Student Number : 0303610W - PhD thesis - School of Chemistry - Faculty of Science / Mesoporous MCM-48 was synthesized and used as a template to synthesize mesoporous carbon (MC) materials. Polystyrene, the carbon source, together with sulfuric acid and toluene were added to the template (160 oC for 6 h) and this procedure generated a low surface area carbon supported/MCM-48 material. A repeat addition and carbonization step was needed to form the precursor carbon/MCM-48 material that was pyrolysed at 900 oC to generate graphitic mesoporous carbon materials. After removal of the silica template, mesoporous carbons were characterized by XRD, HR-TEM, Raman spectroscopy and surface area analysis. The effect of the amount of polystyrene as well as the role of the pyrolysis temperature on the final product was investigated. This synthesis methodology can readily be controlled to produce partially ordered graphitic mesoporous carbon supports with predictable pore width and surface area. The methane selectivity was low (below 6%) and stable, and the overall olefin fraction was found to be good for all the supported catalysts studied. The potassium promoter increased the hydrocarbon chain growth to C68 giving α-1 and α-2 both between 0.79 and 0.90 for all supported catalysts with an exception of MCM-48 supported Fe catalyst that selectively produced hydrocarbons up to C28.

O saco vitelino é uma estrutura única dos vertebrados que evoluiu uma grande diversidade de características de desenvolvimento, estrutural e funcional. É considerado de fundamental importância para a sobrevivência inicial do embrião. Apesar dos intensos estudos sobre a placentação no guinea pig, ainda existem lacunas importantes para a sua compreensão, no que se refere ao saco vitelino. Esta pesquisa visa detalhar a caracterização morfológica e do desenvolvimento do saco vitelino no Cavia porcellus. Dezessete amostras foram coletadas em idades gestacionais de 12 a 55 dias e investigadas mediante histologia convencional, histoquímica, imunohistoquímica incluindo o marcador de proliferação e microscopia eletrônica de transmissão. O material histórico da implantação embrionária foi utilizado da Embryological Collection of the Museum of Natural History Berlim, Germany. Foi observada a presença temporária do saco vitelino parietal e da cavidade do saco vitelino, durante a implantação do blastocisto. O saco vitelino, que recobre a placenta corioalantoica, mostrou uma grande diferença estrutural entre o 12º e 18º dia, como a transição do epitélio pseudoestratificado para projeções, que se assemelham a uma árvore. A membrana de Reichert não está desenvolvida no 12º dia, entretanto verifica-se um espaço preenchido de material extracelular e vacúolos. A membrana de Reichert foi encontrada a partir do 16º dia de gestação e com o desenvolvimento gestacional e se tornou cada vez mais espessa e constituída de material amorfo e acelular. Os dados morfológicos sugerem que a transferência de nutrientes do saco vitelino para o feto foi mais intensa entre o 22º e 40º dia, especialmente no 40º dia. No último terço da gestação, houve uma diminuição dos tecidos de ambos os sacos vitelinos placentário e visceral. Em conclusão, o desenvolvimento do saco vitelino do guinea pig revela grande semelhança com os outros caviomorfos. Os dados sugerem que o saco vitelino nos caviomorfos é fundamental na gestação média, mas, não tanto, para os períodos iniciais da gestação, como é o caso dos outros vertebrados. / The yolk sac is a unique structure of vertebrates that evolved a great diversity of developmental, structural and functional features. It is regarded to have a fundamental role for early embryo survival. Although placentation has been intensely studied in the guinea pig, there are important gaps as in regard to the yolk sac. This research will undergo a detailed morphological and developmental characterization of the yolk sac in Cavia porcellus. Seventeen specimens have been collected at gestational ages from day 12 to 55 and investigated by conventional histology, histochemistry, immunohistochemistry including proliferation marker and transmission electron microscopy. Historical material on earlier stages was used from the Embryological Collection of the Museum of Natural History Berlin, Germany. A temporary parietal yolk sac and yolk sac cavity occurred during the blastocyst implantation. The yolk sac covering the chorioallantoic placenta showed great structural differentiation between day 12 and 18 such as a transition from a pseudostratified epithelium to projections that resemble a tree. A Reicherts membrane was not developed on day 12, but a space filled with extracellular material and vacuoles was present. The membrane was found from day 16 onward and became thick and composed of amorphous and acellular material. The morphological data suggest that a transfer of nutrients from the yolk sac to the fetus was most advanced between 22th and 40th day, especially on the 40th day. In the last third of gestation a decrease in both, placenta and visceral, yolk sacs tissues was observed. In conclusion, the development of the yolk sac in the guinea pig showed similarities to other caviomorphs. Data suggest that the yolk sac in caviomorphs is most important for mid gestation, but not so much for early pregnancy as it is for other vertebrates.

