Evaluation of wetting transparency and surface energy of pristine and aged graphene through nanoscale friction.

Friction on few-layer graphene supported by Si/SiO x and mica substrates was investigated using atomic force microscopy (AFM) and coordinated molecular dynamics (MD) simulations to examine friction hysteresis and wetting transparency. Experiments showed that graphene exhibited higher overall friction, as well as hysteresis, in high humidity conditions, while the hysteresis was not observed under dry conditions. Additionally, as samples were aged under ambient environmental conditions, the work of adhesion increased by an approximate factor of two, likely from oxidation of the surface or adsorption of oxygen containing species from the lab environment. When graphene was supported by mica substrates no significant friction hysteresis was observed. MD simulations showed similar influences to the friction forces with surface energy. Both experiments and simulations suggest that wetting transparency, or a small variation in surface energy resulting from varying the number of graphene layers, contributes to the layer dependent friction forces measured on graphene. [ABSTRACT FROM AUTHOR]