First-principles study of CaCu3B4O12 (B=Co, Rh, Ir)

Employing first-principles density functional theory based calculations we investigated the change in electronic structure of CaCu₃B₄O₁₂ compounds as one moves from 3d (Co) to 4d (Rh) to 5d (Ir) element at B site. Our study sheds light on valences of Cu and B ions as one moves from 3d to 4d to 5d based compounds. The valence of Cu in Co and Rh compound turn out to be that of less known 3+ state, while that in Ir compound turn out to be commonly known 2+ state. Our first-principles study provide microscopic understanding of these different valences of Cu, in terms of changes in the mixing of Cu x ² − y ² and B-a _1g states, driven by changes in the crystal field and spin splitting. The stronger crystal field splitting for 4d and 5d elements compared to 3d at B site drive the low-spin state at Rh and Ir site as opposed to intermediate spin in case of Co.     

Influence of Barrier Material on Spin Splitting Due to Inversion Asymmetry in Heterostructures

Influence of barrier material on the spin splitting of conduction subbands in heterostructures because of structure inversion asymmetry (Bychkov–Rashba splitting) is studied. The spin splitting at a vanishing magnetic field is calculated for two heterostructures: InAs/SiO₂ and InAs/In_0.8Al_0.2As, having the same well material InAs but very different barrier materials. It is demonstrated that the barrier material strongly influences the spin splitting of the ground conduction subband in InAs. The spin splittings for both heterostructures are computed as functions of electron density, we obtain the splitting in InAs/SiO₂ almost twice larger than that in InAs/In_0.8Al_0.2As. The influence of spin-dependent part of the boundary conditions on the spin spin splitting is studied and it is shown that for considered heterostructures it changes the splitting up to 25% of its value. It is emphasized that the Bychkov–Rashba spin splitting is not proportional to the average electric field in heterostructure.     

The search for novel superhard and incompressible materials on the basis of higher borides of <Emphasis Type="Italic">s</Emphasis>, <Emphasis Type="Italic">p</Emphasis>, <Emphasis Type="Italic">d</Emphasis> metals

The state of the art in the search for novel superhard and (or) incompressible materials on the basis of higher borides of s, p, d metals has been briefly reviewed. The information has been considered about experimental and theoretical studies of the following groups of borides: diborides of 4d, 5d heavy metals (Tc, Ru, Rh, Re, Os, and Ir), hexagonal tetraborides with the WB₄-type structure, and AMB₁₄ borides (where A, M are s, p metals) as well as of a number of related systems.     

Strawberry‐like Core–Shell Ag@Polydopamine@BaTiO3 Hybrid Nanoparticles for High‐k Polymer Nanocomposites with High Energy Density and Low Dielectric Loss

Flexible nanocomposites comprising of polymer and high‐dielectric‐constant (high‐k) ceramic nanoparticles are becoming increasingly attractive for dielectric and energy storage applications in modern electronic and electric industry. However, a huge challenge still remains. Namely, the increase of dielectric constant usually at the cost of significant decrease of breakdown strength of the nanocomposites because of the electric field distortion and concentration induced by the high‐k filler. To address this long‐standing problem, by using nano‐Ag decorated core–shell polydopamine (PDA) coated BaTiO₃ (BT) hybrid nanoparticles, a new strategy is developed to prepare high‐k polymer nanocomposites with high breakdown strength. The strawberry‐like BT‐PDA‐Ag based ferroelectric polymer [i.e., poly(vinylideneflyoride‐co‐hexafluroro propylene), P(VDF‐HFP)] nanocomposites exhibit greatly enhanced energy density and significantly suppressed dielectric loss as well as leakage current density in comparison with the nanocomposites with the core–shell structured BT‐PDA. Coulomb‐blockade effect of super‐small nano‐Ag is used to explain the observed performance enhancement of the nanocomposites. The simplicity and scalability of the described approach provide a promising route to polymer nanocomposites for dielectric and energy storage applications.     

