La méthode d'élimination sur le terrain est devenu ces dernières années la méthode de choix pour modéliser la formation des motifs de la microstructure lors de la solidification. Pour monocristaux, accord quantitatif avec des expériences et des solutions analytiques ont été obtenues. La modélisation des polycristaux, qui sont composées de nombreux grains d'une même phase thermodynamique, mais différentes orientations du réseau cristallin, est beaucoup moins avancée. Deux types de modèles ont été proposés: les modèles multi-phase-champ d'utiliser un champ de phase pour chaque grain, et les modèles d'orientation-champ d'utiliser un petit nombre de domaines, mais ont des termes non analytiques dans leur énergie libre fonctionnel. Ce travail examine les divers aspects de la phase de modélisation du champ de polycristaux et est divisé en trois parties. Dans la première, une nouvelle possibilité de décrire l'orientation locale est explorée, en utilisant un paramètre d'ordre tensoriel qui représente automatiquement la symétrie locale du système. Cette approche est testée en phase de développement d'un modèle de champ pour la transition de phase nématique-isotrope dans les cristaux liquides. Le modèle est appliqué pour simuler la solidification directionnelle''''d'un cristal liquide. L'effet du couplage entre l'orientation et la forme nématique interface est étudiée. Les résultats de simulation pour la stabilité d'une interface plane en bon accord avec une analyse de stabilité généralisée, qui tient compte d'une condition nouvelle d'ancrage à l'interface: l'orientation à l'interface nématique est le résultat de l'interaction entre la déformation en vrac et l'anisotropie d'interface. La forme et la stabilité des cellules bien développé est également influencée par cet effet. Numériquement, l'utilisation d'un paramètre d'ordre tensoriel simplifie le traitement des symétries dans le système de manière significative, tandis que les équations de mouvements deviennent beaucoup plus compliquées. Dans la deuxième partie, les joints de grains sont étudiés sur une échelle plus petite longueur, en utilisant un modèle de cristal phase de terrain, où les propriétés élastiques et des dislocations apparaissent naturellement. Avec ce modèle, l'ordre local dans les interfaces est examiné et la stabilité des films liquides entre deux grains solides est étudiée ci-dessous le point de fusion. Cette situation peut être décrite par un potentiel d'interaction entre les deux interfaces solide-liquide, qui est extraite numériquement. Les résultats sont comparés avec un modèle phénoménologique qui se trouve à tenir pour les joints de grains à forte inclinaison, où les dislocations se chevauchent. Pour les joints de grains à faible angle, autour de préfusion dislocation ainsi qu'une brisure de symétrie (paires de dislocations forme) est observée. En conséquence, le potentiel d'interaction devient nonmonotonous, et se compose d'une attraction à longue portée et une répulsion à courte portée. Dans la troisième partie, un nouveau modèle de phase sur le terrain est développé en utilisant une variable d'angle pour décrire l'orientation cristalline. Contrairement aux modèles déjà existants, l'énergie libre est construit sans un terme proportionnel au module du gradient du champ de l'orientation. Au lieu de cela, le gradient de la norme au carré est utilisé, mais il est couplé à la phase du champ avec une fonction de couplage singulier. Diverses simulations référence sont réalisés afin de tester le modèle. Il se trouve qu'elle présente plusieurs artefacts tels que la rotation et le mouvement du grain parasite interface, mais ces effets sont extrêmement petites, telles que le modèle donne des résultats satisfaisants que si la surfusion est très faible. Enfin, les problèmes observés sont analysés et des moyens d'obtenir une meilleure description de la dynamique de l'angle de champ sont discutées.

