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A new blue-emitting phosphor of Ce3+-activated CaLaGa3S6O for white-light-emitting diodes
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Chemical Physics Letters 453 (2008) 197–201 www.elsevier.com/locate/cplett
A new blue-emitting phosphor of Ce3+-activated CaLaGa3S6O for white-light-emitting diodes Ruijin Yu, Jing Wang *, Mei Zhang, Jianhui Zhang, Haibin Yuan, Qiang Su * MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, PR China Received 23 November 2007; in final form 16 January 2008 Available online 21 January 2008
Abstract A new blue-emitting chalcogenide phosphor, Ce3+-activated CaLaGa3S6O, with a high purity crystalline was synthesized by a two-step solid-state reaction. Photoluminescence properties of CaLaGa3S6O:Ce3+ were investigated comparatively with the commercial blue-emitting phosphor BaMgAl10O17:Eu2+. It shows a more perfect and efficient broad absorption band around the 398 nm emission of the commercial near ultraviolet light-emitting diodes (LEDs), and presents a comparable blue-emitting performance. The blue lightemitting LED with the CIE chromaticity coordinates of (0.147, 0.089) was successfully fabricated by precoating CaLaGa3S6O:Ce3+ phosphor onto a 398 nm-emitting InGaN chip. All these results indicate that CaLaGa3S6O:Ce3+ is a promising blue phosphor candidate for white LEDs. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently growing interest was focused on semiconductor white-light-emitting diodes, which have many advantages, such as high efficiency, long lifetime, low power consumption, and environment-friendly characteristics [1,2]. In comparison with the commercial white LEDs fabricated with a blue chip and the yellow phosphor Y3Al5O12:Ce3+, the white LEDs fabricated with near ultraviolet (n-UV) chips and red/green/blue tricolor phosphors can offer a higher efficient solid-state light [3]. Thus, the n-UV phosphor-converted white LED is expected to have great potential applications in the field of solid-state lighting. The current blue phosphor material for solid-state lighting based on near-UV LEDs is mainly BaMgAl10O17:Eu2+ (BAM), but BAM shows a poor absorption band around 400 nm, not well suitable for InGaN chips [4,5]. Therefore, it is necessary to find new blue phosphors that can be effec*
Corresponding authors. Fax: +86 20 84111038 (J. Wang). E-mail addresses: [email protected] (J. Wang), Suqiang@ mail.sysu.edu.cn (Q. Su). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.01.039
tively excited in the UV range especial 400 nm to enhance the luminous efficiency. The chalcogenide has the smaller electronegative value element sulfur compared with the oxide. When Eu2+ or Ce3+ was doped into such chalcogenide hosts, the crystal field splitting of doped ions will be stronger, the absorption of the 4f–5d transitions may extend to the visible (400– 500 nm) area. So the Eu2+ or Ce3+ doped chalcogenide are very appropriate phosphors excited by near-UV or blueemitting diodes for solid-state lighting, such as red-emitting Ca1xSrx(SySe1y):Eu2+ [6], yellow-emitting CaGa2S4:Eu2+ [7,8], and Sr1–xCaxGa2S4:Eu2+ [9] phosphors. The luminescent properties of SrLaGa3S6O:Eu2+ phosphor were first reported by our group, via replacing part of the O in SrLaGa3O7 by S [10]. The phosphor with a broad absorption band in the range of 200–500 nm was expected to be a promising yellowish-green-emitting phosphor for white LEDs. But the pure SrLaGa3S6O host was not obtained according to this one-pot synthesis method. In this Letter, for the first time, a new and interesting blue-emitting phosphor Ce3+-activated CaLaGa3S6O with a high purity crystalline was synthesized. The luminescence
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properties were studied in detail by means of the diffuse reflectance, photoluminescence excitation and emission spectra, concentration quenching and the decay curves. It shows a more perfect and efficient absorption band and a comparable blue-emitting performance, compared with the commercial phosphor BAM. Furthermore, an intense blue light-emitting LED was fabricated by precoating CaLaGa3S6O:Ce3+ phosphor onto a 398 nm-emitting InGaN chip. 2. Experimental The synthesis was carried out by a two-step solid-state reaction. The first step was to prepare the starting sulfide material. The starting sulfide materials b-La2S3, Ga2S3 and c-Ce2S3 were pre-prepared at high temperature in horizontal tube furnaces. b-La2S3 and c-Ce2S3 were prepared from La2O3 (99.99%) and CeO2 (99.99%) under CS2 reducing atmosphere at 1250 °C for 3 h. Ga2S3 was prepared from Ga2O3 (99.99%) under flowing H2S gas at 950 °C for 3 h. The second step is to prepare a series of powder samples of CaLa1–xCexGa3S6O (x = 0.01, 0.03, 0.06, 0.10, 0.15, 0.20) with the starting sulfide materials by a conventional solid-state reaction using the following reaction [11]. CaO þ ð1 xÞ=2La2 S3 þ 3=2Ga2 S3 þ x=2Ce2 S3
950 C;Ar;2 h
!
