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J. Korean Ceram. Soc. > Volume 54(5); 2017 > Article
Kim, Kim, Viswanath, Arunkumar, and Im: Structural and Optical Properties of Yellow-Emitting CaGd2ZrSc(AlO4)3:Ce3+ Phosphor for Solid-State Lighting

Abstract

Single-phase yellow phosphor, CaGd2−xZrSc(AlO4)3:xCe3+ (CGZSA:Ce3+), possessing cubic symmetry with varied Ce3+ concentrations, was synthesized using the solid-state reaction method. The samples were characterized using X-ray diffraction (XRD), excitation spectra, emission spectra, thermal quenching, and decay curves. The cubic phase of CGZSA:Ce3+ phosphor was confirmed via XRD analysis. The photoluminescence spectra of CGZSA:Ce3+ phosphor demonstrated that the phosphor could be excited at the wavelength of 440 nm; a broad yellow emission band was centered at 541 nm. These results indicate that the phosphors are adequately excited by blue light and have the potential to function as yellow-emitting phosphors for applications in white light-emitting diodes.

1. Introduction

Phosphor-converted white light-emitting devices (pc-WLEDs) are considered a promising light source for solid-state lighting, displays, and headlights owing to their high efficiency, low energy consumption, long life, and environmentally benign nature.1) Commercial pc-WLEDs are fabricated by incorporating Y3Al5O12:Ce3+ phosphor (YAG:Ce3+) as an emissive layer under blue LED excitation; this process was initiated by Nichia Corporation.2,3) The dominance of pc-WLEDs in lighting applications is due to the invention of efficient blue LEDs.4) The broadband activators (Eu2+ and Ce3+), especially Ce3+, are the most promising activators for phosphors owing to their broad emission band originating from the 4f-5d transitions.5,6) They can extend the absorption band from ultraviolet (UV) to the visible region in the phosphor hosts, matching the most efficient excitation source, the InGaN blue-LED chip.6)
Garnet-based phosphors are the most commonly utilized phosphor hosts (such as YAG:Ce3+) with the formula unit X3Y2(ZO4)3 belonging to the Ia-3d space group,7) wherein the three crystallographic cation sites, namely X, Y, and Z, form eight-, six-, and four-coordinations to form a dodecahedron, octahedron, and tetrahedron, respectively.8) Moreover, Ce3+-based garnet phosphors have shown interesting optical properties such as broad emission in the visible region, ranging from green (Ca3Sc2(SiO4)3:Ce3+) to yellow (YAG:Ce3+) to orange-red (Lu2CaMg2(Si,Ge)3O12:Ce3+).9-11) The large crystal field splitting of the 2D level of Ce3+ ion in the garnet structure produces emission and excitation at relatively longer wavelengths, especially in the visible region, rendering it an exceptional phosphor host for WLED lighting.10)
A novel aluminate garnet Ca2GdZr2(AlO4)3:Ce3+ phosphor was recently reported with broad absorption ranging from the UV to blue regions and broad cyan emission peaking at 510 nm. Tuning of the emission properties of Ca2MZr2(AlO4)3:Ce3+ (M = Lu3+, Y3+, Gd3+) phosphor from blue (480 nm) to cyan (500 nm) with the increase in the ionic radii of M3+ ion was reported.12) The splitting of the cubic crystal field 2Eg energy level in Ca2MZr2(AlO4)3:Ce3+ increased with the increase in the ionic radii of M3+, which dominated the spectroscopic red-shift emission.12) Based on the Ca2GdZr2(AlO4)3:Ce3+ phosphor, with a maximum emission wavelength of 500 nm, Zr4+ of the Y site was replaced with Sc3+, and Ca2+ of the X site was replaced with Gd3+ to obtain the CaGd2ZrSc(AlO4)3:Ce3+ (CGZSA:Ce3+) phosphor with a maximum emission wavelength of 545 nm. The yellow emission in the CGZSA:Ce3+ phosphor originates from the increase in the splitting of the 2Eg energy level owing to the decrease in the covalent character of Ce-O.12) A new yellow-emitting CaGd2ZrSc(AlO4)3:Ce3+ (CGZSA:Ce3+) garnet phosphor under blue excitation has been reported. However, the effect of Ce3+ concentration on the structural and optical properties of the CGZSA:Ce3+ phosphor has not been reported yet.
In this study, Ce3+-doped aluminate garnet phosphors containing zirconium were synthesized using the solid-state reaction method. To understand the detailed ionic distribution in these garnet crystals, the structure of the representative compound CGZSA:Ce3+ was determined. The photoluminescence (PL) properties of these CGZSA:Ce3+ phosphors were presented and analyzed based on the crystal structure. The thermal stability and quantum efficiency were investigated in detail.