L'utilisation massive des nanomatériaux pose de réels enjeux sanitaires et environnementaux. C'est pourquoi ils sont désormais soumis à une réglementation qui prévoit une traçabilité de ceux-ci depuis leur fabrication jusqu'à leur distribution et l'établissement d'une fiche d'identité de la substance (composition, taille, état d'agglomération, forme, etc.). Une routine de caractérisation de nanoparticules en suspension a ainsi été développée. La Microscopie Électronique en Transmission (MET) a permis d'établir une majorité des paramètres de la fiche d'identité, en combinant à la fois imagerie et spectroscopie (analyses chimiques). La préparation, dont dépendra la qualité des observations, nécessite un développement pour chaque matériau analysé. Pour cela, trois techniques ont été mises au point : le dépôt en voie sèche qui permet une observation directe et simple, la cryogénie qui permet de fixer l'état de la suspension et l'in-situ liquide qui permet d'observer directement la suspension sans changement d'état. Les analyses MET étant locales, une comparaison avec des techniques indirectes a été effectuée par Diffusion Statique (MALS) et Dynamique (DLS) de la Lumière avec et sans fractionnement par couplage flux-force (FFF). Deux matériaux modèles ont été choisis. Le premier est une nanoémulsion de lipides stabilisés par des surfactants, servant de vecteurs à des principes actifs. Une étude de vieillissement par interaction avec des protéines a été menée et de légères variations de taille ont été obtenues. Le second matériau sélectionné est une poudre de nanoparticules de dioxyde de titane, remises en suspension, utilisée dans les crèmes solaires en tant que filtres UV. Ces particules ont été observées avant et après passage en enceinte climatique afin d'observer les effets des rayons UV sur celles-ci. Ceci a confirmé la stabilité des particules. Les protocoles de caractérisation développés au cours de cette thèse peuvent ainsi servir de supports à l'étude d'autres nanoparticules en suspension. / The extensive use of nanomaterials has raised awareness about health issues and their fate in the environment. That is why they are now subject to regulation that has imposed their traceability from their manufacturing to their distribution as the establishment of their characteristics (chemical composition, size, agglomeration state, shape ...). A characterization routine for nanoparticles in suspension was developed. Transmission Electron Microscopy (TEM) fulfills most of the criteria cited before by combining imaging and spectroscopy techniques. Three sample preparation methods were optimized to ensure high quality results : a dry process, rapid freezing to vitrify the sample and the use of an textit{in-situ} liquid TEM holder to prevent any preparation artefact (no phase change). To obtain quantitative analysis, a comparison was made between Dynamic Light Scattering (DLS), Multi-Angle Light Scattering (MALS), with and without a fractionation system (AF4), and TEM. To support this work, two nanomaterials were analyzed. The first one is a nanoemulsion composed of lipid nanoparticles stabilized by surfactants used as nanocarriers for drug delivery. Their stability after protein interaction was investigate and some size variations were observed. The second material is a powder composed of titanium dioxide nanoparticles used as UV filters in sunscreens. These nanoparticles were analyzed before and after interaction with UV radiation in a climatic chamber to confirm their stability. The different protocols developed in this PhD may be used for the analysis of other nanomaterials.

