PERFECT EDGE STRUCTURE OF THE GRAPHENE
A team of researchers led by Prof. Arindam Ghosh from the Indian Institute of Science (IISc), Bengaluru has been successfully able to produce a new type of graphene electrical conductor experimentally. This was theoretically predicted nearly decades ago.
The synthesized graphene is single- or a few layers thick to conduct current along one particular edge — the zig-zag edge. It allows the 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.
In a few-layers-thick graphene, the current which flows along one edge does not experience any kind of resistance. This allows to make use of graphene in power-efficient electronics and ultra-fast quantum information transfer. Experimentally, these results can be conducted even at room temperature.
HOW TO MAKE PARTICULAR ZIG-ZAG EDGE
Since the emergence of graphene in 2004, many groups around all over the world have been trying to access these zig-zag edges. Till now, it was merely impossible to achieve such structures because of the flow of the current through graphene, which flows through both the edge as well as the bulk.
“We succeeded in this endeavor by creating the bulk part of graphene extremely narrow (less than 10 nanometer thick), and hence highly resistive, thus forcing the current to flow through the edge alone,” he added.
“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 zig-zag 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.
Even in highly long graphene, these edge will remain conductive with chemically and structurally pristine.The team used a small metal robot to peel the graphene from bulk pyrolytic graphite.
Amogh Kinikar from the Department of Physics at IISc and the first author of the paper says, “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).”
CHEMICAL ACTIVITY OF EDGE STRUCTURE AND PROPERTIES
The mechanical exfoliation was carried out at room temperature under vacuum. At the time of the exfoliation the electrical conductance was measured before the pristine nature of the edge was affected.
“The edges conduct without any resistance as long as the edges don’t come in contact with any chemicals,” continued 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).”
This is attributed due to the unsatisfied bonds of the carbon atoms and they tend to react with hydrogen present in the air.
Due to the hexagonal structure of the carbon atoms, it is very difficult to exfoliate at 30 degree angle at specific edge with zigzag property.
Prof. H.R.Krishnamurthy from the Department of Physics, IISc and one of the authors of the paper explains, “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.”
As the current research booms on graphene nanoribbons and their use in quantum transfer technology, a high precision of cutting edge technology is required to achieve zig-zag structures in the graphene nanoribbons.
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 added. “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.”
“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.”
The results of the study were published in the journal Nature Nanotechnology.