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2007, 111, 2427-2430 Published on Web 02/16/2007
Er-Doped Lead Borate Glasses and Transparent Glass Ceramics for Near-Infrared Luminescence and Up-Conversion Applications Wojciech A. Pisarski,*,† Tomasz Goryczka,† Joanna Pisarska,‡ and Witold Ryba-Romanowski§ Institute of Materials Science, UniVersity of Silesia, Katowice, Poland, Department of Materials Science, Silesian UniVersity of Technology, Katowice, Poland, and Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław, Poland ReceiVed: January 8, 2007; In Final Form: February 7, 2007
Lead borate based glasses have been analyzed using Raman and infrared spectroscopy. The formation of different borate groups and the direction of BO3 T BO4 conversion strongly depends on the PbO- and/or PbF2-to-B2O3 ratio in chemical composition. PbF2-PbO-B2O3 based glasses containing Er3+ ions have been studied after annealing. The orthorhombic PbF2 crystallites are formed during thermal treatment, which was evidenced by X-ray diffraction analysis. Near-infrared luminescence at 1530 nm and green up-conversion at 545 nm have been registered for samples before and after annealing. The luminescence bands correspond to 4I 4 4 4 3+ ions, respectively. In comparison to the precursor glasses, the 13/2- I15/2 and S3/2- I15/2 transitions of Er luminescence intensities are higher in the studied transparent oxyfluoride glass ceramics. Simultaneously, the half-width of the luminescence lines slightly decreases. It can be the evidence that a small amount of the Er3+ ions is incorporated into the orthorhombic PbF2 phase.
During controlled crystallization of oxyfluoride glasses, also so-called devitrification or ceramming process, the formation of fluoride crystallites embedded in the oxide glass matrix takes place. Rare earth as an optically active ion is usually incorporated into the fluoride crystalline phase. As a consequence, spectral lines are narrowed and luminescence decays from excited states of rare earth ions in transparent glass ceramics are relatively longer in comparison to precursor glasses. This behavior has been observed for the most important Er3+ ions in several transparent glass ceramic (TGC) systems, which are promising for near-infrared (NIR) luminescence at 1.5 µm and up-conversion applications.1-3 However, the luminescence properties of rare earth ions in TGC systems strongly depend on the chemical composition of the host and the thermal treatment conditions. Even a slight component modification in the host matrix or a change of annealing time and temperature influence glass devitrification and the degree of rare earth incorporation into the crystalline phase. It is noted that the Er3+ ions in the composite glass ceramic materials are usually divided into two parts.2 The one part is incorporated into the crystalline fluoride phase. It results in a higher intensity and narrowing of spectral lines and elongation of luminescence decays from excited states of rare earth ions. The other part remains in the glassy matrix, which has no contribution to luminescence characteristics due to the high phonon energy of the oxide host. The local environment and its modification, as well as the concentration and distribution of the optically active ions in the * Corresponding author. Address: University of Silesia, Institute of Materials Science, Bankowa 12, 40-007 Katowice, Poland. Phone: +48 32 359 16 20. E-mail:
[email protected]. † University of Silesia. ‡ Silesian University of Technology. § Polish Academy of Sciences.
