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J. Phys. Chem. C 2010, 114, 18698–18701
Single-Phased White-Light Phosphors Ca9Gd(PO4)7:Eu2+,Mn2+ under Near-Ultraviolet Excitation Chien-Hao Huang,† Wei-Ren Liu,*,‡ and Teng-Ming Chen*,† Phosphors Research Laboratory and Department of Applied Chemistry, National Chiao Tung UniVersity, Hsinchu 30010, Taiwan, R.O.C., and Material and Chemical Research Laboratories, ITRI, Hsichu, Taiwan 300, R.O.C. ReceiVed: July 19, 2010; ReVised Manuscript ReceiVed: September 10, 2010
Single-phased white-light-emitting phosphors Ca9Gd(PO4)7:Eu2+,Mn2+ were synthesized by solid state reactions. Tuning the Eu2+/Mn2+ ratio via the energy transfer varied the emission hue of Ca9Gd(PO4)7:0.007Eu2+,xMn2+ from blue-greenish (0.219, 0.371) to white-light (0.326, 0.328) and eventually to red (0.625, 0.307). The mechanism of transferring energy from a sensitizer Eu2+ to an activator Mn2+ in Ca9Gd(PO4)7:Eu2+,Mn2+ phosphors was demonstrated to be an electric dipole-quadrupole interaction. Combining a near-UV 380 nm chip and a white-emitting Ca9Gd(PO4)7:0.007Eu2+,0.02Mn2+ phosphor produced a white-light near-UV LED, demonstrating CIE chromaticity coordinates of (0.312, 0.327) and a color temperature of 6569 K. 1. Introduction Researchers have recently found extensive applications for white-light-emitting diodes (LEDs) fabricated with a blue InGaN chip and yellow-emitting phosphor Y3Al5O12:Ce3+. The applications of these LEDs include lighting sources, automobile lamps, and backlighting.1-3 The drawback of this combination, however, is a deficiency in the red spectral region4,5 due to a poor color-rendering index (Ra) of 71,6 and a high correlated color temperature (CCT) of 7756 K.7 Single-composition whiteemitting phosphors for near-ultraviolet or ultraviolet excitations have attracted attention for solid state lighting. A comparison of InGaN-based blue chips with YAG:Ce3+ phosphors and a mixture of single-composition emission-tunable phosphors with near-ultraviolet or ultraviolet chip systems shows that the latter has many merits, such as a higher Ra, tunable CCT, and Commission International de I’Eclairage (CIE) chromaticity coordinates. Therefore, near-UV or ultraviolet LEDs with singlephased white-light phosphors can use the energy transfer from a sensitizer to an activator in the same host structure as an alternative to blue chip/YAG:Ce3+ based white-emitting systems, such as SrZn2(PO4)2:Eu2+,Mn2+,8 CaAl2Si2O8:Eu2+,Mn2+,9 Ca10K(PO4)7:Eu2+,Mn2+,10 Ca9Y(PO4)7:Eu2+,Mn2+,11 Ba3MgSi2O8: Eu2+,Mn2+,12 Ba2Ca(BO3)2:Ce3+,Mn2+,13 Sr3B2O6:Ce3+,Eu2+,14 CaGa2S4:Ce3+,Eu2+,15 Sr2LiSiO4F:Ce3+,Eu3+,16 and Ca2BO3Cl: Ce3+,Eu2+.17 This paper first investigates the luminescence properties and energy transfer mechanisms of the Eu2+/Mn2+ in Ca9Gd(PO4)7 host. This study demonstrates single composition white-emitting Ca9Gd(PO4)7:Eu2+,Mn2+ phosphors and the applications to nearUV LEDs. Finally, this study reports the fabrication and analysis of LED devices and the optical properties. 2. Experimental Section 2.1. Sample Preparation. Single-phased white-light phosphors (Ca0.993-xEu0.007Mnx)9Gd(PO4)7 (CGP:0.007Eu2+,xMn2+) * To whom correspondence should be addressed. E-mail: tmchen@ mail.nctu.edu.tw and
[email protected]. † National Chiao Tung University. ‡ Material and Chemical Research Laboratories, ITRI.
