  Raman scattering and photoluminescence (PL) emission are used to investigate
a single layer of tungsten disulfide (WS$_{2}$) obtained by exfoliating n-type
bulk crystals. Direct gap emission with both neutral and charged exciton
recombination is observed in the low temperature PL spectra. The ratio between
the trion and exciton emission can be tuned simply by varying the excitation
power. Moreover, the intensity of the trion emission can be independently tuned
using additional sub band gap laser excitation.
