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In this thesis, the strength of the LBIC system is enhanced in different aspects that includes its feasibility as a non-destructive characterization tool, the signal analysis and development of analytical solution to have better understanding on the defects and inhomogeneities in the quantum dot based hetero-structures for device applications, finally understanding its limits due to the size of the laser beam and interpretation of artefacts in the signal appearance due to the presence of co-devices. Chapter#1 provides the introduction and literature survey of the LBIC system. It covers the importance and area of application of the LBIC. Chapter#2 various tools and instrumentation are discussed briefly for the systems that are developed in the lab as well as standard tools utilised for the material characterization. A LBIC instrumentation a novel colloidal quantum dots (CQD) thin film deposition system is discussed. In the last part along with the standard characterization systems a software tool (semiconductor device simulator) is discussed, which is used to visualize and understand the LBIC profile that is obtained experimentally. Chapter#3 provides the information of colloidal synthesis of PbS and HgxCd1-xTe quantum dots. Device fabrication process is explained step by step for the following devices. p-n junction silicon diodes, PbS-CQD/Si hetero-structures, ITO/PbS-CQD/Al crossbar structures and HgCdTe-CQD/Si hetero-structures. Chapter#4 deals with the major constraints imposed on the LBIC due to the need of Ohmic contacts. To overcome this major limitation, in this work, the origin of the signal is studied with the remote contact geometry for silicon p-n junction devices. It was observed that the signals can be collected with the capacitively coupled remote contacts, where LBIC was ultimately demonstrated as contactless measurement tool without any compromise on the measurements and thus obtained physical parameters. The effect of finite laser beam size is also described, which was found to have effect on the actual dimensions measured with the LBIC images. LBIC utility is further enhanced with the Si/CQD based hetero-structure devices, which are the potential candidates in the evolving device technology to be utilized in various modular systems such as PDs and LED applications. Chapter#5 discusses the origin and possible mechanisms for lateral photo-voltage which is closely monitored in the PbS-CQD/Si hetero-junction device systems. Interestingly, it is observed that there are two different line profiles for n and p type Si substrates. Different mechanisms that give rise to this kind of profiles were found to be distinct and are related to the band alignment of the CQD/Si hetero-structure. It lead to the revelation of an interesting phenomenon and believed to be universally observed irrespective of the materials involved in the formation of hetero-junction. Simulations and experimental results are quite consistent and in agreement with each other, which confirm the underlying physical mechanism that connects the LBIC anomalies with the band alignment. Chapter#6 deals with the spatial variations in the transverse photocurrent in the PbS-CQD film which is studied as a function of applied bias. Analytical equation is setup for the photocurrent in the CQD film under applied bias with the help of available transport mechanism and equations from the literature. The spatial non-uniformity that exists in the photocurrent proved to be the result of spatial inhomoginities in the physical parameters. By correlating the spatial data to the analytical equation, it is shown that the inhomoginities can be predicted. This approach is important for the devices, where monolithic detectors are fabricated by depositing CQD film on Read-Out-Integrated-Circuit (ROIC), where the manifestation of non-uniformity can be understood and probably fixed. Chapter#7 HgCdTe CQD based devices are studied for the purpose of photo-detector applications in MWIR (3  5 μm) region. HgxCd1-xTe Colloidal quantum dots are technologically important due to their wide absorption range that covers different regions of the atmospheric window. HgxCd1-xTe are successfully synthesised, which covers the absorption edge up to ~6.25 m in the IR region. Absorption and photo-response studies are carried out on HgxCd1-xTe/Si hetero-junctions under incident IR radiation. It is observed that the band gap of the quantum dots can be tuned easily by controlling the growth time as a parameter, thus moulded HgxCd1-xTe CQD/Si hetero-structures were found to have good photo-response. Chapter#8 the summary and the future direction and scope of the work is discussed. This includes the interesting observations during this thesis work which are not reported here in details.