10.1021/jp070142g CCC: $37.00
crystalline and noncrystalline part of the host matrix affect the emission parameters like intensity, efficiency, cross section, line width, and lifetime. These parameters play an important role for selection of the Er-doped luminescent materials4-7 as solidstate lasers, fiber amplifiers (EDFA), and NIR-to-visible converters. Oxide and oxyfluoride lead borates belong to the wide family of heavy metal glass systems, which are attractive to study their physicochemical properties. Different structural groups like penta- and diborates containing three and four coordinated boron, ring- and chain-type metaborate units containing nonbridging oxygens, and boroxol ring can be found in borate based glasses. The existence of structurally different borate units in this glass system is favorable to be investigated by spectroscopic methods. Moreover, the fluorescence yield increases with the addition of heavy metal oxide (PbO) or fluoride (PbF2) to the borate matrix.8,9 On the other hand, B2O3 increases the nonradiative multiphonon decay rate of the 4I11/2-4I13/2 transition of Er3+ due to the relatively high phonon energy of the glass host.10 As a consequence, the energy excitation transfers nonradiatively very fast to the 4I13/2 upper laser state and near-infrared laser action at about 1.5 µm due to 4I13/2-4I15/2 transition of Er3+ ions can be observed, when population inversion between the 4I 4 13/2 and I15/2 states occurs. It is important for laser operation of Er3+ in three-level systems, when the 4I11/2 state gets populated first under excitation at the 980 nm line by a laser diode. For that reason, lead borate glasses are interesting for structural and optical investigations. Recently, oxide8 and oxyfluoride9,11 lead borate glasses containing Er3+ have been studied. In this Letter, the oxyfluoride lead borate glasses containing Er3+ ions have been analyzed after annealing. The samples were © 2007 American Chemical Society
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Figure 1. Raman spectra for Er-doped lead borate glasses. For the 1:1 (b) and 4:1 (c) systems, PbO was partially substituted by PbF2 (9%). The inset shows the infrared (FT-IR) spectra recorded for samples in the 1:2, 1:1, 4:1, and 8:1 systems, respectively.
investigated using X-ray diffraction (XRD), Raman, infrared (FT-IR), and luminescence spectroscopy. This problem has not yet been discussed in the literature, to our knowledge. As far as we know, the oxide B2O3-PbO glass ceramics were prepared, where PbMoO4 crystallites are formed during thermal treatment.12 Er-doped lead borate glasses with xPbO-(90 - x)B2O36Al2O3-3WO3-1Er2O3 chemical composition were prepared, where x ) 30, 45, 72, and 80 wt % due to PbO-B2O3 ) 1:2, 1:1, 4:1, and 8:1 systems, respectively. For 1:1 and 4:1 systems, PbO was partially (9%) or totally (45 or 72%) substituted by PbF2. Anhydrous oxides and lead fluoride (99.99% purity, Aldrich) were used as starting materials. A homogeneous mixture was heated in a protective atmosphere of dried argon. Glasses were melted at 900 °C in Pt crucibles and then poured into preheated copper molds and annealed below the glass transition temperature. After this procedure, the samples were slowly cooled to room temperature. Transparent oxyfluoride glass ceramics were obtained by thermal treatment of the precursor glasses at their corresponding crystallization temperatures, determined by a Perkin-Elmer differential scanning calorimeter (DSC). The oxyfluoride glasses were annealed above the glass transition temperature, Tg, under different thermal treatment conditions, at 375, 400, and 425 °C, for 1, 5, 10, and 15 h, respectively. The X-ray diffraction (XRD) patterns were carried out using an INEL diffractometer with Cu KR radiation. The Raman and infrared spectra were measured with a FT-IR Bruker spectrometer using an Ar+ ion laser at a wavelength of 514.5 nm. The luminescence of samples was excited by a Continuum Surelite optical parametric oscillator (OPO), pumped by the third harmonic of a Nd:YAG laser. The luminescence was dispersed by a 1 m double grating monochromator and detected with a photomultiplier with S-20 spectral response. The luminescence spectra were recorded using a Stanford SRS 250 boxcar integrator controlled by a computer. Luminescence decay curves were recorded and stored by a Tektronix TDS 3052 oscilloscope. The Raman spectra recorded for Er-doped lead borate glasses are presented in Figure 1. The inset shows infrared (FT-IR) spectra in the 600-1100 cm-1 range, where infrared bands at about 700 and 850-1050 cm-1 due to BO3 bending and B-O stretching vibration of tetrahedral BO4 units are located.13 Both the Raman and the FT-IR spectra have been analyzed for lead borate glasses, where the PbO- and/or PbF2-to-B2O3 ratio is close
