We report on the synthesis of Bi1−xSbx alloys and the investigation of the relationship between their structural and thermoelectric properties. In order to produce a compound that will work efficiently even above room temperature, Bi1−xSbx alloys were chosen, as they are known to be the best suited n-type thermoelectric materials in the low-temperature regime (200 K). Using a top–down method, we produced nanostructured Bi1−xSbx powders by ball-milling in the whole composition range of 0 < x < 1.0. Nanostructuring of Bi1−xSbx alloys increases the band gap and thus results in an enlargement of the semiconducting composition region (0 ≤ x ≤ 0.5) compared to its bulk counterpart (0.07 ≤ x ≤ 0.22). The enhancement of the band gap strongly affects the transport properties of the alloys, i.e. the electrical conductivity and the Seebeck coefficient. Moreover, nanostructuring reduces the thermal conductivity through the implementation of grain boundaries as phonon-scattering centers, leading to a significant enhancement of the thermoelectric properties. The highest figure-of-merit observed in this study is 0.25 which was found for Bi0.87Sb0.13 at 280 K.

Bi1−x Sb x nanoparticles were prepared by mechanical alloying and compacted using different techniques. The influence of the composition as well as the pressing conditions on the thermoelectric performance was investigated. A strong dependence of the thermoelectric properties on the composition was found, which deviates from the behavior of single crystals. The results indicate a significant change in the band structure of the material induced by the reduced size. The influence of the pressing conditions on the thermoelectric properties also showed composition dependence. The results show that the compacting method has to be chosen carefully.

Bi1-xSbx nanoparticles of different compositions were synthesized in order to investigate the effect of nanostructuring on the thermoelectric figure of merit. Two synthesis pathways, one top down and one bottom up, were carried out. The top down approach was done by ball-milling and the bottom up synthesis by reduction of the metal precursors in organic solvent. Both pathways led to nano sized alloy particles with the rhombohedral crystal structure R (3) over barm. The influence of the reaction and pressing conditions on the structure and the thermoelectric properties was investigated for both pathways. In addition, the influence of the composition on the thermoelectric properties was studied.

Cu2O thin films were grown on sapphire (0001) and MgO (100) substrates by chemical vapor deposition. The crystalline, vibrational and electrical properties of the layers and the amount of incorporated background impurities have been examined. X-ray diffraction measurements revealed, that the polycrystalline films grew in (111) and (100) orientation on sapphire and in (100) orientation on MgO. Raman measurements indicated the presence of CuO inclusions in the films. The electrical properties are dominated by an acceptor level located 150 meV above the valence band. This level may originate from unintentionally incorporated silicon impurities. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A series of samples consisting of alternating stripes of ZnO grown by molecular‐beam epitaxy (MBE) and radio‐frequency (rf) sputtered Ga‐doped ZnO stripes was laterally microstructured with a self‐aligned pattern transfer method. We measured as a function of temperature the Seebeck coefficient S and the electrical resistivity ρ in‐plane of the samples with the transport direction perpendicular to the stripe direction. Throughout the series the bar width and hence the number of interfaces was kept constant, but the interface profile was varied yielding different interface lengths and geometries. The dependence of S, ρ and the power factor S2/ρ on the interface length at room temperature were simulated using an empirical network model and it was demonstrated that the macroscopic transport coefficients are very sensitive to the interface region and that even this rather simple modelling yields useful information about the interface region.

Quantum theory predicts a number of phenomena for materials scaled down to a size where confinement effects occur in one or more dimensions. Numerous devices that are based on these effects have been developed, as for example tunnel diodes, quantum well lasers, etc. An essential component in many of these device concepts are interfaces between conductive materials. To make the devices as efficient as possible in a reproducible way, the interfaces need to be controllable and tunable in their shape, morphology, and transport properties. For multilayer growth, investigations have already shown that a proper control of the quality of the interfaces between the stacked layers is of major importance for the device performance (Fasol et al., 1988; Hillmer et al., 1990). For in-plane interfaces, however, a proper characterization is still missing. To date and to our knowledge, only investigations of grain boundaries have been reported, in which the interfaces were arranged randomly (Schwartz, 1998; Watanabe, 1985; 1993; Watanabe & Tsurekawa, 1999).

Interfaces are a promising way to perturb or even block the propagation of phonons through a solid to reduce its thermal conductivity. Therefore, a better understanding of the influence of the interface-structure on the thermoelectric transport parameters is essential for improving the figure of merit Z of nanostructured thermoelectric materials. With photolithography and wet chemical etching methods we have produced ZnO/ZnO:Al bar structures. Measurements of the Seebeck coefficient perpendicular to these interfaces have been carried out locally as well as globally. The results have been compared to simulations within a simple network model that allows an estimation of the global Seebeck coefficient.

