Emissions of Er³⁺ and Yb³⁺ co-doped SrZrO₃ nanocrystals under near-infrared and near-ultraviolet excitations

In this study, the upconversion (UC) emissions of Er³⁺ and Yb³⁺ co-doped SrZrO₃ nanocrystals (NCs) were investigated in terms of the thermal annealing temperature and concentration of Er³⁺ ions and compared with the emissions under a near-ultraviolet (near-UV) excitation. The NCs were synthesized by the combustion method, and the as-synthesized NCs were post-annealed at high temperatures. The X-ray diffraction patterns revealed that the grain sizes and crystallinity degrees of the samples increased with increasing annealing temperatures. The photoluminescence spectra of our samples exhibited strong green and very weak red emissions with the near-UV excitation, originating from the f-f transitions in the Er³⁺ ions. Interestingly, under near-infrared (near-IR) excitation, we identified sizable visible emissions at 525, 547, and 660 nm in our NCs, which indicated that the UC process successfully occurred in our NCs. These UC emissions were maximized in the NCs with an Er³⁺ concentration of 0.02 and thermal annealing at 1000 ℃. We found that the intensity ratios of red to green emissions increased with increasing annealing temperatures. We discussed the differences in the emissions between near-UV and near-IR excitations.