Band Structure Research Articles

A comparative study of structural, elastic, electronic, thermophysical, and optical properties of cubic binary lave-phase intermetallic compounds of HfX2 (X = cr, mo, and W): An ab initio insight

In the present work, we have made a comparative investigation on the structural, elastic, electronic, thermos-physical, and optical properties of cubic binary lave phase intermetallic compound with the common formula HfX2 (X = Cr, Mo and W) based on Density Functional Theory (DFT) using CASTEP. The result of geometrical optimization has been strongly agreed with the experimental results. The HfW2 compound exhibits larger lattice parameters and volumes than HfMo2 and HfCr2 because of the variation of atomic radii of the exhibiting elements in the studied compounds. The elastic properties show that each of the compounds in the ground state is mechanically stable and has a highly ductile nature. Band structures and energy densities of states have been studied to learn more about electronic properties. These compounds display metallic properties in their electrical band structures. They also have high machinability, a low Debye temperature, low bond hardness, and a remarkably high melting point. Our investigation thoroughly examined the reflectivity, absorption coefficient, refractive index, dielectric function, optical conductivity, and loss function of these metals. The optical absorption, reflectivity spectra, and refractive index of HfX2 (X = Cr, Mo, and W) indicate their potential for use as solar reflectors in the IR-Vis region and UV–Vis absorbers.

Transfer learning relaxation, electronic structure and continuum model for twisted bilayer MoTe2

Large-scale moiré systems are extraordinarily sensitive, with even minute atomic shifts leading to significant changes in electronic structures. Here, we investigate the lattice relaxation effect on moiré band structures in twisted bilayer MoTe2 with two approaches: (a) large-scale plane-wave basis first principle calculation down to 2.88°, (b) transfer learning structure relaxation + local-basis first principles calculation down to 1.1°. We use two types of van der Waals corrections: the D2 method of Grimme and the density-dependent energy correction, and find that the density-dependent energy correction yields a continuous evolution of bandwidth with twist angles. Based on the above results. we develop a complete continuum model with a single set of parameters for a wide range of twist angles, and perform many-body simulations at ν = −1, −2/3, −1/3.

Evaluation of the phonon, optoelectronic, photovoltaic, thermoelectric, photocatalytic and thermodynamic properties of the inorganic perovskite CsPbI2Br using Ab initio calculations

Inorganic CsPbI2Br perovskites, derived from cesium, hold potential for use in solar and optoelectronic devices. This study utilized density functional theory calculations to investigate the structural, phonon, electronic, optical, photovoltaic, thermoelectric, thermodynamic, and photocatalytic properties of CsPbI2Br. The phase stability of our compound is thoroughly researched, and it is confirmed that all are thermodynamically and dynamically stable. Formation energy is employed to confirm thermodynamic stability while phonon dispersion is used to confirm dynamical stability. We have computed the electronic properties of CsPbI2Br, such as PDOS, TDOS, and its band structure. The results indicate that the material is a p-type semiconductor which has a direct gap value of 1.50 eV and 1.92 eV using GGA-PBE and TB-mBJ respectively. Higher absorption coefficient, optical conductivity, reflectivity and other optical properties of the CsPbI2Br compound indicate better absorption in the visible spectrum. This distinguishes this substance as an exceptional candidate for optoelectronic devices. Improved utilization of CsPbI2Br in thermoelectric devices is evident from the power factor PF, Seebeck coefficient, figure of merit, and other thermoelectric properties. CsPbI2Br is advantageous in the solar industry due to its external quantum efficiency, open-circuit voltage of 1.92 V, and short-circuit current of 19.67 mA.cm−2. Additionally, the study examined the compound’s thermodynamic properties and examined how the temperature affected its entropy, free energy, enthalpy and heat capacity. Furthermore, it meets the thermodynamic necessary conditions to initiate the water-splitting reaction; the CsPbI2Br compound demonstrates strong photocatalytic performance. This implies that CsPbI2Br may open up new avenues for thermoelectric and optoelectronic experimentation.

