Institute of Methods for Material Development
A complete characterization of the molecular - electronic and nuclear - structure of matter is a prerequisite for an understanding of the nature of bonding interactions and chemical reactivity. With experimental tools that reveal such information and supporting calculations, we can quantify the charge and energy transfer processes associated with chemical change. Moreover, such information can be directly applied to systematically engineer molecular structure and yield desirable material properties.
Our research focuses on two main themes:
- The characterisation of the electronic and nuclear structure of novel materials that can be used, for instance, in the catalytic transformation of water in an energy-efficient device for the production of solar fuel.
- The development of methods to probe the dynamic electronic and nuclear structure of molecules or materials undergoing physical/chemical changes and energy transfer processes on a few-femtosecond to microsecond timescale. Such spectroscopy measurements span the THz, IR, Vis., UV, EUV and soft X-ray spectral ranges within our research institute.
Molecular Systems: In developing novel materials and optimizing their energy conversion performance, we investigate various model systems - going from atomic species via larger organic (bio) molecules and nanoparticles to solid samples. Our group has a particularly strong interest in aqueous systems – this being a natural condition within a photo- or electrochemical cell, for instance in a water-splitting process – although we also characterize dry samples. Size-ordered studied systems include atomic ions (particularly transition metal ions) and small organic and inorganic molecules in aqueous solution, porphyrines (aq.), diamond and transition-metal nanoparticles (aq.), and novel, functional solid materials.
Development of Spectroscopic Methods: There are multiple important aspects that need to be considered when performing photon and electron measurements to study the electronic and nuclear structure of condensed phase materials.
is highly sensitive to local electronic structure, thus sensing the chemical environment with atomic specificity. Depending on the method used, one detects emitted X-ray photons or emitted photo- or Auger electrons. For an unequivocal interpretation of soft X-ray spectra, it is crucial that the information obtained from complementary photon and electron based spectroscopies is combined. We are fully equipped for such back-to-back studies, although improvements of spectrometer performance and specifications are underway.
Another pressing and challenging experimental development is to apply soft-X-ray photoelectron spectroscopy to the solid (electrode)–water interface. Beyond the introduction of the liquid-microjet technique, this would represent a further technical and scientific break-through in aqueous solution spectroscopy.
The high brilliance at the infrared beam line allows us to perform low throughput measurements that are not possible with conventional broadband infrared sources. Broadband infrared investigations of non-cycling kinetic systems or systems with long recovery dynamics are typically limited to a temporal resolution of a couple of tens of milliseconds when rapid scan FT-IR methods are implemented. We are currently commissioning an instrument, based on a dispersive infrared spectrometer and an advanced 2D photovoltaic detector array, to fully make use of the high brilliance of a synchrotron source. This new spectrometer with a temporal resolution in the microsecond range will find applications where sample-specific spectral and temporal resolutions are required and will allow the dynamics and function of molecules of relevance in biophysics and chemistry to be studied over a broader spectral range.
Investigations in the terahertz spectral range give direct access to the few-meV energy scale and are complementary to angle-resolved photoemission experiments that provide momentum-space resolution as opposed to high energy resolution. High-energy resolution (DE 20 µeV) broadband THz FT-Spectroscopy is an ideal tool for the study of electronic interactions in semiconductors and superconductors, since typical electron-phonon scattering rates and critical energies lie in the few-meV range, corresponding to the energy of a THz photon (the range between 0.1 and 1 THz corresponds to a photon energy range of 0.4 - 4 meV). Significant signal losses generally occur over such long wavelength ranges due to complicated beam paths and/or small sample sizes. Making use of high-brilliance, broadband, coherent synchrotron radiation we can compensate for such losses.
An understanding of a structure-function relationship is a necessary but insufficient requirement for the development of energy efficient molecular devices. In fact, (molecular or material) function is the result of the interplay between electronic and nuclear structure and their evolution in time. Knowledge of the kinetics and dynamics of active molecular and material systems is required to fully understand their function. In such systems, the chemically relevant initial dynamics occur on the femtosecond timescale, with these early events determining device efficiencies. To study such dynamics, we make use of femtosecond duration light pulses and the pump-probe methodology: an ultrashort pump pulse initiates a process and a second time-delayed pulse interrogates the evolution of the excited system. Using such a scheme, we now routinely conduct dynamic studies using ultrashort EUV pulses in our recently completed high harmonic generation (HHG) laser lab at the Free University Berlin (for more see: Instrumentation & Infrastructure). This allows us to study the evolution of valence electronic and nuclear structure in the solution phase and at interfaces.
Beyond these initial experiments, we are developing a new laser based light source to study molecular and material systems in the X-ray spectral region. Making use of newly available laser technologies and the HHG process driven by ultrashort mid-infrared laser pulses, we will produce high-fluence, tunable, few-femtosecond soft X-ray pulses for time-resolved, core level spectroscopy experiments. Such infrastructure will enable us to probe molecular and material function with site-specificity. To date, such experiments have generally been performed at expensive Free Electron Lasers (FELs), with access to such facilities being extremely limited. Hence, the development of more affordable table-top alternatives is of both scientific and economic interest.
... of the processes that are probed through spectroscopy is essential for spectral interpretation, as well as for predicting material properties and mechanistic aspects. To this end, we have recently established a young-investigator theory research group in our institute.