Thermal stability of adsorbed O₂ and H₂O molecules on CVD-graphene

Graphene, owing to its unique physicochemical properties, is being actively investigated as a material for sensitive gas sensors. However, its practical application in electronic devices is limited by the presence of surface-adsorbed molecules. These molecules, acting as dopants, significantly modify the Fermi level and affect the material's resistance, leading to p-type conductivity due to donor-acceptor interactions between the molecules and graphene. This work investigates the effect of thermal annealing in an Ar atmosphere and in high vacuum on the gas-sensing properties of CVD-graphene transferred onto a SiO₂/Si substrate. Before annealing, graphene exhibits a stable and reversible response to oxygen and water vapor. After annealing, an increase in resistance is observed, which is associated with the desorption of surface dopant molecules (O₂, H₂O). Molecular dynamics simulations confirm that the effective desorption of water molecules from defective sites on graphene is significantly enhanced at temperatures above 200 °C, which is in good agreement with experimental results. Re-exposure to these gases leads to a decrease in resistance and an irreversible response. The results obtained highlight the key role of initially adsorbed molecules on the clean graphene surface in its gas sensitivity and demonstrate how thermal cleaning affects the magnitude, dynamics, and reproducibility of the response.