A personal review is presented. We review the renaissance in thermoelectric materials research that started in 1993 with the introduction of the nanostructure concept as a potential method to both increase the power factor and decrease the thermal conductivity and to even do both at the same time. The earliest work was limited to model systems for the demonstration of proof of principle. More recently the focus has evolved into demonstration of embedding the phenomena into bulk samples based on composites and superlattices. We here review this evolution of the nanothermoelectricity field. The resulting current activity is attracting many new researchers, industrial interest and the emergence of new ideas. We now look to the further development of these new ideas, and to the introduction of more new ideas and new approaches, as the field is now approaching the stage of commercial relevance.

The focus of this paper is the characterization of novel thermophotovoltaic (TPV) cell designs which employ a monovalent barrier layer in the p-n junction. The use of a barrier layer enables these cells to operate at longer wavelengths, higher efficiencies, and higher operating temperatures. Initial designs have been made using gallium antimonide (GaSb), which is one of the more common TPV materials. Simulations were performed using Sentaurus by Synopsys to determine barrier materials as well as to optimize the cell. The p-B-n cell was then compared to a simple p-n junction. The simulations show that a p-B-n cell outperforms a typical p-n junction. Additionally, we expect to see increased performance differentials from this device structure when moving to longer wavelength devices.

Nano-alloyed p-type Sb2Te3 and n-type Bi2Te3 thin films were grown on SiO2/Si and BaF2 substrates by molecular beam epitaxy (MBE) in two steps: (i) Repeated deposition of five-layer stacks with sequence Te-X-Te-X-Te (X = Sb or Bi) with elemental layer thicknesses of 0.2 nm on substrates at room temperature, (ii) annealing at 250 °C for two hours at which phase formation of Sb2Te3 or Bi2Te3 occurred. The room temperature MBE deposition method reduces surface roughness, allows the use of non lattice-matched substrates, and yields a more accurate and easier control of the Te content compared to Bi2Te3 thin films, which were epitaxially grown on BaF2 substrates at 290 °C. X-ray diffraction revealed that the thin films were single phase, poly-crystalline, and textured. The films showed grain sizes of 500 nm for Sb2Te3 and 250 nm for Bi2Te3, analyzed by transmission electron microscopy (TEM). The in-plane transport properties (thermopower S, electrical conductivity σ, charge carrier density n, charge carrier mobility μ, power factor S2σ) were measured at room temperature. The nano-alloyed Sb2Te3 thin film revealed a remarkably high power factor of 29 μW cm-1 K-2 similar to epitaxially grown Bi2Te3 thin films and Sb2Te3 single crystalline bulk materials. This large power factor can be attributed to a high charge carrier mobility of 402 cm2 V−1 s-1 similar to high-ZT Bi2Te3/Sb2Te3 superlattices. However, for the nano-alloyed Bi2Te3 thin film a low power factor of 8 μW cm−1 K-2 and a low charge carrier mobility of 80 cm2 V−1 s−1 were found. Detailed microstructure and phase analyses were carried out by energy-filtered TEM in cross-sections. Quantitative chemical analysis by energy-dispersive x−ray spectroscopy (EDS) was also applied. In Bi2Te3 thin films, few nanometer thick Bi-rich blocking layers at grain boundaries and Te fluctuations by 1.3 at.% within the grains were observed. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing.

As many reports show that the superlattice structure could greatly enhance the figure of merit ZT value for the thermoelectric application. We studied the thermal and electrical properties of the InGaN/GaN superlattice structure, and further analyze the thermoelectric features with different superlattice period, doping concentration, and operation temperature. The elastic continuum model and Callaway model have been applied to calculate the phonon dispersion relation and the thermal conductivity, respectively. The electrical properties are obtained by the Boltzmann transport equation with the relaxation time approximation. Simulation results indicate that both the reduced thermal conductivity and enhanced power factor would have the contribution to the enhancement of the figure of merit ZT.

