y e Ma 78.55.�m 42.70.�a e3+ r em ppr ele ove + p color temperature of 7330K upon excitation with 360nm, which is potentially a good candidate as an UV-convertible phosphor for white light-emitting diodes (LEDs). & 2009 Elsevier B.V. All rights reserved. Ds) are owing nd hi high vironm UV LEDs with blue, green and red phosphors have been ngle- d the Eu2+ properties of Ca2BO3Cl:Eu yellow phosphor were reported by ARTICLE IN PRESS Contents lists available at ScienceDirect .el Physic Physica B 404 (2009) 3743–3747 to investigate the preparation and photoluminescence propertiesE-mail address:
[email protected] (Q.Y. Zhang). investigated [12–14]. This approach provides white LEDs with Yang et al. [25] recently. In this regard, we propose that a white light may be produced by sensitizing Eu2+ by a commonly blue- emitting Ce3+ in Ca2BO3Cl host due to the strong crystal field splitting of 5d level. Therefore, the main objective of this work is 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.06.122 � Corresponding author. Tel.: +862087113681; fax: +862087114204. in these kinds of LEDs. In order to improve the white light quality of white LEDs and expend their market size, the combination of The crystal structure of calcium borate chloride (Ca2BO3Cl) was reported by Zak et al. [24] in 1976 and the luminescence 2+ convert phosphors [6–10]. The first white LEDs was commercia- lized based on blue LED chips coated with a yellow-emitting phosphor (Y, Gd)3(Al, Ga)5O12:Ce (YAG:Ce) system by Nichia company in 1996 [11]. However, high color-rendering, color reproducibility and luminous efficiency are still hard to obtain light can be obtained from the Ce3+ and Eu2+ co-doped si phased phosphors by adjusting the ratio of the Ce3+/Eu2+ an resonance-type energy transfer efficiency from Ce3+ to [21–23]. existence [1–5]. This product has attracted potential market demand with many applications, e.g., backlighting, large-screen display, monitors of notebook computers and general illumina- tion. At present, the common way to assemble white LEDs is combining an ultraviolet (UV) or blue LED chip with down- In recent years, there has been more interest in Ce and/or Eu2+ doped luminescent materials due to the fact that Ce3+ and Eu2+ ions posses the electric dipole transitions of f–d with parity allowed, and the influence of crystal-field and nephelauxetic effects which results in broad emission band vary from long- wavelength ultraviolet to yellow [19,20]. More importantly, white 42.79.�e 71.55.Eq Keywords: Phosphor White light-emitting diode Energy transfer Critical distance 1. Introduction White light-emitting diodes (LE fourth generation light source foll ment lamps, fluorescent lamps a lamps, as a consequence of their consumption, long lifetime and en anticipated to be the the incandescent fila- gh-pressure discharge brightness, low power ent friendly situation high color render index and good color uniformity due to the white light are generated only by phosphors. However, the reabsorption of the blue emission by the green or red-emitting phosphors results in low luminous efficiency of the three- converter system. Therefore, it is highly desirable to pay attention to the research on the single-phased white-emitting phosphor with tunable color under the radiation of UV LED chip [15–18]. 