  Momentum resolved electron energy-loss spectroscopy was used to measure the
dispersions of excitons in a free-standing monolayer of WSe2. Besides the
parabolically dispersed valley excitons, a sub-gap dispersive exciton was
observed at specific q values for the first time. The simultaneous STEM-ADF
imaging of the local probed areas suggests that this sub-gap exciton is
attributed to the prolific Se vacancies.
