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 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.
Highlights
Owing to their attractive electronic, optical, and magnetic properties, two-dimensional (2D) van der Waals heterostacks serve as a platform for fundamental studies and display a wide range of potential applications [1, 2]
Our result differ from those reported by Miwa et al [31] for MoS2/graphene heterostacks synthesized in vacuum, where the transition metal dichalcogenides (TMDs) was found to be rotated by 30° with respect to the graphene Brillouin zone (BZ)
A similar distribution of flake orientations was previously observed for chemical vapor deposition (CVD)-grown TMD/graphene [5, 32], which suggests that the vaporphase approach is a suitable technique to obtain heterostructures with azimuthal alignment
Summary
Owing to their attractive electronic, optical, and magnetic properties, two-dimensional (2D) van der Waals (vdW) heterostacks serve as a platform for fundamental studies and display a wide range of potential applications [1, 2]. The optoelectronic potential of this 2D heterostack is being extensively studied, relatively little attention has been paid to its nanotribological properties. Both graphite and bulk WS2 are known as effective lamellar lubricants with hexagonal structures. A number of tribological experiments have been carried out on both graphite and bulk molybdenum disulfide (MoS2), whose structure is analogous to that of WS2, and novel interesting phenomena such as self-retraction and superlubricity have been reported [10,11,12,13]
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