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A potential single-phased white-emitting LiBaBO3:Ce3+, Eu2+ phosphor for white LEDs
JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010, p. 523
A potential single-phased white-emitting LiBaBO3:Ce3+, Eu2+ phosphor for white LEDs LI Panlai (ᴢⳐᴹ), WANG Zhijun (⥟ᖫ)ݯ, YANG Zhiping (ᴼᖫᑇ), GUO Qinglin (䛁ᑚᵫ) (College of Physics Science & Technology, Hebei University, Baoding 071002, China) Received 1 September 2009; revised 4 May 2010
Abstract: Ce3+/Eu2+ co-doped LiBaBO3 phosphor was synthesized by high temperature solid-state reaction method, and its luminescent characteristics were investigated. The hues of the LiBaBO3:Ce3+, Eu2+ phosphor varies from blue to white and eventually to yellow-green by properly tuning the Ce3+/Eu2+ ratio. Under UV excitation, white light was generated by coupling blue and yellow-green emission bands attributed to Ce3+ and Eu2+ emissions, respectively. The luminous efficacy of LiBaBO3:1%Ce3+, 2%Eu2+ calculated from the emission spectrum was about 290 lm/W. Keywords: white LED; LiBaBO3:Eu2+,Ce3+; luminescent characteristics; rare earths
It is easy to fabricate a primitive white LED lamp, which consists of three chips emitting red, green and blue lights. However, different chips need different drive voltages, and they have different thermal and degradation properties, which makes the lamp expensive and its chromaticity vary with temperature and time, thus, its application is restricted. A good choice to assemble low cost white LEDs is to couple a blue or near-ultraviolet (UV) LED with a down converting phosphor. For example, the commercial YAG (yttrium aluminum garnet):Ce yellow phosphor coupled with a blue InGaN LED results in a dichromatic white light emission[1]. In spite of white light that is easily achieved in YAG:Ce based system, the individual degradation rate of the LED and the phosphor will cause chromatic aberration and poor white light performance after long period of working, moreover, there are few yellow phosphors that emit efficiently with 450–470 nm excitation[2–6]. Hence, developing a UV excited single-phased white-emitting phosphor is a good choice for replacing YAG:Ce based dichromatic white LED. Ions possessing d-f or d-d transitions might be good candidates for activators in the phosphors, because their emissions are broadband and could be invisible under the influence of crystal-field and nephelauxetic effects[7]. Moreover, white light can be produced by co-doping these ions in a single host, such as Eu2+/Mn2+ co-activated systems[8–13]. Although there were sweeping studies on Eu2+/Mn2+ co-doped whiteemitting phosphors in the past years, yet, Ce3+/Eu2+ co-doped white-emitting phosphors were rarely reported[14–17]. In certain Eu2+-doped oxide matrices with strong crystal-field splitting and low centroid of the 5d state, yellow-orange emission and UV to blue excitation were commonly observed, while Ce3+ in such oxide lattices showed blue emis-
sion. In this paper, we reported white light obtained by coupling the blue and yellow emissions from Ce3+ and Eu2+ in Ce3+/Eu2+ co-doped single composition oxide host.
1 Experimental LiBaBO3:Ce3+, Eu2+ phosphor was synthesized by solidstate reaction. BaCO3, Li2CO3, H3BO3, CeO2 and Eu2O3 with high purity of 99.99% were mixed in the requisite proportions and calcined at 700 ºC in a reductive atmosphere (H2/N2=5%). The structure was checked by powder X-ray diffraction (XRD, D/max-rA, Cu KĮ, 40 kV, 100 mA, Ȝ= 0.15406 nm). The photoluminescence characteristics were measured by a Shimadzu RF-540 fluorescence spectrophotometer. In order to reduce the measure error, all the samples were cased in the same sample case.
