Exciton dominated optical properties of atomically thin semiconductors

Atomically thin transition metal dichalcogenides (TMDs) are a new family of semiconductors with exciting optical properties for practical applications in optoelectronic devices, as well as intriguing many-body behaviors for fundamental explorations in condensed matter physics. In bulk form they are van der Waals layered materials and have been traditionally used as industrial lubricants. When thinned down to monolayers they surprisingly become direct band gap semiconductors with exceptionally strong light-matter interactions dominated by exciton effects. In these monolayer semiconductors, excitons can be robustly observed even at room temperature, in contrast to conventional bulk semiconductors where the electron-hole binding energies are typically lower than the room temperature thermal energy. The strong excitonic effect is a direct result of both fundamental quantum confinement effects and reduced dielectric screening in a two-dimensional system. This dissertation presents experimental investigations on a few central questions in these materials: how strong is the electron-hole binding effect in the prototypical monolayer semiconductor MoS2 as well as in novel metal-chalcogenolate hybrid materials; how can exciton effects be tuned by carrier doping and controlled by electrostatic gating in real devices; and how can excitons coherently enhance nonlinear optical transitions for the development of novel nonlinear devices and imaging techniques./nThe dissertation starts with an introduction to interband optical transitions including exciton effects in low-dimensional electronic systems. The focus is on providing a many-body physical picture for better understanding experimental phenomena, and not as much on a detailed quantitative treatment. Then three major research works surrounding this topic are included in the dissertation. First, we study carrier-induced quasiparticle band gap and exciton energy renormalization effects in monolayer MoS2. We show that both the quasiparticle band gap and the exciton binding energy can be largely tuned by applying electrostatic gating to a monolayer device. In the second work, we investigate exciton dominated optical properties of a family of “bulk 2D” materials: self-assembled hybrid metal organic chalcogenolate multi quantum wells. Owing to the weak interlayer electronic coupling, the exciton binding energy in these multilayered bulk materials are shown to be close to a true monolayer semiconductor. We also reveal the bright and dark exciton species by cryogenic photoluminescence excitation spectroscopy, providing further evidence of high exciton binding energy in the bulk layered crystal. In the third work, we come back to atomically thin TMDs and study how exciton resonances greatly enhance their nonlinear optical responses, leading to successful continuous-wave generation of nonlinear signals from monolayer TMDs. Finally, in addition to optical spectroscopy studies, we also present two examples of device applications of two-dimensional materials in micro-electromechanical systems (MEMS). In one example we demonstrate micro strain sensors based on the two-dimensional electron gas formed at the interface of AlGaN/GaN. In the second example we demonstrate efficient ultraviolet photodetectors using graphene/diamond heterojunctions. Together these studies provide crucial knowledge on the utilization and control of exciton enhanced linear and nonlinear optical properties of atomically thin semiconductors and related low-dimensional materials, and guidance towards practical optoelectronic device applications.

Advisor: Schuck, Peter J
Lin, Liwei