Synthesis and luminescent characteristics of yellow emitting GdSr2AlO5:Ce3+ phosphor for blue light based white LED

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 5693–5698 www.elsevier.com/locate/ceramint

Synthesis and luminescent characteristics of yellow emitting GdSr2AlO5:Ce3 þ phosphor for blue light based white LED Jin Young Parka, J.H. Leea, G. Seeta Rama Rajub, Byung Kee Moona,n, Jung Hyun Jeonga, Byung Chun Choia, Jung Hwan Kimc b

a Department of Physics, Pukyong National Universiy, Busan 608-737, Republic of Korea Department of Electronics and Radio Engineering, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea c Department of Physics, Dong Eui University, Busan 614-714, Republic of Korea

Received 19 October 2013; accepted 3 November 2013 Available online 12 November 2013

Abstract A series of Ce3 þ ions doped GdSr2AlO5 (GSA) phosphors were synthesized by a citric acid based sol–gel method. The X-ray diffraction patterns confirmed their tetragonal structure after the samples were annealed at 1300 1C, and the scanning electron microscope image showed the closely packed particles. The excitation spectra revealed that the GSA phosphor effectively excited with blue light of 442 nm due to the 4f1-5d1 transition and exhibited yellow emission corresponding to the 5d1-4f1 transition of Ce3 þ ions. The optimum doping concentration of Ce3 þ ions was 5 mol% and the critical distance was calculated to be 17 Å. White LEDs were fabricated by combining blue LED (465 nm) chip with Ce3 þ :GSA phosphor. The CIE chromaticity coordinates (0.34, 0.31) provide their emission potentiality in the white light region. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: GdSr2AlO5:Ce3 þ yellow phosphor; Sol–gel process; Photoluminescence; Temperature dependent luminescence; White-LED

1. Introduction Recently, more and more attention has been paid towards the white light-emitting diodes (WLEDs) due to their lower energy consumption, fast switching, high brightness, environmentally-friendly nature, small in size, long working lifetime and high efficiency, which starts to approach the theoretical limits [1–5]. It is well established that the WLEDs would reduce the universal electricity consumption by around 50% because the electricity is directly converted to light rather than the processes in which light is the by-product of another conversion, such as in traditional lamps [6,7]. It is well known that the WLEDs are fabricated by combining UV-LED with tri-color phosphors, and another method is by uniting the blue LED with yellow phosphors. n Correspondence to: Department of Physics, Pukyong National Universiy, 599-1, Nam-gu, Daeyeon 3-dong, Busan 608-737, Republic of Korea. Tel.: þ 82 51 629 5569; fax: þ82 51 629 5549. E-mail address: [email protected] (B.K. Moon).

Among these, the WLEDs which combined with blue LED and yellow phosphors (Ce3 þ -doped yttrium aluminum garnet; Ce3 þ :YAG) have been widely used because Ce3 þ :YAG shows an efficient broad luminescence in the yellow region at around 530 nm by exciting with blue radiation near 460 nm [8–11]. The Ce3 þ :YAG based WLEDs are used in flashlights, traffic light, and displays, etc., owing to their easy fabrication, low-cost and high brightness. By focusing on these advantages, many researchers are not only developing the Ce3 þ : YAG phosphors with better performance but also continuing the search for other efficient phosphors to replace the of Ce3 þ : YAG phosphors for WLEDs [12–17]. Generally, the most common approach to prepare phosphors is the traditional solid-state reaction (SSR) method [18]. This approach typically requires a high temperature, timeconsuming heating process and subsequent grinding. The grinding process damages the phosphor surfaces, resulting in the loss of emission intensity. In addition, due to insufficient mixing and low reactivity of raw materials, several impurity phases easily co-exist in the product. In recent years, several

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.007

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wet chemical techniques such as the co-precipitation, sol–gel, combustion, hydrothermal, solvothermal and spray-pyrolysis were used to prepare the phosphor precursor [19–24]. Among these synthetic methods, nowadays, the sol–gel process has gained much attention due to its advantages in obtaining the novel chemical compositions with unique properties, excellent purity and relatively low reaction temperature resulting in more homogeneous products, and it is also possible to synthesize phosphors with smaller particle sizes [25]. Using the sol–gel method, ultra-fine and uniform ceramic powders have been prepared by precipitation. These powders of single and multiple component compositions have been produced on a nanoscale particle size for dental and biomedical applications [26]. Upon going through the literature, it is clear that the Ce3 þ : GSA phosphor by sol–gel process has not been reported so far. In this paper, we report on the synthesis of Ce3 þ activated tetragonal GSA host lattice by the sol–gel process, together with the detailed analysis of X-ray diffraction (XRD) patterns, scanning electron microscope (SEM), photoluminescence (PL), temperature dependent PL properties. 2. Experimental

