Proton transport through one-atom-thick crystals

Measurements show that monolayers of graphene and hexagonal boron nitride are unexpectedly highly permeable to thermal protons and that their conductivity rapidly increases with temperature, but that no proton transport is detected for few-layer crystals. Like a proton through graphene A perfect graphene sheet is impermeable to all atoms and molecules: even hydrogen, the smallest of atoms, is not expected to penetrate through graphene's dense electronic cloud within billions of years. This characteristic is thought to extend to other two-dimensional crystals such as hexagonal boron nitride and molybdenum disulphide. Sheng Hu and colleagues now show that, surprisingly, monolayers of graphene and hexagonal boron nitride (but not molybdenum disulphide) are highly permeable to protons. In combination with their stability, this establishes these monolayers as promising candidates for use in many hydrogen-based technologies. Graphene is increasingly explored as a possible platform for developing novel separation technologies 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 . This interest has arisen because it is a maximally thin membrane that, once perforated with atomic accuracy, may allow ultrafast and highly selective sieving of gases, liquids, dissolved ions and other species of interest 2 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 . However, a perfect graphene monolayer is impermeable to all atoms and molecules under ambient conditions 1 , 2 , 3 , 4 , 5 , 6 , 7 : even hydrogen, the smallest of atoms, is expected to take billions of years to penetrate graphene’s dense electronic cloud 3 , 4 , 5 , 6 . Only accelerated atoms possess the kinetic energy required to do this 20 , 21 . The same behaviour might reasonably be expected in the case of other atomically thin crystals 22 , 23 . Here we report transport and mass spectroscopy measurements which establish that monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons under ambient conditions, whereas no proton transport is detected for thicker crystals such as monolayer molybdenum disulphide, bilayer graphene or multilayer hBN. Protons present an intermediate case between electrons (which can tunnel easily through atomically thin barriers 24 ) and atoms, yet our measured transport rates are unexpectedly high 4 , 5 and raise fundamental questions about the details of the transport process. We see the highest room-temperature