Thermoelectric Bi2Te3 based bulk materials are widely used for solid‐state refrigeration and power‐generation at room temperature. For low‐dimensional and nanostructured thermoelectric materials an increase of the thermoelectric figure of merit ZT is predicted due to quantum confinement and phonon scattering at interfaces. Therefore, the fabrication of Bi2Te3 nanowires, thin films, and nanostructured bulk materials has become an important and active field of research. Stoichiometric Bi2Te3 nanowires with diameters of 50–80 nm and a length of 56 μm are grown by a potential‐pulsed electrochemical deposition in a nanostructured Al2O3 matrix. By transmission electron microscopy (TEM), dark‐field images together with electron diffraction reveal single‐crystalline wires, no grain boundaries can be detected. The stoichiometry control of the wires by high‐accuracy, quantitative enegy‐dispersive X‐ray spectroscopy (EDX) in the TEM instrument is of paramount importance for successfully implementing the growth technology. Combined electron diffraction and EDX spectroscopy in the TEM unambiguously prove the correct crystal structure and stoichiometry of the Bi2Te3 nanowires. X‐ray and electron diffraction reveal growth along the [110] and [210] directions and the c axis of the Bi2Te3 structure lies perpendicular to the wire axis. For the first time single crystalline, stoichiometric Bi2Te3 nanowires are grown that allow transport in the basal plane without being affected by grain boundaries.

Short AbstractThe reduction of the thermal conductivity induced by nano-patterning is one of the major approaches for tailoring thermoelectric material properties. Here, we chose two individual bismuth telluride nanowires (NWs), one with a strong diameter variation (NW1) and the other with smooth sidewalls (NW2). We investigated the role of the diameter variation by means of a combined fullthermoelectrical, structural and chemical characterisation on single nanowires. The electrical conductivity of both NWs exceeds the bulk value indicating the presence of a topological surface state. The thermal conductivity of NW2 compares to the bulk, while NW1 is about half that of NW2.Above. HRTEM micrograph of the smooth NW side and the amorphous shell of varying thickness of about (5 ± 1) nm. The inset shows a power spectrum of the NW core.

Thermoelectric (TE) properties of a single nanowire (NW) are investigated in a microlab which allows the determination of the Seebeck coefficient S and the conductivity σ. A significiant influence of the magnetization of a 70 nm ferromagnetic Ni-NW on its power factor S²σ is observed. We detected a strong magneto thermopower effect (MTP) of about 10% and an anisotropic magneto resistance (AMR) as a function of an external magnetic field B in the order of 1%. At T = 295 K and B = 0 T we determined the absolute value of S = -(19 ± 2) µV/K. At zero field the figure of merit ZT ≈ 0.02 was calculated using the Wiedemann-Franz-law for the thermal conductivity. The thermopower S increases considerably as a function of B up to 10% at B = 0.5 T, and with a magneto thermopower of ∂S/∂B ≈ -(3.8 ± 0,5) µV/(K . T). The AMR and MTP are related by ∂s/∂r ≈ -11 ± 1 (∂s = ∂S/S).The TE efficiency increases in a transversal magnetic field (B =0.5T) due to an enhanced power factor by nearly 20%.

The lattice dynamics in an array of 56 nm diameter Bi2Te3 nanowires embedded in a self-ordered amorphous alumina membrane were investigated microscopically using (125)Te nuclear inelastic scattering. The element specific density of phonon states is measured on nanowires in two perpendicular orientations and the speed of sound is extracted. Combined high energy synchrotron radiation diffraction and transmission electron microscopy was carried out on the same sample and the crystallinity was investigated. The nanowires grow almost perpendicular to the c-axis, partly with twinning. The average speed of sound in the 56 nm diameter Bi2Te3 nanowires is ~7% smaller with respect to bulk Bi2Te3 and a decrease in the macroscopic lattice thermal conductivity by ~13% due to nanostructuration and to the reduced speed of sound is predicted.

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