Gas source molecular beam epitaxy (GSMBE) has been used to grow InGaAs/lnP epitaxial layers in selected areas defined by SiO2-masked InP substrates, with the goal of obtaining controlled in-plane variations in the bandgap of the InGaAs wells. The ability to alter the bandgap of the semiconductor spatially over the surface in one growth procedure is desirable for integrating laser, waveguide and detector devices. To form the masked substrates, stripes (ranging in width from 2 pm to 50 pm) were opened up in SiO2 by standard photolithography. The crystal growths were carried out at various substrate temperatures (ranging from 460 °C to 510 °C) and arsenic fluxes (V/lll ratios ranging from 1.2 to 3.4). The properties of the epitaxial layers were investigated by using such analytical techniques as photoluminescence, electroluminescence and transmission electron microscopy (TEM). Photoluminescence measurements performed on waveguide stripes of decreasing width reveal an increasing red-shift of the e1-hh1 transition in InGaAs wells. The maximum red-shift occurred when growing at a high substrate temperature and a low arsenic flux. For example, a decrease in slit width from 50 pm to 10 pm resulted in a 25 meV shift of the photoluminescence peak. From cross-sectional TEM measurements, the wavelength shift observed can be attributed primarily to an increase in thickness of the InGaAs well, due to incorporation of additional indium and gallium migrating from the material on the masked regions. The interfaces in the centre of the stripe region are defect free; however, stacking faults and thickness variations are evident 1-2 pm from the edges. These results are confirmed by scanning photoluminescence, in which the maximum intensity occurs at the centre of the stripe and decreases to zero at the edges. Mapping of the peak wavelength across the stripe reveals a diffusion profile, with the edges being additionally red shifted by 10 nm. Reactive ion etching of the edge and the polycrystalline material results in a much improved spectral photoluminescence scan, in both increased intensity of the bandgap peak and elimination of lower energy peaks assumed to be correlated with edge effects. Finally, a stripe contact light emitting device, with a single 50 A quantum well InGaAs/lnP structure, was fabricated and electrically pumped. The device exhibited spectral peak wavelength shifts between narrow stripes (10 pm) and wide stripes (50 pm) of 22 nm, similar to the value observed by photoluminescence studies. / Thesis / Master of Engineering (ME)

Gas source molecular beam epitaxy (GSMBE) has been used to grow InGaAs/lnP epitaxial layers in selected areas defined by SiO2-masked InP substrates, with the goal of obtaining controlled in-plane variations in the bandgap of the InGaAs wells. The ability to alter the bandgap of the semiconductor spatially over the surface in one growth procedure is desirable for integrating laser, waveguide and detector devices. To form the masked substrates, stripes (ranging in width from 2 pm to 50 pm) were opened up in SiO2 by standard photolithography. The crystal growths were carried out at various substrate temperatures (ranging from 460 °C to 510 °C) and arsenic fluxes (V/lll ratios ranging from 1.2 to 3.4). The properties of the epitaxial layers were investigated by using such analytical techniques as photoluminescence, electroluminescence and transmission electron microscopy (TEM). Photoluminescence measurements performed on waveguide stripes of decreasing width reveal an increasing red-shift of the e1-hh1 transition in InGaAs wells. The maximum red-shift occurred when growing at a high substrate temperature and a low arsenic flux. For example, a decrease in slit width from 50 pm to 10 pm resulted in a 25 meV shift of the photoluminescence peak. From cross-sectional TEM measurements, the wavelength shift observed can be attributed primarily to an increase in thickness of the InGaAs well, due to incorporation of additional indium and gallium migrating from the material on the masked regions. The interfaces in the centre of the stripe region are defect free; however, stacking faults and thickness variations are evident 1-2 pm from the edges. These results are confirmed by scanning photoluminescence, in which the maximum intensity occurs at the centre of the stripe and decreases to zero at the edges. Mapping of the peak wavelength across the stripe reveals a diffusion profile, with the edges being additionally red shifted by 10 nm. Reactive ion etching of the edge and the polycrystalline material results in a much improved spectral photoluminescence scan, in both increased intensity of the bandgap peak and elimination of lower energy peaks assumed to be correlated with edge effects. Finally, a stripe contact light emitting device, with a single 50 A quantum well InGaAs/lnP structure, was fabricated and electrically pumped. The device exhibited spectral peak wavelength shifts between narrow stripes (10 pm) and wide stripes (50 pm) of 22 nm, similar to the value observed by photoluminescence studies. / Thesis / Master of Engineering (ME)

