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In electrides, interstitial anionic electrons (IAEs) in the quantized energy levels at cavities of positively charged lattice framework possess their own magnetic moment and interact with each or surrounding cations, behaving as quasi-atoms and inducing diverse magnetism. Here, we report the reversible structural and magnetic transitions by the substitution of the quasi-atomic IAEs in the ferromagnetic two-dimensional [Gd2C]2+×2e- electride with hydrogens and subsequent dehydrogenation of the canted antiferromagnetic Gd2CHy (y>2.0). It is demonstrated that structural and magnetic transitions are strongly coupled by the presence or absence of the magnetic quasi-atomic IAEs and non-magnetic hydrogen anions in the interlayer space, which dominate exchange interactions between out-of-plane Gd-Gd atoms. Furthermore, the magnetic quasi-atomic IAEs are inherently conserved by the hydrogen desorption from the P3̅1m structured Gd2CHy, restoring the original ferromagnetic state of the R3̅m structured [Gd2C]2+×2e- electride. This variable density of magnetic quasi-atomic IAEs enables the quantum manipulation of floating electron phases on the electride surface.

Chiral fermions (CFs) in condensed matters, distinguished by right (+) or left (−) handedness, hold a promise for emergent quantum devices. Although a chiral anomaly induced current, J chiral = J (+) − J (−), occurs in Weyl semimetals due to the charge imbalance of the CFs, monitoring spatial flow and temporal dynamics of J chiral has not been demonstrated yet. Here, we report real-space imaging and control of J chiral on the topological Dirac semimetal KZnBi at room temperature (RT) by near-field terahertz (THz) spectroscopy, establishing a relation for an electromagnetic control of J chiral . In THz electric and external magnetic fields, we visualize a spatial flow of coherent J chiral in macroscopic length scale and monitor its temporal dynamics in picosecond time scale, revealing its ultralong transport length around 100 micrometers. Such coherent J chiral is further found to be controlled according to field directions, suggesting the feasibility of information science with topological Dirac semimetals at RT.

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 (

Metal nanoparticles (MeNPs) have been used in various industrial applications, owing to their unique physical and chemical properties different from the bulk counterparts. However, the natural oxidation of MeNPs is an imminent hindrance to their widespread applications despite much research efforts to prevent it. Here, a rational approach for non-oxidized bare MeNPs in air, which requires no additional surface passivation treatment is reported. The direct synthetic route uses the [Gd

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.

Structural imperfections such as grain boundaries (GBs) and dislocations are ubiquitous in solids and have been of central importance in understanding nature of polycrystals. In addition to their classical roles, advent of topological insulators (TIs) offers a chance to realize distinct topological states bound to them. Although dislocation inside three-dimensional TIs is one of the prime candidates to look for, its direct detection and characterization are challenging. Instead, in two-dimensional (2D) TIs, their creations and measurements are easier and, moreover, topological states at the GBs or dislocations intimately connect to their lattice symmetry. However, such roles of crystalline symmetries of GBs in 2D TIs have not been clearly measured yet. Here, we present the first direct evidence of a symmetry enforced Dirac type metallic state along a GB in 1T'-MoTe$_2$, a prototypical 2D TI. Using scanning tunneling microscope, we show a metallic state along a grain boundary with non-symmorphic lattice symmetry and its absence along the other boundary with symmorphic one. Our large scale atomistic simulations demonstrate hourglass like nodal-line semimetallic in-gap states for the former while the gap-opening for the latter, explaining our observation very well. The protected metallic state tightly linked to its crystal symmetry demonstrated here can be used to create stable metallic nanowire inside an insulator.

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.

Band engineering is one of core approaches to improve the performance of thermoelectric materials via the Seebeck coefficient enhancement. However, the conclusion that is often found in the literat...

