Open Access
Add to BibSonomyAbstract
Upconversion tri-color luminescence of Er3+/Tm3+/Yb3+ doped BaTiO3 nanocrystals is observed under the excitation of a 980 nm laser diode. Especially, Er3+ emission has a considerable contribution to the blue portion of the UC spectra, different from the ever-reported results, in which blue emission originates only from Tm3+. This realization is beneficial to lower the color separation between blue and green (or red) emissions, in fluorescent labeling. The analysis of excitation power dependence and decay time revealed that blue emission of Er3+ ion is induced by a dual energy transfer upconversion, while Tm3+ plays a role of both emitter and activator.
©2009 Optical Society of America
1. Introduction
For investigating the complex biological processes, such as the infection with larval trematodes [1], polynucleotide structural changes [2], and gene delivery [3], the fluorescent labeling technique has been widely used for imaging the biological units at different wavelength [4]. The rapid development in biology demands the high-sensitivity, high-affinity, sharp emission spectral, and biocompatible fluorescent labels for high detection precision [5,6]. However, traditional labels, such as organic dye, fluorescent protein, and dye doped nanoparticles, always have some intrinsic problems of dye leakage from the matrix, photobleaching, and appreciable quenching. Recently, in order to overcome these problems, the lanthanide rare earth (RE) doped UpConversion (UC) inorganic fluorescent nanoparticles [6–8] were identified and designed, which have a wide range of attractive properties including high sensitivity and photostability, sharp emission lines, long lifetimes, and suitable for time resolved methods [8,9].
To select an efficient host material for UC of RE ion is the first step in achieving viable UC nanocrystals. Patra et al. [10] have recently achieved UC light in the dielectric material BaTiO3 by doping RE ion Er3+ and confirmed its UC efficiency is higher than in TiO2 host. Moreover, barium titanate (BaTiO3) has been paid more attention due to its high dielectric constant, ferroelectric activity, spontaneous polarization and nonlinear optical properties [11–13]. The realization of RE doped UC dielectric and ferroelectric materials should unlock a realm of new possibilities in the field of dielectric-optic or ferroelectric-optic multifunctional materials.
Herein, we report the tri-color UC luminescence (or called UC white light) of Er3+/Tm3+/Yb3+ tri-doped dielectric BaTiO3 nanoparticles. Especially, a considerable contribution of Er3+ emission to the blue portion of the UC spectra was achieved in the tri-doped BaTiO3 nanocrystals which is expected to lower the color separation (halo effect) between the blue and green (or red) emissions [14,15].
2. Experimental
The procedure for the synthesis of UC BaTiO3 nanocrystals was adapted from a recently reported synthesis of un-doped BaTiO3 nanocrystals [16]. It is described briefly as follows: Titanium tetrabutoxide was dissolved in a solution of ethylene glycol slightly acidulated with nitric acid and this solution was heated at 60 °C until it became transparent. Then, an aqueous citric acid solution was added and the heating at the same temperature was continued for 3 h. The mole ratio of citric acid to ethylene glycol was 1:1. After the solution cleared, the required amounts of the aqueous RE nitrate and barium nitrate solutions were simultaneously added with RE mole ratio of 1.6Er3+:6Yb3+ mol%, 0.8Tm3+:6Yb3+ mol%, and 1.6Er3+:0.8Tm3+:6Yb3+ mol% and heated for 2 h at 80 °C. At the end of this heat treatment, a pale-yellow solution was obtained. Then, the viscous solution was heated up to 230 °C for 1 h, and a solidified dark-brown glassy resin was obtained. The resin was converted into powder by grinding. Finally, the precursor powders were directly transferred into a furnace, which had been heated to 700 °C, to anneal for 2h. Three samples are named as S-YE for Yb/Er co-doped, S-YT for Yb/Tm co-doped, and S-YET for Yb/Er/Tm tri-doped, respectively.
