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.
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The
two images above show the energy levels (vertical scale)
and spatial positions (white lines) of electrons within
a three-atom chain. The top image shows the calculated
or theoretical results; the bottom image shows the measured
energy levels in a physical experiment. Electrons are
most likely to be located in the red areas and least
likely in the blue areas. Both images indicate that
the electrons in the outermost atoms (positioned on
the far left and right at the bottom on the vertical
scales) have lower energy than those within the center
atom.
Click
here to download a higher resolution version of this
image. |
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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