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Rapid synthesis of garnet structured aluminosilicate phosphors
Journal of Luminescence 214 (2019) 116537
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Rapid synthesis of garnet structured aluminosilicate phosphors a,*
a
b
P.P. Lohe , D.V. Nandanwar , P.D. Belsare , S.V. Moharil a b c
T
c
Department of Physics, Shri M.M. College of Science, Nagpur, 440009, India Shri Ramdeobaba College of Engineering and Management, Nagpur, 440013, India Department of Physics, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur 440033, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphor converted LED Garnet Combustion synthesis
MY2Al4SiO12:Ce3+ phosphors have been shown to possess several features such as yellow emission and blue excitation, thermal stability, etc. desired for phosphor converted LED. Various methods have been tried earlier to prepare these phosphors. Annealing/synthesis at temperatures as high as 1400 °C was always essential to obtain these compounds. A simple, one step combustion synthesis is described for the preparation of phosphors based on MY2Al4SiO12. The phosphors so prepared had emission intensities comparable to those obtained by conventional solid state reaction. Thus, a method is established to synthesize these compounds at temperatures as low as 500 °C eliminating the problems associated with high temperature processing.
1. Introduction The first YAG: Ce phosphor was synthesized by Blasse and Bril in 1967. After this, it was used as cathode ray tube phosphor namely P46 and P48 [1]. Later, this phosphor was used in low-pressure mercury vapor discharge lamps. It absorbs Hg plasma lines in blue/violet region at wavelengths 405 nm and 436 nm [2,3] and converts to yellow region. The YAG:Ce phosphor was commercially used to improve CRI in HPMV lamps. In 1990, YAG:Ce was promoted as an efficient scintillator material. The radioluminescence property of this phosphor and the higher density analogue LuAG:Ce made this suitable as scintillation detector [4,5]. YAG:Ce was also used as biomarker for fluoroscent bioimaging [6,7]. Ce3+- Yb3+-codoped YAG has been suggested as a NIR emitting downconversion phosphor. It has application as spectrum converter for c-Si solar cell for achieving higher photovoltaic conversion efficiency [8,9]. YAG:Ce3+ has wide applications in white LEDs [10–12]. A thin layer of phosphor absorbs blue light from InGaN chip and converts it into yellow light. The combination of blue with yellow light results in bright white light. This bright light has higher efficiency superior to that of fluoroscent lamps. These type of white LEDs were first reported in 1997 [13]. Though the Blue chip coated YAG:Ce phosphor has many advantages, it has several shortcomings. This type of light has poor colour rendering index due to absence of red component. This cannot be overcome by simply increasing yellow component. Also, correlated colour temperature (CCT) is high for such phosphors. The addition of red component is again necessary to eliminate this drawback. These
*
lacunae are already known and discussed [14–17]. Thermal stability is the next challenge for these kind of LEDs. As the chip area is very small, direct deposition of phosphor may increase its temperature above 400 K. This increase in temperature leads to declined intensity due to thermal quenching. The thermal quenching of phosphor itself is much better [15] and the reduction in intensity is related to the thermally induced concentration quenching [16]. Another aspect of increase in phosphor temperature is the red shift in excitation and emission spectra. Moreover, to obtain higher intensity the diode is operated at higher current. This higher current increases the diode temperature due to the increased non-radiative losses. This in turn causes a red shift of the chip emission and it may disturb the tuning with the Ce3+ excitation spectrum. Several efforts have been made to eliminate some of these drawbacks. Garnet structure induces huge centroid shift which in turn leads to excitation and emission bands at much longer wavelengths than the average [18]. On the other hand, luminescence quenching at lower concentrations, thermal quenching, etc. are undesired features of YAG:Ce phosphor. Modifying the YAG formula keeping the garnet structure intact was the most sought after way for improvement [19]. Substitutions at Y or Al sites had been studied in earlier efforts [20–25]. Kuru et al. had noticed that simultaneous aliovalent substitutions at both Y and Al sites increase the solubility limits [26]. Later, Katelnikovas et al. reported CaY2Al4SiO12:Ce3+ [27] and CaLu2Al4SiO12:Ce3+ [28] phosphors which exhibit blue excitation and yellow/green emissions. MgY2Al4SiO12:Ce3+ [29] also exhibited similar excitation and emission properties. Moreover, Ce3+ → Mn2+ energy transfer was also
Corresponding author. E-mail address: [email protected] (P.P. Lohe).
