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The structural, magnetic and magnetostriction properties in CuFe2-xGaxO4 ceramics synthesized by solid state reaction route and sintered at 1000 °C and 1020 °C are reported here. As per the structural analysis based on the Rietveld refinement of XRD patterns and Raman scattering, the samples crystalized into mixed spinel structures with the coexistence of tetragonal and cubic symmetry. The samples sintered at 1000 °C contained the tetragonal structure as the major structural phase, whereas the cubic phase was dominant in the samples sintered at 1020 °C. The Ga ions were found to preferentially substitute the octahedral sites. For x = 0.15 and 0.20 samples sintered at 1020 °C, a colossal grain growth was observed with grain sizes on the millimeter scale. Analysis of magnetization data in terms of fitting to law of approach to saturation revealed an enhancement of saturation magnetization due to Ga substitution. Magnetostriction strain exhibited a correlation with the occupancy of Cu2+ ions in the octahedral sites, signifying local Jahn–Teller distortion of the lattice as the mechanism behind the magnetostriction. High magnetostriction strain (−88 ppm at 5 kOe) with its rapid increase in the low-magnetic field region demonstrated the potential application of Ga-doped CuFe2O4 for magnetoelectric cofired ceramics. The colossal grain size with relatively low density of grain boundaries played an important role in the rapid rise of magnetostriction curve in low-magnetic field region.

Iron phosphide (FeP) is in the spotlight as a hydrogen evolution reaction (HER) catalyst due to its cost efficiency, good catalytic activity, and stability within a wide pH range. However, there is still a need to synthesize FeP nanoparticles (NPs) with systematic analysis to improve their catalytic activity. Herein, we report FeP NPs synthesized with various phosphorus sources (TOP, trioctylphosphine; TPP, triphenylphosphite; TEAP, tris(diethylamino)phosphine; and TBP, tri-n-butylphosphine) via phosphorization reaction. We clearly demonstrate that the HER activity of the catalyst based on the FeP phase is dependent on the choice of phosphorus source used in the colloidal NP synthesis. Among the samples, FeP NPs synthesized with TPP achieved the highest HER activity with an overpotential of 76 mV at 10 mA cm−2 in 0.5 M H2SO4. Our results reveal a critical aspect of colloidal synthesis to achieve enhanced catalytic activity in the synthesis of transition metal phosphide NPs.

Mesocrystals are assemblies of smaller crystallites and have attracted attention because of their nonclassical crystallization pathway and emerging collective functionalities. Understanding the mesocrystal crystallization mechanism in chemical routes is essential for precise control of size and microstructure, which influence the function of mesocrystals. However, microstructure evolution from the nucleus stage through various crystallization pathways remains unclear. We propose a unified model on the basis of the observation of two crystallization pathways, with different ferric (oxyhydr)oxide polymorphs appearing as intermediates, producing microstructures of magnetite mesocrystal via different mechanisms. An understanding of the crystallization mechanism enables independent chemical control of the mesocrystal diameter and crystallite size, as manifested by a series of magnetic coercivity measurements. We successfully implement an experimental model system that exhibits a universal crystallite size effect on the magnetic coercivity of mesocrystals. These findings provide a general approach to controlling the microstructure through crystallization pathway selection, thus providing a strategy for controlling magnetic coercivity in magnetite systems., Transient metastable intermediates play an important role in the crystallization process. Here, the authors unveil the microstructural changes in magnetite mesocrystals that depend on the intermediate polymorphism and the universal crystallite size effect on the magnetic coercivity of mesocrystals.

In this study, it is demonstrated that the giant self-biased magnetoelectric (SME) response can be achieved from a center-clamped magnetostrictive–piezoelectric laminate composite by employing magnetic tip masses. An asymmetric laminate structure consisting of two different magnetostrictive layers (Metglas and nickel) with opposite signs of piezomagnetic coefficient is introduced to promote structural bending resonance, and the effect of layout change of attaching the magnetic tip masses on SME responses is systematically investigated. The highest SME effect is observed when all magnetic tip masses are loaded on the Metglas layer and their magnetization directions are normal to the Metglas surface. It is proposed that not only the parallel magnetic domains to external magnetic field but also the non-parallel magnetic domains effectively contribute to the total magnetostriction. The fabricated SME laminates exhibit giant SME voltage coefficients ranging from 14.11 to 52.35 V cm−1 Oe−1, depending on the direction of the fields of the tip magnets. These high SME voltage output values and their controllability are promising for precision field sensors, magnetic energy harvesters and field-tunable devices.

