The formation of a superlattice of metallic nanoclusters, consisting of 9 adatoms each, was reported for Tl on the (111)-(7 x 7) surface. It was proposed that mobile adatoms were trapped in attractive potential wells on the faulted half of the (7 x 7) unit cell. Adatom-adatom and adatom-substrate interactions combined, within these basins of attraction, to create the observed metallic bonding situation. On the basis of quasi-equilibrium thermodynamic considerations, the energy difference between adsorption on faulted and unfaulted unit-cell halves that made up the ordered superstructure array, was estimated to be 0.075eV.

Nanodot Formation on the Si(111)-(7 x 7) Surface by Adatom Trapping L.Vitali, M.G.Ramsey, F.P.Netzer: Physical Review Letters, 1999, 83[2], 316-9