We present a photoluminescence study of freestanding and Si/SiO2 supported single-and few-layer MoS2. The single-layer exciton peak (A) is only observed in freestanding MoS2. The photoluminescence of supported single-layer MoS2 is instead originating from the A − (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate is decreasing the indirect band gap energy by up to ≈ 80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.

In this letter we present photoluminescence measurements with different excitation energies on single-layer MoS2 and MoSe2 in order to examine the resonance behavior of the conservation of circular polarization in these transition metal dichalcogenides. We find that the circular polarization of the emitted light is conserved to 100 % in MoS2 and 84 %/79 % (A/A − peaks) in MoSe2 close to resonance. The values for MoSe2 surpass any previously reported value. However, in contrast to previous predictions, the degree of circular polarization decreases clearly at energies less than the two-phonon longitudinal acoustic phonon energy above the resonance.Our findings indicate that at least two competing processes underly the depolarization of the emission in single-layer transition metal dichalcogenides.