Nanostructured Thermoelectrics

We have several research activities focused on the development of nanostructured thermoelectric materials. Thermoelectrics are a class of materials capable of generating electricity from a thermal gradient. The performance of these materials is typically discussed in terms of a dimensionless figure of merit, ZT=S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature, and κ is the thermal conductivity. The main idea behind nanostructured thermoelectric materials is that the lattice thermal conductivity of a material can be reduced by means of phonon scattering at nanoscale interfaces. Towards this end, we are developing new materials via microwave-based synthesis approaches, solution-based synthesis approaches, and via sputtering of nanoscale thermoelectric superlattices. These activities are primarily performed within the Energy Materials In-Situ Laboratory Berlin at Bessy II.

A few examples of our research efforts are presented in the following.

For a full list of publications from our institute, please click here.

Schematic of a thin film printed thermoelectric device, showing p-type and n-type materials connected electrically in series and thermally in parallel. The highlighted image shows an example of one of the hybrid materials produced in our chemistry laboratory.

Reaction mechanism for the reduction of organogermanium halide precursors to elemental germanium nanoparticles. Reaction (1): cleavage of the halogen assisted or preceded by amine coordination. Reaction (2): reduction of Ge(IV) to Ge(II) by H2S. Reaction (3): spontaneous disproportion of the Ge(II) over 140 °C to Ge(0) and Ge(IV).

Phase transitions in Ge and Si superlattices by metal-induced crystallization

Polycrystalline Si and Ge thin films find applications in several technologies, including the semiconductor industry, photovoltaics, and thermoelectrics. The quality of the polycrystalline semiconductors is often an essential factor influencing the performance of these devices. Metal-induced crystallization (MIC) for immiscible alloy systems is a process whereby atoms become very mobile due to bond weakening effects at the metal-semiconductor interface and migrate into metal grain boundaries. Due to thermodynamic effects, these mobile atoms crystallize at a much lower temperature compared to bulk materials. This process continues until, at the right film thicknesses, a complete layer exchange occurs. For compound forming metals, on the other hand, crystallization occurs by formation of compounds and the MIC process can go through a subsequent formation of various compounds for some metals. We apply MIC to group IV systems as thermoelectric superlattices, because MIC has been shown to reduce the crystallization temperature of Si, Ge and SiGe substantially for certain metals, notably Au and Al.

Temperature-resolved XRD measurements during heating of a superlattice structure consisting of 100 bilayers of 10 nm Si0.8Ge0.2 and 0.5 nm Al on Si.