Publication Date: 2024
Advanced Engineering Materials (14381656)26(22)
In this computational study, density functional theory (DFT) is employed to analyze the structural, electronic, elastic, and topological properties of ternary compounds MXY (M = Ti, Sn, Ir, X = Se, Te, Y = Se, Te). The effects of spin–orbit interaction and pressure-induced strain are investigated to understand their influence on the stability, mechanical properties, and electronic behavior, paving the way for potential technological applications. The findings confirm that these compounds are inherently stable in nonmagnetic phases, with spin–orbit interaction critically influencing their energy–volume landscapes. The calculated lattice parameters, ratios of lattice constants, and bulk moduli closely align with existing data, confirming the reliability of our approach. Mechanical assessments reveal distinct behaviors: IrSe2 exhibits the highest stiffness due to pronounced covalent bonding, contrasting with SnTe2's elastic anisotropy and SnSeTe's nearly isotropic properties. Electronically, most compounds show metallic characteristics, except SnSe2, which behaves as a semiconductor with an indirect, pressure-sensitive energy bandgap. Topological analysis under varying hydrostatic pressures indicates band inversions in TiSe2, IrSe2, and SnSeTe, suggesting topological phase transitions absent in other compounds. This study enriches our understanding of these materials and refines the application of DFT in material design. © 2024 The Author(s). Advanced Engineering Materials published by Wiley-VCH GmbH.
Publication Date: 2024
Journal of Materials Science (15734803)59(36)pp. 17079-17095
This study presents a comprehensive analysis of the electronic, mechanical, and optical properties of CrxMo1-xS2, a bulk transition metal dichalcogenide. Using density functional theory with spin–orbit interaction, we employed both the generalized gradient approximation (GGA) and the modified Becke–Johnson potential (mBJ-GGA) to evaluate these properties. Our results confirm that all CrxMo1-xS2 alloys are nonmagnetic and thermodynamically stable, as evidenced by cohesive energy calculations. Mechanical assessments comply with Born’s criteria, further affirming their stability. Interestingly, lower concentrations of Cr, particularly in Cr0.125Mo0.875S2, significantly enhance atomic bond strength and elastic stiffness. Additional mechanical analysis, including the universal elastic anisotropy index, microhardness, machinability index, and Pugh’s criterion, reveals that all alloys are anisotropic and brittle, with Cr0.375Mo0.625S2 and Cr0.625Mo0.375S2 demonstrating superior machinability. On the electronic front, the addition of Cr substantially modifies the MoS2 bandgap and the density of states near the Fermi level. Even at low Cr concentrations, a significant reduction in the energy bandgap is observed, with notable contributions from Cr-dz2 orbitals to the valence and conduction bands. Optically, we examined the dielectric constant ε(ω) components, along with absorption (α(ω)), reflection (R(ω)), and refraction (n(ω)) coefficients in both X and Z directions. An increase in Cr concentration leads to a redshift in these properties, with prominent peaks in the visible light spectrum, especially in the yellow and blue light energies. The thorough examination of electronic, mechanical, and optical properties suggests that CrxMo1-xS2 alloys hold significant potential for various applications in electronic and optical technologies, particularly in areas requiring bandgap engineering. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024.
Publication Date: 2024
Computational Condensed Matter (23522143)39
In this study, the structural, elastic, vibrational, electronic, optical, thermodynamic, and thermoelectric properties of the chalcogenide ternary Y2ZnX4 (Y = In, Ga; X = S, Se) compounds are comprehensively investigated using the pseudopotential plane-wave (PP-PW) and full-potential linearized augmented plane wave (FP-LAPW) methods. The analysis of elastic and vibrational properties reveals the dynamic and mechanical stability of these compounds. The calculated energy band gaps, ranging from 1.47 eV (Ga2ZnSe4) to 2.55 eV (In2ZnS4) in the visible spectrum, decrease as X atoms are substituted with S to Se. All examined compounds demonstrate favorable optical absorption (α > 105 cm−1) in the ultraviolet region. Notably, Ga2ZnSe4 exhibits absorption red-shift towards the visible region at hν = 2.76 eV due to its lower energy band gap, making it a promising candidate for solar cells. The three-dimensional representation of Young's modulus indicates significant deviation from sphericity, revealing anisotropic behavior in all compounds. Pugh's ratio, Poisson's ratio, and Cauchy's pressure analysis suggest ductile behavior in all four chalcogenide ternary compounds. Additionally, all compounds, except In2ZnS4, display auxetic properties. Finally, the calculated thermoelectric properties identify Ga2ZnS4 and In2ZnS4 as promising candidates for high-performance thermoelectric applications, with high Seebeck coefficients of 1848 and 1936 μV/K, respectively, and ZT values approaching unit. © 2024 Elsevier B.V.