2 Results and discussion 2.1 Structure characteristics of LiBaBO3:Ce3+, Eu2+ LiBaBO3 has a monoclinic structure with P21/n space group, and its lattice parameters are a=0.6461 nm, b=0.7107 nm, c=0.7403 nm, and ȕ=117.99°. Fig. 1 shows the XRD pattern of LiBaBO3:Ce3+,Eu2+ phosphor with 1% Ce3+ and Eu2+, respectively. The data agreed well with JCPDS No. 81-1808. Therefore, the host structure was not influenced by doping Ce3+ and Eu2+. 2.2 Photoluminescence characteristics of LiBaBO3:Ce3+, Eu2+ The photoluminescence of LiBaBO3:Ce3+ and LiBaBO3:
Foundation item: Project supported by the National Natural Science Foundation of China (50902042), the Natural Science Foundation of Hebei Province, China (E2009000209, E2010000283), and the Research Foundation of Education Bureau of Hebei Province, China (2009313) Corresponding author: LI Panlai (E-mail: [email protected]; Tel.: +86-312-5079423) DOI: 10.1016/S1002-0721(09)60145-9
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Eu2+ were reported by Pardha Saradhi et al. and our group[18,19]. In this work, spectroscopic properties of the co-doped system LiBaBO3:Ce3+,Eu2+ were investigated, and the emission and excitation spectra of the phosphors are shown in Fig. 2. The excitation wavelength for the emission measurements was 370 nm, for the previous researches showed that it could excite both Ce3+ and Eu2+ in LiBaBO3[18,19]. Fig. 2 displays the emission spectra of LiBaBO3:1%Ce3+, n%Eu2+ (n=1 to 5). The emission spectra of these phosphors display two emission bands which are located at blue and yellow region, corresponding to the emission of Ce3+ and Eu2+, respectively. The Eu2+ emission intensity increases with increasing Eu2+ concentration(n), and reaches maximum as n2, whereas that of Ce3+ emission decreases monotonically from n=1 to 5. The above results can be explained in two aspects. On the one hand, f-d transition is with a large absorption cross section, all the excitation photons might be absorbed by Eu2+ and Ce3+ ions, thereby the relative Ce3+ excited state population can be obtained by (1)
where ı and n are the absorption cross section and excitation state population, respectively, the subscripts denote Ce3+ and Eu2+, and Į is a constant. In this experiment, the Ce3+ concentration was fixed on 1%. Therefore, under the condition of increasing the Eu2+ concentration, the distribution ratio of excitation energy between Eu2+ and Ce3+ is altered. As the Eu2+ concentration increases, En2+ obtains more excitation energy than Ce3+ does, thereby the Eu2+ emission intensity increases. On the other hand, our previous research[18,19] showed that the excitation spectrum of LiBaBO3:Eu2+ partially overlapped the emission spectrum of LiBaBO3:Ce3+, so resonance energy transfer from Ce3+ to Eu2+ in LiBaBO3 may take place, which also leads to increasing Eu2+ emission intensity. The excitation spectra of LiBaBO3:1%Ce3+,2%Eu2+ are shown in the inset of Fig. 2. The excitation spectrum monitored at 436 nm has a broad band peaked at 370 nm, which is similar to the excitation spectrum of LiBaBO3:Ce3+[19]. The excitation spectrum monitored at 507 nm is a broad band ranging from 200–500 nm, and its peak is located at 380 nm, which agrees well to the excitation spectrum of LiBaBO3: Eu2+[18]. 2.3
