Broad Band Gap Research Articles

Electronic, thermal and optical properties of Fm-3m HgF2 material: A DFT approach

In this paper, Fm-3m structure of HgF2 has been studied using the density functional theorem (DFT). The stability of this structure is revealed by the positive phonon frequencies of the phonon dispersion curves and by satisfying Born Huang criteria concerned with elasticity of the structure. For the very first time, the elastic constants C[Formula: see text], C[Formula: see text] and C[Formula: see text] of Fm-3m structure of HgF2 have been explored using thermo_pw package. The ductile nature of HgF2 is evidenced by its [Formula: see text] ratio of 2.56. The linear response approach has been used to evaluate the intrinsic thermodynamic characteristics of HgF2 while examining the temperature fluctuations of the Helmholtz free energy ([Formula: see text], internal energy ([Formula: see text], specific heat at constant volume (Cv), and Debye temperature ([Formula: see text]. The ionic behavior of HgF2 is demonstrated by the broad valence bandgap that is present in the electronic band structure and density of states. Moreover, the hybridization between “s”, “p”, and “d” orbitals in HgF2 is defined by the charge density along the [110] plane. The static refractive index of HgF2 is 1.26. The broad optical investigation demonstrates Fm-3m HgF2’s transparency in the UV and visible spectrums. Additionally, it exhibits considerable UV absorption throughout its range as well as with absorption tail that continues into the visible spectrum. The overall optical parameters of Fm-3m HgF2 show non-linear optical behavior which makes the material to be useful in ultraviolet (UV) photo-sensing devices.

Negligible Bowing Effect of Bandgap and Lattice Constant in a Variety of Compositions using Large Tilt Distortion in a Cesium–Lead Mixed‐Halide System

AbstractHalide perovskites have broad bandgap tunability, making them suitable for diverse applications in optoelectronics and photovoltaics. The optical bandgap of halide perovskites varies almost linearly with the halide ionic size, and therefore, it can be controlled through compositional engineering. However, the mechanism underlying this low‐bandgap variation is not yet fully understood. Thus, this study comprehensively investigates the bandgap bowing of cesium–lead mixed‐halide perovskites using compositional engineering and demonstrates that bandgap bowing is extremely small in a variety of compositions including Cl, Br, and I. Subsequently, through density functional theory calculations, it is suggested that the antibonding character of the valence band maximum, tilt distortion of halide ions, and entropy effect leading to equal participation of various halide ions around Pb are collectively responsible for the small bandgap bowing.

Broad bandgap active metamaterials with optimal time-delayed control

In this paper, a metamaterial with time-delayed feedback control for broadband vibration suppression is investigated. Genetic algorithm is used to optimize the time-delayed feedback controller to achieve low broadband vibration suppression and even full frequency vibration suppression. Time-delayed feedback controller is introduced between the visible and internal masses of the metamaterial. Time-delayed control tunes the bandgap of the metamaterial and achieves broadband tunable vibration suppression. To further broaden the bandgap, a supercell model with time-delayed control is established, and the time-delayed controller is optimized using the genetic algorithm to determine the optimal control gain and time delay. Two optimization schemes with different objectives are designed. Optimization I: Achieve the widest bandgap at the target attenuation rate. Optimization II: Achieve the maximum attenuation rate in the target frequency range. The results show that multiple bandgaps can be coupled to achieve the widest bandgap at the target attenuation rate. The controller is optimized over the full frequency band to achieve the maximum attenuation rate. Numerical results verify that the metamaterial with optimal controller can achieve vibration suppression over the full frequency range.

Sankaranarayanan-Ramasamy (SR) technique L- threonine lithium chloride single crystal: Unidirectional growth, optical and mechanical studies

L-Threonine Lithium Chloride (LTLC), an effective semi-organic NLO material, was successfully produced as a bulk single crystal using the Sankaranarayanan Ramasamy (SR) process. According to a study of a single crystal's X-ray diffraction, LTLC crystallises in an orthorhombic system with space group P212121. From the powder XRD method, unidirectional development along the plane (110) was established. The optical absorption spectra reveals that LTLC has a broad bandgap of 3.75 eV and is very transparent over the whole visible region. On the magnified crystal, photoconductivity measurements show positive photoconductivity. According to the photoluminescence investigation, LTLC emits green light with a wavelength of 535 nm. Vickers microhardness testing was done to determine the crystal's mechanical strength. At different temperatures and frequencies, the dielectric constant and dielectric loss were calculated. In addition to the density functional theory (DFT) calculations such as frontier molecular orbital’s (FMOs), analysis was carried out. Utilizing Kurtz's Nd:YAG Laser, powder testing is another method used to assess the produced crystal's SHG activity.

