V-Based MXene/MAX Heterostructure Composites with Enhancing Dielectric Loss for Broadband Microwave Absorption.

Electromagnetic wave absorption performance is strictly dependent on attenuation and impedance matching, which are directly influenced by the ratio of MXene/MAX in the multilayer structured MXene/MAX composites. However, there is still a challenge to achieve collaborative optimization of dielectric loss and impedance matching by precisely regulating the proportional relationship of MXene/MAX. Herein, V-based MXene/MAX heterostructure composites with different V2C/V2AlC ratios were successfully synthesized by rationally controlling the temperature and time of the hydrothermal reaction. Experimental results indicated that V2C-100 °C-1 harvested the balance between reduced impedance matching and enhanced dielectric losses, which was attributed to the mildly enhanced conduction loss and polarization loss. The first principles indicated that abundant electrons migrate from the V atoms of the MXene to the C atoms of the MAX phase. The charge redistributed and accumulated at the interface, exciting the increase in the dielectric loss of V2C-100 °C-1. As a result, the V2C-100 °C-1 heterostructure composite had an excellent electromagnetic absorption effect with a minimum reflection loss of -50.06 dB and a wide effective absorption bandwidth of 4.0 GHz (12.72-16.72 GHz). This work provides a valuable experience for the development of efficient MXene-based microwave absorbing materials.

Lattice matching and halogen regulation for synergistically induced large Li and Na storage by halogenated MXene V3C2Cl2.

Suffering from the formation of metal-ion dendrites and low storage capacity, MXene materials exhibit unsatisfactory performance in Li and Na storage. In this study, we demonstrate that the MXene V3C2Cl2 structure can induce uniform Li and Na deposition. This is achieved through coherent heterogeneous interface reconstruction and regulated ion tiling by halogen surface termination. The high lattice matching (91% and 99%) between MXenes and Li/Na, along with positive Cl terminal regulation, guides Li/Na ions to nucleate uniformly on the V3C2Cl2 MXene matrix and grow in a planar manner. Cl termination proves effective in regulating Li/Na ions due to its moderate adsorption and diffusion coefficients. Furthermore, upon adsorption onto the Cl-terminated V3C2Cl2 monolayer, Li4 and Na4 clusters undergo dissociation, favoring uniform adsorption over cluster adsorption. V3C2Cl2 MXenes exhibit impressive Li/Na storage capacities of 434.07 mA h g-1 for Li and 217.03 mA h g-1 for Na, surpassing the Li storage capacity of Ti3C2Cl2 by three-fold and the Na storage capacity of V2C by 1.4 times. This study highlights the regulatory role of Cl surface terminals in dendrite formation and Li/Na ion deposition, with potential applications to other metal-ion storage electrodes.

Experimental investigation of the period-adding bifurcation route to chaos in plasma.

Since the characteristic timescales of the various transport processes inside the discharge plasma span several orders of magnitude, it can be regarded as a typical fast-slow system. Interestingly, in this work, a special kind of complex oscillatory dynamics composed of a series of large-amplitude relaxation oscillations and small-amplitude near-harmonic oscillations, namely, mixed-mode oscillations (MMOs), was observed. By using the ballast resistance as the control parameter, a period-adding bifurcation sequence of the MMOs, i.e., from L^{s} to L^{s+1}, was obtained in a low-pressure DC glow discharge system. Meanwhile, a series of intermittently chaotic regions caused by inverse saddle-node bifurcation was embedded between the two adjacent periodic windows. The formation mechanism of MMOs was analyzed, and the results indicated that the competition between electron production and electron loss plays an important role. Meanwhile, the nonlinear time series analysis technique was used to study the dynamic behavior quantitatively. The attractor in the reconstructed phase space indicated the existence of the homoclinic orbits of type Γ^{-}. In addition, by calculating the largest Lyapunov exponent (LLE), the chaotic nature of these states was confirmed and quantitatively characterized. With the decrease in the ballast resistance, the return map of the chaotic state gradually changed from the nearly one-dimensional single-peak structure to the multibranch structure, which indicates that the dissipation of the system decreased. By further calculating the correlation dimension, it was shown that the complexity of the strange attractors increased for higher-order chaotic states.

Symmetry-breaking-induced ferroelectric HfSnX3 monolayers and their tunable Janus structures: promising candidates for photocatalysts and nanoelectronics.

