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Luminescence properties of Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4: A promising phosphor for solid state lighting
Materials Letters 63 (2009) 1275–1277
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Luminescence properties of Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4: A promising phosphor for solid state lighting Haidong Ju, Xingyu Su, Shiqing Xu ⁎, Ying Zhang, Degang Deng, Shilong Zhao, Huanping Wang, Baoling Wang, Liuzheng Sun College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 11 January 2009 Accepted 24 February 2009 Available online 6 March 2009 Keywords: Phosphors Luminescence Substitution Ca12Al10.6Si3.4O32Cl5.4
a b s t r a c t In this article, we synthesized and characterized a novel bluish green phosphor for white light-emitting diodes, Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4. The phosphor shows broad and strong absorption in the region (320– 450 nm), which is essential for improving the efficiency and quality of white light-emitting diodes. When excited at 380 nm, the phosphor shows two emission bands at around 425 and 500 nm. The main emission peak of Eu2+activated Ca12Al10.6Si3.4O32Cl5.4 exhibits red shift in comparison with that of Eu2+-activated Ca12Al14O33, which is due to the introduction of Si and Cl ions. The results show Ca12Al10.6Si3.4O32Cl5.4 is a promising host candidate for the phosphors. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In light source, a recently growing interest is mainly focused on white light-emitting diodes (LEDs) due to their advantages of high efficiency, long lifetime, low power consumption and environment friendly characters [1,2]. Most white LEDs are prepared by the combination of LED chips with phosphors, therefore it is important to search for phosphors with high efficiency and good thermal stability [3–5]. Up to now, there have been many phosphors reported, such as silicates, aluminates, sulfides and oxynitrides/nitrides, but phosphors with excellent optical properties are still very rare [6,7]. To improve optical properties of phosphors, some elements of host materials are usually replaced by other elements, especially by their same group elements, such as Y3(Al, Ga)5O12:Ce, Lu2CaMg(Si, Ge)3O12:Ce [8,9]. However, there are few reports about the substitution of different group elements to improve optical properties of the phosphor. The calcium chloride framework aluminosilicate (Ca12Al10.6Si3.4O32Cl5.4) is a special material with Si substituted for some A1 and Cl replacing some O to keep charge balance and phase stability [10]. According to the best of our knowledge, there have been no investigations regarding calcium chloride framework aluminosilicate phosphor. In this article, we synthesized the new phosphor Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4, which luminescence properties were studied in
⁎ Corresponding author. College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China. Fax: +86 571 28889527. E-mail address: [email protected] (S. Xu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.02.060
detail by means of excitation and emission spectra. Moreover, we also studied the correlativity between the optical properties and the substitution of different group elements in the phosphor. 2. Experimental The Ca12 − xEuxAl10.6Si3.4O32Cl5.4 (x = 0.04–0.24) phosphors were prepared by conventional solid-state reaction method under reductive atmosphere. The starting materials were CaCO3 (A.R.), Al2O3 (A.R.), SiO2 (A.R.), anhydrous CaCl2 (A.R.) and Eu2O3 (99.99%). Stoichiometric amounts of starting materials were thoroughly mixed in an agate mortar by grinding, and sintered at 1000 °C in reductive atmosphere (5% H2/95% N2) for 4 h. The excess calcium chlorides were removed by washing with anhydrous alcohol. The phase purity of the phosphor was checked by powder X-ray diffraction (XRD) analysis with a Thermo ARL XTRA diffractometer with Cu Ka radiation. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed by a Fluorolog Fl3-211-P Spectrometer. 3. Results and discussion Fig. 1 shows the XRD patterns of 16 mol% Eu2+-doped Ca12Al10.6Si3.4O32Cl5.4 that agrees well with the JCPDS Card No. 78-1794. Therefore, we make conclusions that the Ca12Al10.6Si3.4O32Cl5.4 can be synthesized by the high temperature solid-state method. It also indicates the doping Eu2+ ions do not make the phosphor form new phases in the synthesized process. Moreover, Eu2+ atoms probably occupy the Ca2+ sites in the Ca12Al10.6Si3.4O32Cl5.4, because ionic radii
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H. Ju et al. / Materials Letters 63 (2009) 1275–1277
Fig. 1. The XRD pattern of sample Ca12Al10.6Si3.4O32Cl5.4:0.16Eu2+.
