Moiré Exciton Landscape and its Optical Properties in Two-Dimensional Semiconductors

Licentiatavhandling, 2022

In recent years the emergence of atomically thin nanomaterials has led to a new research venue, revealing intriguing properties making them an excellent platform to study many-particle quantum phenomena. Of particular interest is the class of nanomaterials called transition metal dichalcogenides (TMDs), where the strong light-matter coupling reveals promising opportunities in the pursuit of novel optoelectronical devices. Additionally, the two-dimensional nature of TMDs leads to a strong Coulomb interaction, resulting in the formation of excitons, which are tightly bound electron-hole pairs. Due to their large binding energy, they are stable at room temperature and dominate the optical response of these materials. Furthermore, TMD monolayers can be stacked on top of each other to form heterostructures. Introducing a twist angle gives rise to a moiré pattern, allowing for the formation and trapping of highly tunable moiré excitons.

The aim of this thesis is to investigate, on a microscopic footing, the moiré exciton landscape and the many-particle mechanisms governing the optical response of TMD heterostructures. In particular, the focus lies on the impact of the moiré potential on the optical response and how the moiré exciton landscape can be externally tuned. For this purpose, we will shed light on the importance of interlayer hybridization in both twisted and untwisted TMD bilayers. We show how the hybridization can be tuned by applying an external electrical field and can turn materials into indirect semiconductors with dark excitons becoming the lowest states. Overall, the work provides microscopic insights into the twist-angle dependent optical fingerprint of the technologically promising class of atomically thin nanomaterials.