Impact of charge-compensated Fe and Nb co-substitution on BaTiO3: Bandgap and grain size reduction and enhanced bulk photovoltaic power of Al/BFNT/Ag solar cell

The generation of above bandgap photovoltage using bulk ferroelectric materials has become a subject of great interest, however, their photocurrent density is limited by a broad bandgap and poor conductivity. To overcome this limitation, we replaced aliovalent metal ions (Fe3+and Nb5+)at the B-site of robust ferroelectric BaTiO3and fabricated an Al/BaTi1-2xFexNbxO3/Ag photovoltaic device. Both the experimental and the theoretical studies showed that bandgap was lowered to ∼2.55 eV and hence absorption of wide energy range of the solar spectrum was attained. An apt top electrode, reduced bandgap and domain size resulted in greater photocurrent density of 1.46 μA/cm2 and photovoltage of 8.31 V for Al/0.075BFNT/Ag solar cell in unpoled condition. This research suggest that reduced band gap, mixed structural phases and nano-sized domains suffices greatest PV power output while the large polarization and poling are not necessary prerequisites.

Gradient V-Shaped and N-Shaped Seismic Metamaterials.

Seismic metamaterials provide an innovative alternative in earthquake engineering by reducing the hazards from seismic waves without modifying the existing structures. Although many seismic metamaterials have been proposed, a design for a broad bandgap at low frequencies is still in demand. In this study, two novel seismic metamaterials, V- and N-shaped designs, are proposed. We found that by adding a line to the letter V, turning the V-shaped design into an N-shaped design, the bandgap can be broadened. Both the V- and N-shaped designs are arranged in a gradient pattern to combine the bandgaps from metamaterials with different heights. Using only concrete as the base material for the design makes the proposed seismic metamaterial cost effective. Finite element transient analysis and band structures are in good agreement, validating the accuracy of the numerical simulations. Surface waves are effectively attenuated over a broad range of low frequencies using the gradient V- and N-shaped seismic metamaterials.

Photocatalytic removal of organic dyes by titanium doped alumina nanocomposites: Using multivariate factorial and kinetics models

Hazardous containing different groups of dyes from various industries are extremely dangerous and causing unfavorably susceptible dermatitis, skin diseases, transformation, and malignant growth. To overcome the hazards caused by water pollution, eco-friendly composites (TiO2-Al2O3) of titanium oxide (TiO2) and alumina (Al2O3) were synthesized with different proportional ratios (0.25/1, 0.16/1, 0.125/1, and 0.1/1) by kneading method and characterized through X-ray diffraction, scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet-Visible (UV–Vis) spectrophotometry and high resolution transmittance electron microscopy (HR-TEM). Experimental finding showed that the sample TiO2 / Al2O3 with a molar-ratio of 0.1/1 had the higher photodegradation efficacy for malachite green and from the ratio 0.1/1, the higher photodegradation efficiency was achieved for eosin Y. The comparative studies of TiO2-Al2O3 samples were done for basic malachite green and acidic eosin Y and it was estimated that due to adsorbent, Al2O3 surface basicity it showed the highest adsorption for acidic eosin Y and hence had a higher photo degradation removal up to 90% for the eosin Y than for the basic malachite green with 80% photo degradation removal. Reaction mechanism for photodegradation were finest to define by Langmuir isotherms which trailed by pseudo-second order kinetics. The t/qt vs t plots for pseudo-second order kinetic produced high regression coefficients (R2 =0.963), which stated the adsorbate sorption's followed the pseudo second order kinetics. Thermodynamic parameters of dye degradation were calculated by the Van't Hoff equation that indicated the adsorption method was exothermic. The ΔH֯ parameters were found to be -54.44 kJ mol−1. The negative ΔH֯ recommended the exothermic biosorption process. Residuals plots for responses, interaction plots for responses and inter-contour plots were utilized to study the optimum points. Furthermore, accredited to experimental results and a reaction mechanism is planned for the photodegradation of organic dyes with TiO2-Al2O3 photocatalyst. Thus the broad band gap of inert semiconductor TiO2 can be narrower by heterogeneous coupling with Al2O3 through kneading method and showed the excellent photodegradation of organic dyes in the visible region without producing any secondary pollutant.

