Ce travail expérimental cherche à mieux caractériser les effets du cisaillement sur le vieillissement d'un verre colloidal au niveau microscopique. La dynamique des réarrangements d'un tel système est étudiée au moyen d'une technique de diffusion multiple de la lumière (MSDWS) développée au cours de cette thèse. Nous montrons que la cessation d'un cisaillement de forte amplitude est, au niveau microscopique, l'équivalent pour ces systèmes d'une trempe thermique pour d'autres systèmes vitreux. Notre système est en effet rajeuni par un fort cisaillement puis présente un comportement de vieillissent classique à la cessation de celui-ci. De surcroît, nous montrons qu'un cisaillement de faible amplitude permet au système de vieillir plus vite. Nous appelons ce comportement le survieillissement. Ces deux effets antagonistes (rajeunissement et survieillissement) sont aussi observés sur des polymères vitreux soumis à des marches de température. Nous montrons enfin que le modèle SGR fournit une description unifiée de ces comportements similaires pour différents types de perturbation. Ceux-ci mettent en lumière l'élargissement de la distribution de temps de relaxation au sein de ce type de matériaux soumis à des perturbations extérieures.

<p>A colloid of magnesium hydroxide is used to stabilize droplets of monomers before suspension polymerisation. The characteristics of precipitated magnesium hydroxide changes significantly during the first hours. The viscosity is high and flucctuating at first but decreases and becomes low and stable after a few hours. When the colloid ages the primary particles agglomerate into larger particles which increases in size by time due to, among other things, Oswald ripening and aggregation. This can cause problems with poor reproducibility in the production. Therefore, the purpose of this study was finding a way to age the colloid without an increase in particle size and without changing any other features. Several experiments were made to optimize the precipitation procedure and to find out what parameters that are most important. The results showed that the stirring rate and colloidal concentration have big influence.</p><p>Small amounts of chemicals that modifies the surface of the colloid was added. This made the particle size near constant over time and the colloid was aged for two months without increasing in size or changing other properties. This was confirmed by making microspheres that had the same characteristics as microspheres made with a fresh colloid.</p>

Colloidal self assembly is an efficient method for making 3-D ordered nanostructures suitable for materials such as photonic crystals and macroscopic solids for catalysis and sensor applications. Colloidal crystals grown by convective methods exhibit defects on two different scales. Macro defects such as cracks and void bands originate from the dynamics of meniscus motion during colloidal crystal growth while micro defects like vacancies, dislocation and stacking faults are indigenous to the colloidal crystalline structure. This paper analyses the crystallography and energetics of the microscopic defects from the point of view of classical thermodynamics and discusses the strategy for the control of the macroscopic defects through optimization of the liquid-vapor interface. / Singapore-MIT Alliance (SMA)

No description available.

Multijunction solar cells made from a combination of CQDs of differing sizes and thus bandgaps are a promising means by which to increase the energy harvested from the Sun’s broad spectrum. In this dissertation, we first report the systematic engineering of 1.6 eV PbS CQD solar cells, optimal as the front cell responsible for visible wavelength harvesting in tandem photovoltaics. We rationally optimize each of the device’s collecting electrodes—the heterointerface with electron accepting TiO2 and the deep-work-function hole-collecting MoO3 for ohmic contact—for maximum efficiency. Room-temperature processing enables flexible substrates, and permits tandem solar cells that integrate a small-bandgap back cell atop a low thermal-budget larger-bandgap front cell. We report an electrode strategy that enables a depleted heterojunction CQD PV device to be fabricated entirely at room temperature. We develop a two-layer donor-supply electrode (DSE) in which a highly doped, shallow work function layer supplies a high density of free electrons to an ultrathin TiO2 layer via charge-transfer doping. Using the DSE we build all-room-temperature-processed small-bandgap (1 eV) colloidal quantum dot solar cells suitable for use as the back junction in tandem solar cells. We further report in this work the first efficient CQD tandem solar cells. We use a graded recombination layer (GRL) to provide a progression of work functions from the hole-accepting electrode in the bottom cell to the electron-accepting electrode in the top cell. The recombination layers must allow the hole current from one cell to recombine, with high efficiency and low voltage loss, with the electron current from the next cell. We conclude our dissertation by presenting the generalized conditions for design of efficient graded recombination layer solar devices. We demonstrate a family of new GRL designs experimentally and highlight the benefits of the progression of dopings and work functions in the interlayers.

