In spite of the immiscibility of Pb in bulk Cu, atomically resolved sc
anning tunneling microscopy reveals surface alloy formation of Pb depo
sited on Cu(111), even at 300 K. Due to kinetic Limitations at room te
mperature, the incorporation of Pb is restricted to advance from step
edges, while after annealing to 470 K or higher, embedded Pb atoms are
found to be randomly distributed over terraces. At low tunneling volt
ages, standing waves of surface-state electrons scattered by embedded
Pb atoms could be observed. The maximum packing density of the surface
alloy is about 40% (=0.4 ML) of a close-packed Pb overlayer. Thus, de
position above 0.4 ML and subsequent annealing results in hexagonal cl
ose-packed Pb regions, whereas on the nonannealed surface hexagonal cl
ose-packed Pb islands are already found at 0.2 ML. Eventually, at 1 ML
the surface alloy is entirely replaced by a Pb overlayer.