Novel Er3+ doped highly transparent glass-ceramics containing orthorhombic KLu2F7 nanocrystals (24 nm in diameter) have been successfully fabricated for the first time. Their structural, optical and upconversion properties are systematically investigated by XRD, TEM, absorption spectra, upconversion spectra and lifetime measurements. Excited by 980 nm laser, characteristic emissions of Er3+ are detected in all samples. Impressively, the upconversion luminescence of Er3+ is drastically enhanced (1340 and 680 times for green and red emissions, respectively) after crystallization. Laser power dependence of the upconverted emissions and upconversion decay curves were explored to understand the different upconversion mechanisms in precursor glasses and glass-ceramics. Results show that the enrichment of Er3+ into KLu2F7 lattice after crystallization is responsible for the enhanced upconversion.
© 2014 Optical Society of America
Rare earth (RE) ions doped upconversion materials, which possess the ability to convert a longer wavelength radiation to a shorter wavelength radiation via multi-photon process, have been extensively explored in virtue of their potential applications in optical devices, color displays, bio-imaging, etc [1–3]. As is well known, RE fluoride nanocrystals (such as NaYF4) are considered as the most efficient host matrices for upconversion due to their low phonon energy that can minimize the non-radiative relaxation probability and then leads to efficient upconversion luminescence . In recent years, RE ions doped Lu-based fluoride nanocrystals, for instance, BaLuF5, NaLuF4 and LiLuF4, have aroused increasing attention due to their unique upconversion behavior and more efficient upconversion luminescence than that of Y-based counterparts [4–6].
As an important Lu-based fluoride, KLu2F7, with phase transformation from high-temperature monoclinic phase to low-temperature orthorhombic phase at 730 °C, has received scarce attention due to the difficulty to elaborate single phase KLu2F7 . More recently, orthorhombic KLu2F7 single crystals were successfully fabricated by micro-pulling-down method and the scintillation properties of Ce3+ doped KLu2F7 single crystal were also reported . Similar to other Lu-based fluorides, KLu2F7 may be excellent upconversion host matrix. However, there is no report on the study of upconversion properties of KLu2F7-based materials.
On the other hand, RE fluorides based transparent oxyfluoride glass-ceramics, novel nanostructured composites for RE ions doping, have been proven as promising luminescent materials [9–19]. They combine the best characters from both fluoride nanocrystals and oxide glasses, such as low phonon energy environment for RE ions dopant, isotropy, high chemical and mechanical stability, and ability of being processed into various shapes [15,16]. Generally, such materials are generated through controlled crystallization from precursor glasses by heat-treatment. After crystallization, RE ions will abound in the precipitated fluoride nanocrystals preferentially [9–12] and show advantageous luminescent behavior such as high quantum efficiency, long luminescence lifetime and excellent upconversion properties owing to the reduced non-radiative relaxation and enhanced energy transfer (ET) efficiency arising from shorter RE3+-RE3+ distance [13,14].
In this context, we successfully fabricate Er3+-doped transparent oxyfluoride glass-ceramics containing novel orthorhombic KLu2F7 nanocrystals and systematically investigate their upconversion behavior. Excited by near-infrared (NIR) light, the upconversion intensity in glass-ceramics is tremendously increased compared to that in precursor glasses. The different upconversion mechanisms before and after crystallization were discussed in detail. Our research may enrich the understanding of upconversion mechanisms of Er3+ in glass-ceramics.
Glass samples with nominal composition of 54SiO2-10K2CO3-15Al2O3-15KF·2H2O-5LuF3-1ErF3 (in mol %) were prepared by melt-quenching method in air atmosphere. SiO2, Al2O3, K2CO3, KF·2H2O (A.R., all from Sinopharm Chemical Reagent Co., Ltd.) and high purity LuF3, ErF3 (99.99%, from AnSheng Inorganic Materials Co., Ltd.) were used as starting materials. The well ground stoichiometric chemicals were put into a covered alumina crucible and melted at 1550 °C for 1 h in air atmosphere. The melt was poured onto a stainless-steel plate and then pressed by another plate to form precursor glasses (labeled as PG). Subsequently, PG glasses were heat-treated for 2 h at 700 °C to form transparent glass-ceramics, which was labeled as GC700. All samples were polished optically for further characterization.
