Direct Observation of Atomic-Scale Gliding on Hydrophilic Surfaces.

Nanoscale friction behavior on hydrophilic surfaces (HS), influenced by a probe gliding on a confined water layer, has been investigated with friction force microscopy under various relative humidity (RH) conditions. The topographical and frictional responses of the mechanically exfoliated single-layer graphene (SLG) on native-oxide-covered silicon (SiO 2 /Si) and mica were both influenced by RH conditions. The ordinary phenomena at ambient conditions (i.e., higher friction on a HS than on a SLG due to different hydrophilicity), nondistinguishable height, friction of SLG with SiO 2 /Si at high RH (>98%), and the superlubricating behavior of friction on a HS were observed. Furthermore, the subdomain within SLG, consisting of an ice-like water layer intercalated between SLG and SiO 2 /Si, showed friction enhancement. These results suggest that the abundant water molecules at the interface of the probe and a HS can make a slippery surface that overcomes capillary and viscosity effects through the gliding motion of the probe.