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Ultraviolet-to-visible downconversion luminescence in solgel oxyfluoride glass ceramics containing Eu3+:GdF3 nanocrystals Barbara Szpikowska-Sroka,1,* Lidia Żur,1 Rozalia Czoik,1 Tomasz Goryczka,2 Maria Żądło,1 and Wojciech A. Pisarski1 1 2
Institute of Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland
Institute of Materials Science, University of Silesia, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland *Corresponding author: barbara.szpikowska‑[email protected] Received February 7, 2014; revised April 14, 2014; accepted April 15, 2014; posted April 16, 2014 (Doc. ID 206106); published May 23, 2014
GdF3 nanocrystals doped with Eu3 ions in oxyfluoride glass ceramics were prepared by a solgel method. The structural properties were examined by x-ray diffraction measurements. The effects of gadolinium codoping on europium emission in the prepared solgel glasses and glass ceramics have been studied. The emission bands originating from the 5 D0 state of Eu3 ions are enhanced under excitation of Gd3 ions by 273 nm line. The electric dipole 5 D0 → 7 F2 transitions were dominant in the samples before heat treatment, whereas magnetic dipole 5 D0 → 7 F1 transitions had a higher probability in the samples after annealing. The luminescence lifetime for the 5 D0 level of Eu3 ions in the samples after excitation at 273 nm is long lived in comparison to excitation at 393 nm and increased to 190%. Energy transfer from Gd3 to Eu3 was observed. © 2014 Optical Society of America OCIS codes: (140.3490) Lasers, distributed-feedback; (060.2420) Fibers, polarization-maintaining; (060.3735) Fiber Bragg gratings; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/OL.39.003181
Recently, intensive research has been focused on the development of new materials with high emission efficiency enabling conversion of the UV radiation into visible light [1–4]. These compounds, due to their interesting optical properties, play an important role in high-tech optoelectronics [5–8]. The possibility of increased efficiency in the new materials depends on the quantum cutting, which can generate more than one low-energy photons for each absorbed high-energy photon. This phenomenon is well known in the Gd3 ∕Eu3 system, upon excitation of Gd3 at 200 nm. First, a part of the excitation energy is transferred from Gd3 to Eu3 by the cross-relaxation process, and second, the remaining excited energy in the Gd3 ion is transferred to a second Eu3 ion (the directenergy transfer process) [1–3]. Upon excitation of Gd3 at 273 nm, the europium ions are excited by the directenergy transfer process from Gd3 to Eu3 , but this process can significantly affect the increase of emission efficiency [5,9,10]. Materials mentioned above could be obtained by a solgel method. Preparation at room temperature makes it easy to include inorganic and organic additives [11–13]. Lanthanide trifluorides, as an inorganic dopant, are very suitable for luminescent applications due to their optical properties and low phonon energies, reducing the probability of nonradiative transition and therefore leading to an increase of luminescence efficiency [8]. GdF3 , as a host matrix, is a good alternative for other fluoride crystals, due to the presence of the Gd3 ion. The gadolinium ions have a half-filled 4f shell with a stable 8 S7∕2 ground state and can be excited by UV light [9]. The Eu3 ions as optically active dopant are particularly interesting due to the bright reddish-orange light emission. Compounds doped with Eu3 have been extensively used as luminophores in solid-state lasers, plasma display panels or spectroscopic probes [5–7,12,13]. 0146-9592/14/113181-04$15.00/0
In the literature there are many publications on the preparation of codoped Eu/Gd systems (e.g., LiGdF4 , GdF3 , NaGdF4 , CsGd2 F7 , BaF2 , BaGd2 ZnO5 ), in which GdF3 crystals were fabricated by melt quenching, microemulsion, or coprecipitation methods [1–11,14–17]. To the best of our knowledge only a few reports are available on the solgel synthesis of crystals; for example, LiGdF4 , GdOF [1,2,11,12], and GdF3 in silica films [15]. However, there are no reports on the emission of Eu3 and Gd3 in solgel systems before and after heat treatment, as well as the influence of excitation parameters on the luminescence spectra of Eu3 ions and their decays. This work presents new results for Eu3 :GdF3 nanocrystals distributed into solgel glass. We examined the influence of excitation parameters on the energy transfer and some spectroscopic properties, in particular on the luminescence spectra of Eu3 ions and their decays in the absence and presence of Gd3 ions. The samples were