Nuanced dilute doping strategy enables high-performance GeTe thermoelectrics.

In thermoelectrics, doping is essential to augment the figure of merit. Traditional strategy, predominantly heavy doping, aims to optimize carrier concentration and restrain lattice thermal conductivity. However, this tactic can severely hamper carrier transport due to pronounced point defect scattering, particularly in materials with inherently low carrier mean-free-path. Conversely, dilute doping, although minimally affecting carrier mobility, frequently fails to optimize other vital thermoelectric parameters. Herein, we present a more nuanced dilute doping strategy in GeTe, leveraging the multifaceted roles of small-size metal atoms. A mere 4% CuPbSbTe3 introduction into GeTe swiftly suppresses rhombohedral distortion and optimizes carrier concentration through the aid of Cu interstitials. Additionally, the formation of multiscale microstructures, including zero-dimensional Cu interstitials, one-dimensional dislocations, two-dimensional planar defects, and three-dimensional nanoscale amorphous GeO2 and Cu2GeTe3 precipitates, along with the ensuing lattice softening, contributes to an ultralow lattice thermal conductivity. Intriguingly, dilute CuPbSbTe3 doping incurs only a marginal decrease in carrier mobility. Subsequent trace Cd doping, employed to alleviate the bipolar effect and align the valence bands, yields an impressive figure-of-merit of 2.03 at 623 K in (Ge0.97Cd0.03Te)0.96(CuPbSbTe3)0.04. This leads to a high energy-conversion efficiency of 7.9% and a significant power density of 3.44 W cm-2 at a temperature difference of 500 K. These results underscore the invaluable insights gained into the constructive role of nuanced dilute doping in the concurrent tuning of carrier and phonon transport in GeTe and other thermoelectric materials.

Anisotropic Amorphization and Phase Transition in Na2 Ti3 O7 Anode Caused by Electron Beam Irradiation.

Na2 Ti3 O7 is considered one of the most promising anode materials for sodium ion batteries due to its superior safety, environmental friendliness, and low manufacturing cost. However, its structural stability and reaction mechanism still have not been fully explored. As the electron beam irradiation introduces a similar impact on the Na2 Ti3 O7 anode as the extraction of Na+ ions during the battery discharge process, the microstructure evolution of the materials is investigated by advanced electron microscopy techniques at the atomic scale. Anisotropic amorphization is successfully observed. Through the integrated differential phase contrast-scanning transmission electron microscopy technique and density functional theory calculation, a phase transition pathway involving a new phase, Na2 Ti24 O49 , is proposed with the reduction of Na atoms. Additionally, it is found that the amorphization is dominated by the surface energy and electron dose rate. These findings will deepen the understanding of structural stability and deintercalation mechanism of the Na2 Ti3 O7 anode, providing new insight into exploring the failure mechanism of electrode materials.

Interlayer Phonon Coupling from Heavy and Light Sublayers in a Natural Van der Waals Superlattice.

Layered compounds characterized by van der Waals gaps are often associated with relatively weak interlayer particle interactions. However, in specific scenarios, these seemingly feeble forces can exert an impact on interlayer interactions through subtle energy fluctuations, which can give rise to a diverse range of physical and chemical properties, particularly intriguing in the context of thermal transport. In this study, taking a natural superlattice composed of alternately stacked PbS and SnS2 sublayers as a model, we proposed that in a superlattice, there is strong hybridization between acoustic phonons of heavy sublayers and optical phonons of light sublayers. We identified newly generated vibration modes in the superlattice, such as interlayer shear and breathing, which exhibit lower sound velocity and contribute less to heat transport compared to their parent materials, which significantly alters the thermal behaviors of the superlattice compared to its bulk counterparts. Our findings on the behavior of interlayer phonons in superlattices not only can shed light on developing functional materials with enhanced thermal dissipation capabilities but also contribute to the broader field of condensed matter physics, offering insights into various fields, including thermoelectrics and phononic devices, and may pave the way for technological advancements in these areas.

