  One-atom-thick crystals are impermeable to atoms and molecules, but hydrogen
ions (thermal protons) penetrate through them. We show that monolayers of
graphene and boron nitride can be used to separate hydrogen ion isotopes.
Employing electrical measurements and mass spectrometry, we find that deuterons
permeate through these crystals much slower than protons, resulting in a
separation factor of ~10 at room temperature. The isotope effect is attributed
to a difference of about 60 meV between zero-point energies of incident protons
and deuterons, which translates into the equivalent difference in the
activation barriers posed by two dimensional crystals. In addition to providing
insight into the proton transport mechanism, the demonstrated approach offers a
competitive and scalable way for hydrogen isotope enrichment.
