Local electronic and optical phenomena in two-dimensional materials

Abstract

Two-dimensional (2D) materials, consisting of a single layer of atoms, have proven to be a tremendously valuable platform for studying novel physical phenomena. Besides representing the ultimate thickness limit, these materials exhibit new optical, electronic, and mechanical properties often not present in the bulk, rendering them exciting candidate systems for a wide variety of technological applications, ranging all the way from DNA sequencing to solar cells. A major challenge in fully understanding the physical behavior of two-dimensional materials and realizing their technological potential, is that many of the phenomena occur locally and on length scales that are too small to be interrogated with conventional techniques. In this thesis, I present studies of such phenomena in two different 2D materials systems, namely graphene and transition metal dichalcogenides (TMDs), using novel techniques to understand their local behavior. Specifically, we first employ spin defects in diamond as nanoscale noise sensors to locally probe current fluctuations in biased, ultraclean graphene devices. At high electronic drift velocities, we observe a dramatic increase in GHz current noise that grows exponentially across the device. We attribute our observations to an electron-phonon instability driven by Cherenkov amplification of acoustic phonons, which arises when the electronic drift velocity exceeds the speed of sound. Next, we study the optical properties of excitons in twisted TMDs and introduce a new technique for imaging the emergent nanoscale moiré patterns in these devices. By correlating the optical response of the excitons with the local moiré structure, we observe signatures of an exciton array containing two spatially alternating exciton species that can be tuned independently through electrostatic gating. We also study the chiral exciton response in the same system, and demonstrate that twisted TMDs is a promising platform for valleytronic devices. Our work provides new important insights into the local electronic and excitonic behaviors of graphene and TMDs respectively, and demonstrates the immense value of local investigations in 2D materials.