(Bi,Sb)2Se3 alloys are promising alternatives to commercial n-type Bi2(Te,Se)3 ingots for low-mid temperature thermoelectric power generation due to their high thermoelectric conversion efficiency at elevated temperatures. Herein, we report the enhanced high-temperature thermoelectric performance of the polycrystalline Cl-doped Bi2−xSbxSe3 (x = 0.8, 1.0) bulks and their sustainable thermal stability. Significant role of Cl substitution, characterized to enhance the power factor and reduce the thermal conductivity synergetically, is clearly elucidated. Cl-doping at Se-site of both Bi1.2Sb0.8Se3 and BiSbSe3 results in a high power factor by carrier generation and Hall mobility improvement while maintaining converged electronic band valleys. Furthermore, point defect phonon scattering originated from mass fluctuations formed at Cl-substituted Se-sites reduces the lattice thermal conductivity. Most importantly, spark plasma sintered Cl-doped Bi2−xSbxSe3 bulks are thermally stable up to 700 K, and show a reproducible maximum thermoelectric figure of merit, zT, of 0.68 at 700 K.

Electrides, which are ionic crystals composed of excess anionic electrons, are of great interest as an exotic material for fundamental research and practical applications in broad fields of science and technology. However, an inherent chemical instability under ambient conditions at room temperature has been a fatal drawback to be addressed. Here, we report that transition metal-rich monochalcogenides are an emerging class of low-dimensional electrides with excellent chemical and thermal stability in air and water at room temperature through a comprehensive exploration of theoretical prediction and experimental verification. We predict new two-dimensional (2D) electrides crystallized in hexagonal

We report the direct evidence of impacts of the Coulomb interaction in a prototypical Weyl semimetal, MoTe2, that alter its bare bands in a wide range of energy and momentum. Our quasiparticle interference patterns measured using scanning tunneling microscopy are shown to match the joint density of states of quasiparticle energy bands including momentum-dependent self-energy corrections, while electronic energy bands based on the other simpler local approximations of the Coulomb interaction fail to explain neither the correct number of quasiparticle pockets nor the shape of their dispersions observed in our spectrum. With this, we predict a transition between type-I and type-II Weyl fermions with doping and resolve its disparate quantum oscillation experiments, thus highlighting the critical roles of Coulomb interactions in layered Weyl semimetals.

Engineering active sites of metal nanoparticle-based heterogeneous catalysts is one of the most prerequisite approaches for the efficient production of chemicals, but the limited active sites and undesired oxidation on the metal nanoparticles still remain as key challenges. Here, it is reported that the negatively charged surface of copper nanoparticles on the 2D [Ca

Electrides, which have excess anionic electrons, are solid-state sources of solvated electrons that can be used as powerful reducing agents for organic syntheses. However, the abrupt decomposition of electrides in organic solvents makes controlling the transfer inefficient, thereby limiting the utilization of their superior electron-donating ability. Here, we demonstrate the efficient reductive transformation strategy which combines the stable two-dimensional [Gd2C]2+·2e- electride electron donor and cyclometalated Pt(II) complex photocatalysts. Strongly localized anionic electrons at the interlayer space in the [Gd2C]2+·2e- electride are released via moderate alcoholysis in 2,2,2-trifluoroethanol, enabling persistent electron donation. The Pt(II) complexes are adsorbed onto the surface of the [Gd2C]2+·2e- electride and rapidly capture the released electrons at a rate of 107 s-1 upon photoexcitation. The one-electron-reduced Pt complex is electrochemically stable enough to deliver the electron to substrates in the bulk, which completes the photoredox cycle. The key benefit of this system is the suppression of undesirable charge recombination because back electron transfer is prohibited due to the irreversible disruption of the electride after the electron transfer. These desirable properties collectively serve as the photoredox catalysis principle for the reductive generation of the benzyl radical from benzyl halide, which is the key intermediate for dehalogenated or homocoupled products.

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.