FESEM observation was made on each time. The luminescence properties were measured at room temperature using the luminescence spectrophotometer (Photon Technology International (PTI)) with a 60 W xenon arc lamp and the lifetime was measured with a phosphorimeter attachment to the main system with a xenon flash lamp (25 W power). 3. Results and discussion Fig. 1 illustrates the TG/DTA curves of the powder precursor of Ce3 þ :GSA obtained by the sol–gel method. The TG curve shows three distinct weight loss steps up to 900 1C; no further weight loss was registered up to 1200 1C. The DTA curve consists of four exothermic peaks at 398 1C, 450 1C, 637 1C and 930 1C. The first three peaks indicate that the thermal events can be associated with the exhaustion of organic species of the residual nitrogen, and the last exothermic peak is due to the crystallization of GdSr2AlO5 powder from the amorphous component. The crystallization temperature is well in agreement with the XRD analysis. Fig. 2 shows the XRD patterns of Ce3 þ :GSA samples sintered at various temperatures from 1000 to 1400 1C. Although GdSr2AlO5 was crystallized from amorphous component at the

2.1. Synthesis Different concentrations (x ¼ 0–6 mol%) of Gd(1  x)Sr2AlO5: Ce3x þ (GSA) were prepared by sol–gel method by taking the stoichiometric amounts of high purity grade gadolinium nitrate hexahydrate [Gd(NO3)3  6H2O, 99.9%, Aldrich], strontium nitrate [Sr(NO3)2, 99.9%, Aldrich], cerium nitrate hexahydrate [Ce(NO3)3  6H2O, 99.9%, Aldrich], aluminum nitrate pentahydrate (Al(NO3)3  5H2O, 99.9%, Aldrich) as starting materials and citric acid [HOC(COOH)(CH2COOH)2] as a chelating agent. The starting materials were dissolved in distilled water. The mixture was stirred with a magnetic stirrer until the homogeneous solution was formed and citric acid was added into homogeneous solution. It was then heated to 80 1C at a rate of 2 1C/min and maintained for 5 h at that temperature with a magnetic stirring. After opening the cap, the solution was evaporated within 1 h and yellowish wet gel was produced. The gel dried at 120 1C in an oven for a day in ambient atmosphere. The dried powder was sintered at 1300 1C for 10 h in air.

Fig. 1. TG/DTA curves of Ce3 þ :GSA powder precursor.

2.2. Characterization Thermogravimetric/differential thermal analysis (TG/DTA) of the dried powder precursors was carried out on a Material Analysis and Characterization TG-DTA 2000 with a heating rate of 5 1C/min. The phase formation of the sintered powder was analyzed using powder X-ray diffraction (X'PERT PRO X-ray diffractometer) with a CuKα¼ 1.5406 Å and beam voltage of 40 kV and 30 mA beam current. The morphology and sizes were examined using a field emission scanning electron microscopy (FESEM), model JEOL JSM-6700 FESEM. Osmium coating was sprayed on the sample surfaces using Hitachi fine coat ion sputter E-1010 unit to avoid possible charging of specimens before

Fig. 2. XRD patterns of Ce3 þ :GSA phosphors at various sintering temperatures from 1000 to 1400 1C (‘●' indicate GdSrAlO4).

