Tailoring Single-Electron Emission Distributions in the Time-Energy Phase Space.

The precise characterization and control of single-electron wave functions emitted from a single-electron source are essential for advancing electron quantum optics. Here, we introduce a method for tailoring a single-electron emission distribution using energy filtering, enabling selective control of the distribution under various energy barrier conditions of the filter. The tailored electron is studied by reconstructing its Wigner distribution in the time-energy phase space using the continuous-variable tomography method. Our results reveal that the filtering cuts the portion of the distribution below the energy-barrier height of the filter in the time-energy space. While the filtering is demonstrated in a classical regime of the emitted electrons, we expect that this study significantly contributes to the design and implementation of advanced experiments toward quantum information processing based on single electrons.

High-Energy Electron Scattering in Thick Samples Evaluated by Bright-Field Transmission Electron Microscopy, Energy-Filtering Transmission Electron Microscopy, and Electron Tomography.

Energy-filtering transmission electron microscopy (TEM) and bright-field TEM can be used to extract local sample thickness $t$ and to generate two-dimensional sample thickness maps. Electron tomography can be used to accurately verify the local $t$. The relations of log-ratio of zero-loss filtered energy-filtering TEM beam intensity ($I_{{\rm ZLP}}$) and unfiltered beam intensity ($I_{\rm u}$) versus sample thickness $t$ were measured for five values of collection angle in a microscope equipped with an energy filter. Furthermore, log-ratio of the incident (primary) beam intensity ($I_{\rm p}$) and the transmitted beam $I_{{\rm tr}}$ versus $t$ in bright-field TEM was measured utilizing a camera before the energy filter. The measurements were performed on a multilayer sample containing eight materials and thickness $t$ up to 800 nm. Local thickness $t$ was verified by electron tomography. The following results are reported:• The maximum thickness $t_{{\rm max}}$ yielding a linear relation of log-ratio, $\ln ( {I_{\rm u}}/{I_{{\rm ZLP}}})$ and $\ln ( {I_{\rm p}}/{I_{{\rm tr}}} )$, versus $t$.• Inelastic mean free path ($\lambda _{{\rm in}}$) for five values of collection angle.• Total mean free path ($\lambda _{{\rm total}}$) of electrons excluded by an angle-limiting aperture.• $\lambda _{{\rm in}}$ and $\lambda _{{\rm total}}$ are evaluated for the eight materials with atomic number from $\approx$10 to 79.The results can be utilized as a guide for upper limit of $t$ evaluation in energy-filtering TEM and bright-field TEM and for optimizing electron tomography experiments.

Independent trafficking of flavocytochrome b558 and myeloperoxidase to phagosomes during phagocytosis visualised by energy-filtering and energy-dispersive spectroscopy-scanning transmission electron microscopy.

When polymorphonuclear leukocytes (PMNs) phagocytose opsonised zymosan particles (OPZ), free radicals and reactive oxygen species (ROS) are formed in the phagosomes. ROS production is mediated by NADPH oxidase (Nox), which transfers electrons in converting oxygen to superoxide (O2- ). Nox-generated O2- is rapidly converted to other ROS. Free radical-forming secretory vesicles containing the Nox redox center flavocytochrome b558, a membrane protein, and azurophil granules with packaged myeloperoxidase (MPO) have been described. Presuming the probable fusion of these vesicular and granular organelles with phagosomes, the translation process of the enzymes was investigated using energy-filtering and energy-dispersive spectroscopy-scanning transmission electron microscopy. In this work, the primary method for imaging cerium (Ce) ions demonstrated the localisation of H2 O2 generated by phagocytosing PMNs. The MPO activity of the same PMNs was continuously monitored using 0.1% 3,3'-diaminobenzidine-tetrahydrochloride (DAB) and 0.01% H2 O2 . A detailed view of these vesicular and granular structures was created by overlaying each electron micrograph with pseudocolors: blue for Ce and green for nitrogen (N).

Energy Filtering and Thermoelectrics: Artifact or Artifice?