tested before and after heat treatment. Silica xerogels with composition 90SiO2 − 9.5GdF3 − 0.5EuF3 mol: % were prepared by a solgel method following the procedure described in [18,19]. The solution of tetraethoxysilane (TEOS) in ethanol and water with acetic acid as a catalyst was hydrolyzed for 30 min at room temperature (1TEOS − 4C2 H5 OH − 10H2 O − 0.5CH3 COOH). Subsequently, gadolinium(III) and europium(III) acetate hydrate previously dissolved in the mixture of water and trifluoroacetic acid (TFA) was added to the resulting solution 1GdCH3 COO3 EuCH3 COO3 − 5TFA in mol. %). Next the transparent solution was stirred for 1 h. Wet gel was received by leaving the solution in a sealed container at 35°C for 5–6 weeks. This step was required in order to obtain dried samples, known as xerogels. Finally, these xerogels were heat treated at 350°C for 10 h. The furnace temperature was increased at the rate of 10°C∕ min. The process of heating was carried out in air. After annealing, the © 2014 Optical Society of America
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samples were cooled to room temperature in a closed furnace. To examine their optical properties, the Gd3 singly doped samples were prepared using the same procedure. The excitation and emission spectra and their decays were measured using a Horiba Jobin-Yvon FLUOROMAX-4 spectrofluorimeter with a 150 W xenon lamp as a light source. The spectral resolution was 0.1 nm. Decay curves were detected with the accuracy of 1 μs. The nature of samples before and after heat treatment was identified using x-ray diffraction (XRD; X’Pert x-ray diffractometer). An energy level scheme for Gd3 and Eu3 based on the Dieke diagram [10] is presented in Fig. 1. The scheme clearly shows that when Gd3 ions are excited by the UV light of 273 nm (8 S7∕2 → 6 IJ transitions), the excitation energy may be transferred in a nonradiative way to the 6 P7∕2 level or may migrate to the energetically lower levels 5 FJ or 5 IJ of Eu3 [10]. If the energy is transferred by the multiphonon relaxation within Gd3 to the 6 P7∕2 level, then it may migrate to the 5 HJ level of Eu3 , because the 6 PJ (Gd3 ) and 5 HJ (Eu3 ) states are energetically close to each other [9]. In the next step energy may be transferred to the 5 D0 level of Eu3 ions, and then the emission in the visible range following the 5 D → 7 F and 5 D → 7 F transitions can be observed. 0 1 0 2 However, since the 6 P7∕2 level is metastable, it is possible that the excitation energy is promoted by the excited state absorption (ESA) process to 6 GJ levels of Gd3 . In this case the 6 GJ → 6 PJ emission in the 560–640 nm region could be observed. The excitation spectra before heat treatment were monitored at λem 611 nm (5 D0 → 7 F2 transition of Eu3 ) and at λem 311 nm (6 Pj → 8 S7∕2 transition of Gd3 ) in the range from 180 to 590 nm and are presented in Fig. 2. The main observed peaks correspond to transition originating from the ground level 8 S7∕2 to the 6 IJ and also 7 F0 to the 5 L6 and 5 D2 excited states of Gd3 and Eu3 , respectively. The most intense excitation peaks observed at 273 nm and at 393 nm are due to 8 S7∕2 → 6 IJ and 7 F0 → 5 L6 transitions. The sharp band with the maximum at 273 nm is connected to the energy transfer from Gd3 to Eu3 ions. The excitation peaks at about 200 and 250 nm corresponding to the transition from stable ground state 8 S7∕2 to the 6 GJ and 6 DJ have a very low intensity as compared to other registered peaks. The 8S 6 3 at about 200 nm is 7∕2 → GJ transition of Gd
Fig. 1. Energy level scheme for the
Gd3 ∕Eu3
system.
Fig. 2. Excitation spectra of Gd3 ∕Eu3 in xerogels before heat treatment monitored at (a) 312 nm and (b) 611 nm.
beneficial to the quantum cutting in the Gd3 –Eu3 couple system [1–9,14]. Figure 3 presents emission spectra for Eu3 in solgel glasses before and after heat treatment. The emission spectra were registered in the range from 285 to 750 nm, upon excitation at 273 nm to the 6 IJ levels of Gd3 and at 393 nm to the 5 L6 level of Eu3 . Peaks at 579, 590, 611, 648, and 700 nm can be attributed to the 5 D0 → 7 FJ (J 0, 1, 2, 3, 4) transitions of Eu3 . Before heat treatment, after excitation at 393 nm, mainly orange and red emission at 590 and 611 nm, following the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of Eu3 , was observed. The quite interesting phenomena are observed after excitation at 273 nm. The emission band due to 5 D0 → 7 F2 red transition of Eu3 detected after 273 nm excitation is broader than after 393 nm excitation. The broad band at about 619 nm probably consists of two superposed bands
Fig. 3. Emission spectra of Eu3 ∕Gd3 in solgel glasses before and after heat treatment.
