A two-dimensional robust topological insulator with coexisting ferroelectric and valley polarization

Abstract

The quantum spin Hall effect, the valley Hall effect and ferroelectricity are all extensively studied yet distinct properties of insulators and it would be very interesting to see if they could coexist in certain two-dimensional (2D) materials. We present a family of fluorinated methyl-functionalized bismuthene (Bi₂C₂H_6−xF_x) films to achieve the 2D ferroelectric valley topological insulator (FEVTI) state, due to strong spin–orbit coupling (SOC) and non-centrosymmetric layer structures. Bi₂C₂H_6−xF_x films have topological non-trivial band gaps up to 1.08 eV, which are robust against the coverage and distribution of fluorine atoms, and the strongest out-of-plane ferroelectric polarization is up to 0.25 × 10⁻¹⁰ C m⁻¹. Moreover, some Bi₂C₂H_6−xF_x configurations exhibit an extra valley degree of freedom by the valley splitting at the K and K′ points in the Brillouin zone due to the symmetry breaking, resulting in the coexistence of quantum spin and valley Hall states. These phenomena could also be found in other similarly decorated 2D materials, therefore offering unique quantum material platforms for discovering novel physics and exploring innovative applications.