Designing novel two-dimensional (2D) ferroelectric materials by symmetry breaking and studying their mechanisms play important roles in the discovery of new ferroelectric photocatalysts and nanoelectronics. In this study, we have systematically investigated a series of novel ferroelectric 2D HfSnX3 (X = S, Se and Te) monolayers through first-principles calculations. We found that each HfSnX3 monolayer contains a stable ferroelectric phase (FP) and a paraelectric phase (PP). The large polarization (up to 1.64 µC cm-2) in the FP can significantly bend the oxidation reduction potential of water, making HfSnX3 monolayers become excellent ferroelectric photocatalysts. Specifically, by designing a Janus structure to break the symmetry of the PP, we have excitingly obtained a stable Hf2GeSnSe6 (referred to as HGSS) monolayer with triple polarized states. HGSS not only possesses great visible light absorption properties (about 3 × 105 cm-1) as photocatalysts but also successfully solves the dead layer problem previously reported in practical applications. In addition, by constructing a heterostructure with graphene, HGSS has great application in the design of controllable ultrathin p-n junctions. Overall, our study not only predicts a series of potential ferroelectric photocatalytic materials, but also provides valuable insights for designing tunable polarized materials and nanoelectronics.

Structural Engineering Enabled Bimetallic (Ti1- y Nby )2 AlC Solid Solution Structure for Efficient Electromagnetic Wave Absorption in Gigahertz.

Microstructures play a critical role to influence the polarization behavior of dielectric materials, which determines the electromagnetic response ability in gigahertz. However, the relationship between them, especially in the solid-solution structures is still absent. Herein, a series of (Ti1- y Nby )2 AlC MAX phase solid solutions with nano-laminated structures have been employed to illuminate the aforementioned problem. The relationship has been investigated by the lattice distortion constructed via tuning the composition from Ti to Nb in the M-site atomic layer. Experimental characterizations indicated that the dielectric response behaviors between declined conduction loss and boosted polarization loss can be well balanced by niobium atom manipulative solid-solution engineering, which is conducive to impedance matching and electromagnetic absorption performance. Theoretical calculation further proved that the origin of electric dipoles is ascribed to the charge density differences resulting from the altered microscopic atomic distribution. As a result, the Ti1.2 Nb0.8 AlC exhibits the mostly optimized microwave absorption property, in which a minimum reflection loss of -42 dB and an effective absorption bandwidth of 4.3 GHz under an ultra-thin thickness of 1.4 mm can be obtained. This work provides insight into the structural engineering in modifying electromagnetic response performance at gigahertz and which can be expanded to other solid-solution materials.

Multifunctional transmission polarization conversion metasurface based on dislocation-induced anisotropy at the terahertz frequency.

Manipulating the polarization state of terahertz waves is critical for terahertz communication systems. This study proposes a terahertz band polarization conversion metasurface based on dislocation-induced anisotropy. Numerical simulation results revealed that the polarization conversion of orthogonal linearly polarized light, orthogonal circularly polarized light, linearly polarized light to circularly polarized light, and circularly polarized light to linearly polarized light can be realized. Furthermore, the simulation revealed that multifunctional polarization conversion could be achieved by various structures of the bilayer metasurface. Thus, the proposed design can be generalized. The proposed metasurface exhibits considerable potential for applications in terahertz communications.

High-ratio {100} plane-exposed ZnO nanosheets with dual-active centers for simultaneous photocatalytic Cr(VI) reduction and Cr(III) adsorption from water.

The development of an efficient catalyst for the simultaneous removal of Cr(VI) and Cr(III) from water is required to eliminate the risk of Cr(III) reconversion in the photocatalytic Cr(VI) reduction process. ZnO with large regions of high-energy {001} and {101} surfaces is often used to degrade various pollutants due to its high activity. However, the more readily available low-energy facets have relatively limited its applications. Here, we report a new strategy that employs a high proportion of {100} plane-exposed ZnO nanosheets for simultaneous photocatalytic Cr(VI) reduction and Cr(III) adsorption. The mechanism of Zn-O co-exposed on the {100} plane as the dual-active centers to jointly promote Cr(VI) reduction and Cr(III) adsorption was clarified at the atomic level. ZnO nanosheets with a high exposure ratio of the {100} plane achieve a total Cr removal rate of over 90% within 120 min under simulated sunlight irradiation, neutral conditions, and a negligible difference in the band structure.