CaLa1x Cex Ga3 S6 O
The stoichiometric amounts of materials CaO (A.R.), bLa2S3, Ga2S3 and c-Ce2S3 were thoroughly mixed by grinding, and then they were sintered in Ar atmosphere at 950 °C for 2 h in horizontal tube furnaces. The structure of the final products was examined by X-ray powder diffraction using a Rigaku D/max 2200 vpc X-ray Diffractometer with Cu Ka radiation at 40 kV and 30 mA. The XRD patterns were collected in the range 10° 6 2h 6 60°. The diffuse reflection spectra of the samples were measured by a Cary 5000 UV–Vis–NIR spectrophotometer equipped with double out-of-plane littrow monochrometer, using BaSO4 as a standard reference in the measurements. The photoluminescence (PL), photoluminescence excitation (PLE) spectra of CaLaGa3S6O:Ce3+ were measured by a Fluorolog-3 spectrofluorometer (Jobin Yvon Inc/ specx) equipped with a 450 W Xe lamp and double excitation monochromators. The above measurements were carried out at room temperature. The emission spectrum of CaLaGa3S6O:Ce3+ at 10 K, the dependences of emission spectra and decay time on temperature between 40 and 420 K were recorded on an Edinburgh FLS 920 spectrofluorometer, equipped with a 450 W xenon lamp, a 150 W nF900 nanosecond flash lamp with a pulse width of 1 ns and pulse repetition rate of 40–100 kHz and a DE-202S closed cycle helium cryostat (Advanced Research Systems, Inc.). The emission spectra of the LEDs were recorded on an LED-1100 Spectral/Goniometric Analyzer (Labsphere Inc.) under a direct current of 20 mA at room temperature.
3. Results and discussion 3.1. Phase characterization X-ray diffraction (XRD) patterns of the series of phosphors CaLa1–xCexGa3S6O (x = 0.01, 0.03, 0.06, 0.10, 0.15, 0.20) were shown in Fig. 1. The results show that all these compounds are of a single phase of CaLaGa3S6O in line with JCPDS-39-0560, suggesting that the dopant Ce3+ do not significantly influence the XRD patterns of the host. Structurally, CaLaGa3S6O crystallizes into a tetragonal structure in the space group P421m. The structure is constructed by five-membered rings from [Ga(O/ S)4]5 tetrahedrons linked at each corner. The Ca2+ ions and La3+ ions are surrounded octahedrally by eight anions and are distributed randomly in sites of Cs symmetry [11]. The calculated lattice parameters are shown in Table 1. Considering the ionic radii of Ca2+(112 pm), La3+(116 pm) and Ce3+(114 pm) ions in site of CN = 8, it is strongly expected that the Ce3+ ions replace the La3+ ions due to the close radius and the same valence. From the data in Table 1, the unit cell shrinks with increasing Ce3+ concentrations in the range of x = 0.01–0.20, strongly supporting that the Ce3+ ions preferably replace the La3+ ions. In our previous work, one-pot synthesis using oxides as raw materials was used to prepare SrLaGa3S6O host. But the results showed that there are two obvious impurity phases, SrLa2S4 and SrGa2S4, accompanying the main phase SrLaGa3S6O. In the present case, these impurity phases were not detected by the two-step solid-state reaction. It can be concluded that the sulfuration of starting materials including La2O3, Ga2O3 is necessary to obtain pure CaLaGa3S6O phase. 3.2. The luminescence properties of CaLaGa3S6O:Ce3+ The diffuse reflectance and the absorption spectra of CaLaGa3S6O and CaLaGa3S6O:Ce3+ compounds are shown in Fig. 2. The absorption spectra F(R) were obtained from the diffuse reflection spectra by using the Kubelka–Munk function [12]: F ðRÞ ¼ ð1 RÞ2 =2R ¼ K=S where R, K and S are the reflectivity, the absorption coefficient and the scattering coefficient, respectively. It is clear that the CaLaGa3S6O host shows a platform of nearly 80–95% diffuse reflectance in the wavelength range of 500– 800 nm. When Ce3+ ions are doped into the host, it can be seen from the curve (2) of Fig. 2 that there are three obvious broad bands in the range of 200–430 nm. It can be concluded that the obvious absorption bands around 330 and 405 nm are ascribed to 4f ? 5d transition of Ce3+ ions, and the host absorption edge is around 250 nm. The emission and excitation spectra of Ce3+ in CaLaGa3S6O at room temperature are presented in Fig. 3. The excitation spectra (Fig. 3b and c) of Ce3+ in CaLaGa3S6O consist of three broad bands around 286, 332 and 400 nm. They are attributed to the host absorption and the f ? d transitions of Ce3+, and consistent with the
R. Yu et al. / Chemical Physics Letters 453 (2008) 197–201
199
Fig. 1. The XRD patterns of the samples CaLa1–xCexGa3S6O with different Ce3+ concentrations.
