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Synthesis and luminescent properties of high brightness MRE(MoO4)2:Eu3+ (M = Li, Na, K; RE = Gd, Y, Lu) red phosphors for white LEDs
Solid State Sciences 29 (2014) 58e65
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Synthesis and luminescent properties of high brightness MRE(MoO4)2:Eu3þ (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) red phosphors for white LEDs Linlin Li a, Junjun Zhang a, Wenwen Zi a, Shucai Gan a, *, Guijuan Ji a, Haifeng Zou a, Xuechun Xu b a b
College of Chemistry, Jilin University, Changchun 130026, PR China College of Earth Sciences, Jilin University, Changchun 130026, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2013 Received in revised form 6 January 2014 Accepted 12 January 2014 Available online 22 January 2014
A series of Eu3þ-activated double molybdate phosphors MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) have been successfully prepared via the conventional solid-state reaction method. The effects of alkali cations and rare earth ions on the luminescence of MRE(MoO4)2:Eu3þ were investigated in detail. The experimental results show that the emission intensity was found to decrease with increasing the size of alkali cations or decreasing the size of rare earth ions. Under 393 nm light excitation, all compounds exhibited strong red emission at about 613 nm due to 5D0 / 7F2 transition of Eu3þ. Compared with the commercially available red phosphor Y2O3:Eu3þ, the emission intensity of the obtained phosphors is much stronger than that of Y2O3:Eu3þ. Additionally, the excitation spectra of these phosphors implied that the MRE(MoO4)2:Eu3þ phosphors can absorb not only the emission of near UV-LED chips but also that of blue LED chips. The CIE chromaticity coordinates are close to the National Television Standard Committee (NTSC) standard CIE chromaticity coordinate values for red (0.67, 0.33). All the results indicate that these phosphors are promising red-emitting phosphors pumped by near-UV or blue light. Ó 2014 Elsevier Masson SAS. All rights reserved.
Keywords: White LEDs Luminescence Phosphor Rare earth ions
1. Introduction In recent years, white light-emitting diodes (LEDs) as a more preferable replacement for conventional incandescent and fluorescent lamps have attracted more attentions. White LEDs have the advantages of high luminous efficiency, energy-saving, reliability, safety, fast response, environmental-friendly and so on [1,2]. At present, the most common method to achieve white light is based on the combination of an InGaN blue LED chip with a yellowemitting phosphor YAG:Ce3þ [3]. However, several drawbacks appear in practical application, including low luminous efficiency and poor color-rendering index due to the deficiency of red emission [4,5]. Another promising method to obtain white light is using the red, green, and blue light-emitting phosphors to convert the radiation of near UV-LED chips, which is also a current focus in the material and luminescence research [6]. However, the main tricolor phosphors for near UV-LED chips are still some classic phosphors, such as ZnS:(Cuþ, Al3þ) for green, BaMgAl10O17:Eu2þ for blue, and
* Corresponding author. Tel.: þ86 431 88502259. E-mail address: [email protected] (S. Gan). 1293-2558/$ e see front matter Ó 2014 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2014.01.003
Y2O2S:Eu3þ for red [7]. Among these phosphors, the efficiency of Y2O2S:Eu3þ is much lower than that of green and blue phosphors. In addition, the lifetime of Y2O2S:Eu3þ is inadequate under UV irradiation and the phosphor is instable due to releasing of sulfide gas [8,9]. Therefore, in order to improve the stability and efficiency of the phosphor, it is imperative to research and design a new family of red phosphors with high absorption in the near UV or blue region for white LEDs. In search of the efficient red-emitting phosphors, the Eu2þactivated nitride phosphors and Eu3þ-activated double molybdate phosphors have drawn much interest. The Eu2þ-activated nitride phosphors usually have high quantum efficiency, such as CaAlSiN3:Eu2þ and Sr2Si5N8:Eu2þ [10,11], but they were prepared via a complex high-temperature synthetic method. Moreover, as the 4f-5d transition of Eu2þ ion is sensitive to the crystal field and covalency, the Eu2þ doped phosphors exhibit broad emission bands covering the color from blue to red and have low color purity. While the Eu3þ-activated double molybdate phosphors have attracted particular interest, because of their low synthesis temperature, intense red emission, excellent thermal and chemical stability [12e 14]. There are two obvious emission peaks between 550 and 650 nm that ascribed to the 5D0 / 7F1 (w596 nm) and 5D0 / 7F2
L. Li et al. / Solid State Sciences 29 (2014) 58e65
(w613 nm) transitions for Eu3þ ions. It is well known that the magnetic dipole transition 5D0 / 7F1 is insensitive to the site symmetry, because they are parity-allowed. However, the 5 D0 / 7F2 transition belongs to the electric dipole transition due to an admixture of opposite parity 4fn15d states by an odd parity crystal-field component. In addition, 5D0 / 7F2 transition (DJ ¼ 2) is a hypersensitive transition, and the intensity can vary by orders of magnitude, depending on the local environment. When Eu3þ is located at a low symmetry site (without inversion symmetry), it will emit intense red light due to the enhanced 5D0 / 7F2 transition [15]. MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) belong to the family of double molybdate compounds and it also assigned to be isostructural to the CaMoO4. In this crystal, alkali ions and rare earth ions are randomly distributed over the same site of Ca2þ (without an inversion center) in the CaMoO4 host [16]. The random distribution of rare earth ions induces the inhomogeneous broadening of optical spectra when Eu3þ ions are doped in the crystals and occupy the positions of RE3þ [17]. Moreover, MoO2 4 group has strong absorption in the ultraviolet region, so the energy transfer process from MoO2 group to rare-earth ions can easily occur, 4 which can greatly enhance the external quantum efficiency of rareearth ions doped materials [18]. Based on the above investigations and motivated by the attempt to develop efficient red-emitting phosphors excited by near-UV or blue radiation for the application of white LEDs, a series of Eu3þactivated double molybdate phosphors MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) were prepared via solid-state reaction. The effects of alkali cations and rare earth ions on the luminescence of MRE(MoO4)2:Eu3þ were also investigated in this paper.
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2. Experimental section The samples were synthesized by the solid-state reaction technique. The starting materials included M2CO3 (M ¼ Li, Na, K) (A.R. grade), MoO3 (A.R. grade), RE2O3 (RE ¼ Gd, Y, Lu, Eu) (99.99%). For the synthesis of NaY0.85(MoO4)2:0.15Eu3þ, 1.0 mmol Na2CO3, 4.0 mmol MoO3, 0.85 mmol Y2O3 and 0.15 mmol Eu2O3 were weighed and thoroughly mixed in an agate mortar by grinding for 3 h. Then the mixture was transferred to alumina crucible and annealed successively at 900 C for 4 h in air. After that, the alumina crucible was allowed to cool down to room temperature naturally. The sintered cake was grounded, and then the NaY0.85(MoO4)2:0.15Eu3þ phosphor was obtained. The other samples were prepared in a similar manner except using different raw materials. The samples were examined by powder X-ray diffraction (XRD) measurements performed on a Rigaku D/max-II B X-ray diffractometer at a scanning rate of 10 /min in the 2q range from 10 to 80 , with graphite-monochromatic Cu Ka radiation (l ¼ 0.15406 nm). Fourier transform infrared (FT-IR) spectra were recorded at room temperature with a Bruker IFS 66 V instrument. The PL excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W Xe lamp as the excitation source at room temperature. The photoluminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse width ¼ 4 ns; gate ¼ 50 ns) as the excitation source (Continuum Sunlite OPO). All the measurements were performed at room temperature.