https://doi.org/10.1016/j.jlumin.2019.116537 Received 18 February 2019; Received in revised form 3 June 2019; Accepted 4 June 2019 Available online 05 June 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 214 (2019) 116537
P.P. Lohe, et al.
studied in this host [30]. Ji et al. [31] studied the entire series of MY2Al4SiO12:Ce3+ (M = alkaline earth metal) phosphors and found that BaY2Al4SiO12:Ce3+ has yellow emission with much improved thermal quenching. Efforts have been made to understand the changes in the emission spectrum of Ce3+ by modifications of the garnet structure [32]. Solid state reaction method was conventionally used to synthesize MY2Al4SiO12:Ce3+ phosphors. The reaction was carried out at higher temperatures typically 1400 °C. Some other methods were also adapted like sol-gel combustion [27,28] and citrate sol–gel, to simplify the process [33]. However, it showed that as-synthesized powders were amorphous and required high temperature annealing to obtain crystalline powders. Hydrothermal methods [32] also did not prove much useful, in the sense that high temperature treatment was still required. In this paper we report one step combustion synthesis of MY2Al4SiO12:Ce3+ phosphors using mixed fuel approach [34]. Ascombusted powders exhibited desired emission with blue excitation, without any post combustion thermal treatment. Fig. 1. XRD pattern for BaY2Al4SiO12. Experimentally recorded pattern is compared with that calculated using Vesta software.
2. Experimental The samples were synthesized by combustion synthesis. The details have been described earlier [34]. Mixture of urea and Glycine was used as fuel. Patil and co-workers suggested the method of calculation for oxidizer and fuel proportions [35,36]. This method was adopted in the present work. Table 1 gives the details of the ingredients used in syntheses of various phosphors. The calculated amounts of ingredients were thoroughly mixed. Presence of large water of crystallization in aluminium nitrate leads to formation of a thick paste. This thick paste was transferred to a china dish and kept in a preheated furnace at 500 °C. In a short time, the paste foamed and burst into a flame. The burning continued for several seconds. The fluffy powder was obtained as soon as burning ceased. The china dish was immediately taken out from the furnace. X-ray diffraction patterns were recorded on Philips PANalytical X'pert Pro diffractometer. PL characteristics in the range of 200–700 nm, at room temperature were studied using a Hitachi F-7000 spectrofluorimeter, with 1 nm spectral slit width. For comparison some samples were also prepared by conventional solid state reaction. Lifetime measurements were carried out on Horiba Fluorimax 3 spectrofluorimeter using TCSPC software. Horiba 450 nm nano LED was used for excitation. 3. Results and discussion
Fig. 2. Unit cell for BaY2Al4SiO12 showing Ba/Y coordination.
