UC Berkeley

Since the isolation of graphene, a single layer of carbon atoms in honeycomb structure, in 2004, this new material has gotten huge attention from communities in physics, chemistry, materials science, and engineering not only because the charge carriers of graphene show neutrino-like linear energy dispersion as well as chiral behavior near the Dirac point but also because graphene is considered to be a promising candidate for nano- and micro-scale electronic and spintronic device applications.

On the other hand, a hexagonal sheet of boron nitride has a similar honeycomb-like structure, except that the two different sublattices are occupied by boron and nitrogen atoms, respectively. Notwithstanding its structural similarity to graphene, a hexagonal boron nitride sheet is an insulator with a large bandgap and is considered to be useful to optical applications such as ultra-violet lasers.

In this work, we investigate the electronic, optical, and vibrational properties of graphene, hexagonal boron nitride, and related materials such as nanotubes or nanoribbons from first-principles calculations as well as from simple model considerations.

In the first chapter, we briefly review the methodologies used in our work such as density functional theory, the GW approximation, the Bethe-Salpeter equation method, and density functional perturbation theory.

In the following four chapters (2-5), we discuss the calculated spectral features of graphene and compare the results mainly with recent angle-resolved photoemission experiments. In our work, we have explicitly taken into account the effects of electron-electron and electron-phonon

interactions from first-principles. Our calculations reproduce some of the key experimental observations related to many-body effects, including a mismatch between the upper and lower halves of the Dirac cone and the non-trivial energy dependence of carrier linewidths on the binding energy.

The following three chapters (6-8) are on bilayer graphene. In chapters 6 and 7, we discuss the effects of many-body interactions on the dynamics of electrons and phonons in bilayer graphene, in similar ways as in chapters 2 to 5. We show that the interlayer interaction between the two graphene layers change electron-phonon and electron-electron interactions. In chapter 8, we discuss the excitons in biased bilayer graphene. We show that bound excitons qualitatively change the optical response of this novel material.

In the following four chapters (9-12), we discuss the interesting behaviors of charge carriers in graphene subjected to an external periodic potential. For example, we show that the carrier group velocity is anisotropically reduced and that, under certain conditions, electrons can be supercollimated. We also discuss newly generated massless Dirac fermions in graphene superlattices as well as their signatures in quantum Hall conductance measurements.

In chapter 13, we discuss the possibility of generating massless Dirac fermions in a conventional two-dimensional electron gas with an external periodic potential, i.e., a way of making artificial graphene.

In the last four chapters, we discuss several different aspects of boron nitride compounds. In chapter 14, we present the calculated electronic energy bandgaps and effective masses of boron nitride nanoribbons and their changes in response to a transverse electric field. In chapters 15 and 16, we discuss excitons and optical response of boron nitride nanotubes and bulk hexagonal boron nitride, respectively. Finally, in the last chapter, we discuss a novel behavior of electric dipole moment reversal upon hydrogen passivation in boron nitride as well as other III-V or II-VI compound nanostructures.