This thesis describes work to create a variety of thin-film photonic devices based upon liquid crystalline materials. Firstly, a variety of liquid crystal phases are polymer- templated by combining a liquid crystalline material with photo-polymerisable reactive mesogens. Upon photo-curing, a polymer scaffold, which is a template of the original phase, is formed with liquid crystal molecules in interstitial sites. This liquid crystal is removed to yield a polymer scaffold which can be used to template the original phase. Here, polymer-templating is used to template the smectic A liquid crystal alignment onto nematic liquid crystals for the first time; this results in materials with improved contrast ratios and faster response times than conventional nematic devices. Next, a study is performed to compare the electro-optic properties of polymer-templated and polymer-stabilised chiral nematic liquid crystals. The enhanced tuning range of polymer-templated liquid crystals is applied to create a polymer-templated liquid crystal laser and to electrically tune its emission wavelength. Subsequently, thin-film elastomeric liquid crystal lasers are created. The lasing wavelength of these films can be reversibly and selectively tuned without hysteresis by subjecting them to a mechanical stress. Finally, work is performed to study the potential of inorganic materials for use in liquid crystal lasers. Transition metal clustomesogens (liquid crystalline materials that contain highly emissive molybdenum clusters) and inorganic-organic perovskites are considered here. The dispersal and emissive properties of clustomesogens in liquid crystals are studied, and they are used to create circularly polarised light sources with a polarisation that can be controlled using electric fields. Layered structures of inorganic- organic perovskite and liquid crystal are created; these exhibit enhanced amplified spontaneous emission. Then, perovskites are used as the gain materials in distributed feedback lasers for the first time. These lasers may be wavelength-tuned by varying the grating spacing of the structure.

The three-dimensional perovskites are known for their wide range of interesting properties including spectral tunability, charge carrier mobility, solution-based synthesis and many others. Such properties make them good candidates for photovoltaics and photodetectors. Low-dimensional perovskites, on the other hand, are good as light emitters due to the quantum confinement originating from their nanoparticle size. Another class of low-dimensional perovskites, also called low-dimensional-networked perovskites (L-DN), is recently reemerging. Those interesting materials combine the advantages of the nanocrystals and the stability of the bulk. For example, zero-dimensional-networked perovskite (0-DN), a special class of perovskites and the focus of this work, consists of building blocks of isolated lead-halide octahedra that could be synthesized into mm-size single crystal without losing their confinement. This thesis focuses on the synthesis and investigation of the optical properties of the 0-DN perovskites through experimental, theoretical and computational tools. The recent discovery of the retrograde solubility of the perovskites family (ABX3), the basis of the inverse temperature crystallization (ITC), inspired the reinvestigation of the low-dimensional-networked perovskites. The results of the optical characterization showed that the absorption and the corresponding PL spectra were successfully tuned to cover the visible spectrum from 410 nm for Cs4PbCl6, to 520 nm and 700 m for Cs4PbBr6 and Cs4PbI6, respectively. Interestingly, the exciton binding energies (Eb) of the 0-DNs were found to be in the order of few hundred meV(s), at least five times larger than their three-dimensional counterpart. Such high Eb is coupled with a few nanoseconds lifetime and ultimately yielded a high photoluminesce quantum yield (PLQY). In fact, the PLQY of Cs4PbBr6 powder showed a record of 45%, setting a new benchmark for solid-state luminescent perovskites. Computational methods were used to calculate the bandgap and study the corresponding excitonic behavior. However, the unexpected mismatch between the calculated and experimental bandgaps questions the origin of the high luminescence, which to date, remains an area of scientific debate that needs further study. Until then, the high PLQY, together with the spectral tunability, insensitivity to particle size and stability all offer a new avenue for more sustainability in light-emitting materials

