Recent resurgence in thermoelectric (TE) materials research has led to a significant improvement of TE performance and development of some new TE materials designs and concepts. This review provides a summary of some effective techniques for improving TE performance through multi-level microstructure control. A proven approach to elevate figure of merit is via formation of nanocomposites, in which nanophases are dispersed at grain boundaries or within grain. Acting as energy filter or scattering center, nanophases contribute to the increase of Seebeck coefficient and the reduction of thermal conductivity without much degradation of electrical conductivity. For anisotropic TE materials, textured microstructure favors to enhance TE performance along the certain direction. In addition, TE thin film assembled from one-dimensional nanotructured materials is presented as a promising TE film.

Bi2Te3 based alloys have been known as one of the best room-temperature thermoelectric (TE) materials for decades. However, the thermoelectric performance of Bi2Te3 based alloys is sensitive to the inherent microstructure of the material: the pulverized system often exhibits an inferior figure of merit than that of the aligned ingot, due primarily to the inter-grain boundary scattering. So an important question may arise as to whether one can somehow fabricate a beneficial inter-grain boundary. In order to address this question, p-type Bi2Te3 is employed as a test-system in present work. Pulverized Bi0.4Sb1.6Te3 (p-type) powders with selected sizes were put into the autoclave and then hydrothermally treated, where the solution of various alkali metal (Li, Na, K, Rb, Cs) compounds were being used as reaction medium. After the treatment, the as-processed powders were removed, washed and dried, followed by hot pressing into pellet for further TE property measurements. The TE properties were found to be significantly improved as compared to the untreated reference sample. Extensive characterizations including X-ray/electron diffraction, TGA analysis, Raman / Fourier Transform Infrared Spectroscopy, electron microscopy, Rutherford back-scattering and Energy dispersive X-ray analysis were performed on the treated sample. The results revealed that a surface layer (from 10 nm to up to micron in thickness) exhibiting a combined crystalline/amorphous feature was formed on the original bare particles. This layer is believed to be the key factor in the improvement of TE properties of the p-type Bi0.4Sb1.6Te3 material. The synthesis technique will be discussed in detail while some results on the microscopic analysis and TE properties will be presented briefly.

Nanocomposites have been produced by incorporating thermoelectric nanoparticles into a matrix of bulk Bi₂Te₃ material via a hot pressing process. These nanocomposites have been examined by SEM and X-ray powder diffraction. The effects of the incorporation of a variety of nanoparticles upon the resulting thermoelectric properties such as the thermopower, electrical resistivity, thermal conductivity, etc., have been studied in these composites at room temperature and below. The details of the synthesis along with results of the microscopic analysis and thermoelectric properties will be discussed. The potential for improving the figure of merit within the Bi₂Te₃ system by this technique is considered.

Lead chalcogenide dimensional nanocomposites were prepared by densifying nanocrystals synthesized employing an alkaline aqueous solution-phase reaction. The nanocrystal synthesis procedure resulted in high product yields of over 2 g per batch. These nanocrystals were then subjected to Spark Plasma Sintering (SPS) for densification. Transport properties were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements, indicating a strong sensitivity to stoichiometry, surface oxygen adsorption, and porosity. The results for these lead chalcogenide nanocomposites were compared to bulk polycrystalline lead chalcogenides with similar carrier concentrations.

Semiconducting silicides (e.g. CrSi2, β-FeSi2, MnSi1.8, Mg2Si) are promising thermoelectric materials. In addition to their respectable thermoelectric figure-of-merit (ZT up to 0.8), silicides have the advantages of low cost, excellent thermal stability and mechanical strength, and outstanding oxidation resistance, making them suitable for high temperature applications. We have developed general synthetic approaches to high quality single crystal nanowires of silicides to investigate their potential enhancement of thermoelectric properties due to the reduced nanoscale dimension and to explore their applications in thermoelectrics. We will specifically focus on the synthesis and structural characterization of nanowires of chromium disilicide (CrSi2) prepared via a chemical vapor transport (CVT) method. This method complements the more versatile chemical vapor deposition (CVD) of metal carbonyl-silyl single source organometallic precursors we have developed. Structural characterization using electron microscopy, powder X-ray diffraction, and energy dispersive spectroscopy shows that these nanowires are hexagonal CrSi2 single-crystal structures along the <001> growth axis, with diameters ranging from 20−300 nm and length up to 100 μm. The Seebeck coefficient, electrical conductivity, and thermal conductivity of individual CrSi2 nanowires were characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. The obtained thermoelectric figure of merit of the nanowires was close to 0.1 and comparable to the bulk values. This combined Seebeck coefficient and electrical conductivity measurements also provide an effective approach to probing the Fermi level, carrier concentration and mobility in nanowires. We will also discuss our recent results of silicide nanowires of complex Novotny chimney ladder phases and our progress in using individual nanostructures combined well-defined structural characterization to conclusively investigate the complex thermoelectric behaviors of these silicide materials.

