  Further development of the field of all-electric spintronics requires the
successful integration of spin transport channels with spin injector/generator
elements. While with the advent of graphene and related 2D materials high
performance spin channel materials are available, the use of nanostructured
spin generators remains a major challenge. Especially promising for the latter
purpose are 3D topological insulators, whose 2D surface states host massless
Dirac fermions with spin-momentum locking. Here, we demonstrate injection of
spin-polarized current from a topological insulator into graphene, enabled by
its intimate coupling to an ultrathin Bi2Te2Se nanoplatelet within a van der
Waals epitaxial heterostructure. The spin switching signal, whose magnitude
scales inversely with temperature, is detectable up to ~15 K. Our findings
establish topological insulators as prospective future components of spintronic
devices wherein spin manipulation is achieved by purely electrical means.
