The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m-1 K-1 ), a high power factor (11.6 µW cm-1 K-2 ), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe4 tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl+ lone-pair electrons, boosts phonon-phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl+ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

Many strategies have been employed to maintain an "optimal carrier concentration" to achieve a high thermoelectric power factor in n-type PbTe. Here, we show that a high power factor can also be achieved over a broad temperature range without attaining the optimal carrier concentration by alloying n-type PbTe with GaSb. We find that GaSb dissociates into Ga and Sb and dissolves into the PbTe matrix. Ga atoms introduce impurity levels above the valence band of PbTe giving rise to diminished electron-phonon coupling and an increased carrier effective mass leading to a high Seebeck coefficient of -87 mu V K-1 and a high carrier mobility of 810 cm(2) V(-1)s(-1) at 300 K. As a result, a high power factor of 29 mu W cm(-1) K-2 at 323 K is achieved. Phonon band-structure calculations show that Ga is off-centered (0.05 angstrom) from Pb site along the < 111 > direction leading to a reduced lattice thermal conductivity of 0.7 W m(-1) K-1 and a maximum thermoelectric figure of merit of 1.35 at 773 K in PbTe0.997I0.003-2 mol % GaSb. The dissolution of III-V semiconductors in lead telluride is a new finding with broader implications and opens new avenues in controlling thermal and charge transport in advanced thermoelectric materials.

Coarse alpha-Al dendrites and interdendritic networks of lamellar eutectic Si lead to low strength of Al-Si-Mg alloys (e.g. A356). An effective approach involving high-pressure torsion (HPT) processing combined with prior heat treatments is herein proposed to achieve ultra-high strength of A356 alloy. The HPT processing induces the precipitation of nanoscale Si particles, whose number density is strongly related to the heat treatments prior to the HPT. Most importantly, these nanoscale Si particles in turn promote dislocation multiplication and grain refinement, so that the solid solution treated samples with the highest density of nanoscale Si particles after HPT processing, produce the finest microstructures and the highest density of dislocations, thus giving the best yield strength (similar to 440 MPa) and ultimate tensile strength (similar to 560 MPa), with an elongation to failure of 8.8%. As for cast Al-based alloys such as A356 alloy, the number density of nanoscale Si particles provides key information for unravelling the origins of HPT-induced strengthening, and reveals the importance of heat treatments prior to the HPT processing.

PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe 2 (NaPb m SbSe m +2 ) are described and the most promising p‐type PbSe‐based thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPb m SbSe m +2 exhibits unorthodox semiconducting‐like electrical conductivity, despite possessing degenerate carrier densities of ≈10 20 cm −3 . It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe 2 in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe 2 lowers the energy separation between the L‐ and Σ‐valence bands and enhances the power factors under 700 K. The crystallographic disorder of Na + , Pb 2+ , and Sb 3+ moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1–0.55 W m ‐1 K ‐1 between 400 and 873 K without nanostructures. As a consequence, NaPb 10 SbSe 12 achieves maximum ZT ≈1.4 near 900 K when optimally doped. More importantly, NaPb 10 SbSe 12 maintains high ZT across a broad temperature range, giving an estimated record ZT avg of ≈0.64 between 400 and 873 K, a significant improvement over existing p‐type PbSe thermoelectrics.

We demonstrate filled CoSb3 skutterudite materials with excellent thermoelectric (TE) performance that results in one of the highest reported single stage module efficiency. The improvement in TE material performance was obtained by creating isotype n/n "bulk heterojunction" structure through assembly of novel skutterudite nanocrystals with different Yb-doping content. Combination of significant increase in carrier transport through heterojunction structure and reduction in long-range acoustic phonon transmission by two-phase mixture resulted in enhanced power factor and reduced lattice thermal conductivity. As a result, the figure-of-merit (zT) of heterojunction TE material is improved by more than 35% compared with pristine single homogeneous material. Using these improved TE materials, a high module conversion efficiency of similar to 9.15% was obtained when operating between 650 degrees C and 50 degrees C. This is one of the highest conversion efficiency among the practically measured single stage modules.

