Graphene electrode promises stretchy circuits
Carbon honeycomb set to become material of choice for bendy displays.
by; Katharine Sanderson
J. H. Hong said:
Graphene electrodes can now be flexible and transparent.
A transparent, flexible electrode made from graphene could see a one-atom thick honeycomb of carbon — first made just five years ago — replace other high-tech materials used in displays. It could even be used instead of silicon in electronics.
Byung Hee Hong from Sungkyunkwan University in Suwon, Korea, and his colleagues transferred a wafer-thin layer of graphene, etched into the shape needed to make an electrode, onto pieces of polymer. The polymers they used are transparent, and one — polyethylene terephthalate (PET) — can be bent, whereas the other — polydimethylsiloxane (PDMS) — is stretchable. The resulting films conduct electricity better than any other sample of graphene produced in the past.
Until recently, high-quality graphene has been hard to make on a large scale. To produce their graphene, Hong and his colleagues used a technique that is well known in the semiconductor industry — chemical vapour deposition. This involves exposing a substrate to a number of chemicals, often at high temperatures. These chemicals then react on the surface to give a thin layer of the desired product. The results in Hong's case were relatively large, high-quality films of graphene just a few atoms thick and several centimetres wide.
Thinner is better
The team made the electrodes by using nickel as a catalyst on which to react methane and hydrogen. Nickel usually catalyses the formation of thick layers of graphite. But by using a layer of nickel less than 300 nanometres thick and by cooling the sample quickly after the reaction the researchers could produce up to ten single-atom layers of carbon in graphene's signature honeycomb pattern. Their work is published in Nature1. The samples aren't perfect — each layer covers only around two-thirds of the sample — but Hong says he is working to improve this.
The graphene samples can be chemically etched into specific shapes. And when stamped onto the polymer, they can be bent or stretched by as much as 11% without losing their conductivity.
Because the layers of graphene are so thin, the resulting electrodes are transparent, and Hong says that makes the material ideal for use in applications such as portable displays. It could, for instance, be used to replace indium titanium oxide, which is expensive and inflexible. "We are planning to get an investment to build up mass-production facility of the large-scale graphene films," says Hong. His team is also looking at using the graphene electrodes in photovoltaic cells.
Easing the pain
But the electrodes are less likely to be used in bendy electronics — at least in the short term — and more likely to be incorporated in niche applications such as individual ultra-high-frequency transistors, suggests Andre Geim, from the University of Manchester, UK, who was one of the first to make graphene2.
Geim had predicted that chemical vapour deposition would be the best technique for making high-quality graphene films3. The procedure "is of absolutely crucial importance", he says.
"It is very hard to work with small pieces of material. It's really painstaking," Geim adds. The new process, which produces bigger pieces of film, "will tremendously influence the speed of development in this area".
Hong thinks that graphene's most promising application will be to replace the silicon-based materials used in semiconductor technologies. But this would need technological breakthroughs such as the ability to grow larger-scale uniform monolayer graphene films and to modify the conductivity of graphene nanostructures. Such applications could be some time off, says Geim. "It's probably too far beyond the horizon to predict."
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