Ab Initio Density Functional Theory Research Articles

Cs2TiI6 (Cs2TiIxBr6-x) Halide Perovskite Solar Cell and Its Point Defect Analysis.

This work presents a comprehensive numerical study for designing a lead-free, all-inorganic, and high-performance solar cell based on Cs2TiI6 halide perovskite with all-inorganic carrier transport layers. A rigorous ab initio density-functional theory (DFT) calculation is performed to identify the electronic and optical properties of Cs2TiI6 and, upon extraction of the existing experimental data of the material, the cell is designed and optimized to the degree of practical feasibility. Consequently, a theoretical power conversion efficiency (PCE) of 21.17% is reported with inorganic TiO2 and CuI as carrier transport layers. The calculated absorption coefficient of Cs2TiI6 reveals its enormous potential as an alternative low-bandgap material for different solar cell applications. Furthermore, the role of different point defects and the corresponding defect densities on cell performance are investigated. It is found that the possible point defects in Cs2TiI6 can form both the shallow and deep defect states, with deep defect states having a prominent effect on cell performance. For both defect states, the cell performance deteriorates significantly as the defect density increases, which signifies the importance of high-quality material processing for the success of Cs2TiI6-based perovskite solar cell technology.

Electronic Structure and Mechanical Properties of Solvated Montmorillonite Clay Using Large-Scale DFT Method

Montmorillonite clay (MMT) has been widely used in engineering and environmental applications as a landfill barrier and toxic waste repository due to its unique property as an expandable clay mineral that can absorb water easily. This absorption process rendered MMT to be highly exothermic due to electrostatic interactions among molecules and hydrogen bonds between surface atoms. A detailed study of a large supercell model of structural clay enables us to predict long-term nuclear waste storage. Herein, a large solvent MMT model with 4071 atoms is studied using ab initio density functional theory. The DFT calculation and analysis clarify the important issues, such as bond strength, solvation effect, elasticity, and seismic wave velocities. These results are compared to our previous study on crystalline MMT (dry). The solvated MMT has reduced shear modulus (G), bulk modulus (K), and Young’s modulus (E). We observe that the conduction band (CB) in the density of states (DOS) of solvated MMT model has a single, conspicuous peak at −8.5 eV. Moreover, the atom-resolved partial density of states (PDOS) summarizes the roles played by each atom in the DOS. These findings illuminate numerous potential sophisticated applications of MMT clay.

Oxygen effect on the synthesis of vinyl acetate on Pd (100) and Pd/Au (100) surfaces: A periodic DFT study

Vinyl acetate is mainly produced by oxidative acetoxylation of ethylene over the Pd/Au bimetallic heterogeneous catalyst. This process requires the involvement of oxygen, but the role of oxygen has not been well studied. The oxygen effect on the synthesis of vinyl acetate is of considerable economic benefit, as the process conditions can be better optimized. In this study, the O2 adsorption and dissociation on the Pd (100) and Pd/Au (100) surfaces, the effect of the co-adsorbed O atom on the C2H4, CH3COOH and CH3COO adsorptions, and the reaction mechanism have been investigated using ab initio density functional theory (DFT) method. The calculated results suggest that the O adatoms are detrimental to the adsorption of C2H4 and CH3COO, but favorable for CH3COOH. For the reaction, the O adatoms reduce the energy barrier of CH3COOH dehydrogenation and VA formation, and increase the reaction enthalpy (more exothermic). This study provides further mechanistic insight into VA synthesis on the Pd-based catalysts.

Spectroscopic investigations, quantum chemical, drug likeness and molecular docking studies of methyl 1-methyl-4-nitro-pyrrole-2-carboxylate: A novel ovarian cancer drug

