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Long wavelength Ce3+ emission in Y6Si3O9N4 phosphors for white-emitting diodes
Materials Letters 65 (2011) 1176–1178
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
Long wavelength Ce3+ emission in Y6Si3O9N4 phosphors for white-emitting diodes Degang Deng, Shiqing Xu ⁎, Xingyu Su, Qian Wang, Yinqun Li, Gaofeng Li, Youjie Hua, Lihui Huang, Shilong Zhao, Huanping Wang, Chenxia Li 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 24 November 2010 Accepted 14 January 2011 Available online 25 January 2011 Keywords: Phosphors Luminescence White LEDs Y6Si3O9N4:Ce3+
a b s t r a c t Y6Si3O9N4:Ce3+ phosphor was prepared by a solid-state reaction in reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of Y6Si3O9N4:Ce3+. Scanning electron microscopy (SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Photoluminescence (PL) measurements showed that the phosphor can be efficiently excited by near ultraviolet (UV) or blue light excitation, and exhibited bright green emission peaked at about 525 nm. Compared with Ce3+-doped Y4Si2O7N2 phosphors, Ce3+-doped Y6Si3O9N4 phosphors showed longer wavelengths of both excitation and emission. The Y6Si3O9N4:Ce3+ is a potential green-emitting phosphor for white LEDs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A blue-emitting GaN chip pumped YAG:Ce3+ phosphor and a combination of two colors of transmitted blue color and luminescent yellow color make white light [1]. However, white LEDs made by above-mentioned mechanism have the following problems: white emitting color changes with input power and low color rendering index due to two-color mixing. A novel approach has been suggested which utilizes phosphor excited by UV or blue chips to generate white light [2]. Researchers worldwide have investigated many other chemical compounds as suitable phosphors for solid-state lighting. Nitride and oxynitride phosphors are newly developed members of the phosphor family. Until now, most of the nitride and oxynitride phosphors applicable for excitation of UV and blue-LED chips were Ce3+- or Eu2+-doped silicon-based compounds, due to the high covalency of the crystal lattices and strong crystal field of the host lattices [3]. Ce3+- or Eu2+-doped nitride and oxynitride phosphors usually have longer wavelength excitation and emission bands than those of oxide phosphors because of the smaller electro negativity causes stronger nephelauxetic effect (covalancy) and the higher negative charge of N3− makes the crystal field splitting of the d orbital of Ce3+ or Eu2+ larger, according to crystal field theory [4–9]. Recently, the luminescence properties of a series of Ce3+-doped oxynitride compounds in the Y–Si–O–N system (Y 5 (SiO 4 ) 3 N, Y4Si2O7N2, YSiO2N and Y2Si3O3N4) and a modified Ce3+-doped Y2Si3-xAlxO3 + xN4-x melilite compound have been presented [8,9]. Those investigations have shown that blue and green emission of Ce3+
⁎ Corresponding author. Tel.: + 86 571 86835781; fax: + 86 571 28889527. E-mail address: [email protected] (S. Xu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.01.031
can be observed, for example, only Y4Si2O7N2:Ce3+ exhibits a maximum emission band up to 504 nm. In this letter, the Y6Si3O9N4:Ce3+ green-emitting phosphor was synthesized by the conventional high temperature solid state reaction which shows long-wavelength emission peaking 525 nm, and its luminescent properties were also investigated. 2. Experimental The Y6-xCexSi3O9N4 (x = 0–0.8) phosphors were prepared by conventional solid-state reaction method under reductive atmosphere. The starting were materials Y2O3 (99.99%), Si3N4 (A.R.) and CeO2 (99.99%). Stoichiometric amounts of starting materials were thoroughly mixed in an agate mortar by grinding and sintered at 1600 °C in reductive atmosphere (5%H2/95%N2) for 3 h. 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 morphology and size of the calcined particles were observed by scanning electron microscopy (SEM, JSM5601).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 Y6-xSi3O9N4:xCe3+(x = 0–0.8) phosphors. As can be seen, pure phase diffraction peaks of Y6Si3O9N4 are predominant in the XRD patterns which match well with standard card (JCPDF card 30-1461). No other products or starting materials was observed. and a small amount of doped Ce3+ ions has no obvious influence on the structure of the host due to the similar ionic radius of Ce3+ (0.101 nm) and Y3+ (0.90 nm).
