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A reddish orange-emitting stoichiometric phosphor K3Eu(PO4)2 for white light-emitting diodes
Optics & Laser Technology 44 (2012) 39–42
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
A reddish orange-emitting stoichiometric phosphor K3Eu(PO4)2 for white light-emitting diodes Guifang Ju, Yihua Hu n, Li Chen, Xiaojuan Wang, Zhongfei Mu, Haoyi Wu, Fengwen Kang School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Waihuan Xi Road, No. 100, Guangzhou 510006, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 23 February 2011 Received in revised form 9 May 2011 Accepted 11 May 2011 Available online 17 June 2011
A series of Eu3 þ activated K3Y1 xEux(PO4)2 phosphors were synthesized by the solid-state reaction method. The structures and photoluminescent properties of these phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the monoclinic K3Y(PO4)2 or K3Eu(PO4)2. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370–410 nm) light. The orange emission from transition 5D0–7F1 is dominant, and the peak value ratio of 5D0–7F1/5D0–7F2 is 1.44. The emission spectra exhibit strong reddish orange performance (CIE chromaticity coordinates: x ¼ 0.63, y ¼0.36), which is due to the 5D0–7FJ transitions of Eu3 þ ions. The relationship between the structure and the photoluminescent properties of the phosphors was studied. The absence of concentration quenching of Eu3 þ was observed in K3Y1 xEux(PO4)2. K3Eu(PO4)2 has potential application as a phosphor for white light-emitting diodes. & 2011 Elsevier Ltd. All rights reserved.
Keywords: WLED Phosphate Stoichiometric phosphor
1. Introduction White light-emitting diodes (WLEDs) are considered as the third generation (3G) lighting sources because of their excellent properties such as low energy consumption, extremely long life, high durability and mercury free [1–4]. To obtain high brightness WLED, mainly blue emitting LED is used in combination with yellow phosphor to partially downconvert the blue emission to light with longer wavelengths. The yellow emitting YAG:Ce3 þ phosphor is often used [2,4,5]. However, WLEDs that adopt this scheme always suffer from the drawbacks such as halo effect due to the different emission characteristics of the LEDs (directional) and the phosphors (isotropic) [5], low color rendering index (CRI) and high color temperature caused by lack of red component in the spectra [1,2,5–8]. These problems can be solved by another combination scheme, i.e. near ultraviolet (NUV) LED (370–410 nm) with red, green and blue phosphors [1,2,5,7]. However, phosphors (in particular, red phosphors) with excitation spectra matching the NUV excitation are rare. Our research group recently focused on Eu3 þ activated red phosphors, since they have strong absorption bands at 370–410 nm (7F0–5L6,7,5GJ). Moreover, they exhibit a high lumen equivalent, quantum efficiency and stability at the same time. Unfortunately, in Russell–Saunders coupling electric-dipole transitions within the 4f configuration are weak (spin and parity forbidden), even though selection rules are partly lifted by spin– orbit interaction and crystal-field [9]. The intensity of emission depends on the absorption by the phosphor as well as on
n
Corresponding author. Tel.:þ 86 020 39322262; fax: þ86 020 39322265. E-mail address: [email protected] (Y. Hu).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.05.013
radiationless processes in the phosphor [10]. It is well known that high doping of Eu3 þ in a compound leads to strong absorption, whereas very heavy doping causes concentration quenching. This dilemma can be solved by stoichiometric phosphors (i.e. all the rare earth sites in host lattice are occupied by activators.) [11]. Actually, stoichiometric phosphors are not attractive before the invention of GaN LEDs, because strong excitation sources in the NUV or shortwave visible light region are not available. If the excitation level is higher than NUV (e.g. mercury line), the absorption of the phosphor may result from charge transfer band (CTB) or host absorption. These are all allowed transitions. The phosphates with general formula K3R(PO4)2 (R¼ Nd, Eu and Tb) show no concentration quenching of photoluminescence (Nd) [12] or cathodoluminescence (Eu and Tb) [13], despite the high concentration of luminescence centers. But there are no detailed reports on the photoluminescence properties of K3Eu(PO4)2 under NUV excitation. In the present paper, in order to study the concentration quenching of the phosphor, yttrium was introduced to dilute the content of europium. The structures of the prepared samples were characterized by X-ray diffraction (XRD). The main purpose of this work is to directly investigate the spectroscopic properties of K3Eu(PO4)2 under NUV excitation and possible applications.
