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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.

Elemental defect 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 T C 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.

We introduce a new class of electrides with nontrivial band topology by coupling materials database searches and first-principles-calculations-based analysis. ${\mathrm{Cs}}_{3}\mathrm{O}$ and ${\mathrm{Ba}}_{3}\mathrm{N}$ are for the first time identified as a new class of electrides, consisting of one-dimensional (1D) nanorod building blocks. Their crystal structures mimic $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{TiCl}}_{3}$ with the position of anions and cations exchanged. Unlike the weakly coupled nanorods of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{TiCl}}_{3}$, ${\mathrm{Cs}}_{3}\mathrm{O}$ and ${\mathrm{Ba}}_{3}\mathrm{N}$ retain 1D anionic electrons along the hollow interrod sites; additionally, a strong interrod interaction in ${\mathrm{C}}_{3}\mathrm{O}$ and ${\mathrm{Ba}}_{3}\mathrm{N}$ induces band inversion in a 2D superatomic triangular lattice, resulting in Dirac-node lines. The new class of electrides can serve as a prototype for new electrides with a large cavity space that can be utilized for various applications such as gas storage, ion transport, and metal intercalation.

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 [Ca

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.

Structural defects often dominate the electronic- and thermal-transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic-scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti

The self-propagating exothermic chemical reaction with transient thermovoltage, known as the thermopower wave, has received considerable attention recently. A greater peak voltage and specific power are still demanded, and materials with greater Seebeck coefficients have been previously investigated. However, this study employs an alternative mechanism of transient chemical potential gradient providing an unprecedentedly high peak voltage (maximum: 8 V; average: 2.3 V) and volume-specific power (maximum: 0.11 W mm

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 SrTiO

Recent advance in flexible electronics demands development of flexible energy sources. Of particular interests are flexible dye-sensitized solar cells (DSCs). However, a brittle nature of TiO2 materials is one of hurdles to realize flexible DSCs. Here we synthesized flexible photoanodes of TiO2 particles and single-walled carbon nanotubes (SWNTs). Metallic SWNTs provided a greater photovoltaic conversion efficiency than semiconducting SWNTs due to the more efficient electron transport. The metallic SWNTs also constructed effective mechanical network among TiO2 particles providing flexibility and durability. The photoanode was transferred on an indium tin oxide (ITO)-coated polyethylene terephthalate film and characterized for front-illuminated DSCs under the AM 1.5 simulated sunlight. There was only a small decrease in photovoltaic conversion efficiency with bending which was primarily caused by cracking of the ITO layer. Due to this limitation, the TiO2–metallic SWNT photoanode was transferred on a Ti foil and went through up to 1000 bending cycles. The cycled photoanode was assembled for back-illuminated DSCs due to the non-transparent Ti foil. There was no decrease in photovoltaic conversion efficiency even after 1000 bending cycles demonstrating excellent flexibility and durability.

We report an enhancement of the thermoelectric performance in spark-plasma-sintered polycrystalline p-type Bi0.42Sb1.58Te3 by codoping with Ga and Ag at Bi/Sb-site. Through controlled doping of Ga (n-type) and Ag (p-type), electronic transport properties including the electrical conductivity (~988 S/cm at 300 K) and power factor (~3.91 mW m−1 K−2 at 300 K) could be maintained at values comparable to those of pristine Bi0.42Sb1.58Te3, while the lattice thermal conductivity was significantly reduced due to point-defect phonon scattering originating from the mass difference between the host atoms (Bi and Sb) and dopants (Ga and Ag). Through these synergetic effects, a peak ZT of 1.15 was obtained in Bi0.42Sb1.5535Ga0.025Ag0.0015Te3 at 360 K, and ZT could be engineered to be over 1.0 for a wide temperature range (300 K to 420 K).

A high surface area 12CaO·7Al2O3 electride (HT-C12A7:e–) was synthesized by the hydrothermal process and subsequent reduction with CaH2 as a reducing agent, and was then examined as a support for a Ru catalyst in ammonia synthesis. Electrons are successfully incorporated into the cages of C12A7 powder by reduction with CaH2 at 973 K. The concentration of encaged electrons increases with the dehydration temperature of the sample before the CaH2 treatment. The surface area of the resulting material (ca. 20 m2 g–1) is higher than that of the C12A7 electride prepared by conventional solid-phase reaction (SP-C12A7:e–, ca. 1 m2 g–1). The rate of ammonia synthesis over Ru/HT-C12A7:e– is twice that of Ru/SP-C12A7:e–, and there is no significant difference in the apparent activation energy between both catalysts under the optimal conditions. Transmission electron microscopy (TEM) and CO chemisorption measurements revealed that Ru nanoparticles are more highly dispersed on HT-C12A7:e– than that on SP-C12A7:e–, whic...

