Advanced Electron Energy Loss Spectroscopy Investigations of Heterointerfaces for Spintronics Applications

Engineering the structural or chemical architecture of functional materials at the nano or even atomic level enables emergent properties that rely on the interplay between fundamental properties of matter such as charge, spin and local atomic-scale chemistry. A striking illustration of the relevance of this strategy is provided by placing Bi₂Se₃, a topological insulator (TI) with topologically-protected helical two-dimensional surface states and one-dimensional bulk states associated with crystal defects, in close proximity with graphene. The strong spin-orbit interaction and proximity effects result in subtle and controllable electronic band structure changes at and near the interface, with exciting potential for spintronic applications. Here we probe at high energy resolution the interfaces in a system consisting of Bi₂Se₃ films grown by chemical vapor deposition on epitaxial graphene/SiC(0001), where the number of carbon layers can be carefully controlled to tune possible proximity effects between the film and the substrate. All experiments were carried out on a monochromated Nion UltraSTEM100 MC operated at 60kV, with a probe convergence semi-angle of 31mrad and a beam current of approximately 4pA (after monochromation to ~12meV resolution). Chemical mapping confirms the atomic-level chemistry of the layers, while an analysis of the carbon <i>K</i> edge fine structure provides direct insights into the nature of bonding at the SiC/graphene and graphene/Bi₂Se₃ interfaces. Strikingly, the use of a dark-field EELS geometry reveals the emergence of locally resolved fine structure in the ultra-low loss region of the spectrum. In addition to a further direct interrogation of the chemical bonds between the layers, and a demonstration of chemically and atomically sensitive phonon mapping at interfaces in a bulk sample, it is thought these observations can also be linked to the interplay between the various phonon modes and the Dirac plasmons in the TI layers.