2. Experimental 2.1. Synthesis The powder samples of K3Y1 xEux(PO4)2 (x¼ 0, 0.05, 0.1–1.0, in steps of 0.1) were prepared by the solid-state reaction technique at high temperature. The starting materials were K2CO3 (A.R.),
G. Ju et al. / Optics & Laser Technology 44 (2012) 39–42
3.1. XRD phase analysis The XRD patterns of K3Y1 xEux(PO4)2 (x ¼0, 0.5 and 1.0) are shown in Fig. 1. The analysis of XRD patterns confirms that the compounds were obtained as single phase. No extra peak related to the starting materials is observed. K3Y1 xEux(PO4)2 orthophosphates are found to be isotypic to monoclinic K3Y(PO4)2 (PDF#49-0497) or K3Eu(PO4)2 (PDF#49-0504) with space group P21/m (11), z ¼2 [12,14]. There is no detectable phase change within the whole range of Eu3 þ concentration. The radius of Eu3 þ ion is similar to that of Y3 þ ion [15]; therefore, Eu3 þ and Y3 þ can form a continuous solid solution in the host. The lattice parameters of K3Eu(PO4)2 were calculated as follows: a ¼0.9440 nm, b¼0.5622 nm, c¼0.7421 nm, and b ¼90.791. The Eu3 þ ions are connected through O–P–O linkages, and the shortest Eu3 þ –Eu3 þ distance in K3Eu(PO4)2 is about 0.5 nm [12]. 3.2. Photoluminescent properties of K3Y1 xEux(PO4)2 The phosphors K3Y1 xEux(PO4)2 with different doping concentration of Eu3 þ show similar excitation and emission spectra except for their intensities. Different monitor/excitation wavelengths do
10
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(e) PDF#49-0504 K3Eu (PO4)2
F0--5GJ;5L7
Intensity (a.u.)
(d) K3Eu (PO4)2
7 nm
7F -- 5D 0 4
7F -- 5H 0 6
Intensity (a.u.)
(b) K3Y (PO4)2
=591 nm
7
(c) K3Y0.5Eu0.5 (PO4)2
em
7F --5D 0 1 7F --5D 1 1
3. Results and discussion
7F --5D 0 2
The phase purity of the prepared phosphors was investigated by an X-ray diffractometer with Cu Ka radiation (wavelength¼0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The excitation and emission spectra of all the samples were measured by a Hitachi F-7000 Fluorescence Spectrophotometer at room temperature.
F0--5L6
2.2. Measurement
not change the shape and position of spectra either. The relative excitation spectrum of K3Eu(PO4)2 by monitoring 5D0–7F1 (lem ¼591 nm) emission is shown in Fig. 2. It can be seen clearly that the excitation spectrum contains a group of sharp peaks in the visible and ultraviolet region. The sharp peaks are ascribed to the intra-configurational 4f–4f transitions of Eu3 þ in the host lattice: 7 F0 to 5H6, 5D4, 5GJ, 5L7, 5L6, 5D3, 5D2, 5D1 and 7F1 to 5D1 at wavelengths 318, 360, 380, 382, 393, 414, 464, 524 and 533 nm, respectively [16]. At room temperature, the 7F1 level is thermally populated due to the small energy gap of 7F1 and 7F0 (370 cm 1), so the transition of 7F1–5D1 was observed at 533 nm. The strongest excitation band is at 368–410 nm (7F0–5L6,7,5GJ) and its full width at half maximum (FWHM) is about 18 nm, which matches the emission wavelengths of NUV LED chips. The Eu3 þ activated compounds K3Y1 xEux(PO4)2 show strong reddish orange emission under 393 nm excitation. The relative emission spectrum of K3Eu(PO4)2 under 393 nm excitation is given in Fig. 3. The emission spectrum is composed of groups of sharp peaks from the emission of Eu3 þ intra-configurational 4f–4f transitions (5D0–7F0–4) at 580, 591, 618, 655 and 705 nm, respectively. The vibrational energy of PO4 stretching modes is in the region 900–1500 cm 1 [17,18], therefore, no emission from higher Eu3 þ excited levels was detected, even at x ¼0.05, because of efficient multi-phonon relaxation in the phosphors. The spectral energy distribution of the various transitions depend strongly on the host lattice [9,19]. The rare earth ions in this compound locate at sites with Ci symmetry (center of inversion) [12,14]. The spectrum is dominated by the magnetic-dipole transitions from 5 D0 to the 7F1 manifold. This indicates that Eu3 þ ions occupy inversion symmetry sites, which is consistent with the point group Ci. In the case of Ci symmetry, transitions from 5D0 to 7 F0,2–4 are strictly forbidden by selection rules, so that no emission peak corresponding to these transitions is expected. The existence of forced electric-dipole transitions requires loss of inversion symmetry [9], i.e. there is a deviation from inversion symmetry. Magnetic-dipole transitions within the 4f configuration are only spin-forbidden (relaxed by spin–orbit interaction) [9], owing to crystal-field splitting for 7F1 level, three lines are expected for 5D0–7F1 transition. The crystal-field splitting components of 5D0–7F1–4 of Eu3 þ can be observed, but not totally resolved due to the poor resolution of our fluorescence spectrophotometer. The variation of integrated emission intensities of K3Y1 xEux(PO4)2 phosphors with Eu3 þ concentration (x) under 393 nm excitation are
7
(NH4)2HPO4 (A.R.), Y2O3 (4 N), Eu2O3 (4 N) and ethanol (A.R.). All raw materials with stoichiometric amounts were mixed homogeneously with small amounts of ethanol (to help grinding) in an agate mortar. The homogeneous mixture was put into a corundum crucible and calcined at 900 1C in air for 5 h.
