PhD Defense - Xin Liang

Event time: 
Tuesday, April 9, 2019 - 4:00pm to 5:00pm
YQI Seminar Room See map
17 Hillhouse Avenue
New Haven, CT 06511
Event description: 

First Principles Studies of Photoelasticity and Two-dimensional Silica

The thesis presents theoretical studies of photoelasticity and Two-dimensional silica using first principles calculations. The first project in this thesis concerns the elasto-optic effect in solids. The elasto-optic effect, or photoelasticity, describes the linear change of dielectric constant with applied strain and is a universal material property for insulators and semiconductors. Though the elasto-optic responses can be directly computed using first principles (e.g., density functional perturbation theory), little insight into the governing microscopic physical principles is provided by these methods. In this work, we describe a microscopic first principles analysis of photoelasticity in real-space based on Maximally Localized Wannier Functions (MLWFs).  We show that the strain-dependent change of dipole transitions between occupied and unoccupied Wannier functions is the main determinant of photoelasticity. By organizing the dipole transitions into spatially localized shells according to the distances, we find that the photoelasticity is a relatively long-ranged. We believe the long-ranged nature of photoelasticity makes it unlikely to find simple and localized models with very few parameters that can describe photoelasticity with sufficient accuracy. The second project in this thesis investigates the growth of 2D silica and silicate thin films on NixPd1-x(111) alloy substrates. In the past decade, the creation of 2D SiO2 has added a new member to the material class of two-dimensional (2D) Van der Waals (vdW) atomically thin sheet. 2D SiO2 is the thinnest form of silica known with the SiO2 stoichiometry. Apart from being a 2D material, 2D SiO2 is also of interest because of its structural similarities to zeolite catalysts. 2D bilayer SiO2 can serve as a model system that imitates the interior surface of bulk zeolites while its 2D nature permits application of surface microscopy techniques that reach atomic scale resolution. We employ density functional theory (DFT) to study the 2D SiO2 on various metal substrates and demonstrate that epitaxial strain plays an important role in engineering the 2D SiO2 overlayer’s struture. We also focus on the structural competition between crystalline hexagonal 2D SiO2 in commensurate and incommensurate relation to the substrate when epitaxial strain cannot be realized in experiments. The recent creation of NixPd1-x random alloy in experiments is intended to study the strain effect on the morphology of the 2D SiO2 overlayer through the alloy’s tunable lattice constant. However, the application is hindered due to its ability to form silicate overlayer through chemical reaction with the deposited SiO2. We propose a thermodynamically stable Ni silicate structure as a theoretical model for the silicate thin film on the NixPd1-x surface and use DFT to characterize its structural and electronic properties. The next thrust in this effort has been to understand the phase competition between 2D silica and silicate phases on NixPd1-x alloy substrate. First principles calculations suggest that by decreasing the oxygen pressure and increasing Si supply, the 2D SiO2 will become the favorable phase. Experiments show that a decreased oxygen and restricted annealing temperature and time enable the growth of 2D SiO2.