11-03 student typing graphene

[Image above] Typically sharper and clearer diffraction patterns indicate high-quality material, but Ames Laboratory researchers say in the case of graphene, broad patterns may indicate so as well. Credit: PxHere and Arthur Shlain (CC BY 3.0)

As emphasized at the Conference on Glass Problems last week, one of the keys to commercializing a product is quality control. Yet for suppliers of the 2D material graphene, ensuring quality remains a big challenge.

Two papers published in 2018 brought attention to the issue of poor quality and contaminated graphene for sale on the market, an issue due in part to the scarcity of international standards governing production of the material. But another reason for the substandard quality is the need for more techniques that can assess the quality of graphene.

Fortunately, researchers at Ames Laboratory in Iowa think they may have a solution to the latter problem—one that was hiding under our noses the entire time.

Broad diffraction and its relationship to quality

The Ames researchers discovered the solution to reliably assess graphene quality last year when analyzing graphene using low-energy electron diffraction, a technique commonly used in physics to study the crystal structure of solid material surfaces.

Conventionally, “Textbook diffraction states that the more flawless a material is, the sharper and clearer the diffraction spots, and imperfect materials have low intensity, broader diffraction spots,” Ames senior scientist Michael Tringides explains in a press release.

However, in the case of graphene, the Ames researchers observed the opposite—that broad diffraction patterns marked high-quality graphene.

In a paper published in Physical Review B, they explain that the broad diffraction occurs as strong bell-shaped components around the (00) and G(10) spots. The bell-shaped components are not seen in X-ray or helium-scattering scattering experiments, however, which suggests the component’s origin is the spatial localization of graphene electrons within a single layer.

The paper calls out several examples of bell-shaped components appearing in other low-energy electron diffraction studies throughout the years, though at that time no one commented on the phenomenon. “It was a big, noticeable phenomena, and reproducible, and we realized it must be extremely important in some way,” Tringides says.

This September, the Ames researchers published a new paper in The Journal of Physical Chemistry Letters that provides additional information confirming that the bell-shaped components are a measure of high graphene quality.

In particular, the researchers gathered information on graphene from several different types of experiments, including

Spot profile experiments (for determining the role of scattering interference)

These experiments, analyzed as a function of energy, show that measured parameters for the broad and narrow diffraction components do not follow variation of the profile shape expected from scattering interference.

  • Conclusion: The bell-shaped components are not related to scattering interference.

Intercalation experiments (for confirming the origins of bell-shaped components)

These experiments, which decoupled graphene from the silicon carbide substrate, found the bell-shaped components appeared more strongly following intercalation.

  • Conclusion: Further support for the suggestion that the bell-shaped component originates from electron confinement within a single layer.

Annealing experiments (for confirming the relationship between broad diffraction patterns and quality)

These experiments, which recorded the evolution of diffraction patterns from buffer layer to single-layer graphene, found that growth of the bell-shaped components closely mirrors the growth of sharp diffraction spots, which are the generally accepted indicators of quality.

  • Conclusion: Similarity between the evolution of broad and sharp diffraction patterns confirms the relationship between broad diffraction patterns and graphene quality.

The authors emphasize their belief that the findings will extend to other 2D material systems, though that suggestion is still to be tested. Regardless, “the correlation between this strange phenomenon and high-quality graphene is unmistakable,” Tringides says in the press release.

The 2019 paper, published in Physical Review B, is “Diffraction paradox: An unusually broad diffraction background marks high quality graphene” (DOI: 10.1103/PhysRevB.100.155307).

The 2020 paper, published in The Journal of Physical Chemistry Letters, is “High layer uniformity of two-dimensional materials demonstrated surprisingly from broad features in surface electron diffraction” (DOI: 10.1021/acs.jpclett.0c02113).