Table 1 The calculated lattice parameters of CaLa1–xCexGa3S6O (x = 0.01, 0.03, 0.06, 0.10, 0.15, 0.20) Cell parameters ˚ a/A ˚ c/A ˚3 V/A
CaLa1–xCexGa3S6O x = 0.01
x = 0.03
x = 0.06
x = 0.10
x = 0.15
x = 0.20
9.307 6.039 523.10
9.305 6.038 522.85
9.287 6.033 520.33
9.275 6.031 518.92
9.268 6.021 517.30
9.263 6.011 515.69
Fig. 2. The diffuse reflectance and the absorption spectra of CaLaGa3S6O (curve 1) and CaLaGa3S6O:0.10Ce3+ (curve 2) phosphors.
diffuse reflectance spectra (see Fig. 2). Curve (a) is the emission spectrum under 398 nm excitation. A broad emission band at room temperature with a maximum at about
Fig. 3. The excitation (b, kem = 442 nm; c, kem = 478 nm) and emission (a, kex = 398 nm) spectra of CaLaGa3S6O:0.10Ce3+ at room temperature. The upper right inset represents the concentration influence on the emission intensity of CaLa1–xCexGa3S6O. The lower right inset represents the emission spectrum of CaLaGa3S6O:0.10Ce3+ at 10 K.
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442 nm and a shoulder band around 478 nm can be observed. The emission spectrum at 10 K is similar to that at room temperature, which presents well resolved peaks at 438 and 480 nm. [see the lower right inset in Fig. 3]. The energy difference of the sub-bands is about 1998 cm1, which is close to the 2FJ (J = 7/2, 5/2) energy gap of Ce3+ (2000 cm1) in most Ce3+ activated phosphors, indicating that the emission of Ce3+ ions in CaLaGa3S6O is from a single lattice site. The value of the red shift (D) expressed by the energy difference of the lowest 5d excited levels of Ce3+ in present host compared with the free ion of Ce3+, is 24 340 cm1. The Stokes shift is calculated to be 2273 cm1. The influence of the Ce3+ concentration x on the emission intensity of Ce3+-activated CaLaGa3S6O is shown in the upper right inset of Fig. 3. The PL intensity under 398 nm excitation increases with Ce3+ content increasing until a maximum intensity of about x = 0.10 is reached, and then it decreases because of the concentration quenching process. The critical transfer distance (Rc) of Ce3+ in CaLaGa3S6O:Ce3+ phosphor is approximately calculated ˚ according to Ref. [13]. to be 17 A Fig. 4 shows the excitation and emission spectra of CaLaGa3S6O:0.10Ce3+ and the commercial BaMgAl10O17: Eu2+ (BAM) phosphor that is currently used as the blueemitting phosphor for white LED based on n-UV InGaN chip. For these curves, we normalized all parameters those including (emission and excitation) slit width, integrated time, beam intensity. It is comparatively seen that the assynthesized sample (Fig. 4b) rather than BAM (Fig. 4a) exhibits the optimal absorption band around the emission wavelength (398 nm) of n-UV LED (Fig. 4c). Monitoring
the excitation wavelength at 398 nm, the integrated emission intensity of as-synthesized sample is by a factor of about 70% as strong as that of BAM. These results indicate that CaLaGa3S6O:Ce3+ is a potential novel blue-emitting phosphor for LED application if further optimization including starting material, reaction temperature, time, flux, codoped ions, and technical process, etc., is taken to improve the luminescent intensity. The fluorescence decay time of Ce3+ in CaLaGa3S6O: 0.10Ce3+ has been measured at different temperature between 40 and 420 K. The decay curves can be well fitted by a single exponential equation: It = I0 exp(t/s), where It and I0 are the luminescence intensity at time t and time 0, and s is the decay time, respectively. The values of s are listed in Table 2. At 40 and 300 K, the lifetimes are determined to be 17.46 and 15.69 ns, respectively. With the temperature increasing, the lifetime decreases in the time range of 10–18 ns, due to the parity allowed 5d–4f transition of Ce3+ ion with high radiative probability. Generally, as the increase of temperature, the non-radiative processes will be more efficient and consequently the lifetime will be shorter [14]. Therefore the temperature dependence of the lifetime is due to the increase in thermal assisted nonradiative transition probability.
