Fundamental and progress of Bi_2Te_3-based thermoelectric materials

Thermoelectric materials,enabling the directing conversion between heat and electricity,are one of the promising candidates for overcoming environmental pollution and the upcoming energy shortage caused by the over-consumption of fossil fuels.Bi2Te3-based alloys are the classical thermoelectric materials working near room temperature.Due to the intensive theoretical investigations and experimental demonstrations,significant progress has been achieved to enhance the thermoelectric performance of Bi2Te3-based thermoelectric materials.In this review,we first explored the fundamentals of thermoelectric effect and derived the equations for thermoelectric properties.On this basis,we studied the effect of material parameters on thermoelectric properties.Then,we analyzed the features of Bi2Te3-based thermoelectric materials,including the lattice defects,anisotropic behavior and the strong bipolar conduction at relatively high temperature.Then we accordingly summarized the strategies for enhancing the thermoelectric performance,including point defect engineering,texture alignment,and band gap enlargement.Moreover,we highlighted the progress in decreasing thermal conductivity using nanostructures fabricated by solution grown method,ball milling,and melt spinning.Lastly,we employed modeling analysis to uncover the principles of anisotropy behavior and the achieved enhancement in Bi2Te3,which will enlighten the enhancement of thermoelectric performance in broader materials

Evolution of Microstructure in a Cu-Cr in situ Composite Produced by Thermo-Mechanical Processing

This paper studied the microstructure evolution of a deformation-processed Cu-7Cr in situ composite prepared by thermo-mechanical processing. The longitudinal and transverse sectional microstructures were analyzed using an optical microscope and a scanning electronic microscope. In the longitudinal section, the initially randomly distributed Cr dendrites in the as-cast Cu-7Cr alloy were transformed into the fibres aligned parallel to the drawing axis;the Cr dendrites experienced breaking, flattening and rotating, lapping and merging, and homogenizing and refinement during thermo-mechanical processing. In the transverse section, the initially randomly distributed Cr dendrites in the as-cast Cu-7Cr alloy were changed into the curvy ribbon like fibres;the Cr dendrites underwent breaking, flattening and rotating, folding and twisting, and irregularizing and refinement during thermo-mechanical processing.

MBE Growth and Characterization of Strained Hg Te(111)Films on CdTe/GaAs

Strained Hg Te thin films are typical three-dimensional topological insulator materials.Most works have focused on Hg Te(100)films due to the topological properties resulting from uniaxial strain.In this study,strained Hg Te(111)thin films are grown on Ga As(100)substrates with Cd Te(111)buffer layers using molecular beam epitaxy(MBE).The optimal growth conditions for Hg Te films are determined to be a growth temperature of 160℃and an Hg/Te flux ratio of 200.The strains of Hg Te films with different thicknesses are investigated by highresolution x-ray diffraction,including reciprocal space mapping measurements.The critical thickness of Hg Te(111)film on Cd Te/Ga As is estimated to be approximately 284 nm by Matthews'equations,consistent with the experimental results.Reflection high-energy electron diffraction and high-resolution transmission electron microscopy investigations indicate that high-quality Hg Te films are obtained.This exploration of the MBE growth of Hg Te(111)films provides valuable information for further studies of Hg Te-based topological insulators.

Botanical Education: Management of Greening and Maintenance of Modern Landscape

In the management of greening and maintenance of modern garden landscape,the prevention of natural disasters should be emphasized.Adequate fertilizer and water are provided to the plants to meet their growing needs.Pests and diseases are effectively prevented and controlled.

