Graphene on platinum surfaces defies Coulomb's law. Free Technical Library News

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Graphene on platinum surfaces defies Coulomb's law

01.02.2023

Researchers from Basel and Tel Aviv have found that friction changes with speed in specific graphene structures on platinum surfaces, violating Coulomb's law, which states that friction is independent of velocity in the macrocosm.

Materials made from individual atomic layers are highly valued for their low friction properties, useful for reducing friction on hard drives or moving parts of satellites or space telescopes. Composed of a single layer of carbon atoms arranged like a honeycomb, graphene is a prime example and is being tested for its potential as a lubricating layer. Earlier research has shown that graphene tape can slide across a gold surface with almost no friction.

If graphene is applied to a platinum surface, this has a significant effect on the measured friction forces. Now physicists from the Universities of Basel and Tel Aviv have figured out that in this case the friction depends on the speed at which the tip of the atomic force microscope moves across the surface. This conclusion came as a surprise, since friction does not depend on speed according to Coulomb's law, applied in the macrocosm.

When combined with a platinum substrate, graphene no longer forms only a hexagonal honeycomb structure of carbon atoms, but forms superstructures known as Moiré superlattices. Then the surface is no longer absolutely flat and has a certain degree of roughness.

"If we move the AFM tip over this slightly corrugated surface at low speed, we measure a weak and almost constant frictional force," explains Prof. Ernst Mayer from the Swiss Institute for Nanoscience and the Department of Physics at the University of Basel. "But above a certain threshold, friction increases at the rate of the AFM tip," adds first author Dr. Yiming Song. "The larger the moire buildup, the lower the threshold at which friction becomes velocity dependent."

The researchers found that on the ridges of the moiré superstructures, there is more resistance as the tip moves. These ridges experience elastic deformation through the pushing tip before relaxing again when the pressure is high enough. This effect results in large frictional forces that increase with tip speed. Simulation and analytical model confirm the experimental results obtained by this international team of researchers.

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