  Two-dimensional crystals with angstrom-scale pores are widely considered as
candidates for a next generation of molecular separation technologies aiming to
provide extreme, exponentially large selectivity combined with high flow rates.
No such pores have been demonstrated experimentally. Here we study gas
transport through individual graphene pores created by low intensity exposure
to low kV electrons. Helium and hydrogen permeate easily through these pores
whereas larger species such as xenon and methane are practically blocked.
Permeating gases experience activation barriers that increase quadratically
with molecules' kinetic diameter, and the effective diameter of the created
pores is estimated as ~2 angstroms, about one missing carbon ring. Our work
reveals stringent conditions for achieving the long sought-after exponential
selectivity using porous two-dimensional membranes and suggests limits on their
possible performance.
