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- Institut für Physik und Astronomie (26) (remove)
Organische Halbleiter besitzen neue, bemerkenswerte Materialeigenschaften, die sie für die grundlegende Forschung wie auch aktuelle technologische Entwicklung (bsw. org. Leuchtdioden, org. Solarzellen) interessant werden lassen. Aufgrund der starken konformative Freiheit der konjugierten Polymerketten führt die Vielzahl der möglichen Anordnungen und die schwache intermolekulare Wechselwirkung für gewöhnlich zu geringer struktureller Ordnung im Festkörper. Die Morphologie hat gleichzeitig direkten Einfluss auf die elektronische Struktur der organischen Halbleiter, welches sich meistens in einer deutlichen Reduktion der Ladungsträgerbeweglichkeit gegenüber den anorganischen Verwandten zeigt. So stellt die Beweglichkeit der Ladungen im Halbleiter einen der limitierenden Faktoren für die Leistungsfähigkeit bzw. den Wirkungsgrad von funktionellen organischen Bauteilen dar. Im Jahr 2009 wurde ein neues auf Naphthalindiimid und Bithiophen basierendes Dornor/Akzeptor Copolymer vorgestellt [P(NDI2OD‑T2)], welches sich durch seine außergewöhnlich hohe Ladungsträgermobilität auszeichnet. In dieser Arbeit wird die Ladungsträgermobilität in P(NDI2OD‑T2) bestimmt, und der Transport durch eine geringe energetischer Unordnung charakterisiert. Obwohl dieses Material zunächst als amorph beschrieben wurde zeigt eine detaillierte Analyse der optischen Eigenschaften von P(NDI2OD‑T2), dass bereits in Lösung geordnete Vorstufen supramolekularer Strukturen (Aggregate) existieren. Quantenchemische Berechnungen belegen die beobachteten spektralen Änderungen. Mithilfe der NMR-Spektroskopie kann die Bildung der Aggregate unabhängig von optischer Spektroskopie bestätigt werden. Die Analytische Ultrazentrifugation an P(NDI2OD‑T2) Lösungen legt nahe, dass sich die Aggregation innerhalb der einzelnen Ketten unter Reduktion des hydrodynamischen Radius vollzieht. Die Ausbildung supramolekularen Strukturen nimmt auch eine signifikante Rolle bei der Filmbildung ein und verhindert gleichzeitig die Herstellung amorpher P(NDI2OD‑T2) Filme. Durch chemische Modifikation der P(NDI2OD‑T2)-Kette und verschiedener Prozessierungs-Methoden wurde eine Änderung des Kristallinitätsgrades und gleichzeitig der Orientierung der kristallinen Domänen erreicht und mittels Röntgenbeugung quantifiziert. In hochauflösenden Elektronenmikroskopie-Messungen werden die Netzebenen und deren Einbettung in die semikristallinen Strukturen direkt abgebildet. Aus der Kombination der verschiedenen Methoden erschließt sich ein Gesamtbild der Nah- und Fernordnung in P(NDI2OD‑T2). Über die Messung der Elektronenmobilität dieser Schichten wird die Anisotropie des Ladungstransports in den kristallographischen Raumrichtungen von P(NDI2OD‑T2) charakterisiert und die Bedeutung der intramolekularen Wechselwirkung für effizienten Ladungstransport herausgearbeitet. Gleichzeitig wird deutlich, wie die Verwendung von größeren und planaren funktionellen Gruppen zu höheren Ladungsträgermobilitäten führt, welche im Vergleich zu klassischen semikristallinen Polymeren weniger sensitiv auf die strukturelle Unordnung im Film sind.
The characterization of exoplanets is a young and rapidly expanding field in astronomy.
It includes a method called transmission spectroscopy that searches for planetary spectral
fingerprints in the light received from the host star during the event of a transit. This
techniques allows for conclusions on the atmospheric composition at the terminator region,
the boundary between the day and night side of the planet. Observationally a big
challenge, first attempts in the community have been successful in the detection of several
absorption features in the optical wavelength range. These are for example a Rayleighscattering
slope and absorption by sodium and potassium. However, other objects show
a featureless spectrum indicative for a cloud or haze layer of condensates masking the
probable atmospheric layers.
