First-principles Total Energy Research Articles

Transition from indirect to direct band gap in SiC monolayer by chemical functionalization: A first principles study

Abstract First-principles total energy calculations based on the pseudopotential plane-wave method, within the density functional theory, have been carried out to investigate the structural and electronic properties of the pristine and chemically functionalized SiC monolayer. Results show that the SiC monolayer has an indirect band gap, whose value calculated within the PBE and HSE06 theories is 2.492 and 3.264 eV, respectively. Structurally, the chemical functionalization with halogen atoms (F and Cl) generates a buckling in the SiC monolayer. Interestingly, the indirect-to-direct band gap transformation may be achieved by the chemical functionalization as all the fluorinated, chlorinated and Janus functionalized SiC monolayer are direct semiconductors. The electronic band gap of these chemically functionalized monolayers are in the range of [1.502 and 1.978 eV] and [2.508 and 2.998 eV] predicted by PBE and HSE06 functionals, respectively. Our work is expected to pave an effective way to tune the SiC monolayer electronic structure for practical applications.

Multifarious Interfaces, Band Alignments, and Formation Asymmetry of WSe2-MoSe2 Heterojunction Grown by Molecular-Beam Epitaxy.

Monolayer (ML) transition-metal dichalcogenides (TMDs) continue to attract research attention, and the heterojunctions formed by vertically stacking or laterally stitching two different TMDs, e.g., MoSe2 and WSe2, may have many interesting electronic and optical properties and thus are at the center stage of current research. Experimentally realizing such heterojunctions with desired interface morphologies and electronic properties is of great demand. In this work, we report a diverse interface structure in molecular-beam epitaxial WSe2-MoSe2 heterojunction. The corresponding electronic bands show type-II band alignment for both monolayer ML-ML and ML-bilayer lateral junctions irrespective of the presence or absence of step states. Interestingly, a strong anisotropy in lateral heterojunction formation is observed, where sharp interfaces are obtained only when WSe2 deposition precedes MoSe2. Reversing the deposition order leads to alloying of the two materials without a notable boundary. This is explained by a step segregation process as suggested by the first-principles total energy calculations.

Carbon-Related Bilayers: Nanoscale Building Blocks for Self-Assembly Nanomanufacturing

Using a first-principles total energy methodology, we investigated the properties of graphene-like carbon mono and bilayers, functionalized with nitrogen and boron atoms. The resulting stable structures were explored in terms of their potential use as nanoscale two-dimensional building blocks for self-assembly of macroscopic structures. We initially considered graphene monolayers functionalized with nitrogen and boron, but none of them was dynamically stable, in terms of the respective layer phonon spectra. Then, we considered the functionalized graphene-like bilayers (labeled as NCCN, NCNC, BCCB, and NCCB), analyzing their stability, electronic and mechanical properties, and chemical reactivity. We found that while the NCCN, NCNC, and NCCB bilayers were stable, the BCCB one was not. Additionally, the NCCN and NCCB bilayers were explored as potential two-dimensional building blocks for nanostructure self-assembly, which could form stable bulk structures. Particularly, the NCCB bilayer seemed the best choice as a building block, since the resulting 3D crystals, formed by stacking NCCB bilayers, were energetically stable.

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

Energetics and electronic structures of MoS2 nanoribbons

We study the energetics and electronic structures of MoS2 nanoribbons with clean armchair, chiral, and zigzag edges by conducting the first-principle total energy calculations based on the density functional theory. Our calculations showed that the nanoribbon with zigzag edges is the most stable among the ribbons studied here. The ribbons with armchair or near armchair edges are semiconductors with a direct band gap at the Γ point, owing to the large edge relaxation reducing the unsaturated nature of edge atoms, while the ribbons with zigzag and near zigzag edges are metals with the finite density of state at the Fermi level. According to the asymmetric atomic arrangement in ribbons with the chiral and zigzag edges, they have polarity across the ribbon, which monotonically increase with increasing edge angle.