Formation of MY2Al4SiO12 (M = alkaline earth metal) phases following the combustion synthesis was confirmed by XRD. The comparative study of experimental and calculated patterns is as shown in Fig. 1. Excellent agreement is seen. Small difference is observed in relative intensities of some diffraction lines. It may originate due to nonequilibrium nature of the combustion reaction which takes place within a short time. BaY2Al4SiO12 crystallises in the garnet structure (space group I a-3d) with a = b = c = 12.0068 Å. There are 8 formula units in the unit cell. In context of YAG structure, Ba and Si atoms occupy the Y
and Al1 (in the AlO4 tetrahedron) sites, respectively. Y and Ba have 8 coordination while Al has two types of sites with coordinations 4 (Al1) and 6 (Al2) (Fig. 2). Reflectance spectra of BaY2Al4SiO12:Ce samples are shown in Fig. 3. Below 500 nm, reflectance drops (curve a) due to absorption by Ce3+. Another band that is apparent around 250 nm may be due to host absorption. Spectra were similar for combustion synthesized phosphor (curves a and b) and that prepared by the conventional solid sate reaction (curves c and d). Results for PL emission and excitation spectra for BaY2Al4SiO12:Ce3+ are presented in Fig. 4. Typical Ce3+ emission in garnet hosts is observed in the form of a broad band peaking around 533 nm (curve a). Highest intensity is observed for 3 mol % Ce3+ and concentration quenching occurs for higher values. The intensities are of the same order as that obtained for the samples prepared by conventional solid state reaction (curve e). BaY2Al4SiO12: Ce3+ phosphor is thus successfully prepared in one step using the combustion synthesis. In the inset of Fig. 4, peak intensity for 533 nm emission is shown as a function of Ce3+ concentration. The quenching is due to Ce3+ → Ce3+energy transfer. From the concentration quenching curve, critical distance for Ce3+ → Ce3+ energy transfer can be calculated using
Table 1 Combustion mixtures used in preparation of various phosphors. Sr. No.
1 2 3 4
Composition
CaY1.94Ce0.06Al4SiO12 BaY1.94Ce0.06Al4SiO12 MgY1.94Ce0.06Al4SiO12 SrY1.94Ce0.06Al4SiO12
Ingredients mol ratios YN: CeN:
AEN
AlN
Glycine:
Urea
1.94: 1.94: 1.94: 1.94:
1 1 1 1
4 4 4 4
3.33 3.33 3.33 3.33
15 15 15 15
0.06: 0.06: 0.06: 0.06:
YN→Y(NO3)3.6H2O, CeN→Ce(NO3)3.6H2O, AlN→Al(NO3)3.9H2O AEN→Ca (NO3)2.4H2O/Ba(NO3)2/Mg(NO3)2.4H2O/Sr(NO3)2. 2
Journal of Luminescence 214 (2019) 116537
P.P. Lohe, et al.
Fig. 3. Reflectance spectra for BaY2Al4SiO12:Ce3+ prepared by combustion method (curve a). Kubelka-Munk functions F(R) = (1-R)2/2R which are more closely related to absorption coefficient are also plotted (curve b). Spectra for the sample prepared by sold state reaction are similar and shown in curves c and d.
Fig. 5. Colour coordinates for BaY2Al4SiO12:Ce3+ (3 mol.%). 2
F5/2/2F7/2 emissions. In previous work, a distinct shoulder corresponding to the transition to 2F7/2 state was observed. This transition is not obvious in Fig. 4. Emission in the combustion synthesized BaY2Al4SiO12:Ce3+ is green while the sample prepared by solid state reaction is reported [33] to give yellow emission. Ce3+ emission is characterized by fast decay, lifetimes being of the order of several nanoseconds. It is a suitable activator to obtain fastdecaying intense luminescence [41]. Fig. 6 shows luminescence decay for BaY2Al4SiO12:Ce3+. Decay could be fitted to a single exponential. Lifetime was calculated as 36.6 ns. More or less similar results were obtained for SrY2Al4SiO12:Ce3+, CaY2Al4SiO12:Ce3+ and MgY2Al4SiO12:Ce3+ (Fig. 7). XRD patterns of all MY2Al4SiO12 compounds are similar [31], hence not shown here. Excitation spectra are almost similar for all the four MY2Al4SiO12:Ce3+ compounds (Fig. 7, curves e-h), consisting of a dominant broad band around 460 nm CaY2Al4SiO12:Ce3+ emission peaks around 517 nm
Fig. 4. Photoluminescence emission (a–e) and excitation (f) spectra for BaY2Al4SiO12:Ce3+. curves a-d, emission for 460 nm excitation, curve f excitation for 533 nm emission. Ce3+ concentration (mol.%) a > 3, b > 5, c > 6 and d > 2; curve e is the emission spectra of the BaY2Al4SiO12:Ce3+ (3%) sample prepared by the solid state method for 460 nm excitation. Inset shows intensity of 533 nm emission as a function of Ce3+ concentration.
formula [37,38].