Energy-filtered transmission electron microscopy (EFTEM) yields new possibilities for the investigation of Bi2Te3 based nanomaterials. Combined low-loss electron energy-loss spectroscopy (EELS) and energy-dispersive x-ray microanalysis (EDS) and energy-filtered TEM were applied on a Zeiss 912Ω TEM to investigate nanowires, thin films, and bulk materials. Multilayered Bi-Sb-Te nanowires with a diameter of 65 nm and a period of 200 nm and stoichiometric Bi2Te3 nanowires were grown by potential-pulsed electrochemical deposition. Tellurium elemental maps of the multilayered nanowires were obtained by two-window edge-jump ratio images (EJI). EDS chemical analysis showed that small Te fluctuations of 3 at.% yielded significant contrast in EJI. Energy-filtered TEM applied on nano-alloyed Bi2Te3 thin films grown by molecular beam epitaxy (MBE) revealed 10-20 nm thick Bi-rich blocking layers at grain boundaries. Plasmon spectroscopy by EELS was applied on Bi2(Te0.91Se0.09)3 bulk and yielded a plasmon energy of 16.9 eV. Finally, plasmon dispersion was measured for Bi2(Te0.91Se0.09)3 bulk by angle-resolved EELS, which yields a fingerprint of the anisotropy and the dimensionality of the electronic structure of the materials.

A recently discovered synthetic route to new kinetically stable [(MSe)y]m[TSe2]n layered intergrowths has been applied to prepare several different compositions (M = Pb or Sn, T = Ta, Nb, Mo, or W) with m = n = 1, in thin film form. Scanning transmission electron microscopy and synchrotron X-ray diffraction show the nanostructure of these materials is characterized by a combination of in-plane component crystallinity with misregistration and rotational mis-orientation between adjacent layers. Extremely low cross-plane thermal conductivity as low as 0.1 W m-1 K-1 are attributed to the turbostratic nanostructure. By appropriate choice of M and T, we demonstrate that a range of electrical transport properties are possible, from metallic to semiconducting. Annealing (PbSe)0.99WSe2 and (PbSe)1.00MoSe2 specimens in a controlled atmosphere of PbSe or WSe2 is observed to systematically influence carrier properties, and is interpreted in terms of reduction of the concentration of electrically active defects. Considering these observations and the large composition and structural space that can be explored in such [(MSe)y]m[TSe2]n intergrowths, these materials are of interest for further investigation as potential thermoelectric materials.

We investigate the impact of various dopants (Na, Ag, Cd, Zn, Al, Ga, In, Tl, Ge, and Sn) on the electronic structure of Mg2Si by first principles calculations using a hybrid functional that does not need a band gap correction. We find that for Na and Ge in Mg2Si, the impurity-induced states do not affect the density of states at both edges of the valence band and the conduction band. Ag- and Sn affect slightly the density of states at the valence band edge, while Cd and Zn affect slightly the density of state at the conduction band edge. Al and In could modify significantly the density of states at the conduction band edge. Ga introduces states just at the bottom of the conduction band. Tl introduces states in the band gap. This study provides useful information on optimizing the thermoelectric efficiency of Mg2Si.

Indium and cerium added cobalt-antimony based skutterudites with different filling fractions were synthesized using different annealing synthesis parameters. Phase homogeneity and microstructure of the resulting as-cast material were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The skutterudite material was further compacted using a current-assisted short-term sintering device. Temperature dependent measurements of the Seebeck coefficient, electrical and thermal conductivity were carried out on the compacted specimens in the temperature range of 350 K-700 K. Results indicate significant differences in the transport properties between the slowly cooled and quenched as‑cast materials and also with different filling fractions. Based on the measured transport properties the dimensionless figure of merit ZT was calculated for different filling fractions of indium and cerium. Among these compositions a ZTmax of 1.1 at 700 K was obtained.

