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New Carbon Nanomaterial

A simple chemical trick changes graphene into a compound with different electronic properties.
January 30, 2009

Graphene, a single layer of carbon atoms arranged in a honeycomb-like structure, has captured worldwide interest because of its attractive electronic properties. Now, by adding hydrogen to graphene, researchers at the University of Manchester, U.K., have made a new material that could prove useful for hydrogen storage and future carbon-based integrated circuits. While graphene is highly conductive, the new material, called graphane, is an insulator. The researchers can easily convert it back into conductive graphene by heating it to a high temperature.

Versatile graphene: When a highly conductive graphene sheet is exposed to hydrogen atoms (white), they attach to the carbon atoms (black), transforming the material into graphane, an insulator. This is the first evidence that graphene’s properties can be manipulated chemically.

Andre Geim, who led the research and first discovered the nanomaterial in 2004 with Kostya Novoselov, says that the findings suggest that graphene could be used as a base for making entirely new compounds. The hydrogenated compound graphane had been theoretically predicted before, but no one had attempted to create it. “What’s important is that you can make another compound of [graphene] and can chemically tune its electronic properties to what you want so easily,” Geim says.

Adding hydrogen to graphene is just one possibility. Using other chemicals could yield materials with even more appealing properties, such as a semiconductor. “Hydrogenation may not be the end of the exploration; it may be just the beginning,” says Yu-Ming Lin, a nanotechnology researcher at the IBM Thomas J. Watson Research Center, in Yorkstown Heights, NY.

The latest findings are a step toward practical carbon-based integrated circuits, which could be used for low-power, ultrafast logic processors of the future. The findings also open up the possibility of using graphene for hydrogen storage in fuel cells. “Graphene is the ultimate surface because it doesn’t have any bulk–only two faces,” Geim says. This large surface area would make an excellent high-density storage material.

As described in Science, the researchers make graphane by exposing graphene pieces to hydrogen plasma–a mixture of hydrogen ions and electrons. Hydrogen atoms attach to each carbon atom in graphene, creating the new compound. Heating the piece to 450 °C for 24 hours reverts it back to the original state. Geim says that the researchers did not expect to be able to make the new substance so easily.

One of graphene’s promises for electronics is that it can transport electrons very quickly. Transistors made from graphene could run hundreds of times faster than today’s silicon transistors while consuming less power. Researchers are making progress toward such ultrahigh-radio-frequency transistors. But combining the transistors into circuits is a challenge because graphene is not an ideal semiconductor like silicon. Silicon transistors can be switched on and off between two different states of conductivity. Graphene, however, continues to conduct electrons in its off state. Circuits made from such transistors would be dysfunctional and waste a lot of energy.

One way to improve the on-off ratio in graphene transistors and bring them on par with those made of silicon is to cut the carbon sheet into narrow ribbons less than 100 nanometers wide. But making consistently good-quality ribbons is difficult.

Altering the material chemically may be an easier way to tailor its electronic properties and get the properties sought, Geim says. And that means that researchers could fabricate graphene circuits with nanoscale transistors that are smaller and faster than those made from silicon. “Imagine a wafer made entirely of graphene, which is highly conductive,” he says. “[You can] modify specific places on the wafer to make it semiconducting and make transistors at those places.” Areas between the transistors could be converted into insulating graphane, in order to isolate the transistors from each other.

The new work is just a preliminary first step. The researchers still need to thoroughly test the electronic and mechanical properties of graphane. Converting the material into a decent semiconductor might take a lot more chemical tinkering.

Besides, graphene researchers face one big challenge before they can do anything practical: coming up with an easy way to make large pieces of good-quality material in sufficient quantities. “For many applications, one needs a significant amount of material,” says Hannes Schniepp, who studies graphene at the College of William and Mary. “And that’s yet to be demonstrated for graphene or graphane.”

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