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Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications
Journal Pre-proof Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications
Vijay Singh, Sumandeep Kaur, M. Jayasimhadri PII:
S1293-2558(19)30574-6
DOI:
https://doi.org/10.1016/j.solidstatesciences.2019.106049
Reference:
SSSCIE 106049
To appear in:
Solid State Sciences
Received Date:
18 May 2019
Accepted Date:
28 October 2019
Please cite this article as: Vijay Singh, Sumandeep Kaur, M. Jayasimhadri, Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications, Solid
State Sciences (2019), https://doi.org/10.1016/j.solidstatesciences.2019.106049
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Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications Vijay Singh a, *, Sumandeep Kaur b, M. Jayasimhadri b a Department
of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea
b Department
of Applied Physics, Delhi Technological University, Delhi 110 042, India
Abstract Sm3+-doped CaAl4O7 phosphor has been synthesized using the sol-gel method with a varying doping concentration of Sm3+ ions. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were used to study the structural and morphological properties of the material. The UV-VIS-NIR spectrum was also recorded to analyze the optical transitions of CaAl4O7 phosphor doped with Sm3+ ions. The photoluminescence (PL) spectra of the phosphors were also recorded to examine the luminescence properties. The PL spectra exhibit intense orange emission at 602 nm that was obtained for the prepared samples using the 403 nm excitation. The optimized concentration of Sm3+ ions in CaAl4O7 was found to be 0.03 mol, beyond which concentration quenching was observed. The CIE color chromaticity coordinates of all the synthesized phosphors lie in the orange region of the chromaticity diagram. All of these results indicate the candidature of CaAl4O7 phosphor doped with Sm3+ ions for lighting and display applications.
Keywords: Sol-gel; XRD; CaAl4O7; Sm3+ ions; Photoluminescence *Corresponding authors: Email addresses: [email protected] (V. Singh)
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1. Introduction In solid-state lighting, a white-light-emitting device (wLEDs) based on phosphors is one of the promising light sources for potential applications in fluorescent lamps, optical amplifiers, emissive displays, solid-state lasers, radiation dosimetry, X-ray imaging, and sensors [1–4]. The meritorious advantages of wLEDs—such as longer durability, power saving, cost-effectiveness, high luminous efficiency, low driving voltage, fast response time, and environmental friendliness—make them suitable for these applications [5–7]. At present, various attempts have been demonstrated to get efficient wLEDs. One of the approaches to fabricating phosphor-based wLEDs is to assemble an InGaN-blue chip with YAG: Ce yellow phosphor, but the combination of blue and yellow to generate white light exhibits certain limitations, such as high correlated color temperature (CCT), poor color rendering index (CRI) due to the lack of a red component, and blue-yellow color separation [8–10]. Aiming to generate white light with improved CRI, a near-UV excitation source with tricolor (blue, green, and red) phosphor is another approach that can be utilized [11]. Near ultra violet (n-UV) excitable tricolor phosphors have some disadvantages, such as low luminous efficiency due to the blending of different colored phosphors, each of which has a different degradation rate [12]. The efficiency of red (Y2O2S:Eu3+) phosphor is only 1/8 that of blue (BaMgAl10O17:Eu2+) phosphor or green (ZnS:Cu+, Al3+) phosphor [9, 10]. Therefore, it is important to develop new orange-red emitting phosphor to obtain white light with an improved CRI. Considering all lanthanides, samarium (Sm3+) ion is promising because it serves the purpose of enhancing the intensity of a red component in generating white light [13, 14]. Alkaline earth aluminates are chemically stable in the ambient environment and recognized as an excellent material for use in various applications, including construction materials, luminescent materials, catalyst support, components in bio-ceramics, and refractory materials [15–19]. Various 2
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crystallization phases of calcium aluminates exist based on CaO-Al2O3 composition. CaAl4O7 is identified as an attractive host among different aluminates, due to its high optical transparency from UV to NIR range and excellent mechanical and thermal properties with high chemical stability [16]. Owing to the excellent properties of CaAl4O7 as host and excellent emission characteristics of Sm3+ ions as rare earth ion found application in displays and solid state lighting, a series of Sm3+doped CaAl4O7 phosphor has been synthesized via a sol-gel method for which the dopant concentration was varied from 0.01–0.13 mol [20, 21]. In the present article, the structural and morphological properties of the synthesized phosphor have been studied. The optical and luminescence properties were studied in detail for the application of the Sm3+-doped CaAl4O7 phosphor in lighting and display devices. 2. Materials preparation and analysis The sol-gel method was used to prepare a series of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors. In a typical manner for synthesis, all starting materials with high purity were used without further purification. The details of sample composition and starting materials were given in Table 1. In the manner of a typical synthesis, the stoichiometric quantity of starting materials-such as calcium nitrate tetrahydrate (Ca[NO3]2∙4H2O), aluminum nitrate nonahydrate (Al[NO3]3∙9H2O), samarium nitrate hexahydrate (Sm[NO3]3∙6H2O), and the citric acid (citric acid/metal ion 2:1, molar ratio)were first dissolved in 10 ml deionized water under stirring at 500 rpm for 1 hr. A transparent aqueous solution was obtained after stirring, and this was kept at 110°C in the oven to obtain a homogeneous dried gel. The obtained dried gel was sintered at 400°C for 120 min in an air atmosphere. Finally, the resultant brown residual samples were fully ground and annealed at 1000°C for 3 hr in the air to obtain the pure phase of CaAl4O7.
