Structures and magnetic properties of Fe(n)B (n =1-12) clusters

The configurations, electronic structures and magnetic properties of Fe(n)B (n = 1-12) clusters have been calculated within the framework of all-electron density functional theory. The calculated results indicate that the B atom prefers a surface site for all the lowest-energy structures of Fe(n)B with n = 1-9 and 11, while for Fe10B and Fe12B, the B atom is found to occupy a center site forming a B-centered Fe(n) cage. Furthermore, relatively large HOMO-LUMO gaps are found for Fe6B and Fe7B, indicating the chemical inertness of the two isomers. For Fe4B, and Fe12B, the spin magnetic moments of the Fe atom significantly increase, but the spin moments decrease slightly for all the other Fe(n)B clusters.     

First-principles investigation on solution hardening of interstitial impurity elements (O, N) in gamma-TiA1

We have calculated the electronic structures of O-doped and N-doped gamma -TiAl using the first-principles discrete variational method (DVM) with the aim to understand the solution hardening effects of oxygen and nitrogen in gamma -TiAl. Our combination analysis on the electronic density, density of states (DOS) and the local environment total bond orders (LTBO) will show that, X atom (X is O or. N) can strongly bind with its six surrounding atoms via electronic hybridizations of Ti-3d/X-2p and Al-3p/X-2p. As a sequence, there forms a "hard" cohesive region around the impurity atom. A pinning model based on the calculations is proposed to explain the hardening effects. The consistent results are obtained between the present calculation and formal test experiments.     

Theoretical investigation of atomic structure and electronic properties of Ca/Si(110)-(2 × 1) reconstruction

We have presented first-principles total-energy calculations for the adsorption of Ca metals onto a Si(110) surface. The density functional method was employed within its local density approximation to study the atomic and electronic properties of the Ca/Si(110) structure. We considered the (1 × 1) and (2 × 1) structural models for Ca coverages of 0.5 monolayer (ML) and 0.25 ML, respectively. Our total-energy calculations indicate that the (1 × 1) phase is not expected to occur. It was found that Ca adatoms are adsorbed on top of the surface and form a bridge with the uppermost Si atoms. The Ca/Si(110)-(2 × 1) produces a semiconducting surface band structure with a direct band gap that is slightly smaller than that of the clean surface. One filled and two empty surface states were observed in the gap; these empty surface states originate from the uppermost Si dangling bond states and the Ca 4 s states. It is found that the Ca-Si bonds have an ionic nature and complete charge being transferred from Ca to the surface Si atoms. Finally, the key structural parameters of the equilibrium geometry are detailed and compared with the available results for metal-adsorbed Si(110) surface, Ca/Si(001), and Ca/Si(111) structures.     

Exchange Interactions and Ferromagnetism of Pr_(n+1)Co_(3n+5)B_(2n)-type Compounds

In present investigation exchange interactions of Pr_(n+1)Co_(3n+5)B_(2n)-type compounds have been evalu-ated in the light of molecular-field theory. The exchange interactions and ferromagnetism in thesecompounds are discussed in terms of lattice parameters and interatomic distance between Co atom.     

A cluster approach to hydrogen chemisorption on the GaAs(1 0 0) surface

Chemisorption properties of atomic hydrogen on the Ga-rich GaAs(1 0 0), (2×1) and β(4×2) surfaces are investigated using ab initio self-consistent restricted open shell Hartree–Fock (ROHF) total energy calculations with Hay–Wadt (HW) effective core potentials. The effects of electron correlation have been included using many-body perturbation theory through second order with the exception of β(4×2) symmetry due to computational limitations. The semiconductor surface is modeled by finite sized hydrogen saturated clusters. The effects of surface reconstruction have been investigated in detail. We report on the energetics of chemisorption on the (1 0 0) surface layer, including adsorption beneath the surface layer at an interstitial site, and also report on the possible dimer bond breaking at the bridge site. Chemisorption energies, bond lengths, and charge population analysis are reported for all considered sites of chemisorption.     