Layer-structured materials are of central importance in a wide range of research fields owing to their unique properties originating from their two dimensionality and anisotropy. Herein, quasi-2D layer-structured IMnV (I: alkali metals and V: pnictogen elements) compounds are investigated, which are potential antiferromagnetic (AFM) semiconductors. Single crystals of IMnV compounds are successfully grown using the self-flux method and their electronic and magnetic properties are analyzed in correlation with structural parameters. Combined with theoretical calculations, the structural analysis indicates that the variation in the bonding angle between VMnV is responsible for the change in the orbital hybridization of Mn and V, predominantly affecting their anisotropic semiconducting properties. Anisotropy in the magnetic properties is also found, where AFM ordering is expected to occur in the in-plane direction, as supported by spin-structure calculations. Furthermore, a possible ferromagnetic (FM) transition is discussed in relation to the vacancy defects. This study provides a candidate material group for AFM and FM spintronics and a basis for exploring magnetic semiconductors in quasi-2D layer-structured systems.

We report the discovery of a new three-dimensional (3D) topological Dirac semimetal (TDS) material KZnBi, coexisting with a naturally formed superconducting state on the surface under ambient pressure. Using photoemission spectroscopy together with first-principles calculations, a 3D Dirac state with linear band dispersion is identified. The characteristic features of massless Dirac fermions are also confirmed by magnetotransport measurements, exhibiting an extremely small cyclotron mass of m^{*}=0.012 m_{0} and a high Fermi velocity of v_{F}=1.04×10^{6} m/s. Interestingly, superconductivity occurs below 0.85 K on the (001) surface, while the bulk remains nonsuperconducting. The captured linear temperature dependence of the upper critical field suggests the possible non-s-wave character of this surface superconductivity. Our discovery serves a distinctive platform to study the interplay between 3D TDS and the superconductivity.

Copper (Cu) nanoparticles (NPs) have received extensive interest owing to their advantageous properties compared with their bulk counterparts. Although the natural oxidation of Cu NPs can be alleviated by passivating the surfaces with additional moieties, obtaining non-oxidized bare Cu NPs in air remains challenging. Here we report that bare Cu NPs with surface excess electrons retain their non-oxidized state over several months in ambient air. Cu NPs grown on an electride support with excellent electron transfer ability are encapsulated by the surface-accumulated excess electrons, exhibiting an ultralow work function of ~3.2 eV. Atomic-scale structural and chemical analyses confirm the absence of Cu oxide moiety at the outermost surface of air-exposed bare Cu NPs. Theoretical energetics clarify that the surface-accumulated excess electrons suppress the oxygen adsorption and consequently prohibit the infiltration of oxygen into the Cu lattice, provoking the endothermic reaction for oxidation process. Our results will further stimulate the practical use of metal NPs in versatile applications.

Inverted structures of common crystal lattices, referred to as antistructures, are rare in nature due to their thermodynamic constraints imposed by the switched cation and anion positions in reference to the original structure. However, a stable antistructure formed with mixed bonding characters of constituent elements in unusual valence states can provide unexpected material properties. Here, a heavy-fermion behavior of ferromagnetic gadolinium lattice in Gd

Pure quantum electrons render intriguing correlated electronic phases by virtue of quantum fluctuations in addition to an exclusive electron-electron interaction. To realise such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid, which spreads up to ~ 3 Å in the vacuum on the surface of electride crystal. An extremely high electron density and its scant hybridization with underneath atomic orbitals evidence quantum and pure nature of electrons, exhibiting polarized liquid phase demonstrated by spin-dependent measurement. Further, upon reducing the density, the dynamics of quantum electrons drastically changes to that of non-Fermi liquid along with an anomalous band deformation, manifesting a possible transition to a hexatic liquid crystalline phase. Our findings cultivate the frontier of quantum electron systems, which serve as an ideal platform for exploring the correlated electronic phases in a pure manner.

