The ultra-slippery nature of a two-dimensional material called magnetene could be down to quantum effects rather than the mechanics of physical layers sliding across each other, say researchers at the University of Toronto in Canada and Rice University in the US. The result sheds light on the physics of friction at the microscopic scale and could aid the development of reduced-friction lubricants for tiny, implantable devices.
Two-dimensional materials are usually obtained by shaving atomically thin slices from a sample of the bulk material. In graphene, a 2D form of carbon that was the first material to be isolated using this method, the friction between adjacent layers is very low because they are bound together by weak van der Waals forces, and therefore slide past each other like playing cards fanning out in a deck. For magnetene, the bulk material is magnetite, a form of iron oxide with the chemical formula Fe3O4 that exists as a 3D lattice in the natural ore. The bonds between layers are much stronger in magnetene than in graphene, however, so its similarly low-friction nature was a bit of a mystery.
Not just sliding layers
In the new work, which is published in Science Advances, lead author Peter Serles, together with colleagues led by Tobin Filleter, Chandra Veer Singh and Shwetank Yadav, obtained a sample of 2D magnetene by treating magnetite with high-frequency sound waves. This approach separated few-layer samples of magnetene from the bulk material. The team then measured the friction between the sheets using an atomic force microscope probe.
To their surprise, the researchers found that the friction between the tip of the AFM probe and the uppermost layer of magnetene was just as low as it is in the graphene. They therefore suspected that something other than layer sliding was at play. “When you go from a 3D material to a 2D material, a lot of unusual things start to happen due to the effects of quantum physics,” explains Serles. “Depending on what angle you cut the slice, it can be very smooth or very rough. The atoms are no longer as restricted in that third dimension, so they can vibrate in different ways. And the electron structure changes too. We found that all of these together affect the friction.”
To confirm the importance of these quantum phenomena, which include modifications to the charge density of the iron atoms and the emergence of unique low-damping phonon modes (vibrations of the crystal lattice) that are “forbidden” for classical systems, the team compared their experimental results to density functional theory simulations of how the probe slides over the 2D material. They found that the models that included quantum effects best predicted their experimental findings.
Ice-water interface goes viscous
New ultra-low-friction materials
The work could help scientists and engineers design new ultra-low-friction materials in the future. Such materials could be useful as lubricants in small-scale applications like implantable devices, the researchers say. “When you’re dealing with such tiny moving parts, the ratio of surface area to mass is really high,” Filleter explains. “That means things are much more likely to get stuck. What we’ve shown in this work is that it’s precisely because of their tiny scale that these 2D materials have such low friction. These quantum effects wouldn’t apply to larger, 3D materials.”
The researchers say they would now like to better understand the role of the quantum effects, since the same low-friction behaviour has not been observed in other recently-synthesized 2D magnetic iron oxides such as hematene (Fe2O3) or chromiteen (FeCr2O4).
