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}: $\alpha$-quartz, $\alpha$- and $\beta$-cristobalite and stishovite, for GeO_{2}: $\alpha$-quartz, and rutile, for Al_{2}O_{3}: $\alpha$-phase, for Si_{3}N_{4} and Ge_{3}N_{4}: $\alpha$- and $\beta$-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 (0.1–0.5 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 $\alpha$-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 $k$-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 $k\cdot p$ 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 Ag–Au Nanoalloys I: Structure and Electronic Properties

We present a systematic study of the structures and the electronic and magnetic properties of 13-atom Ag–Au 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 Au–Au 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 Kohn–Sham HOMO–LUMO gap of about 0.2 eV and a total spin of 1 μ_B. 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 (I−Au 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 I−Au 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