Graphene nanoribbons on gold: understanding superlubricity and edge effects

Abstract

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.

Lifted graphene nanoribbons on gold: from smooth sliding to multiple stick-slip regimes

“…α, β, λ m , δ are fitting parameters. The values and the λ m = 4.86 nm period oscillation of the lifting-dependent maximum force compares reasonably well with the established static friction trend as a function of the non-lifted GNR length [27], reported in the lower panel of Fig. 8.…”

“…The force peak amplitudes exhibit an oscillation versus effective length of the GNR, mostly due to the imperfect compensation of the moiré superstructure at the two ends of the physisorbed part of the GNR. This effect is also related to the oscillatory behavior of the static friction force versus size reported in the past for totally adhering GNRs [27].…”

“…Firstly, forward and backward configurations imply different effective contact areas between the GNR and the substrate. Since the static friction oscillates widely with GNR contact length L c [27], small differences in the effective contact length can lead in general to quite different static-friction thresholds. As a result, small differences in z 0 may lead to quite different dynamical friction patterns.…”

“…the GNR long axis lies parallel to the Au[−1, 0, 1] crystallographic direction [27]. The atomistic dynamics of the GNR is simulated using the LAMMPS package [28] by means of a REBO force field [29] for C-C and C-H interaction, plus 2-body C-Au and H-Au interactions of the (6-12) Lennard-Jones (LJ) type, as parametrized in Ref.…”

“…The atomistic dynamics of the GNR is simulated using the LAMMPS package [28] by means of a REBO force field [29] for C-C and C-H interaction, plus 2-body C-Au and H-Au interactions of the (6-12) Lennard-Jones (LJ) type, as parametrized in Ref. [27]. In the following we refer to these energy contributions as V REBO and V LJ , respectively.…”