Researchers at Rice University have found that extremely small wrinkles in graphene can alter how the material behaves electrically. The results provide experimental evidence for flexoelectricity, an effect in which uneven bending causes a material to develop an electric charge. The findings are published in Advanced Materials.
The work points to a possible new way to control electricity in materials only a few atoms thick. Instead of changing a material by adding chemicals or combining it with something else, scientists may be able to tune its electrical properties simply by changing its shape. In the future, that strategy could contribute to more sensitive sensors and extremely thin electronic devices.
“Our work shows that even an ordinary wrinkle can become an extraordinary electronic feature when viewed at the atomic scale,” said Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor of Engineering and co-corresponding author of the study. “By demonstrating that geometry alone can reshape electrical behavior in graphene, we open a new pathway for designing materials whose properties can be controlled through structure rather than chemistry.”
Examining Graphene at the Atomic Scale
Graphene consists of a single layer of carbon atoms. For this study, the researchers focused on wrinkles that formed naturally in the material. Some of the bends were squeezed into regions smaller than a billionth of a meter. At that scale, the intense curvature can cause electrons to shift slightly toward one side of the graphene.
“Imagine bending a flexible ruler, except the bend is squeezed into a space smaller than a billionth of a meter,” said Sathvik Ajay Iyengar, a former Rice doctoral student and lead author of the study. “At that scale, the electrons in graphene shift slightly toward one side, creating two opposite electrical sides like the ends of a tiny battery.”
To investigate the effect, the team used specialized microscope probes to map the shape of the wrinkles and measure local electrical energy and current. The researchers also used Raman spectroscopy, a laser-based method that shows how atoms are being stretched or compressed. Computer simulations helped predict how bending should influence the movement of electrons.
By comparing highly curved wrinkles with nearby areas of flat graphene, the researchers were able to separate the electrical effects produced by curvature from other possible influences.
“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.”
Sharp Wrinkles Produce Strong Electrical Effects
The team found that graphene wrinkles behaved somewhat like rows of tiny electrical speed bumps. At their sharply curved tips, the wrinkles altered the local electrical energy. Once about one volt of electricity was applied, the researchers consistently detected an electrical current. The measurements closely matched what the computer models had predicted.
The strength of the response was tied more closely to how sharp each wrinkle was than to how tall it was. The researchers estimated that the resulting polarization was between 100,000 and 10 million times stronger than the polarization seen in much larger flexoelectric systems. Polarization refers to the separation of positive and negative electrical charges inside 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.”
A Prediction From 2008 Gets Experimental Support
The origins of the discovery go back to 2008. At that time, theoretical physicist Vincent Meunier predicted that sharply bending graphene could rearrange its electrons and create an electrical response. Meunier, now the P. B. Breneman Chair and head of the Department of Engineering Science and Mechanics at Pennsylvania State University, is a co-corresponding author of the new study.
When the prediction was first made, however, measuring such an effect across bends only a few atoms wide was extremely challenging.
Years later, Iyengar took another look at data he had collected with Manoj Tripathi, a co-corresponding author with the University of Sussex and now at South Dakota Mines. He noticed unusual electrical signals appearing at the sharpest graphene wrinkles and shared the results with Meunier, who had co-advised his doctoral work.
“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. “Bringing the experiments and atomic-scale calculations together allowed us to test that connection directly.”
A New Route Toward Ultrathin Electronics
The findings give researchers a way to investigate whether deliberately controlling the curvature of graphene wrinkles could be used to adjust the material’s electrical properties. If that approach proves practical, it could eventually help scientists develop more sensitive sensors and ultrathin electronic devices.
Rather than treating wrinkles only as imperfections, researchers may be able to use them as functional features whose geometry helps determine how electricity behaves.
“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, 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 National Science Foundation.