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derived from Eu3 ions (611 nm) and Gd3 ions (622 nm). The emission at 622 nm may indicate a lodged process excitation at 273 nm by the ESA process to 6 GJ levels of Gd3 ions. Moreover, after 273 excitation the 6 PJ → 8 S7∕2 transition of Gd3 at 311 nm is also seen, but its intensity is lower in comparison to the emission of Eu3 ions. This points to the efficient energy transfer from Gd3 to Eu3 . Before heat treatment, the 5 D0 emission upon excitation at 273 nm to 6 IJ levels of Gd3 is indeed much higher compared to the excitation at 393 nm (5 L6 ) of Eu3 . This indicates the occurrence of direct energy transfer and down frequency conversion. The relationship between the intensity of luminescence and excitation parameters has been analyzed by Lepoutre et al. [1], Zhang and Huang [2], Grzyb and Lis [9], and Fujihara et al. [15]. In the studied samples the strongest emission is the electric dipole transition 5 D → 7 F at 611 nm, which confirms that the site occu0 2 pied by Eu3 ions deviates from the symmetric center. A quite different situation was observed for the sample after annealing, where intensities for both 5 D0 → 7 F2 and 5 D0 → 7 F1 transitions are comparable, due to the lower probability of 5 D0 → 7 F2 transition of Eu3 ions located in the symmetric surroundings. The emission of Gd3 has not been observed, which indicates that the effective energy migration from Gd3 to Eu3 occurs. Emission from higher-lying 5 D1 and 5 D2 levels of Eu3 has not been registered. The presence of hydroxyl groups effectively quenches radiative transitions from these states. Before heat treatment the obtained samples exhibit broad emission bands due to the absence of translational symmetry. In contrast, the heat-treated samples at 350°C show a narrow emission band due to 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of the Eu3 . The ratio of the intensity of the red line (electric dipole transition) to the orange line (magnetic dipole transition), known as the luminescence intensity ratio R I5 D0 → 7 F2 ∕ I5 D0 → 7 F1 , is a sensitive function of the local asymmetry around the optically active dopant and the degree of covalent bonds. The previously published results for Tb3 in aluminum nitride system clearly indicate that 5 D3 ∕5 D4 (blue/green) luminescence intensity ratios depend not only on the concentration of the emitting ions but also on the excitation wavelengths and their intensities [20]. In prepared glass samples the red-to-orange luminescence intensity ratios R before heat treatment (SG-Eu) are close to 3.12 and 4.97 for 393 and 273 nm excitation, whereas after heat treatment (SG-Eu-HT) they
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amount to 0.84 and 0.80 for 393 and 273 nm excitations, respectively (Table 1). First, the presence of two superposed emission bands derived from Eu3 ions (611 nm) and Gd3 ions (622 nm) for sample before heat treatment under 273 nm excitation makes an important contribution to the relatively high value of R factor. Second, the R values are considerably higher for samples before annealing, independently on excitation wavelengths, suggesting that Eu3 ions are located in a low symmetry site in amorphous silica. On the contrary, after heat treatment, participation of ionic bonds and local symmetry around the optically active dopant increases as confirmed by the low values of R. The lifetimes for the 5 D0 state of Eu3 in samples before and after heat treatment were also analyzed under different excitation wavelengths (273 and 393 nm) and monitoring 590 nm (5 D0 → 7 F1 ) emission wavelength. The results are shown in Fig. 4. The decay curves for the samples before annealing were well fitted to a single-exponential function but after heat treatment were fitted to a double-exponential function with two different times of decay (Table 1). On this basis we can conclude that after heat treatment some europium ions were located in the crystalline phase. The luminescence lifetime for the 5 D0 level of Eu3 ions depends mainly on radiative probabilities of the 5 D → 7 F transitions. The value of lifetime for the 0 J Eu3 emission upon excitation at 393 nm is 0.19 ms before heat treatment and 0.03 and 1.50 ms after heat treatment. The value of the luminescence lifetime for Eu3 upon excitation at 273 nm increased from 0.37 ms before heat treatment to 0.47 and 2.75 ms after heat treatment. The short emission lifetime of the prepared sample before annealing is due to the presence of the residual
Table 1. Spectroscopic Parameters for Eu3 Ions Sample SG-Eu
λexc nm Transition λem nm 393
SG-Eu
273
SG-Eu-HT
393
SG-Eu-HT
273
5D 5
0
D0 5 D0 5D 0 5 D0 5D 0 5 D0 5 D0
7F
→ 2 → 7 F1 → 7 F2 → 7 F1 → 7 F2 → 7 F1 → 7 F2 → 7 F1
611 590 611 590 611 590 611 590
R
τm ms
3.12
0.19
4.97
0.37
0.84 0.03; 1.50 0.80 0.47; 2.75
Fig. 4. Decay curves for the 5 D0 state of Eu3 excited at 273 or 393 nm and monitored at 590 nm (5 D0 → 7 F1 emission) in sample before and after heat treatment.
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Fig. 5.
