Direct observation of two dimensional magic clusters

Two dimensional magic clusters have been directly observed on the root 3 x root 3R30 degrees reconstructed Ga/Si(111) surface using scanning tunneling microscopy. The magic numbers are n(n + 1)/2, where n (2, 3, 4, or 5) is the number of atoms on each side of these triangular clusters with preferred orientation with respect to the substrate. The root 3 x root 3R30 degrees adatom lattice surrounding the magic clusters exhibits characteristic vacancies. A structural model is proposed to account for the cluster orientation and lattice vacancies as well as the extraordinary abundance and stability of the decamers (n = 4).