Preparation and photoluminescence properties of the Eu2+, Sm3+ co-doped Li2SrSiO4 phosphors

Current Applied Physics 12 (2012) 1045e1051

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Preparation and photoluminescence properties of the Eu2þ, Sm3þ co-doped Li2SrSiO4 phosphors Lijuan Liu a, Panli You a, b, Guangfu Yin a, *, Xiaoming Liao a, Zhongbing Huang a, Xianchun Chen a, Yadong Yao a a b

College of Materials Science and Engineering, Sichuan University, 29# Wangjiang Road, Chengdu 610065, China School of Engineering Technology, Xichang College, Xichang 615013, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 August 2011 Received in revised form 3 January 2012 Accepted 5 January 2012 Available online 15 January 2012

A series of Eu2þ and Sm3þ co-doped Li2SrSiO4 phosphors are prepared by the high temperature solidstate reaction. The morphology, structure and spectroscopic properties of the prepared samples are characterized by scanning electron microscopy, X-ray diffraction, diffuse reflection spectra, photoluminescence spectra and electron paramagnetic resonance spectra, respectively. The effect of Sm3þ doping concentration on the photoluminescence intensity of the prepared samples is also investigated. The results indicate that the crystal structure of Li2SrSiO4 is not changed with the Eu2þ, Sm3þ co-doping. The spherical-like particle size of the obtained product is about 20e30 nm in diameter. When the Sm3þ concentration is 0.3 mol% and the Eu2þ concentration is 0.7 mol%, the phosphors show the maximum emission intensity, which is 50% higher than that of Eu2þ doped Li2SrSiO4. Excited at 420 nm, the phosphor presents a single broad emission band peaking at 558 nm, which is ascribed to the 4f65d1 / 4f7 transitions of Eu2þ and 4G5/2 / 6H5/2 and 4G5/2 / 6H7/2 transitions of Sm3þ. The Commission International de I0 Eclairage chromaticity coordinates of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ are x ¼ 0.28, y ¼ 0.28. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Rare earth luminescence Eu2þ Sm3þ co-doped Photoluminescence Li2SrSiO4

1. Introduction In the context of global energy shortage, energy-saving is an important issue that we are facing on. In the field of lighting, white light emitting diodes (LEDs), a new generation solid source, have been a highlight due to high luminous efficiency, low energy consumption and great potential in environmental protection, and present a trend to replace the traditional incandescent and fluorescent lamps [1]. White LED lighting can be realized by a combination of an InGaN-based blue-LED chip with a YAG:Ce3þ yellow phosphor. However, the white light in this way exhibits some disadvantages, such as lack of red color, high color temperature, low stability and low color index [1,2]. In order to overcome the defects, people have developed many fluorescence systems, which presented good chemical stability and thermal stability, such as M2SiO4:Eu2þ [3e8], M2Si5N8:Eu2þ [9e12], (M ¼ Ba, Sr, Ca), Ca-aSiAlON:Eu2þ [13], and M3MgSi2O8:Eu2þ, Mn2þ [14]. The orthosilicate matrix phosphor possesses wide excitation bands, which can be excited by ultraviolet and blue light. Due to the * Corresponding author. Tel./fax: þ86 28 85413003. E-mail address: [email protected] (G. Yin). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.01.005

advantages mentioned above and the low price of high purity raw SiO2, this kind of phosphor is becoming more and more attractive. Park et al. [15] firstly applied the phosphor of a0 -Sr2SiO4:Eu2þ to ultraviolet chips to obtain white LED. The color coordinates of white LED were x ¼ 0.39, y ¼ 0.41, and the color index was 68 and lumens efficiency was 3.8 lm/W [15]. Moreover, Jong et al. [16] studied the relationship between the colors of Sr2SiO4:Eu2þ phosphor and temperature. Li2SrSiO4 was first synthesized by Haferkorn et al. [17] using a solegel technique. Later, Saradhi and Varadaraju [18] reported Eu2þ doped Li2SrSiO4. Based on the above mentioned investigations, Zhang et al. [19,20] discussed the photoluminescence property of Eu2þ, Ce3þ co-doped Li2SrSiO4 and Ce3þ / Eu2þ energy transfer mechanism in the Li2SrSiO4:Eu2þ, Ce3þ phosphor, and revealed that the Ce3þ / Eu2þ energy transfer would improve the emission intensity of phosphors. The host crystal lattices have little effect on the emission and excitation spectra of Sm3þ because of its characteristic fef transitions. Sm3þ doped phosphor has been widely studied about the characteristic transitions of Sm3þ [21e23]. Some investigations about Eu3þ, Sm3þ co-doped phosphors have been reported [24e30], which revealed that the co-doping of Eu3þ, Sm3þ could

