Mesoscale modeling and computational simulation studies of the self-assembly of heterogeneous colloidal systems.

Abstract

Over the last two decades researchers have advanced the field of colloidal synthesis by developing new synthesis techniques. Colloidal particles are known to self-assemble into various unique architectures. However, there is still no simple rule relating system condition and particle type to achievable self-assembled structures. The goal of this thesis was to use simulation methods to further develop an understanding of how tailoring interparticle interactions and system parameters (such as temperature and concentration) leads to self-assembled structures. The applicability of one specific colloidal system - nanotetrapods - for use as nanoelectronic circuit elements is investigated. The electrical response for MESFET and JFET nanotetrapods was determined through Technology Aided Design Tools, and it was determined that nanotetrapods have the potential to be utilized as circuit elements. Monte Carlo simulations provide insight into how proper tuning of particle-particle and particle-substrate interactions result in the assembly of ordered arrays of electrically gated nanotetrapods. We used lattice energy calculations and normal mode analysis (NMA) to investigate the thermodynamic and mechanical stability of binary, ionic colloidal crystals with size ratio 1.0 : 0.8. Based on these methods, theoretical predictions were made regarding the stable crystal structure as a function of potential interaction parameters. We found the normal mode results are in agreement with lattice energy results, and were compared to molecular dynamics simulations to determine the capacity for self-assembly. We found that not all predicted structures are kinetically accessible. Additionally, we investigated the self-assembly of colloidal crystals for one specific interaction parameter as a function of density and temperature, and found that, in addition to the theoretically predicted crystal structure, a second entropically stabilized crystal structure formed at higher temperatures. The extension of NMA to finite temperature systems was developed without having to couple to slower simulations. Using the Lennard-Jones model, kinetic energy was introduced into the system by randomly displacing particles in a crystal. Temperature was related to these displacements through the equipartition theorem. Upon comparison with published work on the Lennard-Jones spinodal, we determined that NMA reasonably predicts the limit of mechanical stability at low temperatures, but overestimates it at higher temperatures.