Bi₂Se₃ (Bismuth Selenide)

A layered van der Waals chalcogenide, space group R3̄m, built from Se–Bi–Se–Bi–Se quintuple layers bonded by van der Waals gaps. It is one of the canonical three-dimensional topological insulators, featuring a relatively large bulk bandgap (~0.3 eV) and a single Dirac-cone surface state protected by time-reversal symmetry. Its simple surface-state structure makes Bi₂Se₃ a benchmark material for studying topological surface states, spin-momentum locking, and spintronic applications, and it is also a well-known thermoelectric material.