Pseudopotential Method Research Articles

High viscosity ratio multicomponent flow simulations in porous media using a pseudo-potential central moment lattice Boltzmann method

Multicomponent Immiscible flows within porous media are a pivotal concern in engineering applications, especially in contexts such as oil reservoirs, which exhibit a broad spectrum of properties, including varying viscosities. The precise characterization of flow dynamics, particularly when confronted with a dynamic contact interface within intricate and heterogeneous structures, has remained a significant challenge in the field. This study employs an enhanced Pseudo-potential Central Moment Lattice Boltzmann Method to simulate multicomponent immiscible flows within porous media. The model is implemented through an in-house CUDA code (called Dena) to leverage the parallel processing capabilities of the Lattice Boltzmann Method. In order to assess the accuracy and effectiveness of this method, several test cases are examined, including the Capillary Intrusion Test, the Fingering Process, and the Drainage Process within a real porous medium. Comparative analysis against analytical and experimental results substantiates the model's capabilities and accuracy. Furthermore, the impact of viscosity ratio variations among different flow components within porous media is investigated. With the developed approach, it is possible to span a range of viscosity ratios up to 2132. The findings underscore the model's stability and accuracy in simulating high-viscosity ratio multicomponent fluid flows within porous media.

Pressure effects on the structural, electronic, elastic, optical, and vibrational properties of YMg intermetallic compounds: a first-principles study

The properties of YMg in B2 structure have been comprehensively analysed using the first-principles plane-wave pseudopotential method. Specifically, the structural, electronic, elastic, vibrational, and optical properties were investigated using the generalized gradient approximation (GGA) method in the context of density functional theory. The Vienna ab initio simulation package (VASP) was utilized for these calculations. The computed lattice parameter (3.803 Å) and bulk modulus (41.33 GPa) are consistent with the earlier data on ambient pressure. The electronic band structure and energy-dependent density of states reveal the metallic nature of the titled compounds. The Born stability requirements confirmed the mechanical stability. The analysis of Pugh’s and Poisson’s ratios and Cauchy’s pressure reveals that YMg is ductile under the pressures in consideration. According to several anisotropy indices, the compound is noticeably anisotropic both in ambient and under pressure. Our investigation includes an analysis of several fundamental mechanical parameters of the material, including the bulk modulus, the pressure derivative of the Zener anisotropy factor, Poisson’s ratio, isotropic shear modulus and Young’s modulus with a particular focus on their dependence on pressure. We have determined that the elastic constants obtained remain mechanically stable, satisfying the Born Stability conditions even at high pressures of up to 60 GPa. To explore the dynamic stability of YMg, we analysed the material’s phonon dispersion curves. The examined compound displays stability under dynamic conditions from 0 GPa to 30 GPa, as evidenced by its positive vibration frequencies. However, this stability is not sustained under higher pressure, as the compound becomes unstable after 30 GPa to 70 GPa. The electronic band structure and density of states diagrams demonstrate YMg’s metallic properties. At atmospheric pressure (0 GPa), the total density of states (TDOS) near the Fermi level is approximately 1.63 states/eV, with pressure application reducing DOS. The dielectric function, refractive index, and energy loss spectra are examined within the 0–20 eV energy range. YMg exhibits its highest absorption between 4 and 11 eV. The peak optical conductivity is observed around 0.78 eV (equivalent to 1589.5409 nm), while the most significant energy loss occurs at 11.90 eV, roughly corresponding to 2.8 Hz in the ultraviolet spectrum. Moreover, we extensively analyzed the material’s phonon thermodynamic and optical properties, providing insights into its behavior under various conditions. The outcomes acquired at zero pressure are generally coherent with the current theoretical values.

Possibility of formation of stationary structures in relativistically degenerate magnetized quantum plasma with exchange-correlation energy

In the present paper we have studied the possibility of stationary structure formation in ion acoustic wave in a relaivistically degenerate quantum plasma in presence of magnetic field quantum diffraction parameter and localized exchange correlation energy. Recent authors include exchange correlation term in many plasma configurations including quantum and relativistic regime. We have analyzed the applicability of certain mathematical tools like the Sagdeev pseudo-potential method in dealing with the analysis of the formation and properties of large amplitude solitary structures, double layers, shocks etc. The findings of this paper will help future researchers to select analytical methods while studying wave phenomena in plasma .