of Eu2+ and Ca2+ are very close in size, such as Eu2+ (r = 0.117 nm with CN = 6 and r = 0.120 nm with CN = 7) and Ca2+ (r = 0.100 nm with CN = 6, r = 0.106 nm with CN = 7) [11]. The compound Ca12Al10.6Si3.4O32Cl5.4 is very similar to mayenite (Ca12Al14O33) except for Si substituted for some A1 and Cl replacing some O [12]. They all crystallize in pace group I4̅3d, and have some large cages that are surrounded by eight Al (Si) tetrahedrons and two Ca ions. The cages are randomly filled with ‘free’ one of the 33 oxygen anions in mayenite, which is attributed to the larger radius of cage (r = 0.177 nm) than that of O anion (r = 0.132 nm). However, Fig. 2 shows the Cl anions fill the large cages surrounded by eight Al (Si) tetrahedrons and two Ca ions in Ca12Al10.6Si3.4O32Cl5.4 [10]. Cl anions are tightly bound in these cages, and not randomly moved, because
Fig. 2. (A) The coordination of calcium ions in crystal lattice of Ca12Al10.6Si3.4O32Cl5.4 from Ref. [10]; (B) The crystal structure of Ca12Al10.6Si3.4O32Cl5.4 from Ref. [10].
the radius of Cl anions (r = 0.181) is very close to that of the cages (r = 0.171). Moreover, the real chemical composition of the material is Ca12Al10.6Si3.4O32Cl5.4, which is subject to large standard deviation with that of the ideal Ca12Al10Si4O32Cl6. One tenth of Ca2+ ions cannot be coordinated with Cl− anions due to the large standard deviation of chemical composition, but the exact position of these Ca2+ ions in the crystal is not clear at this stage. Fig. 3 shows the excitation spectrum monitored at 500 nm consisting of three broad absorption bands at 340, 380 and 415 nm, which ascribes to the 4f–5d transitions of Eu2+ [13]. However, the excitation spectrum monitored at 425 nm shows two absorption bands in short wavelength region, which peaks at around 290 and 350 nm. The difference of the excitation spectra under different excitation light indicates the phosphor probably have two different light–emitting centers. The strong absorption band at 340–450 nm indicates the phosphor can be excited not only by the UV-LED chip but also by the blue LED chip, which is essential for improving the efficiency and quality of white light-emitting diodes. As can be seen from Fig. 3, the emission spectrum of Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4 consists of two broad bands with a maximum at 500 nm and the other at 425 nm, which is very different to that of Eu2+activated Ca12Al14O33 with only a single emission band (442 nm) [14]. The reason is mainly attributed to Cl replacing some O and Si substituted for some Al in Ca12Al10.6Si3.4O32Cl5.4. Because two emission bands exist in the emission spectrum of Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4 phosphor, Eu2+ ions probably form two corresponding emission centers, Eu (1) and Eu (2). However, Ca2+ ions only occupy one position in the crystal lattice, which is inconsistent with two emission centers that are caused by the Eu2+ substituting for Ca2+ [15]. We presume the sites filled with Ca2+ ions probably have two different crystal field, one with six O, the other with six O and one Cl, because approximately onetenth of the Ca ions cannot coordinate with Cl anions. Because Cl ions with strong coordination effect can strengthen the crystal field splitting of Eu2+ ions, the emission band at 500 nm are probably due to the Eu (1) substituted for some Ca surrounded by six O and one Cl ions, the emission band at 425 nm due to the Eu (2) substituted some Ca surrounded by six O ions [15]. Moreover, compared with Eu2+-activated Ca12Al14O33 with an emission band at 442 nm, the introduction of Si and Cl ions could make the emission band (500 nm) of Ca12Al10.6Si3.4O32Cl5.4 phosphors redshift. The effect of Eu2+-doped concentration on the emission intensity of Ca12Al10.6Si3.4O32Cl5.4 phosphor is also investigated. As shown in Fig. 3, the emission intensity under 380 nm excitation increases
Fig. 3. The excitation and emission spectra of Ca12Al10.6Si3.4O32Cl5.4:0.16Eu2+, the right inset represents the concentration influence on emission intensity.
