Making Graphene More Practical
Researchers at the University of California, Los Angeles, have found a simple way to make large pieces of the carbon material graphene. Graphene, a flat, one-atom-thick sheet of carbon, can transport electrons at very high speeds, making it an attractive material for electronic devices. But producing sufficient quantities of large, uniform single-layer sheets of graphene has been a challenge. So far, processes to make graphene create small quantities of graphene flakes or films made of overlapping pieces.
Using the new method, presented online in Nature Nanotechnology, the researchers report making single-layer 0.6-nanometer-thick pieces that are tens of micrometers wide. Materials-science and engineering professor Yang Yang and his colleagues deposit the sheets on silicon wafers to make prototype field-effect transistors.
Testing at least 50 such transistors, the researchers found that the devices had an output current of a few milliamperes. That is 1,000 times higher than the output current of the devices that others have recently reported while using similar techniques to make graphene. “We believe this is a game-changing approach which will significantly improve graphene electronics in the future,” Yang says.
Electrons flow through graphene sheets tens of times faster than they flow in silicon. The material could lead to electronic devices that are smaller, faster, and less power hungry than are those made of silicon. Thin and transparent, graphene is also a promising replacement for the indium tin oxide electrodes and the silicon thin-film transistors used in flat-panel displays.
The easiest way to make single sheets of graphene is by using adhesive tape to peel graphene flakes off of pieces of graphite, which is a stack of multiple graphene layers. This process results in a very small amount of tiny flakes of graphene. The pieces would have to be much larger for any practical use. “If you can coat an entire silicon wafer with a single sheet of graphene, then you can do lithography or patterning and have little devices,” says James Tour, a chemistry professor at Rice University.
About two years ago, researchers came up with a chemical method that yields larger graphene pieces. They oxidize graphite to make graphite oxide and dissolve it in water. The oxygen atoms pry apart the individual graphene sheets, which get dispersed in the solution. After the researchers deposit the sheets on a substrate, the oxygen is removed using another chemical or by heating.
Manish Chhowalla, a materials-science and engineering professor at Rutgers University, has made one-to-two-nanometer-thick films with this method. He uses vapors of a chemical called hydrazine to remove the oxygen groups from the deposited film. The films, made of slightly overlapping graphene pieces, are a few centimeters wide.
Yang points out that the quality of the sheets made so far has not been very good. Because the graphene sheets are deposited on a substrate first, many oxygen groups get trapped between the sheets and the substrate underneath and are not removed. “These are detrimental to electrical properties,” he says.
Yang and his colleagues have simplified the method. They dissolve graphite oxide pieces in pure hydrazine. This splits apart the individual graphene sheets and gets rid of nearly all the oxygen groups in a single step. The researchers then deposit the pieces on a silicon wafer. They could also deposit the flakes on flexible surfaces. “The main contribution is that they’ve figured out a better way of [removing oxygen groups],” Chhowalla says.
The researchers uniformly cover large areas of silicon wafers about 1.5 centimeters in length and width with graphene sheets. Then they deposit gold electrodes on top of the flakes to make field-effect transistors.
The researchers are working to further improve the quality of the graphene sheets. Pure, flat graphene sheets have a thickness of 0.34 nanometers. The 0.6-nanometer thickness of the sheets that the researchers make implies that a few oxygen groups remain stuck to the graphene. “So it still might not be as good as the graphene you want, but it’s getting close,” Tour says. “It’s certainly good enough for lots of devices.”
Researchers now need to refine the process so that they can cover even larger areas with single graphene sheets, Tour adds. That would be key for practically using graphene in electronic devices. “What you want to be able to do is cover a whole 12-inch wafer with graphene cleanly,” he says. “The Intels won’t touch it until you can do that.”
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