Highly visible active 2D/2D BiOBr/CO32--Bi2O2CO3 Z-scheme heterojunction for photocatalytic removal of 2-hydroxy-1, 4-naphthoquinone

Photocatalysis technology is utilized for the elimination of water pollutants due to its gentle reaction conditions, affordability, cleanliness, and high efficiency. In this study, BiOBr/CO32--Bi2O2CO3 heterojunctions consisting of BiOBr and CO32- self-doped Bi2O2CO3 components were successfully synthesized by a one-pot hydrothermal method. The photocatalysts were subjected to various testing methods for characterization of their morphology, structure, and photoelectrochemical properties. The photocatalytic efficiency of 2-hydroxy-1,4- naphthoquinone (HNQ) was assessed based on its degradation under visible light irradiation. The findings demonstrated a significant enhancement in the photocatalytic activity of the 50 % BiOBr/CO32--Bi2O2CO3 system, resulting in an impressive degradation rate of 83 % for in 90 min. In addition, the material also has good recoverability and cycle stability, after four cycles, the degradation efficiency remains at 78 %. The free radical capture experiment indicated that•O2- is the active species in the photocatalytic degradation of HNQ. A potential reaction mechanism for the photocatalytic process of the BiOBr/CO32--Bi2O2CO3 Z-scheme heterojunction has been proposed based on experiments trapping free radicals and considering the energy band structure of the catalyst. The successful construction of a heterojunction utilizes photogenerated electrons (e-) and holes (h+) in a more rational manner, which accelerates carrier transfer at the interface and improves the photocatalytic performance of the material. This study can offer a novel idea and approach for the synthesis of Bi-based Z-scheme composite photocatalysts.

Electronic structures, thermal properties and molecular dynamics of CsGdCl3 and CsNdCl3 perovskite crystals by first-principles calculations

Lanthanide-based perovskite crystals have been characterized for proposing material design guidelines of perovskite solar cell with stability of photovoltaic performance. The purpose of this study is to characterize band structures, phonon dispersions, molecular dynamics, electronic and thermal conductivities of CsGdCl3 and CsNdCl3 perovskite crystals by first-principles calculation analysis. The CsGdCl3 and CsNdCl3 crystals had wide distributions of 5d orbital of Gd3+ ion, 5d and 4f orbitals of Nd2+ ion near conduction band state as n-type semiconductive behavior. The phonon dispersions and thermal conductivities indicate effects of acoustic phonon on carrier scattering. The CsGdCl3 crystal suppressed carrier scattering as elastic behavior, improving electron diffusion and dynamic stability. The CsNdCl3 crystals expected to cause carrier scattering with acoustic phonon and molecular dynamics. The CsGdCl3 crystal have the superior potential of the performance as semiconductor characteristics equivalent to the performance of the CsPbCl3 crystal.

High photocatalytic activity of g-C3N4/CdZnS/MoS2 heterojunction for hydrogen production

In an effort to improve the light absorption efficiency of the photocatalyst, g-C3N4 nanosheets with visible light response have been prepared by roasting method, and the g-C3N4/CdZnS composite catalyst with 2D/0D architecture has been prepared by hydrothermal method after ultrasonic thinning. The g-C3N4/CdZnS/MoS2 composite catalyst with 2D/0D architecture has been synthesized by secondary hydrothermal method. TEM analysis proves that a close heterogeneous interface has been formed between g-C3N4 and CdZnS, and between CdZnS and MoS2. The ultraviolet diffuse reflectance spectrum shows that MoS2 has a wide light absorption range, which effectively enhances the light utilization ratio of the composite catalyst. The energy band structure diagram and photoelectrochemical test results show that a continuous stepped type II ternary heterojunction is formed among g-C3N4, CdZnS and MoS2, which promotes its separation of photogenerated charges. When the loading mass fraction of g-C3N4 and MoS2 is 4%, the hydrogen evolution rate of g-C3N4/CdZnS/MoS2 composite catalyst is 57.02 mmol g−1 h−1 under visible light, which is exceeding 20% that of g-C3N4/CdZnS and 4.79 times and 44.9 times higher than that of CdZnS and g-C3N4, respectively. The two-dimensional structure of g-C3N4 and MoS2 significantly improves the stability of the composite catalyst. This work offers feasible ways for ternary heterogeneous photocatalyst construction.

