Au overlayer structures on a Ni(110) surface.

Abstract

The atomic arrangement of Au atoms segregated to the (110) surface of a Ni--0.8 at. % Au alloy was studied with low-energy ion scattering (LEIS) in combination with low-energy electron diffraction (LEED). The LEED results have been published elsewhere [E. G. McRae and R. A. Malic, Surf. Sci. 177, 53 (1986)]. From this LEED work and a preliminary analysis of the present data a model for the atomic structures of Au atoms on the Ni(110) surface has been presented. In the present paper, the LEIS data were analyzed quantitatively by comparison with computer simulations. Most features of the models proposed earlier were confirmed by this analysis. We also used the (static) embedded-atom method (EAM) to calculate the positions of Au atoms on Ni(110) for various coverages. After introducing these positions in the computer simulations of the LEIS data, an even better agreement between experiment and calculation was found. At low coverages of less than 0.5 ML, the Au atoms were found to be located in substitutional Ni sites in the first Ni layer. At a Au coverage of 1.28 ML, the Au atoms are at various heights above the Ni surface, and are hexagonally packed within a 7\ifmmode\times\else\texttimes\fi{}7 unit cell with c(2\ifmmode\times\else\texttimes\fi{}4) subunits. For coverages slightly lower than 1.28 ML several other structures are predicted by our EAM calculations. These structures have indeed been found in scanning tunneling microscopy work. In addition, we discuss the observation of one-dimensional melting of the Au overlayer at a temperature of approximately 700 \ifmmode^\circ\else\textdegree\fi{}C, as observed with LEED and LEIS.