2. Experimental Procedure

Different Ce3+ concentrations of CaGd2−xZrSc(AlO4)3:xCe3+ (abbreviated as CGZSA:xCe3+) yellow-emitting phosphors were synthesized using the solid-state reaction method with CaCO3 (Aldrich, 99.99%), Gd2O3 (Aldrich, 99.9%), ZrO2 (Aldrich, 99.99%), Sc2O3 (Kojundo, 99.9%), α-Al2O3 (Kojundo, 99.99%), and CeO2 (Kojundo, 99.99%) as the precursors. The precursors in stoichiometric ratios were mixed using an agate mortar for 30 min, with acetone as the dispersing medium, to obtain homogeneous mixtures. The mixtures were placed in an alumina crucible and sintered at 1450°C for 4 h at a heating rate of 5°C/min under a reducing atmosphere (N2/H2 = 95%: 5%) in a tube furnace. Subsequently, the samples were gradually cooled to room temperature in the furnace and, finally, these samples were ground into powders to obtain a series of samples of CGZSA:xCe3+ with different Ce3+ concentrations (x = 0.01, 0.02, 0.04, 0.05, 0.06, and 0.07).
The composition and phase purity of the samples were identified using X-ray diffraction (XRD). XRD patterns were obtained using CuKα radiation (Philips X’Pert) over the angle range of 10° ≤ 2θ ≤ 100°. Diffraction data were analyzed via Rietveld refinement using the general structure analysis system (GSAS) software.13) Room-temperature PL excitation and emission spectra were obtained using a Hitachi F-4500 fluorescence spectrophotometer in the wavelength range of 300 - 700 nm. The thermal quenching (TQ) characteristics were measured in the temperature range of 25 - 200°C by connecting the Hitachi F-4500 fluorescence spectrometer to an integrated heater, temperature controller, and thermal sensor. At the temperature of −196°C, low-temperature PL spectra were obtained using a Hitachi F-7000 luminescence spectrophotometer under excitation at 325 nm connected to the ARS-cryostat system at the Korea Photonic Technology Institute (KOPTI), Republic of Korea. The fluorescence decay curve was obtained using a Horiba Fluorolog-3 with a 450 nm LED. The quantum yield (QY) of the phosphors was measured using a QY measurement system (Hamamatsu C9920-02) at the KOPTI, Republic of Korea.

