Clusters Of Aluminum Atoms Found To Have Properties Of Other
Elements Reveal A New Form Of Chemistry
January 13, 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.