In 2008, two physicists proposed that something strange should happen when graphene bends almost as much as the atoms allow. They argued that the curvature could unbalance the electrons, producing electrical polarization solely from the shape. If they were right, that meant that a wonder material (graphene has amazing strength, conductivity, flexibility and lightness) could be turned into highly sophisticated ultra-thin electronic devices without the need to add additional material. But would it really work? Strong deformation occurs at distances approaching the scale of individual atoms, where even the most powerful microscopes struggle to determine exactly how strongly a material bends. Now, researchers in the US and UK say tiny wrinkles in graphene may have provided the experimental evidence they were looking for. In a new study, the team reports that extremely steep bends changed the local electrical potential of graphene in ways that closely matched theoretical calculations at the atomic scale. The researchers add that they have now observed flexoelectricity in graphene, meaning that uneven bending creates a separation of electrical charge. However, in one-atom-thick graphene, the researchers argue that the extreme curvature does something more unusual. It changes the way the electron orbitals overlap, redistributing the electrons around the wrinkle. This makes this a case of what researchers call quantum orbital flexoelectricity. It may not mean anything to you yet, but read on and we’ll get to it in a moment. Correct everywhere Small curves in graphene can change the electrical properties of the material, depending on the angle. Exaggerated diagram for illustrative purposes. Credit: ZME Science The path to discovery was not initially easy. Satvik Ajay Iyengar, then a doctoral student at Rice University, was reviewing measurements he had collected with researcher Manoj Tripathi when he noticed unusual electrical signals around the graphene’s most pronounced wrinkles. He took the results to Vincent Meunier, who helped advise him on his doctoral work and also authored a 2008 study, which first introduced the idea of ”electronic flexoelectricity” in low-dimensional systems like graphene. ×Thank you! One more thing… Please check your inbox and confirm your subscription. “When Sathvik showed me the measurements he and Manoj had collected, we realized that the unusual signals could provide an experimental connection to an idea we had predicted many years earlier,” Meunier said in a Rice University statement. “Bringing together the experiments and calculations at the atomic scale allowed us to test that connection directly.” The wrinkles formed where the graphene rested on molybdenum disulfide, another atomically thin material. Because the two materials respond differently to mechanical stress, the graphene was bent into narrow ridges without the researchers having to press it with the tip of a microscope. Previously, a 2021 experiment with graphene nanobubbles, for example, reported flexoelectric behavior after researchers created the bubbles using an electric field from a microscope probe. The new wrinkles form naturally, giving the team a comparatively clean way to examine extreme curvature. Scheme of the graphene wrinkle used in the experiment. The blue graphene sheet bends sharply where it rests on a layer of molybdenum disulfide (yellow), which sits on silica (red). Credit: Advanced Materials The steepest curve The team also found that the curvature of the graphene mattered much more than the height of the wrinkle. At the sharper tips, the electrical behavior changed noticeably, while the taller but less curved wrinkles behaved very similarly to the shorter ones. “The sharpness of the wrinkle turned out to be much more important than its overall size,” Iyengar said. “That tells us that we can potentially tune the electrical behavior by carefully controlling the curvature at the nanoscale.” The effect was surprisingly strong. In the steeper curves, the electrons shifted enough to create a pronounced electrical imbalance, with one side becoming relatively negative and the other relatively positive. The researchers estimated that this polarization is between 100,000 and 10 million times stronger than in much larger flexoelectric systems. Iyengar compares the two sides to opposite ends of a small battery, but without storing or delivering energy as one. The broader promise is that engineers will one day be able to control the behavior of atomically thin electronic components simply by deciding where and how intensely the material is bent. All sorts of interesting and useful properties could emerge in this way, although other currently known useful properties of graphene could also diminish. A 2012 study on graphene wrinkles showed that wrinkles can hinder the movement of electrons through graphene, changing its electrical resistance depending on the shape and direction of the wrinkles. Additionally, a 2021 Nature Nanotechnology study showed that bending molybdenum disulfide unevenly could help it generate an electric current when exposed to light. However, the new result comes with caveats. Researchers cannot yet accurately measure the smallest curvatures on an atom-by-atom basis, and some key quantities depend on order-of-magnitude models and estimates. Nor has anyone built a practical device around this effect. The study was published in the journal Advanced Materials.
