Mediator atoms help graphene self-heal
Graphene and other carbon materials are known to change their structure and even self-heal defects, but the processes involved in these atomic rearrangements often have high energy barriers and so shouldn’t occur under normal conditions. An international team of researchers in Korea, the UK, Japan, the US and France has now cleared up the mystery by showing that fast-moving carbon atoms catalyse many of the restructuring processes.
Graphene – a carbon sheet just one atomic layer thick – is an ideal system for studying defects because of its simple two-dimensional single-element structure. Until now, researchers typically explained the structural evolution of graphene defects via a mechanism known as a Stone-Thrower-Wales type bond rotation. This mechanism involves a change in the connectivity of atoms within the lattice, but it has a relatively large activation energy, making it “forbidden” without some form of assistance.
Using some of the best transmission electron microscopes available, researchers led by Alex Robertson of Oxford University and Kazu Suenaga of AIST Tsukuba found that so-called “mediator atoms” – carbon atoms that do not fit properly into the graphene lattice – act as catalysts to help bonds break and form. “The importance of these rapid, unseen ‘helpers’ has been previously underestimated because they move so fast and have been next-to-impossible to observe,” says co-team leader Christopher Ewels, a nanoscientist at the University of Nantes.
The breakthrough, Ewels says, came when they realized that these usually fast-moving atoms slow down when bound to existing defects in the lattice. “Our technique can be likened to those nature programmes on television in which cameramen often have a hard time filming some of the animals, which can be shy,” he explains. “They therefore sometimes set themselves up in hides next to a watering hole where they know the animals are sure to go, and that’s how they get their film footage.
“In our case, the mediator atoms shoot around too fast for our cameras. By instead imaging and watching pre-existing defects (the watering holes) that occasionally trap mediator atoms (which can linger near the defects for seconds, minutes or even hours), we are able to image them and observe how they influence defect restructuring.”
Since the images were blurred because of the speed of the processes involved, detailed theoretical modelling calculations were required to interpret them. These calculations were done at Seoul National University by Gun-Do Lee and his team and Ewels and his colleagues at the CNRS in Nantes.
Active catalysts
The mediator atoms act as catalysts thanks to their reactive dangling bonds, Ewels explains. By bonding to defective sites and lowering the activation energy of reactions, they trigger a variety of bond-breaking and bond-forming processes. In some cases, the mediators become incorporated into the lattice, kicking out atoms that were already there and causing the ejected atoms to mediate further rearrangements (see image above). “These processes may occur in many other environments, from interstellar carbon chemistry to graphite moderators in nuclear reactors,” Ewels tells Physics World. The new work thus provides an important fundamental understanding of how graphene and related 2D materials structurally rearrange and repair themselves, he adds.
The researchers, who report their results in Science Advances, speculate that analogous mediator atom species may be present in other bulk materials. They now plan to explore this idea further. “We suspect similar process may underlie mechanical deformation processes of many 2D and 3D materials,” Ewels says.
PHYSICSWORLD.COM 4/8/2020
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