Rc=2(3V/4π XcN)1/3
(1)
where xc is the critical concentration (0.03), N is the number of ytrrium sites in the unit cell (16) and V is the volume of the unit cell (1725.84 Å3). Rc comes out to be 19.01 Å. This value is larger than 5 Å, indicating that the exchange interaction is not playing a dominant part for the energy transfer [39,40]. It may be deduced that, the Ce3+ →Ce3+ energy transfer takes place via an electric multi-polar interaction. Excitation spectrum (Fig. 4, curve f) for 533 nm emission contains an intense band around 460 nm which is similar to that observed in Fig. 3. Another weak excitation band is at 340 nm. Again, these features are typical of garnet structure. The overall results for BaY2Al4SiO12:Ce3+ are in good agreement with the literature [31,33]. Colour coordinates were calculated using Radiant imaging Colour Calculator 2.0 software. The results are shown in Fig. 5. The values are x = 0.31, y = 0.60. These fall in the green region and are different from those reported earlier [33]. The changes are due to different ratios of
Fig. 6. Luminescence decay for BaY2Al4SiO12:Ce3+ (3 mol.%). Horiba 450 nm nano LED was used for excitation. 3
Journal of Luminescence 214 (2019) 116537
P.P. Lohe, et al.
[7]
[8] [9]
[10]
[11]
[12]
Fig. 7. Photoluminescence emission (a–d) and excitation (e–h) spectra for SrY2Al4SiO12:Ce3+. a and e, CaY2Al4SiO12:Ce3+, b and f, BaY2Al4SiO12:Ce3+, c and g, MgY2Al4SiO12:Ce3+, d and h.
[13] [14]
(curve a), that of SrY2Al4SiO12:Ce3+, at 528 nm, while that for MgY2Al4SiO12:Ce3+ around 542 nm (curve c). These values are also similar to those reported in the literature. Ce3+ emission band has two components due to split ground state (2F5/2 and 2F7/2). Apparent peak position depends on the relative strengths of these two components which in turn depend on the Ce3+ concentration and local environment. Most relevant facts in context of the present work are that phase pure compounds are obtained in one step combustion and the samples prepared by solid state reaction and combustion have intensities of same order.
[15]
[16] [17] [18] [19]
[20]
4. Conclusions [21] 3+
MY2Al4SiO12: Ce compounds, which have already been shown to be phosphors useful for solid state lighting based on blue emitting chips, are successfully synthesized by one step combustion synthesis using mixed fuel approach. As compared to all previous syntheses described in the literature involving high temperature (around 1400 °C) step, the method reported here consists of a single step carried out at 500 °C, without any subsequent heating. The properties of the combustion synthesized phosphors are comparable with those prepared by the solid state reaction. The emission intensities are limited by concentration quenching due to Ce3+ → Ce3+ energy transfer. Critical distance for this transfer, which takes place via an electric multi-polar interaction, is 19.01 Å.
[22]
[23] [24]
[25]
[26] [27]
Acknowledgements We are thankful to Department of Physics, Sant Gadge Baba Amravati University, for the help provided in recording XRD. Figs. 1 and 2 are prepared using Vesta software [42]. We are grateful to the copyright owners for permitting free use.
