Gallium nitride (GaN) and its related semiconductor alloys are attracting tremendous interest for their wide range of applications in blue and green LEDs, diode lasers, high-temperature and high-power electronics. Nanomaterials such as InGaN/GaN core-shell three-dimensional nanostructures are seen as a breakthrough technology for future solid-state lighting and nano-electronics devices. In a core-shell LED, the active semiconductor layers grown around a GaN core enable control over a wide range of wavelengths and applications. In this thesis the capability for the heteroepitaxial growth of a proof-of-principle core-shell LED is advanced. A design that can be applied at the wafer scale using metalorganic vapor phase epitaxy (MOVPE) crystal growth on highly uniform GaN nanorod (NR) structures is proposed. This project demonstrates understanding over the growth constraints of active layers and dopant layers. The impact of reactor pressure and temperature on the morphology and on the incorporated InN mole fraction was studied for thick InGaN shells on the different GaN crystal facets. Mg doping and effectiveness of the p-n junction for a core-shell structure was studied by extensive growth experiments and characterization. Sapphire and Si substrates were used, and at all the stages of growth and fabrication. The structures were optimized to achieve geometry homogeneity, high-aspect-ratio, incorporation homogeneity for InN and Mg dopant. The three-dimensional nature of NRs and their light emission provided ample challenges which required adaptation of characterization and fabrication techniques for a core-shell device. Finally, an electrically contacted core-shell LED is demonstrated and characterized. Achieving a proof-of-principle core-shell device could be the starting point in the development of nanostructure-based devices and new physics, or in solving technical problems in planar LEDs, such as the polarization of emitted light, the quantum-confined Stark effect, efficiency droop, or the green gap.

Le remplacement du carbone graphite, matériau commercial dans les batteries au lithium ion, par du silicium est un axe privilégié afin d’augmenter la capacité des anodes au sein de ces accumulateurs. En revanche, le silicium micrométrique souffre de puissants effets de dégradation au cours du cyclage. L’expansion volumique des particules lors de la formation des alliages lithiés et la réduction des électrolytes en contact avec la matière active, sous forme de produits de dégradation appelés SEI, induisent une diminution importante de la durée de vie de ces anodes. La communauté scientifique a donc émis l’idée de stabiliser le silicium en diminuant la taille des particules à l’échelle nanométrique, limitant fortement le risque de pulvérisation. De plus, le contact direct entre la matière active et les solvants peut être très largement diminué via la formation d’une coquille de carbone autour des particules de silicium. La problématique est alors la suivante : obtenir un matériau dit « cœur-coquille » à base de silicium nanométrique enrobé de carbone, à l’aide d’un procédé facilement industrialisable.Le Laboratoire Edifices Nanométriques (LEDNA) possède une grande expertise en synthèse de nanomatériaux par pyrolyse laser en phase gaz. Cette méthode de synthèse est souple, possède un rendement de production élevé et offre un contrôle important sur les conditions de réaction. Afin de répondre à la problématique posée, un nouveau réacteur de synthèse à deux étages de réaction a été développé. A l’aide de cette expérience originale, des nanomatériaux à base de silicium cristallin ont été synthétisés, ainsi que leur équivalent enrobé de carbone. Des cœurs de silicium amorphes ont également été enrobé de carbone, permettant l’obtention d’une structure cœur-coquille encore inédite dans la littérature. La microscopie révèle que les matériaux sont sous forme de chainette de particules, une structure obtenue de façon classique par les méthodes de synthèse en phase gaz mais qui se pourrait se révéler bénéfique aux propriétés de conduction électronique et ionique. Les coquilles carbonées caractérisées par spectroscopie Raman révèlent une organisation riche en liaisons sp2 mais peu graphitique. Une étude par spectroscopie des électrons Auger (AES) montre que l’homogénéité de l’enrobage carboné varie selon les matériaux, les plus petits cœurs de silicium bénéficiant d’un meilleur recouvrement. Par diffraction des neutrons, nous avons montré que le silicium amorphe enrobé est très peu sensible à l’oxydation contrairement aux autres matériaux non