The possibility of selecting carrier energies by using suitable potential barriers has played a long-standing role in the physics of thermionic devices. It entered instead the arena of thermoelectricity only in the Nineties through the pioneering work of Rowe and Min and of Nishio and Hirano. Since then, the virtuous use of energy barrier in thermoelectricity has gone through alternating fortunes, with analyses sustaining its capacity as a tool to decouple the adverse interdependency between Seebeck coefficient and electrical conductivity; and papers disproving instead such a possibility. In spite of a yet uncomplete theoretical framework, over the last years an impressive number of papers has been published reporting unusual dependencies of thermopower and conductivity upon carrier densities, mostly in nanocomposites­and attributing them to energy filtering. Aim of this paper is to discuss to which extent and under which physical constraints energy filtering may be actually invoked to explain enhanced power factors­and which alternate possibilities of explanation may be considered instead.

Energy Filtering in Doping Modulated Nanoengineered Thermoelectric Materials: A Monte Carlo Simulation Approach.

Using Monte Carlo electronic transport simulations, coupled self-consistently with the Poisson equation for electrostatics, we explore the thermoelectric power factor of nanoengineered materials. These materials consist of alternating highly doped and intrinsic regions on the scale of several nanometers. This structure enables the creation of potential wells and barriers, implementing a mechanism for filtering carrier energy. Our study demonstrates that by carefully designing the nanostructure, we can significantly enhance its thermoelectric power factor compared to the original pristine material. Importantly, these enhancements stem not only from the energy filtering effect that boosts the Seebeck coefficient but also from the utilization of high-energy carriers within the wells and intrinsic barrier regions to maintain relatively high electronic conductivity. These findings can offer guidance for the design and optimization of new-generation thermoelectric materials through improvements in the power factor.

Enhancing Thermoelectric Performance of n-Type Bi2Te2.7Se0.3 through Incorporation of Amorphous Si3N4 Nanoparticles.

Bi2Te3-based thermoelectric (TE) materials are the state-of-the-art compounds for commercial applications near room temperature. Nevertheless, the application of the n-type Bi2Te2.7Se0.3 (BTS) is restricted by the comparatively low figure of merit (ZT) and intrinsic embrittlement. Here, we show that through dispersion of amorphous Si3N4 (a-Si3N4) nanoparticles both 14% increase in power factor (at 300 K) and 48% decrease in lattice thermal conductivity are simultaneously realized. The increased power factor comes from enhanced thermopower and reduced electrical resistivity while the reduced lattice thermal conductivity originates mainly from scattering of middle- and low-frequency phonons at the incorporated a-Si3N4 nanoparticles. As a result, a large ZTmax = 1.19 (at 373 K) and an average ZTave ∼ 1.12 (300-473 K) with better mechanical properties are achieved for the BTS/0.25 wt % Si3N4 sample. Present results demonstrate that the incorporation of a-Si3N4 is a promising way to improve TE performance.

Room-Temperature Thermoelectric Performance of n-Type Multiphase Pseudobinary Bi2Te3-Bi2S3 Compounds: Synergic Effects of Phonon Scattering and Energy Filtering.

Bismuth telluride-based alloys possess the highest efficiencies for the low-temperature-range (<500 K) applications among thermoelectric materials. Despite significant advances in the efficiency of p-type Bi2Te3-based materials through engineering the electronic band structure by convergence of multiple bands, the n-type pair still suffers from poor efficiency due to a lower number of electron pockets near the conduction band edge than the valence band. To overcome the persistent low efficiency of n-type Bi2Te3-based materials, we have fabricated multiphase pseudobinary Bi2Te3-Bi2S3 compounds to take advantages of phonon scattering and energy filtering at interfaces, enhancing the efficiency of these materials. The energy barrier generated at the interface of the secondary phase of Bi14Te13S8 in the Bi2Te3 matrix resulted in a higher Seebeck coefficient and consequently a higher power factor in multiphase compounds than the single-phase alloys. This effect was combined with low thermal conductivity achieved through phonon scattering at the interfaces of finely structured multiphase compounds and resulted in a relatively high thermoelectric figure of merit of ∼0.7 over the 300-550 K temperature range for the multiphase sample of n-type Bi2Te2.75S0.25, double the efficiency of single-phase Bi2Te3. Our results inform an alternative alloy design to enhance the performance of thermoelectric materials.

Realization of an ultra-low lattice thermal conductivity in Bi2AgxSe3 nanostructures for enhanced thermoelectric performance.