The photoluminescence spectra and the excitation power dependence under the excitation of a 980 laser diode were measured with a R500 spectrophotometer. X-ray diffraction (XRD) data were recorded on a D/max-3c X-ray diffractometer system with graphite monochromatized Cu Kα irradiation (λ = 0.15418 nm). The powder morphology was characterized by transmission electron microscopy (TEM) JEM2100.
3. Results and discussion
3.1 Crystal structure
Figure 1(a) shows the TEM image of S-YET, from which the average diameter of BaTiO3 nanoparticles was measured to be ~20 nm. The insert is a high resolution image which indicates that both of black BaTiO3 core and white RE-rich shell were crystallized in the annealing. X-ray powder diffraction patterns of S-YET is shown in Fig. 1(b), from which the characteristic diffraction peaks of cubic BaTiO3 phase were observed. The precipitation of RE oxide phase was not observed, which implied that RE were doped efficiently into BaTiO3 matrix. This case is in well agreement with the reported results [17,18] that RE cation dopants are highly soluble in BaTiO3. BaTiO3 has the typical configuration of ABO3. Tsur et al [18] has presented that Yb occupy mainly the B-site, and the compensation is mainly via oxygen vacancies, while Er and Tm are both amphoteric dopants (occupy A- and B-site). The calculated crystallite sizes (~19 nm) from XRD patterns by the Scherrer’s equation are in fair with those measured from TEM.
3.2 Fluorescence spectra
Figure 2 shows the room temperature UC fluorescence spectra of BaTiO3 nanoparticles doped with different concentration of RE ions under the excitation power of 115 mW. It is noted that Fig. 2(c) is not the simple overlap of Fig. 2(a) and 2(b), in which it should be emphasized that the 450 nm blue emission peak was remarkably enhanced in S-YET. Tri-color emissions with the intensity ratio of 1.00(red): 1.07(green): 1.38(whole blue) of S-YET, as shown in Fig. 2(c), composed a bright white UC light and shown in the inset of Fig. 2. The pink surrounding the white center arises from the Gaussian profile of laser beam [6].
Fig. 2 The room temperature UC fluorescence spectra of BaTiO3 nanoparticles under excitation of a 980 nm LD (a) S-YE, (b) S-YT, (c) S-YET. The inset is digital image of white light luminescence of S-YET at the excitation power of 115 mW.
Based on the simplified energy level diagram in Fig. 3 , all emission peaks, i.e., 662 nm (red), 542 and 523 nm (green), and 478 and 461 nm (blue), could be easily assigned to the intra-4f transitions 4F9/2—4I15/2 (Er), 4S3/2/2H11/2—4I15/2 (Er), and 1G4—3H6 (Tm), and 1D2—3F4 (Tm) [19–22], respectively, except the 450 nm one which will be confirmed to be due to a dual energy transfer upconversion (DETU) in the following.
Fig. 3 The simplified energy level diagram of Er3+, Tm3+ and Yb3+ ions and the possible excitation and emission UC mechanisms.
