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Eu3+-doped oxyfluoride glass ceramics with LaF3 for white LED color conversion
Optical Materials 41 (2015) 71–74
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Eu2+/Eu3+-doped oxyfluoride glass ceramics with LaF3 for white LED color conversion Sang Hun Lee a, Suk-Rok Bae a, Yong Gyu Choi b, Woon Jin Chung a,⇑ a b
Institute for Rare Metals & Division of Advanced Materials Engineering, Kongju National University, Cheonan, Chungnam 330-717, Republic of Korea Department of Materials Science and Engineering, Korea Aerospace University, Goyang, Gyeonggi 412-791, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 14 November 2014 Keywords: Oxyfluoride Glass–ceramics LED Eu3+ Luminescence
a b s t r a c t SiO2–Na2O–Al2O3–LaF3 glasses doped with Eu2+ and Eu3+ were synthesized to realize an inorganic color converter for white LED using 400 nm UV LED. Among various rare earth ions, Eu2+ and Eu3+ showed prominent emission under 400 nm LED excitation. Carbon and EuF3 content were varied to control the ratio of Eu2+ and Eu3+ during the melting process. When the ratio of Eu2+ and Eu3+ within the glass matrix was properly controlled, color coordinates of the photoluminescence spectra could be adjusted to make white colors under 400 nm LED excitation. The emission intensity was increased with subsequent heat treatment which led to the formation of LaF3 nano-crystals. However, almost no conversion was observed when the glasses were actually mounted on UV-LED to make a white LED. Heavy crystallization of the oxyfluoride glasses was thus investigated to improve its scattering of the light source and color conversion efficiency, and its practical feasibility as an inorganic UV-LED color converter was demonstrated. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Conventional white LEDs (WLEDs) which use organic resins as a color converting material have suffered from short lifetimes, mostly due to the poor chemical and thermal stability of the organic resins, which changes their color coordinate and converting efficiency [1,2]. Inorganic color converters based on ceramics or glasses have thus been extensively studied and applied, to realize high power and high brightness WLEDs [3–6]. Among them, bulk glass phosphors (BGP) which consist of a glass matrix and active elements such as rare earth (RE) ions, transition metal ions and quantum dots, are strong candidates for inorganic color converters. Unlike other inorganic color converters such as phosphor ceramics (PC) [3,4] or phosphor in glass (PiG) [5,6], BGP does not require ceramic phosphors or additional fabrication processes and can be formed into various shapes, which can reduce production costs effectively. Various glasses have been reported for LED color converters [7–11]. Rocha et al. [7] fabricated Eu2+,3+ and Ce3+ doped aluminosilicate glasses and showed their potential as an inorganic UV-LED color converter under 405 nm laser excitation. However, oxide glasses have limited quantum efficiency due to their relatively high ⇑ Corresponding author at: Div. of Advanced Materials Eng., Kongju National University, 1223-24 Cheonan-daero, Seobuk-gu, Cheonan, Chungnam 330-717, Republic of Korea. Tel.: +82 41 521 9377; fax: +82 41 568 5776. E-mail address: [email protected] (W.J. Chung). http://dx.doi.org/10.1016/j.optmat.2014.10.018 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
phonon energy. Oxyfluoride glasses thus have been extensively studied to improve the quantum efficiency of the doped RE ions via fluoride nano-crystals [8–11]. They can provide the oxide glasses with high chemical and thermal stability, as well as low phonon energy to the doped ions. Babu et al. [8] synthesized Dy3+-doped oxyfluoride glasses with PbF2 nano-crystals which show white emission under 451 nm laser excitation, but they used Pb-based materials and laser sources. Luo et al. [9] showed white emission with Dy3+/Ce3+ co-doped oxyfluoride glass with LaF3 nano-crystals but they used 320 nm for excitation, and at that wavelength a high power LED is not commercially available. Recently, Dy3+ and Eu3+ co-doped oxyfluoride glasses with LaF3 nano-crystals under 365 nm LED excitation were reported [10] but the white emission has not been properly adjusted. It should be noted that most of the previous reports on BGPs showed just their photoluminescence (PL) properties under high power laser sources, without practical application to the actual LED chips, even for oxyfluoride glasses. In this study, therefore, we fabricated Eu2+ and Eu3+ co-doped oxyfluoride glass ceramics with LaF3 nano-crystals and examined their practical feasibility as an inorganic color converter for the 400 nm LED, which is commercially available for high power LED. Eu3+-doped oxyfluoride glass with LaF3 has been previously reported for white emission [11] as well as for structural study [12,13] but they only reported PL spectra properties. We adjusted the white emission by controlling the Eu2+ and Eu3+ contents and
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improved their emission intensity with heat treatment and nanocrystal formation. We inspected their PL properties when pumped with a commercial LED as well as when the glass ceramics were actually mounted on a UV-LED to make a WLED. Spectral properties depending on the Eu2+/Eu3+ ratio and their potential for use as an actual color converter after LED mounting, were discussed.
