The group is very active in the field of surface science, which contains topics relevant to heterogeneous catalysis, materials science and interactions at interfaces. Main techniques used to address such questions are photoelectron spectroscopy, x-ray absorption spectroscopy and other surface sensitive methods.

Picture of the molecular beam apparatus (Denecke Group)
Picture of the molecular beam apparatus (Denecke Group)

Research projects

The conversion of biomass to liquid fuels requires catalysts in order to achieve high energy efficiency. But, how do organic molecules adsorb on such transition metal catalysts as used in industrial applications? Utilizing in-situ XPS we want to address basic problems alike. For a high resolution and simultaneously a fast sequence of measurements we use the synchrotron radiation provided by BESSY II in Berlin. Temperature dependent experiments reveal different species present during the reaction at appropriate model catalysts. By comparison of different core levels ( e.g. C1s, O1s, N1s) complementary data can be interpreted. During the reaction mass spectra can be recorded that show the desorbing particles. Additionally, temperature programmed desorption (TPD) experiments are performed.

Density plots of the TPXPS experiments after guaiacol adsorption on Pt(111) at 230 K for the (left) C 1s spectra and (right) O 1s spectra. Graphics: Denecke Group
Density plots of the TPXPS experiments after guaiacol adsorption on Pt(111) at 230 K for the (left) C 1s spectra and (right) O 1s spectra. Graphics: Denecke Group

The molecular beam scattering project investigates the interactions between gas molecules and surfaces on a molecular level. A supersonic nozzle expansion directs a flow of molecules with a narrow, adjustable energy distribution onto a sample surface in ultra-high vacuum. A rotatable mass spectrometer enables the investigation of the angular scattering pattern. By using a chopper, the acquisition of time-of-flight spectra is possible. These methods allow the determination of the following phenomena:

  • scattering mechanism: elastic scattering, inelastic scattering, trapping (thermalization and subsequent desorption) or sticking

  • energy transfer in case of inelastic scattering

  • residence time in case of trapping

The measurement of sticking probabilities is possible due to two flags. Furthermore, low energy electron diffraction (LEED) and auger electron spectroscopy (AES) for sample characterization and thermal desorption spectroscopy (TDS, TPD) is possible.

The project focused recently on the interactions between carbon dioxide and amin-functionalized Ionic Liquids. Ionic Liquids are absorbents with high potential for carbon capture and storage (CCS) applications. A molecular understanding of the gas uptake and release, especially in comparison to the surface structure, helps to design future absorbents.

Systems with multiferroic properties are of interest for research sind the 1960s. Ferroic properties are for example ferromagnetism and ferroelectricity. Therefore, the fabrication and characterization of oxidic heterostructures is in focus of SFB 762 (Sonderforschungsbereich). Primarily the interest is in the magnetic, ferroelectric, multiferroic, semiconductive and isolating properties of such composites are studied in detail. Thin films of a material with one of these ferroic properties (e.g. ferromagnetism) are produced on single-cristalline substrates adhering another ferroic (e.g. ferroelectricity) property. Both ferroic properties are tested towards their interdependence. Modern analytical methods like SQUID (supraconducting quantum interferrence device), MOKE (magnetooptical Kerr effect), XAS (X-ray absorption spectroscopy) as well as the derived XMCD (X-ray magnetic circular dichroism), and AFM (atomic force microscopy) are used to probe these composites. Some of the measurements are carried out at synchrotron radiation facilities such as BESSY II in Berlin.

Wavelength Dispersive X-Ray Fluorescence Spectroscopy (WD-XRF) enables the qualitative and quantitative elemental analysis of solid and liquid materials. With the spectrometer at hand, all elements with atom numbers greater than five (from carbon) can be detected. This method is used in cooperation with other institutes such as the Institute for Chemical Technology (industrial glasses, rice husk ash, diatomite and catalysts) or the Institute for Mineralogy, Crystallography and Materials Science (ancient coins).

Equipment

Here a general overview of the machines that are available in the group. More detailed information about current measurements at the science stations can be found at the Research page.

The molecular beam technique is equipped with a rotatable mass spectrometer and a LEED/AES unit, allowing investigations of the adsorption and desorption behavior of gaseous molecules at different surfaces. In this experiment a directed molecule flux with a narrow, adjustable energy distribution can be dosed onto the surfaces of interest. A more detailed description of actual measurements is given in item molecular scattering at surfaces in the research profile.

Molekularstrahlapparatur mit rotierbaren Massenspektrometer, Foto: AK Denecke
Molekularstrahlapparatur mit rotierbaren Massenspektrometer, Foto: AK Denecke

Different methods of sample modification and characterization are available with the spectrometer VG ESCALAB 220i-XL.

  • XPS (X-ray photoelectron spectroscopy)
  • UPS (Ultraviolet photoelectron spectroscopy)
  • LEED (Low energy electron spectroscopy)
  • AES (Auger electron spectroscopy)
  • SEM (Secondary electron microscopy)
  • PLD (Pulsed laser deposition)
  • EBE (Electron beam evaporation)

Actually, this equipment is employed within the framework of the described research projects and for external service measurements.

ESCALAB 220i-XL. Photo: Denecke Group
ESCALAB 220i-XL. Photo: Denecke Group
Bruker S4 Explorer. Photo: Denecke Group
Bruker S4 Explorer. Photo: Denecke Group

The Bruker S4 Explorer is a high-performance wavelength dispersive X-ray fluorescence spectrometer. An own power press for sample preparation with special wax as well as the usage of Mylar foil allow a wide range of applications from solids to liquids.

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