Manganese‑germanium nanostructure formation on the GaAs(111)–(1 × 1)A surface: Stability and magnetic properties

Spin-polarized first-principles total energy calculations have been developed to explore the Mn and Ge monolayer adsorption and incorporation into the unreconstructed GaAs(111)–(1 × 1)A surface. Surface formation energy results show that a germanium terminated surface is stable in a wide range of chemical potential; in this structure, the first Ga layer is replaced by a Ge layer. Upon increasing the As coverage, it is possible to find a stable structure with a manganese bilayer under an As monolayer. The density of states shows that a Ge terminated structure displays a semiconductor behavior whereas the Mn bilayer is metallic, with evident antiferromagnetic characteristics induced by the Mn atoms. Mn magnetic moments are of the order of ~4 μ B . The magnetic arrangement was confirmed by spin density distribution. Such antiferromagnetic structure may be suggested as a substrate to form magnetic junctions with other magnetic materials. • Ge deposition drives to formation of Ge layers on the GaAs surface. • Mn incorporates to form antiferromagnetic bilayers. • As atoms present magnetic moments induced by proximity to Mn.

Structural and Electronic Properties of Double-Walled Black Phosphorene Nanotubes: A Density Functional Theory Study

The exploration of new low-dimensional systems based on phosphorus atoms is an area of intensive research because of their properties. We use first-principles total energy calculations to investigate the stability and the geometrical and electronic properties of double-walled black phosphorene nanotubes (DWβPNTs). We are dealing with a material that has not been synthesized yet. Therefore, it is important to know if it is stable, so it could be obtained experimentally. To do this, we have calculated the formation energy for several DWβPNTs. The results show that DWβPNTs are more stable than the SWβPNTs. Moreover, some of the systems are even more stable than a black phosphorene monolayer. Also, it is found that for the most stable DWβPNTs, the distance between the two concentric SWβPNTs is similar to the distance between layers of black phosphorus. As a consequence, very small DWβPNTs, in which this distance is not maintained, are not stable. The band gaps of DWβPNTs are smaller than the band gaps of thei...

Determination of second- and third-order elastic constants for energetic materials

Accurate calculations of second-order elastic constants (SOECs) and third-order elastic constants (TOECs) are important for energetic materials to describe the linear and nonlinear mechanical responses quantitatively. Homogeneous deformation method combined with first-principles total energy calculations are performed to calculate a complete set of SOECs and TOECs for cyclotrimethylene trinitramine (RDX), pentaerythritol tetranitrate (PETN) and cyclotetramethylene tetranitramine (δ-HMX). The computed SOECs for RDX, PETN and δ-HMX agree well with previous experiments and theoretical calculations. Bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio deduced from SOECs reveal quantitatively the anisotropic mechanical properties along different crystal orientations. TOECs of these three energetic materials, which can be used to construct constitutive equations and provide input parameters for mesomechanics model and macroscopic simulations, are predicted. In particular, the TOEC results provide guidance for future experimental measurements and lay the foundation for more accurate calculations.

SA85RESTING-STATE BRAIN CONNECTIVITY IN 22Q11.2 DELETION SYNDROME: A MAGNETOENCEPHALOGRAPHY STUDY

The dissolution and diffusion of H isotopes in bcc-Fe are fundamental and essential parameters in the H energy application from nuclear conversion, whereas the relevant data are lacking and relatively dispersive, demonstrating some important factors have been missed during the past studies. Here, we carry out first-principles total energy and vibration spectrum calculations to investigate systematically the interstitial H dissolution and diffusion behaviors in bcc-Fe by considering the temperature effect. Temperature and H chemical potential are two important factors to affect the H dissolution property. In the interstitial lattice, the H dissolution energy referring to the static/temperature-dependent H chemical potential decreases/increases with the increasing temperature. The diffusion activation energy and pre-factor of H also depend on the temperature and increase significantly with the increasing temperature from 300 to 1000 K. The temperature-correction can give a reasonable interpretation for the broad disseminating in the experimental data of H diffusivity in bcc-Fe. Our currently calculated results reveal that phonon vibration energy plays a crucial role in the H dissolution and diffusion with the increasing temperature.