Metal chalcogenides have attracted great attention because of their broad applications. It has been well acknowledged that microstructure can alter the intrinsic properties and performance of metal chalcogenides. The structure-property-performance relationships can be investigated at atomic scale with scanning transmission and transmission electron microscopy (STEM and TEM). Nevertheless, careful specimen preparation is paramount for accurate analyses and interpretations. In this work, we compare the effects of a variety of well-established TEM specimen preparation methods on the observed microstructure of an ingot stoichiometric lead telluride (PbTe). Most importantly, from aberration corrected STEM and first principles calculations, we discovered that argon (Ar) ion milling can lead to surface irradiation damage in the form of Pb vacancy clusters and self-interstitial atom (SIA) clusters. The SIA clusters appear as orthogonal nanoscale features when characterized along the crystal orientation of the rock salt structured PbTe. This obfuscates the interpretation of the intrinsic microstructure of metal chalcogenides, especially lead chalcogenides. We demonstrate that with sufficiently low energy (300 eV) Ar ion cleaning or appropriate high-temperature annealing, the surface damage layer can be properly cleaned and the orthogonal nanoscale features are significantly reduced. This reveals the materials' intrinsic structure and can be used as the standard protocol for future TEM specimen preparation of lead-based chalcogenide materials.

The coarse dendritic microstructure with non-uniform distribution of eutectic Si particles leads to poor ductility of A356 alloy, thus challenging the enhancement of mechanical properties through deformation at room temperature. Here, a new processing protocol was proposed by combining near-rapid solidification with two-step thermo-mechanical treatment (TMT). To significantly improve the ductility of the alloy, near-rapid solidification, in combination with first-step TMT including equal channel angular pressing (ECAP) and intermediate heat treatment, was applied to refine eutectic Si particles and make them evenly distributed in the matrix. On this basis, a high ultimate tensile strength similar to 483 MPa, with relatively good elongation to failure similar to 8.1% can be achieved by the subsequent ageing treatment and composite deformation (ECAP + cryorolling) of second-step TMT. The excellent combination of strength and ductility was mainly attributed to the synergistic effect of high density of dislocations, refined nanoscale Si particles and uniform distributed eutectic Si particles arising from this fabrication processing. This work provides a new idea for the preparation of high strength Al-Si cast aluminum alloy, so that the application of Al-Si cast alloy is promisingly expected to expand.

Understanding the nature of phonon transport in solids and the underlying mechanism linking lattice dynamics and thermal conductivity is important in many fields, including the development of efficient thermoelectric materials where a low lattice thermal conductivity is required. Herein, we choose the pair of synthetic chalcopyrite CuFeS2 and talnakhite Cu17.6Fe17.6S32 compounds, which possess the same elements and very similar crystal structures but very different phonon transport, as contrasting examples to study the influence of lattice dynamics, and chemical bonding on the thermal transport properties. Chemically, talnakhite derives from chalcopyrite by inserting extra Cu and Fe atoms in the chalcopyrite lattice. The CuFeS2 compound has a lattice thermal conductivity of 2.37 Wm-1K-1 at 625 K, while Cu17.6Fe17.6S32 features Cu/Fe disorder and possesses an extremely low lattice thermal conductivity of merely 0.6 Wm-1K-1 at 625 K, approaching the amorphous limit min. Low temperature heat capacity measurements and phonon calculations point to a large anharmonicity and low Debye temperature in Cu17.6Fe17.6S32, originating from weaker chemical bonds. Moreover, Mössbauer spectroscopy suggests that the state of Fe atoms in Cu17.6Fe17.6S32 is in partially disordered structure which induces the enhanced alloy scattering. All the above peculiar features which are absent in CuFeS2, are contributing to an extremely low lattice thermal conductivity of the Cu17.6Fe17.6S32 compound.

Here we report that CdTe alloying and Sb doping increase the density-of-states effective mass and introduce endotaxial nanostructuring in n-type PbTe, resulting in enhanced thermoelectric performance. A prior theoretical prediction for the presence of resonance states in the conduction band of this system, however, could not be confirmed. An amount of 3 mol % CdTe alloying widens the band gap of PbTe by 50%, leading to enhanced carrier effective mass and Seebeck coefficient. This effect is even more pronounced at high temperatures where the solubility of CdTe increases. At 800 K, when the carrier concentration is the same (4 × 1019 cm-3), the Seebeck coefficient of CdTe-alloyed PbTe is -195 μV K-1, 16% higher than that of the Cd-free control sample (-168 μV K-1). Sb doping considerably increases the electron concentration of Pb0.97Cd0.03Te, giving rise to optimized power factors of ∼17 μW cm-1 K-2 at 800 K. More importantly, Sb induces strained endotaxial nanostructures evenly distributed in the matrix. These Sb-rich nanostructures account for the ∼40% reduction in the lattice thermal conductivity over the whole measured temperature range. As a result, a maximum ZT of 1.2 is attained at 750 K in 0.5 mol % Sb-doped Pb0.97Cd0.03Te alloys.