Publication Date: 2023
Journal of Materials Science (15734803)58(24)pp. 10023-10042
This study conducts a thorough examination of the properties of four transition-metal dichalcogenides (TMDCs): WTe2, WSe2, ZrTe2, and NiTe2, using first-principles density functional theory calculations. The results reveal that WSe2 and WTe2 exhibit semiconducting behavior in both bulk and monolayer forms, while ZrTe2 and NiTe2 exhibit metallic behavior in their bulk forms. However, a deviation from metallic behavior is observed in the monolayer form of NiTe2. The study also delves into the optical characteristics of both bulk and monolayer forms, including dielectric function, reflectivity, absorption coefficient, refraction coefficient, and electron energy loss function. These findings provide a comprehensive understanding of the properties of these TMDCs, which can be utilized in the design of advanced optoelectronic devices. Moreover, the observed decrease in absorption coefficient in the monolayer forms of these TMDCs can be leveraged for transparent conductor technology. Overall, this study presents a detailed analysis of the properties of TMDCs, highlighting their potential for technological exploitation in a wide range of optoelectronic applications. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Publication Date: 2023
Physical Chemistry Chemical Physics (14639084)25(17)pp. 12182-12191
Two-dimensional (2D) topological insulators (TIs) hold great promise for future quantum information technologies. Among the 2D-TIs, the TiNI monolayer has recently been proposed as an ideal material for achieving the quantum spin Hall effect at room temperature. Theoretical predictions suggest a sizable bandgap due to the spin-orbit coupling (SOC) of the electrons at and near the Fermi level with a nontrivial 011111111100 011000010100 000000100100 000001001000 000010001000 000010110000 000100100000 000101000000 001001000010 010010000110 011111111110 2 topology of the electronic states, which is robust under external strain. However, our detailed first-principles calculations reveal that, in contrast to these predictions, the TiNI monolayer has a trivial bandgap in the equilibrium state with no band inversion, despite SOC opening the bandgap. Moreover, we show that electron correlation effects significantly impact the topological and structural stabilities of the system under external strains. We employed a range of density functional theory (DFT) approaches, including HSE06, PBE0, TB-mBJ, and GGA+U, to comprehensively investigate the nontrivial topological properties of this monolayer. Our results demonstrate that using general-purpose functionals such as PBE-GGA for studying TIs can lead to false predictions, potentially misleading experimentalists in their efforts to discover new TIs. © 2023 The Royal Society of Chemistry.
Publication Date: 2022
Materials Science and Engineering: B (09215107)281
(Si, Ge, Sn)-based alloys are generally compatible with silicon technology and offer many options to engineer the optical properties. Furthermore, topological phases in some of these alloys have been investigated. In this work, through the calculations of the first-principles, the possibility of realization of topological phases in new SnSi1-xGex alloys (x = 0.0, 0.25, 0.5, 0.75, and 1.0) by hydrostatic pressure has been investigated. Furthermore, the effects of Ge concentration and the hydrostatic pressure on the topological phase of these alloys are studied and the band inversion strength (BIS) as a function of concentration (x) and pressure is plotted in a matrix image. The calculations of electronic band structure and band inversion of these alloys within two generalized gradient approximation (GGA) and Heyd-Scuseria-Ernzerhof screened hybrid (HSE06) functional are compared. It was found that the HSE06 approach was more effective than the GGA approach in improving bandgap and BIS. The results show that the nontrivial topological phase of these alloys in both approaches is due to an s-p band inversion at the Gamma (G) point. Moreover, the calculated topological surface states and ℤ2-index confirm the topological phase transition in these alloys. According to the HSE06 approach, at x ≤ 0.5, pressure changes play an essential role in the topological phase transition, while at x > 0.5, pressure changes affect only the BIS. The SnSi1-xGex alloys are dynamically stable, and it is expected that these alloys can be experimentally synthesized. © 2022 Elsevier B.V.
