Preparation and optical properties of LaSr2AlO5 doped with Ce and Eu

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-00075 Chemical and Biological Sci. 2015, Vol. 60, No. 9, pp. 27-31 This paper is available online at Received December 23, 2015. Accepted December 30, 2015. Contact Nguyen Vu, e-mail address: nguyenvu@ims.vast.ac.vn 27 PREPARATION AND OPTICAL PROPERTIES OF LaSr2AlO5 DOPED WITH Ce AND Eu Nguyen Vu1, Chun-Che Lin2 and Ru-Shi Liu2 1 Institute of Materials Science, Vietnam Academy of Science and Technology 2 Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Abstract. A new host phosphors of LaSr2AlO5 doped with Ce and Eu was synthesized using a solid-state reaction method and the synthesis conditions were investigated. The samples were characterized by X-ra diffraction, scanning electron microscopy, photoluminescent (PL) and photoluminescent excitation (PLE) spectra. LaSr2AlO5:Ce shows a yellow emission at about 570 nm after an excitation at about 450 nm while LaSr2AlO5:Eu presents a red emission. These phosphors could be useful in the production of white l ght-emitting diodes. Keywords: Phosphors, White light-emitting diodes, LaSr2AlO5. 1. Introduction A recent interest in phosphors has led to a rapid development of promising display and illumination technologies. In particular, photoluminescent materials are required for use in plasma display panels (PDPs) and light-emitting diodes (LEDs). New inorganic phosphor materials applied in lighting and display fields are being developed, particularly for use in white light-emitting diodes [1-3]. Rare- arth-activated phosphors have attracted attention in recent times due to their high quantum efficiency, better stability and their suitability for use in optical and light emission applications. Among the rare earth elements, Ce3+ is a low-cost activator which can provide strong absorption of UV and efficient emission to longer wavelengths. Ce3+ is a lanthanide ion with the simplest electronic configuration. Because the 4f - 5d transitions are parity allowed, they have a large absorption cross section and appear as intense bands in spectra. Hence, luminescent materials doped with Ce3+ can efficiently absorb excitation energy. Furthermore, the 5d states of Ce3+ are outer orbital and the coordination surroundings have a prominent influence on their energies so that the 4f - 5d transitions of Ce3+ appear in a large wavelength range that depends on the host lattice. The position of its absorption and emission bands can be controlled by selecting a suitable matrix. Eu has been widely used as a luminescent center in both its valence states, Eu2+ and Eu3+, in different hosts to obtain efficient light emission ranging from blue to red [3]. Eu3+has been recognized as an efficient luminophore. Nguyen Vu, Chun-Che Lin and Ru-Shi Liu 28 Recently, a new yellow-emitting phosphor, LaSr2AlO5:Ce 3+, which could be applicable to white LED, was reported [1]. There, LaSr2AlO5:Ce 3+ exhibits an absorption band in the sub- blue region along with a strong yellow emission centered at about 556 nm. Enhanced yellow-to- orange emission of Si-doped Mg3Y2Ge3O12:Ce 3+ garnet phosphors for warm white LEDs was presented by Jiang [2]. Sr3AlO4F:Ce 3+ phosphor for application in LEDs is also shown in [4, 5]. Eu3+, Ce3+ co-doped CaLaAl3O7 phosphors were synthesized by Singh [6]. However, until now, few papers have been published reporting the synthesis and optical properties of LaSr2AlO5 doped with Eu2+ and/or Eu3+ [7, 8]. In this work, we synthesized a series of C and Eu doped LaSr2AlO5 using a solid-state reaction method. The dependences of structural and optical properties on synthesis conditions were investigated. 