Using Collisional Electron Spectroscopy to Detect Gas Impurities in an Open Environment: CH4-Containing Mixtures.

The collisional electron spectroscopy method for analyzing and determining gaseous impurities was further developed to realize the operation in an open environment. In addition, the method not only facilitates the registration of the impurity components, but also the reactive radicals generated from the discharge reaction. The sandwich-like discharge structure was used to generate a stable, non-local, negative glow equipotential plasma in an open environment, and the I-V characteristic curve of the plasma was collected using an additional sensor electrode. The collisional electron spectroscopy was obtained from the first derivative of the probe current I with respect to the probe potential V by adding a diffusion function to correct it. In addition, our experiment verifies the reliability of the sink theory.

Electronic and optical properties of a novel two-dimensional semiconductor material TlPt2S3: a first-principles study.

Two-dimensional (2D) materials have attracted widespread attention due to their unique physical and chemical properties. Here, by using density functional theory calculations, we suggest a novel 2D TlPt2S3 material whose layered bulk counterpart was synthesized in 1973. Theoretical calculation results indicate that the exfoliating energy of monolayer and bilayer TlPt2S3 is 34.96 meV Å-2 and 36.03 meV Å-2. We systematically studied the electronic and optical properties of monolayer and bilayer TlPt2S3, and revealed that they are indirect band gap semiconductors with band gaps of 2.26 eV and 2.10 eV, respectively. Monolayer and bilayer TlPt2S3 exhibit superior carrier mobility (901.63 cm2 V-1 s-1 and 13635.04 cm2 V-1 s-1 for electron mobility of the monolayer and bilayer, respectively) and photocatalytic performance (as high as 1 × 105 light absorption coefficient in the visible light region). Interestingly, we find that monolayer TlPt2S3 has significant hydrogen evolution performance, while in the bilayer, the electron band distribution shows complete oxygen evolution ability, which indicates that the proposed monolayer and bilayer TlPt2S3 are potential novel 2D materials suitable for photocatalytic water splitting driven by visible light.

Numerical simulation of the bifurcation-remerging process and intermittency in an undriven direct current glow discharge.

As a complex nonlinear medium, gas discharge plasma can exhibit various nonlinear discharge behaviors. In this study, in order to investigate the chaos phenomenon in the subnormal glow region of an undriven direct current glow discharge, a two-dimensional plasma fluid model is established coupled with a circuit model as a boundary condition. Using the applied voltage as control parameter in the simulation, the complete period-doubling bifurcation and inverse period-doubling bifurcation processes in the oscillation region are found, and the influence of the applied voltage on the spatiotemporal distribution of plasma parameters during the bifurcation-remerging process is examined. In addition, the spatial distribution of the plasma parameters of the bifurcation-remerging process is also examined. Also, a series of periodic windows are present in the chaotic region, where the positions and relative order are generally consistent with the universal sequence. Additionally, this study showed that the intermittent chaos appears near the period-3 window, and the bursts appearing in the approximate periodic motion becomes more and more frequent as the control parameters move away from the saddle-node bifurcation point, which shows the typical type-I intermittent chaos characteristics.

Electronically controlled flexible terahertz metasurface based on the loss modulation of strontium titanate.

A flexible terahertz metamaterial is designed to control the transmittance through an external electric field. Two different metallic structures, the split ring (type I structure) and the split ring inside a ring (type II structure), were prepared and voltage was applied through a forked finger electrode. The structures were wrapped in a thin film made by mixing strontium titanate nanopowder with polyimide in a certain ratio. Under normal incidence, the transmittance is controlled by applying a voltage to adjust the imaginary part of the permittivity of strontium titanate. The modulation depth of the type I structure at 1.08 THz is 40.1%, and that of the type II structure at 1.16 THz is 44.7%. The working bandwidths of the two structures are 0.07 THz and 0.42 THz, respectively, and are greatly broadened by combining with the ring. The proposed design enriches the modulation method of the transmission of metamaterials and broadens the application range of flexible terahertz metasurfaces.

Coaxial double-hole PEDOT: PSS electrodes achieving tunable terahertz zoomable convergence.

