Low temperature quenching and high efficiency Tm3+, La3+ or Tb3+ co-doped CaSc2O4:Ce3+ phosphors for light-emitting diodes

Journal of Luminescence 131 (2011) 1770–1775

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Low temperature quenching and high efficiency Tm3 þ , La3 þ or Tb3 þ co-doped CaSc2O4:Ce3 þ phosphors for light-emitting diodes Yibo Chen a,b,c,n, Kok Wai Cheah c, Menglian Gong b a

School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China State Key Laboratory of Optoelectronic Materials and Technologies, Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China c Department of Physics, Hong Kong Baptist University, Hong Kong, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 October 2010 Received in revised form 11 April 2011 Accepted 14 April 2011 Available online 20 April 2011

Low temperature quenching and high efficiency CaSc2O4:Ce3 þ (CSO:Ce3 þ ) phosphors co-doped with Tm3 þ , La3 þ and Tb3 þ ions were prepared by a solid state method and the phase-forming, morphology, luminescence and application properties of these phosphors were investigated. The results showed that co-doping of Tm3 þ , La3 þ and Tb3 þ ions can improve the luminescence properties and decrease temperature quenching of CSO:Ce3 þ phosphor remarkably. High efficiency green-light-emitting diodes were fabricated with the prepared phosphors and InGaN blue-emitting (  460 nm) chips. The good performances of the green-light-emitting LEDs made from co-doped CSO:Ce3 þ phosphors confirm the luminescence enhancement and indicate that Tm3 þ , La3 þ and Tb3 þ co-doped CSO:Ce3 þ phosphors are suitable candidates for the fabrication of high efficiency white LEDs. & 2011 Elsevier B.V. All rights reserved.

Keywords: LED Phosphor CaSc2O4:Ce3 þ Co-doping

1. Introduction Recently semi-conductive white light-emitting diodes (WLEDs) have emerged as the fourth generation of illumination technology [1–4]. It is an obvious tendency that the white LEDs will take a significant place in the lighting domain and replace most of the conventional lamps sooner or later. The currently commercial WLEDs are obtained mainly by combining a 460 nm blue-emitting InGaN chip with a yellow-emitting Ce3 þ doped yttrium aluminum garnet (YAG:Ce3 þ ) phosphor [5]. However, the thermal quenching characteristic of commonly used YAG:Ce3 þ phosphor degrades the white light characteristics at elevated temperature, and such white LEDs encounter the following problems: low color-rendering index, low color reproducibility and low luminous efficiency [6,7]. To resolve these problems, use of red and green phosphors with strong resistance to temperature quenching instead of a yellow phosphor has been proposed [8]. A novel green phosphor, Ce3 þ activated CaSc2O4 (CSO:Ce3 þ ), was prepared by Shimomura et al. [9] and it proved to be suitable and effective phosphor for the purpose mentioned above. In this work, some other rare-earth ions, Tm3 þ , La3 þ and Tb3 þ , were co-doped into the CaSc2O4 host crystal with Ce3 þ , and remarkable

n Corresponding author at: School of Chemistry and Chemical Engineering, Guangzhou University, 230 Waihuanxi Road, Guangzhou 510006, China. Tel.: þ86 20 39366731. E-mail address: [email protected] (Y. Chen).

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.04.030

luminescence enhancement and lower temperature quenching were observed for the co-doped phosphors. The different effects of the different co-doped ions are also discussed. Bright green LEDs were fabricated with the phosphors and blue-emitting InGaN chips.

2. Experimental CaSc2O4:1%Ce3 þ and the co-doped samples CaSc2O4:1%Ce3 þ , x%Tm3 þ , y%La3 þ , z%Tb3 þ (x¼0.5–2 when y¼0, z¼0; y¼ 0.5–2 when x¼0, z¼0; x¼ y¼0.25, 0.5 when z¼0; z¼0.5–2 when x¼0, y¼0) were prepared by a solid-state reaction method. The starting materials are CaCO3 (A.R.), Sc2O3, CeO2, La2O3 and Tb4O7 (99.99%). The stoichiometric mixtures were ground and fired at 1500 1C for 7–10 h under a weak carbon monoxide reductive atmosphere produced by the incomplete burning of activated carbon powder. The samples were ground again and annealed at 1250 1C under a mixture of 5% H2 þ95% N2 atmosphere for at least 4 h to make sure a complete reduction of Ce4 þ to Ce3 þ occurred. Finally, some green–yellow powder samples were obtained. X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku D/max-IIIA diffractometer with Cu Ka radiation ˚ The morphology of the samples was inspected using (l ¼1.5403 A). a JEOL JSM-6330F field emission scanning electron microscope (FESEM), and the samples were gold-coated before the inspection. The photoluminescence signals of the samples were detected using a Hamamatsu R636-10 PMT and recorded in a lock-in amplifier (Standford SR830) system with a 450 W Xe lamp as the light source