Atomic structure of manganese-doped yttrium orthoaluminate

Abstract Using hybrid exchange-correlation functional within density functional theory we have performed first-principle total energy calculations of Mn-doped yttrium orthoaluminate (YAlO3). Its equilibrium atomic structure has been predicted through optimization of coordinates of all atoms using a supercell approach. In our research both Mn3+ and Mn2+ ions have been substituted for the host alumina atom at orthorhombic Pbnm unit cell of YAlO3. F-center has been implemented as charge-compensating defect in case, when Mn2+ dopant is under study. In this study we thoroughly analyze the atomic displacements in seven nearest to Mn ion coordination spheres. Insertion of isoelectronic substitutional point defect Mn3+ leads to modest expansion of distorted oxygen octahedra, while the cage of Y atoms closest to the dopant remains mostly unchanged. Insertion of compensating F-center in the 1st oxygen coordination sphere breaks the symmetry of YAlO3 and heavily distorts the surrounding oxygen octahedron. That is why the Mn2+-F-center bonds is contracted about 32% with respect to reference Mn-O bond of Mn3+-doped orthorhombic YAlO3. Distortion is also well pronounced at Y coordination spheres of Mn2+/YAlO3. The influence of atomic relaxation near the Mn-dopants to the electronic structure of YAlO3 is also discussed.

Structural, electronic, and magnetic properties of the CrN (0 0 1) surface: First-principles studies

Spin-polarized, first-principles total energy calculations have been performed to investigate the structural, electronic and magnetic properties of the chromium nitride (0 0 1) surface. Different approximations were used to treat the non-classical exchange correlation energy: LDA, GGA, LDA + U (3 eV ≤ U ≤ 5 eV) and GGA + U (1 eV ≤ U ≤ 4 eV). It was found that LDA + U with U = 4 eV describes correctly the structural, electronic and magnetic properties of bulk CrN. The CrN (0 0 1) surface has been investigated, employing the surface formation energy (SFE) formalism. For the surface in the cubic phase, three different structures were found to be stable: an ideally terminated surface, one with no nitrogen atoms in the second layer, and one with half of the nitrogen atoms missing in the second layer. In the case of the orthorhombic surface, only the ideal surface is stable in all the range of the chemical potential. Density of States (DOS) calculations show a metallic behavior for all the stable surface structures, with the main contribution around the Fermi level coming from Cr-d orbitals. The effect of nitrogen vacancies is clearly observed with an increase in the metallic behavior in the structures having less nitrogen atoms.

First-principles study on leakage current caused by oxygen vacancies at HfO2/SiO2/Si interface

The relationship between the position of oxygen vacancies in HfO2/SiO2/Si gate stacks and the leakage current is studied by first-principles electronic-structure and electron-conduction calculations. We find that the increase in the leakage current due to the creation of oxygen vacancies in the HfO2 layer is much larger than that in the SiO2 interlayer. According to previous first-principles total energy calculations, the formation energy of oxygen vacancies is smaller in the SiO2 interlayer than that in the HfO2 layer under the same conditions. Therefore, oxygen vacancies will be attracted from the SiO2 interlayer to minimize the energy, thermodynamically justifying the scavenging technique. Thus, the scavenging process efficiently improves the dielectric constant of HfO2-based gate stacks without increasing the number of oxygen vacancies, which cause the dielectric breakdown.

Effect of P impurity on mechanical properties of NiAl Σ5 grain boundary: From perspectives of stress and energy**Project supported by the National Natural Science Foundation of China (Grant Nos. 11404396 and 51201181) and the Subject Construction Fund of Civil Aviation University of China (Grant No. 000032041102).

