Perdew Burke Ernzerhof Research Articles

Strain-induced effects on the optoelectronic properties of ZrSe2/HfSe2 heterostructures.

Two-dimensional semiconductor materials have received much attention in recent years due to their wide variety of applications in the field of nano-optoelectronic devices. In this project, we applied stresses ranging from -6 to +6% to the ZrSe2/HfSe2 heterostructure and systematically investigated its electrical and optical properties. It is discovered that stress can effectively modulate the forbidden bandwidth of the ZrSe2/HfSe2 heterojunction; whereas, under compressive stress, the forbidden bandwidth of the material decreases further until the bandgap is zero, leading to the material's transformation from semiconductor to metal. The forbidden band gap of the ZrSe2/HfSe2 heterojunction increases with increasing horizontal biaxial tensile strain. We discovered that the light absorption performance of this heterostructure is significantly better than that of its similar monomolecular layer and that its light absorption intensity can reach an order of magnitude of 104. Under compressive and tensile stresses, the ZrSe2/HfSe2 heterojunctions exhibit different degrees of red or blue shift. The results indicate that constructing ZrSe2/HfSe2 heterojunctions and applying horizontal biaxial stresses to them can significantly modulate the optoelectronic properties of the materials. ZrSe2/HfSe2 heterojunction is a new type of high-performance photogenerated carrier transport device with a wide range of applications. The calculations in this study are carried out the first principles approach of density functional theory, as implemented in the CASTEP module of Materials-Studio2019. The researchers used an ultrasoft reaction potential to calculate the interactions between the ion core and the electrons and applied the Perdew-Burke-Ernzerhof (PBE) and the generalized gradient approximation (GGA) to perform the calculations. The Monkhorst-Pack technique was employed to create the k-point samples utilized for integration on the Brillouin zone, and the k-point grid was uniformly 6 × 6 × 1. In addition, in order to avoid interactions between the atomic layers affecting the properties and stability of the material, such interactions were prevented by adding a 30 Å vacuum layer. Using a plane-wave energy cutoff of 500 eV and the convergence accuracy of the iterative process was set to 1 × 10-5 eV to ensure the accuracy of the computational results, and in addition. The maximum stress in the lattice was limited to less than 0.05 GPa or the interaction force between neighboring atoms was lower than 0.03 eV/Å. For the calculation of the properties of the optical properties, a k-point grid of 18 × 18 × 1 is used for optimization, and the polarization direction of the material is not taken into account, considering that the material is isotropic. This study proposes to apply the Tkatchenko-Scheffler (TS) dispersion correction method in DFT-D to appropriately represent the interlayer van der Waals interaction forces to solve inaccuracies in the computation of van der Waals interactions via density functional theory.

Effect of shear strain on the electronic and optical properties of Al-doped stanane.

The quasi-metallic properties of stanene limit its prospects in optoelectronic devices. Based on first-principles calculations, a systematic study is conducted on the tuning effects of surface hydrogenation and Al atom doping on the electronic and optical properties of stanene. Surface hydrogenation serves as an ideal way to open the forbidden band of stanene, and Al atom doping further increases hydrogenated stanene (stanane) band gap to 0.460eV. Deformation has a minor impact on the stability of the stanane-Al structure, while shear strain can effectively modulate the band gap engineering of the doped system, reducing the band gap value from 0.460 to 0.170eV. Deformation induces a redshift in the absorption peak and reflectance, also slowing down the rate of decrease in the absorption coefficient, and enhancing the peak value of light reflectance, which is positively correlated with the degree of shear strain. These findings hold promise for expanding the potential application of monolayer stanane in semiconductor devices. All calculations are performed using CASTEP module in Materials Studio based on the density functional theory (DFT). The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) is employed to describe the exchange-correlation energy (Perdew et al., Phys Rev Lett 77(18), 1996). We construct models for both stanene and stanane. The original unit cell of stanene has two Sn atoms, while stanane consists of two Sn atoms and two H atoms, and expand them to a 3 × 3 × 1 supercell with a vacuum layer of 20Å in height to prevent interlayer coupling. After convergence testing, the plane-wave cutoff energy is set to 450eV, and the energy convergence threshold is set to 1 × 10-5eV. The maximum residual stress for each atom is set to 0.01eV/Å. Brillouin zone sampling is performed using a 6 × 6 × 1k-point mesh based on the Monkhorst-Pack method (Monkhorst and Pack, Phys Rev B 13(12), 1976). The k-point accuracy of the density of states and optical properties is 9 × 9 × 1. All calculations are performed using the more advanced OTFG ultrasoft pseudopotential, and structural relaxations are performed using supercells to ensure that the model is fully relaxed. We use the HSE06 functional to calculate the energy band structures of stanane-Al deformed to 0%, 4%, and 8%, resulting in band gap values of 1.465eV, 1.368eV, and 1.016eV, respectively. These values are significantly higher than those obtained using the PBE functional (0.460eV, 0.397eV, and 0.170eV). However, the shapes and trends of the band structures obtained from both PBE and HSE06 calculations are similar. Additionally, the calculation time needed by HSE06 is greatly longer than PBE, which exceeds the capabilities of our computer hardware, and cannot support all subsequent calculations. To investigate the influence of deformations on the variation of band gap values and to conserve computational resources, the subsequent calculations in this study use the PBE functional.

