Date of Award
Doctor of Philosophy (PhD)
Two-dimensional (2D) materials with single or a few atomic layers, such as graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDCs), and the heterostructures or one-dimensional (1D) nanostructures they form, have attracted much attention recently as unique platforms for studying many condensed-matter phenomena and holds great potentials for nanoelectronics and optoelectronic applications. Apart from their unique intrinsic properties which has been intensively studied for over a decade by now, they also allow external control of many degrees of freedom, such as electrical gating, doping and layer stacking. In this thesis, I present a theoretical study of the electronic and optical properties of many different 2D materials and nanostructures using first-principles density functional theory and many-body perturbation theory. I will show what we learn from these theoretical calculations about the relation between the partially extended, partially confined structure and the tunability of their electronic and optical properties with free-carrier doping and electrical gating.
First, we investigate the effect of free-carrier doping on the quasiparticle and exciton properties of 2D material. On one hand, we discuss the origin of the doping-induced band gap renormalization in 2D materials and demonstrate the simplifications that can be made to the theory to allow more efficient calculation. On the other hand, using MoS2 as an example, we study the effect of dynamical screening on the electron-hole interaction and excitonic properties in doped 2D material using the Bethe-Salpeter Equation. Combining them, we show that the quasiparticle band gap of 2D material drops as a non-linear function of doping density by several hundred meV due to the free-carrier screening, but this is offset by the drop in the exciton binding energy and makes the exciton energy remain nearly constant.
Then, we switch gear to study the effect of electrical gating on excitons in bilayer TMDC heterostructures. We reveal the important role of interlayer coupling in deciding the band alignment and excitonic properties. We show that due to the interlayer coupling of valence states, the excitons are superpositions of intralayer and interlayer electron-hole pairs which can be described by a simple tight-binding model. As a result, their dipole oscillator strength and radiative lifetime can be tuned by over an order of magnitude with a practical external gate field of a few V/nm.
Finally, we study the effect of quantum confinement on the formation of magnetism in confined nanostructures. In two one-dimensional structures, graphene nanoribbon and tellurium chain, we find doped free-carriers can have half-metallic ferromagnetic ground state due to the Stoner mechanism. This comes from the quantum-confinement of the electronic state which enhances the density of state and Stoner parameter at the same time. For graphene nanoribbons, we find magnetism in general edge types with large spin polarization energy up to 17 meV/carrier. It can bypass the requirement of specific zigzag edge in previous proposals of graphene nanoribbon magnetism. For tellurium chain, we find magnetic ground state with a significant 6 meV/carrier spin-polarization energy. Due to the strong spin-orbit interaction of tellurium and its unique helical chain structure with chirality, the spins of the magnetic carriers are pinned along a specific direction with an enhanced magnetic anisotropy energy that is larger than the spin-polarization energy, making it of broad interest for spintronics applications.
Chair and Committee
Zohar Zussinov, Alexander Seidel, Erik Henriksen, Rohan Mishra,
Gao, Shiyuan, "Tunable Electronic and Optical Properties of Low-Dimensional Materials" (2018). Arts & Sciences Electronic Theses and Dissertations. 1723.