Table 2 The lifetimes of Ce3+ in CaLaGa3S6O:0.10Ce3+ at different temperatures (kex = 398 nm, kem = 442 nm) Temperature (K) s (ns)
40 17.46
100 17.39
200 16.77
300 15.69
420 11.99
Fig. 4. The excitation spectra of CaLaGa3S6O:0.10Ce3+ (b) and commercial BAM (a) phosphors and emission spectrum of UV-LED (c). The emission spectra of CaLaGa3S6O:0.10Ce3+ (d) and BAM (e) phosphors (kex = 398 nm).
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Fig. 5. The EL spectrum of the fabricated LED with CaLaGa3S6O:0.10Ce3+ phosphor under 20 mA forward-bias current (at the left). The CIE coordinates of CaLaGa3S6O:0.10Ce3+ and BAM (at the right).
3.3. Fabricated LEDs with CaLaGa3S6O:Ce3+ phosphor In order to further investigate the possibility of the as-synthesized phosphor in UV LED, a blue LED was fabricated by combination CaLaGa3S6O:0.10Ce3+ phosphor with a 398 nm-emitting InGaN chip at IF = 20 mA. In Fig. 5 (at the left) is the electroluminescence spectra of the naked n-UV LED and as-fabricated phosphor-converting LED at IF = 20 mA. Two photographs of the lighting n-UV LED and the blue LED with CaLaGa3S6O:Ce3+ are shown in the inset. Intense blue light can be observed with the naked eye, and from two curves, it is seen that the near violet light of the naked n-UV LED chip at 398 nm is absorbed dramatically by CaLaGa3S6O:Ce3+ and simultaneously down-converted into an intensive blue light around 440, 478 nm. Its CIE chromaticity coordinates are calculated to be x = 0.147, y = 0.089, which fall into the blue region and are almost the same as BAM (0.144, 0.072) in the CIE 1931 chromaticity diagram as shown in Fig. 5 (at the right). 4. Conclusions A new blue-emitting phosphor of Ce3+-activated CaLaGa3S6O was synthesized by the two-step solid-state reaction. The photoluminescence spectra show that this phosphor can be excited efficiently by UV–Vis light from 360 to 430 nm and emits intense blue light with two emission bands peaking at about 440 and 478 nm. Furthermore, a blue light-emitting LED was successfully fabricated by precoating CaLaGa3S6O:Ce3+ phosphor onto a 398 nm-
emitting InGaN chip. With further studies being undertaken, this phosphor might be a promising blue phosphor candidate for white LEDs. Acknowledgements This work was supported by the National Nature Science Foundation of China (20501023), the Nature Science Foundation of Guangdong for Doctorial Training base (5300527) and the Science and Technology Project of Guangzhou (2005Z2-D0061). References [1] S. Dalmasso et al., Phys. Status Solidi A 192 (2002) 139. [2] J.-H. Yum, S.-Y. Seo, S. Lee, Y.-E. Sung, J. Electrochem. Soc. 150 (2003) H47. [3] E. Radkov, R. Bompiedi, A.M. Srivastava, A.A. Setlur, C. Becker, Proc. SPIE 5187 (2004) 171. [4] T. Ju stel, H. Nikol, Adv. Mater. 12 (2000) 527. [5] U. Kaufmann et al., Phys. Status Solidi 188 (2001) 143. [6] M. Nazarov, C. Yoon, J. Solid State Chem. 179 (2006) 2529. [7] J. Zhang, M. Takahashi, Y. Tokuda, T. Yoko, J. Ceram. Soc. Jpn. 112 (2004) 511. [8] J.M. Kim, K.N. Kim, S.H. Park, J.K. Park, C.H. Kim, H.G. Jang, J. Korean Chem. Soc. 49 (2005) 201. [9] Y.R. Do, K.Y. Ko, S.H. Na, Y.D. Huh, J. Electrochem. Soc. 153 (2006) H142. [10] X. Zhang, J. Zhang, J. Xu, Q. Su, J. Alloys Compd. 389 (2005) 247. [11] C.L. Teske, Z. Anorg. Allg. Chem. 531 (1985) 52. [12] G. Kortum, G. Schreyer, Z. Naturforsch. 11A (1957) 1018. [13] G. Blasse, Philips Res. Rep. 24 (1969) 131. [14] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin Heidelberg, 1994.