A colloid of magnesium hydroxide is used to stabilize droplets of monomers before suspension polymerisation. The characteristics of precipitated magnesium hydroxide changes significantly during the first hours. The viscosity is high and flucctuating at first but decreases and becomes low and stable after a few hours. When the colloid ages the primary particles agglomerate into larger particles which increases in size by time due to, among other things, Oswald ripening and aggregation. This can cause problems with poor reproducibility in the production. Therefore, the purpose of this study was finding a way to age the colloid without an increase in particle size and without changing any other features. Several experiments were made to optimize the precipitation procedure and to find out what parameters that are most important. The results showed that the stirring rate and colloidal concentration have big influence. Small amounts of chemicals that modifies the surface of the colloid was added. This made the particle size near constant over time and the colloid was aged for two months without increasing in size or changing other properties. This was confirmed by making microspheres that had the same characteristics as microspheres made with a fresh colloid.

<p>This dissertation develops a novel application of the resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) technique toward the end goal of conjugated-polymer-based optoelectronic device fabrication. Conjugated polymers are attractive materials that are being investigated in the development of efficient optoelectronic devices due to their inexpensive material costs. Moreover, they can easily be combined with inorganic nanomaterials, such as colloidal quantum dots (CQDs), so as to realize hybrid nanocomposite-based optoelectronic devices with tunable optoelectronic characteristics and enhanced desirable features. One of the most significant challenges to the realization of optimal conjugated polymer-CQD hybrid nanocomposite-based optoelectronics has been the processes by which these materials are deposited as thin films, that is, conjugated polymer thin film processing techniques lack sufficient control so as to maintain preferred optoelectronic device behavior. More specifically, conjugated-polymer-based optoelectronics device operation and efficiency are a function of several attributes, including surface film morphology, internal polymer chain morphology, and the distribution and type of nanomaterials in the film bulk. Typical conjugated-polymer thin-film fabrication methodologies involve solution-based deposition, and the presence of the solvent has a deleterious impact, resulting in films with poor charge transport properties and subsequently poor device efficiencies. In addition, many next-generation conjugated polymer-based optoelectronics will require multi-layer device architectures, which can be difficult to achieve using traditional solution processing techniques. These issues direct the need for the development of a new polymer thin film processing technique that is less susceptible to solvent-related polymer chain morphology problems and is more capable of achieving better controlled nanocomposite thin films and multi-layer heterostructures comprising a wide range of materials. Therefore, this dissertation describes the development of a new variety of RIR-MAPLE that uses a unique target emulsion technique to address the aforementioned challenges.</p><p>The emulsion-based RIR-MAPLE technique was first developed for the controlled deposition of the conjugated polymers poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[2-methoxy-5-(2'ethylhexyloxy)-1,4-(1-cyanovinylene) phenylene] (MEH-CN-PPV) into homogenous thin films. Therein, it was identified that target composition had the most significant influence on film surface morphology, and by tuning the concentration of hydroxyl bonds in the target bulk, the laser-target absorption depth could be tuned so as to yield more or less evaporative deposition, resulting in films with tunable surface morphologies and optical behaviors.</p><p>Next, the internal morphologies of emulsion-based RIR-MAPLE-deposited MEH-PPV thin films were investigated by measuring their hole drift mobilities using the time-of-flight (TOF) photoconductivity method in the context of amorphous materials disorder models (Bässler's Gaussian Disorder model and the Correlated Disorder model) in order to provide a quantitative measure of polymer chain packing. The polymer chain packing of the RIR-MAPLE-deposited films was demonstrated to be superior and more conducive to charge transport in comparison to spin-cast and drop-cast MEH-PPV films, yielding enhanced hole mobilities.</p><p>The emulsion-based RIR-MAPLE technique was also developed for the deposition of different classes of inorganic nanoparticles, namely un-encapsulated nanoparticles and ligand-encapsulated nanoparticles. These different classes of nanoparticles were identified to have different film growth regimes, such that either rough or smooth films were obtained, respectively. The ligand-encapsulated nanoparticles were then co-deposited with MEH-PPV as conjugated polymer-CQD hybrid nanocomposites, wherein the distributions of the constituent materials in the film bulk were identified to be tunable, from homogeneous to highly clustered. The RIR-MAPLE deposition regime determined the said distributions, that is, if the polymer and CQDs were sequentially deposited from a sectioned target or simultaneously deposited from a single target, respectively. The homogeneous conjugated polymer-CQD nanocomposites were also investigated in terms of their charge transport properties using the TOF photoconductivity technique, where it was identified that despite the enhanced dispersion of CQDs in the film bulk, the presence of a high concentration of CQDs degraded hole drift mobility, which indicates that special considerations must be taken when incorporating CQDs into conjugated-polymer-based nanocomposite optoelectronics.</p><p>Finally, the unique capability of RIR-MAPLE to enable novel conjugated polymer-based optical heterostructures and optoelectronic devices was evaluated by the successful demonstration of a conjugated polymer-based distributed Bragg reflector (DBR), a plasmonic absorption enhancement layer, and a conjugated polymer-based photovoltaic solar cell featuring a novel electron-transporting layer. These optical heterostructures and optoelectronic devices demonstrate that all of the constituent polymer and nanocomposite layers have controllable thicknesses and abrupt interfaces, thereby confirming the capability of RIR-MAPLE to achieve multi-layer, conjugated polymer-based heterostructures and device architectures that are appropriate for enhancing specific desired optical behaviors and optoelectronic device efficiencies.</p> / Dissertation

