Morphology and charge transport in conjugated polymers

To assist rational compound design of organic semiconductors, two problems need to be addressed. First, the material morphology has to be known at an atomistic level. Second, with the morphology at hand, an appropriate charge transport model needs to be developed in order to link charge carrier mobility to structure.rnrnThe former can be addressed by generating atomistic morphologies using molecular dynamics simulations. However, the accessible range of time- and length-scales is limited. To overcome these limitations, systematic coarse-graining methods can be used. In the first part of the thesis, the Versatile Object-oriented Toolkit for Coarse-graining Applications is introduced, which provides a platform for the implementation of coarse-graining methods. Tools to perform Boltzmann inversion, iterative Boltzmann inversion, inverse Monte Carlo, and force-matching are available and have been tested on a set of model systems (water, methanol, propane and a single hexane chain). Advantages and problems of each specific method are discussed.rnrnIn partially disordered systems, the second issue is closely connected to constructing appropriate diabatic states between which charge transfer occurs. In the second part of the thesis, the description initially used for small conjugated molecules is extended to conjugated polymers. Here, charge transport is modeled by introducing conjugated segments on which charge carriers are localized. Inter-chain transport is then treated within a high temperature non-adiabatic Marcus theory while an adiabatic rate expression is used for intra-chain transport. The charge dynamics is simulated using the kinetic Monte Carlo method.rnrnThe entire framework is finally employed to establish a relation between the morphology and the charge mobility of the neutral and doped states of polypyrrole, a conjugated polymer. It is shown that for short oligomers, charge carrier mobility is insensitive to the orientational molecular ordering and is determined by the threshold transfer integral which connects percolating clusters of molecules that form interconnected networks. The value of this transfer integral can be related to the radial distribution function. Hence, charge mobility is mainly determined by the local molecular packing and is independent of the global morphology, at least in such a non-crystalline state of a polymer.Bei der systematischen, Computer gestützten Entwicklung neuer organischer Halbleiter müssen zwei Kernpunkte behandelt werden: Zunächst muss deren Morphologie auf atomarer Ebene bestimmt werden.rnAnschließend wird ein Modell benötigt, welches Ladungsträgermobilitäten für diese Strukturen berechnen kann.rnrnErsteres kann durch Molekulardynamik Simulationen erfolgen, jedoch sind die erreichbaren Längen- und Zeitskalen stark eingeschränkt. Hier bieten systematische Vergröberungsmethoden einen Ausweg, welche im ersten Teil dieser Arbeit vorgestellt werden. Im Rahmen dieser Arbeit wurde ein Software-Paket (Versatile Object-oriented Toolkit for Coarse-graining Applications\u27\u27) entwickelt, welches eine flexible Plattform für die einheitliche Implementierung von Vergröberungsmethoden bietet. Bisher wurden Anwendungen für die Methoden Boltzmann Inversion, iterative Boltzmann Inversion, Monte Carlo Inversion und Force-matching integriert und an vier Referenzsystemen (Wasser, Methanol, Propan und ein einzelnes Hexanmolekül) getestet. Die Vor- und Nachteile der verschiedenen Ansätze werden diskutiert.rnrnDie Modellierung der Ladungsträgerdynamik ist in ungeordneten Systemen eng mit der Konstruktion geeigneter diabatischer Zustände verbunden, zwischen welchen Ladungstransport erfolgen kann. Im zweiten Teil dieser Arbeit wird ein Modell entwickelt, welches Ladungstransport in konjugierten Poylmeren beschreibt. Hierbei wird der Transport durch Springen zwischen konjugierten Segmenten (Bereiche im Polymer, auf welchen Ladungsträger lokalisieren können) beschrieben. Die Transportraten der Sprünge zwischen Mole-külen wird mit der nicht-adiabatischen Marcus-Gleichung berechnet, wohingegen eine adiabatische Ratengleichung für Transport innerhalb der Polymerkette verwendet wird. Die Dynamik der Ladungsträger wird anschließend mit dem kinetischen Monte Carlo Algorithmus simuliert.rnrnIm letzten Teil dieser Arbeit wird mit Hilfe des entwickelten Modells eine Relation zwischen Morphologie und Ladungsmobilität in neutralen und dotierten Zuständen des konjugierten Polymers Polypyrrol etabliert. Es wird gezeigt, dass bei kurzen Ketten die Mobilität der Ladungsträger kaum von der molekularen Ordnung abhängt. Zudem kann die Mobilität anhand des Schwellenwerts des Transferintegrals abgeschätzt werden, welches Moleküle zu einem einzelnen Cluster verbindet. Die Tatsache, dass das Transferintegral eng mit der radialen Verteilungsfunktion verknüpft ist, deutet darauf hin, dass die Ladungsmobilität, zumindest in solch einem nicht-kristallinen Zustand eines Polymers, überwiegend durch die lokale molekulare Packung gegeben ist und damit unabhängig von der globalen Ordnung ist

