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See detailStructure and Lattice Dynamics of Bismuth Telluride Nanostructures
Bessas, Dimitrios ULg

Doctoral thesis (2012)

A brief outline of experimental lattice dynamics related to thermal transport in solids introduces this thesis. The introduction is followed by three chapters dedicated to structure and lattice dynamics ... [more ▼]

A brief outline of experimental lattice dynamics related to thermal transport in solids introduces this thesis. The introduction is followed by three chapters dedicated to structure and lattice dynamics of bulk and nanostructured bismuth telluride, a chapter on lattice instabilities observed in bulk EuTiO3 around room temperature, and a chapter on the development of a resonant ultrasound spectrometer for small samples. First, the lattice dynamics in bulk Bi2Te3 and Sb2Te3 were investigated by nuclear inelastic scattering, diffraction of high energy synchrotron radiation and calorimetry. In combination with earlier inelastic neutron scattering data, the element specific density of phonon states was extracted not only for Te and Sb but also for Bi. The Bi-Te bonding in Bi2Te3 is fundamentally different than the Sb-Te bonding in Sb2Te3. It appears that the Te specific density of phonon states is mostly unaffected upon substitution of Sb with Bi. Phonon polarization analysis was conducted in a Bi2Te3 single crystal. The observed low lattice thermal conductivity in bulk Bi2Te3 and Sb2Te3 results essentially from the small acoustic cut off energy. Second, the lattice dynamics in elemental modulated Sb2Te3 films were studied by nuclear inelastic scattering and diffraction of high energy synchrotron radiation. These studies reveal that the main features in the Sb specific density of phonon states arise from the layered structure. The extracted average speed of sound is practically the same as in bulk Sb2Te3. The impact of the acoustic cut off energy is further highlighted. Third, the phonon confinement in a Bi2Te3 nanowire array was studied by nuclear inelastic scattering, diffraction of high energy synchrotron radiation, scanning as well as transmission electron microscopy. For the first time the element specific density of phonon states was measured on nanowires in two perpendicular orientations. The much lower than in bulk average speed of sound measured in 56 nm diameter nanowires is directly related to the confined dimensions. The related 50 % decrease in macroscopic thermal conductivity is in line with macroscopic measurements on similar nanowires. Fourth, a detailed structural and lattice dynamical investigations of the bulk cubic perovskite EuTiO3 revealed a lattice instability close to room temperature. The low temperature phase is associated with anharmonic europium displacement and has a significant impact in the lattice dynamics. Last, a resonant ultrasound spectrometer for measurements on small samples and thin films was developed. Successful measurements versus temperature of the elastic constants were carried out with sub-percentage accuracy on oriented single crystals smaller than 1 mm3. [less ▲]

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