Fig. 1. XRD patterns of MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ.
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3. Results and discussion 3.1. Phase identification Fig. 1(aec) shows the XRD patterns of MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ. It can be seen from Fig. 1 that the diffraction patterns of the products MRE(MoO4)2 (M ¼ Li, Na; RE ¼ Gd, Y, Lu) can be indexed with a pure tetragonal phase (space group: I41/a), which are mostly consistent with the standard data of scheelite structured LiGd(MoO4)2 (JCPDS card No. 18-0728), NaGd(MoO4)2 (JCPDS card No. 25-0828), LiY(MoO4)2 (JCPDS card No. 17-0773), NaY(MoO4)2 (JCPDS card No. 52-1802) and LiLu(MoO4)2 (JCPDS card No. 23-1192), respectively. For NaLu(MoO4)2, no similar structural data of this compound is available for comparison. However, due to the similar chemical properties and small ionic radius difference between Naþ and Liþ, it is reasonable to assume that the NaLu(MoO4)2 compound is also assigned to be isostructural to the scheelite LiLu(MoO4)2, which is indeed experimentally observed. It can also be found that the KGd(MoO4)2:0.15Eu3þ sample is assigned to the pure KGd(MoO4)2 (JCPDS card No. 52-1694). As for KY(MoO4)2:0.15Eu3þ, almost all peaks belong to KY(MoO4)2 (JCPDS card No. 22-0873), but there are some weak impurity peaks at 25.7 e28.5 , 29.5 , 31.2 e34.5 are assigned to the intermediate products Mo9O26. The KLu(MoO4)2:0.15Eu3þ is more inclined to form the phase that assigned to the isostructural of the KY(MoO4)2, due to the doping of Eu3þ. The weak impurity peaks at 25.7 e28.5 and 31.2 e34.5 are also assigned to the intermediate products Mo9O26. All their cell parameters are presented in Table 1. It can be found that the lattice parameters a, b, c, and V of NaRE(MoO4)2 are slightly bigger than that of LiRE(MoO4)2, but the unit cell volumes of KRE(MoO4)2 are much bigger in comparison with that of NaRE(MoO4)2. This is because LiRE(MoO4)2 and NaRE(MoO4)2 belong to the same crystal structure and Liþ and Naþ have a smaller ionic radius difference. By comparing the lattice parameters of the samples with same alkali cation but different rare earth ions, it can also be seen that the lattice parameters increase systematically with increasing rare earth ionic radius. Moreover, the crystal structure of KRE(MoO4)2 transforms from tetragonal symmetry to triclinic or orthorhombic symmetry, because the ionic radius of potassium ion is too large. KGd(MoO4)2 belong to the P-1 space group, while KY(MoO4)2 and KLu(MoO4)2 are assigned to the Pbna space group. The XRD patterns of NaRE(MoO4)2 (RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ are also given in Fig. 2 for comparison. The XRD patterns are almost the same except that there is a discernible shift in the position of the diffraction peaks, which can be explained by the difference in the ionic radii of these rare earth ions. The ionic radii of these trivalent cations (CN ¼ 8) follow the trend: Gd3þ (1.053 A) > Y3þ (1.019 A) > Lu3þ (0.977 A) [9,19]. With increasing 3þ the ionic radii of RE , the diffraction peaks of NaRE(MoO4)2 shift to
Fig. 2. XRD patterns of NaRE(MoO4)2 (RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ.
a lower degree. However, no impurity lines were observed, which indicates that doping with Eu3þ ions does not form new phases during the synthesis process, and Eu3þ ions have entered the host lattice of NaRE(MoO4)2 without changing the crystal structure dramatically. The FT-IR spectrum could give some key structural information of samples, thus the FT-IR spectra of the NaRE(MoO4)2:0.15Eu3þ (RE ¼ Gd, Y, Lu) are shown in Fig. 3. The absorption bands at about 3430 cm1 and 1645 cm1 are assigned to the OeH stretching vibration and HeOeH bending vibration, respectively. The two bands are the characteristic vibrations of water from air, physically adsorbed on the samples surfaces [20]. The band at 2350 cm1 refers to the CO2 absorbed from the environment. In addition, a strong absorption peak at about 810 cm1 can be assigned to v3 antisymmetric stretching vibration originating from the MoeO stretching vibration in MoO2 4 tetrahedron, and the weak absorption peak at 440 cm1 can be assigned to v2 bending vibration of MoeO [21]. The FT-IR results also confirm that the obtained NaRE(MoO4)2:0.15Eu3þ phosphors possess single-phase scheelite structure.
Table 1 The lattice parameters of Eu3þ-doped MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu). Compound
LiGd(MoO4)2 NaGd(MoO4)2 KGd(MoO4)2 LiY(MoO4)2 NaY(MoO4)2 KY(MoO4)2 LiLu(MoO4)2 NaLu(MoO4)2 KLu(MoO4)2
Crystal system Space group Lattice parameters
Tetragonal Tetragonal Triclinic Tetragonal Tetragonal Orthorhombic Tetragonal Tetragonal Orthorhombic
I41/a I41/a P-1 I41/a I41/a Pbna I41/a I41/a Pbna
a ( A)
b ( A)
c ( A)
V ( A3)
5.1875 5.2377 11.2292 5.1619 5.2020 18.1728 5.1187 5.1638 5.2584
5.1875 5.2377 5.2774 5.1619 5.2020 7.9729 5.1187 5.1638 7.7108
11.3300 11.4421 6.9050 11.2564 11.3633 5.0592 11.0741 11.2793 18.4361
304.89 313.90 345.20 299.93 307.50 733.02 290.15 300.76 745.52
Fig. 3. FT-IR spectra of NaRE(MoO4)2:0.15Eu3þ (RE ¼ Gd, Y, Lu) samples.
L. Li et al. / Solid State Sciences 29 (2014) 58e65
3.2. Luminescence properties of Eu3þ-doped NaY(MoO4)2 phosphors Fig. 4(a) and (b) shows the excitation (lem ¼ 613 nm) and emission (lex ¼ 393 nm) spectra of NaY(MoO4)2 phosphors with different Eu3þ concentrations. By comparing the curves, it can be observed that the peak positions and their regular pattern of each curve are very similar except the intensity. With increasing Eu3þ ions concentration, the excitation and emission intensities all increase firstly and reach a maximum at x ¼ 0.15, then decrease with further increasing Eu3þ ions concentration. The excessive doping leads to concentration quenching of the Eu3þ emission. Concentration quenching may occur because the excitation energy migrates among a large number of centers before being emitted [22]. Thus the energy transfer strongly depends on the distance between the Eu3þ ions. For this reason, it is necessary to obtain the critical distance (RC), which is the critical separation between donors (activators) and acceptors (quenching site), and it can be estimated according to the following equation:
1 3 3V RC ¼ 2 4pxc Z
(1)
where xc is the critical concentration, Z is the number of the Y3þ ions in the unit cell and V is the volume of the unit cell. By taking
Fig. 4. Excitation (a) and emission (b) spectra of NaY(MoO4)2 phosphors with different Eu3þ concentrations.
