<p> Excitons represent the electronic excited state of organic semiconductor and many low-dimensional inorganic semiconductors. In solar energy conversion systems, exciton transport affects how fast the charges reach the electrodes thus governs the performance of photovoltaic cells. In optoelectronic applications such as semiconductor lasers and light-emitting diodes, exciton radiative rate determines the efficiency of luminescence in competition to various nonradiative processes. Therefore, understanding how exciton migrates over space as well as its decay dynamics are vital for the design of highly efficient optoelectronic devices. To interrogate these photophysical processes requires experimental tools with simultaneous high temporal and spatial resolution. In this thesis, I introduce two transient imaging systems (photoluminescence imaging with 300 ps time resolution, and transient absorption microscopy with 200 fs time) that are innovative tools to directly probe excited state dynamics and transport in sub-μm domains. The techniques were applied to a type of promising semiconductor, perovskites, including surface-passivated hybrid perovskite and 2D layered perovskites to explore the fundamental mechanisms that affect exciton transport. The fundamental understanding of excitons shed light on the underlying physics such as exciton delocalization, exciton-exciton interaction, and how these properties affected by the static and dynamic disorders of the material. We further demonstrated a novel twisted superlattice using ultrathin perovskites that confines excitons due to increased density of state from the moiré flat bands. In addition, excitons can be accelerated by strongly interacts photons, forming polariton quasiparticles that possess small effective mass. This is demonstrated by coupling 2D layered perovskites to a plasmonic array. We further showcase the formation of bulk polaritons without an external optical cavity in a self-assembled organic aggregate. Experimental investigation into these intriguing phenomena provide an approach to study fundamental processes such as many-body interaction and quantum coherence. </p>

Desde sua descoberta por S. Iijima em 1991, os nanotubos de carbono têm sido considerados um dos materiais nanoestruturados mais promissores para o desenvolvimento de novos dispositivos eletrônicos em escala nanoscópica. Devido _a sua alta razão entre a área superficial e o volume, esse material se destaca para aplicações como sensores de gás. No presente trabalho, estudamos através de simulações computacionais, a possibilidade de nanotubos de carbono com defeitos de nitrogênio (os chamados nanotubos CNx), poderem ser usados como sensores de moléculas gasosas. Na primeira parte do trabalho foram realizados cálculos de estrutura eletrônica baseados na Teoria do Funcional da Densidade (DFT) para diferentes sistemas formados pelo nanotubo e pela molécula. Através de cálculos de energia de ligação, foi possível identificar quais gases poderiam ou não serem adsorvidos à superfície do nanotubo. Dentre as moléculas investigadas, o monóxido de carbono e a amônia mostraram ser as mais facilmente adsorvidas ao nanotubo. Na segunda parte, foram realizados cálculos das propriedades de transporte utilizando o formalismo das funções de Green fora do equilíbrio (NEGF) recursivo. Foi possível concluir que os nanotubos estudados poderiam ser usados para detectar o monóxido de carbono e a amônia. Todavia, em relação à seletividade, os resultados indicaram que não parece possível distinguir essas duas moléculas, caso o sistema fosse inserido em um ambiente contendo uma mistura desses gases. Ainda, foram feitas simulações de nanotubos contendo defeitos aleatoriamente distribuídos, de forma a levar em conta os fatores de desordem característicos de sistemas mais realistas. / Since their discovery by S. Iijima in 1991, carbon nanotubes have been considered as one of the most promising nanostructured materials for the development of new nanoscopic electronic devices. Due to its high surface area to volume ratio, this material stands out as a candidate for possible gas sensoring applications. In this thesis, we have studied, by means of computational simulations, the possibility of using carbon nanotubes containing nitrogen defects (the so-called CNx nanotubes) as gas sensors. In the first part, we have performed electronic structure calculations based on Density Functional Theory (DFT) of several systems to address the possible binding of different molecules to the nanotube surface. Our results indicate that, among the molecules which were investigated, carbon monoxide and ammonia adsorb more easily to the nanotube surface. In the second part of this thesis, we have performed calculations of the transport properties by means of non-equilibrium Green\'s function formalism (NEGF). The results have shown that the nitrogen-defect carbon nanotubes could be used to detect, mainly carbon monoxide and ammonia molecules. On the other hand, when dealing with the selectivity of this system, it seems to be not possible to distinguish these gases, in the case of inserting the system in a environment containing a mixture of these molecules. Finally, we have simulated carbon nanotubes with defects randomly distributed along its length, in order to take into account disordering factors usually found in more realistic nanosensors.