DFT study of optoelectronic and thermoelectric properties of pure and doped double perovskite Cs2SnI6 for solar cell applications

The global need of energy is raising day by day. To meet this demand double perovskites (DPs) can play a vital role for energy conversion devices. In this letter, we formulated and examined lead-free defect perovskites using a combination of tellurium and tin, denoted as Cs2Sn1-xTexI6 (0, 0.25), utilizing density functional theory which include structural, optical and thermal behavior. Both the optimized volume and lattice parameter exhibit a linear increase with the Te content in Cs2Sn1-xTexI6 (CsSnTeI). The negative value of formation energy for both pure and doped materials along with their Poisson and Pugh ratio confirm the stability of these materials. The band structure analysis reveals its semiconducting behavior, with the band gap expanding proportionally to the increased Te concentration with band gap value of pure (1.20 eV) and doped 91.43 eV) double perovskites. Optical characteristics, including the dielectric function and absorption coefficient, were also analyzed. To expose their potential use of Cs2Sn1-xTexI6 (0, 0.25) in thermoelectric devices, temperature dependent thermoelectric features are investigated reveal that suitability of these materials for energy conversion purposes.

Photoelectric Studies as the Key to Understanding the Nonradiative Processes in Chromium Activated NIR Materials.

In this study, we synthesized a series of Ga1.98-xInxO3:0.02Cr3+ materials with varying x values from 0.0 to 1.0, focusing on their broadband near-infrared emission and photoelectric properties. Interestingly, photocurrent excitation spectra exhibited behavior consistent with the absorption spectra, indicating the promotion of carriers into the band structure by the 4T1, and 4T2 states of Cr3+ ions. This association suggests that photocurrent in this material is influenced not only by valence to conduction band transitions but also by transitions involving Cr3+ dopants. Our investigation of luminescence quenching mechanisms revealed that nonradiative processes were not directly linked to thermally induced relaxation from the excited state 4T2 to the ground state 4A2, as usually suggested in the literature for this type of material. Instead, we linked it to the thermal ionization of Cr3+ ions. Unexpectedly, this process is unrelated to the transfer of electrons from Cr3+ impurities to the conduction band but is associated with the formation of holes in the valence band. This study provided novel evidence of luminescence quenching via the hole-type thermal quenching process in Cr3+-doped oxides, suggesting potential applicability to other transition metal ions and host materials. Finally, we demonstrated the dual-purpose nature of Ga1.98-xInxO3:0.02Cr3+ as a practical emitter for NIR-pc-LEDs and effective photocurrent for UV detectors. This versatility underscores these materials' practicality and broad application potential in optoelectronic devices designed for near-infrared and ultraviolet applications.

Vanadium as a Ti-like mediator boosting electronic transmission in Fe-based MOFs for photocatalytic sterilization

Efficient metal–organic frameworks (MOFs) photocatalytic bactericidal catalysts are urgently needed in water purification. Herein, a Fe-MOF (MIL-88B-NH2(V1Fe5) with promoted electron transport was achieved by vanadium (V) ions doping and V/Fe ratio optimization, showing excellent photocatalytic bactericidal activity against E. coli under visible light irradiation (99.92%). The efficient antibacterial mechanism, V as a Ti-like mediator boosting electronic transmission in MIL-88B-NH2(V1Fe5), was revealed by its band structure, transient photocurrent, electrochemical impedance spectroscopy, and scavenger quenching experiments. The enhancement of photocatalytic bactericidal performance of Fe-MOFs by V–ion-doping was confirmed by two other Fe-MOFs, MIL-53-NH2(V1Fe5) and MIL-101-NH2(V1Fe5), with the same metal ions and ligands, both of which have higher performance than the corresponding undoped MOFs. Among them, MIL-88B-NH2(V1Fe5) exhibits the highest photocatalytic bactericidal activity due to its suitable metal clusters ([M(μ 3-O)] cluster) and topological structure (three-dimensional rhomboid network structure). This work demonstrated the amplification effect of V ion doping on electron transport in Fe-MOFs photocatalysts.

