Direct and indirect optical transitions in bulk and atomically thin MoS2 studied by photoreflectance and photoacoustic spectroscopy

Abstract

Optical transitions in atomically thin MoS2 samples made by sulfidation of a metallic molybdenum layer have been studied by photoreflectance (PR) and photoacoustic (PA) spectroscopy. The obtained spectra are compared with PR and PA spectra of bulk MoS2. It is shown that the absorption edge observed in the PA spectrum shifts to blue when moving from the bulk MoS2 to the atomically thin MoS2 layers, whereas the direct optical transitions at the K point of the Brillouin zone (A and B transitions), which are observed in the PR spectrum, do not shift spectrally in a significant manner. On the other hand, the AH transition, which is related to the direct optical transition at the H point of the Brillouin zone and is typical of bulk MoS2, is not observed for atomically thin MoS2 layers. Moreover, a strong and broad PR resonance related to the band nesting (C transition) is identified in the PR spectra of studied samples. In this case, C and CH transitions are observed for bulk MoS2, while only a C transition is observed for atomically thin MoS2.