28 research outputs found
Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4
An extensive theoretical study is performed for wide bandgap crystalline
oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and
Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}:
-quartz, - and -cristobalite and stishovite, for
GeO_{2}: -quartz, and rutile, for Al_{2}O_{3}: -phase, for
Si_{3}N_{4} and Ge_{3}N_{4}: - and -phases. This work
constitutes a comprehensive account of both electronic structure and the
elastic properties of these important insulating oxides and nitrides obtained
with high accuracy based on density functional theory within the local density
approximation. Two different norm-conserving \textit{ab initio}
pseudopotentials have been tested which agree in all respects with the only
exception arising for the elastic properties of rutile GeO_{2}. The agreement
with experimental values, when available, are seen to be highly satisfactory.
The uniformity and the well convergence of this approach enables an unbiased
assessment of important physical parameters within each material and among
different insulating oxide and nitrides. The computed static electric
susceptibilities are observed to display a strong correlation with their mass
densities. There is a marked discrepancy between the considered oxides and
nitrides with the latter having sudden increase of density of states away from
the respective band edges. This is expected to give rise to excessive carrier
scattering which can practically preclude bulk impact ionization process in
Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure
Anisotropy effects on the plasmonic response of nanoparticle dimers
We present an ab initio study of the anisotropy and atomic relaxation effects on the optical properties of nanoparticle dimers. Special emphasis is placed on the hybridization process of localized surface plasmons, plasmon-mediated photoinduced currents, and electric-field enhancement in the dimer junction. We show that there is a critical range of separations between the clusters (0.1â0.5 nm) in which the detailed atomic structure in the junction and the relative orientation of the nanoparticles have to be considered to obtain quantitative predictions for realistic nanoplasmonic devices. It is worth noting that this regime is characterized by the emergence of electron tunneling as a response to the driven electromagnetic field. The orientation of the particles not only modifies the attainable electric field enhancement but can lead to qualitative changes in the optical absorption spectrum of the system.We thankfully acknowledge financial support by the European Research Council (ERC-2010-AdG Proposal No. 267374 and ERC-2011-AdG Proposal No. 290891), the Spanish Government (Grants MAT2011-28581-C02-01, FIS2013-46159-C3-1-P, and MAT2014-53432-C5-5-R), and the Basque Country Government (Grupos Consolidados IT-578-13).Peer Reviewe
Robustness of the chiral-icosahedral golden shell I-Au 60 in multi-shell structures
International audienceMotivated by the recent theoretical discovery [S.-M. Mullins et al., Nat. Commun. 9, 3352 (2018)] of a surprisingly contracted 60-atom hollow shell of chiral-icosahedral symmetry (I-Au60) of remarkable rigidity and electronegativity, we have explored, via first-principles density functional theory calculations, its physico-chemical interactions with internal and external shells, enabling conclusions regarding its robustness and identifying composite forms in which an identifiable I-Au60 structure may be realized as a product of natural or laboratory processes. The dimensions and rigidity of I-Au60 suggest a templating approach; e.g., an Ih-C60 fullerene fits nicely within its interior, as a nested cage. In this work, we have focused on its susceptibility, i.e., the extent to which the unique structural and electronic properties of I-Au60 are modified by incorporation into selected multi-shell structures. Our results confirm that the I-Au60 shell is robustly maintained and protected in various bilayer structures: Ih-C60@I-Au60, Ih-Au32@I-Au2+60, Au60(MgCp)12, and their silver analogs. A detailed analysis of the structural and electronic properties of the selected I-Au60 shell-based nanostructures is presented. We found that the I-Au60 shell structure is quite well retained in several robust forms. In all cases, the I-symmetry is preserved, and the I-Au60 shell is slightly deformed only in the case of the Ih-C60@I-Au60 system. This analysis serves to stimulate and provide guidance toward the identification and isolation of various I-Au60 shell-based nanostructures, with much potential for future applications. We conclude with a critical comparative discussion of these systems and of the implications for continuing theoretical and experimental investigation
Calculation of optical properties for systems with large supercells
Si and Ge nanocrystals embedded in a wide-gap semiconductor like -SiC are potentially interesting systems for possible electroluminescence applications. A theoretical understanding of these systems is yet to be achieved, both for the optical properties under inclusion of the relevant many-body effects and for the structural and electronic properties. We present first attempts to describe these properties using parameter-free electronic structure calculations and large supercells. A plane-wave-pseudopotential code (VASP ) is used to calculate the electronic structure within density-functional theory (DFT) in local-density approximation (LDA). Ultrasoft non-normconserving pseudopotentials allow the {\it ab-initio} treatment of supercells with up to 512 atoms, even in the case of first-row elements. Each supercell contains one cluster, the remaining space is filled with matrix material. The maximum dot diameters are about 1 nm. Examples are mainly structures made of group-IV materials. In the present talk we focus our attention on three main problems.