GaN-based dilute magnetic semiconductors (DMS) have recently been investigated for use in spintronic devices. In particular, Gd-doped GaN has shown very promising room temperature ferromagnetic behavior and potential for use in spintronics applications. III-Nitride materials have recently had their thermoelectric properties investigated; however this work has not been extended to Nitride-based DMS. Understanding the spin-calorimetric characteristics of GaN-based DMS is important to the successful development of low-power spintronic devices. In this paper the Seebeck and spin-Seebeck effect in MOCVD grown Gd-doped GaN (Gd: GaN) are investigated.

In this work, thin films of Bi2Te3 and Sb2Te3 were synthesized by the nanoalloying approach: Nanoscale layers of the elements Element nanoscale layers of Bi, Sb and Te are stoichiometrically deposited on a cold substrate using a MBE setup and subjected to an annealing process in which a solid state reaction yielding Bi2Te3 and Sb2Te3 takes place. Besides the two binary compounds, nanoscale multilayer (ML) stacks of 9 nm Bi2Te3/9 nm Sb2Te3 were created. The electrical transport properties of the binary compounds were determined in dependence of composition. Compound formation was directly observed in temperature-dependent in-situ XRD scans and was found to start at ∼100 °C. The stability of the Bi2Te3/Sb2Te3 ML nanostructure against temperature-driven interdiffusion during annealing was examined by SIMS and TEM for an annealing temperature of 150 and 250 °C, respectively. A comparative TEM study of the as grown and annealed state is presented.

The surrounding ambient introduces a gaseous boundary to many potential nanotechnology applications such as nanoscale thermoelectric devices and low dimensional thermal control devices. Despite the large surface area to volume ratio of nanostructures, a formal study of the surface scattering effects induced by a gaseous boundary has received little attention. In this work, we consider the perturbing effects to the electron cloud or jellium of conducting nanostructures when submitted to a gaseous interface of varying interaction energies. Specifically, we incorporate the novel experimental method of Dynamic Electron Scattering (DES) to measure the Seebeck coefficient of 30 nm thick Au and Cu metal films in He and Ar atmospheres. The gas particle impact energy is varied by changing the flow speed from stationary (non-moving gas field) to high speed flow over the metal films. The scattering effects of each gas are clearly observable through a Seebeck coefficient increase as the gas impact energy increases. We find the high collision density of He to induce a greater increase in thermopower than the much heavier Ar with lower collision density. The perturbed transport properties of the Au and Cu thin films are explained by kinetic surface scattering mechanisms that dominate the scattering landscape of high surface area to volume ratio materials as suggested by comparative measurements on bulk Cu.

Porous crystalline Si nanowires (PC-SiNW) represent an attractive solution for enhancing the thermoelectric efficiency (ZT) of SiNWs by reducing the lattice thermal conductance (κl). A modified valence force field (MVFF) phonon model along with Landauer’s approach is used to analyze the ballistic κl in PC-SiNWs. A systematic study focusing on the influence of pore size, density, and distribution on the ballistic κl of PC-SiNWs is presented. The model predicts a maximum reduction of ∼19%, ∼23% and ∼30% for 1, 2 and 3 pores, respectively with a constant removal of ∼12% of the atoms in all the cases. The model also predicts a higher reduction of the ballistic κl as the pore separation increases, in the case of 2, 3 and 4 pores, for the same percentage of atoms removed (∼12%) in all the cases. Thus, the presence of a high number of small, well-separated pores suppress κl strongly. This reduction in ballistic κl, in the coherent limit, is attributed to the reduction of the total number of phonon modes and smaller participation of phonon modes (in κl) with increasing number of pores.