Letters to the 1:2, 1:1, 4:1, and 8:1 systems, respectively. The strong peak at about 805 cm-1 has been observed for the 1:2 system, which can be attributed to totally symmetric vibration of the boroxol ring (trigonal deformation mode).14 The intensity of this peak decreases with increasing lead (PbO and/or PbF2) content, and several vibration bands are formed, which correspond to chain- and ring-type metaborate groups, as well as pentaborates and diborates.15 The short-range-order structure of lead borate glasses is changed from trigonal BO3 to pentaborate groups containing three and four coordinated boron atoms. It results in vibration bands located at around 880 and 930 cm-1 for the 4:1 system. Their intensities slightly decrease and additional bands at about 620 and 715 cm-1 related to chainand ring-type metaborate groups containing nonbridging oxygen are observed. Finally, vibration bands at about 576 and 920 cm-1 assigned to diborate groups are present for the sample in the 8:1 system. Similar results have been obtained for Sm-doped lead fluoroborate glasses.16 The data mentioned above are consistent with infrared (FT-IR) measurements. The inset shows two infrared bands in the 600-1100 cm-1 region, which correspond to trigonal BO3 and tetrahedral BO4 groups. The first one is associated to BO3 bending (650-700 cm-1), whereas the second one is related to the B-O stretching vibration of tetrahedral BO4 units (850-1050 cm-1). Bands centered at 1050 cm-1 are related to the antisymmetric stretching mode, whereas the symmetric stretching frequency is located in the 850-900 cm-1 region.17 It is evidently shown that the intensity of the band due to stretching vibration of BO4 units increases when the PbO- and/or PbF2-to-B2O3 ratio is changed from the 1:1 to 4:1 system and then starts to decrease. Addition of heavy metal oxide (PbO) or fluoride (PbF2) to the borate based glass modifies the network structural units, causing a conversion from BO3 to BO4 units. This structural conversion continues up to the 4:1 system. Thus, the highest degree of BO3 f BO4 conversion has been observed. The existence of BO4 units is more suitable for luminescence. The phonon energy for BO4 units is lower than that for BO3 ones. For that reason, luminescence is enhanced because the reduced phonon energy of the host decreases the nonradiative relaxation probability. An increased formation of tetrahedral BO4 units leads to close packing of oxygen atoms around Er3+, resulting in a higher symmetry around the site occupied by rare earth. Our preliminary investigations indicate that the intensity of the near-infrared luminescence band at 1.5 µm and the value of the 4I13/2 lifetime of Er3+ ions are optimally high for the 4:1 system. The luminescence lifetime for the upper 4I13/2 laser level of Er3+ increases from 230 µs (1:1 system) to 400 µs (4:1 system) and then starts to decrease when the PbO-to-B2O3 ratio is changed from the 4:1 to 8:1 system. The luminescence lifetime significantly increases to 600 and 820 µs for the 4:1 system when PbO is partially (9%) or totally (72%) substituted by PbF2. The opposite situation is observed for Er-doped samples, where the PbO-B2O3 ratio is changed from 4:1 to 8:1. Thus, the intensity of the infrared band in the 850-1050 cm-1 region decreases with further increase of the PbO concentration. It results in reduction of BO4 units and consequently suggests a decrease in the symmetry of the site of Er3+ ions. Back conversion from BO4 to BO3 units with the formation of metaborate units takes place above the 4:1 system. Similar to the other Ln-doped (Ln ) Nd, Sm, Dy) lead borate glasses,18 the nonbridging oxygen atoms (NBOs) are formed, which leads to an increase in the distortion of the ligand field at the site of Er3+ ions. For that reason, the luminescence properties of Er3+ ions in lead borate glasses have been limited to the 4:1 system. Especially, Er-
Letters
J. Phys. Chem. B, Vol. 111, No. 10, 2007 2429
Figure 2. X-ray diffraction patterns recorded for Er-doped samples before and after annealing.
