An insight into the luminescence properties of Ce3+ in garnet structured CaY2Al4SiO12:Ce3+ phosphors

Journal Pre-proof An insight into the luminescence properties of Ce3+ in garnet structured CaY2 Al4 SiO12 :Ce3+ phosphors Vijay Singh, D.A. Hakeem, G. Lakshminarayana

PII:

S0030-4026(19)31731-0

DOI:

https://doi.org/10.1016/j.ijleo.2019.163833

Reference:

IJLEO 163833

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Optik

Received Date:

6 September 2019

Accepted Date:

19 November 2019

Please cite this article as: Singh V, Hakeem DA, Lakshminarayana G, An insight into the luminescence properties of Ce3+ in garnet structured CaY2 Al4 SiO12 :Ce3+ phosphors, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163833

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An insight into the luminescence properties of Ce3+ in garnet structured CaY2Al4SiO12:Ce3+ phosphors Vijay Singh a, 1, * [email protected], D. A. Hakeem a, 1, G. Lakshminarayana b

Department of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea b

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Intelligent Construction Automation Center, Kyungpook National University, 80, Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea *Corresponding author:

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Abstract

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E-mail: (V. Singh) 1 Both authors equally contributed to the paper

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In this study, CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors were prepared by using the sol–gel method. The crystal structure, morphology, and luminescence properties were well studied by

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means of X-ray diffraction (XRD), UV-Vis, scanning electron microscope (SEM), and photoluminescence spectroscopy. The XRD patterns revealed that the CaY2Al4SiO12:xCe3+ (0.01 ≤

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x ≤ 0.11) phosphors formed a cubic crystal structure with Ia3d space group, and a = b = c = 12.0089 lattice parameters. The band gap and the particle size of the prepared phosphors, estimated from the UV-Vis spectrum and SEM micrograph, were found to be 3.35 eV and (30–100) nm, respectively. The phosphors showed an intense green emission band upon 449 nm excitation with the average color purity value of around 79 %. It is expected that the CaY2Al4SiO12:xCe3+ (0.01 ≤

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x ≤ 0.11) phosphors can be used as a green component in the WLEDs application.

Keywords: Sol-gel; XRD; Ce3+ ions; CaY2Al4SiO12; Photoluminescence

1. Introduction 1

Recently, the development of inorganic compounds, especially luminescent materials (also called phosphors), has been the subject of extensive research, because of their potential applications that include cathode ray tubes (CRTs), lamp industries, field emission display (FEDs), radiation dosimetry, and white light-emitting diodes (WLEDs) [1-3]. The generation of white light using solid-state devices (i.e. LEDs) has been accepted worldwide as a strategy for energy saving,

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reliability, safety, environmental friendliness, high brightness, fast switching, and color quality [4-6]. Such a strategy promises a higher luminescence performance than the traditional lighting

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sources (fluorescent and incandescent lamp). Thus, WLEDs provide a real chance to replace conventional lighting sources. Currently, commercial white light is achieved by mixing blue

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LED chip (InGaN) and yellow Y3Al5O12:Ce3+ phosphor (YAG:Ce3+), whose luminous efficiency

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is excellent. The efficacy of the YAG:Ce3+ phosphors is much greater than 80 lm/W for the 1 W device, higher than compact fluorescent lamps (CFLs), and comparable to linear fluorescent

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lamps [7]. However, YAG:Ce3+ phosphor possesses a low color rendering index (Ra; <80), high correlated color temperature (CCT; <6000) due to the lack of red component, and insufficient

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thermal stability [7-10]. Moreover, the output color of YAG:Ce3+ is strongly dependent on the temperature and current. With increasing the magnitude of electrical current, the hue of LEDs changes from yellow to white (or blue), which becomes a significant problem in high-power LEDs [11]. To enhance the Ra of the WLEDs, green and red-emitting phosphors are combined

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with the blue LED chip. The Ra value of this type of white LEDs is even > 90. Hence, it is believed that the green phosphors play an important role in fabricating white LEDs with higher Ra [12].

