Recent advances in thermoelectric materials

We examine the effect of strong three-dimensional quantum confinement on the thermopower and electrical conductivity of PbSe nanocrystal superlattices. We show that for comparable carrier concentrations PbSe nanocrystal superlattices exhibit a substantial thermopower enhancement of several hundred microvolts per Kelvin relative to bulk PbSe. We also find that thermopower increases monotonically as the nanocrystal size decreases due to changes in carrier concentration. Lastly, we demonstrate that thermopower of PbSe nanocrystal solids can be tailored by charge-transfer doping.

http://wang.faculty.asu.edu/wp-content/uploads/2012/02/Wang-2008-Enhanced-thermopwer-in-PbSe-NC-QD-superlattices.pdf

Enhanced thermopower in PbSe nanocrystal quantum dot superlattices.

Thermoelectric applications have attracted increasing interest recently due to its capability of converting waste heat into electricity without hazardous emissions. Materials with enhanced thermoelectric performance have been reported in recent two decades. The revival of research for thermoelectric materials began in early 1990s when the size effect is considered. Low-dimensional materials with exceptionally high thermoelectric figure of merit (ZT) have been presented, which broke the limit of ZT around unity. The idea of size effect in thermoelectric materials even inspired the later nanostructuring and band engineering strategies, which effectively enhanced the thermoelectric performance of bulk materials. In this overview, the size effect in low-dimensional thermoelectric materials is reviewed. We first discuss the quantum confinement effect on carriers, including the enhancement of electronic density of states, semimetal to semiconductor transition and carrier pocket engineering. Then, the effect of assumptions on theoretical calculations is presented. Finally, the effect of phonon confinement and interface scattering on lattice thermal conductivity is discussed.

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Size effect in thermoelectric materials

Direct conversion of heat into electricity through advanced thermoelectric (TE) materials has been one of the most attractive solutions to the severe environmental and energy issues facing humanity. In recent years, great progress has been made in improving their dimensionless figure of merit (ZT), which determines the conversion efficiency of TE devices. ZT is related to three “interlocked” factors—the Seebeck coefficient, electrical conductivity, and thermal conductivity. These three factors are interdependent in bulk TE materials, and altering one changes the other two. The difficulty in simultaneously optimizing them caused TE research to stagnate, until great reductions in thermal conductivity were both theoretically and experimentally proven in nanomaterials in 1993. In this review, we first introduce some TE fundamentals and then review the most recently improvements in ZT in different kinds of inorganic and organic TE materials, which is followed by an investigation of the outlook for new directions in TE technology.

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Recent progress in thermoelectric materials

Individual PbSe nanocrystals (NCs) were prepared by a chemical reaction between lead-cyclohexanebutirate and tri-butyl-phosphine/selenium precursors in a tri-octyl-phosphine oxide/tri-butyl-phosphine surfactant, at 118 °C. Increasing precursor concentration, accompanied by an additional heating, up to 150 °C, enabled the formation of NCs assemblies:  a reaction time duration of 10−60 min led to the formation of monodispersed spherical polycrystalline assemblies whose average diameters ranged between 50−500 nm. A reaction time duration of >60 min enabled the formation of an ordered wire-like assembly, with a typical width of 60−150 nm and a length of 1−5 μm. The assemblies are presumably stabilized by the NCs−NCs dipole−dipole interactions, with an estimated interaction energy of −28 kJ/mol. The electrical measurement of a single wire-like assembly revealed a conductivity of 7.0 Ω-1cm-1.

PbSe Nanocrystal Assemblies: Synthesis and Structural, Optical, and Electrical Characterization

In PbSe epitaxial thin films grown by thermal evaporation on KCl(001) substrates and covered with an EuS protective layer, oscillatory dependences of the galvanomagnetic and thermoelectric properties (electrical conductivity σ, the Hall coefficient RH, charge carrier mobility μ, and the Seebeck coefficient S) on the PbSe layer thickness d (3<d<200 nm) were observed at room temperature. Oscillations of the transport properties are associated with quantum size effects due to electron confinement in the PbSe quantum wells.

Quantum size effects in PbSe quantum wells

The effect of oxidation at room temperature on the thermoelectric properties of PbSe/KCl (001) thin films prepared by thermal evaporation was investigated. The dependences of the electrical conductivity, the Hall coefficient, charge carrier mobility, and thermopower on the PbSe layer thickness (d=4–200 nm) were obtained. An inversion of the sign of the dominant carriers from n to p at d∼80 nm was observed under decreasing d. The d dependences of the thermoelectric properties were interpreted, taking into consideration the oxidation processes at the film/air interface within the framework of models considering both n-type and p-type carriers.

K. A. Nasedkin

Papers

Quantum size effects in PbSe quantum wells

Effect of oxidation on the thermoelectric properties of PbSe thin films