Authors: Wenshen Song
Source: Open Scholarship Institutional Repository (Washington University in St. Louis) (article, 2019-06-21)
Field: Materials Science — Materials Chemistry

First-principles Study of Structural and Optical Properties of Novel Materials Novel materials, including two-dimensional (2D) materials, ferroelectric materials, as well as hybrid perovskites materials, have attracted tremendous attention in recent years because of their unique structural symmetries and electronic structures. Among them, 2D materials, such as graphene, black phosphorene, and transition metal dichalcogenides (TMDs), etc., have great potentials for nanoelectronics and optical applications. Particularly, these 2D materials can sustain much larger strain than their bulk counterparts, making strain a unique and efficient tool to tune a wide range of properties of 2D structures. In the first part of this thesis, we explore how strain tunes quasiparticle energy and excitonic effects of typical 2D materials, including black phosphorus and monolayer 2H-phase TMDs, i.e., MoS2, MoSe2, WS2, and WSe2. Beyond the widely studied uniaxial and biaxial strain, we expand the research to arbitrarily axial strains and find complicated variations of quasiparticle band gaps, band-edge energies, direct-indirect gap transitions, and exciton energies. These results provide a complete picture for straining engineering of electronic structures and optical spectra of 2D TMDs.\nThe second part of this thesis focuses on the electronic polarization of non-centrosymmetric layered van der Waals (vdW) metal chalcogen-diphosphates MCDs, especially on CuInP2Se6. We employ first-principles modern-polarization theory to study the electrical polarization ordering and build an electrostatic-energy model to explore the effects of boundary conditions. We find the existence of intrinsic off-plane polarization of CuInP2Se6, with an antiferroelectric (AFE) ground state for monolayer and ferroelectric state (FE) ground state for bulk under the open-circuit boundary condition. However, for close-circuit boundary condition, we find that the ground state is always FE through bulk to monolayer. We also apply Monte Carlo simulations to obtain the FE Curie temperature and electric hysteresis. This study gives hope to overcoming the depolarization effect and realizing ultra-thin FE transistors and memories based on vdW materials.\nIn the last part, we focus on nonlinear optical properties of a novel family of materials, organic-inorganic hybrid perovskites materials. We have developed a high-efficient, large-scale parallel simulation tool (NLOPACK), making it possible to explore the nonlinear optical properties of a family of organic-inorganic hybrid halide perovskites, CH3NH3MX3 (M= Ge, Sn, Pb; X=Cl, Br, I), which contains a large number of atoms and cannot be handled by traditional simulation packages. We employ this code and find significant second harmonic generation (SHG) and linear electro-optic (LEO) effect in the cubic phase of CH3NH3SnI3, in which those effects are comparable with those widely used organic/inorganic nonlinear optical materials. The reason for these enhanced nonlinear optical properties in hybrid perovskites is explained as well. This work will motivate experimental efforts to fabricate hybrid perovskites for low-cost, nonlinear-optical device implementations. Computer science ZnO doping and properties ; Electronic and Structural Properties of Oxides ; 2D Materials and Applications