H. Ju et al. / Materials Letters 63 (2009) 1275–1277
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4. Conclusions In the present work, new bluish green phosphors, Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4 were synthesized by a high temperature solidstate method. The photoluminescence spectra show that the phosphor can be excited efficiently by UV–Vis light (320–450 nm) and emits intense bluish green light with two emission bands at 425 and 500 nm. When the Eu2+ concentration is 16 mol%, the emission intensity of phosphor reaches a maximum. The CIE chromaticity coordinate of phosphors changes from a blue green region to a bluish green region with the increase of excitation wavelength. Compared with Eu2+-activated mayenite, the introduction of Si and Cl atoms brings on the redshift of emission bands in Eu2+-doped Ca12Al10.6Si3.4O32Cl5.4. The new phosphor Eu2+-doped Ca12Al10.6Si3.4O32Cl5.4 has enlarged the range of the potential phosphor candidate for white LEDs. Fig. 4. The emission spectra of Ca12Al10.6Si3.4O32Cl5.4:0.16Eu2+ under different excitation light.
greatly with the increase of the Eu2+ concentration until a maximum intensity is reached at x = 16 mol%. Beyond 16 mol%, the emission intensity begins to decrease, which is attributed to the concentration quenching effect that is mainly caused by an exchange interaction, radiation reabsorption or a multipole–multipole interaction between Eu2+ and Eu2+ [16]. However, the wavelength of the emission peak is not influenced by the Eu2+ concentration. The CIE chromaticity coordinate of the phosphor Ca12Al10.6Si3.4O32Cl5.4 is calculated according to the emission curve, which exactly falls into the bluish green region in the CIE 1931 chromaticity diagram. Fig. 4 shows the CIE chromaticity coordinate translates from blue green region to bluish green region with the increase of excitation wavelength, which is attributed to the emission intensity change of two emission bands under different excitation wavelength. With increasing of excitation wavelength, the intensity of emission band at 500 nm increases, however, the intensity of emission at 425 nm deceases quickly. Therefore, the CIE chromaticity coordinate changes under different excitation wavelength, which offers a method to change the CIE chromaticity coordinate of the phosphor and white LEDs.
Acknowledgements This research is supported by the Project of the National Nature Science Foundation of China (Grant No 60508014 and 50772102), Program for New Century Excellent Talents in University (Grant No NCET-07-0786), the Nature Science Foundation of Zhejiang Province (Grant No R406007), and the Zhejiang Province Science and Technology Program (Grant No 2008C21153). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Nakamura S, Mukai T, Senoh M. Appl Phys Lett 1994;64:1687–9. Schubert EF, Kim JK. Science 2005;308:1274–8. Park JK, Lim MA, Kim CH. Appl Phys Lett 2003;82:683–5. Wu ZC, Shi JX, Wang J, Wu H, Su Q. Mater Lett 2006;60:3499–501. Kim JS, Jeon PE, Choi JC, Park HL, Mho I S. Appl Phys Lett 2004;84:2931–3. Silver J, Withnall R. Chem Rev 2004;104:2833–56. Feldmann C. Adv Funct Mater 2003;13:511–6. Shin I, Massaki K, Ryo I, Masatoshi S. Rare Earths 2003;42:118–9. Setlur A, Heward WJ, Gao Y. Chem Mater 2006;18:3314–22. Feng QL, Glasser FP, Alan-Howie R, Lachowski E. Acta Crystallogr 1988; C44:589–92. Shannon RD. Acta Crystallogr 1976;A32:751–69. Boysen H, Lerch M, Stys A, Senyshyn A. Acta Crystallogr 2007;B63:675–82. Ding W, Wang J, Zhang M, Zhang Q, Su Q. Chem Phys Lett 2007;435:301–5. Zhang J, Zhang Z, Wang T, Hao W. Mater Lett 2003;57:4315–6. Dorenbos P. J Lumin 2003;104:239–60. Liu J, Sun J, Shi C. Mater Lett 2006;60:2830–3.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Luminescence properties of Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4: A promising phosphor for solid state lighting Haidong Ju, Xingyu Su, Shiqing Xu ⁎, Ying Zhang, Degang Deng, Shilong Zhao, Huanping Wang, Baoling Wang, Liuzheng Sun College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 11 January 2009 Accepted 24 February 2009 Available online 6 March 2009 Keywords: Phosphors Luminescence Substitution Ca12Al10.6Si3.4O32Cl5.4