A l'échelle nanométrique, les solides peuvent changer de forme par diffusion de surface, et présentent alors des propriétés de mouillage qui s'apparentent à celles des liquides. Dans cette thèse, nous nous sommes plus particulièrement intéressés au comportement de mouillage des nano-solides sur des substrats nanopatternés, comportant par exemple des piliers ou des tranchées. Sur ces substrats, les nanoparticules (ou ilots) solides peuvent être multi-stables : c'est-à-dire qu'ils peuvent présenter plus d'un état localement stable. Comme les liquides, les solides ont été observés par exemple dans des états dits de Wenzel (pénétrant dans la structure du substrat) ou de Cassie-Baxter (ne pénétrant pas). Grâce à une combinaison de simulations Monte Carlo Cinétiques et de modèles analytiques, nous avons étudié la stabilité de ces états et leur dynamique de transition. Plus particulièrement, avons mis en évidence le rôle de la diffusion de surface et de la nucléation bidimensionnelle sur la dynamique de transition. Nous avons aussi montré que les contraintes élastiques augmentent la stabilité des états de Cassie-Baxter, et mènent à de nouveaux états, avec des morphologies asymétriques ou partiellement empalées dans les nanostructures. Finalement, nous avons proposé de contrôler les transitions de mouillage à l'aide de l'électromigration induite par un faisceau d'électrons. Nos résultats ouvrent la voie vers une nouvelle direction pour les investigations expérimentales / At the nanometer scale, solids can change shape thanks to surface diffusion and therefore display wetting properties that can be likened to those of liquids. This doctoral thesis intends to study particularly the wetting behaviour of nano-solids located on nanopatterned substrates, containing for instance pillars or trenches. Upon these substrates, solid nanoparticles (or islands) can be multi-stable – that is to say they can display more than one locally-stable state. Just like liquids, solids have been observed for example in the context of the so-called Wenzel state (penetrating the very structure of the substrate) and Cassie-Baster state (no penetration). By combining Kinetic Monte Carlo simulations with analytical models, we conducted a study on the stability of these states along with their dynamics of transition. In particular, we highlighted the specific roles that surface diffusion and bidimensional nucleation play in regards to the dynamics of transition. We also demonstrated that elastic constraints increase the stability of Cassie-Baxter states and lead to new states, with either asymmetric morphologies or morphologies that are partially impaled into the nanostructures. Last but not least, we proposed to control wetting transitions using the electromigration brought on by an electron beam. Our results pave the way for a new direction in the field of experimental investigations

Ces travaux concernent l’étude expérimentale des textures se développant dans les ménisques de films smectiques de cristaux liquides thermotropes. Ces fluides complexes nous servent de système modèle pour l’étude des couplages élasto-capillaires qui se manifestent dans certaines conditions par des déformations périodiques de l’interface cristal liquide-air. Dans un premier temps, à partir de l’étude détaillée de la topographie de l’interface par une méthode interférométrique, nous caractérisons les différents types de structures, puis, identifions les principaux paramètres impliqués dans le processus d'apparition des défauts dans le ménisque de films libres suspendus. Ces résultats nous servent de base pour discuter des mécanismes physiques qui génèrent les différents types de défauts observés. Nous regardons en particulier, comment la diminution d’épaisseur de couche associée à une transition de phase va induire une instabilité mécanique responsable de l’apparition des ondulations de l’interface. Dans un deuxième temps, afin d’aller plus loin dans la compréhension des mécanismes, nous avons utilisé des films minces de cristaux liquides déposés sur des substrats solides microstructurés par des plots fabriqués par des techniques photolitographiques. Une telle géométrie permet de générer un ménisque autour de chaque plot et surtout de faire varier un plus grand nombre de paramètres comme par exemple l’ancrage sur le substrat via un traitement chimique de surface. L’ensemble de nos résultats apportent un éclairage nouveau sur la caractérisation et la compréhension des déformations spécifiques aux ménisques de fluides complexes. / The present work is an experimental study of the textures that appear in the meniscus of free standing smectic films with thermotropic liquid crystals. These complex fluids serve as model systems to investigate elasto-capillary phenomena which, under certain conditions, manifest themselves through periodic deformations of the liquid crystalair interface. In the first part of the thesis, we focus our attention on meniscus structures whose interfacial topographies are thoroughly characterized thanks to an in-house optical interferometry technique. Our study allows us to identify the main parameters involved in the development of meniscus structures and to discuss the physical mechanisms that are likely to be responsible for their formation. In particular, we show how a phase transition-induced layer shrinkage triggers a mechanical instability leading to interfacial undulations of the smectic free surface. In the second part of the manuscript, we address the case of thin liquid crystal films deposited on solid patterned solid substrates. The latter consist of regular arrays of microposts fabricated through photolithographic techniques. Such a geometry allows a meniscus to be formed around each micropost and makes it possible to examine the influence of other parameters such as the anchoring conditions on the solid substrate. The results gathered so far are able to shed some light on the characterization and the understanding of the specific deformations and textures that appear in the menisci of complex fluids.

