Significant Improved Luminescence Intensity of Eu 2?+? -Doped Ca 3 SiO 4 Cl 2 Green Phosphor for White LEDs Synthesized Through Two-Stage Method

Transcription

1 Significant Improved Luminescence Intensity of Eu 2?+? -Doped Ca 3 SiO 4 Cl 2 Green Phosphor for White LEDs Synthesized Through Two-Stage Method I. Baginskiy and R. S. Liu J. Electrochem. Soc. 2009, Volume 156, Issue 5, Pages G29-G32. doi: / alerting service Receive free alerts when new articles cite this article - sign up in the box at the top right corner of the article or click here To subscribe to Journal of The Electrochemical Society go to: ECS - The Electrochemical Society

2 Journal of The Electrochemical Society, G29-G /2009/156 5 /G29/4/$23.00 The Electrochemical Society Significant Improved Luminescence Intensity of Eu -Doped Ca 3 SiO 4 Cl 2 Green Phosphor for White LEDs Synthesized Through Two-Stage Method I. Baginskiy a,b and R. S. Liu a,z a Department of Chemistry, National Taiwan University, Taipei 106, Taiwan b Optical Sciences Center, National Central University, Taoyuan 320, Taiwan G29 A two-stage method for the preparation of Eu -doped Ca 3 SiO 4 Cl 2 is reported. It was observed that the emission intensity is significantly enhanced up to 30% by the two-stage method. The critical concentration of Eu was found to be 0.01 mol % and the critical radius was 31.9 Å. The broad excitation band and strong emission indicate that the Ca 3 SiO 4 Cl 2 :Eu phosphor could be a good green-phosphor candidate for creating white light in phosphor-converted white light-emitting diodes LEDs The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted January 9, 2009; revised manuscript received February 2, Published March 11, White-light generation through light-emitting diodes LEDs has a number of advantages over the existing incandescent and halogen lamps in terms of power efficiency and reliability. The first commercially available white LED based on phosphors was produced in 1996; it combines a blue light-emitting In,Ga N with a yellow Y 1 x Gd x 3 Al 1 y Ga y 5 O 12 :Ce 3+ YAG:Ce phosphor. 1 However, this type of white light has a poor color-rendering index because of the color deficiency in the red region. To solve this problem, green and red phosphors in combination with a blue LED and red, green, and blue phosphors with a UV LED were introduced. Both these methods need efficient green phosphors that should have an excitation wavelength matching the emission wavelength of the blue LEDs em : nm or the UV LEDs em : nm. Optical transitions of divalent europium 4f 7 have been investigated in many silicate phosphors. The spectra of Eu -doped compounds are due to electric dipole 4f 7 4f 6 5d transitions which are parity-allowed so that they occur with high transition probabilities. 2,3 The Eu emission color varies from UV to red depending on the host lattice, covalency, the size of the cation, and the strength of the crystal field. 4 Eu -doped Ca 3 SiO 4 Cl 2 is an efficient green-emission phosphor under either UV or blue excitation with quantum efficiency up to 60%. 5 Its structure is composed of alternating layers of Ca 2 SiO 4 and CaCl 2. Here, we report the Eu -doped Ca 3 SiO 4 Cl 2 green phosphor with significant enhanced luminescence performance obtained through the reaction of CaCl 2 and Ca 2 SiO 4 :Eu. The influence of synthesis method on the luminescent properties of Ca 3 SiO 4 Cl 2 :Eu has been investigated. z rsliu@ntu.edu.tw Experimental The Ca 3 x SiO 4 Cl 2 :Eu x phosphors x = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05 mol % were synthesized by a two-stage solid-state reaction method 1. In the first step, the starting materials CaCO 3, Eu 2 O 3, and SiO 2 were mixed together in agate mortar with a required molar ratio. The mixture was placed in a tube furnace and calcined at 1250 C in reducing atmosphere of 25% H 2 /75% N 2 for 4 h to get Ca 2 SiO 4 :Eu. The obtained powder of Ca 2 SiO 4 :Eu was then mixed with calcium chloride CaCl 2 added in a small 10% excess to stoichiometric amount and sintered again at 900 C in 25% H 2 /75% N 2 for 4 h to obtain a final product of Ca 3 SiO 4 Cl 2 :Eu. After cooling, excess CaCl 2 was removed by washing with anhydrous alcohol. For comparison, several samples of Ca 3 x SiO 4 Cl 2 :Eu x x = 0.01, 0.02, and 0.03 were also prepared by the conventional one-stage synthesis method 2, where