Effect of non-metal doping on the optoelectronic properties of ZrS2/ZrSe2 heterostructure under strain: a first-principles study.

In this paper, we systematically studied the effects of non-metallic element (B, C, N, O, F) doping and biaxial stretching on the photoelectric properties of ZrS2/ZrSe2 heterostructures by using the first-principles calculation method based on density functional theory. The results show that the p-type doping is realized by B, C, and N atom doping, and the n-type doping is realized by O and F atom doping. The doping of B and C atoms produces impurity energy levels in the band gap, which affects the conductivity of the heterostructure. The band gap of N and O atom-doped heterostructures increases under tensile strain, but it is still a direct band gap. The analysis of the optical properties of the heterostructures shows that the doping of non-metallic atoms can adjust the optical absorption rate and reflectivity of the heterostructures. Under the action of tensile strain, the optical properties of the doped heterostructures have changed significantly in the low-energy region. This article provides a theoretical basis for the future application of ZrS2/ZrSe2 heterostructures. This paper uses the first-principles calculation method based on density functional theory. The PBE exchange-correlation functional based on generalized gradient approximation (GGA) is selected for the specific calculation, and the crystal structure is geometrically optimized by the ultrasoft pseudopotential method. It is verified that when the cutoff energy of the ZrS2/ZrSe2 heterostructure is 500 eV, the K-point grid is selected to be 10 × 10 × 2 with the lowest energy, so the cutoff energy is selected to be 500 eV. The K-point grid is selected to be 10 × 10 × 2. The convergence limits for structural optimization are as follows: the maximum force between atoms is 0.01 eV/Å, the convergence threshold of the maximum energy change is set to 10-9 eV/atom, and the convergence threshold of the maximum displacement is 0.001 Å. In order to avoid the influence of atomic periodic motion between different atomic layers, a vacuum layer of 20 Å is added in the vertical direction. Considering the interaction of vdW between the interfaces, the DFT-D2 method is used to verify. The optical properties were calculated by the random phase approximation method, and the K-point grid was selected as 12 × 12 × 2.

Strain effects on electronic and dynamical properties of half-Heusler semiconductors: insights from Meta-GGA

In this study, we investigated the effects of mechanical strain, including both tensile and compressive strains, on the electronic properties and dynamical stability of two ternary half-Heusler compounds: TiIrSb and ZrIrSb. We employed the plan wave pseudo-potential method (PW-PP) within the density functional theory (DFT) framework. Our calculations were performed using both the GGA-PBE and Meta-GGA-SCAN approximations. Furthermore, to compute the phonon dispersion, we employed the R2SCAN functional instead of SCAN for both compounds, addressing numerical challenges encountered with the latter. In the absence of strain, our calculations revealed that both compounds exhibit semiconducting behavior, featuring an indirect band gap at identical locations in the Brillouin Zone. Notably, the SCAN functional consistently predicted a larger band gap compared to the corresponding values obtained with PBE for both compounds. Specifically, the band gap expanded significantly, creating a noticeable separation between the valence and conduction bands. For TiIrSb, it increased from 0.84 eV with PBE to 1.05 eV with SCAN, while for ZrIrSb, it increased from 1.41 eV with PBE to 1.71 eV with SCAN. Under the application of strains, both compounds demonstrated an increased band gap under compressive strain, while the application of tensile strain led to a decrease in the band gap, resulting in an indirect-to-direct band gap transition for ZrIrSb. Remarkably, under all strain values, whether tensile or compressive, the SCAN functional consistently exhibited a larger band gap compared to PBE, indicating its accurate description of the material’s electronic structure. The calculated Density of States (DOS) and Partial Density of States (PDOS) reveal that the valence band extremum (VBM) primarily consisted of Ti/Zr-d orbitals, while the conduction band maxima (CBM) predominantly involved strong hybridization between Ti/Zr-d, Ir-d, and Sb-p states. Notably, the SCAN functional predicted higher orbital contributions to Total Density of States (TDOS) compared to the PBE approximation. Importantly, both half-Heusler materials exhibited mechanical and dynamical stability under various strain conditions.

