Stephen Eltinge successfully defends thesis “Ab Initio Insights Into Substrate Effects, Structural Identification, and Excitonic States in 2D Materials”

September 13, 2021

On August 26, 2021, Stephen Eltinge successfully defended the thesis “Ab Initio Insights Into Substrate Effects, Structural Identification, and Excitonic States in 2D Materials”. (Advisor: Sohrab Ismail-Beigi).

Eltinge state, “My thesis reports on computational modeling of two-dimensional materials, focusing particularly on their interactions with substrates. Crystalline materials that are only one atom thick exhibit new phenomena not seen in their three-dimensional “bulk” analogues. Some of these phenomena are enhanced versions of pre-existing properties, such as the “quantum confinement” effect that increases the band gap of low-dimensional semiconductors. Others, such as the quantum spin Hall effect, are intrinsically two-dimensional. In collaboration with experimental groups at Yale, I studied three two-dimensional materials: stanene, the 2D form of tin; borophene, the 2D form of boron; and a thin film of Mg2TiO4. Modern high-performance supercomputer clusters give us the power to study these materials within ever-larger unit cells, incorporating substrates, structural dislocations, and many layers of atomic thin films. I used these tools to elucidate the unprecedentedly large unit cells of borophene on the (100) and (111) surfaces of copper, and to study how the primary mode of optical excitation in Mg2TiO4 changes as a film grows thicker. Overall, my thesis contributes to the body of knowledge concerning three unique materials that have potential applications in ultra-thin, low-power electronic devices, among other areas.”

He will be continuing work in Sohrab Ismail-Beigi’s lab as a postdoctoral associate.

Thesis 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 GW and GW-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.