Lattice structures of the (a) monolayer, (b) bilayer, and (c) T-bilayer, and the band structures of the (d) monolayer, (e) bilayer, and (f) T-bilayer. The monolayer structure has a direct bandgap of ∼2.5 eV at the Γ point. Both types of bilayers have indirect bandgaps, which are ∼1.0 eV and ∼1.9 eV, respectively. The VBM and CBM of the T-bilayer structure are located around the Γ point. However, the CBM of the bilayer structure around the M point is distinctly different from that in the other two cases.