enrobés.Les matériaux ont été utilisés en tant que matériaux d’anode dans des batteries au lithium métal. Une étude par balayage voltamétrique a montré que les matériaux à base de silicium cristallin nécessitent plusieurs balayages avant d’être lithiés jusqu’au cœur. En revanche, le silicium amorphe enrobé subit une lithiation profonde immédiate, phénomène dont la littérature ne fait pas mention faute de pouvoir obtenir ce composite non oxydée selon les méthodes de synthèses traditionnelles. Une étude par spectroscopie d’impédance électrochimique résolue en potentiel a été réalisée afin de déterminer les mécanismes de dégradation de ces électrodes. Nous avons montré que ce phénomène est principalement entretenu par la dissolution des composés de la SEI lors de la délithiation des matériaux. De plus, l’intensité de ce phénomène de dissolution semble corrélée avec la quantité de surface de silicium potentiellement en contact avec l’électrolyte. Enfin, testés galvanostatiquement, les matériaux enrobés de carbone ont démontré des performances très supérieures au carbone graphite. Au régime élevé de 2C, difficilement accessible au matériau d’anode commerciale, le matériau amorphe enrobé a supporté près de 500 cycles en maintenant une capacité et une efficacité coulombique élevée, supérieure à 800 mAh.g-1 et 99,99%. / The replacement of carbon graphite, the commercial anode material in Li-ion batteries, by silicon is one of the most promising strategies to increase the capacity of anode in these devices. However, micrometric silicon suffers from strong degradation effect while cycling. The volume expansion of the lithiated particles and the direct contact between the active material and the solvents induce the continuous formation and pulverization of a solid electrolyte interphase (SEI) leading to the rapid fading of the capacity. Many research groups suggest decreasing the size of the particle to the nanoscale where pulverization of the particles is almost inexistent. Furthermore, the formation of a carbon shell around these silicon nanoparticles is cited as the most efficient way to isolate the material from the direct contact with the solvent. The main issue is to obtain these core shell nanocomposites with a process able to meet industrial requirement.The Nanometric Structure Laboratory (LEDNA) is experimented in the synthesis of nanomaterial thanks to the gas phase laser pyrolysis method. This versatile process is characterized by a high yield of production and permits an efficient control over the reaction parameters. In order to obtain core shell structures, a new reactor has been developed by the combination of two stages of reaction. Thanks to this original setup, crystalline silicon cores covered or not with a carbon shell were achieved in one step for the first time. Likewise, amorphous cores were covered with a carbon shell, leading to the synthesis of a novel nanocomposite. Microscopic study reveals that these materials are obtained in a chain-like structure that can be beneficial to the electronic and ionic conduction properties. The carbonaceous compound were characterized by Raman spectroscopy and appeared to be non-graphitic sp2 rich species known in the literature as basic structural units (BSU). Auger electron spectroscopy study highlights the homogeneity of the carbon covering, in particular over smaller silicon cores. Neutron diffraction showed that the amorphous silicon cores covered with carbon are protected against passive oxidation unlike bare amorphous cores.The nanocomposites were used as anode materials in lithium-metal coin cell configuration. A cyclic voltammetry study highlights that crystalline silicon cores embedded into carbon need many sweeps before their full lithiation whereas amorphous core shell nanocomposites deeply lithiated from the first sweep, a phenomena yet not described in the literature. A potential resolved electronic impedance spectroscopy technic was used to determine the main degradation process of the core shell materials. We showed that the capacity fading can be mainly attributed to SEI dissolution and reformation through cycling, obstructing the porous structure of the electrode and limiting the cyclability. Finally, galvanostatically tested the core-shell nanocomposites reveal enhanced performance compared to graphite carbon. At the high charge/discharge rate of 2C, hardly reachable to the commercial anode material, the amorphous core-shell nanocomposite was cycled up to 500 cycles while maintaining a high capacity of 800 mAh.g-1 and outstanding coulombic efficiency of 99,99 %.

