Terms Of Dielectric Function Research Articles

Effect of strain on the electronic structure and optical properties of Cr-doped monolayer MoS2.

In this paper, the electronic and optical properties of Cr-doped monolayer MoS2 under uniaxial tensile strain are investigated by first-principle calculations. It is shown that uniaxial tensile strain can significantly change the electronic and optical properties of Cr-doped monolayer MoS2, and the bandgap value of the intrinsic MoS2 system gradually decreases with the increase of tensile strain, while the bandgap value of the Cr-doped MoS2 system is relatively stable. However, when the stretching reaches a certain degree, both the intrinsic and doped systems become metallic. From the analysis of the density of states, it is found that new electronic states and energy levels appear in the intrinsic MoS2 system and all Cr-doped monolayer MoS2 systems with the increase of the tensile strain, but the changes in the density of states diagrams of the Cr-doped monolayer MoS2 system are relatively small, which is mainly attributed to the effect of the Cr-doped atoms. The analysis of optical properties displays that the stretched doped system differs from the intrinsic MoS2 system in terms of dielectric function, absorption and reflection, energy loss function, and refractive index. Our results suggest that uniaxial tensile strain can be used as an effective means to modulate the electronic structure and optical properties of Cr-doped monolayer MoS2. These findings provide a theoretical basis for understanding the optoelectronic properties of MoS2 and its doped systems as well as their applications in optoelectronic devices. Based on the first principle of density functional theory framework and the CASTEP module in Materials Studio software (Perdew et al. in Phys Rev Lett 77(18):3865-3868, 1996). The structure of Cr atom-doped MoS2 systems and MoS2 systems were optimized using the generalized gradient approximation plane-wave pseudopotential method (GGA) and Perdew-Burke-Ernzerhof (PBE) generalized functions under 3%, 6%, and 9% tensile deformation, and the corresponding formation energy, bond length, electronic structure, and optical properties of the models were analyzed. The Grimme (J Comput Chem 27(15):1787-1799, 2006) vdW correction with 400 eV cutoff was used in Perdew-Burke-Ernzerhof (PBE) functional to optimize the geometry until the forces and energy converged to 0.02 eV/Å and 1.0e-5eV/atom, respectively. For each model structure optimization, the K-point grid was assumed to be 4×4×1, using the Monkhorst-Pack special K-point sampling method. After the MoS2 supercell convergence test, the plane-wave truncation energy was chosen to be 400 eV. Following geometric optimization, the iterative accuracy converged to no less than 1.0×10-5 eV/atom for total atomic energy and less than 0.02 eV/Å for all atomic forces. We created a vacuum layer of 18 Å along the Z-axis to prevent the impact of periodic boundary conditions and weak van der Waals forces between layers on the monolayer MoS2. In this paper, a total of 27 atoms were used for the 3×3×1 supercell MoS2 system, which consists of 18 S atoms and 9 Mo atoms.

Mechanical and Optical Properties of Topological Semimetal Compound YPtBi

We have reported the mechanical properties of topological semimetals half-Heusler compound YPtBi with LDA and GGA approximation which is implemented in density functional theory. We have calculated elastic parameters which ensure good machinability, covalent bonding, brittleness, low value of Kleinman parameter and high Vickers hardness. Our results reveal the hardness or large resistance of these topological semimetals. Moreover, Born mechanical stability conditions are well fulfilled by the topological semimetal YPtBi. Present study reveals that the low value of bulk modulus and shear modulus wheras high value of Youngs modulus of this topological semimetals which deforms easily with applied external force. We have also calculated optical properties of topological semi-metal YPtBi with both LDA and GGA. Optical properties are calculated in terms of dielectric function and we have calculated dielectric constant, optical reflectivity, absorption co-efficient, optical conductivity, refractive index and electron energy loss in the energy range 0 – 14 eV. We have found higher dielectric constants with GGA in comparison to LDA that imply YPtBi is excellent materials in solar cell applications. Also, YPtBi possess high refractive index in the visible range and it is optically isotropic.

