Ab Initio Density Functional Theory Research Articles

Strain Engineering of Intrinsic Ferromagnetism in 2D van der Waals Materials.

Since the discovery of the low-temperature, long-range ferromagnetic order in monolayers Cr2Ge2Te6 and CrI3, many efforts have been made to achieve a room temperature (RT) ferromagnet. The outstanding deformation ability of two-dimensional (2D) materials provides an exciting way to mediate their intrinsic ferromagnetism (FM) with strain engineering. Here, we summarize the recent progress of strain engineering of intrinsic FM in 2D van der Waals materials. First, we introduce how to explain the strain-mediated intrinsic FM on Cr-based and Fe-based 2D van der Waals materials through ab initio Density functional theory (DFT), and how to calculate magnetic anisotropy energy (MAE) and Curie temperature (TC) from the interlayer exchange coupling J. Subsequently, we focus on numerous attempts to apply strain to 2D materials in experiments, including wrinkle-induced strain, flexible substrate bending or stretching, lattice mismatch, electrostatic force and field-cooling. Last, we emphasize that this field is still in early stages, and there are many challenges that need to be overcome. More importantly, strengthening the guideline of strain-mediated FM in 2D van der Waals materials will promote the development of spintronics and straintronics.

First-principles Calculations Reveal Frictional Advantage for C2N/C6N6 van der Waals Heterostructures.

Friction at the atomic scale is determined for three different carbon nitride structures namely C2N/C2N, C6N6/C6N6 and C6N6/C2N employing ab-initio density functional theory (DFT). The sliding path along the lowest energy corrugations determines the static frictional forces. Both the homo-layer structures (C2N/C2N and C6N6/C6N6) have higher corrugation energy and correspondingly higher static lateral forces with respect to the hetero-layer structure (C2N/C6N6). The corrugation energy for the C2N/C6N6 heterostructure ([[EQUATION]] = 0.29 meV/atom) is one-order lower than C2N/C2N ([[EQUATION]]= 2.08 meV/atom) and C6N6/C6N6 ([[EQUATION]] = 4.37 meV/atom). Such a significantly lower corrugation energy for the heterostructure arises due to the reduced fluctuation in the interfacial charge density along the sliding pathway. Moreover, the change in the interlayer distance along the sliding pathway is only 0.2 Å for the heterostructure while its 0.3 Å and 0.4 Å for C2N and C6N6 homo-layers respectively. The friction coefficients (FL/FN, FL=static lateral force; FN=normal force) decrease with increasing load for all the systems with the lowest value (0.04) for C2N/C6N6 at 2 GPa. The van der Waals heterostructures are, therefore, predicted to be highly efficient lubricant materials for reducing friction at the atomic scale.

Hexagonal MnTe with Antiferromagnetic Spin Splitting and Hidden Rashba–Dresselhaus Interaction for Antiferromagnetic Spintronics

AbstractHexagonal MnTe emerges as a critical component in designing magnetic quantum heterostructures, calling for a detailed study. After finding a suitable combination of exchange–correlation functional and corrections, this study within ab initio density functional theory uncovers an insulating state with a preferred antiferromagnetic (AFM) order. The exchange interaction strengths are computed to estimate the AFM ordering temperature via Monte Carlo calculations. These calculations and symmetry analysis reveal a large spin splitting in the system due to the AFM order without considering spin–orbit interaction, except in the kx‐ky plane. Critically examining the band dispersion and spin textures obtained from these calculations and comparing them with an insightful symmetry analysis and analytical model, a combined Rashba–Dresselhaus interaction in the kx‐ky plane, around the K point of the system, is confirmed. These results and insights would help design heterostructures of MnTe for technological applications.