Y. Chen et al. / Journal of Luminescence 131 (2011) 1770–1775

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Fig. 1. Experimental set-up of high temperature PL measurement device.

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for PLE measurement and a 454 nm Ar laser (Stabilite 2017, SpectraPhysics Co.) for PL measurement. The quantum efficiency measurement was accomplished using an FLS920 Combined Fluorescence Lifetime and Steady State Spectrometer from Edinburgh Inc. with an Xe900 lamp as the light source. The PL measurement at high temperature was done using a USB2000 Fiber Optic Spectrometer (Ocean Optics Inc.), a heater and a monitor; a blue emitting (450–460 nm) F5 LED was used as the light source. The experimental set-up of the PL measurement is shown in Fig. 1. Several green-emitting LEDs were fabricated by combining these phosphors with 460 nm-emitting InGaN chips and we did our best to ensure the added amount of phosphors into each LED was equal. The emission spectra of the LEDs under 20 mA forward bias were recorded on a PMS-80 LED spectrophotocolorimeter (Everfine, China).

3. Results and discussion 3.1. Phase formation and morphology of CaSc2O4:1%Ce3 þ , x%Tm3 þ , y%La3 þ , z%Tb3 þ phosphors The powder XRD patterns of all the phosphors nearly belong to those of a single phase of CaSc2O4 and are in agreement with JCPDS card 20-0234 (Calcium Scandium Oxide, Orthorhombic, Pnam (62)). In Fig. 2, the XRD patterns of CSO:1%Ce3þ (a); CSO:1%Ce3þ , 0.5%Tm3 þ (b); CSO:1%Ce3þ , 0.5%La3 þ (c); CSO:1%Ce3 þ , 0.25%Tm3þ , 0.25%La3þ (d) and CSO:1%Ce3 þ , 0.5%Tb3 þ (e) phosphors are shown as representatives. It can be seen that there are still some Sc2O3 minor peaks in the patterns of CSO:1%Ce3 þ , 0.5%La3þ and CSO:1%Ce3þ , 0.25%Tm3 þ , 0.25%La3þ phosphors. The reason may be ascribed to the starting materials (Sc2O3 and CaO), which do not react completely when added to La2O3 (due its properties such as activity), so some Sc2O3 and CaO exist. The XRD peak of CaO is too weak to figure it out clearly. FESEM observation of the powders is depicted in Fig. 3. It exhibits that the particles are agglomeration in shape and have a good crystallinity due to high sintering temperature. The size of the particles is in the range of 3–8 mm. It can be seen that co-doping other rare-earth ions has little impact on the particles’ size. 3.2. Luminescence properties of CaSc2O4:1%Ce3 þ , x%Tm3 þ , y%La3 þ , z%Tb3 þ phosphors All products obtained show similar profiles in the excitation and the emission spectra, and the excitation and emission spectra

Fig. 2. FESEM graphs of CaSc2O4:0.01Ce3 þ phosphors with or without co-doping ions: a CSO:1%Ce3 þ ; b CSO:1%Ce3 þ , 0.5%Tm3 þ ; c CSO:1%Ce3 þ , 0.5%La3 þ ; d, CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ and e, CSO:1%Ce3 þ , 0.5%Tb3 þ .

