STM STUDIES IN UNDERPOTENTIAL OVERPOTENTIAL METAL-DEPOSITION

The deposition of lead on "real" silver single crystal faces in the so-called underpotential (upd) and overpotential (opd) ranges was studied by in situ STM with atomic resolution. The results are compared with previous electrochemical, morphological, and in situ EXAFS and GIXS investigations. The STM images on bare substrates show regular hexagonal and quadratic lattices for the (111) and (100) planes, respectively, without any reconstructions. At high coverages in the upd range, well-ordered lead adlayers are observed on both substrate orientations. The lead monolayer on Ag(111) was found to be incommensurate, close packed, and rotated by about 4-5-degrees with respect to the substrate. This agrees with previous GIXS measurements and indicates internal compressive strain within the adlayer. In contrast, lead on Ag(100) forms a superlattice structure which has a much lower adatom density than a hexagonal close-packed layer. The symmetry and interatomic distances are close to a c(2 x 2)Pb overlayer. In the opd range, 3-D lead crystallites are formed, On both substrate orientations, the contact plane of the lead crystallites is the close-packed (111) plane. At low overpotentials, the lead crystallites on Ag(111) grow epitaxially, rotated by about 5-degrees with respect to the substrate. On Ag(100), two different orientations of lead crystallites in parallel to both [110] crystallographic directions of the substrate are observed. The nucleation rate of lead depends not only on the overpotential, but also on the initial polarization state in the upd range using an upd-opd transient technique. Nucleation and growth may proceed either inside the imperfect and strained upd-adlayer or on top of it according to the Stranski-Krastanov mechanism.