Real Dielectric Function Research Articles

Study of optoelectronic and thermoelectric properties of K2XSbCl6 (X= In, Au) double perovskites for energy conversion devices: A DFT insight

Density functional theory (DFT) method based on first principles calculations are performed to explore the novel halide perovskites K2InSbCl6 and K2AuSbCl6. This study explores them for the first time to investigate their structural, electronic, optical, and thermoelectric properties. After relax the structure, we computed some parameters such as lattice constant (21.2195, 20.256) bohr, ground states energy (−32681.329187 eV, −59010.453287 eV), and formation energy (−2.06 eV, −1.433 eV) observed for K2XSbCl6 (X = In, Au) compounds respectively. The K2InSbCl6 reveals a direct band gap (1.03 eV), while K2AuSbCl6 shows an indirect band gap (1.01 eV). Hence, both materials express semiconducting behavior. Furthermore, the calculated optical characteristic is favorable for optoelectronics devices. The real dielectric function ε1(ω) for K2XSbCl6 (X = In, Au) 4.0 and 4.25 and the absorption coefficient shows maximum absorption in the visible region. The thermometric response of both compounds is calculated with Boltzmann's theory. To check the material efficiency, considered the thermal parameters, electrical conductivity, thermal conductivity, Seebeck coefficient, and power factor. The zT values of K2InSbCl6 is 0.74 and 0.77 for K2AuSbCl6. The present study of these novel compounds provides an overview of the physical properties of optoelectronics and thermoelectric applications.

Investigation of Structural, Electronic, and Optical Characteristics of a Novel Perovskite Halide, Mg3AsCl3, for Electronic Applications

In the present research article, the structural, electronic, and optical characteristics of a novel perovskite halide, Mg3AsCl3, for a broad range of applications are thoroughly investigated. The characteristics of Mg3AsCl3 perovskite are investigated via the use of the generalized gradient approximation (GGA), Perdew–Burke–Ernzerhof and Heyd–Scuseria–Ernzerhof (HSE06) functionals, which are employed in conjunction with the linear combination of atomic orbital calculator. The structural, electrical, and optical characteristics of the material are investigated using density‐functional theory. In this study of the electronic characteristics, it is found that Mg3AsCl3 exhibits an indirect energy bandgap of 2.49 eV with GGA and 3.54 eV with HSE06. The complex band structure exhibits characteristics that indicate the presence of evanescent waves, which are characterized by a layer separation of 5 Å. The report in this research presents reflectivity values of 0.06328 and 0.02971 with GGA and HSE06 functionals, respectively. The reported values of the extinction coefficient are 3.36487 × 10−5 and 2.07389 × 10−5 calculated with GGA and HSE06 functionals. The value reported for the real dielectric function Re[ε] is 2.79625 with GGA and 2.00658 with HSE06. Overall, in these findings, an overview is given that Mg3AsCl3 holds potential as a candidate for use in a wide range of electronic device applications.

Unveiling the thermoelectric, optical and mechanical performances of Hafnium and Zirconium based Beryllium oxide perovskite

The investigated research study includes ab-initio calculations for Beryllium based oxide perovskite namely XBeO3 (X= Zr and Hf). The study aims to comprehend their mechanical, optical, and thermoelectric properties. The density functional theory is used in conjunction with the full potential linearized augmented plane wave method via WIEN2K code to conduct these studies. ZrBeO3 and HfBeO3 both met the mechanical requirements for stability while having an anisotropic character. Mechanical characteristics demonstrate the brittleness of ZrBeO3 and the ductility of HfBeO3. The determined band profile of both materials shows semiconducting behavior with indirect bandgap. Optical properties are performed within photon energy range of 0–12 eV. ZrBeO3 and HfBeO3 are anticipated to have real dielectric function values of 5.45 and 3.75, respectively. The upward trend of absorption coefficient towards higher energies signifies attenuation in light transmittance through the materials making them strong candidate as UV-absorber. The thermoelectric parameters are finally assessed using the BoltzTrap code and their variances with temperature and chemical potential are also presented. ZrBeO3 and HfBeO3 exhibit intriguing thermoelectric characteristics, with TE efficiency values of 0.24 and 0.4, respectively. Additionally, thermoelectric study shows that ZrBeO3 and HfBeO3 are stable at room temperature and have potential applications in thermoelectric devices.

