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J. Phys. Chem. B 2006, 110, 5950-5954
Preparation Process and Upconversion Luminescence of Er3+-Doped Glass Ceramics Containing Ba2LaF7 Nanocrystals Xianping Fan,*,† Jin Wang,† Xvsheng Qiao,† Minquan Wang,† Jean-Luc Adam,‡ and Xianghua Zhang‡ Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and UMR-CNRS 6512 “Verres & Ceramiques”, Institut de Chimie de Rennes, UniVersite de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France ReceiVed: October 11, 2005; In Final Form: February 8, 2006
The preparation process and upconversion luminescence of the Er3+-doped glass ceramics containing Ba2LaF7 nanocrystals were investigated. The formation of Ba2LaF7 nanocrystals in the glass ceramics was confirmed by X-ray diffraction. Er3+-doped glass ceramics containing Ba2LaF7 nanocrystals exhibited highly efficient upconversion luminescence in comparison with glasses. With the increase of heat treatment temperature the upconversion luminescence intensity increased gradually. The composition of glasses was also found to have significant influence on the crystallization process of glass ceramics. The mixture of Ba2LaF7 and La2O3 nanocrystals and the mixture of La2F3 and La2O3 nanocrystals in the glass ceramics could be obtained by controlling different compositions of glasses. The upconversion luminescence intensity also varied significantly with different nanocrystals in the glass ceramics.
1. Introduction Rare earth (RE) doped materials for upconversion luminescence are attractive for realizing UV-vis compact solid-state lasers and display equipment, pumped by high-power nearinfrared laser diodes.1 The upconversion efficiency depends largely on the structure of the energy levels of RE ions and their local environment.2 The key technique to designing upconversion luminescence materials is to situate the RE ions in a low-phonon-energy environment. Fluorides, chlorides, bromides, and iodides are known to be efficient upconverters.3 This is mainly because multiphonon relaxation is strongly suppressed in these systems due to their low phonon energy. However, the sensitivity to chemicals and the difficulty in fabrication still restrict their applications.4 Oxides possess higher chemical and mechanical stability. In conventional silicate glasses, however, there are few reports of the upconversion phenomenon because of large nonradiative losses due to highenergy vibrations that couple to the RE ions. To satisfy both the low-phonon-energy environment for RE ions and good chemical and mechanical stability for practical use, oxyfluoride glass ceramics have been developed as ideal RE ion hosts used for upconversion devices since 1993.5 Such materials combine the low-phonon-energy environment of fluoride crystals and the high chemical and mechanical stability of oxide glasses. Because of the nanosize of fluoride crystals, these oxyfluoride glass ceramics remain transparent. Until now, four main types of optically transparent oxyfluoride glass ceramics doped with rare earth ions have been achieved: (i) those prepared by Wang and Ohwaki5 and Tick et al.,6 based on the formation of a (Pb,Cd)F2 cubic phase, incorporating Er3+(Tm3+)/Yb3+ or Pr3+, in an aluminosilicate base glass; (ii) formation of LaF3 crystals in a sodium aluminosilicate base glass, which was reported by * Corresponding author. Telephone: 0086-571-87952334. Fax: 0086571-87951234. E-mail:
[email protected]. † Zhejiang University. ‡ Universite de Rennes 1.