Wafer-scale arrayed p-n junctions based on few-layer epitaxial GaTe

Two-dimensional （2D） materials have attracted substantial attention in electronic and optoelectronic applications with the superior advantages of being flexible, transparent, and highly tunable. Gapless graphene exhibits ultra-broadband and fast photoresponse while the 2D semiconducting MoS2 and GaTe exhibit high sensitivity and tunable responsivity to visible light. However, the device yield and repeatability call for further improvement to achieve large-scale uniformity. Here, we report a layer-by-layer growth of wafer-scale GaTe with a high hole mobility of 28.4 cm^2/（V.s） by molecular beam epitaxy. The arrayed p-n ）unctions were developed by growing few-layer GaTe directly on fhree-inch Si wafers. The resultant diodes reveal good rectifying characteristics and a high photovoltaic external quantum efficiency up to 62% at 4.8 μW under zero bias. The photocurrent reaches saturation fast enough to capture a time constant of 22 μs and shows no sign of device degradation after 1.37 million cycles of operation. Most strikingly, such high performance has been achieved across the entire wafer, making the volume production of devices accessible. Finally, several photoimages were acquired by the GaTe/Si photodiodes with reasonable contrast and spatial resolution, demonstrating the potential of integrating the 2D materials with silicon technology for novel optoelectronic devices.

Understanding the structural evolution of Au/WO_(2.7) compounds in hydrogen atmosphere by atomic scale in situ environmental TEM

Hydrogen energy is a resuscitated clean energy source and its sensitive detection in air is crucial due to its very low explosive limit.Metal oxide decorated with noble metal nanoparticles has been used for the enhancement of gas detection and exhibits superior sensitivity.Understanding the intrinsic mechanism of the detection and the enhancement mechanism is thus becoming a fundamental issue for the further development of novel metal/oxide compound gas-sensing materials.However,the correlation between the microstructural evolution,the charge transport and the complex sensing process has not yet been directly revealed and its atomic mechanism is still debatable.In this study,an Au/WO_(2.7) compound was synthesized and exhibited a strongly enhanced gas sensitivity to many reductive gases,especially H2.Aberration-corrected environmental transmission electron microscopy was used to investigate the atomic-scale microstructural evolution in situ during the reaction between H_(2) and Au/WO_(2.7) compound.Swing and sintering processes of the Au particles on the WO_(2.7) surface were observed under heating and gaseous environments,and no injection of hydrogen atoms was suggested.First principle calculations verified the swing and sintering processes,and they can be explained by the enhancement of H2 sensitivity.

Morphological control of SnTe nanostructures by tuning catalyst composition

A method of controlling the morphology of SnTe nanostructures produced by a simple chemical vapor deposition is presented, in which Au-containing catalysts with different Au concentrations are used to induce specific growth behavior. Triangular SnTe nanoplates with a {100} dominated surface and {100}, {111} and {120} side facets were induced by AuSn catalysts, whereas 〈010〉 SnTe nanowires with four nonpolar {100} side-facets were produced using AusSn catalysts. Through detailed structural and chemical characterization, coupled with surface energy calculations, it is found that nanowire growth is thermodynamically controlled via a vapor-solid-solid growth mechanism, whereas nanoplate growth is kinetically controlled via a vapor-liquid-solid growth mechanism. Therefore, this study provides a fundamental understanding of the catalyst＇s role in the growth of IV-VI compound nanostructures.

Structure and quality controlled growth of InAs nanowires through catalyst engineering

In this study, the structure and quality controlled growth of InAs nanowires using Au catalysts in a molecular beam epitaxy reactor is presented. By tuning the indium concentration in the catalyst, defect-free wurtzite structure and defect-free zinc blende structure InAs nanowires can be induced. It is found that these defect-free zinc blende structure InAs nanowires grow along 〈110〉 directions with four low-energy {111} and two {110} side-wall facets and adopt the （111） catalyst/nanowire interface. Our structural and chemical characterization and calculations identify the existence of a catalyst supersaturation threshold for the InAs nanowire growth. When the In concentration in the catalyst is sufficiently high, defect-free zinc blende structure InAs nanowires can be induced. This study provides an insight into the manipulation of crystal structure and structure quality of III-V semiconductor nanowires through catalyst engineering.

