  Single layers of WS2 are direct gap semiconductors with high
photoluminescence (PL) yield holding great promise for emerging applications in
optoelectronics. The spatial confinement in a 2D monolayer together with the
weak dielectric screening lead to huge binding energies for the neutral
excitons as well as other excitonic complexes, such as trions and biexcitons
whose binding energies scale accordingly. Here, we report on the existence of
biexcitons in mechanically exfoliated WS2 flakes from 78 K up to room
temperature. Performing temperature and power dependent PL measurements, we
identify the biexciton emission channel through the superlinear behavior of the
integrated PL intensity as a function of the excitation power density. On the
contrary, neutral and charged excitons show a linear to sublinear dependence in
the whole temperature range. From the energy difference between the emission
channels of the biexciton and neutral exciton, a biexciton binding energy of
65-70 meV is determined.