doped oxyfluoride lead borate glasses in the 4:1 system have been investigated after annealing in detail. Figure 2 presents X-ray diffraction patterns recorded for some Er-doped lead borate glass samples before and after thermal treatment. The sample in the 1:1 system (a) shows a typical diffraction pattern for an amorphous-like structure. Similar behavior is also observed for the sample in the 8:1 system. Several narrowed diffraction lines due to the crystalline ErBO3 phase (PDF-2 card no. 74-1935) have been identified by X-ray diffraction analysis, when the PbO-B2O3 ratio is changed from the 1:1 to 4:1 system (b).19 It corroborates FT-IR measurements, where the broad infrared band, due to B-O stretching vibration of BO4 units, obtained for the 4:1 system without PbF2 is well resolved in contrast to other ones. Additionally, the peak at about 872 cm-1 due to Er-O-B bending has been observed, which can be assigned as the deformation mode of BO3 vibration in lanthanide borates.20 The narrowed diffraction lines due to the crystalline phase have not been observed for the 4:1 system, where PbO was partially or totally substituted by PbF2 (c). Independently on PbF2 content and thermal treatment conditions, the orthorhombic PbF2 phase (PDF-2 card no. P411086) was identified after annealing of lead borate glass in the 4:1 system (d). This situation is quite different in comparison to that obtained for the PbF2-PbO-GeO2 system containing trivalent erbium21 or thulium22 where cubic β-PbF2 crystallites are formed during heat treatment. Figure 3 presents (a) near-infrared (NIR) luminescence at 1.5 µm due to the main 4I13/2-4I15/2 laser transition and (b) green up-conversion due to the 4S3/2-4I15/2 transition of Er3+ ions in lead borate glasses and transparent glass ceramics. For both spectral lines, the luminescence intensity increases, whereas the line width significantly decreases in the case of substitution of PbO by PbF2. Additionally, the up-converted spectrum is shifted in the direction of shorter wavelengths, when PbO is substituted by PbF2. The slightly narrowed and more intense luminescence lines have been obtained for oxyfluoride samples after annealing. It also results in elongation of the luminescence lifetime for the 4I13/2 excited level of Er3+ ions. For the annealed sample, the 4I13/2 lifetime close to 660 µs is longer than values obtained for as-melted glasses with 9% PbF2 (600 µs) and without PbF2 (400 µs). Transparent glass ceramics based on PbF2-PbOB2O3 were successfully prepared; however, both spectroscopic parameters like luminescence line width and lifetime obtained for annealed samples are slightly changed in comparison to precursor glasses. It indicates that the Er3+ ions in small amounts are incorporated into the orthorhombic PbF2 phase. This phenomenon is probably connected with the chemical and
Figure 3. (a) Near-infrared luminescence at 1530 nm due to the 4I13/24 I15/2 transition of Er3+ ions under excitation of the 4F7/2 state (λpump ) 488 nm) and (b) up-conversion due to the 4S3/2-4I15/2 transition of Er3+ ions under excitation of the 4I11/2 state (λpump ) 980 nm), recorded for samples in the 4:1 system before and after annealing.
structural nature of the investigated host matrixes. It can be that borate units bond Er3+ ions and block their movement to the crystalline phase. It is in good agreement with results obtained for oxide lead borate glasses, where the infrared band due to strong Er-O-B bonding and the existence of the ErBO3 crystalline phase was confirmed by infrared (FT-IR) spectroscopy and X-ray diffraction (XRD). In summary, lead borate based glasses have been analyzed using spectroscopic techniques. The formation of different borate groups and the direction of BO3 T BO4 conversion strongly depends on the PbO- and/or PbF2-to-B2O3 ratio in glass composition. Er-doped lead borate glasses have been investigated after annealing. Independently on PbF2 concentration and thermal treatment conditions, the orthorhombic PbF2 phase was stated using X-ray diffraction. Near-infrared luminescence due to the 4I13/2-4I15/2 laser transition and green up-conversion due to the 4S3/2-4I15/2 transition of Er3+ ions have been registered for samples before and after annealing. In comparison to the precursor glasses, the luminescence intensities are higher, whereas the half-width of the luminescence lines slightly decreases in studied transparent oxyfluoride glass ceramics. The borate units bond Er3+ ions and block their movement to the crystalline phase. It indicates that the Er3+ ions in small amounts are incorporated into the PbF2 phase. References and Notes (1) Fan, X.; Wang, J.; Qiao, X.; Wang, M.; Adam, J. L.; Zhang, X. J. Phys. Chem. B 2006, 110, 5950.