[28]
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Anastasiya Latynina, Makoto Watanabe, Daisuke Inomata, Kazuo Aoki, Yoshiyuki Sugahara, Encarnacion Garcia Villora, Kiyoshi Shimamura, Properties of Czochralski grown Ce, Gd:Y3Al5O12 single crystal for white light-emitting diode, J.Alloys.Compds. 553 (2013) 89–92. Y.S. Lin, R.S. Liu, B.M. Cheng, Investigation of the luminescent properties of Tb3 +substituted YAG:Ce, Gd phosphors, J. Electrochem. Soc. 152 (2005) J41–J45. S. Fujita, A. Sakamoto, S. Tanabe, Luminescence characteristics of YAG glass–ceramic phosphor for white LED, IEEE J. Sel. Top. Quantum Electron. 14 (2008) 1387–1391. V.P. Dotsenko, I.V. Berezovskaya, E.V. Zubar, N.P. Efryushina, N.I. Poletaev, Yu.A. Doroshenko, G.B. Stryganyuk, A.S. Voloshinovskii, Synthesis and luminescent study of Ce3+-doped terbium–yttrium aluminum garnet, J.Alloys.Compds. 550 (2013) 159–163. Y. Kuru, E. Onur Savasir, S. Zeynep Nergiz, C. Oncel, M. Ali Gulgun, Enhanced cosolubilities of Ca and Si in YAG (Y3Al5O12), Phys. 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Journal of Luminescence 214 (2019) 116537
P.P. Lohe, et al. [35] J.J. Kingsley, K. Suresh, K.C. Patil, Combustion synthesis of fine-particle metal aluminates, J. Mater. Sci. 25 (1990) 1305–1312. [36] J.J. Kingsley, N. Manickam, K.C. Patil, Combustion synthesis and properties of fine particle fluorescent aluminous oxides, Bull. Mater. Sci. 13 (1990) 179–189. [37] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24 (1969) 131–144. [38] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. 28 (1968) 444–445. [39] D.L. Dexter, A theory of sensitized luminescence in solid, J. Chem. Phys. 21 (1953)
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5
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Rapid synthesis of garnet structured aluminosilicate phosphors a,*
a
b
P.P. Lohe , D.V. Nandanwar , P.D. Belsare , S.V. Moharil a b c
T
c
Department of Physics, Shri M.M. College of Science, Nagpur, 440009, India Shri Ramdeobaba College of Engineering and Management, Nagpur, 440013, India Department of Physics, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur 440033, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphor converted LED Garnet Combustion synthesis
MY2Al4SiO12:Ce3+ phosphors have been shown to possess several features such as yellow emission and blue excitation, thermal stability, etc. desired for phosphor converted LED. Various methods have been tried earlier to prepare these phosphors. Annealing/synthesis at temperatures as high as 1400 °C was always essential to obtain these compounds. A simple, one step combustion synthesis is described for the preparation of phosphors based on MY2Al4SiO12. The phosphors so prepared had emission intensities comparable to those obtained by conventional solid state reaction. Thus, a method is established to synthesize these compounds at temperatures as low as 500 °C eliminating the problems associated with high temperature processing.
1. Introduction The first YAG: Ce phosphor was synthesized by Blasse and Bril in 1967. After this, it was used as cathode ray tube phosphor namely P46 and P48 [1]. Later, this phosphor was used in low-pressure mercury vapor discharge lamps. It absorbs Hg plasma lines in blue/violet region at wavelengths 405 nm and 436 nm [2,3] and converts to yellow region. The YAG:Ce phosphor was commercially used to improve CRI in HPMV lamps. In 1990, YAG:Ce was promoted as an efficient scintillator material. The radioluminescence property of this phosphor and the higher density analogue LuAG:Ce made this suitable as scintillation detector [4,5]. YAG:Ce was also used as biomarker for fluoroscent bioimaging [6,7]. Ce3+- Yb3+-codoped YAG has been suggested as a NIR emitting downconversion phosphor. It has application as spectrum converter for c-Si solar cell for achieving higher photovoltaic conversion efficiency [8,9]. YAG:Ce3+ has wide applications in white LEDs [10–12]. A thin layer of phosphor absorbs blue light from InGaN chip and converts it into yellow light. The combination of blue with yellow light results in bright white light. This bright light has higher efficiency superior to that of fluoroscent lamps. These type of white LEDs were first reported in 1997 [13]. Though the Blue chip coated YAG:Ce phosphor has many advantages, it has several shortcomings. This type of light has poor colour rendering index due to absence of red component. This cannot be overcome by simply increasing yellow component. Also, correlated colour temperature (CCT) is high for such phosphors. The addition of red component is again necessary to eliminate this drawback. These
*
