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The liquid state of solid gold particles.

The liquid state of solid gold particles

Tiny particles of gold, made up of only a few thousand atoms, sometimes behave more like liquids than solids. New experimental evidence suggests that although such particles have a particular, orderly atomic structure at any given moment, they can easily shift from one atomic arrangement to another. Under these conditions, the particle appears to be in a "quasimolten" state, even though the material is at a temperature far below its normal melting point.

"It's both a solid and a liquid," says materials scientist Laurence D. Marks of Northwestern University in Evanston, Ill. "If you look at it for minutes to hours, it's a liquid. But if you look at it on the [much shorter] time scale, say, of a chemical reaction at the surface, it's a solid." Marks and P.M. Ajayan report their findings in the July 17 PHYSICAL REVIEW LETTERS.

The researchers used an intense electron beam to dislodge "ultrafine" gold particles attached to a magnesium oxide surface. Once free, the particles started rapidly and randomly changing their atomic structure. The experiments showed that whereas a large amount of energy was needed to initiate this behavior, only a small amount was required to sustain it.

"We actually turned the beam off for 5 or 10 minutes, and when we turned the beam back on, it was still in the state," Marks says. That observation demonstrates that the particle's fluctuating behavior is a property of the particle itself rather than an electron-beam effect.

Marks and Ajayan also observed that tiny gold particles can sometimes induce the formation of a pillar of material beneath them. "Where the gold touches the magnesium oxide, it strains the material," Marks says. "As a consequence of that strain, some matter gets pulled out toward the particle."

Chemists often use fine particles as catalysts to speed up reactions. Usually, catalysis works by a lock-and-key mechanism in which the particle, having a certain surface structure, is the lock and the incoming molecule the key. "You really have to change how you think about a small particle," Marks says. "Your lock is actually changing in structure. It's varying its code all the time."
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Author:Peterson, I.
Publication:Science News
Date:Jul 29, 1989
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