AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM7
Vacancy-Mediated and Exchange Diffusion in the Pb/Cu(111) Surface Alloy

Tuesday, November 5, 2002, 10:20 am, Room C-110

Session: Diffusion & Growth on Metal Surfaces
Presenter: B.S. Swartzentruber, Sandia National Laboratories
Authors: B.S. Swartzentruber, Sandia National Laboratories
M.L. Anderson, Sandia National Laboratories
M.J. D'Amato, Sandia National Laboratories
P.J. Feibelman, Sandia National Laboratories
Correspondent: Click to Email
To understand the formation and stability of surface alloy systems, it is important to know the fundamental atomic-intermixing and mass-transport processes. Using STM measurements and first-principles DFT calculations we have explored them for the formation and evolution of the Pb/Cu(111) surface alloy. When Pb atoms are deposited on Cu(111), they place exchange with surface Cu atoms forming a 2-d surface alloy phase. At low coverage, thermodynamics favors a random distribution of Pb atoms in the surface layer. However, upon deposition, the Pb atoms take the easy path, forming metastable stripes of high Pb concentration embedded near steps. Over time, equilibrium is reached through the much slower decay of this initially non-uniform concentration profile. We measure the kinetics of mass transport, using STM, by following the motion of individual embedded Pb atoms and also the time evolution of the concentration profile. By analyzing the statistics of the local motion we find the most likely process to be one in which diffusion of the embedded Pb atoms occurs via exchange with thermal surface vacancies. There is also a minority process - occurring much less often - wherein an embedded Pb atom exchanges with a thermal Cu adatom and travels over long distance before re-embedding into the surface layer. Although this minority process rarely occurs, the long length scale over which it transports Pb atoms makes it a crucial ingredient of the overall mass transport process as measured in the decay of the concentration profile. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.