  Adsorbed layers of water are ubiquitously present at surfaces and fill in
microscopic pores, playing a central role in many phenomena in such diverse
fields as materials science, geology, biology, tribology, nanotechnology.
Despite such importance, the crystal structure of nanoconfined water remains
largely unknown. Here we report high-resolution electron microscopy of mono-
and few- layers of water confined between two graphene sheets, an archetypal
example of hydrophobic confinement. Confined water is found to form square ice
at room temperature - a phase with symmetry principally different from the
conventional tetrahedral geometry of hydrogen bonding. The square ice has a
high packing density with a lattice constant of 2.83 angstrom and during TEM
observation assembles in bi- and tri- layer crystallites exhibiting AA
stacking. Our findings are important for understanding of interfacial phenomena
and, in particular, shed light on ultrafast transport of water through
hydrophobic nanocapillaries. Our MD simulations show that square ice is likely
to be common inside hydrophobic nanochannels, independent of their exact atomic
makeup.
