Electronic, optical, and thermoelectric characteristics of (Ae)xFBiS2 (Ae=Sr, Ba, and x=1.7) layered materials useful in optical modulator devices

The recent discovery of superconductivity behavior in the mother BiS2-layered compounds has captivated the attention of several physicists. The crystal structure of superconductors with alternate layers of BiS2 is homologous to that of cuprates and Fe-based superconductors. The full-potential linearized augmented plane-wave (FP-LAPW) technique was utilized to investigate the electronic structures and density of states in the vicinity of the Fermi energy of SrFBiS2 and BaFBiS2 compounds under the electron carriers doping. The introduction of electron doping (carries doping) reveals that the host compounds SrFBiS2 and BaFBiS2 exhibit features indicative of superconductivity. This carrier doping of SrFBiS2 and BaFBiS2 compounds (electron-doped) has a significant impact on the lowest conduction states near the Fermi level for the emergence of the superconducting aspect. The electron doping modifies and induces changes in the electronic structures with superconducting behavior in (Ae)1.7FBiS2(Ae=Sr,Ba) compounds. A Fermi surface nesting occurred under the modification of electrons (carriers) doping in the host compounds SrFBiS2 and BaFBiS2. Furthermore, the optical characteristics of the carrier-doped SrFBiS2 and BaFBiS2 compounds are simulated. Due to the anisotropic behavior, the optical properties of these materials based on BiS2 demonstrate a pronounced polarization dependency. The starting point at zero photon energy in the infrared region is elucidated by considering the Drude features in the optical conductivity spectra of SrFBiS2 and BaFBiS2 compounds, when the electron carriers doping is applied. It was clearly noticed that the spin-orbit coupling (SOC) influences the electronic band structures, density of states, Femi surface, and optical features because of the heavy Bismuth atom, which may disclose fascinating aspects. Further, we conducted simulations to assess the thermoelectric properties of these mother compounds. The two BiS2-layered compounds could be suitable for practical thermoelectric purposes and are highlighted through assessment of electrical conductivity, thermal conductivity, Seebeck coefficient, and power factor. As a result, we propose that the mechanisms of superconducting behavior in BiS2 family may pave new avenues for investigating the field of unconventional superconductivity. It may also provide new insights into the origin of high-Tc superconductivity nature.

Ab-initio study of strain-tunable g-GaN/BN nanoheterostructure for optoelectronic and photocatalytic applications

CONTEXT

Two-dimensional (2D) nanoheterostructures of materials, integrating various phase or materials into a single nanosheet have stimulated large-scale research interest for designing novel two dimensional devices. In contemporary analysis present work, we examined the structural and electronic properties of the isolated 2D BN and GaN monolayers. We have investigated the structural stability and optoelectronic and photocatalytic response of the g-GaN/BN nanoheterostructure along with its response to strain. Nanoheterostructure g-GaN/BN is predicted to be a direct bandgap semiconductor with wide gap of 4.45 eV, whose value can be effectively modulated by applied strain ( ϵ ) , ranging from 4.55 ( ϵ = - 4%) to 3.58 eV ( ϵ = 8%). We also discovered that the tensile strain of 8% can substantially tune the direct bandgap of nanoheterostructure to indirect band gap nature. Even more important, the biaxial tensile strain engineering accentuates an enhancement of optical absorption in the UV region, broadening the light harvesting of the g-GaN/BN nanoheterostructure with the shifting of first absorption peak from 4.64 ( ϵ = - 4%) to 3.71 eV ( ϵ = 8%). Furthermore, strain-tuned band edge potentials arrangement perfectly fits the water reduction and oxidation redox potentials. Our findings portend that the g-GaN/BN nanoheterostructure has application in prospective nanoscale optoelectronic devices and photocatalytic hydrogen evolution system.

