Growth of ultrathin cobalt oxide films on Pt(111)

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 x 3) reconstruction. Structure models are presented for all phases observed, and we argue that some of the moirelike structures might be useful as templates for metal cluster growth.