Band Gap Properties Research Articles

Study on band-gap characteristics and formation mechanism of four-ligament chiral elastic metamaterials

Abstract Chiral elastic metamaterials, owing to their exceptional properties distinct from conventional materials and their superior mechanical performance, exhibit significant potential for applications in vibration reduction, noise suppression, energy absorption, and cushioning. To address the challenge of low-frequency vibration control, this paper proposes a dual-component chiral elastic metamaterial structure with four ligament elements. The study explores the bandgap characteristics and elastic wave propagation behavior of this structure within the 1000 Hz frequency range. By analyzing the vibration modes of the unit cell and calculating the group and phase velocities of elastic waves, the physical mechanism underlying bandgap formation is elucidated. The results demonstrate that the proposed four-ligament chiral elastic metamaterial exhibits excellent bandgap properties, with the bandgap covering more than 80.4% of the frequency range below 1000 Hz. This highlights its capability for low-frequency elastic wave control and offers a theoretical reference for the design of novel vibration reduction and noise suppression structures, as well as for low-frequency elastic wave regulation.

Mn-Rich Induced Alteration on Band Gap and Cycling Stability Properties of LiMnxFe1-xPO4 Cathode Materials.

Olivine-type LiMnxFe1-xPO4 (LMFP) has inherited the excellent heat-stable structure of LiFePO4 (LFP) and the high-voltage property of LiMnPO4 (LMP), which shows great promise as a high-safety, high-energy-density cathode material. In order to combine the high energy density and excellent electrochemical performance, it is essential to consider the Mn/Fe ratio. This paper presents a theoretical investigation of the lattice structure parameters, embedded lithium voltage, local electron density, migration barrier, and lithium ion delithiation and lithiation mechanism of different LiMnxFe1-xPO4 (0.5 ≤ x ≤ 0.8) compounds. In situ-coated LiMnxFe1-xPO4 (0.5 ≤ x ≤ 0.8) composite cathode materials with a size of 100-200 nm were prepared by a hydrothermal method to verify the theoretical study. LiMn0.6Fe0.4PO4/C exhibited a specific capacity of 140.2 and 97.58 mA h·g-1 at 1 and 5C, respectively, and a remarkable capacity retention rate of 88.5% after 200 cycles at 1C. When LiMn0.6Fe0.4PO4/C was assembled into a flexible pouch battery and subjected to long cycle tests at different rates and squeeze and extrusion tests, it demonstrated a capacity retention rate of 99.35% for 100 cycles at 0.2C and 93.2% for 200 cycles at 0.5C. Moreover, the structural evolution of LiMn0.6Fe0.4PO4/C were analyzed in situ XRD, indicating a high stability and the resulted as obtine electrochemical performance, paving the way for optimization of high-energy-density LMFP cathode materials.

Crystal structure, bandgap, photoluminescence and resistivity properties of double perovskite Cs2AgBiCl6 single crystal and its thin film.

Lead-free Cs2AgBiCl6 double perovskite (Cs2AgBiCl6-DP) material, as a substitute for lead halide perovskite materials, has the advantages of environmental friendliness and high stability and has attracted much attention. However, the photoluminescence and conductive properties of Cs2AgBiCl6-DP have not been well studied. In this study, we prepared Cs2AgBiCl6-DP single crystals (SCs) by coordination-dissolution and coordination-precipitation method. Single- and powder-XRD, SEM, EDS, XPS, and EPR characterization were performed to confirm the structural characteristics of Cs2AgBiCl6-DP SCs. The Tauc diagram based on UV-visible (UV-vis) absorption spectroscopy reveals that the optical bandgap of Cs2AgBiCl6-DP SCs is extrapolated to 2.51 eV. Steady-state fluorescence spectra and time-resolved fluorescence spectra show that Cs2AgBiCl6-DP SCs has four fluorescence peaks entered at 443, 615, 650 and 723 nm and a fluorescence lifetime of about 4.16 ns. Cs2AgBiCl6/PMMA thin films were prepared by spin coating suspension (Cs2AgBiCl6 DP and PMMA in acetone solvent). The intensity of emission peak increases with the increase of light intensity at 369 nm. The intensity of emission peak located at 576 nm decreases with increasing incidence wavelength (from 369 to 454 nm) at 10 W m-2. The emission intensity remains basically unchanged under continuous illumination for 9 hours at 369 nm at 5 W m-2, which indicates that the Cs2AgBiCl6-DP thin film has good stability. In addition, the resistivity and block resistance show a negative exponential change with increasing temperature. These results provide some interesting ideas for the fields of photoluminescence and thermistors.

