Christian Dolle, Peter Schweizer and Erdmann Spiecker (from left to right) at their ‘nano workbench’, showing how video game controllers can move line defects in bilayer graphene. Photo: Mingjian Wu.
Christian Dolle, Peter Schweizer and Erdmann Spiecker (from left to right) at their ‘nano workbench’, showing how video game controllers can move line defects in bilayer graphene. Photo: Mingjian Wu.

In the 1940s, scientists first explained how materials can deform plastically via atomic-scale line defects called dislocations. These defects can be understood as tiny carpet folds that allow one part of a material to move relative to another without expending a lot of energy. Many technical applications, such as forging, involve dislocations; they also have important functions in everyday life: for example, they protect lives in the crumple zone of cars by transforming energy into plastic deformation.

Researchers at Friedrich-Alexander University (FAU) Erlangen-Nürnberg in Germany have now found a way of manipulating individual dislocations directly at the atomic scale – a feat only previously dreamt of by materials scientists. Using advanced in situ electron microscopy, researchers in Erdmann Spiecker's group have opened up new ways to explore the fundamentals of plasticity, as they report in a paper in Science Advances.

In 2013, an interdisciplinary group of researchers at FAU found the presence of dislocations in bilayer graphene – a ground-breaking study that was reported in a paper in Nature. The line defects are 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,” explains Spiecker. To further this research, his team of two doctoral candidates, Peter Schweizer and Christian Dolle, knew that in addition to observing these defects, they would also need a way to interact with them.

A powerful microscope is needed to see dislocations. The FAU researchers 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 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 precision. These arms can be outfitted with very fine needles that can be moved over the surface of graphene, although special input devices are needed for high-precision control. “Students often ask us what the gamepads are for,” says Dolle, “but of course they are purely used for scientific purposes.”

At the microscope where the experiments are conducted, there are many scientific instruments – and two video game controllers. “You can't steer a tiny robot arm with a keyboard, you need something that is more intuitive,” Dolle explains. “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 Schweizer. For most materials that would be too much, but graphene is known to be able 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,” Schweizer continues. Using this technique, the researchers could confirm long-standing theories about defect interactions as well as determine new ones. “Without directly controlling the dislocation it would not have been possible to find all these interactions!”

One of the decisive factors for success was the excellent equipment at FAU and its Centre for Nanoanalysis and Electron Microscopy (CENEM). “Without having state-of-the art instruments and the time to try something new this would not have been possible,” says Spiecker. “It's important to grow with new developments, and try to broaden the techniques you have available.”

This story is adapted from material from Friedrich-Alexander University Erlangen-Nürnberg, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.