Despite their distinctive chemical properties, lossy metals are generally avoided in the design of structural colors because the optical losses can degrade the color vibrancy. Herein, we demonstrate a strategy that allows lossy metals supporting near-wavelength dielectric gratings to achieve high color vibrancy by benefiting from the optical loss rather than suffering from it. By exciting the grating rotated 45° relative to the incident field, s-polarized (s-pol) and p-polarized (p-pol) light each excites a spectrally distinct resonance, described by a treatment of coupled waveguide-array modes, that retards the phase over the wavelength. Owing to the birefringence, a cross-polarized reflection spectrum displays two sharp peaks from each component that decreases the monochromaticity. We show that lossy metals can minimize the p-pol contribution, leaving the sharp s-pol response to determine the spectrum and generate high color vibrancy. Through this scheme, we demonstrate that lossy metal substrates including Pt, a catalytically active metal, and W, a CMOS-compatible metal, can achieve larger sRGB gamut coverage ratios of 90% and 69%, respectively, than that of 55% from Ag, while maintaining similar pixel contrast ratios to that of Ag.

PtS2 is a newly developed group 10 2D layered material with high carrier mobility, wide band gap tunability, strongly bound excitons, symmetrical metallic and magnetic edge states, and ambient stability, making it attractive in nanoelectronic, optoelectronic, and spintronic fields. To the aim of application, a large-scale synthesis is necessary. For transition-metal dichalcogenide (TMD) compounds, a thermally assisted conversion method has been widely used to fabricate wafer-scale thin films. However, PtS2 cannot be easily synthesized using the method, as the tetragonal PtS phase is more stable. Here, we use a specified quartz part to locally increase the vapor pressure of sulfur in a chemical vapor deposition furnace and successfully extend this method for the synthesis of PtS2 thin films in a scalable and controllable manner. Moreover, the PtS and PtS2 phases can be interchangeably converted through a proposed strategy. Field-effect transistor characterization and photocurrent measurements suggest that ...

Silver-based nanomaterials have been versatile building blocks of various photoassisted energy applications; however, they have demonstrated poor electrochemical catalytic performance and stability, in particular, in acidic environments. Here we report a stable and high-performance electrochemical catalyst of silver telluride (AgTe) for the hydrogen evolution reaction (HER), which was synthesized with a nanoporous structure by an electrochemical synthesis method. X-ray spectroscopy techniques on the nanometer scale and high-resolution transmission electron microscopy revealed an orthorhombic structure of nanoporous AgTe with precise lattice constants. First-principles calculations show that the AgTe surface possesses highly active catalytic sites for the HER with an optimized Gibbs free energy change of hydrogen adsorption (-0.005 eV). Our nanoporous AgTe demonstrates exceptional stability and performance for the HER, an overpotential of 27 mV, and a Tafel slope of 33 mV/dec. As a stable catalyst for hydrogen production, AgTe is comparable to platinum-based catalysts and provides a breakthrough for high-performance electrochemical catalysts.

Two-dimensional (2D) layered catalysts have been considered as a class of ideal catalysts for hydrogen evolution reaction (HER) because of their abundant active sites with almost zero Gibbs energy change for hydrogen adsorption. Despite the promising performance, the design of stable and economic electrochemical catalyst based on 2D materials remains to be resolved for industrial-scale hydrogen production. Here, we report layered platinum tellurides, mitrofanovite Pt3Te4, which serves as an efficient and stable catalyst for HER with an overpotential of 39.6 mV and a Tafel slope of 32.7 mV/dec together with a high current density exceeding 7000 mA/cm2. Pt3Te4 was synthesized as nanocrystals on a metallic molybdenum ditelluride (MoTe2) template by a rapid electrochemical method. X-ray diffraction and high-resolution transmission microscopy revealed that the Pt3Te4 nanocrystals have a unique layered structure with repeated monolayer units of PtTe and PtTe2. Theoretical calculations exhibit that Pt3Te4 with numerous edges shows near-zero Gibbs free-energy change of hydrogen adsorption, which shows the excellent HER performance as well as the extremely large exchange current density for massive hydrogen production.

