Characterization of defects, modulations and surface layers in topological insulators and structurally related compounds
20 January 2020
Campus Groenenborger, U0.24 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Organization / co-organization:
Department of Physics
Joke Hadermann & Dirk Lamoen
PhD defence Carolien Callaert - Faculty of Science, Department of Physics
Topological insulators, a new class of fascinating materials, are ideally bulk insulating and surface conducting. They are intensely studied due to their special physics and possible future applications, such as in spintronics. Adding defects (point-, line-, planar- and 3D defects), modulating the structure or interfacing the material with another material is a common practice to obtain the desired properties for applications. In this thesis, different kinds of adaptations of topological insulators and structurally related materials were studied. The atomic structure of the materials was determined, because this will affect the properties of the material. Different transmission electron microscopy techniques, complemented with first-principles calculations using the VASP code, were used to characterize the structures.
The first part of the thesis concerns the characterization of the bulk structure of topological insulators. Bi2Se3 and (Bi1-xInx)2Se3 showed atomic mobility around and across the van der Waals gap between the quintuple layers. The topological insulator-normal insulator Sb2(Te1-xSex)3 showed a different substitution order than reported in literature. Some members of the GemBi2nTe(m+3n) series were trigonal layered structures with l-layered (l=7,9,11,5-7) building blocks instead of the five-layered building blocks for Bi2Se3 and others were rock salt structures with planar defects.
The second part of the thesis reveals the structure and chemical composition of the oxidized layers and sublayers of Bi2Te3, Sb2Te3, (Sb0.55Bi0.45)2Te3 and GemBi2nTe(m+3n) and oxidation mechanisms were proposed. Also the structure and chemical composition of the interface between the (approximately) 20 nm thick Fe layer and Bi2Te3 is shown, unfolding the intermediate 3.5 nm amorphous FeTe interface layer, where excessive Bi migrates to the shallow bulk forming septuple layers of Bi3Te4.
In the last part of the thesis, the bulk structures of two structurally related materials, α-GeTe and Fe2Ge3, were solved: α-GeTe, showed a planar defect structure, while Fe2Ge3 was incommensurately modulated.