Synthesis and characterization of colloidal topological (crystalline) insulators

Jara Vliem, j.f.vliem@uu.nl
Sponsor: ERC grant, since October 2020
Supervisors: prof. dr. Daniël Vanmaekelbergh, dr. Ingmar Swart

Topological (crystalline) insulators, Colloidal nanomaterials, Scanning Tunneling Microscopy/Spectroscopy

Topological insulators (TIs) have gained much interest in condensed matter physics due to their unique electronic properties. These materials are characterized by an insulating bulk with topologically protected metallic quantum states at their surface (3D materials) or edge (2D materials). This protection is related to the inversion of the conduction and valence band as a consequence of strong spin-orbit coupling, which results in robust surface states that remain metallic despite the presence of disorder or impurities. The robust surface, combined with exotic effects such as spin-filtering, have resulted in TIs being regarded as highly intriguing materials for material science, nanoelectronics, and spintronics.[1]

Of the verified TIs, Bi2Se3 serves as a model system due to its large inverted bulk bandgap (0.3 eV) and its helical edge state, which manifests as a Dirac cone situated at the Γ point in the Brillouin zone.[2] While extended sheets and bulk crystals of Bi2Se3 have been investigated with scanning tunnelling microscopy (STM) and spectroscopy (STS), reports on the electronic properties of Bi2Se3 nanoplatelets (NPLs) are scarce.[3] However, the small lateral size of NPLs and their specific crystal structure provide a unique opportunity to study the effects of quantum confinement and shape on the NPLs’ opto-electronic properties. We therefore aim to synthesize colloidal Bi2Se3 NPLs and investigate their properties with STM/STS as shown in figure 1a.

In addition to investigating Bi2Se3 NPLs, our goal is to prepare and characterize colloidal topological crystalline insulators (TCIs) of SnTe, Pb1-xSnxSe, and Pb1-xSnxTe (see figure 1b). Such TCIs constitute a subclass of TIs in which the surface states are protected by the point group symmetry of the crystal. We are currently developing a synthesis route using cation exchange and one-pot synthesis techniques to obtain SnTe NPLs and nanocrystals of Pb1-xSnxSe and Pb1-xSnxTe with tunable dimensions and composition.

Figure 1: a) Graphical representation of the STM set-up and the types of results that can be obtained. b) HAADF-STEM image and STEM-EDX element maps of Pb1-xSnx Se nanocubes, showing a homogeneous distribution of Pb and Sn in the particles. Scale bars are 5 nm.

[1] Ramirez, Arthur P., and Skinner, Brian, Nat. Phys. 73, 30-36 (2020)
[2] Zhang, Yi et al., Nat. Phys. 6, 584-588 (2010)
[3] Bhunia, Hrishikesh et al., Phys. Chem. Chem. Phys. 19, 9872-9878 (2017