Theory of Two-dimensional Materials
By:Yuanxi Wang
Published on 2016 by
In this dissertation, I study the synthesis (and post-synthesis modifications), properties, and structural engineering of two-dimensional materials, using atomistic modeling techniques such as empirical forcefields and density functional theory. The dissertation is divided into four parts. The first part consists of four chapters: in Chapters 1-3 I give an overview on density functional theory and how vibrational properties and optical properties can be calculated within the DFT framework; in Chapter 4 a brief review on two-dimensional materials is given.The second part is devoted to the synthesis and post-synthesis modifications of two-dimensional materials. In Chapter 5 I investigate the intercalation of hexagonal boron nitride and graphite using Bronsted acids such as H3PO4. DFT calculations show that dipolar interactions between acids molecules and the 2D sheets drive the intercalation process.In Chapter 6 I study stripe patterns of W and Mo atoms in triangular monolayers of WxMo1-xS2 alloys. DFT calculations suggest that stripe ordering is kinetically driven, governed by fluctuations in local thermodynamic conditions during growth. In addition, I show that the striped phase alloy is electronically isotropic but vibrationally anisotropic, due to the nearly-identical electronic properties but very different atomic masses of Mo and W.The third part is devoted to computation of vibrational and optical properties of molybdenum and tungsten disulfides. In Chapter 7 we investigate the optical signatures of defect-bound excitons in monolayer WS2. I show that the optical transitions between the highest valence band and mid-gap states introduced by sulfur vacancy defects have finite transition matrix elements. These results agrees with the measured photoluminescence spectra of monolayer WS2 grown by chemical vapor deposition, where spatially resolved photoluminescence at low temperatures revealed two bound excitons. To explain the anomaly in the thermal activation energy of the bound exciton, we propose that excitons are bound to negatively charged defects where two channels of radiative recombinations are allowed.Chapter 8 focuses on intervalley scattering processes in monolayer MoS2, as probed by resonance raman spectroscopy, where the incident laser energy is varied across the A and B excitonic levels. We focus on two Raman peaks that are dispersive, a clear signature of second-order processes involving intervalley scattering. Using group theory and DFT calculations, selection rules are established for intervalley scattering processes mediated by acoustic phonons. In Chapter 9 I investigate Second Harmonic Generation (SHG) in monolayer WS2. Its second order nonlinear susceptibility is nearly three orders of magnitude larger than other common nonlinear crystals. Density functional theory calculations suggest that the origin of the large nonlinear susceptibility lies in resonance enhancement and sharp features in the joint density of states.In the final part, I explore possible methods of manipulating the geometry and grain texture of 2D materials. In Chapter 10 I show that when an atomically thin two-dimensional crystal grows on a substrate of non-zero Gaussian curvature, it can develop finite-length grain boundaries that terminate abruptly within a monocrystalline domain. By properly designing the substrate topography, these grain boundaries can be placed at desired locations and at specified misfit angles, as the thermodynamic ground state of a 2D system bound to a substrate. New properties specific to certain grain boundary geometries, including magnetism and metallicity, can thus be engineered into 2D crystals through topographic design of their substrates.
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