3. Results and Discussion

Figure 1(a) shows the results of the Rietveld refinement of the XRD data profiles of CGZSA:0.04Ce3+. The crystal structure of Ca2GdZr2(AlO4)312) (CGZSA) was considered as the starting model for the Rietveld refinements. The residual factor was Rwp = 2.60% and the goodness-of-fit parameter (χ2) was 2.261. When Ce3+ was doped into the CGZSA host lattice, the obtained sample consisted of a single phase with no impurity phases present. The diffraction pattern revealed a general cubic garnet-type structure belonging to the space group of Ia-3d (#230) with cations in special positions (24c, 16a, and 24d sites) and oxygen anions in general positions (96h site). Crystallographic parameters from the Rietveld refinement provided the cell parameters of a = b = c = 12.46417(5) Å. The structural parameters, such as the interatomic distance and the bond valence sums (BVS)14) are listed in Tables 1, 2, and 3. The structure has many shared edges between adjacent polyhedrals. In the CGZSA:0.04Ce3+ structure, the AlO4 tetrahedron and the (Zr/Sc)O6 octahedron share their edges with two and six triangular (Ca/Gd/Ce)O8 dodecahedra, respectively [Fig. 1(b)]. The triangular dodecahedron shares its edges with two tetrahedra, four octahedra, and four other triangular dodecahedra. Tetrahedra and octahedra are linked through the sharing of all corners.15)
When Ce3+ ions are incorporated into the crystal structure of CGZSA, Ce3+ ions may substitute at all cationic sites, i.e., Ca2+, Gd3+, Zr4+, Sc3+, and Al3+. However, considering their corresponding ionic radius and allowed oxygen-coordination number (n), i.e., Ca2+ (1.12 Å, n = 8), Gd3+ (1.053 Å, n = 8), Zr4+ (0.72 Å, n = 6), Sc3+ (0.745 Å, n = 6), and Al4+ (0.39 Å, n = 4), it is difficult for Ce3+ ions (1.01 Å, n = 6 and 1.143 Å, n = 8) to substitute for Zr4+, Sc3+, and Al3+ ions.16) The BVS of the structure refinement of CGZSA:0.04Ce3+ suggested that the BVS for Ca2+ is close to the expected value, and the determined Gd3+/Ce3+ site provides a slightly underbonded location for Gd3+ with BVS = 2.747, whereas Ce3+ is lightly overbonded with BVS = 3.153. This explains the preference of Ca, Gd, and Ce for the 24c site; placing all the Gd and Ce at the 24c site with Ca provides sensible atomic displacement parameters for this site.
Figure 2 shows the powder XRD patterns of the samples with 0.01 ≤ x ≤ 0.07 in CGZSA:xCe3+ and the simulated XRD pattern of CGZSA from the Rietveld refinement data for comparison. The XRD patterns of the samples are consistent with their corresponding simulated XRD patterns. Although the ionic radius of Gd3+ (1.053 Å, n = 8) is smaller than that of Ce3+ (1.143 Å, n = 8), the peak of XRD did not shift as the amount of Gd3+ replaced by Ce3+ increased, as shown in Fig. 2(b). This result shows that the amount of Ce3+ ions was only slightly substituted for and no significant change was observed.
Figure 3(a) presents the excitation and emission spectra of the CGZSA:xCe3+ (x = 0.01 - 0.07) phosphors. The excitation band was observed at 450 nm. The exhibited shapes of the excitation and emission bands decrease in intensity and there is a red-shift of the maximum wavelength with an increase in the Ce3+ concentration due to reabsorption of emitted photons by the activator. Fig. 3(b) shows the excitation and emission spectra of the optimized CGZSA:0.04Ce3+ under excitation at 440 nm at room temperature. The excitation spectrum was separated into two excitation bands of Ce3+-a 5d1 band between 370 and 500 nm, and a 5d2 band level between 300 nm and 360 nm-which are assigned the two lowest 5d levels of Ce3+ and which are attributed to the crystal field splitting. The intensity of the 5d2 band was less than two times that of the 5d1 band owing to symmetry selection rules.17) Similarly, the PL spectrum involves a broad asymmetric emission band related to the spin-allowed d-f transition of Ce3+ with its maximum at the wavelength of 541 nm under excitation at 440 nm.