Impact dynamics of droplets on circular bodies: Exploring the influence of wettability, viscosity, and body dimensions using the lattice Boltzmann method

This study used the multi-relaxation time pseudopotential lattice Boltzmann method to examine the impact dynamics of droplets on circular bodies, focusing on the influence of the surface wettability, the viscosity of droplets by choosing three distinct Reynolds numbers (Re = 100, 300, and 500), and the body diameters. Initially, the study examined impact behavior under non-wetting and wetting conditions, revealing distinct behaviors characterized by dimensional stretch lengths in horizontal and vertical directions. Furthermore, the study evaluated the impact of viscosity by varying the Reynolds number, providing a better understanding of droplet behavior on the solid body. In addition, the effect of changing the diameter of the circular body was examined. This research underlines the importance of surface wettability in the dynamics of contact with the droplets impinging on it. In particular, higher wettability correlates with a monotonic increase in viscosity by acting on the Reynolds number. In addition, the droplet profile responds to changes in Reynolds number, albeit with relatively limited deformation, even in the case of intense interaction with the wetting surface. These results highlight the complex interplay between wettability, droplet dynamics, viscosity, body dimensions, and surface interactions during impact processes.

Half Metallic Properties of the Polymorphs of Rubidium Selenide Compound

Half-metallic ferromagnets denote a specific category of compounds where one spin channel exhibits a gap at the Fermi level, contrasting with the metallic nature of the other spin channel. This distinction results in complete carrier spin polarization, reaching 100% at the Fermi level. A first-principles pseudopotential plane-wave method is utilized to examine the structural, electronic, and magnetic characteristics of RbSe across various polymorphs. Our investigation covers the RbSe compound in the CsCl (B2), NaCl (B1), ZnS (B3), NiAs (B81) and wurtzite (B4) phases. The calculations were done using the quantum ESPRESSO code within the generalized gradient approximation. The lattice parameters, bulk moduli, and their pressure derivatives agrees with prior theoretical data for cubic structures. The electronic band structures and density of states indicate the emergence of half-metallic and magnetic properties in RbSe, which stem from the existence of spin-polarized p orbitals within the Se atom. RbSe shows half-metallic behaviour in all structures studied with an integer magnetic moment of 1μB per formula unit for CsCl, NaCl, ZnS; and 2μB for NiAs and wurtzite. Results showed that most energetically stable phase of all five crystal structures studied is the CsCl-type of RbSe.

Potential enhancements in the photocatalytic activity of the anatase 001 surface by adding cerium impurity

AbstractThis research investigates the structural and electronic properties of titanium dioxide ($$ TiO_2 $$ T i O 2 ) surfaces with the addition of cerium (Ce) impurities using first principles calculations within the framework of density functional theory, employing the pseudopotential method and the computational package Quantum ESPRESSO. Density of states (DOS) calculations reveal that the clean surface exhibits semiconductor behavior. In contrast, the titanium dioxide surface with Ce impurities on the fifth layer generates intermediate states within the energy band gap, resulting in a reduction in the band, with an intraband energy gap of 0.7882 eV induced. Moreover, the inclusion of a Ce atom on the fourth layer of the surface results it is observed that the surface an important characteristic is the appearance of intermediate states in the band gap, which are close to the valence band with a magnetic moment of 0.91 $$ \mu \beta /cell $$ μ β / c e l l . The ntraband gap can enhance the utilization of the visible spectrum. Additionally, the Ce impurity on the $$ TiO_2 $$ T i O 2 surface could promote greater photocatalytic activity in pollutant degradation and increase the oxidative capacity of titanium dioxide.

Structural, elastic, mechanical, electronic, and optical properties of cubic K2Pb2O3 from first-principle study.

Based on first principles, the structure, elasticity, mechanics, electronics, and optical properties of cubic K2Pb2O3 were studied. The structural parameters calculated by this method are close to the previous theoretical results. The elastic constant, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and mechanical stability are studied, and it is shown that cubic K2Pb2O3 is mechanically stable, isotropic, and brittleness. The electrical conductivity and chemical bonding of cubic K2Pb2O3 were analyzed based on the calculated band structure, density of states (DOS), and bond populations. The dispersion of optical functions, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, and loss function, is displayed and analyzed. All computations have been carried out based on density functional theory (DFT) as implemented in the CASTEP code. The norm conservation pseudopotential method is used to exchange correlation functionals within the generalized gradient approximation (GGA).