of CSO:1%Ce3 þ phosphor at room temperature are shown in Fig. 4(a) and (b), respectively. When monitored at 512 nm a broad excitation band appears in 400–500 nm peaking at about 450 nm, which originates from the f–d electron transition of Ce3 þ ion. This indicates that the phosphors are suitable to be excited by a 450–460 nm-emitting InGaN chip. When excited by 450 nm light the samples show intense green emission in a broad range of 500–700 nm centering at about 512 nm with a shoulder on the longer wavelength side due to splitting of the 4f ground state of the Ce3 þ ion, which can be decomposed into two bands by Gauss fitting: one peak at 511 nm attributed to the 5d (2D)-4f (2F7/2) transition of Ce3 þ ions and another peak at 558 nm to the 5d (2D)-4f (2F5/2) transition of Ce3 þ ions (see Fig. 4(b) inside). The energy gap between the two fitted peaks is in accordance with the separation between the two different ground states of Ce3 þ (2000 cm  1 in theory) [10]. All the photoluminescence characteristics indicate that the prepared CSO:Ce3 þ is a good candidate for the fabrication of InGaN-based LEDs. Some other rare-earth ions, Tm3 þ , La3 þ and Tb3 þ , were co-doped with Ce3 þ in the CaSc2O4 host material to see if this can improve the luminescence properties of the CSO:Ce3 þ phosphor. Shimomura et al. [9] pointed out that the activator ion of CSO:Ce3 þ probably replaces the Ca2 þ position of the host crystal because the ionic radius of Ce3 þ (rCe3 þ ¼ 103 pm) is in the same order as those of Ca2 þ (rCa2 þ ¼ 99 pm) and Sc3 þ (rSc3 þ ¼ 81 pm). As for Tm3 þ (rTm3 þ ¼ 87 pm), La3 þ (rLa3 þ ¼ 106pm) and Tb3 þ ions (rTb3 þ ¼ 92 pmr3Tbþ ¼92 pm) in our work, all of the ionic radii of these ions are larger than that of Sc3 þ (rSc3 þ ¼ 81 pm); therefore, these ions will also tend to replace the Ca2 þ site instead of Sc3 þ site in the CaSc2O4 host material when they are co-doped. The effects of these co-doped ions on the photoluminescence intensity of CSO:1%Ce3 þ phosphors were investigated and all the results show a similar trend. The intensity increases first along with increasing co-doped ion content, achieves a maximum and then decreases again. For CSO:1%Ce3 þ phosphor co-doped with Tm3 þ , La3 þ and Tb3 þ ions the optimum concentration of these co-doped ions is found to be 0.5%, and for the CSO:1%Ce3 þ , x%Tm3 þ , y%La3 þ phosphor the highest emission intensity can be obtained when x ¼y¼0.25. The emission spectra of CSO:1%Ce3 þ phosphors co-doped with different ions at their respective optimum concentration as well as the sample without the co-doping

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Relative Intensity / a. u.

Fig. 3. XRD patterns of CaSc2O4:0.01Ce3 þ phosphors with or without co-doping ions: a, CSO:1%Ce3 þ ; b, CSO:1%Ce3 þ , 0.5%Tm3 þ ; c, CSO:1%Ce3 þ , 0.5%La3 þ ; d, CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ and e, CSO:1%Ce3 þ , 0.5%Tb3 þ .

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Fig. 4. Excitation (a, lem ¼ 512 nm) and emission (b, lex ¼ 450 nm) spectra of CaSc2O4:1%Ce3 þ phosphor at room temperature and emission spectrum of CaSc2O4:1%Ce3 þ phosphor at 11 K (inside, lex ¼ 454 nm).

ions are shown in Fig. 5 for a comparison. The emission enhancements brought by different co-doping ions follow the following order with the relative integrated emission intensity in bracket: 0.5%Tb3 þ (255.24)40.25%Tm3 þ , 0.25%La3 þ (159.50)40.5%La3 þ (132.28)4 0.5%Tm3 þ (121.33)4no co-doping ion (100.00). It was found that the highest emission intensity was obtained for the sample co-doped with 0.5%Tb3 þ and the maximum integrated emission intensity was enhanced by more than 1.5 times compared with that of the CSO:1%Ce3 þ sample. The excitation spectrum of the CSO:1%Ce3 þ , 0.5%Tb3 þ sample contains a band at around 276 nm, which is attributed to the allowed transition of Tb3 þ from the 7F6 ground state to the 4f75d1 excited states, but when excited by characteristic absorption wavelength of Tb3 þ (276 nm) no Tb3 þ emission appeared in the emission spectrum, which confirms the energy transfer from Tb3 þ to Ce3 þ . The energy transfer process is proposed as follows: after Tb3 þ is excited into its 5D4 state it transfers the exciting energy to the adjacent Ce3 þ ion and then the electron transition occurs from the Ce3 þ excited state to the ground state [11–13], so the enhanced Ce3 þ emission was observed. As for Tm3 þ it does not emit in the blue region when excited by 276 nm light, so it can also act as intermediate and transfer the exciting energy to Ce3 þ .

