  Group III-V films are of great importance for their potential application in
spintronics and quantum computing. Search for two-dimensional III-V films with
a nontrivial large-gap are quite crucial for the realization of dissipationless
transport edge channels using quantum spin Hall (QSH) effects. Here we use
first-principles calculations to predict a class of large-gap QSH insulators in
functionalized TlSb monolayers (TlSbX2; (X = H, F, Cl, Br, I)), with sizable
bulk gaps as large as 0.22~0.40 eV. The QSH state is identified by Z2
topological invariant together with helical edge states induced by spin-orbit
coupling (SOC). Noticeably, the inverted band gap in the nontrivial states can
be effectively tuned by the electric field and strain. Additionally, these
films on BN substrate also maintain a nontrivial QSH state, which harbors a
Dirac cone lying within the band gap. These findings may shed new light in
future design and fabrication of QSH insulators based on two-dimensional
honeycomb lattices in spintronics.