Density functional theory (DFT) calculation was used to analyse the structural and vibrational properties of Methyl 1-Methyl-4-nitro-pyrrole-2-carboxylate (MMNPC) using the cc-pVTZ basis set. The potential energy surface scan and the most stable molecular structure were optimized using Gaussian 09 program. A potential energy distribution calculation was used to calculate and assign vibrational frequencies using the VEDA 4.0 program package. The Frontier Molecular Orbitals (FMOs) were analysed to determine their related molecular properties. Ab initio density functional theory (B3LYP/cc-pVTZ) method with basis set was used to calculate 13C NMR chemical shift values of MMNPC in the ground state. Fukui function and molecular electrostatic potential (MEP) analysis confirmed the bioactivity of the MMNPC molecule. The charge delocalization and stability of the title compound were studied using natural bond orbital analysis. All experimental spectral values from FT-IR, FT-Raman, UV–VIS, and 13C NMR are in good agreement with the value calculated by the DFT. Molecular docking analysis was carried out to find the MMNPC compound that can be used as a potential drug development candidate for ovarian cancer.

Structural and electronic properties of U4O9 from ab initio and empirical potential calculations

We address here the description of the different phases of ${\mathrm{U}}_{4}{\mathrm{O}}_{9}$ with ab initio density functional theory and molecular dynamics empirical potential calculations. These phases are built from the $\mathrm{U}{\mathrm{O}}_{2}$ fluorite by the addition of oxygen interstitials which assemble in clusters named cuboctahedra. Due to the unit cell size and complexity, their simulation represents a formidable task for numerical approaches. With $\mathrm{DFT}+U$, we study in detail two potential structures for $\ensuremath{\alpha}\ensuremath{-}{\mathrm{U}}_{4}{\mathrm{O}}_{9}$, however, these are just two representatives among the numerous coexisting structures we found. The role of the different valence uraniums (${\mathrm{U}}^{4+}, {\mathrm{U}}^{5+}$, and possibly ${\mathrm{U}}^{6+}$) is highlighted thanks to our quantum-mechanical approach. Temperature effects are then appraised with empirical potentials to describe the evolution of the ${\mathrm{U}}_{4}{\mathrm{O}}_{9}$ structure with temperature. We observe a continuous symmetrization of the structure with increasing temperature. Indeed, below 800 K, it gradually turns cubic and the cuboctahedral oxygen clusters tend to symmetrize. Beyond 800 K, the cuboctahedra start to disappear up to 1400 K, where none can be found in our simulated structures.

Theoretical investigation of substitutionally doped symmetrical finite phenine nanotubes

Ab initio density functional theory calculations were performed to investigate the substitutional doping of boron and nitrogen in symmetrical phenine nanotubes. It was found that the doped structures are energetically favorable and can be grown experimentally. Furthermore, we investigated how electronic and magnetic properties of these nanotubes change in the presence of electron-rich (nitrogen) and electron-deficient (boron) impurities, and found that doping in these tubes resulted in a non-magnetic structure. The doping changes the electronic structure of the tube by reducing the energy gap and the reduction value is sensitive to the nanotube diameter and impurity type. The gap corresponding to the nitrogen doped tubes is in the range of 2.38 eV-2.64 eV, whereas for boron doped tubes, the value lies in 1.91 eV to 2.11 eV, respectively. Moreover, the nitrogen doped phenine nanotubes tend to be n-type semi-conductor. The doped phenine nanotube molecules with a high stability and tunable electronic properties may offer several interesting applications in nano-electronics.

Spin-orbit coupling and electronic properties in Pt2MnGa: an ab-initio study

The electronic and magnetic properties of full-Heusler alloy Pt2MnGa with and without the effect of spin–orbit coupling are studied. The calculations have been carried out using ab initio density functional theory. Both the magnetic spin orders of Pt2MnGa, ferromagnetic and antiferromagnetic, are considered. It is found that the ferromagnetic spin arrangement is the most stable spin order at the ground state, regardless of the incorporation of spin–orbit coupling. The density of states and band structure plots are used to validate the obtained ground state structure, which is further validated by the Bader charge analysis and the charge density distribution of the individual atoms. The obtained results of magnetocrystalline anisotropy energy, total magnetic moment, Curie temperature, and the spin-polarization hint at the possible application of this compound in spintronics devices such as bit-patterned media.

Direct Probing of Lattice-Strain-Induced Oxygen Release in LiCoO2 and Li2 MnO3 without Electrochemical Cycling.