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the experimental and analytic values of xc, Z and V (0.15, 2, and 306.23 A3, respectively), the critical transfer distance of Eu3þ in the NaY(MoO4)2 host is found to be about 12.5 A. Moreover, concentration quenching usually occurs as a result of non-radiative energy transfer among luminescent centers. Non-radiative energy transfer from one Eu3þ to another Eu3þ can be controlled by three different methods: radiation reabsorption, exchange interaction, or electric multipolar interaction [23]. The mechanism of radiation reabsorption comes into effect only when the excitation and the emission spectra have broad overlap. Thus, the small spectra overlap for the NaY(MoO4)2:Eu3þ phosphors indicates that the radiation of reabsorption can be ignored in this case. The exchange interaction needs a large direct or indirect overlap between donor and acceptor, which is responsible for the energy transfer for forbidden transitions and shorter critical distances of less than 5 A. In NaY(MoO4)2:Eu3þ phosphors the RC is much larger than 5 A, hence, the mechanism of exchange interaction is ineffective. As a result, the process of energy transfer of Eu3þ ions in NaY(MoO4)2 would be due to electric multipolar interaction. In addition, it can also be seen that the optimum doped concentration of Eu3þ is 0.15, and it is much higher than other Eu3þ doped phosphors, such as LaBO3:0.05Eu3þ and Y2O3:0.05Eu3þ [24,25]. The high quenching concentration is caused by the scheelite related structure, similar to other Eu3þ doped molybdate. In these compounds, the bond angles of EueOeMo and OeMoeO are more than 100 , leading to a long distance between Eu3þ ions, so it is very difficult to transfer energy for Eu3þ ions [26]. Moreover, the isolated (MoO4)2 groups can also block the energy transfer path. The measured excitation and emission spectra of typical sample NaY(MoO4)2:0.15Eu3þ are shown in Fig. 5. The excitation spectrum is composed of two parts. One is a broad band ranging from 200 to 350 nm, which is responsible for the charge-transfer band (CTB) of O2 / Eu3þ and O2 / Mo6þ. The strong CTB from host MoO2 4 group is favorable for the effective energy transfer and luminescence of Eu3þ. The other part is composed of some sharp lines from 350 to 550 nm, ascribed to the intra-configurational 4fe4f transitions of Eu3þ in the host lattices, and two of the strongest excitation peaks are at about 393 nm (7F0 / 5L6) and 463 nm (7F0 / 5D2), respectively [27,28]. The strong excitation bands located in the wavelength of 350e410 nm and 420e500 nm imply that this kind of phosphor may be a candidate material for application in the field of near UV-excited and blue-excited white LEDs.
Fig. 5. Excitation and emission spectra of typical sample NaY(MoO4)2:0.15Eu3þ.
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From the corresponding emission spectrum of NaY(MoO4)2:0.15Eu3þ phosphor excited at 393 nm, we can see that the emission spectrum shows a weak emission at 591 nm and a strong emission at 613 nm, which are respectively ascribed to the 5 D0 / 7F1 and 5D0 / 7F2 transitions of Eu3þ [29,30]. In the tetragonal scheelite structure NaY(MoO4)2, Y3þ and Naþ are randomly distributed in the same cationic sub-lattice with a symmetry S4 (without an inversion center), and due to the similar ionic size of Y3þ (1.019 A) and Eu3þ (1.066 A), the Eu3þ ions are expected to occupy the Y3þ sites. Therefore, the 613 nm emission corresponding to the 5D0 / 7F2 hypersensitive transition is the dominant peak in Fig. 5. In addition, each emission is observed as two or three subpeaks due to Stark energy splitting, and it is affected by the crystal field around Eu3þ environment in the host lattice [31]. 3.3. Influence of alkali cations and rare earth ions on the luminescent properties of MRE(MoO4)2:Eu3þ Fig. 6(aec) depicts the emission (lex ¼ 393 nm) spectra of (Li/Na/ K)Gd(MoO4)2:0.15Eu3þ, (Li/Na/K)Y(MoO4)2:0.15Eu3þ and (Li/Na/K) Lu(MoO4)2:0.15Eu3þ phosphors, respectively. The insets show the variation trend of relative luminous intensity of 5D0 / 7F2 with different alkali cations at about 613 nm. With atomic number increasing, the eight-coordinated cationic radii increase in the order of Liþ (0.92 A) < Naþ (1.18 A) < Kþ (1.51 A). In Fig. 6, the emission intensity was found to decrease with increasing the size of alkali cations. As the electronegativity of alkali cations is
c(Li)(0.98) > c(Na)(0.93) > c(K)(0.82) and that of O is 3.44, the differences of the electronegativity between alkali cations and O follow the sequence of Dc(LieO)(2.46) < Dc(NaeO)(2.51) < Dc(Ke O)(2.62), and the bond lengths also in the order of LieO < Nae O < KeO, so it is expected that the bond covalency follows the trend of LieO > NaeO > KeO. According to the theory of Judd [32], the higher covalency has the greater transition intensity. On the other hand, in LiRE(MoO4)2:Eu3þ system the Eu3þeO2 distance was found to be the shortest, which could be due to the smaller radii of Liþ as compared to that of Naþ and Kþ. The observed short Eu3þe O2 distance increases the exchange interaction probability for Eu3þ ions to transfer energy. Consequently, the different covalency and the distance of Eu3þeO2 bond may result in the observed trend in the emission spectra [27,31]. In order to investigate the effect of rare earth ions on the luminescent properties of MRE(MoO4)2:Eu3þ we have made an attempt to compare the emission intensity of phosphors with the same alkali cation but different rare earth ions. Fig. 7(aec) shows the emission (lex ¼ 393 nm) spectra of Li(Gd/Y/ Lu)(MoO4)2:0.15Eu3þ, Na(Gd/Y/Lu)(MoO4)2:0.15Eu3þ and K(Gd/Y/ Lu)(MoO4)2:0.15Eu3þ phosphors, respectively. The insets show the variation trend of relative luminescent intensity of 5D0 / 7F2 with different rare earth ions at about 613 nm. It can be seen from Fig. 7 that the emission intensity was found to decrease with decreasing the size of rare earth ions, and all the emission intensity of these three systems LiRE(MoO4)2:Eu3þ, NaRE(MoO4)2:Eu3þ, and KRE(MoO4)2:Eu3þ can be ordered as follows: Gd > Y > Lu. This
Fig. 6. Emission (lex ¼ 393 nm) spectra of (Li/Na/K)Gd(MoO4)2:0.15Eu3þ (a), (Li/Na/K)Y(MoO4)2:0.15Eu3þ (b) and (Li/Na/K)Lu(MoO4)2:0.15Eu3þ (c).
L. Li et al. / Solid State Sciences 29 (2014) 58e65
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Fig. 7. Emission (lex ¼ 393 nm) spectra of Li(Gd/Y/Lu)(MoO4)2:0.15Eu3þ (a), Na(Gd/Y/Lu)(MoO4)2:0.15Eu3þ (b) and K(Gd/Y/Lu)(MoO4)2:0.15Eu3þ (c).
behavior can be explained by the differences in the ionic radii of Gd3þ (1.053 A), Y3þ (1.019 A), and Lu3þ (0.977 A). It is possible that increasing the size of rare earth ions can distort the hosts’ structures and make more sub-lattice variation in these systems. In other words, it can lead to the larger degree of disorder and lower
local symmetry of RE3þ which in the host. Eventually, when Eu3þ ions are doped in the crystals and occupy the positions of RE3þ, the 5 D0 / 7F2 transitions of Eu3þ ions will be improved. According to the above reason, the emission intensity order of these three systems was Gd > Y > Lu.
