Directional and angular locking in the driven motion of Au islands on MoS_{2}

We have performed AFM nanomanipulation experiments on triangular Au islands (with typical linear size of 25-80 nm) previously grown on a MoS2 surface. These islands are found to move along preferential directions, independently of the angle of attack of the scanning probe. A comparison between molecular-dynamics simulations and atomically resolved STM images prove that these directions correspond to the zigzag alignments of the Mo and S atoms on the substrate. This is related to the observed systematic orientation of the islands, which is in turn a consequence of a sharp energy minimum as a function of each island's angular orientation. This directional-locked motion is entirely different from nanomanipulation involving disordered contact interfaces, where the direction of motion is determined by the island geometry and the scan pattern, and roto-translational motion is observed in arbitrary directions. Besides shedding light on the fundamental mechanisms of friction in the considered class of materials, our results could find important applications in the controlled positioning of metal nanoislands as electrodes for molecular electronics

Atomic-scale investigations of ultralow friction on crystal surfaces in ultrahigh vacuum

Controlling friction on the nanometer scale is one of nowadays’ challenges for scientists and engineers. Since the first observation of atomic friction reported by Mate et al. for a tungsten tip sliding on graphite, a lot of progress has been made in the understanding of this phenomenon on the atomic scale. An accurate description of the motion of a sharp tip elastically driven on a crystal surface by a microcantilever was first given by Tomanek et al., who based their interpretation on the Prandtl–Tomlinson model. The lateral (friction) force acting on the tip can be estimated by measuring the angle of torsion of the cantilever. The tip sticks to a given equilibrium position on the surface lattice until the driving force becomes high enough to cause a slip into the closest equilibrium position along the pull direction. The resulting stick-slip motion corresponds to a sawtooth-shaped time evolution of the lateral force with the atomic periodicity of the surface lattice. However, this scenario is observed only if a precise condition is fulfilled. The lateral stiffness of the driving spring must be lower than the curvature of the tip-surface interaction potential. If this is not the case, the tip slides on the surface without abrupt jumps, and a “superlubric” scenario is observed