A series of bar-shaped samples consisting of lateral arrangements of alternating ZnO:Al and ZnO stripes was fabricated by radiofrequency (RF)-sputtering and microfabrication techniques on glass substrates. Throughout the series, the number of interfaces between ZnO and ZnO:Al was varied whilst the material fractions of ZnO:Al and ZnO within the bars were not altered. Lateral thermoelectric transport parameters, i.e., Seebeck effect and electrical resistivity, were measured as a function of temperature for all microstructured samples and two reference samples of ZnO:Al and undoped ZnO. The transport direction through the bar was perpendicular to the stripe direction, such that the electrons and phonons have to pass all interfaces. The transport coefficients of the microstructured samples show clear dependence on the number of interfaces between ZnO and ZnO:Al. Thermoelectric measurements, photoluminescence, and Raman measurements indicate that this is due to diffusion of Al donors along the grain boundaries into the undoped ZnO stripes, which takes place during the fabrication process. Modeling of the dependence of the Seebeck coefficient and the resistivity of the series of samples on the basis of a network model accounting for donor diffusion supports these findings.

In the context of CO 2 neutral and regenerative energy production, the field of thermoelectrics has shifted more and more into the focus of scientific research in the last few years. Particularly a lot of research projects were started in the field of energy autarkic sensor technology and the so called energy harvesting, i.e. the recycling of otherwise lost energy. A potentially huge industrial branch for thermoelectric applications is the automotive industry with a main emphasis on generating electricity out of the waste heat of combustion engines with the help of thermoelectric generators or using Peltier cooling to replace conventional air conditioning in the passenger compartment. In addition, many niche applications are possible, e.g. as sensors for measuring the air pressure of tires etc. The applications of thermoelectric devices are very versatile. We analyse the potential of the state‐of‐the‐art thermoelectric materials SiGe, PbTe, Bi 2 Te 3 , FeSi 2 and potentially ZnO with respect to employment in four types of applications, classified by mobile vs stationary and specialized vs. mass application. The selection criteria comprise efficiency, materials availability, costs, environmental friendliness and toxicity. Based on these criteria, a decision matrix for choosing the appropriate material system for a specific application is defined. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multilayer samples of alternating n-type ZnO and insulating ZnS layers were deposited by radiofrequency (RF) magnetron sputtering on glass substrates. The number of ZnO/ZnS periods was varied throughout the series to increase the number of interfaces, whilst keeping the ratio of total thicknesses of ZnO and ZnS constant. Scanning electron microscopy (SEM) revealed the individual layers, but also a columnar structure. The in-plane Seebeck coefficient S and electric conductivity σ were measured between 50 K and 300 K. The dependence of S and σ on thickness d of the individual ZnO layers can be modeled by introducing a narrow interface layer of high conductivity for d > 100 nm. At lower d, fluctuations of the interfaces lead to additional effects on S and σ which arise due to percolation and can be explained qualitatively in the framework of a network model.

We studied the Seebeck coefficient S of sputtered Zn0.98Al0.02O samples with free carrier concentrations varying from 10(18) to 10(21) cm(-3). The temperature dependence of the Seebeck coefficient at low carrier concentrations exhibits typical semiconductor behavior (S < 0 and pronounced phonon drag below 80K) whereas the metallic as-grown Zn0.98Al0.02O shows a sign reversal of the Seebeck coefficient with decreasing temperature which is related to the not square-root-like density of states of this degenerately doped metallic material. Furthermore, metallic specimens were microstructured by photolithography and wet-chemical etching with a pattern based on a square grid with a unit cell consisting of a centered square-hole. The Seebeck coefficient changed systematically with decreasing size of the unit cell. The change of S is caused by a shift of the Fermi energy due to the creation of additional surface traps at the sidewalls of the micro holes. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We measured the Seebeck coefficients of n-type (Ga,In)(N,As), (B,Ga,In)As, and GaAs epitaxial layers with doping concentrations ranging from 1017to1019cm−3 in the temperature range between 50 and 290K. Despite the significant differences in electronic structure between the nonamalgamation type quaternary alloys and the binary GaAs, the temperature dependence of the Seebeck coefficient for samples of similar doping concentration is almost the same for all three semiconductor systems. The finding can be explained by the similarity of the dispersions of the extended phonon states of the three semiconductor systems in conjunction with a dominant phonon drag contribution to the Seebeck coefficient in the temperature range studied.