Bismuth Selenide is a Tellurium free topological insulator in V-VI compounds with an excellent thermoelectric performance from room temperature to mid-temperature region. Herein, hydrothermally prepared polycrystalline Bi2AgxSe3 nanostructures have been reported for thermoelectric application. The crystal structure identification and morphology with the elemental presence were analyzed by XRD (X-ray diffraction), HR-SEM with EDS (High resolution scanning electron microscope with energy dispersive X-ray), and HR-TEM (High-resolution transmission electron microscope) measurements. The reduced lattice thermal conductivity and enhanced electrical transport properties synergistically boost the thermoelectric properties through the highly-dense stacking faults with the presence of dislocations. The IFFT (Inverse Fast Fourier Transform) pattern reveals the existence of stacking faults and dislocations. These highly dense stacking faults and dislocations act as active phonon scattering centers, which can contribute to effective phonon scattering resultsin extremely low lattice thermal conduction of 0.3 W/mK at 543 K. On the other hand, the involvement of phonon-phonon scattering primarily reduced the lattice thermal conductivity at elevated temperatures. In addition, phonon-carrier scattering was less compared to phonon-phonon scattering at elevated temperature region. Moreover, the enhancement of electrical conductivity and controlled reduction of the Seebeck coefficient plays a vital role in achieving the maximum power factor of 335 µW/mK2 at 543 K due to the energy filtering effect. The synergistic combination of low thermal conduction and the maximum power factor helps to achieve the high peak zT of 0.3 at 543 K.

High Power Factor Nb-Doped TiO2 Thermoelectric Thick Films: Toward Atomic Scale Defect Engineering of Crystallographic Shear Structures.

Donor-doped TiO2-based materials are promising thermoelectrics (TEs) due to their low cost and high stability at elevated temperatures. Herein, high-performance Nb-doped TiO2 thick films are fabricated by facile and scalable screen-printing techniques. Enhanced TE performance has been achieved by forming high-density crystallographic shear (CS) structures. All films exhibit the same matrix rutile structure but contain different nano-sized defect structures. Typically, in films with low Nb content, high concentrations of oxygen-deficient {121} CS planes are formed, while in films with high Nb content, a high density of twin boundaries are found. Through the use of strongly reducing atmospheres, a novel Al-segregated {210} CS structure is formed in films with higher Nb content. By advanced aberration-corrected scanning transmission electron microscopy techniques, we reveal the nature of the {210} CS structure at the nano-scale. These CS structures contain abundant oxygen vacancies and are believed to enable energy-filtering effects, leading to simultaneous enhancement of both the electrical conductivity and Seebeck coefficients. The optimized films exhibit a maximum power factor of 4.3 × 10-4 W m-1 K-2 at 673 K, the highest value for TiO2-based TE films at elevated temperatures. Our modulation strategy based on microstructure modification provides a novel route for atomic-level defect engineering which should guide the development of other TE materials.

Enhanced Thermoelectric Properties of Coated Vanadium Oxynitride Nanoparticles/PEDOT:PSS Hybrid Films.

Thermoelectric (TE) materials transform thermal energy into electricity, which can play an important role for global sustainability. Conducting polymers are suitable for the preparation of flexible TE materials because of their low-cost, lightweight, flexible, and easily synthesized properties. Here, we fabricate organic-inorganic hybrids by combining vanadium oxynitride nanoparticles coated with nitrogen-doped carbon (NC@VNO) and poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS). We find that the electrical conductivity, Seebeck coefficient, and power factor of the NC@VNO/PEDOT:PSS film can be enhanced up to 4158 S/cm, 45.8 µV/K, and 873 µW/mK2 at 380 K, respectively. The large enhancement of the power factor may be due to the facilitation of the interfacial charge transport tunnel between the NC@VNO nanoparticles and PEDOT:PSS. The improvement of the Seebeck coefficient may be due to the energy filter effect as induced by interfacial contact and internal electric field between the NC@VNO nanoparticles and PEDOT:PSS. Our measurement suggests that the high binding energy of pyrrolic-N enhances the Seebeck coefficient, and the high binding energy of oxide-N increases electrical conductivity.

