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Sodium Silicate Glasses for White LEDs,” J. Am. Ceram. Soc. 95, 34–36 ... “Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses,” ...
Enhanced luminescence via energy transfer from Ag+ to RE ions (Dy3+, Sm3+, Tb3+) in glasses JingJing Li, RongFei Wei, XueYun Liu, and Hai Guo* Department of Physics, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China * [email protected]

Abstract: Oxyfluoride glasses containing Ag species and rare earth (RE) ions (Dy3+, Sm3+, Tb3+) were prepared by melt-quenching technique. The type of luminescent species of novel excitation band (230-300 nm peaked at 255 nm) and emission band (300-600 nm peaked at 350 nm) were investigated by absorption, excitation, emission spectra, as well as decay lifetime measurements and can be ascribed to isolated Ag+ ions. Owing to energy transfer from Ag+ to RE ions, significant enhancements of RE ions emission (76 times for Sm3+, 41 times for Dy3+) were observed for nonresonant UV excitation (255 nm). Our research may extend the understanding of interactions between RE ions and Ag species. ©2012 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; (160.2750) Glass and other amorphous materials; (160.5690) Rare-earth-doped materials.

References and Links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

G. Blasse and B. C. Grabmaier, Luminescent Marerials (Springer, 1994). F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen, and X. Liu, “Tuning upconversion through energy migration in core-shell nanoparticles,” Nat. Mater. 10(12), 968–973 (2011). G. Gao, S. Reibstein, M. Peng, and L. Wondraczek, “Tunable dual-mode photoluminescence from nanocrystalline Eu-doped Li2ZnSiO4 glass ceramic phosphors,” J. Mater. Chem. 21, 3156–3161 (2011). D. Chen, Y. Yu, F. Huang, A. Yang, and Y. Wang, “Lanthanide activator doped NaYb1-xGdxF4 nanocrystals with tunable down-, up-conversion luminescence and paramagnetic properties,” J. Mater. Chem. 21, 6186–6192 (2011). G. Seeta Rama Raju, J. Yu, J. Park, H. Jung, and B. Moon, “Photoluminescence and cathodoluminescence properties of nanocrystalline Ca2Gd8Si6O26:Sm3+ phosphors,” J. Am. Ceram. Soc. 95, 238–242 (2012). H. Guo, H. Zhang, J. Li, and F. Li, “Blue-white-green tunable luminescence from Ba2Gd2Si4O13:Ce3+,Tb3+ phosphors excited by ultraviolet light,” Opt. Express 18(26), 27257–27262 (2010). R. Wei, H. Zhang, F. Li, and H. Guo, “Blue-White-Green Tunable Luminescence of Ce3+, Tb3+ Co-Doped Sodium Silicate Glasses for White LEDs,” J. Am. Ceram. Soc. 95, 34–36 (2012). O. Malta, P. Santa-Cruz, G. De Sá, and F. Auzel, “Fluorescence enhancement induced by the presence of small silver particles in Eu3+ doped materials,” J. Lumin. 33, 261–272 (1985). L. Naranjo, C. De Araújo, O. Malta, P. Cruz, and L. Kassab, “Enhancement of Pr3+ luminescence in PbO-GeO2 glasses containing silver nanoparticles,” Appl. Phys. Lett. 87, 241914 (2005). T. Hayakawa, S. Selvan, and M. Nogami, “Field enhancement effect of small Ag particles on the fluorescence from Eu3+-doped SiO2 glass,” Appl. Phys. Lett. 74, 1513–1515 (1999). Y. Wu, X. Shen, S. Dai, Y. Xu, F. Chen, C. Lin, T. Xu, and Q. Nie, “Silver Nanoparticles Enhanced Upconversion Luminescence in Er3+/Yb3+ Codoped Bismuth-Germanate Glasses,” J. Phys. Chem. C 115, 25040– 25045 (2011). M. Eichelbaum and K. Rademann, “Plasmonic enhancement or energy transfer? on the luminescence of gold-, silver-, and lanthanide-doped silicate glasses and its potential for light-emitting devices,” Adv. Funct. Mater. 19, 2045–2052 (2009). M. Eichelbaum, K. Rademann, A. Hoell, D. M. Tatchev, W. Weigel, R. Stößer, and G. Pacchioni, “Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses,” Nanotechnology 19(13), 135701 (2008). H. Guo, J. Li, F. Li, and H. Zhang, “Origin of white luminescence in Ag-Eu Co-doped oxyfluoride glasses,” J. Electrochem. Soc. 158, J165–J168 (2011). H. Guo, X. Wang, J. Chen, and F. Li, “Ultraviolet light induced white light emission in Ag and Eu3+ co-doped oxyfluoride glasses,” Opt. Express 18(18), 18900–18905 (2010). V. K. Tikhomirov, V. D. Rodríguez, A. Kuznetsov, D. Kirilenko, G. Van Tendeloo, and V. V. Moshchalkov, “Preparation and luminescence of bulk oxyfluoride glasses doped with Ag nanoclusters,” Opt. Express 18(21), 22032–22040 (2010).