Chemical Physics Letters 453 (2008) 197–201 www.elsevier.com/locate/cplett
A new blue-emitting phosphor of Ce3+-activated CaLaGa3S6O for white-light-emitting diodes Ruijin Yu, Jing Wang *, Mei Zhang, Jianhui Zhang, Haibin Yuan, Qiang Su * MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, PR China Received 23 November 2007; in final form 16 January 2008 Available online 21 January 2008
Abstract A new blue-emitting chalcogenide phosphor, Ce3+-activated CaLaGa3S6O, with a high purity crystalline was synthesized by a two-step solid-state reaction. Photoluminescence properties of CaLaGa3S6O:Ce3+ were investigated comparatively with the commercial blue-emitting phosphor BaMgAl10O17:Eu2+. It shows a more perfect and efficient broad absorption band around the 398 nm emission of the commercial near ultraviolet light-emitting diodes (LEDs), and presents a comparable blue-emitting performance. The blue lightemitting LED with the CIE chromaticity coordinates of (0.147, 0.089) was successfully fabricated by precoating CaLaGa3S6O:Ce3+ phosphor onto a 398 nm-emitting InGaN chip. All these results indicate that CaLaGa3S6O:Ce3+ is a promising blue phosphor candidate for white LEDs. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently growing interest was focused on semiconductor white-light-emitting diodes, which have many advantages, such as high efficiency, long lifetime, low power consumption, and environment-friendly characteristics [1,2]. In comparison with the commercial white LEDs fabricated with a blue chip and the yellow phosphor Y3Al5O12:Ce3+, the white LEDs fabricated with near ultraviolet (n-UV) chips and red/green/blue tricolor phosphors can offer a higher efficient solid-state light [3]. Thus, the n-UV phosphor-converted white LED is expected to have great potential applications in the field of solid-state lighting. The current blue phosphor material for solid-state lighting based on near-UV LEDs is mainly BaMgAl10O17:Eu2+ (BAM), but BAM shows a poor absorption band around 400 nm, not well suitable for InGaN chips [4,5]. Therefore, it is necessary to find new blue phosphors that can be effec*
Corresponding authors. Fax: +86 20 84111038 (J. Wang). E-mail addresses: [email protected] (J. Wang), Suqiang@ mail.sysu.edu.cn (Q. Su). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.01.039
tively excited in the UV range especial 400 nm to enhance the luminous efficiency. The chalcogenide has the smaller electronegative value element sulfur compared with the oxide. When Eu2+ or Ce3+ was doped into such chalcogenide hosts, the crystal field splitting of doped ions will be stronger, the absorption of the 4f–5d transitions may extend to the visible (400– 500 nm) area. So the Eu2+ or Ce3+ doped chalcogenide are very appropriate phosphors excited by near-UV or blueemitting diodes for solid-state lighting, such as red-emitting Ca1xSrx(SySe1y):Eu2+ [6], yellow-emitting CaGa2S4:Eu2+ [7,8], and Sr1–xCaxGa2S4:Eu2+ [9] phosphors. The luminescent properties of SrLaGa3S6O:Eu2+ phosphor were first reported by our group, via replacing part of the O in SrLaGa3O7 by S [10]. The phosphor with a broad absorption band in the range of 200–500 nm was expected to be a promising yellowish-green-emitting phosphor for white LEDs. But the pure SrLaGa3S6O host was not obtained according to this one-pot synthesis method. In this Letter, for the first time, a new and interesting blue-emitting phosphor Ce3+-activated CaLaGa3S6O with a high purity crystalline was synthesized. The luminescence
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R. Yu et al. / Chemical Physics Letters 453 (2008) 197–201
properties were studied in detail by means of the diffuse reflectance, photoluminescence excitation and emission spectra, concentration quenching and the decay curves. It shows a more perfect and efficient absorption band and a comparable blue-emitting performance, compared with the commercial phosphor BAM. Furthermore, an intense blue light-emitting LED was fabricated by precoating CaLaGa3S6O:Ce3+ phosphor onto a 398 nm-emitting InGaN chip. 2. Experimental The synthesis was carried out by a two-step solid-state reaction. The first step was to prepare the starting sulfide material. The starting sulfide materials b-La2S3, Ga2S3 and c-Ce2S3 were pre-prepared at high temperature in horizontal tube furnaces. b-La2S3 and c-Ce2S3 were prepared from La2O3 (99.99%) and CeO2 (99.99%) under CS2 reducing atmosphere at 1250 °C for 3 h. Ga2S3 was prepared from Ga2O3 (99.99%) under flowing H2S gas at 950 °C for 3 h. The second step is to prepare a series of powder samples of CaLa1–xCexGa3S6O (x = 0.01, 0.03, 0.06, 0.10, 0.15, 0.20) with the starting sulfide materials by a conventional solid-state reaction using the following reaction [11]. CaO þ ð1 xÞ=2La2 S3 þ 3=2Ga2 S3 þ x=2Ce2 S3
950 C;Ar;2 h
!
