Chip keeps atoms in line

By Kimberly Patch, 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 structures."

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 said.

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 digital information.

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," he said.

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 (ORNL).

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
Funding:   Government
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.


August 7/14, 2002

Page One

Ultimate memory demoed

Programming tool makes bugs sing

Nanotubes grown in place

Quantum secrets ride phone lines

Chip keeps atoms in line


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