The problem of wave-function augmentation for non-normconserving pseudopotentials is solved by constructing all-electron wave functions using Bl\"ochl's projector-augmented wave (PAW) method [1]. As an advantage nonlocal contributions to the optical matrix elements do not occur. Spectra are compared with those obtained using normconserving pseudopotentials and the FLAPW method.
The huge supercell size drastically restricts the number of -points. Nevertheless we prefer to use the tetrahedron method for the optical calculations. Unfortunately the increase of the supercell size gives rise to many band crossings which prevent the identification of the same band at the tetrahedron vertices. We present a highly efficient and robust extrapolative Brillouin-zone integration scheme based on second-order perturbation theory (cf. [2]).
We use the simplified treatment of the GW self-energy developed by Cappellini et al. [3] in the beginning of the 90's to calculate the quasiparticle corrections for supercells with several hundreds of atoms . The dielectric constant and the electron density determining the RPA screening are obtained within DFT-LDA. Despite the complications with the augmentation of the Bloch integrals we show agreement with calculations using two-atom cells. Computations for embedded clusters are in progress
The dynamic structure factor of simple metals : a study of the electronic correlation in solids
In this work we compare experimental inelastic X-ray scattering spectra and TDDFT results for sodium and aluminum. The effects due to the real lattice are not sufficient to bring the results significantly closer to the experimental ones than previous results for jellium. We show that an accurate description of electron correlations is also required to provide a reasonable agreement, espeaally in the case of sodium. TDDFT-TDLDA with the inclusion of lifetime effects provides a good agreement for momentum transfer vertical bar q vertical bar smaller than 2 ge, only for larger vertical bar q vertical bar the agreement deteriorates. We will discuss the possibility of constructing an exchange-correlation kernel able to reproduce these lifetime effects
Dynamical response function in sodium and aluminum from time-dependent density-functional-theory
We present a detailed study of the dynamical electronic response in bulk sodium and aluminum within time-dependent density-functional theory (TDDFT). The poor results of the random-phase approximation (RPA) and the time-dependent local-density approximation (TDLDA) in sodium are greatly improved by the approximate inclusion of the finite lifetimes of electrons and holes via a modified independent-particle polarizability, which brings the calculated spectra into good agreement with experiment. For aluminum the changes are less visible, but at some values of momentum-transfer lifetime effects are necessary to obtain qualitatively correct spectra. The double-peak structure in aluminum, induced by band-structure effects, is partially washed out by the inclusion of the finite lifetimes. The latter do not, however, create a double peak by themselves as they do in the case of the homogeneous electron gas. Studying the performance of different time-dependent and nonlocal TDDFT kernels, we conclude that the Gross-Kohn, Corradini et al., and the Hubbard local-field factors improve the spectra compared to the RPA results. However, the results agree less well with experiment than those obtained using TDLDA with added lifetime effects. These results apply to both the loss spectra and the plasmon dispersion
Trends and Properties of 13-Atom AgâAu Nanoalloys I: Structure and Electronic Properties
We present a systematic study of the structures and the electronic
and magnetic properties of 13-atom AgâAu nanoalloys, using
spin-polarized <i>ab initio</i> calculations based on density
functional theory. To this end, we use all possible chemical configurations
of four different initial symmetries as starting structures: icosahedra,
decahedra, cuboctahedra, and the buckled biplanar (BBP) cluster. Mixing
is energetically favored; there is no indication of segregation. We
find a general tendency to minimize the number of AuâAu bonds.
