time, and could have it both ways. “Like
a hot-fudge sundae.”
Small numbers of atoms often form
structures as symmetrical, and almost
as intricate, as those of snowflakes. But
while no two snowflakes, even if they have
the same number of water molecules,
are identical, a small, specific number of
atoms of the same element typically will
assemble into the same, specific shape.
The quintessential example is how 60
carbon atoms form buckyballs. Metal
atoms such as gold, aluminum or tin also
like symmetry. For example, 20 atoms of
gold will assemble into a solid pyramid,
but 16 will form cage-like structures, as
Lai-Sheng Wang, a physical chemist at
Washington State University and at the
Pacific Northwest National Laboratory
in Richland, Wash., and his collaborators have discovered in recent years (SN:
5/20/06, p. 308).
The strange behavior of atoms in
small groupings has been known for a long
time, though only recently have scientists
begun to understand it in detail.
“The whole idea is that small is different,” says Bowen, quoting what he says is
a motto of Uzi Landman of the Georgia
Institute of Technology in Atlanta. The
physical properties of a material, such
as hardness and color, are the same for
a 1-pound lump of the stuff as they are
for a 100-ton chunk. But when you get
to specks made of a few million atoms or
less, properties usually begin to change.
A material such as silicon, which is
usually brittle, can become as ductile
as gold, researchers from the National
Institute of Standards and Technology
reported last November in Applied Physics Letters. Another example is particles
called quantum dots, which fluoresce in
a rainbow of different colors depending
on their size (SN 2/15/03, p. 107).
But with even fewer atoms— a few
hundred or less —the changes become
more dramatic. “If you keep going smaller,
then you enter a region where properties
are erratic,” Bowen says. “Often, one atom
counts.” For example, Wang and his collaborators have shown that tin clusters
behave like conductors or semiconductors, depending on their size; Bowen found
something similar with magnesium.
A job for superatom
For larger clusters, it’s not always clear
when atoms will aggregate into regular
structures or into shapeless blobs with
any number of atoms. “What’s to stop the
cluster from adding a few more atoms?”
asks Roger Kornberg, a structural biologist
at Stanford University’s Medical School.
Last fall in Science, Kornberg and
colleagues described an intricate cluster
they created with exactly 102 gold atoms.
He and his team synthesized their gold
superatoms in a liquid. To control the
clusters’ growth, the team added sulfur-based organic molecules called thiols,
which bind easily to gold. Forty-four thiols bound to each gold cluster’s surface,
preventing the 102-atom clusters from
coalescing to form larger clusters.
What resulted was a superatom (or
maybe a “supermolecule”) with a core of
79 gold atoms arranged into a truncated
under a scanning tunneling microscope, energetic electrons occupy high-energy orbits around
each of a chain of buckyballs (left and center). at even higher energies, the electrons “see”
each buckyball as a single atom, and can flow along the chain as they would in a wire (right).
decahedron: two pyramids with pentagonal bases joined together into a diamond shape, but with the pyramids’ tips
chopped off. Around the core, more gold
atoms formed an unusual pattern, joining
the thiols in shapes looking like handles.
“The thing that surprises me the most,”
says Kornberg’s collaborator Pablo Jadzinsky, “is that the geometry of the cluster
cannot be described in simple words.”
The team determined the cluster’s
structure using X-rays, which required first
coaxing the clusters into forming a crystalline solid. Jadzinsky says that the very fact
that the clusters could form a crystalline
solid means that they are all identical and
that their shapes are fixed. At 1. 5 nanometers, the clusters’ shapes may have fluctuated, as other nanometer-scale shapes
often do (SN: 3/15/03, p. 174).
But the numerology seems sort of
random: Why 102 gold atoms? Why 44
thiols? As it happens, superatom theory
has a good explanation.
Each of the gold atoms donates an
electron to the cluster, just as inside larger
chunks of metal, where mobile electrons
can conduct electricity. Forty-four of
those electrons get immobilized in bonds
between gold atoms and thiols, leaving
58 electrons free to roam. These 58 electrons then orbit the cluster’s core — made
of positive gold ions — just as they would
orbit the nucleus of a stand-alone atom.
And 58 happens to be a “magic number.”
It’s the number of electrons needed to fill
a shell around the superatom, so that it
won’t feel a desire to add or shed electrons,
which would destabilize its structure.
This process is similar to what happens
in noble gases, which are chemically inert
because they have just the right number of
electrons to fill a shell around the atom.
Chemist Royce Murray of the University of North Carolina at Chapel Hill and
his collaborators describe the structure
of a similar, though smaller, gold-thiol
cluster in the March 26 Journal of the
American Chemical Society.
Kornberg says that by tweaking the
conditions in their lab’s vials, he and his
colleagues can obtain clusters of different numbers of gold atoms and thiol molecules, although they haven’t determined
the precise structure in those cases yet.