We report on an unusual structural modulation of a single CoO͑111͒ bilayer grown on Ir͑100͒-͑1 ϫ 1͒ 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 c͑10ϫ 2͒ 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 Å ͑in contrast to 1.25 Å in the CoO bulk͒ and it is by about 1 Å 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.
Density-functional theory (DFT) calculations have been combined with a thermodynamic formalism to determine a phase diagram of lowest-energy structures and compositions of the polar NiO(111) surface in equilibirum with oxygen, hydrogen, and water reservoirs at finite temperatures and pressures. Consistent with experiment we find that for a wide range of oxygen and hydrogen chemical potentials the surface is fully hydroxylated and shows a (1 Â 1) periodicity. At higher temperatures and H-poor conditions water can be removed from the surface and the two (2 Â 2) octopolar reconstructions become the thermodynamically most stable configurations. Other structures, which have been proposed on the basis of experimental data after high-temperature annealing, have to be considered to be kinetically limited metastable phases. In Opoor conditions no reduced surface structures are found to be thermodynamically stable. However, in O-rich environments and at low hydrogen chemical potential the surface can be oxidized by a partial removal of hydrogen or incorporation of additional oxygen. The structural motifs are closely related to the cadmium iodide structure of Ni(OH) 2 and NiO 2 .
The support of epitaxial films frequently determines their crystallographic orientation, which is of crucial importance for their properties. We report a novel way to alter the film orientation without changing the substrate. We show for the growth of CoO on the Ir(100) surface that, while the oxide grows in (111) orientation on the bare substrate, the orientation switches to (100) by introducing a single (or a few) monolayer(s) of Co between the oxide and substrate. This tunability of the orientation of epitaxial films by the appropriate choice of interface chemistry most likely is a general feature.
A substoichiometric monolayer of cobalt oxide has been prepared by deposition and oxidation of slightly less than one monolayer of cobalt on the unreconstructed surface of Ir(100). The ultrathin film was investigated by scanning tunnelling microscopy (STM) and quantitative low-energy electron diffraction (LEED). The cobalt species of the film reside in or near hollow positions of the substrate with, however, unoccupied sites (vacancies) in a 3 × 3 arrangement. In the so-formed 3 × 3 supercell the oxide's oxygen species are both threefold and fourfold coordinated to cobalt, forming pyramids with a triangular and square cobalt basis, respectively. These pyramids are the building blocks of the oxide. Due to the reduced coordination as compared to the sixfold one in the bulk of rock-salt-type CoO, the Co-O bond lengths are smaller than in the latter. For the threefold coordination they compare very well with the bond length in oxygen terminated CoO(111) films investigated recently. The substoichiometric 3 × 3 oxide monolayer phase transforms to a stoichiometric c(10 × 2)-periodic oxide monolayer under oxygen exposure, in which, however, cobalt and oxygen species are in (111) orientation and so form a CoO(111) layer.
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