In this work, we performed transmission spectroscopy by spectrophotometry of three
Hot Jupiter exoplanets. When we began the work on this thesis, optical transmission
spectra have been available for two exoplanets. Our main goal was to advance the current
sample of probed objects to learn by comparative exoplanetology whether certain
absorption features are common. We selected the targets HAT-P-12b, HAT-P-19b and
HAT-P-32b, for which the detection of atmospheric signatures is feasible with current
ground-based instrumentation. In addition, we monitored the host stars of all three objects
photometrically to correct for influences of stellar activity if necessary.
The obtained measurements of the three objects all favor featureless spectra. A variety
of atmospheric compositions can explain the lack of a wavelength dependent absorption.
But the broad trend of featureless spectra in planets of a wide range of temperatures,
found in this work and in similar studies recently published in the literature, favors an
explanation based on the presence of condensates even at very low concentrations in the
atmospheres of these close-in gas giants. This result points towards the general conclusion
that the capability of transmission spectroscopy to determine the atmospheric composition
is limited, at least for measurements at low spectral resolution.
In addition, we refined the transit parameters and ephemerides of HAT-P-12b and HATP-
19b. Our monitoring campaigns allowed for the detection of the stellar rotation period
of HAT-P-19 and a refined age estimate. For HAT-P-12 and HAT-P-32, we derived upper
limits on their potential variability. The calculated upper limits of systematic effects of
starspots on the derived transmission spectra were found to be negligible for all three
targets.
Finally, we discussed the observational challenges in the characterization of exoplanet
atmospheres, the importance of correlated noise in the measurements and formulated
suggestions on how to improve on the robustness of results in future work.
One of the most significant current discussions in Astrophysics relates to the origin of high-energy cosmic rays. According to our current knowledge, the abundance distribution of the elements in cosmic rays at their point of origin indicates, within plausible error limits, that they were initially formed by nuclear processes in the interiors of stars. It is also believed that their energy distribution up to 1018 eV has Galactic origins. But even though the knowledge about potential sources of cosmic rays is quite poor above „ 1015 eV, that is the “knee” of the cosmic-ray spectrum, up to the knee there seems to be a wide consensus that supernova remnants are the most likely candidates. Evidence of this comes from observations of non-thermal X-ray radiation, requiring synchrotron electrons with energies up to 1014 eV, exactly in the remnant of supernovae. To date, however, there is not conclusive evidence that they produce nuclei, the dominant component of cosmic rays, in addition to electrons. In light of this dearth of evidence, γ-ray observations from supernova remnants can offer the most promising direct way to confirm whether or not these astrophysical objects are indeed the main source of cosmic-ray nuclei below the knee. Recent observations with space- and ground-based observatories have established shell-type supernova remnants as GeV-to- TeV γ-ray sources. The interpretation of these observations is however complicated by the different radiation processes, leptonic and hadronic, that can produce similar fluxes in this energy band rendering ambiguous the nature of the emission itself. The aim of this work is to develop a deeper understanding of these radiation processes from a particular shell-type supernova remnant, namely RX J1713.7–3946, using observations of the LAT instrument onboard the Fermi Gamma-Ray Space Telescope. Furthermore, to obtain accurate spectra and morphology maps of the emission associated with this supernova remnant, an improved model of the diffuse Galactic γ-ray emission background is developed. The analyses of RX J1713.7–3946 carried out with this improved background show that the hard Fermi-LAT spectrum cannot be ascribed to the hadronic emission, leading thus to the conclusion that the leptonic scenario is instead the most natural picture for the high-energy γ-ray emission of RX J1713.7–3946. The leptonic scenario however does not rule out the possibility that cosmic-ray nuclei are accelerated in this supernova remnant, but it suggests that the ambient density may not be high enough to produce a significant hadronic γ-ray emission. Further investigations involving other supernova remnants using the improved back- ground developed in this work could allow compelling population studies, and hence prove or disprove the origin of Galactic cosmic-ray nuclei in these astrophysical objects. A break- through regarding the identification of the radiation mechanisms could be lastly achieved with a new generation of instruments such as CTA.