XRD patterns for the studied samples.
hydroxyl groups. These groups cause luminescence quenching by a nonradiative decay mechanism. The concentration of the OH groups during heat treatment is significantly reduced and results in a longer 5 D0 emission lifetime of Eu3 ions. As we previously reported [21], the short luminescence lifetime for samples after heat treatment was observed from Eu3 ions positioned in asymmetric sites, whereas luminescence with a long lifetime was observed from Eu3 ions in a highly symmetric site. The 5 D0 measured lifetime is higher compared to the value obtained by Kondo et al. [10], Lorbeer et al. [6], and Vergeer et al. [7] but is lower than the value obtained by Lepoutre et al. for LiGdF4:Eu3 crystals [11]. The luminescence lifetimes for the 6 PJ → 8 S7∕2 (311 nm) transition of Gd3 upon excitation at 273 nm before and after heat treatment were also determined and are equal to 0.14 and 0.001 ms, respectively. The obtained results are comparable to the value measured by other authors [7,10]. These quite short lifetimes of Gd3 confirm the effective energy transfer from Gd3 to Eu3 . The obtained 5 D0 (Eu3 ) lifetimes collaborated well with emission measurements, where observed reduction of the R factor is connected to the structural transformation from the amorphous phase to the ordered and more ionic environment of Eu3 . To confirm the nature of the studied samples XRD was used (Fig. 5). Based on XRD measurements it can be concluded that the studied systems before heat treatment were fully amorphous. After heat treatment several peaks at the spectrum were observed. Based on a theoretical XRD pattern the broad peaks are assigned to the rhombic GdF3 crystalline phase. Crystal sizes have been estimated using the Scherrer equation and were calculated from FWHM as an average from all diffraction lines. The mean value of the crystal size was 6.3 nm. Moreover, the crystallite size was re-examined with the Williamson–Hall formula, which includes internal stress influence on the line broadening. The lattice deformation was lower than 0.00167% and the estimated particle size amounted to 5.6 nm, which corresponds to the results obtained by Scherrer’s methods.
In summary, the Eu3 :GdF3 nanocrystals have been synthesized by the solgel method and identified by XRD. The spectroscopic parameters of the prepared samples indicate that selective incorporation of Eu3 ions into the fluoride nanocrystals occurred. Because of the overlap of the energy levels between the 6 PJ states of Gd3 and the 5 HJ states of Eu3 , the ultraviolet light excitation at 273 nm was promoted to the energy transfer from Gd3 to Eu3 . Emission originating from the 5 D0 state of Eu3 ions upon excitation at 273 nm is long lived in comparison to the excitation at 393 nm and is increased to 190%. References 1. S. Lepoutre, D. Boyer, and R. Mahiou, J. Lumin. 128, 635 (2008). 2. Q. Y. Zhang and X. Y. Huang, Prog. Mater. Sci. 55, 353 (2010). 3. R. T. Wegh, H. Donker, K. D. Oskam, and A. Meijerink, J. Lumin. 82, 93 (1999). 4. P. Ghosh, S. Tang, and A. V. Mudring, J. Mater. Chem. 21, 8640 (2011). 5. M. Karbowiak, A. Mech, and W. Ryba-Romanowski, J. Lumin. 114, 65 (2005). 6. C. Lorbeer, J. Cybinska, and A. V. Mudring, Chem. Commun. 46, 571 (2010). 7. P. Vergeer, E. van den Pol, and A. Meijerink, J. Lumin. 121, 456 (2006). 8. R. Hua, J. Niu, B. Chen, M. Li, T. Yu, and W. Li, Nanotechnology 17, 1642 (2006). 9. T. Grzyb and S. Lis, J. Rare Earths 27, 588 (2009). 10. Y. Kondo, K. Tanaka, R. Ota, T. Fujii, and Y. Ishikawa, Opt. Mater. 27, 1438 (2005). 11. S. Lepoutre, D. Boyer, S. Fujihara, and R. Mahiou, J. Mater. Chem. 19, 2784 (2009). 12. S. Fujihara and K. Tokumo, J. Fluorine Chem. 130, 1106 (2009). 13. S. Fujihara, in Functionalized Inorganic Fluorides: Synthesis, Characterization & Properties of Nanostructured Solids, A. Tressaud, ed. (Wiley, 2010), Chap. 10, pp. 307– 330. 14. B. Tian, B. Chen, Y. Tian, J. Sun, X. Li, J. Zhang, H. Zhong, L. Cheng, Z. Wuc, and R. Hua, Ceram. Int. 38, 3537 (2012). 15. S. Fujihara, S. Koji, and T. Kimura, J. Mater. Chem. 14, 1331 (2004). 16. B. Liu, Y. Chen, C. Shi, H. Tang, and Y. Tao, J. Lumin. 101, 155 (2003). 17. X. Zhang, T. Hayakawa, M. Nogami, and Y. Ishikawa, J. Alloys Compd. 509, 2076 (2011). 18. A. C. Yanes, J. Del-Castillo, J. Mendez-Ramos, V. D. Rodrıguez, M. E. Torres, and J. Arbiol, Opt. Materials 29, 999 (2007). 19. S. Fujihara, C. Mochizuki, and T. Kimura, J. Non-Cryst. Solids 244, 267 (1999). 20. F. Benz, H. P. Strunk, J. Schaab, U. Künecke, and P. Wellmann, J. Appl. Phys. 114, 073518 (2013). 21. B. Szpikowska-Sroka, L. Żur, R. Czoik, T. Goryczka, A. S. Swinarew, M. Żądło, and W. A. Pisarski, J. Sol-Gel Sci. Technol. 68, 278 (2013).