We report on the underlying mechanism that enables the SnTe-AgSbTe2 system to exhibit superior thermoelectric figure of merit (ZT) compared to its parent compound SnTe. We show that AgSbTe2 alloying has a profound impact on the band structure of SnTe by converging the energies of its light and heavy valence bands, leading to significantly enhanced Seebeck coefficients. We have also unraveled a significant connection between alloying and defect stability in this system, wherein the Sn vacancy concentration increases significantly when Ag and Sb are alloyed on the Sn site. The increased Sn vacancy concentration dramatically reduces the lattice thermal conductivity through both lattice softening and phonon-vacancy scattering to similar to 0.4 W m(-1) K-1 at 800 K. Consequently, a ZT value of 1.2 at 800 K for AgSn5SbTe7 can be achieved by doping I on Te sites. This represents a 300% improvement over pristine SnTe, outperforming many reported SnTe-based thermoelectric materials.

Thermoelectric generators can convert heat directly into usable electric power but suffer from low efficiencies and high costs, which have hindered wide-scale applications. Accordingly, an important goal in the field of thermoelectricity is to develop new high performance materials that are composed of more earth-abundant elements. The best systems for midtemperature power generation rely on heavily doped PbTe, but the Te in these materials is scarce in the Earth's crust. PbSe is emerging as a less expensive alternative to PbTe, although it displays inferior performance due to a considerably smaller power factor S2σ, where S is the Seebeck coefficient and σ is electrical conductivity. Here, we present a new p-type PbSe system, Pb0.98Na0.02Se- x%HgSe, which yields a very high power factor of ∼20 μW·cm-1·K-2 at 963 K when x = 2, a 15% improvement over the best performing PbSe- x%MSe materials. The enhancement is attributed to a combination of high carrier mobility and the early onset of band convergence in the Hg-alloyed samples (∼550 K), which results in a significant increase in the Seebeck coefficient. Interestingly, we find that the Hg2+ cations sit at an off-centered position within the PbSe lattice, and we dub the displaced Hg atoms "discordant". DFT calculations indicate that this feature plays a role in lowering thermal conductivity, and we believe that this insight may inspire new design criteria for engineering high performance thermoelectric materials. The high power factor combined with a decrease in thermal conductivity gives a high figure of merit ZT of 1.7 at 970 K, the highest value reported for p-type PbSe to date.

Core@shell architectures provide a rich platform for designing new geometries composed of various functional nanomaterials. Recent work has shown that Au@MoS2 core@shell structures exhibit strong light matter interactions and promising optoelectronic device performance. However, the role of the core on Au@MoS2 growth dynamics is not well understood, leaving the question of if this unusual structure is extendable to other materials systems unanswered. Herein, we present unambiguous evidence of MoS2 encapsulation of new crystalline and even noncrystalline core materials, including Ag and silica. High-resolution transmission electron microscopy shows intimate contact between each core material and their highly crystalline, conformal MoS2 shells. We propose a generalized growth mechanism for these structures, which is supported by density functional theory energy calculations and implies wider applicability of transition metal dichalcogenide encapsulation to other functional nanoparticles. Further, we demonstrate useful methodology to achieve distinct optical responses, as reflected in the photoluminescence measurements and by discrete dipole approximation calculations. By exploring the role of the core material on synthesis and properties in this architectural platform, we introduce a multiplexed nanoparticle@MoS2 paradigm with numerous viable avenues for future structural and property investigation.

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The introduction of an alkaline earth metal telluride as a second phase in PbTe can lead to very high thermoelectric figure of merit, ZT, as a result of hierarchical structuring, e.g., in the PbTe–SrTe system. However, there are two roadblocks to this strategy: poor solubility and occurrence of incoherent nanoprecipitates in the PbTe matrix, e.g., the PbTe–BaTe system. Here we demonstrate a dual alloying approach by simultaneously alloying CaTe and BaTe in the p-type PbTe matrix to achieve ZTmax ranging up to ∼2.2 at high temperatures. Synergistic enhancement of the Seebeck coefficient via favorable band convergence gives rise to higher power factors up to 34 μW cm–1 K–2 and significant suppression of lattice thermal conductivity, κL, down to ∼0.6 W m–1 K–1 results from large multicenter phonon scattering. Additionally, co-inclusion of Ca and Ba causes unanticipated lattice hardening in otherwise brittle PbTe, essential for practical device applications.

The off-centered Ge leads to the ultralow lattice thermal conductivity and record high average ZT for n-type PbSe.

