Interactions of adsorbates with locally strained substrate lattices

Using scanning tunneling microscopy the effect of local strain at a Ru(001) surface on the adsorption of various adsorbates has been studied. Local strain fields have been produced by Ar-ion implantation and annealing. Thereby the accompanying surface sputter damage is fully healed out with the exception of subsurface cavities filled with argon atoms which have aggregated by bulk diffusion. The resulting nanometer-sized structures contain surface areas of expanded lattice at the tops of the protrusions while around their rim the lattice is compressed relative to the flat surface. Various adsorbates are found to react sensitively to these local lattice distortions. Oxygen atoms adsorb preferentially in the regions of expanded lattice. This preference prevails for all coverages up to the full monolayer with the successive formation of the well-known (2x2)-O, (2x1)-O, (2x2)-3O, and (1x1)-O ordered overlayers on the various parts of the surface. CO at coverages in excess of 0.33 monolayers is found to behave similarly. The experimental results are complemented by investigations of the mixed (O+CO) coadsorbate layer. The reported influence of surface strain on the adsorption energy can be considered as the reverse of strain induction by adsorption, and their direct local demonstration can be used to test theoretical predictions. We also find direct evidence for a compressed lattice zone close to step edges, which extends about 10-20 Angstrom into the terraces. (C) 2001 American Institute of Physics.