14 DAY TRIAL // Scientists have uncovered a class of gold atom clusters that are the first known metallic hollow equivalents of the famous hollow carbon fullerenes known as buckyballs.
The fullerenes are structures that look like a soccer ball made of 60 carbon atoms. Hollow gold cages require fewer atoms, 16, 17 and 18, organized in triangular gem-like configurations. Although they are no more than 6 angstroms across, they nonetheless have room enough to cage another smaller atom.
"This is the first time that a hollow cage made of metal has been experimentally proved," said Lai-Sheng Wang, the paper's lead corresponding author. Wang is an affiliate senior chief scientist at the Department of Energy's Pacific Northwest National Laboratory and professor of physics at Washington State University. He also worked in the Richard Smalley lab that created the first buckyballs, and has spent much of the past decade attempting to find the fullerene's kin in metal.
The main difficulty faced by scientists was that metal clusters tend to compact or flatten. The method used to create such structures is not to actually build them atom by atom, but to create certain macroscopic conditions under which the atoms self-organize in such structures. The difficulty is to find these macroscopic conditions.
At first, scientists aimed at producing larger cages. Some theorized that a cage of 32 gold atoms was possible. But Wang showed that was not the case. They turned their attention instead to clusters smaller than 20 atoms, that were hoped to be 3D - a golden pyramid, no less - but larger than 13 atoms, because structures of less than 13 atoms were known to be flat.
"Au-16 is beautiful and can be viewed as the smallest golden cage," Wang said. He pictures it as having "removed the four corner atoms from our Au-20 pyramid and then letting the remaining atoms relax a little," and thus opening up space in its center.
It and its larger neighbors are stable at room temperature and are known as "free-standing" cages - unattached to a surface or any other body, in a vacuum. "When deposited on a surface, the cluster may interact with the surface and the structure may change."
Wang and his co-workers suspect "that many different kinds of atoms can be trapped inside" these hollow clusters, a process called "doping." "These doped cages may very well survive on surfaces," suggesting a method for influencing physical and chemical properties at smaller-than-nano scales, "depending on the dopants."
Wang's group has not yet attempted to imprison a foreign atom in the hollow Au cages, but they plan to try.
Image credit: Pacific Northwest National Laboratory