Structural analysis of ultra-thin Pd films on W(110)

Abstract

We report on a scanning tunneling microscopy (STM) growth study of ultra-thin Pd films on W(110) in the thickness range up to about three atomic layers. At a substrate temperature of (573±20)K during deposition the initial growth at submonolayer coverages takes place in the step-flow mode, whereby the Pd step edges are preferentially oriented along 〈11¯1〉 directions leading to a pronounced saw-tooth–like appearance. It is found that the Pd monolayer is not smooth but instead exhibits a one-dimensional pattern consisting of alternating straight and zigzag lines along the [001] direction with a periodicity of (7.3±0.5)nm. Atomic resolution STM images reveal that these stripes mark transition regions between two different three-fold–coordinated adsorption sites, i.e. left (L) or right (R) triangular. A structural model is presented which explains the straight and zigzag lines as vacancy rows and Pd grown in the Kurdjumov-Sachs configuration, respectively. In a transition regime between 0.93AL–1.03AL the one-dimensional character converts into triangularly shaped L and R domains up to 100nm in size which are separated by short straight segments approximatelly oriented along 〈11¯1〉 and [001] directions. Growth of the second Pd layer occurs via the nucleation of hexagonal islands and at Pd coverages beyond 2AL the growth mode changes from layer-by-layer to island growth, probably due to the structural transition from the bcc to the fcc crystal structure.