Scanning tunneling studies of topological insulators.

Abstract

The discovery of graphene led to a huge wave of interest in two-dimensional (2D) materials. The overall aim is to reveal the exotic properties of 2D-materials and tailor them for energy saving, storage, and conversion devices such as batteries and electrocatalysts and also for recent technologies such as spintronics, topotronics and twistronics. There is a library of various 2D-materials with novel properties amongst which bismuthene (2D-Bi) and in particular its rectangular allotrope (α-Bi) is of great interest. A large band gap, air-stability and low carrier density of this allotrope enables many real-life applications. More importantly α-Bi is predicted to be a promising room-temperature topological insulator due to its strong spin-orbit coupling. α-Sb is close to α-Bi in terms of morphology but the spin-orbit coupling is different, which could lead to different topological properties. The experimental investigations carried out for this PhD thesis are a step towards filling gaps in our knowledge and finding practical applications for these two materials.

This thesis focused on characterizing the electronic and topological (edge state) properties of various α-Bi structures grown on graphite and α-Sb structures grown on pre-existing Bi/graphite substrates using scanning tunneling microscopy/spectroscopy. Our findings confirm the presence of highly localized edge states for both α-Bi and α-Sb structures and also the role of twist and interaction between the layers in determining the electronic properties of the edges.

2D-heterostructures of α-Sb/α-Bi which have large spatial periodicities allow us to explore the effect of moiré patterns on the electronic and topological characteristics of the system. Moiré patterns allow engineering of both the electronic properties as well as local variations of the topological order of the structures.

Finally we characterized a new phase/structure observed after sequential deposition and found it to be a rectangular allotrope of antimony oxide with promising properties that can be tailored for photovoltaic devices.