R. Mitdank

Temperature-Dependent Thermoelectric Properties of Individual Silver Nanowires

The thermoelectric properties of the Ag NWs are discussed in comparison to the bulk: SAg;Pt(T) was measured with respect to platinum and is in agreement with the bulk, (T) and (T) showed reduced values with respect to the bulk. The latter are both notably dominated by surface scattering caused by an increased surface-to-volume ratio. By lowering T the electron mean free path strongly exceeds the NW’s diameter of 150 nm so that the transition from diusive transport to quasi ballistic one dimensional transport is observed. An important result of this work is that the Lorenz numberL(T) turns out to be independent of surface scattering. Instead the characteristic ofL(T) is determined by the material’s purity. Moreover, (T) and L(T) can be described by the bulk Debye temperature of silver. A detailed discussion of the temperature dependence of L(T) and the scattering mechanisms is given.

Temperature-dependent thermal conductivity in Mg-doped and undoped β-Ga2O3 bulk-crystals

For - only little information exists concerning the thermal properties, especially the thermal conductivity λ. Here, the thermal conductivity is measured by applying the electrical 3ω-method on Czochralski-grown - bulk crystals, which have a thickness of and . At room temperature (RT), the thermal conductivity along the [100]-direction in Mg-doped electrical insulating and undoped semiconducting - is confirmed as for both crystals. The thermal conductivity increases for decreasing temperature down from 25 K to . The phonon contribution of λ dominates over the electron contribution below RT. The observed function is in accord with phonon–phonon–Umklapp scattering and the Debye model for the specific heat at which is about 0.1 times the Debye temperature . Here, a detailed discussion of the phonon–phonon–Umklapp scattering for is carried out. The influence of point defect scattering is considered for .

The effect of a distinct diameter variation on the thermoelectric properties of individual Bi0.39Te0.61 nanowires

The reduction of the thermal conductivity induced by nano-patterning is one of the major approaches for tailoring thermoelectric material properties. In particular, the role of surface roughness and morphology is under debate. Here, we choose two individual bismuth telluride nanowires (NWs), one with a strong diameter variation between 190 nm and 320 nm (NW1) and the other of 187 nm diameter with smooth sidewalls (NW2). Both serve as model systems for which bulk properties are expected if surface properties do not contribute. We investigate the role of the diameter variation by means of a combined full-thermoelectrical, structural and chemical characterization. By transmission electron microscopy the structure, chemical composition and morphology were determined after the thermoelectrical investigation. The NWs showed an oriented growth along the direction and the same composition. The Seebeck coefficients of both NWs are comparable to each other. The electrical conductivity of both NWs exceeds the bulk value indicating the presence of a topological surface state. Whereas the thermal conductivity of NW2 compares to the bulk, the thermal conductivity of NW1 is about half of NW2 which is discussed with respect to its distinct diameter variation.

Evolution of the density of states at the Fermi level ofBi2−yPbySr2−xLaxCuO6+δandBi2Sr2−xLaxCuO6+δcuprates with hole doping

Bi2Sr2-xLaxCuO6+d and Bi2-yPbySr2-xLaxCuO6+d high-Tc superconductors in a wide doping range from overdoped to heavily underdoped were studied by X-ray absorption and photo-emission spectroscopy. The hole concentration p was determined by an analysis of the Cu L3-absorption edge. Besides the occupied density of states derived from photoemission, the un-occupied density of states was determined from the prepeak of the O K-absorption edge. Both, the occupied as well as the unoccupied density of states reveal the same dependence on hole doping, i.e. a continuous increase with increasing doping in the hole underdoped region and a constant density in the hole overdoped region. By comparing these results of single-layer BSLCO with previous results on single-layer LSCO it could be argued that besides the localized holes on Cu sites the CuO2-planes consist of two types of doped holes, from which the so-called mobile holes determine the intensity of the prepeak of the O 1s absorption edge.

Temperature-dependent thermal conductivity and diffusivity of a Mg-doped insulating β-Ga2O3 single crystal along [100], [010] and [001]

The monoclinic crystal structure of β- leads to significant anisotropy of the thermal properties. The 2ω-method is used to measure the thermal diffusivity D in [010] and [001] direction respectively and to determine the thermal conductivity values λ of the [100], [010] and [001] direction from the same insulating Mg-doped β- single crystal. We detect a temperature independent anisotropy factor of both the thermal diffusivity and conductivity values of . The temperature dependence is in accord with phonon–phonon-Umklapp-scattering processes from 300 K down to 150 K. Below 150 K point-defect-scattering lowers the estimated phonon–phonon-Umklapp-scattering values.