METHODS

First principles calculations in this study are performed using density functional theory. Generalized gradient approximation within PBEsol functional employed to address the electron-electron exchange-correlation effects. For avoiding periodic interactions between the layers, we have inserted a vacuum region of thickness 10 Å in the z-direction. For ensuring the convergence accuracy of the computed results, convergence criteria of the iteration process is set to be 0.0001 eV. Local modified Becke-Johnson, a semi local functional, is applied for calculating electronic and optical properties for more accuracy of results. As in layered 2D nanoheterostructure, a factual depiction of the van der Waals interactions cannot be provided by conventional DFT techniques. Accordingly, in order to incorporate these interactions, we had employed the dispersion correction method of Grimme's.

Pressure-driven modification of optoelectronic features of ACaCl3 (A = Cs, Tl) for device applications

Intending to advance the use of halide-perovskites in technological applications, in this research, we investigate the structural, electronic, optical, and mechanical behavior of metal-halide perovskites ACaCl3 (A = Cs, Tl) through first-principle analysis and assess their potential applications. Due to the applied hydrostatic pressure, the interaction between constituent atoms increases, thereby causing the lattice parameter to decrease. The band structure reveals that band gap nature transits from indirect to direct at elevated pressure. Moreover, at high pressure, the electronic band structure shows a notable band gap contraction from the insulator (>5.0 eV) to the semiconductor region, which makes them promising for electronic applications. The charge density map explores the ionic and covalent characteristics of Cs/Tl-Cl and Ca-Cl under pressured and unpressurized environments. Induced pressure enhances the optical conductivity as well as the optical absorption that moves toward the low-energy region (red shift), making ACaCl3 (A = Cs, Tl) advantageous for optoelectronic applications. Additionally, this study reveals that the mechanical properties of ductility and anisotropy were found to be improved at higher pressures than in ambient conditions. Overall, this study will shed light on the technological applications of lead-free halide perovskites in extreme pressure conditions.

Structural, electronic and thermoelectric properties of monolayer TiSe2

CONTEXT AND RESULTS

In this work the first-principles calculations of the structural, electronic and thermoelectric properties of monolayer TiSe2 are presented. The optimized lattice parameter of monolayer TiSe2 shows excellent agreement with the experimental value. The computed band structure and density of states calculations predict metallic nature of monolayer TiSe2 with overlapping of 0.44 eV between the lowest conduction band and top valance band at high symmetry point M. The position of pseudogap formed by Ti-3d orbitals near the Fermi level confirms the mechanical stability of monolayer TiSe2. Due to the influence of positive strain (tensile strain), the Ti-Se bond length increases and the layer height decreases. The applied tensile strain changes the metallic nature of TiSe2 into a semiconductor with opening of bandgap. It has also been observed that the positions of conduction band minima and valance band maxima change with strain. The charge analysis shows that charge transfer from Ti to Se atom increases when tensile strain is applied, while an opposite trend is observed with compression. The computed thermoelectric coefficients i.e. Seeback coefficient, power factor and figure of merit are in good agreement with the experimental data. The temperature dependence of these coefficients is also reported.

COMPUTATIONAL METHOD

The density functional theory based calculations are reported employing the PBE-GGA ansatz using the plane wave-pseudopotential method embodied in the Quantum ESPRESSO package. The self-consistent field calculations are performed over a dense Monkhorst-Pack net of 12 × 12 × 1 k-points. The energy convergence criteria for the self-consistent field calculation were set to 10-6 Ry/atom with a cutoff energy of 90 Ry. The thermoelectric properties are computed by combining the band structure calculations with the Boltzmann transport equation using Boltztrap2 peckage.

Deep Earth Chronicles: High-Pressure Investigation of Phenakite Mineral Be2 SiO4

Beryllium silicate, recognized as the mineral phenakite (Be2 SiO4 ), is a prevalent constituent in Earth's upper mantle. This study employs density-functional theory (DFT) calculations to explore the structural, mechanical, dynamical, thermodynamic, and electronic characteristics of this compound under both ambient and high-pressure conditions. Under ideal conditions, the DFT calculations align closely with experimental findings, confirming the mechanical and dynamical stability of the crystalline structure. Phenakite is characterized as an indirect band gap insulator, possessing an estimated band gap of 7.83 eV. Remarkably, oxygen states make a substantial contribution to both the upper limit of the valence band and the lower limit of the conduction band. We delved into the thermodynamic properties of the compound, including coefficients of thermal expansion, free energy, entropy, heat capacity, and the Gruneisen parameter across different temperatures. Our findings suggest that Be2 SiO4 displays an isotropic behavior based on estimated anisotropic factors. Interestingly, our investigation revealed that, under pressure, the compression of phenakite is not significantly affected by bond angle bending.