D. Deng et al. / Materials Letters 65 (2011) 1176–1178
Fig. 1. XRD patterns of Y6-xSi3O9N4:xCe3+(x = 0, 0.2, 0.4, 0.6, and 0.8).
Fig. 2 shows the scanning electron microscopy (SEM) image of the Y5.6Si3O9N4:0.4 Ce3+ phosphor. The microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Fig. 3 presents the PL spectra of Y6Si3O9N4:Ce3+. The excitation spectrum consists of two bands centered on about 385 nm and 420 nm, these excitation bands are due to the absorption of the incident radiation by Ce3+ ions which leads to the excitation of electrons from the 4f1 ground state (2F5/2, 2F7/2) to the excited 5d1 level (2D) [10]. It suggests that both phosphors can be effectively excited under near UV or UV light excitation. The UV-excited Y6Si3O9N4:Ce3+ (λex = 385, 420 nm) at room temperature yielded a very strong green emission band at 525 nm. Despite there may be different types of Y sites in Y6Si3O9N4:Ce3+, emission spectra at different excitations wavelengths reveal only small difference, this is in good agreement with that of Y4Si2O7N2:Ce3+. Compared with Y4Si2O7N2:Ce3+ [8], Y6Si3O9N4:Ce3+ and YSi3N5:Ce3+ [11], the emission peaks show obvious red shift, the maximum emission position shifts from 500 to 552 nm with an increase nitrogen contents. It can be expected that nitrogen shows less electro negativity comparing with oxygen, which will cause an increase in covalence (nephelauxetic effect) and result in the red-shift of the center of energy gravity. And the ligand-field splitting of the 5d band increases when O2− was replaced by N3−, which is caused by the higher formal charge of N3− compared to O2− [12]. The effect of doped-Ce3+ concentration on the excitation and emission of Y6Si3O9N4:Ce3+ was also investigated. The excitation and emission of Y6Si3O9N4:Ce3+ prepared at various concentration of Ce3+ (x = 0.2–0.8) are showed in Fig. 4. The PL intensity increases with the Ce3+-concentration increasing until a maximum intensity is reached, and then it decreases due to concentration quenching. From Fig. 4, we
Fig. 2. SEM image of Y5.6Si3O9N4:0.4Ce3+ calcined at 1600 °C for 3 h.
1177
Fig. 3. PL spectra of Y5.6Si3O9N4:0.4 Ce3+.
can see that the critical quenching concentration of Ce3+ in the Y6Si3O9N4:Ce3+ phosphor is about of x = 0.4. Concentration quenching usually occurs as a result of a non-radiative energy transfer among luminescent centers. With increasing of Ce3+ concentration, the distance between two neighboring Ce3+ ions becomes short, and thus the probability of energy transfer increase. As shown in Fig. 3, the excitation and emission spectra overlap to some extent, indicating that the radiation re-absorption mechanism may also play a role in the energy transfer. The relative lower optimum Ce3+ concentration for Y6Si3O9N4:Ce3+ phosphors originates from the large spectral overlap between the normalized spectra shapes of excitation and emission spectral [13], since Y6Si3O9N4:Ce3+ phosphors have a lower Stokes shift, as shown in Fig. 3. The lower Stokes shift could be ascribed to the stronger rigidity of the lattice with short crystal lattice constants. 4. Conclusions In the present work, new green phosphor, Ce3+-activated Y6Si3O9N4 were synthesized by a high temperature solid-state method. The photoluminescence spectra show that the phosphor can be excited efficiently by UV–vis light (350–450 nm) and emits intense green light with emission bands at 525 nm. When the Ce3+ concentration is 40 mol%, the emission intensity of phosphors reaches a maximum. Compared with Ce3+-activated Y4Si2O7N2, more N3−
Fig. 4. Excitation and emission spectra of Y6-xSi3O9N4:x Ce3+ with x = 0.2, 0.4, 0.6, 0.8. Excitation wavelength for the emission spectra as 420 nm. The monitoring wavelength for the excitation spectra was 420 nm.