2. Experimental Nominal composition of the glasses was 45SiO2–15Na2O– 15Al2O3–25LaF3 in mol% which was modified from the previous studies [10,14,15] and REF3 (RE = Eu, Dy, Ho, Er or Tm) was doped additionally. The high purity (>99.99%) raw materials were weighed and thoroughly mixed. Carbon was also additionally incorporated, from 15 to 20 wt%, to control the reducing atmosphere during melting and the Eu2+/Eu3+ contents. The conventional melt-quenching method was used to form glasses at 1450 °C for 1 h under ambient atmosphere followed by annealing at 400 °C for 2 h. Heat treatment to form LaF3 nano-crystals was also varied by changing temperature and duration time. The formation of nano-crystals was examined with an X-ray diffractometer (XRD; Rigaku, D/MAX-2500U). A 405 nm UV-LED source (Thorlabs, M405L2) pumped the samples while a UV-LED with 3528 package and 400 nm center wavelength was used to mount the glass ceramics and compose WLED. 0.25 m monochromator (Thermo Oriel, MS257) equipped with a lock-in amplifier (Thermo Oriel, Merlin™) collected the PL emission which was detected with a photomultiplier tube.
3. Results and discussion To identify the proper activator RE-ions, suitable for 400 nm UV-LED color conversion, various candidates were investigated by fabricating oxyfluoride glasses doped with RE ions such as Dy3+, Er3+, Eu2+, Eu3+, Ho3+, Pr3+ and Tm3+. The glass was doped with 3 mol% of REF3 and their PL spectra were examined. As shown in Fig. 1, when pumped with a 405 nm LED, the Eu2+ and Eu3+ -doped glasses showed noticeable emissions compared to other RE-ions. Strong absorption due to the 4f–5d transition of Eu2+ and Eu3+:5D3 state was mostly responsible for the emission spectra. Dy3+ also showed emissions from the Dy3+:4F9/2 state but a weak absorption band near 400 nm resulted in relatively low emission intensity. Other rare earth ions were also examined but showed almost no emission due to their very weak absorption and nonradiative transitions between |4fi states.
Fig. 1. Emission spectra of oxyfluoride glasses doped with various rare earth ions under 405 nm LED excitation with 420 nm cut-off filter applied.
The broad blue to green emission originated from the 4f65d to 4f7 transition of Eu2+ while red emissions came from Eu3+:5D2 ? 7F1,2 transitions. It could thus be anticipated that they would produce a white emission when those emissions were properly combined, by adjusting the emission intensity of the Eu2+ and Eu3+ within the glass matrix. In order to control the emission spectra of Eu2+ and Eu3+ by controlling concentration, carbon content was varied to be 15 to 20 wt% for 3 mol% EuF3 doped oxyfluoride glass, and their emission spectra were monitored under 405 nm LED excitation. As shown in Fig. 2, as carbon content increased, Eu2+ emission increased while the various red emissions from Eu3+:1D2 to 7F1,2,3,4 decreased, indicating the effect of carbon as a reducing agent. However, weak emission from Eu2+ was observed when compared to emissions from Eu3+. The result may imply suppression of Eu2+ ions formation, but it could be caused by the heavy spectral interaction between Eu2+ and Eu3+ which can effectively quench the Eu2+ emission; this will be discussed further. When carbon content was raised to more than 20 wt%, the glass was crystallized as shown in Fig. 2. More effective control of the emission spectra via Eu2+ and Eu3+ was achieved with the EuF3 content when carbon content was fixed at 18 wt%. As depicted in Fig. 3, when the EuF3 content decreased from 3 mol% to 0.5 mol%, the blue to green emission from Eu2+ developed remarkably while the Eu3+ emission decreased. Malchukova et al. [16] observed a similar spectral change in Eu2O3-doped aluminoborosilicate glass. When Eu2O3 concentration was increased they observed an increase of Eu2+ content using electron paramagnetic resonance spectroscopy, as well as a significant decrease of emission intensity from Eu2+. The spectral change with Eu concentration was due to spectral overlap between Eu2+ emission and Eu3+ absorption, and highly effective non-radiative energy transfer from Eu2+:4f65d(eg) to Eu3+. In the present work, however, it was interesting to find that 0.5 mol% of EuF3 showed much higher emission intensity than 3 mol% of EuF3. This can be attributed to the strong absorption cross section at the excitation wavelength (400 nm) [17] and high quantum efficiency (95%) of Eu2+ within the glass matrix [18], while Eu3+ possesses a limited absorption coefficient near 400 nm [17] and low external quantum efficiency of 26.7% in glass [19]. Enhanced non-radiative transitions within Eu3+ ions at high concentration of Eu-ions such as cross relaxation may also contribute to the phenomena. Eu2+ thus seems more suitable for 400 nm LED color conversion. As shown in Fig. 3, color