Publication Date: 2022
Computer Physics Communications (0010-4655)271
We introduce a computational method and a user-friendly code with a terminal-based graphical user interface (GUI), named ElATools, developed to analyze mechanical and anisotropic elastic properties. ElATools enables facile analysis of the second-order elastic stiffness tensor of two-dimensional (2D) and three-dimensional (3D) crystal systems. It computes and displays the main mechanical properties including the bulk modulus, Young's modulus, shear modulus, hardness, p-wave modulus, universal anisotropy index, Chung-Buessem anisotropy index, log-Euclidean anisotropy parameter, Cauchy pressures, Poisson's ratio, and Pugh's ratio, using three averaging schemes of Voigt, Reuss, and Hill. It includes an online and offline database from the Materials Project with more than 13,000 elastic stiffness constants for 3D materials. The program supports output files of the well-known computational codes IRelast, IRelast2D, ElaStic, and AELAS. Four types of plotting and visualization tools are integrated to conveniently interface with GNUPLOT, XMGRACE, view3dscene and plotly libraries, offering immediate post-processing of the results. ElATools provides reliable means to investigate the mechanical stability based on the calculation of six (three) eigenvalues of the elastic tensor in 3D (2D) materials. It can efficiently identify anomalous mechanical properties, such as negative linear compressibility, negative Poisson's ratio, and highly-anisotropic elastic modulus in 2D and 3D materials, which are central properties to design and develop high-performance nanoscale electromechanical devices. Moreover, ElATools can predict the behavior of the sound velocities and their anisotropic properties, such as acoustic phase/group velocities and power flow angles in materials, by solving the Christoffel equation. Six case studies on selected material systems, namely, ZnAu2(CN)4, CrB2, δ-phosphorene, Pd2O6Se2 monolayer, and GaAs, and a hypothetical set of systems with cubic symmetry are presented to demonstrate the descriptive and predictive capabilities of ElATools. Program summary: Title: ElATools Licensing provisions: GNU General Public Licence 3.0 Nature of the problem: Identifying anisotropic elastic properties of 2D and 3D materials, and calculating acoustic phase and group velocities in homogeneous solids. Solution method: Second-order elastic stiffness tensor analysis using transformation law and calculations of the elastic surfaces properties. Solving the Christoffel equation eigenvalue problem using diagonalization and calculations of the sound velocities. Programming language: Fortran 90 Operating system: Unix/Linux/MacOS/Windows by Cygwin: http://www.cygwin.com/ Distribution format: tar.gz Required routines/libraries: LAPACK, BLAS, and Plotly Javascript libraries, GNUPLOT, XMGRACE, view3dscene. Computer: Any system with a Fortran 90 (F90) compiler Memory: Up to 1 GB for any symmetry Run time: Up to 70 seconds for any symmetry, and (400×400) = (θ×ϕ)-mesh in spherical coordinate Documentation: Available at https://yalameha.gitlab.io/elastictools/index.html © 2021 Elsevier B.V.
