Research Article


2018, 11(11): 5946–5956


Superlubricity of epitaxial monolayer WS2 on graphene

Holger Büch1, Antonio Rossi1,2, Stiven Forti1, Domenica Convertino1,2, Valentina Tozzini2, and Camilla Coletti1,3 (*)

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1 Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza S. Silvestro 12, 56127 Pisa, Italy
2 NEST, Istituto Nanoscienze – CNR and Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy
3 Graphene Labs, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

Keywords: superlubricity, graphene, tungsten disulfide, scanning tunneling microscopy (STM), two-dimensional (2D) materials, nanomechanical
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ABSTRACT 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 n/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.
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Superlubricity of epitaxial monolayer WS2 on graphene. Nano Res. 2018, 11(11): 5946–5956

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