X-ray diffraction (XRD) patterns were performed on a Philips X’Pert PRO SUPER X-ray diffraction apparatus with CuKa radiation. Transmittance spectra were measured on a U-3900 ultraviolet-Visible (UV-VIS) spectrophotometer (Hitachi). The microstructure of glass-ceramics was analyzed by a JEM-2010 transmission electron microscopy (TEM) (JOEL Ltd.). Upconversion spectra were measured with a FS920 spectrofluorometer (Edinburgh Instruments) with a 980 nm laser as the excitation source. Decay curves measurements were performed on a SBP 500 monochromater coupled to a R928P photomultiplier with Tektronix TDS5052 oscilloscope. All the measurements were carried out at room temperature.
3. Results and discussion
Figure 1(a) depicts the XRD patterns of PG and GC700 samples. The standard data of JCPDS card No. 27-0459 for orthorhombic KYb2F7 with space group of Pna21 (33) is used as a reference because the unavailability of standard XRD patterns of orthorhombic KLu2F7 and the isostructural character of KLu2F7 to KYb2F7. The cell parameters of KLu2F7 (a = 11.702 Å, b = 13.109 Å and c = 7.711 Å)  are slightly smaller than those of KYb2F7 (a = 11.710 Å, b = 13.240 Å and c = 7.739 Å). As can be seen in Fig. 1(a), PG is amorphous while GC700 shows intense diffraction peaks. Obviously, the XRD patterns of GC700 are almost identical to that of orthorhombic KYb2F7 and shift slightly to larger Bragg angle. Accordingly, the observed phenomenon can be taken as a solid evidence of the formation of orthorhombic KLu2F7 nanocrystals in GC700. The mean crystalline size D of the precipitated KLu2F7 nanocrystals in GC700 estimated by Scherer’s equation  is about 22 nm for GC700.
The transmittance spectra of PG and GC700 in UV-VIS region are given in Fig. 1(b). Clearly, a range of absorption peaks located at 376, 488, 519 and 652 nm are observed, which are assigned to the transitions from 4I5/2 ground state to 4G11/2, 4F7/2, 2H11/2 and 4F9/2 excited states of Er3+, respectively . Besides, due to the small size of precipitated KLu2F7 nanocrystals and their narrow size distribution, the light scattering in glass-ceramics is minimized, and thus GC700 still maintains a fairly good transparency in VIS region .
TEM and high-resolution TEM (HRTEM) images of GC700 are recorded to investigate the microstructures of glass-ceramics. TEM bright-field micrograph given in Fig. 1(c) clearly demonstrates the homogeneously distribution of spherical nanocrystals with a narrow size distribution around 24 nm in diameter among the residual glass phase. The selected area electron diffraction (SAED) patterns [inset of Fig. 1(c)] manifest the polycrystalline diffraction feature of the glass-ceramics. HRTEM image in Fig. 1(d) clearly displays the well-defined lattice structure of precipitated fluoride nanocrystals and the associated interplanar spacing d value is estimated to be 3.184 Å, which can be indexed to (231) crystal plane of orthorhombic KLu2F7 (d(231) = 3.188 Å).
Figure 2(a) shows the upconversion spectra of PG and GC700 (λex = 980 nm). Characteristic emission bands of Er3+ at 525 nm (2H11/2→4I15/2), 545 nm (4S3/2→4I15/2) and 660 nm (4F9/2→4I15/2)  can be observed in all samples. For PG sample, the relatively weak upconversion emission can be easily detected by naked eyes and spectrofluorometer. It is noticeable that the upconversion signal of GC700 is tremendously enhanced after crystallization. Specifically, the upconversion intensity of GC700 increases by 1340 and 680 times for green and red emission compared to that of PG, respectively. Moreover, pronounced Stark splitting in the emission bands of GC700 are observed as well. All above phenomena suggests the successful partition of Er3+ ions into the precipitated nanocrystals with high symmetrical environment and low phonon energy after crystallization.