Fig. 8. Excitation (a) and emission (b) spectra of LiGd(MoO4)2:Eu3þ compared with the red phosphor Y2O3:Eu3þ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Excitation (a) and emission (b) spectra of KLu(MoO4)2:Eu3þ compared with the red phosphor Y2O3:Eu3þ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.4. Comparison of the photoluminescence of MRE(MoO4)2:Eu3þ with Y2O3:Eu3þ Figs. 8 and 9 show the excitation and emission spectra of the LiGd(MoO4)2:0.15Eu3þ and KLu(MoO4)2:0.15Eu3þ compared with the commercially available red phosphor Y2O3:0.05Eu3þ, respectively. For Y2O3, the optimum doped concentration of Eu3þ is 0.05. In MRE(MoO4)2:Eu3þ series, the emission intensity of LiGd(MoO4)2:Eu3þ is the strongest, while that of KLu(MoO4)2:Eu3þ is the weakest. It can be seen from the two figures that both the two samples have much stronger excitation and emission intensity than that of Y2O3:Eu3þ phosphor. The 5D0 / 7F2 transition intensities of the two samples are about nine and five times higher than that of Y2O3:Eu3þ phosphor under near-UV light excitation. Furthermore, according to their excitation spectra, the MRE(MoO4)2:Eu3þ phosphors can absorb not only the emission of near UV-LED chips but also that of blue LED chips. The photoluminescence decay curves for the 5D0 / 7F2 emis-
A0l ¼
D E2 X 64p4 n3 e2 1 ðlÞ 7kU ðlÞ k 5hD50 kU ðlÞ kF7J i c U D FJ U l 0 3hc3 4p3 0 l ¼ 2;4;6
sion of Eu3þ in LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ samples are shown in Fig. 10. It indicates that all the decay curves can be well fitted into a single exponential function as I ¼ I0 exp (t/ s), in which I0 and I are the emission intensities at time ¼ 0 and t, and s is the decay lifetime. The decay lifetimes are determined to be 0.44, 0.64 and 1.04 ms for LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ particles, respectively. It is well known that in order to avoid fluorescence scintillating, the decay lifetime of phosphors used for LEDs should be short enough. It can be seen that the decay lifetime of these two samples are much shorter than that of Y2O3:Eu3þ phosphor. The nature of luminescence behavior of Eu3þ ions can be investigated by analysis of three intensity parameters Ul (where l ¼ 2, 4 and 6) according to the Judd-Ofelt theory of 4fe4f transitions intensities. The parameters can be calculated from emission spectra by the method described by Kodaira [33]. In this method,
the 5D0 / 7F1 transition is taken as internal standard in order to determine the radiative decay rate assigned to the 5D0 / 7F2 (A0-2). Radiative decay of the 5D0 / 7F1 transition is allowed by magnetic dipole and is almost insensible to the crystal field environment around the Eu3þ ion. The U4 and U6 intensity parameters were not included in this study since the 5D0 / 7F4 and 5D0 / 7F6 transitions could not be observed. The Einstein coefficients A0el can be calculated through the following equation:
A0l ¼ A0J ¼ A01
I0J hn01 I01 hn0J
(2)
where I0eJ and hn0eJ represent the area under the emission curve and the energy of 5D0 / 7FJ transition. The value of the A0e1 coefficient of 5D0 / 7F1 transition is estimated to be around 50 s1 [34]. On the basis of the Judd-Ofelt theory, the Einstein coefficients of the spontaneous emission are formulated as:
(3)
where c ¼ n0 ðn20 þ 2Þ=9 is a Lorentz local field correction and an average index of refraction (n0) equal to 1.6 was ðlÞ ðlÞ used. h5U k D0 k:U ðlÞ 7U k F J i2 is the square reduced matrix elements whose value is independent of the chemical environment of ð2Þ ðlÞ the Eu3þ ion and h5U k D0 k:U ð2Þ 7U k F 2 i2 ¼ 0:0032 [35]. The lifetime of excited state (s), non-radiaive (Anrad), and radiative (Arad) rates are related through the following equation: Atot ¼ 1/ s ¼ Anrad þ Arad. The emission quantum efficiency of the emitting 5 D0 level may be calculated from h ¼ Arad/Anrad þ Arad. The values of the U2 intensity parameter and quantum efficiencies of LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ are shown in Table 2. It can be seen that the quantum efficiencies of these two samples are much higher than that of Y2O3:Eu3þ phosphor. Under 393 nm light excitation, the CIE chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) and Y2O3:0.05Eu3þ phosphors are listed in Table 3. The chromaticity
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effects of alkali cations and rare earth ions on the luminescence of MRE(MoO4)2:Eu3þ were studied in detail. The emission intensity was found to decrease with increasing the size of alkali cations or decreasing size of rare earth ions. Under near-UV light excitation, all compounds exhibited strong red emission at about 613 nm due to 5D0 / 7F2 transition of Eu3þ. Compared with the commercially available red phosphor Y2O3:Eu3þ, the emission intensity of these phosphors is much stronger than that of Y2O3:Eu3þ. Moreover, the CIE chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ are closer to the National Television Standard Committee (NTSC) standard CIE chromaticity coordinate values for red. All the results suggest that MRE(MoO4)2:Eu3þ may be efficient red-emitting phosphors for near UV-excited and blue-excited white LEDs.
Acknowledgment
Fig. 10. The decay curves of LiGd(MoO4)2:Eu Y2O3:Eu3þ (c).
3þ
(a), KLu(MoO4)2:Eu
3þ
(b) and
Table 2 Decay rates of radiative (Arad), non-radiative (Anrad), and total (Atot) processes of 5 D0 / 7F2 transitions, luminescence decay lifetimes (s), intensity parameter (U2), and quantum efficiency (h) of LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ phosphor. Compound
Arad (s1) Anrad (s1) Atot (s1) s (ms) U2 (1020 cm2) h (%)
LiGd(MoO4)2:Eu3þ 1229.94 KLu(MoO4)2:Eu3þ 653.10 Y2O3:Eu3þ 224.39
1043.79 909.40 737.15
2272.73 0.44 1562.50 0.64 961.54 1.04
32.8 17.4 5.9
54.12 41.80 23.34
coordinates were calculated based on the corresponding emission spectra, using the CIE 1931 color matching functions. It can be seen that the CIE chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ are closer to the National Television Standard Committee (NTSC) standard CIE chromaticity coordinate values for red (0.67, 0.33) than that of the commercially available Y2O3:Eu3þ phosphor. All the results suggest that MRE(MoO4)2:Eu3þ phosphors may be efficient red-emitting phosphors for white LEDs compared to the conventional Y2O3:Eu3þ phosphor. 4. Conclusion In summary, a series of Eu3þ-activated double molybdate phosphors MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) have been successfully prepared via the solid-state reaction method. The
Table 3 The chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) and Y2O3:Eu3þ. Compound
CIE chromaticity coordinates x
y
LiGd(MoO4)2:Eu3þ NaGd(MoO4)2:Eu3þ KGd(MoO4)2:Eu3þ LiY(MoO4)2:Eu3þ NaY(MoO4)2:Eu3þ KY(MoO4)2:Eu3þ LiLu(MoO4)2:Eu3þ NaLu(MoO4)2:Eu3þ KLu(MoO4)2:Eu3þ Y203:Eu3þ
0.657 0.657 0.656 0.657 0.657 0.656 0.658 0.657 0.653 0.616
0.343 0.343 0.344 0.342 0.342 0.344 0.341 0.343 0.347 0.380
This present work was financially supported by the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, No: OSR-5; and the National Science and Technology Major Projects, No: 2008ZX05018-005.