7F --5D 0 3
40
(a) PDF#49-0497 K3Y (PO4)2 25 nm
10
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30
40 2θ (deg)
50
Fig. 1. XRD patterns of the samples.
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300
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400 450 Wavelength (nm)
500
Fig. 2. Excitation spectrum of K3Eu(PO4)2.
550
5
D0
Intensity (a.u.)
λex = 393 nm
7F 1
5
D0
7F 2
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Integrated intensity
G. Ju et al. / Optics & Laser Technology 44 (2012) 39–42
0.0 0.2 0.4 0.6 0.8 1.0 1.2
5D
0
7F 0
Eu3+ concentration (x)
500
550
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D0
600 650 Wavelength (nm)
7F 3
5D
0
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7F 4
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Fig. 3. Emission spectrum of K2Eu(PO4)2. Inset: photoluminescent intensity of K2Y1 xEux(PO4)2 depends on the concentration (x) of Eu3 þ .
Fig. 4. CIE color space chromaticity diagram of K3Eu(PO4)2 phosphor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5
D0
K3Eu (PO4)2
7F 1
Ca0.76MoO4:0.24Eu3+ 5D
0
7F 2
ex
=393 nm
Intensity (a.u.)
shown in the inset of Fig. 3. It can be seen that the luminescence intensity enhances with the increase in the Eu3þ doping ratio and reaches a maximum at x¼1.0, which indicates that concentration quenching does not take place in Eu3 þ -doped K3Y1 xEux(PO4)2 phosphors. The electric multipole interactions between Eu3þ ions are weak due to the weak oscillator strength of 7F025D0 [20], so only exchange interactions are of importance. However, the critical distance for exchange interactions between Eu3þ ions is about 0.5 nm [21], while the minimum Nd–Nd distance in K3Nd(PO4)2 is about 0.5 nm [12], we suppose that in the case of K3Eu(PO4)2 it is similar. Thus, energy migration over Eu3 þ sublattice cannot take place effectively. Fig. 4 presents the CIE 1931 color space chromaticity diagram to illustrate the chromaticity of K3Eu(PO4)2 phosphor. The CIE coordinates of K3Eu(PO4)2 were measured as x¼0.63, y¼0.36. Their corresponding location has been marked in Fig. 4 with a white star. The CIE coordinates of K3Eu(PO4)2 are in the deep reddish orange area. In the last few years, Eu3 þ activated molybdates, tungstates luminescent materials have been extensively researched [2]. Ca0.76MoO4:0.24 Eu3 þ was also prepared according to the method of Ref. [22]. Fig. 5 gives the emission spectra of the K3Eu(PO4)2 and Ca0.76MoO4:0.24 Eu3 þ . The integrated emission intensity of K3Eu(PO4)2 under 393 nm excitation is about 6.5 times greater than Ca0.76MoO4:0.24 Eu3 þ . In the case of Ca0.76MoO4:0.24 Eu3 þ , the red emission from transition 5D0–7F2 is dominant, which indicates that Eu3 þ ions occupy non-inversion symmetry sites. The 5D0–7F2 emission peak value of K3Eu(PO4)2 is about three times higher than Ca0.76MoO4:0.24 Eu3 þ , even though the transition is parity forbidden in K3Eu(PO4)2.
550
600
650
700
750
Wavelength (nm) Fig. 5. Emission spectra of K3Eu(PO4)2 and Ca0.76MoO4:0.24 Eu3 þ .
reddish orange emission. The CIE chromaticity coordinates were calculated to be (x¼0.63, y¼0.36), which are close to the NTSC standard values (x¼0.67, y¼0.33). The results indicate that the phosphor K3Eu(PO4)2 might find a possible application on NUV InGaN chipbased WLEDs.
4. Conclusions Stoichiometric phosphor K3Eu(PO4)2 was synthesized by the solidstate reaction method, and its photoluminescent properties were investigated under NUV light excitation. The excitation spectrum of K3Eu(PO4)2 consists a strong quasi-broad band at 370–410 nm, corresponding to the transitions 7F0–5L6,7,5GJ. The emission spectrum consists of two groups of strong sharp peaks at about 591 and 618 nm, corresponding to magnetic-dipole transition 5D0–7F1 (M) and electric-dipole transition 5D0–7F2 (E). In our prepared sample K3Eu(PO4)2, the peak value ratio (i.e. M/E) is 1.44, and exhibits intense
Acknowledgments This work is supported by the National Natural Science Foundation of China (nos. 21071034, 20871033). References [1] Pimputkar S, Speck JS, DenBaars SP, Nakamura S. Prospects for LED lighting. Nat Photon 2009;3:180–2.
