Selective Scatterings of Phonons and Electrons in Defective Half-Heusler Nb1- δ CoSb for the Figure of Merit zT > 1.

The recently developed defective 19-electron half-Heusler (HH) compounds, represented by Nb1- δ CoSb, possess massive intrinsic vacancies at the cation site and thus intrinsically low lattice thermal conductivity that is desirable for thermoelectric (TE) applications. Yet the TE performance of defective HHs with a maximum figure of merit (zT) <1.0 is still inferior to that of the conventional 18-electron ones. Here, a peak zT exceeding unity is obtained at 1123 K for both Nb0.7 Ta0.13 CoSb and Nb0.6 Ta0.23 CoSb, a benchmark value for defective 19-electron HHs. The improved zT results from the achievement of selective scatterings of phonons and electrons in defective Nb0.83 CoSb, using lanthanide contraction as a design factor to select alloying elements that can strongly impede the phonon propagation but weakly disturb the periodic potential. Despite the massive vacancies induced strong point defect scattering of phonons in Nb0.83 CoSb, Ta alloying is still found effective in suppressing lattice thermal conductivity while maintaining the carrier mobility almost unchanged. In comparison, V alloying significantly deteriorates the carrier transport and thus the TE performance. These results enlarge the category of high-performance HH TE materials beyond the conventional 18-electron ones and highlight the effectiveness of selective scatterings of phonons and electrons in developing TE materials even with massive vacancies.

Vacuolar Sugar Transporter TMT2 Plays Crucial Roles in Germination and Seedling Development in Arabidopsis.

Vacuolar sugar transporters transport sugar across the tonoplast, are major players in maintaining sugar homeostasis, and therefore play vital roles in plant growth, development, and biomass yield. In this study, we analyzed the physiological roles of the tonoplast monosaccharide transporter 2 (TMT2) in Arabidopsis. In contrast to the wild type (WT) that produced uniform seedlings, the tmt2 mutant produced three types of offspring: un-germinated seeds (UnG), seedlings that cannot form true leaves (tmt2-S), and seedlings that develop normally (tmt2-L). Sucrose, glucose, and fructose can substantially, but not completely, rescue the abnormal phenotypes of the tmt2 mutant. Abnormal cotyledon development, arrested true leaf development, and abnormal development of shoot apical meristem (SAM) were observed in tmt2-S seedlings. Cotyledons from the WT and tmt2-L seedlings restored the growth of tmt2-S seedlings through micrografting. Moreover, exogenous sugar sustained normal growth of tmt2-S seedlings with cotyledon removed. Finally, we found that the TMT2 deficiency resulted in growth defects, most likely via changing auxin signaling, target of rapamycin (TOR) pathways, and cellular nutrients. This study unveiled the essential functions of TMT2 for seed germination and initial seedling development, ensuring cotyledon function and mobilizing sugars from cotyledons to seedlings. It also expanded the current knowledge on sugar metabolism and signaling. These findings have fundamental implications for enhancing plant biomass production or seed yield in future agriculture.

Tunable eg Orbital Occupancy in Heusler Compounds for Oxygen Evolution Reaction*.

Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This study reports the first application of Co2 YZ-type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co2 YZ crystals was synthesized through the arc-melting method and the eg orbital filling of Co was precisely regulated by varying Y and Z sites of the compound. A correlation between the eg orbital filling of reactive Co sites and OER activity was found for Co2 MnZ compounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved for eg orbital filling approaching unity. A similar trend of eg orbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co2 VZ, Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance.

Dirac Nodal Arc Semimetal PtSn4 : An Ideal Platform for Understanding Surface Properties and Catalysis for Hydrogen Evolution.

Conductivity, carrier mobility, and a suitable Gibbs free energy are important criteria that determine the performance of catalysts for a hydrogen evolution reaction (HER). However, it is a challenge to combine these factors into a single compound. Herein, we discover a superior electrocatalyst for a HER in the recently identified Dirac nodal arc semimetal PtSn4 . The determined turnover frequency (TOF) for each active site of PtSn4 is 1.54 H2 s-1 at 100 mV. This sets a benchmark for HER catalysis on Pt-based noble metals and earth-abundant metal catalysts. We make use of the robust surface states of PtSn4 as their electrons can be transferred to the adsorbed hydrogen atoms in the catalytic process more efficiently. In addition, PtSn4 displays excellent chemical and electrochemical stabilities after long-term exposure in air and long-time HER stability tests.

