Tunable giant anisotropic diffusion of water sub-monolayers between graphene layers.

We investigate the in-plane confinement effect of two graphene layers on the diffusion behaviour of water sub-monolayers using molecular dynamics simulations. An unexpected fast diffusion state with giant anisotropy is observed when the two confining graphene walls have certain shifts applied to their relative positions. The phenomenon is mainly attributed to the smooth one-dimensional potential channels produced by the composition effect of the potential energy landscapes of the two graphene walls, and the concerted motion of water molecules due to hydrogen bonding. Unique duality in the diffusion mechanism is observed in the fast diffusion state, as is ballistic motion along the potential channels and Fickian diffusion across such channels. The smooth potential channels can be created in certain directions simply by shifting the confining walls, which provides a novel measure to manipulate the motion of confined molecules in real-time.