Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Ivan I. Oleynik, Ph.D.

Committee Member

Lilia Woods, Ph.D.

Committee Member

Arjan van der Vaart, Ph.D.

Committee Member

Humberto Rodriguez Gutierrez, Ph.D.

Committee Member

Matthias Batzill, Ph.D.

Keywords

2D Excitons, High pressure, Layered materials, Crystal Structure Prediction, Superconductivity

Abstract

Propelled by the emergence of new fundamental physical phenomena in a new class of two-dimensional (2D) atomically thin materials, graphene and other novel two-dimensional (2D) materials are being intensively investigated over the past two decades. In many cases, the single layer compounds have outstanding mechanical and electronic properties compared to those in the bulk crystals. Graphene is the prototypical atomically thin material, however the influence point and extended defects on its fundamental properties is not well understood at the atomic scale. The structural, electronic and vibrational properties of other 2D materials, including chalcogenides beyond transition metal containing systems (e.g. SnS_{2} and SnSe_{2}) were not well understood. New emergent optoelectronic applications of 2D materials require understanding the excitonic effects which are greatly enhanced in 2D compared to 3D bulk materials.

Another key parameter for modifying the fundamental properties of a compound is the application of high pressure. The interesting question is whether the prototypical layered materials such as SnS_{2} and SnSe_{2} would transform to new compounds with different stoichiometries and whether these new materials would exhibit new emergent phenomena such as superconductivity. Recent breakthrough has been recently achieved by demonstrating conventional superconductivity with extremely large critical temperature of \unit[200]{K} in hydrogen sulfide compressed by very high pressures ~200 GPa. Driven by the pursuit of further increasing T_{c} while lowering the compression pressure, a search for new compounds beyond of hydrogen sulfide is warranted.

This dissertation is concerned with application of a suite of atomistic simulation techniques such as first-principles density functional theory (DFT), classical molecular dynamics and evolutionary crystal structure prediction methods to uncover the emergent fundamental physical properties of novel 2D materials such as graphene, tin disulfide (SnS_{2}) and tin diselenide (SnSe_{2}) and to search for new binary and ternary compounds exhibiting superconducting properties. Specifically, the mechanical properties of polycrystalline graphene are investigated with the goal of understanding the effect of the grain boundaries on the strength of graphene. To this end we have developed a novel method for generating large scale polycrystalline samples for use in computer simulations, which are then used in classical molecular dynamics (MD) simulations of nanoindentation experiments. Density functional theory (DFT) is used to provide an accurate description of the layer-dependent electronic, optical, and vibrational properties of novel SnS_{2} and SnSe_{2} 2D materials as well as several other novel 2D compounds within the family of LMDC's. We also extended the effective mass theory to describe the enhancement of excitonic effects in SnS_{2} as a number of layers is decreased down to single layer. Using advanced structural prediction methods, we search for novel Sn_{x}S_{y} and P_{x}S_{y} binary and P_{x}S_{y}H_{z} ternary compounds over a range of pressures up to \unit[200]{GPa}. In all systems, we predict the emergence of several novel compounds, some of which are superconducting with moderate temperatures. This research performed using state-of-the-art simulation techniques demonstrated the power of computer simulations to gain insight into fundamental properties of novel materials and predict new compounds with emergent properties.

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