Low-dimensional thermoelectricity

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.