Quentin Ramasse1,2,Vlado Lazarov3,Demie Kepaptsoglou1,3
SuperSTEM Laboratory1,University of Leeds2,University of York3
Quentin Ramasse1,2,Vlado Lazarov3,Demie Kepaptsoglou1,3
SuperSTEM Laboratory1,University of Leeds2,University of York3
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 Bi<sub>2</sub>Se<sub>3</sub>, 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 Bi<sub>2</sub>Se<sub>3</sub> 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 the specific geometry of the sample offers opportunities for probing directly the spatial localization of electronic states within the graphene layers [1] and at the SiC/graphene and graphene/Bi<sub>2</sub>Se<sub>3</sub> interfaces. Strikingly, the use of a dark-field EELS geometry, using e.g. a set of custom-developed annular apertures, reveals the emergence of locally resolved fine structure in the ultra-low loss region of the spectrum. In addition to a direct interrogation of the chemical bonds between the layers via their vibrational response, these observations are linked to the interplay between the various phonon modes and the Dirac plasmons in the TI layers, whose dispersion is mapped in momentum space with nm spatial sensitivity using a recently developed methodology for nanoscale momentum-resolved spectroscopy [2].<br/>[1] M. Bugnet et al., Phys. Rev. Lett. 128, 116401 (2022)<br/>[2] F.S. Hage et al., Science Advances 4, eaar7495 (2018)