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Abstract
Transparent glass ceramics containing hexagonal NaGdF4: Yb3+, Ho3+ nanocrystals were successfully fabricated via self-crystallization, which was further confirmed by XRD, TEM, HRTEM, and STEM-HADDF, as well as upconversion (UC) emission spectra. Impressively, GC750 exhibited fascinating upconversion luminescence, and the corresponding 5F1/5G6 and 5F2,3/3K8 states of Ho3+ were proven to be thermally coupled energy levels (TCELs), resulting in high temperature-sensitive behaviors based on fluorescence intensity ratio (FIR) for optical thermometry. As a consequence, a high relative sensitivity of 1.43%·K−1 at 390 K was achieved, offering a great potentially application in optical thermometry.
© 2017 Optical Society of America
1. Introduction
Yesterday has witness the bloom of lanthanide doped luminescent materials, attributed to its application in various fields such as solid-state lasers, color display, biolabels, photoactivation, optical fibres, anticounterfeiting, optical thermometry and so on [1–7]. Recently, a specific focus has been on luminescent materials for optical thermometry [8–10]. Luminescent materials, whose characteristic parameters including the absolute emission intensity, the fluorescence intensity ratio (FIR), the lifetime and the peak wavelength could be strongly temperature-sensitive [11–17], is an excellent alternative for non-contact optical thermometry. This method has been demonstrated to be real-time, noninvasive and highly accurate. Hence, it has garnered considerable attention and been expected to replace the traditional temperature sensors based on the thermal expansion of liquid and metal, especially in rigorous environments, e.g., strong magnetic fields, wind tunnel, microfluidic systems and cells with a spatial resolution less than 10 μm.
In particular, FIR-based method plays a key role in the development of optical thermometry. On the basis of two distinguishable emission bands whose temperature-behaviors differ with respect to temperature, the value of FIR varies with temperature, and the change could be used to evaluate temperature. Furthermore, FIR technique is independent of measure conditions, including light scattering, fluctuations of the excitation intensity, reflection and the drifts of the optoelectronic system, as well as the inhomogeneous distribution of the sample. Therefore, temperature evaluation based on FIR technique is reliable. It is worth nothing that FIR-based on the thermally coupled energy levels (TECLs) would be proved to be practical, and lanthanide ions possessing TCELs such as Er3+, Ho3+, Gd3+, Nd3+, Tm3+, Pr3+, Dy3+ have been investigated in the last few years [18–24]. Furthermore, the energy gap between TECLs positively determines the performance of temperature evaluation. In contrast to other lanthanide ions, Ho3+ has a large energy gap between the two corresponding TECLs, therefore, favorable temperature performance would be expected [24].
Notably, glass ceramic is a kind of composite materials, which combines the merits of glass and nanocrystals. Micro/nanocrystals, including fluorides, oxides, sulfides, halides and so on [25–29], could be precipitated with specially designed compositions with subsequent heat-treatment. It is essential that the homogeneous precipitation of the desired nanocrystals and incorporation of the activators into the matrix could be guaranteed, therefore, the development of glass ceramics is still a driving force. More importantly, the oxyfluoride glass ceramics, which precipitated fluoride nanocrystals with low phonon energy, is in favor of upconversion (UC) emission, due to the reduction of adverse multiphonon nonradiative relaxation rate. In this case, oxyfluoride glass ceramics exhibits high optical transparency and chemical/thermal stability of the oxide glass matrix, accompanied with superb optical efficiency as crystals. As a consequence, these distinct advantages of oxyfluoride glass ceramics would be beneficial to UC emission, providing good signal-to-noise ratio for optical thermometry. Among various fluorides, hexagonal NaLnF4 (Ln = Y, Lu, Gd) is considered to be one of the optimum lattices for UC emission. Therefore, the oxyfluoride glass ceramics is a promising candidate for lanthanide doped optical materials, especially for potential application in optical thermometry.