Systematic investigation on the rational design and optimization of bi-based metal oxide semiconductors in photocatalytic applications

To address the global energy shortage and mitigate greenhouse gas emissions on a massive scale, it is critical to explore novel and efficient photocatalysts for the utilization of renewable resources. Bi-based metal oxide (Bi x MO y ) semiconductors composed of bismuth, transition metal, and oxygen atoms have demonstrated improved photocatalytic activity and product selectivity. The vast number of element combinations available for Bi x MO y materials provides a huge compositional space for the rational design and isolation of promising photocatalysts for specific applications. In this study, we have systematically investigated the electronic and optical properties over Bi2O3 and a series of selected Bi x MO y group materials (BiVO4, BiFeO3, BiCoO3, and BiCrO3) by calculating band structure, basic optical property features, mobility and separation of charge carriers. It is clearly noted that the band gap and band edge position of the Bi x MO y group materials can be tuned in a wide range in comparison to Bi2O3. Similarly, the light response of Bi x MO y also can be broadened from the ultraviolet to the visible light region by adjusting the selection of transition metals. Additionally, the analysis of the effective mass of charge carriers of these materials further confirms their possibility in photocatalytic reaction applications because of the appropriate separation efficiency and mobility of carriers. A selection of experimental investigations on the crystal structure, composition, and optical properties of Bi2O3, BiVO4, and BiFeO3 as well as their catalytic performance in the degradation of methylene blue over was also conducted, which agree well with the theoretical predictions.

Revealing Roles of High-Electronegativity Fluorine Atoms in Boosting Redox Potential of Layered Sodium Cathodes.

The development of high-energy-density cathode materials is regarded as the ultimate goal of alkali metal-ion batteries energy storage. However, the strategy of regulating specific capacity is limited by the theoretical capacity, and meanwhile focusing on improving capacity will lead to structural destructions. Herein, a novel perspective is proposed that tuning the electronic band structure by introducing highly electronegative fluoride atoms in NaxTMO2-yFy (0 < x < 1, 0 < y < 2) model compounds to improve redox potential for developing high-energy-density layered oxides. Highly electronegative fluoride atoms is introduced into P2-type Na0.67Fe0.5Mn0.5O2 (NFM), and the thus fluoride NFM (F-NFM) cathode achieved high redox potential (3.0V) and high energy density (446Whkg-1). Proved by structural characterizations, fluorine atoms are successfully incorporated into oxygen sites in NFM lattice. Ultraviolet photoelectron spectroscopy is applied to quantitatively analyze the improved redox potential of F-NFM, which is achieved by the decreased valence band energy in electronic band structure due to the strongly electrophilic fluoride ions. Moreover, fluoride atoms can stabilize the local environment of NFM and improve its redox potential. The work provides a perspective to improve redox potential by tuning the electronic band structure in layered oxides and developing high-energy-density alkali metal-ion batteries.

Hollow Fe‐Doped Ni(OH)2–NiS@Ni(OH)2 Nanorod Array with Regulated Heterostructural Interface and Band Structure for Expediting Alkaline Electrocatalytic Overall Water Splitting

Hollow Fe‐Doped Ni(OH)₂–NiS@Ni(OH)₂ Nanorod Array with Regulated Heterostructural Interface and Band Structure for Expediting Alkaline Electrocatalytic Overall Water Splitting

Regulating band structure, charge transfer and separation, oxygen adsorption and activation by surface ion modification.

