UNIVERSITY PARK, Pa. — Tiny wrinkles could reveal new behaviors in graphene, an exceptionally strong, conductive and thin material used in batteries, computer chips and other electronics. According to a new study published in Advanced Materials , small bends in graphene's two-dimensional (2D), or one-atom-thick, structure can change the material's electrical properties.
The team of physicists and engineers from multiple institutions, including co-corresponding author Vincent Meunier , Department Head of Engineering Science and Mechanics and P. B. Breneman Chair and Professor at Penn State, reported that this wrinkling behavior offers evidence for flexoelectricity. This phenomenon occurs when a material bends unevenly — generating an electric dipole, or a system where two equal and opposite charges are separated by a small distance. By harnessing this characteristic, Meunier explained that scientists may one day control electricity in atomically thin materials — such as graphene — by changing their shape instead of adding new chemicals or materials, potentially powering more responsive sensors and ultrathin electronic devices.
Meunier and his collaborator, Sergei V. Kalinin, who is now at the University of Tennessee, first predicted that sharply bending graphene could rearrange the electrons and produce an electrical response in a paper published in Physical Review B almost 20 years ago. At the time, the prediction was entirely theoretical, but advances in science have now allowed Meunier and his current collaborators to test the effect experimentally.
"One of my driving forces is to try to identify fun ideas to examine that may seem unreachable today, but could be applied years down the line," Meunier said. "To observe this effect practically, you need to have ultra-clean materials, which at the time of the prediction was a bit of a pipe dream. With advancements in fabrication technology, though, that dream is now a reality."
Years later, Sathvik Ajay Iyengar, lead author of the most recent study who earned their doctorate in materials science and nanoengineering from Rice University and is now at the University of California, Berkeley, revisited data he had collected with Manoj Tripathi, a co-corresponding author at the University of Sussex who is now at South Dakota Mines. The team observed that during their experiments, unusual electrical signals were appearing at the sharpest graphene wrinkles. Sathvik brought the findings to Meunier, who had co-advised his doctoral work at Rice University.
"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. "I calculated the effect we had predicted for the specific geometry and compared it to the experiment, and it turns out the fit was almost perfect."
Applying these calculations, the team examined naturally formed wrinkles with bends compressed into spaces only a few atoms wide, where the extreme curvature can shift electrons toward one side of the material. According to Meunier, advancements in materials science allowed the team to apply and test this theoretical effect across incredibly tiny bends.
"Imagine bending a flexible ruler, except the bend is squeezed into a space smaller than a billionth of a meter," Iyengar said. "At that scale, the electrons in graphene shift slightly toward one side, creating two opposite electrical sides like the ends of a tiny battery."
The research team used specialized microscope probes to measure the wrinkles' shape, local electrical energy and electrical current. They also used Raman spectroscopy, a laser-based technique that reveals how atoms are stretched or compressed, along with simulations performed on Penn State's Roar supercomputer, to predict how bending changes the movement of electrons. Comparing sharply curved wrinkles with nearby flat graphene allowed the team to isolate the effects of curvature.
"Earlier studies often examined gentler bends or relied on external pressure, making this subtle effect difficult to separate," Iyengar said. "Comparing the sharply curved wrinkles with flat graphene allowed us to clearly identify the role of extreme curvature."
The researchers found that the wrinkles acted like rows of tiny electrical speed bumps. Their sharply curved tips changed the local electrical energy and consistently produced an electrical current once about one volt of electricity was applied, closely matching predictions made by computer models and mathematical calculations.
The electrical response depended on the sharpness of the wrinkles rather than their height. The researchers estimated that the resulting electrical charge separation, called polarization, was between 100,000 and 10 million times stronger than in much larger bends, or flexoelectric systems. Polarization is the separation of positive and negative electrical charges within a material.
"The sharpness of the wrinkle turned out to be much more important than its overall size," Iyengar said. "That tells us we can potentially tune electrical behavior by carefully controlling curvature at the nanoscale."
That controlled approach could eventually support the development of more sensitive sensors and ultrathin electronic devices.
"Nature already creates these tiny wrinkles for us," Iyengar said. "Understanding how they influence electrical behavior gives scientists another tool for designing future technologies using the structure of a material itself."
Additional authors include James McHugh of the University of Manchester; Jonathan Salvage of the University of Brighton; Robert Vajtai of Rice University; Venkataramana Gadhamshetty of the South Dakota School of Mines and Technology; and co-corresponding author Alan Dalton of the University of Sussex.
The research was supported by the Quad Fellowship, the Sussex Strategy Development Fund, the University of Manchester Dame Kathleen Ollerenshaw Fellowship and the U.S. National Science Foundation under grant numbers OIA-2418752 , OIA-1849206 and DGE-2510643 . This content is solely the responsibility of the authors and does not necessarily represent the views of the funders.
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