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Copper has received considerable attention for conductive nanocomposites as an alternative to costly silver or gold. However, practical application has been impeded by its susceptibility to oxidation in air. Here we report a novel scalable synthesis method of non-oxidized copper nanoparticles (InSituCuNPs) by pre-mixing and in-situ reducing copper formate-(butylamine-octylamine) complex inside soft epoxy matrix. The solid–liquid phase change of the copper formate complex, during the nanocomposite spark-plasma-sintering process, promotes uniform dispersion. Even the outermost atoms of InSituCuNPs are not oxidized since they are surrounded by the thick matrix polymer as soon as in-situ reduced into metallic copper, resulting in high electrical (15,048 Scm−1) and thermal (28.4 Wm−1K−1) conductivities of the nanocomposite. Furthermore, a small addition of 1-dimensional carbon nanotubes decorated with 0-dimensional copper nanoparticles (

The evolutionary magnetism associated with the interlayer spacing in two-dimensional (2D) Y2C electrides has been investigated by first-principles total-energy calculations based on density functional theory. Several structures with different c-axis parameters around the optimized value were taken into our consideration. Mapping of the electron localization function shows that the interstitial electron is strongly localized at the body center position (denoted as the X-site) in the primitive rhombohedral unit cell, serving as an anion which is ionically bonded with the cationic framework of the Y2C layer. As the c-axis parameter decreases, the volume of the X-site is systematically reduced while both the charge and magnetization density for X are increased. It indicates that the compressed inter-layer space effectively increases the degree of localization of interstitial anionic electrons (IAEs) correlated with their enhanced local magnetic moments. We have found that the exchange splitting of the density of states for Y2C becomes more prominent with a decrease in the c-axis parameter as predicted from a pressurized alkali metal system. Accompanied by the calculated magnetization values, it can be concluded that the increased degree of localization for IAEs between cationic framework layers has greatly influenced the Stoner parameter leading to the increased magnetic moment based on the Stoner enhancement mechanism; hence, it plays a key role in the emergence of a spontaneous ferromagnetic electride.

In order to extend the application of graphene oxide (GO) in the area of electronic industries, enhancing the electrical properties of GO as a cost-effective alternative for graphene seems mandatory. Engineering the electrical properties of GO can be achieved in two different approaches: the oxygen functional group reduction and doping GO with chemical dopants. Here, both approaches were utilized to tune the electrical properties of GO toward its application as cathode; first, GO was doped with alkali metal dopants, and later, the doped samples were thermally reduced. Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy were utilized to study the chemical composition of the doped samples. The even distribution of the dopants on the GO surface presented via the EDX elemental map, with no sign of particle development. After doping GO with alkali metals followed by thermal reduction, the sheet resistance of the doped samples was decreased from 311.0 kΩ/sq to as low as 32.1 kΩ/sq. Moreover, the optical properties of GO were effectively engineered via the different doping agents. The ultra-violet photoelectron spectroscopy showed that the shift of the work function of GO was as high as 1.74 eV, after doping followed by thermal reduction.

This study reports in-situ cesium-doped graphene (G/Cs) coating obtained by introducingCs2CO3into the atmospheric pressure chemical vapor deposition (APCVD) furnace dur-ing graphene deposition on copper. The successful Cs-doping of the graphene coating wasconfirmed via X-ray photoelectron spectroscopy (XPS). As compared to the spectra of puregraphene coating, the XPS spectra of the G/Cs coating revealed a shift of the C1s and Cs3d5/2peaks to higher and lower binding energies, respectively; thus, implying the n-type charac-ter of the doping and indicating a charge transfer between Cs and graphene. Raman resultsshow that a pure graphene coating is composed of fewer layers, fewer defects, and largerdomain size than the G/Cs coating. Ultraviolet photoelectron spectroscopy was utilized tostudy the work function of graphene and the G/Cs and revealed that doping graphene withCs dopants reduced the work function of graphene by 1.2 eV. Electrochemical testing during15-day immersion in 0.1 M NaCl indicated the destructive effect of the G/Cs coating on theCu substrate. The results showed that the G/Cs coating exhibits a higher corrosion rate andlower corrosion resistance than even the bare metal itself. This work was supported by Agencia Estatal de Investigación (Ministerio de Ciencia, Innovación y Universidades of Spain, grant No.: ESP2017-82092-ERC (AEI)); SN work is supported by Comunidad de Madrid (Spain) (grant No.: 2018/T2IND/11352); GSA work is supported by the Ministerio de Ciencia, Innovación y Universidades of Spain (grant No.: RYC-2014-15357). The authors thank the National Research Foundation of the Ministry of Education, Republic of Korea (Basic Science Research Program grant No.: 2018R1A2B5A02023190) and Ministry of Education, Science and Technological Development of Serbia (Contract No. 451-03-68/2020-14/200135) for financial support.

The addition of Cu becomes essential in polycrystalline n-type Bi2(Te,Se)3-based alloys since it is known to enhance stability of carrier transport properties as well as thermoelectric performance. However, a way to further optimize is necessary owing to the limited controllability of transport parameters by Cu addition. Herein, we present improved carrier transport properties via I-doping in Cu0.008Bi2Te2.7Se0.3. Weighted mobility and effective mass are increased simultaneously by small amount doping of I at Te/Se-site. As a result, ~15% increased power factor and high average thermoelectric figure of merit (zT) of 0.79 were obtained in a wide temperature range.

