Electronic Band Structure Calculations Research Articles

Miakhs carbon allotropes: Three novel two-dimensional carbon allotropes with sp2 + sp3 hybridized network

MIAKHs carbon allotropes with mixed sp2 + sp3 hybridized network have different properties. Here, three novel two-dimensional materials, namely the MIAKH20, MIAKH28, and MIAKH32 respectively, were predicted based on density functional theory (DFT). MIAKHs carbon allotropes belong to the P4mm (No. 99) space group and their structural properties, mechanical properties, mechanical and dynamical stability, and electronic properties are investigated in this work. The studies of mechanical properties indicate that these 2D materials are mechanically stable and also exhibit the isotropy behavior. In order to examine the thermal stability of these novel 2D materials, MD simulations with Tersoff potential are performed under a NVT ensemble. Our classical molecular dynamics calculations demonstrate that MIAKHs carbon allotropes are dynamically stable. The calculation of electronic band structure of MIAKH32 indicates that this structure is semiconductor with an indirect band gap of 0.53 eV, whereas band structure of MIAKH20, MIAKH28 structures indicates that both materials are metals. This research introduces new 2D polygon-like carbon allotropes and their excellent properties. The obtained results show that these three novel 2D materials can be candidate materials for applications in electromechanical systems and open a new viewpoint for the designs of novel 2D carbon allotropes based on highorder carbon rings.

Computational Prediction of Stacking Mode in Conductive Two-Dimensional Metal-Organic Frameworks: An Exploration of Chemical and Electrical Property Changes.

Conductive two-dimensional metal-organic frameworks (2D MOFs) have attracted interest as they induce strong charge delocalization and improve charge carrier mobility and concentration. However, characterizing their stacking mode depends on expensive and time-consuming experimental measurements. Here, we construct a potential energy surface (PES) map database for 36 2D MOFs using density functional theory (DFT) for the experimentally synthesized and non-synthesized 2D MOFs to predict their stacking mode. The DFT PES results successfully predict the experimentally synthesized stacking mode with an accuracy of 92.9% and explain the coexistence mechanism of dual stacking modes in a single compound. Furthermore, we analyze the chemical (i.e., host-guest interaction) and electrical (i.e., electronic structure) property changes affected by stacking mode. The DFT results show that the host-guest interaction can be enhanced by the transition from AA to AB stacking, taking H2S gas as a case study. The electronic band structure calculation confirms that as AB stacking displacement increases, the in-plane charge transport pathway is reduced while the out-of-plane charge transport pathway is maintained or even increased. These results indicate that there is a trade-off between chemical and electrical properties in accordance with the stacking mode.

To study the structural, electronic and optical properties of predicted stable halide perovskites ABX3

Perovskites have gained great interest in recent times. High efficiency solar materials have been developed from this family. Here in this work, structure, optical and electronic properties of recently artificial intelligence predicted stables perovskites ABX3 are studied through DFT approach. The structural analysis is done through the generalized gradient approximation. For the band structure calculations generalized gradient as well as the modified Becke-Johnson potential are used to investigate electrical and optical properties. The density of states and electronic band structure calculations reveal that the predicted stable halides Perovskites ABX3 have ultra-high to low band gaps. In the absence of experimental results, the calculated results can be valuable for further calculations. In the optical properties of the compounds dielectric function, optical reflectivity, absorption coefficient, optical conductivity, extinction coefficient, refractive index is calculated in different ranges of energy. The maximum value of static dielectric function is observed for KNiI3 and RbIrBr3 are 3.5 and 3 respectively. The highest absorption peak among the all is observed at 26 eV for SrLiF3, while refractive index of that SrLiF3 gives peak at 24 eV then refractive index decreases below the unity as energy increases up to 27.5 eV. The suitable energy bandgaps of the new compounds in the ultraviolet to infrared region certainly appeal for their use in the diverse applications.

First-Principles Study of B16N16 Cluster-Assembled Porous Nanomaterials.

