Metal oxide heteroepitaxy: Stranski-Krastanov growth for iron oxides on Pt(111)

The epitaxial growth of iron oxide films on Pt (111) substrates was investigated by scanning tunneling microscopy and low-energy electron diffraction. The film growth was accomplished by repeated cycles of iron deposition and subsequent oxidation at p (O₂) =10^-6mbar. For oxidation temperatures of 870 K second and third FeO (111) layers grow layer by layer, whereas for oxidation temperatures of 1000 K only one FeO (111) monolayer is formed. On top of the FeO (111) films a homogeneous nucleation of Fe₃O₄ (111) islands takes place, resulting in a Stranski-Krastanov growth for iron oxides on Pt (111). The islands grow in the Fe₃O₄ bulk structure laterally much faster than vertically, forming flat platelets with heights up to 100 and hexagonal and triangular basal planes 1000--5000 in diameter. The islands only expose low index 111 and 211 facet planes, and their growth can be described by an Ostwald ripening mechanism that takes place during each oxidation cycle. Eventually the islands coalesce and form smooth Fe₃O₄ (111) films at least 150 thick. The atomic and mesoscopic surface roughness of these films depends on the growth temperature, where the latter ranges between 40 and 100 on a length scale of 1. By a high-pressure oxidation at p (O₂) =10^-1mbar the Fe₃O₄ (111) films were transformed into well-ordered -Fe₂O₃ (0001) films with similar surface morphologies. In all oxide phases formed the hexagonal oxygen (111) planes are aligned to the Pt (111) substrate surface lattice. The film growth is discussed in terms of surface and interfacial energies, oxidation and growth kinetics, as well as thermodynamic stability ranges of the different oxide phases.