Giant gap quantum spin Hall effect and valley-polarized quantum anomalous Hall effect in cyanided bismuth bilayers

Bismuth (Bi) has attracted a great deal of attention for its strongest spin-orbit coupling (SOC) strength among main group elements. Although quantum anomalous Hall (QAH) state is predicted in half-hydrogenated Bi honeycomb monolayers Bi₂H, the experimental results are still missing. Halogen atoms (X = F, Cl and Br) were also frequently used as modifications, but Bi₂X films show a frustrating metallic character that masks the QAH effects. Here, first-principle calculations are performed to predict the full-cyanided bismuthene (Bi₂(CN)₂) as 2D topological insulator supporting quantum spin Hall state with a record large gap up to 1.10 eV, and more importantly, half-cyanogen saturated bismuthene (Bi₂(CN)) as a Chern insulator supporting a valley-polarized QAH state, with a Curie temperature to be 164 K, as well as a large gap reaching 0.348 eV which could be further tuned by bi-axial strain and SOC strength. Our findings provide an appropriate and flexible material family candidate for spintronic and valleytronic devices.