Luminescence properties and energy transfer in K 2 MgSiO 4 :Ce 3+,Tb 3+ as a green phosphor

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1 Copyright 2016 by American Scientific Publishers All rights reserved. Printed in the United States of America /2016/6/037/008 doi: /mex Luminescence properties and energy transfer in K 2 MgSiO 4 :Ce 3+,Tb 3+ as a green phosphor Yufei Xia, Jian Chen, Yan-Gai Liu, Lefu Mei, Zhaohui Huang, and Minghao Fang School of Materials Science and Technology, Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, China University of Geosciences, Beijing, , China ABSTRACT Series new green K 2 MgSiO 4 :Ce 3+,Tb 3+ phosphors were synthesized by solid-state reaction method. The crystal structure, luminescence and reflectance spectra, energy transfer mechanism and temperature dependent performance have been investigated respectively. The information of crystal structure was detected by X-ray diffraction, and the optimized doping concentration of Ce 3+ ions and Tb 3+ ions were confirmed to 4% mol and 2% mol, respectively. The energy transfer mechanism has been validated to be a resonant type via a dipole dipole interaction with high efficiency of 55.1%. The energy transfer critical distance of this phosphor was calculated by the concentration quenching method (14.08 Å) and spectral-overlap method (15.68 Å). This phosphor shows good thermal stability which maintains high intensity with 80.94% and 67.75% at 100 C and 150 C compared with the initial intensity (25 C). Keywords: Luminescence, Energy Transfer, K 2 MgSiO 4, Phosphor, Silicate. 1. INTRODUCTION The white light-emitting diodes (w-leds) have aroused great attention as for the fourth generation solid-state light sources due to low energy consumption, high brightness and environmental friendly properties. 1 6 Traditionally, the commercial w-leds is assigned by coating the yellowemitting phosphor YAG:Ce 3+ on blue InGaN LED chip, which has a high efficiency (150 lm/w) Aim to achieve low luminous decay, good adaptive capability to environment and long service lifetime, the design of the near-ultraviolet (n-uv) LED which means n-uv LED fabricated with the tricolor emitting phosphors (blue, green and red), which can display extensive spectral distribution over the whole visible-range to obtain high quality white light, has become the alternative way and arouse great research interest It s well known that silicate has good thermal and chemical stability at high temperature caused by their stable crystal structure which was wildly Authors to whom correspondence should be addressed. s: liuyang@cugb.edu.cn, mlf@cugb.edu.cn used in modern industry. The silicate phosphors have aroused a lot of attention due to their good optical stability at high temperature, simple preparation method, broad excitation/emission bands and low fabrication costs K 2 MgSiO 4 has been synthesized by Dollase for the first time since and it utilized as the main phase of slow-release K + fertilizer which is prepared by roasting kaliophilite. The crystal structure of K 2 MgSiO 4 is belong to the space group of Pca2 1 -orthorhombic with layer structure and the lattice parameters are a = (2) Å, b = 5.473(1) Å, c = (3) Å and V = (30) Å 3.Mg and Si atoms form an ordered tetrahedral array which avoids Mg 2+ O Mg 2+ connections and potassium atoms fill all of the intervening large cavity sites. 21 Generally, Tb 3+ ions were usually used as an activator of the green luminescent materials because of the predominant 5 D 4 7 F 5 transition peaking at 545 nm. But, the single doped Tb 3+ ions phosphors were restricted by the low quantum efficiency which ascribe to the f f forbidden transition. Frequently, Ce 3+ ions were used as a sensitizers to transfer energy (in the region of 200 nm 450 nm) to Mater. Express, Vol. 6, No. 1,

