Configuration dependent critical nuclei in the self assembly of magic clusters.

Evidence for the formation of various 2-D structures possessing different numbers of Co-Si magic clusters (size approximately 10.0 +/- 0.5 A), configurations and lifetimes are studied in real time on a Si(111)-(7 x 7) surface at elevated temperature in the STM. Observations of individual cluster diffusion, attachment and detachment dynamics resolve unequivocally the question of self assembly over surface reconstruction. The smallest stable structure consisting of seven individual Co-Si magic clusters arranged in a hexagonal closed packed formation (i = 7) is found to retain sufficient cohesive energy to avoid dissociation. A configuration dependent critical 2-D nuclei (i* = 6) is determined to exist in facilitating the self assembly dynamics.