In recent years, the Surface Plasmon Polariton effect has played an important role for entering the Nano-world. When the metallic materials reach the nanometer level, many special characteristics show up. As the progress of advanced technology development, the equipments which can be operated in nano grade level are more stabilized. Many special surface Plasmonic properties have been discovered through the measurements. This research is to focus on using the Surface Plasmon coupling to excite colloidal quantum dots and observing the emissive behavior of quantum dots. The experiments of changing the distance between the quantum dots and the metal film were performed. The blinking effect disappeared when the quantum dots are very close to the metal film. It showed that some other mechanism is competing with Auger recombination in the quantum dots. The lifetime modification and emission intensity were measured when one quantum dot was placed near a silver cube. The coupling between the surface Plasmon polariton and the quantum dot was discussed.

A further understanding of colloidal mixture behavior under applied electric fields would greatly benefit the design of smart material systems such as electrorheological fluidic devices and microfluidic reconfigurable antennas. This thesis presents a finite element analysis of colloidal mixture electrokinetic behavior. Computations of particle forces as a function of applied frequency and particle shape were performed. An effective medium property method was also studied. Fluidic and electric forces were obtained with various applied excitation frequencies throughout three locations in a coplanar microelectrode domain. This domain consists of two 50 nanometers thick gold electrodes separated by a 30 micrometers gap. The three locations are 1.2 micrometers, 40 micrometers, and 90 micrometers from the gap center. Total force vectors were computed by integrating Maxwell and Cauchy stress tensors to determine whether the particles are pushed toward or away from the electrode gap at frequencies of 10 Hz, 1 kHz, and 100 kHz. It was determined that particles were pushed outside the gap at median frequencies of 1kHz (indicating ac electroosmotic force domination) and began to be pushed back toward the gap at higher frequencies of 100 kHz (indicating dielectrophoretic force intensification). Particle shape effects were examined by calculating the electrical interparticle force between two particles at various incidences with respect to a uniform electric field. Particle attraction occurs when the line between the particle centers is aligned with the electric field; repulsion occurs when this center line is perpendicular. The incidence angle at which the particles switch from attraction to repulsion is defined as θcr. The aspect ratio and particle edge separation distances used in this study were 1, 5,12.92 and 0.25 micrometers, 0.50 micrometers, 2.0 micrometers, respectively. The results indicate that higher aspect ratio particles tend to have smaller θcr values and larger interparticle force magnitudes for given separation distances. Finally, effective dielectric constant simulations utilizing periodic crystalline arrangements of colloidal structure were performed. The results show good agreement with the Maxwell Garnett mixing rule at volume fractions above 30 percent. Less canonical structures of cubic particles were also modeled.

Nanoparticles (such as, carbon nanotubes, carbon black, clay etc.) have one or more dimensions of the order of 100 nm or less. Owing to very high van der Waals force of attraction, these nanoparticles exist in a highly aggregated state. It is often required to break these aggregates to truly experience the “nanosize” effect for any required end use. There are several strategies proposed for dispersing/exfoliating nanoparticles but limited progress has been made towards controlling their dispersion state. The ability to tailor nanoparticle dispersion state in liquid and solid media can ultimately provide a powerful method for tailoring the properties of solution processed nanoparticle-filled polymer composites. This dissertation reports the use of a variety of stimuli-responsive polymers to control the dispersion state of single-walled carbon nanotubes. Stimuli-responsive polymers exhibit conformational transitions as a function of applied stimulus (like pH, temp, chemical etc.). These variations in conformations of the polymer can be used tailor nanotube dispersion state in water and solid composites.The use of pH and temperature responsive polymers to stabilize/disperse single walled carbon nanotubes (SWNTs) in water is presented. Non-covalent functionalization of SWNTs using pH and temperature responsive polymer show tailored dispersion state as a function of pH and temperature, respectively. Carbon nanotube microstructure in these aqueous suspensions was characterized using several techniques (cryo-TEM, viscosity measurements, uv-vis spectroscopy, zeta potential measurements and settling behavior). Furthermore, nanotube dispersion state in aqueous suspensions is preserved to a large extent in the composites formed by drying these suspensions as evidenced by SEM images and electrical conductivity measurements. Based on the results obtained a mechanism is proposed to explain the tailored dispersion of SWNTs as a functions of applied external stimulus (i.e., pH, temperature). Such stimuli-controlled dispersion of carbon nanotubes could have a variety of applications in nanoelectronics, sensing, and drug and gene delivery systems. Furthermore, this dissertation also contains a published study focused on controlling the dispersion state of carbon black (CB) in epoxy composites using clay.