In a sense, graphene is the ultimate balloon. In a classic 2008 experiment, Paul McEuen and colleagues demonstrated that monolayers of the material are completely impermeable to air, argon, and helium.1 They etched microchambers into the surface of an oxidized silicon wafer, filled the chambers with gas, and sealed them with graphene. Then they created a pressure difference so the graphene membranes bulged outwards. Over time, the bulges deflated—but the researchers established that all the escaping gas was seeping into the chambers’ glass walls or through the graphene–substrate seal. Essentially none of it crossed the graphene barrier itself.
Since then, theory and experiment alike have suggested that pristine graphene should block the passage of all atoms and molecules, including the smallest of atoms, hydrogen. Size isn’t the only determining factor: Leonidas Tsetseris and Sokrates Pantelides calculate that boron atoms should pass through graphene more readily than other, smaller atoms—but...