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Using a Miniflex-II (Rigakum, Japan) diffractometer with Cu-Kα radiation (λ=1.5406Å) as X-ray source, the X-ray diffraction (XRD) patterns of the samples were recorded. The XRD patterns were taken with a scan rate of 5°/minute in the 2 range of 10° to 80o. An SEM (S-3400, Hitachi, Japan) was used to obtain morphology details. UV-VIS-NIR measurement was carried out at room temperature on a Cary 5000 Spectrophotometer. Photoluminescence measurements were carried out at room temperature on a Shimadzu RF-5301PC spectrofluorophotometer equipped with Xenon flash lamp. 3. Results and discussion: 3.1 Crystal structure and crystallite size analysis: Fig. 1 shows the XRD patterns of all the Sm3+-doped CaAl4O7 phosphors. The XRD patterns show that the diffraction peaks indexed to hkl planes are completely in accordance with standard data received from the Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 23-1037. The XRD patterns reveal that all the synthesized samples are crystallizes in single-phase monoclinic structure with C2/c space group and lattice parameters of a= 12.89 Å, b= 8.88 Å, c= 5.45 Å, α= γ= 90o, β= 107o and volume of the unit cell is (V= 591.54 Å3) [22]. In a CaAl4O7 lattice, Ca2+ ions occupy crystallographic sites of C2 symmetry surrounded by five oxygen ions, whereas Al3+ ions are distributed over two independent tetrahedral sites, whose surroundings are significantly distorted, giving C1 local symmetries [12, 16]. Moreover, no impurity peaks have been observed in the XRD patterns of any of the doped samples. This affirms the pure phase synthesis of CaAl4O7. The crystallite sizes of all the phosphors have been calculated from the well-known Debye-Scherrer’s formula: D = kλ/ βcosθ, where D is crystallite size, β is full width at half maxima (FWHM), θ is Bragg's diffraction angle, and λ is wavelength used. Table 2 shows a list of the FWHM and crystallite size of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) samples.
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3.2 SEM analysis: SEM images recorded at different resolutions have been used to study the morphology of sol-gelsynthesized phosphor. Fig. 2 shows the morphology of Ca0.97Al4O7:Sm0.03 at different resolutions. Figs. 2a and 2b show the irregular morphology of agglomerated particles. Particles with some agglomeration exhibiting a porous structure of the particles are displayed in Figs. 2c and 2d. The porous structure of the particles might be due to the evolution of gases during the synthesis process [23]. The average particle size is in the range of 1–6 μm. This makes them suitable for various lighting applications [24]. 3.3 UV-VIS-NIR analysis: Fig. 3 shows the diffuse reflectance (DR) spectrum for Ca0.97Al4O7:Sm0.03 phosphor recorded in the UV-VIS-NIR region. The spectrum shows intense band positions at 340, 401, 1093, 1249, 1402, and 1508 nm, which are associated with the characteristic transitions from ground level 6H5/2 to 3H7/2, 4F
7/2,
6F
9/2,
6F
7/2,
6F
5/2,
and 6H15/2 excited levels, respectively [19, 20]. The DR spectrum indicates that
the Sm3+-doped CaAl4O7 phosphor can effectively absorb in n-UV and NIR region. 3.4 Photoluminescence analysis: Fig. 4a shows the photoluminescence excitation spectra of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13). The PLE spectra of these samples have been measured at room temperature by monitoring emission wavelength at 602 nm. These spectra show various narrow and sharp excitation peaks at 305, 315, 332, 344, 361, 374, 403, 418, 435, 443, 449, 462, 473, 482, 487, 501, 528, and 537 nm attributed to transition from ground state 6H5/2 to 4P5/2, 4P3/2, 4G9/2, 4D7/2 + (4K, 4L)17/2 + 4H9/2, 4D3/2 + (4D, 6P)5/2, 4L 4 4 4 4 4 4 4 4 4 4 4 17/2, F7/2, M19/2, I15/2, M17/2, F5/2, I13/2, I11/2, M15/2, I19/2, G7/2, F3/2,
and 4G5/2 excited states,
respectively originating from f-f transitions of Sm3+ ions [25-28]. The excitation spectra for all the
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samples show similar profiles, though there is some intensity variation with the Sm3+ concentration in the host lattice CaAl4O7. Among various excitation peaks, the most intense excitation peak is found at 403 nm corresponding to the transition 6H5/2 → 4F7/2. Therefore, the emission spectra have been recorded at room temperature under 403 nm excitation wavelengths, as shown in Fig. 4b. The emission spectra exhibit emission peaks at 565, 602, and 655 nm corresponding from 4G5/2 → 6H5/2, 6H