A Game Changer: A Multifunctional Perovskite Exhibiting Giant Ferroelectricity and Narrow Bandgap with Potential Application in a Truly Monolithic Multienergy Harvester or Sensor

An ABO₃‐type perovskite solid‐solution, (K_0.5Na_0.5)NbO₃ (KNN) doped with 2 mol% Ba(Ni_0.5Nb_0.5)O_3?δ (BNNO) is reported. Such a composition yields a much narrower bandgap (≈1.6 eV) compared to the parental composition—pure KNN—and other widely used piezoelectric and pyroelectric materials (e.g., Pb(Zr,Ti)O₃, BaTiO₃). Meanwhile, it exhibits the same large piezoelectric coefficient as that of KNN (≈100 pC N^?1) and a much larger pyroelectric coefficient (≈130 µC m^?2 K^?1) compared to the previously reported narrow‐bandgap material (KNbO₃)_1?x ‐BNNO_x . The unique combination of these excellent ferroelectric and optical properties opens the door to the development of multisource energy harvesting or multifunctional sensing devices for the simultaneous and efficient conversion of solar, thermal, and kinetic energies into electricity in a single material. Individual and comprehensive characterizations of the optical, ferroelectric, piezoelectric, pyroelectric, and photovoltaic properties are investigated with single and coexisting energy sources. No degrading interaction between ferroelectric and photovoltaic behaviors is observed. This composition may fundamentally change the working principles of state‐of‐the‐art hybrid energy harvesters and sensors, and thus significantly increases the unit‐volume energy conversion efficiency and reliability of energy harvesters in ambient environments.     

Synergism of Geometric Construction and Electronic Regulation: 3D Se‐(NiCo)Sx/(OH)x Nanosheets for Highly Efficient Overall Water Splitting

The exploration of highly efficient electrocatalysts for both oxygen and hydrogen generation via water splitting is receiving considerable attention in recent decades. Up till now, Pt‐based catalysts still exhibit the best hydrogen evolution reaction (HER) performance and Ir/Ru‐based oxides are identified as the benchmark for oxygen evolution reaction (OER). However, the high cost and rarity of these materials extremely hinder their large‐scale applications. This paper describes the construction of the ultrathin defect‐enriched 3D Se‐(NiCo)S_x/(OH)_x nanosheets for overall water splitting through a facile Se‐induced hydrothermal treatment. Via Se‐induced fabrication, highly efficient Se‐(NiCo)S_x/(OH)_x nanosheets are successfully fabricated through morphology optimization, defect engineering, and electronic structure tailoring. The as‐prepared hybrids exhibit relatively low overpotentials of 155 and 103 mV at the current density of 10 mA cm^?2 for OER and HER, respectively. Moreover, an overall water‐splitting device delivers a current density of 10 mA cm^?2 for ≈66 h without obvious degradation.     

Electric and Mechanical Switching of Ferroelectric and Resistive States in Semiconducting BaTiO3–δ Films on Silicon

Materials that can couple electrical and mechanical properties constitute a key element of smart actuators, energy harvesters, or many sensing devices. Within this class, functional oxides display specific mesoscale responses which often result in great sensitivity to small external stimuli. Here, a novel combination of molecular beam epitaxy and a water‐based chemical‐solution method is used for the design of mechanically controlled multilevel device integrated on silicon. In particular, the possibility of adding extra functionalities to a ferroelectric oxide heterostructure by n‐doping and nanostructuring a BaTiO₃ thin film on Si(001) is explored. It is found that the ferroelectric polarization can be reversed, and resistive switching can be measured, upon a mechanical load in epitaxial BaTiO_3?δ /La_0.7Sr_0.3MnO₃/SrTiO₃/Si columnar nanostructures. A flexoelectric effect is found, stemming from substantial strain gradients that can be created with moderate loads. Simultaneously, mechanical effects on the local conductivity can be used to modulate a nonvolatile resistive state of the BaTiO_3?δ heterostructure. As a result, three different configurations of the system become accessible on top of the usual voltage reversal of polarization and resistive states.     