In this paper, we employ the first-principle total energy method to investigate the effect of P impurity on mechanical properties of NiAl grain boundary (GB). According to “energy”, the segregation of P atom in NiAlΣ5 GB reduces the cleavage energy and embrittlement potential, demonstrating that P impurity embrittles NiAlΣ5 GB. The first-principle computational tensile test is conducted to determine the theoretical tensile strength of NiAlΣ5 GB. It is demonstrated that the maximum ideal tensile strength of NiAlΣ5 GB with P atom segregation is 144.5 GPa, which is lower than that of the pure NiAlΣ5 GB (164.7 GPa). It is indicated that the segregation of P weakens the theoretical strength of NiAlΣ5 GB. The analysis of atomic configuration shows that the GB fracture is caused by the interfacial bond breaking. Moreover, P is identified to weaken the interactions between Al–Al bonds and enhance Ni–Ni bonds.

Origin of polymorphism of the two-dimensional group-IV monochalcogenides

Unlike other two-dimensional (2D) isovalent materials, the 2D group IV monochalcogenides, $MX$ ($M\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{Si}$, Ge, Sn, and Pb; $X\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{S}$, Se, and Te), are found to be either in a black phosphorene-derived distorted NaCl-type ($d$-NaCl) structure or a recently predicted $Pma2$ structure. Both $M$ and $X$ atoms in the $d$-NaCl structure are threefold coordinated, whereas $M$ and $X$ in the $Pma2$ structure are fourfold and twofold coordinated, respectively. Using first-principles total energy and electronic structure calculations and a global structural search technique, we systematically investigated the mechanism underlying the polymorphism of the 2D group-IV monochalcogenides. Our analysis show that the relative stability of the two distinct crystallographic phases depends on the strength of the $M\ensuremath{-}M$ covalent bond and the electronegativity difference between the constituent elements $M$ and $X$. For small cations, the covalency plays more important role, whereas for large cations the Coulomb interaction becomes more dominant. Therefore, the $\mathrm{Si}X$ and $\mathrm{Ge}X$ compounds assume the $Pma2$ structure, whereas the $MX$ compounds with heavy cation elements ($M\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{Sn}$ and Pb) tend to adopt the $d$-NaCl structure.

First-principles energy and vibration spectrum simulations of Cr/V interacting with H in W-based alloy in a fusion reactor

ABSTRACT We perform first-principles total energy and vibration spectrum calculations to study the effect of Cr/V on the H formation and diffusion at temperature range 300–2100 K in dilute W–Cr/W–V alloy. Temperature and H chemical potential are two important factors to affect the H formation energy and migrating energy. The H formation energy referring to the static/temperature-dependent H chemical potential decreases/increases with the temperature. At each given temperature, the presence of Cr in W reduces the H formation energy, while the existence of V in W has little effect on the H formation energy. The diffusion energy barrier and pre-exponential factor strongly depend on the temperature and increase with the temperature from 300 to 2100 K. Both Cr and V additions in W has a large consequences on H migrating energy. The energy barriers at any given temperature can be reduced by ∼0.05 and 0.10 eV in W–Cr and W–V alloys, respectively. The current study reveals that vibration free energy plays a decisive role in the formation energy and migrating energy of H with the temperature, while the thermal expansion energy has little influence on the H formation energy and migrating energy with the temperature.

Finite-temperature H behaviors in tungsten and molybdenum: first-principles total energy and vibration spectrum calculations

We have carried out systematic first-principles total energy and vibration spectrum calculations to investigate the finite-temperature H dissolution behaviors in tungsten and molybdenum, which are considered promising candidates for the first wall in nuclear fusion reactors. The temperature effect is considered by the lattice expansion and phonon vibration. We demonstrate that the H Gibbs energy of formation in both tetrahedral and octahedral interstitial positions depends strongly on the temperature. The H Gibbs energy of formation under one atmosphere of pressure increases significantly with increasing temperature. The phonon vibration contribution plays a decisive role in the H Gibbs energy of formation with the increasing temperature. Using the predicted H Gibbs energy of formation, our calculated H concentrations in both metals are about one or two orders of magnitude lower than the experimental data at temperature range from 900 to 2400 K. Such a discrepancy can be reasonably explained by the defect-capturing effect.

