Hexagonal heterolayers on a square lattice: A combined STM and LEED study of FeO(111) on Pt(100)

Formation of an epitaxial iron oxide monolayer on a Pt(100)-hex substrate was studied by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). High-resolution STM images reveal a sinusoidal height modulation of the top atomic rows along the [011] direction of the original Pt(100)-hex substrate. This modulation is assigned to the buckling of the top oxygen layer due to the interaction with Pt substrate atoms. Two superstructures, described as $\mathrm{FeO}(111)/\mathrm{P}\mathrm{t}(100)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}10)$ and $(2\ifmmode\times\else\texttimes\fi{}9)$ coincidence structures, coexist on the surface. The latter structure results in a much lower Pendry R factor in dynamical LEED analysis than earlier reported for a $c(2\ifmmode\times\else\texttimes\fi{}10)$ structure. Numerous islands with the same surface structure as the terraces develop on the dense FeO overlayer. They are assigned as the $\mathrm{Pt}(100)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ islands formed during the $\mathrm{hex}\ensuremath{\rightarrow}(1\ifmmode\times\else\texttimes\fi{}1)$ reconstruction of the Pt substrate underneath the FeO(111) bilayer. The islands are rectangular and elongated in the direction of hex reconstruction on the original Pt(100). Combined STM and LEED data clearly indicate that anisotropy in the substrate reconstruction leads to anisotropy of the oxide overlayer.