  Despite being only one-atom thick, defect-free graphene is considered to be
completely impermeable to all gases and liquids. This conclusion is based on
theory and supported by experiments that could not detect gas permeation
through micrometre-size membranes within a detection limit of 10^5 to 10^6
atoms per second. Here, using small monocrystalline containers tightly sealed
with graphene, we show that defect-free graphene is impermeable with an
accuracy of eight to nine orders of magnitude higher than in the previous
experiments. We could discern permeation of just a few helium atoms per hour,
and this detection limit is also valid for all other tested gases (neon,
nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen
shows noticeable permeation, even though its molecule is larger than helium and
should experience a higher energy barrier. The puzzling observation is
attributed to a two-stage process that involves dissociation of molecular
hydrogen at catalytically active graphene ripples, followed by adsorbed atoms
flipping to the other side of the graphene sheet with a relatively low
activation energy of about 1.0 electronvolt, a value close to that previously
reported for proton transport. Our work provides a key reference for the
impermeability of two-dimensional materials and is important from a fundamental
perspective and for their potential applications.