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temperature of 1000 1C, the other phase such as Al2O3, GdAlO3, SrO, GdSrAlO4, Gd2O3 phases were remained. At the temperature of 1300 1C the GdSr2AlO5 phosphor was well crystallized into tetragonal phase with a space I4/mcm, indicating that the powders prepared by a sol–gel process are pure in both chemistry and crystalline phase. The calculated lattice constants are a¼ 6.72, b¼ 6.72, and c¼ 10.91 Å, which are very close to the standard JCPDS card [PDF (70-2197)]. Typically, the crystallite size can be estimated using the Scherrer's equation, Dhkl ¼ kλ/ β cosθ, where D is the average grain size, k(0.9) is a shape factor, λ is the X-ray wavelength (1.5406 Å), β is the full width at half maximum (FWHM) and θ is the diffraction angle of an observed peak, respectively. The strongest diffraction peaks are used to calculate the crystallite size, which yield an average value of about 47.6 nm. When the temperature increases above 1400 1C, the GdSrAlO4 phase appears. From these results, Ce3 þ : GdSr2AlO5 phosphors had been stable at the sintering temperature 1300 1C, and above 1400 1C, it decomposed to GdSr2AlO5 and Al2O3 phase. Luminescence efficiency of phosphors depends upon the morphology such as crystallite size, shape, grain boundary, defects and so on [27]. Ce3 þ :GSA phosphors morphology is shown in Fig. 3. When the temperature increases to 1000 1C, the size of particles were begun to agglomerate. While the temperature increases to 1100 and 1200 1C, the net type morphology of the tiny particles was observed, and which starts to coalesce into bigger particles due to Ostwald's ripening. Further, temperature increases to 1300 1C, the combinations of the micron- sized particles with grain boundary lines were observed, and the size of particles were increased with increasing temperature above 1300 1C. It is well known that the closely packed particles prevent the scattering of light and yield the light output efficiently. Fig. 4 shows the excitation and emission spectra of Ce3 þ : GSA phosphors prepared at various sintering temperatures from 1000 to 1400 1C. As known, there is only one electron in

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the 4f state for trivalent Ce ion. The ground state of Ce3 þ is split into 2F5/2 and 2F7/2 with an energy difference of about 2200 cm  1. The first excited state of Ce3 þ is considered as 5d state and the transitions of 4f–5d are parity allowed. In Fig. 4a, the excitation spectra consists of two broad bands peaked at 337 nm and 442 nm due to the transitions from the 4f ground state to crystal field splitting of the 5d states of Ce3 þ ion [28]. When the Ce3 þ :GSA phosphor excited with the blue radiation of wavelength at 442 nm, the Ce3 þ ion of 4f level would be raised to the higher 5d level and would feed afterward to the lower 2D(5d) excited states. The emission spectra of all these samples exhibited a broad band with a band maxima between 560 and 575 nm, which is assigned to the 5d-4f (2F5/2 and 2 F7/2) transitions of Ce3 þ , since Ce3 þ with a 4f electron configuration has two ground states of 2F5/2 and 2F7/2. These transitions of Ce3 þ ions are due to spin orbit coupling under the influence of Oh crystal field [29]. Furthermore, the

Fig. 4. Photoluminescence excitation and emission spectra of Ce3 þ :GSA phosphors at various sintering temperatures.

Fig. 3. The SEM images of Ce3 þ :GSA phosphors prepared at various sintering temperatures; (a) 1000 1C, (b) 1100 1C, (c) 1200 1C, (d) 1300 1C, and (e) 1400 1C.

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emission intensity increases with increasing the sintering temperatures due to improved crystallanity. In order to further understanding of luminescent properties, excitation and emission measurements of Ce3 þ :GSA phosphors as a function of Ce3 þ concentration were carried out (Fig. 5). When the concentration of Ce3 þ ions increased up to 5 mol%, the intensities of excitation and emission bands also increased and then intensity reaches its maximum value. When the concentration of Ce3 þ is further increased above 5 mol%, the excitation and emission intensity begins to decrease due to concentration quenching. The optimum doping concentration of Ce3 þ ions were found to be at 5 mol%. In general, at high levels of doping concentration, a strong interaction occurs between activating ions with reduced distance, resulting in a concentration quenching. A rough estimation of the critical distance for energy transfer (Rc) of Ce3 þ :GSA can be made using the relation given by Blasse to calculate Rc between activator ions of the same kind in host lattice [29]:  Rc  2

3V 4πxc N

1=3 ð1Þ

where Rc corresponds to the mean separation between the nearest Ce3 þ ions at the critical concentration, V is the volume of the unit cell, Z is the number of cations in the unit cell and xc is the critical concentration of activator ions. According to crystal structure of GdSr2AlO5 compound, V ¼ 492.68 Å3, Z ¼ 4 and xc ¼ 0.05, and the calculated Rc is around  17 Å. Fig. 5c shows the peak position at the band maxima and their intensity as a function of Ce3 þ concentration. With increasing the Ce3 þ concentration, emission peak shifted towards the longer wavelength region. Because the ionic radius of Ce3 þ (1.143 Å) is larger than that of Gd3 þ (1.053 Å) ions, Ce3 þ ions substituted for the Gd3 þ site and f–d transition of the Ce3 þ ion is sensitive to host lattice environment. The temperature dependent luminescent properties of phosphors are crucial for testing the suitability of their applications in the development of WLEDs because the thermal quenching property is an important parameter to be considered [30,31]. Generally, the junction temperature of LEDs is about 120 1C and above that temperature, the significant thermal quenching as well as the shifting of emission color is possible. The phosphors have been required high emission at high working temperature for a long term in order to achieve a long lifetime LEDs [32]. Thus, we performed the temperature dependent measurements within the temperature range from 25 to 150 1C for the 5 mol% Ce3 þ :GSA phosphor and compared with Ce3 þ :YAG phosphor. Fig. 6a shows the temperature dependent luminescence spectra of the Ce3 þ :GSA phosphors under the excitation at 442 nm with a step of 5 1C from 25 to 150 1C. Clearly, the emission intensity decreases linearly with increasing the temperature due to the thermal quenching [33]. The emission intensity droped by  70%, and remains 30% of its original intensity at 150 1C. White LEDs were fabricated by combining the blue LED chip with the synthesized GSA:Ce3 þ phosphor, the resulting emission spectrum under a forward bias of 150 mA has been presented in Fig. 6b. The Commission International De I-Eclairage (CIE)