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Ultraviolet-to-visible downconversion luminescence in solgel oxyfluoride glass ceramics containing Eu3+:GdF3 nanocrystals Barbara Szpikowska-Sroka,1,* Lidia Żur,1 Rozalia Czoik,1 Tomasz Goryczka,2 Maria Żądło,1 and Wojciech A. Pisarski1 1 2
Institute of Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland
Institute of Materials Science, University of Silesia, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland *Corresponding author: barbara.szpikowska‑[email protected] Received February 7, 2014; revised April 14, 2014; accepted April 15, 2014; posted April 16, 2014 (Doc. ID 206106); published May 23, 2014
GdF3 nanocrystals doped with Eu3 ions in oxyfluoride glass ceramics were prepared by a solgel method. The structural properties were examined by x-ray diffraction measurements. The effects of gadolinium codoping on europium emission in the prepared solgel glasses and glass ceramics have been studied. The emission bands originating from the 5 D0 state of Eu3 ions are enhanced under excitation of Gd3 ions by 273 nm line. The electric dipole 5 D0 → 7 F2 transitions were dominant in the samples before heat treatment, whereas magnetic dipole 5 D0 → 7 F1 transitions had a higher probability in the samples after annealing. The luminescence lifetime for the 5 D0 level of Eu3 ions in the samples after excitation at 273 nm is long lived in comparison to excitation at 393 nm and increased to 190%. Energy transfer from Gd3 to Eu3 was observed. © 2014 Optical Society of America OCIS codes: (140.3490) Lasers, distributed-feedback; (060.2420) Fibers, polarization-maintaining; (060.3735) Fiber Bragg gratings; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/OL.39.003181
Recently, intensive research has been focused on the development of new materials with high emission efficiency enabling conversion of the UV radiation into visible light [1–4]. These compounds, due to their interesting optical properties, play an important role in high-tech optoelectronics [5–8]. The possibility of increased efficiency in the new materials depends on the quantum cutting, which can generate more than one low-energy photons for each absorbed high-energy photon. This phenomenon is well known in the Gd3 ∕Eu3 system, upon excitation of Gd3 at 200 nm. First, a part of the excitation energy is transferred from Gd3 to Eu3 by the cross-relaxation process, and second, the remaining excited energy in the Gd3 ion is transferred to a second Eu3 ion (the directenergy transfer process) [1–3]. Upon excitation of Gd3 at 273 nm, the europium ions are excited by the directenergy transfer process from Gd3 to Eu3 , but this process can significantly affect the increase of emission efficiency [5,9,10]. Materials mentioned above could be obtained by a solgel method. Preparation at room temperature makes it easy to include inorganic and organic additives [11–13]. Lanthanide trifluorides, as an inorganic dopant, are very suitable for luminescent applications due to their optical properties and low phonon energies, reducing the probability of nonradiative transition and therefore leading to an increase of luminescence efficiency [8]. GdF3 , as a host matrix, is a good alternative for other fluoride crystals, due to the presence of the Gd3 ion. The gadolinium ions have a half-filled 4f shell with a stable 8 S7∕2 ground state and can be excited by UV light [9]. The Eu3 ions as optically active dopant are particularly interesting due to the bright reddish-orange light emission. Compounds doped with Eu3 have been extensively used as luminophores in solid-state lasers, plasma display panels or spectroscopic probes [5–7,12,13]. 0146-9592/14/113181-04$15.00/0
In the literature there are many publications on the preparation of codoped Eu/Gd systems (e.g., LiGdF4 , GdF3 , NaGdF4 , CsGd2 F7 , BaF2 , BaGd2 ZnO5 ), in which GdF3 crystals were fabricated by melt quenching, microemulsion, or coprecipitation methods [1–11,14–17]. To the best of our knowledge only a few reports are available on the solgel synthesis of crystals; for example, LiGdF4 , GdOF [1,2,11,12], and GdF3 in silica films [15]. However, there are no reports on the emission of Eu3 and Gd3 in solgel systems before and after heat treatment, as well as the influence of excitation parameters on the luminescence spectra of Eu3 ions and their decays. This work presents new results for Eu3 :GdF3 nanocrystals distributed into solgel glass. We examined the influence of excitation parameters on the energy transfer and some spectroscopic properties, in particular on the luminescence spectra of Eu3 ions and their decays in the absence and presence of Gd3 ions. The samples were