Methodological contributions to the simulation of charge and energy transport in molecular materials

Diese Arbeit beschäftigt sich mit methodischen Entwicklungen zur Untersuchung von Ladungs- und Energietransportprozessen in molekularen Materialien. Damit ist gemeint, dass neue Ansätzen zur Untersuchung solcher Prozesse eingeführt und getestet, nicht etwa spezielle Prozesse im Detail ergründet, werden. Insbesondere liegt der Fokus auf Methoden zur Untersuchung organischer, halbleitender Materialien mit hohen Ladungsträgermobilitäten oder effizienter Ekzitonendiffusion, wobei die vorgestellten Methoden weitaus breiter anwendbar sind. Zunächst wenden wir eine ursprünglich für den Ladungstransport in DNA-Strängen entwickelte, und später von Heck et al. für organische Halbleiter adaptierte, Methode auf Anthrazenkristalle an. Wir berechnen damit die korrekte Temperaturabhängigkeit der Lochmobilität. Diese ist eng mit dem zugrundeliegenden Transportmechanismus verwoben und kann im Falle von bandartigem Transport, wie in Anthrazen, nicht mit hoppingbasierten Methoden reproduziert werden. Daraufhin führen wir eine Methode zur Berechnung von Ekzitonendiffusionskonstanten in molekularen Materialien auf Basis der direkten Propagation der Ekzitonenwellenfunktion ein. Um solche Rechnungen möglich zu machen, werden unter Ausnutzung der molekularen Struktur Näherungen auf verschiedenen Ebenen eingeführt. Die neue Methode wird, um sie zu testen, auf Ekzitonentransport in Anthrazen angewendet und wir diskutieren dabei auch technische Details, die für die obig angesprochenen Ladungstransportstudien ebenfalls relevant sind. Bei der Propagation der Ekzitonenwellenfunktion müssen viele elektronische Strukturrechnungen angeregter Zustände durchgeführt werden, so dass dazu eine sehr schnelle Methode notwendig ist. Wir verwenden die approximative TD-DFTB Methode, die auf DFT mit einem GGA Funktional basiert. Es ist bekannt, dass GGA Funktionale für ausgedehnte π-Elektronensysteme, wie sie in organischen Halbleitern ständig vorkommen, nicht zuverlässig sind. Innerhalb von DFT lösen sogenannte long-range corrected (LC) Funktionale das Problem. Wir führen LC Funktionale in TD-DFTB ein, was Änderung am Formalismus erfordert. Wir zeigen, dass damit typische Probleme mit π-Systemen und Ladungstransferanregungen gelöst werden, bei tausendfach schnelleren Rechnungen als mit konventionellem TD-DFT. Abschließend beschäftigen wir uns mir der DFTB Methode selbst. LC Funktionale haben einen Parameter, der idealerweise systemspezifisch gewählt wird. Bei jeder Anpassung müssen für DFTB neue Parameter berechnet werden. Ein Satz von atompaarweisen Funktionen, genannt Repulsivpotentiale, erfordern dabei bisher viel Handarbeit. Wir versuchen diesen Vorgang zu automatisieren, indem wir DFTB mit Methoden aus der künstlichen Intelligenz verbinden

Enhancing the undergraduate student expereince via fund raising partnerships. An action research project

This project was developed from practitioner action research and considers the impact of integrating fund-raising activities into the formal curriculum with a target group of undergraduate students. The main aim of this project was to evaluate the impact of developing fund-raising activities as an integral aspect at both module and programme level, grounded within practitioner action research. A core component of this aim was to develop value added structures into established modules to support the student experience and student identity. Furthermore the project sought to broaden, develop and strengthen links with the local wider community recognising the value such interactions can bring. Finally the project set out to develop integration opportunities between the students and organisations in order to develop the students’ awareness of workforce options as future professionals. This study is based within one post-1992 Higher Education Institution (HEI) and consisted of three inter-related but distinct phases. The project involved 60 students during phase one building to some 200 students by phase three. The findings suggest fund-raising activities built into the formal curriculum can be a useful medium for promoting student identity whilst providing a ‘value-added’ component to existing programmes of study

Classifiability of crossed products by nonamenable groups

We show that all amenable, minimal actions of a large class of nonamenable countable groups on compact metric spaces have dynamical comparison. This class includes all nonamenable hyperbolic groups, many HNN-extensions, nonamenable Baumslag-Solitar groups, a large class of amalgamated free products, lattices in many Lie groups, A2-groups, as well as direct products of the above with arbitrary countable groups. As a consequence, crossed products by amenable, minimal and topologically free actions of such groups on compact metric spaces are Kirchberg algebras in the UCT class, and are therefore classified by K-theory

