Realizing fully spin polarized transport in graphene nanoribbons with design of van der Waals vertical heterostructure leads

Zigzag-edged graphene and h-BN nanoribbons with passivated edges in the ground state are either spin-degenerate or spin-unpolarized systems, which are not directly applicable for spin-polarized transport. In this work, based on density functional calculations, we demonstrate that the combination of them to form van der Waals (vdW) heterostructures can be adopted to realize fully spin polarized transport. As an example, the ballistic transport properties of a zigzag-edged graphene nanoribbon (ZGNR) with six zigzag carbon chains is studied first with each lead as a vdW heterostructure formed by attaching a h-BN monolayer to each side of the ZGNR with AA-stacking. A greatly decreased transmission gap (0.28 eV) in one spin and a greatly increased transmission gap (0.82 eV) in the other spin is achieved in the transmission function, resulting in fully spin polarized transport at low bias. This arises from the stagger potential imposed by the h-BN layers, which not only exists in the leads, but also extends to the channel region. It changes the energy of the edge states of different spins localized at different sublattices in an opposite way in the whole device. The transmission gap and the threshold voltage can be further modulated by applying a vertical pressure to the vdW leads to tune the strength of the stagger potential or simply by changing the ribbon width. Finally, the increase of the channel length will greatly reduce the magnitude of the transmission around the Fermi level and the transmission gap will eventually recover the value of the band gap of the pristine ZGNR. These findings not only provide a novel way for achieving fully spin polarized transport in graphene, but also demonstrate the great importance of vdW heterostructures in the design of spintronic devices. (paper)