De nos jours, l’utilisation de liants à faible teneur en clinker est de plus en plus courante. Or la cinétique de réaction des additions minérales utilisées est plus lente que celle du clinker. Si les conditions de cure ne sont pas adaptées, le matériau aura une structure poreuse plus importante, ce qui le rendra plus sensible à la pénétration des agents agressifs extérieurs comme le CO2 ou les Cl-. La carbonatation du béton est l’un des principaux phénomènes pouvant diminuer la durée de vie d’une structure en béton armé. En effet, elle entraine une diminution du pH de la solution interstitielle qui a pour conséquence une dépassivation des armatures, puis la corrosion de ces dernières si les conditions s’y prêtent.L’objectif de cette thèse est d’étudier l’impact de la carbonatation au jeune âge sur des liants à faible teneur en clinker. Pour cela l’étude se compose de deux aspects : le premier est l’étude de l’impact de l’hydratation et de la carbonatation sur les propriétés de transport et les isothermes de sorption hydrique, et le second est l’étude de l’impact du couplage hydratation-séchage-carbonatation sur la microstructure et la structure poreuse. Pour le premier aspect, les propriétés de transport étudiées sont la diffusion de l’O2, la diffusion de la vapeur d’eau, la perméabilité à l’eau liquide qui sont des données d’entrée dans les modèles de carbonatation ainsi que la perméabilité aux gaz qui est un indicateur de durabilité. Pour le second aspect, l’impact du couplage est mesuré par ATG et DRX pour déterminer l’assemblage de phases, de plus de la porosimétrie par intrusion de Mercure et des pesées hydrostatiques sont effectuées pour détecter le changement de la structure poreuse. Les matériaux étudiés sont des pâtes de ciment et des bétons avec un rapport eau/ liant de 0,57 avec l’un des trois liants suivants : CEM I, CEM I +30% de cendres volantes et CEM I +60% de laitiers.Pour le premier aspect, les résultats montrent que pour les pâtes de CEM I le temps de cure a un faible impact sur les isothermes de sorption de vapeur d’eau s’il est compris entre 3 jours et 6 mois. Alors que, pour les pâtes de CEM I + 60% de laitier, l’augmentation du temps de cure accroit significativement la teneur en eau (pour HR=65%, tcure=3 jours et pour tcure=6 mois ) dû à une augmentation de la teneur en C-S-H. D’autre part, la carbonatation entraine une diminution de la teneur en eau de l’échantillon, ainsi que l’amplitude de l’hystérésis. De plus, le gel a une porosité plus grossière. Par ailleurs, la carbonatation entraine une augmentation de la perméabilité aux gaz des matériaux. Pour le second aspect, les résultats montrent que l’utilisation d’additions minérales diminue la résistance à la carbonatation du matériau et que cette résistance augmente avec le temps de cure si l’échantillon contient des additions. La carbonatation de la portlandite, des C-S-H et des aluminates est concomitante. De plus, pour les matériaux aux laitiers, les résultats montrent qu’ils sont plus sensibles à la carbonatation des C-S-H et des aluminates que les CEM I. En effet quand le rapport variation molaire de CaCO3 sur variation molaire de Portlandite est calculé, il vaut 1.8 pour le CEM I et environ 3.5 pour les matériaux aux laitiers. Enfin, la carbonatation entraine une diminution du degré de saturation de l’échantillon. En effet, le degré de saturation à la surface de l’échantillon passe de 50% à 35% après carbonatation pour les échantillons de CEM I et de 50% à 5% pour les échantillons de CEM I + 60% de laitiers. Cette diminution peut s’expliquer par la diminution de la surface spécifique qui est divisée par 2 après carbonatation due à la décalcification des C-S-H. Même si la carbonatation entraine une diminution de porosité cette dernière est trop faible dans ce cas pour contrer cet effet / Nowaday, low clinker content binders are used more and more often. But the kinetics reactions of the supplementary cementitious materials (SCM) are slower than this one of clinker. If the curing conditions are not adapted, material will have a bigger pore structure and becomes more sensitive to the ingress of aggressive species from the environment like the CO2 or Cl-. Carbonation is one of main phenomena which can lead to decrease the life time of reinforced concrete structure. Indeed, it leads to a decrease of pore solution pH which leads to the depassivation of rebar. Then these rebars can be corroded if the conditions are appropriate.The aim of this thesis is to study the impact of carbonation at early age for binder with a low clinker content. This study was composed of two aspects: the first one is focus on the impact of hydration and carbonation on the transport properties and the water vapour sorption isotherms (WVSI), and the second one is focus on the impact of coupling hydration – drying-carbonation on the microstructure and the pore structure. For the first aspect, the studied transport properties was O2 diffusivity, water vapour diffusivity, water liquid permeability which are inputs for carbonation modelling and the intrinsic gas permeability which is a durability factor. For the second aspect, the coupling impact was measured by TGA and DRX to determine the phase assemblage; moreover Mercury intrusion porosimetry (MIP) and hydrostatic weigh were carried out to measure the change in the pore structure. The studied materials