Influence of ice crystal habit and cirrus spatial inhomogeneities on the retrieval of cirrus optical thickness and effective radius

Diese Dissertation untersucht den Einfluss von Eiskristallform und räumlicher Inhomogenität von Zirren auf das Retrieval von optischer Wolkendicke und effektivem Eispartikelradius. Zu diesem Zweck werden flugzeuggetragene spektrale Messungen solarer Strahlung sowie solare und langwellige Strahlungstransfersimulationen durchgeführt. Flugzeuggetragene spektrale aufwärtsgerichtete Radianzen (Strahldichten) sind mit dem SMART-Albedometer (Spectral Modular Airborne Radiation measurement sysTem) während des CIRCLE-2 (CIRrus CLoud Experiment-2) Feldexperiments im Mai 2007 gemessen worden. Basierend auf diesen Radianzdaten werden mittels eines Wolkenretrievalalgorithmus optische Wolkendicken und effektive Eispartikelradien anhand von eindimensionalen Strahlungstransferrechnungen bestimmt. Die Auswirkung der Annahme unterschiedlicher Eiskristallformen auf die retrievten Parameter wird durch Variation der Einfachstreueigenschaften der Eispartikel untersucht. Darüber hinaus wird mittels Strahlungstransferrechnungen auch der Einfluss der Eiskristallform auf den Strahlungsantrieb von Eiswolken ermittelt. Die Frage nach dem relativen Einfluss von räumlicher Wolkeninhomogenität und Eiskristallform wird anhand von dreidimensionalen und independent pixel approximation (IPA) Strahlungssimulationen untersucht. Die Analyse basiert auf einer Modelleiswolke, die aus Daten des NASA (National Aeronautics and Space Administration) TC4 (Tropical Composition, Cloud, and Climate Coupling) Feldexperiments im Sommer 2007 in Costa Rica erzeugt wurde. Lokal gesehen können beide Effekte - Eiskristallform und räumliche Eiswolkeninhomogenität - die gleiche Grössenordnung haben und zu einer Unter- bzw. Überschätzung der retrievten Parameter um 40 – 60% führen. Gemittelt über die ganze Wolke ist jedoch der Einfluss der Eiskristallform viel bedeutender als der von räumlichen Inhomogenitäten.

Solid-state NMR on columnar supramolecular structures

The present work deals with the characterisation of three columnar self-assembled systems, that is, benzene-1,3,5-tricarboxamides, a peripherally thioalkyl-substituted phthalocyanine, and several oligo-(p-phenylenevinylene)s. In order to probe the supramolecular organisation solid-state NMR has been used as the main technique, supported by X-ray measurements, theoretical methods, and thermal analysis. rnrnBenzene-1,3,5-tricarboxamides (BTAs) turned out to be well suited model compounds to study various fundamental supramolecular interactions, such as π-π-interactions, hydrogen bonding, as well as dynamic and steric effects of attached side chains. Six BTAs have been investigated in total, five with a CO-centred amide group bearing different side chains and one with an inverted N-centred amide group. The physical properties of these BTAs have been investigated as a function of temperature. The results indicated that in case of the CO-centred BTAs the stability of the columnar mesophase depends strongly on the nature of the side chains. Further experiments revealed a coplanar orientation of adjacent BTA molecules in the columnar assembly of CO-centred BTAs, whereas the N-centred BTA, showed a deviating not fully coplanar arrangement. These differences were ascribed to distinct hydrogen bonding schemes, involving a parallel alignment of hydrogen bonds in case of CO-centred BTAs and an antiparallel alignment in case of the N-centred counterpart.rnrn The fundamental insights of the supramolecular organisation of BTAs could be partially adapted to an octa-substituted phthalocyanine with thiododecyl moieties. Solid-state NMR in combination with chemical shift calculations determined a tilted herringbone arrangement of phthalocyanine rings in the crystalline phase as well as in the mesophase. Moreover, 1H NMR measurements in the mesophase of this compound suggested an axial rotation of molecules, which is inhibited in the crystalline phase.rnrnAs a third task, the supramolecular assembly of oligo-(p-phenylenevinylene)s of varying length and with different polar head groups have been investigated by a combined X-ray and solid-state NMR study. The results revealed a columnar structure formation of these compounds, being promoted by phase separation of alkyl side chains and aromatic rigid rods. In this system solid-state NMR yielded meaningful insight into the isotropisation process of butoxy and 2-S-methylbutoxy substituted oligo-(p-phenylenevinylene) rods.rn