Le besoin d'énergies propres et renouvelables, et en calculs numériques de plus en plus performant ont été deux des moteurs de la recherche mondiale. Les multiferroiques (matériaux présentant plusieurs ordres ferroiques couplés) ont pendant longtemps été étudiés pour des applications électroniques. Récemment, leur interaction avec la lumière a été considéré pour des applications photovoltaique. Leur grande bande interdite et la faible mobilité de leur porteurs sont néanmoins des freins à la conversion efficace de l'énergie solaire en électricité.Cependant, les matériaux multiferroiques présentent un nombre important de degrés de libertés, et leur interaction avec la lumière ne peut être réduite au seul effet photovoltaique. Ici, l'interaction lumière-multiferroique est d'abord considéré au travers de l'effet de photostriction (changement de longueur sous illumination). Les calculs ab-initio montrent que, dans le bismuth de ferrite, la photostriction peut être comprise comme un effect d'écrantage de la polarisation à l'échelle de la maille primitive, et de l'effet piézoélectrique inverse. Une solution solide de plomb nickel niobium et de titanate de plomb, présentant un fort effet piézoélectrique à sa frontière morphotropique est ensuite synthétisée et caractérisée pour ces propriétés optiques et électriques. Le rôle des défauts dans la grande conductivité des parois de domaines est aussi étudié, et des calculs de la théorie de la fonctionnelle densité montrent que les défauts se forment préférentiellement à la paroi, et y procure une plus grande densité de charges libres. Enfin, nous détaillons les dernières avancées d'un couplage de type spin-orbite, le couplage angulaire magnéto-électrique, et son application à la génération de champs magnétiques par une lumière polarisée circulairement. / The need for clean and renewable energy, as well as constantly improved numerical performances have been two of the most important driving forces in research worldwide. In this light, multiferroic materials, which are materials presenting several ferroic order, have been widely investigated towards their application in electronics and computation, or as sensors. Recently, they have been also considered for their potential use to generate energy through the photovoltaic effect. However, power conversion have remained poor compared to existing technologies such as p-n junction silicon based solar cells, mainly because of their wide bandgap and low mobility of the carriers. Nevertheless, multiferroic materials often present a vast number of degrees of freedom, and their interaction with light cannot be reduced to the sole photovoltaic effect.In this work, we study from first-principles the interaction of light and strain in the multiferroic bismuth ferrite, and find that the so-called photostriction effect originates from a screening of the polarization at the unit cell scale, which results in a photo-induced strain via the action of the converse piezoelectric effect. A solid solution of lead nickel niobium and lead titanate, exhibiting large electromechanical properties at its morphotropic phase boundary, is then synthesized, and its optical and photoinduced properties are studied. Also, the influence of defects at domain walls in the model ferroelectric lead titanate is studied from ab-initio calculations, in order to understand why domain walls exhibit a large conductivity compared to the domains. It is found that defects are more likely to form at the domain wall, and provide it with extra-carriers. Eventually, the advances in a recently considered spin-orbit energy term, the Angular MagnetoElectric coupling (AME), are considered and applied to the Inverse Faraday Effect (IFE), that is the existence of a magnetic field induced by circularly polarized light.