were prepared by a conventional solid state method. A series of CGP:0.007Eu2+,xMn2+ phosphors was prepared from a mixture of CaCO3 (A.R.), Gd2O3 (A.R.), (NH4)2HPO4 (A.R.), Eu2O3 (A.R.), and MnCO3 (A.R.) in the stoichiometric composition of 8.937-9x:1/2:7: 0.063/2:9x. The weighed powder was mixed in an agate mortar and placed in an alumina crucible. This crucible was heated at 1300 °C for 8 h under a reducing atmosphere of 15% H2/85% N2, and cooled slowly to room temperature. 2.2. Sample Characterization. The crystal structure and phase purity of CGP:Eu2+,xMn2+ phosphors were carefully checked by using powder X-ray diffraction (XRD) analysis (Bruker AXS D8) with Cu K radiation (λ ) 1.5418 Å) planes collected between 2θ ) 10° and 80° at 40 kV and 40 mA. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured with a Spex Fluorolog-3 spectrofluorometer (Instruments S.A., NJ, U.S.A.) equipped with a 450 W Xe light source and double excitation monochromators. The samples were excited under 45° incidence, and emitted fluorescence was detected with a Hamamatsu Photonics R928 type photomultiplier perpendicular to the excitation beam. The CIE chromaticity coordinates for all samples were measured with a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan). 3. Results and Discussion 3.1. Crystal Structure. Figure 1 shows the XRD patterns of Ca9Gd(PO4)7 (CGP), CGP:0.007Eu2+, and CGP:0.007Eu2+,0.1Mn2+ phosphors. All the diffraction peaks of the assynthesized samples were consistent with those of Ca9Y(PO4)7 JCPDS No. 46-0402 database.18 XRD results indicate that the structure of CGP host lattice was unchanged upon the doping of Eu2+ ions or of Eu2+/Mn2+ codoping. The Ca9Gd(PO4)7 sample exhibited the same crystal structure as the Ca9Y(PO4)7, namely, an iso-structure. The Ca9Y(PO4)7 has a rhombohedral crystal structure with a space group of R3c (no. 161), and Ca2+ ions have three different coordination numbers. Ca(1), Ca(2), and Ca(3) are two eight-coordinated and one nine-coordinated ions. The ionic radii for eight- and nine-coordinated Ca2+ are 1.12 and 1.18 Å. However, the ionic radii for eight- and nine-
10.1021/jp106693z 2010 American Chemical Society Published on Web 10/08/2010
White-Light Phosphors Ca9Gd(PO4)7:Eu2+,Mn2+
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Figure 1. Powder XRD patterns of Ca9Gd(PO4)7, (Ca0.993Eu0.007)9Gd(PO4)7, (Ca0.893Eu0.007Mn0.1)9Gd(PO4)7, and Ca9Y(PO4)7 standard pattern. Figure 3. CIE chromaticity diagram of CGP:0.007Eu2+,xMn2+ phosphors under 380 nm excitation: (1) x ) 0; (2) x ) 0.01; (3) x ) 0.02; (4) x ) 0.03; (5) x ) 0.05; (6) x ) 0.07; and (7) x ) 0.1. The insets show CGP:0.007Eu2+,xMn2+ phosphors irradiated under 365 nm UV lamp box.
Figure 2. (a) Spectral overlap between the Eu2+ PL spectrum (solid line) and the Mn2+ PLE spectrum (dash line); (b) the emission spectra of (Ca0.993-x)9Gd(PO4)7:0.007Eu2+,xMn2+ phosphors under near-UV 380 nm excitation.
coordinated Eu2+ are 1.25 and 1.3 Å, while that for eightcoordinated Mn2+ is 0.96 Å. These ionic radii results suggest that the Eu2+ and Mn2+ ions randomly occupy the Ca2+ ion sites in the CGP host. 3.2. Luminescence Properties. The significant spectral overlap in Figure 2a indicates that the CGP host may exhibit an energy transfer between sensitizer Eu2+ and activator Mn2+. The PL spectrum of CGP:Eu2+ (solid line) shows a broad band between 400 and 750 nm, which is typically because the 4f65d1 f 4f7 electronic dipole allows transitions of Eu2+ and the