Owing to the similar valence electron structures between the B-N bond and the C-C bond, boron nitride, similar to carbon, can form abundant polymorphs with different frameworks, which possess rich mechanical and electronic properties. Using the hollow, cage-like B16N16 cluster as building blocks, here, we established three new BN polymorphs with low-density porous structures, termed Cub-B16N16, Tet-B16N16, and Ort-B16N16, which have cubic (P4¯3m), tetragonal (P4/nbm), and orthomorphic (Imma) symmetries, respectively. Our density functional theory (DFT) calculations indicated that the existence of porous structure Cub-B16N16, Tet-B16N16, and Ort-B16N16 were not only energetically, dynamically, thermally and mechanically stable, they were even more stable than some known phases, such as sc-B12N12 and Hp-BN. The obtained Pugh's ratio showed that the Cub-B16N16 and Tet-B16N16 structures were brittle materials, but Ort-B16N16 was ductile. The analysis of ideal strength, Young's moduli, and shear moduli revealed that the proposed new phases all exhibited sizable mechanical anisotropy. Additionally, the calculation of electronic band structures and density of states showed that they were all semiconducting with a wide, indirect band gap (~3 eV). The results obtained in this work not only identified three stable BN polymorphs, they also highlighted a bottom-up way to obtain the desired materials with the clusters serving as building blocks.

Tunable Low‐Temperature Thermoelectric Transport Properties in Layered CuCr(S1‐xSex)2 System

AbstractWe have characterized the layered CuCr(S,Se)2 system for the spin‐polarized electronic band structures and low‐temperature thermoelectric transport properties. The electronic band structure calculations reveal semiconducting behavior for CuCrS2, CuCr(S0.5Se0.5)2 and CuCrSe2 with an indirect bandgap of 0.42, 0.30 and 0.10 eV, respectively. The systematically decreased bandgap with increasing Se content is in line with the experimental observations showing a semiconductor‐to‐metal transition with increasing Se‐substitution level in the CuCr(S1‐xSex)2 system because of an increase in the charge carrier density. The p‐type Seebeck coefficient shows a linear temperature dependence for the samples, like in degenerate semiconductors or metals. The remarkably large Seebeck coefficient even in metallic samples is due to a relatively large effective mass of charge carriers. As the thermal conductivity is intrinsically low owing to the layered crystal structure and is further decreased for the Se‐substituted samples because of the increased phonon scattering from point defects, the thermoelectric characteristics are promising. The highest dimensionless figure‐of‐merit values were seen for the x=0.5 sample, e. g., 0.04 at 400 K.

Fully gapped superconductivity and topological aspects of the noncentrosymmetric superconductor TaReSi

We report a study of the noncentrosymmetric TaReSi superconductor by means of the muon-spin rotation and relaxation $(\ensuremath{\mu}\mathrm{SR})$ technique, complemented by electronic band-structure calculations. Its superconductivity, with ${T}_{c}=5.5\phantom{\rule{0.28em}{0ex}}\text{K}$ and upper critical field ${\ensuremath{\mu}}_{0}{H}_{\mathrm{c}2}(0)\ensuremath{\sim}3.4\phantom{\rule{0.28em}{0ex}}\text{T}$, was characterized via electrical-resistivity and magnetic-susceptibility measurements. The temperature-dependent superfluid density, obtained from transverse-field $\ensuremath{\mu}\mathrm{SR}$, suggests a fully gapped superconducting state in TaReSi, with an energy gap ${\mathrm{\ensuremath{\Delta}}}_{0}=0.79\phantom{\rule{0.28em}{0ex}}\text{meV}$ and a magnetic penetration depth ${\ensuremath{\lambda}}_{0}=562\phantom{\rule{0.28em}{0ex}}\text{nm}$. The absence of a spontaneous magnetization below ${T}_{c}$, as confirmed by zero-field $\ensuremath{\mu}\mathrm{SR}$, indicates a preserved time-reversal symmetry in the superconducting state. The density of states near the Fermi level is dominated by the Ta- and Re-$5d$ orbitals, which account for the relatively large band splitting due to the antisymmetric spin-orbit coupling. In its normal state, TaReSi behaves as a three-dimensional Kramers nodal-line semimetal, characterized by an hourglass-shaped dispersion protected by glide reflection. By combining nontrivial electronic bands with intrinsic superconductivity, TaReSi is a promising material for investigating the topological aspects of noncentrosymmetric superconductors.

