Effects of concentration dependent diffusivity on the growth of precursing films of Pb on Cu(1 1 1)

The diffusion of Pb away from partially wetting Pb particles on the Cu(1 1 1) surface to form so-called precursing films has been studied by means of Auger electron spectroscopy at temperatures in the range 353–373 K. These films achieve a maximum thickness of about 0.85 Pb monolayers (0.85 ML) at the particle triple line. The surface concentration profiles of Pb in the vicinity of particles have been analyzed to extract the diffusion coefficient (D) as a function of coverage at various temperatures. D exhibits high values both at low (<0.5 ML) and high (>0.5 ML) Pb coverages, and a minimum near 0.5 ML. This dependence of D on coverage is consistent with the surface structures known to be present on Cu(1 1 1) at various Pb coverages. In particular, the low value of D at intermediate coverage is associated with the incorporation of Pb into the Cu surface in the form of a surface alloy. The time dependence for growth of the precursing film is very similar to the kinetics of precursing film growth in much more complex molecular systems. Our observations of a monolayer precursing film in quasi-equilibrium with a partially wetting particle are consistent with models of the relationship between precursing films and contact angles.