The dependence of upconversion emission intensities (I) on pump power (P) is measured to study the excitation mechanisms accounting for upconversion luminescence. It is well known that in unsaturated condition, I is proportional to the nth power of P and follows formula [21,22],Figs. 2(b) and 2(c) by log-log plots. For PG, the slopes n of linear fittings for emissions at 525 nm (2H11/2→4I15/2), 545 nm (4S3/2→4I15/2) and 660 nm (4F9/2→4I15/2) are 2.14 ± 0.04, 2.13 ± 0.06 and 1.90 ± 0.05, respectively. Meanwhile, the n slopes of GC700 for these emissions are 1.52 ± 0.04, 1.46 ± 0.04 and 1.31 ± 0.02, respectively. These results reveal that two-photon process is responsible for these emissions.
Summarized briefly, the strongly enhanced upconversion intensity and the obvious Stark splitting in GC700 all indicate the preferential incorporation of Er3+ ions into KLu2F7 nanocrystals after crystallization. As a result, the upconversion mechanisms in PG and GC700 will be different from each other. Figure 3(a) illustrates the energy level diagram of Er3+ and proposed upconversion mechanisms accounting for green and red emissions.
Excited by 980 nm laser, Er3+ ions at 4I15/2 ground state are firstly pumped to 4I11/2 intermediate excited state by ground state absorption (GSA). Subsequently, these ions at 4I11/2 level can be populated to higher emitting levels through two most common excitation processes: (1) excited state absorption (ESA), (2) energy transfer upconversion (ETU) . ESA process is a single ion process. While ET process involves two ions and therefore the rate depends strongly on the distance between the active ions .
For PG sample, due to the low concentration and homogeneous dispersion of Er3+ ions in glassy phase, the distance between Er3+ ions is quite long. Thus, ETU can be ruled out and ESA process is responsible for the population of 2H11/2, 4S3/2 and 4F9/2 emitting levels. For green emissions from 2H11/2 and 4S3/2 emitting levels, Er3+ ions at 4I11/2 level are excited to 4F7/2 level by ESA1 process: 4I11/2 + hv → 4F7/2, where hv is the absorbed 980nm photon energy. After that, these ions at 4F7/2 level non-radiatively relax to 2H11/2 and 4S3/2 emitting levels and further radiatively relax down to 4I15/2 ground state to produce emissions at 525 and 545 nm, respectively . For red emission (4F9/2→4I15/2), there are two possible ways to feed the 4F9/2 emitting level. One is multi-phonon non-radiative relaxation from upper 4S3/2 level. The other is ESA2 process: 4I13/2 + hv → 4F9/2. The 4I13/2 intermediate excited state may be populated through a multi-phonon non-radiative process from 4I11/2 level and radiative process from upper emitting levels, such as 4S3/2 level. Finally, Er3+ ions at 4F9/2 level radiatively relax to 4I15/2 ground state to give red emission .
For GC700, owing to the incorporation of Er3+ ions into the precipitated fluoride nanocrystals, the Er3+-Er3+ distance is greatly shortened. In this case, the dominant upconversion mechanism is ETU, which is a more efficient way to promote those Er3+ ions at 4I11/2 level to upper emitting levels to produce much stronger upconversion luminescence. The 2H11/2 and 4S3/2 emitting levels can be pumped by ETU1 process . An excited Er3+ ion at 4I11/2 level relaxes non-radiatively to 4I15/2 level and transfers its excitation energy to a neighboring Er3+ ion at the same level, promoting the later to 4F7/2 level: 4I11/2 + 4I11/2 → 4I15/2 + 4F7/2. While the 4F9/2 red emitting level can be populated by ETU2 process: 4I11/2 + 4I13/2 → 4I15/2 + 4F9/2 . What’s more, the low phonon energy of fluoride nanocrystals can effectively reduce the non-radiatively relaxation of Er3+ ions and further enhance the upconverson efficiency. Accordingly, efficient ETU processes and low non-radiative relaxation rate, deriving from the incorporation of Er3+ ions into the KLu2F7 fluoride nanocrystals, contribute to the extremely strong upconversion luminescence in GC700.
