The initial growth of Co on Cu(001) is atomically identified by scanning tu
nneling microscopy using CO titration and density-functional theory total-e
nergy calculations. Both reveal that at low coverage Co adatoms occupy subs
titutional sites in the Cu substrate surface that act as pinning centers fo
r subsequent island nucleation. The interaction with diffusing adatoms is f
ound to be attractive and stronger for nearest-neighbor Co compared to Cu.
The atomic substitution process actuates a bimodal growth mode as experimen
tally found in the island size distributions. This gives rise to a high den
sity of small Co islands and large Go-decorated Cu islands. [S0163-1829(99)
08943-2].