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.