keeps atoms in line
Technology Research News
As electronic devices become ever smaller,
it is increasingly important to get microscopic amounts of material to
line up, en mass, in the right places.
An international team of scientists has found a way to coax arrays of
evenly-distributed clusters of metal atoms to form automatically on the
surface of a silicon wafer.
The work is a step toward being able to build devices atom-by atom, and
could eventually contribute to technologies that form more closely-packed
information storage materials It could also lead to fabrication processes
that combine electronics like those used in today's computers with optics
like those used in fiber communications systems.
The researchers took advantage of a law of physics that says that under
certain conditions atoms will cluster into particularly stable groups
made up of a specific number of atoms.
Groups of atoms can bond together into different types of structures,
and different patterns are more or less stable depending on the energy
levels of the atoms. The lowest energy level, or preferred bonding pattern,
for a silicon atom is for it to have four nearest neighbors, said Shengbai
Zhang, a senior scientist at the National Renewable Energy Laboratory.
"When every atom in the cluster [has this] bonding chemistry, the cluster
is the most stable," he said.
The researchers found that they could cause atoms of metals, including
indium, manganese and silver, to form these stable groups on the surface
of a silicon wafer. "We optimized the growth conditions and found a small
window at which the clusters start to [become ordered] throughout the
wafer," said Zhang.
The key to the process is closely controlling the temperature, and the
rate that the atoms are deposited onto the silicon so that the atoms have
enough time to hop around and find the low energy positions that result
in the stable groups, Zhang said. "If the rate is too high or temperature
is too low, the atoms will form large clumps. If the temperature is too
high [they will not form] any metal clusters."
The lattice structure of the silicon crystal surface is also important,
he said. "Without the use of the [natural silicon] template that greatly
enhances the stability and size-selectivity of the clusters already formed...
nothing may have worked."
Once the researchers worked out the process, it was "actually very simple"
and could be carried out without much technical difficulty, Zhang said.
The approach can be applied to many different metals and even to metal
alloys, he added.
Different types of metals could form different patterns, said Zhang. The
exact patterns are determined by the interactions between the metal atoms
and the silicon substrate, and the energy within each cell in the silicon
crystal lattice where the clusters form, he said.
The work is a nice extension of a large body of work on the growth of
organized nanostructures on semiconductor surfaces, said Jim Hutchinson,
an associate professor of chemistry and materials science at the University
of Oregon. The researchers have "optimized conditions for the preparation
of new arrays, and [provided] mechanistic insight into the formation of
The work is an "interesting, careful, unique experimental condition,"
said Gabor Somorjai, a chemistry professor at the University of California
at Berkeley. Growing ordered arrays like this is very tricky and subject
to local conditions, however, he said. It may be very difficult to find
the right combination of metals and surfaces to make the clusters consistently,
he said. "It is all possible, but is hard work for many years to come."
These types of nanostructures may eventually be used as chemical catalysts,
high-density data storage media and to make microscopic electronics devices,
said Hutchinson. First, however, the method would have to be extended
to make nanoclusters that are either catalytically active or magnetic,
and in the case of electronic applications, electrically isolate the clusters
from one another and find ways to individually address the clusters, he
The researchers are currently using the nanostructured arrays as templates
to make arrays of bigger dots ten nanometers or larger in diameter. They
are also working on making magnetic arrays that could be used for ultra
high-density information storage. Such arrays could also be used in the
emerging field of spintronics, according to Zhang. Spintronics uses the
spin of electrons rather their charge to represent the ones of zeros of
The research could be applied practically in five to ten years, according
to Zhang. The work is "still basic research. It will take time and
[the] efforts of many to eventually [apply the method] for practical purposes,"
Zhang's research colleagues were Jian-Long Li, Jin-Feng Jia, Xue-Jin Liang,
Xi Liu, Jun-Zhong Wang, and Qikun Xue of the Physics Institute in Beijing,
China, Zhi-Qiang Li and John S. Tse of the Steacie Institute for Molecular
Sciences in Ottawa, Canada, and Zhenyu Zhang of Oak Ridge National Laboratory
They published the research in the February 11, 2002 issue of Physical
Review Letters. The research was funded by International Center for Quantum
and Structures (ICQS) of the Chinese Academy of Sciences (CAS), the Department
of Energy (DOE), the National Science Foundation (NSF) and the National
Research Council of Canada.
Timeline: 5-10 years
TRN Categories: Chemistry; Physics; Materials Science and
Engineering; Nanotechnology; Data Storage Technology
Story Type: News
Related Elements: Technical paper, "Spontaneous Assembly
of Perfectly Ordered Identical-Size Nanocluster Arrays," Physical Review
Letters, February 11, 2002.
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