Cu doping has been known to enhance thermoelectric performance of p-type (Bi,Sb)2Te3 alloys by suppressing bipolar conduction at higher temperatures. However, detailed mechanisms are not provided yet. Herein, we investigate the unchallenged role of Cu doping in (Bi,Sb)2Te3 alloys by means of Bi-rich compositions of Bi0.6Sb1.4Te3 and Bi0.7Sb1.3Te3, which were chosen since they exhibit much inferior thermoelectric performance owing to severe bipolar conduction. It was found that Cu doping increases non-degenerate mobility and density-of-states effective mass of valence band as well as hole concentration. Small amount of Cu doping enhanced maximum thermoelectric figure-of-merit of Bi0.6Sb1.4Te3 composition from 0.25 to 0.98.

Nanoparticles in thermoelectric alloys has been considered as one of the most important ingredients to enhance their thermoelectric figure of merit zT mainly by reducing the lattice thermal conductivity due to intensified phonon scattering. However, the scattering mechanism of phonon with respect to wavelengths, which provides the comprehensive design rules for nanocomposites with enhanced zT, has not been fully understood. Here, we report a critical role of nanoparticles for the lattice thermal conductivity reduction from the theoretical and experimental analysis of the temperature-dependent thermal and electronic transport properties of p-type Ag/Cu nanoparticles-embedded Bi0.5Sb1.5Te3 with respect to their electronic, bipolar, and lattice thermal conductivities. It was found that the introduction of the Ag/Cu nanoparticles reduced the lattice thermal conductivity through the additional phonon scattering based on the changeover between the Rayleigh and geometrical scatterings, indicating the indispensability of nanoparticles to scatter phonons that cannot be scattered effectively by either point defects or grain boundaries.

Compositional tuning is one of the important approaches to enhance the electronic and thermal transport properties of thermoelectric materials since it can generate point defects as well as control the phase evolution behavior. Herein, we investigated the Ti addition effect on the grain growth during melt spinning and thermoelectric transport properties of Hf0.5Zr0.5NiSn0.98Sb0.02 half-Heusler compound. The characteristic grain size of melt-spun ribbons was reduced by Ti addition, and very low lattice thermal conductivity lower than 0.27 W m−1 K−1 was obtained within the whole measured temperature range (300–800 K) due to the intensified point defect (substituted Ti) and grain boundary (reduced grain size) phonon scattering. Due to this synergetic effect on the thermal transport properties, a maximum thermoelectric figure of merit, zT, of 0.47 was obtained at 800 K in (Hf0.5Zr0.5)0.8Ti0.2NiSn0.98Sb0.02.

Structural phase transitions in layered two-dimensional (2D) materials are of significant interest owing to their ability to exist in multiple metastable states with distinctive properties. However...

Incorporation of a foreign element is considered as a promising approach to enhance the performance of thermoelectric materials since this can either improve the power factor by a band structure modification or reduce the thermal conductivity by a phonon scattering strengthening. We fabricated the polycrystalline bulk samples of Cu-incorporated Bi0.5Sb1.5Te3 by melt spinning and spark plasma sintering, and evaluated the electronic and thermal transport properties. From the phase analysis and thermoelectric properties measurement, we found that most of the added excess Cu atoms were substituted at a Sb-site and a small amount of Cu was intercalated at the van der Waals gap between quintuple layers. By the formation of two different point defects (substituted Cu and intercalated Cu), the thermoelectric power factor was enhanced because of the increased density of states effective mass, and simultaneously reduced thermal conductivity originated from the intensified phonon scattering and suppressed bipolar contribution. Maximum thermoelectric figure of merit zT of 1.13 was obtained at 400 K.

Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Ti1–xHfxNiSn1–ySby half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti0.5Hf0.5NiSn0.98Sb0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects.

Herein, we report the enhanced thermoelectric performance of Cu incorporated Bi2Te2.7Se0.3 prepared by melt spinning. The electronic transport properties were significantly improved by Cu incorporation due to the synergetic effect of carrier tuning, band modification, and electron-phonon transport control. The steady dimensionless thermoelectric figure of merit higher than 0.90 was achieved in the wide range of temperature range of 350–450 K for Cu0.008Bi2Te2.7Se0.3, which is ~30% enhancement in comparison with pristine Bi2Te2.7Se0.3. This result can lead to high thermoelectric power generation efficiency 4.2% at ΔT = 180 K.

The thermoelectric transport properties of artificially textured polycrystalline CrSiTe3 samples have been investigated for elucidating their correlations with a crystal structure and the corresponding charge density distribution. We verified that the low mobility of 13–27 cm2 V−1 s−1 attributing poor electronic transport properties is originated from the discrete charge density distribution in the covalently bonded layer. Also, the modulation of thermal conductivity values from 3.5 to 5.4 W m−1 K−1 with varying the texture was confirmed in pristine CrSiTe3. Thus, we suggest that the electronic and thermal transport properties of layer structured CrSiTe3 are strongly correlated with crystallographic features.

Controllable crystal orientation is necessary to obtain high thermoelectric performance in thin films of Bi2Te3 alloys. In the present study, highly (110)-oriented thin films of n-type Bi2Te3-xSex with improved composition controllability are prepared through a simple electrodeposition-based process. Using potential-current co-adjusted pulse electrodeposition (PCP-ED) with adjustments to the zero current during the off-time period enables the fabrication of dense Bi2Te3-xSex thin films with highly (110)-oriented grains by minimizing the ionic gradient (Bi3+, Te2−, Se2−) between the substrate and solution. The power factor of the PCP-ED thin film was much higher than that of the dendritic Bi2Te3-xSex thin film fabricated by constant-potentiostatic electrodeposition (C-ED) because of the simultaneous enhancement of electrical conductivity and Seebeck coefficient. The high power factor of ∼1920 μW/m⋅K2, which is the best value among reported n-type Bi2Te3-based thin films, was obtained at room temperature after low-temperature annealing at 200 °C by exploiting the crystallinity enhancement and carrier concentration optimization.