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improve the emission intensity of the phosphor by energy transfer from Sm3þ to Eu3þ. The doped-Eu2þ phosphor [3e20] have good luminescence property and the extensive application. To our knowledge, there are few reports on the luminescence of Eu2þ, Sm3þ co-doped Li2SrSiO4 phosphor up to now. In this paper, the Eu2þ, Sm3þ co-doped Li2SrSiO4 phosphors are synthesized by the high temperature solid-state reaction. The photoluminescence properties of the prepared Eu2þ, Sm3þ co-doped Li2SrSiO4 are investigated, and the effect of Sm3þ concentration on the photoluminescence properties of the phosphor is also discussed.

2. Experimental details A series of samples of Li2Sr1xySiO4:x mol% Eu2þ (x ¼ 0. 25, 0. 5, 0. 7, 0. 75, 1), y mol% Sm3þ (y ¼ 0, 0.1, 0.3, 0.5, 0.7, 0.9) were synthesized by the high-temperature solid-state reaction. The raw materials were Li2CO3, SrCO3, SiO2 (Chengdu kelon chemical Co., Ltd), Eu2O3 (Shanghai Yingyuan chemical Co., Ltd, 99.9%), and Sm2O3 (Enterprise group chemical reagent Co., Ltd, 99.99%). All the reagents were analytical grade except for Eu2O3 and Sm2O3. These powders were weighed according to stoichiometric ratios, except that the amount of Li2CO3 exceeded 5% to compensate the loss of Liþ. Meanwhile the charge compensation for the substitution of Sr2þ by Sm3þ is achieved by adding equal molar concentration of Liþ. The mixed powders were adequately ground and pre-sintered in air at 600  C for 4 h in muffle furnace. Then the obtained products were ground again and placed in an alumina boat inside a tubular furnace and heated slowly to 900  C with a temperature increased rate of 5  C/min under a reduction atmosphere (5% H2 þ 95% N2). The samples were kept at this temperature for 4 h and then cooled to room temperature with the cooling rate of 5  C/min. Based on the effects of Eu2þ and Sm3þ concentration individually on the luminescence properties of prepared samples, the orthogonal experiments of 2 factors and 3 levels were carried out to optimize the doping concentration of Eu2þ and Sm3þ. The Eu2þ concentrations were 0.7, 0.75 and 0.8 mol% and the Sm3þ concentrations were 0.3, 0.4 and 0.5 mol%, respectively. The structure of the product was analyzed by X-ray powder diffraction (XRD) with CuKa radiation at 40 kV and 25 mA. The XRD patterns were collected in the 2q range from 10 to 80 with a step of 0.02 . The diffuse reflection spectra were measured at room temperature with a UVevisible spectrophotometer (Hitachi U3010) with BaSO4 as reference. The photoluminescence spectra of the compounds were detected by using Hitachi FL-7000 spectrophotometer operating in the range 200e800 nm with a 150 W Xe lamp under a working voltage of 500 V. The scanning speed was 1200 nm/min and the

Fig. 1. XRD pattern of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ.