To our best knowledge, there exists few studies focused on the FIR-based optical thermometry based on the short-length UC emission of Ho3+, especially for glass ceramics. Cao et al. revealed the temperature-dependent upconversion properties of Ho3+ in β-PbF2 nanocrystals [30]. However, the obtained relative sensitivity was not as expected, which may due to the mismatch of the Boltzmann distribution. On the other hand, the β-PbF2 nanocrystals embedded glass ceramics were not environmentally friendly, hindering their future application greatly. In this work, Yb3+/Ho3+ codoped β-NaGdF4 nanocrystals embedded transparent glass ceramics were synthesized successfully with self-crystallation. Typically, the 5F1/5G6 and 5F2,3/3K8 states of Ho3+ are selected as the two thermally coupled levels, whose FIR would vary with temperature and thus Ho3+ can act as an index for probing the surrounding temperature. The purpose of this work is to systematically discuss the FIR-based thermal behaviors of transparent oxyfluoride glass ceramics containing β-NaGdF4: Yb3+, Ho3+ nanocrystals, and its potential application as an extraordinary alternative for luminescence thermometry.
2. Experimental
Specifically, oxyfluoride glass ceramics were fabricated by conventional melting-quenching method with specially designed composition of 70.1SiO2-4.3Al2O3-2.3Na2CO3-2.37Gd2O3-1.2YbF3-0.06HoF3-1.8AlF3-18.5NaF [31,32]. The raw materials of SiO2, Al2O3, Na2CO3, AlF3, NaF (A. R., all from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and high purity Gd2O3, YbF3, HoF3 (99.99%, from AnSheng Inorganic Materials Co., Ltd., Ganzhou, Jiangxi, China) were mixed thoroughly and mechanically ground together. The mixture was melted in an alumina crucible at 1550 °C for 45 min in air atmosphere, and cast into a copper mold preheated at 300 °C to naturally cool down to room temperature. The as-made precursor glass was then maintained at 450 °C for 10 h to completely release internal stresses, which was donated as PG. Subsequently, the PG was heat-treated at 550 °C for 2 h and 750 °C for 2 h continuously to induce homogenous precipitation of hexagonal NaGdF4 nanocrystals among glass matrix, which was donated as GC750.
The crystalline phases of the synthesized samples were characterized by an X-ray diffractometer (Rigaku-TTR-III) using nickel-filtered Cu Kα radiation (λ = 0.15418 nm) in the 2θ range from 10° to 70°. The transmittance spectra were measured employing a SOLID 3700 spectrometer (Shimadzu Ltd., Kyoto, Japan). A high resolution transmission electron microscopy (HRTEM, Talos F200X) equipped with the selected area electron diffraction (SAED) was used to character the microstructure of GC750. Scanning transmission electron microscopy (STEM) operated in the high-angle annular dark-field (HAADF) mode was also performed. Under the excitation of 980 nm diode laser, the UC emission was dispersed and detected by a Jobin-Yvon HRD-1 double monochromator equipped with a Hamamatsu R928 photomultiplier. The signal was analyzed by an EG&G7265 DSP Lock-in Amplifier and stored into computer memories. Decay curves were recorded by a customized ultraviolet to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh). For the measurements of temperature-dependent UC emission, the GC750 was loaded on a copper post, whose temperature was controlled by a temperature controller (OMRON E5CC-800) with a type-K thermocouple and a heating tube. Thermal conductivity was performed by an electronic dilatometer (DIL402PC, Netzsch) and a laser flash apparatus (LFA457, Netzsch). Differential thermal analysis (DTA, STA449C Netzsch) experiment of the precursor glass sample was measured at a heating rate of 10 K/min in air atmosphere in order to follow its thermal behavior.
