GROWTH OF FEOX ON PT(111) STUDIED BY SCANNING-TUNNELING-MICROSCOPY

We have studied the growth of iron oxide films on Pt(111) using scanning tunneling microscopy. The system has been found to depend sensitively on the temperature and oxygen pressure used to form the oxide. As the coverage is varied from below a monolayer to multilayers of approximately four or five layers thick, three different ordered structures are observed as well as regimes in which the iron oxide appears disordered. For coverages less-than-or-equal-to 1 ML, the iron oxide is FeO and a large lattice mismatch between the iron oxide and platinum governs the growth process. The first ordered structure occurs at a monolayer coverage and has a large unit cell where approximately 8 lattice spacings of the FeO fit approximately 9 of the Pt(111) surface. At higher coverages, square-root 3 X square-root 3R30-degrees and 2X2 [W. Weiss, A. Barbieri, M. A. Van Hove, and G. A. Somorjai, Phys. Rev. Lett. 71, 1884 (1993)] ordered structures are observed. These can be ascribed to alpha-Fe2O3 and Fe3O4 phases respectively which are the equilibrium phases for the oxygen pressures and temperatures used in growing the iron oxide. A disordered multilayer growth is also observed.