Isotope- and Thickness-Dependent Friction of Water Layers Intercalated Between Graphene and Mica

The lubricating properties of water have been discussed extensively for millennia. Water films can exhibit wearless high friction in the form of cold ice, or act as lubricants in skating and skiing when a liquid. At the fundamental level, friction is the result of a balance between the rate of energy generation by phonon excitation during sliding and drainage of the energy from the interface by coupling with bulk atoms. Using atomic force microscopy, we found that when H 2 O intercalates between graphene and mica, it increases the friction between the tip and the substrate, dependent on the thickness of the water and graphene layers, while the magnitude of the increase in friction was reduced by D 2 O intercalation. With the help of first-principles density functional theory calculations, we explain this unexpected behavior by the increased spectral range of the vibration modes of graphene caused by water, and by better overlap of the graphene vibration modes with mica phonons, which favors more efficient energy dissipation. The larger increase in friction with H 2 O versus D 2 O shows that the high-frequency vibration modes of the water molecules play a very important role in the transfer of the vibrational energy of the graphene to the phonon bath of the substrate.