lacunae are already known and discussed [14–17]. Thermal stability is the next challenge for these kind of LEDs. As the chip area is very small, direct deposition of phosphor may increase its temperature above 400 K. This increase in temperature leads to declined intensity due to thermal quenching. The thermal quenching of phosphor itself is much better [15] and the reduction in intensity is related to the thermally induced concentration quenching [16]. Another aspect of increase in phosphor temperature is the red shift in excitation and emission spectra. Moreover, to obtain higher intensity the diode is operated at higher current. This higher current increases the diode temperature due to the increased non-radiative losses. This in turn causes a red shift of the chip emission and it may disturb the tuning with the Ce3+ excitation spectrum. Several efforts have been made to eliminate some of these drawbacks. Garnet structure induces huge centroid shift which in turn leads to excitation and emission bands at much longer wavelengths than the average [18]. On the other hand, luminescence quenching at lower concentrations, thermal quenching, etc. are undesired features of YAG:Ce phosphor. Modifying the YAG formula keeping the garnet structure intact was the most sought after way for improvement [19]. Substitutions at Y or Al sites had been studied in earlier efforts [20–25]. Kuru et al. had noticed that simultaneous aliovalent substitutions at both Y and Al sites increase the solubility limits [26]. Later, Katelnikovas et al. reported CaY2Al4SiO12:Ce3+ [27] and CaLu2Al4SiO12:Ce3+ [28] phosphors which exhibit blue excitation and yellow/green emissions. MgY2Al4SiO12:Ce3+ [29] also exhibited similar excitation and emission properties. Moreover, Ce3+ → Mn2+ energy transfer was also
Corresponding author. E-mail address: [email protected] (P.P. Lohe).
https://doi.org/10.1016/j.jlumin.2019.116537 Received 18 February 2019; Received in revised form 3 June 2019; Accepted 4 June 2019 Available online 05 June 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 214 (2019) 116537
P.P. Lohe, et al.
studied in this host [30]. Ji et al. [31] studied the entire series of MY2Al4SiO12:Ce3+ (M = alkaline earth metal) phosphors and found that BaY2Al4SiO12:Ce3+ has yellow emission with much improved thermal quenching. Efforts have been made to understand the changes in the emission spectrum of Ce3+ by modifications of the garnet structure [32]. Solid state reaction method was conventionally used to synthesize MY2Al4SiO12:Ce3+ phosphors. The reaction was carried out at higher temperatures typically 1400 °C. Some other methods were also adapted like sol-gel combustion [27,28] and citrate sol–gel, to simplify the process [33]. However, it showed that as-synthesized powders were amorphous and required high temperature annealing to obtain crystalline powders. Hydrothermal methods [32] also did not prove much useful, in the sense that high temperature treatment was still required. In this paper we report one step combustion synthesis of MY2Al4SiO12:Ce3+ phosphors using mixed fuel approach [34]. Ascombusted powders exhibited desired emission with blue excitation, without any post combustion thermal treatment. Fig. 1. XRD pattern for BaY2Al4SiO12. Experimentally recorded pattern is compared with that calculated using Vesta software.
2. Experimental The samples were synthesized by combustion synthesis. The details have been described earlier [34]. Mixture of urea and Glycine was used as fuel. Patil and co-workers suggested the method of calculation for oxidizer and fuel proportions [35,36]. This method was adopted in the present work. Table 1 gives the details of the ingredients used in syntheses of various phosphors. The calculated amounts of ingredients were thoroughly mixed. Presence of large water of crystallization in aluminium nitrate leads to formation of a thick paste. This thick paste was transferred to a china dish and kept in a preheated furnace at 500 °C. In a short time, the paste foamed and burst into a flame. The burning continued for several seconds. The fluffy powder was obtained as soon as burning ceased. The china dish was immediately taken out from the furnace. X-ray diffraction patterns were recorded on Philips PANalytical X'pert Pro diffractometer. PL characteristics in the range of 200–700 nm, at room temperature were studied using a Hitachi F-7000 spectrofluorimeter, with 1 nm spectral slit width. For comparison some samples were also prepared by conventional solid state reaction. Lifetime measurements were carried out on Horiba Fluorimax 3 spectrofluorimeter using TCSPC software. Horiba 450 nm nano LED was used for excitation. 3. Results and discussion
Fig. 2. Unit cell for BaY2Al4SiO12 showing Ba/Y coordination.
