TY - GEN A1 - Inal, Sahika A1 - Kölsch, Jonas D. A1 - Chiappisi, Leonardo A1 - Janietz, Dietmar A1 - Gradzielski, Michael A1 - Laschewsky, André A1 - Neher, Dieter T1 - Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers N2 - We demonstrate new fluorophore-labelled materials based on acrylamide and on oligo(ethylene glycol) (OEG) bearing thermoresponsive polymers for sensing purposes and investigate their thermally induced solubility transitions. It is found that the emission properties of the polarity-sensitive (solvatochromic) naphthalimide derivative attached to three different thermoresponsive polymers are highly specific to the exact chemical structure of the macromolecule. While the dye emits very weakly below the LCST when incorporated into poly(N-isopropylacrylamide) (pNIPAm) or into a polyacrylate backbone bearing only short OEG side chains, it is strongly emissive in polymethacrylates with longer OEG side chains. Heating of the aqueous solutions above their cloud point provokes an abrupt increase of the fluorescence intensity of the labelled pNIPAm, whereas the emission properties of the dye are rather unaffected as OEG-based polyacrylates and methacrylates undergo phase transition. Correlated with laser light scattering studies, these findings are ascribed to the different degrees of pre-aggregation of the chains at low temperatures and to the extent of dehydration that the phase transition evokes. It is concluded that although the temperature-triggered changes in the macroscopic absorption characteristics, related to large-scale alterations of the polymer chain conformation and aggregation, are well detectable and similar for these LCST-type polymers, the micro-environment provided to the dye within each polymer network differs substantially. Considering sensing applications, this finding is of great importance since the temperature-regulated fluorescence response of the polymer depends more on the macromolecular architecture than the type of reporter fluorophore. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 251 KW - anionic polymerizations KW - dilute aqueous-solutions KW - ether methacrylates KW - n-isopropylacrylamide KW - oligo(ethylene glycol) methacrylate KW - phase-transitions KW - protein interactions KW - solvatochromic fluorophore KW - thermoresponsive polymers KW - to-coil transition Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-95379 SP - 6603 EP - 6612 ER - TY - THES A1 - Lange, Ilja T1 - Energetische Struktur von Schichten organischer Halbleiter und ihr fundamentaler Einfluss auf die physikalischen Eigenschaften organischer elektronischer Bauteile Y1 - 2014 ER - TY - JOUR A1 - Karamzadeh Toularoud, Nasim A1 - Heimann, Sebastian A1 - Dahm, Torsten A1 - Krüger, Frank T1 - Application based seismological array design by seismicity scenario modelling JF - Geophysical journal international N2 - The design of an array configuration is an important task in array seismology during experiment planning. Often the array response function (ARF), which depends on the relative position of array stations and frequency content of the incoming signals, is used as the array design criterion. In practice, additional constraints and parameters have to be taken into account, for example, land ownership, site-specific noise levels or characteristics of the seismic sources under investigation. In this study, a flexible array design framework is introduced that implements a customizable scenario modelling and optimization scheme by making use of synthetic seismograms. Using synthetic seismograms to evaluate array performance makes it possible to consider additional constraints. We suggest to use synthetic array beamforming as an array design criterion instead of the ARF. The objective function of the optimization scheme is defined according to the monitoring goals, and may consist of a number of subfunctions. The array design framework is exemplified by designing a seven-station small-scale array to monitor earthquake swarm activity in Northwest Bohemia/Vogtland in central Europe. Two subfunctions are introduced to verify the accuracy of horizontal slowness estimation; one to suppress aliasing effects due to possible secondary lobes of synthetic array beamforming calculated in horizontal slowness space and the other to reduce the event’s mislocation caused by miscalculation of the horizontal slowness vector. Subsequently, a weighting technique is applied to combine the subfunctions into one single scalar objective function to use in the optimization process. KW - Array Seismology KW - Array design KW - Seismicity modelling Y1 - 2018 SN - 0956-540X SN - 1365-246X VL - 216 IS - 3 SP - 1711 EP - 1727 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Li, Tian-yi A1 - Benduhn, Johannes A1 - Qiao, Zhi A1 - Liu, Yuan A1 - Li, Yue A1 - Shivhare, Rishi A1 - Jaiser, Frank A1 - Wang, Pei A1 - Ma, Jie A1 - Zeika, Olaf A1 - Neher, Dieter A1 - Mannsfeld, Stefan C. B. A1 - Ma, Zaifei A1 - Vandewal, Koen A1 - Leo, Karl T1 - Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells JF - The journal of physical chemistry letters N2 - An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5% is achieved with an external quantum efficiency (EQE) maximum of 68% at 700 nm. Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpclett.9b01222 SN - 1948-7185 VL - 10 IS - 11 SP - 2684 EP - 2691 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Yang, Xiao Hui A1 - Jaiser, Frank A1 - Stiller, Burkhard A1 - Neher, Dieter A1 - Galbrecht, Frank A1 - Scherf, Ullrich T1 - Efficient polymer electrophosphoreseent devices with interfacial layers JF - Advanced functional materials N2 - It is shown that several polymers can form insoluble interfacial layers on a poly (ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer after annealing of the double-layer structure. The thickness of the interlayer is dependent on the characteristics of the underlying PEDOT.PSS and the molecular weight of the polymers. It is further shown that the electronic structures of the interlayer polymers have a significant effect on the properties of red-light-emitting polymer-based electrophosphorescent devices. Upon increasing the highest occupied molecular orbital and lowest unoccupied molecular orbital positions, a significant increase in current density and device efficiency is observed. This is attributed to efficient blocking of electrons in combination with direct injection of holes from the interlayer to the phosphorescent dye. Upon proper choice of the interlayer polymer, efficient red, polymer-based electrophosphorescent devices with a peak luminance efficiency of 5.5 cd A(-1) (external quantum efficiency = 6 %) and a maximum power-conversion efficiency of 5 Im W-1 can be realized. Y1 - 2006 U6 - https://doi.org/10.1002/adfm.200500834 SN - 1616-301X SN - 1616-3028 VL - 16 IS - 16 SP - 2156 EP - 2162 PB - Wiley-VCH CY - Weinheim ER - TY - THES A1 - Mitzscherling, Steffen T1 - Polyelectrolyte multilayers for plasmonics and picosecond ultrasonics T1 - Multischichten aus Polyelektrolyten in der Pikosekundenakustik und Plasmonik N2 - This thesis investigates the application of polyelectrolyte multilayers in plasmonics and picosecond acoustics. The observed samples were fabricated by the spin-assisted layer-by-layer deposition technique that allowed a precise tuning of layer thickness in the range of few nanometers. The first field of interest deals with the interaction of light-induced localized surface plasmons (LSP) of rod-shaped gold nanoparticles with the particles' environment. The environment consists of an air phase and a phase of polyelectrolytes, whose ratio affects the spectral position of the LSP resonance. Measured UV-VIS spectra showed the shift of the LSP absorption peak as a function of the cover layer thickness of the particles. The data are modeled using an average dielectric function instead of the dielectric functions of air and polyelectrolytes. In addition using a measured dielectric function of the gold nanoparticles, the position of the LSP absorption peak could be simulated with good agreement to the data. The analytic model helps to understand the optical properties of metal nanoparticles in an inhomogeneous environment. The second part of this work discusses the applicability of PAzo/PAH and dye-doped PSS/PAH polyelectrolyte multilayers as transducers to generate hypersound pulses. The generated strain pulses were detected by time-domain Brillouin scattering (TDBS) using a pump-probe laser setup. Transducer layers made of polyelectrolytes were compared qualitatively to common aluminum transducers in terms of measured TDBS signal amplitude, degradation due to laser excitation, and sample preparation. The measurements proved that fast and easy prepared polyelectrolyte transducers provided stronger TDBS signals than the aluminum transducer. AFM topography measurements showed a degradation of the polyelectrolyte structures, especially for the PAzo/PAH sample. To quantify the induced strain, optical barriers were introduced to separate the transducer material from the medium of the hypersound propagation. Difficulties in the sample preparation prohibited a reliable quantification. But the experiments showed that a coating with transparent polyelectrolytes increases the efficiency of aluminum transducers and modifies the excited phonon distribution. The adoption of polyelectrolytes to the scientific field of picosecond acoustics enables a cheap and fast fabrication of transducer layers on most surfaces. In contrast to aluminum layers the polyelectrolytes are transparent over a wide spectral range. Thus, the strain modulation can be probed from surface and back. N2 - Diese Doktorarbeit behandelt die Verwendung von Multischichtsystemen aus Polyelektrolyten in den Fachgebieten der Plasmonik und der Pikosekunden-Akustik. Die verwendeten Proben wurden mit dem Spincoater-gestützten Layer-by-Layer-Verfahren hergestellt. Diese Methode ermöglichte die Einstellung Schichtdicke mit einer Präzision von wenigen Nanometern. Im Bereich der Plasmonik wurde die Wechselwirkung von Oberflächenplasmonen stabförmiger Gold-Nanopartikel mit deren Umgebung untersucht. Diese Umgebung bestand aus zwei Phasen: Polyelektrolyte und Luft. Das Volumenverhältnis der Materialien bestimmte die spektrale Position des Oberflächenplasmons. Bei zunehmender Einbettung der Goldpartikel zeigten die gemessenen UV-VIS Spektren eine Rotverschiebung der Plasmonenabsorption. Es wurde ein Modell entwickelt, das die inhomogene Umgebung der Partikel durch eine mittlere dieelekrische Funktion beschreibt. Nachdem die dielektrische Funktion der Goldpartikel in separaten Messungen bestimmt waren, konnte die Lage der Plasmonenabsorption berechnet werden. Die Berechnungen stimmten dabei mit den Messwerten überein. Mit diesem analytischen Modell ist es möglich, die optischen Eigenschaften von metallischen Nanopartikeln in einer inhomogenene Umgebung zu verstehen. Der zweite Teil dieser Arbeit diskutiert die Anwendbarkeit von polyelektrolytischen Multischichten aus PAzo/PAH bzw. Porphyrin-dotiertem PSS/PAH für die Erzeugung von Hyperschallpulsen. Die erzeugten Schallpulse wurden durch zeitaufgelöste Brillouin-Streuung in einem sogenannten pump-probe Aufbau detektiert. Schallerzeugende Schichten aus Polyelektrolyten wurden mit Wandlern aus Aluminium verglichen. Die Messungen zeigten, dass die Polyelektrolyte sehr gut für die Erzeugung von Schallpulsen geeignet sind. Der einfachen Probenpräparation und der guten Effizienz steht jedoch eine geringe Zerstörschwelle gegenüber. AFM-Messungen zeigten besonders bei den PAzo/PAH Multischichten sehr starke Veränderungen in der Struktur. Eine Quantisierung der induzierten Schallamplitude sollte durch eine optische Trennung von Wandler und Propagationmedium erreicht werden. Da die Trennschichten auch eine akustische Abkopplung bewirkten, ließen sich die Schallamplituden nicht bestimmen. Es wurde jedoch festgestellt, dass sich die Effizienz eines Aluminium-Wandlers durch das Aufbringen transparenter Polyelektrolytschichten deutlich steigern lässt. Die Herstellung von Ultraschall-Wandlern aus Polyelektrolyten erweitert die Möglichkeiten der Pikosekunden-Akustik. Zum einen können diese Wandler schnell und kostengünstig direkt auf fast jeder Oberfläche aufgebracht werden. Zum anderen sind Polyelektrolyte in einem breiten Spektralbereich transparent. Das ermöglicht Messungen von der Vorderseite, die bei herkömmlichen Aluminium-Wandlern nicht oder nur schwer realisierbar sind. KW - polyelectrolyte KW - plasmonics KW - picosecond acoustics KW - hypersound KW - nanoparticle KW - Polyelektrolyte KW - Pikosekundenakustik KW - Hyperschall KW - Plasmonik KW - Nanopartikel Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-80833 ER - TY - JOUR A1 - Kurpiers, Jona A1 - Ferron, Thomas A1 - Roland, Steffen A1 - Jakoby, Marius A1 - Thiede, Tobias A1 - Jaiser, Frank A1 - Albrecht, Steve A1 - Janietz, Silvia A1 - Collins, Brian A. A1 - Howard, Ian A. A1 - Neher, Dieter T1 - Probing the pathways of free charge generation in organic bulk heterojunction solar cells JF - Nature Communications N2 - The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-04386-3 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Sarhan, Radwan Mohamed A1 - Koopman, Wouter-Willem Adriaan A1 - Pudell, Jan-Etienne A1 - Stete, Felix A1 - Rössle, Matthias A1 - Herzog, Marc A1 - Schmitt, Clemens Nikolaus Zeno A1 - Liebig, Ferenc A1 - Koetz, Joachim A1 - Bargheer, Matias T1 - Scaling up nanoplasmon catalysis BT - the role of heat dissipation JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Nanoscale heating by optical excitation of plasmonic nanoparticles offers a new perspective of controlling chemical reactions, where heat is not spatially uniform as in conventional macroscopic heating but strong temperature gradients exist around microscopic hot spots. In nanoplasmonics, metal particles act as a nanosource of light, heat, and energetic electrons driven by resonant excitation of their localized surface plasmon resonance. As an example of the coupling reaction of 4-nitrothiophenol into 4,4′-dimercaptoazobenzene, we show that besides the nanoscopic heat distribution at hot spots, the microscopic distribution of heat dictated by the spot size of the light focus also plays a crucial role in the design of plasmonic nanoreactors. Small sizes of laser spots enable high intensities to drive plasmon-assisted catalysis. This facilitates the observation of such reactions by surface-enhanced Raman scattering, but it challenges attempts to scale nanoplasmonic chemistry up to large areas, where the excess heat must be dissipated by one-dimensional heat transport. KW - Gold KW - Raman spectroscopy KW - Silicon KW - Irradiation KW - Lasers Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpcc.8b12574 SN - 1932-7447 VL - 123 IS - 14 SP - 9352 EP - 9357 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Kurpiers, Jona T1 - Probing the pathways of free charge generation and recombination in organic solar cells T1 - Pfade der Generierung und Rekombination freier Ladungsträger in organischen Solarzellen BT - the role of excess energy and dispersive effects BT - die Rolle von Überschussenergie und dispersiven Effekten N2 - Organic semiconductors are a promising class of materials. Their special properties are the particularly good absorption, low weight and easy processing into thin films. Therefore, intense research has been devoted to the realization of thin film organic solar cells (OPVs). Because of the low dielectric constant of organic semiconductors, primary excitations (excitons) are strongly bound and a type II heterojunction needs to be introduced to split these excitations into free charges. Therefore, most organic solar cells consist of at least an electron donor and electron acceptor material. For such donor acceptor systems mainly three states are relevant; the photoexcited exciton on the donor or acceptor material, the charge transfer state at the donor-acceptor interface and the charge separated state of a free electron and hole. The interplay between these states significantly determines the efficiency of organic solar cells. Due to the high absorption and the low charge carrier mobilities, the active layers are usually thin but also, exciton dissociation and free charge formation proceeds rapidely, which makes the study of carrier dynamics highly challenging. Therefore, the focus of this work was first to install new experimental setups for the investigation of the charge carrier dynamics in complete devices with superior sensitivity and time resolution and, second, to apply these methods to prototypical photovoltaic materials to address specific questions in the field of organic and hybrid photovoltaics. Regarding the first goal, a new setup combining transient absorption spectroscopy (TAS) and time delayed collection field (TDCF) was designed and installed in Potsdam. An important part of this work concerned the improvement of the electronic components with respect to time resolution and sensitivity. To this end, a highly sensitive amplifier for driving and detecting the device response in TDCF was developed. This system was then applied to selected organic and hybrid model systems with a particular focus on the understanding of the loss mechanisms that limit the fill factor and short circuit current of organic solar cells. The first model system was a hybrid photovoltaic material comprising inorganic quantum dots decorated with organic ligands. Measurements with TDCF revealed fast free carrier recombination, in part assisted by traps, while bias-assisted charge extraction measurements showed high mobility. The measured parameters then served as input for a successful description of the device performance with an analytical model. With a further improvement of the instrumentation, a second topic was the detailed analysis of non-geminate recombination in a disordered polymer:fullerene blend where an important question was the effect of disorder on the carrier dynamics. The measurements revealed that early time highly mobile charges undergo fast non-geminate recombination at the contacts, causing an apparent field dependence of free charge generation in TDCF experiments if not conducted properly. On the other hand, recombination the later time scale was determined by dispersive recombination in the bulk of the active layer, showing the characteristics of carrier dynamics in an exponential density of state distribution. Importantly, the comparison with steady state recombination data suggested a very weak impact of non-thermalized carriers on the recombination properties of the solar cells under application relevant illumination conditions. Finally, temperature and field dependent studies of free charge generation were performed on three donor-acceptor combinations, with two donor polymers of the same material family blended with two different fullerene acceptor molecules. These particular material combinations were chosen to analyze the influence of the energetic and morphology of the blend on the efficiency of charge generation. To this end, activation energies for photocurrent generation were accurately determined for a wide range of excitation energies. The results prove that the formation of free charge is via thermalized charge transfer states and does not involve hot exciton splitting. Surprisingly, activation energies were of the order of thermal energy at room temperature. This led to the important conclusion that organic solar cells perform well not because of predominate high energy pathways but because the thermalized CT states are weakly bound. In addition, a model is introduced to interconnect the dissociation efficiency of the charge transfer state with its recombination observable with photoluminescence, which rules out a previously proposed two-pool model for free charge formation and recombination. Finally, based on the results, proposals for the further development of organic solar cells are formulated. N2 - Organische Halbleiter sind eine vielversprechende Materialklasse. Ihre besonderen Eigenschaften sind die gute Absorption, das geringe Gewicht und die einfache Verarbeitung zu dünnen Filmen. Daher wird intensiv an der Realisierung organischer Dünnschichtsolarzellen (OPVs) geforscht. Aufgrund der niedrigen Dielektrizitätskonstante organischer Halbleiter sind primäre Anregungen (Exzitonen) jedoch stark gebunden, und es muss ein Typ II-Heteroübergang eingeführt werden, um diese Anregungen in freie Ladungen zu trennen. Daher bestehen die meisten organischen Solarzellen aus mindestens einem Elektronendonator und einem Elektronenakzeptormaterial. Für solche Donator-Akzeptorsysteme sind hauptsächlich drei Zustünde relevant; das Exziton auf dem Donator- oder Akzeptormaterial, der Ladungstransferzustand an der Donator-Akzeptor-Grenzfläche und der ladungsgetrennte Zustand eines freien Elektrons und Lochs. Das Zusammenspiel dieser Zustände bestimmt maßgeblich die Effizienz organischer Solarzellen. Aufgrund der hohen Absorption und der geringen Ladungsträgermobilitäten sind die aktiven Schichten dünn, aber auch die Exzitonendissoziation und die Bildung freier Ladung findet auf kurzen Zeitskalen statt, was die Analyse der Ladungsträgerdynamik erschwert. Im Mittelpunkt dieser Arbeit standen daher zunächst die Installation neuer Versuchsaufbauten zur Untersuchung der Ladungsträgerdynamik in optimierten Solarzellen mit überlegener Empfindlichkeit und Zeitauflösung sowie die Anwendung dieser Methoden auf prototypische photovoltaische Materialien. Im Hinblick auf das erste Ziel wurde ein neuer Aufbau, der optische Methoden wie die zeitaufgelöste Absorptionsspektroskopie (TAS) mit optolektronischen Methoden, wie die zeitaufgelöste Ladungsextraktion (TDCF) kombiniert. Ein wichtiger Teil dieser Arbeit war die Verbesserung der elektronischen Komponenten hinsichtlich der Zeitauflösung und Empfindlichkeit. Zu diesem Zweck wurde ein hochempfindlicher Verstärker zur Ansteuerung und Detektion der Probenantwort in TDCF entwickelt. Dieser Versuchsaufbau wurde im Folgenden auf verschiedene Modellsysteme angewendet, mit besonderem Fokus auf dem Verständnis der Verlustmechanismen, die den Füllfaktor und den Kurzschlussstrom organischer Solarzellen limittieren. Das erste Modellsystem ist ein hybrides Material, das aus anorganischen Quantenpunkten mit organischen Liganden besteht. Messungen mit TDCF ergaben eine schnelle Rekombination freier Ladungsträger, die teilweise durch Fallen unterstützt wurde, während Ladungsextraktionsexperimente unter quasi-konstanter Beleuchtung eine hohe Mobilität zeigten. Mit den gemessenen Parametern konnte die Kennlinie gut mit einem analytischen Modell beschrieben werden. Mit einer weiteren Verbesserung des Aufbaus konnte ein zweites Thema, die detaillierte Analyse der nicht-geminalen Rekombination in einer ungeordneten Polymer-Fulleren-Mischung bearbeitet werden. Die Messungen zeigten, dass hochmobilen Ladungen zu frühen Zeiten eine schnelle, nicht-geminale Rekombination an den Kontakten durchlaufen, was eine scheinbare Feldabhängigkeit der Generierung freier Ladung in TDCF-Experimenten verursachen kann. Andererseits wurde die Rekombination auf längeren Zeitskalen durch dispersive Rekombination in der aktiven Schicht bestimmt, wobei die Ursache der Ladungsträgerdynamik in einer exponentiellen Verteilung der Zustandsdichte liegt. Der Vergleich mit Rekombinationsdaten im stationären Zustand ergab einen sehr schwachen Einfluss nicht-thermalisierter Ladungsträger auf die Rekombinationseigenschaften der Solarzellen unter anwendungsrelevanten Bedingungen. Schließlich wurden temperatur- und feldabhängige Studien zur Erzeugung freier Ladung an drei Donator-Akzeptor-Kombinationen durchgeführt, wobei zwei Donatorpolymere der gleichen Materialfamilie mit zwei verschiedenen Fulleren-Akzeptormolekülen gemischt wurden. Diese besonderen Materialkombinationen wurden ausgewählt, um den Einfluss der Energetik und Morphologie der Mischung auf die Effizienz der Ladungserzeugung zu analysieren. Zu diesem Zweck wurden die Aktivierungsenergien für die Photostromerzeugung für einen weiten Bereich von Anregungsenergien genau bestimmt. Die Ergebnisse beweisen, dass die Bildung freier Ladungen über thermalisierte Ladungstransferzustände erfolgt und keine Aufspaltung mit heißen Exzitonen beinhaltet. Überraschenderweise sind die Aktivierungsenergien vergleichbar mit der thermischen Energie bei Raumtemperatur. Dies führte zu der wichtigen Schlussfolgerung, dass organische Solarzellen nicht aufgrund von hochenergetischen Pfaden gut funktionieren, sondern weil die thermalisierten Ladungstransferzustände schwach gebunden sind. Darüber hinaus wird ein Modell eingeführt, um die Dissoziationseffizienz des Ladungstransferzustands mit seiner mit Photolumineszenz beobachtbaren Rekombination zu verbinden, wodurch ein zuvor vorgeschlagenes Zwei-Pool-Modell für die Bildung und Rekombination von freien Ladungen ausgeschlossen wird. Abschließend werden auf Basis der Ergebnisse Vorschläge zur Weiterentwicklung organischer Solarzellen formuliert. KW - organic solar cells KW - time resolved pump probe spectroscopy Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-429099 ER - TY - THES A1 - Pavlenko, Elena T1 - Hybrid nanolayer architectures for ultrafast acousto-plasmonics in soft matter T1 - Hybride Nanolayer-Architekturen für ultraschnelle Akusto-Plasmonics in weicher Materie N2 - The goal of the presented work is to explore the interaction between gold nanorods (GNRs) and hyper-sound waves. For the generation of the hyper-sound I have used Azobenzene-containing polymer transducers. Multilayer polymer structures with well-defined thicknesses and smooth interfaces were built via layer-by-layer deposition. Anionic polyelectrolytes with Azobenzene side groups (PAzo) were alternated with cationic polymer PAH, for the creation of transducer films. PSS/PAH multilayer were built for spacer layers, which do not absorb in the visible light range. The properties of the PAzo/PAH film as a transducer are carefully characterized by static and transient optical spectroscopy. The optical and mechanical properties of the transducer are studied on the picosecond time scale. In particular the relative change of the refractive index of the photo-excited and expanded PAH/PAzo is Δn/n = - 2.6*10‐4. Calibration of the generated strain is performed by ultrafast X-ray diffraction calibrated the strain in a Mica substrate, into which the hyper-sound is transduced. By simulating the X-ray data with a linear-chain-model the strain in the transducer under the excitation is derived to be Δd/d ~ 5*10‐4. Additional to the investigation of the properties of the transducer itself, I have performed a series of experiments to study the penetration of the generated strain into various adjacent materials. By depositing the PAzo/PAH film onto a PAH/PSS structure with gold nanorods incorporated in it, I have shown that nanoscale impurities can be detected via the scattering of hyper-sound. Prior to the investigation of complex structures containing GNRs and the transducer, I have performed several sets of experiments on GNRs deposited on a small buffer of PSS/PAH. The static and transient response of GNRs is investigated for different fluence of the pump beam and for different dielectric environments (GNRs covered by PSS/PAH). A systematic analysis of sample architectures is performed in order to construct a sample with the desired effect of GNRs responding to the hyper-sound strain wave. The observed shift of a feature related to the longitudinal plasmon resonance in the transient reflection spectra is interpreted as the event of GNRs sensing the strain wave. We argue that the shift of the longitudinal plasmon resonance is caused by the viscoelastic deformation of the polymer around the nanoparticle. The deformation is induced by the out of plane difference in strain in the area directly under a particle and next to it. Simulations based on the linear chain model support this assumption. Experimentally this assumption is proven by investigating the same structure, with GNRs embedded in a PSS/PAH polymer layer. The response of GNRs to the hyper-sound wave is also observed for the sample structure with GNRs embedded in PAzo/PAH films. The response of GNRs in this case is explained to be driven by the change of the refractive index of PAzo during the strain propagation. N2 - Akustische Experimente auf ultraschnellen Zeitskalen ermöglichen die Bestimmung von Tiefeninformationen in Dünnschichtproben. Der Grundgedanke dieser Methode ist die Analyse von Schallwellen, die sich in dem zu untersuchenden Material ausbreiten. Die Schallpulse werden dabei üblicherweise mittels dünner Schichten erzeugt, die dafür auf die Probe aufgebracht werden. Diese Methode ist etabliert für die Untersuchung von harten, anorganischen Materialien, aber weniger entwickelt für weiche, organische Materialien. Die wenigen existierenden Untersuchungen von weichen Materialien mittels ultraschneller Akustik nutzen bisher die Ausdehnung dünner Metallfilme, beispielsweise aus Aluminium oder Titan, für den Umwandlungsprozess von kurzen Lichtpulsen zu Schallwellen. Die deutlich höheren Dichten der Metalle gegenüber der zu untersuchenden weichen Materie führen zu einer geringen Effizienz bei der Einkopplung der Schallpulse in das Material. Weiterhin ist es schwierig, die Metallfilme auf die zu untersuchenden Materialien chemisch aufzubringen. Eine Möglichkeit diese Probleme zu umgehen, ist die Verwendung von Licht-Schallwandlern aus chemisch ähnlicher, weicher Materialien. Hier präsentiere ich die Ergebnisse meiner Untersuchungen von Polymer Filmen, welche Azobenzen als aktiven Bestandteil für die photo-akustische Umwandlung enthalten. Dabei wurden die Filme mittels statischer, sowie auch zeitaufgelöster Spektroskopie untersucht. Mit zeitaufgelösten Brillouin-Streuungs-Experimenten habe ich die Schallgeschwindigkeit in den Polymeren und dem Azobenzen-Schallwandler zu 3.4±0.3 nm/ps bestimmt. Die relative transiente Änderung des Brechungsindex in dem Azobenzenfilm aus optischen Messungen beträgt Δn/n = - 2.6*10‐4. Die Untersuchung der Schallpropagation in verschiedenen Probengeometrien erlaubt es uns, Reflektionen der Schallwellen von verschiedenen Übergängen (Polymer/Quarz, Polymer/Luft) und die Ausbreitung der mechanischen Wellen in weiche (Polymere) und harte (Quarz) angrenzende Materialien zu studieren. Durch Untersuchungen an einer Probe mit Gold-Nano-Stäbchen innerhalb einer Polymerschicht habe ich die Möglichkeit aufgezeigt, die Tiefenposition der Nanopartikel zu bestimmen. Die Ausdehnung des photomechanischen Wandlers wurde mittels zeitaufgelöster Röntgenbeugung zu ε = Δd/d ̴ 5x10-4 bestimmt. Der zweite Teil der Doktorarbeit behandelt die Wechselwirkung von Schallwellen und Gold-Nano-Stäbchen (GNS). GNS werden oft in der Chemie und Biologie als plasmonische Marker eingesetzt. In den meisten Fällen werden die Teilchen dafür mit einer Hülle überzogen, um ihre Agglomeration zu unterdrücken oder um ihnen spezielle Sensoreigenschaften zu geben. Trotz ihrer häufigen Anwendung in teilweise sehr komplexen Geometrien sind die optischen und elastischen Eigenschaften der Hülle der Nanopartikel, sowie deren Wechselwirkung mit der Umgebung wenig erforscht. Um die Wechselwirkung zwischen GNS und Schallwellen zu untersuchen habe ich eine systematische Studie an verschiedenen Probenstrukturen unternommen. Dabei finden wir, dass die viskoelastische Verformung der Polymerhülle um die GNS von der unterschiedlichen Ausdehnung der Fläche unterhalb der Partikel und neben ihnen stammt. Diese Schlussfolgerung wird von einer Simulation ihrer Ausdehnungsdynamik unterstützt. Einen weiteren Beleg liefern Experimente bei denen die Verformung von Polymeren an der Oberfläche dadurch verringert wird, dass die Gold-Nano-Stäbchen mit einer zusätzlichen dünnen Polymerschicht bedeckt werden. KW - ultrafast dynamics KW - plasmonics KW - hypersound KW - azobenzene KW - ultrafast spectroscopy KW - ultraschnelle Dynamik KW - Pump-Probe Spektroskopie KW - Plasmonics KW - Gold-Nanopartikel KW - Azobenzene Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-99544 ER - TY - GEN A1 - Pavlenko, Elena S. A1 - Sander, Mathias A1 - Mitzscherling, Steffen A1 - Pudell, Jan-Etienne A1 - Zamponi, Flavio A1 - Rössle, Matthias A1 - Bojahr, Andre A1 - Bargheer, Matias T1 - Azobenzene – functionalized polyelectrolyte nanolayers as ultrafast optoacoustic transducers N2 - We introduce azobenzene-functionalized polyelectrolyte multilayers as efficient, inexpensive optoacoustic transducers for hyper-sound strain waves in the GHz range. By picosecond transient reflectivity measurements we study the creation of nanoscale strain waves, their reflection from interfaces, damping by scattering from nanoparticles and propagation in soft and hard adjacent materials like polymer layers, quartz and mica. The amplitude of the generated strain ε ∼ 5 × 10−4 is calibrated by ultrafast X-ray diffraction. