Multiscale modeling of two-dimensional materials : structures, properties, and designs

Abstract

Multiscale modeling undertakes to describe a system with multiple models at different scales. In principle, quantum mechanics provides sufficient information. However, the development of a scaled-up model, e.g., molecular dynamics, from quantum mechanics, should be validated against the experiments. Two-dimensional (2D) materials provide excellent platforms to verify theoretical models by directly comparing atomic structures and properties with advanced transmission electron microscopy (TEM) techniques due to their high crystallinity and thin nature. In this thesis, molecular dynamics (MD) models have been developed for the 2D transition metal dichalcogenides (TMDs) such as MoS₂, WS₂, MoSe₂, and WSe₂ from density functional theory (DFT) by focusing on their nonlinearity and failure strains. The structures, crack-tip behaviors, and fracture patterns from the models are directly compared with atomic level in-situ TEM images.

The models have revealed atomic scale mechanisms on the crack-tip behaviors in the single crystals such as roles of sulfur vacancies, geometric interlocking frictions, and the directions of crack propagation. The models have been further validated with more complicated structures from grain boundaries in the WS₂ bilayer and lateral heterostructures, e.g., MoS₂-WSe₂ by the images from ADF-STEM. Also, a method for generation of grain boundary has been proposed for well-stitched grain boundaries based on experimentally observed dislocations and defects. The models and methods have been utilized to understand the chemical reactions for MoS₂ channel growth in WSe₂ and fracture toughness of polycrystalline graphene. Finally, the validated models and methods are utilized to predict the atomic structures of 2D materials with three-dimensional (3D) surfaces, e.g., triply periodic minimal surfaces (TPMS) and corrugated surfaces with non-zero Gaussian curvatures.

The mechanics, failure behaviors, and thermal properties of TPMS graphene are systematically studied from the predicted structures. A recent experiment shows the predicted scaling laws of Young's modulus and strengths agree well with the measurements.