Coexistence of quantum spin Hall effect and intrinsic piezoelectricity in I-doped monolayer Bi4Br4

The quantum spin Hall insulator with intrinsic piezoelectric response has attracted much attention due to its potential applications in topological electronic states and piezoelectric electric coupling fields. Motivated by the comparable chemical properties of Br and I, we construct Janus Bi4BrxI4−x (x = 1, 2, 3) monolayers by tuning the I concentration and systematically investigate their electronic, topological, and piezoelectric properties. First-principles calculations demonstrate that all three Janus structures are dynamically stable, wide-bandgap quantum spin Hall insulators, with nontrivial bandgaps of ∼0.26 eV. Density functional perturbation theory calculations confirm that non-centrosymmetric Bi4Br3I1 and Bi4Br1I3 possess significant in-plane piezoelectric effects, with Bi4Br3I1 exhibiting a notably large piezoelectric strain coefficient of |d11| = 7.092 pm/V. Notably, Bi4BrxI4−x (x = 1, 2, 3) structures have already been synthesized and their properties experimentally verified, underscoring their practical feasibility. These findings establish Janus Bi4BrxI4−x (x = 1, 2, 3) monolayers as a promising platform, enabling the coexistence of nontrivial topological states and strong piezoelectricity, paving the way for next-generation multifunctional quantum devices.