New Inorganic Crystals Spiral Like a Nanoscale Card Deck

One can make a lovely spiral from a deck of cards by simply spinning the fingers. Similarly, researchers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory (Berkeley Lab) have developed new inorganic crystals composed of piles of atomically thin sheets that surprisingly spiral like a nanoscale card deck.

According to the scientists, their unexpected structures, described in a new research published online on June 20^th, 2019, Wednesday, in the journal Nature, may offer exceptional electronic, thermal, and optical properties, including superconductivity.

These helical crystals are composed of stacked layers of germanium sulfide— a semiconductor material like graphene—that promptly create sheets having a thickness of just a few atoms or even one atom. Typically, such “nanosheets” are called “2D materials.”

No one expected 2D materials to grow in such a way. It’s like a surprise gift. We believe that it may bring great opportunities for materials research.

Jie Yao, Assistant Professor, Department of Materials Science and Engineering, UC Berkeley

The shape of the crystals may look like DNA, the helical structure of which is crucial to its function of carrying genetic information; however, their basic structure is, in fact, relatively distinct by nature. In contrast to “organic” DNA that is mainly composed of well-known atoms such as hydrogen, oxygen, and carbon, these “inorganic” crystals are constructed of more far-flung elements of the periodic table, which in this case, are germanium and sulfur. Moreover, while organic molecules mostly take all types of strange shapes owing to the distinctive properties of their main component (that is, carbon), inorganic molecules often take up narrow and straight shapes.

The researchers developed the twisted structures by exploiting a crystal defect known as a screw dislocation—a “mistake” in the orderly crystal structure that offers a slight twisting force to it. This twist, termed as “Eshelby Twist” after the researcher John D. Eshelby, has been applied to develop nanowires that spiral similar to pine trees. However, this research is the first one to use the Eshelby Twist to create crystals constructed of stacked 2D layers of an atomically thin semiconductor.

“Usually, people hate defects in a material—they want to have a perfect crystal,” stated Yao, who is also a faculty scientist at Berkeley Lab. “But it turns out that, this time, we have to thank the defects. They allowed us to create a natural twist between the material layers.”

In a groundbreaking discovery last year, researchers demonstrated that graphene turns superconductive upon stacking two atomically thin sheets of the material and twisting the sheets at the so-called “magic angle.” Other scientists have been successful at stacking two layers simultaneously; by contrast, the new study offers a formula for producing stacked structures that are hundreds of thousands or even millions of layers in a continuously twisting manner.

We observed the formation of discrete steps in the twisted crystal, which transforms the smoothly twisted crystal to circular staircases, a new phenomenon associated with the Eshelby Twist mechanism. It’s quite amazing how interplay of materials could result in many different, beautiful geometries.

Yin Liu, Study Co-First Author and Graduate Student, Department of Materials Science and Engineering, UC Berkeley

The scientists could alter the angle between the layers by tuning the length and conditions of the material synthesis, thus producing a twisted structure that is loose, like an uncoiled Slinky, or compact, like a spring. In addition, while the scientists illustrated the method by culturing helical crystals of germanium sulfide, it could also be used for growing layers of other materials that produce comparable atomically thin layers.

The twisted structure arises from a competition between stored energy and the energy cost of slipping two material layers relative to one another. There is no reason to expect that this competition is limited to germanium sulfide, and similar structures should be possible in other 2D material systems.

Daryl Chrzan, Chair, Department of Materials Science and Engineering, UC Berkeley

Chrzan is also the senior theorist on the paper.

“The twisted behavior of these layered materials, typically with only two layers twisted at different angles, has already showed great potential and attracted a lot of attention from the physics and chemistry communities. Now, it becomes highly intriguing to find out, with all of these twisted layers combined in our new material, if will they show quite different material properties than regular stacking of these materials,” Yao stated. “But at this moment, we have very limited understanding of what these properties could be, because this form of material is so new. New opportunities are waiting for us.”

Additional co-first authors of the paper are Su Jung Kim and Haoye Sun of UC Berkeley and Jie Wang of Argonne National Laboratory. Other authors are Fuyi Yang, Zixuan Fang, Ruopeng Zhang, Bo Z. Xu, Michael Wang, Shuren Lin, Kyle B. Tom, Yang Deng, Robert O. Ritchie, Andrew M. Minor and Mary C. Scott of UC Berkeley; Nobumichi Tamura, Xiaohui Song, Qin Yu, John Turner and Emory Chan of Berkeley Lab and Jianguo Wen and Dafei Jin of Argonne National Laboratory.

The study conducted at Berkeley Lab’s Molecular Foundry and the Advanced Light Source was supported by the U.S. Department of Energy’s Office of Science and Office of Basic Energy Sciences under contract no. DE-AC02-05CH11231. The work was also supported by the U.S. Department of Energy’s Office of Science, Office of Basic Energy Sciences and Materials Sciences and Engineering Division under contract no. DE-AC02-244 05CH11231 within the Electronic Materials Program (KC1201).