The local wavefront modulation technique in the terahertz band is an important basis for the development of terahertz modulation technology. Here, an electrically controlled convergent tunable device based on patterned transparent electrode poly (3, 4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) is realized to locally tune the terahertz wavefront. The device consists of two substrates with circular-hole electrodes and liquid crystal sandwiched between them. The refractive index gradient of liquid crystal in the device can be generated by the coaxial double-hole electrodes, which realize continuous control of significant focusing of the terahertz wave. The test results show that the focal length can be modulated in the range of 3-12 cm with varied external voltage; when it varies from 3 to 8 V, the 1/e2 radius of the spot decreases to 1.3 mm, 0.27 times the initial state, and the spot central intensity magnification increases gradually with the change, up to 3.31 times. The acquisition of the large tunable focal length range of the continuous terahertz zoom device shows that the construction of the gradient refractive index is an important method to regulate the terahertz wavefront by optical means, which greatly promotes the research of terahertz imaging devices.

Low magnetic-field induced high temperature dynamic magnetoelectric coupling performances in Z-type Sr3Co2Fe24O41.

Magnetic-field induced dynamic magnetoelectric coupling effects and polarization performance of Z-type Sr3Co2Fe24O41(SCFO) ceramic has been investigated. Results found that SCFO's transverse tapered magnetic structure can induce electric polarization, and its electric polarization direction will not change under external magnetic effects. First-order dynamic magnetoelectric coupling coefficient (α) and second-order dynamic magnetoelectric coupling coefficient (ß) of SCFO exhibited strong response main in magnetic structural phase transition region. The magnetoelectric structural phase transition position appeared in low magnetic field, and the magnetic moment vector and its corresponding electric polarization vector of SCFO exhibited the most unstable state near its equilibrium position, which is beneficial for inducing strong dynamic magnetoelectric coupling response. When the applied magnetic fields to SCFO increased, the magnetic moment stability near the equilibrium position increased, and the dynamic magnetoelectric coupling response decreased. Results showed that the dynamic magnetoelectric coupling response of SCFO can bearT1= 370 K high temperature. The dynamic magnetoelectric coupling response induced by low magnetic fields in SCFO contributes to its actual application in next generation magnetoelectric information storage devices.

Computational Screening of High Activity and Selectivity TM/g-C3N4 Single-Atom Catalysts for Electrocatalytic Reduction of Nitrates to Ammonia.

Electrocatalytic reduction of nitrates (NO3RR) selectively generating ammonia (NH3) opens up a new idea for treating nitrates in wastewater, which not only reduces nitrates but also obtains the valuable product ammonia. By first-principles calculations, we explore the activity and selectivity for NO3RR to NH3 of TM/g-C3N4 single-atom catalysts. Six TM/g-C3N4 catalysts (TM = Ti, Os, Ru, Cr, Mn, and Pt) are selected by a four-step screening method. Ru/g-C3N4 is the most promising of these six TM/g-C3N4 catalysts because of its lowest energy barrier and extraordinary selectivity. The origin of the NO3RR activity of Ru/g-C3N4 is explained from the viewpoint of NO3- adsorption. In addition, the hydrogen evolution reaction has also been implied to be uncompetitive for the poor adsorption on H atoms. This work provides a screening mechanism for finding new catalysts for NO3RR to NH3, promotes the development of NO3RR, and provides a stimulating impetus for further experimental exploration.

Active polarization-independent plasmon-induced transparency metasurface with suppressed magnetic attenuation.

A tunable polarization-independent plasmon-induced transparency (PIT) metasurface based on connected half-ring and split-ring resonators is proposed to working in the terahertz band. We analyze the PIT effect in metasurfaces comprising of ring resonator and split ring resonator. Due to the magnetic attenuation caused by the reverse current between the two resonators, the relative position of the ring resonator and the split-ring resonator greatly affects the strength of the PIT effect. Magnetic attenuation weakens the dark mode of the split ring resonator. Through simulation and experiment, it is found that connecting the ring resonator and split-ring resonator can avoid magnetic attenuation and achieve a stronger PIT window. Furthermore, the fourfold rotation structure of the connected half-ring and split-ring resonator on silicon substrate achieves an optically controlled polarization-independent PIT effect. The design would provide significant guidance in multifunctional active devices, such as modulators and switches in terahertz communication.

Manganese-doping enhanced local heterogeneity and piezoelectric properties in potassium tantalate niobate single crystals.