CaLa1x Cex Ga3 S6 O
The stoichiometric amounts of materials CaO (A.R.), bLa2S3, Ga2S3 and c-Ce2S3 were thoroughly mixed by grinding, and then they were sintered in Ar atmosphere at 950 °C for 2 h in horizontal tube furnaces. The structure of the final products was examined by X-ray powder diffraction using a Rigaku D/max 2200 vpc X-ray Diffractometer with Cu Ka radiation at 40 kV and 30 mA. The XRD patterns were collected in the range 10° 6 2h 6 60°. The diffuse reflection spectra of the samples were measured by a Cary 5000 UV–Vis–NIR spectrophotometer equipped with double out-of-plane littrow monochrometer, using BaSO4 as a standard reference in the measurements. The photoluminescence (PL), photoluminescence excitation (PLE) spectra of CaLaGa3S6O:Ce3+ were measured by a Fluorolog-3 spectrofluorometer (Jobin Yvon Inc/ specx) equipped with a 450 W Xe lamp and double excitation monochromators. The above measurements were carried out at room temperature. The emission spectrum of CaLaGa3S6O:Ce3+ at 10 K, the dependences of emission spectra and decay time on temperature between 40 and 420 K were recorded on an Edinburgh FLS 920 spectrofluorometer, equipped with a 450 W xenon lamp, a 150 W nF900 nanosecond flash lamp with a pulse width of 1 ns and pulse repetition rate of 40–100 kHz and a DE-202S closed cycle helium cryostat (Advanced Research Systems, Inc.). The emission spectra of the LEDs were recorded on an LED-1100 Spectral/Goniometric Analyzer (Labsphere Inc.) under a direct current of 20 mA at room temperature.
3. Results and discussion 3.1. Phase characterization X-ray diffraction (XRD) patterns of the series of phosphors CaLa1–xCexGa3S6O (x = 0.01, 0.03, 0.06, 0.10, 0.15, 0.20) were shown in Fig. 1. The results show that all these compounds are of a single phase of CaLaGa3S6O in line with JCPDS-39-0560, suggesting that the dopant Ce3+ do not significantly influence the XRD patterns of the host. Structurally, CaLaGa3S6O crystallizes into a tetragonal structure in the space group P421m. The structure is constructed by five-membered rings from [Ga(O/ S)4]5 tetrahedrons linked at each corner. The Ca2+ ions and La3+ ions are surrounded octahedrally by eight anions and are distributed randomly in sites of Cs symmetry [11]. The calculated lattice parameters are shown in Table 1. Considering the ionic radii of Ca2+(112 pm), La3+(116 pm) and Ce3+(114 pm) ions in site of CN = 8, it is strongly expected that the Ce3+ ions replace the La3+ ions due to the close radius and the same valence. From the data in Table 1, the unit cell shrinks with increasing Ce3+ concentrations in the range of x = 0.01–0.20, strongly supporting that the Ce3+ ions preferably replace the La3+ ions. In our previous work, one-pot synthesis using oxides as raw materials was used to prepare SrLaGa3S6O host. But the results showed that there are two obvious impurity phases, SrLa2S4 and SrGa2S4, accompanying the main phase SrLaGa3S6O. In the present case, these impurity phases were not detected by the two-step solid-state reaction. It can be concluded that the sulfuration of starting materials including La2O3, Ga2O3 is necessary to obtain pure CaLaGa3S6O phase. 3.2. The luminescence properties of CaLaGa3S6O:Ce3+ The diffuse reflectance and the absorption spectra of CaLaGa3S6O and CaLaGa3S6O:Ce3+ compounds are shown in Fig. 2. The absorption spectra F(R) were obtained from the diffuse reflection spectra by using the Kubelka–Munk function [12]: F ðRÞ ¼ ð1 RÞ2 =2R ¼ K=S where R, K and S are the reflectivity, the absorption coefficient and the scattering coefficient, respectively. It is clear that the CaLaGa3S6O host shows a platform of nearly 80–95% diffuse reflectance in the wavelength range of 500– 800 nm. When Ce3+ ions are doped into the host, it can be seen from the curve (2) of Fig. 2 that there are three obvious broad bands in the range of 200–430 nm. It can be concluded that the obvious absorption bands around 330 and 405 nm are ascribed to 4f ? 5d transition of Ce3+ ions, and the host absorption edge is around 250 nm. The emission and excitation spectra of Ce3+ in CaLaGa3S6O at room temperature are presented in Fig. 3. The excitation spectra (Fig. 3b and c) of Ce3+ in CaLaGa3S6O consist of three broad bands around 286, 332 and 400 nm. They are attributed to the host absorption and the f ? d transitions of Ce3+, and consistent with the
R. Yu et al. / Chemical Physics Letters 453 (2008) 197–201
199
Fig. 1. The XRD patterns of the samples CaLa1–xCexGa3S6O with different Ce3+ concentrations.