Galaxies are observational probes to study the Large Scale Structure. Their gravitational motions are tracers of the total matter density and therefore of the Large Scale Structure. Besides, studies of structure formation and galaxy evolution rely on numerical cosmological simulations. Still, only one universe observable from a given position, in time and space, is available for comparisons with simulations. The related cosmic variance affects our ability to interpret the results. Simulations constrained by observational data are a perfect remedy to this problem. Achieving such simulations requires the projects Cosmic flows and CLUES. Cosmic flows builds catalogs of accurate distance measurements to map deviations from the expansion. These measures are mainly obtained with the galaxy luminosity-rotation rate correlation. We present the calibration of that relation in the mid-infrared with observational data from Spitzer Space Telescope. Resulting accurate distance estimates will be included in the third catalog of the project. In the meantime, two catalogs up to 30 and 150 Mpc/h have been released. We report improvements and applications of the CLUES' method on these two catalogs. The technique is based on the constrained realization algorithm. The cosmic displacement field is computed with the Zel'dovich approximation. This latter is then reversed to relocate reconstructed three-dimensional constraints to their precursors' positions in the initial field. The size of the second catalog (8000 galaxies within 150 Mpc/h) highlighted the importance of minimizing the observational biases. By carrying out tests on mock catalogs, built from cosmological simulations, a method to minimize observational bias can be derived. Finally, for the first time, cosmological simulations are constrained solely by peculiar velocities. The process is successful as resulting simulations resemble the Local Universe. The major attractors and voids are simulated at positions approaching observational positions by a few megaparsecs, thus reaching the limit imposed by the linear theory.
During this work I built a four wave mixing setup for the time-resolved femtosecond spectroscopy of Raman-active lattice modes. This setup enables to study the selective excitation of phonon polaritons. These quasi-particles arise from the coupling of electro-magnetic waves and transverse optical lattice modes, the so-called phonons. The phonon polaritons were investigated in the optically non-linear, ferroelectric crystals LiNbO₃ and LiTaO₃.
The direct observation of the frequency shift of the scattered narrow bandwidth probe pulses proofs the role of the Raman interaction during the probe and excitation process of phonon polaritons. I compare this experimental method with the measurement where ultra-short laser pulses are used. The frequency shift remains obscured by the relative broad bandwidth of these laser pulses. In an experiment with narrow bandwidth probe pulses, the Stokes and anti-Stokes intensities are spectrally separated. They are assigned to the corresponding counter-propagating wavepackets of phonon polaritons. Thus, the dynamics of these wavepackets was separately studied. Based on these findings, I develop the mathematical description of the so-called homodyne detection of light for the case of light scattering from counter propagating phonon polaritons.
Further, I modified the broad bandwidth of the ultra-short pump pulses using bandpass filters to generate two pump pulses with non-overlapping spectra. This enables the frequency-selective excitation of polariton modes in the sample, which allows me to observe even very weak polariton modes in LiNbO₃ or LiTaO₃ that belong to the higher branches of the dispersion relation of phonon polaritons. The experimentally determined dispersion relation of the phonon polaritons could therefore be extended and compared to theoretical models. In addition, I determined the frequency-dependent damping of phonon polaritons.