3.3 Possible UC channels investigated by decay time and excitation power dependence
Figure 4 shows the measured fluorescence decay of 4I9/2 (Er) and 3H4 (Tm) under 980 nm pulse excitation. It is known that the position of 3H4 state of Tm and 4I9/2 state of Er on the energy level scheme is nearly the same (at around 795 nm). Consequently, we can observed only one luminescence decay curve for both simultaneously and resonantly excited 3H4 (Tm) and 4I9/2 (Er) states in S-YET. However, we can experimentally proved the resonant energy transfer (RET) from 3H4 (Tm) to 4I9/2 (Er). The most important is luminescence decay from the 3H4 state (Tm), which should be decreased due to the depopulation of 3H4 state of Tm ions during the energy transfer process from 3H4 (Tm) to 4I9/2 (Er). The 4I9/2 lifetime of Er should be theoretically increased, but this effect is rather not observed due to efficient multiphonon relaxation (very fast nonradiative decay to the 4I11/2 state). Considering the 3H4 lifetime of Tm considerably (one or two magnitude of order) higher than for 4I9/2 (Er), the contribution of 4I9/2 (Er) to the luminescence decay from both 3H4 (Tm) and 4I9/2 (Er) excited states of S-YET is rather negligible. If it is experimentally observed that the 3H4 lifetime of Tm is remarkably shortened in S-YET relatively to in S-YT, it can be known that the energy transfer has occurred from 3H4 (Tm) to 4I9/2 (Er). In this work, the luminescence decay curves (Fig. 4) were fitted to the double-exponential fitting functions with a long-decay and a short-decay. The luminescence intensity I(t) could be described by the sum of two exponential decay components using following relation
where τ1 and τ2 were short- and long-decay components, parameters A1 and A2 were fitting constants, respectively. Furthermore, the effective lifetime value, τeff, was also calculated by [23]The main decay components, τ1, from fitting procedure and the effective lifetime value τeff are given in Table 1 , which is in well agreement with the above theoretical analysis.Fig. 4 Fluorescence decays of 795 nm emission of (a) 3H4 level of Tm3+, and (b) 4I9/2 (Er) or 3H4 (Tm) level, (c) 4I9/2 level of Er3+.
Table 1. The average lifetimes (τ) obtained from fitting procedure.
Figure 5(a) shows the shape variation of blue emission band with increasing excitation power. It is clear that the intensity of 450 nm peak increases relatively to 478 nm one with increasing excitation power. There are mainly two processes populating the 4F5/2 level of Er for 450 nm emission: (i) excited state absorption (ESA) 4I9/2→4F5/2 by absorbing a 980 nm photon and (ii) ETU 4I9/2,4I11/2→4I15/2,4F5/2 between Er3+ ions. However, our preliminary investigations indicate that the 450 nm peak could not be detected in BaTiO3: Yb, Er nanocrystals as the Er3+ concentration is very low (< 0.6 mol%). Therefore, it is suggested that ETU (ii) is the main one populating the 4F5/2 level of Er3+ in the higher concentration (1.6 mol%) of Er3+ doped BaTiO3 nanocrystals. On the other hand, the luminescence lifetime of the long-lived 4I11/2 (Er) state is considerably higher than that of 3H4 (Tm). Thus, comparing with ETU (iii) 3F2,3H4→3H6,1D2, ETU (ii) should be more efficient for enhancing the 450 nm emission peak (see Fig. 5). By now, combining RET with ETU (ii), a DETU process is clearly presented for producing 450 nm peak as:
This process is also shown in Fig. 3.Fig. 5 (a) The shape variation of blue emission band with increasing the excitation power, and (b) the excitation power dependence of the blue radiation of S-YET and S-YT.
Of course, the 3H4 state of Tm can transfer directly its energy to 4I11/2 state of Er by ETU(iv) 3H4(Tm),4I11/2(Er)→3H6(Tm),4F5/2(Er). However, according to Dexter’s formulation [24], the energy transfer rate is given by
where is the decay time of the donor emission, is the total absorption cross section of the acceptor ion, R the distance between the donor and the acceptor, b and c the parameter dependent on energy transfer type, and and representing the observed shapes of the donor emission band and the acceptor absorption band, respectively. It is quite clear that the energy transfer rate is in inverse proportion to the decay time and the distance R between donors and acceptors. In the case of this paper, of 3H4(Tm) is higher than 4I9/2(Er) and the distance R between Er3+ and Tm3+ ion is larger than that between two Er3+ ions, since the concentration (1.6 mol%) of Er3+ ion is two times of that (0.8 mol%) of Tm3+. Then, it can be inferred that ETU (iv) is not so efficient as ETU (ii) for populating 4F5/2 level of Er3+ ion.The excitation power dependence of the blue radiation is measured, treated by Auzel’s method [25], and present in Fig. 5(b). The n-value for blue band of S-YT and S-YET are lower than the theoretical value (n = 3) that could be strongly ascribed to the saturation effect [26–29]. As an excited level has nearly saturated population, it will play a role similar with the ground state and leads to the lowering of the corresponding n-values. It is noted that the n-value 1.79 of blue radiation of S-YET is lower than that (2.18) of S-YT. According to the elucidation of saturation effect [26–29], there should be some levels with higher saturation degree in S-YET than those of S-YT. Combining the analysis of Figs. 3-5, it can be inferred to be the long-lived 4I11/2 (Er) state. The saturation of the 4I11/2 (Er) state results in two-photon (n = 1.79) process for populating 4F5/2 (Er) level.