Local-Resonance-Induced Dual-Band Topological Corner States of Flexural Waves in a Perforated Metaplate

Despite their proven effectiveness in localizing and steering subwavelength elastic waves, locally resonant elastic topological insulators (TIs) with surface-mounted or embedded resonators are often challenged because of their structural complexity and the difficulty in realizing multiple topological band gaps. Here we present a second-order elastic TI (SETI), made of a perforated metaplate with a series of etched C-shaped slots, to achieve dual-band topological corner states. The etched slots in the metaplate form a type of cantileverlike oscillator, the multimodal bending resonances of which couple with the flexural vibration modes of the metaplate, resulting in multiple locally resonant band gaps. Judiciously configuring these slots in a ${C}_{4v}$-symmetric lattice enables topologically distinct metastructures and creates a SETI. We numerically and experimentally observe the dual-band corner states in two broad topological band gaps. Our platform for observing the topological effects in a perforated metaplate greatly simplifies the fabrication of metastructures for implementing locally resonant elastic TIs, benefiting applications of TIs at the subwavelength scale such as elastic wave trapping and energy amplification.

Review of Recent Advances of ZnO Nanowires Based Sensors Devices

This paper presents the recent advances of ZnO Nanowires Based Sensors Devices. ZnO, an n-type, direct metal oxide semiconductor with a broad band gap, is projected to be the next generation functional nanomaterial for a wide range of sensing applications. Due to their exceptional optoelectronic, physicochemical, and electrical properties, such as low dielectric constant, abundant Zn-O bonds, high luminous transmittance, good physicochemical stability, enormous excitation binding energy, non-toxicity, biocompatibility, large surface area to volume ratio, and others, ZnO and its composites have opened a new era in the fabrication of sensors. The uses of ZnO nanostructures in the fields of environmental monitoring, biomedicine, and optical sensing are outlined in this thorough overview. To gain a better understanding of the function of ZnO in each of these sensors, fundamental sensing mechanisms of ZnO based sensors are explored. Limitations of the current methodologies and the forecast for the future have also been discussed.

Facile synthesis of a novel recyclable dual Z-scheme WO3/NiFe2O4/BiOBr composite with broad-spectrum response and enhanced sonocatalytic performance for levofloxacin removal in aqueous solution

In recent decades, the overuse and misuse of antibiotics have caused serious harm to aquatic ecosystems. This work successfully designed a unique recyclable WO3/NiFe2O4/BiOBr (WNB) composite with dual Z-scheme heterojunction to degrade levofloxacin (LEV) in aqueous solution. The results indicated that the as-prepared WNB composite displayed the greatest sonocatalytic activity for LEV degradation. In particular, WNB showed the highest LEV (at 10 mg/L) removal efficiency (97.97 %) within 75 min under ultrasonic irradiation. The significantly enhanced catalytic activity of WNB was attributed to the unique dual Z-scheme structure, which makes it easier for the transfer and the spatial separation of photo-generated electron-hole (e−-h+) pairs, and dramatically increases the carrier lifespan. Furthermore, the ternary composite comprises three different semiconductors from narrow to broad bandgap, which are suitable for harvesting full-spectrum light. The recycle experiments demonstrated the catalyst's excellent sonocatalytic stability and easy recovery, which is attributed to its magnetic property. Furthermore, based on the intermediates identified during the degradation process, a potential mechanism and pathway for LEV degradation were presented. This work provides a unique perspective on designing efficient sonocatalysts for environmental remediation.