Since the recognition of a significant oxygen-redox contribution to enhancing the capacity of Li transition-metal oxide cathodes, the oxygen release and subsequent structural variations together with capacity fading have been critical issues to achieve better electrochemical performance. As most previous reports dealt with the structural degradation of cathodes after electrochemical cycling, it has been fairly difficult to clarify how substantial the effect of lattice strain on the oxygen release would be while exclusively ruling out any electrochemical influences. By utilizing nanoindentation and mechanical surface polishing of single-crystal LiCoO2 and Li2 MnO3 , we scrutinize the local variations of both the atomic structure and oxygen content. Atomic-column-resolved imaging reveals that local Li-M (M = Co and Mn) disordering and further amorphization are induced by mechanical strain. Moreover, substantial oxygen deficiency in the regions with these structure changes is directly identified by spectroscopic analyses. Ab initio density functional theory calculations also demonstrate energetically favorable formation of oxygen vacancies under shear strain. Providing direct evidence of oxygen release as a consequence of lattice strain, the findings in this work suggest that efficient strain relaxation will be of great significance for longevity of the anion framework in layered oxide cathodes. This article is protected by copyright. All rights reserved.

Electrons Surf Phason Waves in Moiré Bilayers.

We investigate the effect of thermal fluctuations on the atomic and electronic structure of a twisted MoSe2/WSe2 heterobilayer using a combination of classical molecular dynamics and ab initio density functional theory calculations. Our calculations reveal that thermally excited phason modes give rise to an almost rigid motion of the moiré lattice. Electrons and holes in low-energy states are localized in specific stacking regions of the moiré unit cell and follow the thermal motion of these regions. In other words, charge carriers surf phason waves that are excited at finite temperatures. We also show that such surfing survives in the presence of a substrate and frozen potential. This effect has potential implications for the design of charge and exciton transport devices based on moiré materials.

Investigating the Increased CO2 Capture Performance of Amino Acid Functionalized Nanoporous Materials from First-Principles and Grand Canonical Monte Carlo Simulations

Nanoporous materials such as metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) have been identified as key candidates for environmental remediation through catalytic reduction and sequestration of pollutants. Given the prevalence of CO2 as a target molecule for capture, MOFs and COFs have seen a long history of application in the field. More recently, functionalized nanoporous materials have been demonstrated to improve performance metrics associated with the capture of CO2. We employ a multiscale computational approach including ab initio density functional theory (DFT) calculations and classical grand canonical Monte Carlo (GCMC) simulations, to investigate the impact of amino acid (AA) functionalization in three such nanoporous materials. Our results demonstrate a nearly universal improvement of CO2 uptake metrics such as adsorption capacity, accessible surface area, and CO2/N2 selectivity for six AAs. In this work, we elucidate the key geometric and electronic properties associated with improving the CO2 capture performance of functionalized nanoporous materials.

Engineering 2D MoS2 for Enhanced Selectivity and Sensitivity of Selected Green House Gases (CO2, CH4 and N2O): An ab initio Study

2D MoS2 has been identified as a potential material for greenhouse gas (CO2, N2O, and CH4) sensing, however, the underlying principles are not well understood yet such knowledge may offer optimization opportunities for the development of superior devices. Using ab initio Density Functional Theory approach, selected dopants X (X= Cl, O, P, and Se) were judiciously introduced on 2D MoS2 and the response of the modified monolayer to the various greenhouse gases analyzed. The results showed that when the gas molecules are introduced within the periphery of the dopant site, the gases become adsorbed on the dopant with interactions that are characteristic of physisorption due to moderate adsorption energies. Based on the magnitude of adsorption energies, Cl, P, and O doped 2D MoS2 were found to be sensitive and selective to CO2, N2O and CH4 gas molecules respectively. Further, charge transfer analysis of CO2, and N2O adsorbed on Cl and P doped 2D MoS2 monolayer respectively, showed that these gas molecules gain electrons from the doped 2D MoS2 monolayer during adsorption, while CH4 molecule loses electrons to O doped 2D MoS2 surface. In addition, CO2, N2O, and CH4 gas molecules are desorbed from the surfaces of Cl, P, and O doped 2D MoS2 at 432 K, 438 K and 298 K respectively, thus rendering the modified monolayer re-usable detection of greenhouse gases. Cl doped 2D MoS2 monolayer showed superior performance in CO2 detection due to its fast recovery time and moderate thermal treatment temperature.Our results reveal that the competitive adsorption at the X doped 2D MoS2 surfaces between the analyte and the greenhouse gas present in the atmosphere induces a switching in surface conductance that can be related to the greenhouse gases concentration and can be detected electronically, thus disclosing the sensing mechanism and optimization route that can guide future experimental studies.