Crystal Engineering of bright luminescent copper iodide clusters with phosphorus and nitrogen-based ligands

Copper(I) halide clusters are recently considered as good candidate for optoelectronic devices such as OLEDs . Although the copper halide clusters, in particular copper iodide, are very well known since the beginning of the 20th century, only in the late ‘70s the interest on these compounds grew dramatically due their particular photophysical behaviour. These complexes are characterized by a dual triplet emission bands, named Cluster Centred (3CC) and Halogen-to-Ligand charge transfer (3XLCT), the intensities of which are strictly related with the temperature. The CC transition, due to the presence of a metallophylic interactions, is prevalent at ambient temperature while the XLCT transition, located preferentially on the ligand part, became more prominent at low temperature. Since these pioneering works, it was easy to understand the photophysical properties of this compounds became more interesting in solid-state respect to solution with an improvement in emission efficiency. In this work we aim to characterize in SS organocopper(I)iodide compounds to valuate the correlation between the molecular crystal structure and the photophysical properties. It is also considered to hike new strategies to synthesize CuI complexes from the wet reactions to the more green solvent free methods. The advantages in using these strategies are evident but, obtain a single crystal suitable for SCXRD analysis from these batches is quite impossible. The structure solution still remains the key point in this research so we tackle this problem solving the structure by X-ray powder diffraction data. When the sample was fully characterized we moved to design and development of the associated OLED-device. Since copper iodide complexes are often insoluble in organic solvents, the high vacuum deposition technique is preferred. A new non-conventional deposition process have also been proposed to avoid the low complex stability in this practice with an in-situ complex formation in a layer-by layer deposition route.

Mechanical microcontacts of powder particles studied with the quartz crystal microbalance

Within this work, a particle-polymer surface system is studied with respect to the particle-surface interactions. The latter are governed by micromechanics and are an important aspect for a wide range of industrial applications. Here, a new methodology is developed for understanding the adhesion process and measure the relevant forces, based on the quartz crystal microbalance, QCM. rnThe potential of the QCM technique for studying particle-surface interactions and reflect the adhesion process is evaluated by carrying out experiments with a custom-made setup, consisting of the QCM with a 160 nm thick film of polystyrene (PS) spin-coated onto the quartz and of glass particles, of different diameters (5-20µm), deposited onto the polymer surface. Shifts in the QCM resonance frequency are monitored as a function of the oscillation amplitude. The induced frequency shifts of the 3rd overtone are found to decrease or increase, depending on the particle-surface coupling type and the applied oscillation (frequency and amplitude). For strong coupling the 3rd harmonic decreased, corresponding to an “added mass” on the quartz surface. However, positive frequency shifts are observed in some cases and are attributed to weak-coupling between particle and surface. Higher overtones, i.e. the 5th and 7th, were utilized in order to derive additional information about the interactions taking place. For small particles, the shift for specific overtones can increase after annealing, while for large particle diameters annealing causes a negative frequency shift. The lower overtones correspond to a generally strong-coupling regime with mainly negative frequency shifts observed, while the 7th appears to be sensitive to the contact break-down and the recorded shifts are positive.rnDuring oscillation, the motion of the particles and the induced frequency shift of the QCM are governed by a balance between inertial forces and contact forces. The adherence of the particles can be increased by annealing the PS film at 150°C, which led to the formation of a PS meniscus. For the interpretation, the Hertz, Johnson-Kendall-Roberts, Derjaguin-Müller-Toporov and the Mindlin theory of partial slip are considered. The Mindlin approach is utilized to describe partial slip. When partial slip takes place induced by an oscillating load, a part of the contact ruptures. This results in a decrease of the effective contact stiffness. Additionally, there are long-term memory effects due to the consolidation which along with the QCM vibrations induce a coupling increase. However, the latter can also break the contact, lead to detachment and even surface damage and deformation due to inertia. For strong coupling the particles appear to move with the vibrations and simply act as added effective mass leading to a decrease of the resonance frequency, in agreement with the Sauerbrey equation that is commonly used to calculate the added mass on a QCM). When the system enters the weak-coupling regime the particles are not able to follow the fast movement of the QCM surface. Hence, they effectively act as adding a “spring” with an additional coupling constant and increase the resonance frequency. The frequency shift, however, is not a unique function of the coupling constant. Furthermore, the critical oscillation amplitude is determined, above which particle detach. No movement is detected at much lower amplitudes, while for intermediate values, lateral particle displacement is observed. rnIn order to validate the QCM results and study the particle effects on the surface, atomic force microscopy, AFM, is additionally utilized, to image surfaces and measure surface forces. By studying the surface of the polymer film after excitation and particle removal, AFM imaging helped in detecting three different meniscus types for the contact area: the “full contact”, the “asymmetrical” and a third one including a “homocentric smaller meniscus”. The different meniscus forms result in varying bond intensity between particles and polymer film, which could explain the deviation between number of particles per surface area measured by imaging and the values provided by the QCM - frequency shift analysis. The asymmetric and the homocentric contact types are suggested to be responsible for the positive frequency shifts observed for all three measured overtones, i.e. for the weak-coupling regime, while the “full contact” type resulted in a negative frequency shift, by effectively contributing to the mass increase of the quartz..rnThe interplay between inertia and contact forces for the particle-surface system leads to strong- or weak-coupling, with the particle affecting in three mentioned ways the polymer surface. This is manifested in the frequency shifts of the QCM system harmonics which are used to differentiate between the two interaction types and reflect the overall state of adhesion for particles of different size.rn