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However the energy transfer from Tm3 þ to Ce3 þ is not so effective, so the enhancement originated by Tm3 þ is less than that of Tb3 þ . As an optically inert ion, La3 þ cannot improve the intensity of the CSO:1%Ce3 þ phosphor by energy transfer. The enhancement can be ascribed to a little augmentation of the vibration frequency of the crystal lattice originated from the replacement of host ion Ca2 þ (rSc3 þ ¼ 81 pm) with larger La3 þ (rLa3 þ ¼ 106pmr3Laþ ¼106 pm). In this way the non-radiative path is reduced and the radiative path is increased, causing the emission intensity to increase. The emission intensity of CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ is higher than those of both the single co-doped sample with 0.5%Tm3 þ and the single codoped sample with 0.5%La3 þ ; this may be ascribed to the fact that the change of crystal field of Ce3 þ is relatively small when Tm3 þ and La3 þ are added at the same time because the ionic radius of Ca2 þ is bigger than that of Tm3 þ but smaller than that of La3 þ . The emission quantum efficiency Z is defined as the ratio of the number of photons emitted to the number of photons absorbed; Z indicates the high-point of the luminescence properties of phosphors and can be considered as one of the most important standards for evaluation of phosphors. In this work, the

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quantum efficiency enhancement ratios of the co-doped samples comparable to original ones were investigated. Following the method developed by Bril and coworkers [14–17] at Philips Research Laboratories, the values for a given sample can be determined by comparing it with standard phosphors, whose quantum efficiencies have been previously determined by absolute measurements, and the quantum efficiency Zx of a sample is thus determined as follows:    Zx 1rST DFx ¼ ð1Þ 1rx ZST DFST where rST and rx are the amount of exciting radiation reflected by the standard and by the sample, respectively, and ZST is the quantum efficiency of the standard phosphor. The terms DFx and DFST give the integrated intensity of entire spectra for the sample and the standard phosphor, respectively. As to the enhancement ratio DZ%, it can be calculated based on Eq. (1) as follows:   Z Z Z 1r1 DF2 DZ% ¼ 2 1 ¼ 2 1 ¼ 1 ð2Þ Z1 Z1 1r2 DF1 The values of r1, r2, DF1 and DF2 were obtained at the same excitation wavelength, geometric position of the sample and instrumental conditions. Utmost care was taken to ensure a constant and reproducible position for the sample holder and unchanged instrumental conditions throughout all measurements. The reflection coefficient r was established by scanning the emission monochromator through the excitation wavelength region, and the integration of the intensities of the spectra thus obtained (the spectra are shown in Fig. 6(a)). In order to have an absolute r value, MgO was used as a reflectance standard (r ¼0.91) [14]. The values of DF1, DF2 can be determined by integrating the emission intensity over the total spectral range in the emission spectra (the emission spectra of the samples are shown in Fig. 6(b)). From the calculated results, it can be confirmed that the quantum efficiency enhancement ratios of the two higher emission samples CSO:1%Ce3 þ , 0.5%Tb3 þ and CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ compared to CSO:1%Ce3 þ phosphor are 62.67% and 70.28%, respectively. It is well known that the temperature of a LED package rises because of heat generation by the LED itself [18] and the phosphors used for color conversion material of white LED are required to emit luminescence effectively up to 150 1C, so temperature quenching is one of the important technological parameters for phosphors used in white LEDs. The temperaturedependent luminescence properties of the prepared CSO:1%Ce3 þ , 05%Tb3 þ and CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ samples were measured in a temperature range of 30–180 1C using the device

Relative Intensity / a.u.

Relative Intensity / a.u.

