Building a gilded cage

(UNL) scientists researching gold's structure at the nanoscale have discovered hollow cage-like structures made of pure gold atoms.

UNL chemistry professor Xiao Cheng Zeng and colleagues said 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 something like birdcages, can host an atom inside. This property gives them potential to one day 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."

"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 jewellery. 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.

As 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.

Zeng's team was the first to combine quantum chemistry calculations with a powerful computerised 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.