A scanning tunnelling microscopy study of Bi1-xSbx nanostructures

Abstract

Topological Insulators promise a route to develop novel electronic devices with dissipation-less flow of electrons. Theoretical and experimental demonstration of topologically protected electronic states in a host of different materials has resulted in an exponential increase in the research undertaken in this field. The first experimentally verified 3-dimensional topological insulator, Bi1-xSbx exhibits a non-trivial topological phase, within the concentration range of 0.07<x<0.22. The motivation for this project lies in exploring and understanding the impact of miniaturization on the topologically protected states of Bi1-xSbx alloys, when grown on a weakly interacting substrate like HOPG.

Studying the morphology of nanostructured Bi1-xSbx alloys on HOPG also provides a platform to investigate a transition from the ordered morphologies of pure Bi (110) islands to the fractal, disordered morphologies of Sb islands. Scanning Tunnelling Microscopy measurements are used to characterise the morphological and electronic properties of Bi1-xSbx islands. The morphology is similar to the wedding cake like profile of pure Bi (110) islands on HOPG, in which the formation of paired layers with thicknesses of 3, 5, 7 Monolayers (ML) is argued to result from a black-phosphorous like structure. Interesting outcomes include the presence of fingered 3 ML bases and irregular 5 ML stripes. An additional monolayer structure is also observed adjacent to the irregular 5 ML stripes and on top of large 3 ML bases in a select few islands as the Sb concentration is increased, referred to as the (3+1) ML region.

The experimentally acquired electronic spectra for each of the paired layers show similarity to the results obtained for pure Bi (110) islands on HOPG and are in good agreement with the corresponding DFT calculated band structures. Surprisingly, no noticeable difference in the band structures is observed as the Sb content is altered. The preliminary electronic spectra of the (3+1) ML region shows similarity to that of the 3 ML base and is indicative of a low density of states.

Distinct edge states are observed along the Bi1-xSbx island edges, particularly along the straight 7 ML stripe edges. The edge states are characterised by the presence of a distinct peak in the density of states around +0.1 V and observed as distinct, bright edges in the corresponding conductance maps near the Fermi level. In addition to the edge states, the observation of anomalous states in close proximity to the straight 7 ML edges and bright spots within the 5 ML regions are additional, interesting outcomes which merit future experimental studies.