Direct observation of large strain through van der Waals gaps on epitaxial Bi2Te3/graphite: Pseudomorphic relaxation and the role of Bi2 layers on the BixTey topological insulator series

Layered materials can usually grow without strain on top of distinct substrates if the only interaction between them is due to van der Waals forces. In such a scenario it would be expected that the heterointerface made up of weak bounds would not affect the overlayed material significantly for several large lattice-mismatched systems. Here we have studied the first stages of the heteroepitaxial growth of layered bismuth telluride topological insulator on top of highly oriented pyrolitic graphite (HOPG) by molecular beam epitaxy. Samples were investigated by atomic force microscopy (AFM), synchrotron x-ray diffraction (XRD), and micro-Raman spectroscopy. AFM images show hexagonal/triangular flat islands with exposed HOPG areas for the low coverage regime, and the lattice parameter of these Bi2Te3 structures were measured by XRD. The existence of pseudomorphic strain at the initial Bi2Te3 layers was retrieved by both XRD and Raman spectroscopy. We have found evidence that Bi2Te3 layers near the interface are subject to an in-plane compressive strain, leading to a pseudomorphic out-of-plane lattice expansion. Furthermore, the presence of Bi2Te3 islands locally distorts the topmost layer of HOPG, resulting in tensile strain which was measured by Raman spectroscopy. The observed relaxation of 0.1-0.2% for each van der Waals gap is used to calculate elastic constants of Bi-2 bilayers, which are crucial building blocks for the formation of other BixTey topological insulator compounds. Finally, the impact of such a strain in Bi2Te3 electronic structure was investigated by density functional theory calculations. The results show that the band structure of this strained material remains unchanged at the center of the Brillouin zone, confirming the robustness of surface states, but it is consistently affected at the M and K zone edges.