The new way of making graphene with a perfect edge structure was the key to success
Researchers from the Indian Institute of Science (IISc), Bengaluru have been able to experimentally produce a new type of electrical conductor that was theoretically predicted nearly 20 years ago.
A team led by Arindam Ghosh from the Department of Physics, IISc successful produced graphene that is single- or a few-layers thick to conduct current along one particular edge — the zigzag edge. The zigzag edge of graphene layer has a unique property: It allows flow of charge without any resistance at room temperature and above.
“This is the first we found the perfect edge structure in graphene and demonstrated electrical conductance along the edge,” says Prof. Ghosh. The results of the study were published in the journal Nature Nanotechnology.
A few-layers-thick graphene that conducts current along one edge does not experience any resistance and so can lead to realising power-efficient electronics and quantum information transfer, even at room temperature.
Getting an edge
Many groups over the world have been trying to access these edges since the emergence of graphene in 2004, but have been largely unsuccessful because when current flows through graphene, it flows through both the edge as well as the bulk. “We succeeded in this endeavour by creating the bulk part of graphene extremely narrow (less than 10 nanometre thick), and hence highly resistive, thus forcing the current to flow through the edge alone,” he says.
“While the bulk is totally insulating, the edge alone has the ability to conduct because of the unique quantum mechanics of the edge. Because of the zigzag orientation of carbon atoms [resulting from the hexagonal lattice], the electron wave on each carbon atom overlaps and forms a continuous train of wave along the edge. This makes the edge conducting,” explains Prof. Ghosh. The edge will remain conductive even if it is very long but has to be chemically and structurally pristine.
In the past, others researchers had tried making narrow graphene through chemical methods. But the use of chemicals destroys the edges. So the IISc team resorted to mechanical exfoliation to make graphene that are single- and few-layers thick. They used a small metal robot to peel the graphene from pyrolytic graphite. “If you take a metal tip and crash it on graphite and take it back, a part of the graphite will stick to the tip. The peeling was done slowly and gradually (in steps of 0.1 Å),” says Amogh Kinikar from the Department of Physics at IISc and the first author of the paper.
Effect of chemicals
The exfoliation was carried out at room temperature but under vacuum and the electrical conductance was measured at the time of exfoliation before the pristine nature of the edge was affected. The unsatisfied bonds of the carbon atoms make them highly reactive and they tend to react with hydrogen present in the air. “The edges conduct without any resistance as long as the edges don’t come in contact with any chemicals,” says Prof. Ghosh. “It is very easy to passivate [make the surface unreactive by coating the surface with a thin inert layer] the edges to prevent contamination [when narrow graphene is used for commercial purposes].”
As the carbon atoms have a hexagonal structure, exfoliation is by default at 30 degree angle and one of the edges has a zigzag property. “The steplike changes observed for small values of conductance when other variables were changed were surprising. Through theoretical work we were able to link this to edge modes in graphene,” says Prof. H.R.Krishnamurthy from the Department of Physics, IISc and one of the authors of the paper.
There are currently several chemical methods to produce very narrow graphene nanoribbons. But these chemicals tend to destroy the edges. “So the challenge is to produce graphene nanoribbons using chemicals that do not destroy the edges,” Prof. Ghosh says. “We believe that this successful demonstration of the dissipation-less edge conduction will act as great incentive to develop new chemical methods to make high-quality graphene nano-ribbons or nano-strips with clean edges.”