were cement pastes and concretes with water to binder ratio of 0.57 with one of the following binders: CEM I, CEM I +30% PFA and CEM I + 60% GGBS.For the first part, results show that a curing time between 3 days and 6 months has a low impact on the WVSI for the CEM I paste. Whereas, in the CEM I +60% GGBS paste, when the curing time increases, the water content increases (for a RH=65%, tcuring=3 days and for tcuring=6 months ), this is due to the increase of the C-S-H content. Moreover, carbonation leads to decrease the water content and the hysteresis becomes flat. Additionally, carbonation leads to increase the intrinsic gas permeability. For the second part, the results show that the use of SCM decreases the carbonation resistance and this resistance increases with the curing time. Carbonation of Portlandite, C-S-H and aluminates occurs in the same time. Moreover, the CEM I +60% GGBS paste are more sensitive to the carbonation of C-S-H and aluminates than the CEM I paste. Indeed, the molar variation of CaCO3 to the molar variation of Portlandite ratio has a value around 3.5 for the CEM I +60% GGBS and 1.8 for the CEM I. Finally, carbonation leads to decrease the water saturation degree at the surface of sample. Indeed, the degree of saturation at the surface of the sample increases from 50% to 35% after carbonation for the CEM I paste and from 50% to 5% for the CEm I +60% GGBS paste. This decrease can be explained by the decrease of the BET specific surface which is divided by 2 after carbonation. It is due to the decalcification of C-S-H. Although carbonation leads to a decrease of porosity, this one is too small in this case to counter this effect

La plupart des batteries Li-ion aujourd’hui utilisent des électrolytes à base de LiPF6 un sel de lithium connu pour son instabilité chimique au-delà de 60°C car il se dégrade en libérant PF5 et LiF. En présence de traces d’eau il génère en plus des composés oxyfluorophosphorés et du HF qui peut être dommageable à la fois pour les performances et pour le vieillissement de l’accumulateur. Plusieurs sels sont candidats au remplacement de LiPF6, notamment ceux basés sur les anions fluorosulfonylamidures et les anions de Hückel. Ce travail concerne l’étude des propriétés physico-chimiques et de transport des électrolytes à base de 4,5-dicyano-2- (trifluoromethyl)imidazolide de lithium (LiTDI) et bis(fluorosulfonyl)amidure de lithium (LiFSI) pour une utilisation au sein d’accumulateurs de type Li-ion. Dans ce travail il a d’abord été montré que LiTDI n’est que faiblement dissocié dans les mélanges de carbonates d’alkyles utilisés dans les batteries Li-ion tels que le binaire (EC/DMC) ce qui limite sa conductivité. Pour pouvoir remédier à cet inconvénient, une étude des phénomènes de solvatation et d’associations ioniques a été menée et a conduit à proposer un mélange ternaire de solvants (EC/GBL/MP) dans lequel LiTDI est plus dissocié. Le mélange ternaire proposé améliore à la fois les propriétés de transport et les caractéristiques thermiques de l’électrolyte sans compromettre le domaine de stabilité chimique et électrochimique. Enfin, le nouvel électrolyte EC/GBL/MP contenant LiTDI, a été testé en accumulateurs dans les conditions opératoires usuelles (régime C/10 et température ambiante) et sévères (régime 10C et des températures allant de -20 °C à 60 °C). Le problème de corrosion de l’aluminium de LiFSI a aussi été pris en compte. Un électrolyte prometteur à base d’un mélange LiTDI/LiFSI montrant de meilleures performances que chaque sel utilisé séparément dans EC/DMC a été présenté. Les conclusions de cette thèse prouvent que LiTDI ou LiFSI peuvent être utilisés comme sels de lithium dans les électrolytes pour accumulateurs Li-ion. / Most of the Li-ion batteries used in electrical devices contain a solution of LiPF6 in alkylcarbonate solvents with the risk of releasing PF5 at elevated temperatures and HF in the presence of water. Several salts are candidates for the replacement of LiPF6, including those based on fluorosulfonylamides and Hückel anions. This work concerns the study of physicochemical and transport properties of lithium 4,5-dicyano-2- (trifluoromethyl)imidazolide (LiTDI) and lithium bis(fluorosulfonyl)amide (LiFSI) based electrolytes and their use in Li-ion battery. First it was revealed that LiTDI is only weakly dissociated in alkylcarbonate mixtures used in Li-ion batteries such as EC/DMC limiting its conductivity. To overcome this disadvantage, a study of the solvation phenomena and of ionic association within the electrolytes was conducted. This study led to a ternary mixture of solvents (EC/GBL/MP) in which LiTDI is more dissociated. This new solvent mixture improves both the transport properties and the thermal stability of the LiTDI based electrolyte without compromising its chemical and electrochemical stability. Finally, the new LiTDI in EC/GBL/MP electrolyte was tested in NMC/graphite batteries under normal (C/10 rate and room temperature) and severe (10C rate and temperatures varying from - 20 ° C to 60 °C) operating conditions. The aluminium corrosion problem encountered by LiFSI based electrolytes was taken into account and a LiTDI/LiFSI salt mixture based electrolyte showing promising results was presented. The findings of this thesis show that LiTDI or LiFSI can be used as lithium salts in electrolytes for Li-ion batteries.