Computational Study of the Effect of the Size-Dependent Dielectric Functions of Gold Nanomaterials on Optical Properties

The effect of size on the optical properties of gold nanomaterials has been studied using the theoretical Drude–Sommerfield model. The real and imaginary parts of the dielectric function of bulk as a function of wavelength due to free electron contribution and the real and imaginary parts of the dielectric function of nanogold materials as a function of wavelength due to free electron and bond electron contribution are calculated. The real and imaginary parts of the dielectric function of bulk as a function of wavelength due to the free electron contribution graph and The real and imaginary parts of the dielectric function of nanogold materials as a function of wavelength due to free electron and bond electron contributions are plotted. As we observed from the graphs, the real dielectric functions of both bulk and nanogold materials are inversely proportional to wavelength. The imaginary part of the dielectric function of bulk gold materials is independent of wavelength. At high wavelengths, the size of the gold nanomaterial is highly influenced by both real and imagined dielectric functions at high waves. As the wavelength increases, the effect of the size on the dielectric function also increases. The size-dependent dielectric function of nanomaterials is highly influenced by their optical properties and electrical structure.

Growth optimization, optical, and dielectric properties of heteroepitaxially grown ultrawide-bandgap ZnGa2O4 (111) thin film

Ultrawide bandgap ZnGa2O4 (ZGO) thin films were grown on sapphire (0001) substrates at various growth temperatures with a perspective to investigate the electrical and optical characteristics required for high-power electronic applications. Due to the variation in the vapor pressure of Zn and Ga, severe loss of Zn was observed during pulsed laser deposition, which was solved by using a zinc-rich Zn0.98Ga0.02O target. A pure phase single-crystalline ZGO thin film was obtained at a deposition temperature of 750 °C and an oxygen pressure of 1 × 10−2 Torr. The out-of-plane epitaxial relationship between the sapphire and ZGO thin film was obtained from φ-scan. The x-ray rocking curve of the ZGO thin film grown at 750 °C exhibits a full width at half maximum of ∼0.098°, which indicates a good crystalline phase and quality of the thin film. Core-level x-ray photoelectron spectroscopy of ZGO grown at 750 °C indicated that Zn and Ga were in the 2+ and 3+ oxidation states, respectively, and the atomic ratio of Zn/Ga was estimated to be ∼0.48 from the fitted values of Zn-2p3/2 and Ga-2p3/2. The high-resolution transmission electron microscopy images revealed a sharp interface with the thickness of the ZGO film of ∼265 nm, and the signature of minor secondary phases was observed. The bandgap of the ZGO film at different growth temperatures was calculated from the ultraviolet-diffuse reflectance spectroscopy spectra, and its value was obtained to be ∼5.08 eV for the 750 °C grown sample. The refractive index (n) and the extinction coefficient (k) were determined to be ∼1.94 and 0.023 from the ellipsometric data, respectively, and the real dielectric function (ɛr) was estimated to be ∼6.8 at energy 5 eV. The ultrawide bandgap and dielectric function of ZGO recommend its possible potential applications in deep-ultraviolet optoelectronic devices and high-power electronics.

Ab-initio calculation of halogen elements effect on the behavior of new double perovskite Cs2InSbX6 (X = F, Cl, Br, I): 1st part structural and optoelectronic properties

In recent years, there has been a growing fascination with lead-free perovskite materials, particularly in the realms of solar energy and photonics. It is imperative to conduct comprehensive theoretical investigations of these materials to effectively guide experimental pursuits. In this research, we harnessed the power of density functional theory (DFT) calculations, employing the Wien2k code, to delve into the distinctive characteristics of double-perovskite halides Cs2InSbX6 (X = F, Cl, Br, I). The viability and stability of their cubic structures were meticulously assessed, taking into consideration essential metrics such as the tolerance factor, octahedral factor and formation energy. Notably, the pronounced influence of spin–orbit coupling necessitated gap energy adjustments, achieved through a dual-pronged approach utilizing the generalized Perdew–Burke–Ernzerhof gradient approximation (PBE-GGA) and the modified Becke-Johnson (mBJ) exchange potential. The intricacies of band structure and state density were scrutinized using the GGA + mBJ method. The comprehensive findings elucidate that these compounds manifest fundamental electronic bandgaps spanning a range from 0.26 to 2.67 eV. Furthermore, they exhibit remarkably low carrier effective masses and a substantial degree of band dispersion. Of significance is the profound influence of halogen atom substitutions on their optical properties. Notably, a high absorption coefficient in the ultraviolet spectrum and the presence of a negative real dielectric function provide strong evidence for the adoption of metallic properties within a specific energy range. These compelling outcomes underscore the potential of these materials as highly promising candidates for a diverse array of photonic applications.