Synthesis and thermoelectric properties of Rashba semiconductor BiTeBr with intensive texture.

Bismuth tellurohalides with Rashba-type spin splitting exhibit unique Fermi surface topology and are developed as promising thermoelectric materials. However, BiTeBr, which belongs to this class of materials, is rarely investigated in terms of the thermoelectric transport properties. In the study, polycrystalline bulk BiTeBr with intensive texture was synthesized via spark plasma sintering (SPS). Additionally, its thermoelectric properties above room temperature were investigated along both the in-plane and out-plane directions, and they exhibit strong anisotropy. Low sound velocity along two directions is found and contributes to its low lattice thermal conductivity. Polycrystalline BiTeBr exhibits relatively good thermoelectric performance along the in-plane direction, with a maximum dimensionless figure of merit (ZT) of 0.35 at 560 K. Further enhancements of ZT are expected by utilizing systematic optimization strategies.

Passive Self-Sustained Thermoelectric Devices Powering the 24 h Wireless Transmission via Radiation-Cooling and Selective Photothermal Conversion.

The rapid development of the Internet of Things has triggered a huge demand for self-sustained technology that can provide a continuous electricity supply for low-power electronics. Here, a self-sustained power supply solution is demonstrated that can produce a 24 h continuous and unipolar electricity output based on thermoelectric devices by harvesting the environmental temperature difference, which is ingeniously established utilizing radiation cooling and selective photothermal conversion. The developed prototype system can stably maintain a large temperature difference of about 1.8 K for a full day despite the real-time changes in environmental temperature and solar radiation, thereby driving continuous electricity output using the built-in thermoelectric device. Specifically, the large output voltage of >102 mV and the power density of >4.4 mW m-2 could be achieved for a full day, which are outstanding among the 24 h self-sustained thermoelectric devices and far higher than the start-up values of the wireless temperature sensor and also the light-emitting diode, enabling the 24 h remote data transmission and lighting, respectively. This work highlights the application prospects of self-sustained thermoelectric devices for low-power electronics.

High performance magnesium-based plastic semiconductors for flexible thermoelectrics.

Low-cost thermoelectric materials with simultaneous high performance and superior plasticity at room temperature are urgently demanded due to the lack of ever-lasting power supply for flexible electronics. However, the inherent brittleness in conventional thermoelectric semiconductors and the inferior thermoelectric performance in plastic organics/inorganics severely limit such applications. Here, we report low-cost inorganic polycrystalline Mg3Sb0.5Bi1.498Te0.002, which demonstrates a remarkable combination of large strain (~ 43%) and high figure of merit zT (~ 0.72) at room temperature, surpassing both brittle Bi2(Te,Se)3 (strain ≤ 5%) and plastic Ag2(Te,Se,S) and organics (zT ≤ 0.4). By revealing the inherent high plasticity in Mg3Sb2 and Mg3Bi2, capable of sustaining over 30% compressive strain in polycrystalline form, and the remarkable deformability of single-crystalline Mg3Bi2 under bending, cutting, and twisting, we optimize the Bi contents in Mg3Sb2-xBix (x = 0 to 1) to simultaneously boost its room-temperature thermoelectric performance and plasticity. The exceptional plasticity of Mg3Sb2-xBix is further revealed to be brought by the presence of a dense dislocation network and the persistent Mg-Sb/Bi bonds during slipping. Leveraging its high plasticity and strength, polycrystalline Mg3Sb2-xBix can be easily processed into micro-scale dimensions. As a result, we successfully fabricate both in-plane and out-of-plane flexible Mg3Sb2-xBix thermoelectric modules, demonstrating promising power density. The inherent remarkable plasticity and high thermoelectric performance of Mg3Sb2-xBix hold the potential for significant advancements in flexible electronics and also inspire further exploration of plastic inorganic semiconductors.

High-Performance Stretchable Thermoelectric Generator for Self-Powered Wearable Electronics.

Wearable thermoelectric generators (TEGs), which can convert human body heat to electricity, provide a promising solution for self-powered wearable electronics. However, their power densities still need to be improved aiming at broad practical applications. Here, a stretchable TEG that achieves comfortable wearability and outstanding output performance simultaneously is reported. When worn on the forehead at an ambient temperature of 15 °C, the stretchable TEG exhibits excellent power densities with a maximum value of 13.8 µW cm-2 under the breezeless condition, and even as high as 71.8 µW cm-2 at an air speed of 2 m s-1 , being one of the highest values for wearable TEGs. Furthermore, this study demonstrates that this stretchable TEG can effectively power a commercial light-emitting diode and stably drive an electrocardiogram module in real-time without the assistance of any additional power supply. These results highlight the great potential of these stretchable TEGs for power generation applications.