For WLEDs application, the modified YAG phosphors, i.e. (Y, Gd, Lu)3(Al, Ga)5O12:Ce3+ are the most commercially available phosphors. The compositional modification causes the 2

tuning of emission color of the host material. Katelnikovas et al. [13] modified Y3−xAl5O12:Cex3+(YAG:Ce) as Y3−xMg2AlSi2O12:Cex3+ by substituting the Mg2+–Si4+ pair for Al3+, which resulted in the redshift of the Ce3+ emission spectrum, due to a variation in covalence [13]. Lu3−xCexAl5O12 (LuAG:Ce), a contemporary green phosphor, possesses excellent luminescence

and

thermal

properties

compared

to

the

yellow

(Y,

Gd)3(Al,

Ga)5O12:Ce3+ phosphor [14]. However, both the YAG:Ce and LuAG:Ce phosphors contain Y3+

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and Lu3+, which are very expensive. To decrease the usage of Y3+ and Lu3+ elements and the

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synthesis cost of commercial phosphor, the compositional modifications are achieved in several hosts [12, 15, 16]. Katelnikovas et al. [15] modified LuAG:Ce by substituting the Ca2+–Si4+ pair by In

comparison

to

LuAG:Ce,

the

emission

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CaLu2Al4SiO12:Ce3+.

maximum

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the

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CaLu2Al4SiO12:Ce3+ is blue-shifted from (520 to 542) nm by changing the Ce3+ concentration. The quantum efficiencies values of (80 and 86) % are obtained for the CaLu2Al4SiO12:Ce3+

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phosphor doped with (0.25 and 0.5) % of Ce3+ concentration, respectively. The thermal stability of the CaLu2Al4SiO12:Ce3+ phosphor is much worse than those of the LuAG:Ce phosphor. Ji et al.

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[16] substituted Ba2+–Si4+ for Y3+–Al3+ in YAG:Ce to decrease the usage of Y2O3, and obtained BaY2Al4SiO12:Ce3+ phosphor with comparable thermal stability. BaY2Al4SiO12:Ce3+ phosphor successfully preserved the crystal structure of YAG:Ce. Upon doping with Ce3+ ions, BaY2Al4SiO12:Ce3+ absorbs blue light (460 nm), and can efficiently emit yellow light in the

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spectral range (470–700) nm.

Herein, an attempt has been made to substitute Ca2+–Si4+ for Y3+–Al3+ in YAG:Ce3+ to

obtain CaY2Al4SiO12:Ce3+. It is found that the crystal structure of the YAG:Ce3+ does not change by the substitution of Ca2+–Si4+. Strong green emission centered at 519 nm in the (450 to 625) nm range is detected for CaY2Al4SiO12:Ce3+. In this paper, we investigate the structure, 3

luminescence properties, and color purity of the CaY2Al4SiO12:Ce3+ phosphors. It is expected that the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors can be used as a green component in the WLEDs application. 2. Materials preparation and analysis A series of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors were prepared by using the sol–gel method. In a typical synthesis, high purity Ca(NO3)2∙4H2O, Y(NO3)3∙6H2O, Al(NO3)3∙9H2O,

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SiO2, Ce(NO3)3∙6H2O, and citric acid (citric acid/metal ion 2:1, molar ratio) were used as starting

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materials, without further purification. In a typical synthesis, the stoichiometric quantities of metal nitrates and citric acid were firstly dissolved in 10 mL deionized water in a 150 mL glass

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beaker under stirring at 500 rpm. The transparent aqueous solution was obtained after stirring for

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1 h. The resultant transparent solution was kept at 110 °C in oven, until homogeneous dried gel was formed. Then the dried gel obtained was ground and sintered at 400 °C for 2 h. Finally, the

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resultant brown residual sample was fully ground and annealed at 1,100 °C for 3 h in air. The XRD patterns of the samples were recorded using RIGAKU Miniflex-II

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diffractometer. Cu-Kα radiation (λ = 1.5406 Å) was used as X-ray source. The XRD patterns were taken with a scan rate of 5°/min in the 2 range of (10 to 80)°. The morphological details were obtained using SEM (S-3400, Hitachi, Japan). Photoluminescence measurements were carried out at room temperature (RT) by Shimadzu RF-5301PC spectrofluorophotometer,