Formation of MY2Al4SiO12 (M = alkaline earth metal) phases following the combustion synthesis was confirmed by XRD. The comparative study of experimental and calculated patterns is as shown in Fig. 1. Excellent agreement is seen. Small difference is observed in relative intensities of some diffraction lines. It may originate due to nonequilibrium nature of the combustion reaction which takes place within a short time. BaY2Al4SiO12 crystallises in the garnet structure (space group I a-3d) with a = b = c = 12.0068 Å. There are 8 formula units in the unit cell. In context of YAG structure, Ba and Si atoms occupy the Y
and Al1 (in the AlO4 tetrahedron) sites, respectively. Y and Ba have 8 coordination while Al has two types of sites with coordinations 4 (Al1) and 6 (Al2) (Fig. 2). Reflectance spectra of BaY2Al4SiO12:Ce samples are shown in Fig. 3. Below 500 nm, reflectance drops (curve a) due to absorption by Ce3+. Another band that is apparent around 250 nm may be due to host absorption. Spectra were similar for combustion synthesized phosphor (curves a and b) and that prepared by the conventional solid sate reaction (curves c and d). Results for PL emission and excitation spectra for BaY2Al4SiO12:Ce3+ are presented in Fig. 4. Typical Ce3+ emission in garnet hosts is observed in the form of a broad band peaking around 533 nm (curve a). Highest intensity is observed for 3 mol % Ce3+ and concentration quenching occurs for higher values. The intensities are of the same order as that obtained for the samples prepared by conventional solid state reaction (curve e). BaY2Al4SiO12: Ce3+ phosphor is thus successfully prepared in one step using the combustion synthesis. In the inset of Fig. 4, peak intensity for 533 nm emission is shown as a function of Ce3+ concentration. The quenching is due to Ce3+ → Ce3+energy transfer. From the concentration quenching curve, critical distance for Ce3+ → Ce3+ energy transfer can be calculated using
Table 1 Combustion mixtures used in preparation of various phosphors. Sr. No.
1 2 3 4
Composition
CaY1.94Ce0.06Al4SiO12 BaY1.94Ce0.06Al4SiO12 MgY1.94Ce0.06Al4SiO12 SrY1.94Ce0.06Al4SiO12
Ingredients mol ratios YN: CeN:
AEN
AlN
Glycine:
Urea
1.94: 1.94: 1.94: 1.94:
1 1 1 1
4 4 4 4
3.33 3.33 3.33 3.33
15 15 15 15
0.06: 0.06: 0.06: 0.06:
YN→Y(NO3)3.6H2O, CeN→Ce(NO3)3.6H2O, AlN→Al(NO3)3.9H2O AEN→Ca (NO3)2.4H2O/Ba(NO3)2/Mg(NO3)2.4H2O/Sr(NO3)2. 2
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Fig. 3. Reflectance spectra for BaY2Al4SiO12:Ce3+ prepared by combustion method (curve a). Kubelka-Munk functions F(R) = (1-R)2/2R which are more closely related to absorption coefficient are also plotted (curve b). Spectra for the sample prepared by sold state reaction are similar and shown in curves c and d.
