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The ultrasmall Yb/Er: CaF 2 NCs are chosen as an example to certify the upconversion emission intensity can be greatly enhanced through combining various approaches together, including doping Na + ions to promote the growth the particle size and tailor the crystal-field environment of the emitters, control shell thickness to lower the non-radiative decay and tune Yb 3+ concentration in the shell to enhance the absorption of the excitation energy. In comparison with Na + -free 20Yb/2Er: CaF 2 NCs (~ 2 nm), the upconversion luminescence of 20Yb/2Er: CaF 2 @10Yb: CaF 2 NCs (~ 9 nm) containing ~ 0.5 mmol Na + ions in the core and 10% Yb 3+ in the shell with 5 nm thickness intensifies for about 95 times. [ABSTRACT FROM AUTHOR]

Negative thermal quenching effect (TQE) refers to a phenomenon that the luminescence intensity increases as the temperature increases, which is reported in a few rare-earth doped upconversion (UC) nanocrystals (NCs). However, it is still difficult to enhance the negative TQE for those NCs without changing the particle size. Herein, through comparing the temperature dependent UC emission intensity variations between the Yb/Er co-doped CaF 2 and BaF 2 NCs as well as the CaF 2 :Yb/Er and BaF 2 :Yb/Er based core/shell NCs with different shell layers, we verify that the negative TQE can be improved by employing the matrix with lower phonon energy owing to the reduced possibility of the multi-phonon assisted non-radiative relaxation. This novel method for the improvement the negative TQE may found potential applications in anti-counterfeiting and temperature sensing fields. • Yb/Er co-doped CaF 2 and BaF 2 NCs with similar particle size is prepared. • CaF 2 :Yb/Er and BaF 2 :Yb/Er based core/shell NCs with different shells are prepared. • Negative thermal quenching effect are observed in these NCs with similar particle sizes. • Negative thermal quenching effect is evidently modulated by tuning phonon energy. [ABSTRACT FROM AUTHOR]

Lanthanide doped fluoride nanoparticles (NPs) exhibit tunable X-ray excited persistent luminescence (XEPL), which show promising applications in optical imaging, cancer therapy and information storage. Although a few progresses have been achieved in enhancing the XEPL of lanthanide activators, there is no report about the influence of crystallinity on the XEPL intensity. In this work, we verify that the XEPL intensity can be improved by optimizing the NP crystallinity. Our results reveal that better crystallinity benefit the lower non-radiative relaxation possibility but also the restriction of the formation of Frenkel defects. This work provides important information for the development of lanthanide doped fluoride nanoparticles with superior XEPL performances. • The influence of crystallinity on the XEPL intensity was studied for the first time. • Better crystallinity inhibits the formation of Frenkel defects. • It exists optimal crystallinity for stronger XEPL. [ABSTRACT FROM AUTHOR]

The development of multifunctional nanocrystals has attracted growing interest lately in the biological and medical sciences. Herein, the dual-mode luminescence properties and optical thermometric performances of Tm3+, Yb3+: Gd 2 Zr 2 O 7 nanocrystals synthesized by a facile hydrothermal method are investigated systematically. Upon 980 nm excitation, the upconversion (UC) luminescence spectra of the obtained samples consist of blue (485 nm), red (655 and 681 nm), and near-infrared (785 nm) emission bands of Tm3+ ions. Diversely, the X-ray excited luminescence (XEL) spectra contain a strong ultraviolet (UV) emission from Gd3+ ions (314 nm), a weak defect-related emission band (400–600 nm), and several tiny emissions from Tm3+ ions. To realize the optical thermometry in the biological window, the luminescence intensity ratio (LIR) between 3F 2,3 → 3H 6 (681 nm) and 1G 4 →3F 4 (655 nm) transitions is employed for temperature sensing, giving high absolute and relative sensing sensitivities (S a-max = ∼1064 × 10−4 K−1, S r-max = ∼2.32%K−1). Therefore, the present Tm3+, Yb3+: Gd 2 Zr 2 O 7 nanocrystals could be useful in optical thermometry, photodynamic therapy, and X-ray imaging. • Dual-mode luminescence and thermometric properties of Tm3+/Yb3+: Gd 2 Zr 2 O 7 are studied. • UCL spectra consist of blue, red and NIR emissions of Tm3+ upon 980 nm excitation. • UV emission from Gd3+ is predominant under X-ray excitation. • LIR of 681 nm–655 nm shows high sensing sensitivity with S r-max of 2.32% K−1. • Tm3+/Yb3+: Gd 2 Zr 2 O 7 could be used for optical thermometry and photodynamic therapy. [ABSTRACT FROM AUTHOR]