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[2] Ye S, Xiao F, Pan YX, Ma YY, Zhang QY. Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties. Mater Sci Eng R 2010;71:1–34. [3] Murata T, Tanoue T, Iwasaki M, Morinaga K, Hase T. Fluorescence properties of Mn4 þ in CaAl12O19 compounds as red-emitting phosphor for white LED. J Lumin 2005;114:207–12. [4] Smet PF, Korthout K, Van Haecke JE, Poelman D. Using rare earth doped thiosilicate phosphors in white light emitting LEDs: towards low colour temperature and high colour rendering. Mater Sci Eng B 2008;146:264–8. [5] Shen C, Li K. High color rendering index WLED based on YAG:Ce phosphor and CdS/ZnS core/shell quantum dots. Proc SPIE 2009;7516:751609. [6] Jang HS, Im WB, Lee DC, Jeon DY, Kim SS. Enhancement of red spectral emission intensity of Y3Al5O12:Ce3 þ phosphor via Pr co-doping and Tb substitution for the application to white LEDs. J Lumin 2007;126:371–7. [7] Li X, Guan L, Li X, Wen J, Yang Z. Luminescent properties of NaBaPO4:Eu3 þ red-emitting phosphor for white light-emitting diodes. Powder Technol 2010;200:12–5. [8] Guo Y, Sun M, Guo W, Ren F, Chen D. Luminescent properties of Eu3 þ activated tungstate based novel red-emitting phosphors. Opt Laser Technol 2010;42:1328–31. [9] Blasse G, Bril A, Nieuwpoort C. On the Eu3 þ fluorescence in mixed metal oxides, part I—the crystal structure sensitivity of the intensity ratio of electric and magnetic dipole emission. J Phys Chem Solids 1966;27:1587–92. [10] Blasse G. On the Eu3 þ fluorescence in mixed metal oxides, IV. The photoluminescent efficiency of Eu3 þ -activated oxides. J Chem Phys 1966;45:2356–60. [11] Kano T. In: Yen WM, Shionoya S, Yamamoto H, editors. Phosphor handbook. Boca Raton, Florida: CRC Press; 2006. p. 203. [12] Hong HY-P, Chinn SR. Crystal structure and fluorescence lifetime of potassium neodymium orthophosphate, K3Nd(PO4)2, a new laser material. Mater Res Bull 1976;11:421–8.
[13] Kharsika VF, Komissarova LN, Kirichenko AN, Murav’ev EN, Orlovskii VP, Chernyaev AP. Luminescence spectra of Eu3 þ -sctivated potassium lanthanum and potassium gadolinium phosphate vanadates. Inorg Mater 2001;37: 963–7. [14] Jungowska-Hornowska W, Macalik L, Lisiecki R, Godlewska P, Matraszek A, Szczygie" I, et al. Luminescence and vibrational characteristics of the submicro crystals of lanthanum orthophosphates and metaphosphates codoped with Er3 þ and Yb3 þ ions. Mater Chem Phys 2009;117:262–7. [15] Shannon RD. Revised effective ionic radii and systematic studies of interatomie distances in halides and chalcogenides. Acta Crystallogr A 1976;32: 751–67. [16] Li Y, Chang Y, Lin Y, Chang Y, Lin Y. Synthesis and luminescent properties of Ln3 þ (Eu3 þ , Sm3 þ , Dy3 þ )-doped lanthanum aluminum germanate LaAlGe2O7 phosphors. J Alloys Compd 2007;439:367–75. ¨ a¨ J, et al. [17] Szuszkiewicz W, Keller B, Guzik M, Aitasalo T, Niittykoski J, Hols Application of lanthanide (Eu, Nd) spectroscopy as a structural probe of diluted double phosphates. J Alloys Compd 2002;341:297–306. [18] Schwarz L, Kloss M, Rohmann A, Sasum U, Haberlandl D. Investigations of alkaline rare earth orthophosphates M3RE(PO4)2. J Alloys Compd 1998; 275–277:93–5. [19] Blasse G, Bril A. Fluorescence of Eu3 þ -activated sodium lanthanide titanates (NaLn1 xEuxTiO4). J Chem Phys 1968;48:3652–6. [20] Carnall WT, Fields PR, Rajnak K. Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3 þ , Sm3 þ , Eu3 þ , Gd3 þ , Tb3 þ , Dy3 þ and Ho3. J Chem Phys 1968;49:4412–23. [21] Blasse G. Luminescence of inorganic solids: from isolated centers to concentrated systems. Prog Solid State Chem 1988;18:79–171. [22] Liu J, Lian H, Shi C. Improved optical photoluminescence by charge compensation in the phosphor system CaMoO4:Eu3 þ . Opt Mater 2007;29:1591–4.