Fig. 2. Emission spectra of oxyfluoride glasses doped with 3 mol% of EuF3 with varying carbon content under 400 nm LED excitation with 420 nm cut-off filter applied. The inset figure shows the evolution of Eu2+ emission with carbon content.
S.H. Lee et al. / Optical Materials 41 (2015) 71–74
Fig. 3. Emission spectra of oxyfluoride glasses with varying EuF3 content under 400 nm LED excitation with 420 nm cut-off filter applied.
combination via simultaneous emission from Eu2+ and Eu3+ could be achieved when 1 mol% of EuF3 was incorporated. Fig. 4 displays the change of color coordinates for each sample and clearly suggests that white color conversion can be obtained with 1 mol% of EuF3. The emission intensity of the 1 mol% EuF3 doped oxyfluoride glass was further enhanced with heat treatment. As shown in the inset figure of Fig. 4, when the glass was heat treated at 750 °C for varying duration times up to 40 h, the overall emission intensity is highly improved. The emission intensity change can be attributed to a local environment change, from oxygen-dominated to fluorine-dominated structure, induced by the formation of fluoride nano-crystals within the glass matrix, as previously reported [10,20]. XRD was used to examine the glasses and characteristic LaF3 crystal diffraction patterns were observed when the glasses were heat treated (Fig. 5). The preferential incorporation of rare
Fig. 4. CIE color coordinates of the oxyfluoride glasses without heat treatment for varying EuF3 contents: 0.5 (a), 1.0 (b) and 3 mol% (c). CIE color coordinates of oxyfluoride glasses with 1 mol% of EuF3 heat treated at 750 °C for 20 (d) and 40 h (e) are also displayed. The inset figure shows the emission spectra of oxyfluoride glasses under 400 nm LED excitation without and with heat-treatment at 750 °C for 20 and 40 h while EuF3 content was fixed at 1 mol%.
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earth ions into fluoride nano-crystals in oxyfluoride glasses has been already discussed elsewhere [21] and supports the conclusion of spectral change with nano-crystal formation. Enhancing the Eu2+ emission higher than the Eu3+ resulted in the blue shift of the CIE coordinate as depicted in Fig. 4(b), (d) and (e). Different quantum efficiency and sensitivity to the local environment change may be responsible for the difference. The incorporation of Eu2+-ion into the LaF3 crystalline structure requires charge compensation and may also induce spectroscopic change which requires further study. The spectral hole in the Eu2+ emission at 465 nm is mostly due to the Eu3+:7F0 ? 5D2 absorption and further supports the effective energy transfer from Eu2+ to Eu3+. Heat treated oxyfluoride glasses with 1 mol% of EuF3 were thus mounted on a 400 nm LED chip to make a WLED. Electroluminescence (EL) of UV-LED and PL of the mounted glasses were monitored and exhibited in Fig. 6. The glasses maintained their high transparency even after heat treatment. However, unlike the PL spectra monitored from the glass surface, almost no conversion was observed in the presenting emission from the LED chip. High transparency even at pumping wavelength along with the relatively low conversion efficiency of the present 1 mol% EuF3 doped glass ceramics seem to be responsible for the result. Thus, in order to increase the excitation efficiency via scattering within the glass ceramic plate, the glass was heavily crystallized at 760 °C for 5 h. As displayed in Fig. 7, the glass turned greenish white due to light scattering by the grown crystals. When the glass, 2.2 mm in thickness, was mounted on the UV-LED chip, it clearly showed a color converted EL + PL spectrum in which the CIE color coordinate could be also determined. When the thickness was varied from 2.2 to 0.4 mm, the spectra along with the color coordinate of the LEDs were changed, as shown in Fig. 7. It should be noted that the color coordinate of LEDs with a glass ceramic plate having thickness of 0.4–1.6 mm is located within the white color range, suggesting the possible realization of a white LED. The results suggest that the Eu2+/Eu3+-doped glass ceramic with controlled crystallization can be practically used for an inorganic color converter for WLED. However, the present LEDs show greenish to bluish white and need further color adjustment for natural white or warm white. Thus, further study is required to improve its chromaticity for white color adjustment as well as conversion efficiency.