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 297 Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-101996 VL - 8 SP - 13297 EP - 13302 ER - TY - JOUR A1 - Sini, Gjergji A1 - Schubert, Marcel A1 - Risko, Chad A1 - Roland, Steffen A1 - Lee, Olivia P. A1 - Chen, Zhihua A1 - Richter, Thomas V. A1 - Dolfen, Daniel A1 - Coropceanu, Veaceslav A1 - Ludwigs, Sabine A1 - Scherf, Ullrich A1 - Facchetti, Antonio A1 - Frechet, Jean M. J. A1 - Neher, Dieter T1 - On the Molecular Origin of Charge Separation at the Donor-Acceptor Interface JF - Advanced energy materials N2 - Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. Model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force ( energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules. KW - donor-acceptor interfaces KW - energy gradients KW - geometrical deformations KW - nonfullerene acceptors KW - organic photovoltaics KW - photocurrent generation KW - polymer solar cells Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201702232 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 12 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Ullbrich, Sascha A1 - Benduhn, Johannes A1 - Jia, Xiangkun A1 - Nikolis, Vasileios C. A1 - Tvingstedt, Kristofer A1 - Piersimoni, Fortunato A1 - Roland, Steffen A1 - Liu, Yuan A1 - Wu, Jinhan A1 - Fischer, Axel A1 - Neher, Dieter A1 - Reineke, Sebastian A1 - Spoltore, Donato A1 - Vandewal, Koen T1 - Emissive and charge-generating donor-acceptor interfaces for organic optoelectronics with low voltage losses JF - Nature materials N2 - Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes(1,2). Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001% range(3,4). In contrast, the electroluminescence external quantum yield reaches up to 16% in D-A-based organic light-emitting diodes(5-7). Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons(8-11). KW - Electronics, photonics and device physics KW - Optoelectronic devices and components KW - Photonic devices KW - Solar energy and photovoltaic technology Y1 - 2019 U6 - https://doi.org/10.1038/s41563-019-0324-5 SN - 1476-1122 SN - 1476-4660 VL - 18 IS - 5 SP - 459 EP - 464 PB - Nature Publ. Group CY - London ER - TY - GEN A1 - García-Benito, Inés A1 - Quarti, Claudio A1 - Queloz, Valentin I. E. A1 - Hofstetter, Yvonne J. A1 - Becker-Koch, David A1 - Caprioglio, Pietro A1 - Neher, Dieter A1 - Orlandi, Simonetta A1 - Cavazzini, Marco A1 - Pozzi, Gianluca A1 - Even, Jacky A1 - Nazeeruddin, Mohammad Khaja A1 - Vaynzof, Yana A1 - Grancini, Giulia T1 - Fluorination of organic spacer impacts on the structural and optical response of 2D perovskites T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)(2)PbI4 and (Lf)(2)PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1421 KW - fluorinated organic spacer KW - 2D perovskites KW - phase transition KW - temperature dependence KW - excitonic materials Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-512420 SN - 1866-8372 ER - TY - JOUR A1 - Ran, Niva A. A1 - Love, John A. A1 - Heiber, Michael C. A1 - Jiao, Xuechen A1 - Hughes, Michael P. A1 - Karki, Akchheta A1 - Wang, Ming A1 - Brus, Viktor V. A1 - Wang, Hengbin A1 - Neher, Dieter A1 - Ade, Harald A1 - Bazan, Guillermo C. A1 - Thuc-Quyen Nguyen, T1 - Charge generation and recombination in an organic solar cell with low energetic offsets JF - dvanced energy materials N2 - Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J(SC)) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V-OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC61BM), that achieves a high V-OC (0.9 V) with very low energy losses (E-loss = 0.52 eV) from the energy of absorbed photons, a respectable J(SC) (13 mA cm(-2)), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC61BM. These results hold promise that given the appropriate morphology, the J(SC), V-OC, and FF can all be improved, even with very low energetic offsets. KW - energetic offset KW - fill factor KW - morphology KW - organic solar cells KW - recombination Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201701073 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 5 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Hosseini, Seyed Mehrdad A1 - Roland, Steffen A1 - Kurpiers, Jona A1 - Chen, Zhiming A1 - Zhang, Kai A1 - Huang, Fei A1 - Armin, Ardalan A1 - Neher, Dieter A1 - Shoaee, Safa T1 - Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells BT - A Comparative Study of Charge Generation and Extraction JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fundamental questions regarding nanomorphology, interfacial states, charge generation and extraction, and losses in these systems. Herein, we present a comparative study of bulk heterojunction solar cells composed of a recently introduced naphthothiadiazole-based polymer (NT812) as the electron donor and two different acceptor molecules, namely, [6,6]-phenyl-C71-butyric acid methyl ester (PCBM)[70] and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). A comparison between the photovoltaic performance of these two types of solar cells reveals that the open-circuit voltage (Voc) of the NT812:ITIC-based solar cell is larger, but the fill factor (FF) is lower than that of the NT812:PCBM[70] device. We find the key reason behind this reduced FF in the ITIC-based device to be faster nongeminate recombination relative to the NT812:PCBM[70] system. Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpcc.8b11669 SN - 1932-7447 VL - 123 IS - 11 SP - 6823 EP - 6830 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Hosseini, Seyed Mehrdad T1 - Non-Langevin Recombination in Fullerene and Non-Fullerene Acceptor Solar Cells T1 - Nicht-Langevin-Rekombination in Fulleren- und Nicht-Fulleren-Akzeptor-Solarzellen N2 - Organic solar cells (OSCs), in recent years, have shown high efficiencies through the development of novel non-fullerene acceptors (NFAs). Fullerene derivatives have been the centerpiece of the accepting materials used throughout organic photovoltaic (OPV) research. However, since 2015 novel NFAs have been a game-changer and have overtaken fullerenes. However, the current understanding of the properties of NFAs for OPV is still relatively limited and critical mechanisms defining the performance of OPVs are still topics of debate. In this thesis, attention is paid to understanding reduced-Langevin recombination with respect to the device physics properties of fullerene and non-fullerene systems. The work is comprised of four closely linked studies. The first is a detailed exploration of the fill factor (FF) expressed in terms of transport and recombination properties in a comparison of fullerene and non-fullerene acceptors. We investigated the key reason behind the reduced FF in the NFA (ITIC-based) devices which is faster non-geminate recombination relative to the fullerene (PCBM[70]-based) devices. This is then followed by a consideration of a newly synthesized NFA Y-series derivative which exhibits the highest power conversion efficiency for OSC at the time. Such that in the second study, we illustrated the role of disorder on the non-geminate recombination and charge extraction of thick NFA (Y6-based) devices. As a result, we enhanced the FF of thick PM6:Y6 by reducing the disorder which leads to suppressing the non-geminate recombination toward non-Langevin system. In the third work, we revealed the reason behind thickness independence of the short circuit current of PM6:Y6 devices, caused by the extraordinarily long diffusion length of Y6. The fourth study entails a broad comparison of a selection of fullerene and non-fullerene blends with respect to charge generation efficiency and recombination to unveil the importance of efficient charge generation for achieving reduced recombination. I employed transient measurements such as Time Delayed Collection Field (TDCF), Resistance dependent Photovoltage (RPV), and steady-state techniques such as Bias Assisted Charge Extraction (BACE), Temperature-Dependent Space Charge Limited Current (T-SCLC), Capacitance-Voltage (CV), and Photo-Induce Absorption (PIA), to analyze the OSCs. The outcomes in this thesis together draw a complex picture of multiple factors that affect reduced-Langevin recombination and thereby the FF and overall performance. This provides a suitable platform for identifying important parameters when designing new blend systems. As a result, we succeeded to improve the overall performance through enhancing the FF of thick NFA device by adjustment of the amount of the solvent additive in the active blend solution. It also highlights potentially critical gaps in the current experimental understanding of fundamental charge interaction and recombination dynamics. N2 - Organische Solarzellen (OSZ) haben in den letzten Jahren durch die Entwicklung neuartiger Nicht-Fulleren-Akzeptoren (NFA) hohe Wirkungsgrade erzielt. Fulleren-Derivate waren das Herzstück der Akzeptor-Materialien, die in der Forschung zur organischen Photovoltaik (OPV) verwendet wurden. Doch seit 2015 haben neuartige NFAs den Fullerenen den Rang abgelaufen. Allerdings ist das derzeitige Verständnis der Eigenschaften von NFA für OPV noch relativ begrenzt und kritische Mechanismen, die die Leistung von OPV bestimmen, sind immer noch Gegenstand von Diskussionen. In dieser Arbeit geht es um das Verständnis der Reduced-Langevin-Rekombination in Hinblick auf die bauteilphysikalischen Eigenschaften von Fulleren- und Nicht-Fulleren-Systemen. Die Arbeit besteht aus vier eng miteinander verbundenen Studien. Die erste ist eine detaillierte Untersuchung des Füllfaktors (FF), ausgedrückt als Transport- und Rekombinationseigenschaften in einem Vergleich von Fulleren und Nicht-Fulleren-Akzeptoren. Wir untersuchten den Hauptgrund für die geringere FF im NFA-Bauelement (auf ITIC-Basis), nämlich die schnellere nicht-geminate Rekombination im Vergleich zum Fulleren-Bauelement (auf PCBM[70]-Basis). Anschließend wird ein neu synthetisiertes NFA-Derivat der Y-Serie betrachtet, das derzeit die höchste Leistungsumwandlungseffizienz für OSZ aufweist. In der zweiten Studie veranschaulichten wir die Rolle der Unordnung bei der nicht-geminaten Rekombination und der Ladungsextraktion von dicken NFA-Bauelementen (auf Y6-Basis). Infolgedessen haben wir die FF von dickem PM6:Y6 verbessert, indem wir die Unordnung reduziert haben, was zur Unterdrückung der nicht-geminaten Rekombination in Richtung Nicht-Langevin-System führt. In der dritten Arbeit haben wir den Grund für die Dickenunabhängigkeit des Kurzschlussstroms von NFA-Bauelementen aufgedeckt, die durch die außerordentlich lange Diffusionslänge von Y6 verursacht wird. Die vierte Studie umfasst einen umfassenden Vergleich einer Auswahl von Fulleren- und Nicht-Fulleren-Mischungen in Hinblick auf die Effizienz der Ladungserzeugung und Rekombination, um die Bedeutung einer effizienten Ladungserzeugung zum Erzielen einer geringeren Rekombination aufzuzeigen. Zur Analyse der OSCs habe ich transiente Messungen wie das Time Delayed Collection Field (TDCF), Resistance dependent Photovoltage (RPV) sowie stationäre Techniken wie die Bias Assisted Charge Extraction (BACE), Temperature-Dependent Space Charge Limited Current (T-SCLC), Capacitance-Voltage (CV) und Photo-Induce Absorption (PIA) eingesetzt. Die Ergebnisse dieser Arbeit zeichnen ein komplexes Bild zahlreicher Faktoren, die die Rekombination nach dem Prinzip des reduzierten Langèvins und damit die FF und die Gesamtleistung beeinflussen. Dies bietet eine geeignete Plattform zum Identifizieren wichtiger Parameter bei der Entwicklung neuer Mischsysteme. So ist es uns gelungen, die Gesamtleistung zu verbessern, indem wir die FF der dicken NFA-Vorrichtung durch Anpassung der Menge des Lösungsmittelzusatzes in der aktiven Mischungslösung erhöht haben. Außerdem werden potenziell kritische Lücken im derzeitigen experimentellen Verständnis der grundlegenden Ladungswechselwirkung und Rekombinationsdynamik aufgezeigt. KW - Organic solar cells KW - Non-fullerene acceptors KW - Charge recombination KW - Non-Langevin systems KW - Structural and energetic disorder KW - Ladungsrekombination KW - Nicht-Langevin-Systeme KW - Nicht-Fulleren-Akzeptoren KW - Organische Solarzellen KW - Strukturelle und energetische Unordnung Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-547831 ER - TY - JOUR A1 - Roland, Steffen A1 - Yan, Liang A1 - Zhang, Qianqian A1 - Jiao, Xuechen A1 - Hunt, Adrian A1 - Ghasemi, Masoud A1 - Ade, Harald A1 - You, Wei A1 - Neher, Dieter T1 - Charge Generation and Mobility-Limited Performance of Bulk Heterojunction Solar Cells with a Higher Adduct Fullerene JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Alternative electron acceptors are being actively explored in order to advance the development of bulk-heterojunction (BHJ) organic solar cells (OSCs). The indene-C-60 bisadduct (ICBA) has been regarded as a promising candidate, as it provides high open-circuit voltage in BHJ solar cells; however, the photovoltaic performance of such ICBA-based devices is often inferior when compared to cells with the omnipresent PCBM electron acceptor. Here, by pairing the high performance polymer (FTAZ) as the donor with either PCBM or ICBA as the acceptor, we explore the physical mechanism behind the reduced performance of the ICBA-based device. Time delayed collection field (TDCF) experiments reveal reduced, yet field-independent free charge generation in the FTAZ:ICBA system, explaining the overall lower photocurrent in its cells. Through the analysis of the photoluminescence, photogeneration, and electroluminescence, we find that the lower generation efficiency is neither caused by inefficient exciton splitting, nor do we find evidence for significant energy back-transfer from the CT state to singlet excitons. In fact, the increase in open circuit voltage when replacing PCBM by ICBA is entirely caused by the increase in the CT energy, related to the shift in the LUMO energy, while changes in the radiative and nonradiative recombination losses are nearly absent. On the other hand, space charge limited current (SCLC) and bias-assisted charge extraction (BACE) measurements consistently reveal a severely lower electron mobilitiy in the FTAZ:ICBA blend. Studies of the blends with resonant soft X-ray scattering (R-SoXS), grazing incident wide-angle X-ray scattering (GIWAXS), and scanning transmission X-ray microscopy (STXM) reveal very little differences in the mesoscopic morphology but significantly less nanoscale molecular ordering of the fullerene domains in the ICBA based blends, which we propose as the main cause for the lower generation efficiency and smaller electron mobility. Calculations of the JV curves with an analytical model, using measured values, show good agreement with the experimentally determined JV characteristics, proving that these devices suffer from slow carrier extraction, resulting in significant bimolecular recombination losses. Therefore, this study highlights the importance of high charge carrier mobility for newly synthesized acceptor materials, in addition to having suitable energy levels. Y1 - 2017 U6 - https://doi.org/10.1021/acs.jpcc.7b02288 SN - 1932-7447 VL - 121 SP - 10305 EP - 10316 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Sun, Bowen T1 - Energy losses in low-offset organic solar cells T1 - Energieverluste in organischen Solarzellen mit geringer Versetzung BT - from fundamental understanding to characterization considerations BT - von grundlegendem Verständnis bis zu Charakterisierungsüberlegungen N2 - Organic solar cells (OSCs) represent a new generation of solar cells with a range of captivating attributes including low-cost, light-weight, aesthetically pleasing appearance, and flexibility. Different from traditional silicon solar cells, the photon-electron conversion in OSCs is usually accomplished in an active layer formed by blending two kinds of organic molecules (donor and acceptor) with different energy levels together. The first part of this thesis focuses on a better understanding of the role of the energetic offset and each recombination channel on the performance of these low-offset OSCs. By combining advanced experimental techniques with optical and electrical simulation, the energetic offsets between CT and excitons, several important insights were achieved: 1. The short circuit current density and fill-factor of low-offset systems are largely determined by field-dependent charge generation in such low-offset OSCs. Interestingly, it is strongly evident that such field-dependent charge generation originates from a field-dependent exciton dissociation yield. 2. The reduced energetic offset was found to be accompanied by strongly enhanced bimolecular recombination coefficient, which cannot be explained solely by exciton repopulation from CT states. This implies the existence of another dark decay channel apart from CT. The second focus of the thesis was on the technical perspective. In this thesis, the influence of optical artifacts in differential absorption spectroscopy upon the change of sample configuration and active layer thickness was studied. It is exemplified and discussed thoroughly and systematically in terms of optical simulations and experiments, how optical artifacts originated from non-uniform carrier profile and interference can manipulate not only the measured spectra, but also the decay dynamics in various measurement conditions. In the end of this study, a generalized methodology based on an inverse optical transfer matrix formalism was provided to correct the spectra and decay dynamics manipulated by optical artifacts. Overall, this thesis paves the way for a deeper understanding of the keys toward higher PCEs in low-offset OSC devices, from the perspectives of both device physics and characterization techniques. N2 - Organische Solarzellen (OSZ) repräsentieren eine neue Generation von Solarzellen mit einer Vielzahl faszinierender Eigenschaften, darunter geringe Kosten, geringes Gewicht, ästhetisch ansprechendes Erscheinungsbild und Flexibilität. Im Gegensatz zu traditionellen Silizium-Solarzellen erfolgt die Umwandlung von Photonen in Elektronen in OSZ in der Regel in einer aktiven Schicht, die durch das Mischen von zwei Arten organischer Moleküle (Donator und Akzeptor) mit unterschiedlichen Energieniveaus gebildet wird. Der erste Teil dieser Arbeit konzentriert sich auf ein besseres Verständnis der Rolle des energetischen Versatzes und jedes Rekombinationskanals auf die Leistung dieser OSCs mit geringem Versatz. Durch die Kombination fortschrittlicher experimenteller Techniken mit optischer und elektrischer Simulation wurden wichtige Erkenntnisse über die energetischen Versätze zwischen CT und Exzitonen erlangt: 1. Die Stromdichte im Kurzschluss und der Füllfaktor von Systemen mit geringem Versatz werden weitgehend durch feldabhängige Ladungsgenerierung in solchen OSZ mit geringem Versatz bestimmt. Interessanterweise ist deutlich erkennbar, dass eine feldabhängige Ladungsgenerierung aus einer feldabhängigen Exzitonen-Dissociationsausbeute resultiert. 2. Der reduzierte energetische Versatz geht mit einem stark erhöhten bimolekularen Rekombinationskoeffizienten einher, der nicht allein durch die Wiederbevölkerung von Exzitonen aus CT-Zuständen erklärt werden kann. Dies deutet auf die Existenz eines anderen dunklen Zerfallsweges neben CT hin. Der zweite Schwerpunkt der Arbeit lag auf der technischen Perspektive. In dieser Arbeit wurde der Einfluss von optischen Artefakten in der differentiellen Absorptionsspektroskopie auf die Änderung der Probekonfiguration und der aktiven Schichtdicke untersucht. Es wird anhand optischer Simulationen und Experimente ausführlich und systematisch dargelegt und diskutiert, wie optische Artefakte, die durch ein nicht gleichmäßiges Ladungsprofil und Interferenzen verursacht werden, nicht nur die gemessenen Spektren, sondern auch die Zerfalldynamik in verschiedenen Messbedingungen manipulieren können. Am Ende dieser Studie wurde eine generalisierte Methodik auf Basis eines inversen optischen Übertragungsmatrixformalismus bereitgestellt, um die durch optische Artefakte manipulierten Spektren und Zerfalldynamiken zu korrigieren. Insgesamt ebnet diese Arbeit den Weg für ein tieferes Verständnis der Schlüsselaspekte für höhere Wirkungsgrade in OSZ mit geringem Versatz, sowohl aus Sicht der Gerätephysik als auch der Charakterisierungstechniken. KW - organic solar cell KW - organische Solarzelle KW - non-fullerene acceptors KW - Nicht-Fulleren-Akzeptoren KW - charge generation KW - Ladungsgenerierung KW - exciton dissociation KW - Exziton-Dissoziation KW - cavity effects KW - Hohlraumeffekte Y1 - 2024 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-621430 ER - TY - JOUR A1 - Benduhn, Johannes A1 - Tvingstedt, Kristofer A1 - Piersimoni, Fortunato A1 - Ullbrich, Sascha A1 - Fan, Yeli A1 - Tropiano, Manuel A1 - McGarry, Kathryn A. A1 - Zeika, Olaf A1 - Riede, Moritz K. A1 - Douglas, Christopher J. A1 - Barlow, Stephen A1 - Marder, Seth R. A1 - Neher, Dieter A1 - Spoltore, Donato A1 - Vandewal, Koen T1 - Intrinsic non-radiative voltage losses in fullerene-based organic solar cells JF - Nature Energy N2 - Organic solar cells demonstrate external quantum efficiencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared with the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant non-radiative recombination. Here, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer-state energies. This observation is explained by considering non-radiative charge-transfer-state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion efficiency of single-junction organic solar cells would be reduced to about 25.5% and the optimal optical gap increases to (1.45-1.65) eV, that is, (0.2-0.3) eV higher than for technologies with minimized non-radiative voltage losses. Y1 - 2017 U6 - https://doi.org/10.1038/nenergy.2017.53 SN - 2058-7546 VL - 2 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Hofacker, Andreas A1 - Neher, Dieter T1 - Dispersive and steady-state recombination in organic disordered semiconductors JF - Physical review : B, Condensed matter and materials physics N2 - Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed. Y1 - 2017 U6 - https://doi.org/10.1103/PhysRevB.96.245204 SN - 2469-9950 SN - 2469-9969 VL - 96 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Mechau, Norman A1 - Saphiannikova, Marina A1 - Neher, Dieter T1 - Molecular tracer diffusion in thin azobenzene polymer layers JF - Applied physics letters N2 - Translational diffusion of fluorescent tracer molecules in azobenzene polymer layers is studied at different temperatures and under illumination using the method of fluorescence recovery after photobleaching. Diffusion is clearly observed in the dark above the glass transition temperature, while homogeneous illumination at 488 nm and 100 mW/cm(2) does not cause any detectable diffusion of the dye molecules within azobenzene layers. This implies that the viscosity of azobenzene layers remains nearly unchanged under illumination with visible light in the absence of internal or external forces. (c) 2006 American Institute of Physics. Y1 - 2006 U6 - https://doi.org/10.1063/1.2405853 SN - 0003-6951 VL - 89 IS - 25 PB - Elsevier CY - Melville ER - TY - THES A1 - Sajedi, Maryam T1 - Investigation of metal-halide-perovskites by state-of-the-art synchrotron-radiation methods T1 - Untersuchung von Metallhalogenid-Perowskiten mit modernsten Synchrotronstrahlungsmethoden N2 - My thesis chiefly aims to shed light on the favourable properties of LHP semiconductors from the point of view of their electronic structure. Currently, various hypotheses are circulating to explain the exceptionally favourable transport properties of LHPs. Seeking an explanation for the low non-radiative recombination rates and long carrier lifetimes is particularly interesting to the halide perovskites research community. The first part of this work investigates the two main hypotheses that are believed to play a significant role: the existence of a giant Rashba effect and large polarons. The experimental method of ARPES is mainly applied to verify their credibility. The first hypothesis presumes that a giant Rashba effect restricts the recombination losses of the charge carriers by making the band gap slightly indirect. The Rashba effect is based on a strong SOC that could appear in LHPs thanks to incorporating the heavy element Pb in their structure. Earlier experimental work had pointed out this effect at the VBM of a hybrid LHP as a viable explanation for the long lifetimes of the charge carriers. My systematic ARPES studies on hybrid MAPbBr3 and spin-resolved ARPES studies on the inorganic CsPbBr3 disprove the presence of any Rashba effect in the VBM of the reported order of magnitude. Therefore, neither the spin texture nor an indirect band gap character at the VBM in the bulk or at the surface can explain the high efficiency of LHP. In case of existence, this effect is in terms of the Rashba parameter at least a factor of a hundred smaller than previously assumed. The second hypothesis proposes large polaron formation in the electronic structure of LHPs and attributes it to their high defect tolerance and low non-radiative recombination rate. Because the perovskite structure consists of negative and positive ions, polarons of this kind can be expected due to the Coulomb interaction between carriers and the polar lattice at intermediate electron-phonon coupling strength. Their existence is proposed to screen the carriers and defects to avoid recombination and trapping, thus leading to long carrier lifetimes. ARPES results by one group supported this assumption, reporting a 50% effective mass enhancement over the theoretical effective mass for CsPbBr3 in the orthorhombic structure. The current thesis examines this hypothesis experimentally by photon-energy-dependent ARPES spectra and theoretically by GW band calculations of CsPbBr3 perovskites. The investigation is based on the fact that a polaron contribution in charge transport can become evident by an increase of the effective mass as measured by ARPES over the calculated one without polaron effects. However, my experiments on crystalline CsPbBr3 did not imply a larger effective mass for which one could postulate large polarons. In fact, the effective masses determined from ARPES agree with that of theoretical predictions. The second part of my thesis thoroughly investigates the possibility of spontaneously magnetizing LHPs by introducing Mn2+ ions. Mn doping was reported to cause ferromagnetism in one of the most common LHPs, MAPbI3, mediated by super-exchange. The current work investigates the magnetic properties of a wide concentration range of Mn-doped MAPbI3 and triple-cation films by XAS, XMCD, and SQUID measurements. Based on the XAS line shape and a sum-rule analysis of the XMCD spectra, a pure Mn2+ configuration has been confirmed. Negative Curie temperatures are extracted from fitting the magnetization with a Curie-Weiss law. However, a remanent magnetization, which would be an indication of the absence of ferromagnetism down to 2K. As far as the double exchange is concerned, the element-specific XAS excludes a sufficient amount of Mn3+ as a prerequisite for this mechanism. All the findings show no evidence of significant double exchange or ferromagnetism in Mn-doped LHPs. The magnetic behavior is paramagnetic rather than ferromagnetic. In the dissertation's last chapter, orthorhombic features of CsPbBr3 are revealed by ARPES, including an extra VBM at the Γ-point. The VBM of CsPbBr3 shows a temperature-dependent splitting, which decreases by 190 meV between 38K and 300K and tracks a shift of a saddle point at the cubic M-point. It is possible to reproduce the energy shift using an atomic model with a larger unit cell for room temperature, allowing local inversion symmetry breaking. This indicates the importance of electric dipoles for the inorganic LHPs, which may contribute to their high efficiency by breaking inversion symmetry and a Berry-phase effect. N2 - In meiner Dissertation geht es vor allem darum, die vorteilhaften Eigenschaften von LHP ausgehend von ihrer elektronischen Struktur zu beleuchten. Derzeit kursieren zahlreiche Hypothesen, um die außergewöhnlich guten Transporteigenschaften von LHPs zu erklären. Die Suche nach einer Erklärung für die niedrigen strahlungslosen Rekombinationsraten und die langen Ladungsträgerlebensdauern ist von besonderem Interesse für die Forschercommunity der Halogenidperowskite. Der erste Teil dieser Arbeit untersucht die beiden Haupthypothesen, von denen angenommen wird, dass sie eine wichtige Rolle spielen: die Existenz eines riesigen Rashba-Effekts und großer Polaronen. Hier wird hauptsächlich die experimentelle Methode der ARPES angewandt, um ihre Glaubwürdigkeit zu überprüfen. Die erste Hypothese geht davon aus, dass ein riesiger Rashba-Effekt die Rekombinationsverluste der Ladungsträger einschränkt, indem er die Bandlücke leicht indirekt macht. Der Rashba-Effekt basiert auf einer starken Spin-Bahn-Wechselwirkung, die in LHPs, dank der Präsenz des schweren Elements Pb in ihrer Struktur, potenziell auftreten könnte. Eine frühere experimentelle Arbeit hatte diesen Effekt am VBM eines hybriden LHP als mögliche Erklärung für die langen Lebensdauern der Ladungsträger vorgeschlagen. Meine systematischen ARPES-Studien am hybriden MAPbBr3 sowie die winkel- und spinaufgelösten ARPES-Studien am anorganischen CsPbBr3 widerlegen das Vorhandensein eines riesigen Rashba-Effekts im VBM in der angegebenen Größenordnung. Daher können weder die Spin-Textur noch der indirekte Bandlückencharakter am VBM im Volumen oder an der Oberfläche die hohe Effizienz von LHP erklären. Dieser Effekt ist, falls er existiert, mindestens um einen Faktor hundert kleiner als bisher angenommen. Die zweite Hypothese geht von der Bildung großer Polaronen in der elektronischen Struktur von LHPs aus, welche zu ihrer hohen Defekttoleranz und niedrigen strahlungslosen Rekombination srate beitragen soll. Da die Perowskit Struktur aus negativen und positiven Ionen