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Synthesis and luminescent properties of high brightness MRE(MoO4)2:Eu3þ (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) red phosphors for white LEDs Linlin Li a, Junjun Zhang a, Wenwen Zi a, Shucai Gan a, *, Guijuan Ji a, Haifeng Zou a, Xuechun Xu b a b
College of Chemistry, Jilin University, Changchun 130026, PR China College of Earth Sciences, Jilin University, Changchun 130026, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2013 Received in revised form 6 January 2014 Accepted 12 January 2014 Available online 22 January 2014
A series of Eu3þ-activated double molybdate phosphors MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) have been successfully prepared via the conventional solid-state reaction method. The effects of alkali cations and rare earth ions on the luminescence of MRE(MoO4)2:Eu3þ were investigated in detail. The experimental results show that the emission intensity was found to decrease with increasing the size of alkali cations or decreasing the size of rare earth ions. Under 393 nm light excitation, all compounds exhibited strong red emission at about 613 nm due to 5D0 / 7F2 transition of Eu3þ. Compared with the commercially available red phosphor Y2O3:Eu3þ, the emission intensity of the obtained phosphors is much stronger than that of Y2O3:Eu3þ. Additionally, the excitation spectra of these phosphors implied that the MRE(MoO4)2:Eu3þ phosphors can absorb not only the emission of near UV-LED chips but also that of blue LED chips. The CIE chromaticity coordinates are close to the National Television Standard Committee (NTSC) standard CIE chromaticity coordinate values for red (0.67, 0.33). All the results indicate that these phosphors are promising red-emitting phosphors pumped by near-UV or blue light. Ó 2014 Elsevier Masson SAS. All rights reserved.
Keywords: White LEDs Luminescence Phosphor Rare earth ions
1. Introduction In recent years, white light-emitting diodes (LEDs) as a more preferable replacement for conventional incandescent and fluorescent lamps have attracted more attentions. White LEDs have the advantages of high luminous efficiency, energy-saving, reliability, safety, fast response, environmental-friendly and so on [1,2]. At present, the most common method to achieve white light is based on the combination of an InGaN blue LED chip with a yellowemitting phosphor YAG:Ce3þ [3]. However, several drawbacks appear in practical application, including low luminous efficiency and poor color-rendering index due to the deficiency of red emission [4,5]. Another promising method to obtain white light is using the red, green, and blue light-emitting phosphors to convert the radiation of near UV-LED chips, which is also a current focus in the material and luminescence research [6]. However, the main tricolor phosphors for near UV-LED chips are still some classic phosphors, such as ZnS:(Cuþ, Al3þ) for green, BaMgAl10O17:Eu2þ for blue, and
* Corresponding author. Tel.: þ86 431 88502259. E-mail address: [email protected] (S. Gan). 1293-2558/$ e see front matter Ó 2014 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2014.01.003
Y2O2S:Eu3þ for red [7]. Among these phosphors, the efficiency of Y2O2S:Eu3þ is much lower than that of green and blue phosphors. In addition, the lifetime of Y2O2S:Eu3þ is inadequate under UV irradiation and the phosphor is instable due to releasing of sulfide gas [8,9]. Therefore, in order to improve the stability and efficiency of the phosphor, it is imperative to research and design a new family of red phosphors with high absorption in the near UV or blue region for white LEDs. In search of the efficient red-emitting phosphors, the Eu2þactivated nitride phosphors and Eu3þ-activated double molybdate phosphors have drawn much interest. The Eu2þ-activated nitride phosphors usually have high quantum efficiency, such as CaAlSiN3:Eu2þ and Sr2Si5N8:Eu2þ [10,11], but they were prepared via a complex high-temperature synthetic method. Moreover, as the 4f-5d transition of Eu2þ ion is sensitive to the crystal field and covalency, the Eu2þ doped phosphors exhibit broad emission bands covering the color from blue to red and have low color purity. While the Eu3þ-activated double molybdate phosphors have attracted particular interest, because of their low synthesis temperature, intense red emission, excellent thermal and chemical stability [12e 14]. There are two obvious emission peaks between 550 and 650 nm that ascribed to the 5D0 / 7F1 (w596 nm) and 5D0 / 7F2
L. Li et al. / Solid State Sciences 29 (2014) 58e65
(w613 nm) transitions for Eu3þ ions. It is well known that the magnetic dipole transition 5D0 / 7F1 is insensitive to the site symmetry, because they are parity-allowed. However, the 5 D0 / 7F2 transition belongs to the electric dipole transition due to an admixture of opposite parity 4fn15d states by an odd parity crystal-field component. In addition, 5D0 / 7F2 transition (DJ ¼ 2) is a hypersensitive transition, and the intensity can vary by orders of magnitude, depending on the local environment. When Eu3þ is located at a low symmetry site (without inversion symmetry), it will emit intense red light due to the enhanced 5D0 / 7F2 transition [15]. MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) belong to the family of double molybdate compounds and it also assigned to be isostructural to the CaMoO4. In this crystal, alkali ions and rare earth ions are randomly distributed over the same site of Ca2þ (without an inversion center) in the CaMoO4 host [16]. The random distribution of rare earth ions induces the inhomogeneous broadening of optical spectra when Eu3þ ions are doped in the crystals and occupy the positions of RE3þ [17]. Moreover, MoO2 4 group has strong absorption in the ultraviolet region, so the energy transfer process from MoO2 group to rare-earth ions can easily occur, 4 which can greatly enhance the external quantum efficiency of rareearth ions doped materials [18]. Based on the above investigations and motivated by the attempt to develop efficient red-emitting phosphors excited by near-UV or blue radiation for the application of white LEDs, a series of Eu3þactivated double molybdate phosphors MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) were prepared via solid-state reaction. The effects of alkali cations and rare earth ions on the luminescence of MRE(MoO4)2:Eu3þ were also investigated in this paper.
59
2. Experimental section The samples were synthesized by the solid-state reaction technique. The starting materials included M2CO3 (M ¼ Li, Na, K) (A.R. grade), MoO3 (A.R. grade), RE2O3 (RE ¼ Gd, Y, Lu, Eu) (99.99%). For the synthesis of NaY0.85(MoO4)2:0.15Eu3þ, 1.0 mmol Na2CO3, 4.0 mmol MoO3, 0.85 mmol Y2O3 and 0.15 mmol Eu2O3 were weighed and thoroughly mixed in an agate mortar by grinding for 3 h. Then the mixture was transferred to alumina crucible and annealed successively at 900 C for 4 h in air. After that, the alumina crucible was allowed to cool down to room temperature naturally. The sintered cake was grounded, and then the NaY0.85(MoO4)2:0.15Eu3þ phosphor was obtained. The other samples were prepared in a similar manner except using different raw materials. The samples were examined by powder X-ray diffraction (XRD) measurements performed on a Rigaku D/max-II B X-ray diffractometer at a scanning rate of 10 /min in the 2q range from 10 to 80 , with graphite-monochromatic Cu Ka radiation (l ¼ 0.15406 nm). Fourier transform infrared (FT-IR) spectra were recorded at room temperature with a Bruker IFS 66 V instrument. The PL excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W Xe lamp as the excitation source at room temperature. The photoluminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse width ¼ 4 ns; gate ¼ 50 ns) as the excitation source (Continuum Sunlite OPO). All the measurements were performed at room temperature.
Fig. 1. XRD patterns of MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ.