In this work the first observation of new type of liquid crystals is presented. This is ionic self-assembly (ISA) liquid crystals formed by introduction of oppositely charged ions between different low molecular tectonic units. As practically all conventional liquid crystals consist of rigid core and alkyl chains the attention is focused to the simplest case where oppositely charged ions are placed between a rigid core and alkyl tails. The aim of this work is to investigate and understand liquid crystalline and alignment properties of these materials. It was found that ionic interactions within complexes play the main role. Presence of these interactions restricts transition to isotropic phase. In addition, these interactions hold the system (like network) allowing crystallization into a single domain from aligned LC state. Alignment of these simple ISA complexes was spontaneous on a glass substrate. <br><br> In order to show potentials for application perylenediimide and azobenzene containing ISA complexes have been investigated for correlations between phase behavior and their alignment properties. The best results of macroscopic alignment of perylenediimide-based ISA complexes have been obtained by zone-casting method. In the aligned films the columns of the complex align perpendicular to the phase-transition front. The obtained anisotropy (DR = 18) is thermally stable. The investigated photosensitive (azobenzene-based) ISA complexes show formation of columnar LC phases. It was demonstrated that photo alignment of such complexes was very effective (DR = 50 has been obtained). It was shown that photo-reorientation in the photosensitive ISA complexes is cooperative process. The size of domains has direct influence on efficiency of the photo-reorientation process. In the case of small domains the photo-alignment is the most effective. Under irradiation with linearly polarized light domains reorient in the plane of the film leading to macroscopic alignment of columns parallel to the light polarization and joining of small domains into big ones. <br><br> Finally, the additional distinguishable properties of the ISA liquid crystalline complexes should be noted: (I) the complexes do not solve in water but readily solve in organic solvents; (II) the complexes have good film-forming properties when cast or spin-coated from organic solvent; (III) alignment of the complexes depends on their structure and secondary interactions between tectonic units. / In dieser Arbeit wird erstmalig eine neue Klasse von Flüssigkristallen auf Basis ionischer Self-Assembly (ISA) Komplexe beschrieben. Während herkömmliche thermotrope Flüssigkristalle aus steifen, formanisotropen Molekülfragmenten und kovalent gebundenen Flügelgruppen (meist Alkylketten) bestehen, entstehen diese neuartigen supramolekularen Verbindungen durch die Komplexierung gegensätzlich geladener ionischer tektonischer Einheiten und Tenside. <br><br> Ziel der Arbeit war es, die flüssigkristallinen und insbesondere die Orientierungseigenschaften dieser neuen Materialien am Beispiel repräsentativer Modellverbindungen zu untersuchen. Es wurde nachgewiesen, dass die ionischen Wechselwirkungen die thermischen Eigenschaften der Verbindungen entscheidend beeinflussen. So behindern sie den Übergang in die isotrope Phase. Das System wird quasi durch ein Netzwerk ionischer Wechselwirkungen stabilisiert. Makroskopisch orientierte LC Zustände sind offensichtlich Ausgangspunkt für hochgeordnete flüssigkristalline Filme oder gar für die Kristallisation von Monodomänen. In speziellen Fällen erfolgt eine spontane Ausbildung von ISA Monodomänen bereits auf Glassubstraten. <br><br> Mit Hinblick auf potentielle Anwendungen wurden Perylendiimid und Azobenzen enthaltene ISA Komplexe insbesondere hinsichtlich von Zusammenhängen zwischen Phasenverhalten und Orientierungseigenschaften untersucht. Die zone-casting Methode erwies sich als besonders geeignet für die makroskopische Orientierung perylendiimidbasierter ISA Komplexe. In orientierten Filmen richten sich die Kolumnen des Komplexes senkrecht zur Phasenübergangsfront aus. Das dabei erreichte dichroitische Verhältnis (DR=18) ist thermisch stabil. Die untersuchten Azobenzen basierten ISA Komplexe weisen kolumnare LC Phasen auf. Durch Photoalignment mittels linear polarisierten Lichts werden Komplexe sehr effektiv senkrecht bzw. die Columnen der Komplexe parallel zur Polarisation des Lichtes orientiert, wobei sehr hohe DR bis zu 50 erreicht wurden. Weiterhin wurde gezeigt, dass die Photo-Reorientierung photosensitiver ISA Komplexe kooperativ erfolgt. Die Größe der Domänen hat dabei einen entscheidenden Einfluß auf die Effektivität des Photo-Reorientierungsprozesses. So ist der Prozess im Fall kleiner Domänen effektiver. Durch die Bestrahlung mit linear polarisiertem Licht werden die Domänen in der Filmebene reorientiert, was zu einer makroskopischen Ausrichtung der Kolumnen parallel zur Lichtpolarisation und zu einer Vereinigung kleiner Domänen führt.

