Peter Broqvist

Hybrid-functional calculations with plane-wave basis sets: Effect of singularity correction on total energies, energy eigenvalues, and defect energy levels

When described through a plane-wave basis set, the inclusion of exact nonlocal exchange in hybrid functionals gives rise to a singularity, which slows down the convergence with the density of sampled k points in reciprocal space. In this work, we investigate to what extent the treatment of the singularity through the use of an auxiliary function is effective for k-point samplings of limited density, in comparison to analogous calculations performed with semilocal density functionals. Our analysis applies, for instance, to calculations in which the Brillouin zone is sampled at the sole Gamma point, as often occurs in the study of surfaces, interfaces, and defects or in molecular-dynamics simulations. In the adopted formulation, the treatment of the singularity results in the addition of a correction term to the total energy. The energy eigenvalue spectrum is affected by a downwards shift in the energy eigenvalues of the occupied states, while those of the unoccupied states remain unaffected. Analogous corrections also speed up the convergence of screened exchange interactions despite the absence of a proper singularity. Focusing first on neutral systems, both finite and extended, we show that the account of the singularity corrections bears convergence properties which are quantitatively similar to those observed with semilocal density functionals. We emphasize that this is not the case for uncorrected energies, particularly for elongated simulation cells for which qualitatively different trends are found. We then consider differences between total energies of systems differing by their charge state. For systems involving localized electron states, such as ionization potentials and electron affinities of molecular systems or charge transition levels of point defects, the proper account of the singularity correction yields convergence properties which are similar to those of neutral systems. In the case of extended systems, such energy differences provide an alternative way to determine the band edges, but are found to converge more slowly with simulation cells than in corresponding semilocal functionals because of the exchange self-interaction associated to the extra charge.

Supercharged Low-Temperature Oxygen Storage Capacity of Ceria at the Nanoscale.

We provide an explanation for the experimental finding of a dramatically enhanced low-temperature oxygen storage capacity for small ceria nanoparticles. At low temperature, small octahedral ceria nanoparticles will be understoichiometric at both oxidizing and reducing conditions without showing explicit oxygen vacancies. Instead, rather than becoming stoichiometric at oxidizing conditions, such particles are stabilized through oxygen adsorption forming superoxo (O2(-)) ions and become in this way supercharged with oxygen. The supercharging effect is size-dependent and largest for small nanoparticles where it gives a direct increase in the oxygen storage capacity and simultaneously provides a source of active oxygen species at low temperatures.

An SCC-DFTB Repulsive Potential for Various ZnO Polymorphs and the ZnO-Water System.

We have developed an efficient scheme for the generation of accurate repulsive potentials for self-consistent charge density-functional-based tight-binding calculations, which involves energy-volume scans of bulk polymorphs with different coordination numbers. The scheme was used to generate an optimized parameter set for various ZnO polymorphs. The new potential was subsequently tested for ZnO bulk, surface, and nanowire systems as well as for water adsorption on the low-index wurtzite (101̅0) and (112̅0) surfaces. By comparison to results obtained at the density functional level of theory, we show that the newly generated repulsive potential is highly transferable and capable of capturing most of the relevant chemistry of ZnO and the ZnO/water interface.

Enhanced wetting of Cu on ZnO by migration of subsurface oxygen vacancies

Metal adhesion on metal oxides is strongly controlled by the oxide surface structure and composition, but lack of control over the surface conditions often limits the possibilities to exploit this in opto- and micro-electronics applications and heterogeneous catalysis where nanostructural control is of utmost importance. The Cu/ZnO system is among the most investigated of such systems in model studies, but the presence of subsurface ZnO defects and their important role for adhesion on ZnO have been unappreciated so far. Here we reveal that the surface-directed migration of subsurface defects affects the Cu adhesion on polar ZnO(0001) in the technologically interesting temperature range up to 550u2009K. This leads to enhanced adhesion and ultimately complete wetting of ZnO(0001) by a Cu overlayer. On the basis of our experimental and computational results we demonstrate a mechanism which implies that defect concentrations in the bulk are an important, and possibly controllable, parameter for the metal-on-oxide growth.