7/2,
and 6H9/2 transitions, respectively [29, 30]. The most intense emission peak observed at 602
nm for the Sm3+-doped CaAl4O7 phosphor, which corresponds to the 4G5/2 → 6H7/2 transition, is a partially magnetic dipole (MD) and electric dipole transition, where the electric dipole (ED) dominates [31]. The next intense peak, observed at 565 nm, is the MD transition by the Laporte selection rule, which is insensitive to the crystal environment, whereas the purely ED transition at 655 nm is hypersensitive to the lattice environment [32, 33]. The intensity ratio of ED to MD transition describes the asymmetric nature of the local environment around Sm3+ in the host matrix [34]. In the present case, the dominance of the MD transition over the ED transition indicates that Sm3+ ions have occupied sites with high symmetry [35]. The calculated value of ED/MD intensity ratio is in the range 0.44 to 0.45 for the phosphors. The lower values of intensity ratio suggest that the Sm3+ ions have been substituted into symmetric nature [25]. Moreover, the emission spectra for different concentrations of Sm3+ in CaAl4O7 have been examined in order to optimize the concentration of dopant in the host matrix. The intensity of the emission peaks first increases and reaches a maximum point at x = 0.03, where x represents the Sm3+ concentration in host lattice, and beyond that the intensity of the emission peak decreases with a further increase in the concentration of Sm3+ ions. Fig. 5 shows the luminescence intensity variation of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) with Sm3+ ion concentration (x). The phenomenon of concentration quenching causes the decrease in luminescence intensity beyond a certain concentration of dopant. As Sm3+ ion concentration
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increases, the inter-ionic separation between Sm3+ ions reduces, which leads to higher energy transfer among Sm3+ ions and a decrease in luminescence intensity [36]. In Fig. 6, the photographs of the optimized sample under daylight show a fine white powder, whereas when pumped by n-UV light it exhibits an intense orange emission. Furthermore, to confirm the emission of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors, the color chromaticity coordinates have been calculated from the emission spectra of the samples. The calculated value of Commission de I’Eclairage (CIE) coordinates was found to lie in the pure orange region of the CIE chromaticity diagram, as shown in Fig. 7. The CIE coordinates of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphor are listed in Table 3. The CIE coordinates of the optimized sample are close to the coordinates specified by Nichia Corporation developed Amber LED NSPAR 70BS (0.57, 0.42) [31]. The calculated CIE coordinates reveal the utility of Sm3+-doped CaAl4O7 phosphors as orange emitting component in lighting and display devices. 4. Conclusion Calcium aluminates (CaAl4O7) doped with various concentrations of Sm3+ have been synthesized via the sol-gel method. The structural analysis shows the pure phase formation of monoclinic CaAl4O7 with the C2/c space group where doping concentration does not influence the structure of the asprepared samples. The average crystallite size for Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) lies in the range of 30–36 μm, which is appropriate for various display applications. Particles with agglomeration that exhibit porous structures can be seen in the SEM images. The PL spectra of the samples show intense emission at 602 nm, under 403 nm excitation. The optimized concentration of Sm3+ in this present host is 0.03 mol. Beyond that concentration, the quenching effect dominates. The CIE coordinates for Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) are located in the pure orange region of the chromaticity diagram, and the values are close to those of the Amber LED NSPAR 70BS (0.57, 0.42) 7
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developed by the Nichia Corporation. These results indicate the potential for the use of Sm3+-doped CaAl4O7 phosphors as orange emitters in lighting and display applications. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03030003). This paper was supported by the KU Research Professor Program of Konkuk University.
