Superstructure in the termination of CoO(111) surfaces: Low-energy electron diffraction and scanning tunneling microscopy

The surface structures of CoO(111) films epitaxially grown on Ir(100)-(1×1) are investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. A (√3×√3)R30^∘ superstructure is revealed for the films' ground states. It appears for film thicknesses ≥10 both for strained and unstrained films and so most likely applies also to the (111) surface of a bulk CoO crystal. The superstructure is interpreted as a stress-relieving reaction to the switch from rocksalt-type to wurtzite-type stacking below the surface which has been detected earlier.