First-principles calculations investigating structural, electronic optical and elastic properties of ABSe3 (A = Rb, Cs) compounds explored for photovoltaic and optoelectronics applications

In few recent years, great and significant efforts are devoted from researcher all over the world to pursue the revolution of photovoltaic’s materials and their uses in various applications. In the present work, a series of ab-initio simulations based on the density functional theory DFT plane wave pseudo-potential (PW-PP) method and hence performed towards the perselenoborate materials ABSe3 (A = Rb,Cs) for the first time along the three main polarizations of the incident wave directions [100], [010] and [001] with the aim of exploring their structural, electronic, optical and elastic properties. The generalized gradient approximation Perdew–Burke–Ernzerhof (GGA-PBE) carried out with CASTEP code is used for the exchange–correlation potential. The computed results showed that the structural properties of investigated compounds are very good agreement to the available experimental data, showing that the current calculations are quite accurate. Moreover, the density of states and electronic band structure calculations reveal that RbBSe3 (CsBSe3) compounds exhibit direct band gap semiconductor nature 1.66 eV (1,82 eV) respectively within the optimal range band gap 1eV–2eV required for photovoltaic’s applications makes them having great potential to obtain efficient Perovskite solar cell PSC. Additionally, our finding optical properties calculations reveal that the two investigated compounds exhibit strong absorption (prominent absorption peaks up 2,4 × 105cm−1) in UV range, while the real part of the refractive index for RbBSe3 (CsBSe3) was 2.62 (2.60) respectively which might be beneficial for photovoltaic application (top cell in solar cell) and optical applications. Also, high optical conductivity (∼1015sec−1) is found to be observed in visible ultraviolet range (1.7 eV to 30 eV). The lower reflectivity seen by RbBSe3 compared to CsBSe3 in the larger energy spectrum of electromagnetic radiation suggests that RbBSe3 compound is more suitable for electronic applications. Further, once the elastic constants are obtained, the calculated mechanical properties, bulk modulus (B), shear modulus (G), the ratio B/G, Young’s modulus (E), Poisson’s ratio (ν), anisotropy universal (AU) are calculated. Our calculated Young’s modulus indicate that the CsBSe3 is less hardness compared to RbBSe3, while Poisson’s ratio calculated leads them to have a character ionic and RbBSe3 is more ductile than CsBSe3. The calculation value of θ D predicted from elastic constants appears low which is closely related to many physical properties such as specific heat and melting temperature. Finally, the finding results reveal another way of investigating mechanical stability, where both compounds are mechanically stable since all elastic constant computed are perfectly satisfied the Born stability criteria, flexible and brittle. Finally, we hope that all these results will be helpful for designing photovoltaic and optoelectronic devices.