My doctoral research has focused on the development of surface plasmon resonance (SPR) reflectance imaging technique to detect near-field transport properties such as concentration, temperature, and salinity in micro/nano fluidic phenomena in label-free, real-time, and full-field manner. A label-free visualization technique based on surface plasmon resonance (SPR) reflectance sensing is presented for real-time and full-field mapping of microscale concentration and temperature fields. The key idea is that the SPR reflectance sensitivity varies with the refractive index of the near-wall region of the test mixture fluid. The Fresnel equation, based on Kretschmann’s theory, correlates the SPR reflectance with the refractive index of the test medium, and then, the refractive index correlates with the mixture concentration or temperature. The basic operation principle is summarized and the laboratory-developed SPR imaging/analyzing system is described with the measurement sensitivity, uncertainties and detection limitations of the implemented SPR reflectance imaging. Total five proposed uses of SPR reflectance imaging technique are presented: (1) micromixing concentration field development of ethanol penetrating into water contained in a micro-channel, (2) full-field detection of the near-wall salinity profiles for convective/diffusion of saline droplet into water, (3) full-field and real-time surface plasmon resonance imaging thermometry, (4) correlation of near-field refractive index of nanofluids with surface plasmon resonance reflectance, and (5) unveiling hidden complex cavities formed during nanocrystalline self-assembly.

In order to preserve groundwater resources from contamination by volatile organic compounds and to clean up sites contaminated with the compounds, we should understand fate and transport of contaminants in the subsurface systems and physicochemical processes involving remediation technologies. To enhance our understanding, numerical studies were performed on the following topics: (i) multiphase flow and contaminant transport in subsurface environments; (ii) biological transformations of contaminants; (iii) in-situ air sparging (IAS); and, thermal-enhanced venting (TEV). Among VOCs, trichloroethylene (TCE) is one of the most-frequently-detected chemicals in the contaminated groundwater. TCE and its daughter products (cis-1,2-dichloroethylene (cDCE) and vinyl chloride (VC)) are chosen as target contaminants. Density-driven advection of gas phase is generated by the increase in gas density due to vaporization of high-molecular weight contaminants such as TCE in the unsaturated zone. The effect of the density-driven advection on fate and transport of TCE was investigated under several environmental conditions involving infiltration and permeability. Biological transformations of contaminants can generate byproducts, which may become new toxic contaminants in subsurface systems. Sequential biotransformations of TCE, cDCE, and VC are considered herein. Under different reaction rates for two bioreaction kinetics, temporal and spatial concentration profiles of the contaminants were examined to evaluate the effect of biotransformations on multispecies transport. IAS injects clean air into the subsurface below the groundwater table to remediate contaminated groundwater. The movement of gas and the groundwater as a multiphase flow in the saturated zone and the removal of TCE by IAS application were analyzed. Each fluid flow under IAS was examined in terms of saturation levels and fluid velocity profiles in a three-dimensional domain. Several scenarios for IAS systems were simulated to evaluate remedial performance of the systems. TEV was simulated to investigate its efficiency on the removal of a nonaqueous phase liquid in the unsaturated zone under different operational conditions. For numerical studies herein, the governing equations for multiphase flow, multispecies transport, and heat energy in porous media were developed and solved using Galerkin finite element method. A three-dimensional numerical model, called TechFlowMP model, has been developed.

