Graphene nanoribbons on gold : understanding superlubricity and edge effects

We address the atomistic nature of the longitudinal static friction against sliding of graphene
nanoribbons (GNRs) deposited on gold, a system whose structural and mechanical properties have
been recently the subject of intense experimental investigation. By means of numerical simulations
and modeling we show that the GNR interior is structurally lubric (‘superlubric’) so that the static
friction is dominated by the front/tail regions of the GNR, where the residual uncompensated lateral
forces arising from the interaction with the underneath gold surface opposes the free sliding. As
a result of this edge pinning the static friction does not grow with the GNR length, but oscillates
around a fairly constant mean value. These friction oscillations are explained in terms of the GNR-
Au(111) lattice mismatch: at certain GNR lengths close to an integer number of the beat (or moiré)
length there is good force compensation and superlubric sliding; whereas close to half odd-integer
periods there is significant pinning of the edge with larger friction. These results make qualitative
contact with recent state-of-the-art atomic force microscopy experiment, as well as with the sliding
of other different incommensurate systems.