Quantum control of substitutional effects on the S-involved optoelectronic properties of BeSxSe1-X ternary alloys

The research on the quantum control of substitutional effects in BeSxSe1-x ternary alloys opens up exciting opportunities in semiconductor device engineering, photovoltaic, optoelectronic integration, and quantum information processing. By understanding and manipulating the optoelectronic properties of these alloys, researchers can pave the way for advanced technologies with enhanced performance, efficiency, and functionality in various fields. We are investigating how S alloying affects the structural, electronic and optical properties of tin chalcogenide materials. We are assessing the energy band structure and optical properties of BeSxSe1-x (x = 0%, 25%, and 50%) materials using density functional theory (DFT) within a full potential linearized augmented plane wave method (FP-LAPW) framework. The theoretically expected band energy gaps of BeSxSe1-x compounds indicate that S alloying improves their electrical and optical performances, based on the assumptions used in our calculations. The majority of the bonds in the BeSxSe1-x materials (x = 0%, 25%, and 50%) are covalent. All ternary alloys and binary compounds with (x = 0%, 25%, and 50%) exhibit direct (R-Γ) band gap semiconductor behavior, with a band gap energy (Eg). Each alloy system shows a nonlinear increase in the band gap (Eg) with increasing x. As BeSxSe1-x compound belongs to strongly correlated systems, therefore modified Becke–Johnson potential is used to calculate formation enthalpy (ΔHf), and optical properties of the compounds. The intense peaks in the imaginary part of the dielectric function εi(ω) in each spectrum are contributed by chalcogen Se, Be, and S optical excitations. Similar to the trend of band gap variation (Eg) with x, the zero-frequency limit of each imaginary dielectric function as well as the extinction coefficient, refractive index, and real dielectric function, exhibit a corresponding trend of change.

Optical properties and electron characteristics of noble-metal-dielectric oxide nanostructures with covered graphene layer

A comparative study of surface plasmon resonances (SPR) and optical properties of metal/dielectric/graphene multilayer films based on high conductive Ag and Cu metals is presented. For measurements of the generation of surface plasmon polaritons under the resonant condition, we involved the total internal reflection ellipsometry and direct spectroscopic ellipsometry for determining optical properties. SPR was studied in graphene/HfO2/Ag(Cu)/Cr/substrate nano-hetero-structures which contain an additional graphene surface layer for enhancement of their sensing performance. The plasmonic performance of each hybrid nano-hetero-structure is evaluated based on quality factors and propagation length of surface plasmon polaritons. The SPR in the graphene/HfO2/Ag(Cu)/Cr/substrate hybrid nano-hetero-structures demonstrate a strong correlation with the measured macroscopic optical characteristics such as the effective real dielectric function ε1(λ) and optical conductivity σ(λ) and extracted from them effective microscopic parameters: the plasma and relaxation frequencies of free electrons ωP and γ. Our results on graphene/HfO2/Ag(Cu)/Cr/substrate hybrid nano-hetero-structures indicate that the real part of the effective dielectric function ε1(λ) and optical conductivity σ(λ) generally follows effective medium approach and the stronger SPR occurs at higher values of ε1(λSPR), and the ratio of ωP2/γ and lower ones of σ (λSPR). Going forward, it appears that graphene/HfO2/Ag(Cu)/Cr/substrate hybrid nano-hetero-structure designs are more suitable for surface plasmon biomedical sensor applications because they could provide both tunability and low optical loss due to specific electronic properties of graphene.