In this thesis I present studies of some transport and spectroscopic properties for two di erent materials exhibiting large magnetoresistance. Both of these materials are oxides of transition metals, showing exotic magnetic and transport properties. Despite these similarities, they are very different in many other aspects. One of them is an oxide of Manganese, along with a rare-earth metal, and exhibits large magnetoresistance under certain conditions, when doped by an alkaline earth metal. They are known as doped rare-earth manganites. The other material, Sr2FeMoO6, exhibits large magnetoresistance in the parent compound, without any doping, but only in the polycrystalline state. The manganites, on the other hand, show magnetoresistance under appropriate conditions in both single crystal and in polycrystalline state. Moreover, manganites exhibit several Metal-Insulator Transitions (MIT) as a function of doping, temperature and magnetic field. Sr2FeMoO6, on the other hand, is usually always metallic. In the first chapter, a brief introduction is provided regarding different types of magnetoresistance (MR) phenomena observed in different materials, namely Anisotropic MR (AMR), Giant MR (GMR), Collosal MR (CMR), Tunneling MR (TMR), Powder MR (PMR) etc. Out of these, CMR and PMR are found in doped manganites, while Sr2FeMoO6 exhibits PMR only. Next, a brief overview of the structure, properties and theories for both of these materials is provided. For the case of doped manganites, a short introduction is given for a novel two-fluid hamiltonian (called l - b model) which was proposed recently by Ramakrishnan et. al.. This model reproduces several exotic transport and magnetic properties of manganites which were inexplicible by earlier theories. The model was solved within the Dynamical Mean Field Theory (DMFT) framework by Hassan et. al.. A brief description of this DMFT solution is given. Many of the DMFT results for this model have been used in the subsequent chapters. In the second chapter, the hysteresis behaviour of the magnetoresistance and the magnetization (M ) of powdered Sr2FeMoO6 is considered in detail. In a recent experi- ment by Sarma et. al., it was found that this material, when powdered exhibits an exotic variety of PMR. In ordinary PMR, the hysteresis behaviour of the MR is supposed to follow that of M, in the sense that the coercive fields should be identical in both cases. Also, the MR is supposed to be roughly proportional to the square of the magnetization. However, in the experiments by Sarma et. al. on cold-pressed Sr2FeMoO6 powder, it was observed that the M R did not appear to be determined purely by the magnetization. Rather, the coercive fields for the hysteresis of the MR was almost 6 times that of M . Moreover, the quantity M R/M2, instead of remaining constant with changing magnetic field, itself has a hysteresis loop. Apart from establishing the exotic nature of the PMR, the experiment also tries to determine whether the MR originates from intra-grain or inter-grain tunneling. In the second chapter we present a simple toy model to reproduce the experimental results, and provide theoretical explanations. A combination of Monte Carlo and transfer matrix methods are used to simulate the hysteresis behaviour of the M R as well as of M . We show that the observed data can be understood if it is as- sumed firstly that the MR arises predominantly from inter-grain rather than intra-grain tunneling, and that the inter-grain boundaries are themselves magnetic with a coercive field higher than that of the grains. In order to motivate the use of Monte Carlo method for studying hysteresis, a brief survey of main results obtained for some simple models using this technique is also provided. In the third chapter, we study the doping and temperature dependence of core-level photoemission spectra in doped rare-earth manganites. In some recent experiments on Strontium doped (LSMO) and Barium doped (LBMO) samples, it has been observed that the M n2p3/2 core-level spectra shows an intriguing spectral weight transfer over a range of several eV , as a function of doping (x) and temperature (T ), in the ferromagnetic metallic phase. Specifically, there appears a shoulder adjacent to the main peak on the side of lower binding energy, which increases in weight and intensity as the doping increases or the temperature decreases. In LSMO samples, another shoulder was noticed on the higher binding energy side also. Moreover, in data obtained from LBMO samples, the spectra at different temperatures was subtracted from the spectra at/above Tc, and then this difference spectrum was integrated. The integrated weight, when normalized by the weight at the lowest temperature, appears to follow the square of the measured magnetization almost exactly. In order to understand the experimental data, we extended the aforementioned l - b model to include a core-level, and the attractive interaction due to a core-hole on the local valence levels. The impurity problem arising in DMFT, consisting of a single impurity site coupled to a bath, was tailored for the photoemission problem, by including this extra core-level at the impurity site. The hybridization parameters for the bath were determined self-consistently from the DMFT, and then the single particle spectral function for the core-hole was determined. This spectral function is proportional to the photo emission intensity. We found that our calculations reproduced the observed spectral weight transfer as a function of x and T both in trends and in magnitude. The integrated difference spectra weight was found to follow the square of the DMFT magnetization, just as in the experiment. Linear discretization of the conduction bath was used for all the above-mentioned cases. In one particular case, a logarithmic discretization was also undertaken for comparison, and also to obtain the exponents of the edge singularities in the theoretical spectra. In the fourth chapter, the possibility of Anderson Localization in manganites is in- vestigated, using the l - b model. According to this model, a large fraction of the valence electrons are polaronically self-trapped even in the ferromagnetic metallic phase. Due to strong on-site Coulomb interaction, these polarons provide a strongly scattering background, which can localize the mobile-electron band states close to the band edges. Since the fraction of valence electrons which are truly mobile is small, hence the Fermi energy lies close to the lower band edge. Hence, there is a possibility of an Anderson Insulator phase where all charge carriers are localized. To investigate this, we studied the behaviour of the mobility edges as a function of doping. DMFT alone does not include the physics of localization. Hence, in order to obtain the mobility edges, we combined the DMFT results with the Self-consistent Theory of Localization (STL), using a simplified prescription called Potential Well Analogy (PWA) due to Economou et. al.. We found that there is indeed an Anderson Insulator phase in a certain region of doping, which would otherwise have been supposed to be metallic based on purely DMFT results. Finally, we have compared this result, obtained using effective field theories, with an actual real space simulation of the l - b model at T=0. In this case, the mobility edge trajectories were obtained by studying the Inverse Participation Ratio (IPR), as a function of band energy and doping. In the concluding chapter, the principal results presented in this thesis are summa- rized. The limitations of the approach or approximations used are discussed, and future possibilities for overcoming these limitations outlined.