Publication Date: 2021
Journal of Computational Electronics (15698025)20(6)pp. 2300-2307
First-principles electronic, thermoelectric, thermodynamic, and optical calculations of an alkali pnictide compound, Li3Bi, are implemented by WIEN2k, BoltzTraP and Gibbs2 using density functional theory in the presence of spin–orbit coupling. The generalized gradient approximation and modified Becke and Janson functionals with the generalized gradient approximation are utilized for the treatment of exchange and correlation potential. The Li3Bi electronic band structure indicates that this compound is a semiconductor at zero pressure. The energy band gap of this compound closes at a pressure of 6.0 GPa. In contrast, low pressures enhance the energy band gap and reduce the band width of the valence and conduction bands. The pressure and temperature effects on the thermoelectric and thermodynamic performance of this compound are investigated. This results reveal (1) an increase in the power factor values under high temperatures and low pressures, (2) a reduction in the thermal expansion and the specific heat capacity at constant volume and an increase in the Debye temperature under high pressures at constant temperature. Also, the evaluation of optical properties under various hydrostatic pressures shows an increase in the static real part of the dielectric function and the static reflectivity of Li3Bi at a pressure of 6 GPa. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Publication Date: 2021
Materials Science and Engineering: B (09215107)273
We predict the highly stable new full-Heusler order compound Cs(Na, K)2Bi, that can take a diverse set of topological states by strain-engineering. Based on first-principles studies, our findings reveal that the hydrostatic lattice compression, uniaxial compression, and uniaxial tension can transition Cs(Na, K)2Bi to a trivial semiconductor, a normal insulator, a topological insulator, a Weyl semimetal, a Dirac semimetal, and a Nodal Line semimetal. These topological states, induced by various kinds of strain, exhibit a range of interesting optical and electronic transport properties. These results introduce Cs(Na, K)2Bi compounds as promising candidates to make novel topological devices whose properties can be controlled using strain-engineering. © 2021 Elsevier B.V.
Publication Date: 2021
Physica E: Low-Dimensional Systems and Nanostructures (13869477)134
Strain engineering is a useful approach for tuning and advancing the physical features and characteristics of two-dimensional materials, because of their large susceptibility. Also, these materials are appropriate for the nanophotonic and nanoelectronic applications. So in this work, the equilibrium lattice parameter and phonon spectra of TiS monolayer are first calculated and investigated and the dynamic stability of this monolayer is proved at different uniaxial strains. Then the strain dependence of electronic and thermoelectric properties of TiS monolayer is studied by using generalized gradient approximation plus spin-orbit interaction. Furthermore, the effect of different uniaxial strains on the TiS monolayer optical constants is investigated. The noticeable transparency and reflectivity of this monolayer are observed at low energies. © 2021 Elsevier B.V.
Publication Date: 2021
Nanomaterials (20794991)11(10)
Using first‐principles calculations, we predict highly stable cubic bialkali bismuthides Cs(Na, K)2Bi with several technologically important mechanical and anisotropic elastic properties. We investigate the mechanical and anisotropic elastic properties under hydrostatic tension and compression. At zero pressure, CsK2Bi is characterized by elastic anisotropy with maximum and minimum stiffness along the directions of [111] and [100], respectively. Unlike CsK2Bi, CsNa2Bi exhibits almost isotropic elastic behavior at zero pressure. We found that hydrostatic tension and compression change the isotropic and anisotropic mechanical responses of these compounds. Moreover, the auxetic nature of the CsK2Bi compound is tunable under pressure. This compound transforms into a material with a positive Poisson’s ratio under hydrostatic compression, while it holds a large negative Poisson’s ratio of about −0.45 along the [111] direction under hydrostatic tension. An auxetic nature is not observed in CsNa2Bi, and Poisson’s ratio shows completely isotropic behavior under hydrostatic compression. A directional elastic wave velocity analysis shows that hydrostatic pressure effectively changes the propagation pattern of the elastic waves of both compounds and switches the directions of propagation. Cohesive energy, phonon dispersion, and Born–Huang conditions show that these compounds are thermodynamically, mechanically, and dynamically stable, confirming the practical feasibility of their synthesis. The identified mechanisms for controlling the auxetic and anisotropic elastic behavior of these compounds offer a vital feature for designing and developing high‐performance nanoscale electromechanical devices. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
Publication Date: 2021
Journal of Physics and Chemistry of Solids (00223697)154
