Derivatives of synthetic tetrahedrite, Cu12Sb4S13, are receiving increasing attention in the thermoelectric community due to their exploitation of plentiful, relatively nontoxic elements, combined with a thermoelectric performance that rivals that of PbTe-based compounds. However, traditional synthetic methods require weeks of annealing at high temperatures (450-600 °C) and periodic regrinding of the samples. Here we report a solvothermal method to produce tetrahedrite that requires only 1 day of heating at a relatively low temperature (155 °C). This allows preparation of multiple samples at once and is potentially scalable. The solvothermal material described herein demonstrates a dimensionless figure of merit (ZT) vs temperature curve comparable to that of solid-state tetrahedrite, achieving the same ZT of 0.63 at ∼720 K. As with the materials from solid-state synthesis, products from this rapid solvothermal synthesis can be improved by mixing in a 1:1 molar ratio with the Zn-containing natural mineral, tennantite, to achieve 0.9 mol equiv of Zn. This leads to a 36% increase in ZT at ∼720 K for solvothermal tetrahedrite, to 0.85.

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Europium chalcogenide alloys, EuS x Se1–x , have been synthesized both in the solid-state and as colloidal nanoparticles; the composition, structure, magnetism, and optical band gaps have been characterized. The goal was to observe the consequences of selenium concentration on the electronic structure as evidenced by the optical and magnetic properties and whether these properties are maintained in the nanomaterials. Both solid-state and nanoparticle alloys obey Vegard’s law with a systematic change in cell constant as confirmed by the powder X-ray diffraction. The bulk materials form homogeneous alloys that exhibit a linear change in both magnetic and optical properties as a function of composition. A synthetic method to prepare nanoalloys with a wide range of S:Se ratio has been developed. The nanoalloys are homogeneous, and EDS mapping of single nanoparticles indicates relatively uniform S and Se composition across the nanocrystals. The magnetic properties of the nanoparticles appear to parallel those in the solid-state. Although the composition is an effective tool to tune to the optical band gap in the solid-state alloys with a linear change in E g with composition, the nanoparticle optical band gaps appeared to be shifted, which we attribute to the presence of an amorphous selenium phase. The study of the properties of colloidal alloys highlights the importance of the mechanism of nanoparticle formation to control composition and purity.

Design of thermoelectric materials focuses on the optimization of several unfavorably coupled factors: electrical conductivity, Seebeck coefficient, and thermal conductivity. Recent work in thermoelectrics has focused on decreasing lattice thermal conductivity by nanostructuring thermoelectric materials, while recent work in photovoltaics has demonstrated ligand stripping as a means to increased electron mobility in thin films of nanoparticles. In the present work, these two features are combined. A multi-gram scale synthesis of dispersible, lead telluride nanocrystals (25-50 nm) is developed using hot-injection methods in common organic solvents.These nanocrystals (NCs) are ligand-stripped with sulfide (PbTe-S) or iodide (PbTe-I) sources to result in p-type or n-type materials with large Seebeck coefficients at room temperature of 520 µV·K -1 or -540 µV·K -1 , respectively. Sequential stripping with sulfide then iodide (PbTe-SI) resulted in a small Seebeck due to counter-doping. PbTe-S and PbTe-SI are found to generate nanostructured composites by growth of lead sulfide nanocrystals (~50-60 nm) in-situ upon annealing. However, the electrical conductivities are low (<1 S·cm -1 ) due to excess doping during the ligand stripping. Intentional formation of a nanocomposite (PbTe-PbS) is achieved by combining PbTe NCs with 4-6 nm diameter lead sulfide particles via mixing by incipient wetness with a target of 8 mol% lead sulfide. The resulting nanocomposite is n-type with a Seebeck coefficient of -160 µV·K -1 and an electrical conductivity of 42 S·cm -1 at room temperature The lattice thermal conductivities of all materials at room temperature are substantially lower than those of bulk lead telluride (2.0 W m -1 ·K -1 ). However, ZT's are low for all samples (max = 0.03 for PbTe-PbS), attributed primarily to the low electrical conductivities.This work underscores the importance of developing new methods for augmenting electrical conductivity if nanoparticle assemblies are to be practically employed in thermoelectrics.

Giant band-splitting observed in EuS and EuSe nanocrystals by magnetic circular dichroism.