Fig. 5. XRD patterns of oxyfluoride glass and glass ceramics with LaF3 for various heat treatment conditions.
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Fig. 6. EL + PL spectra of 400 nm UV-LEDs mounted with oxyfluoride glass and glass ceramics with 1 mol% of EuF3 for various heat treatment conditions. The inset figure shows an actual photo of the LEDs before and after the LED is turned on.
glasses as a color converting material for UV-LED. Broad blue to green emissions from the Eu2+-ion and red emissions from Eu3+ were combined to achieve white conversion. Little change in emission spectra was observed when the Eu2+/Eu3+ ratio was controlled via carbon content. However, effective spectral control could be achieved via EuF3 content. Emissions from Eu2+ significantly increased while emissions from Eu3+ decreased when EuF3 was decreased from 3 mol% to 0.5 mol%. Efficient non-radiative energy transfer from Eu2+ to Eu3+ was responsible for the phenomena. Oxyfluoride glass with 1 mol% EuF3 successfully showed white emission, and its intensity was further enhanced with heat treatment, which led to LaF3 nano-crystal formation within the matrix. When the glass ceramics were mounted on 400 nm UV-LED, almost no color conversion was observed. Practical color conversion, however, could be achieved when the glass was heavily crystallized to improve excitation efficiency via scattering. Color coordination of the LEDs could be also varied by controlling the glass ceramic thickness, and was located within the white color range. These results suggest the practical feasibility of the current oxyfluoride glass ceramic as a robust inorganic color converting material to realize high-power WLED using 400 nm UV-LED. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2005671). References
Fig. 7. EL + PL spectra and CIE color coordinates of LEDs mounted with heavily crystallized oxyfluoride glass ceramic with 1 mol% of EuF3 and varying glass thickness. The inset figure shows the actual photo of the mounted LED before and after the LED is turned on.
4. Summary Among various RE-ions, Eu2+ and Eu3+ showed noticeable emissions under 405 nm excitation and were used to dope oxyfluoride
[1] N. Narendran, Y. Gu, J.P. Freyssinier, H. Yu, L. Deng, J. Cryst. Growth 268 (2004) 449. [2] M.-H. Chang, D. Das, P.V. Varde, M. Pecht, Mircoelectron. Reliab. 52 (2012) 762. [3] S. Nishiura, S. Tanabe, K. Fujioka, Y. Fujimoto, Opt. Mater. 33 (2011) 688. [4] S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, S. Tanabe, Proc. SPIE 5941 (2005) 594111. [5] Y.K. Lee, J.S. Lee, J. Heo, W.B. Im, W.J. Chung, Opt. Lett. 37 (2012) 3276. [6] S. Yi, W.J. Chung, J. Heo, J. Am. Ceram. Soc. 97 (2014) 342. [7] A.C.P. Rocha, L.H.C. Andrade, S.M. Lima, A.M. Farias, A.C. Bento, M.L. Baesso, Y. Guyot, G. Boulon, Opt. Express 20 (2012) 10034. [8] P. Babu, K.H. Jang, E.S. Kim, L. Shi, H.J. Seo, F. Rivera-López, U.R. RodríguezMendoza, V. Lavín, R. Vijaya, C.K. Jayasankar, L. Rama Moorthy, J. Appl. Phys. 105 (2009) 013516. [9] Q. Luo, X. Qiao, X. Fan, H. Yang, X. Zhang, S. Cui, L. Wang, G. Wang, J. Appl. Phys. 105 (2009) 043506. [10] S.-R. Bae, Y.G. Choi, W.B. Im, K.S. Lee, W.J. Chung, Opt. Mater. 35 (2013) 2034. [11] D. Chen, Y. Yu, P. Huang, H. Lin, Z. Shan, Y. Wang, Acta Mater. 58 (2010) 3035. [12] L.A. Bueno, A.S. Gouveia-Neto, E.B. da Costa, Y. Messaddeq, S.J.L. Ribeiro, J. Phys.: Condens. Matter 20 (2008) 145201. [13] K. Driesen, V.K. Tikhomirov, C. Gorller-Walrand, J. Appl. Phys. 102 (2007) 024312. [14] M.J. Dejneka, J. Non-Cryst. Solids 239 (1998) 149. [15] S. Tanabe, H. Hayashi, T. Hanada, N. Onodera, Opt. Mater. 19 (2002) 343. [16] E. Malchukova, B. Boizot, Mater. Res. Bull. 45 (2010) 1299. [17] P.W. France, in: P.W. Frace (Ed.), Fluoride Glass Optical Fibers, Blackie and Son Ltd., Glasgow, 1990, pp. 132–204. [18] Y. Fujimoto, T. Yanagida, Y. Futami, K. Fukuda, M. Koshimizu, JPS Conf. Proc. 1 (2014) 014024. [19] T. Hayakawa, M. Nogami, Sci. Technol. Adv. Mater. 6 (2005) 66. [20] W.J. Chung, K.H. Kim, B.J. Park, H.S. Seo, J.T. Ahn, Y.G. Choi, J. Am. Ceram. Soc. 93 (2010) 2952. [21] C. Liu, J. Heo, J. Am. Ceram. Soc. 95 (2012) 2100.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Eu2+/Eu3+-doped oxyfluoride glass ceramics with LaF3 for white LED color conversion Sang Hun Lee a, Suk-Rok Bae a, Yong Gyu Choi b, Woon Jin Chung a,⇑ a b
