Edge-pinning effect of graphene nanoflakes sliding atop graphene

Edge effect is one of the detrimental factors preventing superlubricity in laminar solid lubricants. Separating the friction contribution from the edge atom and inner atom is of paramount importance for rational design of ultralow friction across scales in van der Waals heterostructures. To decouple these contributions and provide the underlying microscopic origin at the atomistic level, we considered two contrast models, namely, graphene nanoflakes with dimerized and pristine edges sliding on graphene monolayer based on extensive ab initio calculations. On one hand, we found edge pinning effect by dimerization is obvious for misaligned contact. This case providing local commensuration along edges is reminiscent of Aubry’s pinned phase. The contribution of per dimerized edge carbon atom to the sliding potential energy corrugation is even 1.5 times more than that of an atom in bilayer graphene under commensurate contact. Thus, the dynamic and random edge dimerization during sliding will impact the tribological properties significantly and may be the important sources to account for the marked discrepancies in measured friction parameters [Qu et al., Phys. Rev. Lett. 2020, 125, 126102]. On the other hand, we found the edge contribution to friction is lattice orientation dependent and suppressed in aligned contact. This rationalizes the experimental finding where friction is dominated by interior atoms rather than edge pinning [Liao et al., Nat. Mater. 2022, 21, 47–53]. To eliminate the undesirable edge effects, we adopt strain engineering and edge fluorination to the tribological system constructed here. However, dimerized edges as high frictional pinning sites are robust to both approaches. We hope the detailed atomic information identified here would help for improving superlubric systems.