Ni-induced stepwise capacity increase in Ni-poor Li-rich cathode materials for high performance lithium ion batteries

Li-rich cathode materials have been considered as promising candidates for high-energy lithium ion batteries （LIBs）. In this study, we report a new series of Li-rich materials （Li[Li1/B-2x/BMn2/3-x/3Nix]O2 （0.09 ≤x≤ 0.2）） doped with small amounts of Ni as cathode materials in LIBs, which exhibited unusual phenomenon of capacity increase up to tens of cycles due to the continuous activation of the Li2MnO3 phase. Both experimental and computational results indicate that unlike commonly studied Ni-doped Li-rich cathode materials, smaller amounts of Ni doping can promote the stepwise Li2MnO3 activation to obtain increased specific capacity and better cycling capability. In contrast, excessive Ni will over-activate the Li2MnO3 and result in a large capacity loss in the first cycle. The Lil.25Mn0.625Ni0.12sO2 material with an optimized content of Ni delivered a superior high capacity of -280 mAh.g-1 and good cycling stability at room temperature.

High strength and ductility of titanium matrix composites by nanoscale design in selective laser melting

The use of selective laser melting(SLM)to produce titanium matrix composites(TMCs)with high strength while retaining sufficient tensile ductility suitable for structural applications is emerging as an attractive opportunity in the field of advanced manufacturing.However,the presence of coarse ceramic reinforcements as well as difficulties in optimizing the SLM process is a barrier to the application of TMCs.In this study,we demonstrated the production of TMCs reinforced with in situ high aspect ratio Ti B nanowhiskers by selective laser melting using nanosized BN powder additions.Pure Ti with 2.5 vol.%nanosized BN powder showed promise for producing high performance TMCs with retained ductility.BN acted to produce Ti B nanowhiskers with diameter<50 nm.Further,by controlling post process furnace annealing Ti B retained a low diameter but exhibited a high aspect ratio,up to 400.In addition to Ti B refinement,nanosized BN addition promoted grain refinement during SLM,both acting as a solute to induce nucleation events and,as Ti B is formed,providing nucleation sites leading to an ultrafine grain structure in as printed samples and after annealing.The produced TMCs exhibit high tensile yield strength,up to1392 MPa,while retaining tensile ductility up to 10%.This study has shown how nanoscale design in powder bed fusion additive manufacturing techniques can be used to produce high performance TMCs through a combination of refined grain structure and high aspect ratio Ti B leading to TMCs with significant improvement in strength,isotropic properties and retained tensile ductility.

Mirror-twin induced bicrystalline InAs nanoleaves

In this study, leaf-like one-dimensional InAs nanostructures were grown by the metal-organic chemical vapor deposition method. Detailed structural charac- terization suggests that the nanoleaves contain relatively low-energy {122} or {133} mirror twins acting as their midribs and narrow sections connecting the nanoleaves and their underlying bases as petioles. Importantly, the mirror twins lead to identical lateral growth of the twinned structures in terms of crystallography and polarity, which is essential for the formation of lateral symmetrical nanoleaves. It has been found that the formation of nanoleaves is driven by catalyst energy minimization. This study provides a biomimic of leaf found in nature by fabricating a semiconductor nanoleaf.

Condensed point defects enhance thermoelectric performance of rare-earth Lu-doped GeTe

Heavy rare-earth element doping can effectively strengthen phonon scattering,suppress the lattice thermal conductivity,and enhance the overall thermoelectric performance of GeTe.However,the large electronegativity difference between rare-earth elements(such as La,Eu,and Gd)and Ge refrains the doping limit of rare-earth elements below 1 mol.%in GeTe.Here,compared with other rare earth elements,Lu was found to have a relatively small radius and electronegativity difference with Ge,which can induce a high doping level in GeTe.The result shows that Lu doping effectively reduces the lattice thermal conductivity from 0.77 W^(−1) m K^(−1) of GeTe to 0.35 W m^(−1) K^(−1) of Ge_(0.98)Lu_(0.02)Te at 673 K,and further induces a high zT value of 1.5 in Ge_(0.98)Lu_(0.02)Te at 673 K.Extra Sb alloying optimizes the carrier concentration from 1.02×10^(21) cm^(−3) of Ge_(0.98)Lu_(0.02)Te to 1.77×10^(20) cm^(−3) of Ge0.90Lu0.02Sb0.08Te,which results in a reasonable power factor of 33.82μW cm^(−1) K^(−2) and a low electrical thermal conductivity of 0.75 W m^(−1) K^(−1) at 673 K in Ge_(0.90)Lu_(0.02)Sb_(0.08)Te.Correspondingly,a peak zT of 1.75 at 673 K and an average zT of 0.92 within the temperature range of 303–723 K are obtained in Ge_(0.9)Lu_(0.02)Sb_(0.08)Te.This study indicates that Lu and Sb co-doping can effectively boost the thermoelectric performance of GeTe-based thermoelectric materials.