Thermoelectric properties and figure of merit were evaluated from the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ measured at high temperatures for perovskite-type ceramics of Ba1−xLaxSnO3 with x=0.002, 0.005, and 0.008, which were prepared by a polymerized complex method and a subsequent spark plasma sintering technique. All the polycrystalline dense ceramics showed n-type degenerate semiconducting behavior in the temperature range of 373–1073 K. The La content dependence of the S values revealed successful increase in the electron carriers with the La doping in this x range. The κ values remained almost unchanged with x showing ~9.6 Wm−1 K−1 at room temperature and decreased with increasing temperature. The electronic thermal conductivities calculated by the Wiedemann–Franz law as well as the T−1 dependence of the κ values indicate that the phonon thermal conductivity was dominant. The dimensionless figure of merit ZT increased with increasing temperature for all the ceramics and showed ~0.1 at 1073 K for the ceramics with x=0.002 and 0.005.

We present the results of the pressure dependence of the superconductivity of the inorganic electride 12CaO·7Al2O3:e− (C12A7:e−) single crystal through the ac-susceptibility measurement under high pressure. C12A7:e− has the cage structure and the density of states derived from the cages may play an important role in the superconductivity. The superconducting transition temperature (T c) is ∼ 0.2 K at ambient pressure and monotonically increases up to 1.79 K at 4.7 GPa. The upper critical field H c2 and −dH c2/dT at T c, in proportion to the density of states (DOS) at Fermi energy becomes larger under high pressure. The superconductivity in C12A7:e− is mediated by the electron-phonon interaction, and the increase of Tc with pressure may arise from a peak structure of the DOS of cage conduction band.

The ionic crystal Ca2N is shown to be an electride in terms of [Ca2N]+·e−, with diffusive two-dimensional transport in dense electron layers. The physical properties of electrides — ionic crystals in which electrons behave as anions — significantly depend on the topology of the confining cavity for anionic electrons. Thus, an essential step towards practical electride applications is to discover new confinement spaces with unique topologies. Confined two-dimensional electron layers have previously been achieved by artificially fabricating hetero-interface structures usually of semiconducting materials. Here the authors extend the range of materials demonstrating such behaviour to an electride, dicalcium nitride (Ca2N). This compound has ideal properties for electron confinement: a layered structure with appropriate interlayer spacing and a chemistry that allows for loosely bound electron layers without electron trapping. By providing a new material image for electrides, this work should lead to a series of two-dimensional electrides with unique physical properties. Recent studies suggest that electrides—ionic crystals in which electrons serve as anions—are not exceptional materials but rather a generalized form, particularly under high pressure1,2,3. The topology of the cavities confining anionic electrons determines their physical properties4. At present, reported confining sites consist only of zero-dimensional cavities or weakly linked channels4. Here we report a layered-structure electride of dicalcium nitride, Ca2N, which possesses two-dimensionally confined anionic electrons whose concentration agrees well with that for the chemical formula of [Ca2N]+·e−. Two-dimensional transport characteristics are demonstrated by a high electron mobility (520 cm2 V−1 s−1) and long mean scattering time (0.6 picoseconds) with a mean free path of 0.12 micrometres. The quadratic temperature dependence of the resistivity up to 120 Kelvin indicates the presence of an electron–electron interaction. A striking anisotropic magnetoresistance behaviour with respect to the direction of magnetic field (negative for the field perpendicular to the conducting plane and positive for the field parallel to it) is observed, confirming diffusive two-dimensional transport in dense electron layers. Additionally, band calculations support confinement of anionic electrons within the interlayer space, and photoemission measurements confirm anisotropic low work functions of 3.5 and 2.6 electronvolts, revealing the loosely bound nature of the anionic electrons. We conclude that Ca2N is a two-dimensional electride in terms of [Ca2N]+·e−.

We herein report the electronic and thermal transport properties of p-type Bi0.5Sb1.5Te3polycrystalline bulks with dense pore structure. Dense pore structure was fabricated by vaporization of residual Te during the pressureless annealing of spark plasma sintered bulks of Te coated Bi0.5Sb1.5Te3powders. The lattice thermal conductivity was effectively reduced to the value of 0.35 W m−1 K−1at 300 K mainly due to the phonon scattering by pores, while the power factor was not significantly affected. An enhancedZTof 1.24 at 300 K was obtained in spark plasma sintered and annealed bulks of 3 wt.% Te coated Bi0.5Sb1.5Te3by these synergetic effects.

