  We report on the superlubric sliding of monolayer tungsten disulfide (WS2) on
epitaxial graphene (EG) on silicon carbide (SiC). WS2 single-crystalline flakes
with lateral size of hundreds of nanometers are obtained via chemical vapor
deposition (CVD) on EG and microscopic and diffraction analyses indicate that
the WS2/EG stack is predominantly aligned with zero azimuthal rotation. Our
experimental findings show that the WS2 flakes are prone to slide over graphene
surfaces at room temperature when perturbed by a scanning probe microscopy
(SPM) tip. Atomistic force field based molecular dynamics simulations indicate
that through local physical deformation of the WS2 flake, the scanning tip
releases enough energy to the flake to overcome the motion activation barrier
and to trigger an ultra-low friction roto-translational displacement, that is
superlubric. Experimental observations indicate that after the sliding, the WS2
flakes rest with a rotation of npi/3 with respect to graphene. Atomically
resolved investigations show that the interface is atomically sharp and that
the WS2 lattice is strain-free. These results help to shed light on
nanotribological phenomena in van der Waals (vdW) heterostacks and suggest that
the applicative potential of the WS2/graphene heterostructure can be extended
by novel mechanical prospects.
