PB UPD ON AG(100) AND AU(100) - 2D PHASE-FORMATION STUDIED BY IN-SITUSTM

In situ STM imaging was used to study the morphological and structural aspects of 2D UPD Pb overlayer formation on Ag(100) and Au(100) surfa ces. At low coverages, the electrochemical lead deposition on both sub strates starts at steps leading to a step decoration. On flat terraces the formation of overlayer domains with commensurate Ag(100)-c(2 x 2) or Au(100)-c(2 x 2) Pb and Au(100)-c(3 root 2 x root)R45 degrees Pb s uperlattice structures was observed in the low and medium coverage ran ge. At higher coverages these structures with low adatom density trans form on both substrates to a 2D hcp Pb overlayer. One of the close-pac ked overlayer rows is always found to be in parallel with the [110] or [1 (1) over bar 0] direction of the quadratic substrate lattice. The strong overlayer-substrate attraction leads to an anisotropic compress ion of the Pb monolayer reflected in an additional c(6 x 2) moire supe rstructure observed at high coverages. In this UPD range first-order p hase transitions take place in both systems. On Ag(100) surfaces stabl e 2D Pb islands on top of the compressed Ag(100)-2D hcp Pb monolayer a re observed, whereas on Au(100) substrates a structural phase transiti on occurs. Indications for 2D surface alloy formation were observed af ter extended polarization in both systems, starting predominantly at m onatomic steps of the substrate.