Gas permeation rates of ultrathin graphite sealed SiO₂ cavities

Despite the proven impermeability of graphene toward most standard gases, graphene/graphite sealed SiO₂ cavities always exhibit a nonzero leak rate, and the physical leakage mechanism is still unclear. By measuring leak rates of different gases for the same cavities sealed by ultrathin graphite under identical conditions, we find that the leak rates generally depend on the kinetic diameter of the gas molecules, which implies that the leakage is caused by a molecular sieving mechanism. By comparing different samples, we find that the leak rate of any gas in a particular sample is well predicted by the leak rate of N₂ in that sample. In addition, we observe enhanced leak rates of water-soluble molecules. We infer that the leakage path (i.e., the graphene/graphite-SiO₂ interface) favors hydrophilic species.