the stoichiometric mixture of all components CaCO 3, SiO 2,Eu 2 O 3, and CaCl 2 were mixed together and fired at 900 C in 25% H 2 /75% N 2 for 4 h. The synthesized powders were studied by X-ray powder diffraction XRD with an X Pert PRO advanced automatic diffractometer, operating with 50 kv and 200 ma and using Cu K radiation Å. Photoluminescence PL emission and PL excitation PLE properties were investigated using a FluoroMax-3- FluoroMax-P spectrometer. Results and Discussion The phase purity of Ca 2 SiO 4 and Ca 3 SiO 4 Cl 2 :Eu samples was confirmed by XRD measurements Fig. 1. The morphology of obtained Ca 3 SiO 4 Cl 2 :Eu samples was studied by scanning electron microscopy SEM and shown in Fig. 2A and B for the two- and one-stage route, respectively. The primary particle size lies within a wide region of 2 20 m with a quite-high distribution ratio. But due to synthesis performed in excess of CaCl 2 flux in fact in liquid phase, phosphor particles by both methods are heavily agglomerated with a secondary particle size size of agglomerates up to 50 m depending on grinding procedure. As shown in Fig. 3, the crystal structure of Ca 3 SiO 4 Cl 2 monoclinic system, space group P12 1 /c 1 resembles that of monoclinic -Ca 2 SiO 4 Fig According to previous works, 5,6 crystal structure Ca 3 SiO 4 Cl 2 is composed of alternating layers of CaCl 2 and Ca 2 SiO 4 polyhedrons. There are three independent Ca sites in the Ca 3 SiO 4 Cl 2 differentiated by surrounding Ca 1 SiO 4 Cl 5,Ca 2 SiO 4 3 Cl 3, and Ca 3 SiO 4 4 Cl. Therefore, an activator cation Eu can substitute Ca in all those positions, resulting in several emission bands. The PL and PLE spectra of Ca 2.97 SiO 4 Cl 2 :Eu 0.03 obtained by two- and one-stage methods are shown in Fig. 4a. The broad excitation spectra peaked at 327 nm ranges from 290 to 450 nm, which fits well with the Figure 1. The XRD patterns of 1 Ca 1.99 SiO 4 :Eu 0.01 obtained in this work, 2 standard -Ca 2 SiO 4 from ICSD , 3 Ca 2.97 SiO 4 Cl 2 :Eu 0.03 obtained through the one-stage method, 4 Ca 2.97 SiO 4 Cl 2 :Eu 0.03 obtained through the two-stage method, and 5 standard Ca 3 SiO 4 Cl 2 from ICSD

3 G30 Journal of The Electrochemical Society, G29-G Figure 2. Color online SEM images of Ca 2.97 SiO 4 Cl 2 :Eu 0.03 obtained by A the two-stage route method 1 and B the one-stage route method 2. emission of UV and blue LEDs. It consists of several bands at around 327, 368, and shoulder within nm. Under excitation of ex = 400 nm, Ca 2.97 SiO 4 Cl 2 :Eu 0.03 shows a slightly asymmetric strong emission band centered at 510 nm green and a half width of Figure 4. a Excitation and emission spectra of Ca 2.97 SiO 4 Cl 2 :Eu 0.03 under 400 nm excitation obtained by the two-stage route method 1 and the onestage route method 2 ; inset close-up of blue emission peak at 427 nm; b PL intensity of Ca 2.97 SiO 4 Cl 2 :Eu 0.03 in comparison to commercial green phosphor Ba 2 SiO 4 :Eu under 400 nm excitation. Figure 3. Color online Crystal structures of -Ca 2 SiO 4 and Ca 3 SiO 4 Cl nm. It shows a weak blue band with a peak around 427 nm, which is in good agreement with the reported results. 5 The position of the green emission peak for both Ca 2.97 SiO 4 Cl 2 :Eu 0.03 samples almost completely coincides with that of Ca 2 SiO 4 :Eu. 7 However, the emission intensity of the 510 nm band of Ca 2.97 SiO 4 Cl 2 :Eu 0.03 obtained by the two-stage method method 1 is about 30% higher than that of the sample obtained through the one-stage method method 2. At the same time, the intensity of the blue band at 427 nm is higher for the sample obtained by method 2 inset of Fig. 4a. The broad luminescence originates from the allowed electrostatic dipole 5d 4f transition of Eu ions. However, the 5d state is easily affected by the crystal field of surrounding ions that can split the 5d state in different ways. This causes Eu to emit a wide range of wavelengths of light in different crystal hosts. As noted above, Ca 3 SiO 4 Cl 2 consists of alternating layers of CaCl 2 and Ca 2 SiO 4.It was reported that CaCl 2 :Eu and Ca 2 SiO 4 :Eu phosphors emit at 430 and 505 nm, respectively. 4,7,8 Therefore, the blue emission band in Ca 2.97 SiO 4 Cl 2 :Eu 0.03 at around 427 nm could be attributed to Eu substituted at Ca sites in the CaCl 2 layers. The Ca 2 SiO 4 layers are formed by Ca 3 SiO 4 4 Cl polyhedron, where Ca is surrounded by six oxygen atoms from SiO 4 groups and only one distant Cl ion separated from Ca by 3.13 Å, compared to 2.84 Å in CaCl 2 layers of Ca 3 SiO 4 Cl 2 and 2.74 Å in normal CaCl 2. 6 Hence, the green emission band at around 510 nm should be assigned primarily to Eu ions substituted at Ca sites in the Ca 2 SiO 4 layers of Ca 3 SiO 4 Cl 2. As seen from Fig. 4a, the blue band at 427 nm is completely overlapped with an excitation band of Ca 3 SiO 4 Cl 2 :Eu phosphor that should result in strong energy transfer from Eu in