Study of pressure variation effect on structural, opto-electronic, elastic, mechanical, and thermodynamic properties of SrLiF3

The structural, electronic, elastic, optical and thermodynamic properties of cubic fluoroperovskite SrLiF3 at ambient and high-pressure are investigated by using first-principles total energy calculations within the framework of Generalized Gradient Approximation (GGA), combined with Quasi-harmonic Debye model in which the phonon effects are considered. The pressure effects are determined in the range of 0–50GPa, in which cubic stability of SrLiF3 fluoroperovskite remains valid. The computed lattice parameters agree well with experimental and previous theoretical results. Decrease in lattice constant and bonds length is observed with the increase in pressure from 0 to 50GPa. The effect of increase in pressure on electronic band structure calculations with GGA and GGA plus Tran-Blaha modified Becke–Johnson (TB-mBJ) potential reveals a predominant characteristic associated with widening of bandgap. The influence of pressure on elastic constants and their related mechanical parameters have been discussed in detail. All the calculated optical properties such as the complex dielectric function Ԑ(ω), optical conductivity σ(ω), energy loss function L(ω), absorption coefficient α(w), refractive index n (ω), reflectivity R (ω), and effective number of electrons neff, via sum rules shift towards the higher energies under the application of pressure. Moreover, important thermodynamic properties heat capacities (Cp and Cv), volume expansion coefficient (α), and Debye temperature (θD) are predicted successfully in the wide temperature and pressure ranges.

Theoretical investigation of structural, mechanical and electronic properties of GaAs1-xNx alloys under ambient and high pressure

Using first-principles total energy calculations, we have studied the structural, mechanical and electronic properties of GaAs1-xNx ternary semiconductor alloys with the zinc-blende crystal structure over the whole nitrogen concentration range (with x from 0 to 1) within density functional theory (DFT) framework. To obtain the ideal band gap, we employ the semi-empirical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U). The calculated results illustrate the varying lattice constants and band gap in GaAs1-xNx alloys as functions of the nitrogen concentration x. According to the pressure dependence of the lattice constants and volume, the higher N concentration alloy exhibits the better anti-compressibility. In addition, an increasing band gap is predicted under 20GPa pressure for GaAs1-xNx alloys.

Interface and Doping Effects on Li Ion Storage Behavior of Graphene/Li2O

Graphene/metal oxide nanocomposites have been widely used as the anode materials for Li ion batteries, which exhibit much higher Li storage capacity beyond their theoretical capacity. In order to make clear the Li storage mechanism in graphene/metal oxide, we systematically investigated the interface and (B, N, O, S) doping effects on Li ion storage behavior in graphene/Li2O using first-principles total energy calculations. It is revealed that the doping elements increase the van der Waals interface interaction of graphene/Li2O by changing the electronic structure of graphene through different mechanisms. The Li storage at the graphene/Li2O interface exhibits the synergistic effect resulting from the enhanced interface interaction by the Li insertion at the interface. The p-type and n-type doping induced by B and N dopants in graphene enhance and reduce the Li storage capability of graphene/Li2O, respectively. O and S doping result in the localization of the electronic states in graphene which benefits th...

Evaluation of thermodynamics, formation energetics and electronic properties of vacancy defects in CaZrO3

Using first-principles total energy calculations we have evaluated the thermodynamics and the electronic properties of intrinsic vacancy defects in orthorhombic CaZrO3. Charge density calculations and the atoms-in-molecules concept are used to elucidate the changes in electronic properties of CaZrO3 upon the introduction of vacancy defects. We explore the chemical stability and defect formation energies of charge-neutral as well as of charged intrinsic vacancies under various synthesis conditions and also present full and partial Schottky reaction energies. The calculated electronic properties indicate that hole-doped state can be achieved in charge neutral Ca vacancy containing CaZrO3 under oxidation condition, while reduction condition allows to control the electrical conductivity of CaZrO3 depending on the charge state and concentration of oxygen vacancies. The clustering of neutral oxygen vacancies in CaZrO3 is examined as well. This provides useful information for tailoring the electronic properties of this material. We show that intentional incorporation of various forms of intrinsic vacancy defects in CaZrO3 allows to considerably modify its electronic properties, making this material suitable for a wide range of applications.