3+ Ca2BO3Cl:Ce 3+,Eu2+: A potential tunable phosphors for white light-emitting diod F. Xiao, Y.N. Xue, Q.Y. Zhang � MOE Key Lab of Specially Functional Materials and Institute of Optical Communication a r t i c l e i n f o Article history: Received 12 May 2009 Received in revised form 12 June 2009 Accepted 15 June 2009 PACS: a b s t r a c t Polycrystalline Ca2BO3Cl:C could display tunable colo the ratio of Ce3+ and Eu2+ a Eu2+ was established to be 31 A˚ based on the spectral Ca2BO3Cl:0.06Ce 3+,0.01Eu2 journal homepage: www ellow–white–blue-emitting s terials, South China University of Technology, Guangzhou 510641, PR China ,Eu2+ phosphors were synthesized by a solid-state reaction and which ission from blue to yellow under an ultraviolet (UV) source by adjusting opriately. The mechanism of resonance-type energy transfer from Ce3+ to ctric dipole–dipole natured, and the critical distance was estimated to be rlap and concentration quenching model. A white light was obtained from hosphor with chromaticity coordinates (x ¼ 0.31, y ¼ 0.29) and relative sevier.com/locate/physb a B ARTICLE IN PRESS of Ca2BO3Cl:Ce 3+,Eu2+ phosphor in detail as well as the energy transfer mechanism from Ce3+ to Eu2+. A tunable light from blue through white to yellow was observed, suggesting that this phosphor might be a promising UV-convertible candidate for white LEDs. 2. Experimental In the present work, we synthesized a series of Ca2BO3Cl:x- Ce3+,yEu2+ phosphors by a solid-state reaction method. Stoichio- metric amounts of CaCO3 (A.R.), CaCl2 (A.R.), H3BO3 (A.R.), CeO2 (99.99%), Eu2O3 (99.99%) were mixed thoroughly and pressed into a crucible. The samples were obtained by calcinating the mixture at 500 1C for 2 h and subsequently 900 1C for 4h in an activated carbon reducing atmosphere. The crystal structure of the as-prepared phosphor was examined by X-ray diffractometer (XRD) (Philips Model PW 1830) with Cu-Ka radiation at 40 kV and 40mA. The size and morphology of the samples were characterized by a scanning electron microscopy (SEM) (JEOL JSM-5610LV). Photolumines- cence (PL) and photoluminescence excitation (PLE) spectra were measured on a fluorescence spectrometer (Jobin-Y von TRIAX320) equipped with a 450-W xenon lamp as the excitation source at any significant change in the host structure. Ca2BO3Cl has a monoclinic structure with space group P21/c and lattice parameters a ¼ 3.948 A˚, b ¼ 8.692 A˚, and c ¼ 12.402 A˚. In the crystal lattice, there are two types of seven-fold coordination Ca polyhedra: CaCl3O4 and CaCl2O5, each O atom is surrounded by three Ca atoms, one B atom in the form of a distorted tetrahedral and the Cl atoms are always situated at the corners of the trigonal base, as exhibited in the inset of Fig. 1 (left). Based on the effective ionic radii of cations with different coordination numbers [26], we have suggested that Ce3+ and Eu2+ ions are preferably to substitute Ca2+ for the ionic radii of Ce3+ (1.03 A˚) and Eu2+ (1.09 A˚) are near to that of Ca2+ (0.99 A˚). The inset of Fig. 1 (right) shows the SEM micrograph of Ca2BO3Cl:0.06Ce 3+,0.01Eu2+ phosphor. It is seen clearly that the sample is composed of aggregated particles with size ranging from 0.5 to 2.5mm and appropriate spherical morphology, which is acceptable for the white LED application. F. Xiao et al. / Physica B 404 (2009) 3743–37473744 Fig. 1. XRD profile of Ca2BO3Cl:0.06Ce 