Dejneka;7 (iii) β-PbF2:Er3+ nanocrystals dispersed in the germanate glasses that were reported by Mortier8 and Dantelle et al.9,10 and in the silicate glass that was reported by Kawamoto et al.;11 (iv) formation of glass ceramics containing CaF2 nanocrystals, which was reported by Qiao et al.12 To find new highly efficient upconversion luminescence materials, we developed a novel kind of Er3+-doped transparent glass ceramics containing Ba2LaF7 nanocrystals. The reagents (SiO2, Al2O3, Na2O, NaF, BaF2, La2O3) of this kind of glass ceramics avoided toxic elements Pb and Cd and expensive LaF3. The Ba2LaF7 crystal belongs to cubic crystal series. Rao et al. prepared first Ba2YF7 (and its other rare earth analogues including Ba2LaF7) from the decomposition of YBa2Cu3O3F7 at 800 °C in a vacuum in 1992.13,14 Until now, however, there has been no report of glass ceramics containing Ba2LaF7 nanocrystals. The Er3+-doped Ba2LaF7 exhibited compositional and structural analogy or proximity to the Er3+-doped Ba2YCl7, which possessed highly efficient upconversion luminescence; they were first prepared by Egger et al. in 1996.15 Therefore, the efficient upconversion process of Er3+-doped glass ceramics containing Ba2LaF7 nanocrystals could be expected. In this paper, the formation process of Ba2LaF7 nanocrystals in oxide glass and the upconversion luminescence of the Er3+-doped glass ceramics containing Ba2LaF7 nanocrystals are described. 2. Experimental Section Oxyfluoride glasses were prepared by melting the appropriate batch materials (SiO2, Al2O3, Na2O, NaF, BaF2, La2O3) in covered corundum crucibles in normal atmosphere. Compositions of the glasses are given in Table 1. After melting for 2 h at 1400 °C, the melts were poured onto a brass plate and then pressed by another brass plate. The glasses were transparent except for no. 2 glass, which showed cloudy areas on the sample surface due to excessive fluoride in the glass composition. The obtained glasses were then polished on a UNIPOL-802 precision lapping/polishing machine. Finally, glass ceramics were prepared
10.1021/jp055780i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/08/2006
Luminescence of Er3+-Doped Glass Ceramics
J. Phys. Chem. B, Vol. 110, No. 12, 2006 5951
TABLE 1: Composition of Oxyfluoride Glasses and Experimental Data from DTA Measurements no. 1 no. 2 no. 3 no. 4 no. 5
composition (mol %)
BaF2/La2O3
Tg (°C)
Tx1 (°C)
45SiO2-15Al2O3-12Na2O-21BaF2-7La2O3-0.5ErF3 45SiO2-15Al2O3-16NaF-16BaF2-8La2O3-0.5ErF3 45SiO2-15Al2O3-10Na2O-20BaF2-10La2O3-0.5ErF3 45SiO2-15Al2O3-20NaF-10BaF2-10La2O3-0.5ErF3 50SiO2-20Al2O3-10Na2O-10BaF2-10La2O3-0.5ErF3
3:1 2:1 2:1 1:1 1:1
610
715 775 760 780 790
by heat treatment for 2 h at the different temperatures chosen from differential thermal analysis (DTA) measurements. DTA measurements were carried out in a CDR-1 differential thermal analyzer to establish the glass transition temperature (Tg) and the crystallization peak temperature (Tx). X-ray diffraction measurements were performed with a XD-98 diffractometer with Cu KR radiation at 4°/min scanning rate. Transmittance spectra were measured with a Hitachi U-4100 UV-vis-NIR spectrophotometer. Upconversion luminescence measurements were performed with a Hitachi F-4500 fluorescence spectrophotometer, using 980 nm radiation from a laser diode. The luminescence decay curves were measured with a SP-750 monochromator, a photomultiplier tube, a BOXCAR, and a NCL multichannel data collecting analysis system. The sample was excited by the emission line at 355 nm from the third harmonic of an Xe-lamp pumped Q-switched Nd:YAG laser. All measurements were performed at room temperature. 3. Results and Discussion The compositions of the glasses are given in Table 1. Figure 1 shows the DTA curves of different glasses with indications
Figure 1. DTA curves of the different glasses.
of Tg and Tx. The results are listed in Table 1. To obtain transparent oxyfluoride glass ceramics containing Ba2LaF7 nanocrystals, the no. 1 glass samples were chosen to be heat treated for 2 h at 610, 630, 640, and 650 °C, respectively. Figure 2 shows the X-ray diffraction (XRD) patterns of no. 1 glass (named “G”) and the glass ceramics that were prepared by heat treatment of the no. 1 glasses for 2 h at 610 °C (named “GC610”), 630 °C (named “GC-630”), 640 °C (named “GC-640”), and 650 °C (named as “GC-650”). G and GC-610 are completely amorphous with no diffraction peaks. The diffraction peaks of GC-630, GC-640, and GC-650 can be easily assigned to the cubic Ba2LaF7 phase. With the increase of heat treatment temperature, the diffraction peaks became sharper, which indicated the gradual formation of Ba2LaF7 nanocrystals. Table 2 shows the lattice parameters, unit cell volume, and crystallization fraction of Ba2LaF7 crystallites in glass ceramics that were obtained from XRD in comparison with the standard Powder Diffraction File (PDF) values. The lattice parameters