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D. Deng et al. / Materials Letters 65 (2011) 1176–1178
versus O3− coordinates to Ce3+ brings on the redshift of emission bands in Ce3+-activated Y6Si3O9N4. All the characteristics indicate that Y6Si3O9N4:Ce3+ is a good candidate phosphor that can be applied in white LEDs. Acknowledgements This research is supported by the Project of the National Nature Science Foundation of China (Grant no. 61008042 and 51072190), Program for New Century Excellent Talents in University (Grant no. NCET-07-0786), the Nature Science Foundation of Zhejiang Province (Grant no. Z4100030 and Y4080268), and the Science Technology Project of Zhejiang Province (2009C21020).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Nakainura S, Fasol G. The blue laser diode. Berlin: Springer; 1996. Y. Shimizu, K. Sakano, Y. Noguchi, T. Moriguchi., U. S. Patent 1999; 5,998,925. Xie RJ, Hirosaki N. Sci Technol Adv Mater 2007;8:588–600. Li YQ, Delsing A, de With G, Hintzen HT. Chem Mater 2005;17:3242–8. Yun BG, Miyamoto Y, Yamamoto H. J Electrochem Soc 2007;154:320–5. Li YQ, de With G, Hintzen HT. J Alloy Comp 2004;385:1–11. Xua X, Caia C, Haoa LY, Wanga YC, Li QX. Mater Chem Phys 2009;118:270–2. van Krevel JWH, Hintzen HT, Metselaar R, Meijerink A. J Alloy Comp 1998;268: 272–7. van Krevel JWH, Hintzen HT, Metselaar R. Mater Res Bull 2000;35:747–54. Feldmann C, Justel T, Ronda CR, Schmidt PJ. Adv Funct Mater 2003;13:511–6. Yang HC, Liu Y, Ye S, Qiu JR. Chem Phys Lett 2008;451:218–21. van Krevel JWH, van Rutten JWT, Mandal H, Hintzen HT, Metselaar R. J Solid State Chem 2002;165:19–24. Blasse G, Solid State J. Chem 1986;62:207–11.
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
Long wavelength Ce3+ emission in Y6Si3O9N4 phosphors for white-emitting diodes Degang Deng, Shiqing Xu ⁎, Xingyu Su, Qian Wang, Yinqun Li, Gaofeng Li, Youjie Hua, Lihui Huang, Shilong Zhao, Huanping Wang, Chenxia Li 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 24 November 2010 Accepted 14 January 2011 Available online 25 January 2011 Keywords: Phosphors Luminescence White LEDs Y6Si3O9N4:Ce3+
a b s t r a c t Y6Si3O9N4:Ce3+ phosphor was prepared by a solid-state reaction in reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of Y6Si3O9N4:Ce3+. Scanning electron microscopy (SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Photoluminescence (PL) measurements showed that the phosphor can be efficiently excited by near ultraviolet (UV) or blue light excitation, and exhibited bright green emission peaked at about 525 nm. Compared with Ce3+-doped Y4Si2O7N2 phosphors, Ce3+-doped Y6Si3O9N4 phosphors showed longer wavelengths of both excitation and emission. The Y6Si3O9N4:Ce3+ is a potential green-emitting phosphor for white LEDs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A blue-emitting GaN chip pumped YAG:Ce3+ phosphor and a combination of two colors of transmitted blue color and luminescent yellow color make white light [1]. However, white LEDs made by above-mentioned mechanism have the following problems: white emitting color changes with input power and low color rendering index due to two-color mixing. A novel approach has been suggested which utilizes phosphor excited by UV or blue chips to generate white light [2]. Researchers worldwide have investigated many other chemical compounds as suitable phosphors for solid-state lighting. Nitride and oxynitride phosphors are newly developed members of the phosphor family. Until now, most of the nitride and oxynitride phosphors applicable for excitation of UV and blue-LED chips were Ce3+- or Eu2+-doped silicon-based compounds, due to the high covalency of the crystal lattices and strong crystal field of the host lattices [3]. Ce3+- or Eu2+-doped nitride and oxynitride phosphors usually have longer wavelength excitation and emission bands than those of oxide phosphors because of the smaller electro negativity causes stronger nephelauxetic effect (covalancy) and the higher negative charge of N3− makes the crystal field splitting of the d orbital of Ce3+ or Eu2+ larger, according to crystal field theory [4–9]. Recently, the luminescence properties of a series of Ce3+-doped oxynitride compounds in the Y–Si–O–N system (Y 5 (SiO 4 ) 3 N, Y4Si2O7N2, YSiO2N and Y2Si3O3N4) and a modified Ce3+-doped Y2Si3-xAlxO3 + xN4-x melilite compound have been presented [8,9]. Those investigations have shown that blue and green emission of Ce3+