Au-catalysed free-standing wurtzite structured InAs nanosheets grown by molecular beam epitaxy

In this study,we report the growth of free-standing InAs nanosheets using Au catalysts in molecular beam epitaxy.Detailed structural characterizations suggest that wurtzite structured InAs nanosheets,with features of extensive{1120}surfaces,grown along the<1102>direction and adopted{0001}nanosheet/catalyst interfaces,are initiated from wurtzite structured[0001]nanowires as the inclined epitaxial growth due to relatively higher In concentrations in Au catalysts,and grown from these inclined nanostructures through catalyst-induced axial growth and their enhanced lateral growth under the high growth temperature.Based on the facts that the nanosheets contain large low energy{1120}surfaces and{0001}nanosheet/catalyst interfaces,the growth of our nanosheets is a thermodynamically driven process.This study provides new insights into fabricating free-standing Ⅲ-Ⅴ nanosheets for their applications in future nanoscale devices.

Axiotaxy driven growth of belt-shaped InAs nanowires in molecular beam epitaxy

In this study,we demonstrate the axiotaxy driven growth of belt-shaped InAs nanowires using Au catalysts by molecular beam epitaxy.It is found that,the zinc-blende structured InAs nanowires,with the features of[113]growth direction and extensive{110}side-surfaces,are induced by catalysts in Au–In α phase through the axiotaxy growth,in which the lattice mismatch between the projections of atomic planes onto nanowire/catalyst interfaces is minimized by forming extraordinary tilted interfaces.Our atomic-resolution in situ TEM heating experiments show that the catalysts remained in the solid state of Au–In α phase during the axiotaxy growth,by which the vapor–solid–solid growth mechanism can be confirmed.Through manipulating the growth direction,this unusual growth mechanism can provide a practical pathway to control the morphology of the low-dimensional nanomaterials,from conventional nanowires to belt-shaped nanowires utilizing a significant lateral growth,simply using nanoparticles as catalyst.

Optimization of sodium hydroxide for securing high thermoelectric performance in polycrystalline Sn1−xSe via anisotropy and vacancy synergy

The morphology and composition are two key factors to determine the thermoelectric performance of aqueously synthesized tin selenide(SnSe)crystals;however,their controlling is still under exploring.In this study,we report a high figure-of-merit(ZT)of1.5 at 823 K in p-type polycrystalline Sn1−xSe resulted from a synergy of morphology control and vacancy optimization,realized by carefully tuning the sodium hydroxide(NaOH)concentration during solvothermal synthesis.After a comprehensive investigation on various NaOH concentrations,it was found that an optimized NaOH amount of 10 mL with a concentration of 10 mol L^−1 can simultaneously achieve a large average crystal size and a high Sn vacancy concentration of2.5%.The large microplate-like crystals lead to a considerable anisotropy in the sintered pellets,and the high Sn vacancy level contributes to an optimum hole concentration to the level of2.3×10^19 cm^−3,and in turn a high power factor of7.4μW cm^−1 K^−2 at 823 K,measured along the direction perpendicular to the sintering pressure.In addition,a low thermal conductivity of0.41 W m^−1 K^−1 is achieved by effective phonon scattering at localized crystal imperfections including lattice distortions,grain boundaries,and vacancy domains,as observed by detailed structural characterizations.Furthermore,a competitive compressive strength of52.1 MPa can be achieved along the direction of high thermoelectric performance,indicating a mechanically robust feature.This study provides a new avenue in achieving high thermoelectric performance in SnSe-based thermoelectric materials.