Fluorine impurities at CeO2(111): Effects on oxygen vacancy formation, molecular adsorption, and surface re-oxidation

We investigate the effects of anion doping with fluorine impurities on the chemistry of the CeO2 (111) facet, using the results of DFT + U calculations. We consider three prototypical processes: the formation of oxygen vacancies, the adsorption of O2 and H2O molecules, and the re-oxidation of the surface with fragments of the two molecules. We find that the first two of these processes are not strongly affected, but that the presence of F lowers the energy gained in the re-oxidation of the surface in comparison to the healing of an oxygen vacancy, by 1.47 eV in the case of O2 (provided that the F is part of a cluster) and by 0.92 eV in the case of H2O. Based on these results, we suggest that F could enhance the redox chemistry of ceria by toggling between being in the surface and on the surface, effectively facilitating the release of lattice O by acting as a place holder for it. Finally, we find that the desorption of F as either 12F2 or HF is energetically unfavourable, suggesting that F doped ceria should be stable in the presence of O2 and H2O.

Comparing van der Waals DFT methods for water on NaCl(001) and MgO(001)

In this work, a range of van der Waals type density functionals are applied to the H2O/NaCl(001) and H2O/MgO(001) interface systems to explore the effect of an explicit dispersion treatment. The functionals we use are the self-consistent vdW functionals vdW-DF, vdW-DF2, optPBE-vdW, optB88-vdW, optB86b-vdW, and vdW-DF-cx, as well as the dispersion-corrected PBE-TS and PBE-D2 methods; they are all compared with the standard PBE functional. For both NaCl(001) and MgO(001), we find that the dispersion-flavoured functionals stabilize the water-surface interface by approximately 20%-40% compared to the PBE results. For NaCl(001), where the water molecules remain intact for all overlayers, the dominant contribution to the adsorption energy from density functional theory dispersion stems from the water-surface interactions rather than the water-water interactions. The optPBE-vdW and vdW-DF-cx functionals yield adsorption energies in good agreement with available experimental values for both NaCl and MgO. To probe the strengths of the perturbations of the adsorbed water molecules, we also calculated water dipole moments and found an increase up to 85% for water at the MgO(001) surface and 70% at the NaCl(001) surface, compared to the gas-phase dipole moment.

Band-Filling Correction Method for Accurate Adsorption Energy Calculations: A Cu/ZnO Case Study.

We present a simple method, the band-filling correction, to calculate accurate adsorption energies (Eads) in the low coverage limit from finite-size supercell slab calculations using DFT. We show that it is necessary to use such a correction if charge transfer takes place between the adsorbate and the substrate, resulting in the substrate bands either filling up or becoming depleted. With this correction scheme, we calculate Eads of an isolated Cu atom adsorbed on the ZnO(101̅0) surface. Without the correction, the calculated Eads is highly coverage-dependent, even for surface supercells that would typically be considered very large (in the range from 1 nm × 1 nm to 2.5 nm × 2.5 nm). The correction scheme works very well for semilocal functionals, where the corrected Eads is converged within 0.01 eV for all coverages. The correction scheme also works well for hybrid functionals if a large supercell is used and the exact exchange interaction is screened.

Alignment of Defect Energy Levels at Si‐SiO2 Interface from Hybrid Density Functional Calculations

Keywords: defects ; energy levels ; oxide ; hybrid density functionals ; Silicon Dioxide Reference CSEA-CONF-2009-003View record in Web of Science Record created on 2009-10-14, modified on 2017-05-12

Ceria chemistry at the nanoscale: effect of the environment

We use theoretical simulations to study how oxidative and humid environments affect the chemical composition, shape and structure of ceria nanoparticles. Based on our calculations, we predict that small stoichiometric ceria nanoparticles will have a very limited stability range when exposed to these environments. Instead, we find that reduced ceria nanoparticles are stabilized without changing their inherent shape through the adsorption of oxygen molecules in the form of superoxo species and water in the form of hydroxo species. Based on our results, we propose a redox-cycle for meta-stable ceria nanoparticles without the formation of explicit oxygen vacancies, which is important for understanding the low-temperature oxygen chemistry of ceria at the nanoscale.

Defect Levels of the Ge Dangling Bond Defect

Keywords: Dangling bond defects ; Germanium ; Silicon ; hybrid density functional theory ; charge transition levels ; Interface ; Traps Reference CSEA-CONF-2009-006View record in Web of Science Record created on 2009-10-14, modified on 2017-05-12