W. G. Schmidt

The electronic structure and optical response of rutile, anatase and brookite TiO2.

In this study, we present a combined density functional theory and many-body perturbation theory study on the electronic and optical properties of TiO(2) brookite as well as the tetragonal phases rutile and anatase. The electronic structure and linear optical response have been calculated from the Kohn-Sham band structure applying (semi)local as well as nonlocal screened hybrid exchange-correlation density functionals. Single-particle excitations are treated within the GW approximation for independent quasiparticles. For optical response calculations, two-particle excitations have been included by solving the Bethe-Salpeter equation for Coulomb correlated electron-hole pairs. On this methodological basis, gap data and optical spectra for the three major phases of TiO(2) are provided. The common characteristics of brookite with the rutile and anatase phases, which have been discussed more comprehensively in the literature, are highlighted. Furthermore, the comparison of the present calculations with measured optical response data of rutile indicate that discrepancies discussed in numerous earlier studies are due to the measurements rather than related to an insufficient theoretical description.

Ground‐ and excited‐state properties of DNA base molecules from plane‐wave calculations using ultrasoft pseudopotentials

We present equilibrium geometries, vibrational modes, dipole moments, ionization energies, electron affinities, and optical absorption spectra of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from first principles. The comparison of our results with experimental data and results obtained by using quantum chemistry methods show that in specific cases gradient‐corrected density‐functional theory (DFT‐GGA) calculations using ultrasoft pseudopotentials and a plane‐wave basis may be a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.

Geometrical and optical benchmarking of copper guanidine–quinoline complexes: Insights from TD‐DFT and many‐body perturbation theory†

We report a comprehensive computational benchmarking of the structural and optical properties of a bis(chelate) copper(I) guanidine–quinoline complex. Using various (TD‐)DFT flavors a strong influence of the basis set is found. Moreover, the amount of exact exchange shifts metal‐to‐ligand bands by 1 eV through the absorption spectrum. The BP86/6‐311G(d) and B3LYP/def2‐TZVP functional/basis set combinations were found to yield results in best agreement with the experimental data. In order to probe the general applicability of TD‐DFT to excitations of copper bis(chelate) charge‐transfer (CT) systems, we studied a small model system that on the one hand is accessible to methods of many‐body perturbation theory (MBPT) but still contains simple guanidine and imine groups. These calculations show that large quasiparticle energies of the order of several electronvolts are largely offset by exciton binding energies for optical excitations and that TD‐DFT excitation energies deviate from MBPT results by at most 0.5 eV, further corroborating the reliability of our TD‐DFT results. The latter result in a multitude of MLCT bands ranging from the visible region at 3.4 eV into the UV at 5.5 eV for the bis(chelate) complex. Molecular orbital analysis provided insight into the CT within these systems but gave mixed transitions. A meaningful transition assignment is possible, however, by using natural transition orbitals. Additionally, we performed a thorough conformational analysis as the correct description of the copper coordination is crucial for the prediction of optical spectra. We found that DFT identifies the correct conformational minimum and that the MLCTs are strongly dependent on the torsion of the chelate angles at the copper center. From the results, it is concluded that extensive benchmarking allows for the quantitative analyses of the CT behavior of copper bis(chelate) complexes within TD‐DFT.

Activation of Surface Hydroxyl Groups by Modification of H- Terminated Si(111) Surfaces

Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the bonding geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity and surface stability suggests new directions to functionalize silicon for sensors, photovoltaic devices, and nanoelectronics.

Geometry and electronic structure of InP(001)(2×4) reconstructions

Abstract We investigate the atomic and electronic structure of the InP(001) surface by means of first-principles pseudopotential calculations within density-functional theory. We are able to rule out (4×2) reconstruction models as well as trimer configurations proposed recently as possible equilibrium structures. We predict (2×4) reconstructions characterized by asymmetric In–P or symmetric P dimers for an In-rich and a balanced surface stoichiometry, respectively. For a very limited range of preparation conditions we cannot exclude, however, the existence of further stable (2×4) reconstructions. All favourable equilibrium structures are characterized by filled and empty surface bands close to the bulk valence and conduction band edges, respectively. Details of the geometry and electronic properties for the various possible surface reconstructions are given and compared with the experimental data available.