First‐Principle Density Functional Theory‐Derived Nonleaded KSn1−xGexI3‐Based Perovskite Solar Cells: A Theoretical Study

The lead toxicity and instability of conventional all‐inorganic lead halide perovskites hinder the production of efficient and stable nontoxic perovskite solar cells (PSCs), which prompts the search for viable nonleaded substitutes for photovoltaic (PV) applications. The present study investigates the optoelectronic characteristics of proposed lead‐free orthorhombic perovskites KSn1−xGexI3 (x = 0, 0.5, 1) via first‐principle density functional theory (DFT), to get an insight into their PV applicability. DFT computation with generalized gradient approximation of Perdew–Burke–Ernzerhof exchange–correlation is performed to extract the parameters such as bandgap, mobility, dielectric constant, conduction band minima and valence band maxima of KSnI3, KSn0.5Ge0.5I3, and KGeI3 to demonstrate their capability as a functional layer in PSCs. These characteristics have been utilized to simulate PSCs. The simulated devices FTO/SnO2/KSnI3/Spiro‐OMeTAD/Au, FTO/SnO2/KSn0.5Ge0.5I3/Spiro‐OMeTAD/Au, and FTO/SnO2/KGeI3/Spiro‐OMeTAD/Au exhibit the efficiency of 8.23%, 8.89%, and 4.12%, respectively. Thereafter, the most effective KSn0.5Ge0.5I3‐cell with an efficiency of 8.89% is selected for additional optimization of thickness, defect density, and doping concentration to achieve maximum efficiency. The distinctive green proposes KSn0.5Ge0.5I3‐PSC, with an optimized efficiency of 20.29% stands out among the top PSC technologies. Future research on this subject will focus on synthesizing the proposed novel KSn0.5Ge0.5I3‐PSC and evaluating the performance of the KGeI3‐PSC.

Investigating the molecular crystals of L-Alanine, DL-Alanine, β-Alanine, and Alanine hydrogen chloride: Experimental and DFT analysis of structural and optoelectronic properties

In this study, DFT calculations were employed to assess and contrast the structural, electronic, and optical characteristics of five alanine crystal forms. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) with the Tkatchenko-Scheffler dispersion correction scheme was employed and compared to the PBE functional and the local density approximation (LDA) without dispersion correction. Experimentally, alanine crystal powders were characterized through X-ray diffraction (XRD) and UV–VIS absorption spectrum. Additionally, UV–VIS absorption measurements were performed on L-α-alanine, DL-alanine, β-alanine, and alanine hydrochloride solvated in water. Significant deviations (>10%) were observed between the experimental crystal lattice parameters and those predicted by the GGA and LDA methods. Incorporating dispersion correction into the GGA functional improved prediction accuracy. Additionally, time-dependent DFT (TD-DFT) computations using the polarizable continuum model were conducted to analyze the UV/VIS absorption spectrum of α-, β-, DL-, and hydrogen chloride alanine in water and interpret experimental data. Different alanine crystals exhibited substantial anisotropic electronic and optical properties, including effective masses, absorption, and dielectric function. The L-α-HCl structure displayed the smallest band gap (4.67 eV), suggesting its suitability for applications such as L-α/L-α-HCl/L-α quantum well structures. The main effective masses of electrons and holes were also determined for each crystal for a better understanding of their electronic transport properties.

Insights into structural, electronic, optical properties, and impedance spectroscopy of semi-doped perovskite Nd0.5Ba0.5CoO3: A theoretical and experimental study