As características das células musculares e da junção miotendínea do músculo pterigóideo mediai de ratos adultos e com envelhecimento foram estudadas, empregando-se as técnicas de microscopia de luz e eletrônica de varredura e de transmissão. A microscopia de luz mostrou que as fibras musculares inserem-se na superfície óssea mandibular através de tecido colágeno composto por numerosos fibroblastos. Os cortes examinados sob luz polarizada revelaram a presença de fibras colágenas tipo\' e li\' nesta região. Além disso, puderam-se notar os capilares sangüíneos em grande número. Observações ao microscópio eletrônico de varredura ressaltaram as estriações presentes no músculo esquelético. O tecido conjuntivo que constitui o endomísio foi notado quando as amostras foram tratadas com solução de hidróxido de sódio e foi possível observar os diferentes diâmetros apresentados pelas fibras musculares. A microscopia eletrônica de transmissão evidenciou o alinhamento dos sarcômeros e as concentrações de mitocôndrias junto à lâmina basal na terminação da fibra muscular. O estudo da junção miotendínea dos ratos adultos e com envelhecimento identificou as fibras colágenas e os feixes de miofibrilas formando interdigitações que promovem o aumento da superfície de contato nesta região. Os aspectos observados nos músculos de ratos adultos mostraram-se bastante similares aos encontrados nos ratos com envelhecimento. / The mediai pterygoid muscle cells and myotendinous junction characteristics of adult and aging rats were studied, using light microscopy and scanning and transmission electron microscopy techniques. The Iight microscopy showed the muscle fibers inserting in the mandible surface through collagen tissue and numerous fibroblasts. The samples examined in polarized light revealed the presence of type I and 11I collagen fibers in this site. Furthermore, the numerous blood capillaries could be noticed. Observations at the scanning electron microscope evidenced strias present in the skeletal muscle. The connective tissue that constitutes the endomysium could be noticed when the samples were treated with sodium hydroxide solution and it was possible to observe the muscle fibers different diameters. The transmission electron microscopy revealed the sarcomeres alignment and the mitochondria concentration by the basal lamina in the muscle fiber termination. The adult and aging rats myotendinous junction study showed the collagen fibers and the myofibrils bundles developing finger-like processes that increases the contact areas in this site. The aspects seen in adult rats pterygoid muscles were very similar to the ones seen in the aging rats.

Les fontes de fer sont des alliages eutectiques de fonderie dont la structure consiste en des précipités de graphite dans une matrice riche en fer. Les propriétés mécaniques de ces alliages peuvent être grandement modifiées en agissant sur la matrice et sur la forme des précipités de graphite. Industriellement, les formes de graphite les plus courantes sont lamellaire et sphéroïdal, et l'on passe de l'une à l'autre par un traitement de sphéroïdisation (ajout de 0,02-0,05% massique de cérium ou de magnésium). D'autres morphologies peuvent apparaître en fonction des conditions de refroidissement et de la composition chimique de la fonte liquide. De fait, de très nombreux éléments d'alliage ou à l'état de traces peuvent diminuer l'efficacité du traitement de sphéroïdisation et modifier la forme du graphite. L'objectif de cette étude a été double, d'une part caractériser la structure fine de différentes formes de graphite, d'autre part réaliser des expériences de laboratoire destinées à étudier l'effet de quelques éléments, O, Ce et Sb, sur la croissance du graphite primaire. Dans cette étude, différents types de graphite (sphéroïdal, lamellaire et morcelé) issus de fontes commerciales ont été étudiés par microscopie optique, microscopies électroniques à balayage et en transmission, et spectrométrie de masse des ions secondaires. Leur caractérisation a permis de mettre en évidence certaines similitudes à l'échelle nanométrique malgré les différences de morphologies à l'échelle micrométrique. Des expériences consistant à fondre du fer pur dans des creusets de graphite, avec ou sans ajout délibéré d’antimoine ou de cérium, ont été réalisées. Les échantillons ont été élaborés par chauffage soit sous air soit sous vide primaire, puis refroidis lentement et maintenus au dessus de la température eutectique afin de faire croître de gros précipités de graphite primaire, et enfin rapidement pour figer en une structure fine le liquide résiduel. Les différentes formes de graphite obtenues lors de ces expériences ont fait l'objet des mêmes analyses métallographiques que les fontes industrielles. L’antimoine a favorisé la formation de lamelles de graphite incurvées. Les courbures de ces lamelles ont lieu par des changements dans leur direction de croissance. Le cérium a produit du graphite sous forme de plaquettes courtes et épaisses dans les échantillons élaborés sous air et du graphite sphéroïdal ou explosé dans les échantillons élaborés sous vide. Ainsi, l’un des rôles de cet élément est de désoxyder la fonte, ce qui a été confirmé par l’observation de différents oxydes de cérium dans les échantillons. L’observation de morphologies de graphite particulières dans ces échantillons