arXiv subjects

Ralf Tonner

Publications and source records attributed to Ralf Tonner.

9 recordsLinked to original sources

arXiv2019

Understanding the correlation between electronic coupling and energetic stability of molecular crystal polymorphs: The instructive case of quinacridone

A crucial factor determining charge transport in organic semiconductors is the electronic coupling between the molecular constituents, which is heavily influenced by the relative arrangement of the molecules. This renders quinacridone, with its multiple, structurally fundamentally different polymorphs and their diverse intermolecular interactions an ideal test case for analyzing the correlation between the electronic coupling in a specific configuration and the configuration's energetic stability. To provide an in-depth analysis of this correlation, starting from the $α$-polymorph of quinacridone, we also construct a coplanar model crystal. This allows us to systematically compare the displacement-dependence of the electronic coupling with that of the total energy. In this way, we identify the combination of Pauli repulsion and orbital rehybridization as the driving force steering the system towards a structure in which the electronic coupling is minimal (especially for the valence band and at small displacements). The general nature of these observations is supported by equivalent trends for an analogous pentacene model system. This underlines that the design of high-performance materials cannot rely on the "natural" assembly of the $π$-conjugated backbones of organic semiconductors into their most stable configurations. Rather, it must include the incorporation of functional groups that steer crystal packing towards more favorable structures, where aiming for short-axis displacements or realizing comparably large long-axis displacements appear as strategies worthwhile exploring.

cond-mat.mtrl-sci↗

arXiv2017

Ab initio calculations of the concentration dependent band gap reduction in dilute nitrides

While being of persistent interest for the integration of lattice-matched laser devices with silicon circuits, the electronic structure of dilute nitride III/V-semiconductors has presented a challenge to ab initio computational approaches. The root of this lies in the strong distortion N atoms exert on most host materials. Here, we resolve these issues by combining density functional theory calculations based on the meta-GGA functional presented by Tran and Blaha (TB09) with a supercell approach for the dilute nitride Ga(NAs). Exploring the requirements posed to supercells, we show that the distortion field of a single N atom must be allowed to decrease so far, that it does not overlap with its periodic images. This also prevents spurious electronic interactions between translational symmetric atoms, allowing to compute band gaps in very good agreement with experimentally derived reference values. These results open up the field of dilute nitride compound semiconductors to predictive ab initio calculations.

cond-mat.mtrl-sci↗

arXiv2016

Experimental and Computational Study of Isomerically Pure Soluble Aza-phthalocyanines and Azasubphthalocyanines of Varying Number of Aza Units

Herein, we present a series of isomerically pure, peripherically alkyl substituted, soluble and low aggregating azaphthalocyanines as well as their new, smaller hybrid homologues, the azasubphthalocyanines. The focus lies on the effect of the systematically increasing number of aza building blocks [-N=] replacing the non peripheral [-CH=] units and its influence on physical and photophysical properties of these chromophores. The absolute and relative HOMO-LUMO energies of azaphthalocyanines was analyzed using UV-Vis, CV and compared to density functional theory calculations (B3LYP, TD-DFT). Crystals of substituted subphthalocyanine, N2-Pc*H2 and N4-[Pc*Zn$\cdot$H2O] were obtained out of DCM. For the synthesis of the valuable tetramethyltetraline phthalocyanine building block a new highly efficient synthesis involving a nearly quantitative CoII catalyzed aerobic autoxidation step is introduced replacing inefficient KMnO4/pyridine as oxidant. The lowering of the HOMO level is revealed as the determining factor for the trend in the adsorption energies by electronic structure analysis.

physics.chem-ph↗

arXiv2016

Site-specific reactivity of ethylene at distorted dangling bond configurations on Si(001)

We report differences in adsorption and reaction energetics for ethylene on Si(001) with respect to different dangling bond configurations induced by hydrogen precoverage as obtained via density functional theory calculations. This can help to understand the influence of surface defects and precoverage on the reactivity of organic molecules on semiconductor surfaces in general. Our results show that the reactivity on surface dimers fully enclosed by hydrogen covered atoms is essentially unchanged compared to the clean surface. This is confirmed by our scanning tunnelling microscopy measurements. On the contrary, adsorption sites with partially covered surface dimers show a drastic increase in reactivity. This is due to a lowering of the reaction barrier by more than fifty percent compared to the clean surface, which is in line with previous experiments. Adsorption on dimers enclosed by molecule (ethylene) covered surface atoms is reported to have a highly decreased reactivity, a result of destabilization of the intermediate state due to steric repulsion, as quantified with the periodic energy decomposition analysis (pEDA). Furthermore, an approach for the calculation of Gibbs energies of adsorption based on statistical thermodynamics considerations is applied to the system. The results show that the loss in molecular entropy leads to a significant destabilization of adsorption states.

physics.chem-ph↗

arXiv2016

Comparison of the periodic slab approach with the finite cluster ansatz for metal-organic interfaces at the example of PTCDA on Ag(110)