Modeling and simulations of nanoparticles in liquid crystalline systems

The aim of this work is to investigate, using extensive Monte Carlo computer simulations, composite materials consisting of liquid crystals doped with nanoparticles. These systems are currently of great interest as they offer the possibility of tuning the properties of liquid crystals used in displays and other devices as well as providing a way of obtaining regularly organized systems of nanoparticles exploiting the molecular organization of the liquid crystal medium. Surprisingly enough, there is however a lack of fundamental knowledge on the properties and phase behavior of these hybrid materials, making the route to their application an essentially empirical one. Here we wish to contribute to the much needed rationalization of these systems studying some basic effects induced by different nanoparticles on a liquid crystal host. We investigate in particular the effects of nanoparticle shape, size and polarity as well as of their affinity to the liquid crystal solvent on the stability of the system, monitoring phase transitions, order and molecular organizations. To do this we have proposed a coarse grained approach where nanoparticles are modelled as a suitably shaped (spherical, rod and disk like) collection of spherical Lennard-Jones beads, while the mesogens are represented with Gay-Berne particles. We find that the addition of apolar nanoparticles of different shape typically lowers the nematic–isotropic transition of a non-polar nematic, with the destabilization being greater for spherical nanoparticles. For polar mesogens we have studied the effect of solvent affinity of the nanoparticles showing that aggregation takes places for low solvation values. Interestingly, if the nanoparticles are polar the aggregates contribute to stabilizing the system, compensating the shape effect. We thus find the overall effects on stability to be a delicate balance of often contrasting contributions pointing to the relevance of simulations studies for understanding these complex systems.

Numerical study of homogeneous and heterogeneous crystal nucleation in colloidal systems

