Freestanding Nanolayers of a Wide-Gap Topological Insulator through Liquid-Phase Exfoliation

The layered salt Bi₁₄ Rh₃ I₉ is a weak three-dimensional (3D) topological insulator (TI), that is, a stack of two-dimensional (2D) TIs. It has a wide non-trivial band gap of 210 meV, which is generated by strong spin-orbit coupling, and possesses protected electronic edge-states. In the structure, charged layers of $$ (Bi₄ Rh)₃ I]²⁺ honeycombs and $$ Bi₂ I₈ ]^2- chains alternate. The non-trivial topology of Bi₁₄ Rh₃ I₉ is an inherent property of the 2D intermetallic fragment. Here, the exfoliation of Bi₁₄ Rh₃ I₉ was performed using two different chemical approaches: (a) through a reaction with n-butyllithium and poly(vinylpyrrolidone), (b) through a reaction with betaine in dimethylformamide at 55 °C. The former yielded few-layer sheets of the new compound Bi₁₂ Rh₃ I, while the latter led to crystalline sheets of Bi₁₄ Rh₃ I₉ with a thickness down to 5 nm and edge-lengths up to several ten microns. X-ray diffraction and electron microscopy proved that the structure of Bi₁₄ Rh₃ I₉ remained intact. Thus, it was assumed that the particles are still TIs. Dispersions of these flakes now allow for next steps towards the envisioned applications in nanoelectronics, such as the study of quantum coherence in deposited films, the combination with superconducting particles or films for the generation of Majorana fermions, or studies on their behavior under the influence of magnetic or electric fields or in contact with various materials occurring in devices. The method presented generally allows to exfoliate layers with high specific charges and thus the use of layered starting materials beyond van der Waals crystals.