Institute for Rare Metals & Division of Advanced Materials Engineering, Kongju National University, Cheonan, Chungnam 330-717, Republic of Korea Department of Materials Science and Engineering, Korea Aerospace University, Goyang, Gyeonggi 412-791, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 14 November 2014 Keywords: Oxyfluoride Glass–ceramics LED Eu3+ Luminescence
a b s t r a c t SiO2–Na2O–Al2O3–LaF3 glasses doped with Eu2+ and Eu3+ were synthesized to realize an inorganic color converter for white LED using 400 nm UV LED. Among various rare earth ions, Eu2+ and Eu3+ showed prominent emission under 400 nm LED excitation. Carbon and EuF3 content were varied to control the ratio of Eu2+ and Eu3+ during the melting process. When the ratio of Eu2+ and Eu3+ within the glass matrix was properly controlled, color coordinates of the photoluminescence spectra could be adjusted to make white colors under 400 nm LED excitation. The emission intensity was increased with subsequent heat treatment which led to the formation of LaF3 nano-crystals. However, almost no conversion was observed when the glasses were actually mounted on UV-LED to make a white LED. Heavy crystallization of the oxyfluoride glasses was thus investigated to improve its scattering of the light source and color conversion efficiency, and its practical feasibility as an inorganic UV-LED color converter was demonstrated. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Conventional white LEDs (WLEDs) which use organic resins as a color converting material have suffered from short lifetimes, mostly due to the poor chemical and thermal stability of the organic resins, which changes their color coordinate and converting efficiency [1,2]. Inorganic color converters based on ceramics or glasses have thus been extensively studied and applied, to realize high power and high brightness WLEDs [3–6]. Among them, bulk glass phosphors (BGP) which consist of a glass matrix and active elements such as rare earth (RE) ions, transition metal ions and quantum dots, are strong candidates for inorganic color converters. Unlike other inorganic color converters such as phosphor ceramics (PC) [3,4] or phosphor in glass (PiG) [5,6], BGP does not require ceramic phosphors or additional fabrication processes and can be formed into various shapes, which can reduce production costs effectively. Various glasses have been reported for LED color converters [7–11]. Rocha et al. [7] fabricated Eu2+,3+ and Ce3+ doped aluminosilicate glasses and showed their potential as an inorganic UV-LED color converter under 405 nm laser excitation. However, oxide glasses have limited quantum efficiency due to their relatively high ⇑ Corresponding author at: Div. of Advanced Materials Eng., Kongju National University, 1223-24 Cheonan-daero, Seobuk-gu, Cheonan, Chungnam 330-717, Republic of Korea. Tel.: +82 41 521 9377; fax: +82 41 568 5776. E-mail address: [email protected] (W.J. Chung). http://dx.doi.org/10.1016/j.optmat.2014.10.018 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