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L. Li et al. / Solid State Sciences 29 (2014) 58e65
3. Results and discussion 3.1. Phase identification Fig. 1(aec) shows the XRD patterns of MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ. It can be seen from Fig. 1 that the diffraction patterns of the products MRE(MoO4)2 (M ¼ Li, Na; RE ¼ Gd, Y, Lu) can be indexed with a pure tetragonal phase (space group: I41/a), which are mostly consistent with the standard data of scheelite structured LiGd(MoO4)2 (JCPDS card No. 18-0728), NaGd(MoO4)2 (JCPDS card No. 25-0828), LiY(MoO4)2 (JCPDS card No. 17-0773), NaY(MoO4)2 (JCPDS card No. 52-1802) and LiLu(MoO4)2 (JCPDS card No. 23-1192), respectively. For NaLu(MoO4)2, no similar structural data of this compound is available for comparison. However, due to the similar chemical properties and small ionic radius difference between Naþ and Liþ, it is reasonable to assume that the NaLu(MoO4)2 compound is also assigned to be isostructural to the scheelite LiLu(MoO4)2, which is indeed experimentally observed. It can also be found that the KGd(MoO4)2:0.15Eu3þ sample is assigned to the pure KGd(MoO4)2 (JCPDS card No. 52-1694). As for KY(MoO4)2:0.15Eu3þ, almost all peaks belong to KY(MoO4)2 (JCPDS card No. 22-0873), but there are some weak impurity peaks at 25.7 e28.5 , 29.5 , 31.2 e34.5 are assigned to the intermediate products Mo9O26. The KLu(MoO4)2:0.15Eu3þ is more inclined to form the phase that assigned to the isostructural of the KY(MoO4)2, due to the doping of Eu3þ. The weak impurity peaks at 25.7 e28.5 and 31.2 e34.5 are also assigned to the intermediate products Mo9O26. All their cell parameters are presented in Table 1. It can be found that the lattice parameters a, b, c, and V of NaRE(MoO4)2 are slightly bigger than that of LiRE(MoO4)2, but the unit cell volumes of KRE(MoO4)2 are much bigger in comparison with that of NaRE(MoO4)2. This is because LiRE(MoO4)2 and NaRE(MoO4)2 belong to the same crystal structure and Liþ and Naþ have a smaller ionic radius difference. By comparing the lattice parameters of the samples with same alkali cation but different rare earth ions, it can also be seen that the lattice parameters increase systematically with increasing rare earth ionic radius. Moreover, the crystal structure of KRE(MoO4)2 transforms from tetragonal symmetry to triclinic or orthorhombic symmetry, because the ionic radius of potassium ion is too large. KGd(MoO4)2 belong to the P-1 space group, while KY(MoO4)2 and KLu(MoO4)2 are assigned to the Pbna space group. The XRD patterns of NaRE(MoO4)2 (RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ are also given in Fig. 2 for comparison. The XRD patterns are almost the same except that there is a discernible shift in the position of the diffraction peaks, which can be explained by the difference in the ionic radii of these rare earth ions. The ionic radii of these trivalent cations (CN ¼ 8) follow the trend: Gd3þ (1.053 A) > Y3þ (1.019 A) > Lu3þ (0.977 A) [9,19]. With increasing 3þ the ionic radii of RE , the diffraction peaks of NaRE(MoO4)2 shift to
Fig. 2. XRD patterns of NaRE(MoO4)2 (RE ¼ Gd, Y, Lu) with 15 mol% Eu3þ.
a lower degree. However, no impurity lines were observed, which indicates that doping with Eu3þ ions does not form new phases during the synthesis process, and Eu3þ ions have entered the host lattice of NaRE(MoO4)2 without changing the crystal structure dramatically. The FT-IR spectrum could give some key structural information of samples, thus the FT-IR spectra of the NaRE(MoO4)2:0.15Eu3þ (RE ¼ Gd, Y, Lu) are shown in Fig. 3. The absorption bands at about 3430 cm1 and 1645 cm1 are assigned to the OeH stretching vibration and HeOeH bending vibration, respectively. The two bands are the characteristic vibrations of water from air, physically adsorbed on the samples surfaces [20]. The band at 2350 cm1 refers to the CO2 absorbed from the environment. In addition, a strong absorption peak at about 810 cm1 can be assigned to v3 antisymmetric stretching vibration originating from the MoeO stretching vibration in MoO2 4 tetrahedron, and the weak absorption peak at 440 cm1 can be assigned to v2 bending vibration of MoeO [21]. The FT-IR results also confirm that the obtained NaRE(MoO4)2:0.15Eu3þ phosphors possess single-phase scheelite structure.
Table 1 The lattice parameters of Eu3þ-doped MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu). Compound
LiGd(MoO4)2 NaGd(MoO4)2 KGd(MoO4)2 LiY(MoO4)2 NaY(MoO4)2 KY(MoO4)2 LiLu(MoO4)2 NaLu(MoO4)2 KLu(MoO4)2
Crystal system Space group Lattice parameters
Tetragonal Tetragonal Triclinic Tetragonal Tetragonal Orthorhombic Tetragonal Tetragonal Orthorhombic
I41/a I41/a P-1 I41/a I41/a Pbna I41/a I41/a Pbna
a ( A)
b ( A)
c ( A)
V ( A3)
5.1875 5.2377 11.2292 5.1619 5.2020 18.1728 5.1187 5.1638 5.2584
5.1875 5.2377 5.2774 5.1619 5.2020 7.9729 5.1187 5.1638 7.7108
11.3300 11.4421 6.9050 11.2564 11.3633 5.0592 11.0741 11.2793 18.4361
304.89 313.90 345.20 299.93 307.50 733.02 290.15 300.76 745.52
Fig. 3. FT-IR spectra of NaRE(MoO4)2:0.15Eu3þ (RE ¼ Gd, Y, Lu) samples.
L. Li et al. / Solid State Sciences 29 (2014) 58e65
3.2. Luminescence properties of Eu3þ-doped NaY(MoO4)2 phosphors Fig. 4(a) and (b) shows the excitation (lem ¼ 613 nm) and emission (lex ¼ 393 nm) spectra of NaY(MoO4)2 phosphors with different Eu3þ concentrations. By comparing the curves, it can be observed that the peak positions and their regular pattern of each curve are very similar except the intensity. With increasing Eu3þ ions concentration, the excitation and emission intensities all increase firstly and reach a maximum at x ¼ 0.15, then decrease with further increasing Eu3þ ions concentration. The excessive doping leads to concentration quenching of the Eu3þ emission. Concentration quenching may occur because the excitation energy migrates among a large number of centers before being emitted [22]. Thus the energy transfer strongly depends on the distance between the Eu3þ ions. For this reason, it is necessary to obtain the critical distance (RC), which is the critical separation between donors (activators) and acceptors (quenching site), and it can be estimated according to the following equation:
1 3 3V RC ¼ 2 4pxc Z
(1)
where xc is the critical concentration, Z is the number of the Y3þ ions in the unit cell and V is the volume of the unit cell. By taking
Fig. 4. Excitation (a) and emission (b) spectra of NaY(MoO4)2 phosphors with different Eu3þ concentrations.
