Giving nanowires a DNA-like twist
We see crystals all around us: snowflakes, ice cubes, table salt, gemstones, to name a few. Invisible to the naked eye, but of special interest to scientists, are crystalline "nanowires" — wires with a diameter of a mere few nanometers and a typical length of a micrometer.
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Normally in a rod-like shape, these wires are an interesting area of worldwide research because of their many potential applications, including semiconductors and miniaturized optical and optoelectronic devices.
As reported in a recent Nature paper, scientists at the Center for Nanoscale Materials (CNM), a U.S. Department of Energy (DOE) Office of Science User Facility located at Argonne National Laboratory, played a critical role in the discovery of a DNA-like twisted crystal structure created with a germanium sulfide nanowire, also known as a "van der Waals material." The research was conducted in collaboration with the University of California at Berkeley and Lawrence Berkeley National Laboratory.
The helical DNA-like structure forms spontaneously by giving the nanowire an "Eshelby twist." Co-first author Jie Wang, a former materials scientist in CNM (now at Thorlabs, Inc.), explained that the term "Eshelby twist" refers to its discoverer, John Eshelby.
While a research associate working at the University of Illinois at Urbana-Champaign in the 1950s, Eshelby conducted an important theoretical analysis of "screw dislocation" in a thin rod. Relating the effect to crystals, Wang noted that the "screw dislocation occurs when stress is applied to a rod shape in which the atoms become rearranged in a helical pattern."
When applied to a germanium sulfide nanowire, this twisting causes it to elongate and widen into a helical structure.
"It is amazing that these inorganic germanium sulfide nanowires so closely resemble the organic DNA structure," said co-author Jianguo Wen, a CNM materials scientist. "Nature creates remarkable structures beyond our imagination."
Also important, added CNM scientist and co-author Dafei Jin, was the finding that the nanostructure automatically divides into segments that resemble helically stacked bricks. These brick-like segments arise from the release of energy as the wire diameter grows from tens of nanometers to micrometers.
"The discovered Eshelby twist here offers a new way to engineer nanomaterials," said Wang. "We can tailor these nanowires in many different ways — twist periods from two to twenty micrometers, lengths up to hundreds of micrometers, and radial dimensions from several hundred nanometers to about ten micrometers."
By this means, researchers can adjust the electrical and optical properties of the nanowires to optimize performance for different applications.
"This is an important materials discovery," Wen said. "We are excited to have figured out, using CNM's high-resolution transmission electron microscope, the dislocation structures that drive the nanowires to have an Eshelby twist."
Materials provided by DOE/Argonne National Laboratory. Note: Content may be edited for style and length.
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