A Theory of Nanoparticle Growth and a Theory of Thin Film Nucleation
This dissertation is divided into two parts. The first will discuss a theory that was identified to narrow the size distribution of nanoparticles grown via ion beam synthesis (IBS). Specifically, a simple mathematical argument explains a processing route for the ion beam synthesis of nanoclusters with a narrowed size distribution. The key idea is that growth conditions for which the average nanocluster size is increasing rapidly can lead to narrowed size distributions. Modeling candidate processes using a self-consistent, mean-field theory shows that normalized nanocluster size distributions with full-width at half-maximum of 17% of the average can be attained.
The second part presents a theory for nucleation in confined high-aspect ratio thin films. Specifically, classical nucleation theory is used to consider the solidification of a melt confined between two planar surfaces. The critical nuclei shapes and the associated nucleation energy barriers are computed as a function of the thickness of the film, and the film's relevant bulk and interface energies. The analysis is then repeated for the melting transition, and expressions for the depression and elevation of the melting temperature are found. A nucleus morphology diagram is constructed. This diagram presents the lowest energy morphology of the nuclei, as well as melting points, as a function of the system parameters. Using the nucleus morphology diagram, experimental and system parameters that allow for the desired nucleation behavior can be identified. Furthermore, the nucleus morphology diagram illustrates a region of parameter space where the film is predicted to solidify above its thermodynamic bulk melting temperature, a behavior termed presolidification. The theory is used to predict the temperature at which nucleation of the solid phase and liquid phase are expected for Ge between two glass substrates. Furthermore, a possible route for controlling the orientation of the film is identified as confinement of the film can lead to a large range of solid phase nucleation temperatures, which are a function of the system's interfacial free energies. By controlling the growth temperature, certain orientations may not be able to nucleate thereby reducing the possible number of orientations within a film.