Newswise — University of Nebraska-Lincoln scientists looking to fill gaps in basic understanding about gold's structure at the nanoscale have turned up a full-sized and surprising discovery -- hollow cage-like structures made of pure gold atoms.
UNL chemistry professor Xiao Cheng Zeng and colleagues reported in this week's Proceedings of the National Academy of Sciences' online edition that they have found evidence of the first free-standing hollow cage structure composed of clusters of pure metal atoms, which they've dubbed golden hollow cages.
These structures, many of which look somewhat like bird cages, can host an atom inside. Scientists might someday be able to harness these truly tiny cages to carry useful guest atoms for medical or industrial purposes.
"I'm excited by this discovery. These are the first metal hollow cages," Zeng said. "No one expected the cage structure. It was a shocking surprise."
Zeng, Ameritas University Professor of Chemistry, studies clusters of atoms and other materials in the growing field of nanomaterials science, the study of materials at the smallest scale. For perspective, a nanometer is one billionth of a meter or a few millionth the size of a human hair.
"The holy grail of cluster science is studying these clusters from smallest, or infant, to largest, or adult, from cluster to bulk," Zeng said.
Scientists elsewhere had determined that smaller, or infant, gold clusters with fewer than 14 atoms are generally flat, or pancake-shaped, and that clusters with 19 or 20 atoms are considered adults because they share the pyramidal shape of the bulk gold found in jewelry. The structures for the intermediate-, or adolescent-, sized gold clusters containing 15, 16, 17 or 18 atoms were unclear until Zeng's group launched their quantum chemistry search last year.
"We just wanted to fill in the missing information to determine when these structures start looking like adults or the bulk metal," he said. Scientists speculated a gradual evolution in the structures as gold clusters grew larger - from pancake to larger pancakes to pyramids, from 14 to 20 atoms.
"Nobody expected hollow cages in between," he said.
Scientifically, the discovery is exciting, Zeng said, but it also has practical potential. Since the cages are hollow and have room for an atom, they could be used to deliver useful materials. For example, they might ferry a drug in the human blood stream or serve as a diagnostic tool. Such small particles also might be used as catalysts in generating hydrogen fuel or speed other chemical processes. Zeng's team is studying the golden hollow cages' potential to carry nanomaterials as well as their prospects as catalysts.
Tracking down the structure of literally invisible clusters, which can't be seen even with the most powerful microscope today, requires sophisticated scientific techniques, massive computing power and close collaboration with other leading scientists.
Zeng's team was the first to combine quantum chemistry calculations with a powerful computerized search technique to identify previously unknown nanoscale structures and substances. Using UNL's PrairieFire supercomputer together with computers in the chemistry department, they applied their combined technique to generate many theoretical fingerprints of the gold clusters' structure.
Zeng also worked with physicist Lai-Sheng Wang of the Pacific Northwest National Laboratory and Washington State University who is one of the leading researchers studying gold at the nanoscale. Wang's team provided spectral data or fingerprints of the gold clusters, made by smashing gold with a laser beam. Clusters containing different numbers of atoms produce a unique spectral fingerprint.
By comparing spectral and theoretical fingerprints, UNL researchers identified the structures of the 15-, 16-, 17-, and 18-atom gold clusters. They used computer graphics to display those clusters, which closely matched both the spectral and theoretical fingerprints and revealed the cage-like structures.
"We were shocked when we first saw these cages," Zeng said. He and Satya Bulusu, his graduate research assistant and lead author of the PNAS paper, worked nearly around the clock for several days to confirm their findings.
"I'd call Satya at 3 a.m. and he'd be working in the lab. It was exciting," Zeng recalled. "We're fortunate at UNL to have resources like PrairieFire supercomputer that allow us to do this sort of leading edge research."
The PNAS paper is co-authored by Bulusu and Zeng from UNL's Nebraska Center for Materials and Nanoscience and Wang and Xi Li at Washington State University. Grants from the U.S. Department of Energy, the National Science Foundation's Materials Research Science and Engineering Center at UNL and the Nebraska Research Initiative fund this research.
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PNAS (online, May-2006)