Many of the clusters undergo strong morphology changes. The resulting
structures of lowest energy, independent of the starting geometry,
are distorted biplanar clusters. The cuboctahedra are a rather stable
local minimum against geometry changes following the introduction
of the mixing. All the lowest-energy structures have a KohnâSham
HOMOâLUMO gap of about 0.2 eV and a total spin of 1 ÎŒ<sub>B</sub>. Higher total spin values are found for some of the icosahedra
and decahedra, but they have an energy much higher than that of the
lowest-energy structures of the respective compositions. The quasi-particle
gap is about 3.7 eV across the composition range. It does not vary
appreciably with the composition and structural details of the clusters
Chiral symmetry breaking yields the I-Au60 perfect golden shell of singular rigidity
International audienceThe combination of profound chirality and high symmetry on the nm-scale is unusual and would open exciting avenues, both fundamental and applied. Here we show how the unique electronic structure and bonding of quasi-2D gold makes this possible. We report a chiral symmetry breaking, i.e., the spontaneous formation of a chiral-icosahedral shell (IâAu 60) from achiral (I h) precursor forms, accompanied by a contraction in the Au-Au bonding and hence the radius of this perfect golden sphere, in which all 60 sites are chemically equivalent. This structure, which resembles the most complex of semi-regular (Archimedean) polyhedra (3 4 .5 *), may be viewed as an optimal solution to the topological problem: how to close a 60-vertex 2D (triangular) net in 3D. The singular rigidity of the IâAu 60 manifests in uniquely discrete structural, vibrational, electronic, and optical signatures, which we report herein as a guide to its experimental detection and ultimately its isolation in material forms
Classical versus ab initio structural relaxation: electronic excitations and optical properties of Ge nanocrystals embedded in a SiC matrix
We discuss and test a combined method to efficiently perform ground-state
and excited-state calculations for relaxed structures using both a quantum firstprinciples
approach and a classical molecular-dynamics scheme. We apply this
method to calculate the ground state, the optical properties, and the electronic
excitations of Ge nanoparticles embedded in a cubic SiC matrix. Classical
molecular dynamics is used to relax the large-supercell system. First-principles
quantum techniques are then used to calculate the electronic structure and, in
turn, the electronic excitation and optical properties. The proposed procedure
is tested with data resulting from a full first-principles scheme. The agreement
is quantitatively discussed between the results after the two computational
paths with respect to the structure, the optical properties, and the electronic
excitations. The combined method is shown to be applicable to embedded
nanocrystals in large simulation cells for which the first-principles treatment
of the ionic relaxation is presently out of reach, whereas the electronic, optical
and excitation properties can already be obtained ab initio. The errors incurred
from the relaxed structure are found to be non-negligible but controllable
Structural relaxation effects on the electronic excitations and optical properties of Ge nanocrystals embedded in a SiC matrix
We propose a combined method to eciently perform ground- and excited-state calculations
for relaxed geometries using both a rst-principles approach and a classical molecular-dynamics
scheme. We apply this method to calculate the ground state, the optical properties, and the
electronic excitations of Ge nanoparticles embedded in a SiC matrix. Classical dynamics is used
to relax the large cell system. First-principles techniques are then used to calculate the electronic
structure and, in turn, the electronic excitations and optical properties. The proposed procedure
is tested with data resulting from a full rst-principles scheme. Good qualitative accordance has
been found between the results after the two computational paths regarding the structure, the
optical properties and even the electronic excitations