Effect of crystal structure and dopant concentration on the luminescence of Cr³⁺ in Al₂O₃ nanocrystals

The sol-emulsion-gel method is used for the preparation of Cr ³⁺-doped Al₂O₃ nanocrystals. The luminescence spectra of Cr³⁺ ions in Al₂O₃ nanocrystals are found to be sensitive to the crystal structure and dopant concentration. It is found that a broad band emission in the range of 680-720 nm and the R-lines (R₂, 692.9 and R₁, 694.3 nm) are obtained at 900 °C (θ-Al₂O₃) and 1025 °C (α-Al ₂O₃) heat-treated 0.2 mol% Cr-doped Al₂O ₃ samples respectively. However, for higher concentrations of chromium (2 mol%) R-lines are obtained at 900 °C annealed samples and these lines are broadened and red shifted at 1025 °C annealed sample which is due to Cr-pair (and/or cluster) formation. The analysis suggests that luminescence properties of Cr³⁺ ions are strongly dependent on concentration and crystal phase of the Al₂O₃ nanocrystals.