3+,0.01Eu2+ phosphor and JCPDS card no. 77- 0546. Inset (left) shows the crystal structure of Ca2BO3Cl viewed along a direction nearly parallel to the z axis; Ca, B, O and Cl atoms are shown as blue, gray, red and green spheres, respectively. Inset (right) exhibits SEM images of the as-synthesized Ca2BO3Cl:0.06Ce 3+,0.01Eu2+ phosphor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3.1. Phase identification and morphology Fig. 1 shows the XRD pattern of Ca2BO3Cl:0.06Ce 3+,0.01Eu2+ phosphor. All the diffraction peaks match well with those in Ref. [24] JCPDS card no. 29-0302, indicating that the obtained samples are single phased and the codoped Ce3+ and Eu2+ ions do not cause room temperature. The powder sample was placed at sample channel (1 cm�3mm�2mm). The excitation and emission slits were set at 5 nm, all the spectra were measured at a scan speed of 400nm/min. Emitted light was focused on to the monochromator and monitored at the exit slit by a photon-counting R5108 photomultiplier tube. 3. Results and discussion Fig. 2. PLE and PL spectra of Ca2BO3Cl:0.06Ce 3+ (a) and Ca2BO3Cl:0.01Eu 2+ (b). 3.2. Luminescence properties Fig. 2(a) shows the PLE and PL spectra of Ca2BO3Cl:0.06Ce 3+. The PLE spectrum monitored at 422nm exhibits two excitation bands centered at 281 and 360nm, which are due to the valence to conduction bands of the host-lattice absorption and 4f1-5d1 transition of Ce3+, respectively. Under the excitation of 360nm, the emission spectrum shows a strong broad asymmetric band as expected for Ce3+ with a maximum at around 422nm. The Stoke shift for Ce3+ in Ca2BO3Cl host is about 4080 cm �1, which is located at the range from 1200 (for ScBO3:Ce 3+) to 8000 cm�1 (for SrY2O4:Ce 3+) reported in Ref. [27]. As displayed in Fig. 2(b), it is clear that Eu presents as divalent ion in Ca2BO3Cl host due to the absence of sharp f–f transition lines of Eu3+ in the excitation spectrum and the presence of broad band emission characteristic. The PLE spectrum of Ca2BO3Cl:0.01Eu 2+ phosphor monitored at 573nm shows two bands, a weak band (250–300nm) corresponding to the valence-to-conduction band transition of the Ca2BO3Cl host lattice and a strong broad band located at 410nm ascribed to the 4f–5d transition of the doped Eu2+ ions, respectively. An efficient excitation in the range of 300–500nm matches suitably well with the radiative of UV and blue LED chip. Upon the excitation with 410nm, the emission spectrum has exhibited a broad yellow band centered at 573nm, which is assigned to 4f65d1(t2g)-4f 7(8S7/2) transition of Eu 2+. The low energy of Eu2+ in Ca2BO3Cl host is due to the strong crystal-field ARTICLE IN PRESS F. Xiao et al. / Physica B 404 (2009) 3743–3747 3745 and nephelauxetic effects that are originating from the seven-fold coordination of Ca2+ ions. As shown in Fig. 3, a significant spectral overlap between the emission band of Ce3+ and the excitation band of Eu2+ was observed clearly, which indicates that an effective resonance-type energy transfer is expected in the Ca2BO3Cl host. The inset of Fig. 3 displays clear evidence from Ce3+ to Eu2+ excited at 410nm corresponding to the optimum excitation wavelength of Eu2+ ions. As expected, the yellow emission intensity of Ce3+- and Eu2+- codoped phosphor is higher than that of singly doped Eu2+ sample. Furthermore, the emission spectrum of