Cobalt surface oxides where 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) moiré structure, observed both using 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 moiré pattern is obtained after a final step of annealing at 740 K in oxygen. Insufficient oxidation leads to defects in the moiré pattern, consisting of triangular dislocation loops of different sizes; the smaller ones occupying one half of the moiré 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 in between the islands depending on the thermal treatment. It exhibits the moiré pattern after annealing at 740 K, whereas the zigzag phase was observed after low-temperature annealing. The second monolayer consists of a moiré pattern different from that of the 1st layer, presumably a wurtzite-like structure. Above the 3rd layer, we observe only small 3D islands, which exhibit a band gap. 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 moiré-like structures might be useful as templates for metal cluster growth.
Full text (PDF)
Users with online access to Physical Review B can also access the article at the publisher.
Corresponding author: Maurizio de Santis. Reprints also available from Michael Schmid (schmid).