SUBMONOLAYER GROWTH OF PB ON CU(111): SURFACE ALLOYING AND DE-ALLOYING

In spite of the immiscibility of Pb in bulk Cu, atomically resolved scanning tunneling microscopy reveals surface alloy formation of Pb deposited on Cu(111), even at 300 K. Due to kinetic Limitations at room temperature, 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 voltages, 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, deposition above 0.4 ML and subsequent annealing results in hexagonal close-packed Pb regions, whereas on the nonannealed surface hexagonal close-packed Pb islands are already found at 0.2 ML. Eventually, at 1 ML the surface alloy is entirely replaced by a Pb overlayer.