excitation peaks of CGP:Mn2+ contain several bands centered at 251, 340, 357, 407, and 502 nm (dash line). These lines correspond to the transitions from the 6A1(6S) ground state to the excited states 4T1(4P), 4E(4D), 4T2(4D), [4A1(4G), 4E(4G)], and 4T1(4G) levels.19,20 Figure 2b shows the emission spectra of CGP:0.007Eu2+,xMn2+ phosphors (x ) 0, 0.01, 0.02, 0.03, 0.05, 0.07, and 0.1) under near-UV 380 nm excitation. Doping 0.01 to 0.1 mol of Mn2+ in CGP:0.007Eu2+,xMn2+ phosphors generated blue-greenish and red emission bands centering at 494 nm (4f65d1 f 4f7 transition of Eu2+) and 652 nm (4T1(4G) f 6A1(6S) transition of Mn2+). The intensity of Eu2+ blue-green emission at 494 nm decreased as the Mn2+ content increased to x. The intensity of red emission at 652 nm increased as the Mn2+ content increased to x, reached a maximum at x ) 0.05 mol, and then decreased when x exceeded 0.05. The apparent
decrease in the PL intensity for Mn2+ with x > 0.05 is primarily due to the concentration quenching effect. Figure 3 shows the CIE chromaticity diagram of a singlephased emission-tunable phosphor CGP:0.007Eu2+,xMn2+ under 380 nm excitation. The chromaticity coordinates (x, y) were measured as (0.219, 0.371), (0.257, 0.360), (0.326, 0.328), (0.370, 0.334), (0.491, 0.325), (0.583, 0.311) eventually to (0.625, 0.307) for CGP:0.007Eu2+,xMn2+ phosphors with x ) 0, 0.01, 0.02, 0.03, 0.05, 0.07, and 0.1. These results indicate that changing the Mn2+ concentration can tune the color hue from blue-green (solely 0.007Eu2+, point 1) through white-light (0.007Eu2+/0.02Mn2+, point 3) and eventually to red (0.007Eu2+/ 0.1Mn2+, point 7) in the visible spectral region. The insets of Figure 3 show photographs of CGP:0.007Eu2+,xMn2+ phosphors with different Mn2+ contents in a 365 nm UV lamp box. 3.3. Energy Transfer Mechanism of Ca9Gd(PO4)7:Eu2+, Mn2+ Phosphors. According to Dexter’s energy transfer formula of multipolar interaction, the following relation can be obtained:21
IS0 ∝ CR/3 IS
(1)
where IS0 and IS are the luminescence intensities of the sensitizer Eu2+ with and without activator Mn2+ present, and C is then Mn2+ ion concentration. The plots of (IS0/IS) versus CR/3 with R ) 6, 8, and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions. Panels a-c of Figure 4 illustrate the relationships between (IS0/IS) versus CR/3, revealing a linear behavior only when R ) 8. This implies that the energy transfer from sensitizer Eu2+ to activator Mn2+ follows a nonradiative dipole-quadrupole mechanism, which is similar to the results of previous reports.8-10 3.4. Thermal Quenching and White-Emitting LED Packages by Near-UV Chip. For the application of high-power LEDs, the thermal stability of phosphor is one of the important issues. Temperature dependence of luminescence for CGP: Eu2+,Mn2+ under 380 nm excitation is shown in Figure 5. The activation energy (Ea) can be expressed by:
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Figure 6. The electroluminescent spectra of white LED lamps fabricated using a near-UV 380 nm chip combined with a white-emitting phosphor CGP:0.007Eu2+,0.02Mn2+ driven by a 350 mA current. Figure 4. Dependence of IS0/IS of Eu2+ on (a) C6/3, (b) C8/3, and (c) C10/3.
0.007Eu2+,0.02Mn2+ phosphor may have promising applications for white-light near-UV LEDs. 4. Conclusions
Figure 5. PL spectra of CGP:Eu2+,Mn2+ phosphor excited at 380 nm with different temperatures. The inset shows the normalized PL intensity as a function of temperatures.