Dataset on the crystallographic, vibrational, and electronic properties of 1M-phlogopite K(Mg,Fe)3(Si3Al)O10(OH)2 obtained from Density Functional Theory investigations

The present work reports a dataset on the crystal structure, optical properties (complex dielectric function and refractive index), infrared, reflectance and Raman spectra, and electronic properties (band structure and density of states) of the 1M-polytype of phlogopite [1]. This phyllosilicate presents chemical formula K(Mg,Fe)3(Si3Al)O10(OH)2, with Mg/Fe ratio ≥ 2. The dataset was obtained from Density Functional Theory (DFT) simulations at B3LYP-D* level, i.e., with the hybrid functional B3LYP corrected with an ad hoc DFT-D2 scheme, and all-electron Gaussian-type orbitals basis sets for all atoms in the unit cell. Furthermore, experimental confocal Raman micro-spectrometry data (spectra) collected on a single crystal phlogopite specimen are reported. The quality of the dataset was assessed by comparing the results with available X-ray diffraction and IR/Raman spectroscopy data reported in literature. The reported complete dataset is a reference for future studies in fundamental georesource exploration and exploitation, applied mineralogy, geology, and material science.

Electronic band structure and anisotropic optical properties of bulk and monolayer fullerene networks

We theoretically investigate the local electron density, electronic band structure, density of state, dielectric function, and optical absorption of the bulk and monolayer C₆₀ network structures, based on the latest experimental synthesis [Nature, 2022, 606, 507]. The results show that the ground state electrons are concentrated on the bridge bonds between clusters, the bulk and monolayer C₆₀ network structures have strong absorption peaks in the visible and near infrared regions, and the monolayer quasi-tetragonal phase C₆₀ network structure shows strong polarization dependence. Our results not only provide insights into the physical mechanism of optical absorption of the monolayer C₆₀ network structure, but also reveal potential applications of the C₆₀ network structure in photoelectric devices.

First-principles investigations for the electronic and transport properties of zigzag SiC nanoribbons with Fluorine passivation/adsorption

Nanoribbons with different edge functionalization display interesting electronic properties for various device applications. It requires the necessity of exploring the novel passivating elements commensurate to various technological applications. In this direction, here we have compared the effect of H and F-passivation on the edges of zigzag SiC nanoribbons (ZSiCNR) using density functional theory based calculations. Remarkably, present study reveals that F could be used as an effective passivating element for ZSiCNR similar to widely explored H-passivations. Various possible combinations of F/H are found to have stable structural integrity for practical applications. The effect of F-adatom adsorption is also discussed which present peculiar electronic properties. The half-metallic behavior is observed to be realized via F-adsoprtion which is further confirmed with the transport calculations. The obtained negative differential resistance along the spin dependent electron transport pledges towards wide spread applications of considered ZSiCNR interacting with F.

Four New Sb-based Orthophosphates: Cation Regulation to Investigate Diversified Structural Architecture

In the work, four new Sb-based phosphates, K4(SbO2)5(PO4)3, Rb(SbO2)2PO4, Rb3(SbO2)3(PO4)2 and Cs3(SbO2)3(PO4)2(H2O)1.32, were successfully synthesized by a high-temperature melt method. Among them, Rb(SbO2)2PO4 and Rb3(SbO2)3(PO4)2 are the first reported examples of Rb-containing alkali metal Sb-based phosphates. They show three-dimensional (3D) frameworks composed of [Sb8P4O30]∞ layer for K4(SbO2)5(PO4)3 and [Sb6P2O20]∞ layer for Rb(SbO2)2PO4, and 2D lamellar structure composed of [Sb3P2O10]∞ for Rb3(SbO2)3(PO4)2 and Cs3(SbO2)3(PO4)2(H2O)1.32. A detailed structural comparison shows that the structure dimensions for them transfer from 1D to complex 3D framework with the increase of (Sb+P)/O ratio, which affects performances of the compounds. Optical property and energy band structure calculations were also carried out based on the density functional theory (DFT). The present study enriches the diversity of Sb-based phosphates and paves the way for further explore their optical properties in the future.

