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Luminescence thermometry has recently been gaining popularity in science and technology fields. However, most reported works have used the fluorescence intensity ratio (FIR) to obtain the temperature based on thermally coupled energy levels (TCLs), which have hindered the sensitivity of the technique and limited its applications. Here, NaYF4:Yb/Tm@NaYF4:Yb/Ho microcrystals were prepared using a hydrothermal method. Based on the non-thermally coupled energy levels (NTCLs) of the double luminescent ionic centers Tm3+ and Ho3+, the materials exhibited temperature-dependent luminescence in the range of 304 K to 574 K. The maximum sensitivities were found to be Sa = 0.135 K−1 at 574 K and Sr = 1.101% K−1 at 484 K based on red (Tm3+) and green (Ho3+) emission bands. This study has provided a promising new strategy to break the TCL restriction and has shown the material to have potential applications in luminescence thermometry. [ABSTRACT FROM AUTHOR]

Multicolor luminescence and optical thermometry hold charming potential applications in many fields, which are likely to be achieved in one rationally designed unit. Herein, a triple-wavelength (980 nm, 1550 nm and 808 nm) responsive upconversion microcrystal, using Er3+ ions as sensitizers, is proposed, which can generate tunable emission color from red to yellow and then to green. The modulated population route and energy transfer process of Er3+ are perceived to be the reason for tunable luminescence color upon excitation-wavelength. In addition, based on thermally coupled energy levels (TCLs), i.e., 2H 11/2 /4S 3/2 and non-thermally coupled energy levels (NTCLs), i.e., 2H 11/2 /4F 9/2 of Er3+ ions, the temperature-dependent upconversion properties were investigated. The maximum relative sensing sensitivity (S r) of NaYF 4 :0.5%Tm/20%Er was found to be 1.373 % K−1 at 313 K, outperforming the majority of the same type thermometers. The results indicate that the NaYF 4 :0.5%Tm/20%Er microcrystals serve as a promising candidate for multicolor anticounterfeiting and optical thermometry applications. • Multicolor emission can be obtained from 0.5%Tm20%Er sample. • Based on triple-wavelength responsive, novel information encryption strategies are proposed. • 0.5%Tm20%Er microcrystals possess the maximum relative sensitivity of 1.373%K−1 at 313K under 980 nm excitation. [ABSTRACT FROM AUTHOR]

Manipulating color with lanthanide doped upconversion luminescence materials is an excellent approach for anti-counterfeiting, thanks to their abundant energy levels and convenient NIR-to-VIS luminescence property. However, most of the materials are limited by the single-color emission and low photoluminescence efficiency. Herein, pumping-dependent multicolor upconversion luminescence core-shell microcrystals have been obtained through a facile hydrothermal method. In comparison to nanomaterials, core-shell microcrystals possess a smaller surface area-to-volume ratio and fewer surface defects, leading to enhancing the luminescence. A naked-eye recognizable color can be switched from green to blue with increasing the laser power. Furthermore, the mechanism of tunable color with laser power varying has been systematically studied. Due to emission colors of the core-shell materials with excitation power sensitivity and being distinguished by naked-eye easily, core-shell microcrystals reveal promising applications for anti-counterfeiting. • Core-shell microcrystals have the characteristics of pump dependence and highly efficient luminescence. • A naked-eye recognizable color can be switched from green to blue with increasing the laser power. • Core-shell microcrystals can be used as anti-counterfeiting application through tuning the color. [ABSTRACT FROM AUTHOR]