To build devices that take advantage of graphene’s unique electrical properties, researchers will need large sheets of high quality material. Currently, they typically grow the 2-D material by chemical vapor deposition (CVD) on pieces of copper heated to about 1000 °C. This results in graphene with large crystals, which means there are few crystal boundaries for electrons to bump into which, in turn, leads to good electrical qualities. After growing the film, researchers must somehow transfer the graphene to another surface, such as a silicon wafer or a plastic film. Moving a large sheet of material that’s just an atom thick is challenging, so ideally researchers would just grow the graphene right where it’s needed. But researchers can’t grow graphene directly on many types of surfaces because of the extreme temperatures required.
By designing and building a specialized plasma-blowing device, researchers have now overcome this challenge to bringing graphene into mass production. The new growth technique makes it possible to produce high-quality graphene at less than half the typical temperature, which could make it possible to directly grow the ultrathin electronic material on plastic, silicon circuits, and other surfaces for use in high performance or flexible electronics (Nano Lett. 2019, DOI: 10.1021/acs.nanolett.8b03769).
A process called plasma-enhanced CVD, which uses microwaves to turn gases in a chamber into a reactive plasma, makes it possible to grow graphene at much lower temperatures, around 400 °C. This temperature is compatible with polyimide and other electronic substrates. However, the resulting material is grainy and has poor electrical properties. Researchers led by Jaeho Kim and Hiromoto Itagaki at the National Institute of Advanced Industrial Science and Technology (AIST) found that they could grow higher-quality graphene by more carefully controlling the plasma.
The AIST researchers built an apparatus they call a plasma blower to gain finer control over the plasma’s composition and flow. The device blows methane, argon, and hydrogen gases through a microwave field that energizes the molecules to form a plasma before they hit the surface where the researchers want to grow graphene. The team used computer modeling to study the convection of gases through the blower and on the reactive surface. With this knowledge, they tinkered with the flow of gases and the properties of the microwave radiation to minimize the production of harmful intermediates and optimize the concentration of the stew of electrons and CH and C2 radicals that play an important role in graphene growth.
Blasting a copper substrate with this controlled plasma, the researchers produced high-quality graphene with large crystals at just 400 °C. The optical and electrical qualities of the graphene grown by this method are comparable to those of graphene grown at high temperatures by CVD. So far, the blower produces a plasma about 50 mm across, resulting in graphene strips of the same width. Kim believes it will be possible to grow sheets of graphene a meter wide.
Rodney Ruoff, a chemical engineer at the Ulsan National Institute of Science and Technology, says he’s impressed. “Achieving relatively high-quality graphene at only 400 °C on copper foil is quite interesting,” he says.
Previous attempts to grow graphene under cooler conditions have required a high-temperature processing step to improve the quality of the material, says Deji Akinwande, an electrical engineer at the University of Texas at Austin. “This appears to be a truly low-temperature method,” he says. Ruoff and Akinwande agree that the question now is whether it will work on substrates besides copper, such as the polymer sheets used to make flexible electronics and displays. On that matter, Kim is tight lipped, but says research is in progress.