Spin-polarized flatband and second-order topological phases in fluorinated C3N

Flat band materials provide a fascinating platform for investigating phenomena such as strong correlations, magnetism, topology, and superconductivity. In this study, we introduce fluorination of C3N as an innovative strategy to create a monolayer that features a quasiflat band near the Fermi level. The absorption energy, molecular dynamics simulations, and calculations of mechanical constants indicate that it is energetically, dynamically, and mechanically stable. Additionally, the orbital-projected energy band structure analysis shows that the quasiflat band predominantly arises from the unfluorinated carbon atoms. Further calculations reveal that its quasiflat band is partially occupied when spin freedom is disregarded. According to the Stoner criterion, spin splitting and ferromagnetism would be induced if spin freedom is considered. Subsequent calculations indicate that the spin-up channel hosts a nontrivial second-order topological phase with spin-polarized corner states existing in the zero-dimensional nanodisk. It is suggested that the spin-polarized quasiflat band can be effectively preserved around the Fermi level when placed on the h-BN monolayer. Taking into account the experimentally synthesized C3N and experimentally achievable fluorination for 2D materials, our work introduces a material for investigating the fascinating properties of flat bands and spin-polarized second-order topological states.