besteht, sind solche Polaronen wegen der Coulomb-Wechselwirkung zwischen Ladungsträger und Ionengitter bei mittlerer Stärke der Elektron-Phonon-Kopplung zu erwarten. Es wird angenommen, dass sie die Ladungsträger und Defekte abschirmen, was Rekombination und Trapping verhindert und zu langen Ladungsträgerlebensdauern führt. Die ARPES-Ergebnisse einer Gruppe stützen diese Annahme und zeigen, dass die effektive Masse von CsPbBr3 um 50% höher ist als die theoretische effektive Masse für die orthorhombische Phase. In der vorliegenden Arbeit wird diese Hypothese experimentell mit Hilfe von Photonenenergie-abhängigen ARPES-Spektren und theoretisch mit Hilfe von GW-Bandstrukturberechnungen an CsPbBr3-Perowskiten untersucht. Denn der Beitrag von Polaronen zum Ladungstransport lässt sich durch eine Zunahme der mit ARPES gemessenen effektiven-Masse nachweisen. Meine Experimente an kristallinem CsPbBr3 ergaben jedoch keine erhöhte effektive Masse, für die man große Polaronen postulieren könnte. Tatsächlich stimmen die aus ARPES ermittelten effektiven Massen gut mit den theoretischen Vorhersagen überein. Der zweite Teil meiner Dissertation untersucht die Möglichkeit, LHPs durch den Einbau von Mn2+ Ionen spontan zu magnetisieren. Es wurde berichtet, dass die Mn-Dotierung in einem der häufigsten LHPs - \MAPbI3 - Ferromagnetismus durch "Superaustausch" hervorruft. Außerdem berichteten zwei weitere Arbeiten über Ferromagnetismus sogar bei Raumtemperatur, hervorgerufen jedoch durch Doppalaustausch. In der vorliegenden Arbeit werden die magnetischen Eigenschaften eines weiten Konzentrationsbereichs von Mn-dotiertem MAPbI3 und Dreifachkation-Filmen mittels XAS, XMCD und SQUID-Messungen untersucht. Basierend auf der XAS-Linienform und der Summenregelanalyse der XMCD-Spektren wurde eine reine Mn2+ Konfiguration bestätigt. Negative Curie-Temperaturen werden aus einem Fit der Magnetisierung mit dem Curie-Weiss-Gesetz abgeleitet. Eine remanente Magnetisierung, die auf Ferromagnetismus hindeuten würde, wird jedoch bis hinunter zu 2K nicht beobachtet. Was den Doppelaustausch betrifft, so schließt die elementspezifische XAS eine ausreichende Menge an Mn3+ als Voraussetzung für diesen Mechanismus aus. Nach all diesen Erkenntnissen gibt es keinen Hinweis auf einen signifikanten Doppelaustausch oder Ferromagnetismus in Mn-dotierten LHP. Das magnetische Verhalten ist eher paramagnetisch als ferromagnetisch. Im letzten Kapitel der Dissertation werden orthorhombische ARPES-Strukturen bei CsPbBr3 beobachtet, einschließlich eines zusätzlichen VBM am Γ-Punkt. Das VBM von CsPbBr3 zeigt eine temperaturabhängige Aufspaltung, die zwischen 38K und 300 K um 190 meV abnimmt und von einer Verschiebung eines Sattelpunktes am kubischen M-Punkt stammt. Es ist möglich, die Energieverschiebung mit atomaren Model mit größerer Einzeitszelle für Raumtemperatur, eine Brechung der lokalen Inversionssymmetrie zulässt, zu reproduzieren. Dies deutet auf die Bedeutung elektrischer Dipole für anorganische LHP hin, die zu ihrer hohen Effizienz durch Brechung der Inversionssymmetrie und einen Berry-Phasen-Effekt beitragen könnten. KW - lead halide perovskites (LHP) KW - valence band (VB) KW - valence band maximum (VBM) KW - Brillouin zone (BZ) KW - surface Brillouin zone (SBZ) KW - Spin-orbi coupling (SOC) KW - Brillouin-Zone (BZ) KW - Bleihalogenid-Perowskite (BHP) KW - Oberflächen-Brillouin-Zone (OBZ) KW - Valenzband (VB) KW - Valenzbandmaximum (VBM) KW - Spin-Bahn-Wechselwirkung (SBW) Y1 - 2023 ER - TY - JOUR A1 - Alqahtani, Obaid A1 - Babics, Maxime A1 - Gorenflot, Julien A1 - Savikhin, Victoria A1 - Ferron, Thomas A1 - Balawi, Ahmed H. A1 - Paulke, Andreas A1 - Kan, Zhipeng A1 - Pope, Michael A1 - Clulow, Andrew J. A1 - Wolf, Jannic A1 - Burn, Paul L. A1 - Gentle, Ian R. A1 - Neher, Dieter A1 - Toney, Michael F. A1 - Laquai, Frederic A1 - Beaujuge, Pierre M. A1 - Collins, Brian A. T1 - Mixed Domains Enhance Charge Generation and Extraction in Bulk-Heterojunction Solar Cells with Small-Molecule Donors JF - Advanced energy materials N2 - The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)(2), namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes. KW - charge transport KW - domain purity KW - microscopy KW - mixed domains KW - organic solar cells KW - photovoltaic devices KW - resonant X-ray scattering KW - small molecules KW - transient spectroscopy Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201702941 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 19 PB - Wiley-VCH CY - Weinheim ER - TY - GEN A1 - Grisic, Ana-Marija A1 - Huisinga, Wilhelm A1 - Reinisch, W. A1 - Kloft, Charlotte T1 - P485 Dosing infliximab in Crohn's disease BT - is adjustment for body size justified? T2 - Journal of Crohn's and Colitis N2 - Background: Infliximab (IFX), an anti-TNF monoclonal antibody approved for the treatment of inflammatory bowel disease, is dosed per kg body weight (BW). However, the rationale for body size adjustment has not been unequivocally demonstrated [1], and first attempts to improve IFX therapy have been undertaken [2]. The aim of our study was to assess the impact of different dosing strategies (i.e. body size-adjusted and fixed dosing) on drug exposure and pharmacokinetic (PK) target attainment. For this purpose, a comprehensive simulation study was performed, using patient characteristics (n=116) from an in-house clinical database. Methods: IFX concentration-time profiles of 1000 virtual, clinically representative patients were generated using a previously published PK model for IFX in patients with Crohn's disease [3]. For each patient 1000 profiles accounting for PK variability were considered. The IFX exposure during maintenance treatment after the following dosing strategies was compared: i) fixed dose, and per ii) BW, iii) lean BW (LBW), iv) body surface area (BSA), v) height (HT), vi) body mass index (BMI) and vii) fat-free mass (FFM)). For each dosing strategy the variability in maximum concentration Cmax, minimum concentration Cmin (= C8weeks) and area under the concentration-time curve (AUC), as well as percent of patients achieving the PK target, Cmin=3 μg/mL [4] were assessed. Results: For all dosing strategies the variability of Cmin (CV ≈110%) was highest, compared to Cmax and AUC, and was of similar extent regardless of dosing strategy. The proportion of patients reaching the PK target (≈⅓ was approximately equal for all dosing strategies. Y1 - 2017 U6 - https://doi.org/10.1093/ecco-jcc/jjx002.609 SN - 1873-9946 SN - 1876-4479 VL - 11 IS - 1 SP - S325 EP - S326 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Mayer, Dennis A1 - Lever, Fabiano A1 - Picconi, David A1 - Metje, Jan A1 - Ališauskas, Skirmantas A1 - Calegari, Francesca A1 - Düsterer, Stefan A1 - Ehlert, Christopher A1 - Feifel, Raimund A1 - Niebuhr, Mario A1 - Manschwetus, Bastian A1 - Kuhlmann, Marion A1 - Mazza, Tommaso A1 - Robinson, Matthew Scott A1 - Squibb, Richard J. A1 - Trabattoni, Andrea A1 - Wallner, Måns A1 - Saalfrank, Peter A1 - Wolf, Thomas J. A. A1 - Gühr, Markus T1 - Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy JF - Nature communications N2 - Imaging the charge flow in photoexcited molecules would provide key information on photophysical and photochemical processes. Here the authors demonstrate tracking in real time after photoexcitation the change in charge density at a specific site of 2-thiouracil using time-resolved X-ray photoelectron spectroscopy. The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220-250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states. Y1 - 2022 U6 - https://doi.org/10.1038/s41467-021-27908-y SN - 2041-1723 N1 - Publisher correction: https://doi.org/10.1038/s41467-022-28584-2 VL - 13 IS - 1 PB - Nature Research CY - Berlin ER - TY - JOUR A1 - Liu, Ji-Cai A1 - Ignatova, Nina A1 - Kimberg, Victor A1 - Krasnov, Pavel A1 - Föhlisch, Alexander A1 - Simon, Marc A1 - Gel'mukhanov, Faris T1 - Time-resolved study of recoil-induced rotation by X-ray pump - X-ray probe spectroscopy JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Modern stationary X-ray spectroscopy is unable to resolve rotational structure. In the present paper, we propose to use time-resolved two color X-ray pump-probe spectroscopy with picosecond resolution for real-time monitoring of the rotational dynamics induced by the recoil effect. The proposed technique consists of two steps. The first short pump X-ray pulse ionizes the valence electron, which transfers angular momentum to the molecule. The second time-delayed short probe X-ray pulse resonantly excites a 1s electron to the created valence hole. Due to the recoil-induced angular momentum the molecule rotates and changes the orientation of transition dipole moment of core-excitation with respect to the transition dipole moment of the valence ionization, which results in a temporal modulation of the probe X-ray absorption as a function of the delay time between the pulses. We developed an accurate theory of the X-ray pump-probe spectroscopy of the recoil-induced rotation and study how the energy of the photoelectron and thermal dephasing affect the structure of the time-dependent X-ray absorption using the CO molecule as a case-study. We also discuss the feasibility of experimental observation of our theoretical findings, opening new perspectives in studies of molecular rotational dynamics. Y1 - 2022 U6 - https://doi.org/10.1039/d1cp05000a SN - 1463-9076 SN - 1463-9084 VL - 24 IS - 11 SP - 6627 EP - 6638 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Voroshnin, Vladimir A1 - Tarasov, Artem V. A1 - Bokai, Kirill A. A1 - Chikina, Alla A1 - Senkovskiy, Boris V. A1 - Ehlen, Niels A1 - Usachov, Dmitry Yu. A1 - Gruneis, Alexander A1 - Krivenkov, Maxim A1 - Sanchez-Barriga, Jaime A1 - Fedorov, Alexander T1 - Direct spectroscopic evidence of magnetic proximity effect in MoS2 monolayer on graphene/Co JF - ACS nano N2 - A magnetic field modifies optical properties and provides valley splitting in a molybdenum disulfide (MoS2) monolayer. Here we demonstrate a scalable approach to the epitaxial synthesis of MoS2 monolayer on a magnetic graphene/Co system. Using spin- and angle-resolved photoemission spectroscopy we observe a magnetic proximity effect that causes a 20 meV spin-splitting at the (Gamma) over bar point and canting of spins at the (K) over bar point in the valence band toward the in-plane direction of cobalt magnetization. Our density functional theory calculations reveal that the in-plane spin component at (K) over bar is localized on Co atoms in the valence band, while in the conduction band it is localized on the MoS2 layer. The calculations also predict a 16 meV spin-splitting at the (Gamma) over bar point and 8 meV (K) over bar-(K) over bar' valley asymmetry for an out-of-plane magnetization. These findings suggest control over optical transitions in MoS2 via Co magnetization. Our estimations show that the magnetic proximity effect is equivalent to the action of the magnetic field as large as 100 T. KW - magnetic proximity effect KW - MoS2 KW - monolayer KW - graphene KW - spin-resolved KW - ARPES Y1 - 2022 U6 - https://doi.org/10.1021/acsnano.1c10391 SN - 1936-0851 SN - 1936-086X VL - 16 IS - 5 SP - 7448 EP - 7456 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Abiuso, Paolo A1 - Holubec, Viktor A1 - Anders, Janet A1 - Ye, Zhuolin A1 - Cerisola, Federico A1 - Perarnau-Llobet, Marti T1 - Thermodynamics and optimal protocols of multidimensional quadratic Brownian systems JF - Journal of physics communications N2 - We characterize finite-time thermodynamic processes of multidimensional quadratic overdamped systems. Analytic expressions are provided for heat, work, and dissipation for any evolution of the system covariance matrix. The Bures-Wasserstein metric between covariance matrices naturally emerges as the local quantifier of dissipation. General principles of how to apply these geometric tools to identify optimal protocols are discussed. Focusing on the relevant slow-driving limit, we show how these results can be used to analyze cases in which the experimental control over the system is partial. KW - stochastic thermodynamics KW - thermodynamic control KW - thermodynamic length KW - overdamped brownian systems Y1 - 2022 U6 - https://doi.org/10.1088/2399-6528/ac72f8 SN - 2399-6528 VL - 6 IS - 6 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Stojkoski, Viktor A1 - Sandev, Trifce A1 - Kocarev, Ljupco A1 - Pal, Arnab T1 - Autocorrelation functions and ergodicity in diffusion with stochastic resetting JF - Journal of physics : A, Mathematical and theoretical N2 - Diffusion with stochastic resetting is a paradigm of resetting processes. Standard renewal or master equation approach are typically used to study steady state and other transport properties such as average, mean squared displacement etc. What remains less explored is the two time point correlation functions whose evaluation is often daunting since it requires the implementation of the exact time dependent probability density functions of the resetting processes which are unknown for most of the problems. We adopt a different approach that allows us to write a stochastic solution for a single trajectory undergoing resetting. Moments and the autocorrelation functions between any two times along the trajectory can then be computed directly using the laws of total expectation. Estimation of autocorrelation functions turns out to be pivotal for investigating the ergodic properties of various observables for this canonical model. In particular, we investigate two observables (i) sample mean which is widely used in economics and (ii) time-averaged-mean-squared-displacement (TAMSD) which is of acute interest in physics. We find that both diffusion and drift-diffusion processes with resetting are ergodic at the mean level unlike their reset-free counterparts. In contrast, resetting renders ergodicity breaking in the TAMSD while both the stochastic processes are ergodic when resetting is absent. We quantify these behaviors with detailed analytical study and corroborate with extensive numerical simulations. Our results can be verified in experimental set-ups that can track single particle trajectories and thus have strong implications in understanding the physics of resetting. KW - autocorrelations KW - ergodicity KW - diffusion KW - stochastic resetting Y1 - 2022 U6 - https://doi.org/10.1088/1751-8121/ac4ce9 SN - 1751-8113 SN - 1751-8121 VL - 55 IS - 10 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Maiti, Snehanshu T1 - Magnetohydrodynamic turbulence and cosmic ray transport T1 - Magnetohydrodynamische Turbulenz und Transport kosmischer Strahlung N2 - The first part of the thesis studies the properties of fast mode in magneto hydro-dynamic (MHD) turbulence. 1D and 3D numerical simulations are carried out to generate decaying fast mode MHD turbulence. The injection of waves are carried out in a collinear and isotropic fashion to generate fast mode turbulence. The properties of fast mode turbulence are analyzed by studying their energy spectral density, 2D structure functions and energy decay/cascade time. The injection wave vector is varied to study the dependence of the above properties on the injection wave vectors. The 1D energy spectrum obtained for the velocity and magnetic fields has 𝐸 (𝑘) ∝ 𝑘−2. The 2D energy spectrum and 2D structure functions in parallel and perpendicular directions shows that fast mode turbulence generated is isotropic in nature. The cascade/decay rate of fast mode MHD turbulence is proportional to 𝑘−0.5 for different kinds of wave vector injection. Simulations are also carried out in 1D and 3D to compare balanced and imbalanced turbulence. The results obtained shows that while 1D imbalanced turbulence decays faster than 1D balanced turbulence, there is no difference in the decay of 3D balanced and imbalanced turbulence for the current resolution of 512 grid points. "The second part of the thesis studies cosmic ray (CR) transport in driven MHD turbulence and is strongly dependent on it’s properties. Test particle simulations are carried out to study CR interaction with both total MHD turbulence and decomposed MHD modes. The spatial diffusion coefficients and the pitch angle scattering diffusion coefficients are calculated from the test particle trajectories in turbulence. The results confirms that the fast modes dominate the CR propagation, whereas Alfvén, slow modes are much less efficient with similar pitch angle scattering rates. The cross field transport on large and small scales are investigated next. On large/global scales, normal diffusion is observed and the diffusion coefficient is suppressed by 𝑀𝜁𝐴 compared to the parallel diffusion coefficients, with 𝜁 closer to 4 in Alfvén modes than that in total turbulence as theoretically expected. For the CR transport on scales smaller than the turbulence injection scale 𝐿, both the local and global magnetic reference frames are adopted. Super diffusion is observed on such small scales in all the cases. Particularly, CR transport in Alfvén modes show clear Richardson diffusion in the local reference frame. The diffusion transition smoothly from the Richardson’s one with index 1.5 to normal diffusion as particle’s mean free path decreases from 𝜆∥ ≫ 𝐿 to 𝜆∥ ≪ 𝐿. These results have broad applications to CRs in various astrophysical environments". N2 - Der erste Teil der Arbeit untersucht die Eigenschaften des schnellen Modus in magnetohydrodynamischen (MHD) Turbulenzen. Es werden numerische 1D- und 3D-Simulationen durchgeführt, um eine abklingende Fast-Mode-MHD-Turbulenz zu erzeugen. Die Injektion von Wellenvektoren wird kollinear und isotrop durchgeführt, um Fast-Mode-Turbulenzen zu erzeugen. Die Eigenschaften der Fast-Mode-Turbulenz werden durch die Untersuchung ihrer Energie-Spektraldichte, 2D-Strukturfunktionen und Energieabfall-/Kaskadenzeit analysiert. Die Injektionswellenvektoren werden in verschiedenen Simulationen für unterschiedliche Arten der Injektion variiert, um die Abhängigkeit der oben genannten Eigenschaften von den Injektionswellenvektoren zu untersuchen. Das für die Geschwindigkeits- und Magnetfelder erhaltene 1D-Energiespektrum hat E(k) ∝ k−2. Das 2D-Energiespektrum und die 2D-Strukturfunktionen in parallelen und senkrechten Richtungen zeigen, dass die erzeugte Fast-Mode-Turbulenz von Natur aus isotrop ist. Die Kaskaden-/Zerfallsrate der Fast-Mode-MHD-Turbulenz ist proportional zu k−0.5 für verschiedene Arten der Wellenvektorinjektion. Es werden auch Simulationen in 1D und 3D durchgeführt, um ausgeglichene und unausgeglichene Turbulenzen zu vergleichen. Die Ergebnisse zeigen, dass eine unausgewogene 1D-Turbulenz schneller abklingt als eine ausgeglichene 1D-Turbulenz, während es bei der derzeitigen Auflösung von 512 Gitterpunkten keinen Unterschied im Abklingen von ausgeglichener und unausgewogener 3D-Turbulenz gibt. Der zweite Teil der Arbeit untersucht den Transport kosmischer Strahlung (CR) in angetriebenen MHD-Turbulenzen und ist stark von deren Eigenschaften abhängig. Es werden Testpartikelsimulationen durchgeführt, um die Wechselwirkung von kosmischer Strahlung sowohl mit der gesamten MHD-Turbulenz als auch mit zerlegten MHD-Moden zu untersuchen. Aus den Flugbahnen der Testteilchen in der Turbulenz werden die räumlichen Diffusionskoeffizienten und die Diffusionskoeffizienten für die Streuung im Neigungswinkel berechnet. Die Ergebnisse bestätigen, dass die schnellen Moden die CR-Ausbreitung dominieren, während Alfv´en langsame Moden bei ähnlichen Neigungswinkelstreuungsraten viel weniger effizient sind. Der Querfeldtransport auf großen und kleinen Skalen wird als nächstes untersucht. Auf großen/globalen Skalen wird normale Diffusion beobachtet und der Diffusionskoeffizient wird durch MζA im Vergleich zu den parallelen Diffusionskoeffizienten unterdrückt, wobei ζin Alfv´en-Moden näher bei 4 liegt als in der Gesamtturbulenz, wie theoretisch erwartet. Für den CR-Transport auf Skalen, die kleiner sind als die Turbulenzinjektionsskala L, werden sowohl der lokale als auch der globale magnetische Bezugsrahmen verwendet. Auf solch kleinen Skalen wird in allen Fällen Superdiffusion beobachtet. Insbesondere der CRTransport in Alfv’en-Moden zeigt eine deutliche Richardson-Diffusion im lokalen Bezugssystem. Die Diffusion geht fließend von der Richardson-Diffusion mit dem Index 1,5 zur normalen Diffusion über, wenn die mittlere freie Weglänge der Teilchen, λ∥, von λ∥ ≫ L auf λ∥ ≪ L abnimmt. Diese Ergebnisse haben eine breite Anwendung auf CRs in verschiedenen astrophysikalischen Umgebungen. KW - isotropic fast mode turbulence KW - cascade rate KW - Alfv´en mode MHD turbulence KW - cosmic ray diffusion KW - efficient scattering KW - mean free path KW - Richardson Superdiffusion KW - Alfv´en-Modus MHD-Turbulenz KW - Richardson-Superdiffusion KW - Kaskadenrate KW - Diffusion kosmischer Strahlung KW - effiziente Streuung KW - Isotroper schneller Modus Turbulenzen KW - bedeuten freie Bahn Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-589030 ER - TY - JOUR A1 - Kim, Kyung-Chan A1 - Shprits, Yuri Y. T1 - Statistical Analysis of Hiss Waves in Plasmaspheric Plumes Using Van Allen Probe Observations JF - Journal of geophysical research : Space physics N2 - Plasmaspheric hiss waves commonly observed in high‐density regions in the Earth's magnetosphere are known to be one of the main contributors to the loss of radiation belt electrons. There has been a lot of effort to investigate the distributions of hiss waves in the plasmasphere, while relatively little attention has been given to those in the plasmaspheric plume. In this study, we present for the first time a statistical analysis of the occurrence and the spatial distribution of wave amplitudes and wave normal angles for hiss waves in plumes using Van Allen Probes observations during the period of October 2012 to December 2016. Statistical results show that a wide range of hiss wave amplitudes in plumes from a few picotesla to >100 pT is observed, but a modest (<20 pT) wave amplitude is more commonly observed regardless of geomagnetic activity in both the midnight‐to‐dawn and dusk sector. By contrast, stronger amplitude hiss occurs preferentially during geomagnetically active times in the dusk sector. The wave normal angles are distributed over a broad range from 0° to 90° with a bimodal distribution: a quasi‐field‐aligned population (<20°) with an occurrence rate of <60% and an oblique one (>50°) with a relative low occurrence rate of ≲20%. Therefore, from a statistical point of view, we confirm that the hiss intensity (a few tens of picotesla) and field‐aligned hiss wave adopted in previous simulation studies are a reasonable assumption but stress that the activity dependence of the wave amplitude should be considered. KW - plasmaspheric hiss KW - plasmaspheric plume KW - Van Allen Probes Y1 - 2019 U6 - https://doi.org/10.1029/2018JA026458 SN - 2169-9380 SN - 2169-9402 VL - 124 IS - 3 SP - 1904 EP - 1915 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Qin, Murong A1 - Hudson, Mary A1 - Li, Zhao A1 - Millan, Robyn A1 - Shen, Xiaochen A1 - Shprits, Yuri Y. A1 - Woodger, Leslie A1 - Jaynes, Allison A1 - Kletzing, Craig T1 - Investigating loss of relativistic electrons associated with EMIC Waves at low L values on 22 June 2015 JF - Journal of geophysical research : Space physics N2 - In this study, rapid loss of relativistic radiation belt electrons at low L* values (2.4-3.2) during a strong geomagnetic storm on 22 June 2015 is investigated along with five possible loss mechanisms. Both the particle and wave data are obtained from the Van Allen Probes. Duskside H+ band electromagnetic ion cyclotron (EMIC) waves were observed during a rapid decrease of relativistic electrons with energy above 5.2 MeV occurring outside the plasma sphere during extreme magnetopause compression. Lower He+ composition and enriched O+ composition are found compared to typical values assumed in other studies of cyclotron resonant scattering of relativistic electrons by EMIC waves. Quantitative analysis demonstrates that even with the existence of He+ band EMIC waves, it is the H+ band EMIC waves that are likely to cause the depletion at small pitch angles and strong gradients in pitch angle distributions of relativistic electrons with energy above 5.2 MeV at low L values for this event. Very low frequency wave activity at other magnetic local time can be favorable for the loss of relativistic electrons at higher pitch angles. An illustrative calculation that combines the nominal pitch angle scattering rate due to whistler mode chorus at high pitch angles with the H+ band EMIC wave loss rate at low pitch angles produces loss on time scale observed at L = 2.4-3.2. At high L values and lower energies, radial loss to the magnetopause is a viable explanation. Y1 - 2019 U6 - https://doi.org/10.1029/2018JA025726 SN - 2169-9380 SN - 2169-9402 VL - 124 IS - 6 SP - 4022 EP - 4036 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Smirnov, Artem G. A1 - Kronberg, Elena A. A1 - Latallerie, F. A1 - Daly, Patrick W. A1 - Aseev, Nikita A1 - Shprits, Yuri Y. A1 - Kellerman, Adam C. A1 - Kasahara, Satoshi A1 - Turner, Drew L. A1 - Taylor, M. G. G. T. T1 - Electron Intensity Measurements by the Cluster/RAPID/IES Instrument in Earth's Radiation Belts and Ring Current JF - Space Weather: The International Journal of Research and Applications N2 - Plain Language Summary Radiation belts of the Earth, which are the zones of charged energetic particles trapped by the geomagnetic field, comprise enormous and dynamic systems. While the inner radiation belt, composed mainly of high-energy protons, is relatively stable, the outer belt, filled with energetic electrons, is highly variable and depends substantially on solar activity. Hence, extended reliable observations and the improved models of the electron intensities in the outer belt depending on solar wind parameters are necessary for prediction of their dynamics. The Cluster mission has been measuring electron flux intensities in the radiation belts since its launch in 2000, thus providing a huge dataset that can be used for radiation belts analysis. Using 16 years of electron measurements by the Cluster mission corrected for background contamination, we derived a uniform linear-logarithmic dependence of electron fluxes in the outer belt on the solar wind dynamic pressure. Y1 - 2019 U6 - https://doi.org/10.1029/2018SW001989 SN - 1542-7390 VL - 17 IS - 4 SP - 553 EP - 566 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Dobynde, M. I. A1 - Effenberger, Frederic A1 - Kartashov, D. A. A1 - Shprits, Yuri Y. A1 - Shurshakov, V. A. T1 - Ray-tracing simulation of the radiation dose distribution on the surface of the spherical phantom of the MATROSHKA-R experiment onboard the ISS JF - Life sciences in space research N2 - Space radiation is one of the main concerns for human space flights. The prediction of the radiation dose for the actual spacecraft geometry is very important for the planning of long-duration missions. We present a numerical method for the fast calculation of the radiation dose rate during a space flight. We demonstrate its application for dose calculations during the first and the second sessions of the MATROSHKA-R space experiment with a spherical tissue-equivalent phantom. The main advantage of the method is the short simulation time, so it can be applied for urgent radiation dose calculations for low-Earth orbit space missions. The method uses depth-dose curve and shield-and-composition distribution functions to calculate a radiation dose at the point of interest. The spacecraft geometry is processed into a shield-and-composition distribution function using a ray-tracing method. Depth-dose curves are calculated using the GEANT4 Monte-Carlo code (version 10.00.P02) for a double-layer aluminum-water shielding. Aluminum-water shielding is a good approximation of the real geometry, as water is a good equivalent for biological tissues, and aluminum is the major material of spacecraft bodies. KW - Space radiation KW - Radiation protection KW - Radiation dose calculation KW - GEANT4 modeling KW - Radiation on the ISS KW - MATROSHKA-R Y1 - 2019 U6 - https://doi.org/10.1016/j.lssr.2019.04.001 SN - 2214-5524 SN - 2214-5532 VL - 21 SP - 65 EP - 72 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Zhu, Hui A1 - Chen, Lunjin A1 - Liu, Xu A1 - Shprits, Yuri Y. T1 - Modulation of locally generated equatorial noise by ULF wave JF - Journal of geophysical research : Space physics N2 - In this paper we report a rare and fortunate event of fast magnetosonic (MS, also called equatorial noise) waves modulated by compressional ultralow frequency (ULF) waves measured by Van Allen Probes. The characteristics of MS waves, ULF waves, proton distribution, and their potential correlations are analyzed. The results show that ULF waves can modulate the energetic ring proton distribution and in turn modulate the MS generation. Furthermore, the variation of MS intensities is attributed to not only ULF wave activities but also the variation of background parameters, for example, number density. The results confirm the opinion that MS waves are generated by proton ring distribution and propose a new modulation phenomenon. Y1 - 2019 U6 - https://doi.org/10.1029/2018JA026199 SN - 2169-9380 SN - 2169-9402 VL - 124 IS - 4 SP - 2779 EP - 2787 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Adolfs, Marjolijn A1 - Hoque, Mohammed Mainul A1 - Shprits, Yuri Y. T1 - Storm-time relative total electron content modelling using machine learning techniques JF - Remote sensing N2 - Accurately predicting total electron content (TEC) during geomagnetic storms is still a challenging task for ionospheric models. In this work, a neural-network (NN)-based model is proposed which predicts relative TEC with respect to the preceding 27-day median TEC, during storm time for the European region (with longitudes 30 degrees W-50 degrees E and latitudes 32.5 degrees N-70 degrees N). The 27-day median TEC (referred to as median TEC), latitude, longitude, universal time, storm time, solar radio flux index F10.7, global storm index SYM-H and geomagnetic activity index Hp30 are used as inputs and the output of the network is the relative TEC. The relative TEC can be converted to the actual TEC knowing the median TEC. The median TEC is calculated at each grid point over the European region considering data from the last 27 days before the storm using global ionosphere maps (GIMs) from international GNSS service (IGS) sources. A storm event is defined when the storm time disturbance index Dst drops below 50 nanotesla. The model was trained with storm-time relative TEC data from the time period of 1998 until 2019 (2015 is excluded) and contains 365 storms. Unseen storm data from 33 storm events during 2015 and 2020 were used to test the model. The UQRG GIMs were used because of their high temporal resolution (15 min) compared to other products from different analysis centers. The NN-based model predictions show the seasonal behavior of the storms including positive and negative storm phases during winter and summer, respectively, and show a mixture of both phases during equinoxes. The model's performance was also compared with the Neustrelitz TEC model (NTCM) and the NN-based quiet-time TEC model, both developed at the German Aerospace Agency (DLR). The storm model has a root mean squared error (RMSE) of 3.38 TEC units (TECU), which is an improvement by 1.87 TECU compared to the NTCM, where an RMSE of 5.25 TECU was