Superlubricity (also known as structural superlubricity) is a phenomenon in which the friction between two solid surfaces moving across each other almost vanishes.

The researchers, led by Ernst Meyer of the University of Basel in Switzerland, studied the lubricity of graphene nanoribbons (which are thin strips of graphene) by anchoring the nanostructures onto the tip of a scanning probe microscope (SPM). They then dragged the nanoribbons across a gold surface using the tip (see image). “By measuring the frequency shifts in a cantilever beam supporting the tip, we were able to sense very small external forces,” explains team member Shigeki Kawai of the University of Basel and PRESTO, Japan Science and Technology Agency. The experiments reveal almost perfect, frictionless movement between the two surfaces, and the researchers found that they could move the graphene nanoribbons across lengths of 5 to 50 nanometres using extremely small forces of around 2 to 200 piconewtons.

This superlubric response is caused by the incommensurable contact formed by the crystal lattices of the nanoribbons and gold, the lateral stiffness of graphene and the absence of defects in the ribbons thanks to the bottom-up fabrication technique that was developed by our colleagues in MPIP-Mainz (for the molecules) and in Empa in Switzerland (for the assembly), adds Kawai.

Towards frictionless coatings

The team also used computer-based calculations to investigate the interaction between the surfaces as they moved across one another, and the results from these simulations agree well with the experimental observations.

“In this work, we showed that we can control a complex nanosystem, composed of a gold surface, atomically precise nanoribbons and an ultrasharp tip used to drag the nanoribbons,” says team member Andrea Benassi of Empa and the Technische Universität Dresden in Germany. “Our results could help us better understand how materials contact at the atomic scale, and pave the way to creating frictionless coatings in the future. Out of the lab, full nanoribbon coatings would considerably increase the lifetime of MEMS and NEMS or even larger-scale objects.”

Besides applications, the researchers say they are now focusing on the fundamental issue of actively controlling friction at the nanoscale. “In the present work, we have shown that nanoribbons are superlubricous and we now plan to study how this behaviour is affected by the shape and nature of the ribbons,” says Benassi.

The research is detailed in Science DOI: 10.1126/science.aad3569.