3. Results and discussion
3.1 Structural properties
The crystal structures of the PG and GC750 are investigated by X-ray powder diffraction (XRD), as shown in Fig. 1(a). For the PG, several weak diffraction peaks emerges superimposing on the diffuse hump originating from the glass matrix, suggesting a bit of nanocrystals assigned to hexagonal NaGdF4 have formed directly after melt-quenching. After heat-treatment, the GC750 exhibits distinct peaks of hexagonal NaGdF4 (JCPDS No. 27-0699), and no other diffraction peaks could be found, revealing the formation of the well crystalline NaGdF4 phase. According to the Scherrer formula [33, 34], the mean crystalline size of GC750 is estimated to be about 34 nm. The as-prepared PG and GC750 were both polished at 1 mm thickness for transmittance characterization, as described in Fig. 1(b). A series of absorption peaks located at 448 nm, 537 nm and 644 nm are ascribed to the transitions from the ground state 5I8 to the excited states of Ho3+, which are the 5F1/5G6, 5S2/5F4, and 5F5 states, respectively. Besides, the absorption band centered at 973 nm is attributed to the transition from 2F7/2 to 2F5/2 of Yb3+. As the size of the precipitated crystals is much smaller than the wavelength of visible and near-infrared light, GC750 still maintain a perfect transparency approximately to 90% in visible region after heat-treatment, which may be of great importance for potential application.
Fig. 1 (a) The XRD patterns of PG and GC750, compared with the standard data of β-NaGdF4 reference pattern (JCPDS No. 27-0699). (b)Transmittance spectra of PG and GC750. (c) TEM, (d) SAED pattern and (e) HRTEM of GC750.
The transmission electron microscopy (TEM) image of GC750, as depicted in Fig. 1(c), verifies that nanoparticles sized 30-40 nm are dispersed homogeneously in the glass matrix, with the selected area electron diffraction (SAED) rings indexed to the hexagonal NaGdF4. In the HRTEM image performed by Fig. 1(d), an individual β-NaGdF4 nanoparticle with highly ordered lattice fringes could be observed, confirming the crystalline feature of β-NaGdF4 particles. Furthermore, Fig. 2 displays the performance of GC750 by the scanning transmission electron microscopy (STEM) observations operated in the high-angle annular dark-field (HAADF). It turns out that all the elements of Na, Gd, F, Yb, Ho could be clearly found in the NaGdF4 nanocrystals, while F also exists in the glass matrix. Therefore, these results mentioned above confirm the good crystallization of β-NaGdF4 nanocrystals and the successful incorporation of Yb3+ and Ho3+ into β-NaGdF4.
Fig. 2 (a) STEM-HAADF images of GC750 with associated (b) Na, (c) Gd, (d) F, (e) Yb and (f) Ho elemental mapping.
3.2 UC emission
As presented in Fig. 3, several characteristic emission bands of Ho3+ appear in both PG and GC750 samples under the excitation of 980 nm. These UC emission bands are centered at 540 nm, 642 nm and 750 nm, which is attributed to the transitions from the 5S2/5F4, 5F5, 5I4 states to the ground 5I8 state, respectively. Besides, the UC emissions in short-wavelength range assigned to the high states of 5G4/3K7, 5G5, 5G5’/3H6, 5F2,3/3K8 could be observed in the enlarge UC emission spectra in the inset of Fig. 3. Compared to that of PG, remarkable enhancement of UC emission intensity and narrowed Stark splitting suggest the incorporation of Ho3+ and Yb3+ ions into β-NaGdF4 nanocrystals after heat-treatment. Furthermore, the decay curves of PG and GC750 monitoring at the transition of 5S2/5F4 → 5I8 are plotted in Fig. 4. Under the excitation of 980 nm, the fitting lifetime of GC750 is found to be longer than that of PG, which are 0.86 ms for GC750 and 0.39 ms for PG, respectively. As a consequence, all these results provide a further evidence for the enrichment of Yb3+ and Ho3+ into β-NaGdF4 nanocrystals.
Fig. 3 (a) UC emission spectra of PG and GC750 excited by 980 nm laser. (b) The enlarged UC emission spectra in the range of 350-510 nm.
Fig. 4 Decay curves monitoring at 540 nm (5S2/5F4 → 5I8) of Ho3+ in PG and GC750 samples under the excitation of 980 nm laser.
