LEED I/V determination of the structure of a MoO₃ monolayer on Au(111): Testing the performance of the CMA-ES evolutionary strategy algorithm, differential evolution, a genetic algorithm and tensor LEED based structural optimization

The structure of a thin MoO3 layer on Au(111) with a c(4 × 2) superstructure was studied with LEED I/V analysis. As proposed previously (Quek et al., Surf. Sci. 577 (2005) L71), the atomic structure of the layer is similar to that of a MoO3 single layer as found in regular α-MoO3. The layer on Au(111) has a glide plane parallel to the short unit vector of the c(4 × 2) unit cell and the molybdenum atoms are bridge-bonded to two surface gold atoms with the structure of the gold surface being slightly distorted. The structural refinement of the structure was performed with the CMA-ES evolutionary strategy algorithm which could reach a Pendry R-factor of ∼0.044. In the second part the performance of CMA-ES is compared with that of the differential evolution method, a genetic algorithm and the Powell optimization algorithm employing I/V curves calculated with tensor LEED

Surface Reactivity of Titania–Vanadia Mixed Oxides Under Oxidizing Conditions

The surface structure and reactivity of TiO2(110) with 1–15% of admixed vanadium were studied using thermal desorption spectroscopy, scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy with the methanol partial oxidation as the reactivity test reaction. Prior to the experiments the sample was oxidized at elevated temperatures with O2 of different pressures up to 10−6 mbar. As shown in a preceding publication (Song et al. in Surf Sci 653:181–186, 2016), vanadium leads to an increased reducibility of the mixed oxide, which resulted in the simultaneous presence of reduced and oxidized structural elements at the surface. At low vanadium concentrations (a few percent) small vanadia clusters with V4+ form above the fivefold Ti rows at the surface together with short vanadium decorated strands along [001]. These types of structure promote the partial oxidation of methanol towards formaldehyde at 550 K. At higher vanadium concentrations the vanadia aggregates at the surface contain V5+ in addition to V4+. They produce formaldehyde already at 480 K and below. The oxidized TiO2(110) layer with admixed vanadium releases QMS-detectable amounts of O2 already at a temperature of about 450 K, which is about 80 K below the corresponding temperature for TiO2(110) without vanadium. This is attributed to the increased reducibility of rutile with admixed vanadium

Surface Structure of V₂O₃(0001) – A Combined I/V-LEED + STM Study

Using I/V-LEED and scanning tunneling microscopy we have investigated the surface structure of ~100Å thick V2O3(0001) films on Au(111). Both methods clearly show that the surface is terminated by a layer of vanadyl groups. I/V-LEED quantitative structure determination applied to differently prepared films always leads to a Pendry R-factor for the V=O termination close to 0.11 while the R-factor for a reconstructed O3 termination is always larger than 0.2 and increases with increasing dataset size. These results are at variance with a recent publication by Window et al. [ Phys. Rev. Lett. 114, 2015, 216101 ] in which the authors propose that the V2O3(0001) surface is terminated by a reconstructed O3 structure. Surface oxidation experiments also contradict the conclusions of Window et al. since oxidation leads to a previously identified structure with a (√3x√3)R30° LEED pattern which is not expected for oxidation of an O3 terminated surface. In the course of the I/V-LEED calculations the individual Debye temperatures of the surface atoms were determined as part of the structural optimization procedure. We show that this approach is superior to the kinematical analysis of temperature dependent LEED measurements

Reducing the V₂O₃(0001) surface through electron bombardment – a quantitative structure determination with I/V-LEED

The (0001) surface of vanadium sesquioxide, V2O3, is terminated by vanadyl groups under standard ultra high vacuum preparation conditions. Reduction with electrons results in a chemically highly active surface with a well-defined LEED pattern indicating a high degree of order. In this work we report the first quantitative structure determination of a reduced V2O3(0001) surface. We identify two distinct surface phases by STM, one well ordered and one less well ordered. I/V-LEED shows the ordered phase to be terminated by a single vanadium atom per surface unit cell on a quasi-hexagonal oxygen layer with three atoms per two-dimensional unit cell. Furthermore we compare the method of surface reduction via electron bombardment with the deposition of V onto a vanadyl terminated film. The latter procedure was previously proposed to result in a structure with three surface vanadium atoms in the 2D unit cell and we confirm this with simulated STM images

Transmission electron microscopy study of platinum clusters on Al₂O₃/NiAl(110) under the influence of electron irradiation

Using transmission electron microscopy we have studied the influence of the electron beam in an electron microscope onto platinum clusters deposited on a thin single crystalline γ-Al2O3 film grown by oxidation of NiAl(110). At electron current densities below j≈1 A/cm2 no influence is observed. Movement and coalescence of clusters occur at electron beam current densities between j=2 and some 10 A/cm2. For current densities around j=50 A/cm2 decoration of steps takes place. Further increase to j=100  A/cm2 and above induces drilling of holes into the substrate by clusters. At such current densities also melting of the clusters may occur. Due to the heat capacity of the system the result does not only depend on the electron current density but also on the irradiation time

Growth of well-ordered iron sulfide thin films

In this paper a growth recipe for well-ordered iron sulfide films and the results of their characterisation are presented. The film was studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). XRD data reveal that the film has a NiAs-like structure with Fe vacancies, similar to iron sulfides such as pyrrhotite and smythite, although no indication of any ordering of these vacancies was observed. LEED and STM results show that the film exhibits a 2 × 2 surface reconstruction. XPS data provide additional evidence for a large number of Fe vacancies, and the oxidation states of the Fe and S in the film are analysed

Elucidating Surface Structure with Action Spectroscopy

Surface Action Spectroscopy, a vibrational spectroscopy method developed in recent years at the Fritz Haber Institute is employed for structure determination of clean and H2O-dosed (111) magnetite surfaces. Surface structural information is revealed by using the microscopic surface vibrations as a fingerprint of the surface structure. Such vibrations involve just the topmost atomic layers, and therefore the structural information is truly surface related. Our results strongly support the view that regular Fe3O4(111)/Pt(111) is terminated by the so-called Fetet1 termination, that the biphase termination of Fe3O4(111)/Pt(111) consists of FeO and Fe3O4(111) terminated areas, and we show that the method can differentiate between different water structures in H2O-derived adsorbate layers on Fe3O4(111)/Pt(111). With this, we conclude that the method is a capable new member in the set of techniques providing crucial information to elucidate surface structures. The method does not rely on translational symmetry and can therefore also be applied to systems which are not well ordered. Even an application to rough surfaces is possible

Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

The electronic structure of CaO films of 10–60 monolayer thickness grown on Mo(001) has been investigated with synchrotron-mediated x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Upon annealing or reducing the thickness of the film, a rigid shift of the CaO bands to lower energy is revealed. This evolution is explained with a temperature-induced diffusion of Mo ions from the metal substrate to the oxide and their accumulation in the interface region of the film. The Mo substitutes divalent Ca species in the rocksalt lattice and is able to release electrons to the system. The subsequent changes in the Mo oxidation state have been followed with high-resolution XPS measurements. While near-interface Mo transfers extra electrons back to the substrate, generating an interface dipole that gives rise to the observed band shift, near-surface species are able to exchange electrons with adsorbates bound to the oxide surface. For example, exposure of O2 results in the formation of superoxo species on the oxide surface, as revealed from STM measurements. Mo interdiffusion is therefore responsible for the pronounced donor character of the initially inert oxide, and largely modifies its adsorption and reactivity behavior