Formation mechanism of mirror twin grain boundaries in molecular beam epitaxy grown monolayer WSe2–MoSe2 lateral heterojunctions

Mirror twin grain boundary (MTB) defects, being a special type of high-symmetry one-dimensional (1D) defects in two-dimensional atomically thin transition metal dichalcogenides (TMDCs), have received considerable interest due to their unique structures and intriguing 1D properties. However, formation and distribution of MTBs in hybrid TMDC materials such as heterojunction remain scarcely studied. Herein, we investigate the spatial distribution, lattice registry and formation mechanism of MTBs in molecular beam epitaxy grown monolayer WSe2–MoSe2 lateral heterojunctions using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM). MTBs manifest a much higher density in MoSe2 than in WSe2 domains with a few of them spanning coherently across the domain interface. Compositionally, a Mo-dominant rather than W-dominant configuration was observed in those MTBs located in WSe2 domains and its origin can be attributed to the preferable Mo substitution to W along the MTBs occurring at the later MoSe2 growth period. This proposed mechanism is supported by ab-initio density functional theory calculations and substitution dynamics captured by in-situ ADF-STEM. The present study deepens our understanding of MTBs in heterostructured TMDCs, which may also serve as an excellent platform for the exploration of intriguing 1D physics.

A comparative investigation for identification of N-(4-dimethylamino 3,5-dinitrophenyl)maleimide

This study has identified the characteristic behaviors of N-(4-dimethylamino 3,5-dinitrophenyl)maleimide molecule using ab initio Hartree-Fock (HF) and density functional theory (DFT) based on Becke’s three-parameter hybrid exchange functional combined with Lee-Yang-Parr non-local correlation function (HF/B3LYP and DFT/B3LYP) at 6-311G++(d,p) level of theory for the first time. On this basis, the optimized molecular structures, some thermodynamic features at 300 K, function groups of structures, charge distributions-dipole moments, molecular charge transfer regions, spectroscopic characteristic properties, vibrational frequencies, nuclear magnetic resonance chemical shifts of 13C-NMR and 1H-NMR spectra, and corresponding vibrational assignments have been investigated in detail. Comparisons between some experimental findings and theoretical results are performed to test the reliability of the calculation method preferred in the study. The comparison results in high correlation parameters such as R2 =0.976 and R2 =0.985 for the molecular structures and vibrational frequencies in the DFT and HF calculation levels, respectively. Moreover, the obtained vibrational frequencies and calculated results are in good agreement with the experimental data. Additionally, the simulations of highest/lowest occupied/unoccupied molecular orbital (HOMO and LUMO), molecular electrostatic potential (MEP), and electrostatic potential (ESP) maps have shown that there appear strong non-uniform intra-molecular charge distributions (ICT), electron engagements, lone pairs of electrons, π-π* conjugative effects based on the bond weakening, and intermolecular hydrogen bonding in the compound. Correspondingly, the molecule with the electrophilic reactive and nucleophilic regions has been noted to exhibit kinetical chemical stability. All the discussions have been confirmed by means of the findings of optimized molecular structures and vibrational frequencies belonging to the molecule.

