Spectroscopic ellipsometry thin film and first-principles calculations of electronic and linear optical properties of [(C9H19NH3)2PbI2Br2] 2D perovskite

In this study we report results of first-principles density functional calculations using the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA) for the exchange-correlation energy to calculate electronic and linear optical properties of the (C9H19NH3)(2)PbI2Br2 compound. The linear optical properties, namely, the real epsilon(1)(omega) and imaginary epsilon(2) (omega) parts of dielectric function, the refractive index n (omega) and the extinction coefficient k (omega)) are calculated and compared with experimental spectroscopic ellipsometry spectra. The reflectivity R(omega) and electron energy loss function L (omega) are calculated too. Our calculations performed for band structure and density of states show that the valence band maximum and conduction band minimum are located at Gamma point resulting in a direct band gap of about (Gamma v, Gamma c) of 2.42 eV in good agreement with the experimental data. The investigated compound has a large uniaxial anisotropy of the dielectric function of about 0.0739 and a negative birefringence at zero energy Delta n(0) =-0.11.