Electric and thermal coupled light fields regulating photoelectric sensing performance in photoferroelectrics

Traditional photoelectric semiconductors with single-source energy sensing or hybrid energy sensing integrated with other materials constrain their effectiveness in achieving power stability and device miniaturization. In contrast, photoferroelectrics offer multiple energy responses to electric, thermal, and light fields within a single material, thereby regulating the photoelectric sensing performances. This work proposes a multi-field coupling effect involving electrical, thermal, and light fields to enhance photoelectric sensing performances in electroneutral ion group doped BiFeO3 photoferroelectrics with synergistic improvement in polarization, bandgap, and leakage properties. Notably, the photocurrent output is significantly engineered by applying dual-field modes of pre-poling or thermal coupled light fields compared with the light field sensing solely. More importantly, the responsivity of the optimized photoelectric sensors is increased by nearly five times when pre-poling and thermal fields are applied simultaneously, providing convincible evidence of the sensing enhancement derived from the multi-field coupling effect. This work provides a feasible strategy to improve the photoelectric sensors through multi-field coupling, promoting the application of multifunctional photoelectric devices.

Study on Bandgap of Two‐Dimensional Square Lattice Acoustic Metamaterial with Multiembedded Cores

Acoustic metamaterials have been widely studied for their excellent ability of vibration isolation. In this article, a 2D square lattice acoustic metamaterial structure with multiembedded cores is proposed. Based on Bloch's theorem and finite element method, the band structures and bandgap properties of the structure are calculated and analyzed, and the vibration isolation ability of the structure is investigated. The effects of structural parameters on the bandgap property are analyzed. Thus, the anisotropy of the structure is also analyzed by the contours of phased velocity and group velocity. The results show that the structure has significant bandgap properties and vibration isolation. The second‐order similarity ratio has significant effects on the bandgap property. Via increasing the second‐order similarity ratio, the total bandgap width and low‐frequency bandgap width are widened. In addition, the structure has significant anisotropy, and the energy of elastic wave propagates at the direction of 45° and 135°. The research in this article supplements the design and research of 2D lattice acoustic metamaterials and makes a beneficial exploration for future engineering applications.

Potassium and Boron Co-Doping of g-C3N4 Tuned CO2 Reduction Mechanism for Enhanced Photocatalytic Performance: A First-Principles Investigation.

Graphitic phase carbon nitride (g-C3N4, abbreviated as CN) can be used as a photocatalyst to reduce the concentration of atmospheric carbon dioxide. However, there is still potential for improvement in the small band gap and carrier migration properties of intrinsic materials. K-B co-doped CN (KBCN) was investigated as a promising photocatalyst for carbon dioxide reduction via the Density Functional Theory (DFT) method. The electronic and optical properties of CN and KBCN indicate that doping K and B can improve the catalytic performance of CN by promoting charge migration and separation. In terms of the Gibbs free energy change, the CO2 reduction reaction catalysed by KBCN results in CH3OH, and its optimal pathway is CO2 → *CO2 → *COOH → CO → *OCH → HCHO → *OCH3 → CH3OH. Compared with CN, the doping elements K and B shift the rate-determining step from CO2 → *CO2 to *CO2 → *COOH. The K and B elements co-doping tuned the charge distribution between the catalyst and the adsorbate and reduced the Gibbs free energy of the rate-determining step from 1.571 to 0.861 eV, suggesting that the CO2 reduction activity of KBCN is superior to that of CN. Our work provides useful insights for the design of metallic-nonmetallic co-doped CN for photocatalytic CO2 reduction (CO2PR) reactions.