Nanocomposites can simultaneously enhance the thermoelectric and mechanical properties of thermoelectric materials. Here, we fabricated bulks of Mg1.96Al0.04Si0.97Bi0.03 with monodispersed few-layered reduced graphene oxides utilizing ultrasonic-based wet chemical pulverizing-mixing and spark plasma sintering to improve unfavorable trade-off between thermoelectric performance and mechanical reliability, which is important for commercialization. An unexpected high fracture toughness of ~1.88 MPa m1/2 was observed due to the synergetic effect of the deflection of crack propagation, bridging, and sheet pull-out mechanisms, and a high thermoelectric figure of merit ~0.6 was obtained even for a high content (3 vol.%) of reduced graphene oxides.

Controlling electronic and thermal transport properties simultaneously is an ultimate strategy to accomplish high-performance thermoelectrics. Here, our analysis on carrier transport of a nanograined p-type Bi0.5Sb1.5Te3 thermoelectric alloy clearly reveals that reducing grain size greatly suppresses bipolar conduction by selective suppression of minority carrier (electron) mobility, resulting in both the power factor enhancement and bipolar thermal conductivity reduction. Furthermore, it is shown how reducing grain size affects decreasing lattice thermal conductivity in respect to grain size and phonon wavelength. Therefore, minimizing grain size can enhance thermoelectric performance of Bi0.5Sb1.5Te3 alloy by controlling both electronic and thermal transport properties synergetically.

Magnetism of pure electrons is fundamental for understanding diverse magnetic phenomena in condensed matters but has not been fully investigated in experiments due to the lack of a tractable model system. Such an exotic material necessitates an exclusive magnetic interaction of electrons being devoid of orbital and lattice degrees of freedom. Here, we report the two-dimensional mixed-cation [YGdC]2+∙2e− electride, showing ferrimagnetic nature from the direct exchange interaction of magnetic interstitial electrons in interlayer space. We identify that magnetic interstitial electrons are periodically localized in octahedral and tetrahedral cavities between 2D cationic Y2−xGdx arrays. The mixed configuration of non-magnetic and magnetic cations in cavities induces divergent spin states and interactions of magnetic interstitial electrons, in which their direct exchange interaction overwhelms the interactions with magnetic cations, triggering the ferrimagnetic spin-alignment. This discovery facilitates further exploration of magnetic electrides and nurtures the study of two-dimensional magnetism of layered crystals and electron phases.

An electride, a generalized form of cavity-trapped interstitial anionic electrons (IAEs) in a positively charged lattice framework, shows exotic properties according to the size and geometry of the cavities. Here, we report that the IAEs in layer structured [Gd2C]2+·2e− electride behave as ferromagnetic elements in two-dimensional interlayer space and possess their own magnetic moments of ~0.52 μB per quasi-atomic IAE, which facilitate the exchange interactions between interlayer gadolinium atoms across IAEs, inducing the ferromagnetism in [Gd2C]2+·2e− electride. The substitution of paramagnetic chlorine atoms for IAEs proves the magnetic nature of quasi-atomic IAEs through a transition from ferromagnetic [Gd2C]2+·2e− to antiferromagnetic Gd2CCl caused by attenuating interatomic exchange interactions, consistent with theoretical calculations. These results confirm that quasi-atomic IAEs act as ferromagnetic elements and trigger ferromagnetic spin alignments within the antiferromagnetic [Gd2C]2+ lattice framework. These results present a broad opportunity to tailor intriguing ferromagnetism originating from quasi-atomic interstitial electrons in low-dimensional materials., Ferromagnetic quasi-atomic behavior of interstitial anionic electrons (IAEs) in practical electrides is yet to be discovered experimentally. Here, the authors reveal that IAEs in two-dimensional electride [Gd2C]²+⋅2e- behave as magnetic elements with their own magnetic moment.

Birch reduction of aromatic systems by solvated electrons in alkali metal-ammonia solutions is widely recognized as a key reaction that functionalizes highly stable π-conjugated organic systems. In...

Dislocation arrays embedded in low-angle grain-boundaries have emerged as an effective structural defect for a dramatic improvement of thermoelectric performance by reducing thermal conductivity [1]. A transient liquid-flow assisted compacting process has been employed for p-type Bi 0.5 Sb 1.5 Te 3 material to generate the dislocation arrays at grain-boundaries. The details of underlying formation mechanism are crucial for the feasibility of the process on other state-of-the-art thermoelectric materials but have not been well understood. Here, we report the direct observation of dislocation formation process at grain-boundaries of Sb 2 Te 3 system as a proof-of-concept material. We found that the formation of homointerface between Te-terminated Sb 2 Te 3 matrix phase and Te liquid atomic-layer of secondary phase is a prerequisite factor to achieve the low-energy liquid-solid homointerface at compacting elevated temperature. We further demonstrate from the successful observations of atomic structure in the intermediate state of the compacted pellet that the high self-diffusion rate of Te atoms at the liquid-solid homointerface facilitates an effective grain rearrangement, generating low-energy grain-boundaries embedded with dense dislocation arrays. These results pave the way to improve thermoelectric performance of various materials where dislocation arrays are generated by transient liquid-flow assisted compacting process using precursors with an interface constructed with the same types of atoms.