CIE chromaticity coordinates and luminous efficacy of LiBaBO3:Ce3+, Eu2+
Fig. 3 depicts the chromaticity coordinates for LiBaBO3: 1%Ce3+,n%Eu2+ with various doping Eu2+ concentrations. The chromaticity coordinates (x, y) of these phosphors vary from (0.20, 0.16) (point 1) to (0.31, 0.29) (point 3) and ultimately to (0.42, 0.46) (point 7) corresponding to solely Ce3+ doped, Ce3+ and Eu2+ co-doped, and solely Eu2+ doped phosphors, respectively. It indicates that by properly tuning the ratio of Ce3+/Eu2+, the diversified white light with different hues can be achieved under UV LEDs excitation. We have observed that the white light can be generated by exciting LiBaBO3:1%Ce3+,n%Eu2+ under 370 nm with chromaticity coordinates at (0.31, 0.29) (point 3) and (0.37, 0.35) (point 4), respectively. In order to evaluate the quality Fig. 1 XRD pattern of LiBaBO3:1%Ce3+, 1%Eu2+ phosphor
Fig. 2 Emission spectra of LiBaBO3:1% Ce3+, n% Eu2+ phosphors (the inset: Excitation spectra of LiBaBO3:1% Ce3+, 2% Eu2+ phosphors)
Fig. 3 CIE chromaticity diagram for LiBaBO3:m%Ce3+,n%Eu2+ excited at 370 nm (the corresponding CIE chromaticity coordinates (x, y) are (0.20, 0.16) (1), (0.25, 0.21) (2), (0.31, 0.29) (3), (0.37, 0.35) (4), (0.39, 0.39) (5), (0.39, 0.40) (6) and (0.42, 0.46) (7) (1) m=1, n=0; (2) m=1, n=1; (3) m=1, n=1.5; (4) m=1, n=2; (5) m=1, n=2.5; (6) m=1, n=5; (7) m=0, n=1
LI Panlai et al., A potential single-phased white-emitting LiBaBO3:Ce3+, Eu2+ phosphor for white LEDs
of the single-phased white-emitting LiBaBO3:Ce3+,Eu2+ phosphor, the luminous efficacies of our phosphors excited by a violet light source were calculated and compared with that of the commercial YAG:Ce with blue light excitation. Luminous efficacy of a phosphor for lighting is defined [20] as (2) where V(Ȝ) is the eye sensitivity function, and the greatest value is located at 555 nm. For the commercial YAG:Ce based white emitting system ( the red dot in Fig. 4), the chromaticity coordinates (x, y) is (0.325, 0.332), the color rendering index (CRI) is 81.5, and the color temperature (TC) is 5914 K. According to Eq. (2) and the emission spectrum, the luminous efficacy is about 298.7 lm/W. Similarly, luminous efficacy of LiBaBO3:1%Ce3+, n%Eu2+ can also be obtained. For example, luminous efficacy of LiBaBO3:1%Ce3+, 2%Eu2+ is about 290 lm/W, which is very close to that of YAG:Ce. The results show that the LiBaBO3:1%Ce3+, n%Eu2+ is a promising UV excited single-phased white-emitting phosphor for white LEDs.
3 Conclusions In conclusion, under UV excitation, the emission spectrum of LiBaBO3:Ce3+, Eu2+ consisted of two bands peaked at 436 and 507 nm, corresponding to the emission of Ce3+ and Eu2+, respectively. With changing the Ce3+/Eu2+ ratio, the chromaticity of the phosphor emission varied, white light mixed by blue and yellow emissions could be obtained in Ce3+/Eu2+ co- doped LiBaBO3 with appropriate Ce3+/Eu2+ ratio. The luminous efficacy of LiBaBO3:1%Ce3+, 2%Eu2+ was 290 lm/W, very close to that of YAG:Ce.