Properties of flexible, transparent barium titanate nanoparticle/poly(2-hydroxyethyl methacrylate) hybrid

A transparent BaTiO₃ particle/poly(2-hydroxyethyl methacrylate) (PHEMA) hybrid was synthesized from a Ba–Ti double alkoxide modified with an organic ligand, and the refractive index and ferroelectric properties of the hybrid were studied. The transparent hybrid was flexible and could be shaped not only in the form of thin films on substrates but also in the form of flexible self-standing films. BaTiO₃ particles around 5 nm in diameter were dispersed uniformly in the polymer matrix. The refractive indexes of the hybrid films increased with decreasing wavelength and were dependent on the volume fractions of the BaTiO₃ and polymer phases. The hybrid film synthesized at a molar ratio of BaTiO₃:2-vinlyloxyethanol:H₂O:PHEMA = 1:8:30:5 gave the best result, with a refractive index of 1.55 at 589 nm and an Abbe number of 37.5. A transparent hybrid film on platinized silicone substrates exhibited a polarization–electric field hysteresis loop.     

Phase transition and electrical properties in the Li-modified Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-based lead-free ceramics

Phase transition and electrical properties were demonstrated for a Li-modified Bi_0.5Na_0.5TiO₃-based solid solution. (0.935 − x)Bi_0.5Na_0.5TiO₃ − xBi_0.5Li_0.5TiO₃ − 0.065BaTiO₃ with 0.5 mol% Mn doping was prepared by a conventional solid state reaction method. Close inspection of X-ray diffraction patterns indicated that no characteristic peaks splitting happened, indicating the pseudocubic structure for all the compositions. At a critical composition x of 0.06, optimized performance was obtained with piezoelectric constant d ₃₃ of 176 pC/N, electromechanical coupling factors k _P of 0.33, and k _t of 0.52, respectively. In addition, it was found that the Li substitution could lead to a disruption of long-range ferroelectric order and obtain enhanced frequency dispersion behavior accompanied with the decreasing of the depolarization temperature T _d, which was responsible for the observed weaker ferroelectric polarization and electromechanical response. The composition induced structure evolution was also discussed combined with the Raman spectroscopy.     

Lead-free piezoelectric ceramics with composition of (0.97−x)Na1/2Bi1/2TiO3-0.03NaNbO3-xBaTiO3

Compositions in (Na_1/2Bi_1/2)TiO₃ based ternary system, (0.97 – x) (Na_1/2Bi_1/2)TiO₃-0.03NaNbO₃-xBaTiO₃ (x = 0, 0.01, 0.02, 0.04, 0.05, 0.06, 0.08) are synthesized using conventional solid state reaction method. Influence of BaTiO₃ on crystal structure, dielectric and piezoelectric properties are investigated. All compositions can form single perovskite phase. Powder x-ray diffraction patterns can be indexed assuming a pseudo-cubic structure. Lattice constant increases with the increase of BaTiO₃ concentration. Rhombohedral distortion is observed in poled samples with BaTiO₃ concentration up to 6 mol%. Temperature dependence of dielectric constant and dissipation factor measurement reveals that all compositions experience two phase transitions: from ferroelectric to antiferroelectric and from antiferroelectric to paraelectric. Both transition temperatures, T _c and T _f, are lowered due to introduction of BaTiO₃. Ferroelectric to antiferroelectric phase transition has relaxor characteristics. Piezoelectric properties have relatively higher value around 1 mol% to 4 mol% BaTiO₃. In ceramics with x = 0.02, thickness electromechanical coupling factor (k _t) of 0.51 and piezoelectric charge constant (d ₃₃) of 110 × 10^–12 C/N are obtained. Addition of small amount of BaTiO₃ (x = 0.01, 0.02) improves piezoelectric properties compared to NBT-NN binary system, while T _f remains above 140°C, higher than that of NBT-BT binary system composition with similar piezoelectric properties. This is in favor of the possible application of them as lead-free piezoelectric ceramics.     

Giant Rashba-type spin splitting in bulk BiTeI

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.     

Filtering Spin with Tunnel-Coupled Hole Quantum Wires

Creating spin-polarized currents in nonmagnetic semiconductors is one of the key prerequisites for realizing spintronics devices. We have shown previously that the k-linear Rashba spin splitting present in two-dimensional (2D) electron systems can be utilized in a momentum-selective tunneling geometry to design a spin filter without using magnetic fields or ferromagnetic contacts. Motivated by the fact that spin–orbit effects are typically much stronger in 2D hole systems, we consider quantum wires formed by additional confinement of the lowest (heavy-hole) 2D valence subband. Its k ³-type Rashba term gives rise to a k-linear spin splitting for holes in the quantum wire. Implementation of the spin-filter design is then analogous to the electron case but, in the hole system, requires less momentum selectivity and should therefore be easier to realize.     