In this study, the electronic, structural, and optical properties of semi-doped perovskite Nd0.5Ba0.5CoO3 (NBCO) were investigate using the CASTEP software, which relies on density functional theory (DFT) along with the correlation function of generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE). The mesh parameters of the (super) cell indicate that NBCO adopts an orthorhombic structure in the Pmmm space group, featuring a Coulomb Energy (EC) of approximately −244 eV. DFT and total density of states (DOS) analyses reveal that the as-prepared perovskite manifests metallic characteristics, and a notable covalent interaction is observed between the O-p and Co-d spin states, indicating strong hybridization between these orbitals. Remarkably, NBCO demonstrates 100 % spin polarization (P) at the Fermi level, with a calculated total magnetic moment of approximately 3.44 μB. Moreover, low dielectric loss (tgδ) values suggest facile electron movement, and impedance spectroscopy illustrates a dielectric relaxation phenomenon. DFT calculations predict an orthorhombic crystal structure with higher electron density at cobalt atom positions. Band structure and DOS analyses affirm metallic behavior owing to the robust hybridization between oxygen-p and cobalt-d orbitals. Optical property assessments reveal potential applications in spintronics and optoelectronics, with notable absorption in the ultraviolet–visible range. The conductivity spectra from impedance spectroscopy contribute to understanding temperature-dependent charge transport mechanisms. Dielectric constants decrease with increasing frequency and temperature, while dielectric losses remain low, indicating efficient electron movement. The material adheres to a metallic conduction mechanism as described by Drude's law, and the material's 100 % spin polarization at the Fermi level and a total magnetic moment of 3.44 μB are significant findings. Negative dielectric constants observed at specific frequencies suggest potential applications in metamaterials. Overall, the determined properties position NBCO cobaltite as a promising candidate for high-frequency devices, spintronic components, and sensors.

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.

Impact of Sapphire Step Height on the Growth of Monolayer Molybdenum Disulfide

Although the synthesis of molybdenum disulfide (MoS2) on sapphire has made a lot of progress, how the substrate surface affects the growth still needs to be further studied. Herein, the impact of the sapphire step height on the growth of monolayer MoS2 through chemical vapor deposition (CVD) is studied. The results show that MoS2 exhibits a highly oriented triangular grain on a low-step (0.44–1.54 nm) substrate but nanoribbons with a consistent orientation on a high-step (1.98–3.30 nm) substrate. Triangular grains exhibit cross-step growth, with one edge parallel to the step edge, while nanoribbons do not cross steps and possess the same orientation as the step. Scanning electron microscopy (SEM) reveals that nanoribbons are formed by splicing multiple grains, and the consistency of the orientation of these grains is demonstrated with a transmission electron microscope (TEM) and second-harmonic generation (SHG). Furthermore, our CP2K calculations, conducted using the generalized gradient approximation and the Perdew–Burke–Ernzerhof (PBE) functional with D3 (BJ) correction, show that MoS2 domains prefer to nucleate at higher steps, while climbing across a higher step is more difficult. This work not only sheds light on the growth mechanism of monolayer MoS2 but also promotes its applications in electrical, optical, and energy-related devices.

First principle investigation of tungsten based cubic oxide perovskite materials for superconducting applications: A DFT study

Perovskite will play an important role in the materials study to investigate the different properties. The CASTEP (Cambridge Serial Total Energy Package) code will be used to investigate the material properties. The structural, electronic, thermodynamic, and optical characteristics of novel tungsten-based oxide-perovskite materials WXO3 (where X = Cr and Hg) were investigated by using first principle calculations (CASTEP) with ultra-soft pseudo-potential PW (plane wave) and GGA (generalized gradient approaches)-PBE (Perdew-Burke-Ernzerhof) exchange correlation-functional. According to the calculations, both compounds have a space group of PM3m with a cubic structure. The results we discovered are entirely novel compounds, as demonstrated by computations. According to calculations, the band structure of WXO3 has a zero band gap. The graphical interpretations like DOS (density-of-states), partial DOS, and band structure in both perovskite materials reveal conclusive proof of conductive behavior. The presence of ionic bonds is revealed by a study of bonding characteristics shown by Poisson's ratios of 0.71 and 0.49 for WCrO3 and WHgO3, respectively. The WCrO3 and WHgO3 optical characteristics have also been studied and assessed. The Fermi level will be so highly overlapped that most of the electrons will lie from the conducting valence band VB to the conduction band CB and vice versa. Thermodynamic characteristics are studied, like Debye temperature, sound velocity, melting temperature, and compressibility. These compounds are extensively used in medical devices, sensitive devices, computer parts, and storage systems. Further investigation will be required on these materials with doping any element to enhance the band gap to study the other properties like photocatalytic and solar cell applications.

First-principle study of shear deformation effect on Mg adsorption by monolayer SnS2.

