Robust large gap quantum spin Hall insulators in methyl and ethynyl functionalized TlSb buckled honeycombs

A large bandgap is critical for the applications of quantum spin Hall (QSH) insulators at room temperature. Based on the first-principles calculations, we predict that the methyl and ethynyl functionalized TlSb monolayers, namely, TlSb(CH3)2 and TlSb(C2H)2 films, own QSH states with large bandgaps of 0.13 and 0.272 eV, which possess potential applications at room temperature. For TlSb(CH3)2, the QSH phase arises from the spin-orbit coupling (SOC) induced s-p band inversion, while for TlSb(C2H)2, the QSH phase results from the SOC induced p-p bandgap opening. The QSH effect is further characterized by the Z2 topological invariant and topologically protected edge states. Significantly, the QSH states in TlSb(CH3)2 and TlSb(C2H)2 films are robust against external strain and various methyl/ethynyl coverages, making them especially flexible in the substrate selection. Besides, we find that h-BN is an ideal substrate for TlSb(CH3)2 and TlSb(C2H)2 films to keep QSH states with large bandgaps. Thus, the methyl an...