Purely quantum electron systems exhibit intriguing correlated electronic phases by virtue of quantum fluctuations in addition to electron-electron interactions. To realize such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid, which spreads up to ~ 3 {\AA} in the vacuum on the surface of electride crystal. An extremely high electron density and its weak hybridisation with buried atomic orbitals evidence the quantum and pure nature of electrons, that exhibit a polarized liquid phase as demonstrated by our spin-dependent measurement. Further, upon enhancing the electron correlation strength, the dynamics of quantum electrons changes to that of non-Fermi liquid along with an anomalous band deformation, suggestive of a transition to a hexatic liquid crystal phase. Our findings cultivate the frontier of quantum electron systems, and serve as a platform for exploring correlated electronic phases in a pure fashion., Comment: 29 pages, 4 figures, 10 extended data figures

Inverted structures of common crystal lattices, referred to as antistructures, are rare in nature due to their thermodynamic constraints imposed by the switched cation and anion positions in reference to the original structure. However, a stable antistructure formed with mixed bonding characters of constituent elements in unusual valence states can provide unexpected material properties. Here, we report a heavy-fermion behaviour of ferromagnetic gadolinium lattice in Gd3SnC antiperovskite, contradicting the common belief that ferromagnetic gadolinium cannot be a heavy-fermion system. The specific heat shows an unusually large Sommerfeld coefficient of ~ 1114 mJ⋅mol− 1⋅K− 2 with a logarithmic behaviour of non-Fermi-liquid state. We demonstrate that the heavy-fermion behaviour in the non-Fermi-liquid state appears to arise from the hybridized electronic states of gadolinium 5d-electrons participating in metallic Gd–Gd and covalent Gd–C bonds. These results accentuate unusual chemical bonds in CGd6 octahedra with the dual characters of gadolinium 5d-electrons for the emergence of heavy-fermions.

The evolutionary magnetism associated with the interlayer spacing in two-dimensional (2D) Y

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.

Separation of epitaxial thin films on growth substrate and transferring onto other materials for functional heterostructures have boosted the transformative impact on science and technology. However, this scheme has proved challenging in thin film thermoelectrics, but promised a vast range of applications beyond the limited device configurations of bulk thermoelectrics. Here, we demonstrate that the Bi0.5Sb1.5Te3 (BST) epitaxial thin film on sapphire substrate grown by spontaneous van der Waals epitaxy (vdWE) is exfoliated and transferred onto versatile materials, creating the high-quality freestanding thermoelectric membranes. Unprecedented millimeter-size vdWE BST membranes are produced by etching pseudomorphic Te monolayer on the surface of sapphire substrate in dilute HF solution. The intact exfoliation and direct transfer for vdWE BST membranes ensures a high thermoelectric performance, maintaining the high-quality crystallinity and subsequently showing the remarkable zT value (~0.9 at 300 K) in thin film thermoelectrics. These results represent the realization of long pursued but yet to be demonstrated high-performance thin film thermoelectrics, paving the way for design and fabrication of arbitrary shaped thermoelectric devices.

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.

(Bi,Sb)

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.

Bi-Te-based materials have been used for room-temperature thermoelectric applications. However, n-type Bi2(Te,Se)3 thermoelectric alloys show a limited conversion efficiency, as compared to their p-type counterparts, thus hindering further widespread room-temperature applications. In this study, we investigated the enhanced thermoelectric properties of n-type Bi2(Te,Se)3 materials by the addition of Cu nanoparticles via a conventional high-energy ball milling process. The electrical and thermal transport properties were modulated by changing the amount of Cu nanoparticles. The power factor was enhanced by controlling the carrier, and the total thermal conductivity was reduced mainly due to the reduction in electronic thermal conductivity. Thus, dimensionless thermoelectric figure of merit (zT) at room temperature was enhanced for the Cu-added samples, and the highest zT value of 0.85 at 375 K was achieved in 2% Cu-doped Bi2Te2.7Se0.3. The average zT (zTavg) value between room temperature and 500 K was 0.79 for the 2% Cu-doped Bi2Te2.7Se0.3, which was 20% higher than that of the pristine Bi2Te2.7Se0.3, whereas a zT higher than 0.80 was sustained from room temperature to ~450 K. These results can lead to a high thermoelectric power generation efficiency of 7.6% at ΔT = 200 K.