From the time of the unexpected discovery of the insulating nature of NiO by Verwey half a century ago, Oxide materials have continued to occupy the centre stage of condensed matter physics. The recent discovery of high temperature superconductivity in doped cuprates has given a new impetus to the study of the strongly correlated electron systems. Besides, the occurrence of Colossal Magneto-Resistance (CMR) in doped rare earth manganite has also created renewed interest in these rather old systems. Understanding of the rich and complex phase diagram of these materials and their sensitivity to small perturbations e.g. external magnetic field of a few Tesla, temperature, change in isotope etc. are of great theoretical interest and also these materials have many potential technological applications. A common feature of all these oxide materials is that the transition metal ions have partially filled d-shells. Unlike s and p-electrons which gives rise to hybridized Bloch states, the d-electrons retain their atomic nature in a solid. This gives rise to strong Coulomb interaction among d-electrons which may be comparable or more than its kinetic energy. The strong correlation effects are evident from the experimental fact that the undoped parent compounds are insulators rather than metals as suggested by band theory, which favours a metallic state for systems with one electron per unit cell since this gives rise to partially filled bands (and hence a metallic state). These insulators termed Mott insulators, arise solely due to strong electron-electron correlations as compared to the band insulators which arise due to complete filling of one electron bands thereby giving rise to a gap (band gap)in the excitation spectra. The delicate competition between the kinetic energy and the Coulomb energy for d-electrons is broadly responsible for the wide variety of phenomena like Mott metal-insulator transition (MIT), magnetic transitions, charge ordering, orbital ordering, ferro/antiferroelectricity, and most interestingly the observation of high Tc superconductivity in doped cuprates. In this thesis we will restrict our interest to one such class of oxide materials, namely the doped rare earth manganites. In Chapter 1 we give a brief overview of the structure and basic interactions present in the doped manganites. Also, in the same Chapter we give a brief introduction to the phenomenology of manganites, particularly its phase diagram in the doping and temperature plane and various experimental features, e.g. the wide variety of phase transitions and phenomena particularly the observation of CMR, charge ordering and incipient meso-scale phase separations etc.. Then we briefly introduce a recently proposed microscopic model which is believed to be a minimal model which, for the first time, includes the three most important interactions present in the manganites namely the following -1)coupling of the orbitally degenerate eg electrons to local lattice distortions of Jahn-Teller type which gives rise to two species of electrons. The one denoted by by ℓ is associated with Jahn-Teller effects and hence is localized whereas the other denoted by b is an extended state and propagates through the lattice. 2) The strong Hund’s couplingof ℓ and b electrons to the t2g core spin and 3) the strong Coulomb correlation between the two species of electrons. Additionally, the model includes a new doping dependent ferromagnetic exchange between the t2g core spins which can arise from “virtual double exchange” mechanism which will be discussed in great detail in Chapter 1 . Finally, we give a brief account on Dynamical Mean Field Theory (DMFT) and Numerical Renormalization Group (NRG) as an impurity solver for the single impurity problem arising under single site DMFT approximation. In Chapter 2 we study the effect of inter-site ℓ - b hybridization on the ‘ℓ - b’ model. The single impurity problem arising under DMFT approximation has close connection with the Vigman-Finkelshtein (VF)model. Then we briefly introduce the VF model and bring out its close connection with the impurity problem. We consider both the particle-hole symmetric as well as the U → ∞ particle-hole asymmetric cases. We derive various spectral functions at T = 0K and discuss the nature of fixed points under various circumstances. We explicitly show that for the particle-hole symmetric case the Hamiltonian flows from X-ray edge singularity fixed point to Free Electron fixed point under Renormalization Group transformation. This is evident from the spectral properties of the model. We write down the effective Hamiltonian at the free electron fixed point. For the particle-hole asymmetric case the model flows from X-ray edge singularity fixed point to Free Electron/Strong Coupling fixed point with additional potential scattering terms. We write down the effective Hamiltonian at this fixed point and derive various leading order deviations. We found all of them to be irrelevant in nature also most interestingly the quasi-particles describing the under lying Fermi liquid state are found to be asymptotically non-interacting. We also calculate the Fermi liquid parameter, z, by analyzing the