On the other hand, green and red emissions depend indirectly on the population of Er 4I11/2 level, since the 4I11/2 state absorbs a 980 pump photon to be excited to 4F7/2 level which would relax to 4S3/2 and 2H11/2 levels (for green radiation) and 4F9/2 one (for red radiation), respectively. Therefore, green and red emissions should be changed in S-YET relatively to those in S-YE. Figure 6 shows the excitation power dependence of green and red emissions of (a) S-YE and (b) S-YET, respectively. It is observed that the n-value was enhanced in S-YET for both green and red emissions, but more remarkable for green one (from 1.75 to 1.84). For this phenomenon, it is proposed that the transition from 4I11/2 to 4F7/2 level was lowered in S-YET due to the remarkable enhancement of ETU (ii) 4I9/2,4I11/2→4I15/2,4F5/2. As a result, for the transition from 4I11/2 to 4F7/2 level, the saturation degree of 4I11/2 state of Er was decreased in S-YET relatively to in S-YE which lead to the increase of n-values of green and red emissions in S-YET. Furthermore, the green emission depends mainly on the depopulation of 4F7/2 level, while the red emitting level of 4F9/2 can be populated by many possible processes. Then, it is easy to understand why the n-value of green emission is changed more remarkably. The power dependence investigation of green and red emissions confirmed indirectly the efficient occurring of the above DETU process.
Fig. 6 The excitation power dependence of green and red emissions of (a) S-YE and (b) S-YET, respectively.
3.4 Theoretical interpretation based on Dexter’s equation, defect states, and the OH- influence
Why the above DETU process can occur efficiently in this dielectric material BaTiO3? Firstly, according to the energy transfer rate Eq. (3), it will be interpreted as follows. Equation (3) shows that the energy transfer rate depends on the integration of and . The lager the overlap between and is, the higher the energy transfer rate is. Figure 7(a) shows of transition 4I9/2→4I15/2 and of transition 4I11/2→4F5/2, between which the overlap is very small, due to an energy mismatch of 530 cm−1. As a result, the energy transfer rate is very low in the case of Fig. 7(a). However, as the mismatch having been compensated by Raman Vibration Mode (RVM) [A1, E(TO)] (~515 cm−1) of BaTiO3 (see Fig. 7(b)), the overlap between and was significantly enhanced, which would lead to the remarkable improvement of energy transfer rate in the process of 4I9/2,4I11/2→4I15/2,4F5/2. Therefore, it is concluded that the rich RVM of BaTiO3 matrix [16,20], such as 259 cm−1 [A1(TO)], 305 cm−1 [B1, E(TO + LO)], 515 cm−1 [A1, E(TO)], and 717 cm−1 [A1, E(LO)], is one of the most important facts for the efficient occurring of the above DETU process, since they can supply corresponding phonons to various of energy level mismatch in UC processes.
Fig. 7 (a) The of transition 4I9/2→4I15/2 and of transition 4I11/2→4F5/2, and (b) The of transition 4I9/2→4I15/2 and of transition 4I11/2→4F5/2 with compensation of 515 cm−1 phonon of BaTiO3 lattice.
