Luminescence studies on SrMgAl10O17:Eu, Dy phosphor crystals

ARTICLE IN PRESS Optics & Laser Technology 41 (2009) 81– 84

Contents lists available at ScienceDirect

Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec

Luminescence studies on SrMgAl10O17:Eu, Dy phosphor crystals Tang Wanjun , Chen Donghua, Wu Ming Hubei Key Laboratory for Catalysis and Material Science, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan 430074, China

a r t i c l e in fo

abstract

Article history: Received 1 November 2007 Received in revised form 28 March 2008 Accepted 30 March 2008 Available online 2 June 2008

Using urea as fuel, SrMgAl10O17:Eu, Dy phosphor was prepared by a combustion method. Its luminescence properties under ultraviolet (UV) excitation were investigated. Pure SrMgAl10O17 phase was formed by urea-nitrate solution combustion synthesis at 550 1C. The results indicated that the emission spectra of SrMgAl10O17:Eu, Dy has one main peak at 460 nm and one shoulder peak near 516 nm, which are ascribed to two different types of luminescent Eu2+ centers existing in the SrMgAl10O17 matrix crystal. The blue luminescence emission of SrMgAl10O17:Eu phosphors was improved under UV excitation by codoping Dy3+ ions. The SrMgAl10O17:Eu phosphors showed green afterglow (l ¼ 516 nm) when Dy3+ ions were doped. Dy3+ ions not only successfully play the role of sensitizer for energy transfer in the system, but also act as trap levels and capture the free holes in the spinel blocks. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Combustion synthesis Optical materials and properties Luminescence

1. Introduction Synthetic hexagonal alkaline earth aluminates doped by divalent europium ions are efficient luminescence materials. They show a blue emission that is characterized by high quantum efficiency under UV excitation [1]. They are widely used in plasma display panels (PDPs), field emission displays (FEDs) and fluorescence lamps [2,3]. Hexagonal alkaline earth aluminate phosphors are typically produced by the solid–state reaction method. Combustion synthesis is a novel technique that has been applied to phosphor synthesis in the past few years [4]. Combustion synthesis involves an exothermic reaction between metal nitrates and a fuel. This technique produces highly crystalline powders in the as-synthesized state. In this paper, the Eu2+, Dy3+ codoped SrMgAl10O17 samples were synthesized by a simple combustion process. Their emission and excitation spectra have been investigated and a green afterglow observed. The luminescence performance can be improved greatly when phosphors are doped with suitable auxiliary activators [5]. The role of the codoping with Dy3+ ions in the enhancement of the fluorescence and afterglow from SrMgAl10O17:Eu was studied in detail.

2. Experimental procedure The starting materials were Eu2O3 (4N), Dy2O3 (4N), Sr(NO3)2  4H2O (AR), Mg(NO3)2  6H2O (AR), Al(NO3)3  9H2O (AR)  Corresponding author. Tel./fax: +86 27 67842752.

E-mail address: [email protected] (T. Wanjun). 0030-3992/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2008.03.009

and urea (AR). Eu2O3, Dy2O3 were dissolved in HNO3 solution, and then stoichiometrically weighed metal nitrates and urea were added, mixed with vigorous stirring at 70 1C for 30–40 min. The clear solution was transferred into a porcelain crucible. The crucible containing the solution was introduced into a muffle furnace that had been preheated at 550 1C. With boiling, the solution evaporated and became increasingly more viscous. After a few minutes, combustion ignition took place and voluminous white phosphor powders were obtained. The crystal phases of the prepared particles were analyzed by X-ray diffraction (XRD) pattern measured using a Bruker D8 (Bruker Co. Ltd, German) X-ray diffractometer with graphite monochromatized Cu Ka irradiation (l ¼ 1.5406 A˚). The emission and excitation spectra of all the samples were obtained using a spectrophotometer (Perkin–Elmer LS-55, Perkin–Elmer Co. Ltd., USA) using a Xe flash lamp. The decay curve of afterglow was measured using a ST-86LA (Peking Normal University, China) brightness meter.