Cette étude a pour objectif de comprendre comment les effets de température, de cinétiques de croissance, ou les effets d’environnement peuvent influencer la structure (cristalline ou non cristalline) et la configuration chimique (mélange/ ségrégation/ séparation de phase) de nanoparticules supportées d’Ag et de CoAg. Pour cela, des nanoparticules de CoAg de différentes tailles et compositions ont été préparées par condensation sous ultravide selon différents modes de croissance (co-dépôt ou dépôts séquentiels des deux métaux). Afin d’accéder à l’ensemble des caractéristiques des nanoparticules, des techniques complémentaires ont été couplées: la diffusion de rayons X aux petits et aux grands angles en incidence rasante et les techniques de microscopie en mode d’imagerie haute résolution ou filtrée en énergie. L’analyse préliminaire de particules d’Ag a montré l’existence de structures cristallines et non cristallines (icosaédriques) pour les petites (2-2.5nm) et grandes tailles (6-8nm). Une dominance de la structure décaédrique a été montrée entre ces deux extrêmes. Cette dernière disparaît complètement lors de l’élaboration en température. Pour le système bimétallique Co-Ag, à température ambiante et indépendamment de la taille, de la composition et du mode d’élaboration, les nanoparticules présentent une ségrégation avec une configuration de type coeur d’argent entouré d’une coquille plus ou moins continue à base de Co métallique et d’oxyde de Co. Lorsque les échantillons sont soumis à un traitement thermique, une transition s’opère conduisant à une ségrégation plus importante de type Janus. / This study aims to understand how the temperature, the kinetic growth conditions or the environment can influence the structure (crystalline or non-crystalline) and the chemical order (mixing/ segregation/phase separation) of Ag and CoAg supported nanoparticles. Different samples of CoAg nanoparticles with different sizes and compositions were prepared by condensation under ultrahigh vacuum with different growth modes (co-deposition or sequential deposition of the two metals) and different thermal treatements. To access all the characteristics of the nanoparticles, complementary techniques were coupled: the X ray scattering at small and wide angles under grazing incident X Ray beam and the electron microscopy techniques: (high resolution and energy filtered modes). Preliminary analysis of Ag particles prepared at room temperature showed the existence of crystalline and non-crystalline (icosahedral) structures for small (2-2.5nm) and large sizes (6-8nm). A dominant feature of the decahedral structure was shown between these two extremes. This structure disappears completely when increasing elabration temperature. For the bimetallic Co-Ag system, at room temperature and independently of the size, composition and growth mode, the nanoparticles present a segregated configuration with a silver core surrounded by a more or less continuous shell, based on metallic Co or cobalt oxyde. After heating the samples, a transition takes place, leading to a more important segregation such as Janus one.

This thesis is geared towards using hydrogel nanoparticles in various biotechnological applications. The polymer that was used in making these nanoparticles was poly(N-isopropylacrylamide), which is a thermoresponsive polymer. These particles were used in making fast responsive polymer films, which can be used in optics. It was observed that the rate of deswelling increased as the concentration of the nanoparticles in the film was increased. These particles were also used in making photoresponsive materials. In this case a photoresponsive dye (malachite green) was conjugated to these nanoparticles and in presence of light of appropriate wavelength the particles undergo a phase transition. A core/shell construct was synthesized where the core was composed of degradable cross-links and the shell of composed of non-degradable cross-links. The degradable cross-linker had vicinal diols, which can be cleaved by sodium periodate. Hence after degrading the core, hollow particles were obtained. Zwitterionic particles were made by incorporating a cationic and anionic comonomer. These microgels go from a positively charged state to zwitterionic to negatively charged state on increasing the pH. One of the important potential applications for these microgels is drug delivery. Microgels were used for targeting cancer cells. Folic acid was used as the targeting ligand. The microgels were conjugated with folic acid and were able to target cells that overexpress folate receptors. In one other application core/shell microgels were made which exhibit pore-size dependent permeation of proteins.

