Functionalization And Sensing Properties Of Porous Silicon Nanowires Array

For the monitoring of the environment and the protection of human health,it is becoming more and more important to effectively and accurately detect toxic and harmful gases.At present,the gas sensor is moving toward low cost,mini-portability,high precision,data stabilization,and integrated compatibility.One-dimensional nano-silicon-based room temperature gas sensors are highly promising in developing high-performance,portable,integrated gas sensors and intelligent sensing systems,as a gas sensor sensitive element has many advantages.However,as gas sensors operating at room temperature,the silicon nanowires-based gas sensors have to confront a key problem that is the interference of ambient humidity especially for the severe performance degradation or even invalid in high humidity applications.In this study,the microstructure modulation of porous silicon nanowire arrays gas sensor and the functionalization of its anti-humidity interference improvement were studied at room temperature.Different doping types and concentrations of silicon substrates were etched to form vertical silicon nanowire arrays via Metal-Assisted Chemical Etching.The morphology and formation of silicon substrates were analyzed by the relevant preparation mechanism.This study shows that the P-type silicon nanowire array is superior to the N-type for response value and response/recovery time because the P-type in that the diameter of the porous nanowires is larger than that of the N-type,and the nanopore diameter and depth increase with increasing doping concentration.The effective modulation of the surface wettability of the Si substrate and its influence on the microstructure of the SiNWs array were investigated.The tip cluster and well lateral separation porous SiNWs arrays were efficiently prepared by modulating the surface wettability of the silicon substrate.Based on the discussion of force balance between nanowires,the relationship between the formation of the tip cluster or separation structure of porous SiNWs and the distribution features of Ag nanoclusters on the surface of silicon substrate was clarified.The HF pretreatment creates a hydrophobic surface favorable for deposition of irregular Ag nanoflakes and then for the formation of bundling porous silicon nanowire arrays.The porous silicon nanowire arrays featured by tip-clusters is proved to be highly effective in achieving highly sensitive and rapid response to NO2 gas at room temperature.Finally,for gas sensors operating at room temperature,octadecyltrichlorosilane(OTS)was utilized to modify partially insensitive porous silicon nanowire surface with the Si-O bond,yielding an immunity to high humidity in gas detection process.The modified OTS provides an umbrella-like barrier blocking water molecules attraction on the nanowire surface which disinhibits the main adsorption sites and sensing channels of the sensor.The OTS-modified porous silicon nanowire arrays(OTS/SiNWs)sensor exhibited a practical capability to detect 5 ppb NO2 at humidity of 25%-55% RH and a response of 1.8% to 50 ppb NO2 at 75% RH at room temperature.Moreover,the fast response/recovery characteristics and good reversibility of the OTS/SiNWs sensor was observed even under high humidity conditions.In addition,the OTS/SiNWs sensor displayed excellent stability to humidity variations.The enhanced gas sensing performance is attributed to a marriage of porous structure of porous silicon nanowires and OTS.The simple modification for porous silicon nanowires-based gas sensor will broaden new avenues,not only in chemical trace detection,but also in high humidity health monitoring.