Toward a probe-based method for determining exfoliation energies of lamellar materials

We discuss a potential new measurement application based on nanotribological measurements and simulations of the model lamellar material graphite. While frictional forces always oppose motion, we have observed that friction increases with decreasing load on aged graphite using atomic force microscopy (AFM). This results in an effectively negative nanoscale coefficient of friction. The magnitude of the friction coefficient increases with tip-sample adhesion. Through molecular dynamics and finite element simulations, we have demonstrated that the negative coefficient arises from an increase in out-of-plane deformability of the top layer of graphite with lifting, and is not a result of a variation in atomic corrugation or other material property. Viscoelastic waves which dissipate energy during sliding are more easily generated in the top layer of graphite when it is partially (and reversibly) exfoliated by the AFM tip. As a consequence, the magnitude of the negative friction coefficient is determined by the ratio of the work of adhesion to the exfoliation energy, providing a potential pathway toward the use of friction force microscopy for straightforward determination of the exfoliation energies of lamellar materials.