Novel Janus diamane C₄FCl: a stable and moderate bandgap semiconductor with a huge excitonic effect

Semiconducting two-dimensional Janus materials have drawn increasing attention due to their novel optoelectronic properties. Here, employing first-principles calculations, we systematically explore the stability and electronic and optical properties of Janus diamane C₄FCl. The energetic and dynamical stabilities of C₄FCl have been verified using the cohesive energy and phonon dispersion calculations. It is predicted to possess a direct bandgap of ∼3 eV at the Γ point using the G₀W₀ method. Also, the optical absorption spectrum of C₄FCl is dominated by the enhanced excitonic effects, in which a bright bound exciton with a large binding energy beyond 1 eV can be observed. The light absorption coefficient of C₄FCl for sunlight can be as large as 8 × 10⁴ cm^-1 in the range of visible and near-ultraviolet light, suggesting its potential for optoelectronic applications. These findings enable a deep understanding of the physical properties of novel C₄FCl.