Electronic Band Structure Changes across the Antiferromagnetic Phase Transition of Exfoliated MnPS3 Flakes Probed by µ-ARPES.

Exfoliated magnetic 2D materials enable versatile tuning of magnetization, e.g., by gating or providing proximity-induced exchange interaction. However, their electronic band structure after exfoliation has not been probed, presumably due to their photochemical sensitivity. Here, we provide micrometer-scale angle-resolved photoelectron spectroscopy of the exfoliated intralayer antiferromagnet MnPS3 above and below the Néel temperature down to one monolayer. Favorable comparison with density functional theory calculations enables identifying the orbital character of the observed bands. Consistently, we find pronounced changes across the Néel temperature for bands consisting of Mn 3d and 3p levels of adjacent S atoms. The deduced orbital mixture indicates that the superexchange is relevant for the magnetic interaction. There are only minor changes between monolayer and thicker films, demonstrating the predominant 2D character of MnPS3. The novel access is transferable to other MPX3 materials (M: transition metal, P: phosphorus, X: chalcogenide), providing several antiferromagnetic arrangements.

Development of Nanostructured Bi2Te3 with High Thermoelectric Performance by Scalable Synthesis and Microstructure Manipulations.

Nanostructuring of thermoelectric (TE) materials leads to improved energy conversion performance; however, it requires a perfect fit between the nanoprecipitates' chemistry and crystal structure and those of the matrix. We synthesize bulk Bi2Te3 from molecular precursors and characterize their structure and chemistry using electron microscopy and analyze their TE transport properties in the range of 300-500 K. We find that synthesis from Bi2O3 + Na2TeO3 precursors results in n-type Bi2Te3 containing a high number density (Nv ∼ 2.45 × 1023 m-3) of Te-nanoprecipitates decorating the Bi2Te3 grain boundaries (GBs), which yield enhanced TE performance with a power factor (PF) of ∼19 µW cm-1 K-2 at 300 K. First-principles calculations validate the role of Te/Bi2Te3 interfaces in increasing the charge carrier concentration, density of states, and electrical conductivity. These optimized TE coefficients yield a promising TE figure of merit (zT) peak value of 1.30 at 450 K and an average zT of 1.14 from 300 to 500 K. This is one of the cutting-edge zT values recorded for n-type Bi2Te3 produced by chemical routes. We believe that this chemical synthesis strategy will be beneficial for future development of scalable n-type Bi2Te3 based devices.

Topologically-Enhanced Thermoelectric Properties in Bi2Te3-Based Compounds: Effects of Grain Size and Misorientation.

Topological insulators (TIs) and thermoelectric (TE) materials seem to belong to distinct physical realms; however, in practice, they both share common characteristics. Introducing concepts from TIs into TE materials to enhance their performance and achieve better understanding of electronic transport requires extensive research. Particularly, grain size, misorientation, and grain boundary (GB) character are of utmost importance to attain effective charge carrier transport in TE polycrystals; these factors, however, have not been thoroughly explored. Herein, we investigate the correlation between grain size, misorientation, and lattice strain in Bi2Te3 and its TI signature, aiming to improve its TE performance. We reveal an unusual behavior showing that electron mobility increases upon the increase of grain size, reaching at a maximum value of 495 cm2/V·s for an optimum grain size of 600 nm and most-frequent GB misorientation angle of 60° and then decreases with increasing grain size. It is also indicated that the combined effects of grain size reduction and point defects induce lattice strain in the Bi2Te3-matrix that is essential to trigger the TI contribution to TE transport. This trend is corroborated by first-principles calculations showing that compressive strains form multiple valleys in the valence band and opens the TI band gap. Such a combination of physical phenomena in a well-known TE material is unique and can promote our understanding of the nature of TE transport with implications for TE energy conversion.

Temperature dependence of Fano resonances in CrPS4.

A Fano resonance, as often observed in scattering, absorption, or transmission experiments, arises from quantum interference between a discrete optical transition and a continuous background. Here, we present a temperature-dependent study on Fano resonances observed in photoluminescence from flakes of the layered semiconductor antiferromagnet chromium thiophosphate (CrPS4). Two Fano resonances with a distinctly different temperature dependence were identified. The continuous background that is responsible for the Fano resonances is attributed to the d-d transition of the optically active Cr3+ center, predominantly the spin-forbidden 2Eg → 4A2g transition with contributions of the broad-band 4T2g → 4A2g transition. The discrete states that interfere with this continuous background are suggested to arise from localized atomic phosphorus. A model idea for explaining the individual temperature dependence of the Fano resonances is presented.

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.

