A. Riefer

Structural variety of 5-fluoroarene-2-aminopyrimidine in comparison to 2-aminopyrimidine silver(I) coordination polymers: progress report and overview

A variety of fluoroarene-2-aminopyrimidine (FAP) silver(I) coordination polymers (CPs) has been synthesized based on newly synthesized FAP derivatives, namely 5-(p-methoxytetrafluorophenyl)-2-aminopyrimidine (OFAP) and 5-(p-dimethylaminotetrafluorophenyl)-2-aminopyrimidine (NFAP), and different counterions (OTf−, TFA−, ClO4−, NO3−). Their solid-state assembly as well as optical properties in terms of luminescence and infrared (IR) spectroscopy were investigated. Out of the several structures described herein, we obtained isomorphic CPs to previous studies (5, 9), a CP architecture with a short Ag–Ag distance (8, 3.049 A), but also polymorphic crystals of [Ag(nfap)NO3]n (11a, 11b) and the latter showed differences in color and luminescence emission. Polymorphism gives an unparalleled possibility to investigate the origin of such phenomena since luminescence emission is quite often observed for these silver-hybrid solid-state materials and in several cases Ag⋯Ag interactions are attributed for this phenomenon. We show that this explanation does not necessarily have to be the only one. Therefore we focus also on structural relationships and differences in a comprehensive comparison of our own and other known systems to start a more systematic description of the rich coordination capabilities of FAP and congeneric 2-aminopyrimidine (2-AP) based silver(I) coordination polymers and networks. Density functional theory (DFT) calculations with periodic boundary conditions based on a plane wave basis are used to better understand the electronic structure of these crystalline materials. To complete the picture, steady-state spectroscopy studies (UV-Vis, fluorescence, IR) on all ligands and 2-AP itself were conducted as well as re-examination of the first reported CP of 2-AP and AgI under the above aspects.

Phonon dispersion and zero-point renormalization of LiNbO3 from density-functional perturbation theory.

The vibrational properties of stoichiometric LiNbO3 are analyzed within density-functional perturbation theory in order to obtain the complete phonon dispersion of the material. The phonon density of states of the ferroelectric (paraelectric) phase shows two (one) distinct band gaps separating the high-frequency (∼800 cm(-1)) optical branches from the continuum of acoustic and lower optical phonon states. This result leads to specific heat capacites in close agreement with experimental measurements in the range 0-350 K and a Debye temperature of 574 K. The calculated zero-point renormalization of the electronic Kohn-Sham eigenvalues reveals a strong dependence on the phonon wave vectors, especially near [Formula: see text]. Integrated over all phonon modes, our results indicate a vibrational correction of the electronic band gap of 0.41 eV at 0 K, which is in excellent agreement with the extrapolated temperature-dependent measurements.

Linear and nonlinear optical response of LiNbO 3 calculated from first principles

The dielectric function and second-harmonic generation spectrum of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained within density functional theory. The use of the GW approach to account for quasiparticle effects and the subsequent solution of the Bethe-Salpeter equation lead to a dielectric function in excellent agreement with measured data. The second harmonic generation spectrum calculated within the independent (quasi) particle approximation predicts strong nonlinear coefficients for photon energies above about 1.5 eV. The comparison with measured data suggests that the inclusion of self-energy effects in the nonlinear response improves the agreement with experiment.

LiNb1-xTaxO3 Electronic Structure and Optical Response from First-Principles Calculations

Ab-initio methods are applied to investigate the electronic and optical properties of lithium niobate-tantalate mixed crystals. Thereby the GW approach based on the electronic structure obtained within density functional theory is used to obtain the electron band structure including quasi-particle effects and the linear optical response is gained form the solution of the Bethe-Salpeter equation taking excitonic and local-field effects into account. Nonlinear optical coefficients are evaluated on the level of the independent-particle approximation. It is observed that increasing the tantalum amount widens the band gap and nearly linearly affects the optical birefringence. Also some second harmonic generation coefficients vary strongly with the stoichiometry.

Vibrational fingerprints of LiNbO 3 -LiTaO 3 mixed crystals

The structural and vibrational properties of lithium niobate (LN) — lithium tantalate (LT) mixed crystals (LNT, LiNb 1−x Ta x O 3 ) are investigated over the whole composition range by first-principles simulations. The crystal volume grows roughly linearly from LT to LN, whereby the lattice parameters a and c show minor deviations from the Vegard behavior between the end-compounds, LiNbO 3 and LiTaO 3 . Our calculations in the framework of the density functional theory show the TO 1 , TO 2 and TO 4 -modes to become harder with increasing Nb concentration. TO 3 becomes softer with increasing Nb content, instead. The frequency shifts of the zone center A 1 -TO phonon modes for crystals with different compositions are found to be as large as 30 cm−1. Raman spectroscopy, which is sensitive to the A 1 modes, can be therefore employed to determine the crystal composition.

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.

LiNbO 3 linear and nonlinear optical response from first-principles calculations

The dielectric function and second harmonic generation of ferroelectric LiNbO3 is calculated from first-principles. Thereby we start from the electronic structure calculated within the density functional theory. The use of the GW approach to account for quasiparticle effects and the subsequent solution of the Bethe-Salpeter equation leads to a dielectric function that is in excellent agreement with the available experimental results. Our second harmonic generation calculations rest on the independent particle approximation and predict strong non-linear coefficients, in particular in the energy range starting from 1.5 eV.

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.