Nowadays, studying materials for renewable energy applications are highly de- manded. Hybrid halide perovskites have proven to be promising materials for such technology since their first application in solar cells in 2008, with a power conversion efficiency of 2.7%. Since then, hybrid halide perovskites have proven their superior properties for light-absorbing devices. In this scope, studying the optical properties is ultimately essential. This work investigates the tempera- ture dependence of the optical spectra for formamidinium lead iodide/bromide perovskites (FAPb[IxBr1-x]3 (0 ≤ x ≤ 1) using spectroscopic ellipsometry mea- surements, empirical optical modeling, density functional theory, and molecular dynamics. Five FAPb[IxBr1-x]3 perovskite samples were fabricated by a hybrid processing technique. External Quantum Efficiency measurements reported an energy bandgap range between 1.58 eV and 1.77 eV for the resulted samples. Next, multi-angle spectroscopic ellipsometry measurements were applied with a temperature-controlled stage, allowing the variance of temperature from 25 ◦C to 75 ◦C. The results show a blue shift in the optical spectra at elevated tempera- tures. We then conducted a temperature-dependent empirical model that predicts the optical spectra for the sample of study at higher temperatures using input data of the spectra at room temperature. The model reports low mean squared errors which are less than ≈ 2 around the bandgap, and further development can be applied for better utilization. First-principles investigations were conducted on four FAPb[IxBr1-x]3 per- ovskite unit cells. Structural optimization was applied with assuming fixed angles of the lattice. Atomic configuration was chosen to achieve minimal ground state energies. Ab initio molecular dynamics simulations were applied to each opti- mized structures at target temperatures of 300 K and 350 K using Berendsen thermostat. The simulation time was 4ps with 1fs time step, and the electronic energy bandgap was calculated at each step using PBE functional. The simula- tions reported a rotational motion for the FA molecule that showed to be faster at 350 K, along with higher mean energy bandgap compared to the reported value at 300 K. The optical spectra were extracted using a snapshot from the resulted structures. Similar to the spectroscopic ellipsometry measurements, a temperature induced blue shift was reported. Overall, this work detects and predicts the temperature-dependent optical spectra and confirms the role of the atomic thermal motion. With further devel- opment, higher accuracy can be achieved along with broadening the materials of study for photovoltaic and optoelectronic applications.

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The characteristic polarization response of a ferroelectric material to an applied electric field enables a binary state device in the form of a thin film ferroelectric capacitor that can be used to store digital information. In a high density memory the capacitor is placed on the top of a poly-silicon plug which is connected to the drain of a transistor. Such a configuration poses constraints on the processing conditions of the ferroelectric capacitor in addition to the already existing reliability issues of a ferroelectric capacitor. The current research is an attempt to integrate the ferroelectric capacitor directly into a high density memory structure. Pb_1.1Zr_0.53Ti_0.47O₃ (PZT) and SrBi₂Ta₂O₉ (SBT) are two most promising materials for ferroelectric memory applications. PZT has excellent ferroelectric properties with wide operating temperature range. However, PZT exhibits a considerable loss of switchable polarization with cumulative switching cycles. This phenomenon is known as fatigue and is one of the critical problems affecting the life time of ferroelectric memories. In this research, Ir based electrodes are shown to improve fatigue characteristics of PZT based capacitors not only by enhancing a homogenous growth of perovskite phase of PZT but also by lowering the entrapment of oxygen vacancies at the interface. These Ir electrodes also acted as diffusion barriers for silicon, oxygen and lead. Additionally, Ir electrodes were found to be chemically stable at the processing temperatures of PZT capacitors. These features of Ir based electrodes could help in realization of a practical PZT based high density non volatile random access memories. SBT is an another promising ferroelectric material for ferroelectric memory applications. While SBT has a fatigue free nature, it has a very high processing temperature (>800 °C). Such a high processing temperature limits the choice of electrodes that could be used to integrate the ferroelectric capacitor into the high density memory structure. In this research, an attempt is made to lower the processing temperature and suitable electrodes are chosen accordingly, to enable the integration of SBT based capacitors into high density memories. Lowering the processing temperature was obtained by growing a-b oriented SBT crystallites rather than c-axis oriented crystallites. Additionally, reliability (degradation) and yield of SBT thin film capacitors was found to be correlated to the amount of segregated bismuth oxide in the films. Elimination of secondary phase bismuth oxide was found to result in dramatic improvement in the reproducibility of SBT thin films with a processing temperature close to 750 °C. PtRh based electrodes were found to be quite suitable for integrating SBT capacitors into high density memory structures. These electrodes could withstand a processing temperature of 750 °C while preventing the interdiffusion of silicon, oxygen and bismuth. A solid solution of SBT and Bi₃TiNbO₉ (BTN) is made which reduced the processing temperature of the capacitor material from 750 °C to 650 °C while retaining the excellent fatigue and retention characteristics of SBT. / Ph. D.

