Designing the flexibility of 2D materials


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A new study demonstrates how to create ultra-soft 2D structures that lay the groundwork for the miniaturization of technology to the nanoscale, such as wearable electronics that conform to the flexibility of skin. By controlling the stacking between layers of materials, researchers were able to change the stiffness of two-dimensional materials by engineering the friction between layers.

The cross-disciplinary team at the University of Illinois includes Professor Pinshane Huang from Materials Science and Engineering; two researchers in Mechanical Science and Engineering, Professors Arend van der Zande and Elif Ertekin; Dr. Jaehyung Yu, a postdoctoral researcher who recently graduated from the van der Zande and Ertekin groups; and graduate students Edmund Han in the Huang Group and M. Abir Hossain in the van der Zande Group. All are researchers in the Materials Research Laboratory. The team collaborated with researchers from the National Institute for Materials Science in Japan.

Many next generation technologies, from wearable or stretchable electronics and mechanically reconfigurable quantum systems, to mobile microbots, require electronic materials that can bend and flex out of their original shape while maintaining functionality. However, most electronics today are still flat, stiff, and static.

“If we want to make a device that can change shape like natural cells do, we need electronic devices that really compete with nature in terms of deformability and reconfigurability,” said Han. “We need to take advantage of new 2D nanomaterials to bring together electronics and mechanics.”

“Individual layers of 2D materials are incredibly flexible, around one billion times softer than kitchen plastic wrap with stiffnesses that rival that of cell membranes,” said Prof. van der Zande. “These layers can be vertically stacked—like a stack of papers—to create multilayers of the same or different materials.”

“We know that individual layers of 2D materials can be stacked into aligned or misaligned arrangements, and different stacking arrangements are known to change the electronic properties and friction of 2D materials,” said Hossain. “Our research addresses how to control the bending mechanics of 2D materials.”

“Determining their bending properties is challenging because it requires ultrasensitive measurements with nanoscale precision,” said Dr. Yu. “These criteria are simply too difficult to meet by conventional techniques for measuring bending properties.”

A four-layer stack of graphene (black) and MoS2 (cyan and yellow) bends as it conforms over a hexagonal BN step (red).
A four-layer stack of graphene (black) and MoS2 (cyan and yellow) bends as it conforms over a hexagonal BN step (red).
By simply adding twist between two atomic layers, the researchers were able to reduce the friction between those layers by roughly a thousand-fold, and this reduced friction made it easy for the materials to bend. In addition, they were able to change the bending stiffness of a 2D materials dramatically by arranging the layers in different ways. 

“We have shown that in 2D materials, you can actually design-in the stiffness you want,” said Prof. Ertekin. “For example, we fabricated a structure made out of two layers of graphene and two layers of molybdenum disulfide. Just by changing the order of the layers, we were able to change the stiffness of the stack by 400%.”

“Our design involves carefully separating and stacking individual layers of 2D materials. We then drape these 2D stacks over atom-sized steps—like a rug conforming over a stairway—which allows us to induce controlled nanoscale bends,” said Prof. Huang. “Using an electron microscope, we image and measure the bent shape of these 2D materials, from which we can calculate their bending stiffness.”

The next step for this research is actually fabricating working electronic devices that take advantage of these design-in methods to create ultrasoft, deformable electronics.


The paper, Designing the Bending Stiffness of 2D Material Heterostructures, is published in Advanced Materials.

This research utilized the Thermo Fisher Scientific Themis Z Electron Microscope and the Helios Focused Ion Beam-Scanning Electron Microscope in the Materials Research Laboratory and the use of I-MRSEC shared facilities.

Funding for the project was provided by NSF: I-MRSEC (National Science Foundation, Illinois Materials Research Science and Engineering Center).

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This story was published February 9, 2021.