Interface-dependence of Nucleation and Self-Assembly of Ultrathin Iron Oxide Films

The interface-dependence of heteroepitaxial growth of iron oxide films is investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). We show that the different chemical affinity to the metal substrate (Ru vs. Pt) and the step density (basal vs. vicinal Pt) significantly influence nucleation, heteroepitaxial crystal growth, and adhesion. Repeated Fe deposition-oxidation cycles lead to a Stranski-Krastanov growth mode on all substrates. On Ru(0001), metastable FeO(111) layers with strongly expanded lattice constants with a thickness up to 4 monolayers (ML) can be obtained by one-minute oxidation of the corresponding amount of Fe. Homogeneous nucleation of self-assembled, periodic Fe3O4(111) nanodomains embedded in an ultrathin FeO(111) film occurs on Ru(0001) in ~4 ML thick FeO(111) films. Nucleation of Fe3O4(111) islands below 4 ML on Ru(0001) occurs preferentially at substrate step edges while on Pt(111), no influence of surface defects was observed. On a vicinal Pt substrate, the terrace width and step height triplicates under influence of the wetting FeO(111) film. Differences in the growth behavior are discussed in terms of the involved surface and interface free energies