808 nm excited Nd3+ doped upconversion nanoparticles (UCNPs) have become a new research hotspot due to their reduced thermal effect when applied in a water-rich system. Different from Tm3+-mediated energy migration upconversion of Tb3+, in this work, NaYF4:Yb/Tb@NaYF4:Yb@NaNdF4:Yb@NaYF4 core–shell–shell–shell nanoparticles were designed based on the Yb3+ sensitized cooperative energy-transfer process to realize photon upconversion. NaYF4:Yb served as the migration layer, separating the activator Tb3+ from the sensitizer Nd3+ and minimizing the deleterious cross-relaxation between them. At 40% Yb3+ doping concentration in the NaYF4:Yb layer, bright green luminescence was achieved under 808 nm excitation. Meanwhile, the presence of Yb3+ in the sensitization layer (NaNdF4:Yb) reduced Nd3+ cross-relaxation, leading to improved energy migration efficiency of Nd3+ → Yb3+. Finally, the incorporation of an inert NaYF4 shell resulted in a significant luminescence boost. These nanoparticles can be dispersed in water after surface modification and could be effectively excited after laser penetration of 3 cm of water, indicating the enormous potential of the designed UCNPs in the biological system.
Lead halide perovskites have been very promising for versatile optoelectronic applications, whereas the inherent toxicity and instability of lead halide restrict its wide application. Herein, a kind of high-performance multifunctional phosphor is designed based on heavy concentration Er 3+ -activated lead-free double perovskite Cs 2 NaBiCl 6 (CNBC), which realizes the self-sensitization of Er 3+ ions and emits high-brightness pure green emission upon dual near-infrared (dual-NIR) excitation. The luminescence properties can be further optimized by adding sensitizers (Yb 3+ /Nd 3+ ). Interestingly, the emission intensity of the green region is heightened by 212-fold after Yb 3+ ions doping, which is 40 times higher than that of the current popular phosphors (α-NaYF 4 :2%Er 3+ ,18%Yb 3+ ). Meanwhile, the temperature sensing properties of the thermally coupled levels (Er 3+ : 2 H 11/2 , 4 S 3/2 ) in Cs 2 NaBiCl 6 :40%Er 3+ are also investigated systematically based on the luminescence intensity ratio principle, and the maximum relative sensitivity is calculated to be as high as 1.27% K -1 (980 nm) and 1.57% K -1 (808 nm). Fortunately, the thermal stability and the performance for temperature readout of the studied phosphors are still maintaining a high level after doping sensitizers. The insights provided by this work will broaden the scope of lead-free halide double perovskites in the fields of luminescence and thermometry.
Up conversion luminescent materials Sr 2 YTaO 6 : Er 3+ , Yb 3+ phosphors were prepared by high temperature calcination . In view of the formation, temperature characteristics , and photoluminescence features of the sample, X-ray diffraction and photoluminescence were used. Under the excitation at 980 nm, the series of samples is significantly stimulated by Er 3+ up-conversion luminescence and Yb 3+ sensitization at 524, 550 and 659 nm. According to the diversity of temperature sensing performance from these three highest peaks, a dual-mode temperature sensor on account of fluorescence intensity ratio (thermally coupled level between 2 H 11/2 and 4 S 3/2 , and nonthermally coupled energy levels between 2 H 11/2 and 4 F 9/2 ) is achieved by using a single luminescent center. Moreover, the phosphor has strong thermal stability, which is helpful to improve the accuracy of temperature measurement. Under the temperature measurement of a dual-mode temperature sensor, it is calculated that the optimal absolute sensitivity S a of thermally coupled energy level is 0.78 × 10 −3 K −1 (473 K) and relative sensitivity S r is 1.32 %K −1 (293 K), and the maximum optimal absolute sensitivity S a of non-thermally coupled energy level is 1.02 × 10 −4 K −1 (473 K) and relative sensitivity S r is 0.58 %K −1 (293 K). All results imply that Sr 2 YTaO 6 : Er 3+ , Yb 3+ phosphors are promising materials for self-referenced optical temperature measurement.
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