Computational study of structural phase transformations in ultrathin materials

Abstract

Ultra-thin materials have shown tremendous potential in a wide variety of technological applications, such as flexible electronics, optoelectronics, and transparent devices. In this thesis, computational methods are employed to investigate the structural phase transformations in two different ultra-thin material systems: transition metal dichalcogenide (TMD) monolayers and polymer thin films. In the study of TMD monolayers, we first focus on the mechanical behaviors related to the formation of the atomically thin group VI TMD superlattices.A combination of scaling theory and numerical analysis is employed to investigate strain relaxation mechanisms in misfitting, chemically heterogeneous TMDs. We demonstrate that, in free-standing superlattices, wrinkling of the monolayer is asymptotically preferred over misfit dislocation formation in both binary and ternary superlattices. For substrate-supported monolayers, however, misfit dislocation formation is thermodynamically favored above a critical superlattice width, implying the presence of an upper limit to the thermodynamic stability of coherent, misfitting 2D superlattices. Based on the study of superlattice strain relaxation, we further developed our phase-field model to explore the possibility of phase programming via defects in TMD monolayers. Three different kinds of experimentally available defects, combining with macroscopic in-plane strains, are investigated to facilitate the localization of phase transformations in TMD monolayers. Quantitative relations between the macroscopic strain requirement, phase transformation patterning, and the defect configurations are developed via theoretical analysis and then verified with numerical simulations. To better understand the phase transformations in TMD monolayers, a phase-field crystal model is developed to study the phase transformations between the 2H and 1T structures from an atomistic perspective in TMD monolayers. Geometric analysis and numerical simulations are conducted to find the possible defects induced by the phase transformations. Moreover, the relation between the density of linear defects and the dynamic of the periodic phase transformation is obtained numerically and explained theoretically. With respect to the polymer thin film, the epitaxial crystallization behaviors observed in experiments are investigated. A phase-field model with anisotropic kinetics and interfacial energies is developed. With this phase-field model, both the formation of the spherulite in the polymer droplet and epitaxial needles in the polymer thin film can be reproduced. Additionally, the quantitative relations between the epitaxial needles width, polymer diffusivity, growth rate, and polymer molecular weight are determined.