Investigation into the use of carbon nanotubes networks as gate electrodes in field-effect gas sensors with increased functionality.

Abstract

Gas sensors based on different principles have been developed for the applications of environmental monitoring, industrial processing, aerospace and the human body. Carbon nanotubes (CNTs) demonstrate a detectable electrical properties change upon gas molecules absorption. This has been extensively studied and used in chemiresistors and chemical field-effect transistors. Simultaneous response to different types of gases was reported. The aim of this study is to develop a new type of gas sensor by replacing the gate metal of a field-effect capacitor with a carbon nanotubes network. This novel sensor will combine resistivity measurements with potentiometric measurements, which should ideally lead to increased functionality and selectivity. The substrates (Al/Si/SiO2/Si3N4 or Al/Si/SiO2/Si3N4/LaF3) were fabricated with two gate electrode structures (interdigitated and two-line). CNTs in different solvents were drop or spray coated on to the substrates. Platinum coating on the CNTs was also introduced. The resistive measurements indicated an increase in the resistance of CNTs networks with increased oxygen concentration, and a decrease in the resistance of CNTs networks with increased hydrogen concentration with humidity interference. Evenly distributed CNTs film was achieved by spray coating which allowed the use of CNTs network as gate materials for potentiometric measurements. The potentiometric measurements suggested that the C-V curve shifted towards a lower voltage with an increased hydrogen concentration, and a higher voltage with an increased oxygen concentration. However, the response time was too long for practical applications. Pt deposition significantly improved the hydrogen response rate. A voltage shift of -450 mV with an equilibration time of 2 minutes was achieved when the hydrogen concentration increased from 1 to 100%. The C-V curve shifted by 500 mV with increased oxygen concentration (1-100%).