Joachim Wollschläger

Growth of NiO and MgO films on Ag(100)

The growth morphology of NiO and MgO films deposited in the temperature range 300–350 K has been studied by Spot Profile Analysis of Low Energy Electron Diffraction (SPA-LEED) patterns. The oxide films grow layer-by-layer. They are pseudomorphic during the initial stages. Relaxation via the injection of misfit dislocations with {110} glide planes occurs for coverages larger than a critical coverage (Θc≈5 ML for NiO, Θc=1–2 ML for MgO). The relaxation process leads to the formation of mosaics on the film surface. The tilt angle of the mosaics decreases with increasing oxide thickness due to minimisation of the deformation energy.

Electron diffraction at stepped homogeneous and inhomogeneous surfaces

Defect analysis at surfaces with LEED or RHEED have so far been essentially restricted to steps or superstructure domains. Even if there are no superstructures, variations of the scattering factor influence the spot profile. With the now available high-resolution instruments more details of the spot profile may be observed. Therefore it will be discussed how a careful study of the spot profile at different energies may be used to analyze a surface with simultaneous steps and inhomogeneities. It will be shown how the asperity height is obtained without fitting parameters and how inhomogeneities are analyzed with respect to coverage and size distribution.

Stoichiometry and morphology of MgO films grown reactively on Ag(100)

Abstract The stoichiometry and the morphology of MgO adlayers epitaxially grown on Ag(100) are studied by means of X-ray photoelectron spectroscopy (XPS) and high-resolution spot profile analysis low-energy electron diffraction (SPA-LEED). For comparison, Mg deposited at 200 K is oxidized after and during growth, respectively. The post-oxidized film is dominated by stoichiometric MgO but shows also non-stoichiometric species. Annealing the film to 700 K reduces the latter components. Strong differences have been found for the epitaxy of both kinds of MgO films, depending on the preparation condition. The post-oxidized film shows a clear diffraction pattern with 12-fold symmetry due to MgO(100) grains in three rotational orientations only after annealing to 700 K. Depositing and oxidizing simultaneously the Mg the oxide film shows a better film quality (no metallic Mg, a small MgO2 fraction) and epitaxy with 1×1 already at 200 K.

Stainless steel made to rust: a robust water-splitting catalyst with benchmark characteristics

The oxygen evolution reaction (OER) is known as the efficiency-limiting step for the electrochemical cleavage of water mainly due to the large overpotentials commonly used materials on the anode side cause. Since Ni–Fe oxides reduce overpotentials occurring in the OER dramatically they are regarded as anode materials of choice for the electrocatalytically driven water-splitting reaction. We herewith show that a straightforward surface modification carried out with AISI 304, a general purpose austenitic stainless steel, very likely, based upon a dissolution mechanism, to result in the formation of an ultra-thin layer consisting of Ni, Fe oxide with a purity >99%. The Ni enriched thin layer firmly attached to the steel substrate is responsible for the unusual highly efficient anodic conversion of water into oxygen as demonstrated by the low overpotential of 212 mV at 12 mA cm−2 current density in 1 M KOH, 269.2 mV at 10 mA cm−2 current density in 0.1 M KOH respectively. The Ni, Fe-oxide layer formed on the steel creates a stable outer sphere, and the surface oxidized steel samples proved to be inert against longer operating times (>150 ks) in alkaline medium. In addition Faradaic efficiency measurements performed through chronopotentiometry revealed a charge to oxygen conversion close to 100%, thus underpinning the conclusion that no “inner oxidation” based on further oxidation of the metal matrix below the oxide layer occurs. These key figures achieved with an almost unrivalled-inexpensive and unrivalled-accessible material, are among the best ever presented activity characteristics for the anodic water-splitting reaction at pH 13.

Defects in epitaxial insulating thin films

Defects on thin epitaxial insulator films of NaCl(100), KCl(100), and MgO(100) generated during growth and by electron bombardment are investigated by high-resolution spot profile analysis in low-energy electron diffraction (SPALEED), photoelectron spectroscopy (XPS, UPS), electron energy-loss spectroscopy (EELS), and thermal desorption spectroscopy (TDS). All three insulators contain morphological defects: NaCl overgrows the monatomic Ge(100) steps in a carpet-like mode, whereas KCl grown on NaCl(100) forms a regular array of stacking faults up to three monolayers of KCl, and the MgO film grown on Ag(100) reveals a broadened (1 ? 1) pattern in LEED due to the formation of mosaics. In EELS, surface colour centres, produced by electron bombardment, on NaCl induce losses at 2.1 eV for FS centres and at 1.5 eV for MS centres and on KCl induce losses at 1.6 eV and 1.0 eV, respectively. Close to room temperature, high electron exposures result in additional losses in the band gap due to surface and bulk plasmons of Na and K clusters. A similar anion vacancy defect structure on MgO with losses at 2.1 eV and 3.3 eV can be produced by incomplete desorption of metallic Mg from the MgO(100) surface. TDS experiments show that colour centres increase the binding energy of Kr physisorbed on NaCl(100) by about 25%. The concentration of point defects was determined by titration with noble gases in thermal desorption.