61
the experimental and analytic values of xc, Z and V (0.15, 2, and 306.23 A3, respectively), the critical transfer distance of Eu3þ in the NaY(MoO4)2 host is found to be about 12.5 A. Moreover, concentration quenching usually occurs as a result of non-radiative energy transfer among luminescent centers. Non-radiative energy transfer from one Eu3þ to another Eu3þ can be controlled by three different methods: radiation reabsorption, exchange interaction, or electric multipolar interaction [23]. The mechanism of radiation reabsorption comes into effect only when the excitation and the emission spectra have broad overlap. Thus, the small spectra overlap for the NaY(MoO4)2:Eu3þ phosphors indicates that the radiation of reabsorption can be ignored in this case. The exchange interaction needs a large direct or indirect overlap between donor and acceptor, which is responsible for the energy transfer for forbidden transitions and shorter critical distances of less than 5 A. In NaY(MoO4)2:Eu3þ phosphors the RC is much larger than 5 A, hence, the mechanism of exchange interaction is ineffective. As a result, the process of energy transfer of Eu3þ ions in NaY(MoO4)2 would be due to electric multipolar interaction. In addition, it can also be seen that the optimum doped concentration of Eu3þ is 0.15, and it is much higher than other Eu3þ doped phosphors, such as LaBO3:0.05Eu3þ and Y2O3:0.05Eu3þ [24,25]. The high quenching concentration is caused by the scheelite related structure, similar to other Eu3þ doped molybdate. In these compounds, the bond angles of EueOeMo and OeMoeO are more than 100 , leading to a long distance between Eu3þ ions, so it is very difficult to transfer energy for Eu3þ ions [26]. Moreover, the isolated (MoO4)2 groups can also block the energy transfer path. The measured excitation and emission spectra of typical sample NaY(MoO4)2:0.15Eu3þ are shown in Fig. 5. The excitation spectrum is composed of two parts. One is a broad band ranging from 200 to 350 nm, which is responsible for the charge-transfer band (CTB) of O2 / Eu3þ and O2 / Mo6þ. The strong CTB from host MoO2 4 group is favorable for the effective energy transfer and luminescence of Eu3þ. The other part is composed of some sharp lines from 350 to 550 nm, ascribed to the intra-configurational 4fe4f transitions of Eu3þ in the host lattices, and two of the strongest excitation peaks are at about 393 nm (7F0 / 5L6) and 463 nm (7F0 / 5D2), respectively [27,28]. The strong excitation bands located in the wavelength of 350e410 nm and 420e500 nm imply that this kind of phosphor may be a candidate material for application in the field of near UV-excited and blue-excited white LEDs.
Fig. 5. Excitation and emission spectra of typical sample NaY(MoO4)2:0.15Eu3þ.
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From the corresponding emission spectrum of NaY(MoO4)2:0.15Eu3þ phosphor excited at 393 nm, we can see that the emission spectrum shows a weak emission at 591 nm and a strong emission at 613 nm, which are respectively ascribed to the 5 D0 / 7F1 and 5D0 / 7F2 transitions of Eu3þ [29,30]. In the tetragonal scheelite structure NaY(MoO4)2, Y3þ and Naþ are randomly distributed in the same cationic sub-lattice with a symmetry S4 (without an inversion center), and due to the similar ionic size of Y3þ (1.019 A) and Eu3þ (1.066 A), the Eu3þ ions are expected to occupy the Y3þ sites. Therefore, the 613 nm emission corresponding to the 5D0 / 7F2 hypersensitive transition is the dominant peak in Fig. 5. In addition, each emission is observed as two or three subpeaks due to Stark energy splitting, and it is affected by the crystal field around Eu3þ environment in the host lattice [31]. 3.3. Influence of alkali cations and rare earth ions on the luminescent properties of MRE(MoO4)2:Eu3þ Fig. 6(aec) depicts the emission (lex ¼ 393 nm) spectra of (Li/Na/ K)Gd(MoO4)2:0.15Eu3þ, (Li/Na/K)Y(MoO4)2:0.15Eu3þ and (Li/Na/K) Lu(MoO4)2:0.15Eu3þ phosphors, respectively. The insets show the variation trend of relative luminous intensity of 5D0 / 7F2 with different alkali cations at about 613 nm. With atomic number increasing, the eight-coordinated cationic radii increase in the order of Liþ (0.92 A) < Naþ (1.18 A) < Kþ (1.51 A). In Fig. 6, the emission intensity was found to decrease with increasing the size of alkali cations. As the electronegativity of alkali cations is
c(Li)(0.98) > c(Na)(0.93) > c(K)(0.82) and that of O is 3.44, the differences of the electronegativity between alkali cations and O follow the sequence of Dc(LieO)(2.46) < Dc(NaeO)(2.51) < Dc(Ke O)(2.62), and the bond lengths also in the order of LieO < Nae O < KeO, so it is expected that the bond covalency follows the trend of LieO > NaeO > KeO. According to the theory of Judd [32], the higher covalency has the greater transition intensity. On the other hand, in LiRE(MoO4)2:Eu3þ system the Eu3þeO2 distance was found to be the shortest, which could be due to the smaller radii of Liþ as compared to that of Naþ and Kþ. The observed short Eu3þe O2 distance increases the exchange interaction probability for Eu3þ ions to transfer energy. Consequently, the different covalency and the distance of Eu3þeO2 bond may result in the observed trend in the emission spectra [27,31]. In order to investigate the effect of rare earth ions on the luminescent properties of MRE(MoO4)2:Eu3þ we have made an attempt to compare the emission intensity of phosphors with the same alkali cation but different rare earth ions. Fig. 7(aec) shows the emission (lex ¼ 393 nm) spectra of Li(Gd/Y/ Lu)(MoO4)2:0.15Eu3þ, Na(Gd/Y/Lu)(MoO4)2:0.15Eu3þ and K(Gd/Y/ Lu)(MoO4)2:0.15Eu3þ phosphors, respectively. The insets show the variation trend of relative luminescent intensity of 5D0 / 7F2 with different rare earth ions at about 613 nm. It can be seen from Fig. 7 that the emission intensity was found to decrease with decreasing the size of rare earth ions, and all the emission intensity of these three systems LiRE(MoO4)2:Eu3þ, NaRE(MoO4)2:Eu3þ, and KRE(MoO4)2:Eu3þ can be ordered as follows: Gd > Y > Lu. This
Fig. 6. Emission (lex ¼ 393 nm) spectra of (Li/Na/K)Gd(MoO4)2:0.15Eu3þ (a), (Li/Na/K)Y(MoO4)2:0.15Eu3þ (b) and (Li/Na/K)Lu(MoO4)2:0.15Eu3þ (c).
L. Li et al. / Solid State Sciences 29 (2014) 58e65
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Fig. 7. Emission (lex ¼ 393 nm) spectra of Li(Gd/Y/Lu)(MoO4)2:0.15Eu3þ (a), Na(Gd/Y/Lu)(MoO4)2:0.15Eu3þ (b) and K(Gd/Y/Lu)(MoO4)2:0.15Eu3þ (c).
behavior can be explained by the differences in the ionic radii of Gd3þ (1.053 A), Y3þ (1.019 A), and Lu3þ (0.977 A). It is possible that increasing the size of rare earth ions can distort the hosts’ structures and make more sub-lattice variation in these systems. In other words, it can lead to the larger degree of disorder and lower
local symmetry of RE3þ which in the host. Eventually, when Eu3þ ions are doped in the crystals and occupy the positions of RE3þ, the 5 D0 / 7F2 transitions of Eu3þ ions will be improved. According to the above reason, the emission intensity order of these three systems was Gd > Y > Lu.
