Sub-100-fs formation of dark excitons in monolayer WS2

Abstract

Two-dimensional semiconducting transition metal dichalcogenides (TMDs) are promising for optoelectronic applications due to their strongly bound excitons. While bright excitons have been thoroughly scrutinized, dark excitons are much less investigated as they are not observable with far-field spectroscopy. However, with their non-zero momenta, dark excitons are significant for applications requiring long-range transport or coupling to external fields. We access such dark excitons in WS2 monolayer using transient photoemission electron microscopy with sub-diffraction limited spatial resolution (75 nm) and exceptionally high temporal resolution (13 fs). Image time series of TMD flakes are recorded at several different fluences. We directly observe the ultrafast formation of dark K-Q excitons in monolayer WS2 occurring within 14-50 fs and follow their subsequent picosecond decay. We distinguish exciton dynamics between the interior and edges of the monolayer TMD and conclude that the long-term evolution of dark excitations is defect-mediated while intervalley scattering is not affected.