Substrate-induced structural modulation of a CoO(111) bilayer on Ir(100)

We report on an unusual structural modulation of a single CoO(111) bilayer grown on Ir(100)-(1x1) by oxidation of slightly less than one monolayer of Co deposited on the substrate. Quantitative low-energy electron diffraction and scanning tunneling microscopy in combination with standard x-ray photoelectron spectroscopy and thermal-desorption spectroscopy reveal a cobalt layer next to the substrate covered by an oxygen layer. Both layers' hexagonal atomic arrangements are, however, strongly distorted by the quadratic substrate and form a c(10x2) superstructure on that. The Co layer's buckling amplitudes and atomic bond lengths to Ir atoms are consistent with the hard-sphere radius of metallic Co. The oxide's binding to the substrate appears to be further characterized by two types of oxygen ions. One of them is close to the expected rocksalt-type stacking with respect to the cobalt layer while the other type resides nearly on top of Ir atoms. Its hard-sphere radius is only 0.77 angstrom (in contrast to 1.25 angstrom in the CoO bulk) and it is by about 1 angstrom closer to the substrate than the other type. Being so almost coplanar with the Co layer, it locally forms a hexagonal boron-nitride-type oxide. The oxygen bond to Ir can be interpreted as local pinning of the oxide to the substrate so modulating the entire oxide bilayer.