luminescent properties of green phosphor ca8mg(sio4)4 cl2 :eu2+, dy3+ for led applications

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JOURNAL OF RARE EARTHS 25 (2007) 573 - 577 www.elsevier.comilM4re

Luminescent Properties of Gmen Phosphor C%Mg(SiO, )4 C12 :Eu2 + Dy3 + for LED Applications Fang Ying (5 Huang Xiaowei (X d J \ 3)' ( I . Ndiond Engineering Resemh Center for Rare Ewth Materials, General Research Institute for Non-Femus Metals, m d Grirem Advanced Materials Co., Ltd., Beijing 100088, China; 2. Depwtment of Chemktly, Shangqiu Normal University, Shmgqiu 476000, China)

S)1'2, Zhuang Weidong ( E l l % ) ' * , Hu Yunsheng (4AS&a>', Ye Xinyu (*@?)I,

Received 14 December 2006; revised 25 January 2007

Abstract: The new phosphor calcium magnesium chlorosilicate, codoped with EuZf and Dy3', was synthesized with the help of the high temperature solid state reaction in reducing atmosphere. The excitation and emission spectra were very similar to that of Ca,Mg(Si04),C1,:EuZ', and the Dy" concentration influenced the emission intensity of this phosphor. The intensity of Eu2+ and Dy3+ codoped CMSC was stronger than that of EuZ+ singly doped CMSC. The emission spectrum of the Dy3' ion overlapped the absorption band of the EuZf ion, indicating that an energy transfer from Dy3' to Eu" took place in CMSC:EuZ', Dy3' phosphor. The mechanism of the energy transfer from Dy3+ to Eu2', in this phosphor, might be resonant energy transfer.

Key wonk: calcium magnesium chlorosilicate; phosphor; energy transfer; Eu2+ ; Dy3 + ; rare earths CLC number: 0482.3 Document code: A Article ID: 1002 - 0721 (2007)05 - 0573 - 05

The LED (light emitting diode) has m y advanta- ges such as low enera consumption, high luminosity, and long lifetime, compared with the conventional lamps. Presently, the combination of a blue LED and yellow phosphor Y3Al,0,,:Ce3' is most popular for white-light generation" - 4 1 . However, this device has a lower color rendering index and higher color temperature because of the red deficiency of the YAO phosphor. In recent years, the combination of UV-LED and tri-color phosphor has attracted intensive interest, as it can produce LEDs with full color. To successfully ap- ply phosphor-combined UV-LED to li&ting, fluores- cent materials emitting blue, green, and red lights should have high efficiency under UV-LED excitation.

Ca,Mg(SiO,),CI, (CMSC) crystal structure is a suitable host lattice for luminescent materials with

stable crystal structure and good thermal stability. It has a cubic crystal structure with space group of F&m,unit cell parameter of a = 1.506 nm. In this crystal lattice, there are three types of cation sites, two Ca sites and one M g site. The two Ca sites are the 8-coordinate one and 6-coordinate one, respectively, and the M g site is the 4-coordinate one. The crystal field environments for the three sites are very different's]. CMSC doped with Eu2+ is a green phosphor, and it is known as a suitable UV-LED phosphor, because it has better luminescence characteris- tics under UV-LED excitationL6].

Eu2+ and Dy3+ are good activators for phosphors. They generally act as coactivators, in silicate or aluminate hosts, to produce long afterglow lumines-

* Comsponding autho(E-mai1: [email protected]) Foundation item: Project supported by the National Natural Science Foundation of China (50372086) and MOST of China (2006CB601104) Biography: Fang Ying (1976-), Male, Doctoral candidate

Copyright 02007, by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B.V. All rights reserved.

574 JOURNAL OF RARE EARTHS, Vo125, NOS, Oct. 2007

c e n ~ e [ ~ - ' ' ] . It is generally thought that the Eu2+ ion is the luminescent center. The photoluminescence is considered to be the transition from the 5d to 4f level of Eu2+, and the holes in the traps are responsible for the long afterglow. In this article, Eu2+ and Dy3+ were codoped into the CMSC host, and they pro- duced much stronger luminescence than that of CM SC single doped with Eu", although the long persistence could hardly be observed. The luminescent properties were investigated and energy transfer between Eu2+ and Dy3+ was discussed.

1 Experimental The preparatory materials of CaCO, (A. R.),

MgO(A. R.), SiO, (A. R.), and CaC1, (A . R.) were weighted with an appropriate stoichiometric ratio. A small amount of high purity Eu,O, (99.99%) and Dy203(99.99%) were added, and CaC1, was excessive, about loo%, as flux. After mixing and grinding thoroughly in an agate mortar, the homogeneous mixture was fired at 1100 "c for 2 h in reducing atmosphere. The sintered cake was milled and washed with deionic water to remove the excessive CaCl,, and the desirable phosphor sample was obtained with the help of the drying procedure.

