A Helical Monolayer Ice

Understanding water wetting layers on solid surfaces is essential for many natural and industrial processes. Here we find a helical ice monolayer with every six water molecules helically arranged along the normal of the basal plane by performing an intensive structural search based on ab initio calculations. The helical ice is more stable than all previous models of monolayer and bilayer ices in a wide range of water densities both in vacuum and on weakly interacting substrates due to a stronger network of hydrogen bonds enabled by the helical geometry. More compelling is the fact that this model adequately explains a recent experimental ice monolayer grown on graphite in terms of the lattice parameter, water density, and Moiré pattern. The helical character in the new ice model echoes previously reported helical motifs in one-dimensional ice structures and suggests an unexpected capability of hydrogen bonds in driving the surface reconstruction of ice structures.