Donor-acceptor (D-A) copolymers have revolutionized the field of organic electronics over the last decade. Comprised of a electron rich and an electron deficient molecular unit, these copolymers facilitate the systematic modification of the material's optoelectronic properties. The ability to tune the optical band gap and to optimize the molecular frontier orbitals as well as the manifold of structural sites that enable chemical modifications has created a tremendous variety of copolymer structures. Today, these materials reach or even exceed the performance of amorphous inorganic semiconductors. Most impressively, the charge carrier mobility of D-A copolymers has been pushed to the technologically important value of 10 cm^{2}V^{-1}s^{-1}. Furthermore, owed to their enormous variability they are the material of choice for the donor component in organic solar cells, which have recently surpassed the efficiency threshold of 10%. Because of the great number of available D-A copolymers and due to their fast chemical evolution, there is a significant lack of understanding of the fundamental physical properties of these materials. Furthermore, the complex chemical and electronic structure of D-A copolymers in combination with their semi-crystalline morphology impede a straightforward identification of the microscopic origin of their superior performance. In this thesis, two aspects of prototype D-A copolymers were analysed. These are the investigation of electron transport in several copolymers and the application of low band gap copolymers as acceptor component in organic solar cells. In the first part, the investigation of a series of chemically modified fluorene-based copolymers is presented. The charge carrier mobility varies strongly between the different derivatives, although only moderate structural changes on the copolymers structure were made. Furthermore, rather unusual photocurrent transients were observed for one of the copolymers. Numerical simulations of the experimental results reveal that this behavior arises from a severe trapping of electrons in an exponential distribution of trap states. Based on the comparison of simulation and experiment, the general impact of charge carrier trapping on the shape of photo-CELIV and time-of-flight transients is discussed. In addition, the high performance naphthalenediimide (NDI)-based copolymer P(NDI2OD-T2) was characterized. It is shown that the copolymer posses one of the highest electron mobilities reported so far, which makes it attractive to be used as the electron accepting component in organic photovoltaic cells.\par Solar cells were prepared from two NDI-containing copolymers, blended with the hole transporting polymer P3HT. I demonstrate that the use of appropriate, high boiling point solvents can significantly increase the power conversion efficiency of these devices. Spectroscopic studies reveal that the pre-aggregation of the copolymers is suppressed in these solvents, which has a strong impact on the blend morphology. Finally, a systematic study of P3HT:P(NDI2OD-T2) blends is presented, which quantifies the processes that limit the efficiency of devices. The major loss channel for excited states was determined by transient and steady state spectroscopic investigations: the majority of initially generated electron-hole pairs is annihilated by an ultrafast geminate recombination process. Furthermore, exciton self-trapping in P(NDI2OD-T2) domains account for an additional reduction of the efficiency. The correlation of the photocurrent to microscopic morphology parameters was used to disclose the factors that limit the charge generation efficiency. Our results suggest that the orientation of the donor and acceptor crystallites relative to each other represents the main factor that determines the free charge carrier yield in this material system. This provides an explanation for the overall low efficiencies that are generally observed in all-polymer solar cells.
Diffusion of finite-size particles in two-dimensional channels with random wall configurations
(2014)
Diffusion of chemicals or tracer molecules through complex systems containing irregularly shaped channels is important in many applications. Most theoretical studies based on the famed Fick–Jacobs equation focus on the idealised case of infinitely small particles and reflecting boundaries. In this study we use numerical simulations to consider the transport of finite-size particles through asymmetrical two-dimensional channels. Additionally, we examine transient binding of the molecules to the channel walls by applying sticky boundary conditions. We consider an ensemble of particles diffusing in independent channels, which are characterised by common structural parameters. We compare our results for the long-time effective diffusion coefficient with a recent theoretical formula obtained by Dagdug and Pineda [J. Chem. Phys., 2012, 137, 024107].
Based on extensive Monte Carlo simulations and analytical considerations we study the electrostatically driven adsorption of flexible polyelectrolyte chains onto charged Janus nanospheres. These net-neutral colloids are composed of two equally but oppositely charged hemispheres. The critical binding conditions for polyelectrolyte chains are analysed as function of the radius of the Janus particle and its surface charge density, as well as the salt concentration in the ambient solution. Specifically for the adsorption of finite-length polyelectrolyte chains onto Janus nanoparticles, we demonstrate that the critical adsorption conditions drastically differ when the size of the Janus particle or the screening length of the electrolyte are varied. We compare the scaling laws obtained for the adsorption–desorption threshold to the known results for uniformly charged spherical particles, observing significant disparities. We also contrast the changes to the polyelectrolyte chain conformations close to the surface of the Janus nanoparticles as compared to those for simple spherical particles. Finally, we discuss experimentally relevant physico-chemical systems for which our simulations results may become important. In particular, we observe similar trends with polyelectrolyte complexation with oppositely but heterogeneously charged proteins.