found. This improvement corresponds to a performance increase by 35.6%. The storm-time model outperforms the quiet-time model by 1.34 TECU, which corresponds to a performance increase by 28.4% from 4.72 to 3.38 TECU. The quiet-time model was trained with Carrington averaged TEC and, therefore, is ideal to be used as an input instead of the GIM derived 27-day median. We found an improvement by 0.8 TECU which corresponds to a performance increase by 17% from 4.72 to 3.92 TECU for the storm-time model using the quiet-time-model predicted TEC as an input compared to solely using the quiet-time model. KW - ionosphere KW - relative total electron content KW - geomagnetic storms KW - neural KW - networks KW - NTCM KW - European storm-time model Y1 - 2022 U6 - https://doi.org/10.3390/rs14236155 SN - 2072-4292 VL - 14 IS - 23 PB - MDPI CY - Basel ER - TY - JOUR A1 - Smirnov, Artem A1 - Berrendorf, Max A1 - Shprits, Yuri Y. A1 - Kronberg, Elena A. A1 - Allison, Hayley J. A1 - Aseev, Nikita A1 - Zhelavskaya, Irina A1 - Morley, Steven K. A1 - Reeves, Geoffrey D. A1 - Carver, Matthew R. A1 - Effenberger, Frederic T1 - Medium energy electron flux in earth's outer radiation belt (MERLIN) BT - a Machine learning model JF - Space weather : the international journal of research and applications N2 - The radiation belts of the Earth, filled with energetic electrons, comprise complex and dynamic systems that pose a significant threat to satellite operation. While various models of electron flux both for low and relativistic energies have been developed, the behavior of medium energy (120-600 keV) electrons, especially in the MEO region, remains poorly quantified. At these energies, electrons are driven by both convective and diffusive transport, and their prediction usually requires sophisticated 4D modeling codes. In this paper, we present an alternative approach using the Light Gradient Boosting (LightGBM) machine learning algorithm. The Medium Energy electRon fLux In Earth's outer radiatioN belt (MERLIN) model takes as input the satellite position, a combination of geomagnetic indices and solar wind parameters including the time history of velocity, and does not use persistence. MERLIN is trained on >15 years of the GPS electron flux data and tested on more than 1.5 years of measurements. Tenfold cross validation yields that the model predicts the MEO radiation environment well, both in terms of dynamics and amplitudes o f flux. Evaluation on the test set shows high correlation between the predicted and observed electron flux (0.8) and low values of absolute error. The MERLIN model can have wide space weather applications, providing information for the scientific community in the form of radiation belts reconstructions, as well as industry for satellite mission design, nowcast of the MEO environment, and surface charging analysis. KW - machine learning KW - radiation belts KW - electron flux KW - empirical modeling KW - magnetosphere KW - electrons Y1 - 2020 U6 - https://doi.org/10.1029/2020SW002532 SN - 1542-7390 VL - 18 IS - 11 PB - American geophysical union, AGU CY - Washington ER - TY - JOUR A1 - Walker, Simon N. A1 - Boynton, Richard J. A1 - Shprits, Yuri Y. A1 - Balikhin, Michael A. A1 - Drozdov, Alexander T1 - Forecast of the energetic electron environment of the radiation belts JF - Space Weather: The International Journal of Research and Applications N2 - Different modeling methodologies possess different strengths and weakness. For instance, data based models may provide superior accuracy but have a limited spatial coverage while physics based models may provide lower accuracy but provide greater spatial coverage. This study investigates the coupling of a data based model of the electron fluxes at geostationary orbit (GEO) with a numerical model of the radiation belt region to improve the resulting forecasts/pastcasts of electron fluxes over the whole radiation belt region. In particular, two coupling methods are investigated. The first assumes an average value for L* for GEO, namely LGEO* L-GEO* = 6.2. The second uses a value of L* that varies with geomagnetic activity, quantified using the Kp index. As the terrestrial magnetic field responds to variations in geomagnetic activity, the value of L* will vary for a specific location. In this coupling method, the value of L* is calculated using the Kp driven Tsyganenko 89c magnetic field model for field line tracing. It is shown that this addition can result in changes in the initialization of the parameters at the Versatile Electron Radiation Belt model outer boundary. Model outputs are compared to Van Allen Probes MagEIS measurements of the electron fluxes in the inner magnetosphere for the March 2015 geomagnetic storm. It is found that the fixed LGEO* L-GEO* coupling method produces a more realistic forecast. KW - radiation belt forecasts KW - data based NARMAX modeling KW - verb simulations; KW - geostationary orbit KW - electron flux forecasts Y1 - 2022 U6 - https://doi.org/10.1029/2022SW003124 SN - 1542-7390 VL - 20 IS - 12 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Haas, Bernhard A1 - Shprits, Yuri Y. A1 - Allison, Hayley A1 - Wutzig, Michael A1 - Wang, Dedong T1 - Which parameter controls ring current electron dynamics JF - Frontiers in astronomy and space sciences N2 - Predicting the electron population of Earth's ring current during geomagnetic storms still remains a challenging task. In this work, we investigate the sensitivity of 10 keV ring current electrons to different driving processes, parameterised by the Kp index, during several moderate and intense storms. Results are validated against measurements from the Van Allen Probes satellites. Perturbing the Kp index allows us to identify the most dominant processes for moderate and intense storms respectively. We find that during moderate storms (Kp < 6) the drift velocities mostly control the behaviour of low energy electrons, while loss from wave-particle interactions is the most critical parameter for quantifying the evolution of intense storms (Kp > 6). Perturbations of the Kp index used to drive the boundary conditions at GEO and set the plasmapause location only show a minimal effect on simulation results over a limited L range. It is further shown that the flux at L & SIM; 3 is more sensitive to changes in the Kp index compared to higher L shells, making it a good proxy for validating the source-loss balance of a ring current model. KW - ring current KW - magnetosphere KW - electron lifetimes KW - electrons KW - van allen probes (RBSP) KW - ring current model KW - verb Y1 - 2022 U6 - https://doi.org/10.3389/fspas.2022.911002 SN - 2296-987X VL - 9 PB - Frontiers Media CY - Lausanne ER - TY - GEN A1 - Pick, Leonie A1 - Effenberger, Frederic A1 - Zhelavskaya, Irina A1 - Korte, Monika T1 - A Statistical Classifier for Historical Geomagnetic Storm Drivers Derived Solely From Ground-Based Magnetic Field Measurements T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Solar wind observations show that geomagnetic storms are mainly driven by interplanetary coronal mass ejections (ICMEs) and corotating or stream interaction regions (C/SIRs). We present a binary classifier that assigns one of these drivers to 7,546 storms between 1930 and 2015 using ground‐based geomagnetic field observations only. The input data consists of the long‐term stable Hourly Magnetospheric Currents index alongside the corresponding midlatitude geomagnetic observatory time series. This data set provides comprehensive information on the global storm time magnetic disturbance field, particularly its spatial variability, over eight solar cycles. For the first time, we use this information statistically with regard to an automated storm driver identification. Our supervised classification model significantly outperforms unskilled baseline models (78% accuracy with 26[19]% misidentified interplanetary coronal mass ejections [corotating or stream interaction regions]) and delivers plausible driver occurrences with regard to storm intensity and solar cycle phase. Our results can readily be used to advance related studies fundamental to space weather research, for example, studies connecting galactic cosmic ray modulation and geomagnetic disturbances. They are fully reproducible by means of the underlying open‐source software (Pick, 2019, http://doi.org/10.5880/GFZ.2.3.2019.003) T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 982 KW - geomagnetic observatory data KW - geomagnetic storm drivers KW - historical geomagnetic storms KW - supervised machine learning Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-474996 SN - 1866-8372 IS - 982 SP - 2000 EP - 2015 ER - TY - JOUR A1 - Kühn, Danilo A1 - Müller, Moritz A1 - Sorgenfrei, Nomi A1 - Giangrisostomi, Erika A1 - Jay, Raphael Martin A1 - Ovsyannikov, Ruslan A1 - Martensson, Nils A1 - Sanchez-Portal, Daniel A1 - Föhlisch, Alexander T1 - Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2 JF - Scientific reports N2 - For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in-and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves. Y1 - 2019 U6 - https://doi.org/10.1038/s41598-018-36637-0 SN - 2045-2322 VL - 9 IS - 488 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Woodfield, Emma E. A1 - Horne, Richard B. A1 - Glauert, S. A. A1 - Menietti, J. D. A1 - Shprits, Yuri Y. A1 - Kurth, William S. T1 - Formation of electron radiation belts at Saturn by Z-mode wave acceleration JF - Nature Communications N2 - At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Until now the mechanism of radial diffusion has been assumed but we show here that in-situ acceleration through wave particle interactions, which initial studies dismissed as ineffectual at Saturn, is in fact a vital part of the energetic particle dynamics there. We present evidence from numerical simulations based on Cassini spacecraft data that a particular plasma wave, known as Z-mode, accelerates electrons to MeV energies inside 4 RS (1 RS = 60,330 km) through a Doppler shifted cyclotron resonant interaction. Our results show that the Z-mode waves observed are not oblique as previously assumed and are much better accelerators than O-mode waves, resulting in an electron energy spectrum that closely approaches observed values without any transport effects included. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-07549-4 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Cao, Xing A1 - Ni, Binbin A1 - Summers, Danny A1 - Shprits, Yuri Y. A1 - Gu, Xudong A1 - Fu, Song A1 - Lou, Yuequn A1 - Zhang, Yang A1 - Ma, Xin A1 - Zhang, Wenxun A1 - Huang, He A1 - Yi, Juan T1 - Sensitivity of EMIC wave-driven scattering loss of ring current protons to wave normal angle distribution JF - Geophysical research letters N2 - Electromagnetic ion cyclotron waves have long been recognized to play a crucial role in the dynamic loss of ring current protons. While the field-aligned propagation approximation of electromagnetic ion cyclotron waves was widely used to quantify the scattering loss of ring current protons, in this study, we find that the wave normal distribution strongly affects the pitch angle scattering efficiency of protons. Increase of peak normal angle or angular width can considerably reduce the scattering rates of <= 10 keV protons. For >10 keV protons, the field-aligned propagation approximation results in a pronounced underestimate of the scattering of intermediate equatorial pitch angle protons and overestimates the scattering of high equatorial pitch angle protons by orders of magnitude. Our results suggest that the wave normal distribution of electromagnetic ion cyclotron waves plays an important role in the pitch angle evolution and scattering loss of ring current protons and should be incorporated in future global modeling of ring current dynamics. Y1 - 2019 U6 - https://doi.org/10.1029/2018GL081550 SN - 0094-8276 SN - 1944-8007 VL - 46 IS - 2 SP - 590 EP - 598 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Giangrisostomi, Erika A1 - Ovsyannikov, Ruslan A1 - Sorgenfrei, Nomi A1 - Zhang, Teng A1 - Lindblad, Andreas A1 - Sassa, Yasmine A1 - Cappel, Ute B. A1 - Leitner, Torsten A1 - Mitzner, Rolf A1 - Svensson, Svante A1 - Martensson, Nils A1 - Föhlisch, Alexander T1 - Low Dose Photoelectron Spectroscopy at BESSY II BT - electronic structure of matter in its native state JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - The implementation of a high-transmission, angular-resolved time-of-Right electron spectrometer with a 1.25 MHz pulse selector at the PM4 soft X-ray dipole beamline of the synchrotron BESSY II creates unique capabilities to inquire electronic structure via photoelectron spectroscopy with a minimum of radiation dose. Solid-state samples can be prepared and characterized with standard UHV techniques and rapidly transferred from various preparation chambers to a 4-axis temperature-controlled measurement stage. A synchronized MHz laser system enables excited-state characterization and dynamical studies starting from the picosecond timescale. This article introduces the principal characteristics of the PM4 beamline and LowDosePES end-station. Recent results from graphene, an organic hole transport material for solar cells and the transition metal dichalcogenide MoS2 are presented to demonstrate the instrument performances. Y1 - 2018 U6 - https://doi.org/10.1016/j.elspec.2017.05.011 SN - 0368-2048 SN - 1873-2526 VL - 224 SP - 68 EP - 78 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Pontius, Niko A1 - Beye, Martin A1 - Trabant, Christoph A1 - Mitzner, Rolf A1 - Sorgenfrei, Nomi A1 - Kachel, Torsten A1 - Woestmann, Michael A1 - Roling, Sebastian A1 - Zacharias, Helmut A1 - Ivanov, Rosen A1 - Treusch, Rolf A1 - Buchholz, Marcel A1 - Metcalf, Pete A1 - Schuessler-Langeheine, Christian A1 - Föhlisch, Alexander T1 - Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment JF - Structural dynamics N2 - We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 +/- 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices. (C) 2018 Author(s). Y1 - 2018 U6 - https://doi.org/10.1063/1.5042847 SN - 2329-7778 VL - 5 IS - 5 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Wang, Dedong A1 - Shprits, Yuri Y. A1 - Zhelayskaya, Irina S. A1 - Agapitov, Oleksiy A1 - Drozdov, Alexander A1 - Aseev, Nikita T1 - Analytical chorus wave model derived from van Allen Probe Observations JF - Journal of geophysical research : Space physics N2 - Chorus waves play an important role in the dynamic evolution of energetic electrons in the Earth's radiation belts and ring current. Using more than 5 years of Van Allen Probe data, we developed a new analytical model for upper‐band chorus (UBC; 0.5fce < f < fce) and lower‐band chorus (LBC; 0.05fce < f < 0.5fce) waves, where fce is the equatorial electron gyrofrequency. By applying polynomial fits to chorus wave root mean square amplitudes, we developed regression models for LBC and UBC as a function of geomagnetic activity (Kp), L, magnetic latitude (λ), and magnetic local time (MLT). Dependence on Kp is separated from the dependence on λ, L, and MLT as Kp‐scaling law to simplify the calculation of diffusion coefficients and inclusion into particle tracing codes. Frequency models for UBC and LBC are also developed, which depends on MLT and magnetic latitude. This empirical model is valid in all MLTs, magnetic latitude up to 20°, Kp ≤ 6, L‐shell range from 3.5 to 6 for LBC and from 4 to 6 for UBC. The dependence of root mean square amplitudes on L are different for different bands, which implies different energy sources for different wave bands. This analytical chorus wave model is convenient for inclusion in quasi‐linear diffusion calculations of electron scattering rates and particle simulations in the inner magnetosphere, especially for the newly developed four‐dimensional codes, which require significantly improved wave parameterizations. KW - chorus waves KW - radiation belt electrons KW - ring current electrons KW - analytical model KW - wave-particle interactions KW - diffusion coefficients Y1 - 2019 U6 - https://doi.org/10.1029/2018JA026183 SN - 2169-9380 SN - 2169-9402 VL - 124 IS - 2 SP - 1063 EP - 1084 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Castillo, Angelica M. A1 - Shprits, Yuri Y. A1 - Ganushkina, Natalia A1 - Drozdov, Alexander A1 - Aseev, Nikita A1 - Wang, Dedong A1 - Dubyagin, Stepan T1 - Simulations of the inner magnetospheric energetic electrons using the IMPTAM-VERB coupled model JF - Journal of Atmospheric and Solar-Terrestrial Physics N2 - In this study, we present initial results of the coupling between the Inner Magnetospheric Particle Transport and Acceleration Model (IMPTAM) and the Versatile Electron Radiation Belt (VERB-3D) code. IMPTAM traces electrons of 10-100 keV energies from the plasma sheet (L = 9 Re) to inner L-shell regions. The flux evolution modeled by IMPTAM is used at the low energy and outer L* computational boundaries of the VERB code (assuming a dipole approximation) to perform radiation belt simulations of energetic electrons. The model was tested on the March 17th, 2013 storm, for a six-day period. Four different simulations were performed and their results compared to satellites observations from Van Allen probes and GOES. The coupled IMPTAM-VERB model reproduces evolution and storm-time features of electron fluxes throughout the studied storm in agreement with the satellite data (within similar to 0.5 orders of magnitude). Including dynamics of the low energy population at L* = 6.6 increases fluxes closer to the heart of the belt and has a strong impact in the VERB simulations at all energies. However, inclusion of magnetopause losses leads to drastic flux decreases even below L* = 3. The dynamics of low energy electrons (max. 10s of keV) do not affect electron fluxes at energies >= 900 keV. Since the IMPTAM-VERB coupled model is only driven by solar wind parameters and the Dst and Kp indexes, it is suitable as a forecasting tool. In this study, we demonstrate that the estimation of electron dynamics with satellite-data-independent models is possible and very accurate. KW - Electron populations KW - Radiation belts KW - IMPTAM KW - VERB Y1 - 2019 U6 - https://doi.org/10.1016/j.jastp.2019.05.014 SN - 1364-6826 SN - 1879-1824 VL - 191 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Woodfield, Emma E. A1 - Glauert, Saraha A. A1 - Menietti, J. Douglas A1 - Averkamp, Terrance F. A1 - Horne, Richard B. A1 - Shprits, Yuri Y. T1 - Rapid Electron Acceleration in Low‐Density Regions of Saturn's Radiation Belt by Whistler Mode Chorus Waves JF - Geophysical research letters N2 - Electron acceleration at Saturn due to whistler mode chorus waves has previously been assumed to be ineffective; new data closer to the planet show it can be very rapid (factor of 104 flux increase at 1 MeV in 10 days compared to factor of 2). A full survey of chorus waves at Saturn is combined with an improved plasma density model to show that where the plasma frequency falls below the gyrofrequency additional strong resonances are observed favoring electron acceleration. This results in strong chorus acceleration between approximately 2.5 R-S and 5.5 R-S outside which adiabatic transport may dominate. Strong pitch angle dependence results in butterfly pitch angle distributions that flatten over a few days at 100s keV, tens of days at MeV energies which may explain observations of butterfly distributions of MeV electrons near L = 3. Including cross terms in the simulations increases the tendency toward butterfly distributions. Plain Language Summary Radiation belts are hazardous regions found around several of the planets in our Solar System. They consist of very hot, electrically charged particles trapped in the magnetic field of the planet. At Saturn the most important way to heat these particles has for many years been thought to involve the particles drifting closer toward the planet. This paper adds to the emerging idea at Saturn that a different way to heat the particles is also possible where the heating is done by waves, in a similar way to what we find at the Earth. We use recent information from the Cassini spacecraft on the number and location of particles and also of the waves strength and location combined with computer simulations to show that a particular wave called chorus is excellent at heating the particles where the surrounding number of cold particles is low. Y1 - 2019 U6 - https://doi.org/10.1029/2019GL083071 SN - 0094-8276 SN - 1944-8007 VL - 46 IS - 13 SP - 7191 EP - 7198 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Denton, Richard E. A1 - Ofman, L. A1 - Shprits, Yuri Y. A1 - Bortnik, J. A1 - Millan, R. M. A1 - Rodger, C. J. A1 - da Silva, C. L. A1 - Rogers, B. N. A1 - Hudson, M. K. A1 - Liu, K. A1 - Min, K. A1 - Glocer, A. A1 - Komar, C. T1 - Pitch Angle Scattering of Sub-MeV Relativistic Electrons by Electromagnetic Ion Cyclotron Waves JF - Journal of geophysical research : Space physics N2 - Electromagnetic ion cyclotron (EMIC) waves have long been considered to be a significant loss mechanism for relativistic electrons. This has most often been attributed to resonant interactions with the highest amplitude waves. But recent observations have suggested that the dominant energy of electrons precipitated to the atmosphere may often be relatively low, less than 1 MeV, whereas the minimum resonant energy of the highest amplitude waves is often greater than 2 MeV. Here we use relativistic electron test particle simulations in the wavefields of a hybrid code simulation of EMIC waves in dipole geometry in order to show that significant pitch angle scattering can occur due to interaction with low-amplitude short-wavelength EMIC waves. In the case we examined, these waves are in the H band (at frequencies above the He+ gyrofrequency), even though the highest amplitude waves were in the He band frequency range (below the He+ gyrofrequency). We also present wave power distributions for 29 EMIC simulations in straight magnetic field line geometry that show that the high wave number portion of the spectrum is in every case mostly due to the H band waves. Though He band waves are often associated with relativistic electron precipitation, it is possible that the He band waves do not directly scatter the sub-megaelectron volts (sub-MeV) electrons, but that the presence of He band waves is associated with high plasma density which lowers the minimum resonant energy so that these electrons can more easily resonate with the H band waves. KW - electromagnetic ion cyclotron waves KW - EMIC KW - relativistic electron precipitation KW - pitch angle scattering KW - wave particle interaction KW - radiation belts Y1 - 2019 U6 - https://doi.org/10.1029/2018JA026384 SN - 2169-9402 VL - 124 IS - 7 SP - 5610 EP - 5626 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Kühn, Danilo A1 - Giangrisostomi, Erika A1 - Jay, Raphael Martin A1 - Sorgenfrei, Nomi A1 - Föhlisch, Alexander T1 - The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission JF - New journal of physics : the open-access journal for physics N2 - Pump-probe photoelectron spectroscopy (PES) is a versatile tool to investigate the dynamics of transient states of excited matter. Vacuum space-charge effects can mask these dynamics and complicate the interpretation of electron spectra. Here we report on space-charge effects in Au 4f photoemission from a polycrystalline gold surface, excited with moderately intense 90 ps (FWHM) soft x-ray probe pulses, under the influence of the Coulomb forces exerted by a pump electron cloud, which was produced by intense 40 fs laser pulses. The experimentally observed kinetic energy shift and spectral broadening of the Au 4f lines, measured with highly-efficient time-of-flight spectroscopy, are in good agreement with simulations utilizing a mean-field model of the electrostatic pump electron potential. This confirms that the line broadening is predominantly caused by variations in the take-off time of the probe electrons without appreciable influence of local scattering events. Our findings might be of general interest for pump-probe PES with picosecond-pulse-length sources. KW - space-charge effects KW - mean-field model KW - x-ray photoemission KW - electron spectroscopy KW - pump-probe KW - ARTOF Y1 - 2019 U6 - https://doi.org/10.1088/1367-2630/ab2f5c SN - 1367-2630 VL - 21 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Shprits, Yuri Y. A1 - Vasile, Ruggero A1 - Zhelayskaya, Irina S. T1 - Nowcasting and Predicting the Kp Index Using Historical Values and Real-Time Observations JF - Space Weather: The International Journal of Research and Applications N2 - Current algorithms for the real-time prediction of the Kp index use a combination of models empirically driven by solar wind measurements at the L1 Lagrange point and historical values of the index. In this study, we explore the limitations of this approach, examining the forecast for short and long lead times using measurements at L1 and Kp time series as input to artificial neural networks. We explore the relative efficiency of the solar wind-based predictions, predictions based on recurrence, and predictions based on persistence. Our modeling results show that for short-term forecasts of approximately half a day, the addition of the historical values of Kp to the measured solar wind values provides a barely noticeable improvement. For a longer-term forecast of more than 2 days, predictions can be made using recurrence only, while solar wind measurements provide very little improvement for a forecast with long horizon times. We also examine predictions for disturbed and quiet geomagnetic activity conditions. Our results show that the paucity of historical measurements of the solar wind for high Kp results in a lower accuracy of predictions during disturbed conditions. Rebalancing of input data can help tailor the predictions for more disturbed conditions. KW - Kp index KW - geomagnetic activity KW - empirical prediction KW - solar wind KW - forecast KW - AI Y1 - 2019 U6 - https://doi.org/10.1029/2018SW002141 SN - 1542-7390 VL - 17 IS - 8 SP - 1219 EP - 1229 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Drozdov, Alexander A1 - Aseev, Nikita A1 - Effenberger, Frederic A1 - Turner, Drew L. A1 - Saikin, Anthony A1 - Shprits, Yuri Y. T1 - Storm Time Depletions of Multi-MeV Radiation Belt Electrons Observed at Different Pitch Angles JF - Journal of geophysical research : Space physics N2 - During geomagnetic storms, the rapid depletion of the high-energy (several MeV) outer radiation belt electrons is the result of loss to the interplanetary medium through the magnetopause, outward radial diffusion, and loss to the atmosphere due to wave-particle interactions. We have performed a statistical study of 110 storms using pitch angle resolved electron flux measurements from the Van Allen Probes mission and found that inside of the radiation belt (L* = 3 - 5) the number of storms that result in depletion of electrons with equatorial pitch angle alpha(eq) = 30 degrees is higher than number of storms that result in depletion of electrons with equatorial pitch angle alpha(eq) = 75 degrees. We conclude that this result is consistent with electron scattering by whistler and electromagnetic ion cyclotron waves. At the outer edge of the radiation belt (L* >= 5.2) the number of storms that result in depletion is also large (similar to 40-50%), emphasizing the significance of the magnetopause shadowing effect and outward radial transport. Y1 - 2019 U6 - https://doi.org/10.1029/2019JA027332 SN - 2169-9380 SN - 2169-9402 VL - 124 IS - 11 SP - 8943 EP - 8953 PB - American Geophysical Union CY - Washington ER - TY - GEN A1 - Shprits, Yuri Y. A1 - Horne, Richard B. A1 - Kellerman, Adam C. A1 - Drozdov, Alexander T1 - The dynamics of Van Allen belts revisited T2 - Nature physics N2 - In an effort to explain the formation of a narrow third radiation belt at ultra-relativistic energies detected during a solar storm in September 20121, Mann et al.2 present simulations from which they conclude it arises from a process of outward radial diffusion alone, without the need for additional loss processes from higher frequency waves. The comparison of observations with the model in Figs 2 and 3 of their Article clearly shows that even with strong radial diffusion rates, the model predicts a third belt near L* = 3 that is twice as wide as observed and approximately an order of magnitude more intense. We therefore disagree with their interpretation that “the agreement between the absolute fluxes from the model and those observed by REPT [the Relativistic Electron Proton Telescope] shown on Figs 2 and 3 is excellent.” Previous studies3 have shown that outward radial diffusion plays a very important role in the dynamics of the outer belt and is capable of explaining rapid reductions in the electron flux. It has also been shown that it can produce remnant belts (Fig. 2 of a long-term simulation study4). However, radial diffusion alone cannot explain the formation of the narrow third belt at multi-MeV during September 2012. An additional loss mechanism is required. Higher radial diffusion rates cannot improve the comparison of model presented by Mann et al. with observations. A further increase in the radial diffusion rates (reported in Fig. 4 of the Supplementary Information of ref. 2) results in the overestimation of the outer belt fluxes by up to three orders of magnitude at energy of 3.4 MeV. Observations at 2 MeV, where belts show only a two-zone structure, were not presented by Mann et al. Moreover, simulations of electrons with energies below 2 MeV with the same diffusion rates and boundary conditions used by the authors would probably produce very strong depletions down to L = 3–3.5, where L is radial distance from the centre of the Earth to the given field line in the equatorial plane. Observations do not show a non-adiabatic loss below L ∼ 4.5 for 2 MeV. Such different dynamics between 2 MeV and above 4 MeV at around L = 3.5 are another indication that particles are scattered by electromagnetic ion cyclotron (EMIC) waves that affect only energies above a certain threshold. Observations of the phase space density (PSD) provide additional evidence for the local loss of electrons. Around L* = 3.5–4 PSD shows significant decrease by an order of magnitude starting in the afternoon of 3 September (Fig. 1a), while PSD above L* = 4 is increasing. The minimum in PSD between L* = 3.5–4 continues to decrease until 4 September. This evolution demonstrates that the loss is not produced by outward diffusion. Radial diffusion cannot produce deepening minima, as it works to smooth gradients. Just as growing peaks in PSD show the presence of localized acceleration5, deepening minima show the presence of localized loss. Figure 1: Time evolution of radiation profiles in electron PSD at relativistic and ultra-relativistic energies. figure 1 a, Similar to Supplementary Fig. 3 of ref. 2, but using TS07D model10 and for μ = 2,500 MeV G−1, K = 0.05 RE G0.5 (where RE is the radius of the Earth). b, Similar to Supplementary Fig. 3 of ref. 2, but using TS07D model and for μ = 700 MeV G−1, corresponding to MeV energies in the heart of the belt. Minimum in PSD in the heart of the multi-MeV electron radiation belt between 3.5 and 4 RE deepening between the afternoon of 3 September and 5 September clearly show that the narrow remnant belt at multi-MeV below 3.5 RE is produced by the local loss. Full size image The minimum in the outer