Sb‐doped and GeTe‐alloyed n‐type thermoelectric materials that show an excellent figure of merit ZT in the intermediate temperature range (400–800 K) are reported. The synergistic effect of favorable changes to the band structure resulting in high Seebeck coefficient and enhanced phonon scattering by point defects and nanoscale precipitates resulting in reduction of thermal conductivity are demonstrated. The samples can be tuned as single‐phase solid solution (SS) or two‐phase system with nanoscale precipitates (Nano) based on the annealing processes. The GeTe alloying results in band structure modification by widening the bandgap and increasing the density‐of‐states effective mass of PbTe, resulting in significantly enhanced Seebeck coefficients. The nanoscale precipitates can improve the power factor in the low temperature range and further reduce the lattice thermal conductivity (κ lat ). Specifically, the Seebeck coefficient of Pb 0.988 Sb 0.012 Te–13%GeTe–Nano approaches −280 µV K −1 at 673 K with a low κ lat of 0.56 W m −1 K −1 at 573 K. Consequently, a peak ZT value of 1.38 is achieved at 623 K. Moreover, a high average ZT avg value of ≈1.04 is obtained in the temperature range from 300 to 773 K for n‐type Pb 0.988 Sb 0.012 Te–13%GeTe–Nano.

Ultrafast synthesis of high-quality transition-metal dichalcogenide nanocrystals, such as molybdenum disulfide (MoS2), is technologically relevant for large-scale production of electronic and optoelectronic devices. Here, we report a rapid solid-state synthesis route for MoS2 using the chemically homogeneous molecular precursor, (NH4)2Mo3S13·H2O, resulting in nanoparticles with estimated size down to 25 nm only in 10 s at 1000 °C. Despite the extreme nonequilibrium conditions, the resulting porous MoS2 nanoparticles remain aggregated to preserve the form of the original rod shape bulk morphology of the molecular precursor. This ultrafast synthesis proceeds through the rapid decomposition of the precursor and rearrangement of Mo and S atoms coupled with simultaneous efficient release of massive gaseous species, to create nanoscale porosity in the resulting isomorphic pseudocrystals, which are composed of the MoS2 nanoparticles. Despite the very rapid escape of massive amounts of NH3, H2O, H2S, and S gases from the (NH4)2Mo3S13·H2O mm sized crystals, they retain their original shape as they convert to MoS2 rather than undergo explosive destruction from the rapid escape process of the gases. The obtained pseudocrystals are made of aggregated MoS2 nanocrystals exhibit a Brunauer-Emmett-Teller surface area of ∼35 m2/g with an adsorption average pore width of ∼160 Å. The nanoporous MoS2 crystals are solution processable by dispersing in ethanol and water and can be cast into large-area uniform composite films. Photodetectors fabricated from these films show more than 2 orders of magnitude higher conductivity (∼6.25 × 10-6 S/cm) and photoconductive gain (20 mA/W) than previous reports of MoS2 composite films. The optoelectronic properties of this nanoporous MoS2 imply that the shallow defects that originate from the ultrafast synthesis act as sensitizing centers that increase the photocurrent gain via two-level recombination kinetics.

The broad-based implementation of thermoelectric materials in converting heat to electricity hinges on the achievement of high conversion efficiency. Here we demonstrate a thermoelectric figure of merit ZT of 2.5 at 923 K by the cumulative integration of several performance-enhancing concepts in a single material system. Using non-equilibrium processing we show that hole-doped samples of PbTe can be heavily alloyed with SrTe well beyond its thermodynamic solubility limit of <1 mol%. The much higher levels of Sr alloyed into the PbTe matrix widen the bandgap and create convergence of the two valence bands of PbTe, greatly boosting the power factors with maximal values over 30 μW cm −1 K −2 . Exceeding the 5 mol% solubility limit leads to endotaxial SrTe nanostructures which produce extremely low lattice thermal conductivity of 0.5 W m −1 K −1 but preserve high hole mobilities because of the matrix/precipitate valence band alignment. The best composition is hole-doped PbTe–8%SrTe.

Recent findings about ultrahigh thermoelectric performance in SnSe single crystals have stimulated related research on this simple binary compound, which is focused mostly on its polycrystalline counterparts, and particularly on electrical property enhancement by effective doping. This work systematically investigated the thermoelectric properties of polycrystalline SnSe doped with three alkali metals (Li, Na, and K). It is found that Na has the best doping efficiency, leading to an increase in hole concentration from 3.2 × 10(17) to 4.4 × 10(19) cm(-3) at room temperature, accompanied by a drop in Seebeck coefficient from 480 to 142 μV/K. An equivalent single parabolic band model was found adequate to capture the variation tendency of Seebeck coefficient with doping levels within a wide range. A mixed scattering of carriers by acoustic phonons and grain boundaries is suitable for numerically understanding the temperature-dependence of carrier mobility. A maximum ZT of ∼0.8 was achieved in 1% Na- or K-doped SnSe at 800 K. Possible strategies to improve the mobility and ZT of polycrystals were also proposed.

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