and template-assisted method.In the second chapter,the vertical silicon nanowire arrays were fabricated by MACE etching method.The challenge of applying nanowire arrays to connect to an electrical circuit reliably and effectively was solved by Joule heat welding with tip contact.The fabrication of the vertical silicon nanowire arrays based on the tip contacts can effectively expand the space and nanowire utilization efficiency of the nanowire arrays,which facilitates the diffusion of gas in the two-layer nanowire arrays and improves the gas performance.The experimental results show that the NO2 gas sensor operating at room temperature has a sensitivity of?10 ppb and has good repeatability and response speed.At the same time,this platform can be used to construct a highly ordered array of nano p-n junctions,which can be used to explain the gas sensing mechanism of the p-n junctions in the case of electron flowing through the interfaces and provide direct evidence for the enhancement of the gas sensing performance based on nano p-n junctions.It has the guidance meaning for the design of new architecture of hybrid nanomaterials for gas sensing applications.In the third chapter,ZnO nanorods were grown on the top of the silicon nanowire arrays by hydrothermal method,and an ordered array of nano p-n junctions was fabricated.The sensitive responses for SO2(LOD?50 ppm),NO2(LOD?1 ppm),CO(LOD?50 ppm)and NH3(LOD?10 ppm)were realized at room temperature.Due to the different response performance of the ordered p-n structure at the forward and reverse voltage,recognition of different gases could be achieved.By controlling the amount of the ZnO seed solution for drop coating,the sensing material can be adjusted to form a p-type or n-type gas response mode at a forward voltage.At the reverse voltage,the electrical resistance across the p-n interface becomes the major factor.At this time,the gas response behavior is only affected by the properties of gas,and the different electron-donating(electron-withdrawing)capabilities determine whether the response can occur or not.Therefore,this sensing structure can simultaneously and simply visualize the redox properties of gas and the detecting the electron transfer ability of the gas after being absorbed on the sensing materials,so that the recognition of different gases was realized.In the fourth chapter,ZnO/SiNWs array structure was coated with a layer of reduced graphene oxide.The formed rGO/ZnO/SiNWs structure displayed good electron emission ability as well as the repeatability.The field emission enhancement factor even reached 12000,which is a kind of competitive field electron emission material.The ability to detect multiple gases at low levels of vacuum with good recovery and recognition of electron-withdrawing and electron-donating gases.The.fifth chapter summarizes the above work and puts forward suggestions for improvement.Meanwhile,the prospect of the application of the vertical silicon nanowire array gas sensing platform is also forecasted.