Isotropic pressure-induced electronic band structure of BaTiO3, SrTiO3 and CaTiO3 with its impact on structural and optical properties: ab-initio calculation

ABSTRACT The present theoretical study offers a comprehensive investigation under the application of systematic external static isotropic pressure, phase stability, electronic-band structure and their impact on the optical characteristics of BaTiO3, SrTiO3 and CaTiO3. All the computational works were performed by Cambridge Serial Total Energy Package, using generalised gradient approximation and density functional theory, Perdue-Burke-Ernzerhof, along with the ultra-soft pseudo-potential for the frozen core. According to structural properties, there is no phase transformation and the structure of all compounds remains cubic. The band structure is a perfect tool to observe the nature of a material, such as an insulator, semiconductor or metal. When the pressure is applied, the band gap of BaTiO3, SrTiO3 and CaTiO3 increases and retains their indirect nature. The total/partial/elemental density of states has been estimated for the full description of the band structure. To verify the authenticity of BaTiO3, SrTiO3 and CaTiO3, various optical parameters have been reported such as the real dielectric functions, imaginary dielectric functions, absorption, refractive index, extinction coefficient, energy loss function and reflectivity. All discrepancies in structural parameters, band structure and optical properties occurred due to applied pressure, and BaTiO3, SrTiO3, and CaTiO3 are connected .

First-principles calculations to investigate structural, elastic, electronic, thermodynamic, and thermoelectric properties of CaPd3B4O12 (B = Ti, V) perovskites

This study has explored numerous physical properties of CaPd3Ti4O12 (CPTO) and CaPd3V4O12 (CPVO) quadruple perovskites employing the density functional theory (DFT) method. The calculated lattice constants show inclinable compliance with the experimental results that ensure their structural stability. The mechanical permanence of these two compounds was observed by the Born stability criteria as well. The mechanical and elastic behaviors have been rationalized to investigate elastic constants, bulk, shear, and Young’s modulus, Pugh’s ratio, Poisson’s ratio, and elastic anisotropy indexes. The ductility and anisotropic indexes confirm that both materials are ductile and anisotropic in essence. The band structure of CPTO reveals a 0.88 and 0.46 eV direct narrow band gap while using TB-mBJ and GGA-PBE potentials, respectively, which is an indication of its fascinating semiconducting nature. Whereas, CPVO perovskite exhibits a metallic character. The calculated partial density of states indicates the strong hybridization between Pd-4d and O-2p orbital electrons for CPTO, whereas Pd-4d and V-3d-O-2p for CPVO. The study of the chemical bonding nature and electronic charge distribution graph reveals the coexistence of covalent O-V/Pd bonds, ionic O-Ti/Ca bonds, as well as metallic Ti/V-Ti/V bonding for both compounds. The Fermi surface of CPVO ensures a kind of hole as well as electron faces simultaneously, indicating multifarious band characteristics. The prediction of the static real dielectric function (optical property) of CPTO at zero energy implies its promising dielectric nature. The photoconductivity and absorption coefficient of CPBO display good qualitative compliance with the consequences of band structure computations. The calculated thermodynamic properties manifest the thermodynamical stability for CPBO, whereas phonon dispersions of CPVO exhibit stable phonon dispersion in contrast to slightly unstable phonon dispersion of CPTO. The predicted Debye temperature (θD) has been utilized to correlate its topical features including thermoelectric behaviors. The studied thermoelectric transport properties of CPTO yielded a higher Seebeck coefficient (186 μV/K), power factor (11.9 μWcm−1K−2), and figure of merit (ZT) value of about 0.8 at 800 K, indicating that this material could be a promising candidate for thermoelectric applications.

