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

Abstract

The growth of iron-oxide films on Pt͑111͒ prepared by iron deposition and subsequent oxidation was studied by scanning tunneling microscopy ͑STM͒ and high-resolution low-energy electron diffraction ͑LEED͒. 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 ͑111͒ orientation of the metastable cubic FeO structure up to a thickness of about 2.2 monolayers ͑ML͒. The completion of the second and third FeO layer depends on the precise oxidation temperature, and at coverages of approximately 2 ML three-dimensional Fe 3 O 4 (111) islands start to grow. The FeO͑111͒ 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 (1ϫ1) 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 FeO͑111͒ bilayer with a lattice constant of 3.11 Å and rotated by 1.3°against the platinum substrate is observed. Upon completion of the first layer the film gets compressed leading to a lattice constant of 3.09 Å and a rotation misfit angle of 0.6°. Between 1.5 and 2 ML a coincidence structure rotated by 30°against the platinum substrate forms, and at 2 ML a nonrotated coincidence structure with a lattice constant of 3.15 Å evolves. All these coincidence structures exhibit large periodicities between approximately 22 and 38 Å 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. ͓S0163-1829͑98͒02311-X͔

Morphology and CO adsorption on platinum supported on thin Fe3O4(111) films

“…Indeed, the inset in Fig. 3f shows that the top facets exhibit a superstructure with an ~ 2.6 nm periodicity which is identical to the Moire pattern observed for an FeO(111) monolayer film on Pt(111) [35]. Moreover, the total particles volume measured from STM images turned out to be relatively constant at 300 and 600 K, but showed an ~ 20 % increase upon heating to 850 K. Again, this finding supports the conclusion of Pt particle the encapsulation by an oxide layer.…”

Incommensurate Moiré overlayer with strong local binding: CoO(111) bilayer on Ir(100)

“…The same expansion was found for the two-dimensional Moiré structure of FeO on Pt(111). 14,15 The experimentally determined average lattice parameter in the [011] direction also deviates as little as 0.8% from that of the c(10 × 2) coincidence mesh (3.017Å) and thus is within the accuracy by which DFT and LEED (using a conventional LEED optics) are able to determine equilibrium lattice parameters at all. Hence, the commensurate c(10 × 2) phase is an excellent model structure, exhibiting all the local features of the real oxide structure fairly well.…”

“…Candidates are rock-salt type FeO, MnO, and CoO films of (111) orientation, for which films of ultimate thinness -single hexagonal metal-oxygen bilayers -have been found on a number of metal substrates. [11][12][13][14][15][16][17][18][19][20] In these cases high-order commensurate phases were reported with unit cell dimensions of about ten times that of the substrate, corresponding to about 10% lattice misfits between substrates and oxide films. In the case of (100) oriented substrates, this just leads to close correspondence of row distances in one direction and, by a slight uniaxial distortion, the films achieve one-dimensional commensurability (row-on-row matching).…”

“…As a consequence, slight deviations in size or orientation of the film due to details of the preparation (island sizes, defect density etc.) 15 or simply the limited resolution of the applied real or reciprocal space methods will not lead to totally different notations such as, for example, FeO on Pt(111) (10 × 10), 11 (…”

Surface potential of a polar oxide film: FeO on Pt(111)

“…8,11 The corrugation of this superstructure amounts to about 0.3 Å at bias voltages below 1 V, as can be seen in Fig. 1͑a͒.…”