Titanium Dioxide: A Versatile Earth‐Abundant Optical Material for Photovoltaics

AbstractTitanium dioxide (TiO2) has long been receiving attention as a promising material for enhancing the performance of photovoltaic devices due to its tunable optoelectronic properties. This paper reviews the utilization of TiO2 in recent photovoltaic applications, focusing primarily on its role as an optical material. The fundamental properties of TiO2 are reviewed, such as its wide bandgap and unique property of tunable refractive index. Furthermore, various strategies are discussed to harness the fundamental properties of TiO2 to improve light absorption and charge carrier generation within various photovoltaic devices. Overall, the pivotal role of TiO2 as an Earth‐abundant and non‐critical optical material is highlighted for future advances in the power conversion efficiency and viability of photovoltaic technologies, paving the way for future research and developments aimed at achieving sustainable and cost‐effective solar energy conversion.

Thickness-dependent nanoscale friction across the first-order charge density wave phase transition in 1T-TaS2

The layered charge density wave (CDW) phase transition material 1T-TaS2 has garnered significant attention due to its modulable bandgap and electrical transport properties. These unique properties make 1T-TaS2 highly promising for applications in fields such as optoelectronic devices and microstructure physics devices. In various micro-/nanodevices made from quasi-two-dimensional 1T-TaS2, it is often utilized in thin layers, making the understanding of its frictional properties crucial for practical applications. However, the layer-dependent frictional properties of 1T-TaS2 have not been thoroughly investigated. In this article, we examine the CDW phase transition between the nearly commensurate (NCCDW) and commensurate (CCDW) phases in 1T-TaS2 around 183 K using atomic force microscopy, focusing on the number of layers in the samples. Our results indicate that for thicker samples with more than approximately 17 layers, a friction peak is observed during the NCCDW–CCDW phase transition. In contrast, thinner samples do not exhibit this friction peak, and their friction continuously increases as the temperature decreases. This behavior is attributed to the suppressed NCCDW–CCDW phase transition in thinner samples. These results enhance our understanding of the frictional behavior of 1T-TaS2 in the context of micro-/nano-electromechanical systems. Furthermore, our observations offer a straightforward method to identify the NCCDW–CCDW phase transition, providing an alternative to traditional, more complex techniques such as electrical resistance measurements and Raman spectroscopy.

A new inerter-based acoustic metamaterial MRE isolator with low-frequency bandgap

Abstract Acoustic metamaterials are capable of generating bandgaps at specific frequency ranges, which makes them have good applications in the field of vibration isolation. The bandgaps can be further broadened with active control, nonlinear components and graded structures, such as: controllable stiffness by magnetorheological elastomer (MRE) and graded stiffness. However, the current approaches to reducing the bandgaps have limitations. Both the reduction in structural stiffness and the increase in mass will reduce the overall stability of the acoustic metamaterial. In this research, a novel inerter-based acoustic metamaterial MRE isolator (IAM-MREI) was designed and prototyped to lower the bandgap. Inerters can generate a large equivalent mass with very light weight. Moreover, it is discovered that elements containing quadratic frequency terms are added to the dispersion matrix of the IAM-MREI due to the frequency-independent force applied to the resonators, which is generated by the inerters. By this way, the bandgap calculated by this dispersion matrix is greatly lowered and broadened, which cannot be achieved only with extra equivalent mass. The effects of the inerters on the overall performance of the IAM-MREI was thoroughly investigated and validated both theoretically and experimentally. The evaluation experiments confirmed that the IAM-MREI possesses a low-frequency bandgap and can provide great vibration isolation performance.

Si/AlN p-n heterojunction interfaced with ultrathin SiO2

Ultra-wide bandgap (UWBG) materials offer significant potential for high-power RF electronics and deep ultraviolet photonics. Among these, AlxGa1−xN stands out due to its tunable bandgap (3.4 eV to 6.2 eV) and excellent material properties. However, achieving efficient p-type doping in high aluminum composition AlGaN remains a challenge. This study presents a novel approach by fabricating a p+Si/n-AlN/n+AlGaN heterojunction using semiconductor grafting. Atomic force microscopy (AFM) revealed smooth surfaces for AlN and the nanomembrane, with roughness values of 1.96 nm and 0.545 nm, respectively. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) confirmed a sharp and well-defined Si/AlN interface with minimal defects and strong chemical bonding. X-ray photoelectron spectroscopy (XPS) measurements identified a type-I heterojunction with a valence band offset (ΔEv) of 2.73–2.84 eV and a conduction band offset (ΔEc) of 2.22–2.11 eV. The pn diode devices exhibited linear current–voltage (I-V) characteristics, an ideality factor of 1.92, and a high rectification ratio of 3.3 × 104, with a turn-on voltage of 3.9 V. Temperature-dependent I-V measurements showed stable operation up to 90 °C. The heterojunction’s high-quality interface and impressive electrical performance underscore its potential for advanced AlGaN-based optoelectronic and electronic applications.