In this study, composite devices were fabricated using ferromagnetic FeSiB-based alloys (Metglas) and ferroelectric ceramics, and their magnetic field sensitivity was evaluated. Sintered 0.95Pb(Zr0.52Ti0.48)O3-0.05Pb(Mn1/3Sb2/3)O3 (PZT-PMS) ceramic exhibited a very dense microstructure with a large piezoelectric voltage coefficient (g31 = −16.8 × 10−3 VmN−1) and mechanical quality factor (Qm > 1600). Owing to these excellent electromechanical properties of the PZT-PMS, the laminate composite with a Metglas/PZT-PMS/Metglas sandwich structure exhibited large magnetoelectric voltage coefficients (αME) in both off-resonance and resonance modes. When the length-to-width aspect ratio (l/w) of the composite was controlled, αME slightly varied in the off-resonance mode, resulting in similar sensitivity values ranging from 129.9 to 146.81 VT−1. Whereas in the resonance mode, the composite with small l/w exhibited a large reduction of αME and sensitivity values. When controlling the thickness of the PZT-PMS (t), the αME of the composite showed the largest value when t was the smallest in the off-resonance mode, while αME was the largest when t is the largest in the resonance mode. The control of t slightly affected the sensitivity in the off-resonance mode, however, higher sensitivity was obtained as t increased in the resonance mode. The results demonstrate that the sensitivity, varying with the dimensional control of the composite, is related to the mechanical loss of the sensor. The composite sensor with the PZT-PMS layer exhibited excellent magnetic field sensitivity of 1.49 × 105 VT−1 with a sub-nT sensing limit, indicating its potential for application in high-performance magnetoelectric sensor devices.

Recently a SnS2 based NO2 gas sensor with a 30 ppb detection limit was demonstrated but this required high operation temperatures. Concurrently, SnS2 grown by chemical vapor deposition is known to naturally contain nanoscale defects, which could be exploited. Here, we significantly enhance the performance of a NO2 gas sensor based on SnS2 with nanoscale defects by photon illumination, and a detection limit of 2.5 ppb is achieved at room temperature. Using a classical Langmuir model and density functional theory simulations, we show S vacancies work as additional adsorption sites with fast adsorption times, higher adsorption energies, and an order of magnitude higher resistance change compared with pristine SnS2. More interestingly, when electron–hole pairs are excited by photon illumination, the average adsorption time first increases and then decreases with NO2 concentration, while the average desorption time always decreases with NO2 concentration. Our results give a deep understanding of photo-enhanced gas sensing of SnS2 with nanoscale defects, and thus open an interesting window for the design of high performance gas sensing devices based on 2D materials.

Recently many novel magnetoresistance (MR) phenomena have been reported from studies of two dimensional (2D) materials. Here, we report on the exotic transport behavior of VS2. A large negative and quadratic MR of −10% is observed for an in-plane magnetic field B up to 14 T. Remarkably, when the applied field deviates from the in-plane orientation there is a threshold field, Bc, and the MR shows a plateau of near zero MR. When B < Bc, only a single state exists and the transition between quantum spin states is forbidden. Our work sheds new light on the MR of magnetic 2D materials with localized states and may spur further investigations.

Volume changes associated with the (de)lithiation of a nanostructured Ni3Sn2 coated nickel inverse opal scaffold anode create mismatch stresses and strains between the Ni3Sn2 anode material and its mechanically supporting Ni scaffold. Using in operando synchrotron x-ray diffraction measurements, elastic strains in the Ni scaffold are determined during cyclic (dis)charging of the Ni3Sn2 anode. These strains are characterized using both the center position of the Ni diffraction peaks, to quantify the average strain, and the peak breadth, which describes the distribution of strain in the measured volume. Upon lithiation (half-cell discharging) or delithiation (half-cell charging), compressive strains and peak breadth linearly increase or decrease, respectively, with charge. The evolution of the average strains and peak breadths suggests that some irreversible plastic deformation and/or delamination occurs during cycling, which can result in capacity fade in the anode. The strain behavior associated with cycling of the Ni3Sn2 anode is similar to that observed in recent studies on a Ni inverse-opal supported amorphous Si anode and demonstrates that the (de)lithiation-induced deformation and damage mechanisms are likely equivalent in both anodes, even though the magnitude of mismatch strain in the Ni3Sn2 is lower due to the lower (de)lithiation-induced contraction/expansion.

The chemical shifts, linewidths, and spin-lattice relaxation times for ammonium and water protons in (NH 4 ) 2 CuBr 4 ·2H 2 O were investigated by 1 H magic angle spinning nuclear magnetic resonance (MAS NMR) with a focus on the roles of NH 4 + and H 2 O at high temperatures. The changes in the temperature dependence of the data near T d (=360 K) were related to variations of the H environments; the mechanism above T d was related to hydrogen-bond transfer involving breakage of the weak part of the hydrogen bond. The hysteresis effects for the ammonium and water protons in (NH 4 ) 2 CuBr 4 ·2H 2 O by MAS NMR were described with respect to heating and cooling.