In terms of the ladder-like energy-level diagrams of Yb3+ and Ho3+, possible UC mechanism is plotted in Fig. 5. When excited at 980 nm, Yb3+ at ground 2F7/2 state can absorb energy by ground state absorption to be populated at the 2F5/2 state, and then transfer its energy to Ho3+ in its neighborhood. Meanwhile, Ho3+ can be populated to the 5I6 state by absorbing the energy from Yb3+, and subsequently to the 5F4/5S2 state by absorbing another energy from Yb3+, producing green emission. However, Ho3+ in the intermedia 5I6 state can also relax nonradiatively to the 5I7 state. The 5F5 state, populated by excited state absorption from the 5I7 state, depopulates to the ground 5I8 state, producing red UC emission. Furthermore, the 5G2 state could be populated by a three-photon process, and the subsequent multiphonon nonradiative process is responsible for the emitting of UC emission in short-wavelength range. Due to the low phonon energy of β-NaGdF4, adverse multiphonon nonradiative process could be hindered dominantly, resulting in the prominent green emission. Notably, the energy gap between the 5F1/5G6 and 5F2,3/3K8 states of Ho3+ is appropriate to be TCELs.
Furthermore, the pump power density dependent emission intensities at 484 nm, 540 nm and 642 nm are verified for PG and GC750. As depicted in Fig. 6, a double logarithmic plots are employed, giving n as the slope of the fitting line. It is observed that the slopes corresponding to emission centered at 484 nm under 980 nm excitation are 2.56 for PG and 2.46 for GC750, indicating a three-photon process, while the slopes corresponding to emission at 540 nm and 642 nm are 1.78 and 1.90 for PG and 1.82, 1.89 for GC750 respectively, revealing a two-photon process. These results keep in accordance with the possible UC mechanism presented in Fig. 5.
3.3 Temperature behaviors
The temperature dependent UC emission spectra of GC750 are investigated in detail. Herein, Ho3+ dopants act as a temperature probe, while Yb3+ ions play a role of sensitizers to enhance UC emission of Ho3+. Normalized UC emission spectra of GC750 under the excitation of 980 nm diode laser are performed in Fig. 7(a) at various temperatures from 390 K to 773 K, and the emission band originating from the transition of 5F1/5G6 → 5I8 is doubled for intuitive expression. Remarkably, the emission intensity of the 5F1/5G6 → 5I8 transition increases monotonously with respect to that of 5F2,3/3K8 → 5I8 as the rising of temperature, which is due to the thermal coupling between the two corresponding levels. When thermal equilibrium has been reached, the population of the 5F1/5G6 state and the 5F2,3/3K8 state would obey the Boltzmann distribution, whose ratio could be described as the function with respect to temperature as follows:
where R represents the value of FIR, B is a constant, kB is the Boltzmann constant, ΔE represents the energy gap between the two corresponding TECLs. The FIR (I446/I484) curve is plotted in Fig. 7(b), and can be well fitted with the Eq. (1). The effective energy gap between the 5F1/5G6 and 5F2,3/3K8 states is estimated to be 1515 cm−1, which is larger than that of other lanthanide ions. Thus outstanding temperature performance could be expected.Fig. 7 (a) Normalized UC emission spectra of GC750 under the excitation of 980 nm laser at various temperatures from 390 K to 773 K. (b) Fluorescence intensity ratio data as a function of temperature for GC750. (c) Relative sensitivity and absolute sensitivity curves of GC750 in the temperature range of 390-773 K.
Furthermore, the signal change with respect to temperature is described to evaluate the performance of a luminescent material for optical thermometry, donated as sensitivity S. There exists two different S, including the absolute sensitivity SA defined as the average change with temperature and the relative sensitivity SR defined as the slope of the FIR change with respect to temperature. In comparison with SA, SR is a representative parameter for characterizing the performance of luminescent materials. The sensitivities SA and SR could be respectively given as Eqs. 2- (3):
According to Eq. (2) and Eq. (3), the absolute sensitivity and relative sensitivity curves are exhibited in the temperature range from 390 K to 773 K in Fig. 7(c). It can be seen that the relative sensitivity SR reaches its maximum of 1.43%·K−1 at 390 K. As summarized in Tab. 1, the relative sensitivity of Ho3+ is proved to be superior to that of other lanthanide ions. On the other hand, glass ceramics are of great importance for promising fiber-optic thermometer compared to phosphors. Furthermore, the thermal conductivity is ~0.95 W·m−1·K−1 and glass transition temperature is 853 K. Therefore, the investigation on FIR-based optical thermometry of Ho3+ in β-NaGdF4 crystals is meaningful and potential.