The quaternary alloys BeSiP2xAs2(1−x) with the noncentrosymmetric chalcopyrite structure are studied by means of ab initio density functional theory calculations. The calculations are shown the substitution of Asby P leads to the decrease of the volume of the unit cell and the increase of the energy band gap. Using the Zunger approach, the microscopic origins of the energy band gap bowing are examined in term of volume deformation bVD, charge exchange bCEand structural relaxation bS.The analysis of the total electron density distribution map is shown that the electrons flow from the As, Si and P atoms and accumulate into covalent bonds between the As and Si atoms and between the Si and P atoms. The effect of Pconcentration on the mechanical properties, phonon frequencies and Born effective charges Z∗ of these quaternary alloys are also investigated. The calculated results indicated that these quaternary alloys are mechanical and dynamically stable, have a brittleness character and can be regarded as elastically anisotropic materials. Moreover, the thermodynamic properties of these quaternary alloys including the heat capacity at constant volume CV, the Helmholtz free energy F and entropy S are calculated using the harmonic approximation based on the phonon density of states calculation. Finally, the nonlinear optical coefficient d36 of the quaternary alloys BeSiP2xAs2(1−x) is evaluated and the results confirm that these alloys are remarkable in the nonlinear optics. © 2021 Elsevier Ltd
Publication Date: 2021
Materials Science and Engineering R: Reports (0927-796X)145
The spin-orbit coupling field, an atomic magnetic field inside a Kramers’ system, or discrete symmetries can create a topological torus in the Brillouin Zone and provide protected edge or surface states, which can contain relativistic fermions, namely, Dirac and Weyl Fermions. The topology-protected helical edge or surface states and the bulk electronic energy band define different quantum or topological phases of matters, offering an excellent prospect for some unique device applications. Device applications of the quantum materials rely primarily on understanding the topological properties, their mutual conversion processes under different external stimuli, and the physical system for achieving the phase conversion. There have been tremendous efforts in finding new topological materials with exotic topological phases. However, the application of the topological properties in devices is still limited due to the slow progress in developing the physical structures for controlling the topological phase conversions. Such control systems often require extreme tuning conditions or the fabrication of complex multi-layered topological structures. This review article highlights the details of the topological phases, their conversion processes, along with their potential physical systems, and the prospective application fields. A general overview of the critical factors for topological phases and the materials properties are further discussed to provide the necessary background for the following sections. © 2021
Publication Date: 2021
Physica E: Low-Dimensional Systems and Nanostructures (13869477)127
In this work, some physical properties of ScX (X = Sb and Bi) nano-layers are evaluated based on DFT employing Wien2k code within generalized gradient approximation in presence of spin orbit coupling. The band order and topological phase of ScX (X = Sb, Bi) bulks are investigated and then the topological nature of these bulks is examined by the investigation of surface states topological phase of ScX nano-layers with different thicknesses. The bonding type between the surface atoms of ScX nano-layers is also investigated. Furthermore, the optical properties of ScX nano-layers are investigated and compared. The results show the high contribution of intraband transitions and consequently high reflectivity and negative real part of dielectric function of these nano-layers near zero energy. The different responses of ScX nano-layers to the radiated electromagnetic waves in polarizations of parallel and perpendicular to their surfaces imply the anisotropic optical features of these nano-layers in x (or y) and z directions. © 2020 Elsevier B.V.
Publication Date: 2020
Journal of Physics and Chemistry of Solids (00223697)145
In this study, we investigated some physical properties of AB (A = Sc, Y and B = Sb, Bi) compounds in different structural phases based on density functional theory. The effects of different hydrostatic pressures and biaxial strains on the band orders of AB compounds were investigated. The calculations were conducted with the Wien2k package in the presence of spin orbit coupling. The results showed that the topological d-p band inversion can be induced by appropriate compressive strains and spin orbit coupling. The topological phase of AB compounds was confirmed based on calculations of the Z2 topological invariant. The band inversions and strong topological phases of these compounds may make them suitable for appropriate applications in quantum computation and spintronics. Furthermore, we investigated and compared the optical properties of AB compounds, and the results indicated the high contributions of intraband transitions, high reflection, and negative real part of the dielectric function for these compounds near zero energy. © 2020 Elsevier Ltd
Publication Date: 2020
Journal of Physics and Chemistry of Solids (00223697)143