Table 1 The calculated lattice parameters of CaLa1–xCexGa3S6O (x = 0.01, 0.03, 0.06, 0.10, 0.15, 0.20) Cell parameters ˚ a/A ˚ c/A ˚3 V/A
CaLa1–xCexGa3S6O x = 0.01
x = 0.03
x = 0.06
x = 0.10
x = 0.15
x = 0.20
9.307 6.039 523.10
9.305 6.038 522.85
9.287 6.033 520.33
9.275 6.031 518.92
9.268 6.021 517.30
9.263 6.011 515.69
Fig. 2. The diffuse reflectance and the absorption spectra of CaLaGa3S6O (curve 1) and CaLaGa3S6O:0.10Ce3+ (curve 2) phosphors.
diffuse reflectance spectra (see Fig. 2). Curve (a) is the emission spectrum under 398 nm excitation. A broad emission band at room temperature with a maximum at about
Fig. 3. The excitation (b, kem = 442 nm; c, kem = 478 nm) and emission (a, kex = 398 nm) spectra of CaLaGa3S6O:0.10Ce3+ at room temperature. The upper right inset represents the concentration influence on the emission intensity of CaLa1–xCexGa3S6O. The lower right inset represents the emission spectrum of CaLaGa3S6O:0.10Ce3+ at 10 K.
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442 nm and a shoulder band around 478 nm can be observed. The emission spectrum at 10 K is similar to that at room temperature, which presents well resolved peaks at 438 and 480 nm. [see the lower right inset in Fig. 3]. The energy difference of the sub-bands is about 1998 cm1, which is close to the 2FJ (J = 7/2, 5/2) energy gap of Ce3+ (2000 cm1) in most Ce3+ activated phosphors, indicating that the emission of Ce3+ ions in CaLaGa3S6O is from a single lattice site. The value of the red shift (D) expressed by the energy difference of the lowest 5d excited levels of Ce3+ in present host compared with the free ion of Ce3+, is 24 340 cm1. The Stokes shift is calculated to be 2273 cm1. The influence of the Ce3+ concentration x on the emission intensity of Ce3+-activated CaLaGa3S6O is shown in the upper right inset of Fig. 3. The PL intensity under 398 nm excitation increases with Ce3+ content increasing until a maximum intensity of about x = 0.10 is reached, and then it decreases because of the concentration quenching process. The critical transfer distance (Rc) of Ce3+ in CaLaGa3S6O:Ce3+ phosphor is approximately calculated ˚ according to Ref. [13]. to be 17 A Fig. 4 shows the excitation and emission spectra of CaLaGa3S6O:0.10Ce3+ and the commercial BaMgAl10O17: Eu2+ (BAM) phosphor that is currently used as the blueemitting phosphor for white LED based on n-UV InGaN chip. For these curves, we normalized all parameters those including (emission and excitation) slit width, integrated time, beam intensity. It is comparatively seen that the assynthesized sample (Fig. 4b) rather than BAM (Fig. 4a) exhibits the optimal absorption band around the emission wavelength (398 nm) of n-UV LED (Fig. 4c). Monitoring
the excitation wavelength at 398 nm, the integrated emission intensity of as-synthesized sample is by a factor of about 70% as strong as that of BAM. These results indicate that CaLaGa3S6O:Ce3+ is a potential novel blue-emitting phosphor for LED application if further optimization including starting material, reaction temperature, time, flux, codoped ions, and technical process, etc., is taken to improve the luminescent intensity. The fluorescence decay time of Ce3+ in CaLaGa3S6O: 0.10Ce3+ has been measured at different temperature between 40 and 420 K. The decay curves can be well fitted by a single exponential equation: It = I0 exp(t/s), where It and I0 are the luminescence intensity at time t and time 0, and s is the decay time, respectively. The values of s are listed in Table 2. At 40 and 300 K, the lifetimes are determined to be 17.46 and 15.69 ns, respectively. With the temperature increasing, the lifetime decreases in the time range of 10–18 ns, due to the parity allowed 5d–4f transition of Ce3+ ion with high radiative probability. Generally, as the increase of temperature, the non-radiative processes will be more efficient and consequently the lifetime will be shorter [14]. Therefore the temperature dependence of the lifetime is due to the increase in thermal assisted nonradiative transition probability.