Proton transfer has been intensively researched in the catalysis of reactions involving hydrogen, such as the hydrogen evolution reaction (HER), oxygen evolution reaction, and carbon dioxide reduction. Recently, two-dimensional (2D) materials have gained attention as catalysts for these reactions, and their catalytic effect upon changing the size, shape, thickness, and phase has been studied. However, there are no reports on the role of proton transfer in catalysis by 2D materials. Here, a novel way to enhance the catalytic effect of 2D MoS2 was demonstrated via functionalization with four different organic moieties: phenyl–Me, phenyl–OMe, phenyl–OH, and phenyl–COOH groups. The role of proton transfer in 2D catalysis was carefully investigated via electrochemical kinetic analysis and electrical measurement. The best HER performance was observed with proton-donating COOH-functionalized active materials due to intramolecular proton transfer, which shows potential in hydrogen adsorption engineering using proton transfer. In addition, other molecularly functionalized 2D catalysts, including MoTe2 and graphene, also show proton transfer due to the incorporation of organic moieties, providing enhanced HER performance. Adding organic molecules to two-dimensional materials can reduce the Gibbs free energy of energy-releasing chemical reactions. Hydrogen is a renewable and environmentally-friendly source of energy. Fuel cells release this potential energy and create electricity when a chemical reaction at an electrode oxidizes the hydrogen and leaves just protons. These then generate a current as they cross the cell to a second electrode. Catalysts that increase the rate of this reaction thus improve the performance of the fuel cell. Using electrochemical kinetic analysis and electrical measurement, Hyoyoung Lee from Sungkyunkwan University, Suwon, South Korea and colleagues investigated proton transfer in two-dimensional catalysts to which they had added various phenyl derivatives. They showed that these organic molecules both reduced the energy at which proton transfer begins and improved device stability. In this study, we investigate the effect of surface functional group of 2D materials on the hydrogen evolution reaction (HER) and it shows both band state (ΔGH) and the wettability of 2D catalyst influence on the onset potential. In particular, the COOH functionalized 2D materials demonstrate good catalytic effect and good stability during HER because the COOH moiety increases the polarization of the electrode related to wettability as well as reduces the hydrogen absorption energy of the Mo atom and S atom through proton transfer.

Substitutional doping is known to be effective when used to enhance the thermoelectric figure of merit zT, and this is generally explained as resulting from a reduction in the thermal conductivity caused by an additional atomic-scale defect structure. However, a comprehensive analysis of the substitutional doping effect on the electrical and thermal properties together has not been undertaken, especially when the bipolar thermal conductivity becomes serious. A previous study by the authors also showed that the zT of Bi0.4Sb1.6Te3 thermoelectric alloys was enhanced by indium (In) doping due to the reduction of the total thermal conductivity. Here, we more closely analyze the electrical and thermal transport properties of a series of indium (In)-doped p-type Bi0.4Sb1.6-xInxTe3 (x = 0, 0.003, 0.005, 0.01) using both the single-parabolic-band model and the Debye-Callaway model in an effort to investigate the origin of the observed thermal conductivity reduction more closely. The bipolar contribution to the total thermal conductivity was estimated exclusively based on a two-band model based on a single-parabolic-band model. Furthermore, the lattice thermal conductivity was calculated using the Debye-Callaway model while taking additional In substitutional defects into consideration. The calculations indicated that the significant suppression of bipolar thermal conductivity was achieved as a result of the increased bandgap in Bi0.4Sb1.6Te3 caused by In doping. Additional point defects from In doping also reduced the lattice thermal conductivity, but not as much as the bipolar thermal conductivity did. The study suggests that the suppression of bipolar conduction by means of a bandgap modification can be an effective approach for enhancing zT further via a simple In-doping process in Bi0.4Sb1.6Te3.

Filled skutterudites are promising mid-to-high temperature range thermoelectric materials for power generation, however, a traditional melt-solidification process followed by annealing (TMA) and powder metallurgical sintering requires a long processing time more than 10 days to ensure the structural and compositional homogeniety of materials with a high thermoelectric conversion efficiency zT. To address this, we herein report a simple and efficient synthesis of high-performance n- and p-type filled skutterudites that successfully produces a complete single phase from single to multiple filled materials in a day. The nanograin (∼440 nm) structured bulks are prepared from the combined process of temperature-regulated melt spinning (MS) using ingots and short-time spark plasma sintering (SPS). The controlled phase evolution and transformation by adjusting rapid solidification and densification conditions are demonstrated by a comprehensive analysis including structure refinement and atomic-scale observation, verifying the desired occupancy and random distribution of filling elements, respectively. The maximum zT values of filled skutterudites fabricated here were 1.48 ± 0.17 at 800 K for n-type In0.12Yb0.20Co4.00Sb11.84 and 1.15 ± 0.13 at 750 K for p-type Ce0.91Fe3.40Co0.59Sb12.14, which are comparable to the highest zT values reported for filled skutterudites fabricated by TMA-based processes. Superior reproducibility achieved in shortened processing time enables the present synthetic process to be utilized for commercial manufacturing process that can be readily applied to massive production of bulk filled skutterudites for high-performance thermoelectric power generators.

Manipulation of thermal transport properties by defect engineering is an important but yet unresolved issue in thermoelectrics , since rational design strategies of multiple defect structures for minimizing lattice thermal conductivity are lacking. The key is to comprehend complex interaction between different frequency-dependent phonon scattering processes by multiple defect structures. Herein, individual contributions in lattice thermal conductivity reduction from each defect structure—point defects (0-dimensional, 0D), dislocations (1D), grain boundaries (2D), and nanoparticles (3D)—are characterized based on experimental results of commercial (Bi,Sb) 2Te3 -based alloys fitted to Debye-Callaway model. Then, the cumulative contributions by multiple defect structures are investigated interactively by estimating total phonon relaxation time. Therefore, the interactive role of multiple defect structures in reducing lattice thermal conductivity is elucidated. To extend the approach to other thermoelectric materials , PbTe-based alloys with multiple defect structures of point defects and grain boundaries were modeled as well. This approach enables to provide a thorough design of defect structures for realizing the experimentally attainable low lattice thermal conductivity in thermoelectric materials.