Table 1. FIR-based optical thermometry utilizing different lanthanide ions with the energy gap ΔE and the relative sensitivity SR.
In a certain temperature range, the relative sensitivity for FIR-based optical thermometry is proportional to the energy gap ΔE. That is to say, the large energy gap would lead to high relative sensitivity. However, when the value of ΔE/KBT exceeds 10, the thermal equilibrium could not be reached, leading to mismatch of the Boltzmann distribution. Meanwhile, when the energy gap is too small to distinguish the emission bands of the two corresponding levels, temperature evaluation would deviate from accuracy. Generally, we can conclude that the energy gap of the TECLs for optical thermometry should be in the range from 200 cm−1 to 2000 cm−1. In our work, the energy gap between the TECLs of β-NaGdF4: Yb3+, Ho3+ nanocrystals is estimated to be 1515 cm−1, which cannot only satisfy the requirement for TECLs, but also prove to be a superior candicate for optical thermometry.
4. Summary
The transparent oxyfluoride glass ceramics containing β-NaGdF4 nanocrystals were fabricated via melting-quenching method with self-crystalization, and extensively characterized by XRD, TEM, HRTEM, SEAD, STEM-HADDF to confirm their crystallization. Under the excitation of 980 nm, GC750 exhibits intense UC luminescence of Ho3+. In particular, the FIR of two blue UC emission bands peaked at 446 and 484 nm, originating from the transitions from two thermal coupled energy levels of 5F1/5G6 and 5F2,3/3K8 states to the 5I8 state, is recorded for optical thermometry in the temperature range of 390 K to 773 K. The relative sensitivity for Ho3+ doped β-NaGdF4 nanocrystals reaches 1.43%·K−1 at 390 K, corresponding to an effective energy gap of 1515 cm−1. Therefore, it is expected that the oxyfluoride glass ceramics containing β-NaGdF4: Yb3+, Ho3+ nanocrystals with perfect transparency and high temperature sensitivity has the specific advantage of being applicable in luminescence thermometry.
Funding
Zhejiang Provincial Natural Science Foundation of China (LR15E020001); National Natural Science Foundation of China (Nos. 51572065, 11374291 and 11404321); and 151 talent’s projects in the second level of Zhejiang Province.
References and links
1. S. Gai, C. Li, P. Yang, and J. Lin, “Recent progress in rare earth micro/nanocrystals: soft chemical synthesis, luminescent properties, and biomedical applications,” Chem. Rev. 114(4), 2343–2389 (2014). [CrossRef] [PubMed]
2. J. C. Bünzli, “Lanthanide luminescence for biomedical analyses and imaging,” Chem. Rev. 110(5), 2729–2755 (2010). [CrossRef] [PubMed]
3. N. M. Idris, M. K. G. Jayakumar, A. Bansal, and Y. Zhang, “Upconversion nanoparticles as versatile light nanotransducers for photoactivation applications,” Chem. Soc. Rev. 44(6), 1449–1478 (2015). [CrossRef] [PubMed]
4. X. Huang, S. Han, W. Huang, and X. Liu, “Enhancing solar cell efficiency: the search for luminescent materials as spectral converters,” Chem. Soc. Rev. 42(1), 173–201 (2013). [CrossRef] [PubMed]
5. G. Chen, H. Qiu, P. N. Prasad, and X. Chen, “Upconversion nanoparticles: design, nanochemistry, and applications in theranostics,” Chem. Rev. 114(10), 5161–5214 (2014). [CrossRef] [PubMed]
