Large-gap Quantum Spin Hall Insulators in Chemically Functionalized Bi(110) Bilayers*

For applications of quantum spin Hall (QSH) insulators, the materials need to possess large energy gaps, and they should maintain their non-trivial topological properties when fabricated on substrates. Many QSH insulators have been theoretically proposed, but only a few of them have been experimentally confirmed. Based on first-principles calculations we find that the Bi(110) bilayer, which is metallic in its pristine form, can transform into QSH insulators through hydrogenation and halogenation. Their energy gaps are among the largest, ranging from 0.45 to 0.93 eV, for QSH insulators. A p_z-σ bonding-antibonding splitting mechanism and a charge-transfer mechanism are proposed to understand their electronic structures. These topologically insulating films exhibit excellent dynamical and thermal stability, and could robustly maintain their topological properties against large strains and substrate effects. On MoSe₂ and black phosphorus substrates, the functionalized Bi films form nearly freestanding QSH insulators and show large global energy gaps enough for room temperature utilizations.