De nombreuses techniques de lithographie sont proposées aujourd'hui pour structurer des surfaces à l'échelle micrométrique et nanométrique. Parmi elles, la lithographie colloïdale est intéressante en raison notamment du faible coût du procédé. Dans cette thèse, nous avons développé une nouvelle technique appelée « lithographie électro-colloïdale » qui est basée sur l'utilisation de particules colloïdales soumises à des champs électriques continus et alternatifs. Avec des temps de préparation courts et une instrumentation peu coûteuse, des surfaces structurées polymériques et métalliques ont été produites puis testées pour différentes applications (immobilisation de protéines, réseaux de microélectrodes, surfaces superhydrophobes). / Many lithography techniques have been developed to structure surfaces at the micrometer and sub-micrometer ranges. Among them, colloidal lithography is interesting because the process is inexpensive and does not require the use complex instruments. In this thesis, we have developed a new technique called « electro-colloidal lithography » which is based on the use of colloidal particles organized using alternating and direct electric fields. With short preparation times and inexpensive instruments, polymeric and metallic structured surfaces have been prepared and tested for different applications (protein immobilization, microelectrode arrays, superhydrophobic surfaces)

This thesis is about using nuclear magnetic resonance (NMR) spectroscopy for studying soft materials. Soft materials may be encountered everyday by most readers of this thesis, for example when taking a shower or watching TV. The usefulness of these materials originates from them being soft yet, at the same time, having some kind of a structure. The characteristic length scale of those structures is often on the order of nanometers (10-9 m) and the structure can respond to various external stimuli such as temperature, electric and magnetic fields, or the presence of interfaces. NMR spectroscopy excels when studying soft materials because it is a non-invasive technique with a large spectral resolution. Moreover, different NMR methods allow us to study local molecular dynamics or longer-range translational diffusion. Understanding those latter aspects is very important for the development of dynamic and responsive materials. Papers I-III present our work on assessing molecular adsorption on interfaces in colloidal dispersions. Here, carbon nanotubes (CNTs) or silica particles were the colloidal substrates to which proteins, polymers or surfactants adsorbed. Papers IV-VI concern ionic mobility in liquid crystals (LCs). The influence of material structure on, for example, the anisotropy of diffusion or on the association/dissociation of ions was studied in several LC phases. / QC 20110225

Nanostructured materials are the next generation of high-performance materials, harnessing the novel properties of their nanosized constituents. The controlled assembly of nanosized particles and the design of the optimal nanostructure require a detailed understanding of particle interactions and robust methods to tune them. This thesis describes innovative approaches to these challenges, relating to the determination of Hamaker constants for iron oxide nanoparticles, the packaging of nanopowders into redispersible granules, the tuning of the wetting behavior of nanocrystals and the simulation of collective magnetic properties in arrays of superparamagnetic nanoparticles. The non-retarded Hamaker constants for iron oxides have been calculated from their optical properties based on Lifshitz theory. The results show that the magnitude of vdW interactions in non-polar solvents has previously been overestimated up to 10 times. Our calculations support the experimental observations that oleate-capped nanoparticles smaller than 15 nm in diameter can indeed form colloidally-stable dispersions in hydrocarbons. In addition, a simple procedure has been devised to remove the oleate-capping on the iron oxide nanoparticles, enabling their use in fluorometric assays for water remediation, with a sensitivity more than 100 times below the critical micelle concentration for non-ionic surfactants. Nanosized particles are inherently more difficult to handle in the dry state than larger micron-sized powders, e.g. because of poor flowability, agglomeration and potential toxicity. The rheology of concentrated slurries of TiO2 powder was optimized by the addition of sodium polyacrylate, and spray-dried into fully redispersible micron-sized granules. The polymer was embedded into the granules, where it could serve as a re-dispersing aid. Monte Carlo (MC) simulations have been applied to the collective magnetic behavior of nanoparticle arrays of various thicknesses. The decrease in magnetic susceptibility with the thickness observed experimentally was reproduced by the simulations. Ferromagnetic couplings in the arrays are enhanced by the finite thickness, and decrease in strength with increasing thickness. The simulations indicate the formation of vortex states with increasing thickness, along with a change in their orientation, which becomes more and more isotropic as the thickness increases. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.</p>

