Prof. Dr. Mischa Bonn
The aim of the Molecular Spectroscopy Department is to elucidate how molecules ‘wiggle and jiggle’ at interfaces: i. e. to reveal the dynamics of molecules and intermolecular interactions at interfaces, as well as transport of molecules and charge across those interfaces. This is of fundamental interest, but also highly relevant for biophysics (e. g. water, lipids and proteins at membrane interfaces) and energy conversion (e. g. photocatalytic water splitting at interfaces and charge carrier dynamics across semi-conductor nanostructures).
A brief summary of our research activities:
Elucidating the structure and dynamics of interfacial water.
Water interfaces provide the platform for many important biological, chemical, and physical processes. The interface of water with other materials is ubiquitous, and important for areas as diverse as catalysis, biology, geochemistry, electrochemistry and atmospheric chemistry. Unveiling the microscopic (<1 nm) structure and dynamics of interfacial water is essential for understanding the processes occurring at these various aqueous interfaces. At the interface, the network of very strong intermolecular interactions, hydrogen-bonds, is interrupted and the water is affected by the interface. A central question regarding water at interfaces is the extent to which the structure and dynamics of water molecules are influenced by the interruption of the hydrogen-bonded network and thus differ from those of bulk water. We study interfacial water using advanced laser-based, surface-specific vibrational spectroscopies, that allows for new insights into these important systems.
Vibrational spectroscopy and microscopy on model membranes and proteins at interfaces.
Membranes constitute the highly active partition between living cells and the outside world. They regulate molecular transport, cell adhesion and intercellular signaling. A detailed understanding - and control - of the many biological processes that occur at the membrane surface, such as viral infection and targeted drug delivery, requires insights at the molecular level. Recent developments in experimental techniques have opened avenues for the study of intermolecular interactions and chemical processes at surfaces and interfaces with unprecedented time and spatial resolution, without the need for (fluorescent) labels. We employ Sum Frequency Generation (SFG) and Coherent Anti-Stokes Raman Scattering (CARS) microscopy to address important issues in biological (model) membranes. We investigate, for instance, the structure and dynamics of membrane-bound water, and the interplay of water, lipids and membrane proteins required for membrane function.
Charge carrier dynamics in materials relevant for photovoltaics.
It is challenging to characterize charge carrier movement in semiconductor nanostructures, partly because of the complications of attaching contacts to the sample. Moreover, conductivity measurements at low frequencies are inherently limited in the information content. These drawbacks can be circumvented using freely propagating THz pulses. The rapidly varying electric field contained in these pulses with durations of ~1 picosecond allows for the investigation of key electronic properties of materials such as the electron mean-free path, exciton properties and confinement effects in nanostructured materials. Furthermore, the high time resolution of this technique allows the study of dynamic processes and/or systems far from equilibrium using a pump-probe experimental approach. Using THz spectroscopy, we investigate charge dynamics in building blocks for solar cells: graphene-based materials, conjugated polymers, nanostructured semi-conductors.