Fig. 2. EDS of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ.

excitation and emission slits were 5.0 nm. Scanning electron microscope (SEM) (Hitachi S-4800) was used to observe the micromorphology of the phosphor. The elements analysis was operated by using energy dispersive spectrometer (EDS) coupled with a Hitachi S-4800 and Oxford-IE250 as an attachment. The electron paramagnetic resonance (EPR) spectra were recorded using an EPR spectrometer (Bruker BioSpin GmbH, ER200D-SRC10/12) operating in the X-band frequency (9.464 GHz) with a field modulation frequency of 100 kHz. The magnetic field was scanned from 0 to 4000 G and the microwave power used was 5.160 mW. A powdered glass specimen of 100 mg was taken in a quartz tube for EPR measurements. All the measurements were performed at room temperature. 3. Results and discussion 3.1. XRD analysis and SEM observation The XRD patterns of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ are shown in Fig. 1. Li2SrSiO4 has a hexagonal crystal structure with the P3121 space group, similar to Li2EuSiO4 according to Joint Committee on Powder Diffraction Standards (JCPDS No. 47-0120). The XRD pattern agrees well with that in the references [17,18]. No evident characteristic peaks of other phases are observed, indicating that the doping of Eu2þ or Sm3þ did not cause any significant change in the host structure. The Li2SrSiO4 host has only one type of Sr2þ ion site. The result of elements analysis by EDS is shown in Fig. 2. It indicates the existence of Eu and Sm in this sample (Lithium element cannot be detected as it is not in the measurement range of EDS). The lattice of

Fig. 3. SEM image of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ.

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Fig. 4. Emission spectra of Li2SrSiO4:x mol% Eu2þ (x ¼ 0.25, 0.5, 0.75, 1) and Li2SrSiO4:0.75 mol% Eu2þ, y mol% Sm3þ (y ¼ 0.2, 0.4, 0.6, 0.8, 1).

the crystal structure would be distorted and unstable when the lattice ions are replaced with bigger ions (Eu2þ or Sm3þ). As the ion radius of Sr2þ (0.118 nm) is closer to that of Eu2þ (0.117 nm) or Sm3þ (0.0964 nm), and the ion radius of Si4þ (0.026 nm) ion and Liþ (0.068 nm) is smaller than that of Eu2þ or Sm3þ, the Sr2þ site would be preferentially occupied by Eu2þ or Sm3þ, and the Eu2þ or Sm3þ could exist stably in the structure of this phosphor when they occupied the site of Sr2þ [18], which is consistent with the XRD results. Fig. 3 shows the micro-morphology of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ. It indicates that the diameter of grain is about 20e30 nm with sphere-like shape. Spherical particles possess many advantages, such as high density coating and low scattering etc, which could significantly improve the luminous intensity of phosphors. The cluster pattern can also be observed due to uneven dispersion.

3.2. Effects of Eu2þ and Sm3þ doping concentration on emission intensity In order to find out the optimum doping concentration of Eu2þ and Sm3þ, a series of samples were synthesized. First, the effects of Eu2þ concentration on these phosphors without Sm3þ doping were investigated; the Eu2þ concentration is varied from 0.25 to 1 mol% in a step of 0.25 mol%. In Fig. 4, we could see that with increase of Eu2þ doping concentration, the shape of these spectra did not vary but the emission intensities were enhanced until Eu2þ concentration of 0.75 mol%. Then, when the Eu2þ concentration was 0.75 mol%, the Sm3þ doping concentration was gradually changed from 0.2 to 1 mol% in a step of 0.2 mol%. The result is shown in the inset of Fig. 4, and it can be seen that with increase concentration of Sm3þ the emission intensities increase until the concentration is 0.4 mol%, suggesting that the best Sm3þ concentration is 0.4 mol%. In Fig. 4, an emission peak at 535 nm ascribed to the characteristic of Sm3þ can also be observed for the Sm3þ doped Li2SrSiO4:0.75 mol% Eu2þ. Furthermore, with increasing Sm3þ concentration, the intensity of 535 nm peaks increase until the concentration is 0.6 mol%.

