Les nanostructures métalliques permettent de confiner la lumière à des échelles sub-longueur d’onde grâce à l'excitation de plasmons de surface. Elles ouvrent la voie à de nombreuses applications que ce soit en imagerie, en élaboration de composants photoniques ou en information quantique. Cette thèse porte sur l’étude de nanostructures métalliques, semi-continues ou constituées par des réseaux de trous au désordre contrôlé, et à leur interaction avec des nanocristaux semi-conducteurs colloïdaux particulièrement photostables. En associant plusieurs approches expérimentales complémentaires (spectroscopie en champ lointain, microscopie de champ proche optique, microscopie avec une sonde active de champ proche, caractérisation par microscopie confocale de l’émission de nanocristaux couplés aux surfaces métalliques), nous avons pu mettre en évidence les caractéristiques spécifiques des modes plasmons de ces différentes structures. Pour les réseaux au désordre contrôlé, nous avons en particulier analysé l’apparition progressive de modes localisés intenses et déterminé l’influence de paramètres tels que l’épaisseur de la couche d’or, le diamètre des trous ou la périodicité initiale du réseau. Les résultats expérimentaux obtenus se sont révélés en très bon accord avec les simulations numériques réalisées par FDTD. / Metallic nanostructures allow to confine light at subwavelength scales by the excitation of surface plasmon. They open the way for many applications in imaging, photonic components development and quantum information. This thesis deals with the study of metallic nanostructures, semi-continuous or based on holes gratings with a controlled disorder, and their interaction with colloidal semiconductor nanocrystals that are very photostable. Combining several complementary experimental approaches (far-field spectroscopy, near-field optical microscopy, near-field active probe microscopy, characterization by confocal microscopy of the emission of nanocrystals coupled to the metallic surfaces), we were able to highlight specific characteristics of the plasmon modes of these different structures. For the gratings with a controlled disorder, we have in particular analyzed the emergence of intense localized modes and determined the influence of parameters such as the thickness of the gold layer, the diameter of the holes or the initial periodicity of the grating. The experimental results are in very good agreement with the numerical simulations carried out by FDTD.

Le but de cette thèse et de la recherche associée est une démonstration des avantages d’une combinaison de propriétés inhabituelles de nanostructures plasmoniques avec des aspects parmi les plus intéressants de l’optique non-linéaire. Pour cet effet, la modélisation analytique et numérique a été combiné avec le travail expérimental, qui comprenait la production de nanostructures et les mesures effectuées au moyen de la microscopie confocale non-linéaire résolue en polarisations et de la technique Z-scan modifiée (nommée “f-scan”).Il a été montré que l’anisotropie efficace de génération de seconde-harmonique dans les cristaux plasmoniques (formés par des réseaux rectangulaires de cavités tétraédriques sur une surface d’argent) peut être contrôlée par un choix approprié des paramètres de maille. Il a aussi été montré que cette anisotropie provient principalement d’une structure de bande photonique elle-même anisotrope, présentant une bande interdite plasmonique avec des états plasmoniques en bord de bande, permettant de renforcer le champ électrique local. Les arrangements chiraux bidimensionnels de nanoparticules triangulaires d’or, forment des “meta-molécules” plasmoniques énantiomériques, ont été analysés par microscopie non-linéaire à la lumière polarisée circulairement et par modélisation numérique, révélant un fort effet chiroptique par génération de seconde harmonique en rétro-réflexion. La petite taille des énantiomères uniques permet de créer “des filigranes” (“watermarks”) codés par la chiralité des meta-molécules, qui peuvent être lu par imagerie de la génération de seconde harmonique excitée par un rayon polarisé circulairement. Les caractéristiques quantitatives de la non-linéarité optique du troisième ordre et de l’efficacité d’absorption saturable des solutions aqueuses de fragments de graphène et de graphène dopé par des nanoparticules d’or a été effectuée par une nouvelle technique “f-scan”, qui a été créée et développée par incorporation d’une lentille à distance focale accordable dans une technique de Z-scan traditionnelle. Ces études ont montrées que le graphène présente une absorption saturable ultra-rapide très efficace, qui est parfois convertie en absorption saturable inverse. Il apparaît alors qu’une décoration du graphène par des nanoparticules d’or peut causer une légère amélioration du paramètre d’efficacité d’absorption saturable dans la plage spectrale de leurs résonances plasmoniques. En résumé, cette thèse présente une variété de propriétés optiques non-linéaires apparaissant dans les nanostructures plasmoniques. Différentes possibilités de contrôle de ces propriétés au moyen d’une démarche de nano-ingénierie, soutenue par des modélisations à la fois analytique et numérique ont été démontrées et analysées. Ces travaux ouvrent la voie à la fabrication et à l‘optimisation sur mesure de nouveaux nano-matériaux et nano-dispositifs photoniques reposant sur des effets de nano-plasmonique non-linéaire. / The aim of this thesis and the underlying research work is to demonstrate the benefits emerging from combination of the peculiar properties of plasmonic nanostructures with the most interesting aspects of nonlinear optics. For this purpose, analytical and numerical modeling was combined with