energy level structure of a non-interacting Hamiltonian with effective renormalized parameter. Also, we consider the case of ‘self consistent bath hybridization’ without ℓ - b hybridization for Bethe lattice with infinite coordination. Low energy qualitative features are found to be same but some of the high energy features get qualitatively modified. In Chapter 3 we discuss the transport properties of doped manganites in the insulating phases and also the Hall effect in the metallic phase. In the first part of this chapter we calculate the resistivity based on the ‘ℓ - b’model and try to fit it to the semiconducting form: ρ(T )= ρ0(T /T0)−nexp[Δ(T )/kBT ] and extract the “transport gap”, Δ(T ). This gap can be characterized in terms of the “spectral gap” which can be defined for the ℓ - b model. It is found that the transport gap in the paramagnetic phase can be characterized in terms of the near constant “spectral gap” in this phase whereas the same in the ferromagnetic phase can be characterized in terms of the zero temperature spectral gap. In the last part of this chapter we calculate the Hall resistivity (ρxy) of these materials in the metallic phase. Ρxy is found to be negative and linear in applied field -quite consistent with the experimental findings but this fails to explain the positive linear Hall resistivity at low temperatures and its crossover as a function of field and temperature. We then present a reasonable explanation for this discrepancy and support it by calculating the Hall density of states for a two band “toy model” involving inter species hybridization. In Chapter 4 we calculate the optical conductivity, σ(ω), in ℓ - b model. σ(ω) arises from two independent processes. One of the processes involves ‘b’ electrons only and termed as ‘b - b channel’ and this gives rise to a Drude peak in the low frequency region. another process termed as the ‘ℓ - b channel’ involves hopping of an ℓ-electron to a neighbouring empty site and transforms into a ‘b’like state. This process gives rise to a broad mid-infrared peak. The total conductivity is the sum of contributions from these two incoherent channels. Calculated σ(ω) for metallic systems shows lot of similarities with experimental observations particularly the temperature evolution of the mid-infrared peak and the spectral weight transfer between the two peaks. But for the insulating systems the calculated optical conductivity showed trends similar to more recent experimental observations on some insulating systems (x =0.125) but contradicts with earlier experimental observations on some other insulating system (x =0.1). Finally, in the concluding chapter, we summarize results from all the chapters and also sketch some possible future directions of investigations.

The current trend in electronics manufacturing is to decrease the size of electronic components while attempting to increase processing power and performance. This is leading to increased interest in thinner printed wiring boards and finer line widths and wire pitches. However, mismatches in the thermomechanical properties of materials used can lead to warpage, hindering these goals. Warpage can be problematic as it leads to misalignments during package assembly, reduced tolerances, and a variety of operational failures. Current warpage prediction techniques utilize isotropic volume averaging to estimate effective material properties in layers of copper mixed with interlayer dielectric material. However, these estimates do not provide material properties with sufficient accuracy to predict warpage, as they contain no information about the orientation of the copper traces. This thesis describes the development of a new technique to predict the warpage of a particular substrate. The technique accounts for both the trace pattern planar density and planar orientation in determining effective orthotropic material properties for each layer of a multi-layer substrate. Starting with the trace pattern image, this technique first divides the trace pattern into several smaller areas for a given layer of the substrate and then uses image processing techniques to determine the copper percentage and average trace orientation in each small area. The copper percentage and average trace direction orientation are used in conjunction with the material properties of copper and the dielectric material to calculate the effective orthotropic material properties of each smaller area of the substrate. A finite-element model is then created where each layer is represented as a concatenation of several small areas with independent directional properties, and such a model is then subjected to sequential thermal excursion as seen in the actual fabrication process. The results from the models have been compared against experimental data with a great degree of accuracy. The modeling technique and the results obtained clearly demonstrate the need for the proposed subdivisional orthotropic material property calculations, as opposed to homogeneous isotropic properties typically used for each layer in computational simulations, as these more accurate directional properties are capable of predicting warpage with higher accuracy.