Opening the Bandgap of Metallic Half-Heuslers via the Introduction of d-d Orbital Interactions.

Half-Heusler compounds with semiconducting behavior have been developed as high-performance thermoelectric materials for power generation. Many half-Heusler compounds also exhibit metallic behavior without a bandgap and thus inferior thermoelectric performance. Here, taking metallic half-Heusler MgNiSb as an example, a bandgap opening strategy is proposed by introducing the d-d orbital interactions, which enables the opening of the bandgap and the improvement of the thermoelectric performance. The width of the bandgap can be engineered by tuning the strength of the d-d orbital interactions. The conduction type and the carrier density can also be modulated in the Mg1- x Tix NiSb system. Both improved n-type and p-type thermoelectric properties are realized, which are much higher than that of the metallic MgNiSb. The proposed bandgap opening strategy can be employed to design and develop new half-Heusler semiconductors for functional and energy applications.

Achieving metal-like malleability and ductility in Ag2Te1-x S x inorganic thermoelectric semiconductors with high mobility.

Inorganic semiconductor Ag2Te1-x S x has been recently found to exhibit unexpected plastic deformation with compressive strain up to 30%. However, the origin of the abnormal plasticity and how to simultaneously achieve superb ductility and high mobility are still elusive. Here, we demonstrate that crystalline/amorphous Ag2Te1-x S x (x = 0.3, 0.4, and 0.5) composites can exhibit excellent compressive strain up to 70% if the monoclinic Ag2Te phase, which commonly exists in the matrix, is eliminated. Significantly, an ultra-high tensile elongation reaching 107.3% was found in Ag2Te0.7S0.3, which is the highest one yet reported in the system and even surpasses those achieved in some metals and high-entropy alloys. Moreover, high mobility of above 1000 cm2 V-1 s-1 at room temperature and good thermoelectric performance are simultaneously maintained. A modified Ashby plot with ductility factor versus carrier mobility is thereby proposed to highlight the potential of solid materials for applications in flexible/wearable electronics.

High-Power-Density Wearable Thermoelectric Generators for Human Body Heat Harvesting.

Wearable thermoelectrics has attracted significant interest in recent years. Among them, rigid-structure thermoelectric generators (TEGs) were seldomly employed for wearable applications, although those exhibit significant advantages of high device output performance and impact resistance. Here, we report a type of rigid wearable TEGs (w-TEGs) without ceramic substrates made using a simple cutting-and-bonding method. Owing to the small contact area, the w-TEGs comprising 48-n/p-pairs can be well attached to the human body. The lack of ceramic substrates leaves more space in the height direction, which benefits the wearability in practical applications and high power density. We demonstrated that increasing the height of w-TEGs from 1.38 to 3.14 mm significantly improves the power density by a factor of 10. As a result, the maximum power densities of 7.9 µW cm-2 and 43.6 µW cm-2 for the w-TEGs were realized under the breezeless condition and a wind speed for normal walking, respectively. This work provides a feasible design solution for rigid-structure free-substrate w-TEGs with very high power density, which will speed up the research of wearable thermoelectrics.

Electronic structure and low-temperature thermoelectric transport of TiCoSb single crystals.

Band structure engineering has a strong beneficial impact on thermoelectric performance, where theoretical methods dominate the investigation of electronic structures. Here, we use angle-resolved photoemission spectroscopy (ARPES) to analyze the electronic structure and report on the thermoelectric transport properties of half-Heusler TiCoSb high-quality single crystals. High degeneracy of the valence bands at the L and Γ band maximum points was observed, which provides a band-convergence scenario for the thermoelectric performance of TiCoSb. Previous efforts have shown how crystallographic defects play an important role in TiCoSb transport properties, while the intrinsic properties remain elusive. Using hard X-ray photoelectron spectroscopy (HAXPES), we discard the presence of interstitial defects that could induce in-gap states near the valence band in our crystals. Contrary to polycrystalline reports, intrinsic TiCoSb exhibits p-type transport, albeit defects still affect the carrier concentration. In two initially identical p-type TiCoSb crystal batches, distinct metallic and semiconductive behaviors were found owing to defects not noticeable by elemental analysis. A varying Seebeck effective mass is consistent with the change at the Fermi level within this band convergence picture. This report tackles the direct investigation of the electronic structure of TiCoSb and reveals new insights and the strong impact of point defects on the optimization of thermoelectric properties.