References: [1] Nakamura S, Fasol G. The Blue Laser Diode. Berlin: SpringerVerlag, 1997. 1. [2] Li Panlai, Yang Zhiping, Wang Zhijun, Guo Qinglin, Li Xu. Preparation and luminescence characteristics of Sr3SiO5:Eu2+ phosphor for white LED. Chinese Science Bulletin, 2008, 53(7): 974. [3] Park J K, Kim C H, Park S H, Park H D, Choi S Y. Application of strontium silicate yellow phosphor for white lightemitting diodes. Applied Physics Letters, 2004, 84(10): 1647. [4] Im W B, Kim Y I, Fellows N N, Masui H, Hirata G A, DenBaars S P, Seshadri R. A yellow-emitting Ce3+ phosphor, La1-xCexSr2AlO5, for white light-emitting diodes. Applied Physics Letters, 2008, 93: 091905-1. [5] Jang H S, Jeon D Y. Yellow-emitting Sr3SiO5:Ce3+, Li+ phosphor for white-light-emitting diodes and yellow-light-emitting
525
diodes. Applied Physics Letters, 2007, 90: 041906-1. [6] Yang Z P, Wang S L, Yang G W, Tian J, Li P L, Li X. Luminescent properties of Ca2BO3Cl:Eu2+ yellow-emitting phosphor for white light-emitting diodes. Materials Letters, 2007, 61: 5258. [7] Blasse G Grabmaier B C. Luminescent Materials. Berlin: Springer, 1994. 46. [8] Kim J S, Jeon P E, Choi J C, Park H L, Mho S I, Kim G C. Warm-white-light emitting diode utilizing a single-phase full-color Ba3Mg2Si2O8:Eu2+, Mn2+ phosphor. Applied Physics Letters, 2004, 84(15): 2931. [9] Kim J S, Jeon P E, Park Y H, Choi J C, Park H L, Kim G C, Kim T W. White-light generation through ultraviolet-emitting diode and white-emitting phosphor. Applied Physics Letters, 2004, 85(17): 3696. [10] Yang Zhiping, Liu Yufeng, Wang Liwei, Yu Quanmao, Xiong Zhijun, Xu Xiaoling. Luminescent properties of the single white emitting phosphor Eu2+, Mn2+ co-doped Ca2SiO3Cl2. Acta Physica Sinica, 2007, 56(1): 546. [11] Yang Zhiping, Li Panlai, Wang Zhijun, Guo Qinglin, Li Xu. Luminescence characteristics of the single host white emitting Ca10(Si2O7)3Cl2:Eu2+, Mn2+ phosphor for white LED. Chinese Science Bulletin, 2009, 54(13): 1855. [12] Guo C, Luan L, Ding X, Zhang F, Shi F G, Gao F, Liang L. Luminescent properties of Sr5(PO4)3Cl:Eu2+, Mn2+ as a potential phosphor for UV-LED-based white LEDs. Applied Physics B, 2009, 95: 779. [13] Yang W J, Chen T M. White-light generation and energy transfer in SrZn2(PO4)2:Eu,Mn phosphor for ultraviolet light-emitting diodes. Applied Physics Letters, 2006, 88: 101903-1. [14] Wang Zhijun, Li Panlai, Yang Zhiping, Guo Qinglin, Fu Guangsheng. Energy transfer from Ce3+ to Eu2+ in LiSrBO3 and its potential application in UV-LED-based white LEDs. Chinese Physics Letters, 2009, 26(11): 117802-1. [15] Chang C K, Chen T M. Sr3B2O6:Ce3+, Eu2+: A potential singlephased white-emitting borate phosphor for ultraviolet lightemitting diodes. Applied Physics Letters, 2007, 91: 081902-1. [16] Song Y H, Jia G, Yang M, Huang Y J, Hou H P, Zhang H J. Sr3Al2O5Cl2:Ce3+, Eu2+: A potential tunable yellow-to-whiteemitting phosphor for ultraviolet light emitting diodes. Applied Physics Letters, 2009, 94: 091902-1. [17] Yang W J, Chen T M. Ce3+/Eu2+ codoped Ba2ZnS3: A blue radiation-converting phosphor for white light-emitting diodes. Applied Physics Letters, 2007, 90: 171908-1. [18] Wang Zhijun, Li Panlai, Wang Ying, Yang Zhiping, Guo Qinglin. Luminescence characteristics of LiBaBO3:Eu2+ phosphor for white LED. Acta Physica Sinica, 2009, 58(2): 1257. [19] Li Panlai, Wang Zhijun, Wang Ying, Yang Zhiping, Guo Qinglin, Li Xu, Yang Yanmin, Fu Guangsheng. Luminescence characteristics and crystallographic sites of Ce3+ in LiBaBO3. Acta Physica Sinica, 2009, 58(8): 5819. [20] Zukauskas A, Shur M S, Gaska R. Introduction to Solid State Lighting. New York: Wiley, 2002. 10.
