  Understanding the phase behaviors of nanoconfined water has driven notable
research interests recently. In this work, we examine the structures and
thermodynamics of water encapsulated under a graphene cover. We find layered
water structures up to ~1000 molecules, which is stabilized by the spatial
confinement and pressure induced by the adhesion between graphene cover and
substrate. For monolayer encapsulations, we identify both crystalline lattices
and defects. Free energy analysis shows that these low- entropy orders are
compensated by high formation energies. There exists an order- disorder
transition for this condensed phase at ~480-490 K, with a sharp reduction in
the number of hydrogen bonds and increase in the entropy. These findings offer
fundamental understandings of the encapsulated water, and provide guidance for
practical applications with its presence, for example, in the design of
nanoelectronic devices.