6. G. Wang, Q. Peng, and Y. Li, “Lanthanide-doped nanocrystals: synthesis, optical-magnetic properties, and applications,” Acc. Chem. Res. 44(5), 322–332 (2011). [CrossRef] [PubMed]
7. P. Kumar, K. Nagpal, and B. K. Gupta, “Unclonable security codes designed from multicolor luminescent lanthanide-doped Y2O3 nanorods for anticounterfeiting,” ACS Appl. Mater. Interfaces 9(16), 14301–14308 (2017). [CrossRef] [PubMed]
8. J. Liu, R. V. Deun, and A. M. Kaczmarek, “Optical thermometry of MoS2: Eu3+ 2D luminescent nanosheets,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4(42), 9937–9941 (2016). [CrossRef]
9. L. H. Fischer, G. S. Harms, and O. S. Wolfbeis, “Upconverting nanoparticles for nanoscale thermometry,” Angew. Chem. Int. Ed. Engl. 50(20), 4546–4551 (2011). [CrossRef] [PubMed]
10. E. J. McLaurin, V. A. Vlaskin, and D. R. Gamelin, “Water-soluble dual-emitting nanocrystals for ratiometric optical thermometry,” J. Am. Chem. Soc. 133(38), 14978–14980 (2011). [CrossRef] [PubMed]
11. D. Chen, Z. Wan, and S. Liu, “Highly sensitive dual-phase nanoglass-ceramics self-calibrated optical thermometer,” Anal. Chem. 88(7), 4099–4106 (2016). [CrossRef] [PubMed]
12. X. Y. Li, G. C. Jiang, S. S. Zhou, X. T. Wei, Y. H. Chen, C. K. Duan, and M. Yin, “Luminescent properties of chromium(III)-doped lithium aluminate for temperature sensing,” Sensor. Actuat. Biol. Chem. 202, 1065–1069 (2014).
13. X. Y. Li, X. T. Wei, Y. G. Qin, Y. H. Chen, C. K. Duan, and M. Yin, “The emission rise time of BaY2ZnO5: Eu3+ for non-contact luminescence thermometry,” J. Alloys Compd. 657, 353–357 (2016). [CrossRef]
14. Z. Cao, X. Wei, L. Zhao, Y. Chen, and M. Yin, “Investigation of SrB4O7: Sm2+ as a multimode temperature sensor with high sensitivity,” ACS Appl. Mater. Interfaces 8(50), 34546–34551 (2016). [CrossRef] [PubMed]
15. S. S. Zhou, X. Y. Li, X. T. Wei, C. K. Duan, and M. Yin, “A new mechanism for temperature sensing based on the thermal population of 7F2 state in Eu3+,” Sens. Actuators B Chem. 231, 641–645 (2016). [CrossRef]
16. Y. Gao, F. Huang, H. Lin, J. C. Zhou, J. Xu, and Y. S. Wang, “A novel optical thermometry strategy based on diverse thermal response from two intervalence charge transfer states,” Adv. Funct. Mater. 26(18), 3139–3145 (2016). [CrossRef]
17. H. Suo, C. Guo, J. Zheng, B. Zhou, C. Ma, X. Zhao, T. Li, P. Guo, and E. M. Goldys, “Sensitivity modulation of upconverting thermometry through engineering phonon energy of a matrix,” ACS Appl. Mater. Interfaces 8(44), 30312–30319 (2016). [CrossRef] [PubMed]
18. S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]
19. X. F. Wang, Q. Liu, Y. Y. Bu, C. S. Liu, T. Liu, and X. H. Yan, “Optical temperature sensing of rare-earth ion doped phosphors,” RSC Advances 5(105), 86219–86236 (2015). [CrossRef]
20. S. Zhou, G. Jiang, X. Wei, C. Duan, Y. Chen, and M. Yin, “Pr3+-doped beta-NaYF4 for temperature sensing with fluorescence intensity ratio technique,” J. Nanosci. Nanotechnol. 14(5), 3739–3742 (2014). [CrossRef] [PubMed]
21. F. Vetrone, R. Naccache, A. Zamarrón, A. Juarranz de la Fuente, F. Sanz-Rodríguez, L. Martinez Maestro, E. Martín Rodriguez, D. Jaque, J. García Solé, and J. A. Capobianco, “Temperature sensing using fluorescent nanothermometers,” ACS Nano 4(6), 3254–3258 (2010). [CrossRef] [PubMed]