3. Results and discussion Fig. 1 shows XRD patterns of SrMgAl10O17 and SrMgAl10O17:Eu0.04, Dy0.04 particles prepared by the combustion method. There are no observable differences between the two diffraction patterns, indicating that the pure phase of SrMgAl10O17 (JCPDS No. 26-0879, space group P63/mmc, a ¼ 5.63 A˚, c ¼ 22.47 A˚) has already formed in the combustion step. Furthermore, the little amount of doped rare-earth ions has almost no effect on the SrMgAl10O17 phase composition. In this work, the structure of SrMgAl10O17 with space group P63/mmc was taken as the starting model for synthetic phosphors.

ARTICLE IN PRESS 82

T. Wanjun et al. / Optics & Laser Technology 41 (2009) 81–84

Fig. 1. Powder XRD patterns of SrMgAl10O17:Eu, Dy phosphor.

The luminescence excitation and emission spectra of SrMgAl10O17:Eu, Dy specimens prepared with different dopant contents are shown in Fig. 2. The emission intensity of phosphors was monitored under UV (l ¼ 314 nm) excitation with varying rareearth ions concentration and ratio. Meanwhile, the excitation intensity of phosphors was monitored at l ¼ 460 nm. The prepared phosphors exhibit a blue luminescence emission. No emission beyond 600 nm was observed, indicating that the Eu3+ ions have been effectively reduced to Eu2+ ions during the combustion process [6]. The emission spectrum excited by UV (l ¼ 314 nm) consists of a wide band with a peak at about 460 nm, which corresponds to the 5d–4f transitions of Eu2+ ions. The excitation spectrum of the blue fluorescence (monitored at l ¼ 460 nm) shows two wide bands with their peaks at 250 and 314 nm, respectively, which are due to the crystal field splitting of the Eu2+ d-orbital. Excitation spectra monitored at 460 nm show an optimal excitation band centered at 314 nm. When Dy3+ is doped in the phosphors, the shape of the excitation and emission spectra remains, but the excitation intensity increases in comparison with the undoped case. The luminescent intensity excited by UV increases with the amount of Dy3+ increasing, as shown in Fig. 2a, which gives the luminescent spectra of SrMgAl10O17:0.04Eu2+, xDy3+ phosphors when x is changed. With increasing Dy3+ content at fixed Eu2+ content (0.04 mol) in SrMgAl10O17:Eu2+, Dy3+ phosphor, the emission intensity increases. But when the value of Dy/Eu is over 1, the emission intensity decreases. In general, the excess concentration of activators quenches the photoluminescence. So, it is necessary to experimentally find the critical concentration of Eu2+ or Dy3+ in terms of optimizing the luminous efficiency of SrMgAl10O17:Eux Dyx under the UV excitation. The concentration quenching was observed (Fig. 2b) when the Eu content (x) was 0.04. At this critical concentration, the highest luminescent intensity was observed. In addition to the main peak at 460 nm, a shoulder emission at 516 nm was also seen in the SrMgAl10O17:Eu, Dy phosphors. This shoulder emission was found to be the origin of a green color of afterglow from SrMgAl10O17:Eu, Dy crystals based on afterglow measurements. Divalent Europium ions show a rather short decay time in the range 1–10 ms since the 5d–4f emission of Eu2+ is spin- and parityallowed [7]. However, luminescent materials activated by Eu2+ can show afterglow, especially if additional dopants, e.g. Dy3+, are

Fig. 2. Excitation spectra (lem ¼ 460 nm) and emission spectra (lex ¼ 314 nm) of SrMgAl10O17:Eu, Dy phosphor.

