Ultrathin cobalt oxide films onIr(100)−(1×1)

We report on the formation of ultrathin Co oxide films by oxidation of correspondingly thin Co films epitaxially grown on the unreconstructed---i.e., metastable---$(1\ifmmode\times\else\texttimes\fi{}1)$ phase of Ir(100), on both the clean surface and the surface precovered with half a monolayer of oxygen. As investigated by low-energy electron diffraction patterns and intensities and by atomically resolved images from scanning tunneling microscopy (STM) the oxide forms mainly in its fcc-type rocksalt structure CoO. This is with exception of the submonolayer range and the surface precovered with oxygen for which the ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ spinel-structured phase develops. Rather unusual, the CoO films appear in the polar (111) orientation and so, though the hexagons are slightly distorted, should become increasingly instable with growing thickness. However, STM images recorded under special conditions indicate that there are oxygen vacancies which lead to charge compensation and so may contribute to stabilize the (111) orientation. Also, STM retrieves a considerable buckling of cobalt layers which is inevitable when fcc(111) layers are accommodated on an fcc(100) substrate.