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M. De Santis, A. Buchsbaum, P. Varga, M. Schmid:
"Growth of ultrathin cobalt oxide films on Pt(111)";
Physical Review B, 1284 (2011), 1254301 - 12543010.

Cobalt surface oxides were grown on Pt(111) by depositing Co and dosing with molecular oxygen at temperatures ranging between 300 and 740 K. Oxidation of 1 monolayer (ML) Co results in a two-dimensional (2D) moire ́ structure, observed using both low-energy electron diffraction and scanning tunneling microscopy and interpreted as a polar (oxygen terminated) CoO(111) atomic bilayer. It is expanded by 2.7 ± 0.6% in the surface plane with respect to bulk CoO. An almost-flawless moire ́ pattern is obtained after a final step of annealing at 740 K in oxygen. Insufficient oxidation leads to defects in the moire ́ pattern, consisting of triangular dislocation loops of different sizes; the smaller ones occupy half of the moire ́ cell. Low-temperature annealing (450 K) can be used to create a zigzag phase, which is mainly observed in 1-ML-thick areas after several cycles of Co deposition (1 ML each) and oxidation at 10−7 mbar. The CoO films obtained by deposition/oxidation cycles exhibit Stranski-Krastanov growth; the structure of the 2D layer between the islands depends on the thermal treatment. It exhibits the moire ́ pattern after annealing at 740 K, whereas the zigzag phase was observed after low-temperature annealing. The second monolayer consists of a moire ́ pattern different from that of the first layer, presumably a wurtzite-like structure. Above the third layer, we observe only small three-dimensional islands, which exhibit a bandgap. We have also studied oxidation of surface alloys obtained by depositing Co and annealing. On these surfaces, we found a quasi-(3 × 3) reconstruction. Structure models are presented for all phases observed, and we argue that some of the moire ́like structures might be useful as templates for metal cluster growth.