Superlubricity of epitaxial monolayer WS₂ on graphene

We report the superlubric sliding of monolayer tungsten disulfide (WS₂) on epitaxial graphene (EG) grown on silicon carbide (SiC). Single-crystalline WS₂ flakes with lateral size of hundreds of nanometers are obtained via chemical vapor deposition (CVD) on EG. Microscopic and diffraction analyses indicate that the WS₂/EG stack is predominantly aligned with zero azimuthal rotation. The present experiments show that, when perturbed by a scanning probe microscopy (SPM) tip, the WS₂ flakes are prone to slide over the graphene surfaces at room temperature. 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 trigger an ultralow-friction rototranslational displacement, that is superlubric. Experimental observations show that, after sliding, the WS₂ flakes come to rest with a rotation of nπ/3 with respect to graphene. Moreover, atomically resolved measurements show that the interface is atomically sharp and 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.