Mo-Fe/NbFeSb Thermoelectric Junctions: Anti-Thermal Aging Interface and Low Contact Resistivity.

In recent years, high-performance half-Heusler compounds have been developed as promising thermoelectric materials for power generation. Aiming at practical device applications, one key step is to seek suitable metal electrodes so that low interfacial resistivity is guaranteed under long-term thermal aging. In the previous work, the fresh Mo/Nb0.8Ti0.2FeSb junction was found exhibiting low contact resistivity below 1 µΩ cm2; however, it increased by tens of times under long-term thermal aging, mainly originating from the formation of the high-resistivity FeSb2 phase and the appearance of cracks. Here, the Mo-Fe electrodes are employed to build the junctions with Nb0.8Ti0.2FeSb. The interfacial behavior and contact resistance in these junctions were investigated both before and after the thermal aging. Interestingly, no obvious formation of FeSb2 phase and cracks were observed. As a result, the contact resistivity was below ∼1 µΩ cm2 after 15 days' thermal aging, indicating better connection reliability and lower contact resistivity compared to the Mo/Nb0.8Ti0.2FeSb junction. These findings highlight the applicability of Mo-Fe electrodes and pave the way for NbFeSb-based half-Heusler thermoelectric materials for device applications.

Large Anomalous Hall and Nernst Effects in High Curie-Temperature Iron-Based Heusler Compounds.

The interplay between topology and magnetism has recently sparked the frontier studies of magnetic topological materials that exhibit intriguing anomalous Hall and Nernst effects owning to the large intrinsic Berry curvature (BC). To better understand the anomalous quantum transport properties of these materials and their implications for future applications such as electronic and thermoelectric devices, it is crucial to discover more novel material platforms for performing anomalous transverse transport studies. Here, it is experimentally demonstrated that low-cost Fe-based Heusler compounds exhibit large anomalous Hall and Nernst effects. An anomalous Hall conductivity of 250-750 S cm-1 and Nernst thermopower of above 2 µV K-1 are observed near room temperature. The positive effect of anti-site disorder on the anomalous Hall transport is revealed. Considering the very high Curie temperature (nearly 1000 K), larger Nernst thermopowers at high temperatures are expected owing to the existing magnetic order and the intrinsic BC. This work provides a background for developing low-cost Fe-based Heusler compounds as a new material platform for anomalous transport studies and applications, in particular, near and above room temperature.

Demonstration of valley anisotropy utilized to enhance the thermoelectric power factor.

Valley anisotropy is a favorable electronic structure feature that could be utilized for good thermoelectric performance. Here, taking advantage of the single anisotropic Fermi pocket in p-type Mg3Sb2, a feasible strategy utilizing the valley anisotropy to enhance the thermoelectric power factor is demonstrated by synergistic studies on both single crystals and textured polycrystalline samples. Compared to the heavy-band direction, a higher carrier mobility by a factor of 3 is observed along the light-band direction, while the Seebeck coefficient remains similar. Together with lower lattice thermal conductivity, an increased room-temperature zT by a factor of 3.6 is found. Moreover, the first-principles calculations of 66 isostructural Zintl phase compounds are conducted and 9 of them are screened out displaying a pz-orbital-dominated valence band, similar to Mg3Sb2. In this work, we experimentally demonstrate that valley anisotropy is an effective strategy for the enhancement of thermoelectric performance in materials with anisotropic Fermi pockets.

Thermoelectric Properties of Novel Semimetals: A Case Study of YbMnSb2.

The emerging class of topological materials provides a platform to engineer exotic electronic structures for a variety of applications. As complex band structures and Fermi surfaces can directly benefit thermoelectric performance it is important to identify the role of featured topological bands in thermoelectrics particularly when there are coexisting classic regular bands. In this work, the contribution of Dirac bands to thermoelectric performance and their ability to concurrently achieve large thermopower and low resistivity in novel semimetals is investigated. By examining the YbMnSb2 nodal line semimetal as an example, the Dirac bands appear to provide a low resistivity along the direction in which they are highly dispersive. Moreover, because of the regular-band-provided density of states, a large Seebeck coefficient over 160 µV K-1 at 300 K is achieved in both directions, which is very high for a semimetal with high carrier concentration. The combined highly dispersive Dirac and regular bands lead to ten times increase in power factor, reaching a value of 2.1 mW m-1 K-2 at 300 K. The present work highlights the potential of such novel semimetals for unusual electronic transport properties and guides strategies towards high thermoelectric performance.

