  Topological insulators are new class of quantum materials, which have
insulating energy gaps in bulk, but exhibit gapless edge states or surface
states that are protected by time-reversal symmetry at boundary. It was
theoretically predicted and experimentally confirmed that the binary
tetradymites Bi2Te3, Bi2Se3, and Sb2Te3 are three-dimensional topological
insulators. In this work, we demonstrate by first-principles approach that the
ignored Sb2Se3, although with relatively smaller spin-orbital coupling
strength, can also exhibit topologically protected surface states with a bulk
gap of 0.19 eV, as long as the van der Waals interaction is explicitly included
in the calculations. Detailed analysis of the band structures of Sb2Se3 thin
films indicates that the non-trivial surface state appears at a critical
thickness of six quintuple layers.