Atomic-Resolution Electron Microscopy Unravelling the Role of Unusual Asymmetric Twin Boundaries in the Electron-Beam-Sensitive NASICON-Type Solid Electrolyte.

An atomic-scale understanding of the role of nonperiodic features is essential to the rational design of highly Li-ion-conductive solid electrolytes. Unfortunately, most solid electrolytes are easily damaged by the intense electron beam needed for atomic-resolution electron microscopy observation, so the reported in-depth atomic-scale studies are limited to Li0.33La0.56TiO3- and Li7La3Zr2O12-based materials. Here, we observe on an atomic scale a third type of solid electrolyte, Li1.3Al0.3Ti1.7(PO4)3 (LATP), through minimization of damage induced by specimen preparation. With this capability, LATP is found to contain large amounts of twin boundaries with an unusual asymmetric atomic configuration. On the basis of the experimentally determined structure, the theoretical calculations suggest that such asymmetric twin boundaries may considerably promote Li-ion transport. This discovery identifies a new entry point for optimizing ionic conductivity, and the method presented here will also greatly benefit the mechanistic study of solid electrolytes.

Electron transfer bridge inducing polarization of nitrogen molecules for enhanced photocatalytic nitrogen fixation.

Ammonia (NH3) plays a crucial role in the production of fertilizers, medicines, fibers, etc., which are closely relevant to the development of human society. However, the inert and nonpolar properties of NN seriously hinder artificial nitrogen fixation under mild conditions. Herein, we introduce a novel strategy to enhance the photocatalytic efficiency of N2 fixation through the directional polarization of N2 by rare earth metal atoms, which act as a local "electron transfer bridge." This bridge facilitates the transfer of delocalized electrons to the distal N atom and redirects the polarization of adsorbed N2 molecules. Taking cerium doped BiOCl (Ce-BiOCl) as an example, our results reveal that the electrons transfer to the distal N atom through the cerium atom, resulting in absorbed nitrogen molecular polarization. Consequently, the polarized nitrogen molecules exhibit an easier trend for NN cleavage and the subsequent hydrogenation process, and exhibit a greatly enhanced photocatalytic ammonia production rate of 46.7 µmol g-1 h-1 in cerium doped BiOCl, nearly 4 times higher than that of pure BiOCl. The original concept of directional polarization of N2 presented in this work not only deepens our understanding of the N2 molecular activation mechanism but also broadens our horizons for designing highly efficient catalysts for N2 fixation.

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.

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.

Maldistribution of Chemical Bond Strength Inducing Exceptional Anisotropy of Thermal Conductivity in Non-Layered Materials.

Currently, the efforts to find materials with high κ anisotropy ratios mainly focus on layered materials, however, the limited quantity and lower workability comparing to non-layered ones boost the exploration of non-layered materials with high κ anisotropy ratios. Here, taking PbSnS3 , a typical non-layered orthorhombic compound, as an example, we propose that maldistribution of chemical bond strength can lead to large anisotropy of κ in non-layered materials. Our result reveals that the maldistribution of Pb-S bonds lead to obvious collective vibrations of dioctahedron chain units, resulting in an anisotropy ratio up to 7.1 at 200 K and 5.5 at 300 K, respectively, which is one of the highest ever reported in non-layered materials and even surpasses many classical layered materials such as Bi2 Te3 and SnSe. Our findings can not only broaden the horizon for exploring high anisotropic κ materials but also provide new opportunities for the application of thermal management.

Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Ge1-x-yBixCayTe with Ultrafine Ferroelectric Domain Structure.

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of κL in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced κL by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m-1 K-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 µW cm-1 K-2 in Ge0.85Bi0.09Ca0.06Te. Ultimately, a high ZT of ∼2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

Dynamic evolution mechanism of scanning moiré fringes.

