Effects of Cr³⁺ mol% on the structure and optical properties of the ZnAl₂O₄:Cr³⁺ nanocrystals synthesized using sol-gel process

Zinc aluminate (ZnAl₂O₄) hosts and ZnAl₂O₄:x% Cr³⁺ doped were successfully prepared at a relatively low temperature (~80 °C) using the sol-gel method. The dopant (Cr³⁺) concentrations were varied in the range of x=0-0.03 mol%. The X-ray diffraction (XRD) data revealed that all annealed (at 800 °C) samples consist of the cubic ZnAl₂O₄ structure. The estimated crystallites sizes were in the range of 22-24 nm in diameter. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis confirmed the presence of all expected ions. The surface morphology of the phosphors was influenced by the Cr³⁺ mol%. TEM confirmed that the prepared powder particles are in nanoscale. Ultraviolet-visible spectroscopy (UV-vis) results showed that the Cr³⁺ mol% affects the band gap of the host. The photoluminescence (PL) results showed that the host and the Cr³⁺-doped nanoparticles exhibit violet emission slightly at different peak positions. Slight peak shifts suggests that the luminescence can originate from both the host and Cr³⁺ ion. Emission from the host is attributed to the band-gap defects in the host material, while the emission from the Cr³⁺ is attributed to the ⁴T₁→⁴A₂ transition. At the higher mol% there is an emission at 692 nm, which is attributed to the ²E→⁴A₂ transition in Cr³⁺. The PL and XRD results showed that Cr³⁺ can occupy multiple-sites in the host matrix. The maximum PL intensity was observed from the sample doped with 0.01 mol% of Cr³⁺. The critical distance between Cr³⁺ ions for energy transfer was determined to be 23 Å.