Our research

Materials for catalysis

In heterogeneous catalysis, i.e. the conversion of gaseous substances on solid surfaces, metallic nanoparticles are used on oxidic carrier materials. The catalytic properties of the system are determined by the metal, but interactions between metal and oxide also play an important role if part of the reaction is prepared or takes place on the oxide. Samples that have oxides on metal surfaces offer the possibility of specifically investigating these interactions; the system is inverted for the experiment, the surface becomes an inverse model catalyst. This approach also has the advantage of allowing the use of highly developed methods of surface physics based on the use of electrons. These include photoemission spectroscopy, electron diffraction and electron microscopy. We are currently investigating two different materials for applications in heterogeneous catalysis.

Cerium oxide is one of the few reducible oxides; the cerium ion can occur in the oxide in a tridentate (Ce₂O₃) and/or tetradentate (CeO₂) form and the oxides can transform into each other depending on the ambient conditions. In this way, cerium oxide can act as an oxygen source or store in reactions. This property is already used today in automotive catalysts. We are investigating the dependence of this functional property on the occurring structures in order to identify new approaches for more effective catalysts. As substrates we use the metals that are deposited as metallic nanoparticles in real catalysts.

In a second approach, we investigate metal surfaces in order to understand and improve the properties of the described metal nanoparticles. A current example is the investigation of the oxidation of Pt-Sn surface alloys on crystal surfaces. In order to optimize the chemical properties of the nanoparticles with regard to the reactions taking place on them, other metals are often specifically added to them, which lead to the formation of a surface alloy on the surface of the nanocrystal. As the nano-crystals are very difficult to access experimentally due to their small size and density, a model system is also required here. We use the surfaces of alloy crystals and metallic surface alloys for this purpose. In this example, the addition of tin improves the stability of platinum-based catalysts against carbon deposits and improves the chemical properties of the catalyst. Specifically, we are investigating the oxidation of Pt-Sn surface alloys, using both platinum crystals and crystals of a Pt₃Sn alloy. Both molecular and atomic oxygen are used for oxidation, which is generated from molecular oxygen at extremely high temperatures.

Materials for semiconductor and communication technology

Semiconductor technology has developed rapidly over the past 50 years. The basis for this was the outstanding properties of silicon. However, the structures of modern silicon-based electronic components are now so small that a technological limit seems to have been reached. The search for new semiconductor materials is therefore essential for the further development of electronic systems.

Gallium oxide (Ga₂O3) is a very versatile new material that opens up a wide range of new applications. Due to its extremely high breakdown voltage, it has potential for the production of high-performance components. This requires epitaxial films of the best possible crystal quality. On the other hand, we were able to show in prototypes that defects in the material enable it to be used in so-called non-volatile memories, which could potentially eliminate the separation of computer processors and RAM (random access memory) that has been necessary up to now and thus make considerably faster and more energy-efficient computer cores appear possible.

Since the groundbreaking discovery of graphene with previously unattainable electrical properties, 2-dimensional materials have been the focus of research into new types of semiconductor materials. Due to their adjustable band gap, so-called dichalcogenides play an important role here. For the application of 2-dimensional materials in electronic components, it is important to develop methods that enable effective production processes. In this project, we are investigating how 2-dimensional films can be electronically decoupled from their substrate by means of so-called intercalation. This refers to a targeted change in the interface between the 2-dimensional layer and the substrate on which it was deposited using epitaxy processes.

Vanadium dioxide (VO₂) has a metal-insulator transition in a temperature range that is very interesting for applications. For VO₂ crystals, this transition from a metallic to a non-metallic state is at a temperature of TMI = 67°C. For thin films, however, the transition temperature can be shifted considerably by introducing strains into the material, making applications as a functional material possible, for example for self-reflecting window panes in strong sunlight, but also for electronics. We are investigating the possibilities of specifically changing the transition temperature by using strain layers and selecting suitable crystalline substrates.

Methods

We use low-energy electron microscopy (LEEM), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and electron diffraction (LEED) in our laboratories. These experiments are complemented by a series of one- to two-week measurement periods at synchrotron radiation sources (in Trieste, Barcelona, Hamburg, Lund, etc.), in which we use specialized measurement methods such as photoelectron microscopy (PEEM), angle-resolved photoelectron spectroscopy (ARPES) and X-ray diffraction to obtain additional information to answer physical questions about the relationship between the structure and function of materials.