boundary is reached on the evening of 2 September. After that, the outer boundary moves up, while the minimum decreases by approximately an order of magnitude, clearly showing that this main decrease cannot be explained by outward diffusion, and requires additional loss processes. The analysis of profiles of PSD is a standard tool used, for example, in the study about electron acceleration5 and routinely used by the entire Van Allen Probes team. In the Supplementary Information, we show that this analysis is validated by using different magnetic field models. The Supplementary Information also shows that measurements are above background noise. Deepening minima at multi-MeV during the times when the boundary flux increases are clearly seen in Fig. 1a. They show that there must be localized loss, as radial diffusion cannot produce a minimum that becomes lower with time. At lower energies of 1–2 MeV, which corresponds to lower values of the first adiabatic invariant μ (Fig. 1b), the profiles are monotonic between L* = 3–3.5, consistent with the absence of scattering by EMIC waves that affect only electrons above a certain energy threshold6,7,8,9. In summary, the results of the modelling and observations presented by Mann et al. do not lend support to the claim of explaining the dynamics of the ultra-relativistic third Van Allen radiation belt in terms of an outward radial diffusion process alone. While the outward radial diffusion driven by the loss to the magnetopause2 is certainly operating during this storm, there is compelling observational and modelling2,6 evidence that shows that very efficient localized electron loss operates during this storm at multi-MeV energies, consistent with localized loss produced by EMIC waves. Y1 - 2018 U6 - https://doi.org/10.1038/nphys4350 SN - 1745-2473 SN - 1745-2481 VL - 14 IS - 2 SP - 102 EP - 103 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Shprits, Yuri Y. A1 - Angelopoulos, V. A1 - Russell, C. T. A1 - Strangeway, R. J. A1 - Runov, A. A1 - Turner, D. A1 - Caron, R. A1 - Cruce, P. A1 - Leneman, D. A1 - Michaelis, I. A1 - Petrov, V. A1 - Panasyuk, M. A1 - Yashin, I. A1 - Drozdov, Alexander A1 - Russell, C. L. A1 - Kalegaev, V. A1 - Nazarkov, I. A1 - Clemmons, J. H. T1 - Scientific Objectives of Electron Losses and Fields INvestigation Onboard Lomonosov Satellite JF - Space science reviews N2 - The objective of the Electron Losses and Fields INvestigation on board the Lomonosov satellite ( ELFIN-L) project is to determine the energy spectrum of precipitating energetic electrons and ions and, together with other polar-orbiting and equatorial missions, to better understand the mechanisms responsible for scattering these particles into the atmosphere. This mission will provide detailed measurements of the radiation environment at low altitudes. The 400-500 km sun-synchronous orbit of Lomonosov is ideal for observing electrons and ions precipitating into the atmosphere. This mission provides a unique opportunity to test the instruments. Similar suite of instruments will be flown in the future NSF-and NASA-supported spinning CubeSat ELFIN satellites which will augment current measurements by providing detailed information on pitch-angle distributions of precipitating and trapped particles. KW - Magnetospheric physics KW - Observations KW - Particles precipitating KW - Particles trapped KW - Radiation belts Y1 - 2017 U6 - https://doi.org/10.1007/s11214-017-0455-4 SN - 0038-6308 SN - 1572-9672 VL - 214 IS - 1 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Kim, Kyung-Chan A1 - Shprits, Yuri Y. T1 - Survey of the Favorable Conditions for Magnetosonic Wave Excitation JF - Journal of geophysical research : Space physics N2 - The ratio of the proton ring velocity (VR) to the local Alfven speed (VA), in addition to proton ring distributions, plays a key factor in the excitation of magnetosonic waves at frequencies between the proton cyclotron frequency fcp and the lower hybrid resonance frequency fLHR in the Earth's magnetosphere. Here we investigate whether there is a statistically significant relationship between occurrences of proton rings and magnetosonic waves both outside and inside the plasmapause using particle and wave data from Van Allen Probe-A during the time period of October 2012 to December 2015. We also perform a statistical survey of the ratio of the ring energy (ER, corresponding to VR) to the Alfven energy (EA, corresponding to VA) to determine the favorable conditions under which magnetosonic waves in each of two frequency bands (fcp < f ≤ 0.5 fLHR and 0.5 fLHR < f < fLHR) can be excited. The results show that the magnetosonic waves in both frequency bands occur around the postnoon (12–18 magnetic local time, MLT) sector outside the plasmapause when ER is comparable to or lower than EA, and those in lower-frequency bands (fcp < f ≤ 0.5 fLHR) occur around the postnoon sector inside the plasmapause when ER/EA > ~9. However, there is one discrepancy between occurrences of proton rings and magnetosonic waves in low-frequency bands around the prenoon sector (6–12 MLT) outside the plasmapause, which suggests either that the waves may have propagated during active time from the postnoon sector after being excited during quiet time, or they may have locally excited in the prenoon sector during active time. Y1 - 2018 U6 - https://doi.org/10.1002/2017JA024865 SN - 2169-9380 SN - 2169-9402 VL - 123 IS - 1 SP - 400 EP - 413 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Shprits, Yuri Y. A1 - Kellerman, Adam C . A1 - Aseev, Nikita A1 - Drozdov, Alexander A1 - Michaelis, Ingo T1 - Multi-MeV electron loss in the heart of the radiation belts JF - Geophysical research letters N2 - Significant progress has been made in recent years in understanding acceleration mechanisms in the Earth's radiation belts. In particular, a number of studies demonstrated the importance of the local acceleration by analyzing the radial profiles of phase space density (PSD) and observing building up peaks in PSD. In this study, we focus on understanding of the local loss using very similar tools. The profiles of PSD for various values of the first adiabatic invariants during the previously studied 17 January 2013 storm are presented and discussed. The profiles of PSD show clear deepening minimums consistent with the scattering by electromagnetic ion cyclotron waves. Long-term evolution shows that local minimums in PSD can persist for relatively long times. During considered interval of time the deepening minimums were observed around L* = 4 during 17 January 2013 storm and around L* = 3.5 during 1 March 2013 storm. This study shows a new method that can help identify the location, magnitude, and time of the local loss and will help quantify local loss in the future. This study also provides additional clear and definitive evidence that local loss plays a major role for the dynamics of the multi-MeV electrons. Y1 - 2017 U6 - https://doi.org/10.1002/2016GL072258 SN - 0094-8276 SN - 1944-8007 VL - 44 IS - 3 SP - 1204 EP - 1209 PB - American Geophysical Union CY - Washington ER - TY - GEN A1 - Shprits, Yuri Y. A1 - Zhelavskaya, Irina A1 - Green, Janet C. A1 - Pulkkinen, Antti A. A1 - Horne, Richard B. A1 - Pitchford, David A1 - Glover, Alexi T1 - Discussions on Stakeholder Requirements for Space Weather-Related Models T2 - Space Weather: The International Journal of Research and Applications N2 - Participants of the 2017 European Space Weather Week in Ostend, Belgium, discussed the stakeholder requirements for space weather-related models. It was emphasized that stakeholders show an increased interest in space weather-related models. Participants of the meeting discussed particular prediction indicators that can provide first-order estimates of the impact of space weather on engineering systems. KW - 7924 KW - 7934 KW - 7959 Y1 - 2018 U6 - https://doi.org/10.1002/2018SW001864 SN - 1542-7390 VL - 16 IS - 4 SP - 341 EP - 342 PB - American Geophysical Union CY - Washington ER - TY - THES A1 - Jaiser, Frank T1 - Ladungsträger- und Anregungsdynamik in halbleitenden Polymerschichten mit eingemischten Emittern und Ladungsträgerfallen T1 - Charge and excitation dynamics in semiconducting polymer layers doped with emitters and charge carrier traps N2 - In Leuchtdioden wird Licht durch die Rekombination von injizierten Ladungsträgern erzeugt. Das kann einerseits in anorganischen Materialien geschehen. In diesem Fall ist es notwendig, hochgeordnete Kristallstrukturen herzustellen, die die Eigenschaften der Leuchtdioden bestimmen. Ein anderer Ansatz ist die Verwendung von organischen Molekülen und Polymeren. Auf Grund der Vielseitigkeit der organischen Chemie können die Eigenschaften der verwendeten halbleitenden Polymere schon während der Synthese beeinflusst werden. Außerdem weisen auch diese Polymere die bekannte mechanische Flexibilität auf. Die Herstellung von flexiblen, großflächigen Beleuchtungsquellen und Anzeigelementen ist so möglich. Die erste Leuchtdiode mit einem halbleitenden Polymer als Emitter wurde 1990 hergestellt. Seither hat das Forschungsgebiet eine rasante Entwicklung genommen. Auch erste kommerzielle Produkte sind erhältlich. Im Zuge dieser Entwicklung wurde deutlich, dass die Eigenschaften von polymeren Leuchtdioden – beispielsweise Farbe und Effizienz – durch die Verwendung mehrerer Komponenten in der aktiven Schicht deutlich verbessert werden können. Gleichzeitig ergeben sich neue Herausforderungen durch die Wechselwirkungen der verschiedenen Filmbestandteile. Während die Komponenten oft entweder zur Verbesserung des Ladungstransportes oder zur Beeinflussung der Emission zugegeben werden, muss darauf geachtet werden, dass die anderen Prozesse nicht negativ beeinflusst werden. In dieser Arbeit werden einige dieser Wechselwirkungen untersucht und mit einfachen physikalischen Modellen erklärt. So werden zunächst blau emittierende Leuchtdioden auf der Basis von Polyfluoren untersucht. Dieses Material ist zwar ein sehr effizienter blauer Emitter, jedoch ist es anfällig für chemische Defekte, diese sich nicht vollständig verhindern lassen. Die Defekte bilden Fallenzustände für Elektronen, ihr Einfluss lässt sich durch die Zugabe von Lochfallen unterdrücken. Der zugrunde liegende Prozess, die Beeinflussung der Ladungsträgerbalance, wird erklärt. Im Folgenden werden Mischsystemen mit dendronisierten Emittern, die gleichzeitig eine Falle für Elektronen bilden, untersucht. Hier wird die unterschiedliche Wirkung der isolierenden Hülle auf die Ladungs- und Energieübertragung zwischen Matrix und Farbstoffkern der Dendrimere untersucht. In Mischsystemen haben die Natur der angeregten Zustände sowie die Art und Weise des Ladungsträgertransportes einen großen Einfluss auf diese Transferprozesse. Außerden hat auch hier die Ladungsträgerbalance Auswirkungen auf die Emission. Um den Ladungsträgereinfang in Fallenzuständen zu charakterisieren, wird eine Methode auf Grundlage der Messung des zeitaufgelösten Photostroms in organischen Mischfilmen weiterentwickelt. Die erzielten Ergebnisse zeigen, dass die Übertragung der für geordnete Systeme entwickelten Modelle des Ladungsträgertransportes nicht ohne weiteres auf Polymersysteme mit hoher Unordnung übertragen werden können. Abschließend werden zeitaufgelöste Messungen der Phosphoreszenz in entsprechenden Mischungen aus Polymeren und organometallischen Verbindungen vorgestellt. Auch diese Systeme enthalten üblicherweise weitere Komponenten, die den Ladungstransport verbessern. In diesen Filmen kann es zu einer Übertragung der Tripletts vom Emitter auf die weiteren Filmbestandteile kommen. Bei Kenntnis der in Frage kommenden Wechselwirkungen können die unerwünschten Prozesse vermieden werden. N2 - Light-emitting diodes generate light from the recombination of injected charge carriers. This can be obtained in inorganic materials. Here, it is necessary to produce highly ordered crystalline structures that determine the properties of the device. Another possibility is the utilization of organic molecules and polymers. Based on the versatile organic chemistry, it is possible to tune the properties of the semiconducting polymers already during synthesis. In addition, semiconducting polymers are mechanically flexible. Thus, it is possible to construct flexible, large-area light sources and displays. The first light-emitting diode using a polymer emitter was presented in 1990. Since then, this field of research has grown rapidly up to the point where first products are commercially available. It has become clear that the properties of polymer light-emitting diodes such as color and efficiency can be improved by incorporating multiple components inside the active layer. At the same time, this gives rise to new interactions between these components. While components are often added either to improve the charge transport or to change the emission, it has to made sure that other processes are not influenced in a negative manner. This work investigates some of these interactions and describes them with simple physical models. First, blue light-emitting diodes based on polyfluorene are analyzed. This polymer is an efficient emitter, but it is susceptible to the formation of chemical defects that can not be suppressed completely. These defects form electron traps, but their effect can be compensated by the addition of hole traps. The underlying process, namely the changed charge carrier balance, is explained. In the following, blend systems with dendronized emitters that form electron traps are investigated. The different influence of the insulating shell on the charge and energy transfer between polymer host and the emissive core of the dendrimers is examined. In the blend, the nature of the excited states as well as the method of the charge transport through the layer are of great importance to the transfer. Again, the charge carrier balance influences the emission. To characterize the trapping of charges in trap states, a method based on the measurement of transient photocurrents is enhanced. The results show that models developed for ordered systems can not simply be transferred to polymer systems with a high degree of disorder. Finally, time-resolved measurements of the phosphorescence decay in blends of polymers with organo-metallic compounds are shown. Usually, these systems contain more components that facilitate charge transport. Thus, triplets may be transferred from the phosphorescent dye other components of the film. Knowing the underlying interactions, unwanted processes can be suppressed. KW - OLED KW - Konjugierte Polymere KW - Mehrstoffsystem KW - OLED KW - conjugated polymers KW - multicomponent system Y1 - 2006 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-9484 ER - TY - JOUR A1 - Li, Tian-yi A1 - Benduhn, Johannes A1 - Li, Yue A1 - Jaiser, Frank A1 - Spoltore, Donato A1 - Zeika, Olaf A1 - Ma, Zaifei A1 - Neher, Dieter A1 - Vandewal, Koen A1 - Leo, Karl T1 - Boron dipyrromethene (BODIPY) with meso-perfluorinated alkyl substituents as near infrared donors in organic solar cells JF - Journal of materials chemistry : A, Materials for energy and sustainability N2 - Three furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at similar to 800 nm with absorption coefficient of over 280 000 cm(-1). Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at similar to 790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm(-2). Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor-acceptor interface were comparable in the range of 0.14-0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate V-oc of 0.76 V. Y1 - 2018 U6 - https://doi.org/10.1039/c8ta06261g SN - 2050-7488 SN - 2050-7496 VL - 6 IS - 38 SP - 18583 EP - 18591 PB - Royal Society of Chemistry CY - Cambridge ER - TY - THES A1 - Zeuschner, Steffen Peer T1 - Magnetoacoustics observed with ultrafast x-ray diffraction N2 - In the present thesis I investigate the lattice dynamics of thin film hetero structures of magnetically ordered materials upon femtosecond laser excitation as a probing and manipulation scheme for the spin system. The quantitative assessment of laser induced thermal dynamics as well as generated picosecond acoustic pulses and their respective impact on the magnetization dynamics of thin films is a challenging endeavor. All the more, the development and implementation of effective experimental tools and comprehensive models are paramount to propel future academic and technological progress. In all experiments in the scope of this cumulative dissertation, I examine the crystal lattice of nanoscale thin films upon the excitation with femtosecond laser pulses. The relative change of the lattice constant due to thermal expansion or picosecond strain pulses is directly monitored by an ultrafast X-ray diffraction (UXRD) setup with a femtosecond laser-driven plasma X-ray source (PXS). Phonons and spins alike exert stress on the lattice, which responds according to the elastic properties of the material, rendering the lattice a versatile sensor for all sorts of ultrafast interactions. On the one hand, I investigate materials with strong magneto-elastic properties; The highly magnetostrictive rare-earth compound TbFe2, elemental Dysprosium or the technological relevant Invar material FePt. On the other hand I conduct a comprehensive study on the lattice dynamics of Bi1Y2Fe5O12 (Bi:YIG), which exhibits high-frequency coherent spin dynamics upon femtosecond laser excitation according to the literature. Higher order standing spinwaves (SSWs) are triggered by coherent and incoherent motion of atoms, in other words phonons, which I quantified with UXRD. We are able to unite the experimental observations of the lattice and magnetization dynamics qualitatively and quantitatively. This is done with a combination of multi-temperature, elastic, magneto-elastic, anisotropy and micro-magnetic modeling. The collective data from UXRD, to probe the lattice, and time-resolved magneto-optical Kerr effect (tr-MOKE) measurements, to monitor the magnetization, were previously collected at different experimental setups. To improve the precision of the quantitative assessment of lattice and magnetization dynamics alike, our group implemented a combination of UXRD and tr-MOKE in a singular experimental setup, which is to my knowledge, the first of its kind. I helped with the conception and commissioning of this novel experimental station, which allows the simultaneous observation of lattice and magnetization dynamics on an ultrafast timescale under identical excitation conditions. Furthermore, I developed a new X-ray diffraction measurement routine which significantly reduces the measurement time of UXRD experiments by up to an order of magnitude. It is called reciprocal space slicing (RSS) and utilizes an area detector to monitor the angular motion of X-ray diffraction peaks, which is associated with lattice constant changes, without a time-consuming scan of the diffraction angles with the goniometer. RSS is particularly useful for ultrafast diffraction experiments, since measurement time at large scale facilities like synchrotrons and free electron lasers is a scarce and expensive resource. However, RSS is not limited to ultrafast experiments and can even be extended to other diffraction techniques with neutrons or electrons. N2 - In der vorliegenden Arbeit untersuche ich die Gitterdynamik von magnetisch geordneten und dünnen Filmen, deren Spinsystem mit Femtosekunden-Laserpulsen angeregt und untersucht wird. Die Quantifizierung der laserinduzierten thermischen Dynamik, der erzeugten Pikosekunden-Schallpulse sowie deren jeweiliger Einfluss auf die Magnetisierungsdynamik ist ein schwieriges Unterfangen. Umso mehr ist die Entwicklung und Anwendung von effizienten experimentellen Konzepten und umfangreichen Modellen grundlegend für das Antreiben des zukünftigen wissenschaftlichen und technologischen Fortschritt. In jedem Experiment dieser kummulativen Dissertation untersuche ich das Kristallgitter von Nanometer dünnen Filmen nach der Anregung mit Femtosekunden-Laserpulsen. Die relative Änderung der Gitterkonstante, hervorgerufen durch thermische Ausdehnung oder Pikosekunden-Schallpulse, wird dabei direkt mittels ultraschneller Röntgenbeugung (UXRD) gemessen. Der Aufbau nutzt zur Bereitstellung von ultrakurzen Röntgenpulsen eine lasergetriebene Plasma-Röntgenquelle (PXS). Phononen und Spins üben gleichermaßen einen Druck auf das Gitter aus, welches entsprechend der elastsischen Eigenschaften des Materials reagiert, was das Gitter zu einem vielseitigen Sensor für ultraschenlle Wechselwirkungen macht. Zum einen untersuche ich Materialien mit starken magnetoelastischen Eigentschaften: die stark magnetostriktive Seltenen-Erden-Verbindung TbFe2, elementares Dysprosium oder das technologisch relavante Invar-Material FePt. Zum anderen habe ich eine umfangreiche Studie der Gitterdynamik von Bi1Y2Fe5O12 (Bi:YIG) angestellt, in dem der Literatur zufolge hochfrequente kohärente Spindynamiken durch Femtosekunden-Laseranregung zu beobachten sind. Diese stehenden Spinwellen (SSWs) höherer Ordnung entstehen durch die kohärente und inkohärente Bewegung von Atomen, in anderen Worten Phononen, welche ich durch UXRD vermessen habe. Somit sind wir in der Lage, die experimentellen Beobachtungen der Gitter- und Spindynamik qualitativ und quantitativ zu vereinigen. Dies geschieht durch eine Kombination von Viel-Temperatur- und Anisotropiemodellierung sowie elastische, magnetoelastische, und mikromagnetsiche Modelle. Die gemeinsamen Daten von UXRD und der zeitaufgelösten magnetooptischen Kerr-Effekt Messungen (tr-MOKE), um jeweils die Gitter- und Spindynamik zu messen, wurden in der Vergangenheit noch an unterschiedlichen experimentellen Aufbauten gemessen. Um die Quantifizierung präziser zu gestalten, haben wir in unserer Arbeitsgruppe UXRD und tr-MOKE in einem einzigen Aufbau kombiniert, welcher somit meines Wissens der erste seiner Art ist. Ich half bei dem Entwurf und der Inbetriebnahme des neuen Aufbaus, welcher die gleichzeitige Messung von Gitter- und Spindynamik auf einer ultraschnellen Zeitskala unter identischen Anregungsbedingungen ermöglicht. Außerdem entwickelte ich eine neue Messroutine für Röntgenbeugung, welche die Messzeit von UXRD-Experimenten um bis zu einer Größenordnungen reduziert. Es nennt sich das Schneiden des reziproken Raumes (reciprocal space slicing, RSS) und nutzt den Vorteil von Flächendetektoren die Bewegung von Beugungsreflexen zu detektieren, was von einer Änderung der Gitterkonstante einhergeht, ohne zeitintensive Scans der Beugungswinkel mit dem Goniometer durchzuführen. RSS ist besonders nützlich für ultraschnelle Beugungsexperimente, weil die Messzeit an Großgeräten wie Synchrotrons oder Freie Elektronen Laser eine seltene und teure Ressource ist. Darüber hinaus ist RSS nicht zwangsläufig auf die Anwendung in ultraschnellen Experimenten beschränkt und kann sogar auf andere Beugungsexperimente, wie die mit Neutronen und Elektronen, ausgeweitet werden. KW - ultrafast KW - X-ray diffraction KW - thin films KW - magnetoelasticity KW - ultraschnell KW - Röntgenbeugung KW - dünne Filme KW - Magnetoelastizität Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-561098 ER - TY - THES A1 - Koç, Azize T1 - Ultrafast x-ray studies on the non-equilibrium of the magnetic and phononic system in heavy rare-earths T1 - Ultraschnelle Röntgenuntersuchungen des Nichtgleichgewichts der magnetischen und phononischen Systeme in schweren Seltenen Erden N2 - In this dissertation the lattice and the magnetic recovery dynamics of the two heavy rare-earth metals Dy and Gd after femtosecond photoexcitation are described. For the investigations, thin films of Dy and Gd were measured at low temperatures in the antiferromagnetic phase of Dy and close to room temperature in the ferromagnetic phase of Gd. Two different optical pump-x-ray probe techniques were employed: Ultrafast x-ray diffraction with hard x-rays (UXRD) yields the structural response of heavy rare-earth metals and resonant soft (elastic) x-ray diffraction (RSXD), which allows measuring directly changes in the helical antiferromagnetic order of Dy. The combination of both techniques enables to study the complex interaction between the magnetic and the phononic subsystems. N2 - In dieser Dissertation wird die Relaxationsdynamik des Gitters und der magnetischen Ordnung der zwei schweren, seltenen Erden Dy und Gd nach der Anregung mit femtosekunden Laserpulsen beschrieben. Für diese Untersuchungen wurden dünne Schichten von Dy und Gd bei niedrigen Temperaturen in der antiferromagnetischen Phase von Dy und nahe der Raumtemperatur in der ferromagnetischen Phase von Gd gemessen. Es wurden zwei verschiedene Experimente mittels optischem Anrege- Röntgen Abfrageverfahren durchgeführt, die ultraschnelle Röntgenbeugung mit harten Röntgenstrahlen (UXRD) und die resonante weiche (elastische) Röntgenbeugung (RSXD). Letzteres Verfahren erlaubt es, direkt die Änderungen der helikalen, antiferromagnetischen Ordnung zu messen. Die Kombination beider Techniken ermöglicht es, die komplexe Wechselwirkung zwischen dem magnetischen und dem phononischen Subsystem zu untersuchen. KW - magnetostriction KW - time-resolved x-ray diffraction KW - resonant soft x-ray diffraction KW - magnetism KW - critical exponent KW - heat transport KW - dysprosium KW - gadolinium KW - rare-earth metals KW - non-equilibrium KW - dynamics KW - magnetic and phononic system KW - Magnetostriktion KW - zeitaufgelöste Röntgenbeugung KW - resonante weiche Röntgenbeugung KW - Magnetismus KW - kritischer Exponent KW - Wärmetransport KW - Dysprosium KW - Gadolinium KW - Metalle der seltenen Erden KW - Nichtgleichgewicht KW - Dynamik KW - magnetisches und phononisches System Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-423282 ER - TY - THES A1 - Goldshteyn, Jewgenij T1 - Frequency-resolved ultrafast dynamics of phonon polariton wavepackets in the ferroelectric crystals LiNbO₃ and LiTaO₃ N2 - 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. N2 - Während dieser Arbeit habe ich ein optisches Vier-Wellen-Misch-Experiment aufgebaut, um zeitaufgelöste Femtosekunden-Spektroskopie von Raman-aktiven Gittermoden durchzuführen. Dieser Aufbau erlaubt die Untersuchung selektiv angeregter Phonon Polaritonen. Diese Quasiteilchen entstehen durch die Kopplung von elektromagnetischen Wellen und transversal-optischer Gittermoden, den sogenannten Phononen. Die Phonon Polaritonen wurden in den optisch nichtlinearen, ferroelektrischen Kristallen LiNbO₃ und LiTaO₃ untersucht. Durch die direkte Beobachtung der Frequenzverschiebung der gestreuten, schmalbandigen Abfragepulse konnte die Raman-Wechselwirkung im Abfrage- und Erzeugungsprozess von Phonon Polaritonen nachgewießen werden. Diese experimentelle Methode vergleiche ich mit der Messung mittels ultrakurzen Laserpulsen. Hierbei ist die Frequenzverschiebung wegen der relativ großen Bandbreite der Laserpulse nicht auflösbar. Die Stokes und Anti-Stokes-Intensitäten sind hingegen in einem Experiment mit schmalbandigen Abfragepulsen spektral getrennt. Diese konnten den jeweiligen, entgegengesetzt propagierenden Wellenpaketen der Phonon Polaritonen zugeordnet werden. Deshalb war es moeglich, die Dynamik dieser Wellenpakete einzeln zu untersuchen. Basierend auf diesen Erkenntnissen konnte ich eine mathematische Beschreibung der sogenannten homodynen Detektion des Lichtes für den Fall von Lichtstreuung an entgegengesetzt propagierenden Phonon Polaritonen entwickeln. Desweiteren habe ich die breitbandigen, ultrakurzen Pumppulse mithilfe von zwei Bandpassfiltern so modifiziert, dass zwei spektral unterschiedliche und spektral nicht überlappende Anregepulse zur Verfügung standen. Dadurch wurde die frequenz-selektive Anregung von Polariton-Moden in der Probe ermöglicht. Diese Technik erlaubt mir die Untersuchung auch sehr schwacher Gittermoden in LiNbO₃ und LiTaO₃, die zu den höheren Ästen der Dispersionsrelation der Phonon Polaritonen gehören. Die experimentell bestimmte Dispersionsrelation der Phonon Polaritonen wurde erweitert und mit theoretischen Modellen verglichen. Zusätzlich habe ich die frequenzabhängige Dämpfung der Phonon Polaritonen bestimmt. T2 - Frequenzaufgelöste ultraschnelle Dynamik von Phonon Polariton Wellenpaketen in ferroelektrischen Kristallen LiNbO₃ und LiTaO₃ KW - transient grating KW - time resolved spectroskopy KW - non-linear optics KW - phonon polariton KW - impulsive stimulated Raman scattering KW - transientes Gitter KW - zeitaufgelöste Spektroskopie KW - nichtlineare Optik KW - Phonon-Polariton KW - impulsive stimulierte Raman Streuung Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-71623 ER - TY - THES A1 - Herzog, Marc T1 - Structural dynamics of photoexcited nanolayered perovskites studied by ultrafast x-ray diffraction T1 - Untersuchung der Strukturdynamik photoangeregter Nanoschicht-Perowskite mittels ultraschneller Röntgenbeugung N2 - This publication-based thesis represents a contribution to the active research field of ultrafast structural dynamics in laser-excited nanostructures. The investigation of such dynamics is mandatory for the understanding of the various physical processes on microscopic scales in complex materials which have great potentials for advances in many technological applications. I theoretically and experimentally examine the coherent, incoherent and anharmonic lattice dynamics of epitaxial metal-insulator heterostructures on timescales ranging from femtoseconds up to nanoseconds. To infer information on the transient dynamics in the photoexcited crystal lattices experimental techniques using ultrashort optical and x-ray pulses are employed. The experimental setups include table-top sources as well as large-scale facilities such as synchrotron sources. At the core of my work lies the development of a linear-chain model to simulate and analyze the photoexcited atomic-scale dynamics. The calculated strain fields are then used to simulate the optical and x-ray response of the considered thin films and multilayers in order to relate the experimental signatures to particular structural processes. This way one obtains insight into the rich lattice dynamics exhibiting coherent transport of vibrational energy from local excitations via delocalized phonon modes of the samples. The complex deformations in tailored multilayers are identified to give rise to highly nonlinear x-ray diffraction responses due to transient interference effects. The understanding of such effects and the ability to precisely calculate those are exploited for the design of novel ultrafast x-ray optics. In particular, I present several Phonon Bragg Switch concepts to efficiently generate ultrashort x-ray pulses for time-resolved structural investigations. By extension of the numerical models to include incoherent phonon propagation and anharmonic lattice potentials I present a new view on the fundamental research topics of nanoscale thermal transport and anharmonic phonon-phonon interactions such as nonlinear sound propagation and phonon damping. The former issue is exemplified by the time-resolved heat conduction from thin SrRuO3 films into a SrTiO3 substrate which exhibits an unexpectedly slow heat conductivity. Furthermore, I discuss various experiments which can be well reproduced by the versatile numerical models and thus evidence strong lattice anharmonicities in the perovskite oxide SrTiO3. The thesis also presents several advances of experimental techniques such as time-resolved phonon spectroscopy with optical and x-ray photons as well as concepts for