A DFT study on the role of excitons and electric field-induced symmetry breaking and topological properties of ZrBr

The control of band gap and topological properties is a crucial objective in the field of materials science, particularly for the application of 2D materials such as topological insulators. Zirconium monobromide (ZrBr), a topological insulator with band inversion, holds significant potential for various applications. In this study, we used Density Functional Theory (DFT) with different approximations namely, Generalized Gradient Approximation (GGA) with and without spin orbit coupling (SOC) as well as hybrid functional to investigate the electronic structure and the band gap of this compound. In addition, we introduced the excitonic effect by considering GW plus Bethe–Salpeter equation (GW-BSE) to compute the optical band gap and quasi-particle energy. We found that ZrBr present a low static dielectric function and weak exciton binding energy, generally lower than those of conventional semiconductors. This behavior is due to the reduced Coulomb interaction in the material. To further explore the material’s behavior, we apply an electric field in three directions to observe symmetry breaking and its impact on band gap tuning. This comprehensive analysis aims to explore the understanding and the potential applications of ZrBr as a topological insulator.

Investigation of the physical properties and pressure-induced band gap tuning of Sr3ZBr3 (Z=As, Sb) for optoelectronic and thermoelectric applications: A DFT - GGA and mBJ studies

This study discusses the feasibility of diversifying the scope of application of lead-free Sr3ZBr3 (Z = As, Sb) as promising materials for their enhanced electronic, mechanical, optical, thermodynamic, and thermoelectric properties using first-principles calculation based on Density Functional Theory (DFT). Both compounds have cubic crystal structures, and both materials' dynamic stability is validated by the lack of negative frequencies in their phonon dispersion spectra. Besides ground state physical characteristics, we also examined the impact of hydrostatic pressure (0–21 GPa) on electronic, mechanical, and optical properties. The lattice parameters exhibit a linear decrease from 6.27 to 5.61 Å for Sr3AsBr3 and 6.45 to 5.71 Å for Sr3SbBr3 under increasing pressure, attributed to bond length reduction, which alters their physical properties. Initially observed direct band gap (Γ-Γ) of Sr3AsBr3 and Sr3SbBr3 were 2.35 and 2.30 eV using modified Becke-Johnson (mBJ) functional which reduces to 1.15 and 0.82 eV as the compounds go from 0 to 21 GPa pressure. This study reveals enhanced optical properties under pressure, with broader spectral response, increased sensitivity, and improved dielectric constant. Optical absorption and conductivity also shift toward lower-energy photons. The investigation further shows that the compounds show brittle to ductile transition under high pressure. Their ductility and anisotropy nature are further enhanced with pressure along with other mechanical features. The thermodynamic properties indicate their ability to withstand and perform well at high temperatures and pressures. Lastly, the thermoelectric properties of Sr3ZBr3 (Z = As, Sb) were assessed through electrical and thermal conductivity, Seebeck coefficient, power factor (PF), and figure of merit (ZT) as a function of temperature. Both compounds exhibit high PF and near-unity ZT. These findings underscore their potential as strong candidates for optoelectronic and thermoelectric devices, owing to their direct bandgap and exceptional properties.

Sliding Ferroelectricity Induced Ultrafast Switchable Photovoltaic Response in ε-InSe Layers.