The crystal structure of the sample was checked using an MXP21 VAHF-M21 X X-ray diffractometer, running Cu Ka radiation at 40 kV and 250 mA. The excitation and emission spectra of the sample were detected using a SPEX FluroMax-2 fluorescence spec- trometer.

2 Results and Discussion

2.1 X-my diffmtion analysis Fig. 1 compares the XRD patterns of CMSC:

Eu2+,Dy3+ with that of CMSC: Eu2+. Obviously, both samples exhibit the CMSC phase. The positions

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20 40 60 80 100

204")

Fig. I XRD pattern of CMSC:Eu2', D y 3 + ( l ) and CMSC: Eu2 + (2)

and relative intensities of the diffraction peaks are in good agreement with the results of Ref. [ 5 1 , indicating that the sample is a cubic crystal structure with a space group of F d m , and the doping of Eu2+ and Dy3+ ions does not change the crystal structure.

2.2 Excitation and emission spectra of phosphols

As is known, the divalent Eu ion can emit light from UV to infi-ared depending on the crystal fields from the host. When Eu2+ is doped into CMSC, broad emission occurs[I2]. On the other hand, Dy3+ usually exhibits blue and yellow luminescence when introduced into some hosts"31.

Eu2+ and Dy3+ ions have been codoped into the CMSC host. The excitation and emission spectra at room temperature are presented in Figs. 2 (1 ) and Fig. 3(l), respectively. As shown in Fig. 2, the excitation spectrum (Aern =505 nm) is very broad, from 300 to 470 nm, that is to say, this phosphor can be excited by the UV or blue LED chip. The CMSC:Eu2', Dy' + phosphor exhibits strong broadband emission, as shown in Fig. 3 . The main emission peak is at about 505 nm, which is ascribed to the transition between 5d and 4f of Eu2+ . The 5d state of Eu2+ is

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Excitation spectra of CMSC:Eu*+, D y 3 + ( l ) and CM- SC:Euz + ( 2 )

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Emission spectra of CMSC:Eu2', D y 3 + ( l ) and CMSC: EuZ + ( 2 ) phosphor

Fang Y et ak Luminescent Pmp&ies of Cn,Mg(SiO,),C&:Eu", Dy'+ 575

greatly affected by the crystal field, and it can be spitted by different crystal fields. This makes Eu2' emit different light when the crystal fields change. In CMSC:Eu2', Dy3', Eu2+ occupies two kinds of Ca2' sites: 8-coordinate and 6-coordinate ones. The two sites have different crystal field environments, which leads to two emission bands in CMSC:Eu2+, Dy3': a very strong green band peaked at 505 nm ascribed to Euz+ in the 8-coordinate Ca site, and a weak band peaked at 427 nm assigned to Eu2+ in the 6-coordinate Ca site.

The excitation and emission spectra of CMSC single doped with Eu2+ are also presented in Fig. 2(2) and Fig. 3(2), for comparison. Although the shapes of excitation and emission spectra are very similar to those of CMSC doped with Eu2+ and Dy3', the latter has a higher excitation efficiency and higher emission intensity.

The position of EuZt and Dy3+ in CMSC should be explained comprehensively. Taking the crystal structure into account, the lattice would be distorted and unstable when the M 2 ' ion (0.057 nm) is re- placed with bigger ions (Eu2+ or Dy3+). As the ion radius of Ca2' is closer to that of Eu2+ or Dy3+ , the Ca" site will preferentially be occupied, and Eu2+ mainly occupies the 8-coordinate Ca site (0. 112

The positive ions coordinate with as many negative ions as possible when they come into contact with each other. The polyhedral structure will be de- termined by a radius ratio of positive ion to negative ion. By calculating the value of r f / r - , a conclusion can be drawn that the preferentially selected site occupied by Dy3+ is the 6-coordinate Ca" site.

To further investigate the influence of Dy3+ on the luminescent properties in CMSC, the Dy3' ion is solely implanted into the CMSC host. The excitation and emission spectra are presented in Figs. 4 and 5, respectively. As shown in Fig. 4, the excitation spectrum consists of four efficient sharp peaks, which are ascribed to the f-f transition of the Dy3+ ion. Mean- while, there is a broad excitation band below 300 nm in the spectrum, because of the transfer of an electron from the oxygen to the Dy3 + ion (charge transfer, CT transition). The CT transition is allowed as a pure electronic transition and manifests in the spectra as a broad band with high intensity, and is generally loca- ted in the UV and W V energy range. Additionally, the emission spectrum consists of two emission re- gions: the blue region and the yellow region, which are assigned to the transition of the Dy3+ ion from the F,, excited state to the 6H,,, and 6H13, ground state,

respectively . The intensity of the blue emission is

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Fig. 4 Excitation spectrum of the Dy" ion in CMSC (Aern =