suggère que le rôle du cérium ne se limite pas à la désoxydation de la fonte et des mécanismes d’action de ces éléments ont été discutés. Les caractérisations microstructurales, et en particulier la microscopie électronique en transmission, montrent que le graphite peut s’adapter à différentes conditions de croissance et adopter diverses morphologies dans les fontes. Les résultats de ces travaux ont permis une meilleure compréhension de l’effet des éléments d’addition sur la croissance du graphite et un modèle de croissance permettant de décrire les observations a été proposé. / Cast irons are eutectic foundry alloys with a structure consisting of graphite precipitates within an iron-rich matrix. The mechanical properties of these alloys can be modified by changing the nature of the matrix and the shape of the graphite precipitates. The most usual graphite shapes are lamellar and spheroidal, this modification being achieved through a spheroidisation treatment (adding 0.02-0.05 wt% of cerium or magnesium). Other morphologies can occur depending on the chemical composition of the melt and its cooling conditions. Numerous alloying elements present as traces can reduce the effectiveness of the spheroidisation treatment and modify the shape of graphite. The objectives of this study were to characterize the structure of different graphite types at a fine scale and to carry out laboratory experiments to study the effect of certain elements (oxygen, cerium and antimony) on primary graphite growth. In the present work, various types of graphite found in commercial cast irons were studied using optical microscopy, secondary ion mass spectrometry, scanning and transmission electron microscopy. Characterization of spheroidal, lamellar and chunky graphite was performed in order to reveal their structural features. Even though these precipitates seem very different at lower magnification the results of this investigation emphasize on similarities found between them. Experiments consisting in melting pure iron in graphite crucibles, with or without antimony or cerium additions were carried out. The samples were heated in air or primary vacuum, then slowly cooled and held above the eutectic temperature so as to produce large primary graphite crystals, and finally quenched to produce a fine structure from the remaining liquid. The different graphite morphologies obtained in these experiments underwent the same metallographic analyses as the commercial irons. Antimony favoured the development of curved graphite flakes by promoting frequent changes in the flakes’ growth direction. Cerium produced thick graphite platelets in the samples prepared in air and exploded graphite in the samples prepared in vacuum. One of the main roles of this element is thus to deoxidize the melt, which was confirmed by the presence of several cerium oxides in the samples. Peculiar graphite morphologies were observed in these samples, suggesting that the role of cerium is not limited to deoxidizing the iron and the mechanisms by which this element affects graphite growth were discussed. The microstructural characterizations and mainly transmission electron microscopy show that graphite can adapt to different growth conditions and adopt diverse morpholgies in cast irons. The results of the present work lead to a better understanding of the effect of elements on graphite growth and a model was proposed to describe the experimental observations.

Using current technology, it is now possible to probe material at atomic length scales, increasing our fundamental understanding of material behavior and properties. Metastable β titanium alloys are a subset of titanium alloys with huge potential for the aerospace sector. However, they exhibit atomic transformations which, even after 60 years of research, are still disputed. For example, these alloys are strengthened using the ω phase, but the mechanism by which this phase forms and its stability are still in question. The aim of this PhD project was to investigate the strengthening of metastable Ti-15wt.%Mo by understanding the stability and transformation pathways which make the metastable β titanium alloy class unique. Athermal ω shares the same composition as the β matrix and is formed by rapid cooling from the β phase field. The classical theory of athermal ω formation is based upon a diffusion-less mechanism in which consecutive pairs of {111}β planes collapse together. However, latest high-resolution electron microscope observations have suggested chemical alterations occur as well, which give reason to challenge this classical formation mechanism. Two novel methods were explored to determine the nature of the ω phase: 1) electron imaging of thin material at different collection angles and 2) total X-ray scattering analysis of large volumes of material. Complementary techniques are invaluable since thin foil artefacts were identified. In particular, a new B2 structured phase in the Ti-15wt.%Mo alloy was observed only in thin electron transparent material. Experimental data from the two new methods were compared to simulations. It was found that a frozen phonon description of the ω structure provided a best fit in both scenarios. The results are therefore consistent with the classical theory of ω formation but the collapse of the {111}β planes towards the ω phase is not considered complete.

