  All-electrical (magnetic-material-free) spin injection is one of the
outstanding goals in spintronics. Topological insulators (TIs) have been
recognized as a promising electrically controlled spin source thanks to the
strong spin-orbit coupling and in particular, the spin-momentum locked
topological surface states (TSS) supporting helically spin polarized currents.
Many TI materials such as Bi-based chalcogenides are also layered 2D materials
and can be incorporated into van der Waals (vdW) coupled heterostructures,
opening the possibility of the utilization of TIs for electrical spin injection
into other 2D materials. Here, we demonstrate electrical injection of helically
spin-polarized current into graphene through a 3D TI in a mechanically stacked
heterostructure between Bi2Te2Se (a TI) and chemical vapor deposition
(CVD)-grown graphene, using the spin potentiometric measurement. When a dc
current is flowing from the TI to graphene, we detect a striking step-like
voltage change (spin signal) in both the TI and graphene using ferromagnetic
(FM) probes. The sign of the spin signal can be reversed by reversing the
direction of the dc bias current, and the corresponding amplitude of the spin
signal increases linearly with the bias current, indicative of a
current-induced helical spin polarization in both TI and graphene. In contrast,
the graphene itself exhibits usual nonlocal spin valve signal when the spins
are injected using an FM electrode. We discuss possible origins of the helical
spin polarization injected into the graphene in our TI/graphene heterostructure
that may include TSS as well as the spin-orbit coupled Rashba states. Our
findings show electrical injection of a spin helical current into graphene
through a TI and demonstrate TIs as potential spin sources for future
spintronic devices wherein spin manipulation is achieved electrically.