Secondly, it might be ascribed to the influence of defect states in Perovskite structure of BaTiO3. Many cation dopants are highly soluble in BaTiO3, and are used to engineer the electrical properties of the material [17,18]. Tsur et al [18] has presented that Yb occupy mainly the B-site, and the compensation is mainly via oxygen vacancies. Differently, Er and Tm are both amphoteric dopant (occupy A- and B-site). In this work, it is suggested that Er and Tm occupy mainly the A-site due to the large-scale occupation of B-site by Yb. Based on the investigation of Tsur et al [18], their compensation mechanisms are mainly by electrons or titanium vacancies, and dependent on Ba/Ti ratio. It is believed that these compensating electrons or oxygen and titanium vacancies have made a great impact on the surface layer 4f-5d electrons of the doped RE ions which leads to the enhancement of the energy transfer, especially the resonant energy transfer between Er3+ and Tm3+ ions, due to a fact that the population of 4f-5d states of RE ions is affected by the crystal field of matrix [18,30].
Lastly, the fluorescence quenching, which is induced by vibration modes of carbonates and hydroxyl ions, may be negligible. In many RE doped systems, carbonates and hydroxyl ions constitute the most important channel for fluorescence quenching and decrease of fluorescence lifetimes. Jacinto et al [31] have systematically investigated the effect of OH- radicals on the fluorescent properties of RE doped glasses, and determined the fluorescence quantum efficiency (η) with different OH- contents by an efficient thermal lens technique. Same as previous investigation [10], RVM falling in the range from 1500 to 3500 cm−1 has not been observed which indicated that the nonradiative rate induced by OH- radicals has low influence on the energy transfer processes among the doped RE ions, especially the main ones between Tm3+ and Er3+ ions in this BaTiO3 nanocrystals.
In conclusion, the efficient occurring of the discussed DETU process is ascribed to the good compensation of energy-level mismatching by RVM, low influence by carbonates and hydroxyl ions, and the special defect states of Er3+, Tm3+, and Yb3+ ions with complicated compensation mechanisms in BaTiO3. The more detailed investigation of these facts is currently underway.
4. Conclusion
In conclusion, bright white UC luminescence (tri-color UC) is generated in Er/Tm/Yb tri-doped dielectric BaTiO3 nanocrystals (S-YET) under the excitation of a 980 nm laser diode. Especially, the intense 450 nm blue peak is observed in the UC spectra, which is mainly ascribed to the 4F5/2→4I15/2 transition of Er3+ ion and confirmed to be induced by a DETU process. Tm3+ ion plays a role of both emitter (emit 478 nm blue light) and activator (transfer energy to Er). This UC luminescence of tri-doped dielectric BaTiO3 nanocrystals is expected to lower the color separation (halo effect) between blue and green (or red) emissions.
Acknowledgments
This work is supported by the National 863 Project of China and Hunan Provincial Innovation Foundation For Postgraduate (Grant No. S2008YJSCX12).
References and links
1. D. B. Keeney, C. Lagrue, K. Bryan-Walker, N. Khan, T. L. F. Leung, and R. Poulin, “The use of fluorescent fatty acid analogs as labels in trematode experimental infections,” Exp. Parasitol. 120, 15–20 (2008). [CrossRef] [PubMed]
2. A. A. Deniz, T. A. Laurence, G. S. Beligere, M. Dahan, A. B. Martin, D. S. Chemla, P. E. Dawson, P. G. Schultz, and S. Weiss, “Single-molecule protein folding: diffusion fluorescence resonance energy transfer studies of the denaturation of chymotrypsin inhibitor 2,” Proc. Natl. Acad. Sci. U.S.A. 97(10), 5179–5184 (2000). [CrossRef] [PubMed]
3. J. Suh, M. Dawson, and J. Hanes, “Real-time multiple-particle tracking: applications to drug and gene delivery,” Adv. Drug Deliv. Rev. 57(1), 63–78 (2005). [CrossRef]
4. D. S. Lidke, P. Nagy, R. Heintzmann, D. J. Arndt-Jovin, J. N. Post, H. E. Grecco, E. A. Jares-Erijman, and T. M. Jovin, “Quantum dot ligands provide new insights into erbB/HER receptor-mediated signal transduction,” Nat. Biotechnol. 22(2), 198–203 (2004). [CrossRef] [PubMed]
5. Y. S. Li, X. Y. Zhou, and D. Y. Ye, “Molecular beacons: An optimal multifunctional biological probe”, Biochem. Bioph. Res. Co. (posted 1 may 2008, in press).