For applications in wearable human-device interfaces and optoelectronics, flexible materials capable of supporting spatial and uninterrupted bandgap tunability are of immense value. We demonstrate theoretically and experimentally the wide bandgap tunability of GaSe nanosheets, with simultaneous PL enhancement, via elastic strain engineering at room temperature. The elastic strain gives rise to a continuously variable electronic band structure profile, with a rate of 40 meV/1%, and a 3-fold enhancement in PL intensity is achieved when a uniaxial strain of 1% is introduced. An additional effect is that a new exciton state arises when the strain is raised beyond 0.6%. This work suggests that strain engineering can effectively modulate/control the generation, separation, transport, and recombination of photo-induced charge carriers in GaSe, making it a valuable material for flexible optoelectronic-mechanical applications.

Ferromagnetic layers exhibiting perpendicular magnetic anisotropy (PMA) are essential components in magnetic random access memory cells. Various Co- and Fe-based multilayers (MLs) have been considered because the magnetic properties of MLs can be easily controlled by changing the ferromagnetic and nonmagnetic layer thicknesses and number of bilayers. However, MLs generally suffer from the loss of PMA after high-temperature heat treatments (beyond approximately 400 °C) during device processing, which are associated with back-end-of-line device fabrication. Moreover, ML deposition may limit the throughput of the film mass production. Here, we investigate the PMA stability of sputter-deposited CoFeSiB/Pd ML as well as CoFeSiB–Pd co-deposited alloy (CA) films exposed to various annealing temperatures. Although the ML films exhibited PMA, regardless of the annealing temperature up to 500 °C, the CA films started to exhibit strong PMA after 400 °C, which was maintained up to 500 °C. To understand the magnetic property changes, we performed microstructural analyses. The ML films in the as-deposited state exhibited well-defined layered microstructures and evolved into polycrystalline alloys at 500 °C. In contrast, the CA films in the as-deposited state exhibited an amorphous microstructure but transformed into polycrystalline alloys at 500 °C, similar to their ML counterparts. Both Co and Pd, the major constituents of the films, underwent interdiffusion and formed a Co–Pd ordered phase, which contributed to the appearance of PMA.

We synthesized Sr- and W-doped BaTiO3 (BTO) polycrystals by using a solid-state reaction method. The X-ray diffraction results showed that Sr and W atoms occupied the Ba and Ti sites in tetragonal BTO, respectively, and there were changes in the lattice constants and the volumes in the Sr- and W-doped BTO. We found a change in the latent heat and the Curie temperature (TC) during the transition between the ferroelectric and paraelectric phases while increasing the contents of Sr and W in the Sr- and W-doped BTO. This can be explained by the fact that the doping of Sr and W atoms in BTO prevented a distinct transition between the ferroelectric tetragonal and paraelectric cubic structures by decreasing the c/a ratio to a value close to unity. This study shows a way toward a strategy for modulating a crystal structure by using proper dopants for future applications in ferroelectricity-based devices.

A comprehensive understanding of the effect of polymer chain aggregation-induced molecular ordering and the resulting formation of lower excited energy structures in a conjugated polymer on exciton dissociation and recombination at the interface with a wide-bandgap semiconductor is provided through correlation between structural arrangement of the polymer chains and the consequent electrical and optoelectronic properties. A vertical diode-type photovoltaic test probe is combined with a field effect current modulating device and various spectroscopic techniques to isolate the interfacial properties from the bulk properties. Enhanced energy migration in the quasi-ordered (poly(3-hexylthiophene)) (P3HT) film, processed through vibration-induced aggregation of polymer chains in solution state, is attributed to the presence of the aggregation-induced interchain species in which excitons are allowed to migrate through low barrier energy sites, enabling efficient iso-energetic charge transfer followed by the dow...

The electrodes of a hybrid electrochemical capacitor which utilize the quinone (Q)-hydroquinone (QH2) couple, a prototypical organic redox system known to provide fast and reversible proton-coupled electron-transfer reactions, are deterministically mesostructured via a colloidal templating strategy to provide good ion and electron transport pathways, enabling a high rate performance. Specifically, a conducting polymer, polypyrrole (PPy), is functionalized with a pseudocapacitive material, a Q/QH2-containing catechol derivative, by noncovalent interactions. The mesostructure of this hybrid material is formed into an ordered 3D porous structure by a polystyrene colloidal crystal template-assisted electrosynthesis. The catechol derivative is sufficiently bound to the PPy through noncovalent interactions to provide a volumetric capacitance as high as ≈130 F cm−3 and a capacitance retention of ≈75% over 10 000 charging/discharging cycles. When compared with a randomly structured electrode, the deterministically structured electrode exhibits an improved rate performance due to the mesostructure facilitated electron and ion transport.

