  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. We found the edge contribution to friction is lattice
orientation dependence. In particular, edge pinning effect by dimerization is
obvious for misaligned contact but suppressed in aligned lattice orientation.
The former case providing local commensuration along edges is reminiscent of
Aubry's pinned phase and the contribution of per 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. Furthermore, we
demonstrated that the dimerized edges as high frictional pinning sites are
robust to strain engineering and even enhanced by fluorination. Both structural
and chemical modification in the tribological system constructed here offers
the atomic details to dissect the undesirable edge pinning effect in layered
materials which may give rise to the marked discrepancies in measured friction
parameters from the same superlubric sample or different samples with the same
size and identical preparation.
