Raman scattering and photoluminescence (PL) emission are used to investigate a single layer of tungsten disulfide (WS2) 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.

Optical spectroscopy in high magnetic fields B ≤ 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe2, the exciton emission shifts linearly with the magnetic field and exhibits a splitting that originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac Fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe2 where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model.

Resonant Raman spectra of single layer WS2 flakes are presented. A second order Raman peak (2LA) appears under resonant excitation with a separation from the E 1 2g mode of only 4cm −1 . Depending on the intensity ratio and the respective line widths of these two peaks, any analysis which neglects the presence of the 2LA mode can lead to an inaccurate estimation of the position of the E 1 2g mode, leading to a potentially incorrect assignment for the number of layers. Our results show that the intensity of the 2LA mode strongly depends on the angle between the linear polarization of the excitation and detection, a parameter which is neglected in many Raman studies.