In this study, the effects of different shear deformations on the structural stability, electronic structure, and optical properties of a Mg atom adsorption system of S vacancy defect SnS2 are systematically investigated based on density functional theory. It is shown that the presence of an S-vacancy defect makes the band gap of the SnS2 system significantly smaller than that of the perfect SnS2 system, and the SnS2 system is changed from a direct band gap semiconductor to an indirect band gap semiconductor. The optimal adsorption position of a Mg atom on the S-vacancy SnS2 system is above the S atom where the adsorption energy is the largest and the system is the most stable. The density of states of the adsorption system is predominantly contributed by the S-3p and Sn-5s orbital electrons. The imposition of shear deformation leads to the introduction of certain impurity energy levels in the adsorption system, and the forbidden bandwidth near the Fermi energy level decreases. As compared to the intrinsic SnS2, the absorption and reflection peaks of adsorption systems under different shear deformation are red-shifted and appear in the ultraviolet region. This improves the utilization of the adsorption system for ultraviolet light to a great extent. The model calculations in this paper are performed using the CASTEP module of the Material Studio (MS) software based on the first principles of Density Functional Theory (DFT) (Wei et al. in Physica B 545:99-106, 2018) for plane wave artifacts. Geometrical optimization and computational procedures are used to calculate the exchange-correlation energy using the Perdew-Burke-Ernzerhof (PBE) generalized function (Perdew et al. in Phys Rev B Condens Matter 48:4978, 1993) of the generalized gradient approximation (GGA). The Monkhorst-Pack method (Monkhorst and Pack in Phys Rev B 13:5188-5192, 1976) was used to rationalize the sampling of the highly symmetric k-points in the Brillouin zone. The grid of k-points is set to be 6 × 6 × 1. The plane-wave truncation energy is set to be 400eV. The energy convergence criterion is 1.0 × 10-5eV. The residual stress of all atoms is 0.01eV/Å. A vacuum layer with a thickness of 15Å is set up in the z-direction, which ensures that the interactions of the system along the z-axis between the top and the bottom layers can be ignored during the whole simulation process. We construct a 3 × 3 × 1 SnS2 system containing 27 atoms as the computational model. The intrinsic SnS2 contains 9 Sn atoms and 18 S atoms.

First-principles study on the electronic properties of atom passivating CsPbI2Br surface

The electronic properties of the bulk CsPbI2Br and the passivation of CsPbI2Br (110) surface states are calculated by using first-principles calculations. It is found that the band gap of CsPbI2Br is 1.42[Formula: see text]eV by using the generalized gradient approximation of the Perdew–Burke–Ernzerhof function. The band gap is about 1.96[Formula: see text]eV by using the more complex Heyd–Scuseria–Ernzerhof mixed functional, which is closer to the experimental value of 1.92[Formula: see text]eV. The valence band top of CsPbI2Br bulk is mainly contributed by I-5p orbital and Br-4p orbital, and the conduction band bottom is mainly contributed by Pb-6p orbital. Through the calculation of CsPbI2Br (110) surface states passivated by Cl, F and H atoms, it is found that H atom has the best passivation effect. Its adsorption energy value fluctuates less, it is less sensitive to the adsorption position, and the adsorption is stable. Followed by F atom, its passivation effect is worse than that of H atom, but better than that of Cl atom. Although the passivation position has a certain influence on it, it has little effect. The Cl atom is most affected by the passivation position. The different positions of passivated atoms have a significant impact on adsorption energy. The adsorption stability is poor, and the passivation effect is also poor. Through analyzing the charge density difference and Bader charges, it is found that the H atom gets more electrons from the I atom, which is beneficial to passivate surface states. The electron-acquiring ability of the F atom is inferior to that of the H atom and is superior to that of the Cl atom. The electron-acquiring ability and passivation ability of Cl atom are the weakest among the three elements.

