M. Landmann

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.

Computational approach for structure design and prediction of optical properties in amorphous TiO2 thin-film coatings

We have investigated the structural and electronic properties of amorphous TiO2 using molecular dynamics (MD) simulations based on ab initio density functional theory, a numerically efficient density-functional-based tight-binding approach and classical many-body potentials. The lower level approximations are successively validated by the higher level ones through comparison of the calculated structural and electronic properties. The classical results reproduce all relevant structural features of a-TiO2 as obtained by quantum-mechanical simulation and reproduce the experimentally observed reduced radial distribution function. This gives convincing justification for the use of classical MD in the simulation of ion beam sputtering synthesis of large-area amorphous thin films. Cross-correlation of electronic data with the statistics of disorder-induced under- and over-coordination is derived as a basis for evaluating the optical quality of thin-film coatings.

The atomic structure of ternary amorphous TixSi1−xO2 hybrid oxides

Atomic length-scale order characteristics of binary and ternary amorphous oxides are presented within the framework of ab initio theory. A combined numerically efficient density functional based tight-binding molecular dynamics and density functional theory approach is applied to model the amorphous (a) phases of SiO2 and TiO2 as well as the amorphous phase of atomically mixed TixSi1-xO2 hybrid-oxide alloys over the entire composition range. Short and mid-range order in the disordered material phases are characterized by bond length and bond-angle statistics, pair distribution function analysis, coordination number and coordination polyhedra statistics, as well as ring statistics. The present study provides fundamental insights into the order characteristics of the amorphous hybrid-oxide frameworks formed by versatile types of TiOn and SiOm coordination polyhedra. In a-SiO2 the fourfold crystal coordination of Si ions is almost completely preserved and the atomic structure is widely dominated by ring-like mid-range order characteristics. In contrast, the structural disorder of a-TiO2 arises from short-range disorder in the local coordination environment of the Ti ion. The coordination number analysis indicates a large amount of over and under-coordinated Ti ions (coordination defects) in a-TiO2. Aside from the ubiquitous distortions of the crystal-like coordinated polyhedra, even the basic coordination-polyhedra geometry type changes for a significant fraction of TiO6 units (geometry defects). The combined effects of topological and chemical disorder in a-TixSi1-xO2 alloys lead to a continuos increase in both the Si as well as the Ti coordination number with the chemical composition x. The important roles of intermediate fivefold coordination states of Ti and Si cations are highlighted for ternary a-TixSi1-xO2 as well as for binary a-TiO2. The continuous decrease in ring size with increasing Ti content reflects the progressive loss of mid-range order structure characteristics and the competing roles of network forming and network modifying SiOm and TiOn units in the mixed hybrid oxides.

Consistent Atomic Geometries and Electronic Structure of Five Phases of Potassium Niobate from Density-Functional Theory

We perform a comprehensive theoretical study of the structural and electronic properties of potassium niobate () in the cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral phase, based on density-functional theory. The influence of different parametrizations of the exchange-correlation functional on the investigated properties is analyzed in detail, and the results are compared to available experimental data. We argue that the PBEsol and AM05 generalized gradient approximations as well as the RTPSS meta-generalized gradient approximation yield consistently accurate structural data for both the external and internal degrees of freedom and are overall superior to the local-density approximation or other conventional generalized gradient approximations for the structural characterization of . Band-structure calculations using a HSE-type hybrid functional further indicate significant near degeneracies of band-edge states in all phases which are expected to be relevant for the optical response of the material.

Copper Substrate Catalyzes Tetraazaperopyrene Polymerization

The polymerization of tetraazaperopyrene (TAPP) molecules on a Cu(111) substrate, as observed in recent STM experiments, has been investigated in detail by first principles calculations. Tautomerization is the first step required for the formation of molecular dimers and polymers. The substrate is found to catalyze this tautomerization.

Photo-Excited Surface Dynamics from Massively Parallel Constrained-DFT Calculations

Constrained density-functional theory (DFT) calculations show that the recently observed optically induced insulator-metal transition of the In/Si(111)(8×2)/(4×1) nanowire array (Frigge et al., Nature 544:207, 2017) corresponds to the non-thermal melting of a charge-density wave (CDW). Massively parallel numerical simulations allow for the simulation of the photo-excited nanowires and provide a detailed microscopic understanding of the CDW melting process in terms of electronic surface bands and selectively excited soft phonon modes. Excited-state molecular dynamics in adiabatic approximation shows that the insulator-metal transition can be as fast as 350 fs.

Solving the Scattering Problem for the P3HT On-Chain Charge Transport

The effect of oxygen impurities and structural imperfections on the coherent on-chain quantum conductance of poly(3-hexylthiophene) is calculated from first principles by solving the scattering problem for molecular structures obtained within density functional theory. It is found that the conductance drops substantially for polymer kinks with curvature radii smaller than 17 A and rotations in excess of about 60∘. Oxidation of thiophene group carbon atoms drastically reduces the conductance, whereas the oxidation of the molecular sulfur barely changes the coherent transport properties. Also isomer defects in the coupling along the chain direction are of minor importance for the intrachain transmission.

Submonolayer Rare Earth Silicide Thin Films on the Si(111) Surface

Rare earth induced silicide phases of submonolayer height and 5 × 2 periodicity on the Si(111) surface are investigated by density functional theory and ab initio thermodynamics. The most stable silicide thin film consists of alternating Si Seiwatz and honeycomb chains aligned along the [1\(\overline{1}\) 0] direction, with rare earth atoms in between. This thermodynamically favored model is characterized by a minor band gap reduction compared to bulk Si and explains nicely the measured scanning tunneling microscopy images.

Surface Charge of Clean LiNbO3 Z-Cut Surfaces

The geometry of the polar LiNbO3 (0001) surface is strongly temperature dependent. In this work the surface charge associated to various surface terminations is estimated from first-principles calculations. All stable terminations are found to lower the polarization charge, showing that the surface charge compensation is a major driving force for surface reconstruction.

Lithium Niobate Dielectric Function and Second-Order Polarizability Tensor From Massively Parallel Ab Initio Calculations

The frequency-dependent dielectric function and the second-order polarizability tensor of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained from density-functional theory. The subsequent application of the GW approximation to account for quasiparticle effects and the solution of the Bethe–Salpeter equation yield a dielectric function for the stoichiometric material that slightly overestimates the absorption onset and the oscillator strength in comparison with experimental measurements. Calculations at the level of the independent-particle approximation indicate that these deficiencies are at least partially related to the neglect of intrinsic defects typical for the congruent material. The second-order polarizability calculated within the independent-particle approximation predicts strong nonlinear coefficients for photon energies above 1.5 eV. The comparison with measured data suggests that self-energy effects improve the agreement between experiment and theory. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular for the 21 and 31 tensor components, further improving the agreement with measured data.