650 660 680
TABLE 2: Lattice Parameters, Unit Cell Volume, and Crystallization Fraction of Ba2LaF7 Crystallites Obtained from XRD and Standard PDF Values standard GC-630 GC-640 GC-650
crystn fraction (%)
a
b
c
volume
100 18.1 24.5 31.0
6.088 6.084 6.072 6.066
6.088 6.084 6.072 6.066
6.088 6.084 6.072 6.066
225.644 225.200 223.870 223.207
of Ba2LaF7 crystallites in glass ceramics were found to be smaller than the standard values. The lattice parameters and unit cell volume gradually decreased with increase of the heat treatment temperature. The radius of Er3+ is smaller than that of La3+. In Er3+-doped glass ceramics containing Ba2LaF7 nanocrystals, Er3+ ions can enter Ba2LaF7 nanocrystals by substituting for La3+ ions, which resulted in the shrinkage of the Ba2LaF7 unit cell. With increase of the heat treatment temperature, therefore, Ba2LaF7 nanocrystals grew gradually and more Er3+ ions entered Ba2LaF7 nanocrystals, which resulted in gradual decrease of the lattice parameters and unit cell volume of Ba2LaF7 nanocrystals. From the obtained peak width of the XRD pattern, the size of Ba2LaF7 nanocrystals in glass ceramics can be also calculated by the Debye-Scherrer equation.5,12 The obtained sizes, which were calculated from the strongest peak [111], are about 27 nm for GC-630, 49 nm for GC-640, and 65 nm for GC-650, respectively. Figure 3 shows the transmittance spectra of no. 1 glass and glass ceramics. The glass sample had high transparency. With the increase of the heat treatment temperature, however, Ba2LaF7 nanocrystals became larger and the crystallization fraction became higher. Thus the transmittances of the glass ceramics decreased gradually. Figure 4 shows the upconversion emission spectra (excited at 980 nm) of no. 1 glasses that were heat treated for 2 h at different temperatures. The emission bands can be assigned to 2H 4 4 4 11/2 f I15/2 (525 nm) and S3/2 f I15/2 (545 nm) transitions, respectively. In the glasses, upconversion luminescence can hardly be observed. However, significant upconversion luminescence of Er3+ ions can be observed in the glass ceramics. The emission band corresponding to the 4S3/2 f 4I15/2 transition is split into two distinct peaks. The upconversion emission
Figure 2. XRD patterns of no. 1 glass and glass ceramics that were prepared by heat treatment of no. 1 glasses for 2 h at different temperatures.
5952 J. Phys. Chem. B, Vol. 110, No. 12, 2006
Fan et al.
Figure 3. Transmittance spectra of no. 1 glass and glass ceramics that were prepared by heat treatment of no. 1 glasses for 2 h at different temperatures.
Figure 6. Luminescence decay curves of the 4S3/2 state of Er3+ in no. 1 glass and glass ceramic that was prepared by heat treatment of no. 1 glasses for 2 h at 640 °C.
TABLE 3: Crystal Phases Precipitated in Glass Ceramics Prepared from Different Glasses and Maximum Phonon Energy of Different Hosts
no. 1 no. 2 no. 3 no. 4 no. 5
Figure 4. Upconversion emission spectra of no. 1 glass and glass ceramics that were prepared by heat treatment of no. 1 glasses for 2 h at different temperatures.
Figure 5. Two possible upconversion luminescence mechanisms of Er3+ in the glass ceramics: (a) excited energy absorption; (b) energy transfer upconversion.
intensity increases obviously with increases of the heat treatment temperature. Two possible upconversion luminescence mechanisms of the Er3+ ions have been described by (a) excited energy absorption (ESA) and (b) energy transfer upconversion (ETU),16 which are shown in Figure 5. In these two mechanisms, the 4f electrons of Er3+ ions at the ground state are promoted to the 4I11/2 excited state by excitation with 980 nm light. Then, they can be excited to the 4F7/2 level by excitation state absorption (ESA) of pump photons at 980 nm or by cross relaxation (CR) from excited adjacent Er3+ ions. Finally, the electrons populate the 4S3/2, 2H11/2 levels from 4F7/2 by multiphonon relaxation, which produces green luminescence corresponding to 2H11/2 f 4I15/2 and 4S3/2 f 4I15/2 transitions. Because there are many metastable energy levels between 4S3/2,
BaF2/ La2O3
F/Oa
3:1 2:1 2:1 1:1 1:1
42/34 48/24 40/40 40/30 20/50
treatment temp (°C)
crystalline phases
max phonon energy hωmaxb (cm-1)
630
pure Ba2LaF7 pure Ba2LaF7 Ba2LaF7 and La2O3 LaF3 and La2O3 pure La2O3
BaF2: ∼238 Ba2LaF7: ∼270 LaF3: ∼305 La2O3: ∼411 silicate: ∼1100
650 700 700
a The ratio of F/O is only determined from the extranetwork substances (Na2O, La2O3, NaF, BaF2), excepting the network-forming substancessSiO2 and Al2O3. b From refs 19-21.
