Wash. – 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.
evidence for what their discoverers call “hollow
golden cages” appeared today in the online early
edition of the Proceedings of the National Academy
of Sciences .
fullerene is made up of a sphere of 60 carbon (C)
atoms; gold (Au) requires many fewer—16, 17 and
18 atoms, in triangular configurations more gem-like
than soccer ball. At more than 6 angstroms across,
or roughly a ten-millionth the size of this comma,
they are nonetheless roomy enough to cage a smaller
“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.
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.
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.”
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
PNNL is a DOE
Office of Science laboratory that solves complex
problems in energy, national security, the environment
and life sciences by advancing the understanding
of physics, chemistry, biology and computation. PNNL
employs 4,200 staff, has an annual budget of more
than $725 million, and has been managed by Ohio-based
Battelle since the lab's inception in 1965.