This is an artist’s impression of the new CVD method for producing large, monolayer single-crystal-like graphene films by supplying hydrocarbon molecules to the edge of the growing film. Image: Andy Sproles/Oak Ridge National Laboratory, US Dept. of Energy.
This is an artist’s impression of the new CVD method for producing large, monolayer single-crystal-like graphene films by supplying hydrocarbon molecules to the edge of the growing film. Image: Andy Sproles/Oak Ridge National Laboratory, US Dept. of Energy.

A new method for producing large, monolayer single-crystal-like graphene films more than a foot long relies on harnessing a ‘survival of the fittest’ competition among crystals. The novel technique, developed by a team led by researchers at the US Department of Energy's Oak Ridge National Laboratory (ORNL), may open new opportunities for growing the high-quality two-dimensional (2D) materials necessary for long-awaited practical applications.

Making thin layers of graphene and other 2D materials on a scale required for research purposes is common, but they must be manufactured on a much larger scale for use in future practical applications.

Graphene is touted for its unprecedented strength and high electrical conductivity, and can be made through a couple of well-known approaches: physically extracting individual sheets of graphene from bulk graphite, or growing it atom-by-atom on a catalyst from a gaseous precursor.

The ORNL-led research team used the latter method, known as chemical vapor deposition (CVD), but with a twist. In a paper published in Nature Materials, they explain how localized control of the CVD process allows evolutionary, or self-selecting, growth under optimal conditions, yielding a large, single-crystal-like sheet of graphene.

"Large single crystals are more mechanically robust and may have higher conductivity," said ORNL lead co-author Ivan Vlassiouk. "This is because weaknesses arising from interconnections between individual domains in polycrystalline graphene are eliminated. Our method could be the key not only to improving large-scale production of single-crystal graphene but to other 2D materials as well, which is necessary for their large-scale applications."

Similar to traditional CVD approaches for producing graphene, the researchers sprayed a gaseous mixture of hydrocarbon precursor molecules onto a metallic, polycrystalline foil. However, they carefully controlled the local deposition of the hydrocarbon molecules, supplying them directly to the edge of the emerging graphene film. As the substrate moved underneath, the carbon atoms continuously assembled as a single crystal of graphene up to a foot in length.

"The unencumbered single-crystal-like graphene growth can go almost continuously, as a roll-to-roll and beyond the foot-long samples demonstrated here," said Sergei Smirnov, co-author and a professor at New Mexico State University.

As the hydrocarbons touch down on the hot catalyst foil, they form clusters of carbon atoms that grow over time into larger domains until coalescing to cover the whole substrate. The team previously found that at sufficiently high temperatures, the carbon atoms of graphene did not correlate, or mirror, the substrate's atoms, allowing for non-epitaxial crystalline growth.

Since the concentration of the gas mixture strongly influences how quickly the single crystal grows, supplying the hydrocarbon precursor near the edge of an existing single graphene crystal offers a more effective way to promote its growth than forming new clusters.

"In such a controlled environment, the fastest-growing orientation of graphene crystals overwhelms the others and gets 'evolutionarily selected' into a single crystal, even on a polycrystalline substrate, without having to match the substrate's orientation, which usually happens with standard epitaxial growth," Smirnov said.

The research team found that to ensure optimal growth, it was necessary to create a ‘wind’ that helps to eliminate the cluster formations. "It was imperative that we create an environment where the formation of new clusters ahead of the growth front was totally suppressed, and enlargement of just the growing edge of the large graphene crystal was not hindered," Vlassiouk said. "Then, and only then, nothing stands in the way of the 'fittest' crystalline growth when the substrate is moving."

The team's theoreticians, led by co-author Boris Yakobson, a professor at Rice University, provided a model explaining which crystal orientations possess the unique properties that make them fittest in the run for survival. This model also revealed that the choice of winner may depend on the substrate and the precursors.

"If graphene or any 2D material ever advances to industrial scale, this approach will be pivotal, similar to Czochralski's method for silicon." Yakobson said. "Manufacturers can rest assured that when a large, wafer-size raw layer is cut for any device fabrication, each resulting piece will be a quality monocrystal. This potentially huge, impactful role motivates us to explore theoretical principles to be as clear as possible."

Practical scaling up of graphene using the team's method remains to be seen. But the researchers believe their evolutionary-selection, single-crystal growth method could also be applied to promising alternative 2D materials such as boron nitride, also known as ‘white graphene’, and molybdenum disulfide.

This story is adapted from material from ORNL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.