Conventional perovskite-type ferroelectrics are based on octahedral units of oxygen, and often comprise toxic Pb to achieve robust ferroelectricity. Here, we report the ferroelectricity in a silicate-based compound, Bi2SiO5 (BSO), induced by a structural instability of the corresponding silicate tetrahedral chains. A low-energy phonon mode condenses at ~ 673 K to induce the proper ferroelectric phase transition. Polarization switching was observed in a BSO single crystal with a coercive field of 30 kV/cm and a spontaneous polarization of 0.3 microC/cm2 along a direction normal to the cleavage plane. The in-plane polarization was estimated by first principles calculations to be 23 microC/cm2. The present findings provide a new guideline for designing ferroelectric materials based on SiO4 tetrahedral units, which is ubiquitously found in natural minerals.

We have synthesized a single crystalline Y

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeO

Graphene as a substrate for the van der Waals epitaxy of 2D layered materials is utilized for the epitaxial growth of a layer-structured thermoelectric film. Van der Waals epitaxial Bi0.5 Sb1.5 Te3 film on graphene synthesized via a simple and scalable fabrication method exhibits good crystallinity and high thermoelectric transport properties comparable to single crystals.

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, oxygen vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling oxygen vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structure of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and oxygen vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, oxygen vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films., 28 pages, 12 filgures, published, 2016

We present experimental evidence of enhancement of thermoelectric properties in tellurium (Te) nanoparticle-embedded bismuth antimony telluride (BiSbTe) alloys. Bi0.5Sb1.5Te3 films with a high density of Te particles of 10–20 nm size were prepared by growth of alternating multilayers of ultrathin Te and Bi0.5Sb1.5Te3. As the amount of Te nanoinclusions increased up to ∼15%, the Seebeck coefficient and thermoelectric power factor were increased. Based on the concept of band bending at heterointerfaces as a carrier energy filter, the energy relaxation calculation was made to confirm that the Te nanoinclusions result in a carrier energy filtering effect in p-type bismuth antimony telluride. In addition, thermal conductivities were reduced in the Te-embedded samples, permitting possible further enhancement of the thermoelectric figure of merit. The advantages of Te nanoinclusions in p-type Bi0.5Sb1.5Te3alloys on thermoelectric performance are experimentally realized by both electron- and phonon scattering.

Indium-free and acid-resistant anatase Nb-doped TiO 2 (NTO) electrodes are promising as economical substitutes for high-cost Sn-doped In 2 O 3 (ITO) films used in organic photovoltaics. By rapid-thermal annealing under an ambient vacuum, an insulating amorphous NTO film of low transparency was changed dramatically into a transparent and conductive anatase NTO electrode. Metallic conductivity of the annealed NTO electrode could be attributed to formation of the anatase phase and activation of the Nb dopant. Based on synchrotron X-ray scattering and high-resolution transmission electron microscopy, the electrical properties of the NTO electrode could be correlated with the microstructure of the NTO film. The acid-stability of NTO film also supports its use as a substitute for ITO electrode. Unlike Ga:ZnO and Al:ZnO films, which were easily etched by acidic PEDOT:PSS solution, the NTO film was stable against this reagent. Importantly, the annealing temperature influenced the performance of the organic solar cell fabricated with the NTO electrode. This indicates that activation of Nb dopants and formation of the anatase phase play an important role in the extraction of carrier from the organic layer to the anode electrode.

Solvated electrons in alkali metal-ammonia solutions have attracted attention as a prototype electronic conductor and chemical reducing agent for over a century. However, solvated electrons have not been realized in a high-temperature melt or glass of an oxide system to date. We demonstrated the formation of persistent solvated electrons in both a high-temperature melt and its glass by using the thermally stable electride [Ca(24)Al(28)O(64)](4+)·4e(-) (C12A7:e(-)) and controlling the partial pressure of oxygen. The electrical and structural properties of the resulting melt and glass differ from those of the conventional C12A7:O(2-) oxide, exhibiting metallic and hopping conduction, respectively, and a glass transition temperature that is ~160 kelvin lower than that of C12A7:O(2-) glass. Solvated electrons reside in cage structures in C12A7:e(-) and form a diamagnetic paired state.