To further prove the partition of Er3+ ions into the precipitated fluoride nanocrystals, the measurement of lifetimes for upconversion emissions of Er3+ are performed. Figure 3(b) shows luminescence decay curves for 4S3/2→4I15/2 (545 nm) transition of Er3+ in PG and GC700 excited by a pulsed 980 nm laser. The average lifetime () can be calculated by ,
Novel KLu2F7:Er3+ transparent glass-ceramics with high transparency have been successfully fabricated by melt-quenching technique with further heat-treatment. After crystallization, tremendously enhanced upconversion emissions with obvious Stark splitting, longer luminescence lifetime and increased rise time in decay curve are observed, suggesting the incorporation of Er3+ ions into the precipitated fluoride nanocrystals. For up-conversion mechanisms, ESA processes are dominant in PG sample and ETU processes are dominant in GC700 sample. Our investigation demonstrates that KLu2F7-based transparent glass-ceramics are excellent matrices for upconversion and may find applications in upconversion detectors, optical fibers and solid-state lasers.
This work was supported by NSFC No. 11374269, the Natural Science Foundation of Zhejiang Province (No. LY12E02001), Zhejiang Provincial Key Science and Technology Innovation Team No. 2011R50012, Zhejiang Provincial Key Laboratory No. 2013E10022, and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, No. 2013-skllmd-10).
References and links
3. F. Wang, Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, and X. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature 463(7284), 1061–1065 (2010). [CrossRef] [PubMed]
4. S. Sarkar, B. Meesaragandla, C. Hazra, and V. Mahalingam, “Sub-5 nm Ln³⁺-doped BaLuF₅ nanocrystals: A platform to realize upconversion via interparticle energy transfer (IPET),” Adv. Mater. 25(6), 856–860 (2013). [CrossRef] [PubMed]
5. P. Huang, W. Zheng, S. Zhou, D. Tu, Z. Chen, H. Zhu, R. Li, E. Ma, M. Huang, and X. Chen, “Lanthanide-doped LiLuF4 upconversion nanoprobes for the detection of disease biomarkers,” Angew. Chem. Int. Ed. 53(5), 1252–1257 (2014). [CrossRef]
6. Q. Liu, Y. Sun, T. Yang, W. Feng, C. Li, and F. Li, “Sub-10 nm hexagonal lanthanide-doped NaLuF4 upconversion nanocrystals for sensitive bioimaging in vivo,” J. Am. Chem. Soc. 133(43), 17122–17125 (2011). [CrossRef] [PubMed]
7. H. Tanaka, Y. Furuya, M. Sugiyama, Y. Fujimoto, V. Chani, Y. Yokota, T. Yanagida, Y. Kawazoe, and A. Yoshikawa, “Growth of high-temperature phase KLu2F7 single crystals using quenching process,” J. Cryst. Growth 318(1), 916–919 (2011). [CrossRef]
8. H. Tanaka, Y. Furuya, Y. Yokota, T. Yanagida, A. Yoshikawa, and Y. Kawazoe, “Crystal growth and scintillation properties of Ce doped KLu2F7 single crystal,” in 2010 IEEE Nuclear Science Symposium, (IEEE Nuclear Science Symposium Conference Record, 2010), 220–222.