⁎ Corresponding author. Tel.: + 86 571 86835781; fax: + 86 571 28889527. E-mail address: [email protected] (S. Xu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.01.031
can be observed, for example, only Y4Si2O7N2:Ce3+ exhibits a maximum emission band up to 504 nm. In this letter, the Y6Si3O9N4:Ce3+ green-emitting phosphor was synthesized by the conventional high temperature solid state reaction which shows long-wavelength emission peaking 525 nm, and its luminescent properties were also investigated. 2. Experimental The Y6-xCexSi3O9N4 (x = 0–0.8) phosphors were prepared by conventional solid-state reaction method under reductive atmosphere. The starting were materials Y2O3 (99.99%), Si3N4 (A.R.) and CeO2 (99.99%). Stoichiometric amounts of starting materials were thoroughly mixed in an agate mortar by grinding and sintered at 1600 °C in reductive atmosphere (5%H2/95%N2) for 3 h. 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 morphology and size of the calcined particles were observed by scanning electron microscopy (SEM, JSM5601).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 Y6-xSi3O9N4:xCe3+(x = 0–0.8) phosphors. As can be seen, pure phase diffraction peaks of Y6Si3O9N4 are predominant in the XRD patterns which match well with standard card (JCPDF card 30-1461). No other products or starting materials was observed. and a small amount of doped Ce3+ ions has no obvious influence on the structure of the host due to the similar ionic radius of Ce3+ (0.101 nm) and Y3+ (0.90 nm).
D. Deng et al. / Materials Letters 65 (2011) 1176–1178
Fig. 1. XRD patterns of Y6-xSi3O9N4:xCe3+(x = 0, 0.2, 0.4, 0.6, and 0.8).
Fig. 2 shows the scanning electron microscopy (SEM) image of the Y5.6Si3O9N4:0.4 Ce3+ phosphor. The microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Fig. 3 presents the PL spectra of Y6Si3O9N4:Ce3+. The excitation spectrum consists of two bands centered on about 385 nm and 420 nm, these excitation bands are due to the absorption of the incident radiation by Ce3+ ions which leads to the excitation of electrons from the 4f1 ground state (2F5/2, 2F7/2) to the excited 5d1 level (2D) [10]. It suggests that both phosphors can be effectively excited under near UV or UV light excitation. The UV-excited Y6Si3O9N4:Ce3+ (λex = 385, 420 nm) at room temperature yielded a very strong green emission band at 525 nm. Despite there may be different types of Y sites in Y6Si3O9N4:Ce3+, emission spectra at different excitations wavelengths reveal only small difference, this is in good agreement with that of Y4Si2O7N2:Ce3+. Compared with Y4Si2O7N2:Ce3+ [8], Y6Si3O9N4:Ce3+ and YSi3N5:Ce3+ [11], the emission peaks show obvious red shift, the maximum emission position shifts from 500 to 552 nm with an increase nitrogen contents. It can be expected that nitrogen shows less electro negativity comparing with oxygen, which will cause an increase in covalence (nephelauxetic effect) and result in the red-shift of the center of energy gravity. And the ligand-field splitting of the 5d band increases when O2− was replaced by N3−, which is caused by the higher formal charge of N3− compared to O2− [12]. The effect of doped-Ce3+ concentration on the excitation and emission of Y6Si3O9N4:Ce3+ was also investigated. The excitation and emission of Y6Si3O9N4:Ce3+ prepared at various concentration of Ce3+ (x = 0.2–0.8) are showed in Fig. 4. The PL intensity increases with the Ce3+-concentration increasing until a maximum intensity is reached, and then it decreases due to concentration quenching. From Fig. 4, we
Fig. 2. SEM image of Y5.6Si3O9N4:0.4Ce3+ calcined at 1600 °C for 3 h.