Dielectrophoretic investigation of Bi2Te3 nanowires-a microfabricated thermoelectric characterization platform for measuring the thermoelectric and structural properties of single nanowires

In this article a microfabricated thermoelectric nanowire characterization platform to investigate the thermoelectric and structural properties of single nanowires is presented. By means of dielectrophoresis (DEP), a method to manipulate and orient nanowires in a controlled way to assemble them onto our measurement platform is introduced. The thermoelectric platform fabricated with optimally designed DEP electrodes results in a yield of nanowire assembly of approximately 90% under an applied peak-to-peak ac signal Vpp = 10 V and frequency f = 20 MHz within a series of 200 experiments. Ohmic contacts between the aligned single nanowire and the electrodes on the platform are established by electron beam-induced deposition. The Seebeck coefficient and electrical conductivity of electrochemically synthesized Bi2Te3 nanowires are measured to be -51 μV K(-1) and (943 ± 160)/(Ω(-1) cm(-1)), respectively. Chemical composition and crystallographic structure are obtained using transmission electron microscopy. The selected nanowire is observed to be single crystalline over its entire length and no grain boundaries are detected. At the surface of the nanowire, 66.1 ± 1.1 at.% Te and 34.9 ± 1.1 at.% Bi are observed. In contrast, chemical composition of 64.2 at.% Te and 35.8 at.% Bi is detected in the thick center of the nanowire.

Hole doping in the CuO2-plane of Bi-cuprates studied by XAS: polycrystals and single crystals

The exact shape of the superconducting dome in the Bi-cuprates depends on the proper determination of the hole content in the CuO2 layers. Single crystals of Bi(Pb)-2201 were studied over a wide doping range from the overdoped to the heavily underdoped regime by x-ray absorption spectroscopy (XAS) in order to determine the hole content of the compounds. By analysing the Cu-LIII edge this technique proved to be successful for polycrystalline oxide-based high-Tc superconductors, while for single crystals an unexpected variation of the absorption intensity within the ab-plane on a scale of 10–15% with respect to the angle of the incoming linearly polarized light was observed. Due to it, the procedure of hole content determination on polycrystals cannot be adopted straight forward to single crystals. We will show that this technique also works for single crystals, but is in need of higher experimental effort due to the polarization effects.

Systematic X-ray absorption study of hole doping in BSCCO-phases

Abstract X-ray absorption spectroscopy (XAS) on the O 1s threshold was applied to Bi-based, single crystalline high temperature superconductors (HT c s), whose hole densities in the CuO 2 planes were varied by different methods. XAS gives the intensity of the so-called pre-peak of the O 1s line due to the unoccupied part of the Zhang–Rice (ZR) singlet state. The effects of variation of the number n of CuO 2 -planes per unit cell ( n =1,2,3) and the effect of La-substitution for Sr for the n =1 and 2 phases were studied systematically. Furthermore, the symmetry of the states could be probed by the polarization of the impinging radiation.

Free-standing millimetre-long Bi2Te3 sub-micron belts catalyzed by TiO2 nanoparticles.

Physical vapour deposition (PVD) is used to grow millimetre-long Bi2Te3 sub-micron belts catalysed by TiO2 nanoparticles. The catalytic efficiency of TiO2 nanoparticles for the nanostructure growth is compared with the catalyst-free growth employing scanning electron microscopy. The catalyst-coated and catalyst-free substrates are arranged side-by-side, and overgrown at the same time, to assure identical growth conditions in the PVD furnace. It is found that the catalyst enhances the yield of the belts. Very long belts were achieved with a growth rate of 28 nm/min. A ∼1-mm-long belt with a rectangular cross section was obtained after 8 h of growth. The thickness and width were determined by atomic force microscopy, and their ratio is ∼1:10. The chemical composition was determined to be stoichiometric Bi2Te3 using energy-dispersive X-ray spectroscopy. Temperature-dependent conductivity measurements show a characteristic increase of the conductivity at low temperatures. The room temperature conductivity of 0.20 × 105 S m −1 indicates an excellent sample quality.

X-ray photoconductivity due to trap-sensitive relaxation of hot carriers

We observed the temperature-dependent modulation of the electrical conductivity in ZnO thin films under periodic illumination by soft x rays. At specific temperatures, small variations of the excitation energy near the x-ray absorption edges resulted in large element-specific variations of the conductivity modulation. The emission rate of electrons at traps E1/E2 and E3/E4 in ZnO at these specific temperatures roughly equals the excitation frequency. We conclude that relaxation of electrons, excited from localized core levels into the conduction band, predominantly happens into trap states with the same localization. The experimental results were explained using symmetry selection rules and local transition probabilities.