6. G. Y. Chen, Y. Liu, Y. G. Zhang, G. Somesfalean, Z. G. Zhang, Q. Sun, and F. P. Wang, “Bright white upconversion luminescence in rare-earth-ion-doped Y2O3 nanocrystals,” Appl. Phys. Lett. 91(13), 133103 (2007). [CrossRef]
7. G. S. Yi, H. C. Lu, S. Y. Zhao, Y. Ge, W. J. Yang, D. P. Chen, and L. H. Guo, “Synthesis, Characterization, and Biological Application of Size-Controlled Nanocrystalline NaYF4:Yb,Er Infrared-to-Visible Up-Conversion Phosphors,” Nano Lett. 4(11), 2191–2196 (2004). [CrossRef]
8. J. C. Boyer, F. Vetrone, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4 nanocrystals doped with Er3+, Yb3+ and Tm3+, Yb3+ via thermal decomposition of lanthanide trifluoroacetate precursors,” J. Am. Chem. Soc. 128(23), 7444–7445 (2006). [CrossRef] [PubMed]
9. W. O. Gordon, J. A. Carter, and B. M. Tissue, “Long-lifetime luminescence of lanthanide-doped gadolinium oxide nanoparticles for immunoassays,” J. Lumin. 108(1-4), 339–342 (2004). [CrossRef]
10. A. Patra, C. Friend, R. Kapoor, and P. N. Prasad, “Fluorescence Upconversion Properties of Er3+-Doped TiO2 and BaTiO3 Nanocrystallites,” Chem. Mater. 15(19), 3650–3655 (2003). [CrossRef]
11. A. Jezowski, J. Mucha, R. Pazik, and W. Strek, “Influence of crystallite size on the thermal conductivity in BaTiO3 nanoceramics,” Appl. Phys. Lett. 90(11), 114104 (2007). [CrossRef]
12. P. C. Joshi and S. B. Desu, “Structural, electrical, and optical studies on rapid thermally processed ferroelectric BaTiO3 thin films prepared by metallo-organic solution deposition technique,” Thin Solid Films 300(1-2), 289–294 (1997). [CrossRef]
13. D. A. Tenne, A. Soukiassian, X. X. Xi, H. Choosuwan, R. Guo, and A. S. Bhalla, “Lattice dynamics in BaxSr1− xTiO3 single crystals: A Raman study,” Phys. Rev. B 70(17), 174302 (2004). [CrossRef]
14. V. Sivakumar and U. V. Varadaraju, “Intense Red-Emitting Phosphors for White Light Emitting Diodes,” J. Electrochem. Soc. 152(10), H168–H171 (2005). [CrossRef]
15. Y.-H. Won, H. S. Jang, W. B. Im, D. Y. Jeon, and J. S. Lee, “Tunable full-color-emitting La0.827Al11.9O19.09:Eu2+, Mn2+ phosphor for application to warm white-light-emitting diodes,” Appl. Phys. Lett. 89(23), 231909 (2006). [CrossRef]
16. P. Durán, F. Capel, J. Tartaj, D. Gutierrez, and C. Moure, “Heating-rate effect on the BaTiO3 formation by thermal decomposition of metal citrate polymeric precursors,” Solid State Ion. 141-142(1), 529–539 (2001). [CrossRef]
17. P. Ghosh, S. Sadhu, T. Sen, and A. Patra, “Upconversion emission of BaTiO3:Er nanocrystals,” Bull. Mater. Sci. 31(3), 461–465 (2008). [CrossRef]
18. Y. Tsur, T. D. Dunbar, and C. A. Randall, “Crystal and Defect Chemistry of Rare Earth Cations in BaTiO3,” J. Electroceram. 7(1), 25–34 (2001). [CrossRef]