Enhanced Charge Transport in Ca2MnO4-Layered Perovskites by Point Defect Engineering.

Coupling between thermal and charge transport in crystalline materials has always been one of the greatest challenges in understanding the underlying physics of thermoelectric materials. In this sense, CaO(CaMnO3)m Ruddlesden-Popper layered perovskites, comprising m perovskite subcells separated by CaO planes, exhibit intriguing thermal and electronic transport properties that can be tuned by altering their crystal periodicities. Applying the well-established phonon glass electron crystal (PGEC) concept enables us to increase the transparency of these CaO planes to electron transport at the same time while preserving their opacity to phonon transport. First-principles calculations indicate that the total local potential at CaO planes, where Y substitutes for Ca, is lower by ca. 50% compared to La substitution. Measurements of the electrical conductivity and Seebeck coefficients for Ca2-xRxMnO4 (R = La or Y; x = 0.01, 0.05, 0.1, and 0.15) bulk materials in the range of 300-1000 K confirm that compounds doped with Y exhibit higher electrical conductivity values than their La-doped counterparts. We attribute this to lower polaron hopping energy values (up to 23%) evaluated using the small polaron hopping model. This study introduces an original way to employ the PGEC approach for thermoelectric oxides.

Exfoliated CrPS4 with Promising Photoconductivity.

Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary layered semiconductor chromium thiophosphate (CrPS4 ) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one-step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS4 is also liquid exfoliated, and in combination with colloidal graphene, an ink-jet-printed photodetector is created. This all-printed graphene/CrPS4 /graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 108 (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS4 as active components in light detection, when introduced as ink-printable moieties with a large benefit for manufacturing.

Liquid-Film Assisted Mechanism of Reactive Flash Sintering in Oxide Systems.

Reactive flash sintering in oxide systems is analyzed assuming the formation of a liquid film at the particle contacts at the flash onset temperature. Formation of intermediate phases, as well as phase assemblage, are predicted upon optimal conditions of the electric field and current density. In single-phase impure oxides, the solidus and the solubility limit determine the flash onset temperature. In reacting binary systems, the composition of the liquidus determines primarily the reaction products during the cooling. In multicomponent systems, the oxide with the lowest flash temperature forms the interfacial liquid film, and the solid phase assemblage follows the equilibrium phase diagram. Examples from literature are consistent with reactive flash sintering and flash sintering assisted by a transient liquid film.

Tailoring Thermoelectric Transport Properties of Ag-Alloyed PbTe: Effects of Microstructure Evolution.

Capturing and converting waste heat into electrical power through thermoelectric generators based on the Seebeck effect is a promising alternative energy source. Among thermoelectric compounds, PbTe can be alloyed and form precipitates by aging at elevated temperatures, thus reducing thermal conductivity by phonon scattering. Here, PbTe is alloyed with Ag to form Ag-rich precipitates having a number density controlled by heat treatments. We employ complementary scanning transmission electron microscopy and atom probe tomography to analyze the precipitate number density and the PbTe-matrix composition. We measure the temperature-dependent transport coefficients and correlate them with the microstructure. The thermal and electrical conductivities, as well as the Seebeck coefficients, are found to be highly sensitive to the microstructure and its temporal evolution, e.g., the number density of Ag-rich precipitates increases by ca. 3 orders of magnitude and reaches (1.68 ± 0.92) × 1024 m-3 upon aging at 380 °C for 6 h, which is pronounced by reduction in thermal conductivity to a value as low as 0.85 W m-1 K-1 at 300 °C. Our findings will help to guide predictive tools for further design of materials for energy harvesting.

Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials.

Dislocations have been considered to be an efficient source for scattering midfrequency phonons, contributing to the enhancement of thermoelectric performance. The structure of dislocations can be resolved by electron microscopy whereas their chemical composition and decoration state are scarcely known. Here, we correlate transmission Kikuchi diffraction and (scanning) transmission electron microscopy in conjunction with atom probe tomography to investigate the local structure and chemical composition of dislocations in a thermoelectric Ag-doped PbTe compound. Our investigations indicate that Ag atoms segregate to dislocations with a 10-fold excess of Ag compared with its average concentration in the matrix. Yet the Ag concentration along the dislocation line is not constant but fluctuates from ∼0.8 to ∼10 atom % with a period of about 5 nm. Thermal conductivity is evaluated applying laser flash analysis, and is correlated with theoretical calculations based on the Debye-Callaway model, demonstrating that these Ag-decorated dislocations yield stronger phonon scatterings. These findings reduce the knowledge gap regarding the composition of dislocations needed for theoretical calculations of phonon scattering and pave the way for extending the concept of defect engineering to thermoelectric materials.

