AVS 49th International Symposium
    Surface Science Wednesday Sessions
       Session SS-WeP

Paper SS-WeP22
The Importance of Pb-Vacancy Attraction on Diffusion in the Pb/Cu(001) Surface Alloy

Wednesday, November 6, 2002, 11:00 am, Room Exhibit Hall B2

Session: Surface Science Poster Session
Presenter: M.L. Anderson, Sandia National Laboratories
Authors: M.L. Anderson, Sandia National Laboratories
B.S. Swartzentruber, Sandia National Laboratories
Correspondent: Click to Email
Vacancy-mediated diffusion has recently been shown to be a dominant mass transport mechanism on the Cu(001) surface.@footnote 1@ Not only are vacancies responsible for self-diffusion of surface-layer Cu atoms, but also for diffusion of impurity atoms such as Pd, In, and Pb, embedded in the surface layer. We use atom-tracking STM to measure the diffusion of embedded Pb atoms directly as a function of temperature to extract the diffusion energies. We find a Pb-vacancy attraction that dramatically affects the diffusion statistics. A diffusion event consists of a "burst" of a series of rapid exchanges between an embedded Pb atom and a surface vacancy. The population density of surface vacancies and their migration energy in the Cu surface layer govern the burst-to-burst time interval statistics. Because a single diffusion event entails a number of Pb-vacancy exchanges, the net displacement of Pb is often much more than a single lattice constant. The details of this displacement distribution are measurably affected by the Pb-vacancy interaction. The long length scale of the displacement distribution due to Pb-vacancy attraction is in stark contrast to Pd, which has a repulsive interaction. The energetics extracted from the measurements in comparison with first-principles calculations lead to a better understanding of the driving forces responsible for surface alloy formation and kinetics. 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. @FootnoteText@ @footnote 1@ J. B. Hannon, et al., Phys. Rev. Lett. 79, 2506 (1997); R. van Gastel, et al., Phys. Rev. Lett. 86, 1562 (2001); M. L. Grant, et al., Phys. Rev. Lett. 86, 4588 (2001).