STM, LEED AND AES INVESTIGATION OF MO ENRICHMENT ON NIMO(110) SINGLE-CRYSTAL ALLOY SURFACES

The surface of a Ni-6 at% Mo(110) alloy was enriched in Mo (up to 28 a t%) by preferential ion sputtering of Ni and then annealed for 1 h at different temperatures in the range of 710 to 810 K, STM images reveal ed the presence of non-sputtered [1 (1) over bar 0] atomic segments ho mogeneously distributed on non-perfectly ordered terraces of the alloy , that can be associated with Mo. These segments are from 1.5 to 2 nm long in average, They are located above the (110) plane in the troughs of the (110) surface and are lower than the monoatomic steps possibly as a result of inward relaxation or buckling of the underlying substr ate and/or as a result of a lower density of state in these segments. The interatomic distances in these segments are larger with respect to those of the terraces, Linear segments showing inward relaxation are also evidenced that may correspond to terrace areas enriched in Mo. Th e non-perfectly ordered terraces show evidence of buckling. After cont amination by oxygen as measured by AES, STM revealed the presence of i slands of the (2 x 1)O superstructure similar to that reported on Ni(1 10) and that were developed in the areas not covered by the [1 (1) ove r bar 0] protrusive segments associated with Mo. The (2 X 1) areas are reconstructed and perfectly ordered in contrast with the protrusive s egments associated with Mo. This difference of reconstruction indicate s a selective oxidation of Ni related to easier bond breaking of Ni-Ni than of Mo-Mo or Ni-Mo. After annealing above 810 K, Mo was found to re-dissolve in the bulk and S (bulk impurity) was found to segregate p roducing a series of S superstructures from c(2 X 2) to (3 X 2) not pr esented here.