In this dissertation, we describe a novel method that we call Diffusing Colloidal Probe Microscopy (DCPM), which integrates Total Internal Reflection Microscopy (TIRM) and Video Microscopy (VM) methods to monitor three dimensional trajectories in colloidal ensembles levitated above macroscopic surfaces. TIRM and VM are well established optical microscopy techniques for measuring normal and lateral colloidal excursions near macroscopic planar surfaces. The interactions between particle-particle and particle-substrate in colloidal interfacial systems are interpreted by statistical analyses from distributions of colloidal particles; dynamic properties of colloidal assembly are also determined from particle trajectories. Our studies show that DCPM is able to detect many particle-surface interactions simultaneously and provides an ensemble average measurement of particle-surface interactions on a homogeneous surface to allow direct comparison of distributed and average properties. A benefit of ensemble averaging of many particles is the diminished need for time averaging, which can produce orders of magnitude faster measurement times at higher interfacial particle concentrations. The statistical analyses (Ornstein- Zernike and three dimensional Monte Carlo analyses) are used to obtain particle-particle interactions from lateral distribution functions and to understand the role of nonuniformities in interfacial colloidal systems. An inconsistent finding is the observation of an anomalous long range particle-particle attraction and recovery of the expected DLVO particle-wall interactions for all concentrations examined. The possible influence of charge heterogeneity and particle size polydispersity on measured distribution functions is discussed in regard to inconsistent particle-wall and particle-particle potentials. In the final part of this research, the ability of DCPM is demonstrated to map potential energy landscapes on patterned surfaces by monitoring interactions between diffusing colloidal probes with Au pattern features. Absolute separation is obtained from theoretical fits to measured potential energy profiles and direct measurement by sticking silica colloids to Au surfaces via electrophoretic deposition. Initial results indicate that, as colloidal probe and pattern feature dimensions become comparable, measured potential energy profiles suffer some distortion due to the increased probability of probes interacting with surfaces at the edges of adjacent pattern features. Measurements of lateral diffusion via analysis of mean square displacements also indicated lateral diffusion coefficients in excellent agreement with rigorous theoretical predictions.

Le recyclage des terres rares à partir de déchets électroniques n'a toujours pas trouvé une réalisation industrielle significative. L'une des raisons est le manque de procédures de séparation optimisées en raison de connaissances fondamentales médiocres sur ces systèmes. En raison des similitudes chimiques et physiques de ces métaux, la conception d'une formulation efficace, adaptative et prédictive est toujours hors de portée des possibilités. L'interprétation supramoléculaire de la formation complexe dans la phase organique est de plus en plus importante au cours des dernières années. C'est l'approche la plus prometteuse permettant l'explication de divers phénomènes, tels que la formation de la troisième phase et les signaux forts dans de petites expériences de diffusion et de revenir à des méthodes bien connues de la science des tensioactifs. Notre contribution à une compréhension plus complète dans cette matière est l'analyse du comportement électrodynamique de ces phases et la corrélation de ces résultats avec les résultats des propriétés d'auto-assemblage et du transport de masse dans ces médias. Pour cette étude, nous avons spécifiquement conçu un modèle de référence, en passant un processus d'extraction à ses quatre composants fondamentaux: l'extracteur avant l'extraction (acide Di-(2-éthylhexyl) phosphorique, HDEHP), l'extrait après extraction (son sel de sodium, NaDEHP ), le toluène comme diluant apolaire et de l'eau. Un prisme de phase de Gibbs a été préparé (illustré sur la figure 1), où l'axe z donne le rapport de HDEHP à NaDEHP, ce qui représente le développement d'une extraction. Couvrant le domaine de basse fréquence, la spectroscopie d'impédance a été la méthode de choix afin de déterminer la conductivité dépendant de la fréquence. En utilisant la spectroscopie de relaxation diélectrique, nous révélons des processus dynamiques rapides à haute fréquence. Des mesures combinées SAXS et SANS ont été effectuées pour comparer les tendances électrodynamiques avec les propriétés d'agrégation et les interactions entre groupes. Deux phénomènes ont été identifiés comme responsables du profil de conductivité dans les systèmes micellaires inverse: la formation d'agrégats chargés par la dissimulation et la percolation. Tout au long du système de référence, ces deux processus ont été sondés en fonction de trois variables: la concentration totale d'agent d'extraction, le rapport eau-agent tensioactif et le rapport Na: H. En tant que résultat majeur, l'eau joue un rôle important dans les deux processus. En cas de percolation, les agrégats inverse ne peuvent pas fusionner en l'absence d'eau. Par conséquent, la conductivité électrique est interdite. Dans les systèmes dilués, l'eau facilite le processus de dismutation, entraînant une augmentation de la