Friction Anisotropy of MoS: Effect of Tip-Sample Contact Quality.

Atomic-scale friction measured for a single asperity sliding on 2D materials depend on the direction of scanning relative to the material's crystal lattice. Here, nanoscale friction anisotropy of wrinkle-free bulk and monolayer MoS is characterized using atomic force microscopy and molecular dynamics simulations. Both techniques show 180° periodicity (2-fold symmetry) of atomic-lattice stick-slip friction vs. the tip's scanning direction with respect to the MoS surface. The 60° periodicity (6-fold symmetry) expected from the MoS surface's symmetry is only recovered in simulations where the sample is rotated, as opposed to the scanning direction changed. All observations are explained by the potential energy landscape of the tip-sample contact, in contrast with nanoscale topographic wrinkles that have been proposed previously as the source of anisotropy. These results demonstrate the importance of the tip-sample contact quality in determining the potential energy landscape and, in turn, friction at the nanoscale.