Scanning moiré fringes (SMFs) in scanning transmission electron microscopy (STEM) have a broad application prospect owing to the low-magnification imaging and hereto the low electron irritation damage, especially in defects localization, strain analysis etc. However, the dynamic evolution mechanism of SMFs is still not clear. In this paper, we carry out in-depth study of SMFs with ferroelectric material GeSe as an example. With the help of combination of aberration-corrected STEM imaging and geometrical model, we discuss the evolution of SMFs with variation of scanning step (magnification), and explain its quasiperiodic behavior in the experiments. Our results will deepen the understanding of SMFs, and may widen their applications under the guidance of the new formation mechanism.

Phonon Symphony of Stacked Multilayers and Weak Bonds Lowers Lattice Thermal Conductivity.

Controlling lattice vibrations to obtain intrinsic low thermal conductivity play a critical role in thermal management of electronic and photonic devices, energy converters, and thermal insulation, which necessitates exploring new compounds and a thorough understanding of their chemical structure, bonding, and lattice dynamics. Herein, a new chalcogenide, Ga6 Cr5 Se16 , shows intrinsic low lattice thermal conductivity κlat , which crystallizes in the monoclinic phase (C2/m) with the stacked inverse GaSe4 layers (g'), close-packed Cr3+ Se6 layers (c), GaSe4 layers (g) and loosely-stacked Cr2+ Se6 layers (c') along the c-axis. In this structure, a wide variety of chemical bonding is arranged in each layer, such as covalent Ga-Se, covalent Cr3+ -Se, and weaker Cr2+ -Se bonding, which endow it with a large phonon symphony by strong coupling of soft acoustic and low-lying optical phonons. As a result, Ga6 Cr5 Se16 realizes an intrinsic low κlat of 0.79 W m- 1  K- 1 at 323 K, which is almost four times, or twice lower than that of Cr3 Se4 (2.95 W m- 1  K- 1 ), or Cr2 Se3 (1.56 W m- 1  K- 1 ), Ga2 Se3 (1.36 W m- 1 K- 1 ) at 323 K, respectively. These insights will offer comprehensive understanding of the phonon propagation in complex layered chalcogenides, and also shed useful light on future design of low-κlat solids.

In Situ Investigation of the Phase Transition at the Surface of Thermoelectric PbTe with van der Waals Control.

The structure of thermoelectric materials largely determines the thermoelectric characteristics. Hence, a better understanding of the details of the structural transformation process/conditions can open doors for new applications. In this study, the structural transformation of PbTe (a typical thermoelectric material) is studied at the atomic scale, and both nucleation and growth are analyzed. We found that the phase transition mainly occurs at the surface of the material, and it is mainly determined by the surface energy and the degree of freedom the atoms have. After exposure to an electron beam and high temperature, high-density crystal-nuclei appear on the surface, which continue to grow into large particles. The particle formation is consistent with the known oriented-attachment growth mode. In addition, the geometric structure changes during the transformation process. The growth of nanoparticles is largely determined by the van der Waals force, due to which adjacent particles gradually move closer. During this movement, as the relative position of the particles changes, the direction of the interaction force changes too, which causes the particles to rotate by a certain angle.

Electrical manipulation of skyrmions in a chiral magnet.

Writing, erasing and computing are three fundamental operations required by any working electronic device. Magnetic skyrmions could be essential bits in promising in emerging topological spintronic devices. In particular, skyrmions in chiral magnets have outstanding properties like compact texture, uniform size, and high mobility. However, creating, deleting, and driving isolated skyrmions, as prototypes of aforementioned basic operations, have been a grand challenge in chiral magnets ever since the discovery of skyrmions, and achieving all these three operations in a single device is even more challenging. Here, by engineering chiral magnet Co8Zn10Mn2 into the customized micro-devices for in-situ Lorentz transmission electron microscopy observations, we implement these three operations of skyrmions using nanosecond current pulses with a low current density of about 1010 A·m-2 at room temperature. A notched structure can create or delete magnetic skyrmions depending on the direction and magnitude of current pulses. We further show that the magnetic skyrmions can be deterministically shifted step-by-step by current pulses, allowing the establishment of the universal current-velocity relationship. These experimental results have immediate significance towards the skyrmion-based memory or logic devices.