Effect of bending deformation on the electronic and optical properties of O atoms adsorbed by Be3N2.

In this paper, the optimum coverage of 4.44% and the optimum adsorption sites were determined for the Be3N2 adsorption system of O atoms at different coverages based on density functional theory. The electronic and optical properties of the model were investigated by applying bending deformation to the model at these coverage and adsorption sites. Adsorption of O atoms disrupts the geometrical symmetry of Be3N2, resulting in orbital rehybridization and lowering its band gap. Bending deformation causes the band gap of the adsorbed O atom structure of Be3N2 to first increase and then decrease, resulting in the modulation of its band gap. With increasing bending deformation, the adsorbed system is redshifts, and the degree of redshift increases with increasing bending deformation. All calculations in this paper were performed using the first-principles-based CASTEP module of Materials Studio (MS). The generalized gradient approximation (GGA) plane-wave pseudopotential method and the Perdew-Burke-Ernzerhof (PBE) Perdew et al. Phys Rev Lett 77:3865, 1996 generalized functional were used in the geometry optimization and calculation process to calculate the exchange-correlation potential between electrons. The effect of coverage on the electronic and optical properties of the Be3N2-adsorbed O atom system was investigated by adsorbing different numbers of O atoms on a monolayer of Be3N2. The Be3N2 protocell contains two N atoms and three Be atoms with a space community of P6/MMM (No.191). The original cell was expanded 3 times along the direction of the base vectors a and b in the Be3N2 plane to create a 3 × 3 × 1 monolayer Be3N2 supercell system. A vacuum layer of 15Å is set in the direction of the crystal plane of the vertical monolayer Be3N2 supercell to eliminate interactions between adjacent layers. In the overall energy convergence test of the Be3N2 supercell, the plane wave truncation energy was set to 500eV, and the energy difference between the calculations given in the literature Reyes-Serrato et al. J Phys Chem Solids 59:743-6, 1998 using 550eV was less than 0.01eV, verifying the reliability of the data at a truncation energy of 500eV. The Monkhorst-Pack special k-point sampling method Monkhorst et al. Phys Rev B 13:5188, 1976 was used in the structural calculations, and the grid was set to 3 × 3 × 1. The geometric optimization parameters are set as follows: the self-consistent field iteration convergence criterion is 2.0 × 10-6eV, and the iterative accuracy convergence value is not less than 1.0 × 10-5eV/atom for the total force of each atom and less than 0.03eV/Å for all atomic forces. In addition the high-symmetry k-point path is taken as Γ(0,0,0) → M(0,0.5,0) → K(- 1/3,2/3,0) → Γ(0,0,0) Chen et al, AIP Adv 8:105105, 2018.

Turbulent flow boiling simulation based on pseudopotential lattice boltzmann method with developed wall boundary treatments and unit conversion

The lattice Boltzmann method (LBM) is widely studied for complex flow simulations, including liquid–vapor phase change phenomena. Although previous studies extended the LBM simulation capability from single-phase to two-phase boiling simulations, most of them are restricted to no-flow (pool boiling) or low-flow-rate (laminar flow) boiling conditions owing to the stability problem in collision at low viscosity, representing high Reynolds numbers. However, a high Reynolds number flow is essential for investigating the effects of turbulence on bubble dynamics. Recently, the central moment-based collision method is used to enable high Reynolds numbers for bulk flow; however, extending the method for multiphase-flow simulation with high Reynolds numbers is limited owing to the boundary treatment near the wall. In this study, we augment the previous boundary treatment in LBM simulation with a forcing term, which increases the stability of the simulation even under two-phase conditions. With the improved boundary treatment, the flow boiling phenomena at high Reynolds numbers (up to Re = 107,000) are successfully reproduced. Moreover, in this study, unit conversion is reconsidered with the relationship between conversion factors because the poor outcomes of bubble dynamics are attributed to misinterpreted unit conversion. The new unit conversion method satisfies the necessary conditions for mapping the physical bubble behavior to lattice frame, reproducing bubble dynamics effectively following the Reynolds number, with reasonable spatial and temporal resolution. Consequently, the flow boiling simulation can accurately predict the bubble departure diameter from the real experimental results and the heat transfer trend as a function of the Reynolds number.

