Nicola Peranio

Bi₂Te₃and CoSb₃based nanomaterials were synthesized and their thermoelectric, structural, and vibrational properties analyzed to assess and reduceZT‐limiting mechanisms. The same preparation and/or characterization methods were applied in the different materials systems. Single‐crystalline, ternary p‐type Bi₁₅Sb₂₉Te₅₆, and n‐type Bi₃₈Te₅₅Se₇nanowires with power factors comparable to nanostructured bulk materials were prepared by potential‐pulsed electrochemical deposition in a nanostructured Al₂O₃matrix. p‐type Sb₂Te₃, n‐type Bi₂Te₃, and n‐type CoSb₃thin films were grown at room temperature using molecular beam epitaxy and were subsequently annealed at elevated temperatures. This yielded polycrystalline, single phase thin films with optimized charge carrier densities. In CoSb₃thin films the speed of sound could be reduced by filling the cage structure with Yb and alloying with Fe yielded p‐type material. Bi₂(Te_0.91Se_0.09)₃/SiC and (Bi_0.26Sb_0.74)₂Te₃/SiC nanocomposites with low thermal conductivities andZTvalues larger than 1 were prepared by spark plasma sintering. Nanostructure, texture, chemical composition, as well as electronic and phononic excitations were investigated by X‐ray diffraction, nuclear resonance scattering, inelastic neutron scattering, Mössbauer spectroscopy, and transmission electron microscopy. For Bi₂Te₃materials,ab‐initiocalculations together with equilibrium and non‐equilibrium molecular dynamics simulations for point defects yielded their formation energies and their effect on lattice thermal conductivity, respectively.Current advances in thermoelectric Bi₂Te₃and CoSb₃based nanomaterials are summarized. Advanced synthesis and characterization methods and theoretical modeling were combined to assess and reduceZT‐limiting mechanisms in these materials.

In Bi₂Te₃‐based materials charge‐carrier densities are determined by antisite defects and controlling these defects is a key issue for thermoelectric and topological insulator materials. Bi‐Te thin films with high‐quality thermoelectric properties are deposited using a nano‐alloying approach by molecular beam epitaxy (MBE) and sputtering. The in‐plane transport properties are measured at room temperature as a function of charge‐carrier density. High‐accuracy chemical analysis by wavelength‐dispersive X‐ray spectrometry (WDX) is applied for the first time to these Bi₂Te₃‐based thin films. The acquisition conditions for WDX spectrometry are established using Monte Carlo simulations for the electron trajectories, which guarantees a high lateral resolution and rules out stray radiation generated in the substrate of the films. In contrast to energy‐dispersive X‐ray spectrometry (EDX), which is usually applied, WDX offers unprecedented accuracy for measuring antisite defect concentrations and thus has a high impact on improving the quality of thin films. The charge‐carrier densities are calculated from the WDX results according to the point‐defect model of Miller and Li and the thermopower and electrical conductivity are calculated for different charge‐carrier densities by solving the linearized Boltzmann transport equation. A good quantitative agreement is found for the dependence of the thermopower on stoichiometry, whereas the electrical conductivity is sensitively affected by contaminants.

In Bi₂Te₃ materials the natural nanostructure (nns) with a wavelength of 10 nm can be reproducibly switched ON and OFF by Ar⁺ ion irradiation at 1.5 and 1 keV. Controlled formation of the nns in Bi₂Te₃ materials has potential for reducing its thermal conductivity and could increase the thermoelectric figure of merit.

Quantitative chemical analysis by energy dispersive X‐ray spectrometry (EDX) in a transmission electron microscope (TEM) Zeiss912Ω was applied to p‐type (Bi,Sb)₂Te₃ and n‐type Bi₂(Te,Se)₃ thermoelectric materials. Preliminary results yielded artifacts due to Bi spurious X‐rays and hole‐counts significantly beyond zero. A stray aperture was inserted in the TEM to absorb the stray radiation. With this aperture inserted a high‐accuracy quantitative chemical analysis was established. The hole‐counts decreased by a factor of 5 and the scatter of data decreased by a factor of 4 with respect to no aperture inserted. The mole fractions of Te and Se were 54.4 at% and 5.5 at% and varied by 0.5 at% for n‐type material. A similar behaviour was found for Sb and Bi in p‐type material. The variation in stoichiometry is smaller on the sub‐micrometer scale and increases with increasing length scale for both, n‐type and p‐type Bi₂Te₃. Measurements in the TEM confirmed the inhomogeneous chemical composition found by wavelength dispersive X‐ray spectrometry. The improved accuracy of the quantitative EDX analysis is also important for other compounds with unique physical properties. This was demonstrated on thin foils containing heavy elements and on powders of light elements dispersed on Cu‐grids, i.e., the high‐T_C superconductor Bi₂Sr₂CaCu₂O₈ and the mineral salt hydroxyapatite Ca₁₀(PO₄)₆OH₂.