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Figure captions Fig. 1. Powder XRD pattern of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors Fig. 2. SEM images of Ca0.97Al4O7:Sm0.03 phosphor Fig. 3. UV-VIS-NIR spectrum of Ca0.97Al4O7:Sm0.03 phosphor Fig. 4. Photoluminescence spectra of the Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors (a) Excitation spectrum of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) (λem=602 nm) and (b) Emission spectrum of Ca1xAl4O7:Smx (x =0.01≤x≤0.13) (λexc=403 nm). Fig. 5. Variation in the emission intensity of strong emission (602 nm) as a function of Sm3+ concentration. Fig. 6. Typical photographs of Ca0.97Al4O7:Sm0.03 phosphor (a) phosphor sample under a room light (appearance: white powder) and (b) under UV-365nm (appearance: an orange powder). Fig. 7. CIE chromaticity diagram Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphor. Table captions Table 1: Detailed information of sample composition and starting materials Table 2: FWHM and crystallite size of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors Table 3: CIE color coordinates for Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors
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Table 1: Detailed information of sample composition and starting materials Sample composition Starting materials Ca0.99Al4O7:Sm0.01 Ca=0.4674g Al=3g C.A=3.8420g Sm=0.0044g Ca0.97Al4O7:Sm0.03 Ca=0.4580g Al=3g C.A=3.8420g Sm=0.0266g Ca0.95Al4O7:Sm0.05 Ca=0.4485g Al=3g C.A=3.8420g Sm=0.0444g Ca0.93Al4O7:Sm0.07 Ca=0.4391g Al=3g C.A=3.8420g Sm=0.0622g Ca0.91Al4O7:Sm0.09 Ca=0.4297g Al=3g C.A=3.8420g Sm=0.0800g Ca0.89Al4O7:Sm0.11 Ca=0.4202g Al=3g C.A=3.8420g Sm=0.0977g Ca0.87Al4O7:Sm0.13 Ca=0.4108g Al=3g C.A=3.8420g Sm=0.1155g Ca=Ca(NO3)2·4H2O, Al= Al(NO3)3·9H2O, C A=Citric acid, Sm=Sm(NO3)3·6H2O
Table 2: FWHM and crystallite size of CaAl4O7:Smx (x =0.01≤x≤0.13) phosphors Sample composition Crystalline size (nm) FWHM ( ) Ca0.99Al4O7:Sm0.01 0.2801 30.38 Ca0.97Al4O7:Sm0.03 0.2655 32.06 Ca0.95Al4O7:Sm0.05 0.2358 36.11 Ca0.93Al4O7:Sm0.07 0.2581 32.98 Ca0.91Al4O7:Sm0.09 0.2549 33.5 Ca0.89Al4O7:Sm0.11 0.2675 31.8 Ca0.87Al4O7:Sm0.13 0.2549 33.4
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Table 3: CIE color coordinates for CaAl4O7:Smx (x =0.01≤x≤0.13) phosphors Sample composition X-Coordinate Y-Coordinate Ca0.99Al4O7:Sm0.01 0.53573 0.45556 Ca0.97Al4O7:Sm0.03 0.5386 0.4531 Ca0.95Al4O7:Sm0.05 0.52463 0.46502 Ca0.93Al4O7:Sm0.07 0.51706 0.47145 Ca0.91Al4O7:Sm0.09 0.51526 0.47301 Ca0.89Al4O7:Sm0.11 0.50913 0.47821 Ca0.87Al4O7:Sm0.13 0.50339 0.48314
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Ca0.87Al4O7:Sm0.13
Intesity (Arbitrary Units)
Ca0.89Al4O7:Sm0.11 Ca0.91Al4O7:Sm0.09 Ca0.93Al4O7:Sm0.07 Ca0.95Al4O7:Sm0.05
443
732 820 260 261
333 730
441
151
711 313
331 132 222 602 132
Ca0.99Al4O7:Sm0.01 202
130 221 311 131 420 511 112
221 400
220
111 020
200
311
Ca0.97Al4O7:Sm0.03
JCPDS File No:- 23-1037
10
20
30
40
50
60
70
2 (Degrees)
Fig. 1. Powder XRD patterns of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors
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Fig. 2. SEM images of Ca0.97Al4O7:Sm0.03 phosphor
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110
Ca0.97Al4O7:Sm0.03
Refelection (%)
100 90 80 70 60 50 360
540
720
1080
1260
1440
Wavelength (nm) Fig. 3. UV-VIS-NIR spectrum of Ca0.97Al4O7:Sm0.03 phosphor
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400 320
(a)
240 160 80
500
550
560
exi= 403 nm
480 400 320
(b)
240 160 80
600
650
700
750
co
550
Wavelength (nm)
Sm
3+
500
nc
en
tr
at
io
n
(m
0 X=0.01 X=0.03 X=0.05 X=0.07 X=0.09 X=0.11 X=0.13
)
Wavelength (nm)
Intensity (Arbitrary Units)
450
ol
400
co
350
3+
300
Sm
250
nc
en
tr
at
io
n
(m
0 X=0.01 X=0.03 X=0.05 X=0.07 X=0.09 X=0.11 X=0.13
ol
480
)
Intensity (Arbitrary Units)
560
em= 602 nm
Fig. 4. Photoluminescence spectra of the Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors (a) Excitation spectrum of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) (λem=602 nm) and (b) Emission spectrum of Ca1xAl4O7:Smx (x =0.01≤x≤0.13) (λexc=403 nm).
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600
Intensity (Arbitrary Units)
550 500 450 400 350 300 250 200
0.01
0.03
0.05
0.07
0.09
0.11
0.13
Sm3+ concentration (mol) Fig. 5. Variation in the emission intensity of strong emission (602 nm) as a function of Sm3+ concentration.
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Fig. 6. Typical photographs of Ca0.97Al4O7:Sm0.03 phosphor (a) phosphor sample under a room light (appearance: white powder) and (b) under UV-365nm (appearance: an orange powder).
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Fig. 7. CIE chromaticity diagram Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphor
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Conflict of Interest REF: Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications, submitted by Vijay Singh, Sumandeep Kaur, M. Jayasimhadri
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
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Highlights ►The orange-emitting Sm3+ doped CaAl4O7 phosphor obtained via sol-gel method. ►The Sm3+ emission is quenched at 0.03 mol. ► Synthesized phosphor could be a potential material for lighting and display applications.
