On the Structure of Ultrathin FeO Films on Ag(111)

Abstract

Ultrathin transition metal oxide films exhibit unique physical and chemical properties not observed for the corresponding bulk oxides. These properties, originating mainly from the limited thickness and the interaction with the support, make those films similar to other supported 2D materials with bulk counterparts, such as transition metal dichalcogenides. Ultrathin iron oxide (FeO) films, for example, were shown to exhibit unique electronic, catalytic and magnetic properties that depend on the type of the used support. Ag(111) has always been considered a promising substrate for FeO growth, as it has the same surface symmetry, only ~5% lattice mismatch, is considered to be weakly-interacting and relatively resistant to oxidation. The reports on the growth and structure of ultrathin FeO films on Ag(111) are scarce and often contradictory to each other. We attempted to shed more light on this system by growing the films using different preparation procedures and studying their structure using scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). We observed the formation of a previously unreported Moiré superstructure with 45 Å periodicity, as well as other reconstructed and reconstruction-free surface species. The experimental results obtained by us and other groups indicate that the structure of FeO films on this particular support critically depends on the films’ preparation conditions. We also performed density functional theory (DFT) calculations on the structure and properties of a conceptual reconstruction-free FeO film on Ag(111). The results indicate that such a film, if successfully grown, should exhibit tunable thickness-dependent properties, being substrate-influenced in the monolayer regime and free-standing-FeO-like when in the bilayer form.

2D oxides on metal materials: concepts, status, and perspectives

“…Not illustrated here, we finally note that other cases of tessellation by MO 3 bricks sharing vertexes can be found for FeO phases on both Ag(111) and on Ag(100) 68,69 , where the 1:1 ratio between Fe and O ions is determined by the stabilization of the lower oxidation state of iron under reducing conditions. The morphology of ultrathin ceria on Pt(111) has also been investigated [70][71][72] .…”

Two-Dimensional Iron Oxide on Au(111): Growth Mechanism and Interfacial Properties

“…48 The embedded growth mode on such a soft substrate is correlated with the energies of the as-grown islandedge wetting by the surface, which is generally observed on Au, Cu, or Pd whose step formation energy is low. 49 For example, the embedded growth of FeO on Ag(111) leading to straininduced deformation 27 and the incorporation of MgO films into the top layer on Ag(100). 50 Domain ii is connected to FeO islands along the step edge that exhibits an identical hexagonal pattern as that in domain i, but with a high density of dislocation lines.…”

“…22−26 Regarding the growth mode, for example, researchers observed that the iron oxide thin films on Pt, Ru, and other metal substrates usually show simple layer-by-layer terrace growth, while the iron oxide islands on Ag(111) were embedded in the top silver layer when iron atoms were deposited at high temperature. 27 Ultrathin films of NiO on Ag(100) prepared under a 1 × 10 −6 mbar oxygen atmosphere followed by annealing at 600 K in oxygen also demonstrated embedded growth. 28 To date, many reports about the epitaxial growth mode and the SOMIs of iron oxide films on various metal substrates have been published, showing excellent catalytic activity in various reactions.…”

“…The structure of monolayer FeO was further characterized by surface science tools, low-energy electron diffraction, and scanning tunneling microscopy (STM). − An atomic structure model consisting of Fe 2+ and O 2– was proposed for the first time, exhibiting a uniform layer terminated by close-packed O atoms. During the past several decades, studies on the growth mechanism and structures of iron oxide films have explored the phase engineering of iron oxide thin films on metal supports, such as different growth conditions, temperature, and the interaction between the overlayer and metal substrates. , Differences in the growth behavior of iron oxides on a variety of substrates reveal the impact of strong oxide–metal interactions (SOMIs) on the stabilization mechanism, growth mode, and extraordinary catalytic properties. − Regarding the growth mode, for example, researchers observed that the iron oxide thin films on Pt, Ru, and other metal substrates usually show simple layer-by-layer terrace growth, while the iron oxide islands on Ag(111) were embedded in the top silver layer when iron atoms were deposited at high temperature . Ultrathin films of NiO on Ag(100) prepared under a 1 × 10 –6 mbar oxygen atmosphere followed by annealing at 600 K in oxygen also demonstrated embedded growth …”

Reversible metamorphosis from Fe3O4 to FeO of epitaxial iron oxide films grown on the Fe-p(1 × 1)O surface

“…the preparation of well-dened model systems under highly controlled conditions, is the most appropriate to investigate the detailed atomic structure and chemical composition of oxide surfaces. [5][6][7][8] Thin and ultra-thin Fe oxide lms supported on metallic substrates like Pt, 9,10 Ag, 11,12 Fe, 13,14 , Ni, 15,16 have been investigated by using this approach. Aer these investigations, it has been recognized that also new phases, with stoichiometries and physical properties deviating from those occurring in bulk samples, can be stabilized in epitaxial lms with a thickness of few monolayers.…”