Stefan Heimann

Thermopower engineering of Bi2Te3 without alloying: the interplay between nanostructuring and defect activation

We report on the interplay between nanostructuring and defect activation in dense polycrystalline Bi2Te3 thin films in terms of the thermopower engineering. The Bi2Te3 thin films were prepared at relatively low temperatures (100–160 °C) by atomic layer deposition and their grains showed different sizes in the range of 50–200 nm according to the deposition temperatures. We monitored the conductivity, Seebeck coefficient, and power factor of all samples from the temperature of 50–400 K. By increasing the growth temperature, remarkably, we observed the gradual defect activation from the nominal p-type to n-type in our binary end compound, Bi2Te3 without any alloying. The present results give us an insight on the optimization of thermoelectric materials not only by nanostructuring (i.e., phonon engineering) but also by controlled defect activation (i.e., electron engineering).

Record figure of merit values of highly stoichiometric Sb2Te3 porous bulk synthesized from tailor-made molecular precursors in ionic liquids

We report on the synthesis of Sb2Te3 nanoparticles with record-high figure of merit values of up to 1.5. The central thermoelectric parameters, electrical conductivity, thermal conductivity and Seebeck coefficient, were independently optimized. The critical influence of porosity for the fabrication of highly efficient thermoelectric materials is firstly demonstrated, giving a strong guidance for the optimization of other thermoelectric materials.

Solution-Based Synthesis of GeTe Octahedra at Low Temperature

GeTe octahedra were prepared by reaction of equimolar amounts of GeCl2·dioxane and Te(SiEt3)2 in oleylamine, whereas a slight excess of the Te precursor yielded GeTe octahedra decorated with elemental Te nanowires, which can be removed by washing with TOP. The mechanism of the GeTe formation is strongly influenced by the solvent. The expected elimination of Et3SiCl (dehalosilylation) only occurred in aprotic solvents, whereas Te(SiEt3)2 was found to react with primary and secondary amines with formation of silylamines. Temperature-dependent studies on the reaction in oleylamine showed that crystalline GeTe particles are formed at temperatures higher than 140 °C. XRD, SAED, and HRTEM studies proved the formation of rhombohedral GeTe nanoparticles. These findings were confirmed by a single-crystal and powder X-ray analysis. The rhombohedral structure modification was found, and the structure was solved in the acentric space group R3m.