4 Journal of The Electrochemical Society, G29-G G31 Figure 5. Excitation and emission spectra of Ca 3 SiO 4 Cl 2 :Eu with Eu doping concentrations of 0.005, 0.01, 0.03, and 0.05 mol % under 327 nm excitation; inset dependence of luminescence intensity of Ca 3 SiO 4 Cl 2 :Eu on Eu mol % concentration. the CaCl 2 layer to Eu in the silicate layer. This plays an important role in significant luminescence enhancement in chlorosilicate Ca 3 SiO 4 Cl 2 :Eu phosphors compared to silicate Ca 2 SiO 4 :Eu. Also, from this point of view the higher green emission and lower blue emission in Ca 3 SiO 4 Cl 2 :Eu obtained by method 1 as compared to method 2 can be explained in terms of different ratios between Eu ions in the Ca 2 SiO 4 and CaCl 2 layers. In the case of the two-stage route method 1, the Eu penetrates into CaCl 2 layers by means of diffusion from the existing Ca 2 SiO 4 :Eu structure. In the one-stage route method 2, europium oxide first easily reacts with CaCl 2 flux t m = 775 C, 5 which shifts the Eu -ion distribution ratio in favor of CaCl 2 layers. This results in higher blue emission at 427 nm and lower green emission at 510 nm, which can be assigned to CaCl 2 and Ca 2 SiO 4 layers, respectively. The internal quantum efficiency of Ca 2.97 SiO 4 Cl 2 :Eu 0.03 was measured to be 62% under 400 nm excitation, and PL intensity reaches about 64% of commercially available Ba 2 SiO 4 :Eu, primarily used as a green phosphor in LEDs Fig. 4b. CIE chromaticity coordinates are x = and y = Figure 5 shows PLE and PL spectra of Ca 3 SiO 4 Cl 2 :Eu phosphors with different Eu doping contents acquired at em = 510 and ex = 327 nm, respectively. It was observed that an increase in Eu concentration affects the shape of the excitation spectra. The band at 368 nm and shoulder at around nm increases relative to the dominant peak at 327 nm. Under 327 nm, Ca 2.99 SiO 4 Cl 2 :Eu 0.01 phosphor showed the highest intensity. The shape of emission spectra of Ca 3 SiO 4 Cl 2 :Eu samples with different Eu content does not change as the doping concentration increases. However, the green band shifts insignificantly to the longer-wavelength region from 509 to 511 nm for Ca SiO 4 Cl 2 :Eu and Ca 2.95 SiO 4 Cl 2 :Eu 0.05, respectively. The PL intensity of Ca 3 SiO 4 Cl 2 :Eu phosphor under 327 nm increases until the critical value x c of 0.01 and then decreases inset of Fig. 5 due to a process known as concentration quenching. Liu et al. 5 pointed out a value of 0.02, which could be the result of a different synthesis approach. The probability of energy transfer among Eu ions increases with Eu concentration when the distance between neighboring Eu cations decreases, and it approaches a critical value R c corresponding to x c, nonradiative energy transfer. According to Blasse, the critical transfer distance R c is approximately equal to twice the radius of a sphere with this volume 9 Figure 6. Luminescence spectra of Ca 2.97 SiO 4 Cl 2 :Eu 0.03 at different temperatures; inset dependence of luminescence intensity of 1 Ca 2.97 SiO 4 Cl 2 :Eu 0.03, 2 commercial YAG:Ce 3+, and 3 commercial greenphosphor Ba 2 SiO 4 :Eu on temperature. R c 2 3V 1/3 4 x c Z where x c is the critical concentration, Z is the number of formula units per unit cell, and V is the volume of the unit cell. By taking the values of V = Å 3, Z = 4, and x c = 0.01 and considering three Ca cation per formula unit, the critical transfer distance R c of Eu in Ca 3 SiO 4 Cl 2 :Eu phosphor was calculated to be about 31.9 Å. The nonradiative energy transfer from one Eu ion to another Eu ion often occurs as a result of an exchange interaction, radiation reabsorption, or a multipole multipole interaction. The exchange interaction is generally responsible for the energy transfer of forbidden transitions. 