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 0≤x≤0.30≤x≤0.3 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.