Pb UPD on Ag(100) and Au(100) — 2D phase formation studied by in situ STM

Abstract

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) surfaces. At low coverages, the electrochemical lead deposition on both substrates starts at steps leading to a step decoration. On flat terraces the formation of overlayer domains with commensurate Ag(100)-c(2 × 2) or Au(100)-c(2 × 2) Pb and Au(100)-c(3√2 × √2)R45°Pb superlattice structures was observed in the low and medium coverage range. At higher coverages these structures with low adatom density transform on both substrates to a 2D hcp Pb overlayer. One of the close-packed overlayer rows is always found to be in parallel with the [110] or [1 10] direction of the quadratic substrate lattice. The strong overlayer-substrate attraction leads to an anisotropic compression of the Pb monolayer reflected in an additional c(6 × 2) moiré superstructure observed at high coverages. In this UPD range first-order phase transitions take place in both systems. On Ag(100) surfaces stable 2D Pb islands on top of the compressed Ag(100)-2D hcp Pb monolayer are observed, whereas on Au(100) substrates a structural phase transition occurs. Indications for 2D surface alloy formation were observed after extended polarization in both systems, starting predominantly at monatomic steps of the substrate.