Effects of concentration dependent diffusivity on the growth of precursingfilms of Pb on Cu(111)

The diffusion of Pb away from partially wetting Pb particles on the Cu(111) surface to form so-called precursing films has been studied by means of Au ger electron spectroscopy at temperatures in the range 353-373 K. These fil ms 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 vici nity of particles have been analyzed to extract the diffusion coefficient ( D) as a function of coverage at various temperatures. D exhibits high value s 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 st ructures known to be present on Cu(111) at various Pb coverages. In particu lar, the low value of D at intermediate coverage is associated with the inc orporation of Pb into the Cu surface in the form of a surface alloy. The ti me dependence for growth of the precursing film is very similar to the kine tics of precursing film growth in much more complex molecular systems. Our observations of a monolayer precursing film in quasi-equilibrium with a par tially wetting particle are consistent with models of the relationship betw een precursing films and contact angles. (C) 2001 Elsevier Science B.V. All rights reserved.