The diffusion of Pb, away from partially-wetting Pb particles on the Cu(111) surface, to form so-called precursor films was studied by means of Auger electron spectroscopy at 353 to 373K. The films achieved a maximum thickness of about 0.85 of a Pb monolayer at the particle triple line. The surface concentration profiles of Pb in the vicinity of particles were analyzed in order to deduce the diffusion coefficient as a function of coverage at various temperatures. The diffusion coefficient exhibited high values at low (< 0.5ML) and high (> 0.5ML) Pb coverages; with a minimum near to 0.5ML. This dependence of the diffusion coefficient upon coverage was consistent with the surface structures which were known to be present on Cu(111) at various Pb coverages. In particular, the low value of the diffusion coefficient at intermediate coverages was associated with the incorporation of Pb, into the Cu surface, in the form of a surface alloy. The time-dependence for growth of the precursor film was very similar to the kinetics of precursor film growth in much more complex molecular systems. The observations of a monolayer precursor film, in quasi-equilibrium with a partially-wetting particle, were consistent with models of the relationship between precursor films and contact angles.

Effects of Concentration Dependent Diffusivity on the Growth of Precursing Films of Pb on Cu (111). J.Moon, J.Lowekamp, P.Wynblatt, S.Garoff, R.M.Suter: Surface Science, 2001, 488[1-2], 73-82