Opto-electronic and thermoelectric properties of free-lead inorganic double perovskites Rb/Cs2ScAuI6 for energy devices

Nowadays, the search for renewable and sustainable energies is a global concern worldwide. In this regard, extensive studies are devoted to inorganic perovskites; nonetheless, the latter suffers from several drawbacks, such as lead toxicity. In this paper, structural stability, optoelectronics and thermoelectric characteristics of double perovskites (DPs) Rb/Cs2ScAuI6 have been systematically explained. The phonon dispersion is evaluated for structural existence, and the formation energy is determined to confirm thermodynamic stability. In addition, the elastic constants fulfilled the Born stability criteria, and Pugh’s ratio (B/G) confirms the ductile nature of studied DPs. The obtained results for electronic characteristics of Rb/Cs2ScAuI6 using mBJ+SOC and HSE06 show that the indirect bandgap nature has bandgap values 1.40/1.95 eV and 1.36/1.90 eV, respectively. Further, optical characteristics of DPs have been investigated in terms of dielectric function up to incident photon energy 0–12 eV. Besides, the transport behavior in terms of temperature, carrier concentration, and chemical potential calculated using the BoltzTrap code indicates that both DPs low thermal conductivity, high electrical conductivity, and high Seebeck coefficient.

Elastic, electronic, optical and thermoelectric properties of Ca5Si2N6 and Sr5Ge2N6 ternary nitrides

In order to expand the scope of application of ternary nitrides and explore more thoroughly their potential as novel materials, ab initio calculations founded on density functional theory are executed to examine the mechanical, electronic, optical and transport properties of Ca5Si2N6 and Sr5Ge2N6 nitrides. These compounds are thermodynamically stable established on coherent energy and enthalpy of formation, with Ca5Si2N6 having the best stability. The estimated values of elastic constants and their derived properties suggest that the items under examination are mechanically stable, ductile and elastically anisotropic. Electronic properties have been evaluated using the GGA and TB-mBJ exchange-correlation potentials. The band structure calculated via TB-mBJ reveals that these nitrides are semiconductors, where Ca5Si2N6 has a direct energy gap of 3.55 eV and Sr5Ge2N6 an indirect band-gap of 3.15 eV. By combining Debye temperature and band-gap values, it was concluded that the nitridosilicate compound can be used as phosphor material. According to the optical response, which was examined in terms of dielectric function, complex refractive index, absorption, reflectivity and energy damage function, the potential applications of Ca5Si2N6 and Sr5Ge2N6 have been discussed. Further, thermoelectric properties are computed utilizing the semi-classical Boltzmann transport notion. The obtained results show a large strength operator and depressed thermal conductivity driving to rising figure of merit values, particularly at 300 K (0.978–0.986), signifying that the studied ternary nitrides are favorable nominees for thermoelectric employments at both low and room temperatures.

Direct band gap double perovskite halide Cs2ScInCl6 for optoelectronic applications—A first principle study

Due to their applications in optoelectronic devices, double perovskite halides have recently received a lot of interest. The crystal structure of double perovskite halides is simple and stable, which makes them more fascinating for research in optoelectronic applications. In the present work, Cs2ScInCl6 double perovskite halide has been studied using DFT-based simulations as implied in the WIEN2k package. The goldsmith’s tolerance factor varies and the double perovskite halide is structurally stable in the cubic phase. The ductile nature of Cs2ScInCl6 is varied by greater values than the critical values of Pugh’s and Poisson’s ratio. The direct bandgap is calculated using GGA + SOC and modified Becke-Johnson (mBJ) exchange potentials. The bandgap value of 0.637eV is computed using mBJ exchange calculation. The optical characteristics of Cs2ScInCl6 are investigated in terms of dielectric functions. Our computed optical properties clearly show the optimum absorption of light in the infrared (IR) region, indicating that Cs2ScInCl6 is capable of being used in optoelectronic devices.