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
A reddish orange-emitting stoichiometric phosphor K3Eu(PO4)2 for white light-emitting diodes Guifang Ju, Yihua Hu n, Li Chen, Xiaojuan Wang, Zhongfei Mu, Haoyi Wu, Fengwen Kang School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Waihuan Xi Road, No. 100, Guangzhou 510006, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 23 February 2011 Received in revised form 9 May 2011 Accepted 11 May 2011 Available online 17 June 2011
A series of Eu3 þ activated K3Y1 xEux(PO4)2 phosphors were synthesized by the solid-state reaction method. The structures and photoluminescent properties of these phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the monoclinic K3Y(PO4)2 or K3Eu(PO4)2. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370–410 nm) light. The orange emission from transition 5D0–7F1 is dominant, and the peak value ratio of 5D0–7F1/5D0–7F2 is 1.44. The emission spectra exhibit strong reddish orange performance (CIE chromaticity coordinates: x ¼ 0.63, y ¼0.36), which is due to the 5D0–7FJ transitions of Eu3 þ ions. The relationship between the structure and the photoluminescent properties of the phosphors was studied. The absence of concentration quenching of Eu3 þ was observed in K3Y1 xEux(PO4)2. K3Eu(PO4)2 has potential application as a phosphor for white light-emitting diodes. & 2011 Elsevier Ltd. All rights reserved.
Keywords: WLED Phosphate Stoichiometric phosphor
1. Introduction White light-emitting diodes (WLEDs) are considered as the third generation (3G) lighting sources because of their excellent properties such as low energy consumption, extremely long life, high durability and mercury free [1–4]. To obtain high brightness WLED, mainly blue emitting LED is used in combination with yellow phosphor to partially downconvert the blue emission to light with longer wavelengths. The yellow emitting YAG:Ce3 þ phosphor is often used [2,4,5]. However, WLEDs that adopt this scheme always suffer from the drawbacks such as halo effect due to the different emission characteristics of the LEDs (directional) and the phosphors (isotropic) [5], low color rendering index (CRI) and high color temperature caused by lack of red component in the spectra [1,2,5–8]. These problems can be solved by another combination scheme, i.e. near ultraviolet (NUV) LED (370–410 nm) with red, green and blue phosphors [1,2,5,7]. However, phosphors (in particular, red phosphors) with excitation spectra matching the NUV excitation are rare. Our research group recently focused on Eu3 þ activated red phosphors, since they have strong absorption bands at 370–410 nm (7F0–5L6,7,5GJ). Moreover, they exhibit a high lumen equivalent, quantum efficiency and stability at the same time. Unfortunately, in Russell–Saunders coupling electric-dipole transitions within the 4f configuration are weak (spin and parity forbidden), even though selection rules are partly lifted by spin– orbit interaction and crystal-field [9]. The intensity of emission depends on the absorption by the phosphor as well as on
n
Corresponding author. Tel.:þ 86 020 39322262; fax: þ86 020 39322265. E-mail address: [email protected] (Y. Hu).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.05.013
radiationless processes in the phosphor [10]. It is well known that high doping of Eu3 þ in a compound leads to strong absorption, whereas very heavy doping causes concentration quenching. This dilemma can be solved by stoichiometric phosphors (i.e. all the rare earth sites in host lattice are occupied by activators.) [11]. Actually, stoichiometric phosphors are not attractive before the invention of GaN LEDs, because strong excitation sources in the NUV or shortwave visible light region are not available. If the excitation level is higher than NUV (e.g. mercury line), the absorption of the phosphor may result from charge transfer band (CTB) or host absorption. These are all allowed transitions. The phosphates with general formula K3R(PO4)2 (R¼ Nd, Eu and Tb) show no concentration quenching of photoluminescence (Nd) [12] or cathodoluminescence (Eu and Tb) [13], despite the high concentration of luminescence centers. But there are no detailed reports on the photoluminescence properties of K3Eu(PO4)2 under NUV excitation. In the present paper, in order to study the concentration quenching of the phosphor, yttrium was introduced to dilute the content of europium. The structures of the prepared samples were characterized by X-ray diffraction (XRD). The main purpose of this work is to directly investigate the spectroscopic properties of K3Eu(PO4)2 under NUV excitation and possible applications.