conductivité. / Recycling of rare earths from electronic waste has still not found a significant industrial realization. One reason is the lack of optimized separation procedures due to poor fundamental knowledge on these systems. Due to the chemical and physical similarities of these metals, designing an efficient, adaptive and predictive formulation is still out of scope of possibilities. The supramolecular interpretation of complex-formation in the organic phase has gained an increasing importance in the last years. It is the most promising approach allowing the explanation of diverse phenomena, such as third phase formation and strong signals in small scattering experiments and to revert to methods well known from surfactant science. Our contribution towards a more complete understanding in this matter is the analysis of the electrodynamic behaviour of such phases and the correlation of these findings with the results of self-assembly properties and mass transport in these media.For this study, we specifically designed a reference model, breaking an extraction process down to its four fundamental components: The extractant before extraction (Di-(2-ethylhexyl)phosphoric acid, HDEHP), the extractant after extraction (its sodium salt, NaDEHP), toluene as apolar diluent and water. A Gibbs phase prism has been prepared (illustrated in Figure 1), where the z-axis gives the ratio of HDEHP to NaDEHP, representing the development of an extraction. Covering the low frequency-domain, impedance spectroscopy has been the method of choice in order to determine the frequency-dependent conductivity. Using dielectric relaxation spectroscopy, we reveal fast dynamic processes at high frequencies. Combined SAXS and SANS measurements have been performed to compare the electrodynamic trends with aggregation properties and intercluster interactions.Two phenomena have been identified to be responsible for the conductivity profile in reverse micellar systems: the formation of charged aggregates through dismutation and percolation. Throughout the reference system, these two processes have been probed as function of three variables: total extractant concentration, the water-to-surfactant ratio and the Na:H-ratio. As a major result, water plays a significant role in both processes. In case of percolation, reverse aggregates are not able to merge in the absence of water. Therefore, electrical conductivity is prohibited. In dilute systems, water facilitates the dismutation–process leading to an increase in conductivity.

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Submitted in complete fulfillment for the Degree of Master of Technology: Biotechnology, Durban University of Technology, Durban, South Africa, 2014. / Antibiotic resistance amongst bacterial pathogens is a crisis that has been worsening over recent decades, resulting in serious and often fatal infections that cannot be treated by conventional means. Diseases caused by these drug resistant agents result in protracted illnesses, greater mortality rates and increases in treatment costs. Improvements to existing therapies and the development of novel treatments are urgently required to deal with this escalating threat to human health. One of the more promising strategies to combat antibiotic resistance is the use of metallic nanoparticles. Research into this area has shown that the binding of antibiotics to nanoparticles enhances their antimicrobial effects, reduces side-effects due to requirement of lower dosages of the drug, concentrates the drug at the interaction site with bacterial cells and in certain cases, has re-introduced susceptibility into bacterial strains that have developed drug resistance. Furthermore, these nanoparticles can be used in cancer treatment in similar drug delivery roles. Based on the promising data that demonstrated the synergistic effects of antimicrobial agents with nanoparticles, the aim of our research is to determine the effect of ciprofloxacin-conjugated gold nanoparticles as antimicrobial agents. To achieve this aim our objectives were: (i) to synthesize citrate-capped and ciprofloxacin-conjugated gold nanoparticles; (ii) to determine the physical and chemical characteristics of the ciprofloxacin-nanoparticle hybrid molecule; (iii) to investigate the antimicrobial activity of the conjugated nanoparticles against various species of common pathogens and (iv) to investigate the anti-cancer potential of the citrate-capped nanoparticles against a Caco-2 cell line. In this study, citrate-capped gold nanoparticles were conjugated to the antibiotic, ciprofloxacin, and their antibacterial and anti-cancer activity was evaluated. Initial experiments involved the synthesis and characterization of gold nanoparticles and ciprofloxacin conjugated nanoparticles. The gold nanoparticles were synthesized using the Turkevich citrate reduction technique which has been extensively used in studies thus far. The synthesized nanoparticles were characterized for specific absorbance using a UV-Spectrophotometer. The bond between the nanoparticles and ciprofloxacin was characterized by FTIR. Ultra structural details of the gold nanoparticles were established by TEM. The colloidal stability of the nanoparticles was determined by spectroscopic analysis. The antibacterial activity of the ciprofloxacin-conjugated gold nanoparticles was studied by exposure to pathogenic