Based on the effects of Eu2þ and Sm3þ concentration individually on the emission properties of samples, the orthogonal experiments were carried out. The result of orthogonal experiments was shown in Table 1, and from this table we can draw a conclusion that the best co-doping concentrations of Eu2þ and Sm3þ are 0.7 mol% and 0.3 mol%, respectively. 3.3. Diffuse reflection spectra The diffuse reflection spectra of Li2SrSiO4:0.7 mol% Eu2þ, y mol% Sm3þ (y ¼ 0, 0.3) phosphors are given in Fig. 5. The absorption spectra show two strong absorption bands: one is centered at 260 nm and another broad band from 360 to 525 nm. All the bands are mainly attributed to the 4f7/4f65d1 electronic transition of Eu2þ, which is in agreement with references [18,19]. The broad absorption band wavelength corresponds to the blue LEDs emission wavelength very well. The spectrum of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ indicates that the absorption spectra remain almost unchanged with the Sm3þ ion doping. But the absorption intensity of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ is stronger than that of Li2SrSiO4:0.7 mol% Eu2þ. This implies that the absorption of Li2SrSiO4:Eu2þ phosphor is enhanced due to Sm3þ co-doping. Table 1 Result of orthogonal experiments. lex ¼ 420 nm. Factors Experiment Experiment Experiment Experiment Experiment Experiment Experiment Experiment Experiment Average 1 Average 2 Average 3 Range

1 2 3 4 5 6 7 8 9

Eu2þ concentration

Sm3þ concentration

Emission intensity

0.7 0.7 0.7 0.75 0.75 0.75 0.8 0.8 0.8 1715.667 923.367 887.833 827.834

0.3 0.4 0.5 0.3 0.4 0.5 0.3 0.4 0.5 1296.067 1290.033 940.767 355.300

2402 1111 1634 359.2 1847 563.9 1127 912.1 624.4

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reflectance (%)

90 80 70

0 mol%

60 50

0.3 mol%

40 30 250

300

350

400

450

500

550

600

wavelength (nm) Fig. 5. Diffuse reflectance spectra of Li2SrSiO4:0.7 mol% Eu2þ, y mol% Sm3þ (y ¼ 0, 0.3).

3.4. Photoluminescence properties of Li2SrSiO4:Eu2þ, Sm3þ The photoluminescence excitation and emission spectra of Eu2þ, Sm3þ co-doped Li2SrSiO4 are shown in Fig. 6 and Fig. 7, respectively. The spectrum of Li2SrSiO4:Eu2þ clearly indicates a broad excitation band from 300 to 520 nm corresponding to the 558 nm emission. And there are two stronger excitation bands located at 310 nm and 400 nm in Fig. 6. The inset in Fig. 6 is the emission spectra of Li2SrSiO4, Li2SrSiO4:0.7 mol% Eu2þ and Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ when the excitation wavelength is 254 nm. From these spectra we could see that there existed a peak at the center of 611 nm in Li2SrSiO4:0.7 mol% Eu2þ and Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ but not in Li2SrSiO4, which should be ascribed to the characteristic electric dipole 5D0 / 7F2 transition from Eu3þ [31]. For the typical Eu3þ activated phosphors, they always show strong charge transfer state transition band absorption around 200e300 nm and the characteristic charge transition band of O2/Eu3þ is reported to be 254 nm [31,32]. And the stabilization of Eu2þ in Li2SrSiO4 during the synthesis process requires a strong