Crystallographic image processing (CIP) is a technique first used to aid in the structure determination of periodic organic complexes imaged with a high-resolution transmission electron microscope (TEM). The technique has subsequently been utilized for TEM images of inorganic crystals, scanning TEM images, and even scanning probe microscope (SPM) images of two-dimensional periodic arrays. We have written software specialized for use on such SPM images. A key step in the CIP process requires that an experimental image be classified as one of only 17 possible mathematical plane symmetry groups. The current methods used for making this symmetry determination are not entirely objective, and there is no generally accepted method for measuring or quantifying deviations from ideal symmetry. Here, we discuss the crystallographic symmetries present in real images and the general techniques of CIP, with emphasis on the current methods for symmetry determination in an experimental 2D periodic image. The geometric Akaike information criterion (AIC) is introduced as a viable statistical criterion for both quantifying deviations from ideal symmetry and determining which 2D Bravais lattice best fits the experimental data from an image being processed with CIP. By objectively determining the statistically favored 2D Bravais lattice, the determination of plane symmetry in the CIP procedure can be greatly improved. As examples, we examine scanning tunneling microscope images of 2D molecular arrays of the following compounds: cobalt phthalocyanine on Au (111) substrate; nominal cobalt phthalocyanine on Ag (111); tetraphenoxyphthalocyanine on highly oriented pyrolitic graphite; hexaazatriphenylene-hexacarbonitrile on Ag (111). We show that the geometric AIC procedure can unambiguously determine which 2D Bravais lattice fits the experimental data for a variety of different lattice types. In some cases, the geometric AIC procedure can be used to determine which plane symmetry group best fits the experimental data, when traditional CIP methods fail to do so.

Au début du XXIème siècle, la Microscopie Electronique à Transmission en mode Environnemental (ETEM) est devenue l’une des techniques les plus fiables de caractérisation de nanomatériaux dans des conditions simulant leur vie réelle. L’ETEM est maintenant en mesure de suivre l’évolution dynamique des nanomatériaux dans des conditions variables comme l’exposition à des températures élevées, l’observation en milieux liquide ou gazeux à diverses pressions. Parmi différents domaines de recherche et développement concernés, la catalyse peut bénéficier de manière significative des avancées permises par la microscopie électronique environnementale. Cette thèse, dédiée au développement de l’ETEM au laboratoire MATEIS, a commencé avec l’étude du système catalytique Pd-alumine. Les nanoparticules de Pd déposées sur alpha -Al2O3 et delta-Al2O3 sont très utilisées en physicochimie avec un impact environnemental important : en particulier dans le domaine de l’hydrogénation sélective, pour la synthèse de polymères ou l’hydrogénation de CO2 pour la production de méthane. Nous avons tout d’abord effectué des analyses 2D aux différentes étapes du processus de synthèse du catalyseur : imprégnation du précurseur, séchage et chauffage pour la calcination dans l’air à la pression atmosphérique. La motivation de cette approche a été de comparer des analyses post mortem avec des traitements en ETEM où l’évolution des nanoparticules peut être mesurée in situ et pas seulement « avant » et « après ». De manière générale, les études faites en ETEM en 2D donnent un aperçu limité sur la morphologie des objets et la distribution spatiale des nanoparticules supportées. Nous avons développé une nouvelle approche d’acquisition rapide pour collecter dans des temps très courts des séries d’images sous différents angles de vue pour la tomographie électronique, la rapidité de cette acquisition étant un prérequis pour appréhender correctement la morphologie d’un nano-système au cours de son évolution dynamique in situ. La technique a ensuite été utilisée pour l’étude de plusieurs systèmes où une acquisition tridimensionnelle rapide est indispensable, notamment sur un sujet concernant un enjeu sociétal important, la dépollution des moteurs diesel : l’oxydation de la suie a été étudiée in situ sur des supports à base de zircone entre 400 et 600°C et une pression de 2 mbar d’oxygène à différents degrés de combustion, ce qui a permis d’extraire des données cinétiques telle que l’énergie d’activation du processus. La tomographie électronique rapide a été également appliquée à des matériaux sensibles au faisceau électronique, comme des nanocomposites polymères et des objets biologiques, montrant le large spectre d’applications possibles pour cette technique, qui constitue un pas important vers la caractérisation operando 3D de nanomatériaux en temps réel. / In the beginning of the XXIst century, Environmental Transmission Electron Microscopy has become one of the reliable characterization techniques of nanomaterials in conditions mimicking their real life. ETEM is now able to follow the dynamic evolution of nanomaterials under various conditions like high temperature, liquid or various gas pressures. Among various fields of research, catalysis can benefit significantly from Environmental Microscopy. This contribution starts with the study of the Palladium-Alumina catalytic system. Pd nanoparticles supported by α-Al2O3 and δ-Al2O3 are of an important physicochemical and environmental interest, particularly in the field of selective hydrogenation in petrochemistry, for the synthesis of polymers or CO2 hydrogenation for methane production. We first performed 2D analyses at different steps of the synthesis process, then the same synthesis steps were performed under in situ conditions. The motivation of this approach was to compare post mortem treatments with ETEM observations. In general, 2D data provide limited insights on, for example, the morphology and position of supported nanoparticles. We have then developed a new fast acquisition approach to collect tomographic tilt series in very short times, enabling to reconstruct nano-systems in 3D during their dynamical evolution. Taking advantage of this approach, we have determined the activation energy for soot combustion on YSZ oxidation catalysts for diesel motors from volumetric data extracted from in situ experiments. Fast electron tomography was also applied to electron beam sensitive materials, like polymer nanocomposites and biological materials, showing the wide spectrum of possible applications for rapid 3D characterization of nanomaterials.

