| In
a development that could one day score a touchdown for
better health, chemists Atoms at the ends of self-assembled
atomic chains act like anchors with lower energy levels
than the "links" in the chain, according to
new measurements by physicists at the National Institute
of Standards and Technology (NIST).
The
first-ever proof of the formation of "end states"
in atomic chains may help scientists design nanostructures,
such as electrical wires made "from the atoms
up," with desired electrical properties.
The NIST experiments, described
in the Feb. 4 issue of the journal Science,* involved
measuring and comparing the electronic properties
of gold atoms in short chains assembled on silicon
surfaces. Energy levels of the electrons within the
end atoms of the chains were lower than those of inner
atoms. This condition arises because the structural,
chemical and electronic symmetry of a chain is broken
at each end, and the atoms' electrons are redistributed
to lower the chain's energy. The electronic structure
of atomic chains is comparable to the electronic structure
of bulk crystals, in which surface atoms have different
properties than atoms inside the crystal.
"In the past three decades
the study of surface states on crystals has been a
major endeavor by research groups from all over the
world," says Jason Crain, lead author of the
Science paper. "Our study is the first to show
the formation of localized states at the ends of single
atom chains. The existence of end states will have
implications for future studies of one-dimensional
nanostructures."
The NIST measurements were
made with a scanning tunneling microscope (STM) and
were enabled, in part, by the self-assembly of the
gold chains on a silicon surface. Unlike the metal
surfaces used in previous STM studies of single-atom
chains, the silicon surface behaved as an insulator,
allowing scientists to better isolate the chains and
improve measurements of their atoms' electron energy
levels.
The STM, which has a needle-like
tip that can apply various levels of voltage, was
used to make two types of measurements of numerous
chains composed of three to nine atoms. First, by
maintaining a constant current between the tip and
the gold-on-silicon surface, the STM produced a three-dimensional
image of the surface topography. As the tip scanned
across the sample, it rose and fell with changes in
surface features to maintain a stable current flow.
Then, by holding the STM tip at a constant distance
from the surface, the scientists measured changes
in current as a function of tip voltage. Measures
of conductivity were used to determine the energies
and spatial distribution of electrons in the chains,
which showed differences between the inner and end
atoms.
The project was funded by NIST and the Office of Naval
Research.
As a non-regulatory agency
of the U.S. Department of Commerce's Technology Administration,
NIST develops and promotes measurement, standards
and technology to enhance productivity, facilitate
trade and improve the quality of life.
*J.N. Crain and D.T. Pierce,
"End States in One-Dimensional Atom Chains,"
Science, Feb. 4, 2005.
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