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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Multiscale modeling of semiconductor nanocrystal synthesis in templating media

Kostova, Borislava 01 January 2006 (has links)
Several liquid-phase and vapor-phase techniques are reported in the literature for growing nanostructured materials. The use of templates in the synthesis of nanostructured materials is attractive as it combines precise control of size and shape with easy scale up for industrial production. This work aims to elucidate the underlying mechanisms controlling the growth rate and morphology of II-VI compound semiconductor nanocrystals (also called quantum dots) in templating media by employing theory, modeling, and simulation. The focus of the work is on zinc selenide (ZnSe), which can form nanocrystals emitting in the blue and violet part of the visible spectrum when excited by ultra violet radiation. We developed a lattice Monte Carlo simulation technique to describe the formation of ZnSe quantum dots by reacting diethylzinc with hydrogen selenide in the spherical nanodroplets of a microemulsion formed by self-assembly of a ternary system consisting of an amphiphilic block copolymer, a polar continuous phase (formamide) and a non-polar dispersed phase (heptane) [4]. The stochastic model describes diffusion of diethylzinc molecules in heptane, nucleation of ZnSe through a fast reaction between diethylzinc and hydrogen selenide at the interface between the droplet and the continuous phase, as well as diffusion and coalescence of ZnSe clusters inside the nanodroplet, eventually leading to the formation of a single nanocrystal per nanodroplet. The motion of molecules and clusters in the lattice is programmed according to their diffusivity, which is estimated by the Stokes-Einstein equation. A deterministic diffusion-reaction model describing diethylzinc depletion in a spherical droplet due to a fast interfacial reaction was used to investigate different growth regimes and compare its predictions with those of the stochastic model. In the early stages of the nanocrystal formation process, slow diffusion of hydrogen selenide through the surfactant layer is the rate determining step. The zinc precursor is progressively depleted inside the nanodroplet and its diffusion to the interface becomes the rate controlling step. This transition can be tracked precisely by our stochastic model without any assumptions, but not by the aforementioned deterministic model. The formation of stable clusters (also called "magic clusters") of ZnSe with a fullerene-like close-caged structure has been included in the stochastic simulations. The predicted size variation of the final nanocrystals due to the formation of such clusters has been studied using this stochastic model. (Abstract shortened by UMI.)
2

Phase behavior of polyelectrolyte solutions

Prabhu, Vivek Manohar 01 January 2002 (has links)
Due to the presence of long-ranged electrostatic interactions, polyelectrolyte solutions are characterized by a length scale in addition to the radius of gyration and the correlation length, the Debye screening length. Contrasted to the behavior observed in neutral polymer solutions in which miscibility is controlled by molecular weight and temperature, the inverse-square Debye length additionally controls polyelectrolyte phase behavior. This thesis project experimentally investigated the influence of added barium chloride on both the collective and configurational properties of a model polyelectrolyte, sodium-poly (styrene sulfonate). Regarding the collective properties the crossover from mean field to Ising criticality close to the precipitation phase boundary was measured. This crossover was demonstrated for both salt-dependent and temperature-dependent thermodynamics. A mean field model qualitatively describes the collective behavior in polyelectrolyte solutions as a competition between a short-ranged chemical mismatch, governed by a Flory-Huggins interaction parameter, that disfavors miscibility and a repulsive screened-Coulombic interaction between monomers that favors miscibility. The addition of salt screens the electrostatic interaction such that it becomes short-ranged, leading to the observed precipitation at fixed temperature. Similarly, for a fixed salt concentration, the solvent quality is tuned and precipitation is observed upon lowering temperature. The configurational properties of labeled chains were also examined as a function of molecular weight, polymer concentration, and salt concentration. In solutions without any added salts, we observe scaling laws for low-ionic strength semidilute polyelectrolyte solutions in agreement with the double screening theory. These scaling laws, along with the adequate fits of the labeled chain structure factor with the Debye structure factor, highlight the concept of screening in semidilute solutions and polyelectrolytes obeys Gaussian chain statistics on length scales of the order of a renormalized Kuhn length. Significant coil contraction is measured upon the addition of the multivalent salt. Upon comparing the correlation length, the radius of gyration, and the Debye length, the radius of gyration remains the dominant length scale in the system, until a crossover is observed as the correlation length diverges and surpasses the labeled chain dimension with increased ionic strength. The double screening theory was applied to understand the dependence of size of the labeled chains as functions of polymer concentration and added multivalent salt. It was necessary to include the influence of ion-pairing into a salt-concentration de pendent degree of ionization. Such ion-pair formation is also necessary to calculate phase diagrams with better qualitative agreement with experimental data. These initial efforts should foster strong theoretical and simulation studies and further experimentation in the area of polyelectrolyte solutions.
3

Effects of shape anisotropy on microstructural evolution of diblock copolymers

Panday, Ashoutosh 01 January 2006 (has links)
This dissertation discusses the effects of shape anisotropy on the evolved microstructure of diblock copolymers at various levels. Due to chain connectivity and microphase separation, the diblock copolymers self-assemble into ellipsoidal grains of lamellar and cylindrical morphologies. A grain-structure related phenomenon, Excluded Volume Epitaxy (E.V.E.) is explored in Chapter 2. E.V.E. is a local, inter-grain azimuthal orientational correlations effect, which results from a combination of sporadic nucleation and impingement of growing anisotropic shaped grains. Due to E.V.E., the ellipsoidal grains have a propensity for similar orientations in a local neighborhood, despite complete absence of global orientation in the sample. Simulations and experiments have verified this effect. The Avrami kinetics of anisotropic shaped grains is discussed in Chapter 3. Traditionally Avrami equation is used to model the growth kinetics of volume filling isotropic shaped grains. The probabilistic nature of Avrami kinetics produces a coupling between the grain shape and Poisson distribution. The Poisson-shape coupling remains latent for isotropic grains but becomes operative for anisotropic grains in random orientations scheme leading to inhibited growth kinetics. For unidirectional orientation of anisotropic grains, the growth kinetics remains uninhibited due to absence of Poisson-shape coupling. For two-dimensional case in simultaneous and continuous nucleation regimes, the inhibited kinetics scales as L1/2 where L is the shape anisotropy. The blends of highly shape anisotropic nanoclay, montmorillonite (MMT) and lamellar poly(styrene-b-isoprene), PS-PI are discussed in Appendix A. Annealing and cooling is sufficient to produce long-range lamellar order at 1 wt % clay loading. However at 5 wt % clay loading, shear force is additionally required. This system reveals the effect of shape anisotropy on evolution of long-range order in clay-block copolymer blends. The effect of shape anisotropy of polymer chain in the context of rod-coil block copolymer is explored in Appendix B. Poly(styrene-b-1,3-cyclohexadiene), PS-PCHD self-assembles into core-shell cylinder structure, seen rarely in AB diblock copolymers. A rod-coil free energy model that incorporates liquid crystalline splay distortion energy in the rod phase is presented. In this model, the PCHD block is treated as rod to explain the stability of the core-shell cylinder structure.

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