Generalized Density-Functional Tight-Binding Repulsive Potentials from Unsupervised Machine Learning

We combine the approximate density-functional tight-binding (DFTB) method with unsupervised machine learning. This allows us to improve transferability and accuracy, make use of large quantum chemical data sets for the parametrization, and efficiently automatize the parametrization process of DFTB. For this purpose, generalized pair-potentials are introduced, where the chemical environment is included during the learning process, leading to more specific effective two-body potentials. We train on energies and forces of equilibrium and nonequilibrium structures of 2100 molecules, and test on ∼130 000 organic molecules containing O, N, C, H, and F atoms. Atomization energies of the reference method can be reproduced within an error of ∼2.6 kcal/mol, indicating drastic improvement over standard DFTB

Benchmark and performance of long-range corrected time-dependent density functional tight binding (LC-TD-DFTB) on rhodopsins and light-harvesting complexes

The chromophores of rhodopsins (Rh) and light-harvesting (LH) complexes still represent a major challenge for a quantum chemical description due to their size and complex electronic structure. Since gradient corrected and hybrid density functional approaches have been shown to fail for these systems, only range-separated functionals seem to be a promising alternative to the more time consuming post-Hartree–Fock approaches. For extended sampling of optical properties, however, even more approximate approaches are required. Recently, a long-range corrected (LC) functional has been implemented into the efficient density functional tight binding (DFTB) method, allowing to sample the excited states properties of chromophores embedded into proteins using quantum mechanical/molecular mechanical (QM/MM) with the time-dependent (TD) DFTB approach. In the present study, we assess the accuracy of LC-TD-DFT and LC-TD-DFTB for rhodopsins (bacteriorhodopsin (bR) and pharaonis phoborhodopsin (ppR)) and LH complexes (light-harvesting complex II (LH2) and Fenna–Matthews–Olson (FMO) complex). This benchmark study shows the improved description of the color tuning parameters compared to standard DFT functionals. In general, LC-TD-DFTB can exhibit a similar performance as the corresponding LC functionals, allowing a reliable description of excited states properties at significantly reduced cost. The two chromophores investigated here pose complementary challenges: while huge sensitivity to external field perturbation (color tuning) and charge transfer excitations are characteristic for the retinal chromophore, the multi-chromophoric character of the LH complexes emphasizes a correct description of inter-chromophore couplings, giving less importance to color tuning. None of the investigated functionals masters both systems simultaneously with satisfactory accuracy. LC-TD-DFTB, at the current stage, although showing a systematic improvement compared to TD-DFTB cannot be recommended for studying color tuning in retinal proteins, similar to some of the LC-DFT functionals, because the response to external fields is still too weak. For sampling of LH-spectra, however, LC-TD-DFTB is a viable tool, allowing to efficiently sample absorption energies, as shown for three different LH complexes. As the calculations indicate, geometry optimization may overestimate the importance of local minima, which may be averaged over when using trajectories. Fast quantum chemical approaches therefore may allow for a direct sampling of spectra in the near future

Correction: Benchmark and performance of long-range corrected time-dependent density functional tight binding (LC-TD-DFTB) on rhodopsins and light-harvesting complexes

Correction for ‘Benchmark and performance of long-range corrected time-dependent density functional tight binding (LC-TD-DFTB) on rhodopsins and light-harvesting complexes’ by Beatrix M. Bold et al., Phys. Chem. Chem. Phys., 2020, 22, 10500–10518, https://doi.org/10.1039/C9CP05753F. The authors have recognised two errors in the data for the published version of this article. The first one concerns the reported QM/MM calculations on the Fenna–Matthews–Olson (FMO) complex, and the second error affects all Coulomb couplings presented. In brief, the most important changes are: (i) the ZINDO site energies for the individual pigments are closer to each other and the ZINDO site energy fluctuations are smaller; (ii) the couplings are smaller and now in better agreement with reference data; and (iii) as a consequence, the exciton splitting is decreased. These errors do not change the conclusions of the study but only lead to small corrections of the reported results, as detailed below

Sol−gel processing of water‐soluble carbon nitride enables high‐performance photoanodes **

