Atomic-scale mapping of hydrophobic layers on graphene and few-layer MoS2 and WSe2 in water

The structure and the role of the interfacial water in mediating the interactions of extended hydrophobic surfaces are not well understood. Two-dimensional materials provide a variety of large and atomically flat hydrophobic surfaces to facilitate our understanding of hydrophobic interactions. The angstrom resolution capabilities of three-dimensional AFM are exploited to image the interfacial water organization on graphene, few-layer MoS2 and few-layer WSe2 . Those interfaces are characterized by the existence of a 2 nm thick region above the solid surface where the liquid density oscillates. The distances between adjacent layers for graphene, few-layer MoS2 and WSe2 are similar to 0.50 nm. This value is larger than the one predicted and measured for water density oscillations (similar to 0.30 nm). The experiments indicate that on extended hydrophobic surfaces water molecules are expelled from the vicinity of the surface and replaced by several molecular-size hydrophobic layers.