Charge transfer excitons in atomically thin semiconductors

Transition metal dichalcogenides (TMDs) exhibit a rich exciton landscape. In particular, the emergence of spatially separated charge transfer (CT) excitons have been demonstrated in vertical and lateral TMD heterostructures. Recent experiments have revealed an ultrafast electron and hole transfer process in type-II vertical heterostructures resulting in the formation of tightly bound charge transfer excitons (interlayer excitons). Based on a microscopic theory combined with tr-ARPES measurements, we reveal that phonon-mediated scattering via hybridized dark excitons governs the charge transfer process. We track the time- and momentum-resolved relaxation dynamics of excitons and determine the temperature- and stacking-dependent charge transfer times.

Furthermore, in a joint theory-experiment study we demonstrate the appearance of highly dipolar in-plane CT excitons in lateral TMD heterostructures exhibiting a large energy offset at the atomically thin interface. We reveal an intriguing phonon-mediated interplay of the offset-driven exciton drift across the interface and capture into energetically lower-lying CT excitons at the interface governing their strongly temperature-dependent propagation behaviour.

Finally, we discuss the formation of in-plane moire CT excitons in twisted TMD heterostructures exhibiting periodic moire potentials, which can trap excitons at certain high-symmetry sites. Based on a microscopic theory we investigate the in-plane charge-separation in atomically reconstructed TMD heterostructures and identify three distinct and twist-angle dependent exciton regimes including localized Wannier-like excitons, polarized excitons, and intralayer CT excitons.

The provided insights present an important step forward in microscopic understanding of technologically important charge transfer excitons in atomically thin semiconductors.