phonon energy. Oxyfluoride glasses thus have been extensively studied to improve the quantum efficiency of the doped RE ions via fluoride nano-crystals [8–11]. They can provide the oxide glasses with high chemical and thermal stability, as well as low phonon energy to the doped ions. Babu et al. [8] synthesized Dy3+-doped oxyfluoride glasses with PbF2 nano-crystals which show white emission under 451 nm laser excitation, but they used Pb-based materials and laser sources. Luo et al. [9] showed white emission with Dy3+/Ce3+ co-doped oxyfluoride glass with LaF3 nano-crystals but they used 320 nm for excitation, and at that wavelength a high power LED is not commercially available. Recently, Dy3+ and Eu3+ co-doped oxyfluoride glasses with LaF3 nano-crystals under 365 nm LED excitation were reported [10] but the white emission has not been properly adjusted. It should be noted that most of the previous reports on BGPs showed just their photoluminescence (PL) properties under high power laser sources, without practical application to the actual LED chips, even for oxyfluoride glasses. In this study, therefore, we fabricated Eu2+ and Eu3+ co-doped oxyfluoride glass ceramics with LaF3 nano-crystals and examined their practical feasibility as an inorganic color converter for the 400 nm LED, which is commercially available for high power LED. Eu3+-doped oxyfluoride glass with LaF3 has been previously reported for white emission [11] as well as for structural study [12,13] but they only reported PL spectra properties. We adjusted the white emission by controlling the Eu2+ and Eu3+ contents and
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improved their emission intensity with heat treatment and nanocrystal formation. We inspected their PL properties when pumped with a commercial LED as well as when the glass ceramics were actually mounted on a UV-LED to make a WLED. Spectral properties depending on the Eu2+/Eu3+ ratio and their potential for use as an actual color converter after LED mounting, were discussed.
2. Experimental Nominal composition of the glasses was 45SiO2–15Na2O– 15Al2O3–25LaF3 in mol% which was modified from the previous studies [10,14,15] and REF3 (RE = Eu, Dy, Ho, Er or Tm) was doped additionally. The high purity (>99.99%) raw materials were weighed and thoroughly mixed. Carbon was also additionally incorporated, from 15 to 20 wt%, to control the reducing atmosphere during melting and the Eu2+/Eu3+ contents. The conventional melt-quenching method was used to form glasses at 1450 °C for 1 h under ambient atmosphere followed by annealing at 400 °C for 2 h. Heat treatment to form LaF3 nano-crystals was also varied by changing temperature and duration time. The formation of nano-crystals was examined with an X-ray diffractometer (XRD; Rigaku, D/MAX-2500U). A 405 nm UV-LED source (Thorlabs, M405L2) pumped the samples while a UV-LED with 3528 package and 400 nm center wavelength was used to mount the glass ceramics and compose WLED. 0.25 m monochromator (Thermo Oriel, MS257) equipped with a lock-in amplifier (Thermo Oriel, Merlin™) collected the PL emission which was detected with a photomultiplier tube.
3. Results and discussion To identify the proper activator RE-ions, suitable for 400 nm UV-LED color conversion, various candidates were investigated by fabricating oxyfluoride glasses doped with RE ions such as Dy3+, Er3+, Eu2+, Eu3+, Ho3+, Pr3+ and Tm3+. The glass was doped with 3 mol% of REF3 and their PL spectra were examined. As shown in Fig. 1, when pumped with a 405 nm LED, the Eu2+ and Eu3+ -doped glasses showed noticeable emissions compared to other RE-ions. Strong absorption due to the 4f–5d transition of Eu2+ and Eu3+:5D3 state was mostly responsible for the emission spectra. Dy3+ also showed emissions from the Dy3+:4F9/2 state but a weak absorption band near 400 nm resulted in relatively low emission intensity. Other rare earth ions were also examined but showed almost no emission due to their very weak absorption and nonradiative transitions between |4fi states.
Fig. 1. Emission spectra of oxyfluoride glasses doped with various rare earth ions under 405 nm LED excitation with 420 nm cut-off filter applied.
