  It has been an ultimate but seemingly distant goal of nanofluidics to
controllably fabricate capillaries with dimensions approaching the size of
small ions and water molecules. We report ion transport through ultimately
narrow slits that are fabricated by effectively removing a single atomic plane
from a bulk crystal. The atomically flat angstrom-scale slits exhibit little
surface charge, allowing elucidation of the role of steric effects. We find
that ions with hydrated diameters larger than the slit size can still permeate
through, albeit with reduced mobility. The confinement also leads to a notable
asymmetry between anions and cations of the same diameter. Our results provide
a platform for studying effects of angstrom-scale confinement, which is
important for development of nanofluidics, molecular separation and other
nanoscale technologies.