2 Luminescence properties and energy transfer in K 2 MgSiO 4 :Ce 3+,Tb 3+ Tb 3+ ions because of the existence of an overlap between the excitation spectrum of Tb 3+ ions and the emission spectrum of Ce 3+ ions under excitation of n-uv region. 22 Meanwhile, Ce 3+ ions and Tb 3+ ions co-doped phosphors have become the most meaningful green phosphors which were wildly used in the lighting industry, such as (La 0 55 Ce 0 3 Tb 0 15 )PO 4 (LAP), Ca 2 BO 3 Cl:Ce 3+, Tb 3+, Yb 3+, 23 Sr 3 AlO 4 F:Ce 3+, Tb 3+, 24 Ca 9 Y(PO 4 7 :Ce 3+, Tb 3+, 25 and KCaGd(PO 4 2 :Ce 3+,Tb For our best of known, there is no report focused on the luminescent properties of co-doped Ce 3+ ions and Tb 3+ ions in the host of K 2 MgSiO 4. In the present work, series of K 2 MgSiO 4 phosphors have been synthesized by high temperature solid-state method which has many advantages, such as cheap raw materials, good chemical stability and easy preparation. The energy-transfer mechanism and efficiency were confirmed by experimental results of the photoluminescence emission (PL) spectra and decay curve of the phosphors. The critical distance (R c ) between Ce 3+ and Tb 3+ ions was calculated and the thermal stability was measured by temperature dependent experiment. All the results indicated that the K 2 MgSiO 4 :Ce 3+,Tb 3+ phosphor has utilization potential for white light via adjusting the relative doping concentration ratio of Ce 3+ /Tb 3+ appropriately. 2. EXPERIMENTAL DETAILS 2.1. Sample Preparation K 2 MgSiO 4 host, Ce 3+ and Tb 3+ ions single doped samples K 2 MgSiO 4 :xce 3+ (x = 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), K 2 MgSiO 4 :0.6Tb 3+ and co-doped samples K 2 MgSiO 4 : 0.04Ce 3+, xtb 3+ (x = 0.1, 0.2, 0.4, 0.6, 0.8) were synthesized by traditional high temperature solid-state reaction. K 2 CO 3 (analytical reagent (A. R.)), MgO (A. R.), SiO 2 (A. R.), CeO 2 (A. R.) and Tb 4 O 7 (A. R.) were used to raw materials which well ground in agate mortar according to stoichiometric amounts of reactants. All mixed powders sintered at 1200 C under a reducing atmosphere (H 2 10%, N 2 90%) for 4 h in a horizontal tube furnace. Finally, the products were grounded again after cooling down to room temperature. were measured via a Shimadzu UV-3600 UV-vis-NIR spectrophotometer attached with an integral sphere. 3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis The XRD patterns of K 2 MgSiO 4, K 2 MgSiO 4 :0.04Ce 3+, 0.08Tb 3+ and the standard pattern (JCPDF ) were displayed in Figure 1. All diffraction peaks of these samples show good agreement with the reported JCPDF card even at the highest doping concentration of Tb 3+ ions, which demonstrated that no detectable impurity phase is observed and the doping of Ce 3+ and Tb 3+ ions have been successfully doped in the K 2 MgSiO 4 host lattice without any structural variation. In addition, considering the coordination number, the ionic radius of Ce 3+,Tb 3+, K + and Mg 2+ is at the range of , , and , respectively. Thus, the site of K + will be substituted by Ce 3+ and Tb 3+ because the ionic radius of K + is more similar with Ce 3+ and Tb 3+ than Mg Photoluminescence Properties Figure 2(a) shows the PLE and PL spectra of K 2 MgSiO 4 :0.04Ce 3+. The main excitation peak at 366 nm exhibited in the PLE spectrum ( em = 423 nm) which attribute to the 4f 5d transitions of Ce 3+ ion. Additionally, the emission band were decomposed into two well-separated Gaussian components (the dotted lines in Fig. 2(a)) with an energy gap about 1892 cm 1 between 424 nm (23584 cm 1 ) and 461 nm (21692 cm 1 ), which is good agreement with the theoretical difference between the 2 F 5/2 and 2 F 7/2 levels (about 2000 cm 1 ). Consequently, due to the transition from the 5d excited state to the 2 F 5/2 and 2 F 7/2 ground states of the Ce 3+ ion, a blue emission band from 400 nm to 500 nm with maximum at 423 nm were detected in the PL spectrum ( ex = 423 nm) Sample Characterization The structure of these samples were identified via powder X-ray diffraction on a D8 Advance diffractometer (Germany) with graphite-monochromatized Cu K radiation ( = nm). The photoluminescence emission and photoluminescence excitation (PLE) spectra were measured by a Hitachi F-4600 fluorescence spectrophotometer (Japan) equipped with a 150 W Xe lamp as the excitation source which also can test the temperature dependent luminescence properties after assembled with a computer-controlled electric furnace and a self-made heating attachment. Sample s diffuse reflection spectra Fig. 1. XRD patterns of K 2 MgSiO 4 (a), K 2 MgSiO 4 :0.04Ce 3+ (b), K 2 MgSiO 4 :0.04Ce 3+, 0.08Tb 3+ (c) and the standard data of K 2 MgSiO 4 (JCPDS no ) is exhibited as a reference. 38 Mater. Express, Vol. 6, 2016

3 Luminescence properties and energy transfer in K 2 MgSiO 4 :Ce 3+,Tb 3+ Fig. 3. The reflectance spectra of K 2 MgSiO 4 host (a), K 2 MgSiO 4 : 0.04Ce 3+ (b) and K 2 MgSiO 4 :0.04Ce 3+, 0.02Tb 3+ (c) samples. Fig. 2. The PLE and PL spectra of K 2 MgSiO 4 :0.04Ce 3+ (a), K 2 MgSiO 4 :0.06Tb 3+ (b) and K 2 MgSiO 4 :0.04Ce 3+, 0.02Tb 3+ (c). The PLE and PL spectra of single doped Tb 3+ ions samples are shown in Figure 2(b), a broad band peaking at about 240 nm and 270 nm which are ascribed to the spinallow transition from 4f to 5d state of Tb 3+ ion and the peaking at 330 nm which is correlated to the forbidden f f transition of the Tb 3+ ion were detected in the excitation spectrum monitored at 549 nm. The emission peaks of Tb 3+ ions were found at 485 nm, 545 nm, 580 nm and 620 nm, which are attribute to the 5 D 4 to 7 F J (J = 6, 5, , 3) multiple transitions, respectively. Generally, Ce 3+ ions were co-doped as sensitizers to transfer energy to Tb 3+ ions in order to enhance the emission intensity of Tb 3+ ions, because there is a distinct overlap between the emission band of Ce 3+ ions and the f f absorption band of Tb 3+ ions at around 375 nm to 420 nm, which can be observed from Figures 2(a) and (b). Therefore, it is indicating the possibility of resonance type energy transfer from Ce 3+ to Tb 3+ ions. As shown in Figure 2(c), except for the difference of the relative intensity, the PLE spectrum of K 2 MgSiO Ce 3+, 0.02Tb 3+, monitored at 423 nm, is similar with that monitored at 549 nm, which means the existence of the energy transfer from Ce 3+ to Tb 3+ ions. In order to reveal the absorption characteristics of these samples, the reflectance spectra of K 2 MgSiO 4 host, K 2 MgSiO 4 :0.04Ce 3+ and K 2 MgSiO 4 :0.04Ce 3+, 0.02Tb 3+ are shown in Figure 3. The absorption area of K 2 MgSiO 4 host is at the range of nm. When Ce 3+ ions were introduced into the K 2 MgSiO 4 host, an obvious absorption band between 320 nm to 430 nm with maximum value at 350 nm appeared in the nm n-uv region, the typical f d absorption of Ce 3+ ions, which is stand for the energy absorption. The reflectance spectrum keeps similar shape but much lower than the others when Ce 3+ and Tb 3+ ions co-doped in K 2 MgSiO 4, it s demonstrated the existent of energy transfer between Ce 3+ and Tb 3+ ions. Meanwhile, all the reflectance spectra are coincident with the PLE spectra shown in Figures 2((a), (c)), which illustrate that K 2 MgSiO 4 :Ce 3+,Tb 3+ has the potential for w-leds phosphor because the strong absorption band matching well with n-uv chip. To further investigate the energy transfer course from Ce 3+ to Tb 3+ ions, the optimized concentration of Ce 3+ ions was determined firstly. Figure 