La insistencia de la muerte en la vida: Hegel después de Heidegger

La crítica de Heidegger, según la cual Hegel habría sometido la muerte a las consecuencias lógicas del pensamiento especulativo, y con ello, arrancado el aguijón de su negatividad, pertenece hoy en día al acervo de la confrontación con él. No obstante, leyendo a Hegel después de Heidegger, y a la vista del irresuelto problema de la reflexión de éste sobre la muerte, el concepto de Hegel sobre la muerte se muestra bajo una nueva luz. Puesto que es precisamente el hecho de que él no sustrae desde un principio la negatividad de la muerte al pensamiento lo que le permite conceptualizarla como una dimensión inmanente, evanescente y cambiante en la vida. Sólo con ello se hace comprensible que la muerte no es tan sólo una negación absoluta de la vida, sino también una exigencia incondicional, que rige la vida y que es constitutiva de su “forma” temporal. Palabras clave: Hegel, Heidegger, muerte, ser, nadaAbstract:According to Heidegger’s critique, Hegel would have subjected death to the logical consequences of speculative thought, thus depriving death of the sting of its negativity –such a critique belongs nowadays to the commonplaces of the confrontation with Hegel’s thought. However, if one reads Hegel after Heidegger, and considers the unsolved problems of Heidegger’s thinking on death, Hegel’s concept of death emerges under a new light. Particularly because it was precisely the fact that Hegel did not hide the negativity of death from the consideration of thought, which allowed him to conceive of death as an immanent, pervasive and ever changing dimension of life. Only in this way becomes intelligible that death is not just an absolute negation of life, but also an unconditional claim governing it, and constitutive of life’s temporal “form”. Keywords: Hegel, Heidegger, death, being, nothingness</p

Exploring energy landscapes: from molecular to mesoscopic systems

We review a comprehensive computational framework to survey the potential energy landscape for systems composed of rigid or partially rigid molecules. Illustrative case studies relevant to a wide range of molecular clusters and soft and condensed matter systems are discussed

Exploring energy landscapes: metrics, pathways, and normal mode analysis for rigid-body molecules

We present new methodology for exploring the energy landscapes of molecular systems, using angle-axis variables for the rigid-body rotational coordinates. The key ingredient is a distance measure or metric tensor, which is invariant to global translation and rotation. The metric is used to formulate a generalized nudged elastic band method for calculating pathways, and a full prescription for normal-mode analysis is described. The methodology is tested by mapping the potential energy and free energy landscape of the water octamer, described by the TIP4P potential

Spin flip from dark to bright states in InP quantum dots

We report measurements of the time for spin flip from dark (non-light emitting) exciton states in quantum dots to bright (light emitting) exciton states in InP quantum dots. Dark excitons are created by two-photon excitation by an ultrafast laser. The time for spin flip between dark and bright states is found to be approximately 200 ps, independent of density and temperature below 70 K. This is much shorter than observed in other quantum dot systems. The rate of decay of the luminescence intensity, approximately 300 ps, is not simply equal to the radiative decay rate from the bright states, because the rate of decay is limited by the rate of conversion from dark excitons into bright excitons. The dependence of the luminescence decay time on the spin flip time is a general effect that applies to many experiments.Comment: 3 figure

One-dimensional metal-organic framework photonic crystals used as platforms for vapor sorption.

