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Resolving the Enigma of Bending Graphene – "Everyone disagrees, they were actually right"



  Illustration of bending in two-layer graphene

Illustration of a bend in two-layer graphene. Credit: Blanka Janicek, Pinshane Huang Lab

A new study by engineers at the University of Illinois combines atomic scaling experiments with computer modeling to determine how much energy is required to bend multilayer graphene a question that has escaped scientists since graphene was first isolated. The findings were reported in the journal by Nature Materials on Nov. 11, 2019. . It is considered one of the main components of future technologies.

"What is exciting about this work is that it shows that while not everyone agrees, they were actually right." – Arend van der Zande, Professor of Mechanical Sciences and Engineering

Most of the present graphene studies are aimed at the development of nanoscale electronic devices. Yet researchers say many technologies – from stretchable electronics to tiny robots so small that they cannot be seen with the naked eye – require an understanding of the mechanics of graphene, especially how it bends and bends to unlock their potential.

"The stiffness of the bending of a material is one of its most basic mechanical properties," says Edmund Khan, co-author of the master's and master's degree in materials. "Although we have been studying graphene for two decades, we are still going to solve this very basic property. The reason is that different research groups came up with different answers spanning a range of dimensions.

  Energy required for folding multilayer graphene

Graduate student Edmund Khan, left, Professor Elif Ertekin, graduate student Jaehyung Yu, Prof. Pinschane Y. Huang, Front and Professor Arend M. van der Zande for bending multilayered graphene – a question that has long eluded scientists. Credit: Stephanie Adams

The team discovered why previous research efforts did not agree. "They either bent the material a little, or they bent it a lot," says Johun Yu, a graduate student in mechanical engineering and engineering and a co-author. "But we found that graphene behaves differently in these two situations. When you bend the multilayer graphene a little, it acts more like a hard board or a piece of wood. When you bend it a lot, it acts as a stack of papers in which the atomic layers can slide one after the other. "

" What is exciting about this work is that it shows that while not everyone agrees, in fact everyone is correct, "said Arend van der Zande, Professor of Mechanical Science and Engineering and co-author of the study . "Each group measured something different. What we found is a model to explain all the differences by showing how they all come together through different degrees of bending."

"Because we studied graphene, bent in different quantities, we were able to to see the transition from one mode to another, from rigid to flexible and from n flattening to the sheet. ”- Elif Ertekin, Professor of Mechanical Sciences and Engineering

To transform into isolated graphene, Yu, fabricate separate atomic layers of hexagonal boron nitride, another 2D material, into atomic scale steps, and then stamp the graphene Using a focused ion beam, Han cut a slice of material and depicted the atomic structure with an electron microscope to see where each graphene layer was located.

The team then developed a set of equations and simulations to calculate the flexural strength using the shape of the graphene curve.

By draping multiple layers of graphene through a step of only one to five atoms, the researchers have created a controlled and precise way to measure how the material will bend over the step in different configurations.

"In this simple structure, there are two types of forces involved in bending graphene," says Pinshane Huang, professor of materials science and engineering and co-author of the study. "Attaching or attracting atoms to the surface tries to pull the material down. The harder the material is, the more it will try to pop back, resisting the pull. The form in which graphene takes over the atomic steps encodes all the material stiffness information.

The study systematically controls how accurately the material is bent and how the properties of graphene change.

"Because we investigated graphene bends of varying quantities, we were able to see the transition from one mode to another, from rigid to flexible and from plane behavior," said Elif Ertekin, professor of mechanical science and engineering, who led part of the research on computer modeling. "We have built atom-scale models to show that the reason this can happen is that the individual layers can slip on top of each other. Having this idea, we were able to use the electron microscope to confirm the slip between the layers. "

New results matter for the creation of machines that are small and flexible to interact with cells or biological material

" Cells can change their shape and respond to their environment, and if we want to move in the direction of microbots or systems that have the capabilities of biological systems, we need to have electronic systems that can change their shape and be very soft, "said van der Zande. "By taking advantage of the intermediate sliding, we have shown that graphene can be of an order of magnitude smaller than conventional materials of the same thickness."

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Reference: "Ultrasound compaction when sliding in several layers graphene ”by Edmund Han, Jaehyung Yu, Emil Annevelink, Jangyup Son, Dongyun A. Kang, Kenji Watanabe, Takashi Taniguchi, Elif Ertekin, Pinshane Y. Huang and Arend M. van der Zande, November 11, 2019, Natural Materials .
DOI: 10.1038 / s41563-019-0529-7

The National Science Foundation, through the Illinois Center for Materials Research, supported this study.


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