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Received 17 Feb 2012; revised 1 Apr 2012; accepted 9 Apr 2012; published 18 Apr 2012 23 April 2012 / Vol. 20, No. 9 / OPTICS EXPRESS 10122

17. Y. Dai, X. Hu, C. Wang, D. Chen, X. Jiang, C. Zhu, B. Yu, and J. Qiu, “Fluorescent Ag nanoclusters in glass induced by an infrared femtosecond laser,” Chem. Phys. Lett. 439, 81–84 (2007). 18. E. Borsella, G. Battaglin, M. A. García, F. Gonella, P. Mazzoldi, R. Polloni, and A. Quaranta, “Structural incorporation of silver in soda-lime glass by the ion-exchange process: a photoluminescence spectroscopy study,” Appl.Phys. A. 71, 125–132 (2000). 19. S. Paje, M. García, J. Llopis, and M. Villegas, “Optical spectroscopy of silver ion-exchanged As-doped glass,” J. Non-Cryst. Solids 318, 239–247 (2003). 20. A. Meijerink, M. van Heek, and G. Blasse, “Luminescence of Ag+ in crystalline and glassy SrB4O7,” J. Phys. Chem. Solids 54, 901–906 (1993).

1. Introduction Rare earth (RE) ions doped phosphors have attracted persistent interest during the past decades because of their unique luminescent behaviors and wide applications in illumination, solid-state lasers and so on [1–7]. However, the absorption cross section of RE ions is small due to the parity forbidden f-f transitions, which means RE ions cannot be efficiently excited by ultraviolet (UV) light. Consequently, numerous approaches, such as charge transfer state absorption, f-d absorption and host absorption, were used to enhance the excitation efficiencies of RE ions. Recently, coupling RE ions with Ag nanoparticles (NPs) has been developed as a valuable strategy to improve the luminescence of RE ions [8–11]. It was general accepted that enhanced luminescence occurred due to the local-field surface plasmon resonance (SPR) effect of Ag NPs. SPR, the absorption of which centers at 420 ± 20 nm, is a spectacular optical phenomenon exhibited by metal NPs and regarded as the collective excitation of the conduction band electrons which induces high electromagnetic fields near metal NPs, resulting in boosting excitation efficiency as well as radiative decay rate and help to overcome the drawback of low absorption coefficients of RE ions [12]. On the contrary, some reported enhancement of RE ions luminescence was interpreted by a classical energy transfer from very small molecule-like, non-plamonic Ag particles (MLAg), whereas there is no enhancement effect related to SPR effect [12–15]. Rademann [12] firstly observed ML-Ag in silicate glasses, the excitation and emission of which located at UV region (325-375 nm) and bluish-green region (450-700 nm), respectively. Luminescence enhancement of Eu3+ via energy transfer from ML-Ag in Ag-Eu3+ co-doped oxyfluoride glasses under UV excitation has been described by our group recently [14,15]. More interesting, white emissions were obtained by the combination of emissions from Eu3+ and ML-Ag, suggesting their potential application in W-LEDs. Consequently, systematic studies on the interactions between RE ions and Ag species are very interesting and important to extend their practical applications [16]. Herein, the luminescent properties of RE ions (Dy3+, Sm3+, Tb3+) in oxyfluoride glasses containing Ag species were investigated. Results demonstrate that the novel excitation band (255 nm) and emission band (350 nm) is ascribed to isolated Ag+ ions. Due to energy transfer from Ag+ to RE ions, a great enhancement of RE ions emission was observed for non-resonant UV excitation (255 nm). 