Nucleation on top of a 200x200 nm2 single Pb island shows different morphology (The left part shows few fractal islands and right part many small islands). The left part is 5-layers and the right part is many small islands. The figure to the right is a simulation that shows critical site clusters (ic = 4 on the 5-layer and ic = 1 on the 4-layer part). This shows that the nucleation can be controlled by height.
S. M. Binz, M. Hupalo, and M. C. Tringides, Phys. Rev. B 78, 193407 (2008). "Height-dependent nucleation and ideal layer by layer growth in Pb/Pb(111)/Si(111)"
Z. Kuntová, M. C. Tringides, S. M. Binz, M. Hupalo, and Z. Chvoj, Surf. Sci. 604, 519 (2010). "Controlling nucleation rates in nanostructures with electron confinement"