Summary of research activities
Processes on surface play an important role in many fields. Chemical reactions often proceed with the desired efficiency only on catalyst surfaces; corrosion limits the durability of devices which, however, can be hindered by coatings; the fabrication of semiconductor devices involves growth and diffusion processes on surfaces.
These are only a few examples that illustrate the technological relevance of surfaces. At the Physics Department T30g, one tries to understand the microscopic principles determining the structure of surfaces and their interaction with atoms and molecules. The first step usually consists of electronic structure calculations using density functional theory (DFT, Nobel prize for Chemistry Walter Kohn, 1998). These calculations yield total energies from which equilibrium structures and interaction potential can be derived. At the same time, the analysis of the electronic structure allows the determination of the fundamental mechanisms that lead to the particular structures and processes.
In a second step, the information coming from the DFT calculations is used in the simulation of dynamical processes such as adsorption, dissociation and desorption. Both classical as well as quantum dynamical methods are employed. Thus a complete picture of reactions at surface can be gained.
The modification of the reactivity of metal surfaces by mechanical strain or by deposition of thin films of another material is one of the main research areas in our group. These investigations are performed in close collaborations with experimental colleagues. In bimetallic layer structures, on the one hand the our mechanical strain effects due to the lattice mismatch between both metals. On the other hand, the electronic structure will be modified due to the interaction of both compounds. By a clever choice of the systems addressed by DFT calculations, these two effects can be separated thus leading to a deeper understanding of the factors determining the reactivity. In detail, we study Pt/Ru and Pd/Au layer structures which are of particular interest in heterogeneous catalysis and fuel cell technology, respectively.
Besides, metallic nanostructures and their adsorption properties are studied in order to establish a connection between structure and reactivity. One long-term goal of the theoretical investigation of such realistic systems is to contribute to the design of more efficient catalysts. In addition, systems with a direct industrial relevance are addressed as well. In close collaboration with a lamp manufacturer, the work function change of tungsten electrodes by adsorbate layers is evaluated in order to improve high-pressure discharge cathode lamps. At the other end of the spectrum of our research activities is a project in which we address the electronic structure of amino acids adsorbed on graphite templates thus supporting experimentalists who manipulate these amino acids with the scanning tunneling microscope.
Further emphasis is put on the simulation of dynamical processes on surfaces. We have been able to unravel the microscopic details of the molecular adsorption of oxygen on platinum. This seemingly simple process that is one of the crucial steps occurring in the car-exhaust catalyst had not been fully understood before. In addition, we study the adsorption and desorption of hydrogen. Due to the small mass of hydrogen, quantum dynamical methods are required. In particular, we have addressed the stereodynamics in the dissociation of hydrogen on palladium. An important theoretical research field that is still in its infancy is the treatment of reactions at surface with electronic transitions. We have implemented a mixed quantum-classical scheme that allows the high-dimensional treatment of laser-induced reactions at surfaces.
Besides of the focus on applied problems we are also addressing fundamental physical questions. For example, we have determined the quantum effects in the dynamics of hydrogen. Furthermore, we are developping microscopic models that describe the electronic interaction on surfaces. The close connection between applied research and the determination of fundamental physical and chemical principles represents the specific fascination of theoretical surface science.