Luminescent properties and X-ray photoelectron spectroscopy study of ZnAl₂O₄:Ce³⁺,Tb³⁺ phosphor

Cerium (Ce³⁺)- and terbium (Tb³⁺) co-doped zinc aluminate (ZnAl₂O₄) nanocrystals were successfully prepared by a combustion method using urea as fuel. The as-prepared samples were annealed in hydrogen atmosphere to improve their optical properties and crystallinity. The structure and morphology of the samples analyzed using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) showed that ZnAl₂O₄ crystallized in a well known cubic spinel structure. As deduced from X-ray photoelectron spectroscopy (XPS) data, there was partial inversion of cations in as-prepared (unannealed) samples implying that the ZnAl₂O₄ did not crystallize in a cubic normal spinel. Instead, the XPS data demonstrated possible structural readjustment from inverse to normal spinel after annealing. The excitation and emission data collected when ZnAl₂O₄:Ce ³⁺,Tb³⁺ samples, with different concentrations of Ce ³⁺ and Tb³⁺ ions, were excited at different wavelengths showed that green emission of Tb³⁺ was sensitized by Ce³⁺, i.e. there was energy transfer from Ce³⁺ to Tb³⁺ resulting in improvement of green emission from Tb³⁺. This study therefore, sets out to discuss the sensitizing effect of Ce³⁺ and the effect of annealing on the structure of ZnAl₂O₄:Ce ³⁺,Tb³⁺.