Perovskite-type BaSn1-xCoxO3-δ ceramics were prepared by a solid state reaction, and the electrical conductivity σ and Seebeck coefficient S were measured in the temperature range 373-1073 K to evaluate the thermoelectric power factors S2σ. Dense ceramics with relative densities of ∼98 % were obtained by firing at 1573 K for the BaSn1-xCoxO3-δ with x=0.03-0.15. Temperature dependence of σ and S showed that the BaSn1-xCoxO3-δ ceramics with x=0.03-0.15 were p-type semiconductors. A possible origin of the p-type conduction is hopping of holes between localized states of Co ions. The S2σ values were in the orders of 10-11-10-7 Wm-1K-2 and the highest value was 2.7×10-7 Wm-1K-2 at 1073 K for x=0.15.

The accurate determination of the concentration of electrons incorporated into cages in 12CaO·7Al2O3 (C12A7) has been examined using iodometry. Preliminary experiments confirmed that iodine is reduced in preference to protons by electron-enriched C12A7, even in aqueous solution. The electron concentrations in several C12A7 samples are measured using a procedure where the iodine molecules in an I2−HCl solution are partially reduced by electrons released from the samples, followed by titration of the residual iodine molecules. The accuracy of this procedure is confirmed by the excellent agreement of the results with those from thermogravimetric analysis. The reevaluated maximum electron mobility is nearly twice as high as that reported previously, and the critical electron concentration for the metal−insulator transition is revised slightly because of the accuracy of the iodometry results.

Electrides are ionic compounds in which electrons confined in the interstitial spaces serve as anions and are attractive owing to their exotic physical and chemical properties in terms of their low work function and efficient charge-transfer characteristics. Depending on the topology of the anionic electrons, the surface electronic structures of electrides can be significantly altered. In particular, the electronic structures of two-dimensional (2D) electride surfaces are of interest because the localized anionic electrons at the interlayer space can be naturally exposed to cleaved surfaces. In this paper, we report the electronic structure of 2D Y2C electride surface using scanning tunneling microscopy (STM) and first-principles calculations, which reveals that anionic electrons at a cleaved surface are absorbed by the surface and subsequently resurged onto the surface due to an applied electric field. We highlight that the estranged anionic electrons caused by the electric field occupy the slightly shifted crystallographic site compared with a bulk Y2C electride. We also measure the work function of the Y2C single crystal, and it shows a slightly lower value than the calculated one, which appears to be due to the electric field from the STM junction.

Thermoelectric conversion is an energy harvesting technology that directly converts waste heat from various sources into electricity by the Seebeck effect of thermoelectric materials with a large thermopower (S), high electrical conductivity (σ), and low thermal conductivity (κ). State‐of‐the‐art nanostructuring techniques that significantly reduce κ have realized high‐performance thermoelectric materials with a figure of merit (ZT = S 2∙σ∙T∙κ−1) between 1.5 and 2. Although the power factor (PF = S 2∙σ) must also be enhanced to further improve ZT, the maximum PF remains near 1.5–4 mW m−1 K−2 due to the well‐known trade‐off relationship between S and σ. At a maximized PF, σ is much lower than the ideal value since impurity doping suppresses the carrier mobility. A metal‐oxide‐semiconductor high electron mobility transistor (MOS‐HEMT) structure on an AlGaN/GaN heterostructure is prepared. Applying a gate electric field to the MOS‐HEMT simultaneously modulates S and σ of the high‐mobility electron gas from −490 µV K−1 and ≈10−1 S cm−1 to −90 µV K−1 and ≈104 S cm−1, while maintaining a high carrier mobility (≈1500 cm2 V−1 s−1). The maximized PF of the high‐mobility electron gas is ≈9 mW m−1 K−2, which is a two‐ to sixfold increase compared to state‐of‐the‐art practical thermoelectric materials.

12CaO·7Al2O3 (C12A7) is a typical band insulator, but may be converted to a metallic conductor C12A7:e− by electron-doping via removal of free oxygen ions sitting in the subnanometer-sized cages as the counter ions. Also, C12A7 is known as a fast oxygen ion conductor. These unique features let us expect that it would be possible to dope electrons to C12A7 by removing the free oxygen ions by an external electric field. In this study, we fabricated C12A7/C12A7:e− stacking devices and examined their current–voltage characteristics. The thickness of the top C12A7 layer was controlled by low-temperature oxidation with an aid of an optical model analysis of spectroscopic ellipsometry. We found that the C12A7/C12A7:e− devices exhibited a bistable resistance switching effect with an on-to-off resistance ratio of ∼102 and operated as a resistive random access memory.