Vijay Singh, Sumandeep Kaur, M. Jayasimhadri PII:
S1293-2558(19)30574-6
DOI:
https://doi.org/10.1016/j.solidstatesciences.2019.106049
Reference:
SSSCIE 106049
To appear in:
Solid State Sciences
Received Date:
18 May 2019
Accepted Date:
28 October 2019
Please cite this article as: Vijay Singh, Sumandeep Kaur, M. Jayasimhadri, Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications, Solid
State Sciences (2019), https://doi.org/10.1016/j.solidstatesciences.2019.106049
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications Vijay Singh a, *, Sumandeep Kaur b, M. Jayasimhadri b a Department
of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea
b Department
of Applied Physics, Delhi Technological University, Delhi 110 042, India
Abstract Sm3+-doped CaAl4O7 phosphor has been synthesized using the sol-gel method with a varying doping concentration of Sm3+ ions. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were used to study the structural and morphological properties of the material. The UV-VIS-NIR spectrum was also recorded to analyze the optical transitions of CaAl4O7 phosphor doped with Sm3+ ions. The photoluminescence (PL) spectra of the phosphors were also recorded to examine the luminescence properties. The PL spectra exhibit intense orange emission at 602 nm that was obtained for the prepared samples using the 403 nm excitation. The optimized concentration of Sm3+ ions in CaAl4O7 was found to be 0.03 mol, beyond which concentration quenching was observed. The CIE color chromaticity coordinates of all the synthesized phosphors lie in the orange region of the chromaticity diagram. All of these results indicate the candidature of CaAl4O7 phosphor doped with Sm3+ ions for lighting and display applications.
Keywords: Sol-gel; XRD; CaAl4O7; Sm3+ ions; Photoluminescence *Corresponding authors: Email addresses: [email protected] (V. Singh)
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1. Introduction In solid-state lighting, a white-light-emitting device (wLEDs) based on phosphors is one of the promising light sources for potential applications in fluorescent lamps, optical amplifiers, emissive displays, solid-state lasers, radiation dosimetry, X-ray imaging, and sensors [1–4]. The meritorious advantages of wLEDs—such as longer durability, power saving, cost-effectiveness, high luminous efficiency, low driving voltage, fast response time, and environmental friendliness—make them suitable for these applications [5–7]. At present, various attempts have been demonstrated to get efficient wLEDs. One of the approaches to fabricating phosphor-based wLEDs is to assemble an InGaN-blue chip with YAG: Ce yellow phosphor, but the combination of blue and yellow to generate white light exhibits certain limitations, such as high correlated color temperature (CCT), poor color rendering index (CRI) due to the lack of a red component, and blue-yellow color separation [8–10]. Aiming to generate white light with improved CRI, a near-UV excitation source with tricolor (blue, green, and red) phosphor is another approach that can be utilized [11]. Near ultra violet (n-UV) excitable tricolor phosphors have some disadvantages, such as low luminous efficiency due to the blending of different colored phosphors, each of which has a different degradation rate [12]. The efficiency of red (Y2O2S:Eu3+) phosphor is only 1/8 that of blue (BaMgAl10O17:Eu2+) phosphor or green (ZnS:Cu+, Al3+) phosphor [9, 10]. Therefore, it is important to develop new orange-red emitting phosphor to obtain white light with an improved CRI. Considering all lanthanides, samarium (Sm3+) ion is promising because it serves the purpose of enhancing the intensity of a red component in generating white light [13, 14]. Alkaline earth aluminates are chemically stable in the ambient environment and recognized as an excellent material for use in various applications, including construction materials, luminescent materials, catalyst support, components in bio-ceramics, and refractory materials [15–19]. Various 2
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crystallization phases of calcium aluminates exist based on CaO-Al2O3 composition. CaAl4O7 is identified as an attractive host among different aluminates, due to its high optical transparency from UV to NIR range and excellent mechanical and thermal properties with high chemical stability [16]. Owing to the excellent properties of CaAl4O7 as host and excellent emission characteristics of Sm3+ ions as rare earth ion found application in displays and solid state lighting, a series of Sm3+doped CaAl4O7 phosphor has been synthesized via a sol-gel method for which the dopant concentration was varied from 0.01–0.13 mol [20, 21]. In the present article, the structural and morphological properties of the synthesized phosphor have been studied. The optical and luminescence properties were studied in detail for the application of the Sm3+-doped CaAl4O7 phosphor in lighting and display devices. 2. Materials preparation and analysis The sol-gel method was used to prepare a series of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors. In a typical manner for synthesis, all starting materials with high purity were used without further purification. The details of sample composition and starting materials were given in Table 1. In the manner of a typical synthesis, the stoichiometric quantity of starting materials-such as calcium nitrate tetrahydrate (Ca[NO3]2∙4H2O), aluminum nitrate nonahydrate (Al[NO3]3∙9H2O), samarium nitrate hexahydrate (Sm[NO3]3∙6H2O), and the citric acid (citric acid/metal ion 2:1, molar ratio)were first dissolved in 10 ml deionized water under stirring at 500 rpm for 1 hr. A transparent aqueous solution was obtained after stirring, and this was kept at 110°C in the oven to obtain a homogeneous dried gel. The obtained dried gel was sintered at 400°C for 120 min in an air atmosphere. Finally, the resultant brown residual samples were fully ground and annealed at 1000°C for 3 hr in the air to obtain the pure phase of CaAl4O7.