Theoretical investigation of optoelectronic properties of Al-doped BAs using TB-mBJ approach

For quite a long time, the scientific community has been searching for a material that surpasses the existing material systems in terms of energy conversion efficiency for photovoltaic applications. In this study, the optoelectronic properties of pure and Al-doped boron arsenide B[Formula: see text]Al[Formula: see text]As [Formula: see text] in the zinc blende (ZB) structure were systematically examined using the density functional theory (DFT) and the Modified Becke–Johnson (TB-mBJ) exchange correlation (XC) potential. The Perdew–Burke–Ernzerhof generalized gradient approximation (PBE) functional was used for structural optimization. After considering the ground state lattice constant of 4.82[Formula: see text]Å then calculating the bandgap energy of pure BAs, which are consistent with experimental and theoretical findings, we estimated the basic electronic and optical characteristics of Al-doped boron arsenide, including band structures, electronic density of state, dielectric function, refractive index, extinction coefficient, absorption and optical conductivity. The unusual and interesting optoelectronic features of the investigated compounds revealed in this work offer substantial promise for improving the energy conversion efficiency of solar cells.

Study of structural, elastic, electronic, optical and magnetic properties of Heusler Mn2NiAl: Ab initio calculations

In this paper, we will investigate the structural, electronic, mechanical, magnetic and optical properties of Heusler Mn2NiAl (MNA) compound using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) of the full potential linearized augmented plane-wave method for exchange and use correlations, modified Becke–Johnson and GGA+U Hubbard parameter. The calculated band structure (BS) and density of states (DOS) of MNA showed a metallic (GGA), nearly half-metallic (mBJ) and half metallic (GGA+U) behavior. Moreover, the magnetic computed magnetic moments by GGA+U are higher compared to GGA and mBJ results. Bulk modulus, shear modulus, Voigt and Reuss polycrystalline elastic modulus, Debye temperature, sound velocities, the melting temperatures, [Formula: see text] ratio, Young’s modulus and Poisson’s ratio were obtained. The elastic anisotropy of MNA alloy was analyzed using 2D and 3D figures of directional dependence of Poisson’s ratio, shear modulus, linear compressibility and Young’s modulus. Studies have shown that the Heusler material MNA has magnetic and anisotropic properties and is mechanically stable.

Electric field-driven up-and-down motion of the flexible tail of Al13+ cluster system-a nano-scale flipper.

Dynamic metal nanoclusters have become a hot area of research in the field of nanoscience and nanotechnology due to their potential applications in micro devices. One such dynamic cluster is a quasi-planar ground state (GS) Al13+ cluster which exhibits anelectric field driven up and down flipping motion of the flexible tail which oscillates with respect to the mean plane. A Car-Parrinello molecular dynamics (CPMD) simulation has been carried out to understand the nature of dynamics of the cluster. CPMD simulation study reveals that the flexible tail region of the Al13+ isomeric system (two ground states M1, M2 and a transition state TS connecting them) can be engaged in a systematic up down flipping motion by the application of a transverse electric field. A saw tooth electric field of amplitude 5.19V/nm is sufficient to induce the up-and-down flipping oscillation of the cluster, which has an average oscillation frequency of around 20 THz. AIM, NICS and AdNDP analyses also have been carried out to understand the fluxional nature of the cluster from the electronic structural perspective. Electronic structural analysis of selected optimized intermediate states in the presence of transverse electric field has also been analyzed to correlate the electronic structure with the dynamic nature of the cluster. Single-point energies of all intermediate states between two minima of Al13+ clusters connected through a transition state cluster. Optimized geometries of Al13+ clusters in the presence of electric field of different strengths have been carried out by using the Gaussian 03 package. 6-311 + G(d) basis set and B3LYP hybrid density functional have been utilized for these studies. To establish the flipping motion, Car-Parrinello molecular dynamics (CPMD) has been performed using the cp.x module of the Quantum ESPRESSO 6.3.0 program package using the Perdew-Burke-Ernzerhof (PBE) functional, plane-wave basis set and ultrasoft pseudopotentials. ORTEP-3 and POV ray-3.7 software packages have been used for visualization and graphics generation. Atoms in molecule (AIM), Adaptive Natural Density Partitioning (AdNDP) analysis have been carried out using Multiwfn 3.7 program package.