2D sliding ferroelectric semiconductors have greatly expanded the ferroelectrics family with the flexibility of bandgap and material properties, which hold great promise for ultrathin device applications that combine ferroelectrics with optoelectronics. Besides the induced different resistance states for non-volatile memories, the switchable ferroelectric polarizations can also modulate the photogenerated carriers for potentially ultrafast optoelectronic devices. Here, it is demonstrated that the room temperature sliding ferroelectricity can be used for ultrafast switchable photovoltaic response in ε-InSe layers. By first-principles calculations and experimental characterizations, it is revealed that the ferroelectricity with out-of-plane (OOP) polarization only exists in even layer ε-InSe. The ferroelectricity is also demonstrated in ε-InSe-based vertical devices, which exhibit high on-off ratios (≈104) and non-volatile storage capabilities. Moreover, the OOP ferroelectricity enables an ultrafast (≈3ps) bulk photovoltaic response in the near-infrared band, rendering it a promising material for self-powered reconfigurable and ultrafast photodetector. This work reveals the essential role of ferroelectric polarization on the photogenerated carrier dynamics and paves the way for hybrid multifunctional ferroelectric and optoelectronic devices.

On the structural and bandgap properties of mist-CVD-grown κ-Ga2O3 post continuous temperature annealing

We investigate the continuous annealing of orthorhombic κ-Ga2O3 films on AlN/c-plane sapphire templates grown by Mist Chemical Vapor Deposition (mist-CVD) using in situ high-temperature x-ray diffraction (HT-XRD). Increasing the annealing temperature from 400 to 1100 °C in both vacuum and ambient air reveals a phase transition onset at 825 °C. High-resolution transmission electron microscopy and XRD demonstrated that annealing within the stability window of 650 to 775 °C effectively improves the crystal quality of the κ-Ga2O3 thin film. Optical transmittance and low-loss electron energy loss spectroscopy (EELS) show the pristine film’s bandgaps to be 4.96 and 4.67 eV, respectively, with reduced bandgaps in annealed films due to increased defect density. EELS-derived optical joint density of states indicates that air-annealing fosters sub-bandgap radiative processes, while vacuum annealing suppresses them, qualitatively correlated with the observed photoluminescence intensity variations. The results of this comprehensive high-temperature annealing study offer crucial insight into the influence of annealing ambient conditions on the crystallographic properties of κ-Ga2O3 films and the associated evolution of extended sub-bandgap states.

Integrated convolutional kernel based on two-dimensional photonic crystals.

Optical neural networks (ONNs) exhibit significant potential for accelerating artificial intelligence task processing due to their low latency, high bandwidth, and parallel processing capabilities. Photonic crystals (PhCs) are extensively utilized in integrated optoelectronics because of their unique photonic bandgap properties and precise control of light waves. In this study, we propose an optical reconfigurable convolutional kernel based on PhCs. This kernel can perform convolutional operations on weights by constructing a PhC weight bank. The convolutional kernel demonstrates exceptional performance within the developed optical convolutional neural network framework, successfully realizing various image edge processing tasks. It achieves blind recognition accuracies of 97.81% for the MNIST dataset and 80.31% for the Fashion-MNIST dataset. This study not only demonstrates the feasibility of constructing optical neural networks based on PhCs but to our knowledge, also offers new avenues for the future development of optical computing.

Influence of Zinc Ion Concentration on the Structural, Surface Morphology and Optical Properties of Zinc Selenide Thin Films

ZnSe thin films were deposited on nonconducting glass substrates using different Zn[Formula: see text] ion concentrations. The films were deposited at 80[Formula: see text]C for 2.0[Formula: see text]h via photo-assisted chemical bath technique and annealed for 2.0[Formula: see text]h at 250[Formula: see text]C. X-ray diffraction revealed a hexagonal structure with preferred orientation along the (002) plane and the average crystallite size decreased from 10.5[Formula: see text]nm to 6.8[Formula: see text]nm with increased Zn[Formula: see text] ions. Raman spectra were used to confirm ZnSe phonon modes whose intensity increased with Zn[Formula: see text] ion concentrations although with fluctuation. Optical analysis showed higher absorbance and low transmittance in the visible region than near infrared making the thin films good materials for selective absorber surfaces. The band gap increased from 2.52[Formula: see text]eV to 2.78[Formula: see text]eV as the Zn[Formula: see text] ion concentration varied from 0.05% to 0.25%. The presence of the desired elements was confirmed by the EDS. Photoluminescence studies revealed three emission peaks which were all ascribed to defect state levels in ZnSe and all the samples emitted in reddish color according to CIE color chromaticity analysis. The selective absorption, wide band gap and broad emission properties suggest that the material is promising for optoelectronic applications.

