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Gold on silver: laying down a mixed result.

Layering gold on silver -- a process as ancient as metallurgy itself -- can still conjure up surprises. By carefully observing the deposition of gold atoms on the surface of a silver crystal, researchers have now uncovered a new, unusual kind of growth that leads to the formation of a thin gold film.

These experiments show that instead of forming a blanket atop a silver bed, gold atoms initially prefer snuggling down under the topmost sheet of silver atoms to form an "underlayer." Only after this submerged layer is nearly complete does growth of a gold film occur on top of the upper silver layer.

"We were all surprised by this counter-intuitive and unexpected result," says Shirley Chiang of the IBM Almaden Research Center in San Jose, Calif. Chiang and her colleagues report their findings in the Nov. 30 Physical Review Letters.

The IBM effort was inspired by the puzzling results of an experiment performed two years ago by researchers at Rutgers University in Piscataway, N.J. Their measurements of ions reflected from a gold-coated silver surface suggested that either the gold layer had to be two atoms thick, even when most of the silver surface was still bare, or the gold atoms had somehow mixed with the surface silver atoms.

By combining ion-scattering data with scanning tunneling microscope images of silver surfaces coated with varying amounts of gold, the IBM scientists concluded that gold atoms initially bury themselves beneath a layer of silver atoms. They saw no two-atom-thick gold steps.

These experimental results also confirm theoretical predictions made independently by physicist Che-Ting Chan and co-workers at Ames Laboratory of Iowa State University in Ames. The group's calculations, reported in the Sept. 14 Physical Review Letters, had suggested that the formation of a gold underlayer would lower a silver surface's overall energy.

Because of the high mobility of surface silver atoms at room temperature, says IBM's David E. Fowler, "it seems plausible that the gold might end up underneath."

The IBM researchers also discovered that in later stages of deposition, gold atoms form long, flat, finger-shaped plateaus all pointing in the same direction on the silver surface. These two-dimensional formations, in turn, provide the foundations for three-dimensional gold "islands," which gradually enlarge and coalesce into a solid film.

"That's a behavior that's been talked about but not observed before," Fowler says.
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Title Annotation:IBM research shows that gold laid on silver submerges itself into silver atoms, then accumulates atop surface
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Dec 12, 1992
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