  Two-dimensional (2D) materials are envisaged as ultra-thin solid lubricants
for nano-mechanical systems. So far, their frictional properties at the
nanoscale have been studied by standard friction force microscopy. However,
lateral manipulation of nanoparticles is a more suitable method to study the
dependence of friction on the crystallography of two contacting surfaces.
Still, such experiments are lacking. In this study, we combine atomic force
microscopy (AFM) based lateral manipulation and molecular dynamics simulations
in order to investigate the movements of organic needle-like nanocrystallites
grown by van der Waals epitaxy on graphene and hexagonal boron nitride. We
observe that nanoneedle fragments -- when pushed by an AFM tip -- do not move
along the original pushing directions. Instead, they slide on the 2D materials
preferentially along the needles' growth directions, which act as invisible
rails along commensurate directions. Further, when the nanocrystallites were
rotated by applying a torque with the AFM tip across the preferential sliding
directions, we find an increase of the torsional signal of the AFM cantilever.
We demonstrate in conjunction with simulations that both, the significant
friction anisotropy and preferential sliding directions are determined by the
complex epitaxial relation and arise from the commensurate and incommensurate
states between the organic nanocrystallites and the 2D materials.

Mesoscale and Nanoscale Physics;Soft Condensed Matter;Applied Physics