First-principles investigation on narrow bandgap InSb1−xBix dilute bismide alloys for highly efficient long-wavelength infrared optoelectronics

The electronic structure, optical properties and crystal structure of the dilute bismide InSb1−xBix alloys with (0 ≤ x ≤ 0.125) have been studied using the DFT FP − LAPW method within the novel TB-mBJ and WC-GGA models. The lattice constant increases quasi-linearly in InSb1−xBix for small Bi contents. Based on calculations of the formation energy, we demonstrated the thermodynamic stability of the alloys. The small content of Bi into InSb1−xBix results in a large bandgap narrowing by ∼ 264 meV up to a composition of 0.125 covering the mid- and long wavelength IR ∼ 3.4–12.5 μm spectral range. From DOS analysis, a bandgap reduction in the InSb1-xBix can be explained by both upward movement of the valence band (VB) induced by the resonance interaction of Bi-6p states at the top of VB and the downward moves of In-s and Sb/Bi-p states at the conduction band (CB). A semiconductor-semimetal transition in InSb1−xBix at × ∼ 0.137 was expected. The optical properties in terms of dielectric functions (ε1, ε2), refractive index n, extinction coefficient k, reflectivity R and the absorption coefficient α were calculated for photon energies of 0–8 eV and compared to experimental data. The sharp energy structures appear in various optical spectra associated with interband transitions from occupied In-s, Sb-p and Bi-p orbitals located at VBmax to unoccupied In-s/p, Sb-p and Bi-p states located at CBmin. Good absorbance in the ultraviolet range was observed, which is useful for solar cells design. InSb1−xBix/InSb bismide alloy is an attractive candidate for realizing highly efficient light-emitting sources and photodetectors operating in the long-wavelength infrared spectral range (λLWIR ≥ 8 μm).

Calculations of the Structural, Electronic, Optical, and Elastic Parameters of CdSiX2 (X = P, As) Compounds Based on First‐Principles Theory

Density functional theory is used to investigate the structural, electronic, optical, and mechanical aspects of the chalcopyrites CdSiP2 and CdSiAs2 using both the generalized gradient approximation and the local density approximation. The substitution of As for P alters the lattice constants, elastic constants, and dielectric characteristics considerably. The theoretical calculations correspond reasonably well with experimental data and other theoretical computations. The results of the electronic band structure analysis indicate that all of the compounds investigated are semiconductors with direct bands located at the Γ point. CdSiP2 possesses the highest elastic constants and bulk modulus, whereas CdSiAs2 has the highest values of shear modulus and Young's modulus. Moreover, specific focus is also given to obtaining the orientation‐dependent linear compressibility and Young's modulus. In bulk, CdSiP2 and CdSiAs2 are isotropic, although their Young's modulus is anisotropic. In addition, the optical response of the compounds is examined in the energy range of 0–14 eV in terms of dielectric functions, optical reflectivity, refractive index, extinction coefficient, optical conductivity, and electron energy loss. The computed optical results indicate that all compounds exhibit optical polarization anisotropy, making them suitable for optoelectronic devices.

The study of optical and thermoelectric behaviour of thalium based flouropervoskite (TlSiF3) for photovoltaic and renewable energy applications by DFT

In the present study, structural, electronic, optical, thermodynamic and thermoelectric properties of perovskites TlSiF3 are explored using first-principles calculations based on density functional theory with different exchange-correlation approximations (LDA, PBE-GGA, WC-GGA, PBE-sol GGA, and mBJ-GGA). The computed structure, electronic and charge density plots using PBE-GGA ​+ ​TB-mBJ approximation show that TlSiF3 is an indirect band gap semiconductor (1.73 ​eV gap along with R– M direction) having ionic and covalent bonding. The optical response is analysed and explained in terms of dielectric function, refractive index, extinction coefficient, optical conductivity, reflectivity, and absorption coefficient in the energy range from 0 to 10 ​eV. The thermoelectric transport behaviour of TlSiF3is computed as a function of temperature and chemical potential, μ−EF. Further, the thermoelectric transport behavior of TlSiF3 is computed as a function of temperature and chemical potential μ−EF (eV). It has been observed that highest value of ZT (0.43) is attained at 1000 ​K with a power factor (PF) of 9.3 ​× ​1010WK−2m−1s−1for a fixed chemical potential. Under varying chemical potential at 300K in n-type region, it exhibits ZT value of 0.98, which is required for thermoelectric devices. Furthermore, pressure as well as temperature dependence of the thermodynamic parameters like Debye temperature, heat capacity, bulk modulus, thermal expansion coefficient, heat capacity, have been estimated with the help of GIBBS program based on quasi-harmonic Debye model.

