We report the observation of a thermoelectric power factor in InAs nanowires that exceeds that predicted by a single-band bulk model by up to an order of magnitude at temperatures below about 20 K. We attribute this enhancement effect not to the long-predicted 1D subband effects but to quantum-dot-like states that form in electrostatically nonuniform nanowires as a result of interference between propagating states and 0D resonances.

A large thermoelectric power factor in heavily boron-doped p-type nanograined Si with grain sizes ∼30 nm and grain boundary regions of ∼2 nm is reported. The reported power factor is ∼5 times higher than in bulk Si. It originates from the surprising observation that for a specific range of carrier concentrations, the electrical conductivity and Seebeck coefficient increase simultaneously. The two essential ingredients for this observation are nanocrystallinity and extremely high boron doping levels. This experimental finding is interpreted within a theoretical model that considers both electron and phonon transport within the semiclassical Boltzmann approach. It is shown that transport takes place through two phases so that high conductivity is achieved in the grains, and high Seebeck coefficient by the grain boundaries. This together with the drastic reduction in the thermal conductivity due to boundary scattering could lead to a significant increase of the figure of merit ZT. This is one of the rare observations of a simultaneous increase in the electrical conductivity and Seebeck coefficient, resulting in enhanced thermoelectric power factor.

The electron thermal conductance, , of a dot has been calculated in the regime of weak coupling with two electrode leads within a linear response theory. We discuss the effect of the interplay between the charging energy, the thermal energy, and the confinement in the Coulomb oscillations of . Hence, we consider three energy regions: the quantum limit, where quantum confinement dominates over the thermal energy; the classical regime, where the discreteness of the energy spectrum is screened by the thermal energy; and the intermediate energy region. In the quantum limit, the periodicity of the oscillations of the electron thermal conductance is the same as the Coulomb-blockade oscillations of the conductance, G. Analytical expressions have been obtained for and G in the cases of nondegenerate and for doubly degenerate energy spectrum. The obtained dependence of on the energy level spacing and the thermal energy explicitly shows that quantum confinement is responsible for the fast decrease of the electron thermal conductance of a dot. It is found that degeneracies in the energy spectrum of a dot are opposed to the decrease of the electron thermal conduction due to quantum confinement. It is shown that an external field that raises the degeneracies causes a considerable enhancement in . In the classical and in the intermediate regimes, the electron thermal conductance shows distinct behavior at low and high temperatures. In the classical regime, Coulomb blockade oscillations are shown at low temperatures and simple formulas are obtained for and G. The Wiedermann-Franz law holds at the peaks of and G. The temperature dependence of and G has been calculated up to the limit where transport occurs through two isolated barriers. The relation between and G with increasing thermal energy is discussed.

High optimal thermoelectric efficiencies are theoretically demonstrated in ballistic nanowires with diameter modulation. The physics underlying the good thermoelectric performance of diameter-modulated nanowires is the strong energy dependence of their transmission coefficients. It is shown that the thermoelectric efficiency is directly related to the geometry of the diameter modulation. It becomes evident that geometry optimization can lead to efficient thermoelectric devices based on modulated nanowires.