Les membranes à base de liquides ioniques à conduction protonique (PCIL) sont très prometteuses comme électrolytes des piles à combustible haute température (HT- PEMFC) du fait de leur forte conductivité et stabilité à des températures supérieures à 100°C. L'objectif de cette thèse est de réaliser une étude approfondie sur l'évolution de la morphologie et des propriétés fonctionnelles, des membranes à base de liquides ioniques, avec i) la concentration en PCIL, ii) la méthode d’élaboration et iii) la structure chimique du PCIL. Afin de prouver la potentialité de ces membranes dans le HT-PEMFC, des tests préliminaires en pile sont réalisés et les phénomènes de dégradation des PCIL et des membranes en présence de peroxyde d'hydrogène sont étudiés. La première partie de ce travail est focalisée sur la caractérisation des membranes de Nafion® neutralisées avec le triéthylamine (Nafion-TEA) et gonflées avec triflate de triéthylammonium (TFTEA). Il a été montré que dans le Nafion-TEA sec, les cations présentent une organisation de type « string like » à l'interface hydrophobe-hydrophile. L’introduction du TFTEA dans la membrane Nafion-TEA ne détruit pas sa nano-structuration, mais augmente de manière significative la conductivité ionique du système. La deuxième partie de ce travail nous a permis d'établir que les membranes dopées élaborées par coulée-évaporation présentent une meilleure organisation et une meilleure tenue thermomécanique par rapport à celles obtenues par gonflement. La troisième partie de ce travail est focalisée sur l’étude de l'impact de la nature chimique du PCIL sur la morphologie et les propriétés fonctionnelles des membranes de Nafion-TEA. Il a été démontré que les PCILs avec longues chaînes perfluorées ne modifient pas la nano-structuration du Nafion-TEA. Ceci a un impact fort sur les propriétés de conductivité, de sorption d’eau et sur les propriétés thermomécaniques de la membrane. Dans la dernière partie, des Ionomères aromatiques ont été synthétisés afin de remplacer le Nafion-TEA. Malgré la structure similaire de la chaîne latérale des Ionomères aromatiques et du Nafion®, les membranes à base d’Ionomères aromatiques et TFTEA ne présentent aucune nano-structuration. De plus l’effet plastifiant du TFTEA est plus notable dans le cas des Ionomères aromatiques probablement du fait d’une distribution aléatoire des fonctions ioniques dans la membrane polymère. / The polymer electrolyte membranes based on Proton Conducting Ionic liquids (PCIL) are very promising systems for the high temperature-PEMFC technology owing to their good ionic conductivity and stability at temperatures above 100oC. The objective of this thesis work is to achieve a profound study on the evolution of morphology and consequent functional properties of the PCIL based polymer electrolyte membranes in function of: i). concentration of the PCIL, ii). the method of elaboration and iii). chemical structure of the PCIL. To demonstrate the potential of these membranes in HT-PEMFC, preliminary tests have been carried out in the fuel cell stack and degradation phenomena associated with PCILs and membranes in the presence of hydrogen peroxide have been studied. The first part of this work is focused on the characterization of Nafion® membranes neutralized with triethylamine (Nafion-TEA) and swollen with triethylammonium Triflate (TFTEA). It has been shown that Nafion-TEA exhibits a single layer string-like organization of inter-digited Triethylammonium cations at the hydrophobic-hydrophilic interface when in anhydrous state. The introduction of TFTEA into Nafion-TEA membrane does not destroy its nano-structuration but significantly boosts the anhydrous ionic conductivity and hydrophilicity of the system. The second part of this work has permitted us to establish the fact that doped membranes prepared by casting method have better organization and better thermo-mechanical properties compared to those obtained by swelling method. Third part of this work focuses on the impact of the chemical nature of the PCIL on the morphology and functional properties of Nafion-TEA membranes. It has been demonstrated that the PCILs with long perfluorinated chain length do not modify the nano-structuration of Nafion-TEA membranes at all. This has a strong impact on the ion-conducting, water-sorption and thermo-mechanical properties of the membrane. In the last part, aromatic ionomers were synthesized in order to replace Nafion-TEA in such PCIL based system. Despite the similar structure of the side chain of the synthesized aromatic ionomers and Nafion®, the membranes based on aromatic ionomers and TFTEA do not present any nano-structuration. Moreover, the plasticizing effect of TFTEA is more noticeable in the case of aromatic ionomers probably due to a random distribution functions in the ionic polymer membrane.