Cluster critical size effect during growth on a heterogeneous surface

Nucleation and growth on a complex surface containing a regular network of defects is investigated in the case of Co on Au(111). By means of variable temperature ultrahigh vacuum scanning tunneling microscopy it is shown that both monolayer and bilayer Co clusters can be found on the same surface reconstruction, depending on the nature of the particular nucleation site: two types of point dislocations located at the elbows of the herringbone reconstruction. The existence of a critical size for the structural transition from monolayer to the bilayer islands is inferred on the basis of experimental results, quenched molecular dynamics calculations, and Metropolis Monte Carlo simulations. The resulting phase diagram provides a detailed characterization of the different growth modes (monolayer and bilayer islands) as a function of temperature and coverage.