First principle calculations: The electronic, optical, mechanical, and vibrational properties of TiS3

The computational predictions of transition-metal tri-chalcogenide (TMTCs) were performed using ab initio density functional theory (DFT) to investigate the electronic band structure, the partial density of states (PDOS), optical absorptions, dielectric functions, complex conductivity, reflectivity, refractive index, electron loss, the Poisson’s ratio, Young’s modulus, bulk-to-shear ratio, and phonon dispersion. The bandgap is measured from the valence band maximum (VBM) to the conduction band minimum (CBM) with the G–Z transitions. This suggests that the material is an indirect bandgap semiconductor. The electronic bandgap ([Formula: see text] is significantly improved with nonlocal hybrid functionals, especially in HSE0s, with [Formula: see text] of 1.0[Formula: see text]eV, which is in excellent agreement with the experimental data. However, our data shows that the HF-LDA exchange correlations significantly overestimate the [Formula: see text] with 7.33[Formula: see text]eV. Also, our optical absorption data indicates a high absorption coefficient of about [Formula: see text][Formula: see text]cm[Formula: see text]. The absorption peak of 7.4[Formula: see text]eV indicates TiS3 can be applied in vacuum ultra-violet (VUV) applications. The reflectivity is also shown to be high, with over 90% of light being reflected. The mechanical stability of the monoclinic system can be testified by our elastic coefficients and the phonon dispersions.

Unraveling the atomic and electronic structure of nanocrystals on superconducting Nb(110): Impact of the oxygen monolayer

The niobium surface is almost always covered by a native oxide layer which greatly influences the performance of superconducting devices. Here we investigate the highly stable niobium oxide overlayer of Nb(110), which is characterized by its distinctive nanocrystal structure as observed by scanning tunneling microscopy (STM). Our ab initio density functional theory (DFT) calculations show that a subtle reconstruction in the surface niobium atoms gives rise to rows of fourfold coordinated oxygen separated by regions of threefold coordinated oxygen. The fourfold oxygen rows are determined to be the source of the nanocrystal pattern observed in STM and the two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) are ascribed to the threefold and fourfold oxygens. Furthermore, we find excellent agreement between the DFT calculated electronic structure with scanning tunneling spectroscopy and valence XPS measurements.

Competing Magnetic Interactions and the Role of Unpaired 4f Electrons in Oxygen-Deficient Perovskites Ba3RFe2O7.5 (R = Y, Dy)

Oxygen-deficient perovskite compounds with the general formula Ba3RFe2O7.5 present a good opportunity to study competing magnetic interactions between Fe3+ 3d cations with and without the involvement of unpaired 4f electrons on R3+ cations. From analysis of neutron powder diffraction data, complemented by ab initio density functional theory calculations, we determined the magnetic ground states when R3+ = Y3+ (non-magnetic) and Dy3+ (4f9). They both adopt complex long-range ordered antiferromagnetic structures below TN = 6.6 and 14.5 K, respectively, with the same magnetic space group Ca2/c (BNS #15.91). However, the dominant influence of f-electron magnetism is clear in temperature dependence and differences between the size of the ordered moments on the two crystallographically independent Fe sites, one of which is enhanced by R-O-Fe superexchange in the Dy compound, while the other is frustrated by it. The Dy compound also shows evidence for temperature- and field-dependent transitions with hysteresis, indicating a field-induced ferromagnetic component below TN.

Effects of Alloying Elements on the Solution and Diffusion of Oxygen at Iron Grain Boundary Investigated by First-Principles Study

The effects of alloying elements (Si, Cr, Mo) on the solution and diffusion of oxygen (O) atoms at the grain boundary of iron (Fe) Σ5(310)/[001] are investigated by the simulations of ab initio density functional theory (DFT). It is found that Si, Mo and Cr prefer to segregate to the grain boundary, and further affect the solution and diffusion of O atoms at Fe grain boundaries. The segregated Cr promotes the solution of O, while Si and Mo inhibit the solution of O at the grain boundary. Meanwhile, Cr and Si accelerate the diffusion of O, and Mo retards the diffusion of O in the grain boundary. Further analysis indicates that the effects are closely related to the interactions between the alloying elements and O atoms, which are determined by the competition between the distortion of local structure and the charge transfer between local atoms. Finally, the effects of alloying elements on the O concentration distribution near the grain boundary are explored by employing the Langmuir–McLean models. This work not only provides insights into the effects of alloying elements on the solution and diffusion of O at grain boundaries, but also provides parameters of the atomic interactions for the initial oxidation simulation on a large scale, which relates to the growth of oxide in polycrystalline systems with various grain sizes at experimental temperatures.

