RECENT APPLICATIONS OF SCANNING-TUNNELING-MICROSCOPY FOR THE STUDY OFMETAL UNDERPOTENTIAL DEPOSITION

The application possibilities of electrolytic STM are demonstrated for the study of underpotential deposition of Tl+ and Pb2+ at Chemically polished Ag(111) electrodes in 0.01 M HClO4. In the system Tl+/Ag(111) , the complete first Tl monolayer exhibits a densely packed hexagonal atomic structure with interatomic distances that are slightly compress ed with respect to the Tl bulk phase. An identical atomic structure is observed after the formation of the second monolayer, prior to Tl pha se formation. During long-time polarization of the partially formed fi rst Tl monolayer, marked alterations of the size and shape of the mono atomic terrasses of the substrate are observed. These changes are acco mpanied by the formation of surface domains with a residual Tl coverag e with disordered as well as an expanded hexagonal structure. Subseque nt desorption of this coverage leads to the formation of pit-like feat ures of monoatomic depth in the silver substrate, followed by mergence and disappearance of the individual pits. In the system Pb2+/Ag(111), the complete Pb adsorbate monolayer exhibits also a hexagonal structu re with slightly compressed interatomic distances with respect to the Pb bulk phase. In addition, a larger-periodicity superstructure is obs erved in the STM images, which is interpreted as a Moire pattern produ ced by rotation of the Pb adlayer with regard to the topmost silver la yer, and by differences in the sizes of the Pb and the Ag atoms, as re ported in previous GIXS measurements. Long-time polarisation of the pa rtially formed Pb monolayer is also accompanied by alterations of the size and shape of the atomic terrasses, and by the formation of a resi dual Pb coverage with expanded hexagonal structure. The results of the long-time polarization experiments are compared with the previously i nvestigated voltammetric response in the 2 systems.