Realizing high-temperature superconductivity in borophene with Dirac states assembled by kagome and honeycomb boron layers

Boron's unique electron deficiency allows for the formation of various borophene polymorphs that have the potential for superconductivity. Herein, we have introduced a new route to realize high superconductivity in borophene through kagome and honeycomb boron layers assembly. We have designed a tri-layer borophene, named hP8-B, consisting of two AA-stacked kagome layers with a honeycomb layer serving as the interlayer. hP8-B possesses high superconductivity with an estimated Tc of 35.6 K, holding the highest value in any elemental 2D materials. The high Tc is mainly contributed by the strong coupling of σ-bonding electrons of the kagome layers and in-plane vibrational modes, which differs from that in other superconductive borophene polymorphs. Electronic band structure calculations showcase the existence of a Dirac cone near the Fermi level, indicating that hP8-B may be a potential topological superconductor. The superconductivity of hP8-B can be enhanced to 46.4 K under a biaxial tensile strain of 3% and doping density of 0.0375 holes per atom, due to the softening of the in-plane vibrational modes.

Investigation of the Electronic Structure of the Mg<sub>99.2</sub>Zn<sub>0.2</sub>Y<sub>0.6</sub> Alloy Using X-ray Photoelectron Spectroscopy

We investigated the electronic structure of the Mg99.2Zn0.2Y0.6 alloy using hard and soft X-ray photoemission spectroscopy and electronic band structure calculations to understand the mechanism of the phase stability of this material. Electronic structure of the Mg99.2Zn0.2Y0.6 alloy showed a semi-metallic electronic structure with a pseudo-gap at the Fermi level. The observed electronic structure of the Mg99.2Zn0.2Y0.6 alloy suggests that the presence of a pseudogap structure is responsible for phase stability.

Orbital Hybridization and Hypersensitivity of Eu3+ in YXO4 (X=P, As, V)

A comparative study of Eu3+ ion luminescence in YXO4 (X=P, As, V) with the tetragonal zircon structure is conducted in relation to the intensity of the hypersensitivity 5D0 → 7F2 transition. Both the asymmetry ratio, and the Judd-Ofelt Ω2 intensity parameter increases in the order YPO4 < YAsO4 < YVO4. This correlation is interpreted qualitatively in terms of the covalency and polarizability of (XO4)3−, which increases in the order (PO4)3− < (AsO4)3− < (VO4)3−. The trend is supported by the results of electronic band structure calculations of the three compounds which establish the strength of hybridization between the X cation and the oxygen 2p states. The electronic structure of YAsO4 is calculated to probe the covalence of As–O bonding. The increasing oscillator strength of the Eu3+5D0 → 7F2 transition in going from YPO4 to YAsO4 to YVO4 is consistent with the expectation of ligand dipolar polarization model for hypersensitivity which states that the oscillator strength of the 5D0 → 7F2 transition is proportional to the square of the ligand dipolar polarizability. The connection between the mechanism of hypersensitivity and second harmonic generation (SHG) is presented.

Spin-polarized two-dimensional electron/hole gas at the interface of nonmagnetic semiconducting half-Heusler compounds: Modified Slater-Pauling rule for half-metallicity at the interface

Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.

Structural, electronic, magnetic and thermoelectric properties of Tl2 NbX6 (X = Cl, Br) variant perovskites calculated via density functional theory.

This article presents detailed structural, electronic, magnetic, and thermoelectric properties of two experimentally existing isostructural variant perovskite compounds Tl2 NbX6 (X = Cl, Br) with the help of first principles calculations. As per requirement of stability in the device applications, the structural and thermodynamic stabilities were, respectively verified by tolerance factor and negative formation energies. The structural parameters in ferromagnetic phase were calculated and found in close agreement with the available experimental results. The electronic nature was found as half metallic from spin polarized calculations of electronic band structures and density of states, where the semiconductor nature was found in the spin down states and metallic nature in the spin up states. The magnetic moments of both the compounds were calculated as 1 μB majorly contributed by Nb atom. The Boltzmann transport theory was implemented via BoltzTraP for calculating the spin resolved thermoelectric parameters, such as Seebeck coefficient, electronic and thermal conductivities, and figure of merit. Overall, both the compounds were found suitable for use in spintronics and spin Seebeck effect for energy applications.