Pressure induced structural, electronic and optical properties of wurtzite beryllium monoxide (w-BeO) from first-principle calculations

All-electron density functional calculations of structural, electronic and optical properties of wutrzite beryllium oxide (w-BeO) are carried out at different applied hydrostatic pressure. Calculated lattice constants decrease (a, c), while bulk modulus increases with increasing pressure. Direct fundamental band-gap (Eg) of w-BeO increases with increasing pressure and vice versa and the variation provides first and second order pressure coefficient of band-gap. The calculated volume deformation potential of band-gap of w-BeO is −11.5972 eV. Optical properties of w-BeO at zero and high pressures have been studied in terms of dielectric function ε(ω) and related ancillary optical parameters. Each optical parameter is marginally anisotropic. Calculated components of refractive index at each pressure signify uniaxial characteristics of the w-BeO. Calculated components of static dielectric constant, refractive index and reflectivity decreases, while critical point energy in the spectra of each component of ε2(ω), k(ω), σ(ω) and α(ω) increases with increasing pressure and vice versa. Calculated optical energy-gap (Eopt) of w-BeO increases like Eg with increasing pressure and vice versa. The transmission cutoff energy (ECutoff) is shifted towards the higher energy region with increasing pressure.

Electronic, optical, elastic, mechanical and vibrational properties of hexagonal h-ZnS with density functional theory

In this work, we explored the electronic, optical, elastic and phonon effect in two-dimensional hexagonal Zinc Sulphide (h-ZnS) monolayer. We investigated which aspects of the orbitals of Zinc and Sulphur are the sources of core levels, valence band level and conduction band level in the bandstructure. We wrote the optical parameters in terms of dielectric functions. We observed that the absorption peak represents the optical bandgap. Using the linear response scheme, we determined the phonon dispersions and noticed the transverse-acoustic is the main source in precisely describing the phase transition from the bulk ZnS to h-ZnS monolayer. We also report that h-ZnS has a direct bandgap of 2.58 eV and the material is brittle, based on the B/G ratio. We notice, however, the elastic constants of the h-ZnS are somewhat different from the bulk. We further report that the use of the hybrid functionals is significant, while computationally demanding, in estimating the electronic bandstructure, compared to PBE-GGA, which overestimates the c axis and underestimates an axis.

Insights into the structural, electronic and optical properties of MgA2B4(A = Sc, Y; B = S, Se) spinel compounds: Direct energy band gap materials

Direct bandgap bulk materials are very important for the optical applications. It is therefore important to predict new materials with the desired properties. In the present work, density functional theory is applied to study different physical properties of MgA2B4 (A = Sc, Y; B= S, Se) spinel compounds. Generalized gradient approximation is used to study the structural and elastic parameters while modified Becke Johnson exchange potential is applied to calculate electronic band profiles and optical properties. The studied compounds are stable in cubic structure. Also, energy bandgaps of the compounds are of direct nature. Therefore, these compounds can find useful applications in the optoelectronics devices. Optical properties of these compounds are studied in terms of dielectric function, refractive index, extinction coefficient, optical conductivity and reflectivity. The transport parameters like electrical conductivity and thermal conductivity are also evaluated.