Nanostructuring is a universal strategy to improve the figure of merit (zT) of thermoelectric (TE) materials by the substantial decrease of lattice thermal conductivity due to the intensive scattering of phonons at interfaces. However, nanostructured bismuth antimony telluride alloys with foreign nanosized phases usually suffer from the degradation of carrier mobility at incoherent interfaces. The results reported here describe an approach to generate intrinsic Sb-rich precipitates that have a synchronal effect on micro-grained Bi0.5Sb1.5Te3 alloy. The designed formation of nanosized Sb-rich precipitates by dissolving excess Pb into Bi0.5Sb1.5Te3 matrix intensifies the phonon scattering and modulates the carrier transport simultaneously. By employing density functional theory calculation, transmission electron microscope and atom probe tomography, the role of Pb substituting for Sb site on the evolution of Sb-rich nanophase is clarified and the compositions and crystal structures of the precipitated Sb-rich phases are analyzed, revealing various interface structures. The optimized Bi0.5Sb1.5Te3 + 0.22 wt.% Pb sample shows a maximum zT value of 1.32 at 400 K with an outstanding average zT value of 1.2 over 300 K to 500 K. This work identifies the hidden role of intrinsic Sb-rich precipitates in Bi0.5Sb1.5Te3 alloys and provides an effective way beyond nanostructuring to develop high-performance bismuth antimony telluride thermoelectric alloys.

We report that the spin-alignment of interstitial anionic electrons (IAEs) in two-dimensional (2D) interlayer spacing can be tuned by chemical pressure that controls the magnetic properties of 2D electrides. It was clarified from the isovalent Sc substitution on the Y site in the 2D Y2C electride that the localization degree of IAEs at the interlayer becomes stronger as the unit cell volume and c-axis lattice parameter were systematically reduced by increasing the Sc contents, thus eventually enhancing superparamagnetic behavior originated from the increase in ferromagnetic particle concentration. It was also found that the spin-aligned localized IAEs dominated the electrical conduction of heavily Sc-substituted Y2C electride. These results indicate that the physcial properties of 2D electrides can be tailored by adjusting the localization of IAEs at interlayer spacing via structural modification that controls the spin instability as found in three-dimensional elemental electrides of pressurized potassium...

High pressure offers an intriguing avenue for the change in physical and chemical properties of condensed matters such as superconductivity, structural phase transition, and catalytic activity. However, it is hard to achieve the high-pressure phases of elements and compounds in an ambient condition by chemical pressure due to the limitation in bonding length variation. Here, we report the quasi-high-pressure state of Hf cuboid confined between two-dimensional chalcogen layers in transition-metal-rich dichalcogenide of Hf3Te2, exhibiting the enhanced degree of localization and chemical potential for valence electrons compared to ambient Hf elements. The structural analysis reveals that Hf metals in Hf3Te2 form a local cuboid structure, in which the coordination for central Hf is identical to that of high-pressure body-centered-cubic Hf phase. Density functional theory calculations verify that the compressed cuboid in Hf3Te2 has a key role for an abnormal heat capacity beyond Dulong–Petit limit and reduced ...

Polycrystalline bulks of Sc-, Ti-, and V-doped n-type Cu 0.008 Bi 2 Te 2.7 Se 0.3 were prepared by melt solidification and spark plasma sintering, and their thermoelectric transport properties were investigated. The lattice thermal conductivity of Cu 0.008 Bi 2 Te 2.7 Se 0.3 was slightly reduced by intensified point defect phonon scattering by the substitution of Sc, Ti, and V atoms on the Bi-site. On the other hand, the power factor of Cu 0.008 Bi 2 Te 2.7 Se 0.3 was significantly enhanced by doping of V. Through the experimental and theoretical considerations, it was found that the enhanced power factor by V doping is originated from the increased density of states (DOS) effective mass by modified DOS near at Fermi level. Resultantly, an enhanced zT of 0.85 at 300 K was obtained in 1 at% V-doped Cu 0.008 Bi 2 Te 2.7 Se 0.3 (Cu 0.008 Bi 1.98 V 0.02 Te 2.7 Se 0.3 ).

Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunity to investigate extreme carrier density-driven superconductivity and phase transition in 2D systems. Chemical functionalization and the ionic gating have achieved the high doping density, but their effective ranges have been limited to ∼1 nm, which restricts the use of highly doped 2D semiconductors. Here, we report on electron diffusion from the 2D electride [Ca2N]+·e– to MoTe2 over a distance of 100 nm from the contact interface, generating an electron doping density higher than 1.6 × 1014 cm–2 and a lattice symmetry change of MoTe2 as a consequence of the extreme doping. The long-range lattice symmetry change, suggesting a length scale surpassing the depletion width of conventional metal–semiconductor junctions, was a consequence of the low work function (2.6 eV) with highly mobile anionic electron layers of [Ca2N]+·e–. The combination of 2D electrides and layered materials yields a novel material design i...

We report the enhancement of thermoelectric properties in the complex structured Cu-Bi-S pavonite compounds by optimizing the structural configuration through tuning the Bi-site occupancy, and substitutional doping at interstitial Cu sites by Zn. We verify that electronic transport properties depend on the structural deformation by the Bi site occupancy. Furthermore, we demonstrate that the modification of interstitial site ions enables selective control of thermal conductivity and intrinsically low thermal conductivity can be further suppressed by structural optimization without deteriorating electronic transport properties. We propose that understanding of crystal structure as a basic strategy permits the optimization of thermoelectric properties in the complex structures.

