Atomic imaging of edge structure and growth of a two-dimensional hexagonal ice

The formation and growth of water ice layers on surfaces and of low-dimensional ice under confinement are common occurrences. While structured water adlayers and 2D ice have been imaged, capturing metastable or intermediate edge structures involved in their growth is extremely challenging due to their fragile and short-lived nature.
Here we show that noncontact atomic force microscopy with a CO-terminated tip allows real-space imaging of the edge structures of a 2D bilayer of hexagonal ice grown on an Au(111) surface. We find a new edge type that coexists with the zigzag edge commonly observed in 2D hexagonal crystals, and freeze samples during growth to identify intermediate structures.
When combined with MD simulations, these allow us to reconstruct growth processes that in the case of the zigzag edge involve addition of water molecules to the existing edge and a collective bridging mechanism. Armchair edge growth, in contrast, involves local seeding and edge reconstruction and thus is in stark contrast to conventional views of ice growth. The growth mechanism we have uncovered might also occur at the surface of bilayer hexagonal ice and might support a bilayer-on-bilayer ice growth to 3D ice transformation.