reducing agent (H2), therefore, under the adopted conditions (5% H2 þ 95% N2) Eu2O3 cannot be reduced to Eu2þ completely, some amount of Eu3þ can be revealed in the final products [33,34]. The characteristic emission of Eu3þ is not observed easily at the excitation wavelength of 420 nm, but it could be observed when the excitation wavelength is 254 nm (shown in the inset of Fig. 6), the characteristic excitation of Eu3þ, which demonstrates the existence of Eu3þ. Considering the mentioned above, the excitation band in the range of 370e530 nm with maximum at 400 nm should be the overlap of two bands due to its non-symmetry shape. The one band is the transition band from the 4f7(8S7/2) ground state to the excited state of the seven 7Fj levels in the 4f6 configuration, which is well known from the phosphors doped with Eu3þ. The other band is the 4f / 5d spin allowed transition in the spectra. Although the excitation spectrum cannot distinguished the line transition in the intro-4f configurations, the distinguishable line transitions from the ground state to the seven 7Fj levels in the 4f6 configuration have been observed in the excitation spectrum of Sr3(PO4)2:Eu2þ and SrGa2S4:Eu2þ, which were obtained at the ultra-low temperature [31,35]. The excitation band from 4f7/4f65d1 transitions of Eu2þ is dominant between all the excitation bands and the broad excitation band matches well with blue LEDs emission. Another peak located at 441 nm is observed due to the introduction of Sm3þ ions in Li2SrSiO4:Eu2þ. The excitation band is ascribed to the characteristic of Sm3þ. Another characteristic peak of Sm3þ which located at 401 nm overlaps with that of Eu2þ corresponding to 558 nm emission, thus doping Sm3þ enhances the absorption intensity. Thus, the absorption intensity of the Eu2þ, Sm3þ co-doped Li2SrSiO4 phosphors is significantly enhanced compared with that of Eu2þ doped Li2SrSiO4. The excitation spectrum is consistent with the diffuse reflectance spectra (shown in Fig. 5). Fig. 7 presents the emission of Eu2þ doped Li2SrSiO4 and Eu2þ, 3þ Sm co-doped Li2SrSiO4 with the excitation wavelength 420 nm. It is very clear that the emission spectrum of Li2SrSiO4:Eu2þ exhibits a single broad band from 500 nm extending to 650 nm with the maximum at 558 nm mainly resulted from 4f65d1/4f7 transitions of Eu2þ. The introduction of Sm3þ in Li2SrSiO4:Eu2þ do not change the shape of emission spectrum. But the emission intensity of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ is higher than that of

Fig. 6. Excitation spectra of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ (a) and Li2SrSiO4:0.7 mol% Eu2þ (b).

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3.5. Luminescence properties of Li2SrSiO4:Eu2þ, Sm3þ with different Sm3þ concentrations

Fig. 7. Emission spectra of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ (a) and Li2SrSiO4:0.7 mol% Eu2þ (b).

Li2SrSiO4:0.7 mol% Eu2þ, which results from the emission band of Sm3þ overlapped that of Eu2þ, suggesting the improvement of the emission intensity with Sm3þ co-doping in Li2SrSiO4:Eu2þ phosphor. And the inset figure is the emission spectrum of Sm3þ doped Li2SrSiO4 excited at 420 nm. The characteristic emission of Sm3þ centered at 535 and 598 nm, corresponding to transition 4 G5/2 / 6H5/2 and 4G5/2 / 6H7/2 respectively, overlaps that of Eu2þ when the excitation wavelength is 420 nm and thus improves the emission intensity of this phosphor. The Commission International de I0 Eclairage (CIE) chromaticity for Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ are x ¼ 0.28, y ¼ 0.28, which is shown in Fig. 8, so Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ is a promising phosphor for white LEDs.

Fig. 8. CIE color coordinates of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ (lex ¼ 420 nm).

In order to discuss the effect of different Sm3þ concentrations on the photoluminescence properties of phosphors with 0.7 mol% Eu2þ, a series of experiments were carried out. The emission intensity of Li2SrSiO4:0.7 mol% Eu2þ, y mol% Sm3þ as a function of the Sm3þ concentration is shown in Fig. 9. With the increase of y, the emission intensity at 558 nm increases until y ¼ 0.3 mol%, and decreases when y is over 0.3 mol%. The emission intensity of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ is enhanced 50% higher than that of Li2SrSiO4:0.7 mol% Eu2þ. To explore the reason of the emission intensity first increase and then decrease with Sm3þ concentration increase, the EPR spectra measurements of these phosphors have been carried out. The EPR spectra of Li2SrSiO4:0.7 mol% Eu2þ and Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ are shown in Fig. 10. The shape of the EPR spectrum of Eu2þ ions strongly depends on the microwave frequency and crystal field strength. So, the position of Eu2þ signal in our report does not match with others reports about Eu2þ doped phosphors [36e40]. Because Eu2þ ion possesses an uncoupled electron (4f7, S ¼ 7/2, L ¼ 0, J ¼ 7/2), while Eu3þ does not, the intensity of these EPR spectra denote only the relative concentration of Eu2þ rather than Eu3þ. Due to the doping of Sm3þ ions, the signal of Eu2þ at 989.2473, 1579.668, and 3124.125G increased significantly. Dotsenko et al. [34] put forward the low stability of Eu2þ ions in Li2SrSiO4 and expounded the reason from chemical and physical aspects. Furthermore, Kim et al. [33] reported that the introduction of Ce3þ ions in Li2SrSiO4 causes the stabilization of Li-vacancies, and suppresses the oxidation of Eu2þ ions leading to the enhancement of the emission intensity of phosphor. Similarly, the introduction of Sm3þ ions in Li2SrSiO4 could cause the stabilization of Eu2þ, and limit the oxidation of Eu2þ ions and the decrease of the concentration of Eu2þ, consequently enhances the emission intensity. The result is also confirmed by the inset figure in Fig. 6. It is clear that with the doping of Sm3þ, the intensity of the characteristic emission peak of Eu3þ at 611 nm is weaker than that without Sm3þ doping, that is to say, the concentration of Eu3þ decrease but the concentration of Eu2þ increase when the amount of Eu is a constant value. But the mechanisms of the Eu2þ stabilization need to be further investigated. However, it can be seen clearly from Fig. 9 that 0.3 mol% is the critical concentration of Sm3þ. The emission intensity is decreased

