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

Paper SS3-MoA2
Direct Measurement of Long Jumps of Pd on Cu(001) using Atom-Tracking STM

Monday, October 2, 2000, 2:20 pm, Room 210

Session: Surface Diffusion and Wetting
Presenter: B.S. Swartzentruber, Sandia National Laboratories
Authors: B.S. Swartzentruber, Sandia National Laboratories
M.L. Grant, Carnegie Mellon University
J.B. Hannon, Carnegie Mellon University
Correspondent: Click to Email
When Pd atoms are deposited onto a Cu(001) crystal surface, they readily place exchange with the surface Cu atoms, forming a 2-d surface alloy phase. At low coverage, Pd atoms are distributed randomly in the surface layer. At coverages approaching 0.5 ML, an ordered (2x2) alloy phase forms. The kinetic processes by which the incorporated Pd atoms form ordered structures are currently poorly understood. Using atom-tracking STM, a technique in which the probe tip is locked onto the diffusing species using lateral, XY, feedback, we have directly measured the diffusion of the Pd atoms as a function of temperature. The position of individual Pd atoms is recorded as a function of time with millisecond time resolution, thus recording the diffusion path of the incorporated atom. From these data, the relative binding energies of the sites visited by the atom, as well as the local diffusion barriers, are determined. We find that a large percentage (~25%) of the diffusion events involve displacements that are greater than a single lattice spacing - so-called long jumps. The distribution of jump lengths is well described by a process in which a Pd atom is thermally activated to a metastable binding-energy state, perhaps to a site on the surface, where it can perform several unit-length hops before it comes to rest in another stable binding site. Measuring the details of this atomistic process allows us to better understand the formation, growth, and mass transport kinetics of binary surface alloy structures. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US-DOE under contract DE-AC04- 94AL85000.