Strain induced micro-island formation on Ni/Ru(0001) monolayers

Morphological changes of Ni monolayers on Ru(0001) induced by oxygen adsorption have been investigated using scanning tunneling microscopy. Specifically, the creation of well defined and uniform Ni micro-islands consisting of 3, 6 or 9 Ni atoms is reported and their geometrical structure, as well as their coordination with respect to the substrate lattice determined. The island formation is directly linked to a phase transition of the pseudomorphic Ni layer (lattice constant Δd=dRu-Ru=2.706 A) into a densified, Moire-distorted phase (lattice constant Δd=dNi-Ni=2.492 A) as Ni areas grow to lateral sizes beyond about 200 A. An increasing lateral stress within such densified Ni monolayers induced by the adsorption of oxygen is held responsible for the expulsion of Ni clusters from the monolayer film. The micro-islands represent unusually stable units (with respect to recombination of neighboring islands) which is tentatively attributed to oxygen atoms attached to the trimers and hexagons