COMPUTER SIMULATION OF LI AND BE WETTING LAYERS ON THE SI (100) SURFACE

Within the framework of the of density functionality theory and the pseudopotential method, the atomic and electronic structure of the two-dimensional Li-Si and Be-Si systems forming on the Si (100) surface is calculated, with a metal thickness of one to three monolayers (ML). At the first ML, the formation of ordered silicide wetting layer of Li (with atoms embedded inside the top layer Si) and Be (with atoms embedded between the top two Si layers) was detected. At 2 ML, the systems are modified: Li atoms occupy positions between the top two Si layers, and Be atoms rise at positions above the top Si layer. After that, with a coating thickness of 3 ML, in the case of Li, a continuous disordered wetting layer is formed, and in the case of Be, a wetting layer in the form of disordered chains along the length. At 1 ML, an energy gap appears in the electronic structure of the studied systems in the density of electronic states near the Fermi level, the width of which is 1.02 eV and 0.36 eV, respectively, for Li-Si and Be-Si systems. Then the gap disappears, first for the lithium system (at 2 ML), and then, for the beryllium system (at 3 ML).

First-principles study of electronic and magnetic properties in site dependent Fe-doped chalcopyrite ZnSnAs2semiconductors: Comparison with those of Mn- and Cr-doped ZnSnAs2

The electronic and magnetic properties of Fe-doped ZnSnAs2 within chalcopyrite structures have been studied by first-principles calculations using the density functional theory (DFT) full-potential linearized augmented-plane-wave method and the plane-wave pseudopotential methods. The band gap of ZnSnAs2 was determined to be 0.816 eV using the modified Beck-Jonson (mBJ) exchange potential. When Fe dopants substitutionally replace Sn atoms, half-metallic ferromagnetism is observed in ZnSnAs2 where in the spin-down band at the Fermi level is entirely unoccupied. The magnetic exchange interaction between two Fe atoms substituting Zn and Sn sites in the supercell with different geometries has been studied. The total energy difference (ΔE) between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations suggests that Fe atom occupation of Zn sites in ZnSnAs2 leads to a predominantly AFM state, while the Fe atom occupation of Sn sites prefers the FM state.

Contrasting packing modes for tubular assemblies in chlorosomes

The largest light-harvesting antenna in nature, the chlorosome, is a heterogeneous helical BChl self-assembly that has evolved in green bacteria to harvest light for performing photosynthesis in low-light environments. Guided by NMR chemical shifts and distance constraints for Chlorobaculum tepidum wild-type chlorosomes, the two contrasting packing modes for syn-anti parallel stacks of BChl c to form polar 2D arrays, with dipole moments adding up, are explored. Layered assemblies were optimized using local orbital density functional and plane wave pseudopotential methods. The packing mode with the lowest energy contains syn-anti and anti-syn H-bonding between stacks. It can accommodate R and S epimers, and side chain variability. For this packing, a match with the available EM data on the subunit axial repeat and optical data is obtained with multiple concentric cylinders for a rolling vector with the stacks running at an angle of 21° to the cylinder axis and with the BChl dipole moments running at an angle ß ∼ 55° to the tube axis, in accordance with optical data. A packing mode involving alternating syn and anti parallel stacks that is at variance with EM appears higher in energy. A weak cross-peak at -6 ppm in the MAS NMR with 50 kHz spinning, assigned to C-181, matches the shift of antiparallel dimers, which possibly reflects a minor impurity-type fraction in the self-assembled BChl c.

A DFT study of structural, electronic, mechanical, optical, and hydrogen storage properties of quaternary hydride phase Li4BN3H10

In this paper, we investigated the structural, electronic, mechanical and optical properties of Li4BN3H10 using the ultra-soft pseudopotential method which is based on the density functional theory (DFT) with CASTEP code. The optimized lattice parameters of Li4BN3H10 correlate well with the theoretical calculations and experimental measurements that are currently available. Using elastic constants, the material's stability was verified. Based on the elastic constants, we discovered several mechanical features of the material. Between 0 and 20 eV, optical spectra calculations are made, taking into account the real and imaginary parts of the dielectric function ε(ω), reflectivity R(ω), index of refraction n(ω), coefficients of extinction k(ω) and absorption α(ω). The dielectric function is wide close to the middle ultraviolet (4.13–6.20 eV). The extinction coefficient of the Li4BN3H10 has the ability to worn for implements like Bragg's reflectors, optical and optoelectronic equipments. The optical parameters of Li4BN3H10 disclose that our working constructions have an elevated dielectric constant, with a greatest absorption in the visible range holding out over 1.49 × 105 cm−1 Li4BN3H10 is deemed to be an appropriate material for optoelectronic applications based on calculated optical results. The gravimetric hydrogen storage capacities of Li4BN3H10 compound is 11.06 wt %. These promising capacities exceed the 6.5 wt % target set by the US Department of Energy for 2025.

