Ever wondered how a two-dimensional nanosheet of ice grows on the edges? With a good sense of public relations (submitted to Nature in May but published online January 1 2020, first research article in a new decade) Runze Ma et al. (DOI) have tried to find out. They grew a 2D hexagonal bilayer of ice at 120 Kelvin on a layer of gold inside an STM. Nothing new thus far but with improved resolution of the STM setup it was possible for the first time to visualize the edges.
They were found to exist as equal parts zigzag and armchair. For comparison carbon nanotubes are also hexagonal and can also be of an armchair or a zigzag type. At 120 Kelvin the edges continue to grow but by further freezing to 5K it was possible to observe the dynamics of edge growth. Zigzag edges grow by adding pentagons which then mature into new hexagons. Armchair edges first morph into pentagon-heptagon-pentagon-hexagon structures. They also have more kinks and defects. The authors also speculate on expansion of the results to 3D ice but warn that imaging 3D ice growth this way would be exceedingly difficult.