The dependence of the emission intensity on the Ce3+ substitution is shown in Fig. 3(c). We observed that the optimum substitution of Ce3+ in CGZSA:xCe3+ was x = 0.04. When x exceeds 0.04, a decrease in the relative emission intensity was observed owing to concentration quenching. Generally, concentration quenching is mainly caused by non-radiative energy transfer processes from one Ce3+ ion to another Ce3+ ion. Non-radiative energy transfer usually occurs as a result of exchange interaction, radiation reabsorption, or electric multipolar interaction.18) According to the Dexter theory, the mechanism of radiation re-absorption occurs only when there is a broad overlap of excitation and emission spectra. To further investigate the concentration quenching mechanism of the CGZSA:0.04Ce3+ phosphor, the critical transfer distance (Rc) was roughly estimated. In this case, to further determine the energy transfer mechanism, Rc between Ce3+ activators can be estimated using the following formula: 19)
(1)
RC2(3V4πXCN)1/3
where V corresponds to the volume of the unit cell, N is the number of total Ce3+ sites per unit cell, and Xc is the critical concentration of dopant ions. For the CGZSA:0.04Ce3+ phosphor, on the basis of the structural parameters, we used the values V = 1936.378 Å3, N = 24, and Xc = 0.04 (see Table 1). Rc for the energy transfer in the CGZSA:Ce3+ phosphor was calculated and found to be approximately 15 Å.
Based on the spectral data of the CGZSA:0.04Ce3+ phosphor, we also used the Dexter formula, expressed as follows, to calculate Rc. The formula represents a confined transfer of electric dipole-dipole interaction, and is suitable because we herein assumed dipole-allowed transitions in the case of Ce3+. The probability of transfer of dipole-dipole interaction has been given by Blasse:19,20)
(2)
RC6=0.63×10284.8×10-16PE4fs(E)FA(E)dE
where P is the oscillator or strength of the Ce3+ ion, E is the energy of the maximum spectral overlap, and the integral represents a spectral overlap, which is the product of the normalized spectral shapes of the emission and excitation. The values of E and ∫fs(E)FA(E)dE were derived from the spectral data in Fig. 2(a) as 2.53 eV and 4.8 × 10−2 eV−1, respectively. The value of P corresponding to the broad 4f-5d absorption band was obtained as 10−2 from Blasse. From equation (2), the value of Rc for the energy transfer in the CGZSA:0.04Ce3+ phosphor was calculated and found to be 18 Å, which is close to the value of 15 Å obtained using the concentration quenching data. According to Blasse, the value of Rc for the general exchange interaction is ~ 5 Å.18) Therefore, the exchange interaction can be neglected in the energy transfer within the CGZSA:0.04Ce3+ phosphor.
Via structural analysis, we confirmed that the Ce3+ ions occupied only the Gd site and, via PL measurement at low temperature (−196°C), we confirmed that they generated two peaks in the PL spectra due to the variation of the emission spectra, as shown in Fig. 4. The emission band of the phosphor was deconvoluted into two Gaussians profiles with peaks centered at 528 nm (18938 cm−1) and 581 nm (17222 cm−1). The difference between the two values is 1716 cm−1. Essentially, the Ce3+ ion, with 4f1 ground state configuration of 2F5/2 and 2F7/2, allows two levels whose maxima were isolated in a range of 1600 to 2000 cm−1 as a result of spin-orbit coupling.18) Therefore, using the two Gaussians for the deconvolution of the emission peak resulted in a reasonable value of fitting, as shown in Fig. 4.
The lifetime of the CGZSA:0.04Ce3+ phosphor was calculated by analyzing the decay curves presented in Fig. 5. The normalized decay profile of the CGZSA:0.04Ce3+ phosphor was measured under excitation at 450 nm and by monitoring the emission peak at 541 nm. It was observed that the decay curve fitted well with second-order exponential decay, which can be obtained using the equation:
