Studies of Topological States and Proximity Effects in Functional Materials

Recent advances in instrumentation, such as the introduction of advanced, high-resolution monochromators have allowed for exciting new experiments in the electron microscope. Spectroscopic signatures of optical and acoustical phonons, excitons and defect gap states are now accessible with an atom size probe and in tandem with high precision imaging. This has brought forward electron microscopy and in particular Scanning Electron Microscopy Transmission Electron Microscopy (STEM) in tandem with Electron Energy loss Spectroscopy (EELS) as the ultimate research tool, as it allows to probe simultaneously structural and electronic structure information of materials at the atomic level. Here, we present results on the structure and electronic structure of topological insulator based heterostructures. Topological insulators (TI) are a class of materials that are uniquely insulating in the bulk, while their surface states are metallic and topologically protected by time reversal symmetry. These topologically protected states make TIs suitable for number of applications including quantum computation, spintronics and thermoelectric, but their incorporation TIs in device structures requires the fabrication of these materials as thin films. We study proximity effects between free standing or epitaxial graphene and Bi₂Se₃ at the atomic scale, using momentum resolved vibrational STEM-EELS measurements, and show the evolution of the spectroscopic signatures of phonons and Dirac plasmons across the heterostructures. We will also present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator Bi₂Te₃ on Ge(111) showing that weak van der Waals adhesion between the Bi₂Te3 quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favourable because of the strong hybridization between the Te and Ge.