Superlubricity of epitaxial monolayer WS2 on graphene

TL;DR

This study demonstrates superlubric sliding of monolayer WS2 on epitaxial graphene, revealing ultra-low friction behavior and potential for advanced nanomechanical applications in van der Waals heterostructures.

Contribution

It provides experimental and simulation evidence of superlubricity in WS2/graphene heterostructures, highlighting the atomic-scale mechanisms involved.

Findings

WS2 flakes slide over graphene at room temperature.

Sliding is triggered by local deformation and energy transfer from a scanning probe.

Post-sliding, WS2 flakes rotate by nπ/3 relative to graphene.

Abstract

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.