A critical discussion of different methods and models in Casimir effect

The Casimir-Lifhitz force acts between neutral material bodies and is due to the fluctuations (around zero) of the electrical polarizations of the bodies. This force is a macroscopic manifestation of the van der Waals forces between atoms and molecules. In addition to being of fundamental interest, the Casimir-Lifshitz force plays an important role in surface physics, nanotechnology and biophysics. There are two different approaches in the theory of this force. One is centered on the fluctuations inside the bodies, as the source of the fluctuational electromagnetic fields and forces. The second approach is based on finding the eigenmodes of the field, while the material bodies are assumed to be passive and non-fluctuating. In spite of the fact that both approaches have a long history, there are still some misconceptions in the literature. In particular, there are claims that (hypothetical) materials with a strictly real dielectric function ε(ω) can give rise to fluctuational Casimir-Lifshitz forces. We review and compare the two approaches, using the simple example of the force in the absence of retardation. We point out that also in the second (the ‘field-oriented’) approach one cannot avoid introducing an infinitesimal imaginary part into the dielectric function, i.e. introducing some dissipation. Furthermore, we emphasize that the requirement of analyticity of ε(ω) in the upper half of the complex ω plane is not the only one for a viable dielectric function. There are other requirements as well. In particular, models that use a strictly real ε(ω) (for all real positive ω) are inadmissible and lead to various contradictions and inconsistencies. Specifically, we present a critical discussion of the ‘dissipation-less plasma model’. Our emphasis is not on the most recent developments in the field but on some conceptual, not fully resolved issues.

Density functional study on Ag8-xCuxI8 (0≤x≤8)

Ab-initio calculations are performed to investigate the effect of defects on structural, electronic and optical properties of Ag8-xCuxI8 (0 ≤x ≤ 8). Initially AgI geometrically optimized and Ag atoms are replaced successively by Cu atoms. AgI is thermodynamically most stable, it exhibits low defect formation energy and fast ion conduction. The forbidden energy gap successively decreases with x. There is higher Ag+ ions formation than Cu+ ions. In the optical study, by increasing x, peaks of imaginary and real dielectric functions are shifted towards lower frequency regions, with Ag/Cu defects, there is slight shifting towards higher energy region, gradual increase in refractive index, and the reflectivity peaks are shifted towards higher energy.

Reflectionless excitation of arbitrary photonic structures: a general theory

AbstractWe outline and interpret a recently developed theory of impedance matching or reflectionless excitation of arbitrary finite photonic structures in any dimension. The theory includes both the case of guided wave and free-space excitation. It describes the necessary and sufficient conditions for perfectly reflectionless excitation to be possible and specifies how many physical parameters must be tuned to achieve this. In the absence of geometric symmetries, such as parity and time-reversal, the product of parity and time-reversal, or rotational symmetry, the tuning of at least one structural parameter will be necessary to achieve reflectionless excitation. The theory employs a recently identified set of complex frequency solutions of the Maxwell equations as a starting point, which are defined by having zero reflection into a chosen set of input channels, and which are referred to as R-zeros. Tuning is generically necessary in order to move an R-zero to the real frequency axis, where it becomes a physical steady-state impedance-matched solution, which we refer to as a reflectionless scattering mode (RSM). In addition, except in single-channel systems, the RSM corresponds to a particular input wavefront, and any other wavefront will generally not be reflectionless. It is useful to consider the theory as representing a generalization of the concept of critical coupling of a resonator, but it holds in arbitrary dimension, for arbitrary number of channels, and even when resonances are not spectrally isolated. In a structure with parity and time-reversal symmetry (a real dielectric function) or with parity–time symmetry, generically a subset of the R-zeros has real frequencies, and reflectionless states exist at discrete frequencies without tuning. However, they do not exist within every spectral range, as they do in the special case of the Fabry–Pérot or two-mirror resonator, due to a spontaneous symmetry-breaking phenomenon when two RSMs meet. Such symmetry-breaking transitions correspond to a new kind of exceptional point, only recently identified, at which the shape of the reflection and transmission resonance lineshape is flattened. Numerical examples of RSMs are given for one-dimensional multimirror cavities, a two-dimensional multiwaveguide junction, and a multimode waveguide functioning as a perfect mode converter. Two solution methods to find R-zeros and RSMs are discussed. The first one is a straightforward generalization of the complex scaling or perfectly matched layer method and is applicable in a number of important cases; the second one involves a mode-specific boundary matching method that has only recently been demonstrated and can be applied to all geometries for which the theory is valid, including free space and multimode waveguide problems of the type solved here.