4 displays the PL spectra of K 2 MgSiO 4 :xce 3+ (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.1) phosphors. Distinctly, the variation tendency of intensity is based on the concentration of Ce 3+ ions which is presented as the inset in Figure 4. With the increasing concentration of Ce 3+ ions, the intensity shows a rising trend at the beginning and turn to decrease after over the top at x = 0.04, which attribute to the concentration quenching of Ce 3+ ions. Consequently, x = 0.04 was chosen as the optimum condition for further co-doped research. The PL spectra of K 2 MgSiO 4 :0.04Ce 3+, xtb 3+ phosphors with different Tb 3+ concentration (x = 0.01, 0.02, 0.04, 0.06, 0.08) are presented in Figure 5. Both of the Fig. 4. The PL spectra of K 2 MgSiO 4 :xce 3+ (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.1) phosphors. Mater. Express, Vol. 6,

4 Luminescence properties and energy transfer in K2 MgSiO4 :Ce3+, Tb3+ Fig. 5. The PL spectra of K2 MgSiO4 :0.04Ce3+, xtb3+ (x = 0.01, 0.02, 0.04, 0.06, 0.08) phosphors. emission bands of Ce3+ and Tb3+ ions are found, and the emission intensities of Ce3+ ions decrease with the increasing doping concentration of Tb3+ ions. Meanwhile, as a sensitizer, the emission intensity of Ce3+ ions decreasing apparently with a monotonic tendency compared with the Ce3+ ions single doped phosphor K2 MgSiO4 :0.04Ce3+, which demonstrate the existence of energy transfer from Ce3+ to Tb3+ ions in K2 MgSiO4 host. The variation tendency of intensity is based on the concentration of Tb3+ ions shown in the inset of Figure 5. With the increasing concentration of Ce3+ ions, the intensity shows a rising trend until reach the top at x = 0.02 and then turn to decrease. Figure 6 shows the CIE chromaticity diagram of K2 MgSiO4 :Ce3+, Tb3+ phosphor upon excitation of 365 nm. All these chromaticity coordinates calculated to be around (0.2896, ). The inset shows the digital photograph of K2 MgSiO4 :Ce3+, Tb3+ phosphor under a 365 nm UV lamp, which illustrated K2 MgSiO4 :Ce3+, Tb3+ can be used as a blue-emitting phosphor for w-leds application. Fig. 6. CIE chromaticity diagram of K2 MgSiO4 :Ce3+, Tb3+ phosphor upon excitation of 365 nm and the selected samples digital images excited at 365 nm in UV box. Tb3+ ions, the energy transfer efficiency from Ce3+ to Tb3+ increased obviously and the emission intensity of Ce3+ ions decreased with a monotonous tendency. Simultaneously, the relative emission intensity of Tb3+ ions increased at the beginning and then decreased when x exceeded 0.03 which is attribute to the Ce3+ Tb3+ internal concentration quenching. Therefore, the relative ratio of the emission intensity of Ce3+ and Tb3+ could be adjusted via properly changing the Tb3+ ions concentration. The room-temperature decay curves of the K2 MgSiO4 : 0.04Ce3+, xtb3+ phosphors monitored at 423 nm for Ce Energy Transfer Efficiency and Mechanism In order to investigate the relationship between relative intensity of Ce3+ ions and the energy transfer efficiency from Ce3+ to Tb3+ ions under different Tb3+ ions concentration, the relative intensity of the 4f 5d transition of Ce3+ ions at 423 nm, the 5 D4 7 F5 transition of Tb3+ ions at 549 nm and the energy-transfer efficiency from Ce3+ to Tb3+ in K2 MgSiO4 :0.04Ce3+, xtb3+ (x = 0.01, 0.02, 0.04, 0.06, 0.08) are shown in Figure 7, respectively. The energy-transfer efficiency was calculated as follows: 29 T = 1 IX IO (1) Where T is the energy transfer efficiency, Ix and I0 are the luminescence intensity of the Ce3+ ions with and without of the Tb3+ ions. With the increasing concentration of 40 Fig. 7. Dependence of the Ce3+ emission, Tb3+ emission, and energy transfer efficiency of Ce3+ Tb3+ on Tb3+ doping concentration for K2 MgSiO4 phosphors. Mater. Express, Vol. 6, 2016