2. Experimental Ag and RE ions co-doped glasses were prepared by melt-quenching method. The composition of host glasses is 68SiO2-16BaF2-13K2CO3-3La2O3 (in mol%). High purity SiO2, BaF2, K2CO3, AgNO3 (A.R.) and DyF3, Sm2O3, TbF3 (99.99%) were used as raw materials. Glasses co-doped with 1 wt% AgNO3 and 1 mol% RE ions (labeled as GDyAg, GSmAg and GTbAg, respectively) were prepared. For comparison AgNO3 (1 wt%) and RE ions (1 mol%) single doped glasses (labeled as GAg, GDy, GSm and GTb, respectively) were prepared also. Approximately 15 g batches of raw materials were mixed and melted at 1300 °C for 1h in a covered corundum crucible. The glass melt was poured onto a cold brass plate and then pressed by another plate. At last, samples with thickness of 2 mm were cut and polished for optical experiments.

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Received 17 Feb 2012; revised 1 Apr 2012; accepted 9 Apr 2012; published 18 Apr 2012 23 April 2012 / Vol. 20, No. 9 / OPTICS EXPRESS 10123

Absorption spectra were measured by Hitachi U-3900 ultraviolet-Visible (UV-VIS) spectrophotometer (Tokyo, Japan). Excitation, emission spectra and decay curves were recorded on an Edinburgh FLS920 spectrofluorometer (Netherlands). 3. Results and discussion Figure 1 reveals transmittance spectra and all glasses display a high transparency. For GDyAg and GSmAg, several absorptions assigned to f-f transitions from ground state to excited states were detected. The strongest peaks at 348 nm for GDyAg and 402 nm for GSmAg are related to transitions 6H15/2→4M15/2, 6P7/2 of Dy3+ and 6H5/2→4G7/2 of Sm3+, respectively [1]. In addition, there is no obvious absorption band from SPR of Ag NPs, indicating that there is no Ag NPs in glasses [9, 17]. 100

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Wavelength (nm) Fig. 1. Transmittance spectra of GAg, GDyAg, GSmAg and GTbAg.

Figure 2(a) displays excitation and emission spectra of GDy and GDyAg. Excitation spectra of GDy (λem = 575 nm) include several sharp peaks in 300-500 nm region, which are ascribed to transitions from 6H15/2 ground level to higher energy levels of Dy3+. Obviously, an additional excitation band around 255 nm was observed for GDyAg, indicating this excitation band must be related to Ag species. Under non-resonant excitation of Dy3+ (λex = 255 nm), the emission intensity of Dy3+ in GDy is very weak and even can’t be observed, while that in GDyAg increases greatly and a significant enhanced Dy3+ emission by 41 times was observed. Emission spectra of GDyAg consist of two parts: one broad band at 350 nm and several sharp peaks in 480-670 nm region. Obviously, the sharp peaks at 484 nm, 575 nm and 668 nm are related to 4F9/2→6HJ (J = 15/2, 13/2 and 11/2) transitions of Dy3+, respectively. There is no 350 nm emission band in GDy, indicating ulteriorly that this additional band must be originated from Ag species. Excitation and emission spectra of GSm and GSmAg are shown in Fig. 2(b). Excitation spectra of GSm and GSmAg (λem = 601 nm) consist of several bands at 360, 373, 402 and 467 nm, which are the characteristic f-f transitions of Sm3+. The strongest peak at 402 nm is attributed to 6H5/2→4G7/2 transition. Similar to that of GDyAg, an enhanced excitation band around 255 nm was observed in excitation spectra of GSmAg. Upon 255 nm excitation, GSm showed almost no luminescence of Sm3+. While emission spectra of GSmAg consist of characteristic emission of Sm3+ with the strongest peak at 601 nm (4G5/2→6H7/2). The intensity of Sm3+ emission in GSmAg reaches 76 times stronger than that in GSm. Besides, the emission spectra of GSmAg include not only characteristic emissions of Sm3+, but also a novel emission band around 350 nm.