tested before and after heat treatment. Silica xerogels with composition 90SiO2 − 9.5GdF3 − 0.5EuF3 mol: % were prepared by a solgel method following the procedure described in [18,19]. The solution of tetraethoxysilane (TEOS) in ethanol and water with acetic acid as a catalyst was hydrolyzed for 30 min at room temperature (1TEOS − 4C2 H5 OH − 10H2 O − 0.5CH3 COOH). Subsequently, gadolinium(III) and europium(III) acetate hydrate previously dissolved in the mixture of water and trifluoroacetic acid (TFA) was added to the resulting solution 1GdCH3 COO3 EuCH3 COO3 − 5TFA in mol. %). Next the transparent solution was stirred for 1 h. Wet gel was received by leaving the solution in a sealed container at 35°C for 5–6 weeks. This step was required in order to obtain dried samples, known as xerogels. Finally, these xerogels were heat treated at 350°C for 10 h. The furnace temperature was increased at the rate of 10°C∕ min. The process of heating was carried out in air. After annealing, the © 2014 Optical Society of America
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samples were cooled to room temperature in a closed furnace. To examine their optical properties, the Gd3 singly doped samples were prepared using the same procedure. The excitation and emission spectra and their decays were measured using a Horiba Jobin-Yvon FLUOROMAX-4 spectrofluorimeter with a 150 W xenon lamp as a light source. The spectral resolution was 0.1 nm. Decay curves were detected with the accuracy of 1 μs. The nature of samples before and after heat treatment was identified using x-ray diffraction (XRD; X’Pert x-ray diffractometer). An energy level scheme for Gd3 and Eu3 based on the Dieke diagram [10] is presented in Fig. 1. The scheme clearly shows that when Gd3 ions are excited by the UV light of 273 nm (8 S7∕2 → 6 IJ transitions), the excitation energy may be transferred in a nonradiative way to the 6 P7∕2 level or may migrate to the energetically lower levels 5 FJ or 5 IJ of Eu3 [10]. If the energy is transferred by the multiphonon relaxation within Gd3 to the 6 P7∕2 level, then it may migrate to the 5 HJ level of Eu3 , because the 6 PJ (Gd3 ) and 5 HJ (Eu3 ) states are energetically close to each other [9]. In the next step energy may be transferred to the 5 D0 level of Eu3 ions, and then the emission in the visible range following the 5 D → 7 F and 5 D → 7 F transitions can be observed. 0 1 0 2 However, since the 6 P7∕2 level is metastable, it is possible that the excitation energy is promoted by the excited state absorption (ESA) process to 6 GJ levels of Gd3 . In this case the 6 GJ → 6 PJ emission in the 560–640 nm region could be observed. The excitation spectra before heat treatment were monitored at λem 611 nm (5 D0 → 7 F2 transition of Eu3 ) and at λem 311 nm (6 Pj → 8 S7∕2 transition of Gd3 ) in the range from 180 to 590 nm and are presented in Fig. 2. The main observed peaks correspond to transition originating from the ground level 8 S7∕2 to the 6 IJ and also 7 F0 to the 5 L6 and 5 D2 excited states of Gd3 and Eu3 , respectively. The most intense excitation peaks observed at 273 nm and at 393 nm are due to 8 S7∕2 → 6 IJ and 7 F0 → 5 L6 transitions. The sharp band with the maximum at 273 nm is connected to the energy transfer from Gd3 to Eu3 ions. The excitation peaks at about 200 and 250 nm corresponding to the transition from stable ground state 8 S7∕2 to the 6 GJ and 6 DJ have a very low intensity as compared to other registered peaks. The 8S 6 3 at about 200 nm is 7∕2 → GJ transition of Gd
Fig. 1. Energy level scheme for the
Gd3 ∕Eu3
system.
Fig. 2. Excitation spectra of Gd3 ∕Eu3 in xerogels before heat treatment monitored at (a) 312 nm and (b) 611 nm.
beneficial to the quantum cutting in the Gd3 –Eu3 couple system [1–9,14]. Figure 3 presents emission spectra for Eu3 in solgel glasses before and after heat treatment. The emission spectra were registered in the range from 285 to 750 nm, upon excitation at 273 nm to the 6 IJ levels of Gd3 and at 393 nm to the 5 L6 level of Eu3 . Peaks at 579, 590, 611, 648, and 700 nm can be attributed to the 5 D0 → 7 FJ (J 0, 1, 2, 3, 4) transitions of Eu3 . Before heat treatment, after excitation at 393 nm, mainly orange and red emission at 590 and 611 nm, following the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of Eu3 , was observed. The quite interesting phenomena are observed after excitation at 273 nm. The emission band due to 5 D0 → 7 F2 red transition of Eu3 detected after 273 nm excitation is broader than after 393 nm excitation. The broad band at about 619 nm probably consists of two superposed bands
Fig. 3. Emission spectra of Eu3 ∕Gd3 in solgel glasses before and after heat treatment.