Here, La 2 MgGeO 6 : Bi3+, Er3+ samples were synthesized by the high-temperature solid-phase method, and the up/down-conversion luminescence properties were investigated with the excitation of near-infrared light (980 nm and 1550 nm) and ultraviolet light (322–378 nm). Under the excitation of ultraviolet light, the sample can exhibit the characteristic broadband blue emission (Bi3+ ions) and green emission (Er3+ ions). When excited with near-infrared light, the sample has only typical green and orange up-conversion emission bands. The green and orange luminescence could be achieved by adjusting the excitation wavelength. In addition, the sample can present multicolor luminescence with the change of pump power and temperature. At the same time, the temperature measurement performance of the phosphor was explored by using the fluorescence intensity ratio technique based on the Er3+ ion thermal coupling energy level (2H 11/2 /4S 3/2) and non-thermal coupling energy level (2H 11/2 /4F 9/2). The maximum temperature sensitivity reached 1.23 %K−1 and 0.99 %K−1, respectively. Therefore, La 2 MgGeO 6 : Bi3+, Er3+ samples could be candidates for multicolor anti-counterfeiting and temperature sensors material. • La 2 MgGeO 6 : Bi3+, Er3+ phosphors are used as a dual-mode temperature sensor to discuss the TCEL and NTCEL. • The phosphor can not only be used in optical temperature measurement, but also can achieve anti-counterfeit encryption. • Polychromatic luminescence can be realized by changing excitation wavelength, temperature and other conditions. • Using FIR technology to explore the temperature sensing performance, the phosphor is highly sensitive to temperature. [ABSTRACT FROM AUTHOR]

Trivalent lanthanide ions are widely used for the realization of scintillation, while the narrow transition lines do not benefit the overall integral intensity. In this work, Mn2+ doped LiLuF 4 NPs are employed to realize strong broad emission for the first time. Upon X-ray irradiation, the strong broad emission from 500 to 700 nm corresponding to the Mn2+: 4T 1 →6A 1 transition is observed. Through incorporating Ce3+ and Gd3+ ions, the X-ray excited optical luminescence (XEOL) intensity is greatly improved owing to the introduce of energy transfer pathways of Gd3+→Mn2+ and Ce3+→Gd3+→Mn2+. After coating a LiLuF 4 shell, both the XEOL intensity and X-ray irradiation stability are improved. Our results may promote the exploration of new kinds of scintillators with broad band emissions. • Mn: LiLuF 4 NPs are employed to realize strong broad emission for the first time. • Gd3+ ions are used to improve the XEOL intensity via Gd3+.→Mn2+ energy transfer. • Ce3+ ions are used to improve the XEOL intensity via Ce3+.→Gd3+→Mn2+ energy transfer. • A LiLuF 4 shell is used to improve both the XEOL intensity and X-ray irradiation stability. [ABSTRACT FROM AUTHOR]