Fig. 5. Emission spectra of CaSc2O4:1%Ce3 þ phosphors co-doped with or without Tm3 þ , La3 þ or Tb3 þ : a, CSO:1%Ce3 þ , 0.5%Tb3 þ ; b, CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ ; c, CSO:1%Ce3 þ , 0.5%La3 þ ; d, CSO:1%Ce3 þ , 0.5%Tm3 þ and e, CSO:1%Ce3 þ , lex ¼ 454 nm.

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Fig. 6. Spectra for reflection measurement and the emission spectra of the prepared samples. (a) a, MgO; b, CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ ; c, CSO:1%Ce3 þ and d, CSO:1%Ce3 þ , 0.5%Tb3 þ ;(b) a, CSO:1%Ce3 þ , 0.5%Tb3 þ ; b, CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ and c, CSO:1%Ce3 þ , lex ¼ 450 nm.

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shown in Fig. 1 and the results are shown in Fig. 7(a). It can be seen that after Tm3 þ , La3 þ or Tb3 þ are co-doped both of the co-doped samples are less quenched than CSO:Ce3 þ phosphor, especially above 60 1C. The mechanism of phosphors temperature quenching can be described as follows: with increasing temperature the probability of non-radiative transition caused by thermal activation and release of the luminescent center through the crossing point between the excited state and the ground state increases, so the luminescence is quenched. For CSO:1%Ce3 þ , 0.5%Tb3 þ sample, the energy transfer from Tb3 þ to Ce3 þ may explain the decrease in temperature quenching of the phosphor. For the CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ sample, just as mentioned before, the small enhancement of the vibration frequency of the crystal lattice caused by co-doping La3 þ can increase the energy crossing point slightly, which means increasing of quenching temperature. Actually the temperature quenching of CSO:1%Ce3 þ , 0.5%Tm3 þ was also improved marginally compared to that of CSO:1%Ce3 þ phosphor, which can be explained by the energy transfer from Tm3 þ to Ce3 þ . The decrease of temperature quenching of CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ sample is still attributed to a cooperative effect of Tm3 þ and La3 þ in which the La3 þ co-doping plays a key role. The temperature quenching curve of commercially used YAG:Ce3 þ phosphor is also shown in Fig. 7(b) as a reference. Before  140 1C, our samples and the YAG:Ce3 þ phosphor are comparable; however, when temperature increases above 140 1C, our phosphors show much smaller temperature quenching than the YAG:Ce3 þ phosphor, so the co-doped CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ and CSO:1%Ce3 þ , 0.5%Tb3 þ phosphors are suitable for fabrication of InGaN-based LEDs as an efficient color conversion material with low temperature quenching. 3.3. Fabrication of the LEDs Several green LEDs were fabricated with the prepared CSO phosphors and the emission spectra of the LEDs under 20 mA forward bias are shown in Fig. 8, the performance parameters of the LEDs are listed in Table 1 and the parameters of the original blue chip LED are also included. It can be seen that emitting light of all the prepared LEDs is located in the green–yellow light domain in the CIE diagram (see the CIE chromaticity coordinates

Fig. 8. Emission spectra of the prepared LEDs under 20 mA forward bias: a, CSO:1%Ce3 þ , 0.5%Tb3 þ ; b, CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ ; c, CSO:1%Ce3 þ , 0.5%La3 þ ; d, CSO:1%Ce3 þ , 0.5%Tm3 þ and e, CSO:1%Ce3 þ and the photos of lighting LEDs ( inside) from left to right made from phosphors a–e.

Table 1 Performance parameters of the LEDs made from CSO:Ce3 þ phosphors with or without co-doping. Phosphor sample in LED

CIE chromaticity coordinates (x, y)

Luminous efficiency (lm/W)

CSO:1%Ce3 þ , 0.5%Tb3 þ CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 CSO:1%Ce3 þ , 0.5%La3 þ CSO:1%Ce3 þ , 0.5%Tm3 þ CSO:1%Ce3 þ Original blue chip LED

(0.3887,0.5689) (0.3775,0.5727) (0.3696,0.5501) (0.3773,0.5488) (0.3926,0.5684) (0.1430, 0.0472)

7.12 6.02 5.45 4.83 2.37 8.01

showed in Table 1). The luminous efficiencies of LEDs prepared from the co-doped CSO phosphors are also much higher than that of the CSO:Ce3 þ phosphor (see Table 1) and the enhancement ratio for LED made from CSO:1%Ce3 þ , 0.5%Tb3 þ phosphor was confirmed to be about 200%. So the co-doping of Tm3 þ , La3 þ or Tb3 þ not only enhances the photoluminescence properties of the CSO:Ce3 þ phosphors, but also improves the performances of the corresponding LEDs remarkably.