Fast determination of sample thickness through scanning moiré fringes in scanning transmission electron microscopy.

Sample thickness is an important parameter in transmission electron microscopy (TEM) imaging for interpreting image contrast and understanding the relationship between properties and microstructure. In this study, we introduce a method for sample thickness determination in scanning TEM (STEM) mode based on scanning moiré fringes (SMFs). Focal-series SMF imaging is used and sample thickness can be determined in situ at a medium magnification range, with beam damage and contamination avoided to a large extent. It provides a fast and convenient approach for determining sample thickness in TEM imaging, which is particularly useful for beam-sensitive materials.

Tungsten Nanoparticles Accelerate Polysulfides Conversion: A Viable Route toward Stable Room-Temperature Sodium-Sulfur Batteries.

Room-temperature sodium-sulfur (RT Na-S) batteries are arousing great interest in recent years. Their practical applications, however, are hindered by several intrinsic problems, such as the sluggish kinetic, shuttle effect, and the incomplete conversion of sodium polysulfides (NaPSs). Here a sulfur host material that is based on tungsten nanoparticles embedded in nitrogen-doped graphene is reported. The incorporation of tungsten nanoparticles significantly accelerates the polysulfides conversion (especially the reduction of Na2 S4 to Na2 S, which contributes to 75% of the full capacity) and completely suppresses the shuttle effect, en route to a fully reversible reaction of NaPSs. With a host weight ratio of only 9.1% (about 3-6 times lower than that in recent reports), the cathode shows unprecedented electrochemical performances even at high sulfur mass loadings. The experimental findings, which are corroborated by the first-principles calculations, highlight the so far unexplored role of tungsten nanoparticles in sulfur hosts, thus pointing to a viable route toward stable Na-S batteries at room temperatures.

PD-L1 and HER2 expression in gastric adenocarcinoma and their prognostic significance.

BACKGROUND: The upregulation of programmed death-ligand 1 (PD-L1) and epidermal growth factor receptor 2 (HER2) may play a role in gastric adenocarcinoma (GAC). AIM: To study PD-L1 and HER-2 expression and prognosis in GAC. METHODS: PD-L1 and HER2 expression was determined in tumor tissues of 75 patients with GAC. The correlations between PD-L1, HER2 expression, and clinicopathological factors were analyzed. RESULTS: The positive expression rate for PD-L1 was 57.3% (43/75) and the HER2 over-expression rate was 17.3% (13/75). PD-L1 expression negatively correlated with the grade of GAC differentiation (r =-0.26, P<0.05). Approximately 85% of HER2-positive GACs were found to be PD-L1-positive and PD-L1 expression positively correlated with HER2 overexpression. The TNM stage and combined HER2 and PD-L1 expression were independent prognostic factors affecting the survival of patients with GAC. The median overall survival and recurrence-free survival of groups I (HER2 overexpression and PD-L1 positive), II (HER2 overexpression and PD-L1 negative), III (No HER2 overexpression and PD-L1 positive) and IV (No HER2 overexpression and PD-L1 negative) were (47 (17-77), 15 (0-44), 81 (62-101), and 78 (60-98) months, respectively. CONCLUSION: PD-L1 expression is upregulated in more than half of patients with GAC. Anti-PD-L1 treatment combined with anti-HER2 therapy may benefit patients with locally advanced GAC with HER2 overexpression.

Monodisperse Molybdenum Nanoparticles as Highly Efficient Electrocatalysts for Li-S Batteries.