Recent modification, mechanisms, and performance of zinc oxide-based photocatalysts for sustainable dye degradation

The textile industry, which has crucial social and economic roles, faces sustainability challenges due to the generation of abundant wastewater contaminated by dyes. A wide range of photocatalysts has been developed to degrade dyes to reduce chemical and energy consumption in the treatment of dye-contaminated wastewater. This review mainly focuses on the photocatalytic mechanism and past studies of low-cost zinc oxide (ZnO) photocatalysts with various dopants to degrade dye degradation performance. Unlike previous reviews, this review delves deeply into property changes and possible mechanisms of ZnO-based photocatalysts induced by various dopants. ZnO-based photocatalysts are commonly doped with other semiconductors, metals, and non-metallic dopants. Semiconductor dopants such as titanium dioxide (TiO2), graphitic carbon nitride (g-C3N4), and iron oxide (Fe2O3) significantly modify the band gap and broaden the spectral response range of ZnO photocatalysts. Metallic dopants, including silver (Ag), copper (Cu), and cobalt (Co), introduce plasmonic effects that enhance the electrical conductivity and photocatalytic activity. Conversely, non-metallic dopants, especially carbon nanomaterials and sulfur (S), enable the adjustment of the band gap and surface properties of ZnO. Photocatalytic coating and thin films should be developed for photocatalyst reuse in future works.

Synthesis and Photochemical Properties of Heterometallic Ti-Cu Ring Clusters Constructed from Myo-Inositol Ligand.

Incorporating heterometals into titanium-oxygen clusters represents an effective approach to adjusting the band gap and absorption properties. Herein, a family of heterometallic Ti-Cu clusters was synthesized under solvothermal conditions. The unique structural feature of these clusters is the formation of ring clusters with myo-inositol ligands serving as structure-directing ligands. The myo-inositol ligand, as a typical polyol ligand, demonstrates flexible and distinctive coordination modes capable of chelating up to eight transition metal ions simultaneously. Compounds 4 and 5 show significant absorption in the visible region, indicating that the incorporation of Cu ions can efficiently tune the band gap of titanium-oxygen clusters.

Modulation of bandgap and transport properties by stacking symmetry in bilayer binary materials

The interlayer interactions in layered two-dimensional materials, which are formed by Van der Waals forces, significantly influence the control of electronic and transport properties. The manipulation of the stacking order can modulate these interlayer interactions by altering the atomic arrangement in different layers. In this study, we conducted a systematic examination of the bandgap modulation of bilayer two-dimensional binary materials at high symmetry stackings. A general trend of bandgap modulation has been proposed through a tight-binding model and corroborated by density functional calculations. The mechanisms of bandgap modulation can be leveraged to control the transport properties of devices built with bilayer two-dimensional binary materials. Our research findings offer valuable insights for the control of bandgap in layered two-dimensional materials and their application in transport devices.

Theoretical Study of the Co‐doping Effects at Sr and Fe Sites on the Magnetic and Optical Properties of SrFe12O19

For the first time, the magnetic and optical (bandgap) properties of co‐doped Mg/Dy, Al/Dy, and Mn/Nd (SFO) at Sr and Fe sites are investigated using a microscopic model and the Green's function theory. It is shown that the co‐doping at both Sr and Fe sites can change the behavior of the magnetization which decreases by transition metal or nonmagnetic ion doping at the Fe site (with doping ions larger than the host Fe ions), but by additive co‐doping at the Sr site with rare‐earth ion (which ionic radius is smaller than that of Sr) the magnetization increases with increasing the co‐doping concentration. The bandgap energy decreases in the co‐doped SFO. The mechanism of this co‐doping effect is explained on microscopic level. It is due to the changes of the spin‐exchange interaction constants at the doped sites caused by the appearance of different strain by the doping with two ions. The theoretical results are compared with the existing experimental data and are in good qualitative agreement.