Les trois familles catalytiques à base de perovskites contenant du Ni: massives [[LnFe1-x-yNiyMxO3-δ] (Ln=La, Pr; B=Co, Mn, Ru), sur support [mLnNi0.9Ru0.1О3/nMg-γ-Al2O3] (Ln = La, Pr) et structuré [mLaNi0.9Ru0.1О3/nMg-γ-Al2O3/mousses structurées] ont été synthétisés, caractérisés et testés dans les réactions de vaporeformage de l'éthanol et de glycérol. Les effets de la composition chimique et de la méthode de synthèse sur les propriétés structurelles et texturales, ainsi que sur la réductibilité des échantillons initiaux ont été évalués. L'utilisation préférentielle de Pr, Ni et Ru dans la composition de catalyseur a été démontrée pour toutes les familles. Le rôle essentiel de la modification du support γ-Al2O3 avec ≥ 10%mass de Mg introduit par imprégnation humide pour le catalyseur supporté a également été prouvé. Des catalyseurs de la composition optimale fournissant une activité élevée dans le vaporeformage de l'éthanol et du glycérol à T = 650 °C ont été trouvés: le meilleur catalyseur massif à base du précurseur PrFe0.6Ni0.3Ru0.1O3 fournit une activité élevée pendant au moins 7 h, grâce à la facilité de leur réduction et les propriétés d'oxydoréduction de l'oxyde de praséodyme formé. Les catalyseurs sur support 10-20% PrNi0.9Ru0.1O3/10-15%Mg-γ-Al2O3 fournissent le meilleur rendement en hydrogène (~ 90%) et la stabilité pendant ~ 20 heures. Le catalyseur structuré optimisé à base de la plaquette Ni-Al métallique fournit le rendement stable en hydrogène 80-87% dans l’oxy-vaporeformage d'éthanol dans les mélanges concentrés (concentration d'éthanol de 30%) dans un réacteur pilote pendant 40 heures. Les résultats obtenus rendent ces systèmes catalytiques structurés très prometteurs à utiliser dans les générateurs électrochimiques à base de piles à combustible avec l'utilisation de ressources renouvelables peu coûteuses comme bio-huile. / The three catalytic families based on Ni-containing perovskites: massive [LnFe1-x-yNiyMxO3-δ] (Ln=La, Pr; B=Co, Mn, Ru), supported [mLnNi0.9Ru0.1О3/nMg-γ-Al2O3] (Ln = La, Pr) and structured [mLaNi0.9Ru0.1О3/nMg-γ-Al2O3/structured foams] were synthesized, characterized and tested in the reactions of the ethanol and glycerol steam reforming. The effects of the chemical composition and synthesis method on the structural and textural properties, as well as on reducibility of initial samples were evaluated. The preferred use of Pr, Ni and Ru in the catalyst composition was shown for all families. The essential role of the effective γ-Al2O3 support modification with the ≥10 % wt. of Mg introduced by wetness impregnation for the supported catalyst was also proved. Catalysts of the optimal composition providing a high activity in steam reforming of both ethanol and glycerol at T= 650 °С were found: the best massive catalyst based on the PrFe0.6Ni0.3Ru0.1O3 precursor provides high activity for at least 7 hours, which is explained by the ease of their reduction and the oxidation-reduction properties of the praseodymium oxide formed. Supported 10-20% PrNi0.9Ru0.1O3/10-15%Mg-γ-Al2O3 provide the greatest yield of hydrogen (~ 90%) and stability for ~ 20 hours. Structured catalyst based on the metal Ni-Al platelet provides the yield of hydrogen 80-87% in oxy-steam and steam reforming of ethanol in the concentrated mixtures (ethanol concentration of 30%) in a pilot reactor for 40 hours. The results obtained make these structured catalytic systems very promising to use in electrochemical generators based on fuel cells with the use of inexpensive renewable resource – bio-oil.

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