Ab initio study of electronic, optical and thermoelectric character of intermetallic compounds XGa3 (X = Fe, Ru, Os)

The electronic, optical and thermoelectric response of inter-metallic semiconductor MGa3 (M = Fe, Ru, Os) in the tetragonal phase have been evaluated using PBEsol–mBJ approximation under the framework of DFT. The electronic properties demonstrate the indirect band gap values of these materials. The optical behavior predicted in terms of dielectric function, extinction coefficient, refractive index, optical conductivity, absorption coefficient, energy loss factor, and reflectivity. Moreover, the development of studied materials by changing the temperature is investigated by calculating the thermoelectric properties using BoltzTraP code. Therefore, ab initio calculations of the novel compounds is beneficial for the industry for their promising applications in energy renewable devices.

First principle study of electronic, mechanical, optical and thermoelectric properties of CsMO3 (M = Ta, Nb) compounds for optoelectronic devices

The electronic, mechanical, optical and thermoelectric properties of Cesium based perovskites CsMO3 (M = Nb, Ta) in the cubic phase has been performed through PBEsol-mBJ scheme in the framework of DFT. The electronic band structures and density of states show the studied materials having a direct band gap in the visible range. The mechanical stability and ductile behavior have been analyzed from elastic constants. Moreover, the optical behavior of the studied materials has been analyzed in terms of dielectric functions, refractive index, extinction coefficient, absorption coefficient, optical conductivity, reflectivity and energy loss factor. Finally, the material response with temperature has been elaborated by electrical conductivity, thermal conductivity, Seebeck coefficient, power factor, heat capacity, Hall coefficient, susceptibility and electron density by using BoltzTraP code. This first principle calculation of optical and thermoelectric properties of the novel compounds provides a new route to the experimentalist for the potential application in energy renewable devices.

Modelling the contribution of semi-core electrons to the dielectric function

Many aspects of the interaction of charged particles with matter can be expressed in terms of the dielectric function ε(q,ω). The dielectric function is relatively well known in the optical limit (q=0) but, for example, stopping power calculations require the knowledge of the dielectric function for q≠0. Several approaches have been used to extend the dielectric function to q≠0 for both the valence electrons, using variations of the Lindhard dielectric function and the core levels where an atomic description can be used. The intermediate case of shallow core levels is somewhat problematic. Here collective effects modify the atomic picture, and the hydrogenic approximation of the wave function is less accurate. In this paper we describe a new extension scheme of the contribution to the energy loss function of shallow core levels to q≠0 and show that tor Al and Si it describes the experimental stopping data somewhat better than previous approaches. It also describes reasonably well the proton and electron induced ionization cross section of these shallow core levels. Lastly we investigate the dielectric function in the limit of high momentum, where it can be interpreted as a Compton profile. An approximation scheme in terms of Mermin dielectric function with an amplitude that depends on q works in all cases quite well.

The pressure-induced mechanical and optoelectronic behavior of cubic perovskite PbSnO3 via ab-initio investigations

The density functional theory based full-potential linear-augmented-plane-wave plus local-orbital method has been used to study the physical properties of PbSnO3 in hypothetical cubic perovskite. An external pressure up to 40 GPa has been applied on PbSnO3 to realize the variation in its electronic band structure and the subsequent optical properties. The stability of the PbSnO3 has been investigated by the mechanical properties, the enthalpy of formation and Goldschmidt tolerance factor. Moreover, the Born criteria have been adopted to justify the mechanical stability of the PbSnO3 perovskite. We show that the electronic bandgap of PbSnO3 can be tailored from indirect to direct band gap at high symmetry (X-X) direction at an external pressure of magnitude ~ 26 GPa. The effect of pressure on the optical properties has been studied in terms of dielectric function, absorption, refraction, reflection, and optical loss factor. The application of hydrostatic pressure has shifted the maximum absorption toward the visible range, revealing that PbSnO3 can be used for high- pressure optoelectronic applications.

First-principles study on the physical properties of a layered ZnO with hexagonal α-BN structure

Abstract A layered ZnO with α-BN structure has been studied using first-principles calculations based on density functional theory. The physical properties of the layered ZnO are calculated in terms of dielectric function, infrared reflectance, elastic coefficients, modulus, hardness, and heat capacity. The layered ZnO exhibits a good infrared reflectance with a broad reststrahlen band covering the infrared band below 600 cm−1. The layered ZnO is predicted to be a material behaving in a brittle manner, with a microhardness ~3.6 times higher than that of the wurtzite ZnO. The temperature-dependent thermodynamic functions suggest that the layered ZnO has the thermal properties similar to those of wurtzite ZnO, but having a little higher Debye temperature above room temperature.

