Colloidal Topological Insulators

Abstract

In this dissertation, we have made use of colloidal nanoplatelets to examine the size-dependent properties of Bi2Se3. As a topological insulator (TI), Bi2Se3 has highly conducting, spin-momentum locked, and robust surface states, in combination with a semiconducting bulk. These properties are of interest for fields like quantum computing and spintronics. However, the successful realization of devices based on TIs depends on the ability to understand and control their properties, especially at small scales that are relevant for device fabrication. We have therefore aimed to understand the changes in electronic, optical, and topological properties of Bi2Se3 when its thickness is reduced to a few nanometers. To investigate these effects, we used colloidal nanoplatelets (NPLs), whose tunable lateral dimensions and thickness make them a good model system. The hot-injection synthesis of Bi2Se3 NPLs is described in chapter 3. This method allows for independent control over the NPL thickness and lateral dimensions. In chapter 4, the existence and character of topological edge states in these NPLs are investigated using cryogenic scanning tunneling microscopy/spectroscopy (STM/S). For NPLs with a thickness of 4–6QLs, STM/S shows an ∼8–10nm wide channel of enhanced local density of states encircling the crystal perimeter. This enhanced density of states is absent in thinner, 1–2QL platelets, while the presence of an edge state in the 3QL platelet is ambiguous. Theoretical analyses using an 8-band k·p model and GW-based tight-binding calculations suggest that the observed state is a helical quantum spin Hall state. For 2D Bi2Se3, we find that the band structure contains regions with high contributions from states at the surface QLs, which are called surface-state bands. In chapter 5, we examine charge carrier dynamics in 2D NPLs to understand how these differ from 3D Bi2Se3. We used ultrafast transient absorption spectroscopy for this investigation. When surface-state bands are excited, we observe a relatively long ∼5ps recombination delay. This delay is attributed to the band geometry, which causes a separation of electrons and holes in momentum space and inhibits their fast recombination. Finally, in chapter 6, the investigation of optical properties is expanded by examining electron-phonon coupling in Bi2Se3 NPLs. Femtosecond transient spectroscopy was used to resolve coherent phonon oscillations at a sub-picosecond timescale. The A(1)1g mode is found to couple strongly to a transition at 1.97eV, which coincides with a region of strong absorption. We assign this resonance predominantly to transitions between the VB1 and CB1/2 manifolds along the Γ–K high-symmetry line. Notably, Raman-active Eg modes were absent in our experimental data, which can be explained using a symmetry analysis based on group theory. To summarize, we have shown that bismuth selenide nanoplatelets form a useful model system for investigating size-dependent effects in topological insulators. However, transitioning from these model systems to applications will require the fabrication of higher-quality materials. Moreover, what form these applications will take, and how they can be realized, remains a major open question. It will be exciting to see how the field develops in the future.