Ca2BO3Cl:0.06Ce 3+,0.01Eu2+ presents not only a blue band centered at 422nm of Ce3+ but also a broad yellow band centered at 573nm of Eu2+ under excitation of 360nm (Fig. 4). The excitation spectra monitored at 422 and 573nm is similar to that of the solely Ce3+- and Eu2+-doped Ca2BO3Cl, respectively. In order to investigate the energy transfer process from Ce3+ to Eu2+ in Ca2BO3Cl, the concentration of Ce 3+ is fixed at an optimum Fig. 3. PLE spectrum of Ca2BO3Cl:Eu 2+ and PL spectrum of Ca2BO3Cl:Ce 3+. Inset shows the PL spectra of Ca2BO3Cl:0.01Eu 2+ and Ca2BO3Cl:0.06Ce 3+,0.01Eu2+ (lex ¼ 410nm). Fig. 4. PLE and PL spectra of Ca2BO3Cl:0.06Ce 3+,0.01Eu2+. doping of 0.06 with a varied contents of Eu2+. Fig. 5 shows the PL spectra of Ca2BO3Cl:0.06Ce 3+,yEu2+ phosphors with dopant contents y of 0, 0.005, 0.01, 0.015, 0.02, 0.025. With increasing of Eu2+ dopant concentration, the blue emission intensity of Ce3+ sensitizer decreases gradually due to the enhancement of energy transition from Ce3+ to Eu2+ in the Ca2BO3Cl matrix. However, the yellow emission intensity of Eu2+ was observed to increase with increasing of Eu2+ concentrations and reach a maximum at y ¼ 0.015, beyond which the emission intensity decreases and which could be ascribed to the internal concentration quenching effect of Eu2+ ions. These results could thus provide the energy transfer evidence from Ce3+ to Eu2+ which has earlier been investigated [28,29]. The energy transfer efficiency (ZT) from Ce3+ to Eu2+ can be expressed by [30] ZT ¼ 1� IS IS0 ð1Þ where IS and IS0 are the luminescence intensity of the sensitizer Fig. 5. PL spectra of Ca2BO3Cl:0.06Ce 3+,yEu2+ phosphors (lex ¼ 360nm). Inset shows dependence of the energy transfer efficiency ZT in Ca2BO3Cl:0.06Ce3+,yEu2+ on Eu2+ contents (y). (Ce3+) ion with and without activator (Eu2+) ion present, respectively. The ZT from Ce3+ to Eu2+ in Ca2BO3Cl:0.06Ce3+,yEu2+ are calculated as a function of the Eu2+ concentration (y) and exhibited in the inset of Fig. 5. It is observed clearly that the energy transfer efficiency (ZT) increases gradually with increasing Eu2+ dopant concentration. Based on Dexter’s energy transfer formula of multipolar interaction and Reisfeld’s approximation, the following relation can be obtained [31]: Z0 Z pC n=3 ð2Þ where Z0 and Z are the luminescence quantum efficiency of Ce3+ in the absence and presence of Eu2+, respectively; the Z0/Z values can be approximately calculated by the ratio of related lumines- cence intensities (IS0/IS); C is the concentration of Eu 2+; n ¼ 6 and 8 represent the dipole–dipole and dipole–quadrupole interac- tions, respectively. Fig. 6(a) and (b) shows the plots of IS0/IS�Cn/3. It is noted that when n ¼ 6 and 8, both of the plots exhibit a linear relation. Moreover, because of the Coulombic effect of electric dipole–dipole interaction is larger than electric dipole–quadrupole interaction, the energy transfer from Ce3+ to Eu2+ ions in Ca2BO3Cl host matrix will mainly occur by dipole–dipole interaction on the basis of the theory of energy ARTICLE IN PRESS F. Xiao et al. / Physica B 404 (2009) 3743–37473746 Fig. 6. Dependence of IS0/IS of Ce 3+ on (a) C6/3 and (b) C8/3. Table 1 Comparison of CIE chromaticity coordinates of Ca2BO3Cl:xCe 3+,yEu2+ phosphors. Serial number Ca2BO3Cl:xCe 3+,yEu2+ lex (nm) (x, y) a x ¼ 0.06 y ¼ 0 360 (0.16, 0.06) b x ¼ 