Researchers manipulate individual graphene dislocations on the atomic scale
Materials can deform plastically along atomic-scale line defects called dislocations. Many technical applications such as forging are based on this fundamental process, but the power of dislocations is also exploited in the crumple zones of cars, for instance, where dislocations protect lives by transforming energy into plastic deformation. FAU researchers have now found a way of manipulating individual dislocations directly on the atomic scale.
Using advanced in situ electron microscopy, the researchers in Prof. Erdmann Spiecker's group have opened up new ways to explore the fundamentals of plasticity. They have published their findings in Science Advances.
The thinnest interface with defects
In 2013, an interdisciplinary group of researchers at FAU found dislocations in bilayer graphene—a groundbreaking study that was published in Nature. The line defects were contained between two flat, atomically-thin sheets of carbon—the thinnest interface where this is possible. "When we found the dislocations in graphene, we knew that they would not only be interesting for what they do in the specific material, but also that they could serve as an ideal model system to study plasticity in general," Prof. Spiecker explains. His team of two doctoral candidates sought a way to interact with them.
A powerful microscope is needed to see dislocations. The researchers from Erlangen are specialists in the field of electron microscopy, and are constantly thinking of ways to expand the technique. "During the last three years, we have steadily expanded the capabilities of our microscope to function like a workbench on the nanoscale," says Peter Schweizer. "We can now not only see nanostructures, but also interact with them—for example, by pushing them around, applying heat or an electrical current." At the core of this instrument are small robot arms that can be moved with nanometer-scale precision. These arms can be outfitted with very fine needles that can be moved onto the surface of graphene. Special input devices are needed for high-precision control.
Plasticity at the fingertips
At the microscope where the experiments were conducted, there are many scientific instruments—and two video game controllers. "Students often ask us what the gamepads are for," says Christian Dolle. "But of course, they are purely used for scientific purposes. You can't steer a tiny robot arm with a keyboard, you need something that is more intuitive. It takes some time to become an expert, but then, even controlling atomic-scale line defects becomes possible."
One thing that surprised the researchers at the beginning was the resistance of graphene to mechanical stress. "When you think about it, it is just two layers of carbon atoms—and we press a very sharp needle into that," says Peter Schweizer. For most materials, that would be too much, but graphene is known to withstand extreme stresses. This enabled the researchers to touch the surface of the material with a fine tungsten tip and drag the line defects around. "When we first tried it, we didn't believe it would work, but then we were amazed at all the possibilities that suddenly opened up." Using this technique, the researchers could confirm longstanding theories of defect interactions, as well as find new ones. "Without directly controlling the dislocation, it would not have been possible to find all these interactions," says Dolle.
"Without having state-of-the art instruments and the time to try something new this would not have been possible," Spiecker says. "It's important to grow with new developments, and try to broaden the techniques you have available."