Structural, optical, photoluminescence properties and Ab initio calculations of new Zn2SiO4/ZnO composite for white light emitting diodes

Abstract

Zn2SiO4/ZnO composite have been prepared via simple solid-state chemical reaction. The obtained sample was investigated by X-ray powder diffraction (XRD) using Full prof program refinement, which confirmed the presence of two phases: the willemite (Zn2SiO4) and the zincite (ZnO). Willemite is a trigonal hexagonal phase with R3‾ space group and the lattice parameters a = b = 13.9387 (Å), c = 9.3088 (Å), α = β = 90° and γ = 120°. Then, the reliability factor was χ2 = 2.02. FTIR and Raman analysis confirmed the formation of both ZnO and Zn2SiO4 by the presence of their characteristic vibration modes. Furthermore, UV–visible measurements showed that the composite exhibited a strong absorbance in UV region, and displayed two gap energies (Eg = 3.02 eV and Eg = 5.5 eV) attributed to ZnO and Zn2SiO4, respectively. Moreover, the composite showed a broadening of visible range emissions compared to both Zn2SiO4 and ZnO. The room temperature photo-luminescence spectrum showed white light emissions at 438 and 533 nm attributed to Zn2SiO4 phase, besides those of ZnO. This makes the new Zn2SiO4/ZnO composite promising to be used in white light emitting diodes. Then, the Ab initio calculations were used to calculate the total and partial density of states as a function of the ZnO and Zn2SiO4 energies. The gap energies calculated using the generalized gradient approximation (GGA) are 0.75 and 3.48 eV for ZnO and Zn2SiO4, respectively. Furthermore, the obtained gap values close to Fermi level (EF) demonstrated the semiconductor character. The Beck-Johnson modified exchange potential approximation (mBJ) is used to improve or to correct the result obtained by GGA approximation. In fact, the obtained gap values matched well with the experimental ones: Ea = 2.87 eV and Ea = 5.53 eV for ZnO and Zn2SiO4, respectively.