0.06 y ¼ 0.005 360 (0.25, 0.20) c x ¼ 0.06 y ¼ 0.01 360 (0.31, 0.29) d x ¼ 0.06 y ¼ 0.015 360 (0.38, 0.40) e x ¼ 0.06 y ¼ 0.02 360 (0.38, 0.41) f x ¼ 0.06 y ¼ 0.025 360 (0.40, 0.42) transfer of Dexter [32]. The same results have been reported in previous studies [28–30]. For electric dipole–dipole interaction mechanism, the critical distance (Rc) of energy transfer from Ce 3+ to Eu2+ can be calculated from the following formula [22,31]: R6c ¼ ð3� 1012Þfd Z FsðEÞFAðEÞ E4 dE ð3Þ where fdE0.02 is the electric dipole oscillator strength for the Eu2+. R FS(E)FA(E)/E 4dE represents the spectral overlap between the normalized spectral shapes of Ce3+ emission (FS(E)) and Eu 2+ excitation (FA(E)), and it is reckoned to about 0.0149eV �5 based on the information in Fig. 3. Therefore, the critical energy transfer from sensitizer of Ce3+ to activator of Eu2+ was calculated to be about 31.05 A˚. In addition, Blasse have suggested that the critical distance (Rc) of energy transfer can be estimated by [31] Rc � 2 3V 4pxcZ � �1=3 ð4Þ where V is the volume of the unit cell, Z is the number of host cations in the unit cell, xc is the critical concentration, at which the luminescence intensity of Ce3+ is half that without Eu2+ (ZT ¼ 0.5). According to the crystal structure of the Ca2BO3Cl compound, V ¼ 418.81 A˚, Z ¼ 4 [24] and xc ¼ 0.006. Therefore, Rc was reckoned to be 32.18 A˚, which agrees well with the value based on the spectral overlap between Ce3+ emission and Eu2+ excita- tion. The Commission Internationale de I’Eclairage (CIE) chromati- city coordinates for Ca2BO3Cl:xCe 3+,yEu2+ phosphors which were calculated based on the corresponding emission spectrum are summarized in Table 1 and also represented in Fig. 7. We have g x ¼ 0 y ¼ 0.015 410 (0.46, 0.50) observed that the CIE chromaticity coordinates of solely Ce3+- and Eu2+-doped Ca2BO3Cl phosphors are (x ¼ 0.16, y ¼ 0.07) and (x ¼ 0.46, y ¼ 0.50) corresponding to the hues of blue and yellow, which is represented by point a (solely Ce3+-doped) and point g (solely Eu2+-doped), respectively. With an increase of Eu2+ concentrations, it is noted that the hues of Ca2BO3Cl:xCe 3+,yEu2+ phosphors are tunable from blue through white and eventually to Fig. 7. CIE chromaticity coordinates of Ca2BO3Cl:xCe 3+,yEu2+ phosphors repre- sented as color dot (a–g) (x ¼ 0.06 excited at 360nm, x ¼ 0 excited at 410nm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) yellow on the line as depicted in this figure. In another word, this tunable emission of Ca2BO3Cl:Ce 3+,Eu2+ phosphor integrated with UV LEDs could generate varied white lights. Furthermore, the sample Ca2BO3Cl:0.06Ce 3+,0.01Eu2+ of our system emits white light with CIE coordinates of (x ¼ 0.31, y ¼ 0.29) and its relative color temperature of 7330K which is quite close to the CIE ideal white light (x ¼ 0.33, y ¼ 0.33). The above results implies that the white-emitting Ca2BO3Cl:Ce 3+,Eu2+ phosphor has promising application in UV chip-based white LEDs. 4. Conclusions In summary, Ca2BO3Cl:Ce 3+,Eu2+ samples were prepared by a solid-state reaction under a weak reductive atmosphere. The Ca2BO3Cl:Ce 3+,Eu2+ phosphor shows two broad emission bands centered at 422 (blue) and 573nm (yellow) upon the excitation of 360nm. The energy transfer mechanism from Ce3+ to Eu2+ in Ca2BO3Cl host has been proved to be dominated by an electric