The broad blue to green emission originated from the 4f65d to 4f7 transition of Eu2+ while red emissions came from Eu3+:5D2 ? 7F1,2 transitions. It could thus be anticipated that they would produce a white emission when those emissions were properly combined, by adjusting the emission intensity of the Eu2+ and Eu3+ within the glass matrix. In order to control the emission spectra of Eu2+ and Eu3+ by controlling concentration, carbon content was varied to be 15 to 20 wt% for 3 mol% EuF3 doped oxyfluoride glass, and their emission spectra were monitored under 405 nm LED excitation. As shown in Fig. 2, as carbon content increased, Eu2+ emission increased while the various red emissions from Eu3+:1D2 to 7F1,2,3,4 decreased, indicating the effect of carbon as a reducing agent. However, weak emission from Eu2+ was observed when compared to emissions from Eu3+. The result may imply suppression of Eu2+ ions formation, but it could be caused by the heavy spectral interaction between Eu2+ and Eu3+ which can effectively quench the Eu2+ emission; this will be discussed further. When carbon content was raised to more than 20 wt%, the glass was crystallized as shown in Fig. 2. More effective control of the emission spectra via Eu2+ and Eu3+ was achieved with the EuF3 content when carbon content was fixed at 18 wt%. As depicted in Fig. 3, when the EuF3 content decreased from 3 mol% to 0.5 mol%, the blue to green emission from Eu2+ developed remarkably while the Eu3+ emission decreased. Malchukova et al. [16] observed a similar spectral change in Eu2O3-doped aluminoborosilicate glass. When Eu2O3 concentration was increased they observed an increase of Eu2+ content using electron paramagnetic resonance spectroscopy, as well as a significant decrease of emission intensity from Eu2+. The spectral change with Eu concentration was due to spectral overlap between Eu2+ emission and Eu3+ absorption, and highly effective non-radiative energy transfer from Eu2+:4f65d(eg) to Eu3+. In the present work, however, it was interesting to find that 0.5 mol% of EuF3 showed much higher emission intensity than 3 mol% of EuF3. This can be attributed to the strong absorption cross section at the excitation wavelength (400 nm) [17] and high quantum efficiency (95%) of Eu2+ within the glass matrix [18], while Eu3+ possesses a limited absorption coefficient near 400 nm [17] and low external quantum efficiency of 26.7% in glass [19]. Enhanced non-radiative transitions within Eu3+ ions at high concentration of Eu-ions such as cross relaxation may also contribute to the phenomena. Eu2+ thus seems more suitable for 400 nm LED color conversion. As shown in Fig. 3, color
Fig. 2. Emission spectra of oxyfluoride glasses doped with 3 mol% of EuF3 with varying carbon content under 400 nm LED excitation with 420 nm cut-off filter applied. The inset figure shows the evolution of Eu2+ emission with carbon content.
S.H. Lee et al. / Optical Materials 41 (2015) 71–74
Fig. 3. Emission spectra of oxyfluoride glasses with varying EuF3 content under 400 nm LED excitation with 420 nm cut-off filter applied.
combination via simultaneous emission from Eu2+ and Eu3+ could be achieved when 1 mol% of EuF3 was incorporated. Fig. 4 displays the change of color coordinates for each sample and clearly suggests that white color conversion can be obtained with 1 mol% of EuF3. The emission intensity of the 1 mol% EuF3 doped oxyfluoride glass was further enhanced with heat treatment. As shown in the inset figure of Fig. 4, when the glass was heat treated at 750 °C for varying duration times up to 40 h, the overall emission intensity is highly improved. The emission intensity change can be attributed to a local environment change, from oxygen-dominated to fluorine-dominated structure, induced by the formation of fluoride nano-crystals within the glass matrix, as previously reported [10,20]. XRD was used to examine the glasses and characteristic LaF3 crystal diffraction patterns were observed when the glasses were heat treated (Fig. 5). The preferential incorporation of rare
Fig. 4. CIE color coordinates of the oxyfluoride glasses without heat treatment for varying EuF3 contents: 0.5 (a), 1.0 (b) and 3 mol% (c). CIE color coordinates of oxyfluoride glasses with 1 mol% of EuF3 heat treated at 750 °C for 20 (d) and 40 h (e) are also displayed. The inset figure shows the emission spectra of oxyfluoride glasses under 400 nm LED excitation without and with heat-treatment at 750 °C for 20 and 40 h while EuF3 content was fixed at 1 mol%.