Indium phosphide (InP) is an important III-V compound, with a variety of applications, for example, in light emitting diodes (LED), InP based photonic crystals and in semiconductor lasers, heterojunction bipolar transistors in integrated circuit applications and in transistors for microwave and millimeter-wave systems. The optical and electrical properties of this compound can be further tailored by ion implantation or prospectively by swift heavy ion beams. ¶ Thus knowledge of ion-induced disorder in this material is of important fundamental and practical interest. However, the disorder produced during heavy ion irradiation and the subsequent damage accumulation and recovery in InP is far from being completely understood. In terms of the damage accumulation mechanisms, the conclusions drawn in the numerous studies performed have often been in conflict with one another. A factor contributing to the uncertainties associated with these conflicting results is a lack of information and direct observation of the “building blocks” leading to the ultimate damage created at high ion fluences as an amorphous layer. These building blocks formed at lower fluence regimes by single ion impacts can be directly observed as isolated disordered zones and ion tracks for low energy and swift heavy ion irradiation, respectively. ¶ The primary aim of this work has thus been to obtain a better understanding of the disorder in this material through direct observations and investigation of disorder produced by individual heavy ions in both energy regimes (i.e. elastic and inelastic energy deposition regimes) especially with low ion fluence irradiations. In this thesis the heavy ion induced disorder introduced by low energy Au ions (100 keV Au+) and high energy Au (200 MeV Au+16) ion irradiation in InP were investigated using Transmission Electron Microscopy (TEM), Rutherford Backscattering Spectrometry (RBS/C) and Atomic Force Microscopy (AFM). ¶ The accumulation of damage due to disordered zones and ion tracks is described and discussed for both low energy and swift ion irradiation respectively. ¶ The in-situ TEM annealing of disordered zones created by 100 keV Au+ ion irradiation shows that these zones are sensitive to electron beam irradiation and anneal under electron energies not sufficient to elastically displace lattice atoms, i.e. subthreshold energies for both constituent atoms In and P. ¶ Ion tracks due to swift heavy ion irradiation were observed in this material and the interesting track morphology was described and discussed. The surface nanotopographical changes due to increasing fluence of swift heavy ions were observed by AFM where the onset of large increase in surface roughness for fluences sufficient to cause complete surface amorphization was observed. ¶ In addition to InP, the principle material of this project, a limited amount of TEM observation work has been performed on several other important compounds (apatite and monazite) irradiated by 200 MeV Au+ ions for comparative purposes. Again the observed segmental morphology of ion tracks were shown and possible track formation scenario and structure were discussed and similarities were drawn to the previously observed C60 cluster ion tracks in CaF2 as more knowledge and data base exist about defect dynamics and formation in that material.

