Berlin 2008 – scientific programme
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O: Fachverband Oberflächenphysik
O 75: Oxides and Insulators: Epitaxy and Growth
O 75.8: Talk
Thursday, February 28, 2008, 17:00–17:15, MA 042
Phases of epitaxial cobalt oxide films on Ir(100)-(1×1) — •Kerstin Biedermann, Matthias Gubo, Daniela Hock, Wolfgang Meyer, Lutz Hammer, and Klaus Heinz — Lehrstuhl für Festkörperphysik, Universität Erlangen-Nürnberg, Staudtstr. 7, 91058 Erlangen
The unreconstructed (metastable) Ir(100)-(1×1) surface was used as support for the formation of thin epitaxial films of cobalt oxide. They were prepared by deposition of different amounts of cobalt and simultaneous exposure to an oxygen atmosphere at 320 K followed by annealing at higher temperatures. Dependent on exposure and annealing various structures can be observed. LEED shows that all of them own a hexagonal unit mesh which, however, can be slightly distorted. As a consequence, the oxides formed must consist of polar bilayers.
Ultrathin films (1-2 bilayers) form a c(10×2) superstructure which, according to its appearance in the STM, can be interpreted as rocksalt-type CoO(111) bilayers. Thicker films (≥4 bilayers), when oxygen rich and annealed at up to about 720 K, exhibit an ideal hexagonal unit mesh with a (2×2) superstructure. By further annealing at higher temperatures this transforms (irreversibly) to a (√3×√3)R30∘ phase with the basic hexagonal unit mesh slightly distorted. As monitored by TDS the transformation is accompanied by the loss of about a quarter of the oxygen content. This suggests that structurally this is a transition from spinel-type Co3O4 to rocksalt-type CoO as in fact corroborated by independent quantitative LEED analyses. At about 340 K the (√3×√3)R30∘ structure undergoes a reversible transition to a (1×1) phase.