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Thermoelectric materials are used as solid-state heat pumps and as power generators. The low efficiency of devices based on conventional bulk thermoelectric materials confines their applications to niches in which their advantages in compactness and controllability outweigh that drawback. Recent developments in nanotechnologies have led to the development of thermoelectric nano-materials with double the efficiency of the best bulk materials, opening several new classes of applications for thermoelectric energy conversion technology. We review here first the physical mechanisms that result in the superior thermoelectric performance of low-dimensional solids, compared to bulk thermoelectric materials: they are a reduction of the lattice thermal conductivity, and an increase in the Seebeck coefficient S for a given carrier density. The second part of this review summarizes experimental results obtained on macroscopic arrays of bismuth, antimony, and zinc nanowires with diameters ranging from 200 to 7 nm. We show how size-quantization effects greatly increase S for a given carrier concentration, as long as the diameter of the nanowires remains above 9 nm, below which localization effects start dominating. In a third part, we give data on PbTe nanocomposites, particularly bulk samples containing 30 nm diameter Pb inclusions. These inclusions affect the electron scattering in such a way as to again increase the Seebeck coefficient.

This paper reports the temperature dependence of the resistivity and the longitudinal and transverse magnetoresistance of antimony quantum wires with diameters ranging from 200 down to 10 nm. The samples were prepared in porous anodic alumina host materials using the vapor-phase technique. A theoretical calculation of the band structure of Sb nanowires is presented and a transport model for nanowire systems is used to explain the measured temperature dependence of the resistivity, showing both classical and quantum finite-size effects. The magnetoresistance is quadratic at low fields. In the 200 nm wires, the low-temperature (TϽ50 K) longitudinal magnetoresistance exhibits a maximum at the magnetic field where the cyclotron radius roughly corresponds to the wire radius. Surface scattering dominates below that field, and bulklike scattering dominates above it. In the narrower wires, the low-temperature ͑below 10 K for 50 nm wires and below 40 K for 10 nm wires͒ magnetoresistance shows a steplike feature at the critical magnetic field where the magnetic length equals the wire diameter, as was the case for bismuth wires. This phenomenon is independent of the effective masses, depending only on the geometry of the nanowires and on the magnetic flux in the wire, and it is therefore attributed to a localization effect.

Transverse thermoelectrics utilizing the anomalous Nernst effect (ANE) can be a novel approach to energy sustainability. We investigate the thermoelectric transport properties in ferromagnetic MnBi and observe one of the largest ANEs ever reported. We attribute this giant ANE to the coexistence of ferromagnetism and the heavy Bi atom. Our discovery proposes an alternative recipe to generate large ANE, which introduce a large spin-orbit coupling to ferromagnetic systems.

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