The H.E.S.S. array is a third generation Imaging Atmospheric Cherenkov Telescope (IACT) array. It is located in the Khomas Highland in Namibia, and measures very high energy (VHE) gamma-rays. In Phase I, the array started data taking in 2004 with its four identical 13 m telescopes. Since then, H.E.S.S. has emerged as the most successful IACT experiment to date. Among the almost 150 sources of VHE gamma-ray radiation found so far, even the oldest detection, the Crab Nebula, keeps surprising the scientific community with unexplained phenomena such as the recently discovered very energetic flares of high energy gamma-ray radiation. During its most recent flare, which was detected by the Fermi satellite in March 2013, the Crab Nebula was simultaneously observed with the H.E.S.S. array for six nights. The results of the observations will be discussed in detail during the course of this work. During the nights of the flare, the new 24 m × 32 m H.E.S.S. II telescope was still being commissioned, but participated in the data taking for one night. To be able to reconstruct and analyze the data of the H.E.S.S. Phase II array, the algorithms and software used by the H.E.S.S. Phase I array had to be adapted. The most prominent advanced shower reconstruction technique developed by de Naurois and Rolland, the template-based model analysis, compares real shower images taken by the Cherenkov telescope cameras with shower templates obtained using a semi-analytical model. To find the best fitting image, and, therefore, the relevant parameters that describe the air shower best, a pixel-wise log-likelihood fit is done. The adaptation of this advanced shower reconstruction technique to the heterogeneous H.E.S.S. Phase II array for stereo events (i.e. air showers seen by at least two telescopes of any kind), its performance using MonteCarlo simulations as well as its application to real data will be described.
Despite remarkable progress made in the past century, which has revolutionized our understanding of the universe, there are numerous open questions left in theoretical physics. Particularly important is the fact that the theories describing the fundamental interactions of nature are incompatible. Einstein's theory of general relative describes gravity as a dynamical spacetime, which is curved by matter and whose curvature determines the motion of matter. On the other hand we have quantum field theory, in form of the standard model of particle physics, where particles interact via the remaining interactions - electromagnetic, weak and strong interaction - on a flat, static spacetime without gravity. A theory of quantum gravity is hoped to cure this incompatibility by heuristically replacing classical spacetime by quantum spacetime'. Several approaches exist attempting to define such a theory with differing underlying premises and ideas, where it is not clear which is to be preferred. Yet a minimal requirement is the compatibility with the classical theory, they attempt to generalize. Interestingly many of these models rely on discrete structures in their definition or postulate discreteness of spacetime to be fundamental. Besides the direct advantages discretisations provide, e.g. permitting numerical simulations, they come with serious caveats requiring thorough investigation: In general discretisations break fundamental diffeomorphism symmetry of gravity and are generically not unique. Both complicates establishing the connection to the classical continuum theory. The main focus of this thesis lies in the investigation of this relation for spin foam models. This is done on different levels of the discretisation / triangulation, ranging from few simplices up to the continuum limit. In the regime of very few simplices we confirm and deepen the connection of spin foam models to discrete gravity. Moreover, we discuss dynamical, e.g. diffeomorphism invariance in the discrete, to fix the ambiguities of the models. In order to satisfy these conditions, the discrete models have to be improved in a renormalisation procedure, which also allows us to study their continuum dynamics. Applied to simplified spin foam models, we uncover a rich, non--trivial fixed point structure, which we summarize in a phase diagram. Inspired by these methods, we propose a method to consistently construct the continuum theory, which comes with a unique vacuum state.