Dielectric properties,thermal expansion and heat capacity of (1 − x)Na0.5Bi0.5TiO3–xBaTiO3 single crystals (x = 0, 0.02, 0.025, 0.0325 and 0.05)

High-quality and large-size lead-free (1 − x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ single crystals (x = 0, 0.025, 0.0325 and 0.05) were grown using Czochralski method. The obtained samples were of pure perovskite structure with rhombohedral symmetry at room temperature. Thermal expansion, heat capacity, ferroelectric and dielectric properties were measured in a wide temperature range. The broad anomalies observed in thermal expansion and heat capacity were corresponded to structural, ferroelectric and dielectric anomalies, related to temperature features of polar regions and formation of a long-range order ferroelectric phase. The Burns temperature was found to increase with increasing BaTiO₃ content. At low-frequency (100 Hz–100 kHz) the samples showed diffuse phase transitions. The obtained results were discussed in terms of local electric and strain fields caused by a difference in ionic radii between (Na,Bi) and Ba ions.     

Three-scale analysis of BaTiO<Subscript>3</Subscript> piezoelectric thin films fabrication process and its experimental validation

A three-scale analysis of crystal growth process is newly proposed based on the first-principles calculation and on the finite element analysis in order to generate a new biocompatible piezoelectric thin film. Crystal growth process of lead-free BaTiO₃ thin films was designed and experimentally generated on SrTiO₃(100), (110), (111), and MgO(100) substrates using the radio-frequency magnetron sputtering method. Crystal structures of BaTiO₃ were measured by X-ray diffraction (XRD) θ/2θ scan. We used Pt for the electrode and measured piezoelectric strain constants d ₃₃ using the ferroelectric measurement system. As a result, analytical crystal orientation fractions on SrTiO₃(110) and (111) substrates had good quantitative agreement with experimental ones, and ones on SrTiO₃(100) and MgO(100) substrates corresponded with these experimental crystal structures. Furthermore, analytically determined piezoelectric strain constants d ₃₃ qualitatively showed a good agreement with experimental ones. Especially, for SrTiO₃(100) and MgO(100) substrates, the differences of d ₃₃ depending on orientation fractions were analyzed by the three-scale simulation accurately. Consequently, it is confirmed that the three-scale analysis is a useful simulation tool to design new biocompatible piezoelectric thin films.     

Multi‐Nonvolatile State Resistive Switching Arising from Ferroelectricity and Oxygen Vacancy Migration

Resistive switching phenomena form the basis of competing memory technologies. Among them, resistive switching, originating from oxygen vacancy migration (OVM), and ferroelectric switching offer two promising approaches. OVM in oxide films/heterostructures can exhibit high/low resistive state via conducting filament forming/deforming, while the resistive switching of ferroelectric tunnel junctions (FTJs) arises from barrier height or width variation while ferroelectric polarization reverses between asymmetric electrodes. Here the authors demonstrate a coexistence of OVM and ferroelectric induced resistive switching in a BaTiO₃ FTJ by comparing BaTiO₃ with SrTiO₃ based tunnel junctions. This coexistence results in two distinguishable loops with multi‐nonvolatile resistive states. The primary loop originates from the ferroelectric switching. The second loop emerges at a voltage close to the SrTiO₃ switching voltage, showing OVM being its origin. BaTiO₃ based devices with controlled oxygen vacancies enable us to combine the benefits of both OVM and ferroelectric tunneling to produce multistate nonvolatile memory devices.     

Dielectric properties of (Na0.5Bi0.5)1 − xMexTiO3 ceramics near morphotropic phase boundary