5 Luminescence properties and energy transfer in K 2 MgSiO 4 :Ce 3+,Tb 3+ ions emission were measured and the lifetime as well as energy transfer efficiency are shown in Figure 7, respectively. The lifetime was calculated by the nonexponential fitting method follow by this equation: 24 0 = I t tdt I t dt (2) 0 Where is the lifetime, I t is the intensity at time t. Based on the above Eq. (2), the lifetime of Ce 3+ ions were calculated to 66.82, 54.09, 35.52, and ns for K 2 MgSiO 4 :0.04Ce 3+, xtb 3+ with x = 0, 0.1, 0.2, 0.4, 0.6 and 0.8, respectively. Obviously, with the increasing doping concentration of Tb 3+, the lifetime of Ce 3+ ions decrease monotonously which is the symbol of energy transfer efficiency rising from Ce 3+ to Tb 3+.The energy transfer efficiency was estimated via the following equation: 24 T = 1 x (3) 0 Where 0 and x are the lifetimes of the sensitizer with and without the activator, and T is the calculation of energy transfer efficiency. The energy transfer efficiency from the sensitizer Ce 3+ ions to the activator Tb 3+ ions is given in the inset of Figure 8, which shows good agree with the results calculated by PL intensity, it demonstrate that the lifetime of Ce 3+ ions decreasing with the increasing concentration of Tb 3+ ions. 11 The critical distance R c was calculated based on the concentration quenching method. Generally, concentration quenching is attributing to the energy consume because of the resonance between atoms. The average distance R Ce Tb between Ce 3+ ions and Tb 3+ ions can be expressed via the formula given by Blasse: 32 [ ] 3V 1/3 R C 2 (4) 4 x c N Here N is the number of Z ions in the unit cell, V is the unit cell volume and x is the total concentration of Ce 3+ ions and Tb 3+ ions. For K 2 MgSiO 4 host, N = 8and V = Å 3. The optimized concentration (x c = = 0 1), where the total concentration of Ce 3+ ions and Tb 3+ ions when the energy transfer efficiency is 50%, was used in the above equation. Consequently, the critical distance (R c ) of energy transfer can be reckoned to Å. Aim for further researching the exchange and multipolar interactions from Ce 3+ ions to Tb 3+ ions, the energy transfer mechanism were investigated by Dexter s energy transfer formula: 33 ln ( 0 ) C and 0 Cn/3 (5) Where 0 and are the luminescence quantum efficiency of Ce 3+ ions in the absence and presence of Tb 3+ ions, respectively. C is the total concentration of Ce 3+ ions and Tb 3+ ions. The relationship of ( 0 / C Ce 3+ +Tb 3+ corresponds to the exchange interaction, and n = 6, 8, and 10 stand for dipole dipole (d d), dipole quadrupole (d q), and quadrupole quadrupole (q q) interactions, respectively. The value of 0 / can be approximately estimated by the following relation: ln ( I0 I ) C and I S0 I S C n/3 (6) I S0 and I S are the luminescence intensity of Ce 3+ ions in absence and presence of Tb 3+ ions present. The relationships between I S0 /I S and C Ce 3+ +Tb 3+ as well as I S0 /I S and C n/3 are presented in Figure 9. The Ce 3+ +Tb 3+ R2 value for Figure 9(b) of the linear fit was calculated to be , which is most similar to the linear relation when n = 6, suggesting that the energy transfer from Ce 3+ to Tb 3+ follows a dipole dipole mechanism in K 2 MgSiO 4 :Ce 3+,Tb 3+ phosphor. Therefore, as the energy transfer system dominant by the dipole dipole interaction, the critical distance R c could be calculated by the following equation: 18 R 6 c = Q A E 4 f S E F A E de (7) Fig. 8. Photoluminescence decay curves of Ce 3+ in K 2 MgSiO 4 :0.04Ce 3+, xtb 3+ phosphors. (Excited at 370 nm, monitored at 413 nm). where Q A is the absorption coefficient of Ce 3+ (Q A = ), E (in ev) represent the corresponding energy, which determined by the maximum value in the overlap area between the emission spectrum of Ce 3+ ions and the excitation spectrum of Tb 3+ ion. FS E FA E de stand for the spectral overlap between the normalized shapes of Ce 3+ ions emission FS E and Tb 3+ ions excitation FA E. Hence, the R c of Ce 3+ Tb 3+ was estimated to be Å, which is marching well with the result reckoned by concentration quenching method. Mater. Express, Vol. 6,

