Microscopic Modeling of Exciton Propagation and Dissociation in Two-Dimensional Materials

Licentiate thesis, 2021

Atomically thin materials have been in the spotlight of research during the last decade due to their exceptional properties, providing a platform for the study of novel physical phenomena. In particular, transition metal dichalcogenides (TMDs) have emerged as promising atomically thin materials for future optoelectronic applications owing to their strong light-matter interaction and their high tunability. Furthermore, the strong Coulomb interaction in TMDs leads to the formation of tightly-bound electron-hole pairs -- excitons -- that dominate optics, dynamics and transport properties. Therefore, an accurate microscopic description of excitons in TMDs is essential for their technological application.

The aim of this thesis is to microscopically investigate the underlying many-particle mechanisms behind the main processes in optoelectronic devices, such as optical generation and relaxation of excitons as well as their propagation and dissociation into unbound electron-hole pairs. Based on the density matrix formalism, we develop equations of motion describing the dynamics in a system of interacting electrons, phonons, and photons. We investigate the density-dependence of the optical absorption and the thermalization of excitons into so-called dark states. We shed light on exciton propagation, revealing the microscopic mechanisms behind the appearance of spatial rings (halos) in the photoluminescence at strong excitation. Moreover, we tackle the problem of exciton dissociation, providing insights on the prominent role of dark excitons, and examine the tunability and optimal conditions for the efficient operation of TMD-based optoelectronic devices. Finally, we provide microscopic insights on charge separation in WS2-graphene heterostructures.
Our theoretical work, together with experimental support, contributes to the understanding of the many-particle mechanisms that govern the performance of TMD-based optoelectronic devices.