dipole–dipole interaction. A white light was generated from Ca2BO3Cl:0.06Ce 3+, 0.01Eu2+ phosphor with CIE chromaticity coordinates of (x ¼ 0.31, y ¼ 0.29) and relative color temperature of 7330K. Moreover, a tunable color emission from blue through white and ultimately to yellow was achieved by adjusting the Ce3+- and Eu2+-dopant concentrations. The present study demon- strates Ca2BO3Cl:Ce 3+, Eu2+ as a promising single-phased tunable white-emitting phosphor for their applications towards the development of white LEDs. ARTICLE IN PRESS Acknowledgments The authors would like to acknowledge the support from the NSFC (Grant no. 50872036). References [1] J.K. Park, R.J. Choi, K.N. Kim, C.H. Kim, Appl. Phys. Lett. 87 (2005) 031108. [2] N. Kimura, K. Sakuma, S. Hirafune, K. Asano, N. Hirosaki, R.-J. Xie, Appl. Phys. Lett. 90 (2007) 051109. [3] H.S. Jang, H. Yang, S.W. Kim, J.Y. Han, S.-G. Lee, D.Y. Jeon, Adv. Mater. 20 (2008) 2696. [4] H.Y. Jiao, Y.H. Wang, J. Electrochem. Soc. 156 (2009) J117. [5] S. Nakamura, M. Senoh, T. Mukai, Appl. Phys. Lett. 62 (1993) 2390. [6] W.B. Im, Y.-II. Kim, N.N. Fellows, H. Masui, G.A. Hirata, S.P. DenBaars, R. Seshadri, Appl. Phys. Lett. 93 (2008) 091905. [7] M.S. Shur, A. Zukauskas, Proc. IEEE 93 (2005) 1691. [8] Y.-S. Tang, S.-F. Hu, W.-C. Ke, C.C. Lin, N.C. Bagkar, R.-S. Liu, Appl. Phys. Lett. 93 (2008) 131114. [9] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931. [10] Z.D. Hao, J.H. Zhang, X. Zhang, X.Y. Sun, Y.S. Luo, S.Z. Lu, Appl. Phys. Lett. 90 (2007) 261113. [11] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1996. [12] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [13] G.B. Kumar, S. Buddhudu, Physica B 403 (2008) 4164. [14] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, Appl. Phys. Lett. 82 (2004) 3696. [15] Q.Y. Zhang, C.H. Yang, Y.X. Pan, Nanotechnology 18 (2007) 145602. [16] W.-J. Yang, T.-M. Chen, Appl. Phys. Lett. 88 (2006) 101903. [17] C.F. Guo, L. Luan, Y. Xu, F. Gao, L.F. Liang, J. Electrochem. Soc. 155 (2008) J310. [18] Y.H. Won, H.S. Jang, W.B. Im, D.Y. Jeon, J.S. Lee, Appl. Phys. Lett. 89 (2006) 231909. [19] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [20] R.-S. Liu, Y.-H. Liu, N.C. Bagkar, Appl. Phys. Lett. 91 (2007) 061119. [21] C.-K. Chang, T.-M. Chen, Appl. Phys. Lett. 91 (2007) 081902. [22] Y.H. Song, G. Jia, M. Yang, Y.J. Huang, H.P. You, H.J. Zhang, Appl. Phys. Lett. 94 (2009) 091902. [23] N. Lakshminarasimhan, U.V. Varadaraju, J. Electrochem. Soc. 152 (2005) H152. [24] Z. Zak, F. Hanic, Acta Cryst. Sect. B 32 (1976) 1784. [25] Z.P. Yang, S.L. Wang, G.W. Yang, J. Tian, P.L. Li, X. Li, Mater. Lett. 61 (2007) 5258. [26] R.D. Shannon, Acta Cryst. Sect. A 32 (1976) 751. [27] G. Blasse, A. Bril, J. Chem. Phys. 47 (1967) 5139. [28] U. Caldin˜o, J. Phys. Condens. Matter 15 (2003) 7127. [29] Y. Tan, C. Shi, J. Phys. Chem. Solids. 60 (1999) 1805. [30] W.-J. Yang, T.-M. Chen, Appl. Phys. Lett. 90 (2007) 171908. [31] G. Blasse, Philips Res. Rep. 24 (1969) 131. [32] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. F. Xiao et al. / Physica B 404 (2009) 3743–3747 3747 Ca2BO3Cl:Ce3+,Eu2+: A potential tunable yellow-white-blue-emitting phosphors for white light-emitting diodes 1. Introduction Experimental Results and discussion Phase identification and morphology Luminescence properties Conclusions Acknowledgments References