()
ln
Ea Io ) ln A I kT
(1a)
where Io and I are luminescence intensity of CGP:Eu2+,Mn2+ phosphor (by integrating the area of etch spectrum) at room temperature and testing temperature, respectively; A is constant; and k is Boltzmann’s constant (8.617 × 10-5 eV/K). The Ea was obtained to be 0.1372 eV/K. The inset displayed the thermal quenching of Eu2+ and Mn2+ emission intensity in CGP. Above 100 °C, the normalized emission intensity of Eu2+ and Mn2+ was 0.6 and 0.87, respectively. The slight intensity decay indicates that CGP:Eu2+,Mn2+ phosphor could be applied for high-powder LED application. Figure 6 shows the electroluminescent spectrum of white LED lamps fabricated with a near-UV 380 nm chip combined with a single-phased white-emitting phosphor CGP:0.007Eu2+,0.02Mn2+ driven by a 350 mA current. The electroluminescent spectrum clearly shows three emission bands at 380, 490, and 652 nm due to the near-UV chip, Eu2+ emission, and Mn2+ emission. The white-light LED lamp package was fabricated by integrating a mixture of transparent silicon resin and whiteemitting CGP:0.007Eu2+,0.02Mn2+ phosphor dropped on a nearUV 380 nm chip. The optical properties of the white-light LED show a correlated color temperature of 6569 K and CIE color coordinates of (0.312, 0.327). The inset of Figure 6 shows a photograph of the white-light LED lamp under a forward bias of 350 mA. These results indicate that the CGP:
This study reports the synthesis of a novel single-phase whitelight-emitting near-UV LED phosphor Ca9Gd(PO4)7:Eu2+,Mn2+ using a conventional solid-state reaction. The energy transfer from sensitizer Eu2+ to activator Mn2+ in Ca9Gd(PO4)7 host was a resonant type via a nonradiative dipole-quadrupole mechanism. A white-light near-UV LED was fabricated by using a near-UV 380 nm chip pumped by a single-phase white-light Ca9Gd(PO4)7:0.007Eu2+,0.02Mn2+ phosphor driven by a 350 mA current, producing a white light with a correlated color temperature of 6569 K and color coordinates of (0.312, 0.327). These results indicate that Ca9Gd(PO4)7:Eu2+,Mn2+ is a promising single-composition phosphor for application involving whitelight near-UV LEDs. Acknowledgment. This research was supported by th Industrial Technology Research Institute (ITRI) under contract No. 9301XS1J31 and in part by the National Science Council of Taiwan under contract No. NSC98-2113-M-009-005-MY3 (T.-M.C.). Supporting Information Available: Materials and methods, table giving a comparison of CIE chromaticity coordinates, and figures showing SEM image, Particle size distribution, diffuse reflectance spectra, temperature dependence emission spectra, CIE coordinates, electroluminescence spectrum, and decay curves. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shur, M. S.; Zˇukauskas, A. Proc. IEEE 2005, 93, 1691. (2) Uchida, Y.; Taguchi, T. Opt. Eng. 2005, 44, 124003. (3) Wang, F.; Xue, X.; Liu, X. Angew. Chem., Int. Ed. 2008, 47, 906. (4) Setlur, A. A.; Heward, W. J.; Gao, Y.; Srivastava, A. M.; Chandran, R. G.; Shankar, M. V. Chem. Mater. 2006, 18, 3314. (5) Batentschuk, M.; Osvet, A.; Schierning, G.; Klier, A.; Schneider, J.; Winnacker, A. Radiat. Meas. 2004, 38, 539. (6) Jang, H. S.; Im, W. B.; Lee, D. C.; Jeon, D. Y.; Kim, S. S. J. Lumin. 2007, 126, 371. (7) Jang, H. S.; Won, Y. H.; Jeon, D. Y. Appl. Phys. B: Lasers Opt. 2009, 95, 715. (8) Yang, W. J.; Chen, T. M. Appl. Phys. Lett. 2006, 88, 101903. (9) Yang, W. J.; Luo, L.; Chen, T. M.; Wang, N. S. Chem. Mater. 2005, 17, 3883. (10) Liu, W. R.; Chiu, Y. C.; Yeh, Y. T.; Jang, S. M.; Chen, T. M. J. Electrochem. Soc. 2008, 156, J165.
White-Light Phosphors Ca9Gd(PO4)7:Eu2+,Mn2+ (11) Huang, C. H.; Chen, T. M.; Liu, W. R.; Chiu, Y. C.; Yeh, Y. T.; Jang, S. M. ACS Appl. Mater. Interfaces 2010, 2, 259. (12) Kim, J. S.; Lim, K. T.; Jeong, Y. S.; Jeon, P. E.; Choi, J. C.; Park, H. L. Solid State Commun. 2005, 135, 21. (13) Guo, C.; Luan, L.; Xu, Y.; Gao, F.; Liang, L. J. Electrochem. Soc. 2008, 155, J310. (14) Chang, C. K.; Chen, T. M. Appl. Phys. Lett. 2007, 91, 081902. (15) Najafov, H.; Kato, A.; Toyota, H.; Iwai, K.; Bayranov, A.; Iida, S. Jpn. J. Appl. Phys. 2002, 41, 1424.
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18701 (16) Sivakumar, V.; Varadaraju, U. V. J. Electrochem. Soc. 2009, 156, J179. (17) Xiao, F.; Xue, Y. N.; Zhang, Q. Y. Phys. B 2009, 404, 3743. (18) JCPDS No. 00-46-0402. (19) Huang, C. H.; Kuo, T. W.; Chen, T. M. ACS Appl. Mater. Interfaces 2010, 2, 1395. (20) Huang, C. H.; Chen, T. M. Opt. Express 2010, 18, 5089. (21) Dexter, D. L.; Schulman, J. H. J. Chem. Phys. 1954, 22, 1603.
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