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Bulk water exists in many forms, including liquid, vapour and numerous crystalline and amorphous phases of ice, with hexagonal ice being responsible for the fascinating variety of snowflakes1,2. Much less noticeable but equally ubiquitous is water adsorbed at interfaces and confined in microscopic pores. Such low-dimensional water determines aspects of various phenomena in materials science, geology, biology, tribology and nanotechnology3-8. Theory suggests many possible phases for adsorbed and confined water9-17, but it has proved challenging to assess its crystal structure experimentally17-23. Here we report high-resolution electron microscopy imaging of water locked between two graphene sheets, an archetypal example of hydrophobic confinement. The observations show that the nanoconfined water at room temperature forms 'square ice'-a phase having symmetry qualitatively different from the conventional tetrahedral geometry of hydrogen bonding between water molecules. Square ice has a high packing density with a lattice constant of 2.83 Å and can assemble in bilayer and trilayer crystallites. Molecular dynamics simulations indicate that square ice should be present inside hydrophobic nanochannels independently of their exact atomic nature.
Many molecular dynamics(MD) studies9-17 have explored the structure of low-dimensional water and predicted a great variety of phases, with results sensitive to modelled conditions and sometimes appearing conflicting. For example, buckled monolayer ice10 and flat hexagonal ice14,16,23, respectively, were found inside hydrophilic and hydrophobic nanochannels below roomtemperature, whereas no in-plane order was observed for water inside simulatedmica (hydrophilic) and graphite (hydrophobic) nanochannels at and above room temperature13,15. A close analogue of planar square ice was seen in MD simulations of water inside carbon nanotubes9,11,12,18, where water molecules form a monolayer that can be viewed as a sheet of square ice rolled up into a quasi-onedimensional cylinder. Neutron studies18 revealed features consistent with the existence of such 'ice nanotubes', which melted above 50 K. Twodimensional (2D) ices have also been found experimentally on the surfaces of mica and graphite19-23. The studies using scanning probe microscopy20-22 and electron crystallography19,23 showthat near-surface water can formcorrelated, solid-like layers. As for their intralayer structure, information is only available for2Dices grown below150 K,which were found to be hexagonal, with in-plane coordination similar to that of bulk ices19,23.
For this study,we deposited a graphene monolayer on a standard transmission electron microscopy (TEM) grid, exposed it to a small amount of...