The structural, electronic, optical, mechanical and vibrational properties of Te2W with HSE03 functional

The structural, electronic, optical, mechanical and vibrational properties of hexagonal Di-Tellurium-Tungsten (Te2W) are exploited with a hybrid Heyd–Scuseria–Ernzerhof 2003 (HSE03) functional and semi-local general gradient approximation (GGA) scheme using ab initio density functional theory (DFT) calculations. The structural and electronic properties have been analyzed with the band structure and electron orbitals from the partial density of states (PDOS). Our reflectivity data from optical spectra explain its applications over mechanical engineering such as lubricant oils, further with the fact that the refractive index results obtained suggest the possibility of deploying Te2W material over optical displays. Our work finalizes by testifying the physical and mechanical stability, with our DFT-calculated elastic coefficients along with its criteria, mechanical moduli and phonon data provided. In general, our non-local hybrid functional corrects the band structure and bandgap energy [Formula: see text] extremely well, over the traditional semi-local exchange-correlation functional.

Temperature controlled synthesis of boron carbon nitride nanosheets and study of their bandgap modulation and nonlinear optical properties

This work reports a novel mechanism to synthesize boron carbon nitride (BCN) nanosheets with varying carbon concentration where the whole process is controlled by temperature variation. The morphology, layer numbers, crystallinity, chemical composition and bonding of synthesized BCN nanosheets were confirmed by Field emission scanning electron microscopy (FESEM), Atomic force microscopy (AFM), Transmission electron microscopy (TEM), X-Ray photoelectron spectroscopy (XPS) and Energy Dispersive X-Ray Analysis (EDX). Molecular configuration and bonding vibration of the nanosheets and their linear optical and 3rd-order nonlinear optical properties were analyzed by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, UV–Vis spectroscopy and Z-scan technique respectively. The bandgap estimated from UV–Vis absorption data matches well with the theoretical values calculated by ab initio density functional theory (DFT) thus confirming the formation of BCN nanosheets. The thermal treatment of hBN in the graphitic environment is demonstrated to control the tuning of the morphological, structural, linear, and nonlinear optical characteristics of synthesized BCN nanosheets which currently pave the way of new opportunities for the development of tunable bandgap-based electronics and photonic devices.

Dynamic and static insights on the interaction mechanism between heavy metal contaminants in MSW incineration flue gas and modified carbon materials: Aggregation of PbO and CdO

Clarifying the interaction mechanism between heavy metals and adsorbent is the foundation to trigger the efficiency upgrades of adsorption technology. However, the existing studies ignore the coupling effect between heavy metal pollutants, which leads to a mismatch between theory and practice. Ab-initio molecular dynamics and density functional theory calculations are combined to uncover the dynamical interactions of mixed Pb/Cd species over the transition metal-modified carbon materials (CMs), discovering a new mechanism for enhanced heavy metal removal. Among various mixtures, PbO/CdO generates a stable and compact quadrilateral structure, greatly favorable for the nucleation and agglomeration of gas-phase heavy metals. According to the bond length tracking and electronic analyses, PbO plays an aggregating role and CdO plays a stabilizing role, resulting in dynamic stabilization of the tetragonal aggregates. Due to the different charge transfer and hybridization strengths between the surface-active sites and aggregates, the aggregates can only be stably trapped by Fe-modified CM. In addition, the Fe-modified CM exhibits excellent aggregation properties in the wide temperature region and in the low molecular environment. Overall, Fe-modified CM achieves synergistic removal of PbO and CdO, accommodating the environment of wide temperatures and low heavy metal concentrations, which provides an inspiration for the adsorbent design.

A Density Functional Theory (DFT) Perspective on Optical Absorption of Modified Graphene Interacting with the Main Amino Acids of Spider Silk.

We investigated the possible adsorption of each of the main building blocks of spider silk: alanine, glycine, leucine, and proline. This knowledge could help develop new biocompatible materials and favors the creation of new biosensors. We used ab initio density functional theory methods to study the variations in the optical absorption, reflectivity, and band structure of a modified graphene surface interacting with these four molecules. Four modification cases were considered: graphene with vacancies at 5.55% and fluorine, nitrogen, or oxygen doping, also at 5.55%. We found that, among the cases considered, graphene with vacancies is the best candidate to develop optical biosensors to detect C=O amide and differentiate glycine and leucine from alanine and proline in the visible spectrum region. Finally, from the projected density of states, the main changes occur at deep energies. Thus, all modified graphene's electronic energy band structure undergoes only tiny changes when interacting with amino acids.