Fig. 5. Colour coordinates for BaY2Al4SiO12:Ce3+ (3 mol.%). 2
F5/2/2F7/2 emissions. In previous work, a distinct shoulder corresponding to the transition to 2F7/2 state was observed. This transition is not obvious in Fig. 4. Emission in the combustion synthesized BaY2Al4SiO12:Ce3+ is green while the sample prepared by solid state reaction is reported [33] to give yellow emission. Ce3+ emission is characterized by fast decay, lifetimes being of the order of several nanoseconds. It is a suitable activator to obtain fastdecaying intense luminescence [41]. Fig. 6 shows luminescence decay for BaY2Al4SiO12:Ce3+. Decay could be fitted to a single exponential. Lifetime was calculated as 36.6 ns. More or less similar results were obtained for SrY2Al4SiO12:Ce3+, CaY2Al4SiO12:Ce3+ and MgY2Al4SiO12:Ce3+ (Fig. 7). XRD patterns of all MY2Al4SiO12 compounds are similar [31], hence not shown here. Excitation spectra are almost similar for all the four MY2Al4SiO12:Ce3+ compounds (Fig. 7, curves e-h), consisting of a dominant broad band around 460 nm CaY2Al4SiO12:Ce3+ emission peaks around 517 nm
Fig. 4. Photoluminescence emission (a–e) and excitation (f) spectra for BaY2Al4SiO12:Ce3+. curves a-d, emission for 460 nm excitation, curve f excitation for 533 nm emission. Ce3+ concentration (mol.%) a > 3, b > 5, c > 6 and d > 2; curve e is the emission spectra of the BaY2Al4SiO12:Ce3+ (3%) sample prepared by the solid state method for 460 nm excitation. Inset shows intensity of 533 nm emission as a function of Ce3+ concentration.
formula [37,38].
Rc=2(3V/4π XcN)1/3
(1)
where xc is the critical concentration (0.03), N is the number of ytrrium sites in the unit cell (16) and V is the volume of the unit cell (1725.84 Å3). Rc comes out to be 19.01 Å. This value is larger than 5 Å, indicating that the exchange interaction is not playing a dominant part for the energy transfer [39,40]. It may be deduced that, the Ce3+ →Ce3+ energy transfer takes place via an electric multi-polar interaction. Excitation spectrum (Fig. 4, curve f) for 533 nm emission contains an intense band around 460 nm which is similar to that observed in Fig. 3. Another weak excitation band is at 340 nm. Again, these features are typical of garnet structure. The overall results for BaY2Al4SiO12:Ce3+ are in good agreement with the literature [31,33]. Colour coordinates were calculated using Radiant imaging Colour Calculator 2.0 software. The results are shown in Fig. 5. The values are x = 0.31, y = 0.60. These fall in the green region and are different from those reported earlier [33]. The changes are due to different ratios of
Fig. 6. Luminescence decay for BaY2Al4SiO12:Ce3+ (3 mol.%). Horiba 450 nm nano LED was used for excitation. 3
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[7]
[8] [9]
[10]
[11]
[12]
Fig. 7. Photoluminescence emission (a–d) and excitation (e–h) spectra for SrY2Al4SiO12:Ce3+. a and e, CaY2Al4SiO12:Ce3+, b and f, BaY2Al4SiO12:Ce3+, c and g, MgY2Al4SiO12:Ce3+, d and h.
[13] [14]
(curve a), that of SrY2Al4SiO12:Ce3+, at 528 nm, while that for MgY2Al4SiO12:Ce3+ around 542 nm (curve c). These values are also similar to those reported in the literature. Ce3+ emission band has two components due to split ground state (2F5/2 and 2F7/2). Apparent peak position depends on the relative strengths of these two components which in turn depend on the Ce3+ concentration and local environment. Most relevant facts in context of the present work are that phase pure compounds are obtained in one step combustion and the samples prepared by solid state reaction and combustion have intensities of same order.
[15]
[16] [17] [18] [19]
[20]
4. Conclusions [21] 3+
MY2Al4SiO12: Ce compounds, which have already been shown to be phosphors useful for solid state lighting based on blue emitting chips, are successfully synthesized by one step combustion synthesis using mixed fuel approach. As compared to all previous syntheses described in the literature involving high temperature (around 1400 °C) step, the method reported here consists of a single step carried out at 500 °C, without any subsequent heating. The properties of the combustion synthesized phosphors are comparable with those prepared by the solid state reaction. The emission intensities are limited by concentration quenching due to Ce3+ → Ce3+ energy transfer. Critical distance for this transfer, which takes place via an electric multi-polar interaction, is 19.01 Å.
[22]
[23] [24]
[25]
[26] [27]
Acknowledgements We are thankful to Department of Physics, Sant Gadge Baba Amravati University, for the help provided in recording XRD. Figs. 1 and 2 are prepared using Vesta software [42]. We are grateful to the copyright owners for permitting free use.
[28]
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