Optical excitations of defects in realistic nanoscale silica clusters: comparing the performance of density functional theory using hybrid functionals with correlated wavefunction methods

Optical excitations of low energy silica (SiO(2))(4) clusters obtained by global optimization, as opposed to constructed by hand, are studied using a range of theoretical methods. By focusing on the lowest energy silica clusters we hope to capture at least some of the characteristic ways by which the dry surfaces of silica nanosystems preferentially terminate. Employing the six lowest energy (SiO(2))(4) cluster isomers, we show that they exhibit a surprisingly wide range of geometries, defects, and associated optical excitations. Some of the clusters show excitations localized on isolated defects, which are known from previous studies using hydrogen-terminated versions of the defect in question. Other clusters, however, exhibit novel charge-transfer excitations in which an electron transfers between two spatially separated defects. In these cases, because of the inherent proximity of the constituent defects due to the small cluster dimensions, the excitation spectrum is found to be very different from that of the same defects in isolation. Excitation spectra of all clusters were calculated using time-dependent density functional theory (TD-DFT) and delta-SCF DFT (DeltaDFT) methods employing two different hybrid density functionals (B3LYP and BB1K) differing essentially in the amount of incorporated Hartree-Fock-like exchange (HFLE). In all cases the results were compared with CASPT2 calculated values which are taken as a benchmark standard. In line with previous work, the spatially localized excitations are found to be well described by TD-DFT/B3LYP but which gives excitation energies that are significantly underestimated in the case of the charge-transfer excitations. The TD-DFT/BB1K combination in contrast is found to give generally good excitation energies for the lowest excited states of both localized and charge-transfer excitations. Finally, our calculations suggest that the increased quality of the predicted excitation spectra by adding larger amounts of HFLE is mainly due to an increased localization of the excited state associated with the elimination of spurious self-interaction inherent to (semi-)local DFT functionals.     

Octahedrality versus tetrahedrality in stoichiometric ceria nanoparticles

We predict that tetrahedral Ce(n)O(2n) nanoparticles <2 nm in size become more stable than those experimentally observed at larger sizes with truncated octahedral morphologies, based on global optimisation and density functional calculations.     

Density functional studies of model cerium oxide nanoparticles

Density functional plane-wave calculations have been performed to investigate a series of ceria nanoparticles (CeO2-x)(n), n Ce3+ reduction have been accounted for through the use of an effective on-site Coulomb repulsive interaction within the so-called DFT+U approach. Twelve nanoparticles of up to 2 nm in diameter and of both cuboctahedral and octahedral forms are chosen as representative model systems. Energetic and structural effects of oxygen vacancy formation in these nanoparticles are discussed with respect to those in the bulk and on extended surfaces. We show that the average interatomic distances of the nanoparticles are most significantly affected by the creation of oxygen vacancies. The formation energies of non-stoichiometric nanoparticles (CeO2-x)(n) are found to scale linearly with the average coordination number of Ce atoms; where x < 0 species, containing partially reduced O atoms, are less stable. The stability of octahedral ceria particles at small sizes, and the predicted strong propensity of Ce cations to acquire a reduced state at lower coordinated sites, is supported by interatomic potential-based global optimisations probing the low energy isomers of the Ce19O32 nanoparticle.     

Columnar-to-disk structural transition in nanoscale (SiO2)N clusters

Extensive large-scale global optimizations refined by ab initio calculations are used to propose (SiO2)N N = 14-27 ground states. For N < 23 clusters are columnar and show N-odd-N-even stability, energetically and electronically. At N = 23 a columnar-to-disk structural transition occurs reminiscent of that observed for SiN. These transitions differ in nature but have the same basis, linking the nanostructural behavior of an element (Si) and its oxide (SiO2). Considering the impact of devices based on the nanoscale manipulation of the result is of potential technological importance.     

Potentiometrische p-Bestimmung

Ohne Zusammenfassung     

Steady streaming around a spherical drop displaced from the velocity antinode in an acoustic levitation field

The steady (acoustic) streaming associated with a sphericaldrop displaced from the velocity antinode of a standing waveis studied. The ratio of the particle size to the acoustic wavelengthis treated as small but non-zero, and the solution is developedin the form of a two-term expansion in terms of the correspondingsmallness parameter. The drop viscosity is assumed to be muchhigher than that of the surrounding fluid, which is the casefor a drop in a gas medium. There are essentially three distinctregions where the steady streaming flow is analysed: insidethe drop (internal circulation), in the Stokes shear-wave layerat the surface on the gas side, and the gas outside the Stokeslayer (the outer streaming region). Solutions for the internalcirculation and the outer streaming are obtained in the limitof small Reynolds number. Despite the gas-to-liquid viscosity ratio being small, the outerstreaming may be dramatically affected by the fact that thesphere is liquid as opposed to solid. The parameter that measuresthe effect of liquidity is essentially the viscosity ratio dividedby the relative (to the particle size) thickness of the Stokeslayer. The case of a solid sphere is recovered by letting thisparameter go to zero.     

Tandem inverse electron demand Diels-Alder, retro-Diels-Alder and intramolecular Diels-Alder sequences: one-pot synthesis of diaza-polycycles

A new and straightforward methodology is described for the construction of complex nitrogen-containing polycycles from substituted 1,2,4-triazines and enamines, prepared in situ from carbonyl components and allylic amines. The transformation proceeds via a pericyclic reaction cascade (inverse electron demand Diels-Alder then retro-Diels-Alder then intramolecular Diels-Alder) forming the polycyclic systems in good to excellent yield and with high diastereoselectivity in a one-pot procedure. The scope and limitations of the methodology are described as are preliminary studies to extend its synthetic utility.