ALBUQUERQUE, N.M. – Coffee beans spilled upon a table form no pattern, they’re a
mess, their distribution dictated by the laws of chance. The same was generally
believed true of atoms deposited upon a substrate. Now, the first vision of a
peaceable kingdom in which deposited atoms form orderly, controllable 2-D
nanopatterns has been observed by researchers at the Department of Energy’s
Sandia National Laboratories.

Pattern control at this level means that nanotemplates could be formed to fine-tune
the device characteristics of self-assembling nanostructures. Possibly, characteristics
could be tailored for devices like photonic lattices, an advanced method for controlling
light and of wide interest to the huge telecommunications industry.

The work, described in the Aug. 30 Nature, produced real-time video of atoms
self-arranging themselves in the manner long predicted by a variety of theorists but
contrary to ordinary intuition. Thus, such theories generally had been treated with a
great deal of skepticism, says Sandia physicist Norm Bartelt: “There was no obvious
route for atoms to arrange themselves in predicted patterns.”

Says Sandia researcher Richard Plass, “Kinetics say that 10,000 moving atoms
should go anywhere. Nobody really expected an assembly would arise.”

Observation of the real-time assembly process, along with control over physical
factors that influence that process, offer a means of finding out far more about the
conditions under which atoms self-assemble than any theory could predict, and thus,
how to influence that assembly into more desirable structures.

“There are many control knobs we can turn to create new patterns,” says Bartelt.
Among them are temperature and material composition.

The researchers observed atoms of lead deposited on a copper substrate forming,
first, lead dots, then lead stripes, and then reverse dots – copper becoming the dot
material – as more lead is added.

“The work – which to our knowledge is the first unambiguous observation of the
expected sequence of domain patterns – helps understand the new physics that
manifests itself at these small length scales,” says Sandia project lead Gary Kellogg.
“New materials with highly specialized properties necessary to meet defense and
consumer needs can be fabricated only by tailoring the structure of the material on the
nanometer scale. This work provides insight into how nature does this, and how
humans can do the same.”

Sandia researchers were able to record real-time, real-space images using a
low-energy electron microscope (LEEM) that show exactly how the nanostructures are
generated, self-assemble, and transform. “The close agreement between experiment
and theory allows us to probe the key inter-atomic force parameters involved in the
process,” says Kellogg.

Theorists long have believed that competing attractive and repulsive inter-atomic
interactions can lead to the spontaneous formation of ordered patterns in widely
varying chemical and physical systems. Potentially, such patterns could be used as
templates for nanostructure fabrications.

“There are precedents for people using these patterns for further growth of quantum
dots,” says Bartelt. “They can be the starting point of controllable patterns that extend
into three dimensions.”

Though models have clearly predicted the possibility of controlling any pattern’s
geometry and order, depending on temperature and amount of secondary metal
introduced, experimental verification of these models had remained elusive till now.

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Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed
Martin company, for the U.S. Department of Energy. With main facilities in
Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in
national security, energy and environmental technologies, and economic
competitiveness.

Sandia Media Relations Contact: Neal Singer, 505-845-7078, nsinger@sandia.gov
Sandia Technical Contact: Norm Bartelt, 925-294-3061, bartelt@sandia.gov

Contact: Neal Singer

nsinger@sandia.gov

505-845-7078