Nanoanalytics and Interfaces
In consequence of our society’s growing energy needs and the increasing environmental pollution, alternative cost-efficient and environmentally-friendly energy concepts are needed. A popular approach is the conversion of energy from a natural or renewable source, such as the sun or hydrogen. Nanostructured materials are suitable for this application and used as catalyst support, electrode material or active layer. Different materials are combined and the resulting interfaces are often crucial for the properties of the devices. The aim of our research group is to determine the atomic arrangement and chemical composition of interfaces and of defects in nanostructured materials. This information is correlated to optical, electronic and chemical properties.
In part, the nanostructures are grown in our group via wet chemical synthesis routes. Additionally, we collaborate with several research groups. To characterize the samples, we mostly use electron microscopy, in particular transmission electron microscopy (TEM). High-resolution TEM (HRTEM) and high-angle annular dark-field (HAADF) scanning TEM (STEM) are applied to investigate the atomic structure. Since the stability of nanostructured materials is of great interest, dynamical processes are also studied. For example, phase transformations and diffusion processes can be investigated down to the atomic scale using in-situ TEM heating experiments. Besides bright-field and dark-field imaging, electron tomography allows determining the three-dimensional morphology of the nanostructures. The chemical composition of individual nanometer-sized particles and grains is analyzed by energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). EELS measurements are also used to determine optical properties (e.g. band gap) and to get insight into the electronic structure (bonding characteristics, oxidation state). The latter information is obtained by analyzing the electron energy-loss near-edge structure (ELNES) of individual, element-specific edges. Often, a combination of different techniques is required to obtain the desired information.
The results are correlated to the synthesis conditions and to the properties (optical, electronic and chemical). Finding these correlations allows developing strategies to modify and improve these properties and to increase the stability. The studied materials systems range from oxides, nitrides, carbides to polymers and composites and find applications in photovoltaics, fuel cells and photo-catalysis. Furthermore, thin films, which are used as contacts and protection layers, are in the focus of the research group.