http://www.science.psu.edu/alert/Castleman1-2005.htm
Clusters of Aluminum Atoms Found to Have Properties of
Other Elements Reveal a New Form of Chemistry
13 January 2005—A research team has discovered
clusters of aluminum atoms that have chemical
properties similar to single atoms of metallic and
nonmetallic elements when they react with iodine. The
discovery opens the door to using 'superatom
chemistry' based on a new periodic table of cluster
elements to create unique compounds with distinctive
properties never seen before. The results of the
research, headed jointly by Shiv N. Khanna, professor
of physics at Virginia Commonwealth University and A.
Welford Castleman Jr., the Evan Pugh Professor of
Chemistry and Physics and the Eberly Family
Distinguished Chair in Science at Penn State
University, will be reported in the 14 January 2005
issue of the journal Science.
"Depending on the number of aluminum atoms in the
cluster, we have demonstrated 'superatoms' exhibiting
the properties of either halogens or alkaline earth
metals," says Castleman. "This result suggests the
intriguing potential of this chemistry in nanoscale
synthesis." The discovery could have practical
applications in the fields of medicine, food
production and photography.
The researchers examined the chemical properties,
electronic structure, and geometry of aluminum
clusters both theoretically and experimentally in
chemical compounds with iodine atoms. They found that
a cluster of 13 aluminum atoms behaves like a single
iodine atom, while a cluster of 14 aluminum atoms
behaves like an alkaline earth atom. "The discovery of
these new iodine compounds, which include aluminum
clusters, is critical because it reveals a new form of
'superatom' chemistry," said Khanna. "In the future,
we may apply this chemistry, building on our previous
knowledge, to create new materials for energy
applications and even medical devices."
To make their discovery, the research team replaced
iodine atoms with the aluminum clusters in naturally
occurring chains or networks of iodine atoms and
molecules known as polyiodides. When the researchers
substituted the iodine atom with the aluminum cluster,
Al13, they observed that the entire chemistry of the
compound changed--causing the other iodine molecules
to break apart and bind individually to the cluster.
The researchers then were able to bind 12 iodine atoms
to a single Al13 cluster, forming a completely new
class of polyiodides. "Our production of such a
species is a stirring development that may lead to new
compounds with a completely new class of chemistry and
applications," says Castleman. "Along with the
discovery that Al14 clusters appear to behave
similarly to alkaline earth atoms when combined with
iodine, these new results give further evidence that
we are really on our way to the development of a
periodic table of the 'cluster elements'."
The researchers conducted experimental reactivity
studies that indicate that certain aluminum-cluster
superatoms are highly stable by nature. The team's
related theoretical investigations reveal that the
enhanced stability of these superatoms is associated
with a balance in their atomic and electronic states.
While the clusters resemble atoms of other elements in
their interactions, their chemistry is unique,
creating stable compounds with bonds that are not
identical to those of single atoms.
Using stable clusters provides a possible route to an
adaptive chemistry that introduces the
aluminum-cluster species into nanoscale materials,
tailoring them to create desirable properties. "The
flexibility of an Al13 cluster to act as an iodine
atom shows that superatoms can have synthetic utility,
providing an unexplored 'third dimension' to the
traditional periodic table of elements," said Khanna.
"Applications using Al13 clusters instead of iodine in
polymers may lead to the development of improved
conducting materials. Assembling Al13I units may
provide aluminum materials that will not oxidize, and
may help overcome a major problem in fuels that burn
aluminum particles."
The theoretical investigations for this project were
conducted by Khanna with N.O. Jones, a graduate
student in the physics department at Virginia
Commonwealth University, and the experimental work was
conducted by Castleman with Denis Bergeron and Patrick
J. Roach, graduate students in the chemistry
department at Penn State.
This research was supported by the U. S. Air Force
Office of Scientific Research and the U. S. Department
of Energy.
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