2H
11/2 levels and the ground state, the upconversion luminescence of Er3+ ions is usually baffled by the multiphonon relaxation. The multiphonon relaxation probability depends primarily upon the energy gap between two successive levels and the phonon energy of the host.17,18 The lower the phonon energy of host is, the smaller the multiphonon relaxation probability is. Due to a much smaller phonon energy of Ba2LaF7 nanocrystals than that of the silicate glass host, as shown in Table 3, the upconversion luminescence of Er3+ ions in the Ba2LaF7 nanocrystals will be stronger than that in the glasses. From XRD patterns it was found that the crystal size and the crystallization fraction in the glass ceramics increased with the increase of the heat treatment temperature, which implied that more Er3+ ions have been surrounded by the Ba2LaF7 nanocrystals. Thus the upconversion luminescence intensity increased significantly with the increase of the heat treatment temperature. Figure 6 shows luminescence decay curves of the 4S3/2 state of Er3+ in the no. 1 glass and glass ceramic by monitoring the 4S 4 3+ 3/2 f I15/2 emission of Er . The decay curves of the glass can be fitted to a single-exponential function, yielding the lifetime of 20.14 ( 1.44 µs. The decay curves of the glass ceramics were fitted to a double-exponential function, yielding the average lifetime of 191.52 ( 14.15 µs. The results clearly show that fluoride nanocystal precipitation in the glass host led to greatly longer 4S3/2 lifetimes. The different ratios of BaF2/La2O3, 3:1, 2:1 and 1:1 (as given in Table 1), have been used to investigate the effect of glass compositions on the formation process of nanocrystals in the glass ceramics. In addition, the amounts of fluorides were also varied through changing the amounts of NaF and Na2O. From
Luminescence of Er3+-Doped Glass Ceramics
Figure 7. XRD patterns of no. 1-no. 5 glass ceramics.
Figure 1 it can be seen that the glass transition temperature (Tg) and the first and second crystallization temperatures (Tx1 and Tx2) varied significantly with glass compositions. Two distinct crystallization peaks were present in all glasses. The first crystallization temperature, Tx1, at lower temperature can be attributed to the crystallization of a fluoride and/or lanthanide oxide. The second crystallization temperature, Tx2, at higher temperature can be attributed to the crystallization process of several mixed silicate oxides and alumina oxide that have been confirmed by XRD measurements. From the Tg and Tx1 values of five glasses presented in Table 1, it is observed that both Tg and Tx shift to higher temperature with the increase of oxides in the glasses. This may be due to the higher fusion temperatures of oxides compared to fluorides. Figure 7 shows the corresponding XRD patterns of glass ceramics that were prepared from no. 1-no. 5 glasses. Table 3 shows the crystal phases precipitated in glass ceramics. The composition of glasses has a significant influence on the crystallization process. When the ratio of BaF2/La2O3 is 3 (no. 1 glass ceramic), only Ba2LaF7 nanocrystals can be formed. When the ratio of BaF2/La2O3 decreases to 2, La2O3 nanocrystals would precipitate together with Ba2LaF7 nanocrystals if no other fluorides were added to the glass (no. 3 glass ceramic). If more fluorides were added by substituting NaF for Na2O, however, pure Ba2LaF7 nanocrystals could still be obtained (no. 2 glass ceramic). When the ratio of BaF2/La2O3 was 1, only La2O3 nanocrystals could be formed if no other fluorides were added to the glass (no. 5 glass ceramic). If more fluorides were added by substituting NaF for Na2O and Al2O3, however, LaF3 nanocrystals would precipitate together with La2O3 nanocrystals (no. 4 glass ceramic). The ionic radius of La3+ (1.19 Å) is smaller than that of Ba2+ (1.50 Å), and La3+ also has a higher valence than Ba2+. Ba2+ seemed not be easy to crystallize in comparison with La3+. Only when amounts of BaF2 were rather high (e.g., ratio of BaF2/La2O3 was up to 2), Ba2+ could crystallize with La3+ and F- to form Ba2LaF7 nanocrystals. Therefore, the increasing ratio of BaF2/La2O3 would result in formation of Ba2LaF7 nanocrystals. Figure 8 shows the upconversion emission spectra of the no. 1-no. 5 glass ceramics. The emission band around 545 nm is assigned to 4S3/2 f 4I15/2 transitions. This band splits into two distinct subpeaks due to the crystal field effect. The emission band around 525 nm is assigned to 2H11/2 f 4I15/2 transitions. It was obvious that the upconversion luminescence intensity of Er3+-doped glass ceramics depended significantly on the maximum phonon energy of precipitated fluoride nanocrystals in the glasses. Table 3 also shows the maximum phonon energy of Ba2LaF7, LaF3, and La2O3 crystals. The maximum phonon energy can be found to be La2O3 > LaF3 > Ba2LaF7. Similar luminescence intensity in both no. 1 and no. 2 glass ceramics
J. Phys. Chem. B, Vol. 110, No. 12, 2006 5953
Figure 8. Upconversion emission spectra of no. 1-no. 5 glass ceramics.
