  Two-dimensional dumbbell (DB) stanene has been proposed as a promising
candidate material for realizing quantum spin Hall effect (QSHE) by Tang et al
[P. Tang, P. Chen, W. Cao, H. Huang, S. Cahangirov, L. Xian, Y. Xu, S. C.
Zhang, W. Duan, A. Rubio. Phys. Rev. B, 90, 121408 (2014)]. However, the small
bulk-gap limits its possible applications at room temperature. Based on
first-principles calculations, we predict that its band gap can be enhanced to
148 meV under methyl-functionalization, which can be further tuned by applying
lattice strain. The QSHE is confirmed by s-px,y band inversion, topological
invariant Z2 = 1, and helical gapless edge within bulk band gap. Notably, the
characteristic properties of edge states, such as the large Fermi velocity and
Dirac cone, can be modulated by edge modification. The effects of substrates on
topological properties are explored when it is grown on various substrates,
like SiC, h-BN, and Bi2Te3 sheets. These findings provide significant guidance
for future fabrication and realistic applications of QSHE based on stanene in
spintronics.