2. Content 2.1. Experiments La0.975Ce0.025Sr2AlO5 (LSA), EuSr2AlO5 (ESA) and La0.975Eu0.025Sr2AlO5 (LESA) samples were prepared using a solid-state reaction method with La2O3, CeO2, Eu2O3, SrCO3, and Al2O3 serving as starting materials. These raw materials were mixed in stoichiometric ratio using an agate mortar and pestle for 30 min. Then, all the samples were heated in a tube- type furnace at 1400 to 1500 oC for 4 h in a reducing atmosphere of 5 % H2 / 95 %N2 (LSA1- LSA9 and LESA1) or in air (ESA1 and LESA2). X-ray diffraction (XRD) was performed using a PHILIPS X'pert PRO diffractometer with CuKα (1.5418Å) radiation. Photoluminescent (PL) and photoluminescent excitation (PLE) spectra were obtained at room temperature using a Spex Fluorolog-3 spectrophotometer with 450 W Xe light sources. 2.2. Results and dicussion Figure 1 presents XRD patterns of LSA samples with different synthesis conditions. It can be seen that samples heated at 1500 oC (LSA4 & LSA5) are single phase like standard phase EuSr2AlO5, while samples heated at 1400 oC exhibit some impurity phases. The Bragg reflections of LSA samples shift to low angles compared with EuSr2AlO5. The result are almost same when the conditions of the LSA4 sample were repeated for the LSA9 sample. 10 20 30 40 50 60 70 0 1 2 3 LSA5 LSA4 LSA3 EuSr 2 AlO 5 standard 850OC, 2h; 1400OC, 4h; 1500OC, 4h 1500OC, 4h 850OC, 2h; 1400OC, 4h In te n si ty ( n o rm .) 2(degree) 1400OC, 4hLSA2 Figure 1. XRD patterns of LSA samples with different synthesis conditions Preparation and optical properties of LaSr2AlO5 doped with Ce and Eu 29 Because there is no JCPDS card for LaSr2AlO5, we compared the XRD patterns of the LSA samples with the EuSr2AlO5 standard. The longer La 3+ ionic radii relative to that of Eu3+ causes a shift of Bragg reflections in the LSA samples compared with the EuSr2AlO5 standard. For comparison, we synthesized a EuSr2AlO5 sample by solid state reaction and kept it at 1500 oC for 4 h. The result shows 2  peaks from the EuSr2AlO5 prepared sample (heated at 1500 oC, 4 h) that are the same as that of the EuSr2AlO5 standard (Figure 2). 35 40 45 50 55 60 65 70 0.0 0.4 0.8 In te n si ty ( n o rm .) 2(degree) ESA1: prepared EuSr2AlO5: Standard Figure 2. XRD patterns of the EuSr2AlO5 prepared sample (ESA1) heated at 1500 o C for 4 h and the EuSr2AlO5 standard Figure 3 shows the XRD patterns of La0.975Eu0.025Sr2AlO5 samples (LESA1 and LESA2). These samples were heated at 1500 oC for 4 h in H2/N2 (LESA1) and in air (LESA2) atmospheres. We can see that the single phase, like the standard phase of EuSr2AlO5 in La0.975Eu0.025Sr2AlO5 samples, is also observed. 20 30 40 50 60 70 0 1 2 LESA1 LESA2 1500OC, 4h, in air 1500OC, 4h, in H 2 /N 2 EuSr 2 AlO 5 standard In te n si ty ( n o rm .) 