Comprehensive DFT investigation of ternary thallium tetragonal crystals: assessing their viability for solar cell applications

To ascertain the suitability of the TlInX2 (X = S, Se, Te) tetragonal structures for photovoltaic applications, first-principles calculations were carried out using the pseudopotential plane wave method to assess the structural, electronic, elastic, and optical characteristics of the considered compounds via the DFT software CASTEP. For all three compounds, our calculated structural parameters were in excellent agreement with both the experimental and previous theoretical results. Our calculations provided the first predicted elastic constants for the TlInS2, TlInSe2 and TlInTe2 compounds. The three tetragonal systems were mechanically stable and exhibited pronounced and noticeable elastic anisotropy. Analysis of the optical properties revealed that TlInX2 (X = S, Se, Te) exhibited distinct absorption in the ultraviolet radiation range and pronounced optical anisotropy. The calculated bandgap values obtained using the HSE06 hybrid functional were 1.68 eV, 1.38 eV, and 1.36 eV for TlInS2, TlInSe2 and TlInTe2, respectively. These findings are of significant interest because they categorize all three compounds as promising candidates for use in photovoltaic cell applications.

First-principles study of the magnetic and optical properties of PtSe2 doped with halogen elements F, Cl, and Br

The electronic structure, magnetic and optical properties of halogen-doped two dimensional PtSe2 are investigated by using the first-principles ultra-soft pseudopotential plane wave method based on density functional theory. It is shown that the doped PtSe2 is more stable under Bottom-Se conditions than under Top-Se conditions, and the higher the doping concentration (C d), the lower the band gap. At C d = 5.56%, the Cl- and Br-doped PtSe2 are transformed from a non-magnetic semiconductor to a magnetic n-type semiconductor with a magnetic moment (M B) of 1 μB; while neither the F-doped PtSe2 nor the pristine PtSe2 is magnetic. When C d = 11.1%, the F-doped PtSe2 at the first neighborhood becomes magnetic metal with M B = 1.39 μB; while that doped at the second nearest neighbor retains a semiconductor with M B = 0. Thus Cl- and Br-doped PtSe2, as well as the first-neighbor F-doped PtSe2 can be well applied in spintronic devices. The optical properties are enhanced for all three doping systems with an obvious peak appearing in the infrared light region. Absorption and reflectivity curve still has a peak in the infrared light region.

Biaxial tensile-compressive deformation on the electronic structure and optical properties of doped with different concentrations F–MoTe2: A first-principles study

Under the framework of density functional theory based on the first principles, the plane wave pseudopotential method was used to investigate the effects of biaxial stretch-compression deformation on the electrical structure and optical characteristics of molybdenum telluride systems doped with various fluorine atom concentrations. Electrical structure, density of states, charge transfer, and optical characteristics were calculated and thoroughly investigated in the molybdenum ditelluride system. The results demonstrate that the band gap shrinks with increasing fluorine atom doping concentration of fluorine atoms, and n-type doping occurs for different doping concentrations of halogen fluorine atoms. The semiconductor-quasi-metal transition occurs in the MoTe2 system when the doping concentration reaches 16.67%. After that, the fluorine atoms with 4% doping concentration were deformed by stretching and compression, and the semiconductor started to transition to quasi-metal as the amount of stretching increased. The deformation of the system changed considerably when the stretching amount was 14%. Compression induces a direct-to-indirect bandgap transition, transforming semiconductors into metals. Significant changes in electronic energy band diagrams and density of states diagrams. In terms of optical qualities, the absorption spectra of the entire system are blue-shifted during stretching and compression. The reflectance spectra are red-shifted and then blue-shifted.

