Small spirals could have big impact

This microscopic, twisting spiral was “grown” by depositing sheets of 2D material over a substrate curved slightly by slipping a nanoparticle underneath.

201024 spiral
CREDIT: Song Jin Lab

It exhibits interesting, tuneable, superconductive properties, according to the chemists who created it, making it an important development in the intriguing world of twistronics.

“This is the current frontier of 2D material research,” says Song Jin, from University of Wisconsin-Madison, US. “In the last few years, scientists have realised that when you make a small twist between atomic layers – usually a few degrees – you create very interesting physical properties, such as unconventional superconductivity.”

In a paper in the journal Science, Jin and colleagues describe their way to control the growth of twisting, microscopic spirals of materials that are just one atom thick, allowing them to build continuously twisting stacks of materials.

Standard practice, says first author Yuzhou Zhao, is to mechanically stack two sheets of thin materials on top of each other and control the twist angle between them by hand. But when you grow these 2D materials directly, you cannot control the angle because the interactions between the layers are very weak.

The alternative, Jin says, is to look outside the world of Euclidean geometry, with its flat planes, straight lines and right angles, and think about curves.

His team twisted spirals by taking advantage of an imperfection in growing crystals called screw dislocations. In 2D materials, the dislocations provide a step up for following layers of the structure as it spirals like a parking ramp with all layers throughout the stack connected, aligning the orientation of every layer.

In order to grow a non-Euclidean spiral structure and make the spirals twist, the researchers changed the foundation that their spirals grew from.

Instead of growing crystals on a flat plane, Zhao placed a nanoparticle, like a particle of silicon oxide, under the spiral’s centre. During the growth process, the particle disrupts the flat surface and creates a curved foundation for the 2D crystal to grow on.

What they found, they say, is that instead of an aligned spiral where the edge of each layer lies parallel to the previous layer, the 2D crystal forms a continuously twisting, multilayer spiral that twists predictably from one layer to the next.

The angle of the interlayer twist arises from a mismatch between the flat (Euclidean) 2D crystals and the curved (non-Euclidean) surfaces they grow on.

Zhao then developed a simple mathematical model to predict the twist angles of spirals, based on the geometric shape of the curved surface, and his modelled spiral shapes matched well with the grown structures.

When the spirals were examined under an electron microscope, the images showed that atoms in neighbouring twisted layers formed an expected overlapping interference pattern called a moiré pattern.

“We now can follow a rational model rooted in mathematics to create a stack of these 2D layers with a controllable twist angle between every layer, and they’re continuous,” Zhao suggests.

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