As a most promising environmental technology, the substantial enhancement of photocatalytic efficiency is still a big challenge for practical applications. In this work, the surface of Bi2O2CO3 (BOC) nanotubes are modified by Cl and I. The as-obtained samples at different hydrothermal temperatures (T) are designated as T-X-BOC (X = Cl, I). X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) prove that Cl and I merely chemically adsorb on the BOC surface, rather than dope into the crystal lattice. The surface modification of Cl and I slightly increases light absorption range, while significantly promotes the photoelectron migration from bulk to the surface that greatly enhances the carrier separation efficiency. Density functional theory (DFT) calculations further prove that surface Cl and I have adjusted band structure and surface charge distribution. Besides, the surface Cl and I favor the O2 adsorption and trap the surface photoelectrons, thus promoting the formation of •O2-; while the surface Cl and I impede the surface adsorption of H2O, thus refraining the generation of •OH. In the degradation of rhodamine B (RhB), holes and •O2- radicals play the crucial role. Under ultraviolet light irradiation (λ < 420nm) for 45min, the RhB degradation ratios over 150-Cl-BOC (94%) and 150-I-BOC (85%) are 4.2 and 3.7 times higher than that of original BOC (18%), respectively. This work demonstrates that the simple surface halogenation modification greatly improves the photocatalytic activity.

First-principles calculations to investigate Structural, Electronic, mechanical and Optical Properties of SrAlO3 Compound

In this study, density functional theory calculations were used to investigate the structural, electronic, mechanical, and optical properties of SrAlO3 perovskites. Calculations were performed using WEIN2K with the Generalized Gradient Approximation (GGA) and modified Becke-Johnson (MBJ) methods. The present compound shows large band gap around 5.67 eV for GGA method and 6.70 eV for MBJ method. The band structure (BS) and partial density of states (PDOS) reveal the predominant involvement of the oxygen-p orbital in the valence band, while the Al and Sr-s, p and d orbital contribute significantly to the conduction band, that play a significant role in determining the electronic and optical properties. Using the GGA and MBJ methods, the optical properties of SrAlO3 have been determined in terms of optical conductivity, dielectric constant, absorption coefficient, energy loss, optical reflectivity, and refractive index. It has been noted that the features present in the optical absorption spectra obtained experimentally match those of the simulated spectra featuring an oxygen vacancy. We use elastic constants to describe a material's mechanical properties. Under normal conditions, these materials' elastic constants demonstrate their mechanical stability. We meticulously determined the elastic constants by graphing the directional variations of compressibility, Young's modulus, and shear modulus. Our findings indicate that these chemicals have high elastic constants. Furthermore, we collect and interpret the phonon spectra. Complete phonon dispersion simulations validate the dynamic stability of the SrAlO3.

Pressure dependence of superconducting behavior of solid Br and I: A first-principles study

Based on the first-principles calculations, this study employs the HSE06 functional for the first time to understand the high-pressure behavior of diatomic molecular crystal bromine and iodine. It meticulously replicates the metallization process of iodine and bromine from aspects including bond length, charge density, band structure, and density of states. Furthermore, we also investigate the pressure-dependent superconducting properties of monoatomic phases of bromine and iodine, probing into the reasons behind the discrepancies in Tc values among different phases.

Possible gapless helical edge states in hydrogenated graphene

Electronic band structures in hydrogenated graphene are theoretically investigated by means of first-principle calculations and an effective tight-binding model. It is shown that regularly designed hydrogenation to graphene gives rise to a large band gap about 1 eV. Remarkably, by changing the spatial pattern of the hydrogenation, topologically distinct states can be realized, where the topological nontriviality is detected by C2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C_2$$\\end{document} parity indices in bulk and confirmed by the existence of gapless edge/interface states as protected by the mirror and sublattice symmetries. The analysis of the wave functions reveals that the helical edge states in hydrogenated graphene with the appropriate design carry pseudospin currents that are reminiscent of the quantum spin Hall effect. Our work shows the potential of hydrogenated graphene in pseudospin-based device applications.

