On edges: Team captures growth of Nebraska Ice

Chemistry


Posted January 7, 2020 by Scott Schrage

Scott Frost treaded the sideline as the Huskers faced Colorado in Boulder, watching the Blackshirts try to stomp out a Buffalo comeback in a game ultimately decided by just 3 points.

About 500 miles due east — a span traced by the shallow river now linked to his career-defining achievement — Xiao Cheng Zeng was computer-chipping away the secrets of two-dimensional ice in Hamilton Hall.

The year? 2019. And 1997.

“I remember that year so well, because that’s the last national championship we won (in football),” said Zeng, Chancellor’s University Professor of chemistry. “My computer was very slow at the time, not even as fast as your phone. I couldn’t do 3D (computations), so I went to 2D.”

The decision proved fateful and fortunate. In December of that year, Zeng co-authored a paper that predicted the existence of a nanoscopic wonder: the first 2D form of ice and, according to his model, the first to contract rather than expand upon freezing. When he learned that the Oto Indians had named his adopted state after the Platte River — “Nebraska” meaning “flat water” in the Oto language — Zeng coined his 2D discovery “Nebraska Ice.”

Twenty-two years later, Zeng and colleagues at his undergraduate alma mater, Peking University, have now captured the real-time growth of Nebraska Ice — and discovered a surprise around the edges.

The first successful attempt to synthesize Nebraska Ice, in 2000, occurred amid extreme conditions: at minus 40 degrees Fahrenheit, under a pressure of more than 7,000 pounds per square inch, between two plates separated by a space roughly 100,000 times finer than a human hair. Confining its water molecules to such an infinitesimal alley allowed the ice to grow, but it also shielded prying eyes and inquiring minds from that growth.

This time, researchers in Peking managed to grow Nebraska Ice on an uncovered field of gold. Using an especially sophisticated microscope — one with a probe hovering just above the ice, recording the various forces of its hydrogen and oxygen atoms to generate the nanoscale equivalent of a topographic map — the researchers then imaged the configuration of its crystals.

That configuration can shift as the ice grows, which the researchers learned by pulling off a seeming oxymoron — freezing the ice itself — after lowering the temperature to minus 450 degrees Fahrenheit. At that point, even individual molecules barely move and the ice’s edges go still.

“We wanted to know how the thing changes from infancy to adulthood,” Zeng said.

Freezing Nebraska Ice at various stages of its development effectively enabled the team to take snapshots that captured the pattern of its lattice-like framework and the orientation of its edges. To date, the observed crystals of two-dimensional ices had always formed hexagons, with their edges resembling either zig-zags or a so-called armchair pattern. And in some cases, that’s exactly what Zeng’s colleagues observed.

But the team discovered that some of those hexagonal crystals can also morph into pentagons or heptagons, resulting in edges that temporarily take on topological defects sometimes observed in other 2D materials. Based on Zeng’s simulations, the traditional hexagonal pattern holds just behind the leading edge of the ice. When water molecules approach a zig-zag edge, pentagons tend to emerge first; when those molecules instead encounter an armchair edge, a 5-7-5 configuration — pentagon-heptagon-pentagon — initially emerges before settling into a 5-6-5-6 pattern.

“That was very unexpected,” said Zeng, who modeled the phenomenon with help from the university’s Holland Computing Center. “Everyone thought there would be hexagons dominating at that leading edge.”

Zeng said the new study also ranks as the second independent experiment to support the existence of Nebraska Ice, a widely accepted threshold for officially recognizing and naming it with a Roman numeral. As the first 2D ice to reach that standard, he said, it has a strong claim to No. 1 — even if it had to wait a bit longer for its title than the ’97 Huskers.

The team published its findings in the journal Nature and received funding in part from the U.S. National Science Foundation.