The importance of critical size cluster is seen also on ring(5 layer)+vacancy(4-layer)+center island(5-layer). Higher island density on unstable (4-layer) vs stable (5-layer) islands.
S. M. Binz, M. Hupalo, and M. C. Tringides, J. Appl. Phys. 105, 094307 (2009). "Quantum size effect dependent critical size cluster and finite size effects"