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high‐quality, binder‐free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water‐based sol−gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol−gel process capitalizes on the use of a water‐soluble PHI precursor that allows formation of a non‐covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm −2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero‐bias conditions and represents the very first example of a ‘soft matter’‐based PEC system capable of bias‐free photoreforming. The robust binder‐free films derived from sol−gel processing of water‐soluble PCN thus constitute a new paradigm for high‐performance ‘soft matter’ photoelectrocatalytic systems and pave the way for further applications in which high‐quality PCN films are required.Completely unbiased : Robust binder‐free films derived from sol−gel processing of a water‐soluble polymeric carbon nitride precursor exhibit unprecedented performance in photoelectrocatalytic reforming of alcohols, including effective operation under bias‐free conditions

Cobaloxime complex salts : synthesis, patterning on carbon nanomembranes and heterogeneous hydrogen evolution studies

Cobaloximes are promising, earth-abundant catalysts for the light-driven hydrogen evolution reaction. Typically, these cobalt(III) complexes are prepared in situ or employed in their neutral form, e.g. [Co(dmgH 2 )(py)Cl], even though related complex salts have been reported previously and could in principle offer improved catalytic activity as well as more efficient immobilization on solid support. Here we report an interdisciplinary investigation into complex salts [Co(dmgH) 2 (py) 2 ] + [Co(dmgBPh 2 ) 2 Cl 2 ] - , TBA + [Co(dmgBPh 2 ) 2 Cl 2 ] - and [Co(dmgH) 2 (py) 2 ] + BArF - . We describe their strategic syntheses from commercially available complex [Co(dmgH) 2 (py)Cl] and demonstrate that these double and single complex salts are potent catalysts for the light-driven hydrogen evolution reaction. We also show that scanning electrochemical cell microscopy can be used to deposit arrays of catalysts [Co(dmgH) 2 (py) 2 ] + [Co(dmgBPh 2 ) 2 Cl 2 ] - and [Co(dmgH) 2 (py)Cl] on supported and free-standing amino-terminated ~ 1 nm thick carbon nanomembranes (CNMs). Photocatalytic H 2 evolution at such arrays was quantified with Pd microsensors using scanning electrochemical microscopy, thus providing a new approach for catalytic evaluation and opening up novel routes for the creation and analysis of “designer catalyst arrays”, nano-printed in a desired pattern on a solid support

Effects of crystallographic anisotropy on fracture development and acoustic emission in quartz

Transgranular microcracking is fundamental for the initiation and propagation of all fractures in rocks. The geometry of these microcracks is primarily controlled by the interaction of the imposed stress field with the mineral elastic properties. However, the effects of anisotropic elastic properties of minerals on brittle fracture are not well understood. This study examines the effects of elastic anisotropy of quartz on the geometry of brittle fracture and related acoustic emissions (AE) developed during indentation experiments on single crystals at ambient pressure and temperature. A Hertzian cone crack developed during blunt indentation of a single crystal of flawless Brazilian quartz parallel to the c axis shows geometric deviation away from predictions based on the isotropic case, consistent with trigonal symmetry. The visible cone crack penetration depth varies from 3 to 5 mm and apical angle from 53 to 40. Electron backscatter diffraction (EBSD) mapping of the crack tip shows that fracturing initiates along a ~40 μm wide process zone, comprising damage along overlapping en echelon high-index crystallographic planes, shown by discrete bands of reduced electron backscatter pattern (EBSP) quality (band contrast).Coalescence of these surfaces results in a stepped fracture morphology. Monitoring of AE during indentation reveals that the elastic anisotropy of quartz has a significant effect on AE location and focal mechanisms. Ninety-four AE events were recorded during indentation and show an increasing frequency with increasing load. They correspond to the development of subsidiary concentric cracks peripheral to the main cone crack. The strong and complex anisotropy in seismic velocity (~28% Vp, ~43% Vs with trigonal symmetry) resulted in inaccurate and high uncertainty in AE locations using Geiger location routine with an isotropic velocity model. This problem was overcome by using a relative (master event) location algorithm that only requires a priori knowledge of the velocity structure within the source volume. The AE location results correlate reasonably well to the extent of the observed cone crack. Decomposition of AE source mechanisms of the Geiger relocated events shows dominantly end-member behavior between tensile and compressive vector dipole events, with some double-couple-dominated events and no purely tensile or compressive events. The same events located by the master event algorithm yield greater percentage of vector dipole components and no double-couple events, indicating that AE source mechanism solutions can depend on AE location accuracy, and therefore, relocation routine that is utilized. Calculations show that the crystallographic anisotropy of quartz causes apparent deviation of the moment tensors away from double-couple and pure tensile/compressive sources consistent with the observations. Preliminary modeling of calcite anisotropy shows a response distinct from quartz, indicating that the effects of anisotropy on interpreting AE are complex and require detailed further study