Valley-polarized exciton dynamics in a 2D semiconductor heterostructure

Stacking to prolong valley lifetime In the material MoSe2, which, like graphene, has a two-dimensional honeycomb crystal lattice, the electronic structure has two “valleys.” Electrons can be distinguished by the valley they reside in, making them act as potential information carriers. However, electrons easily lose this information by scattering into the other valley. Rivera et al. placed single layers of MoSe2 and WSe2 on top of each other and shone circularly polarized light on the structure. The light caused excitons—pairs of electrons and holes—to form so that the hole and electron came from the same valley but different layers. The valley-specific character of such excitons persisted far longer than would be possible in a single layer of either material. Science, this issue p. 688 Photoluminescence measurements are used to deduce a valley lifetime of 40 nanoseconds in heterostructures of MoSe2 and WSe2. Heterostructures comprising different monolayer semiconductors provide an attractive setting for fundamental science and device technologies, such as in the emerging field of valleytronics. We realized valley-specific interlayer excitons in monolayer WSe2-MoSe2 vertical heterostructures. We created interlayer exciton spin-valley polarization by means of circularly polarized optical pumping and determined a valley lifetime of 40 nanoseconds. This long-lived polarization enables the visualization of the expansion of a valley-polarized exciton cloud over several micrometers. The spatial pattern of the polarization evolves into a ring with increasing exciton density, a manifestation of valley exciton exchange interactions. Our work introduces van der Waals heterostructures as a promising platform from which to study valley exciton physics.