Fig. 2: Optical dielectric constant and crystal structure of Bi2Se2.4 thin film.

a The increase in εopt and αe caused by the transition from amorphous to crystalline, where the inset is a schematic diagram of the transformation of saturated bonds to unsaturated bonds. b The XRD spectrum, c Raman spectrum, and d HRTEM image of crystalline Bi2Se2.4 thin film