A DFT investigation of mechanical, optical and thermoelectric properties of double perovskites K2AgAsX6 (X = Cl, Br) halides

Focusing on renewable energy device applications, we explored double perovskites K2AgAsX6 (X = Cl, Br) halides for the first time. We used WIEN2k (all-electron method) and BoltzTrap code to investigate optoelectronic and thermoelectric properties for renewable energy device applications. For structural and elastic calculations, we employed PBEsol generalized gradient approximations and found that both halides in the cubic phase are structurally stable. Further, Pugh's and Poisson's predicted values indicate that both halides are ductile. Furthermore, we also calculate the formation energy to check the thermodynamical stability of both halides. For accurate calculations of optoelectronic properties, we applied Tran-Bhala modified Becke and Johnson potential (TB-mBJ). Electronic band structure calculations indicate both halides are indirect bandgap nature having values of 2.1 eV and 1.6 eV, respectively, and might be potential candidates for solar cell applications. We also discussed the optical parameters within the incident photon energy range of 0–12 eV. We found strong absorption in the visible region for K2AgAsX6 (X = Cl, Br) halides indicating both halides are appropriate solar cell devices. Lastly, we calculate thermoelectric parameters against temperature and chemical potential. A probing figure of merit shows halides is also potential candidates for thermoelectric device applications.

DFT calculations on physical properties of the lead-free halide-based double perovskite compound Cs2CdZnCl6

Lead-free halide double perovskite materials have recently attracted considerable interest from the scientific community due to their vast potential in optoelectronic applications without toxicity issues. Herein, we report a new lead-free halide double perovskite, namely Cs2CdZnCl6, based on DFT calculations. We found that Cs2CdZnCl6 is mechanically stable in the cubic nonmagnetic phase. The Electronic band structure calculations revealed that this compound exhibits a direct semiconductor band gap of 1.43 eV. The dielectric function ε, reflectivity R, refractive index n, absorption coefficient α, and optical conductivity σ are analyzed up to 13.0 eV. Furthermore, it is found that the studied compound has a significant photoresponse in the ultraviolet light region. Finally, the findings suggest that Cs2CdZnCl6 is a promising material for optoelectronic applications.

Two-dimensional [formula omitted] (X = Si, Ge; Y = S, Se, Te) family with potential application in photocatalysis

Photocatalysis is an attractive route for energy harvesting and pollution mitigation, however, designing an optimal catalyst is rather challenging. Motivated by fundamental interest and potential applications, the investigations of two-dimensional (2D) photocatalyst are important subjects in solar energy conversion. In this work, the stability, electronic, and optical properties of 2D Cr2X2Y6 (X = Si, Ge; Y = S, Se, Te) family are investigated to explore their photocatalytic activity for water splitting. Density functional theory (DFT) and ab-initio molecular dynamics (AIMD) calculations were employed. The electronic band structure calculations indicate that all these monolayers, except Cr2Ge2Te6 and Cr2Si2Te6, have an adequate bandgap, as well as suitable band-edge positions for photocatalytic water splitting reactions, specifically under acidic conditions. Moreover, the studied materials show strong capability of harvesting sunlight, that is essential for photocatalytic activity. Remarkably, Cr2Ge2Se6 and Cr2Si2Se6 monolayers exhibited high solar-to-hydrogen energy conversion efficiencies of 14.89% and 13.26%, respectively. The presented results pave the way for designing other electronically optimized layered photocatalysts for water splitting.