Im Rahmen dieser Arbeit wurden Computersimulationen von Keimbildungs- und Kris\-tallisationsprozessen in rnkolloidalen Systemen durchgef\"uhrt. rnEine Kombination von Monte-Carlo-Simulationsmethoden und der Forward-Flux-Sampling-Technik wurde rnimplementiert, um die homogene und heterogene Nukleation von Kristallen monodisperser Hart\-kugeln zu untersuchen. rnIm m\"a\ss{ig} unterk\"uhlten Bulk-Hartkugelsystem sagen wir die homogenen Nukleationsraten voraus und rnvergleichen die Resultate mit anderen theoretischen Ergebnissen und experimentellen Daten. rnWeiterhin analysieren wir die kristallinen Cluster in den Keimbildungs- und Wachstumszonen, rnwobei sich herausstellt, dass kristalline Cluster sich in unterschiedlichen Formen im System bilden. rnKleine Cluster sind eher l\"anglich in eine beliebige Richtung ausgedehnt, w\"ahrend gr\"o\ss{ere} rnCluster kompakter und von ellipsoidaler Gestalt sind. rn rnIm n\"achsten Teil untersuchen wir die heterogene Keimbildung an strukturierten bcc (100)-W\"anden. rnDie 2d-Analyse der kristallinen Schichten an der Wand zeigt, dass die Struktur der rnWand eine entscheidende Rolle in der Kristallisation von Hartkugelkolloiden spielt. rnWir sagen zudem die heterogenen Kristallbildungsraten bei verschiedenen \"Ubers\"attigungsgraden voraus. rnDurch Analyse der gr\"o\ss{ten} Cluster an der Wand sch\"atzen wir zus\"atzlich den Kontaktwinkel rnzwischen Kristallcluster und Wand ab. rnEs stellt sich heraus, dass wir in solchen Systemen weit von der Benetzungsregion rnentfernt sind und der Kristallisationsprozess durch heterogene Nukleation stattfindet. rn rnIm letzten Teil der Arbeit betrachten wir die Kristallisation von Lennard-Jones-Kolloidsystemen rnzwischen zwei ebenen W\"anden. rnUm die Erstarrungsprozesse f\"ur ein solches System zu untersuchen, haben wir eine Analyse des rnOrdnungsparameters f\"ur die Bindung-Ausrichtung in den Schichten durchgef\"urt. rnDie Ergebnisse zeigen, dass innerhalb einer Schicht keine hexatische Ordnung besteht, rnwelche auf einen Kosterlitz-Thouless-Schmelzvorgang hinweisen w\"urde. rnDie Hysterese in den Erhitzungs-Gefrier\-kurven zeigt dar\"uber hinaus, dass der Kristallisationsprozess rneinen aktivierten Prozess darstellt.

Transport Properties and Novel Sensing Applications of Organic Semiconducting Crystals

The present thesis is focused on the study of Organic Semiconducting Single Crystals (OSSCs) and crystalline thin films. In particular solution-grown OSSC, e.g. 4-hdroxycyanobenzene (4HCB) have been characterized in view of their applications as novel sensors of X-rays, gamma-rays, alpha particles radiations and chemical sensors. In the field of ionizing radiation detection, organic semiconductors have been proposed so far mainly as indirect detectors, i.e. as scintillators or as photodiodes. I first study the performance of 4HCB single crystals as direct X-ray detector i.e. the direct photon conversion into an electrical signal, assessing that they can operate at room temperature and in atmosphere, showing a stable and linear response with increasing dose rate. A dedicated study of the collecting electrodes geometry, crystal thickness and interaction volume allowed us to maximize the charge collection efficiency and sensitivity, thus assessing how OSSCs perform at low operating voltages and offer a great potential in the development of novel ionizing radiation sensors. To better understand the processes generating the observed X-ray signal, a comparative study is presented on OSSCs based on several small-molecules: 1,5-dinitronaphthalene (DNN), 1,8-naphthaleneimide (NTI), Rubrene and TIPS-pentacene. In addition, the proof of principle of gamma-rays and alpha particles has been assessed for 4HCB single crystals. I have also carried out an investigation of the electrical response of OSSCs exposed to vapour of volatile molecules, polar and non-polar. The last chapter deals with rubrene, the highest performing molecular crystals for electronic applications. We present an investigation on high quality, millimeter-sized, crystalline thin films (10 – 100 nm thick) realized by exploiting organic molecular beam epitaxy on water-soluble substrates. Space-Charge-Limited Current (SCLC) and photocurrent spectroscopy measurements have been carried out. A thin film transistor was fabricated onto a Cytop® dielectric layer. The FET mobility exceeding 2 cm2/Vs, definitely assess the quality of RUB films.