Ion doping, an effective way to modify the nature of materials, is beneficial for the improvement of material properties. Mn doping exhibits gain of piezoelectric properties in KTa1-x Nb x O3 (KTN). However, the impact mechanism of Mn ions on properties remains unclear. Here, the effects of Mn doping on local heterogeneity and piezoelectric properties in KTN are studied. The electric field-induced strain of Mn-doped KTN is ∼0.25% at 10 kV cm-1, 118% higher than that of pristine KTN. Meanwhile, as a result of Mn doping, the dielectric permittivity was tripled and the ferroelectricity was modified. The changes in A1(2TO), B1 + E(3TO) and E(4TO) vibrations characterized by Raman spectra indicate increased local polarization, weak correlation of dipoles and distorted lattices in Mn-doped KTN, respectively. First-principles calculations demonstrate stronger local heterogeneity introduced by Mn dopants, which weakens the dipole correlations and reduces domain sizes. As a result, the decreased domain sizes, combined with the larger ratio of lattice parameters c and a of the Mn-contained portion, are responsible for the higher piezoelectricity. This work reveals the impact on properties of KTN from Mn dopants and the prominent role of local heterogeneity in improving piezoelectricity, being valuable for the optimization and design of material properties.

Photovoltaic properties in an orthorhombic Fe doped KTN single crystal.

Since the domain wall photovoltaic effect (DW-PVE) is reported in BiFeO3 film, the investigations on photovoltaic properties in ferroelectrics have appealed more and more attention. In this work, we employed two Fe doped KTa1-xNbxO3 (Fe:KTN) single crystals in tetragonal phase and orthorhombic phase, respectively, possessing similar net polarization along [001]C direction, to quantize the contribution on photovoltaic properties from bulk photovoltaic effect (BPVE) and DW-PVE in Fe:KTN. The results show that there are significant enhancements of open-circuit voltages (VOC = -6.0 V, increases over 440%) and short-circuit current density (JSC = 18.5 nA cm-2, increases over 1580%) in orthorhombic Fe:KTN with engineer-domain structure after poled, corresponding to 14.2 mV and 2.2 mV for the single domain wall and bulk region under illumination of 405 nm light (100 mW). It reveals that DW-PVE plays a major role in KTN-based ferroelectrics, indicating an orthorhombic Fe:KTN single crystal is one of the potential photovoltaic materials.

Super terahertz phase shifter achieving high transmission and large modulation depth.

We propose an industrial-grade liquid-crystal-based terahertz (THz) 2π-phase shifter with predictable ultra-high amplitude transmittance. The phase retardation reaches 360.5° at 1.68 THz by analyzing the birefringence of liquid crystal (LC), and the amplitude transmittance in 0.3-1.5 THz is over 83%. More than 91.5% transmittance can be reached by decreasing the scattering of the THz wave in the dynamic deflection process of LC molecules, and that is close to the transmittance limit of quartz-based devices. This millimeter-thick phase shifter reaches full phase modulation and ultra-high transmittance in a broad THz band, is easy to be integrated in a quasi-optical system with a compact size and can be utilized as a wave plate, even an element in a THz phased array.

Ultra-large electric field-induced strain in potassium sodium niobate crystals.

Electromechanical coupling in piezoelectric materials allows direct conversion of electrical energy into mechanical energy and vice versa. Here, we demonstrate lead-free (K x Na1-x )NbO3 single crystals with an ultrahigh large-signal piezoelectric coefficient d 33* of 9000 pm V-1, which is superior to the highest value reported in state-of-the-art lead-based single crystals (~2500 pm V-1). The enhanced electromechanical properties in our crystals are realized by an engineered compositional gradient in the as-grown crystal, allowing notable reversible non-180° domain wall motion. Moreover, our crystals exhibit temperature-insensitive strain performance within the temperature range of 25°C to 125°C. The enhanced temperature stability of the response also allows the materials to be used in a wider range of applications that exceed the temperature limits of current lead-based piezoelectric crystals.

Formation of inverse electron distribution function and absolute negative conductivity in nonlocal plasma of a dc glow discharge.

The inversion of the electron energy distribution function (EEDF) at low energies and the absolute negative electron conductivity are predicted and confirmed by numerical modeling of a direct current glow discharge in argon. It is shown that, in contrast to the local approximation used earlier for searching the inverse EEDF, in a real gas-discharge plasma, the formation of the EEDF is significantly affected by the terms with spatial gradients in the Boltzmann kinetic equation. In analogy with the inverse population of excited states in lasers, such a medium will amplify electromagnetic waves.