2. Experimental 2.1. Synthesis The powder samples of K3Y1 xEux(PO4)2 (x¼ 0, 0.05, 0.1–1.0, in steps of 0.1) were prepared by the solid-state reaction technique at high temperature. The starting materials were K2CO3 (A.R.),
G. Ju et al. / Optics & Laser Technology 44 (2012) 39–42
3.1. XRD phase analysis The XRD patterns of K3Y1 xEux(PO4)2 (x ¼0, 0.5 and 1.0) are shown in Fig. 1. The analysis of XRD patterns confirms that the compounds were obtained as single phase. No extra peak related to the starting materials is observed. K3Y1 xEux(PO4)2 orthophosphates are found to be isotypic to monoclinic K3Y(PO4)2 (PDF#49-0497) or K3Eu(PO4)2 (PDF#49-0504) with space group P21/m (11), z ¼2 [12,14]. There is no detectable phase change within the whole range of Eu3 þ concentration. The radius of Eu3 þ ion is similar to that of Y3 þ ion [15]; therefore, Eu3 þ and Y3 þ can form a continuous solid solution in the host. The lattice parameters of K3Eu(PO4)2 were calculated as follows: a ¼0.9440 nm, b¼0.5622 nm, c¼0.7421 nm, and b ¼90.791. The Eu3 þ ions are connected through O–P–O linkages, and the shortest Eu3 þ –Eu3 þ distance in K3Eu(PO4)2 is about 0.5 nm [12]. 3.2. Photoluminescent properties of K3Y1 xEux(PO4)2 The phosphors K3Y1 xEux(PO4)2 with different doping concentration of Eu3 þ show similar excitation and emission spectra except for their intensities. Different monitor/excitation wavelengths do
10
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50
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(e) PDF#49-0504 K3Eu (PO4)2
F0--5GJ;5L7
Intensity (a.u.)
(d) K3Eu (PO4)2
7 nm
7F -- 5D 0 4
7F -- 5H 0 6
Intensity (a.u.)
(b) K3Y (PO4)2
=591 nm
7
(c) K3Y0.5Eu0.5 (PO4)2
em
7F --5D 0 1 7F --5D 1 1
3. Results and discussion
7F --5D 0 2
The phase purity of the prepared phosphors was investigated by an X-ray diffractometer with Cu Ka radiation (wavelength¼0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The excitation and emission spectra of all the samples were measured by a Hitachi F-7000 Fluorescence Spectrophotometer at room temperature.
F0--5L6
2.2. Measurement
not change the shape and position of spectra either. The relative excitation spectrum of K3Eu(PO4)2 by monitoring 5D0–7F1 (lem ¼591 nm) emission is shown in Fig. 2. It can be seen clearly that the excitation spectrum contains a group of sharp peaks in the visible and ultraviolet region. The sharp peaks are ascribed to the intra-configurational 4f–4f transitions of Eu3 þ in the host lattice: 7 F0 to 5H6, 5D4, 5GJ, 5L7, 5L6, 5D3, 5D2, 5D1 and 7F1 to 5D1 at wavelengths 318, 360, 380, 382, 393, 414, 464, 524 and 533 nm, respectively [16]. At room temperature, the 7F1 level is thermally populated due to the small energy gap of 7F1 and 7F0 (370 cm 1), so the transition of 7F1–5D1 was observed at 533 nm. The strongest excitation band is at 368–410 nm (7F0–5L6,7,5GJ) and its full width at half maximum (FWHM) is about 18 nm, which matches the emission wavelengths of NUV LED chips. The Eu3 þ activated compounds K3Y1 xEux(PO4)2 show strong reddish orange emission under 393 nm excitation. The relative emission spectrum of K3Eu(PO4)2 under 393 nm excitation is given in Fig. 3. The emission spectrum is composed of groups of sharp peaks from the emission of Eu3 þ intra-configurational 4f–4f transitions (5D0–7F0–4) at 580, 591, 618, 655 and 705 nm, respectively. The vibrational energy of PO4 stretching modes is in the region 900–1500 cm 1 [17,18], therefore, no emission from higher Eu3 þ excited levels was detected, even at x ¼0.05, because of efficient multi-phonon relaxation in the phosphors. The spectral energy distribution of the various transitions depend strongly on the host lattice [9,19]. The rare earth ions in this compound locate at sites with Ci symmetry (center of inversion) [12,14]. The spectrum is dominated by the magnetic-dipole transitions from 5 D0 to the 7F1 manifold. This indicates that Eu3 þ ions occupy inversion symmetry sites, which is consistent with the point group Ci. In the case of Ci symmetry, transitions from 5D0 to 7 F0,2–4 are strictly forbidden by selection rules, so that no emission peak corresponding to these transitions is expected. The existence of forced electric-dipole transitions requires loss of inversion symmetry [9], i.e. there is a deviation from inversion symmetry. Magnetic-dipole transitions within the 4f configuration are only spin-forbidden (relaxed by spin–orbit interaction) [9], owing to crystal-field splitting for 7F1 level, three lines are expected for 5D0–7F1 transition. The crystal-field splitting components of 5D0–7F1–4 of Eu3 þ can be observed, but not totally resolved due to the poor resolution of our fluorescence spectrophotometer. The variation of integrated emission intensities of K3Y1 xEux(PO4)2 phosphors with Eu3 þ concentration (x) under 393 nm excitation are
7
(NH4)2HPO4 (A.R.), Y2O3 (4 N), Eu2O3 (4 N) and ethanol (A.R.). All raw materials with stoichiometric amounts were mixed homogeneously with small amounts of ethanol (to help grinding) in an agate mortar. The homogeneous mixture was put into a corundum crucible and calcined at 900 1C in air for 5 h.