Fig. 9. Emission spectra of Li2SrSiO4:0.7 mol% Eu2þ, y mol% Sm3þ (y ¼ 0, 0.1, 0.3, 0.5, 0.7, and 0.9).

intensity(a.u.)

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8.0x10

5

6.0x10

5

4.0x10

5

2.0x10

5

References

B A

0.0 -2.0x10

5

-4.0x10

5

0

500

1000

1500

2000

2500

3000

3500

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magnetic field(Guass) Fig. 10. EPR spectra of (A) Li2SrSiO4:0.7 mol% Eu2þ and (B) Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ.

with the continuous increase of Sm3þ concentration. It may be explained as followings: The main factor for decreasing emission intensity is nonradiative energy transfer between Eu2þ ions [18]. According to the previous studies [41,42], the probability of energy transfer between the Eu2þ ions is due to distance-dependent multipoleemultipole interaction. When the distance between Eu2þ ions is beyond the critical distance, the emission intensity will be decreased due to concentration quenching. Because the radius of Sm3þ is smaller than that of Sr2þ and Eu2þ, the doping of Sm3þ make the distance among Eu2þ ions get smaller. When the concentration of Sm3þ ions increases, the distance between the Eu2þ ions becomes smaller, leading to the probability of more energy transfer among the Eu2þ ions [7] and results in concentration quenching.

4. Conclusions A series of the Eu2þ and Sm3þ co-doped Li2SrSiO4 phosphors are prepared by the high temperature solid-state reaction. The result of SEM shows the grain size is 20e30 nm. When the Sm3þ concentration is 0.3 mol% and the Eu2þ concentration is 0.7 mol%, the phosphors show the maximum emission intensity. The doping of Sm3þ in Li2SrSiO4:0.7 mol% Eu2þ could improve the emission intensity of this phosphor. And the emission intensity of Eu2þ, Sm3þ co-doped Li2SrSiO4 is 50% higher than that of Eu2þ doped Li2SrSiO4. The Eu2þ, Sm3þ co-doped Li2SrSiO4 phosphors have broad excitation band (300e520 nm) with yellow emission wavelength of 558 nm. Excited at 420 nm, the phosphor presents a single broad emission band peaking at 558 nm. The CIE chromaticity coordinates of Li2SrSiO4:0.7 mol% Eu2þ, 0.3 mol% Sm3þ are x ¼ 0.28, y ¼ 0.28. This indicates white light could be obtained by integrating the phosphor with blue LEDs chips.

Acknowledgments This project was financially sponsored by the National Natural Science Foundation of China (project No. 60871062 and 50873066). The supports of Key Technologies Research and Development Program of Sichuan Province (2010SZ0088 & 2008SZ0021) are also acknowledged with gratitude. We thank also the National Engineering Research Center for Biomaterials & Engineering Research Center in Biomaterials, Sichuan University for the assistance with the microscopy work.

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