Grain boundary scattering provides an avenue by which to effectively lower the thermal conductivity in pulverized thermoelectric materials, however, the “bare” inter-grain boundary often simultaneously degrades the electrical conductivity and thermopower. Thus a controlled inter-grain boundary would be very beneficial in order to improve the thermoelectric performance of the system. But the question is how to engineer such a boundary.In this talk we present a proof-of-principle investigation on the pulverized p-Bi2Te3 (Bi0.4Sb1.6Te3) system by means of electrical resistivity, thermopower, thermal conductivity, specific heat, Hall coefficient, Raman/Infrared spectroscopy, X-ray/electron diffraction, electron microscopy and compositional analysis. Utilizing the alkaline hydrothermal treatment and nano-coating techniques recently developed at Clemson, we fabricate a thin layer on the surface of fine p-Bi2Te3 grains. The interface layer, ~ few tens nm thick and formed right at the inter-grain boundary in a hotpress-densified sample, enabled us to “decouple” and individually optimize the various thermoelectric properties. As a result, the hydrothermally treated and pulverized sample possessed ZT values comparable to those of a commercial ingot but with a better so called “compatibility factor” as well as better mechanical properties. In view of the concept of material design, this process helps achieve a new level of control as a tuning parameter with which to optimize the figure of merit ZT and compatibility factor. In principle, this strategy can be readily applied to other existing thermoelectric materials. This presentation will focus on the resulting thermoelectric properties and microscopic analysis and the synthesis techniques will be discussed in detail elsewhere.

Several theoretical studies suggested that Bi-based and III-V nanowire structures may possess enhanced thermoelectric figure of merit, ZT. It was found in our earlier measurements employing a suspended microdevice that the thermoelectric properties of individual bismuth telluride, InSb, and CrSi2 nanowires are largely influenced by the crystalline quality, chemical composition and surface roughness of the nanowires. In addition, a major problem for thermoelectric measurements of individual nanowires especially bismuth telluride nanowires is the presence of a stable native oxide that prohibits electrical contact to be made directly to the nanowires. Focused electron or ion beam induced deposition of Pt on the nanowire was used in our previous work to make electrical contact to the nanowire. Care was needed to prevent the nanowire from being contaminated by ions present during the Pt deposition process. Furthermore, it has been suggested that the presence of the surface oxide or surface contamination can result in high surface charge state densities that can dominate the intrinsic transport properties of nanowire and thin film thermoelectric materials. In fact, it was found that annealing in a hydrogen environment can significantly enhance the thermoelectric properties of bismuth telluride films.In this work, we investigate the effect of in situ hydrogen annealing on the thermoelectric properties of individual bismuth telluride nanowires. The thermoelectric measurement method is based on an improved design of our microfabricated suspended device. The current measurement does not require Pt deposition on the nanowire for making electrical contact. Instead, it was found that ohmic contact between the nanowire and the underlying pre-patterned Pt electrodes on the suspended devices can be made by annealing the nanowire at about 480 K while hydrogen is flown into the evacuated sample space of a cryostat. Our measurement results show that that the thermal and electrical conductances and ZT of the nanowires are increased upon hydrogen annealing. In addition, both the contact thermal and electrical resistances are eliminated from the measured thermal conductivity, electrical conductivity, or Seebeck coefficient by using a unique four-probe thermoelectric measurement method. Transmission Electron Microscopy (TEM) and Energy Dispersion Spectroscopy (EDS) measurements are performed on the same nanowires assembled on the suspended device so as to correlate the structural characteristics to the measured thermoelectric properties of the nanowires. High resolution TEM results reveal highly crystalline structure of the hydrogen-annealed bismuth telluride nanowires.

Indium antimonide (InSb) is a narrow bandgap semiconductor with one of the smallest effective mass values among semiconductors and very high mobility. It is commonly used in infrared detectors and magnetic field sensors. The thermoelectric properties of bulk InSb crystals have been characterized by Yamaguchi et al. in the 10-723 K temperature range, with highest figure of merit (ZT) of 0.6 found at 673 K [1]. A theoretical calculation by Mingo has predicted that quantum confinement of electrons and diffuse phonon-surface scattering in InSb nanowires can result in enhanced ZT compared to the bulk value [2, 3]. Ye et al. has developed a vapor-liquid-solid (VLS) method to synthesize single crystal InSb nanowires. In an previous measurement, we observed that the obtained VLS InSb nanowires possess higher electrical conductivity and lower Seebeck coefficient than bulk crystals, most probably due to tellurium or oxygen impurities in the nanowire [4]. In this work the impurity concentration in the VLS InSb nanowires is minimized by using pure InSb wafers as source materials for VLS growth in high vacuum environment. The crystal structure and chemical composition of the obtained nanowires are analyzed using High Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive X-ray Spectroscopy (EDAX). Temperature-dependant thermopower and electrical conductance are probed using a nanofabricated device where a top gate and substrate back gate voltage can be used to tune the Fermi level of the system via the field effect. Experiments are conducted to investigate the effect of in situ hydrogen annealing and surface passivation on the thermoelectric properties of the InSb nanowires