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Optical manipulation of the exciton charge state in single layer tungsten disulfide

The layered transition metals from group VI [dichalcogenides] such as molybdenum disulfide MoS2), tungsten disulphide (WS2) or tungsten diselenide (WSe2) have recently attracted a lot of attention as alternative candidates for exploring single atomic layer structures. The particularity of these materials is the indirect band gap, which changes to a direct gap for a monolayer structure. Direct gap emission is characterized by an exciton doublet (A and B) which are split by spin orbit coupling.

The magneto - optical methods are very powerful for investigation electronic properties of such materials as emission occurs in visible part of the spectrum. Additional application of the magnetic field allows playing with the spin degree of freedom.

We show that in a single layer of tungsten disulfide (WS$_2$) obtained by the exfoliation of n-type bulk crystals, we observe both charged and neutral exciton recombination in the photoluminescence (PL) emission spectra. Additionally, by simply changing the intensity of the laser excitation, we can tune the ratio between the trion and exciton emission which demonstrates our ability to tune the density of 2D carriers with light. Moreover, using additional sub band gap laser excitation, the trion emission intensity can be independently tuned.

Fig.1 Typical micro - PL spectra measured as a function of the 532nm excitation power at 4K. (a),(b) results obtained for two different flakes. Spectra are offset vertically for clarity