Optically probing the band offset in rhombohedrally stacked MoS₂bilayer

Stacking van der Waals materials in different orders enables a new opportunity to realize two-dimensional (2D) ferroelectricity, where the stacking-induced out-of-plane spontaneous polarization can be electrically switched by interlayer sliding. In rhombohedrally stacked MoS₂ bilayer, such spontaneous polarization leads to an interlayer potential with a type-II band alignment at the K/K' point of Brillouin zone. Since the conduction and valence band offsets are not equal, the A exciton is split by about 10 meV, which has been observed through optical spectroscopy of an artificially stacked bilayer MoS₂. Here we study the band alignment of a natural MoS₂ bilayer with rhombohedral stacking by performing optical reflection and photoluminescence spectroscopy of A/B excitons and their charged species while precisely controlling the doping and displacement field in the bilayer. As a result, we can quantitatively measure the conduction and valence band offset individually as well as the 2D spontaneous polarization density. Our study provides a detailed understanding about the bandstructure of rhombohedral MoS₂ bilayer, which may contribute to the potential application of 2D ferroelectricity.