a b s t r a c t In this article, we synthesized and characterized a novel bluish green phosphor for white light-emitting diodes, Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4. The phosphor shows broad and strong absorption in the region (320– 450 nm), which is essential for improving the efficiency and quality of white light-emitting diodes. When excited at 380 nm, the phosphor shows two emission bands at around 425 and 500 nm. The main emission peak of Eu2+activated Ca12Al10.6Si3.4O32Cl5.4 exhibits red shift in comparison with that of Eu2+-activated Ca12Al14O33, which is due to the introduction of Si and Cl ions. The results show Ca12Al10.6Si3.4O32Cl5.4 is a promising host candidate for the phosphors. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In light source, a recently growing interest is mainly focused on white light-emitting diodes (LEDs) due to their advantages of high efficiency, long lifetime, low power consumption and environment friendly characters [1,2]. Most white LEDs are prepared by the combination of LED chips with phosphors, therefore it is important to search for phosphors with high efficiency and good thermal stability [3–5]. Up to now, there have been many phosphors reported, such as silicates, aluminates, sulfides and oxynitrides/nitrides, but phosphors with excellent optical properties are still very rare [6,7]. To improve optical properties of phosphors, some elements of host materials are usually replaced by other elements, especially by their same group elements, such as Y3(Al, Ga)5O12:Ce, Lu2CaMg(Si, Ge)3O12:Ce [8,9]. However, there are few reports about the substitution of different group elements to improve optical properties of the phosphor. The calcium chloride framework aluminosilicate (Ca12Al10.6Si3.4O32Cl5.4) is a special material with Si substituted for some A1 and Cl replacing some O to keep charge balance and phase stability [10]. According to the best of our knowledge, there have been no investigations regarding calcium chloride framework aluminosilicate phosphor. In this article, we synthesized the new phosphor Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4, which luminescence properties were studied in
⁎ Corresponding author. College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China. Fax: +86 571 28889527. E-mail address: [email protected] (S. Xu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.02.060
detail by means of excitation and emission spectra. Moreover, we also studied the correlativity between the optical properties and the substitution of different group elements in the phosphor. 2. Experimental The Ca12 − xEuxAl10.6Si3.4O32Cl5.4 (x = 0.04–0.24) phosphors were prepared by conventional solid-state reaction method under reductive atmosphere. The starting materials were CaCO3 (A.R.), Al2O3 (A.R.), SiO2 (A.R.), anhydrous CaCl2 (A.R.) and Eu2O3 (99.99%). Stoichiometric amounts of starting materials were thoroughly mixed in an agate mortar by grinding, and sintered at 1000 °C in reductive atmosphere (5% H2/95% N2) for 4 h. The excess calcium chlorides were removed by washing with anhydrous alcohol. The phase purity of the phosphor was checked by powder X-ray diffraction (XRD) analysis with a Thermo ARL XTRA diffractometer with Cu Ka radiation. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed by a Fluorolog Fl3-211-P Spectrometer. 3. Results and discussion Fig. 1 shows the XRD patterns of 16 mol% Eu2+-doped Ca12Al10.6Si3.4O32Cl5.4 that agrees well with the JCPDS Card No. 78-1794. Therefore, we make conclusions that the Ca12Al10.6Si3.4O32Cl5.4 can be synthesized by the high temperature solid-state method. It also indicates the doping Eu2+ ions do not make the phosphor form new phases in the synthesized process. Moreover, Eu2+ atoms probably occupy the Ca2+ sites in the Ca12Al10.6Si3.4O32Cl5.4, because ionic radii
1276
H. Ju et al. / Materials Letters 63 (2009) 1275–1277
Fig. 1. The XRD pattern of sample Ca12Al10.6Si3.4O32Cl5.4:0.16Eu2+.