This thesis considers various strongly correlated quantum phases in solid state and cold atom spin systems. In the first part we focus on phases emerging in multi-orbital materials. We study even-parity spin-triplet superconductivity originating from Hund's coupling between t2g orbitals and investigate the effect of spin-orbit interaction on spin-triplet and spin-singlet pairing. Various aspects of the pairing state are discussed against the backdrop of the spin-triplet superconductor Sr2RuO4. Motivated by the remarkable phenomena observed in the bilayer compound Sr3Ru2O7, which point to the formation of an electronic nematic phase in the presence of critical fluctuations, we investigate how such a broken symmetry state emerges from electronic interactions. Since the broken x-y symmetry is revealed experimentally by applying a small in-plane magnetic field component, we examine nematic phases in a bilayer system and the role of the in-plane magnetic field using a phenomenological approach. In addition, we propose a microscopic mechanism for nematic phase formation specific to Sr3Ru2O7. The model is based on a realistic multi-orbital band structure and local and nearest neighbour interactions. Considering all t2g-orbital derived bands on an equal footing, we find a nematic quantum critical point and a nearby meta-nematic transition in the phase diagram. This finding harbours important implications for the phenomena observed in Sr3Ru2O7. The second part is devoted to the study of the anisotropic bilinear biquadratic spin-1 Heisenberg model, where the existence of an unusual direct phase transition between a spin-nematic phase and a dimerized valence bond solid phase in the quasi-1D limit was conjectured based on Quantum Monte Carlo simulations. We establish the quasi-1D phase diagram using a large-N Schwinger boson approach and show that the phase transition is largely conventional except possibly at two particular points. We further discuss how to realize and to detect such phases in an optical lattice.

This thesis considers various strongly correlated quantum phases in solid state and cold atom spin systems. In the first part we focus on phases emerging in multi-orbital materials. We study even-parity spin-triplet superconductivity originating from Hund's coupling between t2g orbitals and investigate the effect of spin-orbit interaction on spin-triplet and spin-singlet pairing. Various aspects of the pairing state are discussed against the backdrop of the spin-triplet superconductor Sr2RuO4. Motivated by the remarkable phenomena observed in the bilayer compound Sr3Ru2O7, which point to the formation of an electronic nematic phase in the presence of critical fluctuations, we investigate how such a broken symmetry state emerges from electronic interactions. Since the broken x-y symmetry is revealed experimentally by applying a small in-plane magnetic field component, we examine nematic phases in a bilayer system and the role of the in-plane magnetic field using a phenomenological approach. In addition, we propose a microscopic mechanism for nematic phase formation specific to Sr3Ru2O7. The model is based on a realistic multi-orbital band structure and local and nearest neighbour interactions. Considering all t2g-orbital derived bands on an equal footing, we find a nematic quantum critical point and a nearby meta-nematic transition in the phase diagram. This finding harbours important implications for the phenomena observed in Sr3Ru2O7. The second part is devoted to the study of the anisotropic bilinear biquadratic spin-1 Heisenberg model, where the existence of an unusual direct phase transition between a spin-nematic phase and a dimerized valence bond solid phase in the quasi-1D limit was conjectured based on Quantum Monte Carlo simulations. We establish the quasi-1D phase diagram using a large-N Schwinger boson approach and show that the phase transition is largely conventional except possibly at two particular points. We further discuss how to realize and to detect such phases in an optical lattice.