Rise and fall of Mott insulating gaps in YNiO3 paramagnets as a reflection of symmetry breaking and remaking

The ${\mathrm{YNiO}}_{3}$ nickelate is a paradigm $d$-electron oxide that manifests the intriguing temperature-mediated sequence of three phases transitions from (i) magnetically ordered insulator to (ii) paramagnetic (PM) insulator and then to (iii) PM metal. Such phenomena raised the question of the nature of the association of magnetism and structural symmetry breaking with the appearance in (i) and (ii) and disappearance in (iii) of insulating band gaps. It is demonstrated here that first-principles mean-field--like density-functional theory (DFT), driven by molecular dynamics temperature evolution, can describe not only the origin of the magnetically long-range ordered insulating phase (i), but also the creation of an insulating paramagnet (ii) that lacks spin- long-range order, and of a metallic paramagnet (iii) as temperature rises. This approach provides the patterns of structural and magnetic symmetry breaking at different temperatures, in parallel with band gaps obtained when the evolving geometries are used as input to DFT electronic band-structure calculations. This disentangles the complex interplay among spin, charge, and orbital degrees of freedom. Analysis shows that the success in describing the rise and fall of the insulating band gaps along the phase transition sequence is enabled by allowing sufficient flexibility in describing diverse local structural and magnetic motifs as input to DFT. This entails the use of sufficiently large supercells that allow expressing structural disproportionation of octahedra, as well as a description of PM phases as a distribution of local magnetic moments (rather than using a single averaged moment). It appears that the historic dismissal of mean-field--like DFT as being unable to describe such Mott-like transitions was premature, as it was based on consideration of averaged crystallographic unit cells, a description that washes out local symmetry-breaking motifs. The magnetically ordered insulating ${\mathrm{YNiO}}_{3}$ phase (i) and the PM insulating phase (ii) result in DFT from allowing symmetry breaking, evident already by considering the athermal internal energy. In contrast, the PM metallic phase (iii) is formed thermally by smearing out thus weakening symmetry breaking. Analysis of snapshots of the different forms of structural vs magnetic symmetry breaking shows that only the loss of the polymorphous distribution of magnetic moments existing in (ii) causes the fall of the band gap, resulting in the metallic state in (iii). The interesting conclusion is that such a description of the rise [in phases (i) and (ii)] and fall [in phase (iii)] of the insulating gap does not rely on the traditional Mott-like strong correlation understanding, but on breaking and remaking of magnetic and structural symmetries reflected in energy lowering.

Structural, electronic and thermoelectric properties of SnTe with dilute co-doping of Ag and Cu

The low bandgap semiconductor, SnTe is receiving significant attention as a thermoelectric material because of its low toxicity and environment-friendly nature. In this study, we report the effect of co-doping dilute concentrations of Ag and Cu ions in SnTe that suppresses the Sn vacancies leading to optimized thermoelectric properties. Samples of nominal chemical composition Sn1.03–2xAgxCuxTe (x = 0, 0.01, 0.02, 0.04) were prepared by the solid-state route. The Rietveld refinement of powder XRD of these compounds showed a fcc (Fm3¯m) structure with no other impurity phases. Diffuse reflectance IR spectroscopy showed an increase in the bandgap upon Ag-Cu co-doping in SnTe, associated with valence band convergence. Electronic band structure calculations confirmed an increase in the bandgap along with a reduction in the energy difference between light and heavy valence bands having maxima at the L and Σ points. Partial density of states (P-DOS) calculations showed that Ag-Cu doping in SnTe does not contribute towards the formation of resonant energy levels. The Seebeck coefficient S of the Sn1.01Ag0.01Cu0.01Te reached a maximum value of ∼ 95 μV/K at 783 K, compared to 86 μV/K of pure SnTe. The power factor increased with the doping concentration, reaching ∼ 10.8 μWK−2cm−1 at 783 K for x = 0.04. The lattice thermal conductivity κL decreased on Ag-Cu co-doping, with κL = 0.44 Wm−1K−1 above 750 K for x = 0.04, which is close to the Cahill model. The combination of higher power factor and reduced lattice thermal conductivity in the Ag-Cu co-doped samples resulted in enhanced ZT = 0.24–0.29 at 773 K.

