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Graphene: Softer through bending

11-19 few layer graphene

[Image above] What is the bending stiffness of graphene? Depends on how much you bend it. Credit: Blanka Janicek (Pinshane Huang Lab), University of Illinois

Can you believe it’s been almost 10 years since Andre Geim and Konstantin Novoselov received the 2010 Nobel Prize in Physics for graphene?

Since Geim, Novoselov, and their collaborators unambiguously produced and identified graphene in 2004, major advances have been made in both how the material is studied (from simulation to live action movies) and how it is produced (for example, 3D printing).

Yet for all the research on graphene over the past two decades, some of its most fundamental properties still are not completely understood. For example, bending stiffness.

“The bending stiffness of a material is one of its most fundamental mechanical properties,” Edmund Han, materials science and engineering graduate student at the University of Illinois at Urbana-Champaign, says in a university press release. “Even though we have been studying graphene for two decades, we have yet to resolve this very fundamental property.”

The reason for this uncertainty comes from the fact that different research groups have come up with different values for bending stiffness—values that span across orders of magnitude.

“For monolayer graphene, literature values for its bending stiffness range from 0.83 to 10,000 eV,” write Han and his collaborators from the University of Illinois at Urbana-Champaign and the National Institute for Materials Science in Japan in a recent paper. “Furthermore, the reported bending stiffness of bilayer graphene ranges across two orders of magnitude, from 3.4 to 160 eV, while values for trilayer graphene range from 7 to 690 eV.”

In the paper, the researchers aimed to understand why such discrepancies in bending stiffness measurements exist. To do so, they fabricated heterostructures of few-layer graphene and draped it over atomically sharp steps of hexagonal boron nitride, which allowed them to systematically vary the thickness and degree of curvature in the graphene.

Through testing numerous curvature configurations and building atomic-scale models, they discovered why previous research efforts disagreed on bending stiffness. “They were either bending the material a little or bending it a lot,” Jaehyung Yu, mechanical science and engineering graduate student at the University of Illinois at Urbana-Champaign, says in the press release. “But we found that graphene behaves differently in these two situations.”

“When you bend multilayer graphene a little, it acts more like a stiff plate or a piece of wood,” Yu continues. “When you bend it a lot, it acts like a stack of papers where the atomic layers can slide past each other.”

In the press release, University of Illinois at Urbana-Champaign professor Arend van der Zande explains how exciting it is that despite the supposed disagreements, the results show everyone was actually correct. “Every group was measuring something different,” he says. “What we have discovered is a model to explain all the disagreement by showing how they all relate together through different degrees of bending.”

In the paper’s conclusion, the researchers note that though they focused on the properties of graphene, their conclusions should generalize to other van der Waals-bonded materials. Additionally, “these results will be important for the design of new classes of highly curved nanosystems such as nanoelectromechanical systems, stretchable electronics and origami structures made from 2D materials,” they add.

The paper, published in Nature Materials, is “Ultrasoft slip-mediated bending in few-layer graphene” (DOI: 10.1038/s41563-019-0529-7).