Mar 12 2020
When compared to steel, graphene can be approximately six times lighter and 200 times stronger. These properties have made it the most preferred material in the manufacturing sector. At the University of Illinois at Urbana-Champaign, researchers recently discovered more properties of graphene sheets that could be beneficial to the industry.
Atomistic configuration of twisted bilayer graphene. Image Credit: Department of Aerospace Engineering, The Grainger College of Engineering.
Soumendu Bagchi, a doctoral student, and his guide Huck Beng Chew from the Department of Aerospace Engineering collaborated with Harley Johnson from Mechanical Sciences and Engineering and discovered the behavior of twisted sheets of graphene and their stability at various temperatures and sizes.
We concentrated on two graphene sheets stacked on top of each other but with a twist angle. We did atomistic simulations at different temperatures for different sizes of graphene sheets. Using insights from these simulations, we developed an analytical model—you can plug in any sheet size, any twist angle, and the model will predict the number of local stable states it has as well as the critical temperature required to reach each of those states.
Soumendu Bagchi, Doctoral Student, Department of Aerospace Engineering, The Grainger College of Engineering, The University of Illinois at Urbana-Champaign
Bilayer graphene occurs in an untwisted Bernal-stacked configuration, said Bagchi. This configuration is also the repeated stacking sequence of crystalline hexagonal graphite. Upon twisting bilayer graphene, it tends to untwist back to its initial state since that is the most stable state and arrangement of atoms.
When the twisted atomic structure is heated, it tends to rotate back, but there are certain magic twist angles at which the structure remains stable below a specific temperature. And, there is a size dependency as well.
Soumendu Bagchi, Doctoral Student, Department of Aerospace Engineering, The Grainger College of Engineering, The University of Illinois at Urbana-Champaign
“What’s exciting about our work is, depending upon the size of the graphene sheet, we can predict how many stable states you will have, the magic twist angles at these stable states, as well as the range of temperatures required for twisted graphene to transition from one stable state to another,” added Bagchi.
Chew stated that manufacturers have been making efforts to create graphene transistors, and twisted bilayers of graphene have interesting electronic properties. While manufacturing graphene transistors, it is essential to know what temperature will stimulate the material to realize a specific mechanical response or rotation.
“They’ve known that a graphene sheet has certain electronic properties, and adding a second sheet at an angle yields new unique properties. But a single atomic sheet is not easy to manipulate. Fundamentally, this study answers questions about how twisted graphene sheets behave under thermal loading, and provides insights into the self-alignment mechanisms and forces at the atomic level,” stated Bagchi.
“This could potentially pave the way for manufacturers to achieve fine control over the twist angle of 2D material structures. They can directly plug in parameters into the model to understand the necessary conditions required to achieve a specific twisted state,” added Bagchi.
Bagchi stated that the 2D properties of such materials have not been studied previously. This study is an elementary study that started as a different project, when he chanced upon something strange.
He noticed that the graphene sheets showed some temperature dependence. We wondered why it behaved this way—not like a normal material. In normal materials, the interface is typically very strong. With graphene, the interface is very weak allowing the layers to slide and rotate. Observing this interesting temperature dependency wasn’t planned. This is the beauty of discovery in science.
Huck Beng Chew, Department of Aerospace Engineering The Grainger College of Engineering, The University of Illinois at Urbana-Champaign
This study was funded by the AFOSR Aerospace Materials for Extreme Environment Program and a grant from the National Science Foundation.