Two-body interatomic potentials in the Morse potential form have been developed for bismuth telluride, and the potentials are used in molecular dynamics (MD) simulations to predict the thermal conductivity of Bi2Te3 bulk, nanowires and few-quintuple thin films. The density functional theory with local density approximations is first used to calculate the total energies for many artificially distorted Bi2Te3 configurations to produce the energy surface. Then by fitting to this energy surface and other experimental data, the Morse potential form is parameterized. Molecular dynamics simulations are then performed to predict the thermal conductivity of bulk Bi2Te3 at different temperatures, and the results agree with experimental data well. We also predicted the thermal conductivity of Bi2Te3 nanowires with diameter ranging from 3 to 30 nm with both smooth (SMNW) and rough (STNW) surfaces. It is found that when the nanowire diameter decreases to the molecular scale (below 10 nm, or the so called "quantum wire"), the thermal conductivity shows significant reduction as compared to bulk value. We find the dimensional crossover behavior of thermal transport in few quintuple layer (QL) thin films at room temperature, and we attribute it to the interplay between phonon Umklapp scattering and boundary scattering. Also, nanoporous films show significantly reduced thermal conductivity compared to perfect thin films, indicating that they can be very promising thermoelectric materials.

Nanofluids, consisting of nanometric particles suspended in a base fluid, have become a new alternative for improving heat management technology. Silver, which is known to exhibit pretty high electrical and thermal conductivity among metals, has been selected for this research. At present, we are focused on the study of the size- and shape-controlled synthesis conditions of silver nanocrystals in polyol media. Control of crystal size and shape at the nanoscale were achieved by suitable selection of the synthesis conditions and the presence of habit-controlling agents like chloride ions. Silver nanostructures (faceted crystals, wires, rods) were remarkable monodisperse in size and their dimension could be controlled in the 30-50nm range (particles) and 24-127nm in thickness for rods or wires.

With the event of nanotechnology, the field of thermoelectric (TE) materials has been re-invigorated with many recent advances towards materials with high thermoelectric efficiency (dimensionless figure of merit, ZT). The realization of such materials opens up new avenues to the creation of devices that can be used in freon-less refrigeration, micro-electronic cooling, or for harnessing lost heat energy from sources such as car engines. In our own research work, we have successfully devised a synthetic technique towards nanoparticles composed of bismuth, antimony, and tellurium that has proven highly versatile in tuning both the composition and shape/structure of the resulting nanoparticles. The ability to control the nanoparticle composition and shape/structure are highly important as these are critical parameters that dictate the resulting devices TE activity. In a modified polyol synthetic technique, it was found that many complex composition, shape, and structure combinations could be obtained for the nanoparticles including Bi-Sb nanodiscs with controllable size, a heterostructure composed of Sb2Te3 nanodiscs deposited on Te nanowires, or small particles deposited on a (Bi0.5Sb0.5)2Te3 wire, just to name a few. By simply changing the capping ligands used in the synthesis, the nanoparticles resulting composition, morphology and structure could be changed, leading to a straightforward route towards TE nanoparticles with interesting properties. This presentation focuses on our recent study of the synthesis of bismuth, antimony, and tellurium composite nanoparticles with applications in thermoelectric materials in terms of understanding the underlying mechanisms of the synthetic technique, and characterization of the resulting nanomaterial properties.

We present a theoretical investigation of the thermoelectric power factor enhancement in metal/semiconductor nanocomposites by the energy dependent electron scattering from ionized nanoparticles. The metal nanoparticles embedded in semiconductors can be ionized to donate electrons to the matrix, which will result in a Coulomb potential tail around the nanoparticles. Here we show the significant effect of slowly varying potential tails on thermoelectric properties of the nanocomposites. The Coulomb potential is different from that of the conventional ionized impurities due to the finite size of the ionized particles, and the fact that the nanoparticles can give multiple electrons to the matrix. Detailed calculations for scattering rates and thermoelectric coefficients are presented for ErAs semi-metallic nanoparticles in InGaAs semiconductors. The partial wave method is used to consider the exact potential profile around nanoparticles and Boltzmann transport equation is used to calculate the transport coefficients. We find that an increase by 15~30% in power factor can be achieved over a wide temperature range in these material systems in addition to the thermal conductivity reduction to further enhance ZT.