19. W. F. Krupke and J. M. Poindexter, “Energy Levels of Er 3+ in LaF3 and Coherent Emission at 1.61 mu,” J. Chem. Phys. 41(5), 1225 (1964). [CrossRef]
20. C. Jacinto, M. V. D. Vermelho, E. A. Gouveia, M. T. de Araujo, P. T. Udo, N. G. C. Astrath, and M. L. Baesso, “Pump-power-controlled luminescence switching in Yb3+/Tm3+ codoped water-free low silica calcium aluminosilicate glasses,” Appl. Phys. Lett. 91(7), 071102 (2007). [CrossRef]
21. P. A. Tanner, C. S. K. Mak, W. M. Kwok, K. L. Phillips, and M. F. Joubert, “Luminesence from the 3P2 State of Tm3+,” J. Phys. Chem. B 106(14), 3606–3611 (2002). [CrossRef]
22. W. A. Pisarski, T. Goryczka, J. Pisarska, and W. Ryba-Romanowski, “Er-doped lead borate glasses and transparent glass ceramics for near-infrared luminescence and up-conversion applications,” J. Phys. Chem. B 111(10), 2427–2430 (2007). [CrossRef] [PubMed]
23. C. Jacinto, S. L. Oliveira, L. A. O. Nunes, J. D. Myers, M. J. Myers, and T. Catunda, “Normalized-lifetime thermal-lens method for the determination of luminescence quantum efficiency and thermo-optical coefficients: Application to Nd3+-doped glasses,” Phys. Rev. B 73(12), 125107 (2006). [CrossRef]
24. D. J. Dexter, “Theory of Concentration Quenching in Inorganic Phosphors,” J. Chem. Phys. 21, 836 (1953). [CrossRef]
25. F. Auzel, “Materials and devices using double-pumped-phosphors with energy transfer,” Proc. IEEE 61(6), 758–786 (1973). [CrossRef]
26. F. Pandozzi, F. Vetrone, J. C. Boyer, R. Naccache, J. A. Capobianco, A. Speghini, and M. Bettinelli, “A spectroscopic analysis of blue and ultraviolet upconverted emissions from Gd3Ga5O12:Tm3+, Yb3+ nanocrystals,” J. Phys. Chem. B 109(37), 17400–17405 (2005). [CrossRef]
27. H. Sun, Z. Duan, G. Zhou, C. Yu, M. Liao, L. Hu, J. Zhang, and Z. Jiang, “Ch. Yu, M. Liao, L. Hu, J. Zhang, and Z. Jiang, “Structural and up-conversion luminescence properties in Tm3+/Yb3+-codoped heavy metal oxide–halide glasses,” Spectrochim. Acta [A] 63(1), 149–153 (2006). [CrossRef]
28. S. Xu, H. Ma, D. Fang, Z. Zhang, and Z. Jiang, “Upconversion luminescence and mechanisms in Yb3+-sensitized Tm3+-doped oxyhalide tellurite glasses,” J. Lumin. 117(2), 135–140 (2006). [CrossRef]
29. I. R. Martin, V. D. Rodriguez, V. Lavin, and U. R. Rodriguez-Mendoza, “Infrared, blue and ultraviolet upconversion emissions in Yb3+–Tm3+-doped fluoroindate glasses,” Spectrochim. Acta [A] 55(5), 941–945 (1999). [CrossRef]
30. S. Saha, P. S. Chowdhury, and A. Patra, “Luminescence of Ce3+ in Y2SiO5 nanocrystals: Role of crystal structure and crystal size,” J. Phys. Chem. B 109(7), 2699–2702 (2005). [CrossRef]
31. C. Jacinto, S. L. Oliveira, L. A. O. Nunes, T. Catunda, and M. J. V. Bell, “Thermal lens study of the OH− influence on the fluorescence efficiency of Yb3+-doped phosphate glasses,” Appl. Phys. Lett. 86(7), 071911 (2005). [CrossRef]