Pressure-induced ferroelectric-to-superconductor transition in SnPS3

Ferroelectricity and superconductivity are prominent yet distinct quantum phenomena; materials exhibiting both of these phenomena are rare and of great fundamental and practical interest. Here, combining ab initio calculations and in situ measurements, we show that ferroelectric ${\mathrm{SnPS}}_{3}$ turns into a superconductor under pressure at 31.7 GPa, accompanied by a structural phase transition. Electronic band structure calculations reveal a partial flat band near the Fermi energy in compressed ${\mathrm{SnPS}}_{3}$, suggesting correlation effects as a possible origin of the observed superconducting state. The discovery of a pressure-induced ferroelectric-to-superconductor transition in ${\mathrm{SnPS}}_{3}$ raises the prospect of establishing this intriguing quantum phenomenon among a large class of metal phosphorous trichalcogenides, thereby broadening the material basis to elucidate the underlying physics.

Insight into the structural, electronic, optical, and elastic properties of niobium carbide

ABSTRACT The structural parameters, electronic, optical, and elastic properties of Niobium Carbide (NbC) compound within four different structures: rock-salt (RS), wurtzite (WZ), cesium chloride (CsCl), and Nickel Arsenide (NiAs) are examined using the Full-Potential Linearized-Augmented Plane Wave (FP-LAPW). The exchange–correlation potential (VXC) has been treated by Perdew, Burke, and Ernzerhof's generalized gradient approximation (PBE-GGA) when structural properties, transition pressure, and elastic properties are estimated. For electronic properties, in addition to (PBE-GGA), the modified Becke–Johnson (mBJ-GGA) was used for increased accuracy. Present results show that the RS of NbC is the most stable among the four examined structures with the lowest equilibrium energy. The calculated lattice constants are in good agreement with the former calculations. The elasticity and the formation energy calculations show that NbC is stable within the four studied structures. The electronic band structure calculations of NbC show that it has a metallic nature in the four considered structures.

Understanding the electronic, structural, optical, photovoltaic and thermoelectric properties of Cs2GeSnBr6 by first-principles and SCAPS-1D simulation

The electronic, optical, structural, thermoelectric and photovoltaic properties of Cs2GeSnBr6 are investigated by using density functional theory and SCAPS-1D simulation. There is a lack of imaginary frequency in the phonon dispersion band structure implies that Cs2GeSnBr6 is stable against lattice distortion. Also, Goldsmith tolerance factor (τ), octahedral factor (µ), and convex-hull calculations demonstrated that Cs2GeSnBr6 can be synthesized. In addition, electronic band structure calculation using GGA-PBE+mBJ+SOC shows that Cs2GeSnBr6 has a direct band gap value of 1.01 eV. Further, optical calculations revealed the higher value of absorption coefficient (α) and optical conductivity (σ) of Cs2GeSnBr6. Motivated by this, we have modelled the ITO/ZnO/Cs2GeSnBr6/Au photovoltaic cell which shows a maximum power conversion efficiency of 18.74%. Moreover, the obtained large ZT value Cs2GeSnBr6 which is originating from its good electrical conductivity with poor thermal conductivity.

First-principles study of structural, elastic, electronic and optical properties of cubic perovskite AgXF3 (X = Be, Mg, Ca and Sr) compounds

The structural, elastic, electronic and optical properties of cubic fluoroperovskite crystal AgXF3 (X=Be, Mg, Ca and Sr) were studied using the first-principles method. The optimized lattice constant a0 increases by varying the cation from Be to Sr, which is consistent with previous reports. The calculations of electronic band structure show that AgBeF3, AgMgF3 and AgCaF3 have indirect band gaps, while AgSrF3 has direct band gaps. The band gaps of AgBeF3, AgMgF3, AgCaF3 and AgSrF3 are 1.820, 1.901, 2.074 and 1.795 eV, respectively, which all exhibit semiconducting properties. It is further found that as the cation changes from Be to Sr, the top of the valence band approaches the Fermi level. The nature of chemical bonds in these compounds is discussed by the electron localization function. The optical properties of AgXF3 (X=Be, Mg, Ca and Sr), including the reflectance, absorption, refractive index, dielectric function, conductivity and loss function have been studied, and the wide range of absorption energies makes these materials promising for significant applications in semiconductor fields.