Table 2 The lifetimes of Ce3+ in CaLaGa3S6O:0.10Ce3+ at different temperatures (kex = 398 nm, kem = 442 nm) Temperature (K) s (ns)
40 17.46
100 17.39
200 16.77
300 15.69
420 11.99
Fig. 4. The excitation spectra of CaLaGa3S6O:0.10Ce3+ (b) and commercial BAM (a) phosphors and emission spectrum of UV-LED (c). The emission spectra of CaLaGa3S6O:0.10Ce3+ (d) and BAM (e) phosphors (kex = 398 nm).
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Fig. 5. The EL spectrum of the fabricated LED with CaLaGa3S6O:0.10Ce3+ phosphor under 20 mA forward-bias current (at the left). The CIE coordinates of CaLaGa3S6O:0.10Ce3+ and BAM (at the right).
3.3. Fabricated LEDs with CaLaGa3S6O:Ce3+ phosphor In order to further investigate the possibility of the as-synthesized phosphor in UV LED, a blue LED was fabricated by combination CaLaGa3S6O:0.10Ce3+ phosphor with a 398 nm-emitting InGaN chip at IF = 20 mA. In Fig. 5 (at the left) is the electroluminescence spectra of the naked n-UV LED and as-fabricated phosphor-converting LED at IF = 20 mA. Two photographs of the lighting n-UV LED and the blue LED with CaLaGa3S6O:Ce3+ are shown in the inset. Intense blue light can be observed with the naked eye, and from two curves, it is seen that the near violet light of the naked n-UV LED chip at 398 nm is absorbed dramatically by CaLaGa3S6O:Ce3+ and simultaneously down-converted into an intensive blue light around 440, 478 nm. Its CIE chromaticity coordinates are calculated to be x = 0.147, y = 0.089, which fall into the blue region and are almost the same as BAM (0.144, 0.072) in the CIE 1931 chromaticity diagram as shown in Fig. 5 (at the right). 4. Conclusions A new blue-emitting phosphor of Ce3+-activated CaLaGa3S6O was synthesized by the two-step solid-state reaction. The photoluminescence spectra show that this phosphor can be excited efficiently by UV–Vis light from 360 to 430 nm and emits intense blue light with two emission bands peaking at about 440 and 478 nm. Furthermore, a blue light-emitting LED was successfully fabricated by precoating CaLaGa3S6O:Ce3+ phosphor onto a 398 nm-
emitting InGaN chip. With further studies being undertaken, this phosphor might be a promising blue phosphor candidate for white LEDs. Acknowledgements This work was supported by the National Nature Science Foundation of China (20501023), the Nature Science Foundation of Guangdong for Doctorial Training base (5300527) and the Science and Technology Project of Guangzhou (2005Z2-D0061). References [1] S. Dalmasso et al., Phys. Status Solidi A 192 (2002) 139. [2] J.-H. Yum, S.-Y. Seo, S. Lee, Y.-E. Sung, J. Electrochem. Soc. 150 (2003) H47. [3] E. Radkov, R. Bompiedi, A.M. Srivastava, A.A. Setlur, C. Becker, Proc. SPIE 5187 (2004) 171. [4] T. Ju stel, H. Nikol, Adv. Mater. 12 (2000) 527. [5] U. Kaufmann et al., Phys. Status Solidi 188 (2001) 143. [6] M. Nazarov, C. Yoon, J. Solid State Chem. 179 (2006) 2529. [7] J. Zhang, M. Takahashi, Y. Tokuda, T. Yoko, J. Ceram. Soc. Jpn. 112 (2004) 511. [8] J.M. Kim, K.N. Kim, S.H. Park, J.K. Park, C.H. Kim, H.G. Jang, J. Korean Chem. Soc. 49 (2005) 201. [9] Y.R. Do, K.Y. Ko, S.H. Na, Y.D. Huh, J. Electrochem. Soc. 153 (2006) H142. [10] X. Zhang, J. Zhang, J. Xu, Q. Su, J. Alloys Compd. 389 (2005) 247. [11] C.L. Teske, Z. Anorg. Allg. Chem. 531 (1985) 52. [12] G. Kortum, G. Schreyer, Z. Naturforsch. 11A (1957) 1018. [13] G. Blasse, Philips Res. Rep. 24 (1969) 131. [14] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin Heidelberg, 1994.