Structural, mechanical, optoelectronic and thermoelectric properties of Cs1-xAxPbI3 (A=K, Rb) perovskites: Density functional investigation

The structural, mechanical, thermodynamic, electrical, optical, and thermoelectric properties of Cs1-xAxPbI3 (A = K, Rb) perovskites within GGA approximations have been studied. The calculations are performed using the QUANTUM-ESPRESSO computational package based on the density functional theory (DFT) and the pseudo-potential method. The calculated elasticity parameters illustrated that Cs1-xAxPbI3 (A = K, Rb) perovskites have mechanical stability conditions at ambient pressure. The calculation of the elastic modulus shows that KPbI3 and Cs0.75 K0·25PbI3 with Young's modulus equal to 16.11 GPa and 17.00 GPa have the lowest and highest hardness respectively. The estimated BGH ratio values were greater than 1.75 for all perovskites, reflecting the flexibility of the compounds. Calculation of band structure confirmed the semiconductor behavior with a direct band gap in the range of 1.38 eV–1.51 eV. Our investigation of charge carrier mobility revealed that electrons have greater carrier mobility than holes in all Cs1-xAxPbI3 (A = K, Rb) perovskites. The optical absorption peaks of Cs1-xAxPbI3 (A = K, Rb) perovskites are located in the visible area, indicating their effective use in optical and optoelectronic devices. Calculations of the Seebeck coefficient revealed the n-type semiconductor nature of all perovskites. Thermoelectric parameters, including the Seebeck coefficient, electrical conductivity divided by the relaxation time, and thermal conductivity divided by the relaxation time, all increase as temperature increases. The thermoelectric properties of Cs1-xAxPbI3 (A = K, Rb) perovskite were improved by increasing the concentration of potassium and rubidium. The maximum electronic figure of merit of all Cs1-xAxPbI3 (A = K, Rb) perovskites at room temperature was 0.99, which makes them good candidates for application in thermoelectric devices.

Strain-induced modification in thermal properties of monolayer 1T-ZrS2 and ZrS2/ZrSe2 heterojunction.

This paper systematically analyzes the phonon dispersion curves of single-layer ZrS2, ZrSe2, and ZrS2/ZrSe2 heterostructures under different strains. The phonon spectra and thermal parameters of the three structures were obtained based on the density functional perturbation theory method. The upper limits of strain that different monolayers and heterojunctions can withstand were studied. The monolayers ZrSe2 and ZrS2 can withstand up to 8% biaxial tensile strain, and the ZrS2/ZrSe2 heterojunction can withstand up to 6% biaxial tensile strain. In addition, the van der Waals force of the heterojunction may cause phonon tearing in the vertical direction. The application of biaxial tensile strain can adjust the thermal properties of the system to a large extent, which is similar to the strain effect in the pristine case. When the temperature rises, the entropy enthalpy of the three structures also gradually increases, the free energy gradually decreases, and the heat capacity of the system gradually increases until it tends to be stable. Taking single-layer ZrS2 as an example, we analyzed the change curve of thermal properties of single-layer ZrS2 under tensile strain. The results show that with the gradual increase of strain, the crystal's entropy, enthalpy, and free energy change differently. In addition, the heat capacity increases slowly under high temperatures. When all systems reach the limit strain, the phonon spectrum appears to have an imaginary frequency, and the thermal properties decrease significantly. This paper uses the first-principle calculation method based on density functional theory, and the PBE exchange-correlation function based on generalized gradient approximation (GGA) is selected for a specific calculation. The density functional perturbation theory (DFPT) calculates the full kinetic matrix. Because the lattice constants of ZrS2 and ZrSe2 are similar and have similar periodicity, the corresponding unit cells are used for structural optimization and property calculation. The Brillouin zone is integrated using the K points generated by the Monkhorst-pack method. For single-layers ZrS2 and ZrSe2, 8 × 8 × 1K-point grid is selected, and for ZrS2/ZrSe2 heterojunction, 8 × 8 × 2K-point grid is selected. A vacuum layer of 30Å was added in the vertical direction to avoid interlayer interaction. The non-conservative pseudopotential method is used to optimize the structure, and the optimization convergence is set as follows: the cutoff energy is set to 700eV, the convergence threshold of the maximum force between atoms is 0.01eV/Å, the convergence threshold of the maximum energy change is set to 1 × 10-9eV, and the convergence threshold of the maximum displacement is 0.001Å.