Lithium-sulfur (Li-S) batteries have attracted widespread attention due to their high theoretical energy density. However, their practical application is still hindered by the shuttle effect and the sluggish conversion of lithium polysulfides (LiPSs). Herein, monodisperse molybdenum (Mo) nanoparticles embedded onto nitrogen-doped graphene (Mo@N-G) were developed and used as a highly efficient electrocatalyst to enhance LiPS conversion. The weight ratio of the electrocatalyst in the catalyst/sulfur cathode is only 9%. The unfilled d orbitals of oxidized Mo can attract the electrons of LiPS anions and form Mo-S bonds during the electrochemical process, thus facilitating fast conversion of LiPSs. Li-S batteries based on the Mo@N-G/S cathode can exhibit excellent rate performance, large capacity, and superior cycling stability. Moreover, Mo@N-G also plays an important role in room-temperature quasi-solid-state Li-S batteries. These interesting findings suggest the great potential of Mo nanoparticles in building high-performance Li-S batteries.

Néel-Type Elliptical Skyrmions in a Laterally Asymmetric Magnetic Multilayer.

Magnetic skyrmions, topological-chiral spin textures, have potential applications in next-generation high-density and energy-efficient spintronic devices for information storage and logic technologies. Tailoring the detailed spin textures of skyrmions is of pivotal importance for tuning skyrmion dynamics, which is one of the key factors for the design of skyrmionic devices. Here, the direct observation of parallel aligned elliptical magnetic skyrmions in Pt/Co/Ta multilayers with an oblique-angle deposited Co layer is reported. Domain wall velocity and spin-orbit-torque-induced out-of-plane effective field analysis demonstrate that the formation of unusual elliptical skyrmions is correlated to the anisotropic effective perpendicular magnetic anisotropy energy density (Keff u ) and Dzyaloshinskii-Moriya interaction (DMI) in the film plane. Structural analysis and first-principles calculations further show that the anisotropic Keff u and DMI originate from the interfacial anisotropic strain introduced by the oblique-angle deposition. The work provides a method to tune the spin textures of skyrmions in magnetic multilayers and, thereby, a new degree of freedom for the design of skyrmionic devices.

Revealing the origin of dislocations in Pb1-xSb2x/3Se (0 < x ≤ 0.07).

Defect engineering is an effective route to improve the performance of thermoelectric materials, including Sb doped PbSe, but the formation mechanism of defects remains unclear. In the thermoelectric material Pb1-xSb2x/3Se (0 < x ≤ 0.07), a large number of dislocations have been reported, and they enhance intermediate-frequency phonon scattering, thereby improving the zT value. However, the microstructural origin of dislocations remains unclear. In this paper, via a combination of atomic resolution scanning transmission electron microscopy and density functional theory, we successfully revealed the microstructure of Pb1-xSb2x/3Se (x = 0-0.07) for in-depth understanding of the formation mechanism of dislocations. Plenty of zinc blende (ZB) nanostructures are found in the PbSe matrix with a rock salt (RS) structure, and the theoretical calculations confirm its viability from the point of view of formation energy. A similar ZB structure is identified in the dislocation cores of Sb-doped materials as well, and thus the formation mechanism of dislocations is discussed for this PbSe system. This result provides important guidance to understand the structural evolution in compounds with a RS structure, especially in high-performance lead chalcogenide thermoelectric materials.

Direct visualization of spatially correlated displacive short-range ordering in Nb0.8CoSb.

Whether the atomic arrangement has a long-range order bifurcates solid-state matter into two major categories: crystalline and amorphous, between which lies a short-range order, a frontier research topic of fundamental and application implications. To date, it is still challenging to extract the details of short-range order from the corresponding diffuse diffraction pattern due to the phase problem. Here, we employed the high-angle annular dark field (HAADF) imaging technique to pinpoint the short-range order encoded in the one-of-a-kind diffuse the diffraction bands of defective half-Heusler Nb0.8CoSb. Utilizing a protocol based on two limiting cases, we found that the native Nb vacancies up to 20% are dominantly displacive short-range ordered yet spatially correlated. To the best of our knowledge, this is the first time that a dominantly displacive short-range order is reported at the atomic scale. These results are vital for an in-depth understanding and engineering of the thermodynamics and transport properties of the materials with abundant native defects, including but not limited to defective half-Heusler compounds.