The first part of my PhD work is about the lattice vibrations in silicon nanowires. First-principles calculations based on the linear response are performed to investigate the quantum confinement effect in lattice vibrations of silicon nanowires (SiNW). The radial breathing modes (RBM) are found in our calculations, which have a different size-dependent frequency shift compared with the optical modes. They are well explained by the elastic model. Finally, the relative activity of the Raman scattering in the smallest SiNW is calculated. The RBM can be clearly identified in the Raman spectrum, which can be used to estimate the size of nanowires in experiment. In the second part of my PhD work, we focus on the electron-hole pair (exciton) in semiconductor nanowires and its influence on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire with a diameter of 1.2 nm. Using plane wave and pseudopotentials, the quasiparticle states are calculated within the so-called GW approximation, and the electron-hole interaction is evaluated with the Bethe-Salpeter Equation (BSE). The enhanced excitonic effect is found in the absorption spectrum. The third part of my work is about the electronic structure in Si/Ge core-shell nanowires. The electronic band structure is studied with first-principles methods. Individual conduction and valence bands are found in the core part and the shell part, respectively. The band offsets are determined, which give rise to the spatial separation of electron and hole charge carriers in different regions of the nanowires. This allows for a novel-doping scheme that supplies the carriers into a separate region in order to avoid the scattering problem. This is the key factor to create high-speed devices. With the confinement effect, our results show important correction in the band offset compared with the bulk heterostructure. Finally, an optimum doping strategy is proposed based on our band-offset data.

Aim of this work is to construct dye sensitized solar cells employing core shell type nanocrystalline FTO/TiO2 photoanodes. Fluorine doped tin dioxide (FTO) nanoparticles were synthesized under hydrothermal conditions. Homogeneously precipitated SnO2 nanoparticles were dispersed in aqueous solutions containing NH4F as fluorine source and heat treated at 180oC for 24 hours. X-Ray analysis revealed that particles show rutile type cassiterite structure. Particles had 50 m2/g specific surface area measured by BET. Particle size was around 15-20 nm verified by XRD, BET and SEM analysis. Electrical resistivity of the powders measured with four point probe technique was around 770 ohm.cm for an F/Sn atomic ratio of 5, which showed no further decrease upon increasing the fluorine content of solutions. Thick films were deposited by screen printing technique and SEM studies revealed that agglomeration was present in the films which decreased the visible light transmission measured by UV-Visible spectrophotometry. TiO2 shell coating was deposited by hydrolysis of ammonium hexafluorotitanate and TiCl4 aqueous solutions. Efficiency of FTO nanoparticles was enhanced upon surface treatment where best result was 4.61 % for cells treated with TiCl4. Obtained photocurrent of 22.8 mA/cm2 was considered to be very promising for the future work. Enhancement v in efficiency was mostly attributed to suppressed recombination of photoelectrons and it is concluded that improved efficiencies can be obtained after successful synthesis of FTO nanoparticles having lower resistivity values and deposition of homogeneous shell coatings.