Atomically smooth and single crystalline Ge(111)/cubic-Pr2O3(111)/Si(111) heterostructures: Structural and chemical composition study

Engineered wafer systems are an important materials science approach to achieve the global integration of single crystalline Ge layers on the Si platform. Here, we report the formation of single crystalline, fully relaxed Ge(111) films by molecular beam epitaxial overgrowth of cubic Pr oxide buffers on Si(111) substrates. Reflection high-energy electron diffraction, scanning electron microscopy, and x-ray reflectivity show that the Ge epilayer is closed, flat, and has a sharp interface with the underlying oxide template. Synchrotron radiation grazing incidence x-ray diffraction and transmission electron microscopy reveal the type-A/B/A epitaxial relationship of the Ge(111)/cubic Pr2O3(111)/Si(111) heterostructure, a result also corroborated by theoretical ab initio structure calculations. Secondary ion mass spectroscopy confirms the absence of Pr and Si impurities in the Ge(111) epilayer, even after an annealing at 825 °C.

The influence of lattice oxygen on the initial growth behavior of heteroepitaxial Ge layers on single crystalline PrO2(111)∕Si(111) support systems

A combined structure and stoichiometry study on the growth behavior of single crystalline Ge(111) layers on PrO2(111)∕Si(111) heterostructures is presented. Ex situ x-ray diffraction techniques indicate that the interaction between Ge and PrO2(111) results in a complete reduction of the buffer oxide to a cubic Pr2O3(111) film structure. In situ reflection high energy electron diffraction, x-ray and ultraviolet photoelectron spectroscopy studies demonstrate that this chemical reduction of the oxide support occurs during the initial Ge growth stage. The interaction of PrO2 with Ge results in the formation of an amorphous Ge oxide layer by the diffusion of lattice oxygen from the dielectric to the forming semiconductor deposit. After the complete conversion of PrO2 to cubic Pr2O3, the supply of reactive lattice oxygen is exhausted and the continuous Ge deposition reduces the initially formed amorphous GeO2-like film to GeO. The sublimation of volatile GeO uncovers the single crystalline cubic Pr2O3(111) film...

Static Magnetic Proximity Effect in Pt/NiFe2O4 and Pt/Fe Bilayers Investigated by X-Ray Resonant Magnetic Reflectivity.

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity is measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be (0.6±0.1)  μB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02  μB per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.

Oxidized Mild Steel S235: An Efficient Anode for Electrocatalytically Initiated Water Splitting

The surface of steel S235 was oxidized by Cl2 gas and checked for its electrocatalytic efficiency regarding oxygen formation in aqueous solution. If exposed to humid Cl2 gas for 110 min, steel S235 became an electrocatalyst that exhibits an overpotential for the oxygen evolution reaction (OER) of 462 mV at 1 mA cm(-2) at pH 7. The OER activity of the same sample at pH 13 was moderate (347 mV overpotential at 2.0 mA cm(-2) current density) in comparison with OER electrocatalysts developed recently. Potential versus time plots measured at a constant current demonstrate the sufficient stability of all samples under catalysis conditions at pH 7 and 13 for tens of hours. High-resolution X-ray photoelectron spectra could be reasonably resolved with the proviso that Fe2 O3 , FeO(OH), MnO(OH), and Mn2 O3 are the predominant Fe and Mn species on the surface of the oxidized steel S235.

Stabilization of the ceria ι-phase (Ce7O12) surface on Si(111)

In this work a 250 nm CeO2(111) film grown on a hex-Pr2O3(0001)/Si(111) system is annealed at 660 °C for 30 min to form the ι bulk phase of Ce7O12 as controlled by x-ray photoelectron spectroscopy. The (111) surface of the stabilized ι phase is characterized via high-resolution low-energy electron diffraction. The ι-phase surface exhibits a (7×7)R19.1° superstructure with two mirror domains. This structure is attributed to a periodic ordering of oxygen vacancies compared to the fluorite structure of CeO2.