The structural, electronic, optical and elastic properties of CsCdxPb1−xCl3alloys are studied using the plane-wave pseudopotential method under the framework of density functional theory (DFT). The Perdew–Burke–Ernzerhof functional (GGA−PBE)is employed to treat the exchange and correlation potentials. The supercell approach (SC)and the virtual crystal approximation (VCA)are also used to model the CsCdxPb1−xCl3alloys. Because of the symmetry constraint in the VCAapproach, the calculated lattice parameters of these alloys only within SCapproach show linear behavior in agreement with the Vegard's law. Using the approach of Zunger and co-workers, the microscopic origin of the energy band gap bowing of these alloys is investigated in terms of the volume deformation, charge exchange and structural relaxation. The energy band gap of these alloys is also predicted to be suitable for solar absorber applications. The real and imaginary parts of the macroscopic dielectric function (MDF)of CsCdxPb1−xCl3alloys are studied using the single-particle picture within random phase approximation (RPA). The calculated static dielectric constant of these alloys increases when Cdconcentration decreases. To study the excitonic effects (EXC), the macroscopic dielectric function of these alloys is also calculated through the solution of the Bethe-Salpeter equation (BSE)using Tamm-Dancoff approximation within many-body perturbation theory. The variation of some polycrystalline structural properties of these alloys such as the Bulk modulus, Young's modulus, Poisson's ratio, Pugh's ratio, shear and Zener's anisotropy parameters as a function of the Cdconcentration are studied using perturbation density functional theory (PDFT). The calculated results show that these alloys are mechanically stable and rather elastically anisotropic. This study provides a detailed theoretical analysis of the CsCdxPb1−xCl3alloys and can give helpful guidance for further relevant research. © 2020
Publication Date: 2020
Physica E: Low-Dimensional Systems and Nanostructures (13869477)122
The investigations of quantum spin Hall effect and the edge states manipulation of two dimensional topological insulators are very salient for the practical applications and fundamental sciences. The high thermoelectric efficiency of these materials has also been confirmed, recently. So, in this study the first-principles calculations are implemented based on density functional theory in the presence of spin orbit interaction, to evaluate the topological phase transition and thermoelectric performance of XBi (X = Sc, Y) monolayers under in-plane strains. It is found that the compressive in-plane strains and spin orbit interaction, which host considerable effects on the electronic structure, can cause the topologically nontrivial phase and high thermoelectric performance. The topological state of XBi monolayers is confirmed with the Z2 topological invariant calculation. These results provide these monolayers for the novel thermoelectric and nanoelectronics quantum devices and topological phenomena. Also some optical properties of XBi monolayers are calculated and investigated under in-plane strains. The results show the considerable transparency and reflectivity of these monolayers near zero energy. © 2020 Elsevier B.V.
Publication Date: 2020
Journal of Physics Condensed Matter (09538984)32(25)
Topological insulators with novel surfaces or edge states are the topological nature sequel of bulk electronic wave functions of these materials. The observed signatures in the electronic structure of topological insulators can make them excellent candidates for thermoelectric materials. Low dimensional materials such as phosphorene and Bi2Te3 nanowire have been confirmed to be desirable for the design of devices with high thermoelectric performance. So in this work, the phonon modes, formation energy and cohesive energy of LaX (X = Sb, Bi) monolayers are first calculated and investigated. Then the band order of these monolayers is investigated by the band structure calculations and the topological phase of these monolayers is proved by using the calculation of Z 2 topological invariant. The energy band gap and the band inversion strength of these monolayers are evaluated under in-plane strains. Also, the effect of different temperatures and in-plane strains on the thermoelectric performance of LaX monolayers is studied. The results show the high thermoelectric efficiency and d-p topological band inversion of these monolayers under compressive strains. © 2020 IOP Publishing Ltd.
Publication Date: 2020
Journal of Magnetism and Magnetic Materials (03048853)503
Based on ab initio calculations, we have studied structural, electronic and optical properties of MoX2(X = S, Se) metal dichalcogenides and their nano-layers (NLs) according to the density functional theory using Wien2k code. The generalized gradient approximation (GGA) and GGA Engel-vosko are adopted to perform the exchange-correlation calculations. The equilibrium lattice parameters of MoS2 and MoSe2 compounds are calculated. The mechanical stability of these compounds is proved using Born condition. Moreover, the ductility and brittleness of MoX2 compounds are studied. The topological phase of MoS2 and MoSe2 bulks and their NLs is studied utilizing band order, Z2 invariant and surface density of states. The optical properties of MoX2 bulks and NLs are studied. The Penn model is satisfied for both MoS2 and MoSe2 compounds by studying their real part of static dielectric function. The increase of the thickness of these NLs leads to the changes in the absorption, reflectivity and energy loss function of these NLs in some specific energy ranges. © 2020 Elsevier B.V.
