Gas Sensor Based On Epitaxial Graphene For NO2 Sensing

Abstract

Graphene based gas sensor has been a major trend in the field of scientific research for gas detection. The stable two-dimensional structure, outstanding electronic properties and compatibility with CMOS processing makes it an ideal candidate for a cheap, effective ultra-sensitive gas sensor. The purpose of this thesis is to investigate the sensitivity of the epitaxial graphene based gas sensor at low concentrations of NO2 gas. It also studies the effect of humidity and temperature on the sensitivity of the sensor for its practical application. An epitaxial graphene layer was grown on 4H-SiC substrate at temperature suitable for monolayer graphene. The graphene layer was characterized using Atomic force Microscope (AFM) for surface morphology and Auger Electron Spectroscopy (AES) for number of graphene layers. Six sensors samples were fabricated using the grown epitaxial graphene layer for evaluating the stability in the sensitivity of the sensors to NO2 exposure. The sensitivity was calculated as the relative change in their resistance on the detection of a test gas. Initially, the drifts of the sensors were measured and the effects of temperature and humidity on their drifts were also evaluated. They were then exposed to four different lower concentrations of NO2 gas i.e. 0.1ppb, 0.2ppb, 0.5ppb and 1ppb. The sensitivities of the sensors were found to be proportional to the concentration of NO2 gas. The effect of humidity on the sensitivity of the sensor has been outlined by comparing the sensor responses with exposure to NO2 in dry carrier gas (Relative Humidity, RH=0.02%) and the NO2 gas in humidified carrier gas (RH=50%). It has been observed that increase in humidity causes improvement in the sensitivity of the sensors at low NO2 concentrations. There was 2.725% improvement in the sensitivity at 1ppb NO2 concentration under humid condition. Similarly, the effect of temperature on the sensitivity has been ascertained by comparing the measurements at room temperature and at an elevated temperature. At the elevated temperature (110oC), the sensitivities of the sensors decreased compared to their responses at room temperature (20oC). There was 2.86% decrease in the sensitivity at 1ppb NO2 concentration at elevated temperature.