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Scientists grow spirally new material

  ** Crystal with twist: Scientists breed new material in a spiral
The researchers at UC Berkeley and Berkeley Lab created a new crystal made up of a spiral stack of atomically thin germanium sulfide layers. Picture credits: UC Berkeley Picture by Yin Liu

With a simple turn of the finger you can turn a card game into a beautiful spiral. In the same way, scientists from the University of California, Berkeley, and the Lawrence Berkeley National Laboratory (Berkeley Lab) have produced new inorganic crystals from stacks of atomically thin layers that unexpectedly wind like a nanoscale map deck.

Their surprising structures, reported in a new study published online Wednesday, June 20, in the journal Nature reveal unique optical, electronic and thermal properties, according to the researchers including superconductivity.

These helical crystals consist of stacked layers of germanium sulfide, a semiconductor material that, like graphene, easily forms layers that are only a few atoms or even a single atom thick. Such "nanosheets" are commonly referred to as "2-D materials."

"Nobody expected 2-D materials to grow like that, it's like a surprise gift," said Jie Yao, an assistant professor of materials science and technology at UC Berkeley. "We believe that this brings great opportunities for materials research."

While the shape of the crystals resembles DNA, whose helical structure is critical for the transmission of genetic information, its underlying structure is indeed quite different. In contrast to "organic" DNA, which is mainly composed of known atoms such as carbon, oxygen and hydrogen, these "inorganic" crystals consist of more distant elements of the periodic table ̵

1; in this case sulfur and germanium. And while organic molecules often take on all sorts of crazy forms due to the unique properties of their primary component carbon, inorganic molecules tend to be more straight and narrow structures.

To create the twisted structures, the team used a crystal defect. This is called a screw dislocation, a "defect" in the ordered crystal structure that gives some twisting power. Named after the scientist John D. Eshelby, this "Eshelby Twist" made nanowires that wind like pine trees. However, this study marks the first time that the Eshelby Twist has made crystals from stacked 2-D layers of an atomically thin semiconductor.

  ** Crystal with a twist: Scientists breed spirally new material
The helical crystals can give surprising new properties such as superconductivity. Picture credits: UC Berkeley Picture by Yin Liu

"Usually, people hate defects in a material – they want a perfect crystal," said Yao, who also works as a faculty scientist at the Berkeley Lab. "But it turns out we have to thank the defects this time because they allowed us to create a natural twist between the layers of material."

In an important discovery last year, scientists reported that graphene in two atoms becomes superconducting, thin sheets of material are stacked and rotated at the so-called "magic angle". While other researchers have managed to stack two layers simultaneously, the new work provides a recipe for the synthesis of stacked structures hundreds of thousands or even millions of layers continuously twisting.

"We observed the formation of discrete layers of steps in the twisted crystal that turns the smooth twisted crystal into a spiral staircase, a new phenomenon associated with the Eshelby-Twist mechanism," said Yin Liu, co-lead author of the work and Ph.D. student for materials science and engineering at UC Berkeley. "It's quite amazing how the interaction of materials can lead to many beautiful geometries."

By adjusting the conditions and the length of the material synthesis, the researchers were able to change the angle between the layers and create a twisted structure that is narrow, such as spring or loose, like a slinky developed. While the research team demonstrated the technique by growing helical crystals from germanium sulfide, it could probably be used to grow layers of other materials that form similarly thin atomic layers.

"The twisted structure results from a competition between stored energy and the energy cost of moving two layers of material relative to each other," said Daryl Chrzan, chairman of the Department of Materials Science and Engineering and senior theoretician on paper. "There is no reason to believe that this competition is limited to germanium sulfide, and similar structures should be possible in other 2D material systems."

Different perspectives, have already shown great potential and attracted much attention in physics and chemistry. Now it will be extremely exciting to find out if all these twisted layers in our new material have very different materials properties than the regular stacking of these materials, "Yao said." However, we currently have very limited knowledge of what these properties could be this material form is so new. New opportunities are waiting for us. "

The research team discovers a completely incomplete rotation of nanowire growth

Further information:
Yin Liu et al. Helical van der Waals crystals with discretized Eshelby phrase, Nature (2019). DOI: 10.1038 / s41586-019-1308-y

Provided by
University of California, Berkeley

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Crystal with twist: Scientists make new material spiral (2019, June 20)
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