Publication Date: 2019
Journal of Electronic Materials (03615235)48(12)pp. 7977-7990
The Born effective charge tensors, high-frequency dielectric constants, phonon frequencies at Г symmetry point, and phonon dispersion curves of MoS2xSe2(1−x) (x = 0, 0.25, 0.5, 0.75, and 1) alloys are calculated based on density functional perturbation theory using optimized lattice parameters. The structural stability, stiffness, ductility, and plasticity of these alloys are explored in detail by calculating the polycrystalline structural properties. The calculated Born effective charges and Poisson’s ratio indicate the existence of a combination of ionic and covalent bonding between the transition metal and chalcogens in each layer. Based on the calculated phonon frequencies, the temperature dependence of the specific heat at constant volume, entropy, and Helmholtz free energy of MoS2xSe2(1−x) (x = 0, 0.25, 0.5, 0.75, and 1) alloys are calculated in the harmonic approximation. Structurally, the spatial inversion symmetry in the MoS2xSe2(1−x) (x = 0.25, 0.5, and 0.75) alloys is broken. This leads MoS2xSe2(1−x) (x = 0.25, 0.5, and 0.75) alloys to exhibit novel nonlinear optical properties that do not exist in the 2H-MoX2 (X = S, Se) compounds. So, the nonlinear optical properties are calculated by applying the 2n + 1 theorem to an electric-field-dependent energy functional. It is found that MoS2xSe2(1−x) (x = 0.25, 0.5, and 0.75) alloys exhibit remarkable large nonlinear optical susceptibility and electro-optic coefficients and would be promising candidates for use in nonlinear optical applications. Van der Waals interactions are included in all the first-principles calculations, to correctly describe the interaction between adjacent layers. The calculated lattice parameters, electronic energy bandgaps, phonon frequencies, and high-frequency dielectric constants of 2H-MoX2 (X = S, Se) compounds are in good agreement with available theoretical and experimental results. © 2019, The Minerals, Metals & Materials Society.
Publication Date: 2019
Computational Condensed Matter (23522143)21
The structural, electronic, thermodynamic, vibrational, elastic, linear and nonlinear optical properties of AlxGa1-xP ternary alloys are studied using plane-wave pseudopotential method based on the density functional theory. The alchemical mixing method is used to construct an alloy by mixing the pseudopotentials in an appropriate way. The local density approximation is used for the exchange and correlation potentials calculations using the ABINIT code. The Born effective charges, phonon frequencies, longitudinal-transvers optical splitting and thermal properties are calculated using density functional perturbation theory. By applying the many-body effects within the Bethe-Salpeter approach using the Tam-Dancoff approximation, the frequency-dependent macroscopic dielectric function is calculated. The nonlinear dielectric (electronic) susceptibility, the electro-optic tensor and the Raman tensor are investigated in the framework of density functional perturbation theory using the 2n+1 theorem. Moreover, due to density functional theory band gap problem, the band-gap correction is performed within the one-shot GW approximation of many-body perturbation theory. The calculated results of AlP and GaP compounds are in acceptable agreement with available theoretical and experimental results, so the present study on the AlxGa1-xP alloys would be helpful for future experimental and theoretical investigations. © 2019 Elsevier B.V.
Publication Date: 2019
Indian Journal of Physics (09731458)93(11)pp. 1427-1436
The WIEN2K package based on the density functional theory within the generalized gradient approximation (GGA) and GGA plus Hubbard parameter was applied to explore the structural, electronic, magnetic properties and topological phase of CeNiSb bulk and nano-layer (NL), in the presence of spin orbit coupling. Also the topological phase, band order and the linear coefficient of electronic specific heat of CeNiSb bulk and nano-layer are studied. The electronic charge distribution at this nano-layer surface, within GGA and GGA + U approaches, is calculated. © 2019, Indian Association for the Cultivation of Science.
