Influence of Composition, Doping Concentration and Annealing Temperatures on Optical Properties of Eu³⁺-Doped ZnO–SiO₂ Nanocomposites

We report the influence of Eu³⁺ doping concentration, ZnO content as well as annealing temperature on optical properties and micro structures of ZnO–SiO₂ nanocomposites. Thin layers of ZnO–SiO₂ nanocomposites with various compositions doped with different Eu³⁺ concentrations were prepared by sol–gel method and spin-coating process. The structure and morphology of the thin films as a function of annealing temperature were studied and analyzed by X-ray diffraction (XDR) and scanning electron microscope (SEM). Upon annealing temperatures above 1000 °C, we observed a coexistence of hexagonal structure of ZnO and willemite structure of Zn₂SiO₄. The Eu³⁺-related photoluminescence (PL) spectra vary with doping concentration, ZnO composition and the heat treatment process. The domination of emission peak at 613 nm assigned for ⁵D₀-⁷F₂ transition of Eu³⁺ ions indicates a majority of optically active Eu³⁺ ions locating at low symmetry sites in the ZnO–SiO₂ hosting materials in the thin films. Energy transfer processes upon photon absorptions within ZnO nanocrystals and Eu³⁺ ions are investigated via photoluminescence excitation spectra in the range of excitation wavelengths from 250 to 450 nm. The decrease of Eu³⁺-related PL intensity happens in coincidence with the formation of the willemite structure of Zn₂SiO₄ at higher annealing temperatures.