An exhaustive analysis of the structural, electronic, magnetic, mechanical, thermal, thermoelectric, optical and thermodynamic properties of AMnBi2 (A = Ca and Sr) via DFT+U and Monte Carlo simulations

Ab initio calculations are used in the present study to analyze the structural, magnetoelectronic, mechanical, thermal, thermoelectric, optical and thermodynamic properties of AMnBi2 compounds (where A = Ca and Sr). The calculations use the density functional theory involving the Full Potential –Linearized Augmented Plane. Wave and the Monte Carlo simulation. We have used the Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional for modelling the exchange-correlation potential with Hubbard correction (GGA + U) was employed. The bulk modulus, its first derivative, and the optimal lattice parameters have been found. Both compounds display metallic nature. The electronic transport coefficients have been calculated. The entropy, thermal expansion coefficient, Debye temperature, and heat capacities Cv and Cp are calculated. The elastic constants at zero pressure, the bulk modulus B, and the shear modulus G have been evaluated. The Young's modulus and Poisson's ratio of two compounds have been performed. The Debye temperature is also approximated from the average sound velocity. The Néel temperature, magnetization, and hysteresis cycles are found.

First-principles calculations on electronic, superconducting, mechanical, and thermodynamic properties for the superhard compound Tc2C under pressure

This study presents a theoretical investigation on the electronic, superconducting, mechanical, and thermodynamic properties of the superhard compound Tc2C up to 100 GPa. The investigation is conducted via the first-principles plane-wave pseudopotential method. Furthermore, to treat the exchange-correlation potential for the total energy calculations, the revised Perdew–Burke–Ernzerhof formulation stemming from the generalized gradient approximation is employed. The results suggest that Tc2C is a potential hard compound due to the robust covalent bonding between Tc and C atoms. Moreover, the electronic structure and electron–phonon coupling reveal that Tc2C is a superconducting material with Tc of 0.59 K at 0 GPa. Additionally, using the quasi-harmonic Debye model, the variation of crucial structural parameters, such as the volume thermal expansion coefficient, heat capacity, and isothermal bulk modulus and its pressure derivatives, is predicted in terms of temperature and pressure. The results demonstrate that temperature and pressure have contrasting effects on the volume thermal expansion coefficient and heat capacity, and the effect of temperature is larger than that of pressure, especially at low temperatures. The isothermal bulk modulus decreases with increasing temperature but increases with increasing pressure. The first and second pressure derivatives of the isothermal bulk modulus for Tc2C are not the assumed constants in the equation of state study at high pressures and temperatures. To our knowledge, this study presents the first quantitative theoretical prediction of the pressure and temperature dependence of the thermodynamic parameters for Tc2C, which is essential for understanding the basic physical properties of similar transition metal carbides.

Construction of SrTiO3/TiO2 heterojunction by in-situ ion exchange method to enhance photochemical performance

Construction of heterojunctions has been shown to be one of the most promising methods for the preparation of highly efficient catalysts, because it is feasible and effective for the separation of electron-hole pairs, prolonging the lifetime of photogenerated electrons. In this study, TiO2 was grown on fluorine-doped tin oxide glass (FTO), and then a strontium source was added to obtain heterojunctions between SrTiO3 and TiO2 using in-situ ion exchange hydrothermal method. The in-situ grown SrTiO3/TiO2 (TST) composite has a better absorption performance, a narrower band gap, and a lower carrier recombination rate compared to the pure TiO2 sample, which improves the photoelectrochemical performance. The photocurrent density of TST-10 (hydrothermally treated for 10 h) has the largest value of 0.06 mA·cm−2, which is about three times higher than that of pure TiO2. Then, the first-principles density functional theory (DFT) calculation was used to further explore the photocatalytic mechanism of TST. The exchange correlation energy was based on the Generalised Gradient Approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional calculation method to study the electronic and optical properties of SrTiO3/TiO2 heterostructures. The results show that the carrier transfer rate and the optical absorption coefficient of the material increase significantly after the establishment of the in-situ heterojunction. This study offers insight into the heterostructures’ interface significance on photocatalytic activity.

