Making high-performance computer circuits from graphene requires starting with high-quality films of the material on an insulating surface. But defects usually crop up when graphene is grown directly on or transferred to an insulator from another growth substrate. A novel method to grow inches-wide single-crystal graphene directly on an insulating sapphire substrate could be a way forward (Nat. Mater. 2022, DOI: 10.1038/s41563-021-01174-1).
Graphene is made at industrial scale today using chemical vapor deposition (CVD) to grow the material from the bottom up on copper foils. CVD-grown graphene is polycrystalline, and cannot shuttle electrons and positively-charged holes as quickly as single-crystal material—and speed is essential for high performance electronic devices. Plus, when the CVD flakes are transferred to insulating substrates to make devices, metal atoms often contaminate the graphene, and mechanical stress wrinkle it.
The new technique gives centimeters-wide, wrinkle-free single sheets of graphene with no overlapping layers. Such high-quality graphene grown directly on an insulating surface should pave the way for high-performance electronics and optoelectronics devices made from graphene, says Rodney S. Ruoff, a chemist at the Ulsan National Institute of Science and Technology. The technique could potentially be used to grow other types of 2D materials, he says.
Last year, Ruoff and his colleagues reported a way to grow flat, single-crystal sheets on copper-nickel foils (Nat. Mater. 2021, DOI: 10.1038/s41586-021-03753-3). Ruoff worked with Bo Tian and Xixiang Zhang of King Abdullah University of Science and Technology, and their colleagues to continue improving the process. They start by placing a 5 cm wide copper foil on a clean sapphire substrate. Heating five times at high temperatures for cycles ranging from 5–25 hours results in a single-crystal copper film fused to the sapphire.
They put the substrate into a CVD system and pump methane gas over it at high temperature. As the methane decomposes, its carbon atoms diffuse through the copper film into the 2.15Å gap between the copper and sapphire. There, the carbon atoms combine, growing a graphene layer at the interface, Ruoff says.
Finally, the researchers dip the sample into liquid nitrogen at -196°C for 30 min followed by rapid heating. Pressure from the nitrogen atoms makes the copper film detach from the sapphire, so it can be pulled off with tweezers. The team was able to make 1 cm2 graphene pieces on the sapphire. Making larger ones is possible, says Tian, but would require more time and energy.
“The graphene quality looks very high, and the lack of wrinkles is a very appealing feature for future electronics applications,” says Sergei Smirnov, a chemist at New Mexico State University. “These wrinkles are difficult to avoid using transfer methods.”
Oliver J. Burton, an engineer at the University of Cambridge, agrees that this work overcomes the challenge of making high-quality graphene on an insulating surface.
But the core limitation of the method is the hours-long heating cycles needed to make the single-crystal copper films, “which are simply too long for industrial production times.”
Tian and Ruoff admit that the long times are a drawback, but with automation and engineering, they hope that others can speed things up. “People in the industry are very clever,” Tian says.