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Using a Miniflex-II (Rigakum, Japan) diffractometer with Cu-Kα radiation (λ=1.5406Å) as X-ray source, the X-ray diffraction (XRD) patterns of the samples were recorded. The XRD patterns were taken with a scan rate of 5°/minute in the 2 range of 10° to 80o. An SEM (S-3400, Hitachi, Japan) was used to obtain morphology details. UV-VIS-NIR measurement was carried out at room temperature on a Cary 5000 Spectrophotometer. Photoluminescence measurements were carried out at room temperature on a Shimadzu RF-5301PC spectrofluorophotometer equipped with Xenon flash lamp. 3. Results and discussion: 3.1 Crystal structure and crystallite size analysis: Fig. 1 shows the XRD patterns of all the Sm3+-doped CaAl4O7 phosphors. The XRD patterns show that the diffraction peaks indexed to hkl planes are completely in accordance with standard data received from the Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 23-1037. The XRD patterns reveal that all the synthesized samples are crystallizes in single-phase monoclinic structure with C2/c space group and lattice parameters of a= 12.89 Å, b= 8.88 Å, c= 5.45 Å, α= γ= 90o, β= 107o and volume of the unit cell is (V= 591.54 Å3) [22]. In a CaAl4O7 lattice, Ca2+ ions occupy crystallographic sites of C2 symmetry surrounded by five oxygen ions, whereas Al3+ ions are distributed over two independent tetrahedral sites, whose surroundings are significantly distorted, giving C1 local symmetries [12, 16]. Moreover, no impurity peaks have been observed in the XRD patterns of any of the doped samples. This affirms the pure phase synthesis of CaAl4O7. The crystallite sizes of all the phosphors have been calculated from the well-known Debye-Scherrer’s formula: D = kλ/ βcosθ, where D is crystallite size, β is full width at half maxima (FWHM), θ is Bragg's diffraction angle, and λ is wavelength used. Table 2 shows a list of the FWHM and crystallite size of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) samples.
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3.2 SEM analysis: SEM images recorded at different resolutions have been used to study the morphology of sol-gelsynthesized phosphor. Fig. 2 shows the morphology of Ca0.97Al4O7:Sm0.03 at different resolutions. Figs. 2a and 2b show the irregular morphology of agglomerated particles. Particles with some agglomeration exhibiting a porous structure of the particles are displayed in Figs. 2c and 2d. The porous structure of the particles might be due to the evolution of gases during the synthesis process [23]. The average particle size is in the range of 1–6 μm. This makes them suitable for various lighting applications [24]. 3.3 UV-VIS-NIR analysis: Fig. 3 shows the diffuse reflectance (DR) spectrum for Ca0.97Al4O7:Sm0.03 phosphor recorded in the UV-VIS-NIR region. The spectrum shows intense band positions at 340, 401, 1093, 1249, 1402, and 1508 nm, which are associated with the characteristic transitions from ground level 6H5/2 to 3H7/2, 4F
7/2,
6F
9/2,
6F
7/2,
6F
5/2,
and 6H15/2 excited levels, respectively [19, 20]. The DR spectrum indicates that
the Sm3+-doped CaAl4O7 phosphor can effectively absorb in n-UV and NIR region. 3.4 Photoluminescence analysis: Fig. 4a shows the photoluminescence excitation spectra of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13). The PLE spectra of these samples have been measured at room temperature by monitoring emission wavelength at 602 nm. These spectra show various narrow and sharp excitation peaks at 305, 315, 332, 344, 361, 374, 403, 418, 435, 443, 449, 462, 473, 482, 487, 501, 528, and 537 nm attributed to transition from ground state 6H5/2 to 4P5/2, 4P3/2, 4G9/2, 4D7/2 + (4K, 4L)17/2 + 4H9/2, 4D3/2 + (4D, 6P)5/2, 4L 4 4 4 4 4 4 4 4 4 4 4 17/2, F7/2, M19/2, I15/2, M17/2, F5/2, I13/2, I11/2, M15/2, I19/2, G7/2, F3/2,
and 4G5/2 excited states,
respectively originating from f-f transitions of Sm3+ ions [25-28]. The excitation spectra for all the
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samples show similar profiles, though there is some intensity variation with the Sm3+ concentration in the host lattice CaAl4O7. Among various excitation peaks, the most intense excitation peak is found at 403 nm corresponding to the transition 6H5/2 → 4F7/2. Therefore, the emission spectra have been recorded at room temperature under 403 nm excitation wavelengths, as shown in Fig. 4b. The emission spectra exhibit emission peaks at 565, 602, and 655 nm corresponding from 4G5/2 → 6H5/2, 6H
7/2,
and 6H9/2 transitions, respectively [29, 30]. The most intense emission peak observed at 602
