Direct evidence for efficient ultrafast charge separation in epitaxial WS/graphene heterostructures.

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS/graphene heterostructure might provide a platform for efficient optical spin injection into graphene.