AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS3-MoA

Paper SS3-MoA3
Bronze Formation through the Motion of Tin Islands on Cu(111)

Monday, October 2, 2000, 2:40 pm, Room 210

Session: Surface Diffusion and Wetting
Presenter: A.K. Schmid, Sandia National Laboratories
Authors: A.K. Schmid, Sandia National Laboratories
N.C. Bartelt, Sandia National Laboratories
R.Q. Hwang, Sandia National Laboratories
Correspondent: Click to Email
When Sn atoms are deposited on top of a Cu (111) surface at room temperature, they exchange into the surface to form a surface bronze alloy. One might have expected that this process would be rather simple, the understanding of which would be largely complete when one understood how a single Sn atom exchanged with a single Cu atom. We find, however, that the alloying occurs by an entirely unanticipated cooperative process that is considerably more intriguing than this. Using a combination of atomic resolution scanning tunneling microscopy (STM) and low-energy electron microscopy (LEEM) we find that shortly after Sn deposition large, 100,000 atom 2-D Sn islands coalesce on the Cu surface. These islands proceed to run across the surface. As they move, Sn atoms within the islands randomly exchange with Cu atoms in the surface. The exchanged Cu atoms are ejected from the Sn islands in the form of ordered 2-D bronze crystals. Sn islands consistently move away from their trail towards unalloyed regions of the Cu surface. We are able to trace the motion of the Sn islands to a simple atomic fact: Sn atoms on top of the Cu surface are strongly repelled by Sn atoms already incorporated into the Cu. The islands thus lower the free energy of the surface by moving. We find that island velocity is independent of size, consistent with a model in which the mobility of the Sn islands is determined by diffusion through the interior of the islands.