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Received 17 Feb 2012; revised 1 Apr 2012; accepted 9 Apr 2012; published 18 Apr 2012 23 April 2012 / Vol. 20, No. 9 / OPTICS EXPRESS 10124

Figure 2(c) presents excitation and emission spectra of GTb and GTbAg. Excitation spectra (λem = 543 nm) of GTb contain two parts: one broad band in 230-300 nm region and some sharp peaks in 300-400 nm region. The former is attributed to 4f8→4f75d1 transition and the latter are due to 4f-4f transitions of Tb3+. Excitation spectra of GTbAg also display an intense band around 255 nm. Upon 255 nm excitation, emission spectra compose of typical 5 D3,4→7FJ (J = 3-6) transitions of Tb3+, among which 5D4→7F5 transition at 543 nm is predominant. Compared to GTb, Tb3+ emission intensity of GTbAg was enhanced. And in addition to characteristic emission peaks of Tb3+, a broad band emission centered at 350 nm was also detected in GTbAg. Based on above discussions, we can conclude that the additional excitation band at 255 nm (230-290 nm) and emission band at 350 nm (300-450 nm) are originated from Ag species. Significant enhancements of RE ions emission (76 times for Sm3+, 41 times for Dy3+) were observed for non-resonant UV excitation (255 nm). Such phenomena indicate the existence of energy transfer from Ag species to RE ions. (a)

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Wavelength (nm) Fig. 2. (color online) Excitation and emission spectra of (a) GDy and GDyAg; (b) GSm and GSmAg; (c) GTb and GTbAg; (d) GAg.

In order to investigate the nature of new excitation at 255 nm and emission at 350 nm, GAg was prepared and its excitation (λem = 350 nm) and emission (λex = 255 nm) spectra are displayed in Fig. 2(d). Obviously, excitation spectra exhibit a broadband centered at 255 nm. And a broadband at 300-600 nm peaked at 350 nm is observed in emission spectra. The shape and position of broad excitation and emission bands are consistent with aforementioned bands in Figs. 2(a)-2(c). Similar broad excitation around 245 nm and emission around 340 nm related to electronic transition between 4d10 (1S0) ground state and 4d95s1 (3D1) excited state of isolated Ag+ ions were reported in soda-lime silicate glasses [18,19]. Therefore, the broad excitation at 255 nm and emission at 350 nm may be assigned to Ag+ ions.

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Received 17 Feb 2012; revised 1 Apr 2012; accepted 9 Apr 2012; published 18 Apr 2012 23 April 2012 / Vol. 20, No. 9 / OPTICS EXPRESS 10125

Intensity (a.u.)

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Time (µs) Fig. 3. Decay curves of 601 nm emission excited by 402 nm (I) and 255 nm (II) light for GSmAg; decay curve of 350 nm emission (λex = 255 nm) for GAg (III).