We present a comparative study of metal-organic interface properties obtained from dispersion corrected density functional theory calculations based on two different approaches: the periodic slab supercell technique and cluster models with 18 to 290 Ag atoms. Fermi smearing and fixing of cluster borders are required to make the cluster calculation feasible and realistic. The considered adsorption structure and energy of a PTCDA molecule on the Ag(110) surface is not well reproduced with clusters containing only two metallic layers. However, clusters with four layers of silver atoms and sufficient lateral extension reproduce the adsorbate structure within 0.02 Å and adsorption energies within 10\% of the slab result. A consideration of the computational effort shows that the cluster approach is a competitive alternative to methods using periodic boundary conditions and of particular interest for research at surface defects and other systems that do not show periodic symmetry.

physics.chem-ph↗

arXiv2016

Extent of hydrogen coverage of Si(001) under chemical vapor deposition conditions from ab initio approaches

The extent of hydrogen coverage of the Si(001)c(4x2) surface in the presence of hydrogen gas has been studied with dispersion corrected density functional theory. Electronic energy contributions are well described using a hybrid functional. The temperature dependence of the coverage in thermodynamic equilibrium was studied computing the phonon spectrum in a supercell approach. As an approximation to these demanding computations, an interpolated phonon approach was found to give comparable accuracy. The simpler ab initio thermodynamic approach is not accurate enough for the system studied, even if corrections by the Einstein model for surface vibrations are considered. The on-set of H2 desorption from the fully hydrogenated surface is predicted to occur at temperatures around 750 K. Strong changes in hydrogen coverage are found between 1000 and 1200 K in good agreement with previous reflectance anisotropy spectroscopy experiments. These findings allow a rational choice for the surface state in the computational treatment of chemical reactions under typical metal organic vapor phase epitaxy conditions on Si(001).

physics.chem-ph↗

arXiv2016

Electron-Vibron Coupling at Metal-Organic Interfaces from Theory and Experiment

We study the significance and characteristics of interfacial dynamical charge transfer at metal-organic interfaces for the organic semiconductor model system 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) on Ag(111) quantitatively. We combine infrared absorption spectroscopy and dispersion-corrected density functional theory calculations to analyze dynamic dipole moments and electron-vibron coupling at the interface. We demonstrate that interfacial dynamical charge transfer is the dominant cause of infrared activity in these systems and that it correlates with results from partial charge and density of states analysis. Nuclear motion generates an additional dynamic dipole moment but represents a minor effect except for modes with significant out-of-plane amplitudes.

physics.chem-ph↗

arXiv2016

Configuration Dependence of Band Gap Narrowing and Localization in Dilute GaAs_{1-x} Bi_x Alloys

Anion substitution with bismuth (Bi) in III-V semiconductors is an effective method for experimental engineering of the band gap Eg at low Bi concentrations, in particular in gallium arsenide (GaAs). The inverse Bi-concentration dependence of Eg has been found to be linear at low concentrations x and dominated by a valence band-defect level anticrossing between As and Bi occupied p levels. This dependence breaks down at high concentrations where empirical models accounting only for the As-Bi interaction are not applicable. Predictive models for the valence band hybridization require a first-principle understanding which can be obtained by density functional theory with the main challenges being the proper description of Eg and the spin-orbit coupling. By using an efficient method to include these effects, it is shown here that at high concentrations Eg is modified mainly by a Bi-Bi p orbital interaction and by the large Bi atom-induced strain. This points to the role of different atomic configurations obtained by varying the experimental growth conditions in engineering arsenide band gaps, in particular for telecommunication laser technology.

cond-mat.mtrl-sci↗

arXiv2015

A periodic Energy Decomposition Analysis (pEDA) method for the Investigation of Chemical Bonding in Extended Systems

The development and first applications of a new periodic energy decomposition analysis (pEDA) scheme for extended systems based on the Kohn-Sham approach to density functional theory are described. The pEDA decomposes the binding energy between two fragments (e.g. the adsorption energy of a molecule on a surface) into several well-defined terms: preparation, electrostatic and dispersion interaction, Pauli repulsion and orbital relaxation energies. The pEDA presented here for an AO-based implementation can handle restricted and unrestricted fragments for 0D to 3D systems considering periodic boundary conditions with and without the determination of fragment occupations. For the latter case, reciprocal space sampling is enabled. The new method gives comparable results to established schemes for molecular systems and shows good convergence with respect to the basis set (TZ2P), the integration accuracy and k-space sampling. Four typical bonding scenarios for surface adsorbate complexes were chosen to highlight the performance of the method representing insulating (CO on MgO(001)), metallic (H$_2$ on M(001), M = Pd, Cu) and semiconducting (CO and C$_2$H$_2$ on Si(001)c(4x2)) substrates. These examples cover the regimes of metallic, semiconducting and insulating substrates as well as bonding scenarios ranging from weakly interacting to covalent (shared electron and donor acceptor) bonding. The results presented lend confidence, that the pEDA will be a powerful tool for the analysis of surface-adsorbate binding in the future, enabling the transfer of concepts like ionic and covalent binding, donor-acceptor interaction, steric repulsion and others to extended systems.

physics.chem-ph↗