Over the past few years, thermoelectric (TE) materials have been receiving an increasing amount of attention owing to their promising potential for energy conversion and thermal management applications. Thermal characterisation techniques offer a powerful tool in investigating and optimizing the TE device performance. In addition, they can provide a better understanding of the underlying fundamental principles such as Peltier effects at the interfaces of the active medium. In this paper, we present the design and thermal characterisation of integrated highspeed microcoolers based on SiGe superlattices. The electrode metalisation is laid out as a coplanar waveguide, enabling to supply electrical pulses with short rise times to the coolers. We employ a variety of CCD-based transient thermoreflectance imaging methods to perform an extensive dynamic thermal analysis. These techniques provide 2-D temperature maps of the chip surface with ∼100ns temporal and submicron spatial resolution without the need to scan the sample. Net cooling in the 2 degree range is observed, with response times well below 1μs. This is almost two orders of magnitude faster compared to the best in the literature. The obtained images also confirm the previous observations that the Peltier cooling term responds faster than the Joule heating term, in agreement with their expected locality and associated thermal mass. This provides potential to study ultrafast electron-phonon interactions during Peltier effects.

The carrier concentration and electronic transport properties in Bi2-xSbxTe3 alloy can be tuned by varying the Bi to Sb ratio, for high thermoelectric figure of merit. The concentration of intrinsic antisite defects in these alloys is also known to change with Bi to Sb ratio. Here we report the thermoelectric figure of merit of Sn doped Bi0.5Sb1.5Te3 alloy. Different atomic percentages of Sn was substituted at Bi/Sb site in Bi0.5Sb1.5Te3 alloy, synthesized by planetary ball milling. The electrical conductivity decreases with increasing Sn doping but for higher Sn content the electrical conductivity increases compared to undoped alloy. The Seebeck coefficient changes in accordance to electrical conductivity, resulting in small decrease in power factor for highest Sn doping. The lattice thermal conductivity shows a systematic decrease, with increasing Sn concentration resulting in a significant thermal conductivity reduction. Hence an increase in thermoelectric figure of merit could be achieved for the highest Sn (3at%) doping in Bi0.5Sb1.5Te3 alloy as compared to undoped alloy.

Nanostructuring has opened new ways to increase the thermoelectric performance of a host of materials, mainly by decreasing their thermal conductivity κ while preserving the Seebeck coefficient S and electrical conductivity σ. The thermoelectric properties of degenerated polycrystalline silicon films with nanocavities (NCs) have been studied as a function of annealing temperature upon isochronous annealings in argon carried out every 50°C in the range 500 – 1000°C which were used to modify the shape of the NCs. We found that presence of the NCs had no negative effect on the electronic properties of the system. The measured values of S and σ were close to those previously reported for the blank polycrystalline silicon films with the same doping level. The thermal conductivity was also found to be close to the value measured on the blank sample, about half of the reported value in polycrystals. This led to a power factor of 15.2 mWm-1K-2 and a figure of merit of 0.18 at 300 K.

In the past two decades, there has been growing interests in the design and improvement of thermoelectric (TE) materials and devices largely due to their potential use in technologies such as: 1) the conversion of waste heat to electricity, 2) solid-state refrigeration and heating, 3) biomedical batteries, and 4) power sources for both ground and space-based electronics.1 Recent research has suggested that by using nanotechnology (i.e. nanostructuring / nanoengineering) large advances can be gained in controlling interfaces to hinder thermal transport while allowing electrical movement. Thin film structuring of thermoelectric materials potentially offers several advantages over bulk thermoelectric materials especially for cooling applications. Furthermore, others have advocated that by making thermoelectric materials very small, one can achieve an enhanced ZT (the thermoelectric figure of merit) due to quantum confinement effects.2-5 The structure and physical properties of doped fullerene materials were investigated for use as electrically conducting phonon blocking layers. The synthesis and thermal properties of ZnxC60 thin films are reported. Preliminary results have shown the formation of amorphous fullerides structures with thermal conductivities as low as 0.13 Wm-1K-1. Physical and structural measurements (e.g. Electron Microscopy, Electron Diffraction, and Raman Spectroscopy) will be reported detailing the unique structure-property relationships in these materials.