Functionalisation of CVD graphene for gas sensing

Abstract

Due to several of its physical properties, graphene is well suited to use within gas sensing devices. The extremely large surface-to-volume ratio and atomic thinness means any adsorbate on the graphene surface will strongly modify the electrical properties. Also, graphene typically presents high carrier mobilities and a low density of states close to its Dirac point further improving its sensor characteristics. In combination, these factors have given rise to extensive research into graphene as a candidate for gas sensing, it has been found that even at the modest charge exchange, interaction between a graphene sheet and adsorbates can produce a measurable variation in the graphene’s conductivity and shift in the Dirac point of the graphene channel. This work reports upon gas sensing performance of monolayer graphene grown by Chemical Vapour Deposition (CVD). The electrical properties are characterised during exposure to gaseous analytes via analysis of their electrical behaviour as graphene field effect transistor (GFET) devices. Results are presented for adsorption of NH3 and NO2 gas leading to electron donation and withdrawing effects respectively. Both Fermi level shift and charge mobility change upon the gas adsorption is used to fingerprint sensor response for these two analytes. We present results that indicate atmospheric adsorption is responsible for strong changes in graphene sensor recovery and that this effect is reversable with exposure to high vacuum conditions. In addition, this work illustrates the potential for improvement upon current graphene gas sensing devices via wet chemical oxygen functionalisation. The oxygen functionalised CVD graphene is characterised using Raman Spectroscopy, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy. The electrical properties are then characterised before and after oxidation via analysis of gate dependent GFET measurements and the during gas exposure by resistivity measurements. It is demonstrated that even for a relatively low concentration of introduced oxygen groups (~1.65×1010 𝑐𝑐𝑐𝑐−2) the oxygen functionalised CVD graphene sensors show a response of up to 600x that of the comparable non-functionalised sensor. The response time for oxidised CVD graphene is also measured and found to be two times faster than pristine CVD graphene and is shown to be capable of detecting low gas concentrations of NO2 with a limit of detection of ~41ppb.