bacteria (Staphyloccocus aureus, E. coli, Klebsiella pneumoniae, Enterocococcus spp., Enterobacter spp., and Psuedomonas spp.). MIC values were measured to give indication of antimicrobial effect. These bactericidal properties of the conjugate nanoparticles were further investigated by electron microscopy. To evaluate the action of the citrate capped gold nanoparticles on cancer cells, we exposed Caco-2 cells to various concentrations of the nanoparticles and its effect was evaluated by measuring the viability of the cells. The results showed that 0.5 mM trisodium citrate reduced gold chloride to yield gold nanoparticles, which were spherical and 15 to 30 nm (by TEM characterization) and had an absorption maxima of 530 nm. The ciprofloxacin conjugated nanoparticles had an absorption maxima of 667nm. The colloidal stability, which is used to assess whether the synthesized particles will retain their integrity in solution showed that citrate-capped GNPs were most stable at 37°C over a 14 day storage period while ciprofloxacin-conjugated GNPs were found to be most stable at 4°C over a 14 day period. The FTIR results showed that chemical bonding in the conjugated nanoparticles occurs between the pyridone moiety of ciprofloxacin and the nanoparticle surface. The antimicrobial results of ciprofloxacin-conjugated GNPs had a significantly improved killing response compared to ciprofloxacin on both Gram positive and Gram negative bacteria. The citrate-capped GNPs are shown to exert a similar cytotoxic effect to gemcitabine on the Caco-2 cell line at a concentration of 0.5 mM. These results indicate that combining gold nanoparticles and ciprofloxacin enhances the antimicrobial effect of the antibiotic. The conjugate nanoparticles increase the concentration of antibiotics at the site of bacterium-antibiotic interaction, and thus enhance the binding and entry of antibiotics into bacteria. This has great implications for treatment of infection, as these antibiotic-conjugated nanoparticles can be incorporated into wound dressings, be administered intravenously as drug delivery agents, be engineered to possess multiple functionalities in addition to antibacterial activity and act as dual infection tracking and antimicrobial agents. Likewise, in this study, gemcitabine, an anticancer drug and gold nanoparticles were shown to kill cancer cells. In addition to their use in photothermal therapy and as drug delivery agents, the nanoparticles themselves possess anti-cancer activity against the Caco-2 cells. Thus, they have potential to act alone as a form of cancer treatment if functionalized with certain targeting agents that are specific to cancer cells, reducing the side-effects that come with regular chemotherapeutic drugs. It can be concluded that ciprofloxacin-conjugated gold nanoparticles enhance antibacterial effects of the antibiotic ciprofloxacin against bacterial cells and citrate-capped gold nanoparticles have anti-cancer activity against the Caco-2 cell line.

The dynamics of colloidal particles driven over optical potential energy landscapes is studied. Experiments are conducted using colloids driven by solvent flow or piezo-stage, optical tweezers, magnetic fields, and video-microscopy. Firstly, the properties of optical traps and potential energy landscapes are determined using driven colloidal particles and clusters. The trap stiffness and potential depth of single Gaussian traps are measured directly. It is shown that the nature of optical potential energy landscapes may be fully engineered and predicted using a sum of single Gaussian potentials. Next, the motion of colloidal particles driven by a constant force over a periodic optical potential energy landscape is considered. The average particle velocity is found as a function of the driving velocity, and the wavelength of the optical potential energy landscape, which is found to be sinusoidal at small trap spacings. The critical driving velocity required for a particle to move across the landscape is determined as a function of the wavelength. Brownian motion is found to have a significant effect on the critical driving velocity, but a negligible effect at high driving velocity. Subsequently, the dynamic mode locking caused by adding a modulation to the driving force is studied. This synchronisation manifests as a `Devil's staircase' in the average particle velocity as a function of driving velocity. The amplitude and frequency dependence of the mode locked steps are studied. Furthermore, particle trajectories are examined, and phase portraits show locked (unlocked) states as closed (open) loops in phase space. A state diagram of mode locked steps is constructed. Finally, driven systems of magnetically interacting colloidal particles are examined in potential energy landscapes. The critical driving velocity of a chain of coupled particles driven by a constant force is found to depend strongly on the chain length and the magnetic field. Secondly, a mobile density wave (kink) in an optically pinned chain of coupled particles is exposed to a constant and modulated drive. The kink is found to behave as a quasi-particle, exhibiting analogous dynamic mode locking behaviour to the single particle case. Finally, the mode locking of a finite mobile chain is considered, and found to be affected by the chain flexibility, which is a function of the magnetic field.