The South Korean case of deploying rural broadband via fiber networks by implementing universal service obligation and public-private partnership based project

Despite broadband being essential infrastructure for conducting basic socio-economic activities and reducing inequality and the digital divide, expanding broadband coverage in rural areas remains a significant challenge in many countries due to high deployment costs. South Korea, a developed country in terms of fixed broadband penetration, implemented two policies for rural broadband via fiber access networks simultaneously during the COVID-19 pandemic. One is the universal broadband service at 100 Mbps speed introduced in 2020, and the other is the rural broadband project in terms of a public-private partnership (PPP) that proceeded from August 2020 to 2022. While both policies contribute to reducing the broadband gap between the urban and the rural in the country, it is still important to find the optimal cost-effective combination of the plural rural policies as much as possible from the cost perspective. Accordingly, this study proposes an investment cost estimation model for deploying rural broadband via fiber networks in South Korea and evaluates whether implementing two rural broadband policies in the country is cost-effective. The proposed model is designed according to the bottom-up approach, which involves building an efficient network with the latest technologies; thus, it gives a more accurate and reliable estimation of the investment cost of building rural broadband in the South Korean territory. The significant findings of this study are twofold. First, the PPP-based project could make the participating internet service providers (ISPs) deploy broadband at a cost-effective level, similar or lower to the universal service, and then recover the investment cost per building within a reasonable period. Thus, the result implies that governments need to consider the PPP-based project as an effective measure to deploy rural broadband, in addition to the introduction of universal service. Implementing multiple broadband policies to expand rural broadband coverage could be a reasonable and practical approach to improving broadband availability. Second, the governments still needs to make efforts to solve the complicated difficulties in rural areas in South Korea where rural-related broadband policies are still challenging to implement. This study confirmed that in some regions, the PPP-based project was still inferior to universal service in terms of average investment cost. In contrast, universal service had significant constraints to providing broadband because of the end-users cost burden, resulting in low broadband adoption. Therefore, the government needs to consider some measures to increase broadband availability and broadband adoption, such as widening the rural areas under the jurisdiction of participating ISPs, decreasing the proportion of participating ISPs bearing the investment cost of deploying broadband, and improving the cost burden of the end-users of universal service in some limited areas.

Quantitatively regulating the ketone structure of triazine-based covalent organic frameworks for efficient visible-light photocatalytic degradation of organic pollutants: Tunable performance and mechanisms.

As promising visible-light-responsive photocatalysts, triazine-based covalent organic frameworks (CTFs) still suffer from broad bandgap and high electron-hole recombination. As such, different contents of electron-rich ketone group were introduced to CTFs (X % keto-CTF), aiming to clarify the mechanism of quantitatively regulating ketone for enhanced visible-light photocatalytic performance of CTFs. As ketone content increased, the bandgap narrowed, electron-hole recombination decreased, charge transfer and quantum yield increased. As a result, keto-CTF outperformed other keto-CTFs in visible-light photocatalytic degradation of tetracycline, and apparent rate constant of TC (kobs) was 3.69 times higher than that of CTF. Importantly, ketone tuning induced varied types and concentrations of reactive species. Integrated with quantitative structure-activity relationships (QSARs) analysis and density functional theory (DFT) calculations, this study unravels how ketone content regulates bandgap structure of CTF, affects the contribution of varied reactive species, and quantitatively enhances the photocatalytic performance of CTFs. It also provides novel insights into the precise design and synthesis of CTFs-based catalyst structures for high-efficient visible-light photocatalytic degradation of organic pollutants.

Working principle and characteristic analysis of SiC MOSFET

Power semiconductor devices are frequently used in the electronic and power industries. As the third-generation broadband gap power semiconductor devices, silicon carbide devices have attracted extensive attention. This paper introduces the internal and external structure of SiC MOSFET in detail and analyzes the basic working principle of SiC MOSFET and the electrical characteristics of each stage in the conductive process. By analyzing the theoretical calculation method of threshold voltage under body effect and the relationship diagram drawn by specific experimental test results, it is proved that the increase of body effect will lead to the decrease of the threshold voltage. Two main reasons for BTI characteristics are explained: the energy band shift of the SiC/SiO2 interface is small, and there are many charge traps at SiC/SiO2 interface. Finally, the theoretical calculation method of threshold voltage and the influence principle of threshold voltage drift caused by SiC/SiO2 interface charge: interface trap Qit, oxide trap Qot, movable ion Qm and fixed charge Qf are analyzed. The research on the basic principle and important characteristics of SiC MOSFET in this paper have important reference significance for improving the reliability of SiC MOSFET devices.

