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Research Reveals Halogen
Characteristics of Cluster of Metal Atoms
A stable cluster of aluminum atoms, Al13, acts as a single entity
in chemical reactions, demonstrating properties similar to those
of a halogen, reports a research team led by A.
Welford Castleman Jr., Evan Pugh Professor of Chemistry and Physics and the Eberly
Family Distinguished Chair in Science at Penn
State, in a paper
published in the journal Science. Experimental results and theoretical
calculations indicate that the cluster chemically resembles a “superhalogen” atom,
retaining its properties during the reaction and in reaction products.
Other team members include Denis E. Bergeron of the Penn State
departments of chemistry and physics and Shiv N. Khanna of the
Virginia Commonwealth University department of Physics. One implication
of the research is the possibility of using such clusters as building
blocks in nanoscale fabrication.
The project focused on experimental
evidence of the existence of a very stable cluster anion, Al13I-,
prepared by the gas-phase reaction of aluminum clusters with HI
gas. Mass-spectrometric analysis indicated that the reaction produced
relatively few products, the most abundant corresponding to Al13I-.
Energy calculations to determine the bonding mechanism between
the aluminum cluster and the iodine atom indicate that the extra
electron is localized on the Al13 cluster, meaning that the cluster
maintains its integrity throughout the reaction. Because the cluster
has a greater electron affinity in the compound, or attraction
to the free electron, than does iodine, it can be considered a “superhalogen.”
“One
of the themes of our research is using the clusters as building
blocks for new nanoscale materials,” says Castleman. “In
many cases, people have worked from the top down; that is, subdividing
matter to get it smaller and smaller. We’re trying to work
with atoms and molecules and put them together—working our
way from the bottom up. If we can retain the properties of aggregates,
as we put them together, perhaps we will be able to construct new
nanoscale materials.” The key to using the aggregates as building
blocks is that they retain their individual properties during the
reaction and do not coalesce into a large aggregate.
One goal of
the research is to test the Jellium model of stable clusters, which
treats metal atoms in a small system as positive cores surrounded
by the valence electrons. The model predicts certain closed-shell
arrangements with high stability, called magic clusters. In the
Jellium model, the cluster’s atomic nuclei and inner
electrons are seen as a single, spherical, positively charged
core, surrounded by valence electrons in electronic shells similar
to those of atoms. Essentially, the magic clusters can be viewed
as superatoms, capable of forming compounds.
“When we started
looking at reactions, Al13 turned out to be very interesting for
several reasons,” says Castleman. “It
behaves very much like a halogen, somewhere between iodine and
bromine in the way it wants to bind an electron. If we could put
an iodine atom in contact with Al133 , the Al13 has
a little higher electron affinity than iodine, which could allow
the Al13 to retain the electron, thereby bonding the Al133 and I together.”
Experimental
observations indicated that the stability of the Al13I- ion is
comparable to that of BrI-, a well-known and very stable molecular
halogen ion. The ability of a cluster of aluminum atoms to behave
like a halogen opens up the prospect that Al13 and other magic
clusters can retain their properties as building blocks for assembling
new materials.
“This superhalogen is not disrupted even in
the presence of the very reactive iodine atom in close proximity,
but still keeps its properties,” says Castleman. “Now
that we have shown that this is possible, we see potential ways
to make other clusters,
maybe involving other metals or alloys. It should be possible to
construct something in the Jellium framework that would have the
properties not only of a halogen, but of other types of atoms as
well. For example, the Al13- ion itself resembles a
rare gas atom because it is so unreactive. Ideally, we could have
a whole series of clusters—a ‘three dimensional’ periodic table,
not of elements but rather of clusters simulating the properties
of the elements.” The goal is to use these clusters as building
blocks to tailor the design and formation of nanoscale materials
with selected properties.
This research was supported by the U.S.
Air Force Office of Scientific Research and the U.S.
Department of Energy.
Steve Miller
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