6 Luminescence properties and energy transfer in K2 MgSiO4 :Ce3+, Tb3+ 6/3 8/3 10/3 Fig. 9. Dependence of Is0 /Is of Ce3+ on (a) C Ce3+ +Tb3+, (b) C Ce 3+ +Tb3+, (c) C Ce3+ +Tb3+ and (d) C Ce3+ +Tb Temperature Dependence Properties Temperature dependent PL spectra and the emission intensity versus temperature of K2 MgSiO4 :0.04Ce3+, 0.02Tb3+ phosphors ( em = 365 nm) were displayed in Figures 10(a), (b). Proverbially, it s very important that phosphor has excellent thermal quenching property to ensure durable chromaticity, brightness and conversion efficiency of white light output at around 150 C in the white LED application The temperature dependent PL spectra of K2 MgSiO4 :0.04Ce3+, 0.02Tb3+ phosphor Fig shows outstanding thermal stable property, which illustrated that the phosphor maintain high intensity with 80.94% and 67.75% at 100 C and 150 C compared with the initial intensity (25 C), respectively. The decreasing of emission intensity with temperature rising is due to the interaction of electron-phonon in the ground state and excited state of luminescence center, and this nonradiative transition leading decrease of emission intensity which is depend on temperature. 10 Temperature dependent emission spectra and the emission intensity versus temperature of K2 MgSiO4 :0.04Ce3+, 0.02Tb3+ phosphor. Mater. Express, Vol. 6, 2016

7 Luminescence properties and energy transfer in K 2 MgSiO 4 :Ce 3+,Tb CONCLUSION A new green K 2 MgSiO 4 :Ce 3+,Tb 3+ phosphor has been prepared successfully without any impurity and the energy transfer from Ce 3+ to Tb 3+ ions in the K 2 MgSiO 4 has been demonstrated to be the dipole dipole interaction with high efficiency of 55.1%. The energy transfer critical distance of this phosphor was calculated to be about Å and Å according to the concentration quenching method and spectral-overlap method, respectively. In addition, this phosphor maintains high intensity with 80.94% and 67.75% at 100 C and 150 C compared with the initial intensity (25 C). Acknowledgment: This work was sponsored by National Natural Science Foundation of China (Grant No ), the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET ) and the Fundamental Research Funds for the Central Universities (Grant No ). References and Notes 1. H. K. Liu, L. B. Liao, and Z. 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8 Luminescence properties and energy transfer in K 2 MgSiO 4 :Ce 3+,Tb M. M. Shang, D. L. Geng, D. M. Yang, X. Kang, Y. Zhang, and J. Lin; Luminescence and energy transfer properties of Ca 2 Ba 3 (PO 4 3 Cl and Ca 2 Ba 3 (PO 4 3 Cl:A (A = Eu 2+ /Ce 3+ /Dy 3+ /Tb 3+ ) under UV and lowvoltage electron beam excitation; Inorg. Chem. 52, 3102 (2013). 32. G. Blasse; Energy transfer in oxidic phosphors; Phys. Lett. 28, 444 (1968). 33. D. L. Dexter; A theory of sensitized luminescence in solids; J. Chem. Phys. 21, 836 (1953). 34. D. L. Geng, G. G. Li, M. M. Shang, D. M. Yang, Y. Zhang, Z. Y. Cheng, and J. Lin; Color tuning via energy transfer in Sr 3 In(PO 4 3 :Ce 3+ /Tb 3+ /Mn 2+ phosphors; J. Mater. Chem. 22, (2012). 35. X. M. Zhang, W. L. Li, K. H. Jang, and H. J. Seo; Luminescence enhancement by energy transfer from Ce 3+ ions in Ba 1 6 Ca 0 4 P 2 O 7 :Ce 3+, Tb 3+ phosphor; Curr. Appl. Phys. 12, 299 (2012). Received: 8 June Revised/Accepted: 1 August Mater. Express, Vol. 6, 2016