Bringing together several materials into a single nanoparticle is an attractive way to design systems that exhibit diverse physical and chemical properties. Cobalt nanoparticles are extensively used in magnetic separation, ferrofluids, and magnetic storage media. The deposition of gold nanoparticles onto cobalt core significantly affects their optical properties due to the introduction of surface Plasmon. Here the synthesis of gold nanoparticles decorated cobalt-silica nanoparticles are reported for the first time. Their optical and magnetic properties and capacity as a surface enhanced Raman scattering (SERS) substrate were investigated. This nano-material is of particular interest as a dual agent allowing both magnetic separation and SERS detection. The synthesis involves three steps: i) synthesis of Co nanoparticles / ii) deposition of a silica shell around the Co core and introduction of amine functional groups on the surface / iii) decoration of the surface with gold nanoparticles. Co nanoparticles were prepared in an inert atmosphere in the presence of capping and reducing agents. Size of the cobalt nanoparticles was varied by changing the concentration of the capping agent. Since cobalt particles are easily oxidized, they were coated with silica shell both to prevent oxidation and allow further functionalization. Silica coating of the particles were performed in water/ethanolic solution of tetraethyl orthosilicate (TEOS). Thickness of silica coating was controlled by varying the concentrations of TEOS. Besides, by adding 3-aminopropyl-triethoxysilane (APTS) to the reaction medium, primarily amine groups were introduced on the silica surface. For further modifications citrate stabilized gold nanoparticles were appended onto the surface of amine modified core-shell cobalt-silica nanoparticles. Gold decorated magnetic core-shell structures were used as SERS substrate with Raman dyes / brilliant cresyl blue (BCB) and rhodamine 6G (R6G). They were also utilized for preconcentration and SERS detection of 4-mercapto benzoic acid (4-MBA). Gold nanoparticles on the silica and thiol group on the 4-MBA were very selective to each other, thus, 4-MBA could be attached on to gold surface and it could be easily separated magnetically from the reaction medium and identified by Raman spectroscopy. Characterization of the cobalt, cobalt-silica and gold modified cobalt-silica nanoparticles was done by Field Emission Scanning Electron Microscopy (FE-SEM), Scanning-Transmission Electron Microscopy (S-TEM), Energy-Dispersive X-ray Spectroscopy (EDX), UV-Vis spectrometry, and Raman microscope system.

Poly(3,4-ethylenedioxythiophene), PEDOT, belongs to the group of conducting polymers and is characterized by its high stability, a moderate band gap and its optical transparency in the conductive state. A large disadvantage of conducting polymers, and also PEDOT, is their poor solubility. One way to achieve processible materials is the synthesis of colloidal particles. Thus, this work focuses on the development of conductive particles by preparing composite structures. Polymeric colloids like latex particles and microgels were used as templates for the oxidative polymerization of EDOT. Depending on template structure completely different composite morphologies with variable properties were obtained. It was found that modification with PEDOT did not only cause conductive particles for application as humidity sensor materials, but also candidates for further functionalization with gold nanoparticles (Au-NPs). Due to a multi-stage synthesis route it was possible to achieve polystyrene(core)-PEDOT(shell)-particles decored with Au-NPs. Microgels acting as “micro reactors” for the incorporation of PEDOT and Au-NPs were also used for preparing multifunctional composites for catalytic applications. / Poly(3,4-ethylendioxythiophen), PEDOT, gehört zur Gruppe der leitfähigen Polymere und zeichnet sich durch seine hohe Stabilität, eine moderate Bandlücke und seine optische Transparenz im dotierten Zustand aus. Ein Nachteil leitfähiger Polymere, wie auch von PEDOT, ist deren schlechte Löslichkeit. Die Synthese kolloidaler Partikel bietet jedoch eine Möglichkeit dieses Problem zu umgehen. In diesem Zusammenhang richtete sich der Fokus dieser Arbeit auf die Darstellung leitfähiger Partikel in Form von Kompositstrukturen. Polymerkolloide, wie Latex- und Mikrogelpartikel, sind als Template eingesetzt worden, in deren Gegenwart PEDOT durch eine oxidative Polymerisation synthetisiert wurde. In Abhängigkeit von der Struktur des Templats sind unterschiedliche Kompositmorphologien mit steuerbaren Eigenschaften erhalten worden. Auf diese Weise wurden neben Materialien für die Feuchtigkeitssensorik leitfähige Kompositpartikel hergestellt, die zusätzlich mit Gold-Nanopartikeln (Au-NP) funktionalisiert werden konnten. Durch ein mehrstufiges Syntheseverfahren sind somit Polystyrol(Kern)-PEDOT(Schale)-Partikel mit Au-NP-funktionalisierter Oberfläche synthetisiert worden. Mikrogelpartikel, die als „Mikroreaktoren“ für die Inkorporation von PEDOT- und Au-NP dienten, wurden ebenfalls eingesetzt, um multifunktionale Komposite mit katalytischen Eigenschaften herzustellen.

