A field effect gas sensor based on self-standing nanowire arrays

In this thesis we have introduced a novel gas sensor that works by fingerprinting the ionization breakdown characteristics of the unknown gases. This technique provides excellent selectivity; it is well established that at a constant temperature and pressure, every gas displays a unique breakdown electric field. In addition, because this technique does not involve adsorption or desorption of gases, the sensor exhibits a fast response and is not limited by considerations of reversibility and recovery. We employed vertically-aligned, self-standing arrays of gold nanowires (AuNWs) at one of the electrodes of a miniature parallel-plate ionization cell. The AuNW arrays were synthesized using a tailored version of the electrochemical template-assisted technique, resulting freestanding structures with controlled sparseness. Individual AuNWs, owing to their nanoscale tips, generate very high non-linear electric fields near their tips that provoke the breakdown process due to formation of a corona and allow self-sustaining discharges to be created at relatively low voltages. In an alternative approach, we exploited the tunneling field ionization characteristics of the gases to identify the unknown gas type and measure its concentration. The phenomenon of field ionization consists of electron tunneling from gas atoms (molecules) through a potential barrier into a vacant energy level of the conduction band of a metal. It has been widely used in field-ion microscopy and mass spectrometry. However, the electric fields required to field-ionize gaseous species are in the range of 2-5 V/Å, orders of magnitude higher than the breakdown fields. With the provision of low voltage operation, this method can be utilized in detection of gases at very low concentrations. In this research, we also fabricated sensors using AuNWs with particular tip geometry and composition capable of field-ionizing gas particles at sub-10V voltages. The devices were successfully characterized using several gas specie