GaAs(001): Surface Structure and Optical Properties

The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Abstract Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H+/4 e− process, while oxygen can be fully reduced to water by a 4 e−/4 H+ process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2 −. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

GaP(001) and InP(001): Reflectance anisotropy and surface geometry

We have investigated the optical anisotropy of GaP(001) and InP(001) surfaces. The samples were prepared by homoepitaxial metalorganic vapor phase epitaxy growth and either directly transferred into ultrahigh vacuum (UHV) or in situ capped and, after transfer, decapped in UHV by thermal desorption of a P/As capping layer. Symmetry, composition, and surface optical anisotropy were characterized by low-energy electron diffraction, Auger electron spectroscopy, and reflectance anisotropy spectroscopy. We observe (2×1)/(2×2)-like reconstructions for the very P-rich and (2×4) reconstructions for the more cation-rich surfaces. No (4×2) reconstruction could be prepared, independent of the preparation method. A comparison of the reflectance anisotropy between GaP(001) and InP(001) surfaces shows similar line shapes for the very cation-rich (2×4) surfaces. For less cation-rich surfaces, however, we observe distinct differences between the spectra of the two systems. In both cases, different line shapes in the refle...

Bis-μ-oxo and μ-η2:η2-peroxo dicopper complexes studied within (time-dependent) density-functional and many-body perturbation theory

Based on the equilibrium geometries of [Cu2(dbdmed)2O2]2+ and [Cu2(en)2O2]2+ obtained within density‐functional theory, we investigate their molecular electronic structure and optical response. Thereby results from occupation‐constrained as well as time‐dependent DFT (ΔSCF and TDDFT) are compared with Greens function‐based approaches within many‐body perturbation theory such as the GW approximation (GWA) to the quasiparticle energies and the Bethe‐Salpeter equation (BSE) approach to the optical absorption. Concerning the ground‐state energies and geometries, no clear trend with respect to the amount of exact exchange in the DFT calculations is found, and a strong dependence on the basis sets is to be noted. They affect the energy difference between bis‐μ‐oxo and μ‐η2:η2‐peroxo complexes by as much as 0.8 eV (18 kcal/mol). Even stronger, up to 5 eV is the influence of the exchange‐correlation functional on the gap values obtained from the Kohn‐Sham eigenvalues. Not only the value itself but also the trends observed upon the bis‐μ‐oxo to μ‐η2:η2‐peroxo transition are affected. In contrast, excitation energies obtained from ΔSCF and TDDFT are comparatively robust with respect to the details of the calculations. Noteworthy, in particular, is the near quantitative agreement between TDDFT and GWA+BSE for the optical spectra of [Cu2(en)2O2]2+.

The Cu2O2 torture track for a real-life system: [Cu2(btmgp)2O2](2+) oxo and peroxo species in density functional calculations.

Density functional theory (DFT) calculations of the equilibrium geometry, vibrational modes, ionization energies, electron affinities, and optical response of [Cu2(btmgp)2(μ‐O)2]2+ (oxo) and [Cu2(btmgp)2(μ‐η2:η2‐O2)]2+ (peroxo) are presented. Comprehensive benchmarking shows that the description of the oxo–peroxo energetics is still a torture track for DFT, but finds the molecular geometry to be comparatively robust with respect to changes in the exchange‐correlation functionals and basis sets. Pure functionals favor the oxo core found experimentally, whereas hybrid functionals shift the bias toward the peroxo core. Further stabilization of peroxo core results from relaxing the spin degrees of freedom using the broken‐symmetry (BS) approach. Dispersion effects, conversely, tend to favor the oxo configuration. Triple‐zeta basis sets are found to represent a sensible compromise between numerical accuracy and computational effort. Particular attention is paid to the modification of the electronic structure, optical transitions, and excited‐state energies along the transition path between the oxo and peroxo species. The excited‐state potential energy surface calculations indicate that two triplet states are involved in the transition that stabilize the BS solution. Charge decomposition and natural transition orbital analyses are used for obtaining microscopic insight into the molecular orbital interactions. Here, the crucial role of guanidine π‐interactions is highlighted for the stabilization of the Cu2O2 core.