We present the fabrication of one-dimensional photonic crystals (Bragg stacks) based on a microporous metal–organic framework material and mesoporous titanium dioxide. The Bragg stack heterostructures were obtained using two complementary synthesis approaches utilizing the bottom-up assembly of heterogeneous, i.e. two-component photonic crystal multilayer structures. Zeolitic imidazolate framework ZIF-8 and mesoporous titanium dioxide were chosen as functional components with different refractive indices. While ZIF-8 is intended to impart molecular selectivity, mesoporous TiO2 is used to ensure high refractive index contrast and to guarantee molecular diffusion within the Bragg stack. The combination of micro- and mesoporosity within one scaffold endows the 1D-MOF PC with characteristic adsorption properties upon exposure to various organic vapors. In this context, the sorption behavior of the photonic material was studied as a function of partial pressure of organic vapors. The results show that the multilayered photonic heterostructures are sensitive and selective towards a series of chemically similar solvent vapors. It is thus anticipated that the concept of multilayer heterogeneous photonic structures will provide a versatile platform for future selective, label-free optical sensors

On noise treatment in radio measurements of cosmic ray air showers

Precise measurements of the radio emission by cosmic ray air showers require an adequate treatment of noise. Unlike to usual experiments in particle physics, where noise always adds to the signal, radio noise can in principle decrease or increase the signal if it interferes by chance destructively or constructively. Consequently, noise cannot simply be subtracted from the signal, and its influence on amplitude and time measurement of radio pulses must be studied with care. First, noise has to be determined consistently with the definition of the radio signal which typically is the maximum field strength of the radio pulse. Second, the average impact of noise on radio pulse measurements at individual antennas is studied for LOPES. It is shown that a correct treatment of noise is especially important at low signal-to-noise ratios: noise can be the dominant source of uncertainty for pulse height and time measurements, and it can systematically flatten the slope of lateral distributions. The presented method can also be transfered to other experiments in radio and acoustic detection of cosmic rays and neutrinos.Comment: 4 pages, 6 figures, submitted to NIM A, Proceedings of ARENA 2010, Nantes, Franc

The wavefront of the radio signal emitted by cosmic ray air showers

Analyzing measurements of the LOPES antenna array together with corresponding CoREAS simulations for more than 300 measured events with energy above $10^{17}\,$eV and zenith angles smaller than $45^\circ$, we find that the radio wavefront of cosmic-ray air showers is of approximately hyperbolic shape. The simulations predict a slightly steeper wavefront towards East than towards West, but this asymmetry is negligible against the measurement uncertainties of LOPES. At axis distances $\gtrsim 50\,$m, the wavefront can be approximated by a simple cone. According to the simulations, the cone angle is clearly correlated with the shower maximum. Thus, we confirm earlier predictions that arrival time measurements can be used to study the longitudinal shower development, but now using a realistic wavefront. Moreover, we show that the hyperbolic wavefront is compatible with our measurement, and we present several experimental indications that the cone angle is indeed sensitive to the shower development. Consequently, the wavefront can be used to statistically study the primary composition of ultra-high energy cosmic rays. At LOPES, the experimentally achieved precision for the shower maximum is limited by measurement uncertainties to approximately $140\,$g/cm$^2$. But the simulations indicate that under better conditions this method might yield an accuracy for the atmospheric depth of the shower maximum, $X_\mathrm{max}$, better than $30\,$g/cm$^2$. This would be competitive with the established air-fluorescence and air-Cherenkov techniques, where the radio technique offers the advantage of a significantly higher duty-cycle. Finally, the hyperbolic wavefront can be used to reconstruct the shower geometry more accurately, which potentially allows a better reconstruction of all other shower parameters, too.Comment: accepted by JCA