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derived from Eu3 ions (611 nm) and Gd3 ions (622 nm). The emission at 622 nm may indicate a lodged process excitation at 273 nm by the ESA process to 6 GJ levels of Gd3 ions. Moreover, after 273 excitation the 6 PJ → 8 S7∕2 transition of Gd3 at 311 nm is also seen, but its intensity is lower in comparison to the emission of Eu3 ions. This points to the efficient energy transfer from Gd3 to Eu3 . Before heat treatment, the 5 D0 emission upon excitation at 273 nm to 6 IJ levels of Gd3 is indeed much higher compared to the excitation at 393 nm (5 L6 ) of Eu3 . This indicates the occurrence of direct energy transfer and down frequency conversion. The relationship between the intensity of luminescence and excitation parameters has been analyzed by Lepoutre et al. [1], Zhang and Huang [2], Grzyb and Lis [9], and Fujihara et al. [15]. In the studied samples the strongest emission is the electric dipole transition 5 D → 7 F at 611 nm, which confirms that the site occu0 2 pied by Eu3 ions deviates from the symmetric center. A quite different situation was observed for the sample after annealing, where intensities for both 5 D0 → 7 F2 and 5 D0 → 7 F1 transitions are comparable, due to the lower probability of 5 D0 → 7 F2 transition of Eu3 ions located in the symmetric surroundings. The emission of Gd3 has not been observed, which indicates that the effective energy migration from Gd3 to Eu3 occurs. Emission from higher-lying 5 D1 and 5 D2 levels of Eu3 has not been registered. The presence of hydroxyl groups effectively quenches radiative transitions from these states. Before heat treatment the obtained samples exhibit broad emission bands due to the absence of translational symmetry. In contrast, the heat-treated samples at 350°C show a narrow emission band due to 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of the Eu3 . The ratio of the intensity of the red line (electric dipole transition) to the orange line (magnetic dipole transition), known as the luminescence intensity ratio R I5 D0 → 7 F2 ∕ I5 D0 → 7 F1 , is a sensitive function of the local asymmetry around the optically active dopant and the degree of covalent bonds. The previously published results for Tb3 in aluminum nitride system clearly indicate that 5 D3 ∕5 D4 (blue/green) luminescence intensity ratios depend not only on the concentration of the emitting ions but also on the excitation wavelengths and their intensities [20]. In prepared glass samples the red-to-orange luminescence intensity ratios R before heat treatment (SG-Eu) are close to 3.12 and 4.97 for 393 and 273 nm excitation, whereas after heat treatment (SG-Eu-HT) they
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amount to 0.84 and 0.80 for 393 and 273 nm excitations, respectively (Table 1). First, the presence of two superposed emission bands derived from Eu3 ions (611 nm) and Gd3 ions (622 nm) for sample before heat treatment under 273 nm excitation makes an important contribution to the relatively high value of R factor. Second, the R values are considerably higher for samples before annealing, independently on excitation wavelengths, suggesting that Eu3 ions are located in a low symmetry site in amorphous silica. On the contrary, after heat treatment, participation of ionic bonds and local symmetry around the optically active dopant increases as confirmed by the low values of R. The lifetimes for the 5 D0 state of Eu3 in samples before and after heat treatment were also analyzed under different excitation wavelengths (273 and 393 nm) and monitoring 590 nm (5 D0 → 7 F1 ) emission wavelength. The results are shown in Fig. 4. The decay curves for the samples before annealing were well fitted to a single-exponential function but after heat treatment were fitted to a double-exponential function with two different times of decay (Table 1). On this basis we can conclude that after heat treatment some europium ions were located in the crystalline phase. The luminescence lifetime for the 5 D0 level of Eu3 ions depends mainly on radiative probabilities of the 5 D → 7 F transitions. The value of lifetime for the 0 J Eu3 emission upon excitation at 393 nm is 0.19 ms before heat treatment and 0.03 and 1.50 ms after heat treatment. The value of the luminescence lifetime for Eu3 upon excitation at 273 nm increased from 0.37 ms before heat treatment to 0.47 and 2.75 ms after heat treatment. The short emission lifetime of the prepared sample before annealing is due to the presence of the residual
Table 1. Spectroscopic Parameters for Eu3 Ions Sample SG-Eu
λexc nm Transition λem nm 393
SG-Eu
273
SG-Eu-HT
393
SG-Eu-HT
273
5D 5
0
D0 5 D0 5D 0 5 D0 5D 0 5 D0 5 D0
7F
→ 2 → 7 F1 → 7 F2 → 7 F1 → 7 F2 → 7 F1 → 7 F2 → 7 F1
611 590 611 590 611 590 611 590
R
τm ms
3.12
0.19
4.97
0.37
0.84 0.03; 1.50 0.80 0.47; 2.75
Fig. 4. Decay curves for the 5 D0 state of Eu3 excited at 273 or 393 nm and monitored at 590 nm (5 D0 → 7 F1 emission) in sample before and after heat treatment.
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Fig. 5.