A time-dependent density functional theory study on the absorption spectra of Cu(n) clusters

The structures and absorption spectra of neutral Cu(n) (n = 3-9) clusters, are studied in the framework of the time-dependent density functional theory (TDDFT) using all-electron calculations. The obtained ground state structures are respectively planar for Cu(n) (n = 2-6) and 3-dimensional for Cu(n) (n = 7-9). The calculations of transition energies, oscillator strengths and dipole moment of absorption spectra have been calculated and investigated in a large energy interval, which allow us to determine the role of d electrons. The spectral analysis shows that 3d electrons are responsible for spectrum behavior of Cu(n) (n = 3-8) in addition to the contribution of 4s electrons, while only 3d electrons are primarily responsible for spectroscopic patterns of Cu9. The results are entirely different from the absorption spectra of Ag(n) with the same number of atoms.     

Electronic structure, charge density, and chemical bonding properties of C11H8N2O o-methoxydicyanovinylbenzene (DIVA) single crystal

A comprehensive theoretical density functional theory (DFT) study of the electronic crystal structure, bonding properties, electron charge density of C₁₁H₈N₂O o-methoxydicyanovinylbenzene (DIVA) single crystals were performed. The exchange and correlation potential was described within a framework of the local density approximation (LDA) by Ceperley-Alder and gradient approximation (GGA) based on exchange–correlation energy optimization to calculate the total energy. In addition, we have used Engel–Vosko generalized gradient approximation (EV-GGA) and the modified Becke–Johnson potential (mBJ) for the electronic crystal structure, bonding properties, electron charge density calculations. There is systematically increasing in the energy gap from 2.25 eV (LDA), 2.34 eV (GGA), 2.50 eV (EV-GGA), 2.96 eV (mBJ). Our calculations show that this crystal possess direct energy gap. Furthermore, the electronic charge density space distribution contours in the (1 1 0) crystallographic plane clarifies the nature of chemical bonding.     

Formation Energies of the Lithium Intercalations in MoS2

First-principles calculations have been performed to study the lithium intercalations in MoS2. The formation energies, changes of volumes, electronic structures and charge densities of the lithium intercalations in MoS2 are presented. Our calculations show that during lithium intercalations in MoS2, the lithium intercalation formation energies per lithium atom are between 2.5 eV to 3.0 eV. The volume expansions of MoS2 due to lithium intercalations are relatively small     

Theoretical investigations on low energy surfaces and nanowires of MgH(2)

The phase stability, chemical bonding, and electronic structure of MgH(2) nanowires and possible low energy surfaces of α-MgH(2) thin films have been investigated using the ab initio projected augmented plane-wave method. Structural optimizations based on total energy calculations predicted that, for the α-MgH(2) phase, the (101) surface is more stable among the possible low energy surfaces. The electronic structure study reveals that the nanowires also have nonmetallic character similar to that of the bulk and thin film phases. Bonding analysis shows that the character of chemical bonding in nanowires has been considerably changed compared with that in bulk phases. Similarly, the bond distances in the surfaces of nanowires are found to be higher than in the bulk material, suggesting that it is possible to remove hydrogen from the nanowires considerably more easily than from bulk crystals.     

Measurement and Monte Carlo modelling of the JRC 241Am-Li(alpha,n) source spectrum

The neutron energy spectrum of the JRC 241Am-Li(alpha,n) radionuclide source has been measured at the PTB in Germany using various spectrometry systems, such as Bonner spheres, proton recoil counters and NE213 liquid scintillators. The source photon spectrum has been measured and the neutron contamination due to traces of beryllium through the 9Be(alpha,n)12C reaction has been determined. The effects associated with source encapsulation and the materials it contains have been investigated previously, using Monte Carlo simulations, the various structures seen in the spectra have been explained and the anisotropy of fluence distributions in 4pi have been calculated and compared to measurements. These extensive high resolution spectrometry measurements have been coupled to Monte Carlo calculations to provide a realistic spectrum and new spectrum-averaged fluence-to-dose equivalent conversion coefficients for the actual 241Am-Li source.     