A potential single-phased white-emitting LiBaBO3:Ce3+, Eu2+ phosphor for white LEDs LI Panlai (ᴢⳐᴹ), WANG Zhijun (⥟ᖫ)ݯ, YANG Zhiping (ᴼᖫᑇ), GUO Qinglin (䛁ᑚᵫ) (College of Physics Science & Technology, Hebei University, Baoding 071002, China) Received 1 September 2009; revised 4 May 2010
Abstract: Ce3+/Eu2+ co-doped LiBaBO3 phosphor was synthesized by high temperature solid-state reaction method, and its luminescent characteristics were investigated. The hues of the LiBaBO3:Ce3+, Eu2+ phosphor varies from blue to white and eventually to yellow-green by properly tuning the Ce3+/Eu2+ ratio. Under UV excitation, white light was generated by coupling blue and yellow-green emission bands attributed to Ce3+ and Eu2+ emissions, respectively. The luminous efficacy of LiBaBO3:1%Ce3+, 2%Eu2+ calculated from the emission spectrum was about 290 lm/W. Keywords: white LED; LiBaBO3:Eu2+,Ce3+; luminescent characteristics; rare earths
It is easy to fabricate a primitive white LED lamp, which consists of three chips emitting red, green and blue lights. However, different chips need different drive voltages, and they have different thermal and degradation properties, which makes the lamp expensive and its chromaticity vary with temperature and time, thus, its application is restricted. A good choice to assemble low cost white LEDs is to couple a blue or near-ultraviolet (UV) LED with a down converting phosphor. For example, the commercial YAG (yttrium aluminum garnet):Ce yellow phosphor coupled with a blue InGaN LED results in a dichromatic white light emission[1]. In spite of white light that is easily achieved in YAG:Ce based system, the individual degradation rate of the LED and the phosphor will cause chromatic aberration and poor white light performance after long period of working, moreover, there are few yellow phosphors that emit efficiently with 450–470 nm excitation[2–6]. Hence, developing a UV excited single-phased white-emitting phosphor is a good choice for replacing YAG:Ce based dichromatic white LED. Ions possessing d-f or d-d transitions might be good candidates for activators in the phosphors, because their emissions are broadband and could be invisible under the influence of crystal-field and nephelauxetic effects[7]. Moreover, white light can be produced by co-doping these ions in a single host, such as Eu2+/Mn2+ co-activated systems[8–13]. Although there were sweeping studies on Eu2+/Mn2+ co-doped whiteemitting phosphors in the past years, yet, Ce3+/Eu2+ co-doped white-emitting phosphors were rarely reported[14–17]. In certain Eu2+-doped oxide matrices with strong crystal-field splitting and low centroid of the 5d state, yellow-orange emission and UV to blue excitation were commonly observed, while Ce3+ in such oxide lattices showed blue emis-
sion. In this paper, we reported white light obtained by coupling the blue and yellow emissions from Ce3+ and Eu2+ in Ce3+/Eu2+ co-doped single composition oxide host.
1 Experimental LiBaBO3:Ce3+, Eu2+ phosphor was synthesized by solidstate reaction. BaCO3, Li2CO3, H3BO3, CeO2 and Eu2O3 with high purity of 99.99% were mixed in the requisite proportions and calcined at 700 ºC in a reductive atmosphere (H2/N2=5%). The structure was checked by powder X-ray diffraction (XRD, D/max-rA, Cu KĮ, 40 kV, 100 mA, Ȝ= 0.15406 nm). The photoluminescence characteristics were measured by a Shimadzu RF-540 fluorescence spectrophotometer. In order to reduce the measure error, all the samples were cased in the same sample case.
