Nanotribological Effect of Water Layers Intercalated between Exfoliated MoS2 and Mica

When the two-dimensional material molybdenum disulfide (MoS2) is exfoliated on a hydrophilic substrate such as mica, a layer of water is intercalated at the interface at a relative humidity over 20%. This intercalated water layer increases friction at a microscopic scale by providing an additional excitation channel. Using atomic force microscopy, we quantitatively examined various frictional effects from the intercalated water layer. A general tendency of friction dependence on the number of MoS2 and water layers followed the case of graphene exfoliated on mica, despite different structure, layer thickness, hydrophilicity, and water growth mode in the intercalated water layer. These phenomena reveal a universal trend of frictional behavior in confined water, indicating that the physical and electronic properties of the atomic layer covering intercalated water layers do not play an important role.