2(degree) Figure 3. XRD patterns of La0.975Eu0.025Sr2AlO5 samples (LESA1 and LESA2) Photoluminescent (PL) and photoluminescent excitation (PLE) spectra of La0.975Ce0.025Sr2AlO5 (LSA) samples with different synthesis conditions are presented in Figure 4. PL spectra of LSA samples are broad with a maximum at about 570 nm and a maximum PLE spectra at about 450 nm. The PL and PLE of LSA4 and LSA9 are almost the same. Both LSA4 and LSA9 have a higher PL intensity than LSA5 and LSA2. The results allow us to conclude that the LSA samples can absorb light in the blue region and emit light in the yellow range which is interesting for white light-emitting diodes. Nguyen Vu, Chun-Che Lin and Ru-Shi Liu 30 400 450 500 550 600 650 700 0 1000000 2000000 3000000 4000000 Excitation em= 570 nm Emission ex= 440 nm LSA9Em LSA4Em LSA5Em LSA2Em In te n si ty ( a .u ) Wavelength (nm) LSA9Exc LSA4Exc LSA5Exc LSA2Exc Figure 4. PL and PLE spectra of LSA samples with different synthesis conditions: LSA4 (1500 o C, 4 h), LSA9 (1500 o C, 4 h), LSA5 (850 o C, 2 h, then 1400 o C, 4 h, and then 1500 o C, 4 h ), LSA2 (1400 o C, 4 h) 560 600 640 680 720 0 300000 600000 In te n si ty ( a .u ) Wavelength (nm) ESA1_EM LESA1-EM LESA2_EM 620 nm 616 702 578 588 602 610 608  exc =392 nm Figure 5. PL spectra of ESA1, LESA1 and LESA2 samples under 392 nm excitation 320 340 360 380 400 420 440 0 500000 1000000  e m = 6 2 0 n m  em =616 nm ESA1_EX LESA1_EX LESA2_EX In te n si ty ( a .u ) Wavelength (nm) 392 nm Figure 6. PLE spectra of ESA1, LESA1 and LESA2 samples Preparation and optical properties of LaSr2AlO5 doped with Ce and Eu 31 The EuSr2AlO5 (ESA1) and La0.975Eu0.025Sr2AlO5 (LESA1, LESA2) samples show red emission under both 254 and 365 nm excitations by UV illumination. Figures 5 and 6 represent the PL and PLE spectra of ESA1, LESA1 and LESA2 samples, respectively. The PL spectra show 5D0 - 7FJ (J = 0 - 4) transitions of Eu 3+ nder 392 nm excitation. Position peaks are almost the same, but 620 and 610 nm peaks in the EuSr2AlO5 sample shift to 616 and 608 nm, respectively, in La0.975Eu0.025Sr2AlO5 samples. The PLE spectra of ESA1, LESA1 and LESA2 show some peaks from 363 to 414 nm which correspond to transitions from ground state (7F0) to excited states of Eu 3+. Because EuSr2AlO5 has a higher concentration of Eu3+, its absorption has a higher intensity, with excitation at 392 nm, than that of La0.975Eu0.025Sr2AlO5. But there is broad band below 350 nm that shows higher intensity for La0.975Eu0.025Sr2AlO5 than for EuSr2AlO5. 3. Conclusion Single phase like standard phase of EuSr2AlO5 can be obtained for La0.975Ce0.025Sr2AlO5, EuSr2AlO5 and La0.975Eu0.025Sr2AlO5 prepared using the SSR method when heated at 1500 oC for 4 h. The highest PL intensity can also be achieved in this condition with one heat treatment. Under excitation at about 450 nm, the LaSr2AlO5:Ce sample shows a yellow emission at about 570 nm due to the transition of Ce3+ from the 5d1 excited state to 2F5/2 and 2F7/2. LaSr2AlO5:Eu presents a red emission corresponding to the 5D0 and 7FJ (J = 0 - 4) states of Eu3+. Acknowledgments. This work was made possible thanks to the support of the National Science Council of Taiwan, National Taiwan University and Institute of Materials Science (VAST). REFERENCES [1] W. B. Im, N. N. Fellows, S. P. DenBaars, R. Seshadri an Y. Kim, 2009. Chem. Mater., 21, 2957. [2] Z. Jiang, Y. Wang, Z and L.Wang, 2010. J. Electrochem. Soci. 157, J155. [3] S. Chawla, A. Yadav Mater, 2010. Chem. Phys.122, 582. [4] W. Chen, H. Liang, H. Ni, Pei He and Q. Su, 2010. J. Electrochem. Soc. 157, J159. [5] W.B. Im, S. Brinkley, J. Hu, A. Mikhailovsky, S. P. DenBaars, and R. Seshadri, 2010. Chem. Mater. 22, 2842. [6] V. Singh, V. V. R. K. Kumar, R. P. S. Chakradhar and H. Y. Kwak, 2010. 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