Nanodot formation on the Si(111)-(7 x 7) surface by adatom trapping

The formation of a superlattice of metallic nanoclusters consisting of nine adatoms each is reported for thallium on the Si(111)-(7 X 7) surface. It i s proposed that mobile adatoms are trapped in attractive potential wells on the faulted half of the (7 X 7) unit cell. Adatom-adatom and adatom-substr ate interactions combine within these basins of attraction to create the ob served metallic bonding situation. From quasiequilibrium thermodynamic cons iderations, the energy difference between adsorption on the faulted and unf aulted unit cell halves, that yields the ordered superstructure array, is e stimated to 0.075 eV.