Metal oxides are widely used in many applications such as thermoelectric, solar cells, sensors, transistors, and optoelectronic devices due to their outstanding mechanical, chemical, electrical, and optical properties. For instance, their high Seebeck coefficient, high thermal stability, and earth abundancy make them suitable for thermoelectric power generation, particularly at a high-temperature regime. In this article, we review the recent advances of developing high electrical properties of metal oxides and their applications in thermoelectric, solar cells, sensors, and other optoelectronic devices. The materials examined include both narrow-band-gap (e.g., Na
 x
 CoO2, Ca3Co4O9, BiCuSeO, CaMnO3, SrTiO3) and wide-band-gap materials (e.g., ZnO-based, SnO2-based, In2O3-based). Unlike previous review articles, the focus of this study is on identifying an effective doping mechanism of different metal oxides to reach a high power factor. Effective dopants and doping strategies to achieve high carrier concentration and high electrical conductivities are highlighted in this review to enable the advanced applications of metal oxides in thermoelectric power generation and beyond.

Metal oxides for thermoelectric power generation and beyond

Nanostructured Bi1–xSbx alloys constitute a convenient system to study charge transport in a nanostructured narrow-gap semiconductor with promising thermoelectric properties. In this work, we developed the colloidal synthesis of monodisperse sub-10 nm Bi1–xSbx alloy nanocrystals (NCs) with controllable size and compositions. The surface chemistry of Bi1–xSbx NCs was tailored with inorganic ligands to improve the interparticle charge transport as well as to control the carrier concentration. Temperature-dependent (10–300 K) electrical measurements were performed on the Bi1–xSbx NC based pellets to investigate the effect of surface chemistry and grain size (∼10–40 nm) on their charge transport properties. The Hall effect measurements revealed that the temperature dependence of carrier mobility and concentration strongly depended on the grain size and the surface chemistry, which was different from the reported bulk behavior. At low temperatures, electron mobility in nanostructured Bi1–xSbx was directly prop...

Bi(1)-(x)Sb(x) alloy nanocrystals: colloidal synthesis, charge transport, and thermoelectric properties.

Bismuth antimony (Bi1−xSbx) is one of the most important materials systems for fundamental materials science, condensed matter physics, low temperature thermoelectrics, infrared applications, and beyond. The bulk materials have been studied for many decades. Recently, nanoscience and nanotechnology has been introduced to this materials system, which has brought much more interest and attention for both research and applications. The present article reviews the up-to-date achievements on electronic properties of nanostructured Bi1−xSbx, and points out future directions for scientific research in this field.

Electronic properties of nano-structured bismuth-antimony materials

Evaluation of Polaron Transport in Solids from First‐principles

Polycrystalline Bi-Sb alloys have been synthesized over a wide range of antimony concentration (8 at% to 20 at%) by solid state reaction method. In depth structural analysis using X-Ray diffraction (XRD) and temperature dependent resistivity measurement of synthesized samples have been performed. XRD data confirmed single phase nature of polycrystalline samples and revealed that complete solid solution is formed between bismuth and antimony. Rietveld refinement technique, utilizing MAUD software, has been used to perform detail structural analysis of the samples and lattice parameters of synthesized Bi-Sb alloys have been estimated. Lattice parameter and unit cell volume decreases monotonically with increasing antimony content. The variation of lattice parameters with antimony concentration depicts a distinct slope change beyond 12 at% Sb content sample. Band gap has been estimated from the thermal variation of resistivity data, with the 12% Sb content sample showing maximum value. It has been observed that, with increasing antimony concentration the transition from direct to indirect gap semiconductor is intimately related to the variation of the estimated lattice parameters. Band diagram for the polycrystalline Bi-Sb alloy system has also been proposed.

Sb concentration dependent structural and resistive properties of polycrystalline Bi-Sb alloys

Thermal and electronic properties of Bi1-xsbx alloys

Doping induced high temperature transport properties of Ca1−xGdxMn1−xNbxO3 (0 ≤x ≤ 0.1)

In this paper we present thermopower (TEP) studies on polycrystalline bismuthx2013;antimony (Bix2013;Sb) alloys (containing 10, 12 and 15 at% antimony) as a function of pressure and temperature The pressure and temperature dependence of thermopower of these alloys are dominated by the complex band structure (light hole L band and heavy hole Tamp;H bands) The band structures of Bi rich Bix2013;Sb alloys depend on the Sb concentrations and also on pressure and temperature The complexity of band structure leads to an unusual behavior of thermopower at high pressures and temperatures The present range of Bix2013;Sb alloy compositions belongs to the regime of degenerate narrow band gap semiconductors (which are the best choice of materials for low temperature thermoelectric cooling applications) Pressure and temperature reduces the inter band gaps and eventually these materials behave as semimetals Smaller magnitude of thermoelectric power and its decreasing magnitude with temperature confirm the decrease in the band gap energies between direct gap (Lax2013;Ls) and also indirect band gap (Lax2013;T) The present experimental results give clear evidence to a temperature-induced crossover from direct gap semiconductor (Lax2013;Ls) to an indirect band gap involving T and or H and La bands With the application of pressure, the temperature corresponding to this crossover shifts to lower temperature These experimental results are discussed qualitatively based on the energy band structure proposed for these alloy systems involving both heavy hole T and H bands

Effect of pressure and temperature on thermopower of Bi–Sb alloys

Ternary alloys of thermoelectric materials Bi–Sb–Te and Bi–Se–Te of molecular formula, Bi0·5Sb1·5Te3 (p type) and Bi0·36Se0·064Te0·576 (n type), were prepared by mechanical alloying method. The preparation of materials by mechanical alloying method has effectively reduced the thermal conductivity by generating a large number of induced grain boundaries with required degree of disorder. The process of frequent milling was adapted for grain refinement. Substantial reduction in thermal conductivity was achieved due to nano-structuring of these alloys. Thermal conductivity values were found to be very low at room temperature, 0·5 W/mK and 0·8 W/mK, respectively for p and n type materials.

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V. Shubha

Papers

Thermal and electronic properties of Bi1-xsbx alloys

Doping induced high temperature transport properties of Ca1−xGdxMn1−xNbxO3 (0 ≤x ≤ 0.1)

Effect of pressure and temperature on thermopower of Bi–Sb alloys

Reduction in thermal conductivity of Bi–Te alloys through grain refinement method