Alloying at Surfaces by the Migration of Reactive Two-Dimensional Islands

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Science  24 Nov 2000:
Vol. 290, Issue 5496, pp. 1561-1564
DOI: 10.1126/science.290.5496.1561

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We have studied the formation kinetics of the copper-tin alloy bronze when tin is deposited on the (111) surface of copper at room temperature. Low-energy electron microscopy and atomic-resolution scanning tunneling microscopy reveal that bronze forms on the surface by a complicated, unanticipated cooperative mechanism: Ordered two-dimensional tin islands containing several hundred thousand atoms spontaneously sweep across the surface, leaving bronze alloys in their tracks. We propose that this process, driven by surface free energy, is a version of the “camphor dance” observed on liquid surfaces, and should be a general mechanism of surface alloying when surface diffusion is faster than exchange into the substrate.

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