Luminescent and structural properties of manganese-doped zinc aluminate spinel nanocrystals

Manganese-doped zinc aluminate spinel (ZnAl₂O₄:Mn; Mn=0–6.0 mol%) phosphor nanoparticles were prepared by the sol–gel process. The effects of thermal annealing and dopant concentration on the structure, microstructure and luminescence of the powder phosphors were investigated. The X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) results confirmed that a single-phase spinel started to crystallize at around 600 °C for the investigated powders. On heating at 600–1200 °C, the powders had the average crystallite sizes of around 12–33 nm. The crystallite size and lattice constant increased as the doping level of Mn increased. FT-IR spectra exhibited only absorption bands of the AlO₆ octahedral groups, which suggested that the powder phosphors mainly crystallized in a normal spinel structure. Scanning electron microscopy (SEM) investigations showed the primary particle sizes were around 20–25 nm for the powders annealed at 1000 °C, and less than ca. 50 nm for those annealed at 1200 °C. Photoluminescence (PL) spectra under UV or visible light excitation exhibited a strong green emission band centered at 510 nm, corresponding to the typical ⁴T₁(⁴G)—⁶A₁(⁶S) transition of tetrahedral Mn²⁺ ions. The most intense PL emission was obtained by exciting at 458 nm. The PL intensity was significantly enhanced by the improved crystallinity and diminished OH^? groups. Optimum brightness occurred at a doping of 3.0 mol% Mn.