of Eu2+ and Ca2+ are very close in size, such as Eu2+ (r = 0.117 nm with CN = 6 and r = 0.120 nm with CN = 7) and Ca2+ (r = 0.100 nm with CN = 6, r = 0.106 nm with CN = 7) [11]. The compound Ca12Al10.6Si3.4O32Cl5.4 is very similar to mayenite (Ca12Al14O33) except for Si substituted for some A1 and Cl replacing some O [12]. They all crystallize in pace group I4̅3d, and have some large cages that are surrounded by eight Al (Si) tetrahedrons and two Ca ions. The cages are randomly filled with ‘free’ one of the 33 oxygen anions in mayenite, which is attributed to the larger radius of cage (r = 0.177 nm) than that of O anion (r = 0.132 nm). However, Fig. 2 shows the Cl anions fill the large cages surrounded by eight Al (Si) tetrahedrons and two Ca ions in Ca12Al10.6Si3.4O32Cl5.4 [10]. Cl anions are tightly bound in these cages, and not randomly moved, because
Fig. 2. (A) The coordination of calcium ions in crystal lattice of Ca12Al10.6Si3.4O32Cl5.4 from Ref. [10]; (B) The crystal structure of Ca12Al10.6Si3.4O32Cl5.4 from Ref. [10].
the radius of Cl anions (r = 0.181) is very close to that of the cages (r = 0.171). Moreover, the real chemical composition of the material is Ca12Al10.6Si3.4O32Cl5.4, which is subject to large standard deviation with that of the ideal Ca12Al10Si4O32Cl6. One tenth of Ca2+ ions cannot be coordinated with Cl− anions due to the large standard deviation of chemical composition, but the exact position of these Ca2+ ions in the crystal is not clear at this stage. Fig. 3 shows the excitation spectrum monitored at 500 nm consisting of three broad absorption bands at 340, 380 and 415 nm, which ascribes to the 4f–5d transitions of Eu2+ [13]. However, the excitation spectrum monitored at 425 nm shows two absorption bands in short wavelength region, which peaks at around 290 and 350 nm. The difference of the excitation spectra under different excitation light indicates the phosphor probably have two different light–emitting centers. The strong absorption band at 340–450 nm indicates the phosphor can be excited not only by the UV-LED chip but also by the blue LED chip, which is essential for improving the efficiency and quality of white light-emitting diodes. As can be seen from Fig. 3, the emission spectrum of Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4 consists of two broad bands with a maximum at 500 nm and the other at 425 nm, which is very different to that of Eu2+activated Ca12Al14O33 with only a single emission band (442 nm) [14]. The reason is mainly attributed to Cl replacing some O and Si substituted for some Al in Ca12Al10.6Si3.4O32Cl5.4. Because two emission bands exist in the emission spectrum of Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4 phosphor, Eu2+ ions probably form two corresponding emission centers, Eu (1) and Eu (2). However, Ca2+ ions only occupy one position in the crystal lattice, which is inconsistent with two emission centers that are caused by the Eu2+ substituting for Ca2+ [15]. We presume the sites filled with Ca2+ ions probably have two different crystal field, one with six O, the other with six O and one Cl, because approximately onetenth of the Ca ions cannot coordinate with Cl anions. Because Cl ions with strong coordination effect can strengthen the crystal field splitting of Eu2+ ions, the emission band at 500 nm are probably due to the Eu (1) substituted for some Ca surrounded by six O and one Cl ions, the emission band at 425 nm due to the Eu (2) substituted some Ca surrounded by six O ions [15]. Moreover, compared with Eu2+-activated Ca12Al14O33 with an emission band at 442 nm, the introduction of Si and Cl ions could make the emission band (500 nm) of Ca12Al10.6Si3.4O32Cl5.4 phosphors redshift. The effect of Eu2+-doped concentration on the emission intensity of Ca12Al10.6Si3.4O32Cl5.4 phosphor is also investigated. As shown in Fig. 3, the emission intensity under 380 nm excitation increases
Fig. 3. The excitation and emission spectra of Ca12Al10.6Si3.4O32Cl5.4:0.16Eu2+, the right inset represents the concentration influence on emission intensity.
