The real part of the dielectric function ( 1 (w)) is related to the energy stored by the medium when a material is exposed to an electromagnetic spectrum. The optical spectra in 1 ( ) (Fig. ) for FBNF mainly arise approximately in between 4.6 eV to 15.2 eV with the early sharpest peaks at 8.79 eV and 12.55 eV in E‖x. Meanwhile, such sharpest peaks occur at 7.71 eV and 11.59 eV in E‖z, significantly different from h-BN. The value of 1 ( ) is positive throughout the whole energy range in the in-plane direction but a negative value is detected in between the 11.95 eV to 12.45 eV for out-of-plane direction in FBNF monolayer which is unprecedented in h-BN. The corresponding negative frequency is known as plasma frequency ( ), which is very important to understand many exotic optical phenomena . The static dielectric function ( 1 (0)), the value of 1 ( ) at zero photon energy, is 1.5 (≈ h-BN) for inplane direction but higher (1.65) in out-of-plane direction.