Anisotropic molecules confined on a spherical or other curved surface can display coupled positional and orientational orderings, which make possible applications in physics, chemistry, biology, and material science. Therefore, controlling the order of such system has attracted much attention recently. Several distinct conformations of rod-like or chain-like molecules confined on a spherical surface have been predicted, including states such as tennis-ball, rectangle, and cut-and-rotate splay. These conformations have four +1/2 defects and are suggested to dominate over the splay conformation that has two +1 defects. For the purpose of investigating the conformations of 2-fold anisotropic molecules confined on the spherical surface, the author of this thesis utilizes the Onsager model to study the system of rigid rods and conducts Monte Carlo simulations on the bead-bond model to research the system of semiflexible polymer chains. At low surface coverage density, no particular pattern of the molecules would form. However, coupled positional and orientational ordering begins to emerge beyond a transition density. On the basis of the numerical solutions of the Onsager model of rigid rods, the splay conformation is shown to be the only stable state. On the other hand, Monte Carlo simulations on a polymer system indicate that the ordered state always accompanies the tennis-ball symmetry. With comparison to the continuous isotropic-nematic transition of a fluid of hard rods embedded in a flat two-dimensional space, the disorder-order transition for both the system of rigid rods and the system of polymer chains confined on the spherical surface has first-order phase-transition characteristics.

The successful applications of nanoparticles require the ability to tune their properties by controlling size and shape at the nanoscale. In metal nanomaterial research, the optical properties have been of interest especially because of the applications to medical diagnostics and nanooptics. It is important to prepare nanoparticles of well-defined shape and size for properly characterizing the optical properties. We describe improved seed mediated synthesis of gold nanorods (GNRs) producing a high yield of NRs with low polydispersity and few byproducts. The efficient separation of GNRs from mixture of shapes is achieved by understanding the hydrodynamics of nanoparticles undergoing centrifugation. The optical properties of resulting refined GNRs are compared to predictions of existing theories, and the main parameters affecting them are discussed. GNRs with well defined aspect ratios are introduced into a polyvinyl alcohol matrix by means of solution-casting techniques. The film is drawn to induce the uniaxial alignment of GNRs to be used as color polarizing filters. We prepare GNR polarizing filter with different peak positions ranging from visible to near infra red by using different aspect ratio of NRs. To utilize GNRs to make nanoscale devices, spatial organization is required. We characterize the self-assembly of GNRs observed on a TEM grid. The drying process is accompanied by complex hydrodynamic and thermodynamic events, which create rich range of patterns observed. Being anisotropic in shape, the rods can form liquid crystal (LC) assemblies above a certain concentration. We observed LC phase of GNRs by resorting to an evaporation of aqueous NR solution. The convective flow caused by the solvent evaporation carries NRs from the bulk solution to solid-liquid-air interface, which makes the solution locally very concentrated driving the phase transition of NRs. We calculate the order parameter from various assemblies observed, and compare the observed phase behavior to the one expected on the basis of theory.

In this study, we want to realize the influence of temperature and mechanical rubbing on the surface free energy and wettability of liquid crystal(LC) of polyimide(PI) thin films. In different surface conditions, we use the LC that was dropped on to the surface of the sample and become stable to explore the wettability of the surface. We also observe the phenomenon and influence of the LC on the PI with increasing surface temperature. We utilize hot stage and contact angle measuring system to obtain the surface free energy to realize the surface of the PI and measure the contact angle of LC in different temperature to realize the wettability of the PI. We found that the increase of the surface temperature leads to the decrease in the surface free energy of the PI and the contact angle of the FLC on the PI. The increase of the surface temperature causes the stabilization in the surface polar energy and the LC contact angle of the unrubbed PI. But the increase of the surface temperature causes the increase in the surface polar energy and the decrease of the LC contact angle of the rubbed PI. In the anti-parallel-direction to the rubbing direction the rubbed PI exhibits better wettability.

