Microscopic Theory of Charge Complexes in Atomically-Thin Materials

Doktorsavhandling, 2023

Atomically-thin materials have emerged as the most promising two-dimensional platform for future optoelectronic applications and for the study of quantum many-body physics. In particular, transition metal dichalcogenides (TMDs) exhibit strong Coulomb interaction, resulting in the formation of tightly-bound electron-hole complexes that dominate optics, dynamics, and transport. In the neutral regime, excitons -- bound electron-hole pairs -- constitute the dominating many-particle species from low to moderate photoexcitation densities. In the presence of doping, however, excitons can bind to additional charges and form trions. In order to achieve an efficient and controllable implementation of TMDs in novel devices, understanding the fundamental properties of excitons and trions in these materials is crucial.

The aim of this thesis is to provide a microscopic understanding of the underlying many-particle mechanisms in TMD optoelectronic devices. Based on the density-matrix formalism, we describe the dynamics in a system of interacting electrons, holes, phonons, and photons. We model the excitonic features of optical absorption spectra and reveal how they are influenced by the excitation density. We unveil the formation dynamics of dark excitons after photoexcitation and resolve the main pathways of phonon-assisted dissociation. Furthermore, we tackle exciton diffusion, tracing the emergence of photoluminescence halos back to the large heating and thermal drift of excitons at strong excitation. Finally, we consider doped TMDs and investigate the trion dynamics, including diffusion and photoluminescence. In particular, we predict so far unobserved luminescence signatures that could shed light on the internal structure of trions.

Overall, this work provides microscopic insights into many-particle processes governing the optics, dynamics, and transport in atomically thin semiconductors.