Enhanced downconversion luminescence of NaLuF4:Yb3+,Er3+ micrometer hexagonal prismatic crystals under 980 nm excitation.

• Improvement of downconversion luminescence by adjusting the co-doping ratio of Yb3+ and Er3+ in NaLuF 4 :Yb3+,Er3+ micrometer prismatic crystals. • Defects in the crystals were removed by treating the microcrystals with ion exchange and high-temperature annealing methods to further improve the downconversion luminescence performance. • The crystalline quality and densification of the treated crystals were enhanced, and the diffraction peaks were closer to those of standard β-NaLuF 4. • The downconversion luminescence of the treated sample was significantly enhanced, and its luminescence intensity could reach 2.5 times of that of the untreated sample. NaLuF 4 :Yb3+,Er3+ micrometer prismatic crystals are potential materials for the preparation of optical waveguide devices. However, structural defects and OH– groups are generated inside and on their surfaces during growth processes. The structural defects and OH– groups lead to a large depletion of excitation energy and severely quench the downconversion luminescence of Er3+. In addition, the Yb3+/Er3+ co-doping ratio significantly affects the downconversion luminescence of the microcrystals, which in turn affects the performance of the optical waveguide amplifiers based on the microcrystals. By adjusting the co-doping ratio of Yb3+ and Er3+ in the microcrystals and removing the defects in the crystals, their downconversion luminescence performance was significantly improved. The results showed that the downconversion luminescence under the excitation from a 980 nm laser reached the maximum value when the doping ratio of Yb3+ and Er3+ was 20:1.5. In addition, the OH– groups and structural defects in the micrometer crystals were significantly eliminated by ion-exchange and high-temperature annealing treatments, and the crystalline quality was significantly improved. After ion-exchange and high-temperature annealing treatments, the downconversion luminescence enhancement of NaLuF 4 :Yb3+,Er3+ microcrystals was 2.5 times that of the untreated samples. By changing the Yb3+/Er3+ co-doping ratio in the NaLuF 4 :Yb3+/Er3+ crystals, a clear pattern exists between the down-conversion luminescence and the co-doping ratio of the samples. Meanwhile, the lattice defects and OH– groups in the crystals can be removed by ion exchange and high-temperature annealing methods, which can further enhance the down-conversion luminescence. By observing the scanning electron micrographs before and after the reaction, it can be found that the size and surface morphology of the samples have changed greatly, and the crystalline quality and densification of the crystals of the treated samples have been improved. The XRD diffraction peaks of the treated sample (110) are shifted to a large angle, which is closer to β-NaLuF 4. Meanwhile, the upconversion luminescence, downconversion luminescence, and downconversion energy level lifetimes of the samples before and after the treatment can be found to be significantly changed under the excitation at 980 nm. [Display omitted] [ABSTRACT FROM AUTHOR]