Rare-Earth Nd Inducing Record-High Thermoelectric Performance of(GeTe)_(85)(AgSbTe_(2))_(15)

As a promising midtemperature thermoelectric material with both higher thermoelectric performance and mechanical property,Tellurium Antimony Germanium Silver(TAGS-x),written as(GeTe)_(x)(AgSbTe_(2))_(1-x),especially(GeTe)_(0.85)(AgSbTe_(2))_(0.15)(TAGS-85),has attracted wide attention.Herein,we innovatively use Nd doping to synergistically decrease the carrier concentration to the optimal level leading to enhanced dimensionless figure of merit,zT.Our density-functional theory calculation results indicate that Nd-doping reduced carrier concentration should be attributed to the enlargement of band gap.The optimized carrier concentration results in an ultrahigh power factor of~32μWcm^(-1)K^(-2)at 727 K in Ge_(0.74)Ag_(0.13)Sb_(0.11)Nd_(0.02)Te.Simultaneously,the lattice thermal conductivity of Ge_(0.74)Ag_(0.13)Sb_(0.11)Nd_(0.02)Te retained as low as~0.5 at 727 K.Ultimately,a record-high zT of 1.65 at 727 K is observed in the Ge_(0.74)Ag_(0.13)Sb_(0.11)Nd_(0.02)Te.This study indicates rare-earth Nd doping is effective in boosting the thermoelectric performance of TAGS-85 and approached a record-high level via synergistic effect.

Optimal array alignment to deliver high performance in flexible conducting polymer-based thermoelectric devices

Flexible thermoelectric devices(F-TEDs)show great potentials to be applied in curved surface for power generation by harvesting low-grade energy from human body and other heat sources.However,their power generation efficiency is constrained by both unsatisfactory constituent materials performance and immature device design.Here,we used an optimal alignment of vertically-aligned poly(3,4-ethylenedioxythiophene)polystyrene sulfonate(PEDOT:PSS)arrays to assemble a 2.7×3.2 cm^(2)F-TEDs,exhibiting a maximum power output of 10.5μW.Such a high performance can be ascribed to the outstanding power factor of 198μW m^(-1)K^(-2)by the synergetic effect of both high charge mobility and optimal oxidation level and the optimized array alignment that maximizes the temperature difference utilization ratio across the TE legs.Particularly,optimized leg distance of 6 mm and leg length of 12 mm are determined to realize a high temperature difference utilization ratio of over 95%and a record-high output power density of 1.21μW cm^(-2)under a temperature difference of 30 K.Further,reliable bending(1000 cycles)and stability(240 h)tests indicate the outstanding mechanical robustness and environmental stability of the developed F-TEDs.This study indicates our reasonable device design concept and facile material treatment techniques secure high-performance F-TEDs,serving as a reference for other flexible energy harvesting devices with wide practical applications.

Immunological and metabolic characteristics of the Omicron variants infection

The Omicron variants of SARS-CoV-2,primarily authenticated in November 2021 in South Africa,has initiated the 5th wave of global pandemics.Here,we systemically examined immunological and metabolic characteristics of Omicron variants infection.We found Omicron resisted to neutralizing antibody targeting receptor binding domain(RBD)of wildtype SARS-CoV-2.Omicron could hardly be neutralized by sera of Corona Virus Disease 2019(COVID-19)convalescents infected with the Delta variant.Through mass spectrometry on MHC-bound peptidomes,we found that the spike protein of the Omicron variants could generate additional CD8+T cell epitopes,compared with Delta.These epitopes could induce robust CD8+T cell responses.Moreover,we found booster vaccination increased the cross-memory CD8+T cell responses against Omicron.Metabolic regulome analysis of Omicron-specific T cell showed a metabolic profile that promoted the response of memory T cells.Consistently,a greater fraction of memory CD8+T cells existed in Omicron stimulated peripheral blood mononuclear cells(PBMCs).In addition,CD147 was also a receptor for the Omicron variants,and CD147 antibody inhibited infection of Omicron.CD147-mediated Omicron infection in a human CD147 transgenic mouse model induced exudative alveolar pneumonia.Taken together,our data suggested that vaccination booster and receptor blocking antibody are two effective strategies against Omicron.