(3)
I(t)=I0+A1exp(-t/τ1)+A2exp(-t/τ2)
where I(t) is the luminescence intensity, t is the time, A1 and A2 are constants, and τ1 and τ2 are the decay times of the exponential components. The decay profile of the CGZSA:0.04Ce3+ phosphor shows rapid components (τ1 = 5.1 ns) and slow time (τ2 = 37.2 ns). Further, 5d→4f fluorescence transitions of the Ce3+ ions are the allowed electric dipole-dipole and their fluorescence lifetimes are in the time range of 10 - 100 ns owing to local changes of the crystal field.21) The presence of the fast component indicates the existence of a non-radiative process from the excited Ce3+ to the quenching centers, such as structural defects and local distortions in crystals.22) The slow component is in accordance with the intrinsic lifetime of Ce3+ in the CGZSA: 0.04Ce3+ phosphor.
The TQ property is an important parameter for the practical applications of phosphors. Fig. 6(a) shows the thermal quenching characteristics of the CGZSA:0.04Ce3+ phosphor in the temperature range of 25 to 200°C. It can be observed that the emission intensity of CGZSA:0.04Ce3+ decreases rapidly with the increase in temperature, and only approximately 46% of the emission intensity recorded at room temperature remains at 100°C. Furthermore, it can be observed that, with the increase in temperature, the emission intensity decreases gradually and the emission band changes from two apparent asymmetric broad peaks to one definite broadband. In order to explain the large TQ behavior of the CGZSA:0.04Ce3+ phosphor, two possible models in Ce3+-doped phosphors are considered, as follows. One model, the well-known non-radiative relaxation model, is explained by using a configurational coordinate diagram (CCD), wherein the excited luminescence center is thermally activated through phonon interaction and released through the crossing point between the excited and ground states. This non-radiative transition probability via thermal activation is strongly dependent on the temperature, and results in a decrease in the emission intensity.23) Owing to the increasing phonon interaction and the non-radiative transition with the increase in temperature, the spectral overlap between the excitation band and the first emission band increases. The other model is the thermal ionization model.24) Thermal ionization refers to a thermally activated electron transfer process to the conduction band.
Further, in order to investigate the TQ characteristics, the activator energy (Ea) was calculated using the Arrhenius equation, shown below:25)
(4)
I(T)=I01+Aexp(-EkT)
where I0 is the initial intensity, I(T) is the intensity at a given temperature T, A is a constant, E is the activation energy for TQ, and k is the Boltzmann constant. Through the best fit using the Arrhenius equation, the activation energy (E) was obtained as 0.22 eV for the CGZSA:0.04Ce3+ phosphor, as shown in Fig. 6(b). The activation energy of the YAG:Ce3+ phosphor was determined to be approximately 0.77 eV.24) This indicates that the CGZSA:0.04Ce3+ phosphor has a cross-point in the CCD lower than that of the YAG: Ce3+ phosphor. The thermal emission stability decreases owing to the reduction of the energy displacement between the host conduction band and the Ce3+ 5d band levels. Moreover, the host, which includes Gd ions, has a large non-radiative transition in the garnet structure owing to the weak crystal structure of the host and, hence, the energy relaxation from the excited state to the ground state has a large loss due to lattice vibration.26) Further, the value of QY of the CGZSA:0.04Ce3+ phosphor, measured at room temperature, was 20%. The value of QY is also related to ΔEa; hence, it would be lower in the phosphor with stronger luminescence TQ.