Materials in smaller scales exhibit promising properties that are useful for wide variety of applications. Semiconductor quantum wells and quantum dots are two main examples of low-dimensional systems, where the quantum wells act as two-dimensional systems and the quantum dots act as zero-dimensional systems. Alternatively, semiconductor nanowires act as one-dimensional materials, and they exhibit promising and device applicable properties. These nanowires are relatively new class of materials compared to the quantum wells and the quantum dots. The semiconductor nanowires are expected to be the building blocks for future nanoelectronic and nano-optoelectronic device technology. Vapor-liquid-solid (VLS) mechanism is a widely used mechanism for the growth of semiconductor nanowires, where metal nanoparticles are used as the catalysts for the nanowires growth. This growth mechanism offers a flexibility to control the size, morphology and location of the semiconductor nanowires. In the VLS growth, changing the chemical composition of vapor constituents produce consequent compositional modulation in each nanowire. The compositional modulation along the nanowire axis produces axial nanowire heterostructures and in radial direction produces radial nanowire heterostructures. Such compositional modulation within an individual nanowire enables the designing of band structure of a nanowire and thereby allows the fabrication of single nanowire devices. These nanowire heterostructures show many potential properties and consequent applications. Although the semiconductor nanowire heterostructures are promising semiconductor nanostructures, the fundamental growth mechanisms of axial and radial nanowire heterostructures have not been explored sufficiently due to their complex nature of the growth. In this regard, this PhD thesis addresses the fundamental issues associated with axial and radial nanowire heterostructures. For such fundamental investigations, this PhD work chooses InAs/GaAs nanowire heterostructure system due to its potential applications. In fact, InAs/GaAs nanowire heterostructures are the first reported axial nanowire heterostructure system. However, no detailed investigations were reported on this system so far. The detailed nucleation and growth mechanisms associated with InAs/GaAs nanowire heterostructure system are explored in this thesis using electron microscopy investigations. This objective is achieved in the following steps. • InAs/GaAs nanowire heterostructures are grown using Au nanoparticles and metal-organic chemical vapor deposition (MOCVD) method. To determine the axial and radial growth evolution of InAs on GaAs nanowires, different InAs/GaAs nanowire heterostructures are produced by depositing InAs for different durations on GaAs nanowires. The GaAs nanowires are initially grown for 30 min and then the InAs is deposited on these nanowires for 1, 3, 5 and 30 min. • These InAs/GaAs nanowire heterostructures are subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. These investigations determine that, in the initial stages of the InAs axial growth (1 min), the Au particles move sidewards and subsequently downwards by maintaining an interface with the GaAs nanowire. Such a movement of Au catalysts is attributed to lower Au/GaAs interfacial energy than Au/InAs. The detailed TEM investigations show that this Au movement depends upon the crystallographic nature of the GaAs nanowire. The Au particle is always tend to move towards {112}B sidewall of the GaAs nanowire rather than its {112}A sidewalls. Increase in InAs growth duration shows that InAs branches evolve from GaAs-InAs core-shell structures. Such evolution is observed in following steps: (1) the movement of Au particle terminates when it encounters the radially grown InAs on GaAs nanowires; (2) further growth of InAs leads to the InAs nanowire growth from those terminated Au nanoparticles in the form of branches. • The TEM observations of InAs/GaAs nanowire heterostructures show that, in the initial stages of InAs radial growth on GaAs nanowires, InAs nucleates preferentially in the concave regions of the non-planar sidewalls of the GaAs nanowire. The further growth of InAs leads to the preferential formation of InAs shell structure at the regions of concave regions. Such heterogeneous formation of shell structure resembles InAs nanoring structures around GaAs nanowire cores. InAs growth on the planar {112} sidewalls of GaAs nanowires with hexagonal cross sections shows different growth phenomena to the above described InAs nanorings formation. In this case, InAs preferentially nucleates on {112}A sidewalls of the GaAs nanowires and with further deposition of InAs, the complete shell structure of InAs form with {110} sidewalls on the GaAs nanowire cores. • In addition to the above mentioned investigations, to observe the growth evolution of GaAs on InAs nanowires, GaAs is grown for 3 and 30 min on InAs nanowires. The TEM investigations of these nanostructures show that the axial GaAs/InAs hetero-interface contains an InGaAs transition segment in contrast to the sharp InAs/GaAs (InAs on GaAs) hetero-interface. The different nature of hetero-interfaces is attributed to the different affinities between Au catalysts and Ga or In. The radial growth of GaAs on InAs nanowires show that the GaAs shell has grown in wurtzite structure around the wurtzite structured InAs nanowire cores. Overall, through the extensive SEM and TEM investigations, this PhD thesis addresses the fundamental issues related to the growth of axial and radial nanowire heterostructures. Such fundamental investigations are expected to advance the processing and application prospective of the semiconductor nanowires and their associated heterostructures.