First- Principle Calculation of the Electronic and Optical Properties of Nanolayered ZnO Polymorphs by PBE and mBJ Density Functionals

First principle calculations of nanolayered ZnO polymorphs (Wurzite–, Zinc blende–, Rocksalt–structures) in the scheme of density functional theory were performed with the help of full potential linear augmented plane wave (FP-LAPW) method. The exchange - correlation potential is described by generalized gradient approximation as proposed by Perdew–Burke–Ernzrhof (GGA–PBE) and modified Becke–Johnson (mBJ) approximation. The electronic behavior and the optical properties of the structures are investigated and compared to experimental data, where available. The electronic structure of w–ZnO and z–ZnO revealed a 3.01 eV and 2.59 eV direct energy gap in (Γ→Γ) direction by applying mBJ potential. In contrast to w– and z–ZnO the electronic structure of r–ZnO shows an indirect 2.81 eV energy gap in (Γ→L) direction. Reflectivity, transmittance and refractive index spectra for three nano layered of ZnO phases in Uv – visible region have been calculated. The electron effective mass values at the bottom of conduction band were evaluated for the three geometries.