Nucleation has been studied on atomic scale for Ag deposition on a Pt(111) surface at low temperature (50-120 K) by means of variable-temperature scanning tunneling microscopy. From island density versus coverage data, the transition from the initial steps of nucleation to growth and coalescence has been studied as a function of temperature. The data show that dimers are stable nuclei for $T\le 110$ K, which facilitates quantitative comparison with rate equations from nucleation theory. The migration barrier $E_m=157\mathrm{}\pm 10$ meV is determined for Ag adatoms on Pt(111) and compared with theoretical values.

• Received 23 February 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.1955