2 Results and discussion 2.1 Structure characteristics of LiBaBO3:Ce3+, Eu2+ LiBaBO3 has a monoclinic structure with P21/n space group, and its lattice parameters are a=0.6461 nm, b=0.7107 nm, c=0.7403 nm, and ȕ=117.99°. Fig. 1 shows the XRD pattern of LiBaBO3:Ce3+,Eu2+ phosphor with 1% Ce3+ and Eu2+, respectively. The data agreed well with JCPDS No. 81-1808. Therefore, the host structure was not influenced by doping Ce3+ and Eu2+. 2.2 Photoluminescence characteristics of LiBaBO3:Ce3+, Eu2+ The photoluminescence of LiBaBO3:Ce3+ and LiBaBO3:
Foundation item: Project supported by the National Natural Science Foundation of China (50902042), the Natural Science Foundation of Hebei Province, China (E2009000209, E2010000283), and the Research Foundation of Education Bureau of Hebei Province, China (2009313) Corresponding author: LI Panlai (E-mail: [email protected]; Tel.: +86-312-5079423) DOI: 10.1016/S1002-0721(09)60145-9
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JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010
Eu2+ were reported by Pardha Saradhi et al. and our group[18,19]. In this work, spectroscopic properties of the co-doped system LiBaBO3:Ce3+,Eu2+ were investigated, and the emission and excitation spectra of the phosphors are shown in Fig. 2. The excitation wavelength for the emission measurements was 370 nm, for the previous researches showed that it could excite both Ce3+ and Eu2+ in LiBaBO3[18,19]. Fig. 2 displays the emission spectra of LiBaBO3:1%Ce3+, n%Eu2+ (n=1 to 5). The emission spectra of these phosphors display two emission bands which are located at blue and yellow region, corresponding to the emission of Ce3+ and Eu2+, respectively. The Eu2+ emission intensity increases with increasing Eu2+ concentration(n), and reaches maximum as n2, whereas that of Ce3+ emission decreases monotonically from n=1 to 5. The above results can be explained in two aspects. On the one hand, f-d transition is with a large absorption cross section, all the excitation photons might be absorbed by Eu2+ and Ce3+ ions, thereby the relative Ce3+ excited state population can be obtained by (1)
where ı and n are the absorption cross section and excitation state population, respectively, the subscripts denote Ce3+ and Eu2+, and Į is a constant. In this experiment, the Ce3+ concentration was fixed on 1%. Therefore, under the condition of increasing the Eu2+ concentration, the distribution ratio of excitation energy between Eu2+ and Ce3+ is altered. As the Eu2+ concentration increases, En2+ obtains more excitation energy than Ce3+ does, thereby the Eu2+ emission intensity increases. On the other hand, our previous research[18,19] showed that the excitation spectrum of LiBaBO3:Eu2+ partially overlapped the emission spectrum of LiBaBO3:Ce3+, so resonance energy transfer from Ce3+ to Eu2+ in LiBaBO3 may take place, which also leads to increasing Eu2+ emission intensity. The excitation spectra of LiBaBO3:1%Ce3+,2%Eu2+ are shown in the inset of Fig. 2. The excitation spectrum monitored at 436 nm has a broad band peaked at 370 nm, which is similar to the excitation spectrum of LiBaBO3:Ce3+[19]. The excitation spectrum monitored at 507 nm is a broad band ranging from 200–500 nm, and its peak is located at 380 nm, which agrees well to the excitation spectrum of LiBaBO3: Eu2+[18]. 2.3
