Ultrafast Spin and Charge Dynamics in Monolayer WSe₂-Graphene Heterostructures

Monolayer transition metal dichalcogenides (TMDs) have garnered much attention due to their long spin/valley lifetimes and ability to optically excite spin/valley polarization. Additionally, one of the great strengths of TMDs is their ability to compliment other materials, such as graphene, by acting as a means of optical spin injection or proximity coupling. Recently proximity mediated charge transfer and optical spin injection has been demonstrated in TMD/graphene heterostructures. However, the spin transfer dynamics across a TMD/graphene interface remain largely unexplored.

Here we use time-resolved Kerr rotation (TRKR) microscopy to image the spatial dependence of spin/valley dynamics in monolayer WSe₂/graphene heterostructures. While the bare WSe₂ demonstrates long-lived spin valley lifetimes, spatial maps reveal a quenching of spin-valley signal at the WSe₂/graphene interfaces. Time delay scans show this quenched lifetime to be up to 3 orders of magnitude lower than in bare WSe₂. These interfaces also exhibit quenched photoluminescence and enhanced photoconductivity, demonstrating efficient charge transfer from WSe₂ to graphene. Consequently, we attribute the ultrafast spin/valley quenching to spin transfer by conducted charge carriers.