Development of low voltage gas ionization tunneling sensor based on p-type ZnO nanostructures

In this article, we report design, fabrication, and characterization of a gas field ionization-tunneling sensor (GFITS) based on zinc oxide (ZnO) nanowires. The device that operates at very low voltages is made up of two parallel plates separated by a narrow gap. ZnO nanowires are grown on one of the plates and used as the anode of this capacitive device. The nanowires that were synthesized using electrochemical technique on silicon or gold substrates, amplify the electric field between the two plates and reduce the ionization voltage of the gas molecules. Electrons from the gas atoms tunnel through the potential barrier of the gas atoms into the tips of nanowires. The generated tunneling current can be used to identify unknown gases. Nanowires with different aspect ratios and various morphologies were used to assemble the device, which was then tested for several gases. Distinct I-V characteristics for gases like Ar, He, and N-2 at low pressures were achieved. Our observations show that nanowires grown on gold substrates do not have vertically parallel structures, rather they grow in the form of flower shapes and the devices made of those samples operate at much lower voltages compared to those made of parallel nanowires grown on semiconductor substrates. To investigate the effect of geometrical field enhancement on the operating voltage of the sensor, the electric field enhancement of nanowires has been simulated using COMSOL Multiphysics. The results show that the enhancement factor of flower-like nanostructures of ZnO is much higher than those of freestanding nanowires. (C) 2019 Elsevier B.V. All rights reserved.