incorporated into the host lattice. Interestingly, the SrMgAl10O17 phosphor doped only with Eu2+ shows a small but certain amount of afterglow. The afterglow spectra of SrMgAl10O17:Eu, Dy with different compositions are shown in Fig. 3. The afterglow intensity of SrMgAl10O17:Eu, Dy phosphors was measured under UV (l ¼ 334 nm) excitation. Meanwhile, the excitation spectra of phosphors were monitored under l ¼ 516 nm. The afterglow spectra of SrMgAl10O17:Eu2+, Dy3+ were measured after the excitation source was switched off at 1 ms. Under UV excitation all specimens yielded a green afterglow with the peak wavelength at 516 nm. The optimal concentration of Eu2+ is 0.04 and the optimal ratio of Eu to Dy is 1 for maximum intensity of afterglow (Fig. 3a). An important result of the present work is that we observed green afterglow in the SrMgAl10O17:Eu2+, Dy3+ phosphors. Fig. 4 shows the decay curves of afterglow of the specimens. These specimens were irradiated by the UV light for 10 min. After the light source was removed, all of the specimens showed a rapid decay and subsequently an afterglow. When the value of Eu/Dy is

ARTICLE IN PRESS T. Wanjun et al. / Optics & Laser Technology 41 (2009) 81–84

83

Fig. 4. Afterglow decay curve of SrMgAl10O17:Eu, Dy phosphors.

Fig. 3. Afterglow excitation spectra (lem ¼ 512 nm) and emission spectra (lex ¼ 340 nm) of SrMgAl10O17:Eu, Dy phosphor.

1, the initial luminescence intensity is higher and the decay time is longer than that of the others. The decay curves had been analyzed by curve fitting and it was found that the curve could be fitted perfectly using the equation [8] (shown in Fig. 4) I ¼ A1exp(t/t1)+A2exp(t/t2), where I is the afterglow intensity; A1 and A2 are the two constants, t is defined as the decay time for the exponential components. Using the fitting function provided by Microcal Origin, the parameters of A1, t1(s), A2, and t2(s) calculated are 0.54, 7.47, 0.08, and 38.16, respectively. The emission spectra of SrMgAl10O17:Eu2+, Dy3+ vary largely between luminescence (Fig. 2) and afterglow (Fig. 3). The luminescence emission spectra of SrMgAl10O17:Eu2+, Dy3+ measured under UV excitation show a main band at 460 nm and a shoulder band at 516 nm. Meanwhile, its afterglow emission spectrum measured under UV excitation shows only a band at 516 nm. Thus, the environment of an activator for luminescence differs from that for afterglow. Detailed information on the crystal parameters of SrMgAl10O17 is very important for understanding the underlying mechanism of its luminescence properties. It has been shown by using Sm2+ as

probe that BaMgAl10O17 structure offers two or three different types of sites for Sm2+ [9]. Since the ionic radii and the chemical properties for Sm2+ and Eu2+ are nearly identical, and the SrMgAl10O17 and BaMgAl10O17 particles have the same crystal structure [2]; one may make a similar conclusion regarding the sites of Eu2+ in SrMgAl10O17. Since the crystal field can affect the 4f65d1 electronic states of 2+ Eu , host lattices with Eu2+ ions on different crystallographic sites show more than one emission band. The structure of strontium magnesium hexaaluminate (SrMgAl10O17) phase (JCPDS No. 26-0879, space group P63/mmc, a ¼ 5.63 A˚, c ¼ 22.47 A˚) is well established [10]. SrMgAl10O17 consists of one conduction layer (SrO) and two spinel blocks (MgAl10O16) stacking alternatively in the c-direction forming a layer structure. The b–alumina structure contains corner-sharing AlO4 tetrahedra and 9-coordinated Sr2+. A small Mg2+ ion is known to be incorporated in the spinel block by replacing trivalent Al3+. Therefore, there are at least two sites available for incorporating Eu2+ in the SrMgAl10O17 lattice. Because the radius of Sr2+ (0.121 nm) is very similar to that of Eu2+ (0.120 nm), Eu2+ ions mainly occupy Sr2+ sites in the conduction layer and form the corresponding emission center, which peaks at about 460 nm. As a result, SrMgAl10O17:Eu shows a strong blue emission with a peak at 460 nm as a consequence of 4f6d1-4f7 electron transition in Eu2+ [11]. A small quantity of Eu2+ ions are assumed to incorporate into the spinel blocks (MgAl10O16) instead of the conduction layer (SrO) of SrMgAl10O17, which produce a new shoulder band at l ¼ 516 nm. The Eu2+ ions located at these two different sites should emit in different energy ranges. Codoping with Dy3+ ions did not change the wavelength position or the bandwidth of the luminescence and afterglow spectra. The luminescence of SrMgAl10O17:Eu2+ was improved largely by codoping with Dy3+. Doped Dy3+ ions may play the role of an auxiliary activator, which consequently results in large enhancement in the luminescence intensity of SrMgAl10O17:Eu. In the spinel blocks, the incorporation of the Dy3+ ion forms a highly dense trapping level located at a suitable depth in relation to the thermal release rate at room temperature, thus producing the obvious afterglow [12].