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earth ions into fluoride nano-crystals in oxyfluoride glasses has been already discussed elsewhere [21] and supports the conclusion of spectral change with nano-crystal formation. Enhancing the Eu2+ emission higher than the Eu3+ resulted in the blue shift of the CIE coordinate as depicted in Fig. 4(b), (d) and (e). Different quantum efficiency and sensitivity to the local environment change may be responsible for the difference. The incorporation of Eu2+-ion into the LaF3 crystalline structure requires charge compensation and may also induce spectroscopic change which requires further study. The spectral hole in the Eu2+ emission at 465 nm is mostly due to the Eu3+:7F0 ? 5D2 absorption and further supports the effective energy transfer from Eu2+ to Eu3+. Heat treated oxyfluoride glasses with 1 mol% of EuF3 were thus mounted on a 400 nm LED chip to make a WLED. Electroluminescence (EL) of UV-LED and PL of the mounted glasses were monitored and exhibited in Fig. 6. The glasses maintained their high transparency even after heat treatment. However, unlike the PL spectra monitored from the glass surface, almost no conversion was observed in the presenting emission from the LED chip. High transparency even at pumping wavelength along with the relatively low conversion efficiency of the present 1 mol% EuF3 doped glass ceramics seem to be responsible for the result. Thus, in order to increase the excitation efficiency via scattering within the glass ceramic plate, the glass was heavily crystallized at 760 °C for 5 h. As displayed in Fig. 7, the glass turned greenish white due to light scattering by the grown crystals. When the glass, 2.2 mm in thickness, was mounted on the UV-LED chip, it clearly showed a color converted EL + PL spectrum in which the CIE color coordinate could be also determined. When the thickness was varied from 2.2 to 0.4 mm, the spectra along with the color coordinate of the LEDs were changed, as shown in Fig. 7. It should be noted that the color coordinate of LEDs with a glass ceramic plate having thickness of 0.4–1.6 mm is located within the white color range, suggesting the possible realization of a white LED. The results suggest that the Eu2+/Eu3+-doped glass ceramic with controlled crystallization can be practically used for an inorganic color converter for WLED. However, the present LEDs show greenish to bluish white and need further color adjustment for natural white or warm white. Thus, further study is required to improve its chromaticity for white color adjustment as well as conversion efficiency.
Fig. 5. XRD patterns of oxyfluoride glass and glass ceramics with LaF3 for various heat treatment conditions.
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Fig. 6. EL + PL spectra of 400 nm UV-LEDs mounted with oxyfluoride glass and glass ceramics with 1 mol% of EuF3 for various heat treatment conditions. The inset figure shows an actual photo of the LEDs before and after the LED is turned on.
glasses as a color converting material for UV-LED. Broad blue to green emissions from the Eu2+-ion and red emissions from Eu3+ were combined to achieve white conversion. Little change in emission spectra was observed when the Eu2+/Eu3+ ratio was controlled via carbon content. However, effective spectral control could be achieved via EuF3 content. Emissions from Eu2+ significantly increased while emissions from Eu3+ decreased when EuF3 was decreased from 3 mol% to 0.5 mol%. Efficient non-radiative energy transfer from Eu2+ to Eu3+ was responsible for the phenomena. Oxyfluoride glass with 1 mol% EuF3 successfully showed white emission, and its intensity was further enhanced with heat treatment, which led to LaF3 nano-crystal formation within the matrix. When the glass ceramics were mounted on 400 nm UV-LED, almost no color conversion was observed. Practical color conversion, however, could be achieved when the glass was heavily crystallized to improve excitation efficiency via scattering. Color coordination of the LEDs could be also varied by controlling the glass ceramic thickness, and was located within the white color range. These results suggest the practical feasibility of the current oxyfluoride glass ceramic as a robust inorganic color converting material to realize high-power WLED using 400 nm UV-LED. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2005671). References
Fig. 7. EL + PL spectra and CIE color coordinates of LEDs mounted with heavily crystallized oxyfluoride glass ceramic with 1 mol% of EuF3 and varying glass thickness. The inset figure shows the actual photo of the mounted LED before and after the LED is turned on.
4. Summary Among various RE-ions, Eu2+ and Eu3+ showed noticeable emissions under 405 nm excitation and were used to dope oxyfluoride
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