nm for the Sm3+-doped CaAl4O7 phosphor, which corresponds to the 4G5/2 → 6H7/2 transition, is a partially magnetic dipole (MD) and electric dipole transition, where the electric dipole (ED) dominates [31]. The next intense peak, observed at 565 nm, is the MD transition by the Laporte selection rule, which is insensitive to the crystal environment, whereas the purely ED transition at 655 nm is hypersensitive to the lattice environment [32, 33]. The intensity ratio of ED to MD transition describes the asymmetric nature of the local environment around Sm3+ in the host matrix [34]. In the present case, the dominance of the MD transition over the ED transition indicates that Sm3+ ions have occupied sites with high symmetry [35]. The calculated value of ED/MD intensity ratio is in the range 0.44 to 0.45 for the phosphors. The lower values of intensity ratio suggest that the Sm3+ ions have been substituted into symmetric nature [25]. Moreover, the emission spectra for different concentrations of Sm3+ in CaAl4O7 have been examined in order to optimize the concentration of dopant in the host matrix. The intensity of the emission peaks first increases and reaches a maximum point at x = 0.03, where x represents the Sm3+ concentration in host lattice, and beyond that the intensity of the emission peak decreases with a further increase in the concentration of Sm3+ ions. Fig. 5 shows the luminescence intensity variation of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) with Sm3+ ion concentration (x). The phenomenon of concentration quenching causes the decrease in luminescence intensity beyond a certain concentration of dopant. As Sm3+ ion concentration
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increases, the inter-ionic separation between Sm3+ ions reduces, which leads to higher energy transfer among Sm3+ ions and a decrease in luminescence intensity [36]. In Fig. 6, the photographs of the optimized sample under daylight show a fine white powder, whereas when pumped by n-UV light it exhibits an intense orange emission. Furthermore, to confirm the emission of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors, the color chromaticity coordinates have been calculated from the emission spectra of the samples. The calculated value of Commission de I’Eclairage (CIE) coordinates was found to lie in the pure orange region of the CIE chromaticity diagram, as shown in Fig. 7. The CIE coordinates of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphor are listed in Table 3. The CIE coordinates of the optimized sample are close to the coordinates specified by Nichia Corporation developed Amber LED NSPAR 70BS (0.57, 0.42) [31]. The calculated CIE coordinates reveal the utility of Sm3+-doped CaAl4O7 phosphors as orange emitting component in lighting and display devices. 4. Conclusion Calcium aluminates (CaAl4O7) doped with various concentrations of Sm3+ have been synthesized via the sol-gel method. The structural analysis shows the pure phase formation of monoclinic CaAl4O7 with the C2/c space group where doping concentration does not influence the structure of the asprepared samples. The average crystallite size for Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) lies in the range of 30–36 μm, which is appropriate for various display applications. Particles with agglomeration that exhibit porous structures can be seen in the SEM images. The PL spectra of the samples show intense emission at 602 nm, under 403 nm excitation. The optimized concentration of Sm3+ in this present host is 0.03 mol. Beyond that concentration, the quenching effect dominates. The CIE coordinates for Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) are located in the pure orange region of the chromaticity diagram, and the values are close to those of the Amber LED NSPAR 70BS (0.57, 0.42) 7
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developed by the Nichia Corporation. These results indicate the potential for the use of Sm3+-doped CaAl4O7 phosphors as orange emitters in lighting and display applications. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03030003). This paper was supported by the KU Research Professor Program of Konkuk University.
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Figure captions Fig. 1. Powder XRD pattern of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors Fig. 2. SEM images of Ca0.97Al4O7:Sm0.03 phosphor Fig. 3. UV-VIS-NIR spectrum of Ca0.97Al4O7:Sm0.03 phosphor Fig. 4. Photoluminescence spectra of the Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors (a) Excitation spectrum of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) (λem=602 nm) and (b) Emission spectrum of Ca1xAl4O7:Smx (x =0.01≤x≤0.13) (λexc=403 nm). Fig. 5. Variation in the emission intensity of strong emission (602 nm) as a function of Sm3+ concentration. Fig. 6. Typical photographs of Ca0.97Al4O7:Sm0.03 phosphor (a) phosphor sample under a room light (appearance: white powder) and (b) under UV-365nm (appearance: an orange powder). Fig. 7. CIE chromaticity diagram Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphor. Table captions Table 1: Detailed information of sample composition and starting materials Table 2: FWHM and crystallite size of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors Table 3: CIE color coordinates for Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors
