Interlayer excitons in bilayer MoS2 with strong oscillator strength up to room temperature

Coulomb bound electron-hole pairs, excitons, govern the optical properties of semiconducting transition-metal dichalcogenides like MoS2 and WSe2. We study optical transitions at the K point for 2H homobilayer MoS2 in density functional theory including excitonic effects and compare them with reflectivity measurements in high-quality samples encapsulated in hexagonal BN. In both calculated and measured spectra we find a strong interlayer exciton transition in energy between A and B interlayer excitons, observable for T = 4-300 K, whereas no such transition is observed for the monolayer in the same structure in this energy range. The interlayer excitons consist of an electron localized in one layer and a hole state delocalized over the bilayer, which results in the unusual combination of high oscillator strength and a static dipole moment. We also find signatures of interlayer excitons involving the second-highest valence band (B) and compare absorption calculations for different bilayer stackings. For homotrilayer MoS2 we also observe interlayer excitons and an energy splitting between different intralayer A excitons originating from the middle and outer layers, respectively.