Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Ismail-Beigi, Sohrab

Abstract

The 21st century has seen enormous growth in the study of two-dimensional (2D) materials, beginning with the isolation of graphene but rapidly expanding to include a wide variety of other compounds. Due to their size, 2D materials have immediate appeal for applications in nanoscale electronics. At the same time, uniquely low-dimensional phenomena such as the quantum spin Hall effect, quantum confinement, and 2D superconductivity are of interest to basic physics researchers. This dissertation presents ab initio investigations of three 2D materials. First, we discuss the binding of stanene on various substrates. Stanene, the buckled monolayer form of tin, is predicted to be a 2D topological insulator with symmetry-protected helical edge states. We investigate the effects of strain, chemical functionalization, and substrate–overlayer interactions on the topological band structure of stanene, showing that Al2O3 is an ideal substrate for synthesizing a potential quantum spin Hall insulator. Next, we examine the polymorphic structure of borophene sheets, the monolayer form of boron. We report on research that revealed the complex atomic structure of borophene on the Cu(111) and Cu(100) surfaces, including the crucial role played by simulated scanning tunneling microscopy (STM) data. We discuss the effect of modulation by the substrate on the occurrence of Dirac cones in the borophene band structure. Finally, we discuss the potential for Mg2TiO4 films to host long-lived, strongly bound interlayer excitons. At the DFT level, we obtain the band structure of Mg2TiO4 films grown on MgO and show how the polar films have a band offset favorable for interlayer exciton formation. Motivated by this work, we present ?? and ??-BSE calculations of quasiparticle energies, exciton binding energies, and optical absorption spectra. These calculations more clearly characterize the suite of excitons that exist in Mg2TiO4 and shed light on the importance of film thickness in controlling their relative binding energies. The materials studied in this dissertation are diverse in chemical identity and properties, but are unified by their 2D structure and the crucial role played by their growth substrates, which are discussed throughout.

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