Fig. 2
Different exciton types in atomically thin nanomaterials and related heterostructures. a Excitons are Coulomb-bound electron-hole pairs (ovals in the picture). Momentum-forbidden dark excitons consist of electrons and holes located at different valleys in the momentum space. Spin-forbidden dark excitons consist of electrons and holes with opposite spin. These states cannot be accessed by light due to the lack of required momentum transfer and spin-flip, respectively. Electrons and holes in localized excitons are trapped into an impurity-induced potential. b Interlayer excitons appear, where electrons and holes are located in different layers
