  A large bulk band gap is critical for the applications of quantum spin hall
(QSH) insulators in spintronics at room temperature. Based on first-principles
calculations, we predict that the methyl-functionalized III-Bi monolayers,
namely III-Bi-(CH3)2 (III=Ga, In, Tl) thin films, own QSH states with band gap
as large as 0.260, 0.304 and 0.843 eV, respectively, making them suitable for
room-temperature applications. The topological characteristics are confirmed by
s-px,y band inversion, topological invariant Z2, and the topologically
protected edge states. Noticeably, for GaBi/InBi-(CH3)2 films, the s-px,y band
inversion occurred in the progress of spin-orbital coupling (SOC), while for
TlBi(CH3)2 film, the s-px,y band inversion happened in the progress of chemical
bonding. Significantly, the QSH states in III-Bi-(CH3)2 films are robust
against the mechanical strains and various methyl coverages, making these films
particularly flexible to substrate choice for device applications. Besides, the
h-BN substrate is an ideal substrate for III-Bi-(CH3)2 films to realize large
gap nontrivial topological states.. These findings demonstrate that the
methyl-functionalized III-Bi films may be good QSH effect platforms for
topological electronic devices design and fabrication in spintronics.