4. Conclusions A SrMgAl10O17:Eu, Dy phosphor was synthesized via a combustion process from metal nitrates and urea as fuels. The

ARTICLE IN PRESS 84

T. Wanjun et al. / Optics & Laser Technology 41 (2009) 81–84

X-ray diffraction results indicate that the doped and pure SrMgAl10O17 samples were successfully obtained by combustion reactions. The fluorescence intensity of SrMgAl10O17:Eu blue phosphor can be improved by codoping the host lattice with Dy3+. Incorporation of Dy3+ also results in the occurrence of afterglow, which can be fitted by a biexponential decay function. It is possible to suggest that SrMgAl10O17:Eu, Dy has some adequate features, and can be used as a kind of luminescent material in solid-state light resources. References [1] No¨tzold D, Wulff H, Jilg S, Kantz L, Schwarz L. Structure and optical properties under VUV/UV excitation of Eu2+-doped alkaline earth aluminate phosphors. Phys Stat Sol A 2006;203:919–29. [2] Youl Jung KY, Lee HW, Kang YC, Park SB, Yang YS. Luminescent properties of (Ba,Sr)MgAl10O17:Mn, Eu green phosphor prepared by spray pyrolysis under VUV excitation. Chem Mater 2005;17:2729–34. [3] Dawson B, Ferguson M, Marking G, Diaz AL. Mechanisms of VUV damage in BaMgAl10O17:Eu2+. Chem Mater 2004;16:5311–7.

[4] Park S, Kang S. Combustion synthesis of Eu2+-activated BaMgAl10O17 phosphor. J Mater Sci 2003;14:223–8. [5] Zhang J, Zhang Z, Tang Z, Tao Y, Long X. Luminescent properties of the BaMgAl10O17:Eu2+, M3+ (M ¼ Nd, Er) phosphor in the VUV region. Chem Mater 2002;14:3005–8. [6] Chang H, Lenggoro IW, Ogi T, Okuyama K. Direct synthesis of barium magnesium aluminate blue phosphor particle via a flame route. Mater Lett 2005;59:1183–7. [7] Justel T, Bechtel H, Mayr W, Wiechert DU. Blue-emitting BaMgAl10O17:Eu with a blue body color. J Lumin 2003;104:137–43. [8] Katsumata T, Nabae T, Sasajima K, Komuro S, Morikawa T. Effects of composition on the long phosphorescent SrAl2O4:Eu2+, Dy3+ phosphor crystals. J Electrochem Soc 1997;144(9):L243–5. [9] Ellens A, Zwaschka F, Kummer F, Meijerink A, Raukas M, Mishra K. Sm2+ in BAM: fluorescent for the number of luminescing sites of Eu2+ in BAM. J Lumin 2001;93:147–53. [10] Iyi N, Gobbels M. Crystal structure of the new magnetoplumbite-related compound in the system SrO–Al2O3–MgO. J Solid State Chem 1996; 122:46–52. [11] Dorenbos P. Energy of the first 4f7-4f65d transition of Eu2+ in inorganic compounds. J Lumin 2003;104:239–60. [12] Matsuzawa T, Aoki Y, Takeuchi N, Murayama Y. A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+, Dy3+. J Electrochem Soc 1996; 143:2670–3.