  Single-layer WS$_2$ is a direct-gap semiconductor showing strong excitonic
photoluminescence features in the visible spectral range. Here, we present
temperature-dependent photoluminescence measurements on mechanically exfoliated
single-layer WS$_2$, revealing the existence of neutral and charged excitons at
low temperatures as well as at room temperature. By applying a gate voltage, we
can electrically control the ratio of excitons and trions and assert a residual
n-type doping of our samples. At high excitation densities and low
temperatures, an additional peak at energies below the trion dominates the
photoluminescence, which we identify as biexciton emission.
