  Transition metal dichalcogenides hold promise for applications in novel
optoelectronic devices. There is therefore a need for materials that can be
obtained in large quantities and with well understood optical properties. In
this report, we present a thorough photoluminescence (PL) investigations of
monolayer tungsten disulphide obtained via liquid phase exfoliation. As shown
by microscopy studies, the exfoliated nanosheets have dimensions of tens of
nanometers and thickness of 2.5 monolayers on average. The monolayer content is
about 20%. Our studies show that at low temperature the photoluminescence is
dominated by excitons localized on nanosheet edges. As a consequence, the PL is
strongly sensitive to environment and exhibits an enhanced splitting in
magnetic field. As the temperature is increased, the excitons are thermally
excited out of the defect states and the dominant transition is that of the
negatively charged exciton. Furthermore, upon excitation with a circularly
polarized light, the PL retains a degree of polarization reaching 50% and
inherited from the valley polarized photoexcited excitons. The studies of PL
dynamics reveal that the PL lifetime is on the order of 10 ps, probably limited
by non-radiative processes. Our results underline the potential of liquid
exfoliated TMD monolayers in large scale optoelectronic devices.