experimental work, which included nanofabrication and measurements performed by means of polarization-resolved nonlinear confocal microscopy and by modified Z-scan technique (called "f-scan").It has been shown that the effective anisotropy of the second-harmonic generation in plasmonic crystals (formed by rectangular arrays of tetrahedral recesses in silver surface) can be controlled by proper choice of lattice constants. It also has been shown that this anisotropy arises mainly from the anisotropic photonic band structure, exhibiting plasmonic band gap with plasmonic band edge states, enabling enhancement of the local electric field.Two-dimensional chiral arrangements of triangular gold nanoparticles, forming plasmonic enantiomeric "meta-molecules", have been studied by nonlinear microscopy operating with circularly polarized light and by numerical modeling, revealing strong chiroptical effect in backscattered second-harmonic radiation. Small size of individual enantiomers allows to create "watermarks", encoded by the chirality of meta-molecules, which can be readout by imaging of second-harmonic generation excited by circularly polarized laser beam.Quantitative characterization of the third-order optical nonlinearity and saturable absorption efficiency of aqueous solutions of graphene and gold-nanoparticle decorated graphene has been performed by novel "f-scan" technique, which has been created and developed by incorporation of a focus-tunable lens into traditional Z-scan. These studies have shown that the graphene exhibits very efficient ultrafast saturable absorption, which is occasionally suppressed by reverse saturable absorption. Moreover, it turns out that decoration of graphene by gold nanoparticles may cause a slight improvement of the saturable absorption efficiency parameter within spectral range of their plasmon resonances.In summary, the following thesis presents various nonlinear optical properties of plasmonic nanostructures. Different possibilities of controlling these properties by means of nano-engineering, supported by analytical and numerical modeling, is also analyzed and demonstrated. This work opens up new perspectives for fabrication and rational design of novel photonic nano-materials and nano-devices based on nonlinear nanoplasmonic phenomena.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

Solid basic metal oxides have been extensively studied for biodiesel production via transesterification, researches are now focused on attaining high catalytic activity and durability towards one-step alkali transesterification, as well as high stability towards high free fatty acids (FFAs) and water content in oils for simultaneous esterification and transesterification, to enable their commercialization in industry. This work encompasses the design and characterization of three mixed metal oxide systems, with a detailed evaluation of their potential application in catalyzing transesterification of camelina oil to yield biodiesel. Na0.1Ca0.9TiO3 nanorods were synthesized via a simple alkaline hydrothermal pathway with ethanol as a co-solvent. Owing to their high basic strength of 11<H_<15, 92.7% biodiesel conversion was reached at mild reaction conditions. However, the catalyst showed poor recycle performance, probably attributed to the leaching of active species during transesterification, as revealed by X-ray photoelectron spectroscopy (XPS). A new class of mesoporous Zn/MgO catalyst was synthesized by a simple alkaline hydrothermal method. Zn/MgO calcinated at 600 ℃ exhibited 88.7% biodiesel conversion at 120 ℃ with 3% w/w catalyst, 24:1 methanol to oil molar ratio for 8 h. The catalyst could be reused for five runs without significant loss of activity (≥84.0% biodiesel conversion). The excellent catalyst performance is possibly attributed to its high surface area and large mesopores. The higher surface basic sites density as compared to mesoporous MgO, as indicated by higher total basicity determined from benzoic titration and an increased lattice O2- percentage as revealed from XPS, attributing to its superior catalytic activity. A series of nano-sized MgO-ZnO catalysts with precise stoichiometry were successfully prepared by a simple EDTA complexing approach. Mg0.5Zn0.5 calcinated at 600 ℃ gave a maximum biodiesel conversion of 89.3% at 120 ℃ with 3% w/w catalyst, 24:1 methanol to oil molar ratio for 8 h. Its superior catalytic performance to MgO is mainly associated with the high basic sites density as determined from benzoic titration and XPS. The biodiesel conversion retained over 83.0% for five runs. The enhanced catalyst activity and stability might be contributed by the incorporation of Zn2+ for Mg2+ in MgO lattice and a high homogeneous distribution of MgO particles on ZnO, with the formation of Mg-O-Zn bond as evidenced by Fourier transform infrared spectroscope (FTIR) and XPS. The catalyst also demonstrated high tolerance to FFAs (10% w/w) and water (2% w/w) content, which make it desirable for direct conversion of oils with high FFAs level to biodiesel in a single-step process. Lastly, a Zn/La2O3 catalyst was synthesized by a simple hydrothermal pathway. It exhibits a higher basic strength than La2O3, as evidenced by the slightly lower O1s binding energy determined by XPS, leading to a higher catalytic activity. The enhanced catalytic activity and stability is likely contributed by the incorporation of Zn2+ for La3+ in the lattice. Using 1% w/w Zn/La2O3 as catalyst, the highest biodiesel conversion of 92.7% was obtained at 120 ℃ for 16 h with 36:1 methanol to oil molar ratio. The effective catalyst displayed a biodiesel conversion greater than 84.0% for four runs. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

No description available.

