Robust large-gap quantum spin Hall insulators in methyl-functionalized III-Bi buckled honeycombs

Based on first-principles calculations, we predict that the methyl-functionalized III-Bi monolayers, namely III-Bi-$({\mathrm{CH}}_{3}){}_{2}$ (III = Ga, In, Tl) films, own quantum spin hall (QSH) states with band gaps 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 $s\ensuremath{-}{p}_{x,y}$ band inversion, topological invariant ${Z}_{2}$, and topologically protected edge states. Noticeably, for GaBi/InBi-$({\mathrm{CH}}_{3}){}_{2}$, the $s\ensuremath{-}{p}_{x,y}$ band inversion occurred in the process of spin-orbital coupling (SOC), while for $\mathrm{TlBi}({\mathrm{CH}}_{3}){}_{2}$, the $s\ensuremath{-}{p}_{x,y}$ band inversion happened in the process of chemical bonding. Significantly, the nontrivial topological states in III-Bi-$({\mathrm{CH}}_{3}){}_{2}$ films are robust against the mechanical strain and various methyl coverage, making them particularly flexible to substrate choice for device applications. Besides, we find the $h$-BN is an ideal substrate for III-Bi-$({\mathrm{CH}}_{3}){}_{2}$ 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.