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Au16, the world's smallest hollow gold cage.
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"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. The experiments
were buttressed and the clusters' geometry deciphered from theoretical
calculations led by Professor Xiao Cheng Zeng of the University of
Nebraska and co-corresponding author.
Wang, who worked in the Richard Smalley lab that
gave the world buckyballs, is part of a large cluster of researchers
who have spent much of the past decade attempting to find the
fullerene's kin in metal. But their search has proved difficult
because of metal clusters' tendency to compact or flatten.
Experiments at the PNNL-based W. R. Wiley
Environmental Molecular Sciences Laboratory elicited the photoelectron
spectra of clusters smaller than Au32, which had been theorized as the
gold-cage analog to C60 but ruled out by Wang's group in an experiment
that showed it as being a compact clump.
They instead turned their attention to clusters
smaller than 20 atoms, which earlier work by Wang's group showed were
3-D - a golden pyramid, no less - but larger than 13 atoms, known to
be flat. The spectra and calculations showed that clusters of 15 atoms
or fewer remained flat but that all but one possible configuration of
16, 17 and 18 atoms open in the middle. At 19 atoms, the spaces fill
in again to form a near-pyramid.
"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 Au20 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. |