the implementation of x-ray diffraction setups at standard synchrotron beamlines with largely improved time-resolution for investigations of ultrafast structural processes. This work forms the basis for ongoing research topics in complex oxide materials including electronic correlations and phase transitions related to the elastic, magnetic and polarization degrees of freedom. N2 - Diese publikationsbasierte Dissertation ist ein Beitrag zu dem aktuellen Forschungsgebiet der ultraschnellen Strukturdynamik in laserangeregten Nanostrukturen. Die Erforschung solcher Vorgänge ist unabdingbar für ein Verständnis der vielseitigen physikalischen Prozesse auf mikroskopischen Längenskalen in komplexen Materialien, welche enorme Weiterentwicklungen für technologische Anwendungen versprechen. Meine theoretischen und experimentellen Untersuchungen betrachten kohärente, inkohärente und anharmonische Gitterdynamiken in epitaktischen Metal-Isolator-Heterostrukturen auf Zeitskalen von Femtosekunden bis Nanosekunden. Um Einsichten in solche transienten Prozesse in laserangeregten Kristallen zu erhalten, werden experimentelle Techniken herangezogen, die ultrakurze Pulse von sichtbarem Licht und Röntgenstrahlung verwenden. Ein zentraler Bestandteil meiner Arbeit ist die Entwicklung eines Linearkettenmodells zur Simulation und Analyse der laserinitiierten Atombewegungen. Die damit errechneten Verzerrungsfelder werden anschließend verwendet, um die Änderung der optischen und Röntgeneigenschaften der betrachteten Dünnfilm- und Vielschichtsysteme zu simulieren. Diese Rechnungen werden dann mit den experimentellen Daten verglichen, um die experimentellen Signaturen mit errechneten strukturellen Prozessen zu identifizieren. Dadurch erhält man Einsicht in die vielseitige Gitterdynamiken, was z.B. einen kohärenten Transport der Vibrationsenergie von lokal angeregten Bereichen durch delokalisierte Phononenmoden offenbart. Es wird gezeigt, dass die komplexen Deformationen in maßgeschneiderten Vielschichtsystemen hochgradig nichtlineare Röntgenbeugungseffekte auf Grund von transienten Interferenzerscheinungen verursachen. Das Verständnis dieser Prozesse und die Möglichkeit, diese präzise zu simulieren, werden dazu verwendet, neuartige ultraschnelle Röntgenoptiken zu entwerfen. Insbesondere erläutere ich mehrere Phonon-Bragg-Schalter-Konzepte für die effiziente Erzeugung ultrakurzer Röntgenpulse, die in zeitaufgelösten Strukturanalysen Anwendung finden. Auf Grund der Erweiterung der numerischen Modelle zur Beschreibung von inkohärenter Phononenausbreitung und anharmonischer Gitterpotentiale decken diese ebenfalls die aktuellen Themengebiete von Wärmetransport auf Nanoskalen und anharmonischer Phonon-Phonon-Wechselwirkung (z.B. nichtlineare Schallausbreitung und Phononendämpfung) ab. Die erstere Thematik wird am Beispiel der zeitaufgelösten Wärmeleitung von einem dünnen SrRuO3-Film in ein SrTiO3-Substrat behandelt, wobei ein unerwartet langsamer Wärmetransport zu Tage tritt. Außerdem diskutiere ich mehrere Experimente, die auf Grund der sehr guten Reproduzierbarkeit durch die numerischen Modelle starke Gitteranharmonizitäten in dem oxidischen Perowskit SrTiO3 bezeugen. Diese Dissertation erarbeitet zusätzlich verschiedene Weiterentwicklungen von experimentellen Methoden, wie z.B. die zeitaufgelöste Phononenspektroskopie mittels optischer Photonen und Röntgenphotonen, sowie Konzepte für die Umsetzung von Röntgenbeugungsexperimenten an Standard-Synchrotronquellen mit stark verbesserter Zeitauflösung für weitere Studien von ultraschnellen Strukturvorgängen. KW - ultraschnelle Röntgenbeugung KW - Phononen KW - epitaktisch KW - ultrafast x-ray diffraction KW - phonons KW - epitaxial Y1 - 2012 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-62632 ER - TY - JOUR A1 - Liebig, Ferenc A1 - Sarhan, Radwan Mohamed A1 - Prietzel, Claudia Christina A1 - Thünemann, Andreas F. A1 - Bargheer, Matias A1 - Koetz, Joachim T1 - Undulated Gold Nanoplatelet Superstructures BT - In Situ Growth of Hemispherical Gold Nanoparticles onto the Surface of Gold Nanotriangles JF - Langmuir N2 - Negatively charged flat gold nanotriangles, formed in a vesicular template phase and separated by an AOT-micelle-based depletion flocculation, were reloaded by adding a cationic polyelectrolyte, that is, a hyperbranched polyethylenimine (PEI). Heating the system to 100 degrees C in the presence of a gold chloride solution, the reduction process leads to the formation of gold nanoparticles inside the polymer shell surrounding the nanoplatelets. The gold nanoparticle formation is investigated by UV-vis spectroscopy, small-angle X-ray scattering, and dynamic light scattering measurements in combination with transmission electron microscopy. Spontaneously formed gold clusters in the hyperbranched PEI shell with an absorption maximum at 350 nm grow on the surface of the nanotriangles as hemispherical particles with diameters of similar to 6 nm. High-resolution micrographs show that the hemispherical gold particles are crystallized onto the {111} facets on the bottom and top of the platelet as well as on the edges without a grain boundary. Undulated gold nanoplatelet superstructures with special properties become available, which show a significantly modified performance in SERS-detected photocatalysis regarding both reactivity and enhancement factor. Y1 - 2018 U6 - https://doi.org/10.1021/acs.langmuir.7b02898 SN - 0743-7463 VL - 34 IS - 15 SP - 4584 EP - 4594 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Tchoumba Kwamen, Christelle Larodia A1 - Rössle, Matthias A1 - Leitenberger, Wolfram A1 - Alexe, Marin A1 - Bargheer, Matias T1 - Time-resolved X-ray diffraction study of the structural dynamics in an epitaxial ferroelectric thin Pb(Zr0.2Ti0.8)O-3 film induced by sub-coercive fields JF - Applied physics letters N2 - The electric field-dependence of structural dynamics in a tetragonal ferroelectric lead zirconate titanate thin film is investigated under subcoercive and above-coercive fields using time-resolved X-ray diffraction. The domain nucleation and growth are monitored in real time during the application of an external field to the prepoled thin film capacitor. We propose the observed broadening of the in-plane peak width of the symmetric 002 Bragg reflection as an indicator of the domain disorder and discuss the processes that change the measured peak intensity. Subcoercive field switching results in remnant disordered domain configurations. Published under license by AIP Publishing. Y1 - 2019 U6 - https://doi.org/10.1063/1.5084104 SN - 0003-6951 SN - 1077-3118 VL - 114 IS - 16 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Stete, Felix A1 - Koopman, Wouter-Willem Adriaan A1 - Bargheer, Matias T1 - Signatures of strong coupling on nanoparticles BT - revealing absorption anticrossing by tuning the dielectric environment JF - ACS Photonics N2 - In the strong coupling regime, exciton and plasmon excitations are hybridized into combined system excitations. The correct identification of the coupling regime in these systems is currently debated, from both experimental and theoretical perspectives. In this article we show that the extinction spectra may show a large peak splitting, although the energy loss encoded in the absorption spectra clearly rules out the strong coupling regime. We investigate the coupling of J-aggregate excitons to the localized surface plasmon polaritons on gold nanospheres and nanorods by fine-tuning the plasmon resonance via layer-by-layer deposition of polyelectrolytes. While both structures show a characteristic anticrossing in extinction and scattering experiments, the careful assessment of the systems’ light absorption reveals that strong coupling of the plasmon to the exciton is not present in the nanosphere system. In a phenomenological model of two classical coupled oscillators, a Fano-like regime causes only the resonance of the light-driven oscillator to split up, while the other one still dissipates energy at its original frequency. Only in the strong-coupling limit do both oscillators split up the frequencies at which they dissipate energy, qualitatively explaining our experimental finding. KW - hybrid nanoparticles KW - exciton plasmon coupling KW - layer-by-layer deposition KW - strong coupling KW - absorption measurements Y1 - 2017 U6 - https://doi.org/10.1021/acsphotonics.7b00113 SN - 2330-4022 VL - 4 SP - 1669 EP - 1676 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Mattern, Maximilian A1 - Pudell, Jan-Etienne A1 - Laskin, Gennadii A1 - Reppert, Alexander von A1 - Bargheer, Matias T1 - Analysis of the temperature- and fluence-dependent magnetic stress in laser-excited SrRuO3 JF - Structural dynamics N2 - We use ultrafast x-ray diffraction to investigate the effect of expansive phononic and contractive magnetic stress driving the picosecond strain response of a metallic perovskite SrRuO3 thin film upon femtosecond laser excitation. We exemplify how the anisotropic bulk equilibrium thermal expansion can be used to predict the response of the thin film to ultrafast deposition of energy. It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale. We find a very large negative Grüneisen constant describing the large contractive stress imposed by a small amount of energy in the spin system. The temperature and fluence dependence of the strain response for a double-pulse excitation scheme demonstrates the saturation of the magnetic stress in the high-fluence regime. KW - Thin films KW - Thermodynamic properties KW - Bragg peak KW - Ultrafast X-ray diffraction KW - Thermal effects KW - Phonons KW - Magnetism KW - Lattice dynamics KW - Lasers KW - Perovskites Y1 - 2020 U6 - https://doi.org/10.1063/4.0000072 SN - 2329-7778 VL - 8 IS - 2 PB - AIP Publishing LLC CY - Melville, NY ER - TY - JOUR A1 - Zeuschner, Steffen Peer A1 - Wang, Xi-Guang A1 - Deb, Marwan A1 - Popova, Elena A1 - Malinowski, Gregory A1 - Hehn, Michel A1 - Keller, Niels A1 - Berakdar, Jamal A1 - Bargheer, Matias T1 - Standing spin wave excitation in Bi BT - YIG films via temperature-induced anisotropy changes and magneto-elastic coupling JF - Physical review : B, Condensed matter and materials physics N2 - Based on micromagnetic simulations and experimental observations of the magnetization and lattice dynamics after the direct optical excitation of the magnetic insulator Bi : YIG or indirect excitation via an optically opaque Pt/Cu double layer, we disentangle the dynamical effects of magnetic anisotropy and magneto-elastic coupling. The strain and temperature of the lattice are quantified via modeling ultrafast x-ray diffraction data. Measurements of the time-resolved magneto-optical Kerr effect agree well with the magnetization dynamics simulated according to the excitation via two mechanisms: the magneto-elastic coupling to the experimentally verified strain dynamics and the ultrafast temperature-induced transient change in the magnetic anisotropy. The numerical modeling proves that, for direct excitation, both mechanisms drive the fundamental mode with opposite phase. The relative ratio of standing spin wave amplitudes of higher-order modes indicates that both mechanisms are substantially active. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevB.106.134401 SN - 2469-9950 SN - 2469-9969 VL - 106 IS - 13 PB - American Physical Society CY - College Park ER - TY - THES A1 - Kiel, Mareike T1 - Static and ultrafast optical properties of nanolayered composites : gold nanoparticles embedded in polyelectrolytes T1 - Statische und ultraschnelle optische Eigenschaften von nanogeschichteten Kompositmaterialien. Gold-Nanopartikel in Polyelektrolytschichten. N2 - In the course of this thesis gold nanoparticle/polyelectrolyte multilayer structures were prepared, characterized, and investigated according to their static and ultrafast optical properties. Using the dip-coating or spin-coating layer-by-layer deposition method, gold-nanoparticle layers were embedded in a polyelectrolyte environment with high structural perfection. Typical structures exhibit four repetition units, each consisting of one gold-particle layer and ten double layers of polyelectrolyte (cationic+anionic polyelectrolyte). The structures were characterized by X-ray reflectivity measurements, which reveal Bragg peaks up to the seventh order, evidencing the high stratication of the particle layers. In the same measurements pronounced Kiessig fringes were observed, which indicate a low global roughness of the samples. Atomic force microscopy (AFM) images veried this low roughness, which results from the high smoothing capabilities of polyelectrolyte layers. This smoothing effect facilitates the fabrication of stratified nanoparticle/polyelectrolyte multilayer structures, which were nicely illustrated in a transmission electron microscopy image. The samples' optical properties were investigated by static spectroscopic measurements in the visible and UV range. The measurements revealed a frequency shift of the reflectance and of the plasmon absorption band, depending on the thickness of the polyelectrolyte layers that cover a nanoparticle layer. When the covering layer becomes thicker than the particle interaction range, the absorption spectrum becomes independent of the polymer thickness. However, the reflectance spectrum continues shifting to lower frequencies (even for large thicknesses). The range of plasmon interaction was determined to be in the order of the particle diameter for 10 nm, 20 nm, and 150 nm particles. The transient broadband complex dielectric function of a multilayer structure was determined experimentally by ultrafast pump-probe spectroscopy. This was achieved by simultaneous measurements of the changes in the reflectance and transmittance of the excited sample over a broad spectral range. The changes in the real and imaginary parts of the dielectric function were directly deduced from the measured data by using a recursive formalism based on the Fresnel equations. This method can be applied to a broad range of nanoparticle systems where experimental data on the transient dielectric response are rare. This complete experimental approach serves as a test ground for modeling the dielectric function of a nanoparticle compound structure upon laser excitation. N2 - Im Rahmen dieser Arbeit wurden Gold-Nanopartikel/Polyelektrolyt Multischichtstrukturen hergestellt, strukturell charakterisiert und bezüglich ihrer optischen Eigenschaften sowohl statisch als auch zeitaufgelöst analysiert. Die Strukturen wurden mithilfe der Dip-coating oder der Spin-coating Methode hergestellt. Beide Methoden ermöglichen das Einbetten einzelner Partikellagen in eine Polyelektrolytumgebung. Typische Strukturen in dieser Arbeit bestehen aus vier Wiederholeinheiten, wobei jede aus einer Nanopartikelschicht und zehn Polyelektrolyt-Doppellagen (kationisches und anionisches Polyelektrolyt) zusammengesetzt ist. Die Stratizierung der Gold-Nanopartikellagen wurde mittels Röntgenreflektometrie-Messungen im Kleinwinkelbereich nachgewiesen, welche Bragg Reflexionen bis zur siebten Ordnung aufzeigen. Das ausgeprägte Kiessig Interferenzmuster dieser Messungen weist zudem auf eine geringe globale Rauheit hin, die durch Oberflächenanalysen mit einem Rasterkraftmikroskop bestätigt werden konnte. Diese geringe Rauheit resultiert aus den glättenden Eigenschaften der Polyelektrolyte, die die Herstellung von Multilagensystemen mit mehreren Partikellagen erst ermöglichen. Die Aufnahme eines Transmissionselektronenmikroskops veranschaulicht eindrucksvoll die Anordnung der Partikel in einzelne Schichten. Durch photospektroskopische Messungen wurden die optischen Eigenschaften der Strukturen im UV- und sichtbaren Bereich untersucht. Beispielsweise wird eine Verschiebung und Verstärkung der Plasmonenresonanz beobachtet, wenn eine Goldnanopartikellage mit transparenten Polyelektrolyten beschichtet wird. Erst wenn die bedeckende Schicht dicker als die Reichweite der Plasmonen wird, bleibt die Absorption konstant. Die spektrale Reflektivität jedoch ändert sich auch mit jeder weiteren adsorbierten Polyelektrolytschicht. Die Reichweite der Plasmonenresonanz konnte auf diese Art für Partikel der Größe 10 nm, 20 nm und 150 nm bestimmt werden. Die Ergebnisse wurden im Kontext einer Effektiven Mediums Theorie diskutiert. Die komplexe dielektrische Funktion einer Multilagenstruktur wurde zeitabhängig nach Laserpulsanregung für einen breiten spektralen Bereich bestimmt. Dazu wurden zuerst die Änderungen der Reflektivität und Transmittivität simultan mittels der Pump-Probe (Anrege-Abtast) Spektroskopie gemessen. Anschließend wurden aus diesen Daten, mithilfe eines Formalismus, der auf den Fresnelschen Formeln basiert, die Änderungen im Real- und Imaginärteil der dielektrischen Funktion ermittelt. Diese Methode eignet sich zur Bestimmung der transienten dielektrischen Funktion einer Vielzahl von Nanopartikelsystemen. Der rein experimentelle Ansatz ermöglicht es, effektive Medien Theorien und Simulationen der dielektrischen Funktion nach Laserpulsanregung zu überprüfen. KW - Nanopartikel KW - Polyelektrolyte KW - Dielektrische Funktion KW - Anrege-Abtast Spektroskopie KW - nanoparticles KW - polyelectrolytes KW - dielectric function KW - pump-probe spectroscopy Y1 - 2012 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-61823 ER - TY - JOUR A1 - Liebig, Ferenc A1 - Henning, Ricky A1 - Sarhan, Radwan Mohamed A1 - Prietzel, Claudia Christina A1 - Bargheer, Matias A1 - Koetz, Joachim T1 - A new route to gold nanoflowers JF - Nanotechnology N2 - Catanionic vesicles spontaneously formed by mixing the anionic surfactant bis(2-ethylhexyl)sulfosuccinate sodium salt with the cationic surfactant cetyltrimethylammonium bromide were used as a reducing medium to produce gold clusters, which are embedded and well-ordered into the template phase. The gold clusters can be used as seeds in the growth process that follows by adding ascorbic acid as a mild reducing component. When the ascorbic acid was added very slowly in an ice bath round-edged gold nanoflowers were produced. When the same experiments were performed at room temperature in the presence of Ag+ ions, sharp-edged nanoflowers could be synthesized. The mechanism of nanoparticle formation can be understood to be a non-diffusion-limited Ostwald ripening process of preordered gold nanoparticles embedded in catanionic vesicle fragments. Surface-enhanced Raman scattering experiments show an excellent enhancement factor of 1.7 . 10(5) for the nanoflowers deposited on a silicon wafer. KW - catanionic vesicles KW - gold cluster KW - gold nanoflowers KW - crystal growth KW - HRTEM KW - SEM Y1 - 2018 U6 - https://doi.org/10.1088/1361-6528/aaaffd SN - 0957-4484 SN - 1361-6528 VL - 29 IS - 18 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Sander, Mathias T1 - Ultrafast tailored strain fields in nanostructures T1 - Ultraschnelle massgeschneiderte Dehnungsfelder in Nanostrukturen N2 - This publication based thesis, which consists of seven published articles, summarizes my contributions to the research field of laser excited ultrafast structural dynamics. The coherent and incoherent lattice dynamics on microscopic length scales are detected by ultrashort optical and X-ray pulses. The understanding of the complex physical processes is essential for future improvements of technological applications. For this purpose, tabletop soruces and large scale facilities, e.g. synchrotrons, are employed to study structural dynamics of longitudinal acoustic strain waves and heat transport. The investigated effects cover timescales from hundreds of femtoseconds up to several microseconds. The main part of this thesis is dedicated to the investigation of tailored phonon wave packets propagating in perovskite nanostructures. Tailoring is achieved either by laser excitation of nanostructured bilayer samples or by a temporal series of laser pulses. Due to the propagation of longitudinal acoustic phonons, the out-of-plane lattice spacing of a thin film insulator-metal bilayer sample is modulated on an ultrafast timescale. This leads to an ultrafast modulation of the X-ray diffraction efficiency which is employed as a phonon Bragg switch to shorten hard X-ray pulses emitted from a 3rd generation synchrotron. In addition, we have observed nonlinear mixing of high amplitude phonon wave packets which originates from an anharmonic interatomic potential. A chirped optical pulse sequence excites a narrow band phonon wave packet with specific momentum and energy. The second harmonic generation of these phonon wave packets is followed by ultrafast X-ray diffraction. Phonon upconversion takes place because the high amplitude phonon wave packet modulates the acoustic properties of the crystal which leads to self steepening and to the successive generation of higher harmonics of the phonon wave packet. Furthermore, we have demonstrated ultrafast strain in direction parallel to the sample surface. Two consecutive so-called transient grating excitations displaced in space and time are used to coherently control thermal gradients and surface acoustic modes. The amplitude of the coherent and incoherent surface excursion is disentangled by time resolved X-ray reflectivity measurements. We calibrate the absolute amplitude of thermal and acoustic surface excursion with measurements of longitudinal phonon propagation. In addition, we develop a diffraction model which allows for measuring the surface excursion on an absolute length scale with sub-Äangström precision. Finally, I demonstrate full coherent control of an excited surface deformation by amplifying and suppressing thermal and coherent excitations at the surface of a laser-excited Yttrium-manganite sample. N2 - Diese publikations basierte Dissertation enthält sieben veröffentlichte Artikel und ist ein Beitrag zum Forschungsfeld der laserangeregten ultraschnellen Strukturdynamik. Dabei wird die kohärente und inkohärente Gitterdynamik mit Hilfe von ultrakurzen optischen Pulsen sowie Röntgenpulsen auf mikroskopischer Längenskala untersucht. Das Verständnis dieser komplexen physikalischen Prozesse ist essenziell für die Verbesserung von zukünftigen technologischen Anwendungen. Hierfür wurde die Strukturdynamik von longitudinal akustischen Schallwellen und Wärmetransport mit Hilfe von verschieden Messinstrumenten, basierend auf Labor und Synchrotronstrahlungsquellen, untersucht. Die untersuchten Effekte umfassen Zeitskalen von einigen hundert Femtosekunden bis hin zu mehreren Mikrosekunden. Der Hauptteil meiner Dissertation beruht auf der Untersuchungen von definiert angeregten Phonon-Wellenpakten, die sich in Perowskit Nanostrukturen ausbreiten. Die Kontrolle wird entweder durch Laseranregung einer nanostruktieren Doppelschichtprobe oder durch eine zeitlich versetzte Laserpulsfolge erreicht. Dabei wird die Einheitszelle senkrecht zu den Gitterebenen auf ultraschnellen Zeiten modifiziert. Daraus folgt eine ultraschnelle Modulation der Röntgenbeugungs Effizienz, die als Phonon Braggschalter verwendet wird, um harte Röntgenpulse von Synchrotrons der dritten Generation zu verkürzen. Zudem haben wir die nichtlineare Mischung von Phonon-Wellenpaketen mit hoher Amplitude beobachtet, die der Anharmonizität des interatomaren Potential herrührt. Durch eine gechirpte optische Laserpulsfolge wird ein schmalbandiges Phonon-Wellenpaket mit definiertem Impuls und definierter Energie angeregt. Dabei wird die Erzeugung der zweiten Harmonischen mittels ultraschneller Röntgenbeugung untersucht. Die Phononkonversion findet hierbei durch die hohe Phononamplitude statt, die die akustischen Eigenschaften des Kristalls verändert. Dieser Prozess führt zum Aufsteilen der Wellenfront und folglich zur Erzeugung der höheren Harmonischen des Phonon-Wellenpakets. Außerdem habe ich ultraschnelle Schallpulse parallel zur Richtung der Probenoberfläche demonstriert. Dabei werden zwei sogenannte transiente Gitteranregungen verwendet, die räumlich und zeitlich zueinander versetzt sind, um thermische Gradienten und akustische Oberflächenmoden kohärent zu kontrollieren. Die Amplitude der kohärenten und inkohärenten Oberflächenausdehnung kann mit Hilfe von Röntgenreflektivität getrennt betrachtet werden. Zusätzlich haben wir ein Beugungsmodel entwickelt, mit dem wir die Oberflächenausdehnung auf einer absoluten Längenskale mit sub-Ängström Präzision kalibrieren. Schließlich zeige ich volle kohärente Kontrolle von der angeregten Oberflächenausdehnung durch Verstärkung und Unterdrückung von thermischen und kohärenten Anregungen auf der Oberfläche einer dünnen, laserangeregten Yttriummanganat Schicht. KW - Ultrafast X-ray diffraction KW - strain KW - acoustic waves KW - Ultraschnelle Röntgenbeugung KW - akustische Wellen KW - Dehnung Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-417863 ER - TY - GEN A1 - Mitzscherling, Steffen A1 - Cui, Qianling A1 - Koopman, Wouter-Willem Adriaan A1 - Bargheer, Matias T1 - Dielectric function of two-phase colloid–polymer nanocomposite N2 - The plasmon resonance of metal nanoparticles determines their optical response in the visible spectral range. Many details such as the electronic properties of gold near the particle surface and the local environment of the particles influence the spectra. We show how the cheap but highly precise fabrication of composite nanolayers by spin-assisted layer-by-layer deposition of polyelectrolytes can be used to investigate the spectral response of gold nanospheres (GNS) and gold nanorods (GNR) in a self-consistent way, using the established Maxwell–Garnett effective medium (MGEM) theory beyond the limit of homogeneous media. We show that the dielectric function of gold nanoparticles differs from the bulk value and experimentally characterize the shape and the surrounding of the particles thoroughly by SEM, AFM and ellipsometry. Averaging the dielectric functions of the layered surrounding by an appropriate weighting with the electric field intensity yields excellent agreement for the spectra of several nanoparticles and nanorods with various cover-layer thicknesses. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 305 Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-102695 SP - 29465 EP - 29474 ER - TY - THES A1 - Willig, Lisa T1 - Ultrafast magneto-optical studies of remagnetisation dynamics in transition metals T1 - Ultraschnelle Magneto-Optische Studien der Remagnetisierungsdynamik von Übergangsmetallen N2 - Ultrafast magnetisation dynamics have been investigated intensely for two decades. The recovery process after demagnetisation, however, was rarely studied experimentally and discussed in detail. The focus of this work lies on the investigation of the magnetisation on long timescales after laser excitation. It combines two ultrafast time resolved methods to study the relaxation of the magnetic and lattice system after excitation with a high fluence ultrashort laser pulse. The magnetic system is investigated by time resolved measurements of the magneto-optical Kerr effect. The experimental setup has been implemented in the scope of this work. The lattice dynamics were obtained with ultrafast X-ray diffraction. The combination of both techniques leads to a better understanding of the mechanisms involved in magnetisation recovery from a non-equilibrium condition. Three different groups of samples are investigated in this work: Thin Nickel layers capped with nonmagnetic materials, a continuous sample of the ordered L10 phase of Iron Platinum and a sample consisting of Iron Platinum nanoparticles embedded in a carbon matrix. The study of the remagnetisation reveals a general trend for all of the samples: The remagnetisation process can be described by two time dependences. A first exponential recovery that slows down with an increasing amount of energy absorbed in the system until an approximately linear time dependence is observed. This is followed by a second exponential recovery. In case of low fluence excitation, the first recovery is faster than the second. With increasing fluence the first recovery is slowed down and can be described as a linear function. If the pump-induced temperature increase in the sample is sufficiently high, a phase transition to a paramagnetic state is observed. In the remagnetisation process, the transition into the ferromagnetic state is characterised by a distinct transition between the linear and exponential recovery. From the combination of the transient lattice temperature Tp(t) obtained from ultrafast X-ray measurements and magnetisation M(t) gained from magneto-optical measurements we construct the transient magnetisation versus temperature relations M(Tp). If the lattice temperature remains below the Curie temperature the remagnetisation curve M(Tp) is linear and stays below the M(T) curve in equilibrium in the continuous transition metal layers. When the sample is heated above phase transition, the remagnetisation converges towards the static temperature dependence. For the granular Iron Platinum sample the M(Tp) curves for different fluences coincide, i.e. the remagnetisation follows a similar path irrespective of the initial laser-induced temperature jump. N2 - Ultraschnelle Magnetisierungsdynamiken wurden in den letzten zwei Jahrzehnten intensiv untersucht. Hingegen der Wiederherstellungsprozess der Magnetisierung nach einer ultraschnellen Demagnetisierung wird selten experimentell untersucht und im Detail diskutiert. Der Fokus dieser Arbeit liegt auf der Untersuchung der Magnetisierung auf langen Zeitskalen nach der Anregung durch einen Laserpuls. Dazu werden zwei ultraschnelle zeitaufgelöste Techniken verwendet, um die Relaxierung von dem magnetischen System und dem System des Gitters nach Anregung mit einem hochenergetischen ultrakurzen Laserpuls zu untersuchen. Das magnetische System wurde untersucht mithilfe von Messungen des zeitaufgelösten magneto-optischen Kerr Effekts. Der experimentelle Aufbau wurde im Rahmen dieser Arbeit entwickelt. Die Gitterdynamik wurde mittels ultraschneller Röntgendiffraktometrie aufgenommen. Die Kombination beider Techniken liefert ein besseres Verständnis von den Prozessen involviert in Magnetisierungsrelaxation aus einem Nicht-Gleichgewichtszustand. Drei unterschiedliche Probensysteme wurden im Rahmen dieser Arbeit untersucht: Dünne Nickel Schichten umgeben von nicht-magnetischen Schichten, eine kontinuierliche Schicht aus einer Eisen Platin Legierung und eine Probe bestehend aus Eisen Platin Nanopartikeln eingebettet in einer Kohlenstoffmatrix. Die Studie der Remagnetisierung zeigt einen generellen Trend für alle diese Systeme auf: Der Remagnetisierungsprozess kann mit zwei Zeitabhängikeiten beschrieben werden. Eine erste exponentielle Zeitabhängigkeit, die mit zunehmender Menge an absorbierter Energie verlangsamt wird bis ein lineares Verhalten beobachtet wird. Darauf folgend gibt es eine zweite exponentielle funktionale Abhängigkeit in der Zeit. Im Falle einer geringen Energieabsorption ist die erste Remagnetisierung schneller als die zweite. Mit steigender Fluenz wird die Remagnetisierung in der ersten Zeitabhängigkeit verlangsamt und kann als lineare Funktion beschrieben werden. Wenn der durch den Pump Puls induzierte Temperatursprung in der Probe ausreichend groß ist, wird ein Phasenübergang in die paramagnetische Phase beobachtet. In dem Remagnetisierungsprozess wird dieser Übergang durch einen deutlich sichtbaren Wechsel zwischen linearem und exponentiellen Verhalten sichtbar. Mithilfe der Kombination aus der von Rötngendaten gewonnener Gittertemperatur Tp(t) und der Magnetisierung M(t) erhalten wir die zeitliche Abhängigkeit der Magnetisierung von der Gittertemperatur M(Tp). Falls die Gittertemperatur unter der Curietemperatur bleibt, ist die Remagnetisierungskurve M(Tp) linear und bleibt unterhalb der statischen Gleichgewichtskurve M(T) für die kontinuierlichen Übergangsmetalle. Wenn die Probe über den Phasenübergang geheizt wird, nähert sich die Remagnetisierung der statischen Kurve an. Die transiente Remagnetisierungskurven M(Tp) der granularen Eisen Platin Probe folgen immer einem der statischen Magnetisierungskurve ähnlichen Verlauf, unabhängig von dem laser-induzierten Temperatursprung. KW - ultrafast dynamics KW - remagnetization KW - magnetization dynamics KW - transition metals KW - condensed matter KW - ultraschnelle Dynamik KW - magneto-optics KW - Magneto-Optik KW - Remagnetisierung KW - Magnetisierungsdynamik KW - Übergangsmetalle KW - kondensierte Materie Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-441942 ER - TY - JOUR A1 - Pudell, Jan-Etienne A1 - Sander, M. A1 - Bauer, R. A1 - Bargheer, Matias A1 - Herzog, Marc A1 - Gaál, Peter T1 - Full Spatiotemporal Control of Laser-Excited Periodic Surface Deformations JF - Physical review applied N2 - We demonstrate full control of acoustic and thermal periodic deformations at solid surfaces down to subnanosecond time scales and few-micrometer length scales via independent variation of the temporal and spatial phase of two optical transient grating (TG) excitations. For this purpose, we introduce an experimental setup that exerts control of the spatial phase of subsequent time-delayed TG excitations depending on their polarization state. Specific exemplary coherent control cases are discussed theoretically and corresponding experimental data are presented in which time-resolved x-ray reflectivity measures the spatiotemporal surface distortion of nanolayered heterostructures. Finally, we discuss examples where the application of our method may enable the control of functional material properties via tailored spatiotemporal strain fields. Y1 - 2019 U6 - https://doi.org/10.1103/PhysRevApplied.12.024036 SN - 2331-7019 VL - 12 IS - 2 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Stete, Felix A1 - Schossau, Phillip A1 - Bargheer, Matias A1 - Koopman, Wouter-Willem Adriaan T1 - Size-Dependent coupling of Hybrid Core-Shell Nanorods BT - Toward Single-Emitter Strong-Coupling JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Owing to their ability of concentrating electromagnetic fields to subwavelength mode volumes, plasmonic nanoparticles foster extremely high light-matter coupling strengths reaching far into the strong-coupling regime of light matter interaction. In this article, we present an experimental investigation on the dependence of coupling strength on the geometrical size of the nanoparticle. The coupling strength for differently sized hybrid plasmon-core exciton-shell nanorods was extracted from the typical resonance anticrossing of these systems, obtained by controlled modification of the environment permittivity using layer-by-layer deposition of polyelectrolytes. The observed size dependence of the coupling strength can be explained by a simple model approximating the electromagnetic mode volume by the geometrical volume of the particle. On the basis of this model, the coupling strength for particles of arbitrary size can be predicted, including the particle size necessary to support single-emitter strong coupling. