A vertical tip–tip contact silicon nanowire array for gas sensing
Novel chemiresistive gas sensors based on a vertical tip–tip contact silicon nanowire (TTC-SiNW) array were constructed. The welding of TTC-SiNWs after joule heating treatment was confirmed by a current–voltage curve (I–V curve). The TTC-SiNW structure not only resolved the problem of electrode contact encountered in conventional nanowire sensors, but also elongated the nanowire length to increase the void space for fast gas diffusion. The TTC-SiNW sensor comprising the same two types of SiNW arrays responded to NO2 very sensitively. The LOD for the p–p and n–n contact SiNW arrays is around 150 ppb and 3 ppb (S/N = 3), respectively. Furthermore, the highly oriented nano-junction formed on the TTC structure provided solid evidence to clarify the contribution of the nanojunction to gas sensing behavior. The TTC-SiNW sensor with a p–n junction displays a significant rectification effect. The sensitive response towards NO2 (LOD is about 18 ppb) was observed at a reverse bias voltage, whereas the response at a forward bias voltage was insignificant. Finally, the mechanism of gas sensing behavior on different types of TTC structures was proposed.