A Practical Approach to Evaluate Lattice Thermal Conductivity in Two-Phase Thermoelectric Alloys for Energy Applications.

Modelling of the effects of materials' microstructure on thermal transport is an essential tool for materials design, and is particularly relevant for thermoelectric (TE) materials converting heat into electrical energy. Precipitates dispersed in a TE matrix act as phonon-scattering centers, thereby reducing thermal conductivity. We introduce a practical approach to tailor a definite precipitate size distribution for a given TE matrix, and implement it for PbTe. We evaluate vibrational properties from first principles, and develop an expression for phonon relaxation time that considers both matrix vibrational properties and precipitate size distribution. This provides us with guidelines for optimizing thermal conductivity.

Role of Nanostructuring and Microstructuring in Silver Antimony Telluride Compounds for Thermoelectric Applications.

Thermoelectric (TE) materials are of utmost significance for conversion of heat flux into electrical power in the low-power regime. Their conversion efficiency depends strongly on the microstructure. AgSbTe2-based compounds are high-efficiency TE materials suitable for the mid-temperature range. Herein, we explore an Ag16.7Sb30Te53.3 alloy (at %) subjected to heat treatments at 380 °C for different durations aimed at nucleation and coarsening of Sb2Te3-precipitates. To characterize the Sb2Te3-precipitation, we use a set of methods combining thermal and electrical measurements in concert with transmission electron microscopy and atom probe tomography. We find correlations between the measured TE transport coefficients and the applied heat treatments. Specifically, the lowest electrical and thermal conductivity values are obtained for the as-quenched state, whereas the highest values are observed for alloys aged for 8 h. In turn, long-term heat treatments result in intermediate values of transport coefficients. We explain these findings in terms of interplay between precipitate formation and variations in the matrix composition, highlighting the importance of thermal stability of the material under service conditions.

Effects of Rapid Solidification on Phase Formation and Microstructure Evolution of AgSbTe2-Based Thermoelectric Compounds.

We report on rapid solidification of an Ag(16.7)Sb(30.0)Te(53.3) compound using planar flow casting to stabilize the δ-AgSbTe2 single phase and avoid precipitation of the interconnected Sb2Te3 phase, which leads to deterioration of thermoelectric properties. Rapidly solidified samples are in form of flakes with different thickness (60­400 µm). Precipitation of Sb2Te3 phase is fully inhibited in thin flakes (thickness below 100 µm), which consist of an homogeneous δ-AgSbTe2 matrix, whereas isolated Sb2Te3 precipitates, dispersed throughout the δ-AgSbTe2 matrix, were found in thick flakes (thickness above 100 µm). The lattice parameter of the δ-AgSbTe2 phase progressively increases with the cooling rate, indicating progressive supersaturation of the matrix for high degree of supercooling. Bulk specimens were prepared by hot pressing of the rapidly solidified flakes to evaluate thermoelectric properties. After sintering of the rapidly solidified flakes, the differential scanning calorimetry (DSC) traces indicates partial decomposition of the non equilibrium δ-AgSbTe2 into the stable phases. Measurements of the thermoelectric transport properties indicate the positive effects of rapid solidification on thermal conductivity and Seebeck coefficient and its negative effect on electrical conductivity, suggesting an operative way to improve thermoelectric performance.

Atom-probe tomography of nickel-based superalloys with green or ultraviolet lasers: a comparative study.

Recent developments in the technology of laser-pulsed local-electrode atom-probe (LEAP) tomography include a picosecond ultraviolet (UV) laser system having a 355 nm wavelength and both external and in-vacuum optics. This approach ensures focusing of the laser beam to a smaller spot diameter than has heretofore been obtained using a green (532 nm wavelength) picosecond laser. We compare the mass spectra acquired, using either green or UV laser pulsing, from nickel-based superalloy specimens prepared either electrochemically or by lifting-out from bulk material using ion-beam milling in a dual-beam focused ion beam microscope. The utilization of picosecond UV laser pulsing yields improved mass spectra, which manifests itself in higher signal-to-noise ratios and mass-resolving power (m/Δm) in comparison to green laser pulsing. We employ LEAP tomography to investigate the formation of misoriented defects in nickel-based superalloys and demonstrate that UV laser pulsing yields better accuracy in compositional quantification than does green laser pulsing. Furthermore, we show that using a green laser the quality of mass spectra collected from specimens that were lifted-out by ion milling is usually poorer than for electrochemically-sharpened specimens. Employing UV laser pulsing yields, however, improved mass spectra in comparison to green laser pulsing even for ion-milled microtips.