Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/AdvMater_19_1043_2007.pdf

New Directions for Low-Dimensional Thermoelectric Materials**

In a typical thermoelectric device, a junction is formed from two different conducting materials, one containing positive charge carriers (holes) and the other negative charge carriers (electrons). When an electric current is passed in the appropriate direction through the junction, both types of charge carriers move away from the junction and convey heat away, thus cooling the junction. Similarly, a heat source at the junction causes carriers to flow away from the junction, making an electrical generator. Such devices have the advantage of containing no moving parts, but low efficiencies have limited their use to specialty applications, such as cooling laser diodes. The principles of thermoelectric devices are reviewed and strategies for increasing the efficiency of novel materials are explored. Improved materials would not only help to cool advanced electronics but could also provide energy benefits in refrigeration and when using waste heat to generate electrical power.

http://science.sciencemag.org/content/sci/285/5428/703.full.pdf

Thermoelectric Cooling and Power Generation

Semiconductor nanowires represent an important class of nanostructure building block for photovoltaics as well as direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting. In addition, we have also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials. It is important to note that the engines that generate most of the world's power typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled, the impact globally would be enormous. Our silicon nanowire thermoelectric technology could have a significant impact in alternative energy generation.

Semiconductor nanowires for energy conversion.

The field of thermoelectrics has progressed enormously and is now growing steadily because of recently demonstrated advances and strong global demand for cost-effective, pollution-free forms of energy conversion. Rapid growth and exciting innovative breakthroughs in the field over the last 10-15 years have occurred in large part due to a new fundamental focus on nanostructured materials. As a result of the greatly increased research activity in this field, a substantial amount of new data--especially related to materials--have been generated. Although this has led to stronger insight and understanding of thermoelectric principles, it has also resulted in misconceptions and misunderstanding about some fundamental issues. This article sets out to summarize and clarify the current understanding in this field; explain the underpinnings of breakthroughs reported in the past decade; and provide a critical review of various concepts and experimental results related to nanostructured thermoelectrics. We believe recent achievements in the field augur great possibilities for thermoelectric power generation and cooling, and discuss future paths forward that build on these exciting nanostructuring concepts.

Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features

This review presents an overview of the thermal properties of mesoscopic structures. The discussion is based on the concept of electron energy distribution, and, in particular, on controlling and probing it. The temperature of an electron gas is determined by this distribution: refrigeration is equivalent to narrowing it, and thermometry is probing its convolution with a function characterizing the measuring device. Temperature exists, strictly speaking, only in quasiequilibrium in which the distribution follows the Fermi-Dirac form. Interesting nonequilibrium deviations can occur due to slow relaxation rates of the electrons, e.g., among themselves or with lattice phonons. Observation and applications of nonequilibrium phenomena are also discussed. The focus in this paper is at low temperatures, primarily below $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where physical phenomena on mesoscopic scales and hybrid combinations of various types of materials, e.g., superconductors, normal metals, insulators, and doped semiconductors, open up a rich variety of device concepts. This review starts with an introduction to theoretical concepts and experimental results on thermal properties of mesoscopic structures. Then thermometry and refrigeration are examined with an emphasis on experiments. An immediate application of solid-state refrigeration and thermometry is in ultrasensitive radiation detection, which is discussed in depth. This review concludes with a summary of pertinent fabrication methods of presented devices.

/pdf/opportunities-for-mesoscopics-in-thermometry-and-10z63yzsh2.pdf

Opportunities for mesoscopics in thermometry and refrigeration: Physics and applications

Recently, it has been shown theoretically that it may be possible to increase the thermoelectric figure of merit ($Z$) of certain materials by preparing them in the form of two-dimensional quantum-well structures. Using $\mathrm{PbTe}/{\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ multiple-quantum-well structures grown by molecular-beam epitaxy, we have performed thermoelectric and other transport measurements as a function of quantum-well thickness and doping. Our results are found to be consistent with theoretical predictions and indicate that an increase in $Z$ over bulk values may be possible through quantum confinement effects using quantum-well structures.

Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit

Experimental investigations have been performed on the thermoelectric properties on the multiple-quantum-well structures of PbTe/Pb/sub 1-x/Eu/sub x/Te grown by molecular beam epitaxy. Our results are found to be consistent with theoretical predictions and indicate that an increase in Z over bulk values may be possible through quantum confinement effects using quantum-well structures.

The electrical conductivity, Seebeck coefficient, and power factor of $n$- and $p$-type PbTe were calculated over the temperature range of 300--700 K and carrier concentration range of $1\ifmmode\times\else\texttimes\fi{}{10}^{17}\ensuremath{-}4\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$ by using the Boltzmann transport equation and relaxation time approximation and were compared to measurement results. A three-band model for PbTe was used and included scattering from screened polar optical phonons as well as the deformation potential of acoustic and optical phonons. Nonparabolicity and anisotropy of the principal conduction and valence bands at the $L$ point in the Brillouin zone were taken into account, and the calculated transport properties were in excellent agreement with measurement results over the full range of carrier concentrations and temperatures. The Lorenz number was also calculated and seen to vary from 0.54 to 0.89 of ${L}_{0}$ $(2.44\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{W}\text{ }\ensuremath{\Omega}\text{ }{\text{K}}^{\ensuremath{-}2})$ depending on the carrier concentration and temperature. The measured in-plane transport properties of epitaxially grown $n$- and $p$-type PbTe/PbSe nanodot superlattice (NDSL) samples with $\ensuremath{\sim}13%$ PbSe by volume fraction are also presented and discussed in comparison to the baseline homogeneous PbTe. The presence of the PbSe nanodots in a PbTe matrix was observed to reduce the carrier mobility by $\ensuremath{\sim}25%--35%$ while having no effect on the Seebeck coefficient, thus resulting in a reduced power factor for PbTe/PbSe NDSLs compared to PbTe. The properties of the NDSL samples were also observed to have the same temperature dependence as PbTe.

Carrier concentration and temperature dependence of the electronic transport properties of epitaxial PbTe and PbTe/PbSe nanodot superlattices

: Electrical power densities of up to 33 W/sq cm and up to 12 W/sq cm were obtained for n-type and p-type PbTeSe-based stand-alone thermoelectric devices, respectively, at modest temperature gradients of 200 deg C (Tcold=25 deg C) These large power densities were enabled by greatly improving electrical contact resistivities in the thermoelectric devices Robust electrical contacts with contact resistivities as low as 39x10(exp 6) ohm-sq cm and 40x10(exp 6) ohm-sq cm for n- and p-type telluride-based-materials, respectively, were developed by investigating several metallization schemes and contact layer doping/alloy combinations, in conjunction with a novel contact application process This process exposes heated semiconductor surfaces to an atomic hydrogen flux under high vacuum for surface cleaning (oxide and carbon removal), followed immediately by an in-situ electron-beam evaporation of the metal layers

T. C. Harman

Papers

Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit

Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit

Carrier concentration and temperature dependence of the electronic transport properties of epitaxial PbTe and PbTe/PbSe nanodot superlattices

High-Output-Power Densities from MBE-grown n- and p-Type PbTeSe-based Thermoelectrics via Improved Contact Metallization