Halogenated organic compounds are important anthropogenic chemicals widely used in chemical industry, biology, and pharmacology; however, the persistence and inertness of organic halides cause human health problems and considerable environmental pollution. Thus, the elimination or replacement of halogen atoms with organic halides has been considered a central task in synthetic chemistry. In dehalogenation reactions, the consecutive single-electron transfer from reducing agents generates the radical and corresponding carbanion and thus removes the halogen atom as the leaving group. Herein, we report a new strategy for an efficient chemoselective hydrodehalogenation through the formation of stable carbanion intermediates, which are simply achieved by using highly mobile two-dimensional electrons of inorganic electride [Ca2N]+·e– with effective electron transfer ability. The consecutive single-electron transfer from inorganic electride [Ca2N]+·e– stabilized free carbanions, which is a key step in achieving t...

Growth of perovskite oxide thin films on Si in crystalline form has long been a critical obstacle for the integration of multifunctional oxides into Si-based technologies. In this study, we propose pulsed laser deposition of a crystalline SrTiO3 thin film on a Si using graphene substrate. The SrTiO3 thin film on graphene has a highly (00l)-oriented crystalline structure which results from the partial epitaxy. Moreover, graphene promotes a sharp interface by highly suppressing the chemical intermixing. The important role of graphene as a 2D substrate and diffusion barrier allows expansion of device applications based on functional complex oxides., 23 pages, 5 figures

We have synthesized a single crystalline Y2C electride of centimeter-scale by floating-zone method and successfully characterized its anisotropic electrical and magnetic properties. In-plane resistivity upturn at low temperature together with anisotropic behavior of negative magnetoresistance is ascribed to the stronger suppression of spin fluctuation along in-plane than that along the c-axis, verifying the existence of magnetic moments preferred for the c-axis. A superior magnetic moment along the c-axis to that along the in-plane direction strongly demonstrates the anisotropic magnetism of Y2C electride containing a magnetically easy axis. It is clarified from the theoretical calculations that the anisotropic nature of the Y2C electride originates from strongly localized anionic electrons with an inherent magnetic anisotropy in the interlayer spaces.

Transition metal oxides have been extensively studied and utilized as efficient catalysts. However, the strongly correlated behavior which often results in intriguing emergent phenomena in these materials has been mostly overlooked in understanding the electrochemical activities. Here, we demonstrate a close correlation between the phase transitions and oxygen evolution reaction (OER) in a strongly correlated SrRuO3. By systematically introducing Ru-O vacancies into the single-crystalline SrRuO3 epitaxial thin films, we induced phase transition in crystalline symmetry which resulted in corresponding modification in the electronic structure. The modified electronic structure significantly affect the electrochemical activities, so a 30% decrease in the overpotential for the OER activity was achieved. Our study suggests that a substantial enhancement in the OER activity can be realized even within single material systems, by rational design and engineering of their crystal and electronic structures., 31 pages, 18 figures, 2 tables

Water- and acid-stable electride realizes a sustainable electrocatalytic reaction as an efficient solid-state electron source., Electrides have emerged as promising materials with exotic properties, such as extraordinary electron-donating ability. However, the inevitable instability of electrides, which is caused by inherent excess electrons, has hampered their widespread applications. We report that a self-passivated dihafnium sulfide electride ([Hf2S]2+∙2e−) by double amorphous layers exhibits a strong oxidation resistance in water and acid solutions, enabling a persistent electrocatalytic hydrogen evolution reaction. The naturally formed amorphous Hf2S layer on the cleaved [Hf2S]2+∙2e− surface reacts with oxygen to form an outermost amorphous HfO2 layer with ~10-nm thickness, passivating the [Hf2S]2+∙2e− electride. The excess electrons in the [Hf2S]2+∙2e− electride are transferred through the thin HfO2 passivation layer to water molecules under applied electric fields, demonstrating the first electrocatalytic reaction with excellent long-term sustainability and no degradation in performance. This self-passivation mechanism in reactive conditions can advance the development of stable electrides for energy-efficient applications.

The van der Waals (vdW) interaction in two-dimensional (2D)-layered materials affects key characteristics of electronic devices, such as the contact resistance, with a vertical heterostructure geometry. While various functionalizations to manipulate the properties of 2D materials have shown issues such as defect generation or have a limited spatial range for the methods, engineering the vdW interaction in nondestructive ways for device applications has not been tried or properly achieved yet. Here, we introduce the proximity engineering of the vdW interaction in multilayered graphene, which is observed as modified interlayer distances and deviated stacking orders by Raman spectroscopy. A 2D electride, [Ca2N]+·e-, possessing a low-work function of 2.6 eV, was used to trigger an avalanche of electrons over tens of graphene layers, exceeding the conventional spatial-range limit (∼1 nm) by screening with a carrier density of 1014 cm-2. Our proximity engineering reduces the vdW interaction in a nondestructive way and achieves a promising graphene-metal contact resistance of 500 Ω·μm without using complicated edge contacts, which demonstrates a way to use moderately decoupled graphene layers for device applications.