4. Conclusions CSO:Ce3 þ phosphors with or without co-doping ions Tm3 þ , La3 þ and Tb3 þ were prepared by a solid state method. Tm3 þ , La3 þ and Tb3 þ co-doping not only enhances the photoluminescence, and decreases the temperature quenching of the phosphor, but also increases the performances of the LEDs fabricated with the phosphors. High efficiency green-light-emitting diodes were fabricated with the prepared phosphors. Tm3 þ , La3 þ and Tb3 þ co-doped CSO:Ce3 þ phosphors are suitable candidates for the fabrication of high efficiency white LEDs.

Fig. 7. Temperature quenching curves of the prepared phosphors and commercially used YAG:Ce3 þ phosphor: (a) K stands for CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ ; . stands for CSO:1%Ce3 þ , 0.5%Tb3 þ ; ’ stands for CSO:1%Ce3 þ and (b) K stands for CSO:1%Ce3 þ , 0.25%Tm3 þ , 0.25%La3 þ ; . stands for CSO:1%Ce3 þ , 0.5%Tb3 þ ; ’ stands for commercial YAG:Ce3 þ , lex ¼460 nm.

Acknowledgment This work is financially supported by Foundation for Distinguished Young Talents in Higher Education of Guangdong, China

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(Grant no. LYM10107) and the Project (ITS/477/09) of Hong Kong Innovation and Technology Commission. References [1] R. Mueller-Mach, G.O. Mueller, M.R. Krames, T. Trottier, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 339. [2] J.H. Yum, S.-Y. Seo, S. Lee, Y.-E. Sung, J. Electronchem. Soc. 150 (2003) H47. [3] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Chol, H.L. Park, G.C. Kim, T.W. Kim, Appl. Phys Lett. 85 (2004) 3696. [4] J.K. Park, C.H. Kim, S.H. Park, H.D. Park, S.Y. Chol, Appl. Phys. Lett. 84 (2004) 1647. [5] S. Lee, S. Seo, J. Electrochem. Soc. 149 (2002) J85. [6] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, W.C. Lai, J.M. Tsai, G.C. Chi, R.K. Wu, IEEE Photon. Technol. Lett. 15 (2003) 18.

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[7] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [8] Y.D. Huh, J.H. Shim, Y. Kim, Y.R. Do, J. Electrochem. Soc. 150 (2003) H57. [9] Y. Shimomura, T. Kurushima, N. Kijima, J. Electrochem. Soc. 154 (2007) J234. [10] L.Y. Cai, X.D. Wei, H. Li, Q.L. Liu, J. Lumin. 129 (2008) 165. [11] M.T. Jose, A.R. Lakshmanan, Opt. Mater. 24 (2004) 651. [12] B.Y. Zhang, T.M. Chen, C.R. Chen, G.Y. Hong, Y.M. Li, Chin, J. Lumin. 16 (1995) 139. [13] H.P. You, G.Y. Hong, Chin, J. Lumin. 18 (1997) 191. [14] A. Bril, Absolute efficiencies of phosphors with ultraviolet and cathode-ray excitation, in: H. Kallman, G. Spruch (Eds.), Luminescence of Organic and Inorganic Materials, Willey, Eindhoven, 1962, p. 479. [15] A. Bril, A.W. De Jager-Veenis, J. Res. Natl. Bur. Stand 80A (1976) 401. [16] A. Bril, A.W. De Jager-Veenis, J. Electrochem. Soc. 123 (1976) 396. [17] A.W. De Jager-Veenis, A. Bril, Philips J. Res. 33 (1978) 124. [18] C.Z. Wu, D.H. Chen, J. Light. Eng. 20 (2009) 38 in Chinese.