Publication Date: 2019
Journal of Physics and Chemistry of Solids (00223697)132pp. 213-221
The structural and electronic properties of InAsxSb1-x ternary alloys are studied using plane-wave pseudopotential method based on the density functional theory. The Born effective charge, elastic constants, phonon dispersion curves, linear and nonlinear optical properties of these compounds are calculated using density functional perturbation theory. Based on the calculated phonon frequency, the specific heat at constant volume and entropy are obtained within the harmonic approximation. Moreover, due to the density functional theory band gap problem, the band gap correction is performed within the one-shot GW approximations. The distinguished role of In 4d states on the electronic energy band gap and nonlinear properties of these alloys are extensively explored by the comparison between the calculated results of Hartwigsen-Goedecker-Hutter and Trouiller-Martins-type pseudopotentials. The calculated results show that InAsxSb1-x alloys are mechanically stable and can be a good candidate for high-performance nonlinear optical material. The calculated results of InAs and InSb compounds are in acceptable agreement with available theoretical and experimental results, so the calculated results of InAsxSb1-x alloys will be helpful for future experimental and theoretical investigations. © 2019 Elsevier Ltd
Publication Date: 2019
Computational Condensed Matter (23522143)19
The first principle investigations of the structural, electronic, linear and nonlinear response properties of the zinc-blende ZnSe and ZnTe are performed based on the density functional theory using a plane-wave pseudopotential approach. The Born effective charges, piezoelectric tensor, phonon frequencies, LO–TO splitting and thermal properties of these compounds are calculated using a linear response method within the density functional theory framework. The macroscopic dielectric function is calculated in the response of the many body effects within the Bethe-Salpeter approach using the Tam-Dancoff approximation. The nonlinear response properties to atomic displacements and electric fields are investigated within the density functional perturbation theory framework based on the 2n+1 theorem as implemented by the ABINIT software. After the calculation of the Kohn–Sham electronic band structures, the correction of the energy band gap is computed using the GW approximation within the many-body perturbation theory framework (MBPT). The results are in excellent agreement with the available experimental and other theoretical results. © 2019 Elsevier B.V.
Publication Date: 2019
Solar Energy (0038092X)184pp. 372-377
On the basis of density functional calculations and using Bader's atom in molecule theory, this article presents quantitative microscopic analyses on the bonding properties of amorphous silicon (a-Si) which could reflect in the observable mechanical and electronic behaviors of this material. In addition, the occurrence and strength of quantum confinement of charge carriers in a composition of silicon crystal nano slabs (SiNSs) embedded in hydrogenated a-Si (a-Si:H) semiconductor are studied. It is shown that the strongest confinement effect happens for Si slabs limited in [1 0 0] direction. The band gap tunability with the width of SiNSs is exhibited and a scaling law is investigated for the size dependent behavior of energy states. It is demonstrated and argued why in these systems the confinement of holes is stronger than electron confinement. The computational methodology used to passivate a-Si defects by hydrogen is also detailed. © 2019
Publication Date: 2019
Indian Journal of Physics (09731458)93(5)pp. 571-582
Our previous study of XFeSi (X = La, Gd, Tb) bulks shows that Fe atom has negligible contribution to the magnetic properties of these compounds. In this paper, the contribution of Fe atom to the magnetic properties of XFeSi (X = La, Gd, Tb) nano-layers is investigated. The calculated results are performed based on the density functional theory. The exchange–correlation potential is calculated using generalized gradient approximation and generalized gradient approximation plus Hubbard parameter. The structural, electronic and magnetic properties of XFeSi (X = La, Gd, Tb) nano-layers are investigated in the presence of spin–orbit coupling. The calculated results are compared with the corresponding results of their bulks. Furthermore, the thermodynamic properties of these nano-layers are investigated using the quasi-harmonic Debye model. The bulk modulus, Debye temperature, specific heat at constant pressure and volume and thermal expansion coefficient of these nano-layers are calculated and compared with the corresponding results of their bulks. © 2018, Indian Association for the Cultivation of Science.