CIE chromaticity coordinates and luminous efficacy of LiBaBO3:Ce3+, Eu2+
Fig. 3 depicts the chromaticity coordinates for LiBaBO3: 1%Ce3+,n%Eu2+ with various doping Eu2+ concentrations. The chromaticity coordinates (x, y) of these phosphors vary from (0.20, 0.16) (point 1) to (0.31, 0.29) (point 3) and ultimately to (0.42, 0.46) (point 7) corresponding to solely Ce3+ doped, Ce3+ and Eu2+ co-doped, and solely Eu2+ doped phosphors, respectively. It indicates that by properly tuning the ratio of Ce3+/Eu2+, the diversified white light with different hues can be achieved under UV LEDs excitation. We have observed that the white light can be generated by exciting LiBaBO3:1%Ce3+,n%Eu2+ under 370 nm with chromaticity coordinates at (0.31, 0.29) (point 3) and (0.37, 0.35) (point 4), respectively. In order to evaluate the quality Fig. 1 XRD pattern of LiBaBO3:1%Ce3+, 1%Eu2+ phosphor
Fig. 2 Emission spectra of LiBaBO3:1% Ce3+, n% Eu2+ phosphors (the inset: Excitation spectra of LiBaBO3:1% Ce3+, 2% Eu2+ phosphors)
Fig. 3 CIE chromaticity diagram for LiBaBO3:m%Ce3+,n%Eu2+ excited at 370 nm (the corresponding CIE chromaticity coordinates (x, y) are (0.20, 0.16) (1), (0.25, 0.21) (2), (0.31, 0.29) (3), (0.37, 0.35) (4), (0.39, 0.39) (5), (0.39, 0.40) (6) and (0.42, 0.46) (7) (1) m=1, n=0; (2) m=1, n=1; (3) m=1, n=1.5; (4) m=1, n=2; (5) m=1, n=2.5; (6) m=1, n=5; (7) m=0, n=1
LI Panlai et al., A potential single-phased white-emitting LiBaBO3:Ce3+, Eu2+ phosphor for white LEDs
of the single-phased white-emitting LiBaBO3:Ce3+,Eu2+ phosphor, the luminous efficacies of our phosphors excited by a violet light source were calculated and compared with that of the commercial YAG:Ce with blue light excitation. Luminous efficacy of a phosphor for lighting is defined [20] as (2) where V(Ȝ) is the eye sensitivity function, and the greatest value is located at 555 nm. For the commercial YAG:Ce based white emitting system ( the red dot in Fig. 4), the chromaticity coordinates (x, y) is (0.325, 0.332), the color rendering index (CRI) is 81.5, and the color temperature (TC) is 5914 K. According to Eq. (2) and the emission spectrum, the luminous efficacy is about 298.7 lm/W. Similarly, luminous efficacy of LiBaBO3:1%Ce3+, n%Eu2+ can also be obtained. For example, luminous efficacy of LiBaBO3:1%Ce3+, 2%Eu2+ is about 290 lm/W, which is very close to that of YAG:Ce. The results show that the LiBaBO3:1%Ce3+, n%Eu2+ is a promising UV excited single-phased white-emitting phosphor for white LEDs.
3 Conclusions In conclusion, under UV excitation, the emission spectrum of LiBaBO3:Ce3+, Eu2+ consisted of two bands peaked at 436 and 507 nm, corresponding to the emission of Ce3+ and Eu2+, respectively. With changing the Ce3+/Eu2+ ratio, the chromaticity of the phosphor emission varied, white light mixed by blue and yellow emissions could be obtained in Ce3+/Eu2+ co- doped LiBaBO3 with appropriate Ce3+/Eu2+ ratio. The luminous efficacy of LiBaBO3:1%Ce3+, 2%Eu2+ was 290 lm/W, very close to that of YAG:Ce.
