Spectroscopic Investigation of Plasma-Fluorinated Monolayer Graphene and Application for Gas Sensing

Large-area monolayer fluorinated graphene (FG) is synthesized by a controllable SF6 plasma treatment. The functional groups of FG are elucidated by various spectroscopies, including Raman spectroscopy, X-ray photoemission spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Raman results suggest that the defects are introduced into the monolayer graphene during the fluorination process. The fluorine content can be varied by the plasma treatment and can reach the maximum (∼24.6 atom % F) under 20 s of plasma treatment as examined by XPS measurement. The angle-dependent NEXAFS results reveal that the fluorine atoms interact with the graphene matrix to form covalent C-F bonds, which are perpendicular to the basal plane of FG. FG is applied as a gas-sensing material and owns much better performance for ammonia detection compared to pristine graphene. On the basis of our density functional theory simulation results, the fast response/recovery behavior and high sensitivity of the FG gas sensor are attributed to enhanced physical absorption due to the C-F covalent bonds on the surface of FG.