research topics

SnIP-related 1D-Semiconductors

Within our new project we’ll synthesize low-dimensional semiconducting materials with double-helical SnIP structure, delaminate them to nano-fibers and characterize them afterwards. Nanofibers or 1D-semiconductors like that will be tested afterwards concerning their potential usage as solar cells, sensors or field-effect transistors. Enlarging the SnIP structure family with SnIP homologues will enable us to derive structure-correlations for new inorganic, double-helix compounds. In sum, we are searching for ultra-thin, inorganic and chiral counterparts to the well-established double-helix compounds in biology and polymer science.

 

Ion Conductors

Today ion conductors play a key role in high-technology devices like solid state batteries, chemical sensors, solar cells or solid oxide fuel cells. Within the SFB 458 "Ionenbewegung in Materialien mit ungeordneten Strukturen- vom Elementarschritt zum makroskopischen Transport" the development of ion conducting materials and the understanding of the transport phenomena of these materials are of great interest. We focus our research activities on the synthesis and characterisation of new ion conducting copper, silver and lithium compounds as well as the optimisation of present ion conducting materials by chemical modification of the non mobile part of the structures. A wide range of analytic, spectroscopic and diffraction methods will be applied to the materials in order to get a detailed insight in the chemical and physical properties. Starting with the crystal structure determination and the determination of the electrical properties we try to optimise the materials in terms of long time stability, conductivity and polymorphism. Various methods like DSC (Differential scanning calorimetry), DTA (Differential thermal analysis) are used to get detailed information about the thermal properties. Spectroscopic methods like IR and Raman spectroscopy and solid state NMR techniques are additional applications to support the optimisation process.


Thermoelectrics and Energy Materials

The discovery of the new class of coinage metal polychalcogenide halides uncovered a new set of materials with promising properties. An extremely low thermal diffusivity in combination with high mobility and redox activity of structure units in the solid state are common features of this compounds. A linear and mobile chalcogen chain is responsible for a reversible switching of the electric properties from a p-conducting via an n-conducting state back to p-conduction by a simple change of temperature in one compound. Such a reversible redox-reaction (formation and breaking of covalently-bonded Te dumbells) within the chalcogenide substructure can have a certain impact in data storage applications due to its local resistivity switch in the sub-nano regime. The high mobility of the coinage metal and the polychalcogenide substructure lead to an highly efficient scattering of phonons and therefore an extremely low thermal diffusivity and thermal conductivity. Some compounds reach the thermal conductivity of nanostructured materials and superlattices known to be the best available thermoelectrics at the moment. Our activities will be focused on the optimization of the thermoelectric properties to challenge the problems in energy production in the future. There are still major aspects in terms of the efficiency and the stability of thermoelectrics, especially in bulk samples without nano-structuring, which need to be solved soon.

Polymorphism and phase transition

Polymorphism is a common feature in solid state ionics. It is of fundamental interest to learn something about the formation, the stability and the kinetics of phases and their phase transitions. In cooperation with PD Dr. P. Schmidt, TH Dresden, and Dr. R. Weihrich, University Regensburg, we will try to create a bottom up approach with the aid of new synthesis strategies, the modulation of phase formation and growth and the examination of thermodynamics and kinetics of crystal formation and crystal growth. We will focus on reactions via the gas phase and try to explain.

Polyphosphides

Recently we started to discover new semiconducting transition metal - main group polyphoshides. Variations in the polyphosphide substructure from 0 dimensional molecular units, polymeric 1 dimensional strands and 2 dimensional layers can be stabilized combining group 11 and group 14/15 elements. New building units like M3Sn heteroclusters (M = Au) and the first inorganic material with covalent Sb-P interactions have been prepared by main group halide mineralization reactions at elevated temperatures. One key finding is the new and effective route to black phosphorus which makes this allotrope now available for large scale applications. Due to their physical properties these polyphosphides are potential candidates for electrochemical applications.

Non-harmonic refinement of crystal structures

In the scientific field of structure determination based on X-ray or neutron diffraction the refinement of atomic positions using a non harmonic approach is a method to examine systems showing high dynamic or static disorder. Disorder is a major aspect in the field of solid state ionics and is directly connected to ion dynamics and transport phenomena in solid electrolytes. Based on the potential of this method, like the calculation of joint probability density functions (jpdf) and one particle potentials (opp), we try to get a better insight in the structural features of materials and their physical properties. Directly connected to this method is the 3D - visualisation and analysis of the jpdf data using modern visualization programs.

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