We report a simple and effective method to obtain the magnetoelectric (ME) effect at zero magnetic bias field (HDC = 0), i.e., the self-biased ME (SME) effect, using an ME laminate composite clamped at its center with its free ends loaded with magnetic tip masses. The method exploits the shifting of the magnetic hysteresis loop of the ferromagnetic (FM) layer of the laminate induced by a preapplied magnetic field (Hp) along the longitudinal direction. The optimum magnetic-field strength corresponding to the maximum of the ME voltage coefficient (αME) vs HDC curve was calculated using equations derived from correlations of magnetic coefficients. In experiments involving the laminate with an FM/ferroelectric/FM symmetrical structure, the strength of Hp was tuned to the optimum value, shifting the αME vs HDC curve along the HDC axis enough to obtain the maximum αME at HDC = 0 (αSME). To further enhance αSME, an asymmetric configuration of the laminate was designed using two different FM materials having piezomagnetic coefficients with opposite signs. The ME laminate with the asymmetrical structure exhibited a large αSME of 55.7 V cm−1 Oe−1 at its bending resonance frequency.

Given the dominant portion of oxygen reduction reactions at the electrodes, the development of an advanced electrode is critical for high-performance solid oxide fuel cells operating at intermediate temperatures. Here, Sm 0.5 Sr 0·5 CoO 3-δ and Gd 0.2 Ce 0·8 O 1.9 composite nanofibers are fabricated by the one-step electrospinning process and evaluated as the electrode of intermediate-temperature solid oxide fuel cells (IT-SOFCs), showing the 36% improved electrode performance compare to the SSC nanofiber-based cathodes at 650 °C for 100 h. The simultaneous phase formation with precursor-based one-step electrospinning ensures that the desired structural properties, including maximized heterointerfaces, increased grain boundary density, and tensile strain state, are achieved to promote oxygen reduction reactions at the cathodes. These results can provide design guidelines to utilize nanostructured electrodes by engineering the structural properties, and thus, to develop high-performance IT-SOFCs.

Doping effects in devices based on two-dimensional (2D) materials have been widely studied. However, detailed analysis and the mechanism of the doping effect caused by encapsulation layers has not been sufficiently explored. In this work, we present experimental studies on the n-doping effect in WSe2 field effect transistors (FETs) with a high-k encapsulation layer (Al2O3) grown by atomic layer deposition. In addition, we demonstrate the mechanism and origin of the doping effect. After encapsulation of the Al2O3 layer, the threshold voltage of the WSe2 FET negatively shifted with the increase of the on-current. The capacitance-voltage measurements of the metal insulator semiconductor (MIS) structure proved the presence of the positive fixed charges within the Al2O3 layer. The flat-band voltage of the MIS structure of Au/Al2O3/SiO2/Si was shifted toward the negative direction on account of the positive fixed charges in the Al2O3 layer. Our results clearly revealed that the fixed charges in the Al2O3 encapsulation layer modulated the Fermi energy level via the field effect. Moreover, these results possibly provide fundamental ideas and guidelines to design 2D materials FETs with high-performance and reliability.

GaSe layers with thicknesses ranging from a monolayer to 100 nm are successfully mechanically exfoliated for use in gas sensing. In combination with density functional theory calculations, general guidelines to determine the number of layers using Raman spectra are presented. With decreasing layer numbers, quantum confinement induces a red-shift for out-of-plane modes and a blue-shift for in-plane modes. The relative Raman shifts of the out-of-plane vibrational modes A′1(11) and A′1(22) grow exponentially with decreasing stack thickness from 100 to 1 layers. Moreover, the change in first-order temperature coefficient (χ) also increases exponentially as the number of layers is reduced, with the value of the first-order temperature coefficient of the A′1(22) mode of monolayer GaSe (≈−1.99 × 10−2 cm−1 K−1) being almost double that of 100 layer GaSe (≈−1.22 × 10−2 cm−1 K−1). Finally, the exfoliated GaSe is used for gas sensing and shows high sensitivity, displaying a minimum detection limit of 4 ppm for NH3 at room temperature, confirming the potential of mechanically exfoliated GaSe in high-sensitivity gas sensors.