  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.
