The very early stages of Pb deposition on Si(111)-(7 x 7) surfaces were investigated by means of scanning tunnelling microscopy. The combination of variable-temperature scanning with unusually long periods of imaging time permitted the observation that single Pb atoms were highly mobile within each half-(7 x 7) unit cell. Individual jumps of single atoms between different half-cells had to be resolved, as well as the formation of atom pairs. An activation energy of 0.64eV (figure 2) was deduced for the diffusion of single atoms between different half-cells. The decrease in the density of single atoms (and a concomitant change in the density of larger clusters) was consistent with the results of continuous observation. This demonstrated that only a very small effect upon the dynamics of single Pb atoms could be attributed to the scanning motions of the tip.

J.M.Gómez-Rodríguez, J.J.Sáenz, A.M.Baró, J.Y.Veuillen, R.C.Cinti: Physical Review Letters, 1996, 76[5], 799-802