Single crystals of 12CaO·7Al2O3 (C12A7) have been grown by the Czochralski method with three different growth directions of , , and . Employment of a 2.0% oxygen-containing nitrogen atmosphere and use of an iridium crucible were the critical factors for obtaining the single crystal. The crystal exhibited orange color resulting from the incorporation of Ir4+ ions of ∼ 5 × 1017 cm− 3, presumably occupying at Ca2+ sites. High resolution X-ray diffraction analyses indicated that the C12A7 single crystal shows the tetragonal symmetry belonging to the space group I 4 ― 2 m .

[Ca24Al28O64]4+(O2-)2-x(e-)2x (0 ≤ x ≤ 2, C12A7:e-) crystal shows a metal−insulator transition at x ∼ 1. Varied concentrations of electrons (Ne) are introduced in the subnanometer-sized crystallogr...

Pore-free ⟨110⟩ oriented 12CaO·7Al2O3 (C12A7) single crystals with sizes up to 7 mm (Phi) × 40 mm are grown by controlling the melt states in a floating zone (Fz) method. Control is achieved by increasing the melt temperature to ∼1495 °C and using a high-density C12A7 polycrystalline feed rod, which reduces oxygen bubbles in the melt almost completely due to a distinct decrease in O2-solubility with temperature increase. The optimization of the melt temperature, coupled with ⟨110⟩ oriented single crystal as a seed and the decreased growth speed, allows high-quality crystal growth. The grown crystal exhibits a good internal transparency (above 98% in the visible region) and does not contain magnetic impurities such as Ir, which are inevitably incorporated in Czochralski grown crystals. The crystal can be used as a precursor for superconductive C12A7:e−.

Crystal structure refinements of undoped and 14 at.% F-doped LaFeAsO ( T c =20 K) were performed over the temperature region of 25 to 300 K by synchrotron X-ray diffraction. The electronic structures of the crystals were also examined using density functional theory (DFT). Undoped LaFeAsO was observed to undergo a crystallographic transition from the tetragonal ( P 4/ n m m ) to orthorhombic ( C m m a ) phases at ∼160 K. This crystallographic transition was accompanied with an antiferromagnetic spin ordering at ∼140 K. The DFT calculation results suggest that both transitions were caused by the “Cooperative Jahn–Teller effect”. In contrast, an F-doped sample maintains the non-magnetic tetragonal phase down to 25 K. F-ion substitutions for the O sites changed the bond length and angle in the distorted LaO tetrahedron, whereas weaker geometric effects were observed in the FeAs layer. Further, the F-doping induced large decreases in the c -axis length and the La–As distance, suggesting an enhancement of the ...

It was reported that a mixed light metal oxide compound, 12CaO� 7Al2O3 (C12A7), which is known as a constituent of aluminous cement, became a superconductor at an ambient pressure by the exclusive replacement of extra-framework oxygen ions in subnanometer-sized crystallographic cages with electrons. Temperature dependences of resistivity, magnetic susceptibility, and magnetic field dependent resistivity of single-crystals and thin films revealed superconducting transition at temperature (Tc )o f� 0:4 K and a critical magnetic field of � 30 mT. Tc varies 0.2–0.4 K with the electron concentration. The interaction of the anionic electrons in the free space (cages) with the cationic framework may be responsible for the emergence of the superconducting state. [doi:10.2320/matertrans.MBW200717]

We report herein on Ruddlesden-Popper-type doped Sr n+1 Ti n O 3n+1 (n=2) as a potential candidate for n-type thermoelectric (TE) oxides. The TE properties of 5 at.% La-, Nd-, and Nb-doped Sr 3 Ti 2 O 7 polycrystalline ceramics were investigated and the origin of Seebeck coefficient was discussed from the viewpoint of the symmetry of TiO 6 octahedra. A significant reduction in the lattice thermal conductivity was observed by the enhancement of phonon scattering at SrO/(SrTiO 3 ) n interfaces originating from the inherent superlattice structure, and the maximum dimensionless figure of merit, ZT∼0.15, at a 1000K value was obtained in 5 at.% La-doped Sr 3 Ti 2 O 7 .