Using a high-temperature solid-phase method, a series of La 2 MgGeO 6 : Bi3+, Eu3+ samples with different doping amounts of Bi3+ and Eu3+ were obtained. The crystal structure, morphology, luminescence characteristics, temperature-varying luminescence spectrum, fluorescence attenuation curve, and energy transfer mechanism of the samples were explored. In La 2 MgGeO 6 : Bi3+, Eu3+ phosphors, Bi3+ and Eu3+ ions occupy the La3+ ion position. Under ultraviolet (UV) excitation, the emission spectra of LMGO: Bi3+ and the excitation spectra of LMGO: Eu3+ have significant spectral overlap, indicating that there is energy transfer between Bi3+ and Eu3+. By adjusting the doping amounts of Eu3+, adjustable luminescence can be obtained in the co-doped samples. According to the varying temperature-dependence trends of Bi3+ and Eu3+ ions, fluorescence intensity ratio technology was used to explore the temperature-sensing characteristics of Bi3+ and Eu3+ ions. The maximum absolute sensitivity and relative sensitivity were 0.074%·K−1 and 2.389%·K−1, respectively. In addition, by monitoring the lifetime decay time of Bi3+ with temperature, the maximum temperature sensitivity of the La 2 MgGeO 6 : Bi3+ samples was 4.09%·K−1. The above results indicate that La 2 MgGeO 6 : Bi3+, Eu3+ samples can be used as potential luminescent materials for temperature detection. • La 2 MgGeO 6 : Bi3+, Eu3+ is explored to be a very promising optical thermometric materials. • The LMGO: Bi3+, Eu3+ shows high signal discriminability in the blue and red regions. • The LMGO: Bi3+, Eu3+ phosphors can circumvent the drawbacks of TCL-based phosphors. • The fluorescence intensity ratio and decay life of the material are highly sensitive to temperature. • The thermal-quenching mechanism of Bi3+ and Eu3+ were discussed. [ABSTRACT FROM AUTHOR]

A series of Ca 2- x Sr x MgSi 2 O 7 : Ce3+, Eu2+ (x = 0, 0.5, 1, 1.5, 2) phosphors were synthesized on the basis of high temperature solid-state reaction strategy. Its phase composition, optical spectral properties, fluorescence lifetime and temperature sensitivity were investigated in detail. Under excitation at 342 nm, the emission spectra of Ca 2 MgSi 2 O 7 : Ce3+, Eu2+ exhibit a purple emission band (Ce3+ ions) and a green emission band (Eu2+ ions). The emission band of Ce3+ overlaps with the excitation band of Eu2+, indicating that there may be energy transfer (ET) between the two ions. The decreasing fluorescence lifetime of Ce3+ with increasing Eu2+ concentration further proves the existent of ET process in Ca 2 MgSi 2 O 7 : Ce3+, Eu2+ phosphor. In addition, Ca 2- x Sr x MgSi 2 O 7 : Ce3+, Eu2+ solid solution was designed and synthesized by Sr2+ substituting Ca2+. As Sr2+ gradually replaced Ca2+, the blue-shift of Ce3+ and Eu2+emission band were caused by the change of crystal field. With the increase of temperature, the fluorescence intensity of Ce3+ and Eu2+ emission bands decreased slowly and rapidly, respectively. Hence, in the Ce3+/Eu2+ co-activated Ca 2- x Sr x MgSi 2 O 7 phosphor, Ce3+ ions and Eu2+ ions can be used as fluorescence intensity ratio (FIR) signals. The maximum relative sensitivity can reach 2.43% K−1 (at 293 K) under 342 nm excitation. The above results indicate that Ca 2- x Sr x MgSi 2 O 7 : Ce3+, Eu2+ phosphors are potential candidates for optical thermometer materials. • The Ca 2- x Sr x MgSi 2 O 7 : Ce3+, Eu2+ (x = 0, 0.5, 1, 1.5, 2) phosphors are promising optical thermometric materials. • The emission peaks of Ce3+ and Eu2+ blue-shift due to the change of crystal field caused by Sr2+ replacing Ca2+. • FIR based on dual luminescence center can overcome the disadvantage of fluorescence intensity ratio based on TCL. • The thermal-quenching mechanism of Ce3+ and Eu2+ were discussed. [ABSTRACT FROM AUTHOR]