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equipped with Xenon flash lamp. 3. Results and discussion

3.1 Crystal structure and crystallite size analysis Figure 1 shows the X-ray diffraction (XRD) measurement of the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors that was carried out using Cu-K (l =1.5405 Å) radiation. All the peaks in the 4

XRD patterns can be indexed to a cubic phase with Ia3d space group and lattice parameters as a = b = c = 12.0089, which is in line with the JCPDS Card No. 33-0040 of pure cubic YAG:Ce3+ garnet. All the XRD patterns of the CaY2Al4SiO12:Ce3+ phosphors are similar, with the exception of their intensity. This may have been caused by the difference in the ionic radius of the eight coordinated Ce3+ (1.14 Å) and Ca2+ (1.12 Å) ions. This indicates that the desired

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CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors have been well crystallized. However, the few extra impurity peaks of the Ca2Al2SiO7, as indicated by (*), are detected in the XRD pattern of

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the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors. Similar secondary peaks are reported in the literature for CaY2Al4SiO12:Tb3+ and CaY2Al4SiO12:Gd3+ [17, 18]. The crystallite size (D) of the

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prepared CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphor is calculated by Scherrer’s formula:

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D = (0.9λ)/(β𝑐𝑜𝑠θ)

where λ is the wavelength of radiation (0.15418 nm), θ is the angle of the Bragg diffraction peak,

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and β is the full width at half maximum of the diffraction peak (in radian). The crystallite sizes of the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors are calculated by considering the XRD

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peaks corresponding to the (211), (420), (640), and (642) planes. The calculated crystallite sizes were found to be 25.93, 27.69, 25.96, 29.24, 25.49, and 26.70 nm for 0.01, 0.03, 0.06, 0.09, 0.11 and 0.13 mol Ce3+ content, respectively.

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3.2 SEM analysis

Since the photoluminescence properties of the phosphor depend upon the shape, size, crystallinity, defects, and grain boundaries, it is important to study the morphology of the CaY2Al4SiO12:Ce3+ phosphors. Figure 2 shows the SEM image of the CaY2Al4SiO12:0.05Ce3+ phosphor. At relatively lower magnification (50 m), the particles appear to have irregular and 5

inhomogeneous morphology, which may be due to the agglomeration of particles. The irregular and inhomogeneous morphology is caused by the non-uniform distribution of temperature during the sintering process. The average particle size of the agglomerates is around (30–100) nm, as can be seen from the inset of Figure 2.

3.3 UV-VIS spectrum of CaY2Al4SiO12:0.05Ce3+ phosphor

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Figure 3 shows the UV-Visible reflectance spectra of CaY2Al4SiO12:0.05Ce3+ recorded at RT. A

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broad band at ~455 nm is detected in the reflectance spectra of CaY2Al4SiO12:0.05Ce3+, which is predominantly contributed by the 4f–5d transition of the Ce3+ ions [19]. To determine the optical

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bandgap of the CaY2Al4SiO12:0.05Ce3+ phosphor under study, the optical absorption coefficient

𝐹(𝑅) =

(1−𝑅)2 2𝑅

= 𝛼

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(α) is calculated using the following Kubelka-Munk function:

……………………………………………………. (1)

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where, R denotes the reflectance of the CaY2Al4SiO12:0.05Ce3+ phosphor. Finally, the optical band gap (Eg) for CaY2Al4SiO12:0.05Ce3+ phosphor is estimated by the following formula [20]:

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𝐹(𝑅)ℎ  𝐵(ℎ − 𝐸𝑔)𝑛 ……………………………………………..………. (2) where, B is constant, F(R) is the absorption coefficient, hν is the photon energy, and n denotes the type of electronic transition. It is known that n = 1/2 for the allowed direct, n = 2 for the allowed indirect, n = 3/2 for the forbidden direct, and n = 3 for the forbidden indirect transitions.

