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.”