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Stormy weather at crystal surfaces.

Atoms on the surfaces of microscopic gold crystals appear to be in constant motion. Not only do they hop from site to site, but they also continually shuttle between a crystal's orderly columns and clouds of atoms hovering near certain surfaces. These recent observations result from the combination of a high-resolution electron microscope and a video-recording system that magnifies gold crystals about 20 million times and, on an atomic scale, tracks their growth as it happens.

"The motion of atomic columns and the existence of atom clouds revealed here may have important consequences for crystal growth, surface science and catalysis studies," say David J. Smith of arizona State University in Tempe and his colleagues at the University of Lund in sweden. Their report appears in the Sept. 5 NATURE.

The researchers use a powerfule electron beam to bombard 55-atom clusters of gold scattered across a carbon film. These tiny crystals turn out to be unstable, and some crystals begin to grow at the expense of others. A TV monitor allows the scientists to watch the rapid changes in crystal shape and orientation.

It's like watching living atoms, says Smith. "You can sit and look at one small particle for 10 minutes," he says. "You may get 29 different shapes in 30 seconds, and then it will sit still for a while, and then it goes on. We also see different effects according to how big the particles are." In addition, the ever-changing cloud shapes seem to show the pathways that atoms follow out of or into the lattice columns. In some instances, parts of a cloud look like miniature tornadoes directed toward particular crystal columns.

"It may well be that column hopping and changes of cloud shape are an indication of how atoms locate the most favorable lattice position during crystal growth," the researchers say. Using the same equipment, it should also be possible to monitor the way in which a variety of atoms interact with a metal surface. This an important question in the study of how catalysts work.

Once concern about the research is that the motion observed may be due to the effect of the electron beam rather than a characteristic of crystal behavior. "We're on a fact-finding mission," says Smith. "How general is the phenomenon that we have observed?" So far, the researchers have seen similar although not identical behavior at platinum crystal surfaces.
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Title Annotation:crystal growth
Publication:Science News
Date:Sep 28, 1985
Words:398
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