Fig. 8. Excitation (a) and emission (b) spectra of LiGd(MoO4)2:Eu3þ compared with the red phosphor Y2O3:Eu3þ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
64
L. Li et al. / Solid State Sciences 29 (2014) 58e65
Fig. 9. Excitation (a) and emission (b) spectra of KLu(MoO4)2:Eu3þ compared with the red phosphor Y2O3:Eu3þ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.4. Comparison of the photoluminescence of MRE(MoO4)2:Eu3þ with Y2O3:Eu3þ Figs. 8 and 9 show the excitation and emission spectra of the LiGd(MoO4)2:0.15Eu3þ and KLu(MoO4)2:0.15Eu3þ compared with the commercially available red phosphor Y2O3:0.05Eu3þ, respectively. For Y2O3, the optimum doped concentration of Eu3þ is 0.05. In MRE(MoO4)2:Eu3þ series, the emission intensity of LiGd(MoO4)2:Eu3þ is the strongest, while that of KLu(MoO4)2:Eu3þ is the weakest. It can be seen from the two figures that both the two samples have much stronger excitation and emission intensity than that of Y2O3:Eu3þ phosphor. The 5D0 / 7F2 transition intensities of the two samples are about nine and five times higher than that of Y2O3:Eu3þ phosphor under near-UV light excitation. Furthermore, according to their excitation spectra, the MRE(MoO4)2:Eu3þ phosphors can absorb not only the emission of near UV-LED chips but also that of blue LED chips. The photoluminescence decay curves for the 5D0 / 7F2 emis-
A0l ¼
D E2 X 64p4 n3 e2 1 ðlÞ 7kU ðlÞ k 5hD50 kU ðlÞ kF7J i c U D FJ U l 0 3hc3 4p3 0 l ¼ 2;4;6
sion of Eu3þ in LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ samples are shown in Fig. 10. It indicates that all the decay curves can be well fitted into a single exponential function as I ¼ I0 exp (t/ s), in which I0 and I are the emission intensities at time ¼ 0 and t, and s is the decay lifetime. The decay lifetimes are determined to be 0.44, 0.64 and 1.04 ms for LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ particles, respectively. It is well known that in order to avoid fluorescence scintillating, the decay lifetime of phosphors used for LEDs should be short enough. It can be seen that the decay lifetime of these two samples are much shorter than that of Y2O3:Eu3þ phosphor. The nature of luminescence behavior of Eu3þ ions can be investigated by analysis of three intensity parameters Ul (where l ¼ 2, 4 and 6) according to the Judd-Ofelt theory of 4fe4f transitions intensities. The parameters can be calculated from emission spectra by the method described by Kodaira [33]. In this method,
the 5D0 / 7F1 transition is taken as internal standard in order to determine the radiative decay rate assigned to the 5D0 / 7F2 (A0-2). Radiative decay of the 5D0 / 7F1 transition is allowed by magnetic dipole and is almost insensible to the crystal field environment around the Eu3þ ion. The U4 and U6 intensity parameters were not included in this study since the 5D0 / 7F4 and 5D0 / 7F6 transitions could not be observed. The Einstein coefficients A0el can be calculated through the following equation:
A0l ¼ A0J ¼ A01
I0J hn01 I01 hn0J
(2)
where I0eJ and hn0eJ represent the area under the emission curve and the energy of 5D0 / 7FJ transition. The value of the A0e1 coefficient of 5D0 / 7F1 transition is estimated to be around 50 s1 [34]. On the basis of the Judd-Ofelt theory, the Einstein coefficients of the spontaneous emission are formulated as:
(3)
where c ¼ n0 ðn20 þ 2Þ=9 is a Lorentz local field correction and an average index of refraction (n0) equal to 1.6 was ðlÞ ðlÞ used. h5U k D0 k:U ðlÞ 7U k F J i2 is the square reduced matrix elements whose value is independent of the chemical environment of ð2Þ ðlÞ the Eu3þ ion and h5U k D0 k:U ð2Þ 7U k F 2 i2 ¼ 0:0032 [35]. The lifetime of excited state (s), non-radiaive (Anrad), and radiative (Arad) rates are related through the following equation: Atot ¼ 1/ s ¼ Anrad þ Arad. The emission quantum efficiency of the emitting 5 D0 level may be calculated from h ¼ Arad/Anrad þ Arad. The values of the U2 intensity parameter and quantum efficiencies of LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ are shown in Table 2. It can be seen that the quantum efficiencies of these two samples are much higher than that of Y2O3:Eu3þ phosphor. Under 393 nm light excitation, the CIE chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) and Y2O3:0.05Eu3þ phosphors are listed in Table 3. The chromaticity
L. Li et al. / Solid State Sciences 29 (2014) 58e65
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effects of alkali cations and rare earth ions on the luminescence of MRE(MoO4)2:Eu3þ were studied in detail. The emission intensity was found to decrease with increasing the size of alkali cations or decreasing size of rare earth ions. Under near-UV light excitation, all compounds exhibited strong red emission at about 613 nm due to 5D0 / 7F2 transition of Eu3þ. Compared with the commercially available red phosphor Y2O3:Eu3þ, the emission intensity of these phosphors is much stronger than that of Y2O3:Eu3þ. Moreover, the CIE chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ are closer to the National Television Standard Committee (NTSC) standard CIE chromaticity coordinate values for red. All the results suggest that MRE(MoO4)2:Eu3þ may be efficient red-emitting phosphors for near UV-excited and blue-excited white LEDs.
Acknowledgment
Fig. 10. The decay curves of LiGd(MoO4)2:Eu Y2O3:Eu3þ (c).
3þ
(a), KLu(MoO4)2:Eu
3þ
(b) and
Table 2 Decay rates of radiative (Arad), non-radiative (Anrad), and total (Atot) processes of 5 D0 / 7F2 transitions, luminescence decay lifetimes (s), intensity parameter (U2), and quantum efficiency (h) of LiGd(MoO4)2:Eu3þ, KLu(MoO4)2:Eu3þ and Y2O3:Eu3þ phosphor. Compound
Arad (s1) Anrad (s1) Atot (s1) s (ms) U2 (1020 cm2) h (%)
LiGd(MoO4)2:Eu3þ 1229.94 KLu(MoO4)2:Eu3þ 653.10 Y2O3:Eu3þ 224.39
1043.79 909.40 737.15
2272.73 0.44 1562.50 0.64 961.54 1.04
32.8 17.4 5.9
54.12 41.80 23.34
coordinates were calculated based on the corresponding emission spectra, using the CIE 1931 color matching functions. It can be seen that the CIE chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ are closer to the National Television Standard Committee (NTSC) standard CIE chromaticity coordinate values for red (0.67, 0.33) than that of the commercially available Y2O3:Eu3þ phosphor. All the results suggest that MRE(MoO4)2:Eu3þ phosphors may be efficient red-emitting phosphors for white LEDs compared to the conventional Y2O3:Eu3þ phosphor. 4. Conclusion In summary, a series of Eu3þ-activated double molybdate phosphors MRE(MoO4)2 (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) have been successfully prepared via the solid-state reaction method. The
Table 3 The chromaticity coordinates of MRE(MoO4)2:0.15Eu3þ (M ¼ Li, Na, K; RE ¼ Gd, Y, Lu) and Y2O3:Eu3þ. Compound
CIE chromaticity coordinates x
y
LiGd(MoO4)2:Eu3þ NaGd(MoO4)2:Eu3þ KGd(MoO4)2:Eu3þ LiY(MoO4)2:Eu3þ NaY(MoO4)2:Eu3þ KY(MoO4)2:Eu3þ LiLu(MoO4)2:Eu3þ NaLu(MoO4)2:Eu3þ KLu(MoO4)2:Eu3þ Y203:Eu3þ
0.657 0.657 0.656 0.657 0.657 0.656 0.658 0.657 0.653 0.616
0.343 0.343 0.344 0.342 0.342 0.344 0.341 0.343 0.347 0.380
This present work was financially supported by the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, No: OSR-5; and the National Science and Technology Major Projects, No: 2008ZX05018-005.
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