Transitions in Xenes between excitonic, topological and trivial insulator phases: Influence of screening, band dispersion and external electric field

Using a variational approach, the binding energies E_bEb of the lowest bound excitons in Xenes under varying electric field are investigated. The internal exciton motion is described both by Dirac electron dispersion and in effective-mass approximation, while the screened electron-hole attraction is modeled by a Rytova-Keldysh potential with a 2D electronic polarizability \alpha_{2D}α2D. The most important parameters as spin-orbit-induced gap E_gEg, Fermi velocity v_FvF and \alpha_{2D}α2D are taken from ab initio density functional theory calculations. In addition, \alpha_{2D}α2D is approximated in two different ways. The relation of E_bEb and E_gEg is ruled by the screening. The existence of an excitonic insulator phase with E_b>E_gEb>Eg sensitively depends on the chosen \alpha_{2D}α2D. The values of E_gEg and \alpha_{2D}α2D are strongly modified by a vertical external electric bias UU, which defines a transition from the topological into a trivial insulator at U=E_g/2U=Eg/2, with the exception of plumbene. Within the Dirac approximation, but also within the effective mass description of the kinetic energy, the treatment of screening dominates the appearance or non-appearance of an excitonic insulator phase. Gating does not change the results: the prediction done at zero electric field is confirmed when a vertical electric field is applied. Finally, Many-Body perturbation theory approaches based on the Green’s function method, applied to stanene, confirm the absence of an excitonic insulator phase, thus validating our results obtained by ab initio modeling of \alpha_{2D}α2D.

Enhanced Hydrogen Bonds of the (H2O)n (n = 4-8) Clusters Confined in Uranyl Peroxide Cluster Na20(UO2)20(O2)30.

Water is a basic resource and an essential component of living organisms. It often exhibits some novel properties under confinement. The water clusters (H2O)n (n = 4-8) confined in the cavity of uranyl peroxide cluster Na20(UO2)20(O2)30 (U20) have been computationally investigated by using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations in this study. The results show that the confined water clusters can form hydrogen bonds with the internal oxygen atoms (Ouranyl) of U20, and their conformations changed significantly. The average lengths (2.553-2.645 Å) of hydrogen bonds in confined (H2O)n are shorter than those (2.731-2.841 Å) in the corresponding free water clusters. Moreover, these confined hydrogen bonds show better hydrogen bond patterns according to the quantified indices. The natural bond orbital (NBO) calculations determine that there is electron transferring from the U20 to its interior (H2O)n. It is the main reason for enhancing hydrogen bond interactions among the confined water molecules because their oxygen atoms are more negatively charged and their hydrogen atoms are more positively charged. The quantum theory of atoms in molecules (QTAIM) and interacting quantum atoms (IQA) analyses indicate that the confined hydrogen bonds are more covalent, based on the significant electron density ρ(r) and local energy density H(r) at the bond critical points (BCPs), and the stronger energies of interatomic exchange interactions (Vxc). These findings may help to promote the communication of confined water clusters and enrich the understating of confined hydrogen bonds.

Thermochemical and kinetic investigation of the hydrazine borane as an alternative to hydrazine: The N2H4BH3 + H reactional system

The properties of hydrazine borane (N2H4BH3) were studied to assess its potential as an alternative fuel for use in space missions. The reactions of N2H4BH3 with atomic hydrogen were explored through three different elementary steps, and the thermochemical properties were determined using ab initio methods and density functional theory. The hydrogen abstraction from the borane group had the lowest adiabatic barrier height and all reactions were exothermic. The rate coefficient for hydrogen abstraction from the borane group was 1.4×10−13cm3molecule−1s−1 at 350 K, and N2H4BH3 was found to have better kinetic stability than hydrazine at intermediate and low temperatures.

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.