22. D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek, and M. Samoc, “Neodymium(III) doped fluoride nanoparticles as non-contact optical temperature sensors,” Nanoscale 4(22), 6959–6961 (2012). [CrossRef] [PubMed]
23. Z. M. Cao, S. S. Zhou, G. C. Jiang, Y. H. Chen, C. K. Duan, and M. Yin, “Temperature dependent luminescence of Dy3+ doped BaYF5 nanoparticles for optical thermometry,” Curr. Appl. Phys. 14(8), 1067–1071 (2014). [CrossRef]
24. S. S. Zhou, S. Jiang, X. T. Wei, Y. H. Chen, C. K. Duan, and M. Yin, “Optical thermometry based on upconversion luminescence in Yb3+/Ho3+ co-doped NaLuF4,” J. Alloys Compd. 588, 654–657 (2014). [CrossRef]
25. W. Xu, X. Gao, L. Zheng, Z. Zhang, and W. Cao, “Short-wavelength upconversion emissions in Ho3+/Yb3+ codoped glass ceramic and the optical thermometry behavior,” Opt. Express 20(16), 18127–18137 (2012). [CrossRef] [PubMed]
26. A. Sarakovskis and G. Krieke, “Upconversion luminescence in erbium doped transparent oxyfluoride glass ceramics containing hexagonal NaYF4 nanocrystals,” J. Eur. Ceram. Soc. 35(13), 3665–3671 (2015). [CrossRef]
27. K. Driesen, V. K. Tikhomirov, C. Gorller-Walrand, V. D. Rodriguez, and A. B. Seddon, “Transparent Ho3+-doped nano-glass-ceramics for efficient infrared emission,” Appl. Phys. Lett. 88(7), 073111 (2006). [CrossRef]
28. W. Wisniewski, A. Keshavarizi, T. Zscheckel, and C. Rüssel, “EBSD-based phase identification in glass-ceramics of the Y2O3-Al2O3-SiO2 system containing α- and β-Y2Si2O7,” J. Alloys Compd. 699, 832–840 (2017). [CrossRef]
29. C. G. Lin, S. X. Dai, C. Liu, B. A. Song, Y. S. Xu, F. F. Chen, and J. Heo, “Mechanism of the enhancement of mid-infrared emission from GeS2-Ga2S3 chalcogenide glass-ceramics doped with Tm3+,” Appl. Phys. Lett. 100(23), 231910 (2012). [CrossRef]
30. V. Seznec, H. L. Ma, X. H. Zhang, V. Nazabal, J. L. Adam, X. S. Qiao, and X. P. Fan, “Preparation and luminescence of new Nd3+ doped chloro-sulphide glass-ceramics,” Opt. Mater. 29(4), 371–376 (2006). [CrossRef]
31. D. Chen, Z. Wan, Y. Zhou, P. Huang, J. Zhong, M. Ding, W. Xiang, X. Liang, and Z. Ji, “Bulk glass ceramics containing Yb3+/Er3+: β-NaGdF4 nanocrystals: Phase-separation-controlled crystallization, optical spectroscopy and upconverted temperature sensing behavior,” J. Alloys Compd. 638, 21–28 (2015). [CrossRef]
32. A. Herrmann, M. Tylkowski, C. Bocker, and C. Rüssel, “Preparation and luminescence properties of glass–ceramics containing Sm3+-doped hexagonal NaGdF4 crystals,” J. Mater. Sci. 48(18), 6262–6268 (2013). [CrossRef]
33. J. K. Cao, L. P. Chen, W. P. Chen, D. K. Xu, X. Y. Sun, and H. Guo, “Enhanced emissions in self-crystallized oxyfluride scintillating glass ceramics containing KTb2F7 nanocryatsls,” Opt. Mater. Express 6(7), 2201–2206 (2016). [CrossRef]
34. F. F. Hu, J. K. Cao, X. T. Wei, X. Y. Li, J. J. Cai, H. Guo, Y. H. Chen, C. K. Duan, and M. Yin, “Luminescence properties of Er3+-doped transparent NaYb2F7 glass-ceramics for optical thermometry and spectral conversion,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4(42), 9976–9985 (2016). [CrossRef]