Artificial intelligence: A powerful paradigm for scientific research.

Artificial intelligence (AI) coupled with promising machine learning (ML) techniques well known from computer science is broadly affecting many aspects of various fields including science and technology, industry, and even our day-to-day life. The ML techniques have been developed to analyze high-throughput data with a view to obtaining useful insights, categorizing, predicting, and making evidence-based decisions in novel ways, which will promote the growth of novel applications and fuel the sustainable booming of AI. This paper undertakes a comprehensive survey on the development and application of AI in different aspects of fundamental sciences, including information science, mathematics, medical science, materials science, geoscience, life science, physics, and chemistry. The challenges that each discipline of science meets, and the potentials of AI techniques to handle these challenges, are discussed in detail. Moreover, we shed light on new research trends entailing the integration of AI into each scientific discipline. The aim of this paper is to provide a broad research guideline on fundamental sciences with potential infusion of AI, to help motivate researchers to deeply understand the state-of-the-art applications of AI-based fundamental sciences, and thereby to help promote the continuous development of these fundamental sciences.

High-Performance Mg3Sb2-x Bi x Thermoelectrics: Progress and Perspective.

Since the first successful implementation of n-type doping, low-cost Mg3Sb2-x Bi x alloys have been rapidly developed as excellent thermoelectric materials in recent years. An average figure of merit zT above unity over the temperature range 300-700 K makes this new system become a promising alternative to the commercially used n-type Bi2Te3-x Se x alloys for either refrigeration or low-grade heat power generation near room temperature. In this review, with the structure-property-application relationship as the mainline, we first discuss how the crystallographic, electronic, and phononic structures lay the foundation of the high thermoelectric performance. Then, optimization strategies, including the physical aspects of band engineering with Sb/Bi alloying and carrier scattering mechanism with grain boundary modification and the chemical aspects of Mg defects and aliovalent doping, are extensively reviewed. Mainstream directions targeting the improvement of zT near room temperature are outlined. Finally, device applications and related engineering issues are discussed. We hope this review could help to promote the understanding and future developments of low-cost Mg3Sb2-x Bi x alloys for practical thermoelectric applications.

Metallic n-Type Mg3 Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance.

Mg3 (Sb,Bi)2 alloys have recently been discovered as a competitive alternative to the state-of-the-art n-type Bi2 (Te,Se)3 thermoelectric alloys. Previous theoretical studies predict that single crystals Mg3 (Sb,Bi)2 can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n-type Mg3 (Sb,Bi)2 single crystals. Here, the first thermoelectric properties of n-type Te-doped Mg3 Sb2 single crystals, synthesized by a combination of Sb-flux method and Mg-vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T-1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te-doped Mg3 Sb2 single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge-carrier scattering is crucial for developing high-performance thermoelectric materials and indicates that single-crystalline Mg3 (Sb,Bi)2 solid solutions can exhibit higher zT compared to polycrystalline samples.

Violation of the T -1 Relationship in the Lattice Thermal Conductivity of Mg3Sb2 with Locally Asymmetric Vibrations.

Most crystalline materials follow the guidelines of T -1 temperature-dependent lattice thermal conductivity (κ L ) at elevated temperatures. Here, we observe a weak temperature dependence of κ L in Mg3Sb2, T -0.48 from theory and T -0.57 from measurements, based on a comprehensive study combining ab initio molecular dynamics calculations and experimental measurements on single crystal Mg3Sb2. These results can be understood in terms of the so-called "phonon renormalization" effects due to the strong temperature dependence of the interatomic force constants (IFCs). The increasing temperature leads to the frequency upshifting for those low-frequency phonons dominating heat transport, and more importantly, the phonon-phonon interactions are weakened. In-depth analysis reveals that the phenomenon is closely related to the temperature-induced asymmetric movements of Mg atoms within MgSb4 tetrahedron. With increasing temperature, these Mg atoms tend to locate at the areas with relatively low force in the force profile, leading to reduced effective 3rd-order IFCs. The locally asymmetrical atomic movements at elevated temperatures can be further treated as an indicator of temperature-induced variations of IFCs and thus relatively strong phonon renormalization. The present work sheds light on the fundamental origins of anomalous temperature dependence of κ L in thermoelectrics.