PhD defence: Colloidal Topological Insulators

PLEASE NOTE: The candidate gives a layman's talk, therefore the livestream will start fifteen minutes earlier.

The material bismuth selenide (Bi2Se3) is a topological insulator. These materials have a conductive surface and an insulating interior (bulk). Due to the robustness of these surface states, topological insulators are promising materials for future electronics. Since such applications generally make use of small components, it is important to investigate the properties of micro and nanoscale Bi2Se3 crystals.

In this dissertation, the changes in optical and electronic properties of Bi2Se3 are examined when the thickness of the material is decreased to a few nanometers. For this purpose, a new chemical method has been developed to grow thin Bi2Se3 crystals with control over the thickness and lateral size. These crystals were used to demonstrate that Bi2Se3 at a thickness of 4 to 6nm has conductive edge states and behaves as a 2D topological insulator, while crystals thicker than 6nm possess a fully conductive surface and show the properties of a 3D topological insulator.

The transition from a 3D to a 2D topological insulator may also influence the optical properties of Bi2Se3. Therefore, the absorption of light in the crystals has been investigated. We show that light absorption by electrons in the edge states proceeds differently than in the bulk. The absorption of light also causes atomic vibrations in the crystal that influence thermal con duction. Our results indicate that vibrations in the thickness direction of the crystals are more easily observed than vibrations in the lateral direction.

This shows that the electronic and optical properties of Bi2Se3 can be influenced by accurately controlling the thickness and size of the crystals.