1177
Fig. 3. PL spectra of Y5.6Si3O9N4:0.4 Ce3+.
can see that the critical quenching concentration of Ce3+ in the Y6Si3O9N4:Ce3+ phosphor is about of x = 0.4. Concentration quenching usually occurs as a result of a non-radiative energy transfer among luminescent centers. With increasing of Ce3+ concentration, the distance between two neighboring Ce3+ ions becomes short, and thus the probability of energy transfer increase. As shown in Fig. 3, the excitation and emission spectra overlap to some extent, indicating that the radiation re-absorption mechanism may also play a role in the energy transfer. The relative lower optimum Ce3+ concentration for Y6Si3O9N4:Ce3+ phosphors originates from the large spectral overlap between the normalized spectra shapes of excitation and emission spectral [13], since Y6Si3O9N4:Ce3+ phosphors have a lower Stokes shift, as shown in Fig. 3. The lower Stokes shift could be ascribed to the stronger rigidity of the lattice with short crystal lattice constants. 4. Conclusions In the present work, new green phosphor, Ce3+-activated Y6Si3O9N4 were synthesized by a high temperature solid-state method. The photoluminescence spectra show that the phosphor can be excited efficiently by UV–vis light (350–450 nm) and emits intense green light with emission bands at 525 nm. When the Ce3+ concentration is 40 mol%, the emission intensity of phosphors reaches a maximum. Compared with Ce3+-activated Y4Si2O7N2, more N3−
Fig. 4. Excitation and emission spectra of Y6-xSi3O9N4:x Ce3+ with x = 0.2, 0.4, 0.6, 0.8. Excitation wavelength for the emission spectra as 420 nm. The monitoring wavelength for the excitation spectra was 420 nm.
1178
D. Deng et al. / Materials Letters 65 (2011) 1176–1178
versus O3− coordinates to Ce3+ brings on the redshift of emission bands in Ce3+-activated Y6Si3O9N4. All the characteristics indicate that Y6Si3O9N4:Ce3+ is a good candidate phosphor that can be applied in white LEDs. Acknowledgements This research is supported by the Project of the National Nature Science Foundation of China (Grant no. 61008042 and 51072190), Program for New Century Excellent Talents in University (Grant no. NCET-07-0786), the Nature Science Foundation of Zhejiang Province (Grant no. Z4100030 and Y4080268), and the Science Technology Project of Zhejiang Province (2009C21020).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Nakainura S, Fasol G. The blue laser diode. Berlin: Springer; 1996. Y. Shimizu, K. Sakano, Y. Noguchi, T. Moriguchi., U. S. Patent 1999; 5,998,925. Xie RJ, Hirosaki N. Sci Technol Adv Mater 2007;8:588–600. Li YQ, Delsing A, de With G, Hintzen HT. Chem Mater 2005;17:3242–8. Yun BG, Miyamoto Y, Yamamoto H. J Electrochem Soc 2007;154:320–5. Li YQ, de With G, Hintzen HT. J Alloy Comp 2004;385:1–11. Xua X, Caia C, Haoa LY, Wanga YC, Li QX. Mater Chem Phys 2009;118:270–2. van Krevel JWH, Hintzen HT, Metselaar R, Meijerink A. J Alloy Comp 1998;268: 272–7. van Krevel JWH, Hintzen HT, Metselaar R. Mater Res Bull 2000;35:747–54. Feldmann C, Justel T, Ronda CR, Schmidt PJ. Adv Funct Mater 2003;13:511–6. Yang HC, Liu Y, Ye S, Qiu JR. Chem Phys Lett 2008;451:218–21. van Krevel JWH, van Rutten JWT, Mandal H, Hintzen HT, Metselaar R. J Solid State Chem 2002;165:19–24. Blasse G, Solid State J. Chem 1986;62:207–11.