Measuring Spatially-Resolved Potential Drops at Semiconductor Hetero-Interfaces Using 4D-STEM.

Characterizing long-range electric fields and built-in potentials in functional materials at nano to micrometer scales is of supreme importance for optimizing devices, e.g., the functionality of semiconductor hetero-structures or battery materials is determined by the electric fields established at interfaces which can also vary spatially. In this study, momentum-resolved four-dimensional scanning transmission electron microscopy (4D-STEM) is proposed for the quantification of these potentials and the optimization steps required to reach quantitative agreement with simulations for the GaAs/AlAs hetero-junction model system are shown. Using STEM the differences in the mean inner potentials (∆MIP) of two materials forming an interface and resulting dynamic diffraction effects have to be considered. This study shows that the measurement quality is significantly improved by precession, energy filtering and a off-zone-axis alignment of the specimen. Complementary simulations yielding a ∆MIP of 1.3 V confirm that the potential drop due to charge transfer at the intrinsic interface is ≈0.1 V, in agreement with experimental and theoretical values found in literture. These results show the feasibility of accurately measuring built-in potentials across hetero-interfaces of real device structures and its promising application for more complex interfaces of other polycrystalline materials on the nanometer scale.

Enhanced Thermoelectric Performance of Yb-Filled Skutterudite with Bottom-Up Formed CoSi2 Nanoparticles.

It is known that Yb-filled skutterudite with excellent thermoelectric performance is promising for a power generation device in the intermediate temperature region. Here we created a new approach to obtain nanostructured materials by adding Si to Co-overstoichiometric Yb-filled skutterudite through high-energy ball milling, which embedded bottom-up formed CoSi2 nanoparticles into grain-refining Yb0.25Co4Sb12, synergistically resulting in the enhanced thermoelectric properties and room-temperature hardness. On one hand, the abundant grain boundaries and phase interfaces effectively blocked the propagation of medium-low frequency phonons, resulting in a lower lattice thermal conductivity. On the other hand, phase interfaces barrier nicely screened a portion of low-energy electrons, leading to an improved power factor. As a result, an enhanced peak ZT value of ∼1.43 at 823 K and a promising average ZT of ∼1.00 between 300 and 823 K were achieved in the Yb0.25Co4Sb12/0.05CoSi2 sample. Meanwhile, such nanostructures also enhanced the hardness through the collective contributions of second phase and fine grain strengthening, which made skutterudite more competitive in practical application.

Recent Advances on Thermoelectric Silicon for Low-Temperature Applications.

Silicon is the most widely used functional material, as it is geo-abundant and atoxic. Unfortunately, its efficiency as a thermoelectric material is very poor. In this paper, we present and discuss advances of research on silicon and related materials for thermoelectric applications, mostly focusing on the comparison between the two strategies deployed to increase its performance, namely either reducing its thermal conductivity or, in polycrystalline materials, increasing its power factor. Special attention will be paid to recent results concerning silicon thin films. The enhancement of Si performances has motivated efforts to develop integrated heat microharvesters operating around room temperature, which will be reviewed also in view of their applications to power wireless sensors for the Internet of Things.

Effects of Co-doping and Microstructure on Charge Carrier Energy Filtering in Thermoelectric Titanium-Doped Zinc Aluminum Oxide.

ZnO is a promising thermoelectric (TE) material for high-temperature applications; however, its TE performance is limited by strong coupling between electrical and thermal transport. In this study, we synthesized Al and Ti co-doped ZnO by a solid-state reaction and air sintering at 1500 °C and analyzed the microstructure to establish its correlation with TE properties. The TE transport properties were measured between room temperature and 800 °C, and electronic properties were calculated from first principles calculations. Herein, we introduced second and third phases into a ZnO-based matrix to enhance its power factor (PF) by charge carrier energy filtering by applying co-doping with Al and Ti. Although multiphase materials usually do not exhibit high PF, in this study, it is observed that three-phase ZnO-based materials exhibit higher PF values compared to the two-phase materials. We observed unusual behavior, in which the Seebeck coefficient (S ) and electrical conductivity (σ) values increased simultaneously with temperature for Zn1-x-yAlxTiyO, originating from energy filtering of charge carriers due to both co-doping and the peculiar multiphase structure. High σ values were associated with the increase of electron concentration, while high S values were due to Fermi energy tuning and heavier effective masses initiated by Al and Ti co-doping. Besides increasing the PF, the multiphase structure played an essential role in reducing lattice thermal conductivity due to phonon scattering by the Umklapp, point defect, and second-phase mechanisms. Our approach yielded an increase of the TE figure of merit upon formation of a three-phase 2 wt % Ti-doped Zn0.98Al0.02O compound of ca. 10 times compared to the corresponding value attained for its two-phase ZnAl0.02O counterpart.

