Indirect Interlayer Bonding in Graphene-Topological Insulator van der Waals Heterostructure: Giant Spin-Orbit Splitting of the Graphene Dirac States

van der Waals (vdW) heterostructures of two-dimensional materials exhibit properties and functionalities that can be tuned by stacking order and interlayer coupling. Although direct covalent bonding is not expected at the heterojunction, the formation of an interface nevertheless breaks the symmetries of the layers, and the orthogonal requirement of the wave functions can lead to indirect interfacial coupling, creating new properties and functionalities beyond their constituent layers. Here, we fabricate graphene/topological insulator vdW heterostructure by transferring chemical vapor deposited graphene onto Bi2Se3 grown by molecular beam epitaxy. Using scanning tunneling microscopy/spectroscopy, we observe a giant spin-orbit splitting of the graphene Dirac states up to 80 meV. Density functional theory calculations further reveal that this splitting of the graphene bands is a consequence of the breaking of inversion symmetry and the orthogonalization requirement on the overlapping wave functions at the interface, rather than simple direct bonding. Our findings reveal two intrinsic characteristics the symmetry breaking and orthogonalization of the wave functions at the interface-that underlines the properties of vdW heterostructures.