Surfactant-induced surface restructuring: (4×4)-Pb/Cu(111)

The atomic structure of a monolayer of Pb on Cu(111) was investigated by an in situ combination of low-energy electron diffraction, scanning tunnelling microscopy and Auger electron spectroscopy. The pronounced (4×4) super-structure observed is found to be due to a (in surface projection) hexagonally close-packed but vertically buckled Pb layer which induces a substantial complex restructuring of the first three Cu layers. In this interface to the copper bulk no intermixing of Pb and Cu atoms is observed. Surprisingly, the overall buckling amplitude of the first Cu layer is even larger than that of the lead layer and the positional height of Pb atoms residing on top of Cu atoms is lower than for all other atoms. The structural results agree qualitatively with recent effective-medium-theory calculations. The reconstruction induced by the lead adatoms shows that the substrate reacts in a structurally flexible way to their presence. It is proposed that this might be related to the surface-active (`surfactant') role that Pb plays in homogeneous and heterogeneous epitaxial growth on Cu(111).