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpcc.8b04204 SN - 1932-7447 VL - 122 IS - 31 SP - 17976 EP - 17982 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Erler, Alexander A1 - Riebe, Daniel A1 - Beitz, Toralf A1 - Löhmannsröben, Hans-Gerd A1 - Grothusheitkamp, Daniela A1 - Kunz, Thomas A1 - Methner, Frank-Jürgen T1 - Characterization of volatile metabolites formed by molds on barley by mass and ion mobility spectrometry JF - Journal of mass spectrometr N2 - The contamination of barley by molds on the field or in storage leads to the spoilage of grain and the production of mycotoxins, which causes major economic losses in malting facilities and breweries. Therefore, on-site detection of hidden fungus contaminations in grain storages based on the detection of volatile marker compounds is of high interest. In this work, the volatile metabolites of 10 different fungus species are identified by gas chromatography (GC) combined with two complementary mass spectrometric methods, namely, electron impact (EI) and chemical ionization at atmospheric pressure (APCI)-mass spectrometry (MS). The APCI source utilizes soft X-radiation, which enables the selective protonation of the volatile metabolites largely without side reactions. Nearly 80 volatile or semivolatile compounds from different substance classes, namely, alcohols, aldehydes, ketones, carboxylic acids, esters, substituted aromatic compounds, alkenes, terpenes, oxidized terpenes, sesquiterpenes, and oxidized sesquiterpenes, could be identified. The profiles of volatile and semivolatile metabolites of the different fungus species are characteristic of them and allow their safe differentiation. The application of the same GC parameters and APCI source allows a simple method transfer from MS to ion mobility spectrometry (IMS), which permits on-site analyses of grain stores. Characterization of IMS yields limits of detection very similar to those of APCI-MS. Accordingly, more than 90% of the volatile metabolites found by APCI-MS were also detected in IMS. In addition to different fungus genera, different species of one fungus genus could also be differentiated by GC-IMS. KW - APCI KW - fungus KW - gas chromatography KW - ion mobility spectrometry KW - mass KW - spectrometry KW - mold KW - soft X-ray Y1 - 2020 U6 - https://doi.org/10.1002/jms.4501 SN - 1076-5174 SN - 1096-9888 VL - 55 IS - 5 SP - 1 EP - 10 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Pranav, Manasi A1 - Hultzsch, Thomas A1 - Musiienko, Artem A1 - Sun, Bowen A1 - Shukla, Atul A1 - Jaiser, Frank A1 - Shoaee, Safa A1 - Neher, Dieter T1 - Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells JF - APL materials : high impact open access journal in functional materials science N2 - Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high-performance donor-acceptor systems. Here, we probe the origin of field-dependent free-charge generation and photoluminescence in wnon-fullereneacceptor (NFA)-based organic solar cells using the polymer PM6 and the NFA Y5-a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artifacts from non-geminate recombination of photogenerated and dark charge carriers near the electrodes. We then correlate free charge generation with steady-state photoluminescence intensity and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low-offset system is entirely controlled by the field-dependent dissociation of local excitons into charge-transfer states. (c) 2023 Author(s). Y1 - 2023 U6 - https://doi.org/10.1063/5.0151580 SN - 2166-532X VL - 11 IS - 6 PB - AIP Publishing CY - Melville ER - TY - JOUR A1 - Hovhannisyan, Karen V. A1 - Nemati, Somayyeh A1 - Henkel, Carsten A1 - Anders, Janet T1 - Long-time equilibration can determine transient thermality JF - PRX Quantum N2 - When two initially thermal many-body systems start to interact strongly, their transient states quickly become non-Gibbsian, even if the systems eventually equilibrate. To see beyond this apparent lack of structure during the transient regime, we use a refined notion of thermality, which we call g-local. A system is g-locally thermal if the states of all its small subsystems are marginals of global thermal states. We numerically demonstrate for two harmonic lattices that whenever the total system equilibrates in the long run, each lattice remains g-locally thermal at all times, including the transient regime. This is true even when the lattices have long-range interactions within them. In all cases, we find that the equilibrium is described by the generalized Gibbs ensemble, with three-dimensional lattices requiring special treatment due to their extended set of conserved charges. We compare our findings with the well-known two-temperature model. While its standard form is not valid beyond weak coupling, we show that at strong coupling it can be partially salvaged by adopting the concept of a g-local temperature. Y1 - 2023 U6 - https://doi.org/10.1103/PRXQuantum.4.030321 SN - 2691-3399 VL - 4 IS - 3 PB - American Physical Society CY - College Park ER - TY - GEN A1 - Schrape, Oliver A1 - Balashov, Alexey A1 - Simevski, Aleksandar A1 - Benito, Carlos A1 - Krstić, Miloš T1 - Master-Clone placement with individual clock tree implementation BT - a Case on Physical Chip Design T2 - 2018 IEEE Nordic Circuits and Systems Conference (NORCAS): NORCHIP and International Symposium of System-on-Chip (SoC) N2 - A hybrid design approach of the hierarchical physical implementation design flow is presented and demonstrated on a fault-tolerant low-power multiprocessor system. The proposed flow allows to implement selected submodules in parallel with contrary requirements such as identical placement and individual block implementation. The overall system contains four Leon2 cores and communicates via the Waterbear framework and supports Adaptive Voltage Scaling (AVS) functionality. Three of the processor core variants are derived from the first baseline reference core but implemented individually at block level based on their clock tree specification. The chip is prepared for space applications and designed with triple modular redundancy (TMR) for control parts. The low-power performance is enabled by contemporary power and clock management control. An ASIC is fabricated in a low-power 0.13 mu m BiCMOS technology process node. KW - Hierarchical Design KW - Physical Implementation KW - Clock Tree Implementation Y1 - 2018 SN - 978-1-5386-7656-1 PB - IEEE CY - New York ER - TY - JOUR A1 - Chen, Junchao A1 - Lange, Thomas A1 - Andjelkovic, Milos A1 - Simevski, Aleksandar A1 - Krstić, Miloš T1 - Prediction of solar particle events with SRAM-based soft error rate monitor and supervised machine learning JF - Microelectronics reliability N2 - This work introduces an embedded approach for the prediction of Solar Particle Events (SPEs) in space applications by combining the real-time Soft Error Rate (SER) measurement with SRAM-based detector and the offline trained machine learning model. The proposed approach is intended for the self-adaptive fault-tolerant multiprocessing systems employed in space applications. With respect to the state-of-the-art, our solution allows for predicting the SER 1 h in advance and fine-grained hourly tracking of SER variations during SPEs as well as under normal conditions. Therefore, the target system can activate the appropriate mechanisms for radiation hardening before the onset of high radiation levels. Based on the comparison of five different machine learning algorithms trained with the public space flux database, the preliminary results indicate that the best prediction accuracy is achieved with the recurrent neural network (RNN) with long short-term memory (LSTM). Y1 - 2020 U6 - https://doi.org/10.1016/j.microrel.2020.113799 SN - 0026-2714 VL - 114 PB - Elsevier CY - Oxford ER - TY - THES A1 - Damle, Mitali T1 - Gas distribution around galaxies in cosmological simulations T1 - Gasverteilung um Galaxien in kosmologischen Simulationen N2 - The evolution of a galaxy is pivotally governed by its pattern of star formation over a given period of time. The star formation rate at any given time is strongly dependent on the amount of cold gas available in the galaxy. Accretion of pristine gas from the Intergalactic medium (IGM) is thought to be one of the primary sources for star-forming gas. This gas first passes through the virial regions of the galaxy before reaching the Interstellar medium (ISM), the hub of star formation. On the other hand, owing to the evolutionary course of young and massive stars, energetic winds are ejected from the ISM to the virial regions of the galaxy. A bunch of interlinked, complex astrophysical processes, arising from the concurrent presence of both infalling as well as outbound gas, play out over a range of timescales in the halo region or the Circumgalactic medium (CGM) of a galaxy. It would not be incorrect to say that the CGM has a stronghold over the gas reserves of a galaxy and thus, plays a backhand, yet, rather pivotal role in shaping many galactic properties, some of which are also readily observable. Observing the multi-phase CGM (via spectral-line ion measurements), however, remains a non-trivial effort even today. Low particle densities as well as the CGM’s vast spatial extent, coupled with likely deviations from a spherical distribution, marr the possibility of obtaining complete, unbiased, high-quality spectral information tracing the full extent of the gaseous halo. This often incomplete information leads to multiple inferences about the CGM properties that give rise to multiple contradicting models. In this regard, computer simulations offer a neat solution towards testing and, subsequently, falsifying many of these existing CGM models. Thanks to their controlled environments, simulations are able to not only effortlessly transcend several orders of magnitude in time and space, but also get around many of the observational limitations and provide some unique views on many CGM properties. In this thesis, I focus on effectively using different computer simulations to understand the role of CGM in various astrophysical contexts, namely, the effect of Local Group (LG) environment, major merger events and satellite galaxies. In Chapter 2, I discuss the approach used for modeling various phases of the simulated z = 0 LG CGM in Hestia constrained simulations. Each of the three realizations contain a Milky Way (MW)–Andromeda (M31) galaxy pair, along with their corresponding sets of satellite galaxies, all embedded within the larger cosmological context. For characterizing the different temperature–density phases within the CGM, I model five tracer ions with cloudy ionization modeling. The cold and cool–ionized CGM (H i and Si iii respectively) in Hestia is very clumpy and distributed close to the galactic centers, while the warm-hot and hot CGM (O vi, O vii and O viii) is tenuous and volume-filling. On comparing the H i and Si iii column densities for the simulated M31 with observational measurements from Project AMIGA survey and other low-z galaxies, I found that Hestia galaxies produced less gas in the outer CGM, unlike observations. My carefully designed observational bias model subsequently revealed the possibility that some MW gas clouds might be incorrectly associated with the M31 CGM in observations, and hence, may be partly responsible for giving rise to the detected mismatch between simulated data and observations. In Chapter 3, I present results from four zoom–in, major merger, gas–rich simulations and the subsequent role of the gas, originally situated in the CGM, in influencing some of the galactic observables. The progenitor parameters are selected such that the post–merger remnants are MW–mass galaxies. We generally see a very clear gas bridge joining the merging galaxies in case of multiple passage mergers while such a bridge is mostly absent when a direct collision occurs. On the basis of particle–to–galaxy distance computations and tracer particle analysis, I found that about 33–48 percent of the cold gas contributing to the merger–induced star formation in the bridge originated from the CGM regions. In Chapter 4, I used a sample of 234 MW-mass, L* galaxies from the TNG50 cosmological simulations, with an aim of characterizing the impact of their global satellite populations on the extended cold CGM properties of their host L* halos. On the basis of halo mass and number of satellite galaxies (N_sats ), I categorized the sample into low and high mass bins, and subsequently into bottom, inter and top quartiles respectively. After confirming that satellites indeed influence the extended cold halo gas density profiles of the host galaxies, I investigated the effects of different satellite population parameters on the host halo cold CGMs. My analysis showed that there is hardly any cold gas associated with the satellite population of the lowest mass halos. The stellar mass of the most massive satellite (M_*mms ) impacted the cold gas in low mass bin halos the most, while N_sats (followed by M_*mms ) was the most influential factor for the high mass halos. In any case, how easily cold gas was stripped off the most massive satellite did not play much role. The number of massive (Stellar mass, M* > 10^8 M_solar) satellites as well as the M_*mms associated with a galaxy are two of the most crucial parameters determining how much cold gas ultimately finds its way from the satellites to the host halo. Low mass galaxies are found rather lacking on both these fronts unlike their high mass counterparts. This work highlights some aspects of the complex gas physics that constitute the basic essence of a low-z CGM. My analysis proved the importance of a cosmological environment, local surroundings and merger history in defining some key observable properties of a galactic CGM. Furthermore, I found that different satellite properties were responsible for affecting the cold–dense CGM of the low and high-mass parent galaxies. Finally, the LG emerged as an exciting prospect for testing and pinning down several intricate details about the CGM. N2 - Die zeitliche Entwicklung der Sternenentstehung in einer Galaxie ist ein bestimmender Faktor für deren Entwicklung. Dabei ist die Sternenentstehungsrate stark abhängig von der in der Galaxie verfügbaren Menge an kaltem Gas. Die Akkretion von Gas aus dem intergalaktischen Medium (IGM) wird als eine der wichtigsten Quellen für das Gasreservoir angesehen, aus dem sich junge Sterne bilden. Bei diesem Prozess passiert das Gas zunächst die virialisierten äußeren Regionen der Milchstraße bevor es das Interstellare Medium (ISM) erreicht, der wichtigste Ort für die galaktische Sternentstehung. Im Gegensatz dazu tragen energiereiche Winde Gas zurück in die virialisierten Außenbereiche der Galaxie. Diese entstehen aufgrund der spezifischen Evolutionsprozesse von besonders jungen und massereichen Sternen in der galaktischen Scheibe. Durch das Zusammenspiel von einfallendem und das die Galaxie verlassendem Gas entsteht eine Vielzahl von astrophysikalischen Prozessen welche auf unterschiedlichsten Zeitskalen sowie in der Haloregion der Galaxie und dem zirkumgalaktischen Medium (CGM) von besonderer Wichtigkeit sind. Es kann behauptet werden, dass das CGM maßgeblich über die Gasreserven der Galaxie entscheidet und daher eine elementare Rolle in der Bestimmung vieler galaktischer Eigenschaften spielt von denen mache direkt beobachtbar sind. Die Beobachtung des CGM in seinen vielen unterschiedlichen Gasphasen (durch die Spektrallinienanalyse mehrerer Ionenspezies) gestaltet sich auch heute noch als kompliziert. Die geringen Teilchendichten und die schiere Größe im Zusammenspiel mit Abweichungen von sphärischer Geometrie erschweren es, vollständige, repräsentative und hochqualitative spektrale Datensätze zu erhalten welche das volle Ausmaß das galaktischen Halos in Betracht ziehen. Diese unvollständige Informationslage führt oft zu unterschiedlichen Interpretationen der Eigenschaften des CGM welche sich in verschiedenen, sich mitunter widersprechenden Modellen, widerspiegeln. In diesem Zusammenhang bieten Computersimulationen eine elegante Lösung, um viele der CGM Modelle zu testen und schließlich zu verifizieren oder falsifizieren. Die kontrollierte Umgebung erlaubt es, das CGM mühelos auf unterschiedlichsten Größenordnungen in Raum und Zeit zu untersuchen aber auch observationstechnische Limitationen zu umgehen, um ein einzigartiges Bild der Eingenschaften des CGM zu erhalten. In dieser Arbeit fokussiere ich mich auf die effektive Nutzung von verschiedenen Computersimulationen, um die Rolle des CGM im verschiedenen astrophysikalischen Kontexten zu verstehen. Im Kapitel 2 diskutiere ich den Ansatz, welcher für das Modellieren derunterschiedlichen Gasphasen des CGM in der Lokalen Gruppe (LG) bei z = 0 in den ”constrained” Simulationen des Hestia Projekts angewandt worden iii ist. Jede der drei Realisierungen enthält ein Milchstraßen-M31 Paar zusammen ihren Satellitengalaxien. Alle zusammen sind dabei eingebettet in den größeren kosmologischen Kontext. Für die Charakterisierung der unterschiedlichen Temperatur-Dichte Phasen im CGM habe ich eine Gruppe von fünf Ionen gewählt welche das Vorhandensein der Phasen anzeigen. Für jede der Zellen in der Simulation habe ich das cloudy post-processing Toolkit angewandt und die entsprechenden Anteile der Ionen im Gas bestimmt. Das kalte und kühle CGM (entsprechend charakterisiert durch H i beziehungsweise Si iii) zeigt sich sehr klumpig und ist nahe an den galaktischen Zentren verteilt während das warm-heiße CGM (charakterisiert durch O vi, O vii, O viii) dünn verteilt und volumenfüllend ist. Durch den Vergleich der Säulendichten für H i und Si iii aus den Simulationen zusammen mit Beobachtungsdaten der AMIGA Durchmusterung und Studien über andere Galaxien mit geringer Rotverschiebung habe ich herausgefunden, dass hestia weniger Gas in den Außenbereichen des CGM produziert als es die Beobachtungsdaten suggerieren. Mein sorgfältig entworfenes Modell für den Beobachtungsbias hat die Möglichkeit aufgezeigt, dass in Beobachtungen mache der Milchstraße zugehörigen Gaswolken als M31-zugehörig missinterpretiert werden könnten. Im Kapitel 3 präsentiere ich Ergebnisse von vier zoom-in, major merger und gasreichen Simulationen unter dem Gesichtspunkt der Rolle des Gases, welches ursprünglich dem CGM zugehörig ist und dessen Einfluss auf einige galaktische Observablen. Die initialen Parameter sind so ausgewählt, dass die den Verschmelzungen entspringenden Galaxien eine vergleichbare Masse wie die der Milchstraße besitzen. Im Allgemeinen sehen wir eine klare Brücke von Gas im Falle von Verschmelzungen welche mehrere separate Annäherungen durchlebten. Im Vergleich dazu fehlt diese Brücke in den Fällen einer direkten Kollision. Auf der Grundlage von particle-to-galaxy Distanz Berechnungen und tracer particle Analysen habe ich herausgefunden, dass rund 33–48 Prozent des kalten Gases aus dem CGM zur Sternenentstehung, welche in Folge der Kollision erfolgt, beiträgt. In Kapitel 4 habe ich eine Stichprobe aus 234 L* Galaxien, jeweils mit der Masse der Milchstraße, aus der Kosmologischen Simulation TNG50 genutzt, um den Einfluss der globalen Begleitgalaxienpopulation auf die Eigenschaften des ausgedehnten und kalten CGM der Zentralhalos zu bestimmen. Auf der Basis der Halomasse habe ich die Galaxienhalos in Bins niedriger und hohern Masse eingeteilt. Dabei ist jeder dieser Bins wiederum unterteilt in das untere, mittlere und obere Quartil in Abhängigkeit der Anzahl der Begleitgalaxien (N_sats) im jeweiligen Halo. Nach der Bestätigung dass Begleitgalaxien in der Tat die Gasdichteprofile ihrer Zentralgalaxie beeinflussen, habe ich die Effekte von verschiedenen Populationsparametern der Begleitgalaxien auf die CGM der jeweiligen Zentralgalaxien untersucht. Meine Analyse zeigt, dass nahezu kein kaltes Gas mit der Population der Satellitengalaxien in den Halos mit der geringsten Masse assoziiert ist. Das Gas der Halos im masseärmeren Bin ist primär beeinflusst durch die stellare Masse der massereichsten Satelliten-galaxie (M_*mms), wohingegen N_sats (gefolgt von M _*mms) die Masse des kalten Gases der massereichsten Zentralhalos am signifikantesten beeinflusst hat. Unabhängig davon schien es nicht von Relevanz zu sein wie einfach das Gas von der massereichsten Satellitengalaxie abgetragen werden kann. Die Anzahl der der massereichen (M* > 10^8 M_solar) Satellitengalaxien, sowie die mit einer Galaxie assoziierten M_*mms zeigten sich als zwei der Wichtigsten Parameter um zu verstehen wie das kalte Gas von den Satellitengalaxien in den Halo transferiert wird. Im Falle von masseärmeren Galaxien scheinen sich diese in beiden Aspekten von ihren massereichen Gegenstücken zu unterscheiden und zeigen keine besondere Abhängingkeit. Diese Arbeit behandelt einige Aspekte der komplexem physikalischen Aspekte von astrophysikalischen Gasen welche die Basis für die Untersuchung des CGM bei geringen Rotverschiebungen bildet. Meine Analyse zeigt die Wichtigkeit des Kosmologischen Umfelds, die lokale Umgebung, sowie die Verschmelzungshistorie indem sie fundamentale Observablen des galaktischen CGM beeinflussen. Des weiteren habe ich herausgefunden, dass verschiedene Satelliteneigenschaften für die Beeinflussung des kalt–dichten CGM der masseärmeren und massereichen Muttergalaxien verantwortlich waren. Schließlich stellte sich heraus, dass die LG eine vielversprechendes Beispiel zum Testen und Festhalten mehrerer komplizierter Details über das CGM darstellt. KW - circumgalactic medium KW - cosmological simulations KW - local group KW - major mergers KW - satellite galaxies KW - zirkumgalaktischen Medium KW - kosmologische Computersimulationen KW - lokalen Gruppe KW - Major mergers KW - Begleitgalaxien Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-590543 ER - TY - JOUR A1 - Davidzon, Iary A1 - Ilbert, Olivier A1 - Faisst, Andreas L. A1 - Sparre, Martin A1 - Capak, Peter L. T1 - An Alternate Approach to Measure Specific Star Formation Rates at 2 < z < 7 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We trace the specific star formation rate (sSFR) of massive star-forming galaxies (greater than or similar to 10(10)M(circle dot)) from z similar to 2 to 7. Our method is substantially different from previous analyses, as it does not rely on direct estimates of star formation rate, but on the differential evolution of the galaxy stellar mass function (SMF). We show the reliability of this approach by means of semianalytical and hydrodynamical cosmological simulations. We then apply it to real data, using the SMFs derived in the COSMOS and CANDELS fields. We find that the sSFR is proportional to (1 + z)(1.1) (+/-) (0.2) at z > 2, in agreement with other observations but in tension with the steeper evolution predicted by simulations from z similar to 4 to 2. We investigate the impact of several sources of observational bias, which, however, cannot account for this discrepancy. Although the SMF of high-redshift galaxies is still affected by significant errors, we show that future large-area surveys will substantially reduce them, making our method an effective tool to probe the massive end of the main sequence of star-forming galaxies. KW - galaxies: evolution KW - galaxies: high-redshift KW - galaxies: star formation Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aaa19e SN - 0004-637X SN - 1538-4357 VL - 852 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Padash, Amin A1 - Aghion, Erez A1 - Schulz, Alexander A1 - Barkai, Eli A1 - Chechkin, Aleksei V. A1 - Metzler, Ralf A1 - Kantz, Holger T1 - Local equilibrium properties of ultraslow diffusion in the Sinai model JF - New journal of physics N2 - We perform numerical studies of a thermally driven, overdamped particle in a random quenched force field, known as the Sinai model. We compare the unbounded motion on an infinite 1-dimensional domain to the motion in bounded domains with reflecting boundaries and show that the unbounded motion is at every time close to the equilibrium state of a finite system of growing size. This is due to time scale separation: inside wells of the random potential, there is relatively fast equilibration, while the motion across major potential barriers is ultraslow. Quantities studied by us are the time dependent mean squared displacement, the time dependent mean energy of an ensemble of particles, and the time dependent entropy of the probability distribution. Using a very fast numerical algorithm, we can explore times up top 10(17) steps and thereby also study finite-time crossover phenomena. KW - Sinai diffusion KW - clustering KW - local equilibrium Y1 - 2022 U6 - https://doi.org/10.1088/1367-2630/ac7df8 SN - 1367-2630 VL - 24 IS - 7 PB - IOP Publishing CY - Bristol ER - TY - JOUR A1 - McKenna, Russell A1 - Pfenninger, Stefan A1 - Heinrichs, Heidi A1 - Schmidt, Johannes A1 - Staffell, Iain A1 - Bauer, Christian A1 - Gruber, Katharina A1 - Hahmann, Andrea N. A1 - Jansen, Malte A1 - Klingler, Michael A1 - Landwehr, Natascha A1 - Larsén, Xiaoli Guo A1 - Lilliestam, Johan A1 - Pickering, Bryn A1 - Robinius, Martin A1 - Tröndle, Tim A1 - Turkovska, Olga A1 - Wehrle, Sebastian A1 - Weinand, Jann Michael A1 - Wohland, Jan T1 - High-resolution large-scale onshore wind energy assessments BT - a review of potential definitions, methodologies and future research needs JF - Renewable energy N2 - The rapid uptake of renewable energy technologies in recent decades has increased the demand of energy researchers, policymakers and energy planners for reliable data on the spatial distribution of their costs and potentials. For onshore wind energy this has resulted in an active research field devoted to analysing these resources for regions, countries or globally. A particular thread of this research attempts to go beyond purely technical or spatial restrictions and determine the realistic, feasible or actual potential for wind energy. Motivated by these developments, this paper reviews methods and assumptions for analysing geographical, technical, economic and, finally, feasible onshore wind potentials. We address each of these potentials in turn, including aspects related to land eligibility criteria, energy meteorology, and technical developments of wind turbine characteristics such as power density, specific rotor power and spacing aspects. Economic aspects of potential assessments are central to future deployment and are discussed on a turbine and system level covering levelized costs depending on locations, and the system integration costs which are often overlooked in such analyses. Non-technical approaches include scenicness assessments of the landscape, constraints due to regulation or public opposition, expert and stakeholder workshops, willingness to pay/accept elicitations and socioeconomic cost-benefit studies. For each of these different potential estimations, the state of the art is critically discussed, with an attempt to derive best practice recommendations and highlight avenues for future research. KW - onshore wind KW - resource assessments KW - social acceptance KW - planning constraints KW - research priorities Y1 - 2022 U6 - https://doi.org/10.1016/j.renene.2021.10.027 SN - 0960-1481 VL - 182 SP - 659 EP - 684 PB - Elsevier CY - Amsterdam ER - TY - THES A1 - Engbert, Ralf T1 - Nichtlineare Dynamik kognitiv-motorischer Prozesse BT - tempoinduzierte Übergänge bei der Produktion von Polyrhythmen N2 - Die Produktion von Polyrhythmen ist ein wichtiger experimenteller Zugang für die Untersuchung der menschlichen Motorik. Durch Variation des Tempos (externer Kontrollparameter) bei rhythmischen Bewegungsabläufen können qualitative Übergänge in der Koordinationsdynamik induziert werden. Diese Übergänge lassen sich mit der Methode der symbolischen Dynamik in experimentellen Zeitreihen nachweisen und sind ein wichtiger Hinweis darauf, dass die untersuchten Bewegungsabläufe nichtlinearen Kontrollprozessen unterliegen. Die theoretische Beschreibung bimanueller Rhythmusproduktion mit gekoppelten Differenzengleichungen führt auf ein Modell