Effects of Pb²⁺ ions concentration on the structure and PL intensity of Pb-doped ZnAl₂O₄ nanocrystals synthesized using sol-gel process

Undoped and Pb²⁺-doped ultrafine cubic zinc aluminate (ZnAl ₂O₄) hosts were successfully prepared at a relatively low temperature (~80 °C) using the sol-gel method. The concentration of Pb ²⁺ was varied from 0 to 5 mol%. The TGA showed that the minimum annealing temperature required to obtain single phase ZnAl₂O ₄ must be above 400°C. The XRD data revealed that all the annealed samples were single phase crystalline structures and the estimated crystallites size were in the range of 21-30 nm in diameter. The FTIR results suggest that heat-treating can destroy some of the bonds. The surface morphology of the phosphors was influenced by the Pb²⁺ mol%. Undoped and Pb²⁺-doped ZnAl₂O₄ nanoparticles exhibit the violet emission at slightly different positions. The slight peak shifts suggests the possibilities that the luminescence centre can either be due to the defects level in the host or Pb²⁺ ions. The emission peaks at 390 and 399 nm are ascribed to the typical UV transitions ³P_0,1 → ¹S₀ in Pb²⁺ ion. At the higher Pb²⁺ mol%, the luminescence quenching behaviour occurs, which suggests that doping with Pb²⁺ ions is accompanied by the introduction of new defect sites that enhance non-radiative recombination of the excited electrons.