Symmetry-breaking-induced ferroelectric HfSnX3 monolayers and their tunable Janus structures: promising candidates for photocatalysts and nanoelectronics

Designing novel two-dimensional (2D) ferroelectric materials by symmetry breaking and studying their mechanisms play important roles in the discovery of new ferroelectric photocatalysts and nanoelectronics. In this study, we have systematically investigated a series of novel ferroelectric 2D HfSnX3 (X = S, Se and Te) monolayers through first-principles calculations. We found that each HfSnX3 monolayer contains a stable ferroelectric phase (FP) and a paraelectric phase (PP). The large polarization (up to 1.64 & mu;C cm(-2)) in the FP can significantly bend the oxidation reduction potential of water, making HfSnX3 monolayers become excellent ferroelectric photocatalysts. Specifically, by designing a Janus structure to break the symmetry of the PP, we have excitingly obtained a stable Hf2GeSnSe6 (referred to as HGSS) monolayer with triple polarized states. HGSS not only possesses great visible light absorption properties (about 3 x 10(5) cm(-1)) as photocatalysts but also successfully solves the dead layer problem previously reported in practical applications. In addition, by constructing a heterostructure with graphene, HGSS has great application in the design of controllable ultrathin p-n junctions. Overall, our study not only predicts a series of potential ferroelectric photocatalytic materials, but also provides valuable insights for designing tunable polarized materials and nanoelectronics.