References: [1] Nakamura S, Fasol G. The Blue Laser Diode. Berlin: SpringerVerlag, 1997. 1. [2] Li Panlai, Yang Zhiping, Wang Zhijun, Guo Qinglin, Li Xu. Preparation and luminescence characteristics of Sr3SiO5:Eu2+ phosphor for white LED. Chinese Science Bulletin, 2008, 53(7): 974. [3] Park J K, Kim C H, Park S H, Park H D, Choi S Y. Application of strontium silicate yellow phosphor for white lightemitting diodes. Applied Physics Letters, 2004, 84(10): 1647. [4] Im W B, Kim Y I, Fellows N N, Masui H, Hirata G A, DenBaars S P, Seshadri R. A yellow-emitting Ce3+ phosphor, La1-xCexSr2AlO5, for white light-emitting diodes. Applied Physics Letters, 2008, 93: 091905-1. [5] Jang H S, Jeon D Y. Yellow-emitting Sr3SiO5:Ce3+, Li+ phosphor for white-light-emitting diodes and yellow-light-emitting
525
diodes. Applied Physics Letters, 2007, 90: 041906-1. [6] Yang Z P, Wang S L, Yang G W, Tian J, Li P L, Li X. Luminescent properties of Ca2BO3Cl:Eu2+ yellow-emitting phosphor for white light-emitting diodes. Materials Letters, 2007, 61: 5258. [7] Blasse G Grabmaier B C. Luminescent Materials. Berlin: Springer, 1994. 46. [8] Kim J S, Jeon P E, Choi J C, Park H L, Mho S I, Kim G C. Warm-white-light emitting diode utilizing a single-phase full-color Ba3Mg2Si2O8:Eu2+, Mn2+ phosphor. Applied Physics Letters, 2004, 84(15): 2931. [9] Kim J S, Jeon P E, Park Y H, Choi J C, Park H L, Kim G C, Kim T W. White-light generation through ultraviolet-emitting diode and white-emitting phosphor. Applied Physics Letters, 2004, 85(17): 3696. [10] Yang Zhiping, Liu Yufeng, Wang Liwei, Yu Quanmao, Xiong Zhijun, Xu Xiaoling. Luminescent properties of the single white emitting phosphor Eu2+, Mn2+ co-doped Ca2SiO3Cl2. Acta Physica Sinica, 2007, 56(1): 546. [11] Yang Zhiping, Li Panlai, Wang Zhijun, Guo Qinglin, Li Xu. Luminescence characteristics of the single host white emitting Ca10(Si2O7)3Cl2:Eu2+, Mn2+ phosphor for white LED. Chinese Science Bulletin, 2009, 54(13): 1855. [12] Guo C, Luan L, Ding X, Zhang F, Shi F G, Gao F, Liang L. Luminescent properties of Sr5(PO4)3Cl:Eu2+, Mn2+ as a potential phosphor for UV-LED-based white LEDs. Applied Physics B, 2009, 95: 779. [13] Yang W J, Chen T M. White-light generation and energy transfer in SrZn2(PO4)2:Eu,Mn phosphor for ultraviolet light-emitting diodes. Applied Physics Letters, 2006, 88: 101903-1. [14] Wang Zhijun, Li Panlai, Yang Zhiping, Guo Qinglin, Fu Guangsheng. Energy transfer from Ce3+ to Eu2+ in LiSrBO3 and its potential application in UV-LED-based white LEDs. Chinese Physics Letters, 2009, 26(11): 117802-1. [15] Chang C K, Chen T M. Sr3B2O6:Ce3+, Eu2+: A potential singlephased white-emitting borate phosphor for ultraviolet lightemitting diodes. Applied Physics Letters, 2007, 91: 081902-1. [16] Song Y H, Jia G, Yang M, Huang Y J, Hou H P, Zhang H J. Sr3Al2O5Cl2:Ce3+, Eu2+: A potential tunable yellow-to-whiteemitting phosphor for ultraviolet light emitting diodes. Applied Physics Letters, 2009, 94: 091902-1. [17] Yang W J, Chen T M. Ce3+/Eu2+ codoped Ba2ZnS3: A blue radiation-converting phosphor for white light-emitting diodes. Applied Physics Letters, 2007, 90: 171908-1. [18] Wang Zhijun, Li Panlai, Wang Ying, Yang Zhiping, Guo Qinglin. Luminescence characteristics of LiBaBO3:Eu2+ phosphor for white LED. Acta Physica Sinica, 2009, 58(2): 1257. [19] Li Panlai, Wang Zhijun, Wang Ying, Yang Zhiping, Guo Qinglin, Li Xu, Yang Yanmin, Fu Guangsheng. Luminescence characteristics and crystallographic sites of Ce3+ in LiBaBO3. Acta Physica Sinica, 2009, 58(8): 5819. [20] Zukauskas A, Shur M S, Gaska R. Introduction to Solid State Lighting. New York: Wiley, 2002. 10.