Dans cette thèse, deux applications de nanomatériaux sont présentées : la première concerne l’utilisation de la technique LSPR pour la détection d’interactions moléculaires et la seconde est consacrée à l’utilisation de surfaces semi-conductrices de TiO2 et de ZnO pour la photo-catalyse pour la dépollution environnementale. D’abord, nous avons présenté la préparation et la caractérisation d’interfaces plasmoniques d’ITO/Au NSs/ITO. Celles-ci montrent des signaux optiques aussi bien pour des couches minces qu’épaisses d’ITO. Elles permettent une détection LSPR à courte et à longue portée. La chimie « click » a été utilisée pour greffer le CBPQT4+ fonctionnalisé alcynyl sur une interface ITO/Au NSs/ITO terminée « azide ». Les interfaces obtenues ont été utilisées pour démontrer la possibilité d’enregistrer la formation et la rupture de complexes « hôte-invité » (CBPQT+4/TTF) utilisant la technique LSPR. Dans la seconde partie, consacrée à la photocatalyse, nous nous sommes intéressés à deux types de traitements de TiO2 et de ZnO dans le but d’améliorer leur efficacité photocatalytique sous irradiation visible. Nous avons, préparé et caractérisé des lames de quartz recouvertes d’une couche mince de TiO2 de différentes épaisseurs avec et sans nanostructures d’or sous la couche mince de TiO2. La performance photocatalytique de ces interfaces n’est pas exceptionnelle pour la dégradation de la rhodamine B mais elles sont très stables. La dernière partie de la thèse concerne la préparation et la caractérisation de nanostructures de ZnO et de ZnO dopé à l’iode. La substitution des ions d’oxygène par des ions d’iode introduit des niveaux donneurs dans la bande interdite de ZnO qui sont suffisamment superficiels à température ambiante. L’activation d’une transition, dans la bande interdite dans le cas des substrats de ZnO dopé à l’iode, explique les meilleures performances photocatalytiques de ces substrats de la rhodamine B sous irradiation visible. Ces résultats sont très prometteurs pour diverses applications photocatalytiques des substrats à base de ZnO. / In this thesis, two applications of nanomaterials are presented: the first concerns the use of the LSPR for the detection of molecular interactions and the second is devoted to the use of TiO2 and ZnO semi-conductive surfaces for photocatalysis in view of environmental remediation. The first part concerns the fabrication and characterization of plasmonic interfaces of ITO/Au NSs/ITO. These surfaces showed optical signals for thin as well as for thick ITO films. These interfaces allowed short and long-range LSPR sensing. The “click” chemistry was used to covalently link an alkynyl-functionalized CBPQT4+ unit to an azide-terminated LSPR interface. Tetrathiafulvalene was used as a model guest molecule to demonstrate the possibility to follow the complexation/decomplexation events by monitoring the change in the LSPR signal. The second part is devoted to photocatalysis. It is focused on the investigation of the effect of two different treatments of TiO2 and ZnO on their photocatalytic efficiency under visible light irradiation. We have first prepared and characterized quartz slides coated with a thin layer of TiO2 with different thicknesses with and without gold nanostructures under the thin layer of TiO2. The photocatalytic performance of these interfaces is not exceptional for the degradation of rhodamine B under visible light irradiation but these interfaces have the advantage of being very stable. The last part of the thesis consists on the preparation of ZnO and iodine-doped ZnO substrates by a hydrothermal chemical deposition, and evaluation of their photocatalytic performance. The substitution of oxygen ions by iodine ions introduces donor levels in the bandgap of ZnO, which are sufficiently shallow at room temperature. The activation of a sub-band gap transition in iodine-doped ZnO nanostructured substrates explains the better performance of these substrates for the photodegradation of rhodamine B under visible light irradiation. These results are very promising in view of various photocatalytic applications of the ZnO-based substrates.