Persistent surface states with diminishing gap in MnBi2Te4/Bi2Te3 superlattice antiferromagnetic topological insulator

Magnetic topological quantum materials (TQMs) provide a fertile ground for the emergence of fascinating topological magneto-electric effects. Recently, the discovery of intrinsic antiferromagnetic (AFM) topological insulator MnBi₂Te₄ that could realize quantized anomalous Hall effect and axion insulator phase ignited intensive study on this family of TQM compounds. Here, we investigated the AFM compound MnBi₄Te₇ where Bi₂Te₃ and MnBi₂Te₄ layers alternate to form a superlattice. Using spatial- and angle-resolved photoemission spectroscopy, we identified ubiquitous (albeit termination dependent) topological electronic structures from both Bi₂Te₃ and MnBi₂Te₄ terminations. Unexpectedly, while the bulk bands show strong temperature dependence correlated with the AFM transition, the topological surface states with a diminishing gap show negligible temperature dependence across the AFM transition. Together with the results of its sister compound MnBi₂Te₄, we illustrate important aspects of electronic structures and the effect of magnetic ordering in this family of magnetic TQMs.

Electronic properties and vibrational spectra of (NH4)2M″(SO4)2·6H2O (M = Ni, Cu) Tutton's salt: DFT and experimental study

The complex crystals of the family of the Tutton's salt have been investigated through the numerous experimental and theoretical studies to understand their physical properties and their potential technological applications. In spite of the more than 60 years of research, there are very few studies about the electronic properties of Tutton's salt. In our present work, we have calculated the stability, electronic properties and the first theoretical study of band structure of the three different crystals of the Tutton's salt, ammonium nickel sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O), ammonium nickel-copper sulfate hexahydrate ((NH₄)₂Ni_0.5Cu_0.5(SO₄)₂·6H₂O) and ammonium copper sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O) with the help of periodic ab-initio calculations based on density functional theory (DFT). In addition to this, the internal Raman and FTIR modes of the ionic fragments [Ni(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺ NH₄⁺ and SO₄^2- of the sample crystals were obtained by employing the ab initio and the orientation of the molecular vibrations of the ionic fragments have been presented in picturized form. Furthermore, the Raman and FTIR spectroscopy of the sample crystals were measured in the range 100-4000 cm^-1 and 400-4000 cm^-1 respectively, and the internal vibrational modes obtained from experimental measurement have been compared with those obtained from DFT calculations.

Formation of Boron-Carbon Nanosheets and Bilayers in Boron-Doped Diamond: Origin of Metallicity and Superconductivity

The insufficient data on a structure of the boron-doped diamond (BDD) has frustrated efforts to fully understand the fascinating electronic properties of this material and how they evolve with doping. We have employed X-ray diffraction and Raman scattering for detailed study of the large-sized BDD single crystals. We demonstrate a formation of boron-carbon (B-C) nanosheets and bilayers in BDD with increasing boron concentration. An incorporation of two boron atoms in the diamond unit cell plays a key role for the B-C nanosheets and bilayer formation. Evidence for these B-C bilayers which are parallel to {111} planes is provided by the observation of high-order, super-lattice reflections in X-ray diffraction and Laue patterns. B-C nanosheets and bilayers minimize the strain energy and affect the electronic structure of BDD. A new shallow acceptor level associated with B-C nanosheets at ~37 meV and the spin-orbit splitting of the valence band of ~6 meV are observed in electronic Raman scattering. We identified that the superconducting transitions occur in the (111) BDD surfaces only. We believe that the origin of Mott and superconducting transitions is associated with the two-dimensional (2D) misfit layer structure of BDD. A model for the BDD crystal structure, based on X-ray and Raman data, is proposed and confirmed by density functional theoretical calculation.