A 3D crystal structure can be transformed into a layered structure by dimensional manipulation, expanding the 2D material library., The discovery of new families, beyond graphene, of two-dimensional (2D) layered materials has always attracted great attention. However, it has been challenging to artificially develop layered materials with honeycomb atomic lattice structure composed of multicomponents such as hexagonal boron nitride. Here, through the dimensional manipulation of a crystal structure from sp3-hybridized 3D-ZnSb, we create an unprecedented layered structure of Zintl phase, which is constructed by the staking of sp2-hybridized honeycomb ZnSb layers. Using structural analysis combined with theoretical calculation, it is found that the 2D-ZnSb has a stable and robust layered structure. The bidimensional polymorphism is a previously unobserved phenomenon at ambient pressure in Zintl families and can be a common feature of transition metal pnictides. This dimensional manipulation of a crystal structure thus provides a rational design strategy to search for new 2D layered materials in various compounds, enabling unlimited expansion of 2D libraries and corresponding physical properties.

Elemental doping is a universal strategy in controlling the functionalities of materials that strongly correlate with naturally formed atomic defects. Element-resolved chemical mapping for atomic defects has answered numerous problems on the relations between defective structures and properties. However, tracking small amounts of dopants in multicomponent bulks and clarifying their doping behaviors remain challenging. Using advanced X-ray spectroscopy, the excess Cu doping behavior in ternary Bi2Te2·7Se0.3 bulk alloy is visualized with the unprecedented detectability from sub-one atomic percent of concentration. The low content of 0.2 at.% Cu preferentially occupies the Bi site, while Cu atoms are found in three crystallographic sites of Bi2Te3 structure and van der Waals gap at high Cu content of 1.2 at.%. These behaviors explain the nontrivial role of Cu dopants on carrier generation processes and relevant thermoelectric properties of Bi2Te2·7Se0.3. The atomic-level identification should also stimulate the elucidation of diverse properties in doped nanomaterials and quantum phenomena in doped topological insulators.

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. Experimentally, however, 2D electrides have been only found in a couple of layered nitrides and carbides. Here, we report new thermodynamically stable alkaline-earth based 2D electrides by using a first-principles global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation. The method was applied to binary compounds consisting of alkaline-earth elements as cations and group VA, VIA, or VIIA nonmetal elements as anions. We revealed that the stability of a layered 2D electride structure is closely related to the cation/anion size ratio; stable 2D electrides possess a sufficiently large cation/anion size ratio to minimize electrostatic energy among cations, anions, and anionic electrons. Our work demonstrates a new avenue to the discovery of thermodynamically stable 2D electrides beyond experimental material databases and provides new insight into the principles of electride design.

Quasi-two-dimensional (2D) metallicity in a ternary layered transition metal chalcogenide of single crystalline Ti2PTe2 is studied by transport prop-erties measurements and electronic structure analyses. The logarithmic dependence of resistivity in the low temperature region appears as a charac-teristic feature of the quasi-2D transport in a weakly disordered metallic system. A large Sommerfeld coefficient value of 6.94 mJ/mol·K2 and negative temperature dependence of the Hall coefficient indicate that electron-electron interaction is dominant in a phase-breaking mechanism originating from bipolar behavior. Angle-resolved photoemission spectroscopy combined with theoretical calculations successfully proves that quasi-2D elec-tron pockets encompassing a small three-dimensional hole pocket mainly determine the quasi-2D metallic transport properties. We also adopt an orbital character calculation to verify that Ti character is dominant in valence bands, revealing that 2D Ti-networks form quasi-2D conducting ...

Polycrystalline bulks of n-type Cu0.008Bi2Te2.7Se0.3 were prepared to investigate the controllability of its thermoelectric transport properties by using the compaction conditions of spark plasma sintering (SPS). The 00l crystal orientation to the press direction of the SPSed bulks was easily improved by increasing the applied pressure at 500 °C. The thermoelectric figure of merit, ZT values (0.72 − 0.75 at 300 K), of all samples were almost the same, however, both the electronic and the thermal transport properties could be tuned significantly by adjusting the sintering pressure. This result highlights the feasibility of using pressure-induced sintering as a fabrication technology for Bi2Te3-based polycrystalline bulks with high mechanical reliability, which is an effective means of optimizing the electrical and the thermal conductivities for maximizing the efficiencies of the thermoelectric cooling and the power generation modules.

We investigated the thermoelectric properties of Al and Bi co-doped Mg2Si polycrystalline bulks fabricated using a solid state reaction combined with the spark plasma sintering technique. Through controlled doping of Al and Bi, the power factor could be enhanced due to an increase in the Seebeck coefficient benefiting from an enhancement of the density of states effective mass. The lattice thermal conductivity was reduced due to intensified point-defect phonon scattering originating from the mass difference between the host Si atoms and Bi dopants. By these synergetic effects, the dimensionless figure of merit (ZT) of 1.02 was obtained at 873 K.

Doping with foreign atom has been shown to be an effective way to enhance the dimensionless figure of merit ZT of Bi2Te3-based thermoelectric raw materials. Herein, we report that doping with Nb is effective in enhancing the Seebeck coefficient of n-type Cu0.008Bi2Te2.7Se0.3 polycrystalline bulks. Considering compensation of the Seebeck coefficient due to decrease of the electrical conductivity in Nb-doped compositions, the absolute value of Seebeck coefficient rather increased benefiting from an enhancement of the density of states (DOS) effective mass m* from 1.09m 0 (Cu0.008Bi2Te2.7Se0.3) to 1.21m 0 − 1.27m 0 (Cu0.008Bi2−x Nb x Te2.7Se0.3) due to a DOS engineering effect. The values of ZT were 0.84 at 300 K and 0.86 at 320 K for Cu0.008Bi1.99Nb0.01Te2.7Se0.3. This compositional tuning approach highlights the possibility of further enhancement of ZT for n-type Bi2Te3-based compounds by using a combination of nanostructuring technologies to reduce the thermal conductivity.