9. D. Chen, Y. Yu, P. Huang, H. Lin, Z. Shan, and Y. Wang, “Color-tunable luminescence of Eu3+ in LaF3 embedded nanocomposite for light emitting diode,” Acta Mater. 58(8), 3035–3041 (2010). [CrossRef]
10. C. Liu and J. Heo, “Electron energy loss spectroscopy analysis on the preferential incorporation of Er3+ ions into fluoride nanocrystals in oxyfluoride glass-ceramics,” J. Am. Ceram. Soc. 95(7), 2100–2102 (2012). [CrossRef]
11. V. K. Tikhomirov, L. F. Chibotaru, D. Saurel, P. Gredin, M. Mortier, and V. V. Moshchalkov, “Er3+-doped Nanoparticles for Optical Detection of Magnetic Field,” Nano Lett. 9(2), 721–724 (2009). [CrossRef] [PubMed]
12. D. Chen, Y. Yu, P. Huang, F. Weng, H. Lin, and Y. Wang, “Optical spectroscopy of Eu3+ and Tb3+ doped glass ceramics containing LiYbF4 nanocrystals,” Appl. Phys. Lett. 94(4), 041909 (2009). [CrossRef]
13. J. Yang, H. Guo, X. Liu, H. Noh, and J. Jeong, “Down-shift and up-conversion luminescence in BaLuF5:Er3+ glass-ceramics,” J. Lumin. 151, 71–75 (2014). [CrossRef]
14. Y. Wei, X. Liu, X. Chi, R. Wei, and H. Guo, “Intense upconversion in novel transparent NaLuF4:Tb3+, Yb3+ glass-ceramics,” J. Alloy. Comp. 578, 385–388 (2013). [CrossRef]
15. X. Liu, Y. Wei, R. Wei, J. Yang, and H. Guo, “Elaboration, Structure, and Luminescence of Eu3+-Doped BaLuF5-Based Transparent Glass-Ceramics,” J. Am. Ceram. Soc. 96(3), 798–800 (2013). [CrossRef]
16. A. Herrmann, M. Tylkowski, C. Bocker, and C. Rüssel, “Cubic and hexagonal NaGdF4 crystals precipitated from an aluminosilicate glass: preparation and luminescence properties,” Chem. Mater. 25(14), 2878–2884 (2013). [CrossRef]
17. S. Ye, B. Zhu, J. Luo, J. Chen, G. Lakshminarayana, and J. Qiu, “Enhanced cooperative quantum cutting in Tm3+- Yb3+ codoped glass ceramics containing LaF3 nanocrystals,” Opt. Express 16(12), 8989–8994 (2008). [CrossRef] [PubMed]
18. J. P. Zhang, D. C. Yu, F. F. Zhang, M. Y. Peng, and Q. Y. Zhang, “Sequential three-photon near-infrared quantum cutting in transparent fluorogermanate glass-ceramics containing LaF3:Tm3+ nanocrystals,” Opt. Mater. Express 4(1), 111–120 (2014). [CrossRef]
19. W. J. Zhang, D. C. Yu, J. P. Zhang, Q. Qian, S. H. Xu, Z. M. Yang, and Q. Y. Zhang, “Near-infrared quantum splitting in Ho3+:LaF3 nanocrystals embedded germanate glass ceramic,” Opt. Mater. Express 2(5), 636–643 (2012). [CrossRef]
20. E. I. Ardashnikova, M. P. Borzenkova, and A. V. Novoselova, “Transformations in binary potassium fluoride and rare-earth element series,” Russ. J. Inorg. Chem. 25, 1501–1505 (1980).
21. H. Guo, N. Dong, M. Yin, W. Zhang, L. Lou, and S. Xia, “Visible upconversion in rare earth ion-doped Gd2O3 nanocrystals,” J. Phys. Chem. B 108(50), 19205–19209 (2004). [CrossRef]
22. M. Pollnau, D. R. Gamelin, S. R. Lüthi, H. U. Güdel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61(5), 3337–3346 (2000). [CrossRef]