H. Ju et al. / Materials Letters 63 (2009) 1275–1277
1277
4. Conclusions In the present work, new bluish green phosphors, Eu2+-activated Ca12Al10.6Si3.4O32Cl5.4 were synthesized by a high temperature solidstate method. The photoluminescence spectra show that the phosphor can be excited efficiently by UV–Vis light (320–450 nm) and emits intense bluish green light with two emission bands at 425 and 500 nm. When the Eu2+ concentration is 16 mol%, the emission intensity of phosphor reaches a maximum. The CIE chromaticity coordinate of phosphors changes from a blue green region to a bluish green region with the increase of excitation wavelength. Compared with Eu2+-activated mayenite, the introduction of Si and Cl atoms brings on the redshift of emission bands in Eu2+-doped Ca12Al10.6Si3.4O32Cl5.4. The new phosphor Eu2+-doped Ca12Al10.6Si3.4O32Cl5.4 has enlarged the range of the potential phosphor candidate for white LEDs. Fig. 4. The emission spectra of Ca12Al10.6Si3.4O32Cl5.4:0.16Eu2+ under different excitation light.
greatly with the increase of the Eu2+ concentration until a maximum intensity is reached at x = 16 mol%. Beyond 16 mol%, the emission intensity begins to decrease, which is attributed to the concentration quenching effect that is mainly caused by an exchange interaction, radiation reabsorption or a multipole–multipole interaction between Eu2+ and Eu2+ [16]. However, the wavelength of the emission peak is not influenced by the Eu2+ concentration. The CIE chromaticity coordinate of the phosphor Ca12Al10.6Si3.4O32Cl5.4 is calculated according to the emission curve, which exactly falls into the bluish green region in the CIE 1931 chromaticity diagram. Fig. 4 shows the CIE chromaticity coordinate translates from blue green region to bluish green region with the increase of excitation wavelength, which is attributed to the emission intensity change of two emission bands under different excitation wavelength. With increasing of excitation wavelength, the intensity of emission band at 500 nm increases, however, the intensity of emission at 425 nm deceases quickly. Therefore, the CIE chromaticity coordinate changes under different excitation wavelength, which offers a method to change the CIE chromaticity coordinate of the phosphor and white LEDs.
Acknowledgements This research is supported by the Project of the National Nature Science Foundation of China (Grant No 60508014 and 50772102), Program for New Century Excellent Talents in University (Grant No NCET-07-0786), the Nature Science Foundation of Zhejiang Province (Grant No R406007), and the Zhejiang Province Science and Technology Program (Grant No 2008C21153). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Nakamura S, Mukai T, Senoh M. Appl Phys Lett 1994;64:1687–9. Schubert EF, Kim JK. Science 2005;308:1274–8. Park JK, Lim MA, Kim CH. Appl Phys Lett 2003;82:683–5. Wu ZC, Shi JX, Wang J, Wu H, Su Q. Mater Lett 2006;60:3499–501. Kim JS, Jeon PE, Choi JC, Park HL, Mho I S. Appl Phys Lett 2004;84:2931–3. Silver J, Withnall R. Chem Rev 2004;104:2833–56. Feldmann C. Adv Funct Mater 2003;13:511–6. Shin I, Massaki K, Ryo I, Masatoshi S. Rare Earths 2003;42:118–9. Setlur A, Heward WJ, Gao Y. Chem Mater 2006;18:3314–22. Feng QL, Glasser FP, Alan-Howie R, Lachowski E. Acta Crystallogr 1988; C44:589–92. Shannon RD. Acta Crystallogr 1976;A32:751–69. Boysen H, Lerch M, Stys A, Senyshyn A. Acta Crystallogr 2007;B63:675–82. Ding W, Wang J, Zhang M, Zhang Q, Su Q. Chem Phys Lett 2007;435:301–5. Zhang J, Zhang Z, Wang T, Hao W. Mater Lett 2003;57:4315–6. Dorenbos P. J Lumin 2003;104:239–60. Liu J, Sun J, Shi C. Mater Lett 2006;60:2830–3.