2430 J. Phys. Chem. B, Vol. 111, No. 10, 2007 (2) Liu, F.; Ma, E.; Chen, D.; Yu, Y.; Wang, Y. J. Phys. Chem. B 2006, 110, 20843. (3) Qiao, X.; Fan, X.; Wang, M. Appl. Phys. Lett. 2006, 89, 111919. (4) Patra, A.; Friend, C. S.; Kapoor, R.; Prasad, P. N. J. Phys. Chem. B 2002, 106, 1909. (5) Vetrone, F.; Boyer, J. C.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. J. Phys. Chem. B 2002, 106, 5622. (6) Saitoh, A.; Matsuishi, S.; Se-Weon, C.; Nishii, J.; Oto, M.; Hirano, M.; Hosono, H. J. Phys. Chem. B 2006, 110, 7617. (7) Pisarski, W. A.; Pisarska, J.; Ryba-Romanowski, W. Chem. Phys. Lett. 2003, 380, 604. (8) Chen, Q.; Ferraris, M.; Milanese, D.; Menke, Y.; Monchiero, E.; Perrone, G. J. Non-Cryst. Solids 2003, 324, 12. (9) Kassab, L. R. P.; Courrol, L. C.; Seragioli, R.; Wetter, N. U.; Tatumi, S. H.; Gomes, L. J. Non-Cryst. Solids 2004, 348, 94. (10) Hocde, S.; Jiang, S.; Peng, X.; Peyghambarian, N.; Luo, T.; Morrell, M. Opt. Mater. 2004, 25, 149. (11) dos Santos, E. A.; Courrol, L. C.; Kassab, L. R. P.; Gomes, L.; Wetter, N. U.; Vieira, N. D., Jr.; Ribeiro, S. J. L.; Messaddeq, Y. J. Lumin. 2007, 124, 200.
Letters (12) Kashchieva, E. P.; Ivanova, V. D.; Jivov, B. T.; Dimitriev, Y. B. Phys. Chem. Glasses 2000, 41, 355. (13) Pisarski, W. A.; Pisarska, J.; Ma¸ czka, M.; Ryba-Romanowski, W. J. Mol. Struct. 2006, 792-793, 207. (14) Krogh-Moe, J. Phys. Chem. Glasses 1965, 6, 46. (15) Pan, Z.; Henderson, D. O.; Morgan, S. H. J. Chem. Phys. 1994, 101, 1767. (16) Souza Filho, A. G.; Mendes Filho, J.; Melo, F. E. A.; Custodio, M. C. C.; Lebullenger, R.; Hernandes, A. C. J. Phys. Chem. Solids 2000, 61, 1535. (17) Ross, S. D. Spectrochim. Acta., Part A 1972, 28, 1555. (18) Saisudha, M. B.; Ramakrishna, J. Phys. ReV. B 1996, 53, 6186. (19) Pisarski, W. A.; Goryczka, T.; Wodecka-Dus´, B.; Płon´ska, M.; Pisarska, J. Mater. Sci. Eng., B 2005, 122, 94. (20) Weir, C. E.; Lippincott, E. R. J. Res. Natl. Bur. Stand. (U.S.) 1961, 65A, 173. (21) Gouveia-Neto, A. S.; da Costa, E. B.; Bueno, L. A.; Ribeiro, S. L. L. J. Alloys Compd. 2004, 375, 224. (22) Ryba-Romanowski, W.; Dominiak-Dzik, G.; Solarz, P.; Klimesz, B.; Z˙ elechower, M. J. Non-Cryst. Solids 2004, 345-346, 391.