Full-potential study of Fe2NiZ (Z = Al,Si, Ga,Ge)

The first-principles calculations using full-potential in the stable F-43m phase have been performed to investigate the structural, elastic, magnetic, nature of chemical bonding and electronic properties of Fe₂-based inverse Heusler alloys. The structural stability and the lattice constants match well with the experimental results. We have further reported other mechanical, elastic and thermophysical properties for the first time of these Fe₂NiZ (Z = Al, Si, Ga, Ge) materials. Cauchy's pressure and Pugh's index of ductility label these materials as ductile. The spin magnetic moment distributions show that these materials are ferromagnetic in stable F-43m phase. Further, spin resolved electronic structure calculations show that the discrepancies in magnetic moments of Fe-I and Fe-II depend upon the hybridization of Fe with the main group element. The charge density distribution plots present a clear picture of the stronger covalent bonding in Fe₂NiSi and the decreasing trend of covalent bonding in these materials. The main group electron concentration is predominantly responsible in establishing the magnetic properties, formation of magnetic moments and the magnetic order for these alloys. Spin resolved band structure calculations show that these materials are metallic in stable F-43m phase at ambient conditions.     

First-principles Investigation on Solution Hardening of Interstitial Impurity Elements (O,N) inγ-TiAl

1. Introduction7-titanium aluminides (7-TiAl) are considered asthe top candidate materials for structural applica-nons with high operational temperature[1]. However,similarly to the most illtermetallic compounds, thepromising applications of 7-TiAl are limited by theproblem of poor ductility at ambient condition. Dueto this reason, many researches have been carried outin recent years to overcome such a demerit[2]. As themost conventional and effective way in altering the microstructure of ma…     

Electronic band structure calculations on thin films of the L21 full Heusler alloys X2YSi (X, Y = Mn, Fe, and Co): Toward spintronic materials

To design half-metallic materials in thin film form for spintronic devices, the electronic structures of full Heusler alloys (Mn₂FeSi, Fe₂MnSi, Fe₂FeSi, Fe₂CoSi, and Co₂FeSi) with an L2₁ structure have been investigated using density functional theory calculations with Gaussian-type functions in a periodic boundary condition. Considering the metal composition, layer thickness, and orbital symmetries, a 5-layered Co₂FeSi thin film, whose surface consists of a Si layer, was found to have stable half-metallic nature with a band gap of ca. 0.6 eV in the minority spin state. Using the group theory, the difference between electronic structures in bulk and thin film conditions was discussed.     

Theoretical investigation on the heats of formation and the interactions among the isocyano groups in polyisocyanoprismanes C(6)H(6-n)(NC)(n) (n=1-6)

A series of polyisocyanoprismanes, C(6)H(6-n)(NC)(n) (n=1-6), has been designed computationally. We have calculated the heats of formation (HOFs) of the title compounds by using density functional theory (DFT) with 6-31G** basis set. We chose [3]prismane C(6)H(6)-D(3h) as a reference compound in the process of designing isodesmic reactions. The relationship between the HOFs and the molecular structures is discussed. The results have shown that the HOFs of the title compounds gradually increase with increasing number of isocyano groups. On average, the contribution of one isocyano group to the heat of formation is about 232.3 kJ/mol and 234.1 kJ/mol at the B3LYP and B3P86 levels, respectively. The relative stabilities of the title compounds are discussed in terms of the calculated HOFs, the energy gaps between the frontier orbitals, and the bond dissociation energies. The interactions of the isocyano groups in these polyisocyanoprismanes are also discussed. The results have not only shown that these compounds may be used as high-energy-density materials, but also provide some useful information for further syntheses.     

Formaldehyde on TiO2 anatase (1 0 1): A DFT study

The adsorptions of formaldehyde molecule on the stoichiometric anatase TiO₂ (1 0 1) surface have been studied by first principles calculations. Four types of adsorption have been investigated at 0.25 ML coverage. Two of them are chemical adsorptions and the other two are physical adsorptions. For the chemical adsorptions, C, O atoms in the formaldehyde molecule form two bonds with the O_2c/O_3c and Ti_5c on the anatase (1 0 1) surface. The CO bond in the formaldehyde molecule is elongated and a dioxymethylene structure forms in the two chemical adsorptions. The OTi_5c interaction can be found in the two physical adsorptions and it is the only contacting point at the interface. No serious internal distortion in the formaldehyde molecule can be found in the physical adsorptions. The LDOS and the difference of the charge density are calculated to investigate the interface bonds of the adsorption. As the adsorption coverage increase, the molecules on the surface repel each other and weaken the adsorptions. For example, the chemical adsorption may become physical adsorption at high coverage.     