mit nichtlinearer Fehlerkontrolle. Es ist eine wichtige Eigenschaft der Kontrollprozesse, dass sie mit zeitverzögerter Rückkopplung arbeiten. Neben deterministischen Steuerungsmechanismen ist die Motorik des Menschen ausserdem von Fluktuationen auf zwei Ebenen gekennzeichnet, der kognitiven Kontrollebene und der Ebene der motorischen Systeme. Daher ist die Koordination von Bewegungen das Ergebnis von Wechselwirkungen zwischen nichtlinearen, zeitverzögerten Kontrollprozessen und stochastischen Fluktuationen. KW - Nichtlineare Dynamik KW - kognitive Prozesse KW - Motorik KW - Polyrhythmen Y1 - 1997 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0000157 ER - TY - THES A1 - Aseev, Nikita T1 - Modeling and understanding dynamics of charged particles in the Earth's inner magnetosphere T1 - Modellierung und Untersuchung der Dynamik geladener Teilchen in der inneren Magnetosphäre der Erde N2 - The Earth's inner magnetosphere is a very dynamic system, mostly driven by the external solar wind forcing exerted upon the magnetic field of our planet. Disturbances in the solar wind, such as coronal mass ejections and co-rotating interaction regions, cause geomagnetic storms, which lead to prominent changes in charged particle populations of the inner magnetosphere - the plasmasphere, ring current, and radiation belts. Satellites operating in the regions of elevated energetic and relativistic electron fluxes can be damaged by deep dielectric or surface charging during severe space weather events. Predicting the dynamics of the charged particles and mitigating their effects on the infrastructure is of particular importance, due to our increasing reliance on space technologies. The dynamics of particles in the plasmasphere, ring current, and radiation belts are strongly coupled by means of collisions and collisionless interactions with electromagnetic fields induced by the motion of charged particles. Multidimensional numerical models simplify the treatment of transport, acceleration, and loss processes of these particles, and allow us to predict how the near-Earth space environment responds to solar storms. The models inevitably rely on a number of simplifications and assumptions that affect model accuracy and complicate the interpretation of the results. In this dissertation, we quantify the processes that control electron dynamics in the inner magnetosphere, paying particular attention to the uncertainties of the employed numerical codes and tools. We use a set of convenient analytical solutions for advection and diffusion equations to test the accuracy and stability of the four-dimensional Versatile Electron Radiation Belt (VERB-4D) code. We show that numerical schemes implemented in the code converge to the analytical solutions and that the VERB-4D code demonstrates stable behavior independent of the assumed time step. The order of the numerical scheme for the convection equation is demonstrated to affect results of ring current and radiation belt simulations, and it is crucially important to use high-order numerical schemes to decrease numerical errors in the model. Using the thoroughly tested VERB-4D code, we model the dynamics of the ring current electrons during the 17 March 2013 storm. The discrepancies between the model and observations above 4.5 Earth's radii can be explained by uncertainties in the outer boundary conditions. Simulation results indicate that the electrons were transported from the geostationary orbit towards the Earth by the global-scale electric and magnetic fields. We investigate how simulation results depend on the input models and parameters. The model is shown to be particularly sensitive to the global electric field and electron lifetimes below 4.5 Earth's radii. The effects of radial diffusion and subauroral polarization streams are also quantified. We developed a data-assimilative code that blends together a convection model of energetic electron transport and loss and Van Allen Probes satellite data by means of the Kalman filter. We show that the Kalman filter can correct model uncertainties in the convection electric field, electron lifetimes, and boundary conditions. It is also demonstrated how the innovation vector - the difference between observations and model prediction - can be used to identify physical processes missing in the model of energetic electron dynamics. We computed radial profiles of phase space density of ultrarelativistic electrons, using Van Allen Probes measurements. We analyze the shape of the profiles during geomagnetically quiet and disturbed times and show that the formation of new local minimums in the radial profiles coincides with the ground observations of electromagnetic ion-cyclotron (EMIC) waves. This correlation indicates that EMIC waves are responsible for the loss of ultrarelativistic electrons from the heart of the outer radiation belt into the Earth's atmosphere. N2 - Die innere Magnetosphäre der Erde ist ein sehr dynamisches System, das hauptsächlich vom äußeren Sonnenwind beeinflusst wird, der auf das Magnetfeld unseres Planeten einwirkt. Störungen im Sonnenwind, wie z.B. koronale Massenauswürfe und sogenannte Korotierende Wechselwirkungsbereiche, verursachen geomagnetische Stürme, die zu deutlichen Veränderungen der Populationen geladener Teilchen in der inneren Magnetosphäre führen - Plasmasphäre, Ringstrom und Strahlungsgürtel. Satelliten, die in Regionen mit erhöhten energetischen und relativistischen Elektronenflüssen betrieben werden, können durch tiefe dielektrische Ladung oder Oberflächenladungen bei schweren Weltraumwetterereignissen beschädigt werden. Die Vorhersage der Dynamik der geladenen Teilchen und die Abschwächung ihrer Auswirkungen auf die Infrastruktur sind heutzutage von besonderer Bedeutung, insbesondere aufgrund unserer zunehmenden Abhängigkeit von Weltraumtechnologien. Die Dynamik von Teilchen in der Plasmasphäre, des Ringstrom und in den Strahlungsgürteln sind durch Kollisionen und kollisionsfreie Wechselwirkungen mit elektromagnetischen Feldern, die durch die Bewegung geladener Teilchen induziert werden, stark gekoppelt. Mehrdimensionale numerische Modelle vereinfachen die Betrachtung von Transport-, Beschleunigungs- und Verlustprozessen dieser Partikel und ermöglichen es uns, vorherzusagen, wie die erdnahe Weltraumumgebung auf Sonnenstürme reagiert. Die Modelle beruhen zwangsläufig auf einer Reihe von Vereinfachungen und Voraussetzungen, die sich auf die Modellgenauigkeit auswirken und die Interpretation der Ergebnisse erschweren. In dieser Dissertation quantifizieren wir die Prozesse, die die Dynamik der Elektronen in der inneren Magnetosphäre steuern. Dabei richten wir den Fokus insbesondere auch auf die Unsicherheiten der verwendeten numerischen Codes. Wir verwenden eine Reihe praktischer analytischer Lösungen für Advektions- und Diffusionsgleichungen, um die Genauigkeit und Stabilität des 4-dimensionalen ''Versatile Electron Radiation Belt'' Codes (VERB-4D Code) zu testen. Wir zeigen, dass die im Code implementierten numerischen Schemata zu den analytischen Lösungen konvergieren und der Code sich unabhängig vom angenommenen Zeitschritt stabil verhält. Wir demonstrieren, wie die Genauigkeit des numerischen Schemas für die Konvektionsgleichung die Ergebnisse von Ringstrom- und Strahlungsgürtelsimulationen beeinflussen kann, und dass es von entscheidender Beteutung ist, numerische Schemata höherer Ordnung zu verwenden, um numerische Fehler im Modell zu reduzieren. Mit dem ausführlich getesteten VERB-4D Code modellieren wir die Dynamik der Ringstromelektronen während des Sturms vom 17. März 2013. Wir zeigen, dass die Diskrepanzen zwischen dem Modell und Beobachtungen oberhalb von 4.5 Erdradien durch Unsicherheiten in den äußeren Randbedingungen erklärt werden können und dass die Elektronen durch die globalen elektrischen und magnetischen Felder von der geostationäre Umlaufbahn zur Erde transportiert wurden. Wir untersuchen weiterhin, wie die Simulationsergebnisse von den Eingabemodellen und Parametern abhängen. Wir zeigen, dass das Modell besonders empfindlich für das globale elektrische Feld und die Lebensdauer der Elektronen unterhalb von 4.5 Erdradien ist. Außerdem quantifizieren wir auch die Auswirkungen von radialer Diffusion und subauroralen Polarisationsströmen. Wir haben einen datenassimilativen Code entwickelt, der mithilfe des Kalman-Filters ein Konvektionsmodell für den Transport und den Verlust energetischer Elektronen mit den Satellitendaten der Van Allen Probes kombiniert. Wir zeigen, dass die Verwendung eines Kalman-Filters Modellunsicherheiten im elektrischen Konvektionsfeld, in der Lebensdauer der Elektronen und in den Randbedingungen korrigieren kann. Weiterhin zeigen wir, wie der Innovationsvektor - die Differenz zwischen Beobachtungen und Modellvorhersagen - verwendet werden kann, um physikalische Prozesse zu identifizieren, die im Modell der Dynamik der energetischen Elektronen fehlen. Außerdem berechnen wir radiale Profile der Phasenraumdichte ultrarelativistischer Elektronen mithilfe von Van Allen Probes-Messungen. Wir analysieren die Form der Profile und zeigen, dass die Entstehung neuer lokaler Minima in den radialen Profilen mit den Bodenbeobachtungen von EMIC-Wellen übereinstimmt. Diese Korrelation legt nahe, dass EMIC-Wellen für den Verlust ultrarelativistischer Elektronen vom Herzen des äußeren Strahlungsgürtels in die Erdatmosphäre verantwortlich sind. KW - ring current electrons KW - radiation belts KW - mathematical modeling KW - wave-particle interactions KW - data assimilation KW - Ringstromelektronen KW - Strahlungsgürtel KW - mathematische Modellierung KW - Wellen-Teilchen Wechselwirkungen KW - Datenassimilation Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-479211 ER - TY - JOUR A1 - Born, Artur A1 - Johansson, Fredrik O. L. A1 - Leitner, Torsten A1 - Bidermane, Ieva A1 - Kuehn, Danilo A1 - Martensson, Nils A1 - Föhlisch, Alexander T1 - The degree of electron itinerancy and shell closing in the core-ionized state of transition metals probed by Auger-photoelectron coincidence spectroscopy JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Auger-photoelectron coincidence spectroscopy (APECS) has been used to examine the electron correlation and itinerance effects in transition metals Cu, Ni and Co. It is shown that the LVV Auger, in coincidence with 2p photoelectrons, spectra can be represented using atomic multiplet positions if the 3d-shell is localized (atomic-like) and with a self-convoluted valence band for band-like (itinerant) materials as explained using the Cini-Sawatzky model. For transition metals, the 3d band changes from band-like to localized with increasing atomic number, with the possibility of a mixed behavior. Our result shows that the LVV spectra of Cu can be represented by atomic multiplet calculations, those of Co resemble the self-convolution of the valence band and those of Ni are a mixture of both, consistent with the Cini-Sawatzky model. KW - spectra KW - Ni Y1 - 2022 U6 - https://doi.org/10.1039/d2cp02477b SN - 1463-9076 SN - 1463-9084 VL - 24 IS - 32 SP - 19218 EP - 19222 PB - Royal Society of Chemistry CY - Cambridge ER - TY - THES A1 - Matuschek, Hannes T1 - Applications of reproducing Kernel Hilbert spaces and their approximations Y1 - 2015 ER - TY - JOUR A1 - Kroh, Daniel A1 - Eller, Fabian A1 - Schötz, Konstantin A1 - Wedler, Stefan A1 - Perdigón-Toro, Lorena A1 - Freychet, Guillaume A1 - Wei, Qingya A1 - Dörr, Maximilian A1 - Jones, David A1 - Zou, Yingping A1 - Herzig, Eva M. A1 - Neher, Dieter A1 - Köhler, Anna T1 - Identifying the signatures of intermolecular interactions in blends of PM6 with Y6 and N4 using absorption spectroscopy JF - Advanced functional materials N2 - In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the-art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in the blend films and accounts for the different solar cell efficiencies reported with the two blends. KW - charge-transfer states KW - Frank-Condon analysis KW - morphology KW - organic solar cells Y1 - 2022 U6 - https://doi.org/10.1002/adfm.202205711 SN - 1616-301X SN - 1616-3028 VL - 32 IS - 44 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kniepert, Juliane A1 - Paulke, Andreas A1 - Perdigón-Toro, Lorena A1 - Kurpiers, Jona A1 - Zhang, Huotian A1 - Gao, Feng A1 - Yuan, Jun A1 - Zou, Yingping A1 - Le Corre, Vincent M. A1 - Koster, Lambert Jan Anton A1 - Neher, Dieter T1 - Reliability of charge carrier recombination data determined with charge extraction methods JF - Journal of applied physics N2 - Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k(2) = 1.7 x 10(-17)m(3) s(-1). BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that surface recombination is of minor importance despite the small active layer thickness. Published under license by AIP Publishing. Y1 - 2019 U6 - https://doi.org/10.1063/1.5129037 SN - 0021-8979 SN - 1089-7550 VL - 126 IS - 20 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Schwarze, Martin A1 - Schellhammer, Karl Sebastian A1 - Ortstein, Katrin A1 - Benduhn, Johannes A1 - Gaul, Christopher A1 - Hinderhofer, Alexander A1 - Perdigón-Toro, Lorena A1 - Scholz, Reinhard A1 - Kublitski, Jonas A1 - Roland, Steffen A1 - Lau, Matthias A1 - Poelking, Carl A1 - Andrienko, Denis A1 - Cuniberti, Gianaurelio A1 - Schreiber, Frank A1 - Neher, Dieter A1 - Vandewal, Koen A1 - Ortmann, Frank A1 - Leo, Karl T1 - Impact of molecular quadrupole moments on the energy levels at organic heterojunctions JF - Nature Communications N2 - The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the p-p-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor-acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers. Y1 - 2019 U6 - https://doi.org/10.1038/s41467-019-10435-2 SN - 2041-1723 VL - 10 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Collado-Fregoso, Elisa A1 - Pugliese, Silvina N. A1 - Wojcik, Mariusz A1 - Benduhn, Johannes A1 - Bar-Or, Eyal A1 - Perdigón-Toro, Lorena A1 - Hörmann, Ulrich A1 - Spoltore, Donato A1 - Vandewal, Koen A1 - Hodgkiss, Justin M. A1 - Neher, Dieter T1 - Energy-gap law for photocurrent generation in fullerene-based organic solar cells BT - the case of low-donor-content blends JF - Journal of the American Chemical Society N2 - The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C-60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state. Y1 - 2019 U6 - https://doi.org/10.1021/jacs.8b09820 SN - 0002-7863 VL - 141 IS - 6 SP - 2329 EP - 2341 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Rodriguez Zuluaga, Juan T1 - Electric and magnetic characteristics of equatorial plasma depletions T1 - Elektrische und magnetische Beschreibung von äquatorialen Plasma-Verarmungen BT - an observational assessment using the Swarm mission BT - eine empirische Beurteilung mit der Satellitenmission Swarm N2 - Near-Earth space represents a significant scientific and technological challenge. Particularly at magnetic low-latitudes, the horizontal magnetic field geometry at the dip equator and its closed field-lines support the existence of a distinct electric current system, abrupt electric field variations and the development of plasma irregularities. Of particular interest are small-scale irregularities associated with equatorial plasma depletions (EPDs). They are responsible for the disruption of trans-ionospheric radio waves used for navigation, communication, and Earth observation. The fast increase of satellite missions makes it imperative to study the near-Earth space, especially the phenomena known to harm space technology or disrupt their signals. EPDs correspond to the large-scale structure (i.e., tens to hundreds of kilometers) of topside F region irregularities commonly known as Spread F. They are observed as depleted-plasma density channels aligned with the ambient magnetic field in the post-sunset low-latitude ionosphere. Although the climatological variability of their occurrence in terms of season, longitude, local time and solar flux is well-known, their day to day variability is not. The sparse observations from ground-based instruments like radars and the few simultaneous measurements of ionospheric parameters by space-based instruments have left gaps in the knowledge of EPDs essential to comprehend their variability. In this dissertation, I profited from the unique observations of the ESA’s Swarm constellation mission launched in November 2013 to tackle three issues that revealed novel and significant results on the current knowledge of EPDs. I used Swarm’s measurements of the electron density, magnetic, and electric fields to answer, (1.) what is the direction of propagation of the electromagnetic energy associated with EPDs?, (2.) what are the spatial and temporal characteristics of the electric currents (field-aligned and diamagnetic currents) related to EPDs, i.e., seasonal/geographical, and local time dependencies?, and (3.) under what conditions does the balance between magnetic and plasma pressure across EPDs occur? The results indicate that: (1.) The electromagnetic energy associated with EPDs presents a preference for interhemispheric flows; that is, the related Poynting flux directs from one magnetic hemisphere to the other and varies with longitude and season. (2.) The field-aligned currents at the edges of EPDs are interhemispheric. They generally close in the hemisphere with the highest Pedersen conductance. Such hemispherical preference presents a seasonal/longitudinal dependence. The diamagnetic currents increase or decrease the magnetic pressure inside EPDs. These two effects rely on variations of the plasma temperature inside the EPDs that depend on longitude and local time. (3.) EPDs present lower or higher plasma pressure than the ambient. For low-pressure EPDs the plasma pressure gradients are mostly dominated by variations of the plasma density so that variations of the temperature are negligible. High-pressure EPDs suggest significant temperature variations with magnitudes of approximately twice the ambient. Since their occurrence is more frequent in the vicinity of the South Atlantic magnetic anomaly, such high temperatures are suggested to be due to particle precipitation. In a broader context, this dissertation shows how dedicated satellite missions with high-resolution capabilities improve the specification of the low-latitude ionospheric electrodynamics and expand knowledge on EPDs which is valuable for current and future communication, navigation, and Earth-observing missions. The contributions of this investigation represent several ’firsts’ in the study of EPDs: (1.) The first observational evidence of interhemispheric electromagnetic energy flux and field-aligned currents. (2.) The first spatial and temporal characterization of EPDs based on their associated field-aligned and diamagnetic currents. (3.) The first evidence of high plasma pressure in regions of depleted plasma density in the ionosphere. These findings provide new insights that promise to advance our current knowledge of not only EPDs but the low-latitude post-sunset ionosphere environment. N2 - Der erdnahe Weltraum stellt eine bedeutende wissenschaftliche und technologische Herausforderung dar. Insbesondere in niedrigeren magnetischen Breitengraden unterstützen die horizontale Geometrie des Magnetfelds und seine geschlossenen Feldlinien das Vorhandensein eines speziellen elektrischen Stromsystems, abrupte Änderungen der elektrischen Felder und das Auftreten von Plasmairregularitäten. Von besonderem Interesse sind regionale Unregelmäßigkeiten im Zusammenhang mit äquatorialen Plasma-Verarmungen (EPDs, Abkürzung aus dem Englischen für „equatorial plasma depletions”). Sie stören trans-ionosphärischer Funkwellen, welche zur Positionierung, Kommunikation und Erd-beobachtung eingesetzt werden. Die schnelle Entwicklung von Satellitenmissionen macht das Verständnis der erdnahen Weltraumphänomene zu einer Priorität, insbesondere derjenigen, welche die Weltraumtechnologie schädigen oder ihre Signale stören können. Die EPDs und die damit verbundenen Plasmairregularitäten sind seit Beginn des Weltraumzeitalters eines der am häufigsten untersuchten Phänomene. EPDs sind großflächigen Strukturen (d. h. zehn bis hundert Kilometer), die auf Spread F Ereignisse zurückgeführt werden können. Sie äußern sich als mit dem Hintergrund-Magnetfeld ausgerichtete Kanäle verarmter Plasmadichte, welche in niedrigen Breiten in der Ionophäre nach Sonnenuntergang auftreten. Obwohl die klimatologische Variabilität des Auftretens von EPDs bezüglich der Jahreszeit, geografischen Länge, Ortszeit und des Sonnenzyklus wohl bekannt sind, trifft dies nicht für ihre Tag-zu-Tag-Variabilität zu. Die spärlichen Beobachtungen von bodengestützten Instrumenten, wie Radargeräten, und die wenigen gleichzeitigen Messungen ionosphärischer Parameter von weltraumgestützten Instrumenten auf erdnahen Umlaufbahnen haben Wissenslücken hinterlassen, die für das Verständnis der Variabilität von EPDs essentiell sind. In dieser Dissertation habe ich von einzigartigen Beobachtungen der im November 2013 gestarteten ESA Satellitenkonstellationsmission „Swarm“ profitiert, um drei Probleme zu bearbeiten, die neue und signifikante Ergebnisse zum aktuellen Wissen über EPDs enthüllten. Ich habe Swarms Messungen der Elektronendichte, des magnetischen und des elektrischen Feldes verwendet, um Folgendes zu beantworten: (1.) In welche Richtung breitet sich die mit den EPDs verbundene elektromagnetische Energie aus? (2.) Was sind die räumlichen und zeitlichen Eigenschaften der elektrischen Ströme (feldgerichtete und diamagnetische Ströme) in Bezug auf EPDs, d. h. wie hängen sie von der geografischen Länge, Jahreszeit und Lokalzeit ab? (3.) Unter welchen Bedingungen findet der mit EPDs verbundene Ausgleich zwischen magnetischem Druck und Plasmadruck statt? Die Ergebnisse zeigen, dass: (1.) Die mit EPDs verbundene elektromagnetische Energie bevorzugt interhemisphärische Strömungen, das heißt, der zugehörige Poynting-Fluss strömt von einer magnetischen Hemisphäre zur anderen und die Strömungsrichtung variiert mit geografischer Länge und Jahreszeit. (2.) Die feldgerichteten Ströme an den Rändern von EPDs sind interhemisphärisch. Im Allgemeinen schließen sie sich in der Hemisphäre mit der höchsten Pedersen-Leitfähigkeit. Die derartige hemisphärische Präferenz zeigt eine Abhängigkeit bezüglich der Jahreszeit/geografischen Länge. Die diamagnetischen Ströme erhöhen oder verringern den magnetischen Druck innerhalb der EPDs. Diese beiden Effekte beruhen auf Variationen der Plasmatemperatur innerhalb der EPDs, die von der geografischen Länge und der Lokalzeit abhängt. (3.) EPDs weisen einen höheren oder niedrigeren Plasmadruck als ihre Umgebung auf. In Niederdruck-EPDs werden die Plasmadruckgradienten meist durch Variationen der Plasmadichte hervorgerufen, sodass Temperaturschwankungen vernachlässigbar sind. Hochdruck-EPDs deuten auf hohe innere Temperaturen hin, etwa das Zweifache der Umgebungstemperatur. Aufgrund ihres häufigeren Auftretens in der Nähe der Südatlantischen Magnetfeldanomalie wird vermutet, dass solche hohen Temperaturen auf den Einfall hochenergetischer Teilchen zurückzuführen sind. In einem breiteren Kontext zeigt diese Dissertation auf, wie spezielle Satellitenmissionen mit hohem Auflösungsvermögen die Spezifikation der ionoshärischen Elektrodynamik in niedrigen Breiten und das Verständnis von EPDs verbessern, was wertvoll für aktuelle und zukünfte Kommunikatoins-, Positionierungs- sowie Erdbeobachtungsmissionen ist. Die Beiträge dieser Arbeit stellen gleich mehrere "Premieren" in der EPD-Forschung dar: (1.) Der erste empirische Nachweis interhemisphärischer elektromagnetischer Energieflüsse und feldgerichteter Ströme. (2.) Die erste raum-zeitliche Beschreibung von EPDs auf der Grundlage ihrer assoziierten feldgerichteten und diamagnetischen Ströme. (3.) Der erste Nachweis hohen Plasmadrucks in Regionen verminderter Plasmadichte in der Ionosphäre. Diese Forschungsergebnisse liefern neue Erkenntnisse, die nicht nur unser derzeitiges Wissen über EPDs, sondern auch jenes über die ionosphärische Domaine in niedrigen Breiten nach Sonnenuntergang fördert. KW - equatorial plasma depletions KW - electric and magnetic fields KW - spread F KW - ionosphere KW - swarm mission KW - äquatorialen Plasma-Verarmungen KW - elektrische und magnetische Felder KW - Spread F KW - Ionosphäre KW - Satellitenmission Swarm Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-445873 ER - TY - INPR A1 - Engbert, Ralf A1 - Scheffczyk, Christian A1 - Krampe, Ralf-Thomas A1 - Rosenblum, Mikhael A1 - Kurths, Jürgen A1 - Kliegl, Reinhold T1 - Tempo-induced transitions in polyrhythmic hand movements N2 - We investigate the cognitive control in polyrhythmic hand movements as a model paradigm for bimanual coordination. Using a symbolic coding of the recorded time series, we demonstrate the existence of qualitative transitions induced by experimental manipulation of the tempo. A nonlinear model with delayed feedback control is proposed, which accounts for these dynamical transitions in terms of bifurcations resulting from variation of the external control parameter. Furthermore, it is shown that transitions can also be observed due to fluctuations in the timing control level. We conclude that the complexity of coordinated bimanual movements results from interactions between nonlinear control mechanisms with delayed feedback and stochastic timing components. T3 - NLD Preprints - 41 Y1 - 1997 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-14380 ER - TY - JOUR A1 - Stolterfoht, Martin A1 - Caprioglio, Pietro A1 - Wolff, Christian Michael A1 - Marquez, Jose A. A1 - Nordmann, Joleik A1 - Zhang, Shanshan A1 - Rothhardt, Daniel A1 - Hörmann, Ulrich A1 - Amir, Yohai A1 - Redinger, Alex A1 - Kegelmann, Lukas A1 - Zu, Fengshuo A1 - Albrecht, Steve A1 - Koch, Norbert A1 - Kirchartz, Thomas A1 - Saliba, Michael A1 - Unold, Thomas A1 - Neher, Dieter T1 - The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells JF - Energy & environmental science N2 - Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V-OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the V-OC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V-OC of the device. Importantly, the V-OC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the V-OC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the V-OC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces. Y1 - 2019 U6 - https://doi.org/10.1039/c9ee02020a SN - 1754-5692 SN - 1754-5706 VL - 12 IS - 9 SP - 2778 EP - 2788 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Zhang, Shanshan A1 - Hosseini, Seyed Mehrdad A1 - Gunder, Rene A1 - Petsiuk, Andrei A1 - Caprioglio, Pietro A1 - Wolff, Christian Michael A1 - Shoaee, Safa A1 - Meredith, Paul A1 - Schorr, Susan A1 - Unold, Thomas A1 - Burn, Paul L. A1 - Neher, Dieter A1 - Stolterfoht, Martin T1 - The Role of Bulk and Interface Recombination in High-Efficiency Low-Dimensional Perovskite Solar Cells JF - Advanced materials N2 - 2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH3(CH2)(3)NH3)(2)(CH3NH3)(n-1)PbnI3n+1 perovskite cells with different numbers of [PbI6](4-) sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (V-OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C-60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device V-OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements. KW - 2D perovskites KW - interface recombination KW - perovskite solar cells KW - photoluminescence KW - V-OC loss Y1 - 2019 U6 - https://doi.org/10.1002/adma.201901090 SN - 0935-9648 SN - 1521-4095 VL - 31 IS - 30 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Caprioglio, Pietro A1 - Zu, Fengshuo A1 - Wolff, Christian Michael A1 - Prieto, Jose A. Marquez A1 - Stolterfoht, Martin A1 - Becker, Pascal A1 - Koch, Norbert A1 - Unold, Thomas A1 - Rech, Bernd A1 - Albrecht, Steve A1 - Neher, Dieter T1 - High open circuit voltages in pin-type perovskite solar cells through strontium addition JF - Sustainable Energy & Fuels N2 - The incorporation of even small amounts of strontium (Sr) into lead-base hybrid quadruple cation perovskite solar cells results in a systematic increase of the open circuit voltage (V-oc) in pin-type perovskite solar cells. We demonstrate via absolute and transient photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C-60 electron-transporting layer. The resulting solar cells exhibited a V-oc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, reducing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high V-oc values and efficiencies in Sr-containing quadruple cation perovskite pin-type solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer. Y1 - 2019 U6 - https://doi.org/10.1039/c8se00509e SN - 2398-4902 VL - 3 IS - 2 SP - 550 EP - 563 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Le Corre, Vincent M. A1 - Stolterfoht, Martin A1 - Perdigón-Toro, Lorena A1 - Feuerstein, Markus A1 - Wolff, Christian Michael A1 - Gil-Escrig, Lidon A1 - Bolink, Henk J. A1 - Neher, Dieter A1 - Koster, L. Jan Anton T1 - Charge Transport Layers Limiting the Efficiency of Perovskite Solar Cells: How To Optimize Conductivity, Doping, and Thickness JF - ACS Applied Energy Materials N2 - Perovskite solar cells (PSCs) are one of the main research topics of the photovoltaic community; with efficiencies now reaching up to 24%, PSCs are on the way to catching up with classical inorganic solar cells. However, PSCs have not yet reached their full potential. In fact, their efficiency is still limited by nonradiative recombination, mainly via trap-states and by losses due to the poor transport properties of the commonly used transport layers (TLs). Indeed, state-of-the-art TLs (especially if organic) suffer from rather low mobilities, typically within 10(-5) and 10(-2) cm(-2) V-1 s(-1), when compared to the high mobilities, 1-10 cm(-2) V-1 s(-1), measured for perovskites. This work presents a comprehensive analysis of the effect of the mobility, thickness, and doping density of the transport layers based on combined experimental and modeling results of two sets of devices made of a solution-processed high-performing triple-cation (PCE approximate to 20%). The results are also cross-checked on vacuum-processed MAPbI(3) devices. From this analysis, general guidelines on how to optimize a TL are introduced and especially a new and simple formula to easily calculate the amount of doping necessary to counterbalance the low mobility of the TLs. KW - perovskite solar cells KW - transport layers KW - conductivity KW - doping KW - charge transport Y1 - 2019 U6 - https://doi.org/10.1021/acsaem.9b00856 SN - 2574-0962 VL - 2 IS - 9 SP - 6280 EP - 6287 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Würfel, Uli A1 - Perdigón-Toro, Lorena A1 - Kurpiers, Jona A1 - Wolff, Christian Michael A1 - Caprioglio, Pietro A1 - Rech, Jeromy James A1 - Zhu, Jingshuai A1 - Zhan, Xiaowei A1 - You, Wei A1 - Shoaee, Safa A1 - Neher, Dieter A1 - Stolterfoht, Martin T1 - Recombination between Photogenerated and Electrode-Induced Charges Dominates the Fill Factor Losses in Optimized Organic Solar Cells JF - The journal of physical chemistry letters N2 - Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. However, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers, and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implies that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit. Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpclett.9b01175 SN - 1948-7185 VL - 10 IS - 12 SP - 3473 EP - 3480 PB - American Chemical Society CY - Washington ER -