Thermoelectric and optical properties of the SrS graphene by DFT

ABSTRACT The mechanical, electrical, optical and thermoelectric properties of SrS graphene-like structure are discussed using the density functional theory and FP-Lapw method. To calculate the electrical properties, the generalized gradient approximation, Kohn–Sham, and Green function Waves function approximations are used in our correlation potential, and the optical calculations are performed based on the Time-dependent density functional theory (TDDFT), Bethe-Salpeter equation, and Random Phase approximation approximations. The structural calculations suggest the phonon stability of SrS, with an observed optical phonon gap in the range of 5–9 THz, indicating that the bonds are ionic. The electronic calculations show that the SrS graphene-like has a direct bandgap of 2.6 eV and also the static value of its real dielectric function has the maximum value with RPA, and minimum value with BSE approximations. In the BSE and TDDFT approximations, the optical behaviour of SrS graphene-like shows optical stability in the infrared, visible and UV edge regions. It has an excellent thermoelectric quality at room temperature, and remarkably its thermoelectric behaviour is stable even beyond 500 K, hence it is a good choice for power generators.

Optical properties of magnetron sputtered nanocrystalline gold nitride thin films

The significance of this work is the direct synthesis of gold nitride films for surface plasmonic applications. Nanocrystalline gold nitride thin films were fabricated by reactive radio frequency (RF) magnetron sputtering on Phynox alloy substrates at room temperature and 250 °C for 1 and 2 h under a fixed power density of 0.3 W cm−2. The deposition rate is 50 nm h−1. X-ray diffraction (XRD) patterns reveal the formation of AuN2 phase with an orthorhombic crystal structure of gold nitride and was further confirmed and supported by Selected Area Electron Diffraction (SAED) patterns and High-Resolution Transmission Electron Microscopy (HR-TEM) images. Specular reflectance in the visible and near-infrared region is between 20 to 70%. Optical constants and dielectric functions of the films were obtained by employing the Drude-Lorentz model in the range of 1.37–4.1 eV. The real dielectric function crosses zero for values between 1.80–2.22 eV indicating that AuN2 films are suitable material for surface plasmonic applications. Field Emission Scanning Electron Microscopy (FE-SEM) images reveal the nano-porous microstructure of the films. Nanoindentation data show that films hardness values are in the range of 4–7 GPa, which are significantly higher compared with hardness values reported by others.

A theoretical investigation of ThCr2Si2-type Pd-based superconductors XPd2Ge2 (X = Ca, Sr, La, Nd)

ThCr2Si2-type Pd-based superconductors CaPd2Ge2, SrPd2Ge2, LaPd2Ge2, and NdPd2Ge2 have the critical temperature of 1.69 K, 3.12 K, 1.12 K, and 1.30 K respectively. Using the first-principles investigation based on the density function theory, we have investigated the physical properties including structural, mechanical, electronic, chemical bonding, optical and thermodynamic properties of these compounds. Our optimized lattice parameters are good accord with the experimental lattice parameters. The analysis of the mechanical properties ensures that these compounds exhibit ductile in nature and show anisotropic behavior. The study of the band structure and density of states (TDOS and PDOS) reveals the metallic nature of XPd2Ge2 (X = Ca, Sr, La, Nd). At the Fermi level the major contribution comes from Ge-4p states for CaPd2Ge2, SrPd2Ge2, LaPd2Ge2, and from Nd-4f states for NdPd2Ge2. The investigated chemical bonding and total charge density reveal the ionic, covalent and metallic bonding exist in four compounds. The observed optical functions of these compounds reveal the high reflectivity in the low energy region which indicates that these compounds might be used as coating materials to reduce solar heating. The large negative value of the real dielectric function also ensures the metallic behavior of these phases. We have also calculated the Debye temperature, melting temperature and minimum thermal conductivity. Finally, we have calculated the superconducting properties of all compounds.

Image potential in lossy dielectrics

The interaction of a point charge with surrounding media (the image potential) is analyzed using the quantum-mechanical description of dielectrics that consistently takes losses in media into account. We have derived a real dielectric function (without an imaginary part) appropriate for lossy dielectrics which, compared to the lossless one, significantly changes the image potential in the standard (first-order) calculations. However, when one adds to the Hamiltonian all higher-order terms caused by the losses, one finds that the image potential remains the same, as if there were no losses in media. Our calculations are approximative when analyzing complicated systems with various lossy dielectrics, but in some simple and essential cases (e.g., a dielectric plate) we are able to give the exact proof.

