Surface diffusion of Pb on Cu(110) at low coverage: competition between exchange and jump

We have studied the surface diffusion of Pb on Cu(110) at low coverage by Rutherford backscattering spectrometry (RBS) in the 500–800 K temperature range. The spread, after annealing, of a deposit achieved at room temperature is measured along the and [001] directions, providing the corresponding diffusion coefficients. The activation energy is found to be close to 0.6 eV along both directions: the ratio of the two diffusion coefficients is nearly constant and equal to 2.4 in the temperature range studied. These results are interpreted in the frame of a model which takes into account the fact that, at low coverages, Pb atoms form a disordered surface alloy above room temperature. Thus, for a Pb atom, an elementary diffusion event is a multi-process consisting successively in a de-insertion (exchange with a Cu adatom), some jumps along the ‘open' direction, and a reinsertion in the surface plane (exchange with a Cu surface plane atom). The model predicts that, at low temperature, the diffusion becomes much more anisotropic as Pb adatoms undergo a great number of jumps before their insertion. This behaviour also accounts for the formation of two-dimensional Pb clusters which have been observed on various Cu surfaces by scanning tunneling microscopy experiments on low temperature deposits