Minute Contamination Can Wreck Graphene Production

When Christopher DiMarco was studying the mechanical properties of graphene at Columbia University more than six years ago, he found himself growing increasingly frustrated. 
 
“Chris had worked out some beautiful theory about those properties and was trying to do some experiments to test his theory,” said James Hone, a mechanical engineering professor at Columbia and DiMarco’s doctoral advisor. “He took over our graphene chemical vapor deposition (CVD) furnace to look at the different growth parameters based on temperature, different pressures, and flows of different types of gases to see how it changed the grains. He was very systematic and precise—yet, in the end, he couldn’t find patterns. 
 
“Nothing was reproducible. It nearly drove him crazy.”
 
DiMarco, along with Hone and collaborators at five other institutions, have discovered how to make consistent, defect-free graphene. In a paper published in May 2024 in the journal, Nature, they showed that the key is removing contamination from a very common element: oxygen.

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Graphene is often called a “wonder material” thanks to its remarkable tensile strength and electrical conductivity, and industry has been working to scale up graphene production for more than a decade. But CVD, a process commonly used to synthesize the material, results in “highly variable” results, Hone said. 
 
“There’s always been a lack of reproducibility in the field—and it’s been hard to document because you can’t do the same test and show what exactly is different,” he said. “Yet, there was a lot of lore about having to season the quartz tubes we use in the right way, or only use the top half of the tanks of gas before getting new ones, or that growth will work really well in the winter but not in the summer when it’s more humid.”
 
The CVD process creates graphene by passing a carbon-containing gas, like methane, over a copper surface at a temperature high enough to break carbon atoms away from gas molecules. Those atoms then rearrange to form the layer of desired graphene material. 
 
“It’s a fairly simple process,” Hone said. “The methane sticks to the copper and the copper acts as a catalyst to crack the methane. At that point, the hydrogen basically goes away, and you get atomic carbon which sticks to the surface, links up, and forms graphene. As the graphene grows, the copper is no longer exposed and it stops growing. It’s what we call a self-limiting process.”

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But while Hone’s lab had been using CVD to synthesize large-area graphene for more than a decade, the researchers were still stymied about the lack of reproducibility. After a visit to the University of Montreal to Richard Martel’s laboratory several years ago, Hone recounted the difficulty DiMarco was having with his research—and his own irritation regarding the variance in material quality from grow session to grow session.
 
“I told him we’ve been growing graphene for a long time and that we were frustrated that sometimes it works well and sometimes it doesn’t,” he said. “Things aren’t reproducible and there is a hidden variable we can’t figure out. [Martel] immediately said, ‘I know what’s going on. It’s oxygen!’”
 
Martel had published a few papers demonstrating that oxygen could slow the graphene growth process. That pivotal conversation helped to redirect DiMarco’s work—and allowed Hone and his colleagues to uncover a fundamental flaw in traditional CVD growth. They quickly learned that even a part per million of oxygen is enough to disrupt the growth of graphene. He and his colleagues then conducted a series of painstaking experiments to understand how oxygen affected CVD, and why oxygen-free CVD (OF-CVD) methods can produce higher quality graphene samples.

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“What oxygen does is, at the edge of a growing grain where you have dangling bonds, it chews things up,” said Hone. “As you can imagine, if you are trying to make grains grow together, the amount of copper that is exposed starts to decrease because you are covering it with graphene. Your growth rate starts to go down. But if you have oxygen in the system, you have this kind of competition where the grain never quite closes up which affects the quality. You get these little voids and pinholes in the material.”
 
The paper in Nature, “Reproducible graphene synthesis by oxygen-free chemical vapour deposition,” was authored by researchers at Columbia and Montreal, as well as Infinite Potential Laboratories in Waterloo, Ont., the U.S. National Institute of Standards and Technology, Howard University, and the National Institute for Materials Science of Japan.

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Hone said this work is a milestone to help the field move closer toward large-scale production of graphene. And, moving forward, the group will develop methods to transfer higher quality graphene to functional substrates like silicon for electronics or other applications. While Hone cautioned there is still other work to be done to scale up production in terms of speed, he is optimistic that his team’s work has helped to move the field forward.
 
“In order to make progress, you have to be on a solid foundation,” said Hone. “We didn’t realize we weren’t on a solid foundation for a long time. This work shows what’s really been going on and we now have a process we can take to industry and say, ‘If you reach these pressures and these temperatures, this is the growth rate. We know what it’s going to be.’”
 
Kayt Sukel is a technology writer in Houston.