Upconversion luminescence enhancement and temperature sensing behavior of F− co-doped Ba3Lu4O9:Er3+/Yb3+ phosphors

Abstract

A series of Ba₃Lu₄O₉:Er³⁺/Yb³⁺ (EYBLO) phosphors co-doped with F⁻ ions were synthesized by a simple solid-state reaction method. The results showed that the primary rhombohedral structure was maintained and the crystal lattice began to shrink when F⁻ ions were introduced into the host matrix to occupy the O²⁻ site. The agglomerations and the impurities (OH⁻ and CO₂) with higher phonon energy on the sample surface could be minimized and the sample crystallinity could be improved. Under 980 nm laser diode excitation, the green and red UC emissions of the EYBLO:0.4F⁻ sample show nearly 5- and 7.5-fold enhancements in contrast to F⁻-free EYBLO. The upconversion luminescence can be finely tuned from yellow to red light to some extent by increasing F⁻ concentration. Based on pump-power dependence and decay lifetime analysis, the energy level diagram was illustrated and the upconversion energy-transfer mechanism was discussed. The green and red emission enhancements are attributed to the modification of the local crystal field of Er³⁺ ions and reduction of crystal site symmetry. The cross-relaxation and back-energy-transfer processes play an important role to enhance the red/green UC emission intensity ratios. The fluorescence intensity ratio technique was employed to investigate the temperature sensing behavior of the synthesized phosphors. The temperature sensing properties can be enhanced by doping of F⁻ ions, and the maximum sensitivity is found to be 44.57 × 10⁻⁴ K⁻¹ at 523 K. It is promising to provide an alternative approach for enhancing UC luminescence and the temperature sensitivity in oxide matrixes and then obtain high-quality optical temperature-sensing materials by simply co-doping F⁻ ions.