Investigation of the electronic, optical, elastic, mechanical and vibrational properties of CuI using HSE03

Ab Initio density functional theory (DFT) simulations have been employed to systematically explore the electronic, optical, elastic, mechanical and vibrational properties. In this study, we revealed that [Formula: see text]-CuI has a wide direct bandgap energy of 3.21 eV, is pure covalent and brittle. We also found that the core level is made up of I s electron orbitals, the valence band is constructed with I p orbitals, and the Cu s orbital states mainly contribute to the conduction band minimum (CBM). The reflectivity of CuI is reported to be low (35.9% for the light reflected), showing high material absorption. A high absorption coefficient of [Formula: see text] is also reported. The elastic and mechanical properties can further confirm the mechanical stability of the CuI system, derived from DFT-calculated elastic constants and phonon dispersion from density functional perturbation theory (DFPT) calculations.

Comprehensive ab initio study of effects of alloying elements on generalized stacking fault energies of Ni and Ni3Al

Excellent high-temperature mechanical properties of Ni-based single-crystal superalloys (NSCSs) are attributed to the yield strength anomaly of ${\mathrm{Ni}}_{3}\mathrm{Al}$ that is intimately related to generalized stacking fault energies (GSFEs). Therefore, clarifying the effects of alloying elements on the GSFEs is of great significance for alloys design. Here, by means of ab initio density functional theory calculations, we systematically calculated the GSFEs of different slip systems of Ni and ${\mathrm{Ni}}_{3}\mathrm{Al}$ without and with alloying elements using the alias shear method. We obtained that for Ni, except for magnetic elements Mn, Fe, and Co, most of the alloying elements decrease the unstable stacking fault energy (${\ensuremath{\gamma}}_{\text{USF}}$) of the $[01\overline{1}](111)$ and $[11\overline{2}](111)$ slip systems and also decrease the stable stacking fault energy (${\ensuremath{\gamma}}_{\text{SF}}$) of the $[11\overline{2}](111)$ slip system. Interestingly, the reduction effects exhibit a strong correlation with the inverse of atom radii. For ${\mathrm{Ni}}_{3}\mathrm{Al}$, most of the alloying elements in groups IIIB--VIIB show a strong Al site preference. Except for Mn and Fe, the elements in groups VB--VIIB and the first column of group VIII increase the values of ${\ensuremath{\gamma}}_{\text{USF}}$ of different slip systems of ${\mathrm{Ni}}_{3}\mathrm{Al}$, which makes the slip deformation and dislocation emits difficult. On the other hand, the elements in groups IIIB--VIIB also increase the value of ${\ensuremath{\gamma}}_{\text{SF}}$, and thus reduce the stability of the antiphase boundary, complex stacking fault, and superlattice intrinsic stacking fault of ${\mathrm{Ni}}_{3}\mathrm{Al}$. We found that Re is an excellent strengthening alloying element that significantly increases the slip barrier of the tailing slip process for Ni, and also enhances the slip barrier of the leading slip process of three slip systems for ${\mathrm{Ni}}_{3}\mathrm{Al}$. W and Mo exhibit similar effects as Re. We predicted that Os, Ru, and Ir are good strengthening alloying elements as well, since they show the strengthening effects on both the leading and the tailing slip process for Ni and ${\mathrm{Ni}}_{3}\mathrm{Al}$. This work established an exhaustive dictionary of the effects of various alloying elements on the GSFEs of both Ni and ${\mathrm{Ni}}_{3}\mathrm{Al}$ phases, which would help to guide the design of next-generation high-performance NSCSs.

Bulk Modulus, Elastic Constants, and Force Constants of Interatomic Bonds of II–III2–VI4 Compounds

Ab initio density functional perturbation theory is used to calculate the elastic constants of CdGa2S4, CdGa2Se4, CdGa2Te4, and ZnGa2Se4 thiogallates. The values of the bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are determined. The force constants of interatomic bonds in CdGa2Te4 and ZnGa2Se4 compounds are calculated. Regularities in the dependence of the frequencies of Raman‐active phonons on the atomic masses are established.