Solid state chemistry of binary and ternary chalcogenides with topological surface states

Abstract

Whenever a new kind of electronic state of matter is discovered, it triggers a new cycle of prosperity in the field of solid state physics. For example, the discovery of superconductors in metals and later oxides, not only inspired scientists to explore further into materials that they once thought were fully understood, but also eventually changed people's daily life. Similarly, the new field of topological insulators has gained enough momentum and is ready to bring a new boom to materials studies. Topological insulators are a group of semiconductors that are insulating in the interior while conducting on the surfaces or edges. In these materials, the twisting of bulk electronic states (primarily due to spin-orbit coupling) results in bizarre surface or edge states. While physicists and theorists are studying the fundamental principles, solid state chemists and material scientists play a different role. We can synthesize the candidate materials predicted by physicists, and more importantly, contribute new ideas based on chemical intuition.

In this thesis, I present the synthesis and structural and physical characterizations of several binary and ternary chalcogenides with non-trivial topological surface states. In Chapter 1, the history of the discovery of topological insulators is briefly reviewed and two strategies to find new candidates are discussed. In Chapter 2, the structure family of tetradymite, which contains the most currently known topological insulators, is introduced. The optimization of the properties of Bi2Te2Se (a topological insulator with the best physical properties so far) using defect chemistry and compositional tuning, the discovery of a new topological insulator - Bi2Te1.6S1.4, as well as the structural and physical properties of a semiconductor - (Bi0.12In0.88)2Se3 are discussed. In Chapter 3, the search of topological insulator candidates is extended to tetradymite-related derivative structures and the discovery of a semi-metal with non-trivial topological surface states from the Bi2-Bi2Se3 infinitely adaptive homologous series is presented. In Chapter 4, I discuss two methods to incorporate ferromagnetism into topological insulator systems - bulk intergrowth and heterostructure fabrication - in order to study the relationship between topological surface states and ferromagnetism. In Chapter 5, I present the crystallography for a group of Cu-containing bismuth selenides.