Growth and structure of ultrathin FeO films on Pt(111) studied by STM and LEED

Abstract

The growth of iron-oxide films on Pt(111) prepared by iron deposition and subsequent oxidation was studied by scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction (LEED). Despite a 10% lattice mismatch to the substrate, an epitaxial growth of well-ordered films is observed. The oxide starts to grow layer by layer in a (111) orientation of the metastable cubic FeO structure up to a thickness of about 2.2 monolayers (ML). The completion of the second and third FeO layer depends on the precise oxidation temperature, and at coverages of approximately 2 ML three-dimensional ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ islands start to grow. The FeO(111) layers consist of hexagonal close-packed iron-oxygen bilayers that are laterally expanded when compared to bulk FeO and slightly rotated against the platinum substrate. They all exhibit oxygen-terminated unreconstructed $(1\ifmmode\times\else\texttimes\fi{}1)$ surface structures. With increasing coverage several structural film changes occur, and four coincidence structures with slightly different lateral lattice constants and rotation misfit angles against the platinum substrate are formed. In the submonolayer regime an FeO(111) bilayer with a lattice constant of 3.11 \AA{} and rotated by 1.3\ifmmode^\circ\else\textdegree\fi{} against the platinum substrate is observed. Upon completion of the first layer the film gets compressed leading to a lattice constant of 3.09 \AA{} and a rotation misfit angle of 0.6\ifmmode^\circ\else\textdegree\fi{}. Between 1.5 and 2 ML a coincidence structure rotated by 30\ifmmode^\circ\else\textdegree\fi{} against the platinum substrate forms, and at 2 ML a nonrotated coincidence structure with a lattice constant of 3.15 \AA{} evolves. All these coincidence structures exhibit large periodicities between approximately 22 and 38 \AA{} that are visible in the STM images up to the third FeO layer surface. The LEED patterns exhibit characteristic multiple scattering satellite spots. The different coincidence structures reflect lowest-total-energy arrangements, balancing the contributions of substrate-overlayer interface energies and elastic energies within the strained oxide overlayer for each coverage.