M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / In this work, the primary investigation has been on the development of the laser pyrolysis setup and its optimization for the synthesis of nano-size VO2-x films. More specifically the focus was on making VO2-x depositions using various laser pyrolysis parameters and establish in this way (1) an optimum laser wavelength threshold for the photon induced dissociation of the molecular precursors while the thermal contribution was kept minimal by using low power density (laser energy of 30 W) and (2) the lower threshold for pure thermal contributions by working with wavelengths far from resonance in order to minimize pure photon induced contributions. The interest in synthesizing nano-size VO2-x materials stems from the low metal-insulator transition temperature at near room temperature with opto-electronic and thermo-electronic properties that can be used in specialised applications. A large number of samples were synthesized under various conditions and annealed under argon atmosphere for 17 hours. XRD analysis identified the VO2 (B) and/or β-V2O5 vanadium oxide phases characteristic for certain samples grown under optimum conditions. Raman spectroscopy also confirmed these vanadium oxide phases with bands observed at 175, 228, 261, 303, 422 and 532 cm-1. SEM analysis revealed a plethora of different nanostructures of various size and shapes. The particles have a range of sizes between 55 nm to 185 nm in diameter. The particles showed morphologies which included nano-rods, nanospheres and nano-slabs. An interesting phenomenon was observed on the samples synthesized with high power density, which was observed and reported by Donev iii et al. EDS analysis on the particles was also used to probe the elemental composition of the sample. Optical studies were performed on the samples which showed transitions in the visible and infrared region in accordance with the ones observed in the international literature using different nano-synthesis methods.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. October 2013. Johannesburg, South Africa. / We report the role of a double step heat treatment process in the synthesis of novel GaN nanostructures (NSs) using a two stage furnace following a catalyst free vapour-solid growth mechanism. Morphological analysis revealed that GaN NSs were composed of rod-like structures with average diameter of 250 nm and accumulated particulates of GaN with diameter of ~ 12 – 16 nm providing enhanced surface area. The wurtzite phase of GaN nanorods of agglomerated nanoclusters was synthesized at temperatures as low as 750 °C. An X-ray photoelectron spectroscopic study confirmed formation of GaN. The surface areas of the GaN NSs were high at ~20 m2/g with respect to that expected for solid nanorod structures. The GaN NSs were of high crystallinity and purity as revealed by structural studies. Raman spectral analysis showed stronger intensity of the A1(LO) mode with respect to that for E2(high) mode indicating the high electronic quality of the sample. A photoluminescence study revealed the dominant presence of a defect band around 1.7-2.1 eV corresponding to nitrogen di-vacancies. Subsequent annealing in NH3 has demonstrated a compensation of the defect state and evolution of a band edge peak with possible hydrogen compensation of surface states. We also report the role of activated carbon on Ga2O3 to make GaN/C nanostructure composites using a single stage furnace. TEM analysis showed that GaN/C nanostructures gave different morphologies with different ratios of GaN/C. The surface areas of these materials showed an increase as the ratio of activated carbon was increased. PXRD showed that a ratio of Ga2O3: C of 1:0.5 (w/w) was sufficient to form GaN. TGA revealed that the ratios of Ga2O3: C of 1:0.5 – 1:2 gave materials that were thermally stable. Raman spectra showed that the material had excellent electronic properties. The material with a Ga2O3/C 1:2 ratios showed a poor gas response due to the change in reference value of resistance with the variation of hydrogen concentration. iv This study also provides the first investigation of GaN as a catalyst support in hydrogenation reactions. The GaN NSs were synthesized via chemical vapour deposition (CVD) in a double stage furnace (750 ºC) while nitrogen doped carbon spheres (NCSs) were made by CVD in a single stage furnace (950 ºC). TEM analysis revealed that the GaN NSs were rod-like with average diameters of 200 nm, while the NCSs were solid with smoother surfaces, and with diameters of 450 nm. Pd nanoparticles (1 and 3% loadings) were uniformly dispersed on acid functionalized GaN NSs and NCSs. The Pd nanoparticles had average diameters that were influenced by the type of support material used. The GaN NSs and NCSs were tested for the selective hydrogenation of cinnamaldehyde in isopropanol at 40 and 60 °C under atmospheric pressure. A comparative study of the activity of the nanostructured materials revealed that the order of catalyst activity was 3% Pd/GaN >3% Pd/NCSs > 1% Pd/NCSs > 1% Pd/GaN. However, 100% selectivity to hydrocinnamaldehyde (HCALD) was obtained with 1% Pd/GaN at reasonable conversion rates.