We present photoluminescence and resonant Raman measurements of atomically thin layers of molybdenum disulfide (MoS2) in the energy range of the A and B optical transitions. We found a Stokes shift of the transition energies in the resonant Raman measurements compared to the photoluminescence. The Stokes shift increases for the bilayer compared to the single layer. Furthermore, in resonance with the A transition, the Raman intensity of the A1g mode of the bilayer is six times larger than that of the single layer.
Schematic crystal structure of molybdenum disulfide. (a) Top view (slightly tilted). (b) Side view.