Broadband Rayleigh wave attenuation utilizing an inertant seismic metamaterial

In this work, a concept of constructing inertant seismic metamaterials for Rayleigh wave attenuation using inerter-based vibration absorbers (IDVAs) is proposed. The dynamic behaviors of the vibration absorbers are controlled by an inertant mechanical network, while three typical oscillator configurations are considered in the design of the proposed inertant seismic metamaterial. An expanded effective medium method is developed to obtain the dispersion of Rayleigh waves, which is separated into three branches by two broad Rayleigh wave band gaps at low frequencies. The influences of the inertance-to-mass ratio as well as the stiffness ratio on the Rayleigh wave propagation are thoroughly discussed. The results show that the multiple-resonance IDVAs can efficiently open low frequency band gaps of the inertant seismic metamaterial without having to increase the mass of the resonators. Band gaps of the low-frequency Rayleigh waves can be further merged to form an extra wide one by adding dissipative mechanisms, which further enhance the attenuation of the Rayleigh waves. Specifically, by analyzing the corresponding displacement fields, it is observed that the seismic energy propagating as Rayleigh surface waves is converted to that propagating as bulk waves not concentrated to the surface, which offers a protected area behind the inertant seismic metamaterials for the facilities located at the surface of the half space. This work provides a new approach for seismic vibration isolations utilizing inertant seismic metamaterials that can be applied in protecting large infrastructures or civil engineering architectures.

Regulating energy band structures of triazine covalent organic frameworks with electron-donating/withdrawing substituents for visible-light-responsive photocatalytic tetracycline degradation and Cr(VI) reduction

Environmental contaminations have raised soaring concerns about human health worldwide. Developing metal-free photocatalysts as green agents to solve these problems is urgent. Covalent organic frameworks (COFs) are considered a promising platform for the molecule-level design of visible-light-responsive photocatalysts due to their tailored coordination/electronic structures and excellent charge carrier mobility. However, COFs without substituents (e.g., COFs-H) still suffer from broad bandgaps and low electron-hole separation efficiency. In this work, we introduced electron-donating/withdrawing substituents on COFs-H to fine-tune the bandgap and photocatalytic performance of COFs. Theoretical and experimental studies revealed that all substituents narrowed the bandgap of COFs and enhanced the electron-hole separation efficiency. Electron-withdrawing/donating substituents significantly alter the energy level of COFs-R, improving the redox capacities of photo-generated holes and electrons for tetracycline (TC) degradation and Cr(VI) reduction. The large difference in electrostatic potential between the two monomers in COFs-R enhances the charge carrier generation and intramolecular electron transfer intrinsically. This work unravels how substituents with different electronic effects regulate the energy band structures and photo-redox capacities of COFs. It further provides new insight into the precise regulation of COFs toward highly efficient visible-light-driven photocatalytic remediation of organic contaminants and heavy metal ions.

Topologically tunable local-resonant origami metamaterials for wave transmission and impact mitigation

Origami metamaterials can control the propagation and energy transmission of elastic waves via the flexible design of microstructure topology, which has great application prospects in the field of structural impact resistance. In this paper, by integrating the Bloch theorem and the local resonance (LR) mechanism, a tunable hybrid origami metamaterial (HOM) is proposed to achieve low-frequency and broad bandgaps (BGs) for realizing wave transmission and impact mitigation. The elastodynamic wave equation of the HOM is established based on the Mindlin plate theory and discretized with the higher-order spectral elements, and band structures of the HOM are obtained through the semi-analytical periodic spectral plate (PSP-SAFE) methods. Effective dynamic properties of the HOM are numerically calculated to explain wave transmission behavior. Interestingly two complete BGs in the low-frequency range are found, and the proposed metamaterial can broaden the low-frequency bandgaps by coupling the polarization BGs and local resonance BGs. The influences of three significant parameters on two complete BGs within the region of interest are then discussed. Furthermore, the wave mitigation capacity of locally resonant HOM is investigated under impact pulses. It is noted that the reaction peak force of the HOM possesses an approximately 25.7% better reduction than that of origami structures with no mass inclusion. Especially when the external excitation frequency is consistent with local-resonance BGs, the transmission rate shows dramatically decrease due to the dramatic dissipation of energy in space. The locally resonant hybrid origami metamaterial shows promising potential for applying low-frequency wave attenuation and impact mitigation.