Theoretical investigation of band gap and optical properties of ZnO1−xTex alloys (x = 0, 0.25, 0.5, 0.75 and 1)

In this study, the special quasi-random structure (SQS) approach has been considered for structural, electronic and optical properties of rock-salt (RS) and zinc-blende (ZB) phases of ZnO1−xTex (x=0, 0.25, 0.5, 0.75 and 1) using density functional theory. The Wu–Cohen generalized gradient approximation (GGA) has been employed for optimizing the lattice parameters (a0) and bulk moduli (B0) in both phases which show reasonable agreement with numerous theoretical and experimental results. To compute the band gaps with high degree of precision, we employed Engel–Vosko GGA and modified Becke and Johnson local density approximation (mBJLDA) functionals. In the RS phase, metallic nature of the compounds under investigation is evident for 0.25<x<1, whereas direct band gap appears at all concentrations in ZB phase. The density of states (total and partial) are presented to comprehensively analyze the electronic structure of all compounds. Contrary to earlier theoretical studies, the mBJLDA band gap values for the end binaries appear to be significantly improved showing overall better agreement with the experimental data. The optical properties of ZnO1−xTex alloys in the ZB phase are also discussed in terms of dielectric function which show their potential utilization in optoelectronic devices.

First principle study of CsSrM3 (M=F, Cl)

Structural, elastic, chemical bonding and optoelectronic properties of materials are important to recognize their applications in technology. In the present work CsSrM3 (M=F, Cl) are investigated to obtained their structural, elastic, chemical bonding and optoelectronic properties using the highly accurate full potential linearized augmented plane wave method. The lattice constant increases while bulk modulus decreases going from F to Cl in CsSrM3. Both the compounds are found to have indirect bandgaps. CsSrF3 is found to have a mixed bonding nature, while it is purely ionic bonding in CsSrCl3. Optical responses of the materials are calculated in terms of dielectric functions, refractive indexes, reflectivities, energy loss functions, optical conductivities and absorption coefficients. Both the materials can be used for high frequency optical and optoelectronic devices, also they can be used for transparent coatings.

Optical characterization of rocksalt Pb1–xSnx Te alloys

AbstractThe optical properties in terms of dielectric function ϵ (ħω) = ϵ1(ħω) + iϵ2(ħω) of rocksalt Pb1–x Snx Te alloys (0 ≤ x ≤ 1) are investigated experimentally by spectroscopic ellipsometry and theoretically by first‐principles electronic band structure calculation. We find a strong optical response in the 0.5–2.0 eV region arising from optical absorption around the LW‐line of the Brillouin zone. The response peak of the imaginary part of the dielectric functions at E = 1.6–1.8 eV shifts towards lower energies for high Sn compositions as a consequence of narrower W‐point band‐gap Eg(W) for the Sn‐rich alloys. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Infrared ellipsometry characterization of porous silicon Bragg reflectors

We investigate porous silicon Bragg reflectors in a nondestructive manner using variable angle-of-incidence infrared spectroscopic ellipsometry. In addition to the thickness, volume porosity, inhomogeneity, and optical anisotropy, properties of the solid content of the porous material are investigated in terms of dielectric function and surface chemistry. The material was found to have positive birefringence. The high sensitivity of the technique is employed to detect and identify infrared resonant absorptions related to different Si-H as well as Si-O-Si vibrational modes. The average electrical resistivity of the solid content of the porous material is determined to be 0.03 Omega cm, which is larger than the corresponding bulk value of 0.019 Omega cm. Furthermore the average carrier concentration in the porous material shows a decrease from 6.2 x 10(18) cm(-3) to 4 x 10(18) cm(-3).