Adsorption and sensing performances of greenhouse gases (CO2，CH4，SF6, N2O) on Ni modified GeSe surface

The greenhouse effect has drawn significant recent attention due to its serious impact on Earth's climate and ecosystems. Therefore, this study investigates the adsorption and sensing properties of four greenhouse gases (CH4, CO2, SF6, and N2O) on the Ni-GeSe monolayer using first principles of DFT. The doping behavior of Ni on intrinsic GeSe monolayer is initially studied and the most stable doping structure is selected. Then the adsorption energies, adsorption distances, charge transfers, energy band structures, molecular frontier orbitals and recovery times of the selected Ni-GeSe structure with the four greenhouse gases are analyzed and compared. The results show that Ni modification can improve the surface activity of the intrinsic GeSe monolayers, thereby improving their adsorption and sensing properties. Ni-GeSe has weak adsorption of CH4 and CO2, while strong chemisorption of SF6 and N2O, making it suitable as an adsorbent for these two gases. Analysis of the recycling performance of Ni-GeSe monolayers shows that Ni-GeSe has a good recovery time for N2O at a temperature of 398K, suggesting its potential as an efficient low-power resistive N2O gas sensor. And gas sensors based on GeSe and its modified materials can be theoretically guided by these calculations.

Emerging Roles of Graphitic Carbon Nitride-based Materials in Biomedical Applications.

Graphite carbon nitride (g-C3N4) is a two-dimensional conjugated polymer with a unique energy band structure similar to graphene. Due to its outstanding analytical advantages, such as relatively small band gap (2.7 eV), low-cost synthesis, high thermal stability, excellent photocatalytic ability, and good biocompatibility, g-C3N4 has attracted the interest of researchers and industry, especially in the medical field. This paper summarizes the latest research on g-C3N4-based composites in various biomedical applications, including therapy, diagnostic imaging, biosensors, antibacterial, and wearable devices. In addition, the application prospects and possible challenges of g-C3N4 in nanomedicine are also discussed in detail. This review is expected to inspire emerging biomedical applications based on g-C3N4.

Enhanced Thermoelectric Properties for Eco-friendly CaTiO3 by Band Sharpening and Atomic-Scale Defect Phonon Scattering

CaTiO3-based compounds have emerged as a promising thermoelectric material, renowned for their environmentally benign, thermally stable, and cost-efficient merits. Nonetheless, the pristine CaTiO3 manifests inherently low electronic transport properties. Herein, the thermoelectric properties of Ca1-xDyxTiO3 (x = 0, 0.05, 0.10, 0.15, 0.20) compounds are systematically investigated. The electrical transport properties are markedly enhanced by synergistic optimization of the carrier concentration, mobility, and density-of-states effective mass. Density functional theory (DFT) results demonstrate that the conduction band tends to sharper and the lighter band participates in carrier transport after Dy doping. The large discrepancy in atomic mass results in considerable mass fluctuations, which give rise to intense phonon scattering. Benefitting from the modulated band structure and reduced thermal conductivity, the highest ZT of 0.31 is achieved at 1073 K, enhanced by 287.5% in contrast with pristine CaTiO3 (ZTmax = 0.08). The defect and energy band modulation strategies proposed to optimize thermoelectric performance are applicable to other thermoelectric materials. This investigation inspires the exploration of high-performance and eco-friendly high temperature thermoelectric material.

Insight view of thermal, mechanical and structural behavior of novel double perovskites Ba2XSbO6 (X = Sc, Y): A DFT based study

The study of double perovskite structures Ba2XSbO6 (X = Sc, Y) is carried out using the first principles algorithms within the framework of Density Functional Theory (DFT). The results showed that these compounds possess a cubic perovskite structure with the lattice space group Fm-3m generally represented by the group No. 225. The band structure of these compounds confirms their insulating nature. The calculated values of elastic parameters determined the brittle nature of Ba2ScSbO6 and Ba2YSbO6 compounds. Positive frequencies of the phonon dispersion curve indicates that these double perovskites are dynamically stable, with one acoustic and ten optical phonon modes having the combination of Raman active, Infrared active and optically inactive modes. The transport properties of these materials including electrical and thermal conductivity, as well as their thermoelectric power have been investigated. Additionally, the figure of merit was calculated to assess the suitability of double perovskites for thermoelectric devices and advanced technological applications.