Alexander Y. Smolyanitsky

Anomalous Friction in Suspended Graphene

Since the discovery of the Amontons law and with support of modern tribological models, friction between surfaces of three-dimensional materials is known to generally increase when the surfaces are in closer contact. Here, using molecular dynamics simulations of friction force microscopy on suspended graphene, we demonstrate an increase of friction when the scanning tip is retracted away from the sample. We explain the observed behavior and address why this phenomenon has not been observed for isotropic 3D materials.

Effects of surface compliance and relaxation on the frictional properties of lamellar materials

We describe the results of atomic-level stick–slip friction measurements performed on chemically-modified graphite, using atomic force microscopy (AFM). Through detailed molecular dynamics simulations, coarse-grained simulations, and theoretical arguments, we report on complex indentation profiles during AFM scans involving local reversible exfoliation of the top layer of graphene from the underlying graphite sample and its effect on the measured friction force during retraction of the scanning tip. In particular, we report nearly constant lateral stick–slip magnitudes at decreasing loads, which cannot be explained within the standard framework based on continuum mechanics models for the contact area. We explain this anomalous behavior by introducing the effect of local compliance of the topmost graphene layer, which varies when interaction with the AFM tip is enhanced. Such behavior is not observed for non-lamellar materials. We extend our discussion toward the more general understanding of the effects of the top layer relaxation on the friction force under positive and negative loads. Our results may provide a more comprehensive understanding of the effectively negative coefficient of friction recently observed on chemically-modified graphite.

Effects of thermal rippling on the frictional properties of free-standing graphene

With use of simulated friction force microscopy, we study the effect of varying temperature on the frictional properties of suspended graphene. In contrast with the theory of thermally activated friction on the surfaces of three-dimensional materials, kinetic friction is demonstrated to both locally increase and decrease with increasing temperature, depending on sample size, scanning tip diameter, scanning rate, and the externally applied normal load. We attribute the observed effects to the thermally excited flexural ripples intrinsically present in graphene, demonstrating a unique case of temperature-dependent dynamic roughness in atomically thin membranes.

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.