We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 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 WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge transfer 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 WS2 and graphene bands as revealed by high resolution ARPES. In combination with spin-selective excitation using circularly polarized light the investigated WS2/graphene heterostructure might provide a new platform for efficient optical spin injection into graphene.
@article{arxiv.1904.01379,
title = {Direct evidence for efficient ultrafast charge separation in epitaxial WS$_2$/graphene heterostructure},
author = {S. Aeschlimann and A. Rossi and M. Chávez-Cervantes and R. Krause and B. Arnoldi and B. Stadtmüller and M. Aeschlimann and S. Forti and F. Fabbri and C. Coletti and I. Gierz},
journal= {arXiv preprint arXiv:1904.01379},
year = {2019}
}