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Metal oxide surfaces, thin films, and nanostructures

STM_LEED_CoO

STM and LEED view of a single layer of CoO on Ir(100). The unit cell is approximately a c(10×2) however the details of the LEED pattern (insert) tell, that this is not the correct unit cell. The oxide overlayer forms a one-dimensional moire pattern.

Since many oxides are insulators or large-gap semiconducturs they are not accessible to surface science methods that employ electrons as probes. We therefore prepare  oxide films that are only a few monolayers thick on metal substrates where electrons can tunnel through the oxide. In such systems the morphology and resulting atomic structure of the oxide is determined by the interaction at the metal-oxide interface and by growth parameters.  We employ LEED, STM, and DFT to unravel the influence of such parameters on the atomic structure of the films.

We will extend this work towards the preparation and investigation of well defined interfaces between oxides that may exhibit new electronic, magnetic or even superconducting properties.

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The idea of this project is to identify the role of metal metal-oxide interfaces in catalysing basic oxidation reactions (CO to CO2, H2 to H2O, hydrocarbons to H2O and CO(2)). The approachuses LEED, STM, and TDS in ultra-high vacuum (UHV) and aims at clarifying adsorbate adsorption sites, reduced oxide sites, intermediate reoxidation configurations etc.) occurring prior to or during the chemical reactions. The insights from these experiments might become helpful for the design of catalysts that remove dangerous substances from the (exhaust) air or allow to produce climate-neutral fuels.

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Doping allows significant control over properties of semiconductors. While this is common for Si and other commercially used semiconductors much less is known about doped oxides. The importance of understanding atomic scale doping in these materials arises from their application as (photo-)catalysts and electrodes in batteries and solar cells. The idea is to introduce metal atoms into the surface of a homogeneous oxide and to characterize locally their influence on electronic properties (gap states, electrostatic potential) by STM/STS.

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Transition-metal oxides cover a wide range of crystal structures and oxidation states. They can be metals or insulators and the magnetic properties may show ferro-, ferri-, or antiferromagn­etic behavior. The theoretical description is challenging due to localized and strongly-correlated electrons. Experiments have to deal with demanding sample preparations due to the variety of oxidation states and crystal structures. Studies of the electronic structure usually require well-ordered surfaces prepared under ultrahigh-vacuum conditions. Electron spectroscopies can only be applied to insulating oxides, if they are prepared as thin films on a metallic substrate.

We use two-photon photoemission to study the unoccupied electronic conduction bands on thin oxide films on metal substrates in addition to the valence bands known from conventional photoelectron spectroscopy. The characterization of the electronic structure of the oxide films is important for the investigation of the electronic properties of adsorbed molecules in the funCOS project.

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FunCOS

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