Les protéines d’adhésions homophiles N-cadhérine jouent un rôle majeur dans le développement du cerveau, notamment en agissant sur la croissance et la plasticité synaptique. Au cours de ma thèse, j’ai étudié le rôle de la N-cadhérine dans ces deux processus en utilisant des neurones issus de cultures primaires déposés sur des substrats micropatternés. Ces substrats sont recouverts de N-cadhérine purifiée afin d’induire des adhésions N-cadhérines sélectives au niveau de micro-motifs régulièrement espacés. Mes deux premières études sont basées sur le modèle d’embrayage moléculaire, décrivant le processus par lequel la motilité du cytosquelette d’actine se couple aux adhésions au niveau de la membrane cellulaire afin de générer des forces de traction aux zones de contact avec le substrat, permettant ainsi l’avancée cellulaire (Giannone et al., 2009). Plusieurs études ont mis en avant l’existence d’un tel modèle (Mitchison et Kirschner, 1988 ; Suter et Forscher, 1998), cependant le mécanisme exact permettant d’expliquer ce couplage mécanique de l’actine aux protéines d’adhésions reste mal connu. Via des techniques de pinces optiques, des travaux précédemment menés dans l’équipe ont prouvé l’existence d’un couplage entre le flux d’actine et les adhésions N-cadhérine permettant la migration du cône de croissance (Bard et al., 2008). Cette technique n’a cependant pas permis la visualisation directe de l’engagement d’un tel mécanisme. Nous avons donc couplé l’utilisation des substrats micro-patternés à la microscopie haute résolution sptPALM/TIRF afin de visualiser directement la dynamique des protéines impliquées dans l’embrayage moléculaire. Dans le premier article, j’ai montré pour la première fois l’existence d’interactions transitoires entre le flux d’actine et les adhésions N-cadhérines au niveau du cône de croissance, reflétant un embrayage glissant à l’échelle de la molécule unique (Garcia et al., en préparation). Dans le second article, en travaillant sur des neurones plus matures, nous avons pu montrer l’engagement d’un embrayage moléculaire trans-synaptique entre adhésions N-cadhérines et flux d’actine permettant la stabilisation du filopode dendritique et ainsi sa transition en épine mature (Chazeau/Garcia et al., en préparation). J’ai également participé à une troisième étude dans laquelle j’ai observé l’effet des substrats micropatternés recouverts de N-cadhérine, sur la synaptogenèse. J’ai ainsi pu prouver que la N-cadhérine déposée sur les micro-motifs, stimule la croissance dendritique et axonale et joue un rôle prépondérant dans la maturation morphologique des neurones. Cependant, la N-cadhérine est incapable d’induire la formation de synapses contrairement aux protéines d’adhésion neurexine/neuroligine ou SynCam (Czöndör et al., 2013). / The homophilic adhesion molecule N-cadherin plays major roles in brain development, notably affecting axon outgrowth and synaptic plasticity. During my PhD work, I addressed the role of N-cadherin in these two processes, using primary neurons cultured on micro-patterned substrates. These substrates are coated with purified N-cadherin to trigger selective N-cadherin adhesions in a spatially controled manner. My two first studies are based on the “molecular clutch” paradigm, by which the actin motile machinery is coupled to adhesion at the cell membrane to generate forces on the substrate and allow cells to move forward (Giannone et al., 2009). Many publications have provided evidence for such a mechanism (Mitchison et Kirschner, 1988 ; Suter et Forscher, 1998), but the exact mechanisms underlying the molecular coupling between the actin retrograde flow and adhesion proteins remain elusive. The team previously inferred, using optical tweezers, that a molecular clutch between the actin flow and N-cadherin adhesions drives growth cone migration (Bard et al., 2008), but could not achieve a direct visualization of the engagement process with this technique. Here, we combined the use of micropattern substrates with high resolution microscopy sptPALM/TIRF to visualize directly the dynamics of the main proteins involved in the molecular clutch. In my first paper, I reveal for the first time transient interactions between the actin flow and N-cadherin adhesions in growth cones, reflecting a slipping clutch process at the individual molecular level (Garcia et al., in preparation). In a second study, working with more mature neurons, we revealed that engagement of a molecular clutch between trans-synaptic N-cadherin adhesions and the actin flow underlies the stabilization of dendritic filopodia into mature spines (Chazeau/Garcia et al., in preparation). I also participated to a third study, where I observed the effect of N-cadherin coated substrates on synaptogenesis. I showed that, although N-cadherin on micro-patterned substrates stimulated axonal and dendritic elongation and played a major role in morphological maturation, it was not able to induce synapse formation like neurexin/neuroligin or SynCAM adhesions (Czöndör et al., 2013).

In this thesis, patterning methods to fabricate various functional particle patterns on substrates were developed, with the main aim of modifying the properties and functions of the substrates. Two classes of model substrates were selected; topographically patterned and smooth substrates. For the first model system, i.e., topographically patterned substrates, replication molding was used to topographically pattern substrates of different materials. The topographically patterned substrates, including TiO2, block-copolymer substrates (PS-b-P2VP and PS-b-P4VP), and microrings (TiO2 and Au), were then used to assemble silica (SiO2) microparticles for functional applications. By the assembly of microparticles on topographically patterned substrates, the wettability of the former could be reversibly switched from hydrophobic to hydrophilic. Moreover, a platform for the preparation of Janus particles by orthogonal functionalization of the top and bottom sides of microparticles assembled on topographically patterned substrates was developed. Clusters of superparamagnetic nanoparticles were stamped on the second class of model substrates, i.e., smooth silanized silicon substrates. A capillary stamping approach combined with an external permanent magnetic field or electromagnets was realized to print magnetic nanoparticle-based inks. In this way, ordered arrays of clusters of magnetic nanoparticles were produced.