Electrochemical intercalation and exfoliation of (Sb2Te3)2(Sb2)2.

An electrochemical method for exfoliating (Sb2Te3)2(Sb2)2, a van der Waals material, topological insulator, and the first member of the homologous series of topological superlattices (Sb2Te3) m (Sb2) n with m = n has been demonstrated. The objective of this work was to determine whether electrochemical exfoliation can yield pristine few or single layer 2D materials in this material system. To this end, polycrystalline ingots of (Sb2Te3)2(Sb2)2 were prepared from stoichiometric mixtures of elemental Sb and Te heated in vacuum. Diffraction and scanning transmission electron microscopy data confirmed the 2/2 stacking sequence. Cathodic exfoliation was performed and studied using 4 separate cations, H+, Na+, Li+, and tetrapropylammonium (TPA+). Delamination of the surface of (Sb2Te3)2(Sb2)2 electrodes was consistently observed at negative applied biases. These materials were collected and characterized via scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. The delaminated materials were layered, consistent with exfoliation of the parent ingot. The key findings were that materials prepared by electrochemical exfoliation were less prone to oxidation in air than layer stacks obtained by mechanical exfoliation and that cathodic exfoliation performed with larger cations yielded a more homogeneous, single-crystalline structure as compared to the smaller cations. These differences indicated the selectivity of cleavage planes for electrochemical exfoliation vs. mechanical exfoliation. The significance of these findings will be to prepare thin materials that help understand how the electronic properties of topological insulators evolve with the number of layers.