The very early stages of Pb deposition on (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 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