To further testify this point, decay curves of luminescence were measured and are shown in Fig. 3. Curve (I) of 601 nm emission (λex = 402 nm) for GSmAg can be approximately fitted by a single exponential function with a lifetime of 1.98 ms, which is associated with transition related to Sm3+. Curve (II) for GSmAg monitored at 601 nm (λex = 255 nm) exhibits a bi-exponential decay profile. The estimated lifetimes of slow and fast components are 2.0 ms and 16.5 µs, respectively. The slow decay is assigned to Sm3+ also. Curve (III) of GAg for 350 nm emission (λex = 255 nm) is fast and corresponding lifetime was about 15.7 µs, in accordance with the fast feature of curve (II). It was reported in Refs [[18, 20]]. that the decay time of single Ag+ ions doped glasses is about 14 µs, besides, the shape of decay curves is similar to curve (III). Therefore, lifetime measurement had proved that the additional emission at 350 nm is originated from single Ag+ ions undoubtedly. 40

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Co-doped with Ag , significant enhancement of RE ions emission under non-resonant UV excitation (240-275 nm) was observed (Fig. 2), evidencing efficient energy transfer from Ag+ to RE ions. The energy level diagram of Ag+-RE ions and possible energy transfer process are schematically presented in Fig. 4. Apparently, the excited states of RE ions contributing to

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Received 17 Feb 2012; revised 1 Apr 2012; accepted 9 Apr 2012; published 18 Apr 2012 23 April 2012 / Vol. 20, No. 9 / OPTICS EXPRESS 10126

luminescence are all in an energy range below the excited state energies of Ag+ ions. Hence, these states can be in principal populated by energy transfer from Ag+. Excited by UV light (240-275 nm), Ag+ ions are excited from ground state to excited state. An excited Ag+ ion relaxes from excited state to ground state nonradiatively and transfers the excitation energy to a neighboring RE ion, promoting it from ground state to excited state. After then, RE ions in excited level relax from luminescent level to ground level radiatively and result in characteristic emissions of RE ions.

Fig. 5. (color online) (a) Emission spectra of GTbSmAg (λex = 255 nm); (b) CIE chromaticity diagram related to white light emission of GDyAg and GTbSmAg (λex = 255 nm).

It should be mentioned that in emission spectra of GDyAg, the emission of Ag+ located at blue region and that of Dy3+ at orange-red region. Combining these two emissions, white emission may be achieved. Simultaneity, by combing blue emission of Ag+, green emission of Tb3+ and red emission of Sm3+, white emission can be also achieved. For this purpose, Ag+, Sm3+ and Tb3+ co-doped sample (GTbSmAg) was prepared and its emission spectra are presented in Fig. 5(a). It is clear that emission spectra consist of three parts: 3D1→1S0 transition of Ag+, 5D4→7F5 transition of Tb3+ and 4G5/2→6H7/2, 9/2 transitions of Sm3+. Luminescence colors of samples are characterized by Commission International de I’Eclairage (CIE) chromaticity diagram and shown in Fig. 5(b). The color is white for GDyAg (X = 0.320, Y = 0.312) and GTbSmAg (X = 0.325, Y = 0.342) excited by 255 nm light. Their CIE coordinate are close to the standard equal energy white light illuminate (X = 0.333, Y = 0.333). The inset of Fig. 5(b) gives luminescent photos (λex = 254 nm) for GDyAg and GTbSmAg. Perfect white light was observed, indicating that our glasses are very promising materials for the fabrication of W-LEDs. 4. Conclusion The main results of investigation on luminescent properties of Ag+-RE co-doped oxyfluoride glasses are listed below. (1) The additional novel excitation band (255 nm) and emission band (350 nm) is originated from isolated Ag+. (2) Energy transfer from Ag+ to RE ions was firstly observed. (3) RE ions luminescence was significantly enhanced due to energy transfer from Ag+ under non-resonant UV excitation (240-275 nm). (4) White emission can be obtained in GDyAg and GTbSmAg glasses. Our research may extend the understanding of interactions between RE ions and Ag species and Ag+-RE co-doped glasses may be applied as W-LEDs phosphors. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 10904131).

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Received 17 Feb 2012; revised 1 Apr 2012; accepted 9 Apr 2012; published 18 Apr 2012 23 April 2012 / Vol. 20, No. 9 / OPTICS EXPRESS 10127