Structural, electronic and magnetic properties of the CuFeO[formula omitted] multiferroic compound

The structural, electronic and magnetic properties of the delafossite material CuFeO2 were studied using Density Functional Theory (DFT) combined with Monte Carlo simulation (MCS). The structural stability was verified in this work and showed that this material is antiferromagnetic (AFM). The density of states (DOS) carried out using the generalized Perdew–Burke–Ernzerhof gradient (PBE-GGA), Tran-Balaha modified Becke–Johnson exchange potentials (TB-mBJ; nmBJ) and the GGA＋ U show that the compound has a metallic behavior in PBE-GGA and semiconductor behavior in TB-mBJ, nmBJ and GGA＋ U approximations. The obtained partial magnetic moment was used to study the magnetic behavior by Monte Carlo simulation, particularly, the evolution of magnetization as a function of temperature and phase transition. The results of MCS showed the presence of two-phase transitions: ferromagnetic–antiferromagnetic (FM-AFM) first-order phase transition and antiferromagnetic–paramagnetic (AFM-PM) second-order phase transition. The Neel temperatures found are TN2=11.2 K for the first transition and TN1=16.6 K for the second transition.

Quantifying the Accuracy of Density Functionals on Transition Metal Bulk and Surface Properties.

Density functional theory would be exact when the exact exchange-correlation (xc) functional would be known, but since it is regretfully not known, dozens of xc functionals have been developed in the past decades, with some of them better suited for describing certain systems and/or properties. For transition metals (TMs), recent systematic studies assessing bulk properties─shortest interatomic bond distance, δ, cohesive energy, Ecoh, and bulk modulus, B0─and surface features─surface energy, γ, work function, ϕ, and interlayer distances, δij─of 27 TM bulks and 81 TM surfaces, highlighted that generalized gradient approximation (GGA) based xc functionals are, overall, better suited than other types of xc functionals for the TMs bulk and surfaces description, such as Perdew-Burke-Ernzerhof (PBE) or Vega-Viñes (VV). Still, some basic local density approximation xc functionals were not assessed, such as the Hedin-Lundqvist (HL) and Perdew-Zunger (PZ), or GGAs such as the revised Perdew-Burke-Ernzerhof (revPBE) or the Armiento-Mattsson (AM05). Here, we expand the analysis by not only including them but also the recent meta-GGA strongly constrained appropriately normed (SCAN) xc functional, characterized by fulfilling all 17 mathematical conditions an xc must comply, plus the Bayesian error estimation functional (BEEF) xc, a functional parametrized over a large and diverse set of experimental results using machine learning. The present results reveal that none of the xc studied excel neither PBE nor VV, yet AM05 and SCAN performance is quite acceptable, while BEEF xc probably needs more shells of parametrization to reach competitive accuracy levels.

Surface functionalization of graphene-like boron arsenide monolayer: a first-principles study

In this work, the effects of hydrogen (H) and oxygen (O) adsorption on the electronic and magnetic properties of graphene-like boron arsenide (BAs) monolayer are investigated using first-principles calculations. Pristine monolayer is a non-magnetic two-dimensional (2D) material, exhibiting direct gap semiconductor character with band gap of 0.75 (1.18) eV as calculated by generalized gradient approximation with Perdew–Burke–Ernzerhof (HSE06) functional. Four high-symmetry adsorption sites are considered, including on-top of B atom (), on-top of As atom (), on-top of hollow site (), and on-top of bridge site (). Using the criterion of adsorption energy, it is found that and sites are favorable adsorption sites for H and O adatom, respectively. The analysis of electronic interactions indicate the charge transfer from host BAs monolayer to both adatoms. H adsorption conducts to the emergence of magnetic semiconductor nature in BAs monolayer with a total magnetic moment of 1.00 . Herein, the magnetism is originated mainly from H adatom and its neighbor As atoms. In contrast, the non-magnetic nature of BAs monolayer is preserved upon absorbing O atoms. In this case, the energy gap exhibits a slight reduction of 4%. Further, the effects of adatom coverage are also analyzed. The presented results suggest an effective modification of ground state electronic properties, as well as induction of new feature-rich properties to make new multifunctional 2D materials from non-magnetic BAs monolayer.