12CaO · 7Al2O3 (C12A7) electride, which is synthesized by replacing free oxygen ions in cages with electron anions, has distinct advantages over electrides reported so far in respect of thermal and chemical stability and flexible preparation of various sample forms including single crystal, thin film, polycrystalline bulk and powder. These advantages, together with the fact that the concentration of the electron anions is controlled in a wide range according to a synthetic process, make the C12A7 electride attract growing attentions from both scientific and practical points of views. This paper reviews several chemical and physical synthetic processes of the C12A7 electride including thermal treatment of C12A7 under metal vapor and reducing gas atmospheres, hot Ar+ ion implantation, solidification of the strongly reduced C12A7 melt, and crystallization of the reduced glass in vacuum. Each process, having its own suitability for a specific form of the electride, has unique advantages such as a completeness of anion replacement, mass production capability and controllability of the electron-doped area. Electronic and optical properties of the resulting electrides prepared by the different process are briefly discussed in terms of the feature of the processes.

A class of intermetallics of TiNiSn half-Heusler compound with MgAgAs structure type is currently of interest as a potential high temperature thermoelectric material. The ternary TiNiSn compound has showed promising thermoelectric properties, a high Seebeck coefficient and low electrical resistivity. The present study reports the effect of Hf alloying on Ti site, Pt and Pd alloying on Ni site, and Sb doping on Sn site for the optimization of thermoelectric properties of TiNiSn-based compounds. Also, to achieve a low thermal conductivity, a powder metallurgy technique is used for the fabrication of the compounds. These efforts result in the dimensionless figure of merit, ZT as 0.78 for the hot-pressed (Ti0.95Hf0.05)Ni(Sn0.99Sb0.01) sample at 770 K with a large power factor (4.1 mW/mK2), which makes these materials very attractive for potential power generation applications.

Enhancement of the Seebeck coefficient (S ) without reducing the electrical conductivity (sigma) is essential to realize practical thermoelectric materials exhibiting a dimensionless figure of merit (ZT=S2 x sigma x T x kappa-1) exceeding 2, where T is the absolute temperature and kappa is the thermal conductivity. Here, we demonstrate that a high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO(3) yields unusually large |S|, approximately five times larger than that of SrTiO(3) bulks, while maintaining a high sigma2DEG. In the best case, we observe |S|=850 microV K-1 and sigma2DEG=1.4 x 10(3) S cm-1. In addition, by using the kappa of bulk single-crystal SrTiO(3) at room temperature, we estimate ZT approximately 2.4 for the 2DEG, corresponding to ZT approximately 0.24 for a complete device having the 2DEG as the active region. The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements.

The two dimensional electride, dicalcium nitride, is successfully used as electron transfer agent in the simple and highly efficient hydrogenation of alkynes and alkenes.

Thermoelectrics, which transports heat for refrigeration or converts heat into electricity directly, is a key technology for renewable energy harvesting and solid-state refrigeration. Despite its importance, the widespread use of thermoelectric devices is constrained because of the low efficiency of thermoelectric bulk alloys. However, boundary engineering has been demonstrated as one of the most effective ways to enhance the thermoelectric performance of conventional thermoelectric materials such as Bi2 Te3 , PbTe, and SiGe alloys because their thermal and electronic transport properties can be manipulated separately by this approach. We review our recent progress on the enhancement of the thermoelectric figure of merit through boundary engineering together with the processing technologies for boundary engineering developed most recently using Bi2 Te3 -based bulk alloys. A brief discussion of the principles and current status of boundary-engineered bulk alloys for the enhancement of the thermoelectric figure of merit is presented. We focus mainly on (1) the reduction of the thermal conductivity by grain boundary engineering and (2) the reduction of thermal conductivity without deterioration of the electrical conductivity by phase boundary engineering. We also discuss the next potential approach using two boundary engineering strategies for a breakthrough in the area of bulk thermoelectric alloys.

Artificial van der Waals heterostructures with two-dimensional (2D) atomic crystals are promising as an active channel or as a buffer contact layer for next-generation devices. However, genuine 2D heterostructure devices remain limited because of impurity-involved transfer process and metastable and inhomogeneous heterostructure formation. We used laser-induced phase patterning, a polymorph engineering, to fabricate an ohmic heterophase homojunction between semiconducting hexagonal (2H) and metallic monoclinic (1T') molybdenum ditelluride (MoTe2) that is stable up to 300°C and increases the carrier mobility of the MoTe2 transistor by a factor of about 50, while retaining a high on/off current ratio of 10(6). In situ scanning transmission electron microscopy results combined with theoretical calculations reveal that the Te vacancy triggers the local phase transition in MoTe2, achieving a true 2D device with an ohmic contact.