High Thermoelectric Performance SnTe with a Segregated and Percolated Structure.

A nanostructure has a significant role in enhancing the power factor and preventing the heat propagation for thermoelectric materials. Herein, we propose a unique segregated and percolated (SP) microphase-separated structure to enhance the thermoelectric performance of SnTe. The SP structure is composed of insoluble SnTe and AgCuTe, in which AgCuTe with ultralow lattice thermal conductivity undergoes a solid-phase welding during a spark plasma sintering process and forms continuous percolated layers at the interface of isolated SnTe. The SP structure achieved a simultaneous scattering for low energy holes due to the energy offset of the valence band maximum between SnTe and AgCuTe and for phonons due to the noncoherent interfaces between SnTe and AgCuTe, resulting in a high Seebeck coefficient of ∼219.4 µV/K and a low lattice thermal conductivity of ∼1.1 W m-1 K-1 at 800 K for (SnTe)0.55(AgCuTe)0.45. The thermoelectric performance was further enhanced by means of the cosubstitution of In and Mn for Sn in the SnTe lattice, inducing resonance levels and extra phonon scattering. As a result, the SP structure combined with In/Mn codoping enable us to achieve a low lattice thermal conductivity of 0.47 W m-1 K-1, a peak ZT of ∼1.45 at 800 K, and a high average ZT of ∼0.73 (400-800 K) for (Sn0.98In0.01Mn0.01Te)0.75(AgCuTe)0.25.

Optimized Thermoelectric Performance of Carbon Nanoparticle-Carbon Nanotube Heterostructures by Tuning Interface Barrier Energy.

Herein, thermoelectric carbon nanoparticle (CNP)-carbon nanotube (CNT) heterostructures are introduced as a promising flexible thermoelectric material. The optimal barrier energy between the CNP and CNT increases the Seebeck coefficient (S) of the heterostructures through the energy filtering effect. For optimized thermoelectric performance, the CNP-CNT barrier energy can be effectively tuned by controlling the work function of the CNPs. The optimized p-type CNP-CNT heterostructures exhibited S and power factor (PF) of 50.6 ± 1.4 µV K-1 and 400 ± 26 µW m-1 K-2, respectively. The n-type CNP-CNT heterostructures, optimized for another work function of the CNPs, exhibited S and PF of up to -37.5 ± 3.4 µV K-1 and 214 ± 42 µW m-1 K-2, respectively. The energy harvesting capability of a thermoelectric generator prepared using p- and n-type CNP-CNT heterostructures with optimized barrier energies is demonstrated. The thermoelectric generator with 10 p-type and 9 n-type thermoelectric elements exhibited a maximum output power of 0.12 µW from a ΔT of 5 K. This work shows a facile strategy for synthesizing thermoelectric CNP-CNT heterostructures with optimized energy filtering effects. Application to the thermoelectric device on a paper substrate is also discussed.

Recent Progress in Multiphase Thermoelectric Materials.

Thermoelectric materials, which directly convert thermal energy to electricity and vice versa, are considered a viable source of renewable energy. However, the enhancement of conversion efficiency in these materials is very challenging. Recently, multiphase thermoelectric materials have presented themselves as the most promising materials to achieve higher thermoelectric efficiencies than single-phase compounds. These materials provide higher degrees of freedom to design new compounds and adopt new approaches to enhance the electronic transport properties of thermoelectric materials. Here, we have summarised the current developments in multiphase thermoelectric materials, exploiting the beneficial effects of secondary phases, and reviewed the principal mechanisms explaining the enhanced conversion efficiency in these materials. This includes energy filtering, modulation doping, phonon scattering, and magnetic effects. This work assists researchers to design new high-performance thermoelectric materials by providing common concepts.

