Insight into Tetrahedral Materials and Two-Dimensional Materials from Advanced Atomistic Simulations
Open AccessThis dissertation investigates tetrahedral and two-dimensional materials for a variety of their physical properties, by employing a range of atomistic modeling approaches, including density functional theory (DFT), molecular dynamics, and Monte Carlo (MC) method. The two material systems of interest encompass some of the most critical materials to modern society, including water/ice, group IV semiconductors, and transition metal dichalcogenides. To facilitate our modeling work, method development and implementation were carried out to enable large-scale modeling capacity. Such effort includes (1) massive parallelization of forward flux sampling that demonstrates a parallel efficiency of 84% on 200,000 CPU cores, a nearly 1,000-fold increase over the previous implementation, (2) implementation of new algorithm for performing ring-statistics of tetrahedral networks that runs over two million particles, and (3) Monte Carlo code that samples configurational space in group IV alloy. Empowered by these tools, we explore tetrahedral and two-dimensional materials for their structures, phase behaviors, and optoelectronic properties. Using forward flux sampling method, we investigate the role of surface geometry on heterogeneous ice nucleation, particularly under confinement, which leads to our discovery of anomalous stability of two-dimensional ice. Such findings can be connected to another study of this dissertation on interlayer electronic coupling in arbitrarily stacked MoS2 bilayers -- a truly two-dimensional material, where our DFT calculation identities that the coupling is controlled by interlayer sulfur-sulfur interaction. Finally, by combining DFT with Monte Carlo sampling, we demonstrate that partial atomic ordering exists in GeSn alloy, an emerging group IV semiconductor alloy of high promise for mid-infrared photonic application. This finding challenges the commonly adopted assumption that GeSn is an ideal solid solution, and has important implications in understanding and predicting the optoelectronic properties of group IV alloys.
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