Molecular transport through capillaries made with atomic scale precision

Nanoscale pores and capillaries have been studied intensively because of their importance in many natural phenomena and use in numerous applications1. Significant progress has been achieved in fabricating artificial capillaries with nanometer dimensions, which led to the emergence of new research areas including nanofluidics2-4. However, it remains extremely challenging to control capillary sizes at this spatial scale, especially because of surface roughness. Here we report ultimately narrow and smooth capillaries that can be viewed as if individual atomic planes were removed from a bulk crystal, leaving behind flat voids of a chosen height. The capillaries are fabricated by van der Waals assembly5 of atomically flat materials using two-dimensional crystals6 as spacers in between. To demonstrate the technology, we use graphene and its multilayers as archetypal two-dimensional materials and study water transport through channels ranging in height from a single atomic plane to many dozens of them. The unexpectedly fast flow (up to 1 m/s) is attributed to high capillary pressures (1,000 bar) combined with large slip lengths. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement, which we associate with an increased structural order in nanoconfined water. Our work opens a venue for making capillaries and cavities with sizes tunable to angstrom precision and permeation properties controlled through a wide choice of atomically flat materials available for channel walls.