86 research outputs found
chiral modification of platinum ab initio study of the effect of hydrogen coadsorption on stability and geometry of adsorbed cinchona alkaloids
Hydrogen coadsorption affects the orientation and stability of cinchona alkaloids used for the chiral modification of platinum and thus can influence their enantiodifferentiating ability
Real-space electronic-structure calculations: Combination of the finite-difference and conjugate-gradient methods
We present a scheme for a rapid solution of a general three-dimensional Schrödinger equation. The Hamiltonian operator is discretized on a point grid using the finite-difference method. The eigenstates, i.e., the values of the wave functions in the grid points, are searched for as a constrained (due to the orthogonality requirement) optimization problem for the eigenenergies. This search is performed by the conjugate-gradient method. We demonstrate the scheme by solving for the self-consistent electronic structure of the diatomic molecule P2 starting from a given effective electron potential. Moreover, we show the efficiency of the scheme by calculating positron states in low-symmetry solids.Peer reviewe
Electron-positron Car-Parrinello methods: Self-consistent treatment of charge densities and ionic relaxations
A calculation method based on the two-component density-functional theory is presented for electron systems with a localized positron. Electron-ion and positron-ion interactions are described by norm-conserving pseudopotentials and full ionic potentials, respectively. The electron and positron densities are solved self-consistently using a plane-wave expansion for electron and a real-space grid method for positron wave functions, respectively. The forces on ions exerted by a positron trapped at an open-volume defect and the ensuing ionic relaxations are determined using first-principles methods. In the case of semiconductors, the self-consistent solution of electron and positron densities as well as the ionic positions are found to depend strongly on the treatment of the electron-positron correlation in constructing the effective potentials. We consider several approximations to the correlation energy while demonstrating the method by calculations for a positron trapped by a Ga vacancy in GaAs. Especially, the effects on the observable positron annihilation characteristics, i.e., positron lifetimes, core annihilation line shapes, and two-dimensional angular correlation maps are discussed.Peer reviewe
Nitrogen Doping of Amorphous Carbon Surfaces
The surface properties of amorphous carbon ( a−C) are studied using first-principles electronic structure methods. The effect of nitrogen doping near the surface and, in particular, the effect of nitrogen on the work function is studied by doing a series of nitrogen substitutions near the surface. It is found that the work function is reduced by nitrogen doping of the a−C surface at “on top of the surface” sp1 and sp2 sites. Nitrogen doping by low energy ion bombardment is suggested as a doping method to minimize work function of the a−C surfaces.Peer reviewe
Structure of CAl12
The structures of an isolated CAl12 cluster and a solid composed of CAl12 clusters have been studied using the Car–Parrinello method, based on the density functional theory and the local density approximation. We have compared the results of using the ultrasoft Vanderbilt pseudopotential with those of both a traditional pseudopotential and a linear combination of atomic orbitals method. We have confirmed the high stability of the cluster in its icosahedral structure. However, we show that the cluster‐assembled solid is unstable against melting of the clusters, as previously found for SiAl12.Peer reviewe
Epitaxial Synthesis of Blue Phosphorene
Phosphorene is a new two-dimensional material composed of a single or few
atomic layers of black phosphorus. Phosphorene has both an intrinsic tunable
direct band gap and high carrier mobility values, which make it suitable for a
large variety of optical and electronic devices. However, the synthesis of
single-layer phosphorene is a major challenge. The standard procedure to obtain
phosphorene is by exfoliation. More recently, the epitaxial growth of
single-layer phosphorene on Au(111) has been investigated by molecular beam
epitaxy and the obtained structure has been described as a blue-phosphorene
sheet. In the present study, large areas of high-quality monolayer phosphorene,
with a band gap value at least equal to 0.8 eV, have been synthesized on
Au(111). Our experimental investigations, coupled with DFT calculations, give
evidence of two distinct phases of blue phosphorene on Au(111), instead of one
as previously reported, and their atomic structures have been determined.Comment: This paper reports on the epitaxial synthesis of blue phosphoren
Structure and stability of graphene nanoribbons in oxygen, carbon dioxide, water, and ammonia
We determine, by means of density functional theory, the stability and the
structure of graphene nanoribbon (GNR) edges in presence of molecules such as
oxygen, water, ammonia, and carbon dioxide. As in the case of
hydrogen-terminated nanoribbons, we find that the most stable armchair and
zigzag configurations are characterized by a non-metallic/non-magnetic nature,
and are compatible with Clar's sextet rules, well known in organic chemistry.
In particular, we predict that, at thermodynamic equilibrium, neutral GNRs in
oxygen-rich atmosphere should preferentially be along the armchair direction,
while water-saturated GNRs should present zigzag edges. Our results promise to
be particularly useful to GNRs synthesis, since the most recent and advanced
experimental routes are most effective in water and/or ammonia-containing
solutions.Comment: accepted for publication in PR
Structure, Stability, Edge States and Aromaticity of Graphene Ribbons
We determine the stability, the geometry, the electronic and magnetic
structure of hydrogen-terminated graphene-nanoribbons edges as a function of
the hydrogen content of the environment by means of density functional theory.
Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum
pressures. Under more standard conditions, the most stable structures are the
mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with
one di- and two mono-hydrogenated sites. At high hydrogen-concentration
``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in
analogy to the spontaneous breaking of graphene into small-width nanoribbons
observed experimentally in solution. The stability and the existence of exotic
edge electronic-states and/or magnetism is rationalized in terms of simple
concepts from organic chemistry (Clar's rule)Comment: 4 pages, 3 figures, accepted for publication by Physical Review
Letter
X-ray Linear Dichroism in cubic compounds: the case of Cr3+ in MgAl2O4
The angular dependence (x-ray linear dichroism) of the Cr K pre-edge in
MgAl2O4:Cr3+ spinel is measured by means of x-ray absorption near edge
structure spectroscopy (XANES) and compared to calculations based on density
functional theory (DFT) and ligand field multiplet theory (LFM). We also
present an efficient method, based on symmetry considerations, to compute the
dichroism of the cubic crystal starting from the dichroism of a single
substitutional site. DFT shows that the electric dipole transitions do not
contribute to the features visible in the pre-edge and provides a clear vision
of the assignment of the 1s-->3d transitions. However, DFT is unable to
reproduce quantitatively the angular dependence of the pre-edge, which is, on
the other side, well reproduced by LFM calculations. The most relevant factors
determining the dichroism of Cr K pre-edge are identified as the site
distortion and 3d-3d electronic repulsion. From this combined DFT, LFM approach
is concluded that when the pre-edge features are more intense than 4 % of the
edge jump, pure quadrupole transitions cannot explain alone the origin of the
pre-edge. Finally, the shape of the dichroic signal is more sensitive than the
isotropic spectrum to the trigonal distortion of the substitutional site. This
suggests the possibility to obtain quantitative information on site distortion
from the x-ray linear dichroism by performing angular dependent measurements on
single crystals
- …