Ceramic samples of modified ferroelectric sodium-bismuth titanate (Na_0.5Bi_0.5)_0.87 Me_0.13TiO₃ (Me = Pb, Sr and Pb + Sr), were prepared using conventional solid state reaction techniques. The studies of powder X-ray diffraction of the obtained compounds revealed their rhombohedral symmetry at room temperature, the increase of lattice constant and the increase of rhombohedral lattice distortion (except for the material with Sr dopand, where distortion decreases). Temperature (at room temperature to 400°C) and frequency (at 20 Hz to 1 MHz) dielectric measurements reveal that A-site cations addition of Pb and/or Sr have resulted in the increase of relative electric permittivity. However, the temperature T _m (when the electric permittivity is a maximum) increases after Pb or Sr doping and it decreases after (Pb + Sr) doping. The pyroelectric and current loop measurements have shown that all samples were ferroelectric. The results of these measurements also allowed us to determine the temperature variation of the remanent and spontanous polarizations. The polarizations are found to decrease after Pb or Sr doping and increase after (Pb + Sr) doping. The piezoelectric constants (d ₃₃ and d ₃₁) and electromechanical coupling factors (k ₃₃ and k ₃₁) were obtained from resonance-antiresonance measurements method. The best piezoelectric and electromechanical properties have NBT doped by Pb. This ceramic may be good candidate for device applications. The diffuse ferroelectric phase transition of the investigated materials, similarly as for pure NBT, has been revealed. The properties of these materials (especially in diffuse phase transition range) can be explained by the behaviour of polar regions.     

Competition Between Rashba Effect and Exchange Interaction in a Hg0.98Mn0.02Te Magnetic 2DEG

Magnetotransport measurements are used to study the peculiar properties of a modulation doped n-type Hg_0.98Mn_0.02Te magnetic 2DEG. The Rashba effect and giant Zeeman spin splitting are observed simultaneously and can be separated by temperature and gate voltage dependent Shubnikov-de Haas oscillations. The Rashba effect is found to be the dominant term up to 5 T.     

Piezoelectric BNT-BT<Subscript>0.11</Subscript> thin films processed by sol–gel technique

0.89(Na_0.5Bi_0.5)TiO₃–0.11BaTiO₃, (BNT-BT_0.11) thin film was fabricated by sol–gel/spin coating process, on platinized silicon wafer. Perovskite structure with random orientation of crystallites has been obtained at 700 °C. Piezoelectric activity of BNT-BT_0.11 thin film was detected using piezoresponse force microscopy (PFM). Effective piezoelectric coefficient d _33eff of such film, recorded at 5 V applied dc voltage, was ~29 pm/V, which is similar to other BNT-BT _x thin films. The complex refractive index and dielectric function of BNT-BT_0.11 thin films were also investigated. The high leakage current density significantly influences the dielectric, ferroelectric, and piezoelectric properties of the BNT-BT_0.11 films.     

Analysis of the (5d2+5d6s)–5d6p Transition Arrays of Os VII and Ir VIII,and the 6s 2S–6p 2P Transitions of Ir IX

The spectra of osmium and iridium were photographed in the 300 Å to 1600 Å region on a 3 m normal incidence spectrograph using a triggered spark source. The (5d² + 5d6s)–5d6p transition arrays of Os VII and Ir VIII were analyzed. All levels of these three configurations in both spectra have been established. There are 77 lines in Os VII and 71 lines in Ir VIII classified. The parametric least squares fitting calculations are used to interpret both spectra. The 6s ²S_1/2–6p ²P_1/2,3/2 transitions in Ir IX have also been identified.     

Growth Mechanism of Metal Clusters on a Graphene/Ru(0001) Template

Using first‐principles calculations combined with scanning tunneling microscopy experiments, we investigated the adsorption configurations, electronic structures and the corresponding growth mechanism of several transition metal (TM) atoms (Pt, Ru, Ir, Ti, Pd, Au, Ag, and Cu) on a graphene/Ru(0001) moiré template (G/Ru(0001)) at low coverage. We find that Pt, Ru, Ir, and Ti selectively adsorb on the fcc region of G/Ru(0001) and form ordered dispersed metal nanoclusters. This behavior is due to the unoccupied d orbital of the TM atoms and the strong sp³ hybridization of carbon atoms in the fcc region of G/Ru(0001). Pd, Au, Ag, and Cu form nonselective structures because of the fully occupied d orbital. This mechanism can be extended to metals on a graphene/Rh(111) template. By using Pt as an example, we provide a layer by layer growth path for Pt nanoclusters in the fcc region of the G/Ru(0001). The simulations of growth mechanism agree well with the experimental observations. Moreover, they also provide guidance for the selection of suitable metal atoms to form ordered dispersed metal nanoclusters on similar templates.