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Table 1: Detailed information of sample composition and starting materials Sample composition Starting materials Ca0.99Al4O7:Sm0.01 Ca=0.4674g Al=3g C.A=3.8420g Sm=0.0044g Ca0.97Al4O7:Sm0.03 Ca=0.4580g Al=3g C.A=3.8420g Sm=0.0266g Ca0.95Al4O7:Sm0.05 Ca=0.4485g Al=3g C.A=3.8420g Sm=0.0444g Ca0.93Al4O7:Sm0.07 Ca=0.4391g Al=3g C.A=3.8420g Sm=0.0622g Ca0.91Al4O7:Sm0.09 Ca=0.4297g Al=3g C.A=3.8420g Sm=0.0800g Ca0.89Al4O7:Sm0.11 Ca=0.4202g Al=3g C.A=3.8420g Sm=0.0977g Ca0.87Al4O7:Sm0.13 Ca=0.4108g Al=3g C.A=3.8420g Sm=0.1155g Ca=Ca(NO3)2·4H2O, Al= Al(NO3)3·9H2O, C A=Citric acid, Sm=Sm(NO3)3·6H2O
Table 2: FWHM and crystallite size of CaAl4O7:Smx (x =0.01≤x≤0.13) phosphors Sample composition Crystalline size (nm) FWHM ( ) Ca0.99Al4O7:Sm0.01 0.2801 30.38 Ca0.97Al4O7:Sm0.03 0.2655 32.06 Ca0.95Al4O7:Sm0.05 0.2358 36.11 Ca0.93Al4O7:Sm0.07 0.2581 32.98 Ca0.91Al4O7:Sm0.09 0.2549 33.5 Ca0.89Al4O7:Sm0.11 0.2675 31.8 Ca0.87Al4O7:Sm0.13 0.2549 33.4
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Table 3: CIE color coordinates for CaAl4O7:Smx (x =0.01≤x≤0.13) phosphors Sample composition X-Coordinate Y-Coordinate Ca0.99Al4O7:Sm0.01 0.53573 0.45556 Ca0.97Al4O7:Sm0.03 0.5386 0.4531 Ca0.95Al4O7:Sm0.05 0.52463 0.46502 Ca0.93Al4O7:Sm0.07 0.51706 0.47145 Ca0.91Al4O7:Sm0.09 0.51526 0.47301 Ca0.89Al4O7:Sm0.11 0.50913 0.47821 Ca0.87Al4O7:Sm0.13 0.50339 0.48314
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Ca0.87Al4O7:Sm0.13
Intesity (Arbitrary Units)
Ca0.89Al4O7:Sm0.11 Ca0.91Al4O7:Sm0.09 Ca0.93Al4O7:Sm0.07 Ca0.95Al4O7:Sm0.05
443
732 820 260 261
333 730
441
151
711 313
331 132 222 602 132
Ca0.99Al4O7:Sm0.01 202
130 221 311 131 420 511 112
221 400
220
111 020
200
311
Ca0.97Al4O7:Sm0.03
JCPDS File No:- 23-1037
10
20
30
40
50
60
70
2 (Degrees)
Fig. 1. Powder XRD patterns of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors
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Fig. 2. SEM images of Ca0.97Al4O7:Sm0.03 phosphor
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110
Ca0.97Al4O7:Sm0.03
Refelection (%)
100 90 80 70 60 50 360
540
720
1080
1260
1440
Wavelength (nm) Fig. 3. UV-VIS-NIR spectrum of Ca0.97Al4O7:Sm0.03 phosphor
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400 320
(a)
240 160 80
500
550
560
exi= 403 nm
480 400 320
(b)
240 160 80
600
650
700
750
co
550
Wavelength (nm)
Sm
3+
500
nc
en
tr
at
io
n
(m
0 X=0.01 X=0.03 X=0.05 X=0.07 X=0.09 X=0.11 X=0.13
)
Wavelength (nm)
Intensity (Arbitrary Units)
450
ol
400
co
350
3+
300
Sm
250
nc
en
tr
at
io
n
(m
0 X=0.01 X=0.03 X=0.05 X=0.07 X=0.09 X=0.11 X=0.13
ol
480
)
Intensity (Arbitrary Units)
560
em= 602 nm
Fig. 4. Photoluminescence spectra of the Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphors (a) Excitation spectrum of Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) (λem=602 nm) and (b) Emission spectrum of Ca1xAl4O7:Smx (x =0.01≤x≤0.13) (λexc=403 nm).
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600
Intensity (Arbitrary Units)
550 500 450 400 350 300 250 200
0.01
0.03
0.05
0.07
0.09
0.11
0.13
Sm3+ concentration (mol) Fig. 5. Variation in the emission intensity of strong emission (602 nm) as a function of Sm3+ concentration.
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Fig. 6. Typical photographs of Ca0.97Al4O7:Sm0.03 phosphor (a) phosphor sample under a room light (appearance: white powder) and (b) under UV-365nm (appearance: an orange powder).
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Fig. 7. CIE chromaticity diagram Ca1-xAl4O7:Smx (x =0.01≤x≤0.13) phosphor
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Conflict of Interest REF: Luminescence properties of orange emitting CaAl4O7:Sm3+ phosphor for solid state lighting applications, submitted by Vijay Singh, Sumandeep Kaur, M. Jayasimhadri
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
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Highlights ►The orange-emitting Sm3+ doped CaAl4O7 phosphor obtained via sol-gel method. ►The Sm3+ emission is quenched at 0.03 mol. ► Synthesized phosphor could be a potential material for lighting and display applications.