492 nm) (excitation peaks (1) 325 nm; (2) 350 nm; (3) 365 nm; and (4) 387 nm)

100-

80-

60 - 40 - 20 - 07

I 400 450 500 550 600 650 700

Wavelength!nm

Fig 5 Emission spectrum of Dy3+ ion in CMSC(A,, =350 nm)

stronger than that of the yellow emission. Obviously, the codopant only displays the lumi-

nescence from the Eu", and the emission of the Dy3+ ion cannot be observed. The excitation spectrum has no change of shape, but the intensity increa- ses compared with the CMSC:Eu2+, thus the emission is strengthened. It can be considered that Dy3+ absorbs the excitation energy, and then transfers it to Eu2+ and produces a stronger emission, which implies that Dy3+ sensitizes the luminescence of Eu2+.

2 3 Effect of Dy3+ concentration on luminescent pmperties of CMSC:Eu2+, Dy3+

Fig. 6 shows the effect of Dy3+ concentration on the luminescent properties of CMSC:0.1Eu2+, xDy3+ with the concentration of Eu2+ remaining at 0. 1 mol% . It can be seen that the emission intensity rises with an increase in Dy3+ concentration, from 0 to 0.02 mol%, and reaches the maximum when the concentration is 0.02, and then the intensity falls gadually with the Dy3' concentration increasing from 0.02 up to 0.06 mol%. As Dy3+ is in low concentration, the interaction between the Dy3+ and E d + ions is very lim- ited, and the emission intensity rises. Subsequently, the concentration quench of Dy3+ occurs, resulting in a de- crease in emission intensity.

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2.4 Enerpy tiansfer between Eu" and Dy3+ The rare earth ion is a discrete luminescent cen-

ter. Energy transfer is often caused by interaction of two centers. Dexters theory of resonant energy transfer elucidates that two optical centers within a certain distance may be in resonance and may transfer the excitation energy from a sensitizer to an activator. The close proximity of the centers enables them to be con- nected by electrostatic interaction or by the quantum mechanical exchange interaction. For resonant energy transfer, it is necessary that the transition energies of the sensitizer and activator be equal. Besides the close distance between the sensitizer and activator ions, the spectral overlap requires the resonance energy trans-

It is mentioned earlier that the Dy3' ion preferentially occupies the 6-coordinated Ca" site in the * CMSC lattice. Dy3+ and EuZf ions in the 8-coordinated Ca" site are very close to each other. The 4F,, -+6H,,, emission band of the Dy3+ ion overlaps the absorption band of the E d ' ion (Fig. 7). There is a pathway in the dipolar-dipolar interaction of Dy3'+ Eu2+. It also offers the possibility of irradiation reabsorption. But irradiation reabsorption depends on en-

fer[ 13, 141

0.bO 0.bl 0.b2 0:03 0 . b 0:OS 0.b6

Content of Dy3+

Fig. 6 Effect of Dy3+ concentration on luminescent intensity of CM SC:O.I Eu2 + , xDy ' +

120 4 I

I * I 200 300 400 500 600 700

Wavclengthhm

Fig 7 Spectra of the absorption of the Eu2+ ion (1) and the e- mission of the Dy3+ ion (2 ) , indicating the overlap between the absorption and emission bands

ergy transfer by the photon. Transfer distance is ei- ther large or small. It can be easily found that the mechanical mixture of CMSC: 0. 2Eu2' and CMSC: 0. 04Dy3' has a lower emission intensity than CM- SC:0.1Eu2', 0.02Dy3+. The relative emission intensity of the former is lower than that of CMSC: 0. 1Eu2+, whereas, that of the latter is much stronger than CMSC:0.1Eu2+. Therefore, it indicates that the energy transfer efficiency is related to the distance of

. The mechanism of the energy transfer from Dy3+ to Euz+ in this phosphor may not be irradiation reabsorption, but resonant energy transfer, which improves the luminescent efficiency to some extent.

~~3 + and &2+ [Is, 161

3 Conclusion In summary, a new phosphor CMSC: ELI'+,

Dy3+ was synthesized by a solid-state reaction at high temperature in reducing atmosphere. This phosphor could be efficiently excited by ultraviolet or blue light LED chip. The emission spectrum exhibited a stronger broadband emission with the main peak at 505 nm, which was attributed to Eu2+ emission. Dy3 + concentration influenced the emission intensity of this phosphor. The intensities of Euz+ and Dy3+ codoped CMSC were stronger than that of CMSC singly doped with E d + . It could be considered that nonradiative energy transfer from Dy3' to Eu2+ took place in the CMSC:Eu2',Dy3+ phosphor. Eu" ion was sensitized by the Dy3+ ion because of the resonant energy transfer.

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luminescent properties of green phosphor ca8mg(sio4)4 cl2 :eu2+, dy3+ for led applications

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