7F --5D 0 3
40
(a) PDF#49-0497 K3Y (PO4)2 25 nm
10
20
30
40 2θ (deg)
50
Fig. 1. XRD patterns of the samples.
60
300
350
400 450 Wavelength (nm)
500
Fig. 2. Excitation spectrum of K3Eu(PO4)2.
550
5
D0
Intensity (a.u.)
λex = 393 nm
7F 1
5
D0
7F 2
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Integrated intensity
G. Ju et al. / Optics & Laser Technology 44 (2012) 39–42
0.0 0.2 0.4 0.6 0.8 1.0 1.2
5D
0
7F 0
Eu3+ concentration (x)
500
550
5
D0
600 650 Wavelength (nm)
7F 3
5D
0
700
7F 4
750
Fig. 3. Emission spectrum of K2Eu(PO4)2. Inset: photoluminescent intensity of K2Y1 xEux(PO4)2 depends on the concentration (x) of Eu3 þ .
Fig. 4. CIE color space chromaticity diagram of K3Eu(PO4)2 phosphor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5
D0
K3Eu (PO4)2
7F 1
Ca0.76MoO4:0.24Eu3+ 5D
0
7F 2
ex
=393 nm
Intensity (a.u.)
shown in the inset of Fig. 3. It can be seen that the luminescence intensity enhances with the increase in the Eu3þ doping ratio and reaches a maximum at x¼1.0, which indicates that concentration quenching does not take place in Eu3 þ -doped K3Y1 xEux(PO4)2 phosphors. The electric multipole interactions between Eu3þ ions are weak due to the weak oscillator strength of 7F025D0 [20], so only exchange interactions are of importance. However, the critical distance for exchange interactions between Eu3þ ions is about 0.5 nm [21], while the minimum Nd–Nd distance in K3Nd(PO4)2 is about 0.5 nm [12], we suppose that in the case of K3Eu(PO4)2 it is similar. Thus, energy migration over Eu3 þ sublattice cannot take place effectively. Fig. 4 presents the CIE 1931 color space chromaticity diagram to illustrate the chromaticity of K3Eu(PO4)2 phosphor. The CIE coordinates of K3Eu(PO4)2 were measured as x¼0.63, y¼0.36. Their corresponding location has been marked in Fig. 4 with a white star. The CIE coordinates of K3Eu(PO4)2 are in the deep reddish orange area. In the last few years, Eu3 þ activated molybdates, tungstates luminescent materials have been extensively researched [2]. Ca0.76MoO4:0.24 Eu3 þ was also prepared according to the method of Ref. [22]. Fig. 5 gives the emission spectra of the K3Eu(PO4)2 and Ca0.76MoO4:0.24 Eu3 þ . The integrated emission intensity of K3Eu(PO4)2 under 393 nm excitation is about 6.5 times greater than Ca0.76MoO4:0.24 Eu3 þ . In the case of Ca0.76MoO4:0.24 Eu3 þ , the red emission from transition 5D0–7F2 is dominant, which indicates that Eu3 þ ions occupy non-inversion symmetry sites. The 5D0–7F2 emission peak value of K3Eu(PO4)2 is about three times higher than Ca0.76MoO4:0.24 Eu3 þ , even though the transition is parity forbidden in K3Eu(PO4)2.
550
600
650
700
750
Wavelength (nm) Fig. 5. Emission spectra of K3Eu(PO4)2 and Ca0.76MoO4:0.24 Eu3 þ .
reddish orange emission. The CIE chromaticity coordinates were calculated to be (x¼0.63, y¼0.36), which are close to the NTSC standard values (x¼0.67, y¼0.33). The results indicate that the phosphor K3Eu(PO4)2 might find a possible application on NUV InGaN chipbased WLEDs.
4. Conclusions Stoichiometric phosphor K3Eu(PO4)2 was synthesized by the solidstate reaction method, and its photoluminescent properties were investigated under NUV light excitation. The excitation spectrum of K3Eu(PO4)2 consists a strong quasi-broad band at 370–410 nm, corresponding to the transitions 7F0–5L6,7,5GJ. The emission spectrum consists of two groups of strong sharp peaks at about 591 and 618 nm, corresponding to magnetic-dipole transition 5D0–7F1 (M) and electric-dipole transition 5D0–7F2 (E). In our prepared sample K3Eu(PO4)2, the peak value ratio (i.e. M/E) is 1.44, and exhibits intense
Acknowledgments This work is supported by the National Natural Science Foundation of China (nos. 21071034, 20871033). References [1] Pimputkar S, Speck JS, DenBaars SP, Nakamura S. Prospects for LED lighting. Nat Photon 2009;3:180–2.