Prediction of new crystal structures under extreme conditions

One of the most important challenges in chemistry and material science is the connection between the contents of a compound and its chemical and physical properties. In solids, these are greatly influenced by the crystal structure.rnrnThe prediction of hitherto unknown crystal structures with regard to external conditions like pressure and temperature is therefore one of the most important goals to achieve in theoretical chemistry. The stable structure of a compound is the global minimum of the potential energy surface, which is the high dimensional representation of the enthalpy of the investigated system with respect to its structural parameters. The fact that the complexity of the problem grows exponentially with the system size is the reason why it can only be solved via heuristic strategies.rnrnImprovements to the artificial bee colony method, where the local exploration of the potential energy surface is done by a high number of independent walkers, are developed and implemented. This results in an improved communication scheme between these walkers. This directs the search towards the most promising areas of the potential energy surface.rnrnThe minima hopping method uses short molecular dynamics simulations at elevated temperatures to direct the structure search from one local minimum of the potential energy surface to the next. A modification, where the local information around each minimum is extracted and used in an optimization of the search direction, is developed and implemented. Our method uses this local information to increase the probability of finding new, lower local minima. This leads to an enhanced performance in the global optimization algorithm.rnrnHydrogen is a highly relevant system, due to the possibility of finding a metallic phase and even superconductor with a high critical temperature. An application of a structure prediction method on SiH12 finds stable crystal structures in this material. Additionally, it becomes metallic at relatively low pressures.

Polar Codes for error correction: Analysis and Decoding Algorithms

I Polar Codes sono la prima classe di codici a correzione d’errore di cui è stato dimostrato il raggiungimento della capacità per ogni canale simmetrico, discreto e senza memoria, grazie ad un nuovo metodo introdotto recentemente, chiamato ”Channel Polarization”. In questa tesi verranno descritti in dettaglio i principali algoritmi di codifica e decodifica. In particolare verranno confrontate le prestazioni dei simulatori sviluppati per il ”Successive Cancellation Decoder” e per il ”Successive Cancellation List Decoder” rispetto ai risultati riportati in letteratura. Al fine di migliorare la distanza minima e di conseguenza le prestazioni, utilizzeremo uno schema concatenato con il polar code come codice interno ed un CRC come codice esterno. Proporremo inoltre una nuova tecnica per analizzare la channel polarization nel caso di trasmissione su canale AWGN che risulta il modello statistico più appropriato per le comunicazioni satellitari e nelle applicazioni deep space. In aggiunta, investigheremo l’importanza di una accurata approssimazione delle funzioni di polarizzazione.

Millennial and sub-millennial scale climatic variations recorded in polar ice cores over the last glacial period

Since its discovery in Greenland ice cores, the millennial scale climatic variability of the last glacial period has been increasingly documented at all latitudes with studies focusing mainly on Marine Isotopic Stage 3 (MIS 3; 28–60 thousand of years before present, hereafter ka) and characterized by short Dansgaard-Oeschger (DO) events. Recent and new results obtained on the EPICA and NorthGRIP ice cores now precisely describe the rapid variations of Antarctic and Greenland temperature during MIS 5 (73.5–123 ka), a time period corresponding to relatively high sea level. The results display a succession of abrupt events associated with long Greenland InterStadial phases (GIS) enabling us to highlight a sub-millennial scale climatic variability depicted by (i) short-lived and abrupt warming events preceding some GIS (precursor-type events) and (ii) abrupt warming events at the end of some GIS (rebound-type events). The occurrence of these sub-millennial scale events is suggested to be driven by the insolation at high northern latitudes together with the internal forcing of ice sheets. Thanks to a recent NorthGRIP-EPICA Dronning Maud Land (EDML) common timescale over MIS 5, the bipolar sequence of climatic events can be established at millennial to sub-millennial timescale. This shows that for extraordinary long stadial durations the accompanying Antarctic warming amplitude cannot be described by a simple linear relationship between the two as expected from the bipolar seesaw concept. We also show that when ice sheets are extensive, Antarctica does not necessarily warm during the whole GS as the thermal bipolar seesaw model would predict, questioning the Greenland ice core temperature records as a proxy for AMOC changes throughout the glacial period.