can be observed due to the same Ba2LaF7 crystalline phase. For the no. 3 glass ceramic, the mixture of La2O3 and Ba2LaF7 crystalline phase resulted in weaker upconversion luminescence intensity in comparison with those of the no. 1 and no. 2 glass ceramics. The upconversion luminescence intensity of the no. 4 glass ceramic decreased further due to a lower ratio of F-/O2-. For the no. 5 glass ceramic, there was only a La2O3 crystalline phase and the upconversion luminescence could not be observed. 4. Conclusions A new kind of oxyfluoride glass ceramic containing Ba2LaF7 nanocrystals was prepared. The formation of Ba2LaF7 nanocrystals was confirmed by XRD. Er3+-doped glass ceramics containing Ba2LaF7 nanocrystals exhibited efficient upconversion luminescence in comparison with that of glasses. With the increase of the heat treatment temperature the upconversion luminescence intensity increased gradually. The composition of glasses has a significant influence on the crystallization process of glass ceramics. The pure Ba2LaF7 nanocrystals, a mixture of Ba2LaF7 and La2O3 nanocrystals, a mixture of La2F3 and La2O3 nanocrystals, or pure La2O3 nanocrystals can be obtained by controlling different compositions of glasses. The upconversion luminescence intensity also varied significantly with different nanocrystals in glass ceramics. Acknowledgment. The authors gratefully acknowledge support for this research from the National Nature Science Foundation of China (No. 50472062). References and Notes (1) Ferber, S.; Gaebler, V.; Eichler, H. J. Opt. Mater. 2002, 20, 211. (2) Chivian, J. S.; Case, W. E.; Eden, D. D. Appl. Phys. Lett. 1979, 35, 124. (3) Hehlen, M. P.; Kramer, K.; Gudel, H. U.; Mcfarlane, R. A.; Schwartz, R. N. Phys. ReV. B 1994, B49, 12475. (4) Riseberg, L. A.; Moos, H. W. Phys. ReV. 1968, 174, 429. (5) Wang, Y.; Ohwaki, J. Appl. Phys. Lett. 1993, 63, 3268. (6) Tick, P. A.; Borreli, N. F.; Cornelius, L. K.; Newhouse, M. A. J. Appl. Phys. 1995, 78, 6367. (7) Dejneka, M. J. J. Non-Cryst. Solids 1998, 239, 149. (8) Mortier, M.; Goldner, P.; Chateau, C.; Genotelle, M. J. Alloys Compd. 2001, 323-324, 245. (9) Dantelle, G.; Mortier, M.; Vivien, D.; Patriarche, G. Chem. Mater. 2005, 17 (8), 2216. (10) Dantelle, G.; Mortier, M.; Vivien, D.; Patriarche, G. J. Mater. Res. 2005, 20 (2), 472. (11) Kawamoto, Y.; Kanno, R.; Qiu, J. J. Mater. Sci. 1998, 33, 63. (12) Qiao, X. S.; Fan, X. P.; Wang, J.; Wang, M. Q. J. Non-Cryst. Solids 2005, 351, 357. (13) Rao, U. R. K.; Tyagi, A. K.; Muraldharan, K. V. J. Mater. Sci. Lett. 1992, 11, 435.
5954 J. Phys. Chem. B, Vol. 110, No. 12, 2006 (14) Tyagi, A. K.; Rao, U. R. K.; Iyer, R. M. J. Mater. Sci. Lett. 1993, 12, 1663. (15) Egger, P.; Rogin, P.; Riedener, T.; Gudel, H. U.; Wickleder, M. S.; Hulliger, J. AdV. Mater. 1996, 8, 668. (16) Auzel, F. Chem. ReV. 2004, 104, 139. (17) Weber, M. J. Phys. ReV. 1968, 171, 283.
Fan et al. (18) Reisfeld, R.; Boehm, L.; Eckstein, Y.; Lieblich, N. J. Lumin. 1975, 10, 193. (19) Reisfeld, R.; Jørgensen, C. K. Laser and Excited States of Rare Earth; Springer-Verlag: New York, 1977; p 99. (20) Oostra, D. J.; Hartog, H. W. Phys. ReV. B 1984, 29 (5), 8840. (21) Gerald, B.; Dacol, F. H. Phys. ReV. B 1990, 41 (7), 4747.
