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Eshelby twist induces helical, multilayered van der Waals crystals

By Rachel Nuwer July 24, 2019
twisted GeS crystals to other substrates
Scanning electron microscope images show the twisted GeS crystals after the transfer to other substrates. Scale bar, 4 µm. Credit: Nature

In the 1950s, John D. Eshelby showed that a defect called a screw dislocation can twist the lattice planes in a crystal into a helical arrangement. By introducing the Eshelby twist to GeS nanowires, researchers have now synthesized helical van der Waals crystals whose several to many atomic layers twist upwards like a spiraling deck of cards. They report their work in a recent issue of Nature.

“Twisting is a very powerful approach that can bring new phenomena and new properties to the materials,” says Jie Yao, an assistant professor in the Department of Materials Science and Engineering at the University of California, Berkeley, and senior author of the article. “By twisting layers, we introduce an additional periodicity that’s able to completely change behaviors of electrons in the crystals.”

Prior to this new work, other research teams were able to twist conventional nanowires and also to twist two layers of van der Waals-bonded graphene. The latter work, led by Pablo Jarillo-Herrero, a physicist at the Massachusetts Institute of Technology, unexpectedly produced superconductivity. Yao and his colleagues are the first known to twist a multilayer van der Waals material in a controllable way, opening possibilities for twisted van der Waals structures with tailored topologies.

“One thing we’re very happy about is there is no rocket science or very complex techniques required to grow such crystals,” Yao says. “In other words, the barrier for using our technique is not that high.”

Yao’s research team introduced screw dislocation-driven growth to the van der Waals materials field. This classical method induces vertical growth in crystals by harnessing naturally occurring defects. To apply the method to van der Waals materials, the researchers heated powdered GeS crystals to induce chemical vapor transport. The evaporated crystal vapors traveled on inert argon gas to a downstream substrate, ranging from quartz to silicon nitride. Cooler temperatures then caused the vapor to settle.

The researchers adjusted various parameters—including temperature, pressure, flow rate, and time—to encourage optimum conditions for twisted crystal growth. Of the thousands of GeS crystals that began to grow on the substrate, the researchers identified hundreds with the desired defect that causes twisted growth. “At this point we cannot dictate that a specific crystal will be twisted,” Yao says. “But we can drive the growth conditions toward the direction in which we have a larger percentage of twisted crystals.”

The MIT team previously demonstrated that the angle of twist is critical for causing unique properties, including superconductivity. Yao and his colleagues were able to control the twisting rate and angle of their crystals by applying the half-century-old theory developed by  Eshelby,  whose research laid the foundations for the field of defect mechanics. According to Eshelby, the twisting rate of a cylindrical structure strongly depends on the material’s cross-sectional area size, with structures with smaller areas twisting faster than larger ones. Yao and his colleagues quantitatively confirmed that they could control the twisting rate of their helical van der Waals crystals by changing the diameter of the materials. One way they accomplished this was by introducing gold droplet catalysts, upon which the crystals grew. The larger the diameter of the droplet, the larger the diameter of the corresponding crystal.

“This paper is really exciting, for sure,” says Yi Cui, a materials scientist at Stanford University, who was not involved in the research. “Twisting two-dimensional material stacks can generate new, exciting physical and chemical properties, including superconductivity.”

The new method will allow researchers to stack several, hundreds, or even thousands of nanometer-thin layers with a constant twisting angle, Yao says. Now that the method is established, he and his colleagues are eager to begin characterizing how electrons and photons will behave in such structures. Photons propagated through cylindrical optical fibers currently form the backbone of the internet, so Yao and his colleagues are intrigued by the question, for example, of whether photons propagating in a helical structure could lead to interesting, unexpected applications for information processing.

“New interactions between the twisted layers will definitely have very strong influences on the microscopic particles,” Yao says. “Our knowledge of these influences is still limited, but I believe there will be very interesting phenomena showing up based on this new materials platform.”

Read the abstract in Nature.