Topotactic anion exchange in Bi₂Te₃: layer-specific substitution and its plasmonic effects

We demonstrate the site-selective transformation of Bi₂Te₃, with two crystallographically inequivalent anions, into ordered Bi₂Te₂Se by anion exchange, in which Se exclusively replaces the octahedrally coordinated middle layer of Te in the quintuple-layer structure, preserving both the structure and morphology. Cross-sectional HAADF-STEM imaging confirms this site-specific Se incorporation. First-principles calculations support the thermodynamic stability of the ordered Bi₂Te₂Se configuration, with the kinetics also favouring the replacement of the central Te with Se. Low-loss EELS reveals a blue shift in the bulk plasmonic resonance upon exchange. This spectral evolution validates the formation of an ordered electronic band structure, establishing a robust strategy for the deterministic tuning of collective excitations in quintuple-layer materials.