Recent advances on covalent organic frameworks (COFs) as photocatalysts: different strategies for enhancing hydrogen generation.

The excessive use of traditional fossil fuels has led to energy and environmental pollution problems. Solar-driven hydrogen generation has attracted much attention in recent years owing to its environmental friendliness and economic feasibility. So far, a series of photocatalysts have been advanced. Unfortunately, these photocatalysts face some issues including poor sunlight harvesting ability, weak photo-corrosion resistance, broad band gap, bad stability, inferior hydrogen evolution rate and so on. It just so happens that COFs have emerged to provide an opportunity for settling these issues. Covalent organic frameworks (COFs), a novel family of porous materials with regular porosity and tunable physicochemical structures, have been extensively explored as photocatalysts for hydrogen production. Moreover, their photocatalytic activities are highly structurally dependent. In this review, we mainly focus on the linkage chemistry and disparate strategies for boosting COF-based photocatalytic hydrogen generation performance in detail. The prospects and obstacles confronted in the development of COF-based photocatalysts and proposals to settle dilemmas are also discussed.

Synergy of vacancy engineering and surface plasmon modification for improving the photocatalytic performance of La2Ti2O7 with a broad band gap

A novel Z-scheme AgCl/Ag/OV-La2Ti2O7 photocatalyst with Ag clusters as the electron transfer medium was successfully synthesized for improving its photocatalytic activity.

Broadband and Student Performance Gaps after the Covid-19 Pandemic

Broadband and Student Performance Gaps after the Covid-19 Pandemic

Research progress of perovskite/crystalline silicon tandem solar cells with efficiency of over 30%

Double junction tandem solar cells consisting of two absorbers with designed different band gaps show great advantage in breaking the Shockley-Queisser limit efficiency of single junction solar cell by differential absorption of sunlight in a wider range of wavelengths and reducing the thermal loss of photons. Owing to the advantages of adjustable band gap and low cost of perovskite cells, perovskite/crystalline silicon tandem solar cells have become a research hotspot in photovoltaics. We systematically review the latest research progress of perovskite/crystalline silicon tandem solar cells. Focusing on the structure of perovskite top cells, intermediate interconnection layers and crystalline silicon bottom cells, we summarize the design principles of high-efficiency tandem devices in optical and electrical aspects. We find that the optical and electrical engineering of each layer structure in perovskite/crystalline silicon tandem solar cells goes through the whole process of device preparation. We also summarize the challenges of limiting the further improvement of the efficiency of the perovskite/crystalline silicon tandem solar cells and the corresponding improvement measures, which covers the following respects: 1) Improving the balance between <i>V</i><sub>oc</sub> and <i>J</i><sub>sc</sub> of the broadband perovskite cell through additive engineering and interface engineering; 2) improving the bandgap matching between the electrical layers and reducing the carrier transport barrier through adjusting the work function or conductivity of layers; 3) improving the photocurrent coupling between sub-cells and the photocurrent of tandem solar cells by using light engineering and conformal deposition technology of perovskite cells. At present, there have been many technologies to improve the stability of perovskite solar cells, such as additive engineering and interface engineering, but the problem has hardly been solved. Therefore, improving the stability of broadband gap perovskite solar cells to the level of crystalline silicon solar cells will become an important challenge to limit its large-scale application. In terms of efficiency, the mass production efficiency of perovskite/crystalline silicon tandem solar cells is far lower than that of the laboratory level. One of the reasons is that it is difficult to achieve low-cost and deposition of uniform large area perovskite solar cells. Therefore improving the stability of broadband gap perovskite solar cells and developing low-cost large-area perovskite deposition technology will become extremely critical. Finally we look forward to the next generation of higher efficient low-cost tandem solar cells. We believe that with the increasing demand for higher efficiency photovoltaic devices, the triple junction solar cells based on the perovskite/crystalline silicon stack structure will become the future photovoltaics.