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By using Eq. (2), the indirect energy bandgap of CaY2Al4SiO12:0.05Ce3+ phosphor is calculated by plotting the (F(R) ∗ hν)2 as a function of hν (eV) (Fig. 3 (b)). The obtained linear region of the plot is extrapolated, so that it coincides with the x-axis. It is found that the extrapolated line coincides with the x-axis at 3.28 eV, which is the indirect band gap of the CaY2Al4SiO12:0.05Ce3+ phosphor. The calculated bandgap value is closer to the Y36

3+

xAl2Ga3O12:xTb

, Y3-yAl2Ga3O12:yEu3+ and Y3-xTb0.5Al2Ga3O12:zEu3+ garnet phosphors viz 4.30,

4.25, 4.22, respectively [21].

3.4 Photoluminescence analysis Figures 4 (a) and (b) show the excitation and emission spectra of the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphor monitored at (519 and 449) nm, respectively. The excitation spectra of

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CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphor showed a broad and intense band at 449 nm

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related to the 4f–5d1 (E1) transition of Ce3+ ions. However, the UV excitation band E2 of Ce3+, which is generally observed between (300 and 400) nm for the different garnets, is either

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negligible, or absent. Similar behavior of the E2 band of Ce3+ has been reported in different

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garnet phosphors [22-24]. Hence, undoubtedly the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors are more useful for the creation of blue LED excited garnet convertors.

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Upon excitation at 449 nm, the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors show a broad green emission band in the range (450 to 625) nm, corresponding to the transition from the

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lowest component of the 5d state to the 2F5/2 and 2F7/2 levels of the Ce3+. Obviously, the incorporation of the Ce3+ ions in CaY2Al4SiO12:xCe3+ phosphors have affected the luminescence intensity of the 519 nm band, as can be seen in Fig. 4 (b). The luminescence intensity increases with increase in the Ce3+ concentration and reaches up to 0.05 mol concentration; however, with

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further increase in the Ce3+ ions concentration, the emission intensity is quenched. The quenching of the emission generally depends upon the distance between the activators. The general rule is, the higher the Ce3+ ion concentration, the lower the distance between Ce3+ ions. It is therefore important to calculate the critical distance between the activators, which is expressed by the formula given below [25]: 7

Rc ≈ 2 (

3V

4π𝑥𝑐 𝑁

1/3

)

………………………………………… (3)

where, V is the volume of the unit cell, xc is the critical concentration of activator ions, and N is the number of formula units per unit cell. For CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors, V= 1,731.8 Å3, N = 8, and the critical concentration of activator ions is found to be xc= 0.05. Hence, the critical distance for energy transfer (Rc) between Ce3+ ions is calculated to be 10.11 Å.

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To evaluate the color performance of the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors, the Commission Internationale de I’Eclairage (CIE) chromaticity coordinates (x, y) and color

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purity were calculated by using the CIE calculator software from the emission spectra. Table I

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shows the calculated color coordinates (x, y) (see Figure 6) and color purity values. The calculated CIE color co-ordinates fall under the green region of the CIE diagram, which affirms

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that the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors emit green emission. In addition, the

significantly higher. 4. Conclusion

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average color purity of the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors is 79 %, which is

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The CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors were prepared by using the sol–gel method. The phase formation was affirmed by the XRD analysis, which indicates the successful formation of cubic garnet structure (JCPDS Card No. 33-0040). It was found that the crystal

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structure of the YAG:Ce3+ does not change by the substitution of Ca2+–Si4+. Strong green emission centered at 519 nm in the (450 to 625) nm range corresponding to the transition from the lowest component of the 5d state to the 2F5/2 and 2F7/2 levels of the Ce3+ is detected. With increase in the Ce3+ content, the quenching of emission is observed at 0.05 mol Ce3+ concentration. The critical distance between the Ce3+ activators at 0.05 mol Ce3+ concentration is 10.11 Å. The CIE color co-ordinates of the phosphors fall under the green region with average 8

purity of 79 %. It is expected that the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors can be used as a green component in the WLEDs application.

