Multilayer Two-Dimensional Water Structure Confined in MoS_2

The conflicting interpretations (square vs rhomboidal) of the recent experimental visualization of the two-dimensional (2D) water confined in between two graphene sheets by transmission electron microscopy measurements, make it important to clarify how the structure of two-dimensional water depends on the constraining medium. Toward this end, we report here molecular dynamics (MD) simulations to characterize the structure of water confined in between two MoS_2 sheets. Unlike graphene, water spontaneously fills the region sandwiched by two MoS_2 sheets in ambient conditions to form planar multilayered water structures with up to four layer. These 2D water molecules form a specific pattern in which the square ring structure is formed by four diamonds via H-bonds, while each diamond shares a corner in a perpendicular manner, yielding an intriguing isogonal tiling structure. Comparison of the water structure confined in graphene (flat uncharged surface) vs MoS_2 (ratchet-profiled charged surface) demonstrates that the polarity (charges) of the surface can tailor the density of confined water, which in turn can directly determine the planar ordering of the multilayered water molecules in graphene or MoS_2. On the other hand, the intrinsic surface profile (flat vs ratchet-profiled) plays a minor role in determining the 2D water configuration.