XRD patterns for the studied samples.
hydroxyl groups. These groups cause luminescence quenching by a nonradiative decay mechanism. The concentration of the OH groups during heat treatment is significantly reduced and results in a longer 5 D0 emission lifetime of Eu3 ions. As we previously reported [21], the short luminescence lifetime for samples after heat treatment was observed from Eu3 ions positioned in asymmetric sites, whereas luminescence with a long lifetime was observed from Eu3 ions in a highly symmetric site. The 5 D0 measured lifetime is higher compared to the value obtained by Kondo et al. [10], Lorbeer et al. [6], and Vergeer et al. [7] but is lower than the value obtained by Lepoutre et al. for LiGdF4:Eu3 crystals [11]. The luminescence lifetimes for the 6 PJ → 8 S7∕2 (311 nm) transition of Gd3 upon excitation at 273 nm before and after heat treatment were also determined and are equal to 0.14 and 0.001 ms, respectively. The obtained results are comparable to the value measured by other authors [7,10]. These quite short lifetimes of Gd3 confirm the effective energy transfer from Gd3 to Eu3 . The obtained 5 D0 (Eu3 ) lifetimes collaborated well with emission measurements, where observed reduction of the R factor is connected to the structural transformation from the amorphous phase to the ordered and more ionic environment of Eu3 . To confirm the nature of the studied samples XRD was used (Fig. 5). Based on XRD measurements it can be concluded that the studied systems before heat treatment were fully amorphous. After heat treatment several peaks at the spectrum were observed. Based on a theoretical XRD pattern the broad peaks are assigned to the rhombic GdF3 crystalline phase. Crystal sizes have been estimated using the Scherrer equation and were calculated from FWHM as an average from all diffraction lines. The mean value of the crystal size was 6.3 nm. Moreover, the crystallite size was re-examined with the Williamson–Hall formula, which includes internal stress influence on the line broadening. The lattice deformation was lower than 0.00167% and the estimated particle size amounted to 5.6 nm, which corresponds to the results obtained by Scherrer’s methods.
In summary, the Eu3 :GdF3 nanocrystals have been synthesized by the solgel method and identified by XRD. The spectroscopic parameters of the prepared samples indicate that selective incorporation of Eu3 ions into the fluoride nanocrystals occurred. Because of the overlap of the energy levels between the 6 PJ states of Gd3 and the 5 HJ states of Eu3 , the ultraviolet light excitation at 273 nm was promoted to the energy transfer from Gd3 to Eu3 . Emission originating from the 5 D0 state of Eu3 ions upon excitation at 273 nm is long lived in comparison to the excitation at 393 nm and is increased to 190%. References 1. S. Lepoutre, D. Boyer, and R. Mahiou, J. Lumin. 128, 635 (2008). 2. Q. Y. Zhang and X. Y. Huang, Prog. Mater. Sci. 55, 353 (2010). 3. R. T. Wegh, H. Donker, K. D. Oskam, and A. Meijerink, J. Lumin. 82, 93 (1999). 4. P. Ghosh, S. Tang, and A. V. Mudring, J. Mater. Chem. 21, 8640 (2011). 5. M. Karbowiak, A. Mech, and W. Ryba-Romanowski, J. Lumin. 114, 65 (2005). 6. C. Lorbeer, J. Cybinska, and A. V. Mudring, Chem. Commun. 46, 571 (2010). 7. P. Vergeer, E. van den Pol, and A. Meijerink, J. Lumin. 121, 456 (2006). 8. R. Hua, J. Niu, B. Chen, M. Li, T. Yu, and W. Li, Nanotechnology 17, 1642 (2006). 9. T. Grzyb and S. Lis, J. Rare Earths 27, 588 (2009). 10. Y. Kondo, K. Tanaka, R. Ota, T. Fujii, and Y. Ishikawa, Opt. Mater. 27, 1438 (2005). 11. S. Lepoutre, D. Boyer, S. Fujihara, and R. Mahiou, J. Mater. Chem. 19, 2784 (2009). 12. S. Fujihara and K. Tokumo, J. Fluorine Chem. 130, 1106 (2009). 13. S. Fujihara, in Functionalized Inorganic Fluorides: Synthesis, Characterization & Properties of Nanostructured Solids, A. Tressaud, ed. (Wiley, 2010), Chap. 10, pp. 307– 330. 14. B. Tian, B. Chen, Y. Tian, J. Sun, X. Li, J. Zhang, H. Zhong, L. Cheng, Z. Wuc, and R. Hua, Ceram. Int. 38, 3537 (2012). 15. S. Fujihara, S. Koji, and T. Kimura, J. Mater. Chem. 14, 1331 (2004). 16. B. Liu, Y. Chen, C. Shi, H. Tang, and Y. Tao, J. Lumin. 101, 155 (2003). 17. X. Zhang, T. Hayakawa, M. Nogami, and Y. Ishikawa, J. Alloys Compd. 509, 2076 (2011). 18. A. C. Yanes, J. Del-Castillo, J. Mendez-Ramos, V. D. Rodrıguez, M. E. Torres, and J. Arbiol, Opt. Materials 29, 999 (2007). 19. S. Fujihara, C. Mochizuki, and T. Kimura, J. Non-Cryst. Solids 244, 267 (1999). 20. F. Benz, H. P. Strunk, J. Schaab, U. Künecke, and P. Wellmann, J. Appl. Phys. 114, 073518 (2013). 21. B. Szpikowska-Sroka, L. Żur, R. Czoik, T. Goryczka, A. S. Swinarew, M. Żądło, and W. A. Pisarski, J. Sol-Gel Sci. Technol. 68, 278 (2013).