Optical properties of (Ge2)1−x (InP)x solid solutions

The optical properties of (Ge₂)_1?x (InP)_x solid solutions have been studied by means of ab initio calculations of the electron band structure performed using a self-consistent relativistic scalar full-potential linear muffin-tin orbital method and the density functional theory within the framework of a local charge density approximation.     

Homogeneous 2D MoTe2 p–n Junctions and CMOS Inverters formed by Atomic‐Layer‐Deposition‐Induced Doping

Recently, α‐MoTe₂, a 2D transition‐metal dichalcogenide (TMD), has shown outstanding properties, aiming at future electronic devices. Such TMD structures without surface dangling bonds make the 2D α‐MoTe₂ a more favorable candidate than conventional 3D Si on the scale of a few nanometers. The bandgap of thin α‐MoTe₂ appears close to that of Si and is quite smaller than those of other typical TMD semiconductors. Even though there have been a few attempts to control the charge‐carrier polarity of MoTe₂, functional devices such as p–n junction or complementary metal–oxide–semiconductor (CMOS) inverters have not been reported. Here, we demonstrate a 2D CMOS inverter and p–n junction diode in a single α‐MoTe₂ nanosheet by a straightforward selective doping technique. In a single α‐MoTe₂ flake, an initially p‐doped channel is selectively converted to an n‐doped region with high electron mobility of 18 cm² V^?1 s^?1 by atomic‐layer‐deposition‐induced H‐doping. The ultrathin CMOS inverter exhibits a high DC voltage gain of 29, an AC gain of 18 at 1 kHz, and a low static power consumption of a few nanowatts. The results show a great potential of α‐MoTe₂ for future electronic devices based on 2D semiconducting materials.     

Computational studies on nitratoethylnitramine (NENA), its tautomers and charged forms

An energetic material, nitratoethylnitramine (NENA), its tautomers and also its charged forms are considered quantum chemically, using various basis sets at the levels of ab initio and density functional theories (DFT). NENA has been found to be sensitive to negative charge development, resulting in rupture of ONO(2) bond. Also conformational and molecular dynamics (MD) studies have been performed on NENA. Various geometrical parameters, energies and infrared spectra have been obtained and discussed. Also, calculations indicate that s-cis conformation of NENA is slightly more stable than the s-trans and the tautomers of it have very comparable total energy values to NENA. On the other hand, on the basis of homolytic bond dissociation energies (BDE) for ONO(2) bond in the structures, it is clear that the presence of the tautomers in the bulk of NENA somewhat should decrease its sensitivity.     

The stability and optical gap of zinc oxide clusters (ZnO)n (n = 2-18)

The stability and the optical band gap of the Zinc Oxide clusters (ZnO)n (n = 2-18) are investigated by using density functional theory (DFT) and the time-dependent density functional theory (TD-DFT), respectively. The differences between the HOMO-LUMO gap (delta(h-l)) and the optical gap (delta(opt)) are dramatic for small clusters (2 < or = n < or = 5). As the increasing of the cluster size, the differences become small. The results indicate that the stability and the optical gap are related to the sizes and symmetries of the clusters. Further, it is shown that the structures have much greater impact on the optical gap, there is the dipole-forbidden transition in the optical gap for high symmetric structures.     

Quantum chemical study on 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) and some of its constitutional isomers

Presently, certain isomeric compounds of NTO and their tautomers have been investigated by performing density functional theory (DFT) calculations at B3LYP/6-31G(d,p) and ROB3P86/6-311G(d,p) levels and also ab initio calculations at RHF/6-311G(d,p) level. The optimized geometries, vibrational frequencies, electronic structures and some thermodynamical values for the presently considered NTO isomers have been obtained in their ground states. Also, detonation performances were evaluated by the Kammlet-Jacobs equations, based on the calculated densities and heat of formation values. The homolytic bond dissociation energies (BDEs) (at ROB3P86/6-311G(d,p) level) of NNO(2) and CNO(2) for the molecules were calculated. Moreover, aromatic character of NTO and its isomers and tautomers were investigated by performing NICS calculations using the gauge invariant atomic orbital (GIAO) approach at the B3LYP/6-31G(d,p) and B3LYP/cc-pVDZ levels.