The synthesis of nanocrystals is unique compared to the formation of larger micron-sizesspecies as the final crystal sizes are not much larger than the primary nuclei. As a consequencethe final outcome of a nanocrystal synthesis i.e mean crystal size, concentrationand standard deviation is almost solely determined by the end of the nucleation phase. Directingthe growth of crystals beginning from aggregates of only tens of atoms into maturemonodisperse nanocrystals requires that the governing kinetics are strictly controlled at everymoment of the reaction. To effect this task various different ligands need to be employed,each performing a particular function during both nucleation and growth. (For complete abstract open document)

Les nanocristaux d'AgInS2-ZnS sont des candidats prometteurs pour le développement de nano-luminophores non-toxiques et performants. Grâce à leur taille et à leur forte absorption, ces nano-luminophores permettent l'exploitation d'effets nano-optiques pouvant augmenter leur efficacité à l'absorption ou à l'émission. Ce document présente, dans un premier temps, une méthode d'analyse qui couple la mesure du rendement quantique à celle du temps de vie de luminescence et permet l'étude des contributions radiatives et non-radiatives des différents mécanismes de luminescence des nanocristaux d'AgInS2-ZnS. En modifiant la taille, la chimie de surface et la structure du cœur de ces nanocristaux, nous construisons un modèle global expliquant le rôle de leur composition et soulignant l'importance de leur surface. De nouvelles stratégies sont identifiées pour optimiser ces nanomatériaux. Leur application conjointe permet d'envisager des rendements quantiques proches de 90%. Dans un second temps, une méthode de simulation numérique générale a été développée pour prédire l'effet produit par le couplage nano-optique entre une particule plasmonique et un luminophore. Cette méthode a été appliquée au cas des structures cœur/coquille/coquille (métal/isolant/AgInS2-ZnS) et les configurations optimales du système ont été déterminées. Une nanostructure particulièrement performante permettant de combiner les effets du couplage à l'absorption et à l'émission a été identifiée. Une méthode de synthèse de ces nanostructures est développée. Les résultats expérimentaux obtenus sont en accord à la fois avec la compréhension de la fluorescence des nanocristaux d'AgInS2-ZnS et la prédiction obtenue par simulation. / AgInS2-ZnS nanocrystals are promising materials for the development of non-toxic, highly efficient nano-phosphors. Their size and strong absorption allow them to exploit nano-optical effects potentially enhancing both their absorption and emission processes. This work presents a method combining quantum yield measurements with time resolved emission spectroscopy allowing for the study of both radiative and non-radiative properties of each recombination pathways. Modifying the size, surface chemistry, and core structure of the nanocrystals, we construct a global model explaining the role of their composition and emphasizing the critical aspect of their surface. New strategies are identified to increase the internal quantum yield of these materials. Combining these approaches, it is now possible to expect 90% efficiencies. In a second step, a simulation method was developed to predict the nano-optical effects induced by a plasmonic nanostructure on a given phosphor. We applied this method on core/shell/shell (metal/insulator/AgInS2-ZnS) nanostructures and theoretically determined optimal configurations of the system. A particularly efficient nanostructure achieving coupling on both absorbed and emitted light is identified. Hybrid plasmonic nanostructures are synthesized. Their performances are in accordance with both our understanding of the fluorescence mechanisms of AgInS2-ZnS nanocrystals and the predictions made via simulation.