Acknowledgements This paper was supported by the KU Research Professor Program of Konkuk University. This

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research was also supported by Basic Science Research Program through the National Research

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Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03030003).

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Figure caption Figure 1. Powder XRD pattern of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors. Figure 2. SEM image of CaY2Al4SiO12:0.05Ce3+ phosphor. Figure 3. (a) UV-VIS spectrum and (b) plot of (F(R) ∗ hν)2 Vs hν of CaY2Al4SiO12:0.05Ce3+ phosphor.

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Figure 4. Photoluminescence spectra of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors (a) Excitation spectrum of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) (λem=519 nm) and (b) Emission spectrum of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) (λexc= 449 nm).

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Figure 5. Variation in the emission intensity of strong emission (519 nm) as a function of Ce3+ concentration.

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Figure 6. CIE chromaticity diagram of CaY2Al4SiO12:xCe3+ where x = 0.01(●), x = 0.03(■), x = 0.05 (♦), x = 0.07 (▲), x = 0.09 (▼), and x = 0.11 (◀).

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Table I: CIE chromaticity coordinates and color purity of the CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors monitored at 449 nm wavelengths.

Phosphors

λem = 449 nm

Color purity: (Sign) y

x = 0.01

0.2471

0.6455

79.71 (●)

x = 0.03

0.2591

0.6396

77.57 (■)

x = 0.05

0.2593

0.6396

77.57 (♦)

x = 0.07

0.2487

0.6489

x = 0.09

0.2513

0.6431

x = 0.11

0.2514

0.6499

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x

80.42 (▲)

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78.86 (▼)

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80.49 (◀)

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CaY2Al4SiO12:0.11Ce

Intensity (a.u.)

CaY2Al4SiO12:0.09Ce

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CaY2Al4SiO12:0.07Ce

CaY2Al4SiO12:0.05Ce

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932

840 842 664

800

CaY2Al4SiO12:0.01Ce

-p

631 444 640 552 642

532

521 440

**

422 431

400

321

220

211

420

CaY2Al4SiO12:0.03Ce

12

24

36

re

JCPDS File No:-33-0040

48

60

72

lP

2 (Degrees)

Jo

ur na

Figure 1. Powder XRD patterns of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors

16

of ro -p

Jo

ur na

lP

re

Figure 2. SEM image of CaY2Al4SiO12:0.05Ce3+ phosphor

17

of ro -p re lP ur na Jo Figure 3. (a) UV-VIS spectrum and (b) plot of (F(R) ∗ hν)2 Vs hν of CaY2Al4SiO12:0.05Ce3+ phosphor 18

(a) em = 519 nm

CaY2Al4SiO12:0.01Ce

Intensity (a.u.)

CaY2Al4SiO12:0.03Ce CaY2Al4SiO12:0.05Ce CaY2Al4SiO12:0.07Ce

300

350

400

450

-p

250

ro

of

CaY2Al4SiO12:0.09Ce CaY2Al4SiO12:0.11Ce

500

re

Wavelength (nm)

lP 500

550

CaY2Al4SiO12:0.01Ce CaY2Al4SiO12:0.03Ce CaY2Al4SiO12:0.05Ce CaY2Al4SiO12:0.07Ce CaY2Al4SiO12:0.09Ce CaY2Al4SiO12:0.11Ce

ur na

Jo

Intensity (a.u.)

(b) exc = 449 nm

600

650

700

Wavelength (nm)

Figure 4. Photoluminescence spectra of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) phosphors (a) Excitation spectrum of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) (λem=519 nm) and (b) Emission spectrum of CaY2Al4SiO12:xCe3+ (0.01 ≤ x ≤ 0.11) (λexc= 449 nm) 19

of ro -p re

Jo

ur na

lP

Figure 5. Variation in the emission intensity of strong emission (519 nm) as a function of Ce3+ concentration

20

of ro -p re lP

Jo

ur na

Figure 6. CIE chromaticity diagram of CaY2Al4SiO12:xCe3+ where x = 0.01(●), x = 0.03(■), x = 0.05 (♦), x = 0.07 (▲), x = 0.09 (▼), and x = 0.11 (◀).

21