The crystal chemistry of complex structured pavonite homologue Cux+yBi5−yS8 (1.2 ≤ x ≤ 1.4, 0.4 ≤ y ≤ 0.55) compounds with various crystallographic atomic sites was investigated in the context of their thermoelectric properties. We clarified the origins of the electronic and thermal transport properties of Cux+yBi5−yS8 compounds based on the change in the composition, which is strongly correlated with the occupancy of each atomic site. Ab initio calculations revealed that the narrow gap n-type semiconducting nature of Cux+yBi5−yS8 compounds originates from the presence of interstitial Cu ions. Structural refinements combined with transport measurements revealed that asymmetrical disorders of interstitial Cu ions have a large anisotropic thermal displacement factor, leading to an intrinsically low value (∼0.49 W m−1 K−1) and temperature-independent behavior of lattice thermal conductivity. Comprehensive structural analysis provided an elemental doping strategy focusing on interstitial sites. Thermoelectric properties were significantly enhanced by the simultaneous increase of power factor and decrease of lattice thermal conductivity. It is noted that structural factors, such as occupancy and thermal displacement parameter, of interstitial sites among the various crystallographic sites should be considered as primary characteristics in the crystal chemistry of complex structured crystals. Correspondingly, a peak ZT for the system was obtained in Cu1.576Zn0.024Bi4.6S8, which showed ∼30% enhancement over that of the pristine Cux+yBi5−yS8 compound.

The recent observation of extremely large magnetoresistance (MR) in the transition-metal dichalcogenide MoTe2 has attracted considerable interest due to its potential technological applications as well as its relationship with novel electronic states predicted for a candidate type-II Weyl semimetal. In order to understand the origin of the MR, the electronic structure of MoTe2−x (x = 0.08) is systematically tuned by application of pressure and probed via its Hall and longitudinal conductivities. With increasing pressure, a monoclinic-to-orthorhombic (1 T′ to Td) structural phase transition temperature (T*) gradually decreases from 210 K at 1 bar to 58 K at 1.1 GPa, and there is no anomaly associated with the phase transition at 1.4 GPa, indicating that a T = 0 K quantum phase transition occurs at a critical pressure (Pc) between 1.1 and 1.4 GPa. The large MR observed at 1 bar is suppressed with increasing pressure and is almost saturated at 100% for P > Pc. The dependence on magnetic field of the Hall and longitudinal conductivities of MoTe2−x shows that a pair of electron and hole bands are important in the low-pressure Td phase, while another pair of electron and hole bands are additionally required in the high-pressure 1 T′ phase. The MR peaks at a characteristic hole-to-electron concentration ratio (nc) and is sharply suppressed when the ratio deviates from nc within the Td phase. These results establish the comprehensive temperature-pressure phase diagram of MoTe2−x and underscore that its MR originates from balanced electron-hole carrier concentrations.

Transition metal oxide thin films show versatile electrical, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of the magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO3 thin films using pulsed laser epitaxy. Using epitaxial stabilization technique with atomically flat polycrystalline SrTiO3 substrate, epitaxial polycrystalline SrRuO3 thin film with crystalline quality of each grain comparable to that of single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart., Comment: 24 pages, 5 figures

Elemental defects in transition metal oxides is an important and intriguing subject that result in modifications in variety of physical properties including atomic and electronic structure, optical and magnetic properties. Understanding the formation of elemental vacancies and their influence on different physical properties is essential in studying the complex oxide thin films. In this study, we investigated the physical properties of epitaxial SrRuO3 thin films by systematically manipulating cation and/or oxygen vacancies, via changing the oxygen partial pressure (P(O2)) during the pulsed laser epitaxy (PLE) growth. Ru vacancies in the low-P(O2)-grown SrRuO3 thin films induce lattice expansion with the suppression of the ferromagnetic TC down to ~120 K. Sr vacancies also disturb the ferromagnetic ordering, even though Sr is not a magnetic element. Our results indicate that both A and B cation vacancies in an ABO3 perovskite can be systematically engineered via PLE, and the structural, electrical, and magnetic properties can be tailored accordingly., 22 pages, 9 figures, 1 table

Demands on high-quality layer structured two-dimensional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for a 2D chalcogenide film on a three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of a high-quality 2D chalcogenide (Bi0.5Sb1.5Te3) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in situ formation of a pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and ea...

Cation substitutional doping is an effective approach to modifying the electronic and thermal transports in Bi2Te3-based thermoelectric alloys. Here we present a comprehensive analysis of the electrical and thermal conductivities of polycrystalline Pb-doped p-type bulk Bi0.48Sb1.52Te3. Pb doping significantly increased the electrical conductivity up to ~2700 S/cm at x = 0.02 in Bi0.48-xPbxSb1.52Te3 due to the increase in hole carrier concentration. Even though the total thermal conductivity increased as Pb was added, due to the increased hole carrier concentration, the thermal conductivity was reduced by 14–22% if the contribution of the increased hole carrier concentration was excluded. To further understand the origin of reduction in the thermal conductivity, we first estimated the contribution of bipolar conduction to thermal conductivity from a two-parabolic band model, which is an extension of the single parabolic band model. Thereafter, the contribution of additional point defect scattering caused by Pb substitution (Pb in the cation site) was analyzed using the Debye–Callaway model. We found that Pb doping significantly suppressed both the bipolar thermal conduction and lattice thermal conductivity simultaneously, while the bipolar contribution to the total thermal conductivity reduction increased at high temperatures. At Pb doping of x = 0.02, the bipolar thermal conductivity decreased by ~30% from 0.47 W/mK to 0.33 W/mK at 480 K, which accounts for 70% of the total reduction.