We demonstrate snake-like motion of graphene nanoribbons atop graphene and hexagonal boron nitride ( h-BN) substrates using fully atomistic nonequilibrium molecular dynamics simulations. The sliding dynamics of the edge-pulled nanoribbons is found to be determined by the interplay between in-plane ribbon elasticity and interfacial lattice mismatch. This results in an unusual dependence of the friction-force on the ribbon's length, exhibiting an initial linear rise that levels-off above a junction-dependent threshold value dictated by the pre-slip stress distribution within the slider. As part of this letter, we present the LAMMPS implementation of the registry-dependent interlayer potentials for graphene, h-BN, and their heterojunctions that were used herein, which provides enhanced performance and accuracy.

The importance of many-body dispersion effects in layered materials subjected to high external loads is evaluated. State-of-the-art many-body dispersion density functional theory calculations performed for graphite, hexagonal boron nitride, and their hetero-structures were used to fit the parameters of a classical registry-dependent interlayer potential. Using the latter, we performed extensive equilibrium molecular dynamics simulations and studied the mechanical response of homogeneous and heterogeneous bulk models under hydrostatic pressures up to 30 GPa. Comparison with experimental data demonstrates that the reliability of the many-body dispersion model extends deep into the subequilibrium regime. Friction simulations demonstrate the importance of many-body dispersion effects for the accurate description of the tribological properties of layered materials interfaces under high pressure.

We demonstrate that robust superlubricity can be achieved via both biaxial and uniaxial tensile strains in a substrate using molecular dynamics simulation. Above a critical strain, the friction is no longer dependent on the relative orientation between the surfaces mainly due to the complete lattice mismatch. Importantly, the larger the size of the flake is, the smaller the critical biaxial strain is.

Negative friction coefficient, where friction is reduced upon increasing normal load, is predicted for superlubric graphite/hexagonal boron nitride heterojunctions. The origin of this counterintuitive behavior lies in the load-induced suppression of the moiré superstructure out-of-plane distortions leading to a less dissipative interfacial dynamics. Thermal induced enhancement of the out-of-plane fluctuations leads to unusual increase of friction with temperature. The highlighted frictional mechanism is of general nature and is expected to appear in many layered materials heterojunctions.Energy dissipation, wear, and the ensuing failure of moving components are problems encountered in many human activities. Reducing friction is of particular importance in microscopic and nanoscopic mechanical devices [1] , where the damaging consequences of local heating are amplified by the large surface-to-volume ratio. Standard lubrication schemes may fail in these extremely confined conditions, which calls for novel alternative solutions [2], such as dry solid coatings. In this approach, frictional forces are reduced thanks to the effective cancellation of lateral interactions occurring between incommensurate rigid crystalline surfaces. This phenomenon, often termed structural superlubricity, was first proposed theoretically a few decades ago [3] as a way to achieve extremely low friction coefficients [4][5][6][7]. Despite the fact that structural superlubricity has been observed in different material contacts [8][9][10][11][12], its implementation in practical solid/solid lubrication schemes remains a challenging task. Nevertheless, recent breakthrough demonstrations [13][14][15][16][17], based on van-der-Waals heterojunctions, suggest that this goal may be within our reach.Among the family of layered compounds, junctions formed between graphite and hexagonal boron nitride (h-BN) have been predicted as promising candidates to achieve robust superlubricity [18,19]. Figure 1: (a) The model heterojunction: a graphene monolayer aligned over a four-layers thick h-BN substrate with rigid bottom layer. Carbon atoms are colored according to their vertical position with respect to the average basal plane of the graphene layer, where blue and red correspond to the maximum and minimum values attained, respectively. The scale bar is the same as in panel c. (b) Colored map of the average carbon-carbon distance (in units of the equilibrium bond-length) after relaxation of the graphene layer over h-BN at zero normal load. (c) The corresponding colored map of the out-ofplane distortions of graphene. (d) Schematics of the simulations setup: the graphene layer is attached to a rigid stage moving at fixed velocity of stage along the x direction. Driving forces are exerted via identical springs of elastic constant ∥ acting only parallel to the substrate. No forces are exerted by the springs in the normal direction. The perpendicular position of the stage is shifted vertically for clarity of the presentation. Carbon, boron, and nitrogen atoms ar...

The low-temperature mechanical response of a single porphyrin molecule attached to the apex of an atomic force microscope (AFM) tip during vertical and lateral manipulations is studied. We find that approach-retraction cycles as well as surface scanning with the terminated tip result in atomic-scale friction patterns induced by the internal reorientations of the molecule. With a joint experimental and computational effort, we identify the dicyanophenyl side groups of the molecule interacting with the surface as the dominant factor determining the observed frictional behavior. To this end, we developed a generalized Prandtl-Tomlinson model parametrized using density functional theory calculations that includes the internal degrees of freedom of the side group with respect to the core and its interactions with the underlying surface. We demonstrate that the friction pattern results from the variations of the bond length and bond angles between the dicyanophenyl side group and the porphyrin backbone as well as those of the CN group facing the surface during the lateral and vertical motion of the AFM tip.