A novel ionic liquid crystal (ILC) system (C18IMCNBr) with a liquid crystal alignment used as an electrolyte for a dye-sensitized solar cell (DSSC) showed the higher short-circuit current density (Jsc) and the higher light-to-electricity conversion efficiency than the system using the non- alignment liquid crystalline ionic liquid (C18IMCNBr),due to the higher conductivity of liquid crystal alignment. The larger Jsc and efficiency value of liquid crystal alignment supported that the higher conductivity of liquid crystal alignment is attributed to the enhancement of the exchange reaction between iodide species. As a result of formation of the two-dimensional electron conductive pathways organized by the localized I3- and I- at liquid crystal alignment layers, the concentration of polyiodide species exemplified by Im- (m =5,7, ...) was higher in alignment C18IMCNBr. However, in the two-dimensional electron conductive pathways of C18IMCNBr, more collision frequencies between iodide species (I-,I3-, and Im-) could be achieved than that in the three-dimensional space of C18IMCNBr, which could lead to the promotion of the exchange reaction between iodide species, the contribution of a two-dimensional structure of the conductive pathway through the increase of collision frequency between iodide species was proposed.

We proposed a flexible electronics with functional poly (arylene ether)s and discotic liquid crystal. Firstly, we provided a series of the PAE polymer with remarkable thermal stability and high optical transmittance. The PAEs were synthesized via nucleophilic displacement as polymerization on a 2-trifluoromethyl-activated bisfluoro monomer, which reacted with bisphenols. Thermal analysis indicated the PAEs possessed a high glass transition point of ~300¢J and the decomposition temperature Td=500¢J at a weight loss of 5%. Additionally, high transmittance of 85%, low dielectric constant of ~2.0, and well mechanical property of the PAE films were experimentally verified, as a high potential substrate for flexible electronics. Two kind of device structure has been prepared, were organic thin film solar cell and Dye sensitized solar cell. Furthermore, we provided the self-assembled triphenylene-based liquid crystal and its polymer derivative to apply for photovoltics. The 2, 3, 6, 7-tetra-6-octyloxydibenzo[a,c]phenazine-11-carboxylic group and the polymer of merging with disc-unit and polyacrylamide. Both show a highly isotropic phase transition point of ~270¢J and the decomposition temperature Td~450¢J at a weight loss of 5%. The specific absorption in visible light region was at 200 - 450 nm. A distinct self-arrangement of columnar array was investigated by optical textures. The self-arranged pathway enhanced carrier mobility due to £k-£k conjugation in hexagonal column stacking. Finally, the PAEs and DLC materials applied to thin film solar cell (ITO/PEDOT:PSS/DLC-PAM/P3HT:PCBM/Al) as hole transporting layer. The photo-conversion efficiency was strong depending on organic compounds, such as molecular structure, photo-physic and chemical properties. On the basis of integrated characteristics, it suggested a high potential as flexible electronics for photovoltics.

In this study, we want to realize the influence of temperature on the ferroelectric liquid crystal flow and rubbing strength on the surface free energy of the polyimide thin film. The rubbing strength is determined by pile impress and the number of rubbing. The surface energy of the samples is measured. On the other hand the ferroelectric liquid crystal flow on the polyimide thin film was observed using a polarizing microscope. We found that the polyimide film treated by rubbing the surface properties will become non-uniformity. When the number of rubbing increased to a certain extent, polar energy is not changed, white disperse energy is continued to show fluctuations. In liquid crystal flow observation, we found that when the polyimide thin film temperature is low, the flow of ferroelectric liquid crystal is relatively slow, and its flow direction is in rubbing direction. When the polyimide thin film temperature is high, ferroelectric liquid crystal has two-dimensional flow. In addition to the direction of the original rubbing direction, but also increased the flow in the direction of the vertical rubbing.