4. Conclusions

Yellow CGZSA:xCe3+ phosphors were successfully prepared using a solid-state reaction. The phase purity was characterized using XRD analysis. Under excitation at 440 nm, the CGZSA:0.04Ce3+ phosphor showed a bright yellow emission peak at approximately 541 nm. The PL properties, QY, and thermal stability of the CGZSA:0.04Ce3+ phosphor were investigated in detail to evaluate their use in LEDs. Therefore, it can be concluded that the yellow CGZSA:xCe3+ phosphor is a promising material that can be employed in pc-WLEDs for solid-state lightning.

Acknowledgments

This work was financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1A2B3011967).

Fig. 1
(a) Rietveld refinement of the powder X-ray diffraction profile of CaGd2ZrSc(AlO4)3:0.04Ce3+ phosphor. Data (points), fit (lines), difference profile, and expected reflection positions are displayed. (b) Schematic of the crystal structure of the CaGd2ZrSc(AlO4)3:0.04Ce3+ phosphor viewed in the [111] direction.
jkcs-54-5-422f1.gif
Fig. 2
(a) X-ray diffraction pattern of CaGd2−xZrSc(AlO4)3:xCe3+ (x = 0.01 - 0.07) phosphors. As a reference, the simulated XRD for CaGd2ZrSc(AlO4)3:0.04Ce3+ is shown. (b) Magnified XRD pattern in the region between 31.5° and 32.5° for the CaGd2−xZrSc(AlO4)3:xCe3+ phosphor as a function of Ce3+ concentration.
jkcs-54-5-422f2.gif
Fig. 3
(a) Excitation and emission spectra of CaGd2−xZrSc(AlO4)3:xCe3+ (x = 0.01 - 0.07) phosphors. (b) Excitation and emission spectra of the CaGd2ZrSc(AlO4)3:0.04Ce3+ phosphor. (c) Relative emission intensity as a function of Ce3+ substitution, x.
jkcs-54-5-422f3.gif
Fig. 4
Deconvoluted PL emission spectra obtained using two Gaussian equations for the CaGd2ZrSc(AlO4)3:0.04Ce3+ phosphor at −196°C (λex = 375 nm).
jkcs-54-5-422f4.gif
Fig. 5
Decay curve of Ce3+ emission in the CaGd2ZrSc(AlO4)3:0.04Ce3+ phosphor under excitation at 450 nm, monitored at 541 nm.
jkcs-54-5-422f5.gif
Fig. 6
(a) PL spectra of the CaGd2ZrSc(AlO4)3:0.04Ce3+ phosphor under various temperatures (25 - 200°C) and (b) plots of fitted activation energy for thermal quenching.
jkcs-54-5-422f6.gif
Table 1
Rietveld Refinement and Crystal Data of CaGd2ZrSc(AlO4)3:0.04Ce3+ Obtained Using X-ray Diffraction
Formula CaGd1.96Ce0.04ZrSc(AlO4)3
radiation type CuKα
T/K 295
2θ range (degree) 10 - 100
symmetry cubic
space group Ia-3d (#230)
a, b, c 12.46417(5)
volume/Å3 1936.378(23)
Z 8
Rp 1.74%
Rwp 2.60%
χ2 2.261
Table 2
Refined Structural Parameters for CaGd2ZrSc(AlO4)3:0.04Ce3+ Derived from Rietveld Refinement Using X-ray Powder Diffraction Patterns at Room Temperaturea
atom Wyck. x y z gb 100 × Uisoc2
Ca 24c ¼ 0 0.330 0.603
Gd 24c ¼ 0 0.656 0.603
Ce 24c ¼ 0 0.014 0.603
Zr 16a ¼ ¾ ¼ 0.500 0.525
Sc 16a ¼ ¾ ¼ 0.500 0.525
Al 24d ¼ 0 1 0.396
O 96h 0.3447(4) 0.4652(23) 0.0530(28) 1 0.426

a The numbers in parentheses are the estimated standard deviations of the last significant figure.

b Constraint on occupancy: g(Ca) + g(Gd) + g(Ce) = 1 and g(Zr) + g(Sc) = 1.

c Constraint on isotropic thermal factor: Uiso(Ca) = Uiso(Gd) = Uiso(Ce) and Uiso(Zr) = Uiso(Sc).

Table 3
Selected Bond Lengths (Å) and Bond Valence Sums for CaGd2ZrSc(AlO4)3:0.04Ce3+a
dodecahedron
Gd/Ca/Ce-O 2.4080(34)
Gd/Ca/Ce-O3) 2.4080(34)
Gd/Ca/Ce-O9) 2.4080(34)
Gd/Ca/Ce-O11) 2.4080(34)
Gd/Ca/Ce-O7) 2.5217(32)
Gd/Ca/Ce-O1) 2.5217(32)
Gd/Ca/Ce-O10) 2.5217(32)
Gd/Ca/Ce-O13) 2.5217(32)
BVS of dodecahedron Ca = 2.017, Gd = 2.747, Ce = 3.153

octahedron

Zr/Sc-O1) 2.090(5)
Zr/Sc-O2) 2.090(5)
Zr/Sc-O3) 2.090(5)
Zr/Sc-O4) 2.090(5)
Zr/Sc-O5) 2.090(5)
Zr/Sc-O6) 2.090(5)
BVS of octahedron Zr = 3.970, Sc = 3.128

tetrahedron

Al-O 1.759(4)
Al-O8) 1.759(4)
Al-O9) 1.759(4)
Al-O12) 1.759(4)
BVS of tetrahedron Al = 2.956

a Symmetry transformations used to generate equivalent atoms: 1)+y+3/4, +x+1/4, −z+1/4, 2)x+3/4, −z+3/4, −y+3/4, 3)+z+1/4, −y+1/4, +x+3/4, 4)+x+3/4, +z+3/4, +y+3/4, 5)z+1/4, +y+1/4, −x+3/4, 6)y+3/4, −x+1/4, +z+1/4, 7)z+1/2, −x, +y+1/2, 8)z+1/4, −y+3/4, +x+3/4, 9)x+1/2, +y, −z, 10)+z, −x, −y+1/2, 11)z+1/4, −y+1/4, −x+1/4, 12)+z+1/4, −y+3/4, −x+1/4, 13)y+3/4, +x+1/4, +z+3/4

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