Significantly Enhanced Thermoelectric Performance of Graphene through Atomic-Scale Defect Engineering via Mobile Hot-Wire Chemical Vapor Deposition Systems.

Over the years, numerous studies have attempted to develop two-dimensional (2D) materials for improving both the applicability and performance of thermoelectric devices. Among the 2D materials, graphene is one of the promising candidates for thermoelectric materials owing to its extraordinary electrical properties, flexibility, and nontoxicity. However, graphene synthesized through traditional methods suffers from a low Seebeck coefficient and high thermal conductivity, resulting in an extremely low thermoelectric figure of merit (ZT). Here, we present an atomic-scale defect engineering strategy to improve the thermoelectric properties of graphene using embedded high-angle tilt boundary (HATB) domains in graphene films. These HATB domains serve as both energy filtering sites to filter out lower-energy charge carriers and scattering sites for phonons. Compared to the conventionally grown chemical vapor deposited graphene, the graphene with HATB domains shows an improved Seebeck coefficient (50.1 vs 21.1 µV K-1) and reduced thermal conductivity (382 vs 952 W m-1K-1), resulting in a ZT value that is ∼7 times greater at 350 K. This defect engineering strategy is promising not only for graphene-based materials but also for 2D materials, in general, where further research and optimization could overcome the limitations of conventional bulk thermoelectric materials in energy-harvesting systems.

Significant Enhancement in the Seebeck Coefficient and Power Factor of p-Type Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) through the Incorporation of n-Type MXene.

Thermoelectric (TE) materials are important for sustainable development because they can directly convert heat into electricity. Compared with inorganic TE materials, conductive polymers have demonstrated unique benefits and their irreplaceability. But their TE properties, particularly the Seebeck coefficient, must be greatly enhanced for practical application. In this work, MXene (Ti3C2Tx), an n-type two-dimensional material, is blended into p-type poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The Seebeck coefficient of the composites increases with the increasing MXene loading at the MXene loading below 33 wt % and then decreases with further increasing of the MXene loading. MXene can enhance the Seebeck coefficient from 23 up to 57.3 µV K-1 and the power factor from 44.1 up to 155 µW m-1 K-2. For the first time, enhancement in the Seebeck coefficient of a p-type TE polymer by an n-type filler has been achieved. Enhancement in the Seebeck coefficient is ascribed to energy filtering of charge carriers by the internal electric field arising from the electron transfer from MXene to PEDOT:PSS. The internal electric field can filter the charge carriers with low energy and thus enhance the Seebeck coefficient.

Ecofriendly Highly Robust Ag8SiSe6-Based Thermoelectric Composites with Excellent Performance Near Room Temperature.

Bi2(TeSe)3 is a dominant n-type thermoelectric material used in commercial applications. However, its toxicity and rarity hamper further large-scale industrial applications. Herein, we develop a Ag8SiSe6-based composite as a promising n-type semiconductor with the advantages of nontoxicity and elemental abundance. Ag8SiSe6 composites with Ag2Se and Si nanoprecipitation are fabricated by a unique precipitation reaction sensitive to the hot pressing process. The energy-filtering effect between these phases optimizes electrical resistivity (∼14.59 µΩ·m) and the Seebeck coefficient (above -150 µV·K-1) of the composites, resulting in a maximum power factor of ∼1772 µW·m-1·K-2(@125 °C), which is the highest value in an argyrodite system near room temperature. Nanoprecipitation of Ag2Se and Si can also scatter more phonons and further reduce the lattice thermal conductivity to 0.20 W·m-1·K-1. As a result, a maximum ZT value of ∼0.9 (@125 °C) and an average ZT value of ∼0.7 (25-200 °C) are obtained in the composite with 12 vol % Ag2Se and 0.23 vol % Si, which is sintered at 525 °C. These thermoelectric properties are comparable to those of a commercial n-type Bi2Te3 compound. In addition, the Ag8SiSe6 composite has robust mechanical properties (Vickers hardness of >110 HV and bending strength of 70.6 MPa), much better than those of other thermoelectric compounds, because of which Ag8SiSe6 has great commercial application as an alternative to Bi2Te3-based compounds.