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[2] Ye S, Xiao F, Pan YX, Ma YY, Zhang QY. Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties. Mater Sci Eng R 2010;71:1–34. [3] Murata T, Tanoue T, Iwasaki M, Morinaga K, Hase T. Fluorescence properties of Mn4 þ in CaAl12O19 compounds as red-emitting phosphor for white LED. J Lumin 2005;114:207–12. [4] Smet PF, Korthout K, Van Haecke JE, Poelman D. Using rare earth doped thiosilicate phosphors in white light emitting LEDs: towards low colour temperature and high colour rendering. Mater Sci Eng B 2008;146:264–8. [5] Shen C, Li K. High color rendering index WLED based on YAG:Ce phosphor and CdS/ZnS core/shell quantum dots. Proc SPIE 2009;7516:751609. [6] Jang HS, Im WB, Lee DC, Jeon DY, Kim SS. Enhancement of red spectral emission intensity of Y3Al5O12:Ce3 þ phosphor via Pr co-doping and Tb substitution for the application to white LEDs. J Lumin 2007;126:371–7. [7] Li X, Guan L, Li X, Wen J, Yang Z. Luminescent properties of NaBaPO4:Eu3 þ red-emitting phosphor for white light-emitting diodes. Powder Technol 2010;200:12–5. [8] Guo Y, Sun M, Guo W, Ren F, Chen D. Luminescent properties of Eu3 þ activated tungstate based novel red-emitting phosphors. Opt Laser Technol 2010;42:1328–31. [9] Blasse G, Bril A, Nieuwpoort C. On the Eu3 þ fluorescence in mixed metal oxides, part I—the crystal structure sensitivity of the intensity ratio of electric and magnetic dipole emission. J Phys Chem Solids 1966;27:1587–92. [10] Blasse G. On the Eu3 þ fluorescence in mixed metal oxides, IV. The photoluminescent efficiency of Eu3 þ -activated oxides. J Chem Phys 1966;45:2356–60. [11] Kano T. In: Yen WM, Shionoya S, Yamamoto H, editors. Phosphor handbook. Boca Raton, Florida: CRC Press; 2006. p. 203. [12] Hong HY-P, Chinn SR. Crystal structure and fluorescence lifetime of potassium neodymium orthophosphate, K3Nd(PO4)2, a new laser material. Mater Res Bull 1976;11:421–8.
[13] Kharsika VF, Komissarova LN, Kirichenko AN, Murav’ev EN, Orlovskii VP, Chernyaev AP. Luminescence spectra of Eu3 þ -sctivated potassium lanthanum and potassium gadolinium phosphate vanadates. Inorg Mater 2001;37: 963–7. [14] Jungowska-Hornowska W, Macalik L, Lisiecki R, Godlewska P, Matraszek A, Szczygie" I, et al. Luminescence and vibrational characteristics of the submicro crystals of lanthanum orthophosphates and metaphosphates codoped with Er3 þ and Yb3 þ ions. Mater Chem Phys 2009;117:262–7. [15] Shannon RD. Revised effective ionic radii and systematic studies of interatomie distances in halides and chalcogenides. Acta Crystallogr A 1976;32: 751–67. [16] Li Y, Chang Y, Lin Y, Chang Y, Lin Y. Synthesis and luminescent properties of Ln3 þ (Eu3 þ , Sm3 þ , Dy3 þ )-doped lanthanum aluminum germanate LaAlGe2O7 phosphors. J Alloys Compd 2007;439:367–75. ¨ a¨ J, et al. [17] Szuszkiewicz W, Keller B, Guzik M, Aitasalo T, Niittykoski J, Hols Application of lanthanide (Eu, Nd) spectroscopy as a structural probe of diluted double phosphates. J Alloys Compd 2002;341:297–306. [18] Schwarz L, Kloss M, Rohmann A, Sasum U, Haberlandl D. Investigations of alkaline rare earth orthophosphates M3RE(PO4)2. J Alloys Compd 1998; 275–277:93–5. [19] Blasse G, Bril A. Fluorescence of Eu3 þ -activated sodium lanthanide titanates (NaLn1 xEuxTiO4). J Chem Phys 1968;48:3652–6. [20] Carnall WT, Fields PR, Rajnak K. Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3 þ , Sm3 þ , Eu3 þ , Gd3 þ , Tb3 þ , Dy3 þ and Ho3. J Chem Phys 1968;49:4412–23. [21] Blasse G. Luminescence of inorganic solids: from isolated centers to concentrated systems. Prog Solid State Chem 1988;18:79–171. [22] Liu J, Lian H, Shi C. Improved optical photoluminescence by charge compensation in the phosphor system CaMoO4:Eu3 þ . Opt Mater 2007;29:1591–4.