Optoelectronic response and excitonic properties of monolayer MoS2

Ab initio, electronic energy bands of MoS2 single layer are reported within the local density functional approximation. The inclusion of spin orbit coupling reveals the presence of two excitons A and B. We also discuss the change of physical properties of MoS2 from multilayer and bulk counterparts. The nature of the band gap changes from indirect to direct when the thickness is reduced to a single monolayer. The imaginary and real dielectric functions are investigated. Refractive index and birefringence are also reported. The results suggest that MoS2 is suitable for potential applications in optoelectronic and photovoltaic devices. The ab initio study is essential to propose the crucial parameters for the analytical model used for A-B exciton properties of the monolayer MoS2. From a theoretical point of view, we consider how the exciton behavior evolves under environmental dielectrics.

Structural, optoelectronic, and thermodynamic properties of Y $$_x$$ x Al $$_{1-x}$$ 1 - x N semiconducting alloys

The structural, electronic, optical, and thermodynamic properties of Y\(_x\)Al\(_{1-x}\)N alloys were computed using first-principles calculations. The effects of exchange and correlation have been considered by means of the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof parametrization. In addition, the Tran–Blaha-modified Becke–Johnson potential (TB-mBJ) was applied to give a better description of the band-gap energies and optical spectra. The lattice parameters, bulk modulus, and band-gap energy show nonlinear dependence on concentration x. Results for rock-salt Y\(_x\)Al\(_{1-x}\)N alloys show that the band gap undergoes an indirect (\(\Gamma \rightarrow X\))-to-direct (\(\Gamma \rightarrow \Gamma \)) transition at a given yttrium composition, followed by a direct (\(\Gamma \rightarrow \Gamma \))-to-indirect (\(\Gamma \rightarrow X\)) transition in a higher yttrium concentration. For wurtzite Y\(_x\)Al\(_{1-x}\)N alloys, the band gap presents a direct (\(\Gamma \rightarrow \Gamma \))-to-indirect (\(M\rightarrow \Gamma \)) transition at a given yttrium composition, followed by an indirect (\(M\rightarrow \Gamma \))-to-indirect (\(M\rightarrow \Sigma \)) transition in a higher yttrium concentration. The real dielectric function, imaginary dielectric function, refractive index, and extinction coefficient were calculated using the TB-mBJ potential. Using a regular solution model, slightly lower mixing enthalpies for wurtzite Y\(_x\)Al\(_{1-x}\)N alloys were found. The mixing enthalpy for a given concentration differs depending on structures, and on the interaction between atoms of constituents. The effect of temperature on the volume, bulk modulus, Debye temperature, and the heat capacity for Y\(_x\)Al\(_{1-x}\)N alloys was analyzed using the quasi-harmonic Debye model. Results show that the heat capacity is fairly sensitive to composition as temperature increases.

Tuning the Dielectric Properties of Organic Semiconductors via Salt Doping

Enhancing the dielectric permittivity of organic semiconductors may open new opportunities to control charge generation and recombination dynamics in organic solar cells. The potential to tune the dielectric permittivity of organic semiconductors by doping them with redox inactive salts was explored using a combination of organic synthesis, electrical characterization, and time-resolved infrared spectroscopy. The addition of the salt, LiTFSI (lithium bis(trifluoro-methyl-sulfonyl)imide), to a conjugated polymer specifically designed to incorporate ions into its bulk phase increased the density of holes and enhanced the static dielectric permittivity of the polymer blend by more than an order of magnitude. The frequency and phase dependence of the real dielectric function demonstrates that the increase in dielectric permittivity resulted from dipolar motion of bound ion pairs or clusters of ions. Interestingly, the increases in the hole density and dielectric permittivity were associated with enhancement of the hole mobility by 2 orders of magnitude relative to the undoped polymer. The charge recombination lifetime also increased by an order of magnitude in the blend with a fullerene electron acceptor when ions were added to the polymer. The findings indicate that ion doping enables organic semiconductors with large increases in low frequency dielectric permittivity and that these changes result in improved charge transport and suppressed charge recombination on the microsecond time scale.