10,11 The emission of Eu ions can be ascribed to allowed 4f 5d transition. Therefore, the exchange and reabsorption interaction cannot account for the energy transfer of Eu in Ca 3 SiO 4 Cl 2 :Eu. The process of energy transfer between Eu ions in Ca 3 SiO 4 Cl 2 :Eu phosphor is due to the electric multipole multipole interaction as suggested by Dexter. 10 The investigation of thermoluminescence is important in order to understand the stability of PL properties against increased temperature. The PL intensity of Ca 3 SiO 4 Cl 2 :Eu phosphor decreases with increasing temperature, as shown in Fig. 6. As can be seen from the inset in Fig. 6, the thermal stability of Ca 3 SiO 4 Cl 2 :Eu is higher than that of Ba 2 SiO 4 :Eu, commonly used as a green phosphor in LEDs, but lower than YAG:Ce 3+ phosphor in the temperature range of C. The emission intensity is thermally quenched at a higher temperature due to the thermal relaxation through the crossing point between the excited state and the ground state in the configurational coordinate diagram. A nonraditive transition occurs because of the interaction between the electron and thermally active phonon. 4,12 The probability of thermal activation is strongly dependent on the temperature, resulting in the decrease of emission intensity according to the following equation 13 I T = 1+cexp E 2 a kt where I 0 is initial PL intensity, I T is intensity at different temperatures, E a is activation energy of thermal quenching, c is a constant for a certain host, and k is the Boltzmann constant ev. According to Eq. 2, the activation energy E a was calculated to be ev. The blue emission band shows a hardly noticeable 1 nm redshift with increasing of the temperature in ac- I 0 1

5 G32 Journal of The Electrochemical Society, G29-G cordance with Varshini theory. 14 The green emission peak of Ca 3 SiO 4 Cl 2 :Eu shows the shift to shorter wavelengths blueshift with increasing of the temperature from 510 to 501 nm. As seen from Fig. 4a, the emission band at 510 nm is slightly asymmetric, indicating that it consists of two overlapped bands, presumably from Eu ion in CaCl 2 and Ca 2 SiO 4 layers. In the case of closely located energy levels, the blueshift can be explained in terms of back tunneling from the excited states of the lower-energy emission band to the excited states of the higher energy emission band by assistance of thermally active phonon. 4 Conclusions We have synthesized a green phosphor Ca 3 SiO 4 Cl 2 :Eu by a two-stage approach with enhanced luminescence properties. The enhanced luminescence over the conventional one-stage method is up to 30%. Under UV and blue irradiation, it emits a green band centered at 510 nm. The critical concentration of Eu was found to be 0.01 mol % and the critical radius was 31.9 Å. However, under excitation of nm more suitable for LEDs, the Ca 3 SiO 4 Cl 2 :Eu phosphor with 0.03 mol % doping concentration has the highest emission intensity. With increasing of the temperature, the emission of Ca 3 SiO 4 Cl 2 :Eu shows the blueshift and thermal quenching of the luminescence. The broad excitation band and strong emission indicate that Ca 3 SiO 4 Cl 2 :Eu could be a good green-phosphor candidate for creating white light in phosphorconverted white LEDs. Acknowledgments The authors thank the National Science Council contract no. NSC M MY3 and the Ministry of Economic Affairs contract no. 96-EC-17-A-07-S1-043, Taiwan. National Taiwan University assisted in meeting the publication costs of this article. References 1. S. Nakamura and G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, p. 216, Springer, Berlin P. Dorenbos, J. Lumin., 104, A G. Blasse and A. Bril, Philips Res. Rep., 23, A J. S. Kim, J. S. Kim, Y. H. Park, S. M. Kim, J. C. Choi, and H. L. Park, Solid State Commun., 133, A J. Liu, H. Lian, J. Sun, and C. Shi, Chem. Lett., 34, A V. R. Czaya and G. Bissert, Acta Crystallogr., A27, G. Blasse, W. L. Wanmaker, J. W. Vrugt, and A. Bril, Philips Res. Rep., 23, A T. Kobayasi, S. Mroczkowski, J. F. Owen, and L. H. Brixner, J. Lumin., 21, A G. Blasse, Philips Res. Rep., 24, A D. L. Dexter, J. Chem. Phys., 21, M. Zhang, J. Wang, Q. Zhang, W. Ding, and Q. Su, Mater. Res. Bull., 42, A J. S. Kim, A. K. Kwon, Y. H. Park, J. C. Choi, H. L. Park, and G. C. Kim, J. Lumin., , A B. Henderson and G. F. Imbusch, Optical Spectroscopy of Inorganic Solids, Clarendon Press, Oxford Y. P. Varshini, Physica (Amsterdam), 34, A