Fig. 5. Photoluminescence spectra of Ce3 þ :GSA phosphors at different concentrations of Ce3 þ ions; (a) excitation spectra, (b) emission spectra, and (c) the peak maxima position and relative emission intensity.

chromaticity coordinates for Ce3 þ :GSA phosphors were calculated using the blue LED with the excitation at 480 nm, as shown in Fig. 6c. The digital photograph image of fabricated LED before and after packaging of GSA phosphor presented in Fig. 6c. We have observed that the 5 mol% Ce3 þ doped GSA phosphor exhibits excellent CIE coordinates of (0.34, 0.31), which are quite

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Fig. 6. (a) The emission intensities of Ce3 þ :GSA phosphors as a function of temperature. (b) Emission spectrum of white LED (blue LED þGSA phosphor). (c) CIE color coordinates (1) Ce3 þ :GSA phosphor, (2) blue LED (465 nm) þGSA phosphor, and (3) blue LEDþYAG phosphor. The inset of (c) shows the photographs of blue emission from blue LED chip and white light emission from the combination of blue LED and GSA phosphor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

close to that of the CIE of Ce3 þ :YAG: phosphor (0.292, 0.325) and white light condition (0.33, 0.33). So, we suggest that Ce3 þ : GSA phosphors are suitable for the generation natural white light emission [34].

4. Conclusion In summary, Ce3 þ doped GdSr2AlO5 phosphors were successfully prepared by a sol–gel method. A tetragonal XRD phase and closely packed particles were observed after sintering at 1300 1C. The luminescence properties of the novel Ce3 þ doped GSA phosphors exhibited a yellow emission peak at around 565 nm under the blue excitation wavelength. From the PL investigation, it was observed that the reported phosphor materials would be a promising yellow emitting host which was assigned to a transition from the upper 5d1 to the ground 4f1 state of Ce3 þ ions. Among the different concentrations, 5 mol% Ce3 þ doped GSA showed the highest luminescence intensity. The CIE coordinates of Ce3 þ doped GSA phosphors combined with blue-light exhibited good chromaticity coordinates (0.34, 0.31) in the white region. Hence, we are able to suggest that the Ce3 þ :GSA phosphors

are promising materials for their application in the development of white light for indoor illuminations. Acknowledgment This work was supported by a Research Grant of Pukyong National University (2013 Year) and GdSr2AlO5:Ce3+ supplied by the Display and Lighting Phosphor Bank at Pukyong National University. References [1] N. Narendran, Y. Gu, J.P. Freyssinier, H. Yu, L. Deng, Solid-state lighting: failure analysis of white LEDs, J. Crystal Growth 268 (2004) 449. [2] J.Y. Park, H.C. Jung, G.Seeta Rama Raju, B.K. Moon, J.H. Jeong, J.H. Kim, Tunable luminescence and energy transfer process between Tb3 þ and Eu3 þ in GYAG:Bi3 þ , Tb3 þ , Eu3 þ phosphors, Solid State Sci. 12 (2010) 719. [3] G.S.R. Raju, J.Y. Park, H.C. Jung, E. Pavitra, B.K. Moon, J.H. Jeong, J.H. Kim, Excitation induced efficient luminescent properties of nanocrystalline Tb3 þ /Sm3 þ :Ca2Gd8Si6O26 phosphors, J. Mater. Chem. 21, 2011, p. 6136. [4] G.S.R. Raju, J.Y. Park, H.C. Jung, R. Balakrishnaiah, B.K. Moon, J.H. Jeong, Blue and green emissions with high color purity from

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