A Novel Garnet Phosphor for White LEDs - ACS Publications

Article pubs.acs.org/JPCC

Synthesis, Structure, and Photoluminescence Properties of Ce3+Doped Ca2YZr2Al3O12: A Novel Garnet Phosphor for White LEDs Xicheng Wang and Yuhua Wang* Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, Department of Material Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, P. R. China ABSTRACT: A novel garnet phosphor Ca2YZr2Al3O12:Ce3+ (CYZA:Ce3+) has been successfully designed and synthesized via the solid state method. The crystal structure, morphology, as well as their photoluminescence properties were investigated in detail. Under nearultraviolet (n-UV) excitation, CYZA:Ce3+ exhibits cyan to green emission with the maxima from 484 to 503 nm when varying the Ce3+ concentration. The internal quantum efficiency of the optimal sample is 56%. The concentration quenching of Ce3+ emission occurs via the energy transfer among the nearest-neighbor ions. The thermal stability is superior compared with the Ba2SiO4:Eu2+ commercial phosphor. These results suggest that CYZA:Ce3+ can be considered as a potential candidate for white LEDs.

1. INTRODUCTION During the past decades, white light-emitting diodes (W-LEDs) have received great attention owing to their excellent properties such as long lifetime, high energy efficiency, and ecofriendliness.1,2 They are considered to be the next generation solid-state lighting source to replace the conventional incandescent and fluorescent lamps.3 At present, in practical application white light is most commonly obtained by fabricating the blue LED chips together with the yellowemitting phosphors.4,5 However, this method shows a disadvantage of low color rendering index due to the lack of blue-green emission and red emission. In order to solve this problem, extensive efforts have been made by using nearultraviolet (n-UV) chips combining with a mixture of red, green, and blue phosphors.6 Nevertheless, it has poor efficiency caused by the large Stokes shift between excitation and emission in the n-UV excitable phosphor. Thus, it is necessary to find novel phosphors matching well with the n-UV LED chips; i.e., phosphors should have high efficiency and high stability under n-UV excitation. In recent decades, garnet has been extensively investigated as a host material due to its outstanding physical and chemical stabilities. Ce3+-doped garnet phosphors exhibit excellent luminescence properties, such as the most frequently used yellow phosphor YAG:Ce3+, green phosphors Ca3Sc2Si3O12:Ce3+, Lu3Al5O12:Ce3+ (for blue LED chips), etc.7−9 Garnet structure belongs to the Ia3̅d space group with a general formula {C}3[B]2(A)3O12, where cations of C, B, and A © XXXX American Chemical Society

are surrounded by eight, six, and four nearest O anions, forming coordination polyhedron of dodecahedron, octahedron, and tetrahedron, respectively. C is usually occupied by rare earth metal, alkaline earth metal, and alkali metal, for which Ce3+ substitution may occur. It is worth noting that the emission color of Ce3+ varies from green to yellow and even to yelloworange in different hosts with different cation compositions.10 Among these Ce3+-doped garnet phosphors, however, there are rare reports about phosphors emitting blue to green colors, which are essential parts to realize full-color emission in WLEDs as well. To find novel efficient blue-green emitting phosphors applied in n-UV LEDs from readily available raw materials, a new garnet Ca2YZr2Al3O12 (CYZA) has been designed as the host for Ce3+ ions doping in this study. Meanwhile, Huang and Zhuang et al. respectively discovered that Ce3+ ions exhibited highly efficient photoluminescence in its analogues of Ca2GdZr2Al3O12 and Ca2LaZr2Ga3O12.11,12 Our study is motivated by the following two aspects: on one hand, investigating interplay between structure and luminescence properties in depth; on the other hand, exploring its potential application in W-LEDs. The CYZA is synthesized by the solidstate method, and the crystal structure has been ascertained via the means of Rietveld refinement. The morphology and Received: February 15, 2015 Revised: June 22, 2015

A

DOI: 10.1021/acs.jpcc.5b01552 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C photoluminescence properties of CYZA have also been investigated in detail.

2. EXPERIMENTAL SECTION Preparation. The CYZA:Ce3+ samples were synthesized through a conventional solid-state method. Briefly, the starting raw materials CaCO3 (A.R.), Y2O3 (A.R.), ZrO2(A.R.), Al2O3(A.R.), and CeO2 (99.995%) were weighed in stoichiometric proportions, and 3 wt % CaF2 was added as the flux. The raw materials were finely ground and then sintered in boron nitride (BN) crucibles at 1400 °C for 5 h under flowing gas of N2−NH3 in a tube furnace. The products were then cooled to room temperature in the furnace, ground, and pulverized for further measurements. Characterization. The phase purity of samples was analyzed by X-ray diffraction (XRD) using a Bruker D2 PHASER X-ray diffractometer with graphite monochromator using Cu Kα radiation (λ = 1.54056 Å), operating at 30 kV and 15 mA. Rietveld refinement13 was performed using the software GSAS.14,15 The morphology of the sample was obtained using scanning electron microscopy (SEM; Hitachi S-4800). The element composition was analyzed using an EDX detector attached to a scanning electron microscope (SEM, TESCSN MIRA 3 XMU). Electron diffraction and high-resolution transmission electron microscopy (HRTEM) were performed with an FEI Tecnai F30 transmission electron microscope (TEM) operated at 300 kV. Reflectance spectra were measured on a PE lambda950 UV−vis spectrophotometer, and the BaSiO4 white power was used as the reference. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at room temperature by a FLS-920T fluorescence spectrophotometer (Edinburgh Instruments) equipped with a 450 W Xe light source and double excitation monochromators. Hightemperature luminescence intensity measurements were performed by an aluminum plaque with cartridge heaters; the temperature was obtained by thermocouples inside the plaque and controlled with a standard TAP-02 high-temperature fluorescence controller (Orient KOJI instrument Co., Ltd.). The quantum efficiency was obtained using a Fluorlog-3 spectrofluorometer equipped with a 450 W xenon lamp (Horiba Jobin Yvon).

Figure 1. XRD patterns for CYZA:xCe3+ samples with different Ce3+ concentrations (0 ≤ x ≤ 0.07).

Figure 2. Rietveld refinement XRD patterns of the CYZA host.

Table 1. Refined Structural Data of CYZA

3. RESULTS AND DISCUSSION Phase and Crystal Structure. Figure 1 depicts the XRD patterns for the CYZA samples with different concentrations of Ce3+ (0 ≤ x ≤ 0.07). The diffraction data of the CYZA samples can be indexed to the Ia3̅d space group of the cubic system. No diffraction peaks of impurities or raw materials are observed. The crystal structure of CYZA was analyzed using the Ca3Hf2(FeAlSi)O12 garnet compound as the starting model.16 Figure 2 presents the comparison between experimental and calculated diffraction patterns and the difference profile of Rietveld refinement for the CYZA host as well. The refinement is convergent well with low residual factors Rwp = 7.98% and Rp = 6.44%. The final refined crystallographic data are listed in Table 1. The cell parameters were obtained to be a = b = c = 12.4826(2) Å and V = 1945.00(3) Å3. The crystallographic site coordinates, site occupancy factors, and equivalent isotropic displacement parameters are listed in Table 2. Ca2+ and Y3+ are randomly distributed on the 24c site with the Ca:Y ratio of 2:1.

formula

Ca2YZr2Al3O12

crystal system space group lattice parameters a = b = c (Å) α = β = γ° cell volume (Å3) Z calculated density R-factors Rwp Rp

cubic Ia3̅d (No. 19) 12.4826(2) 90 1945.00(3) 8 4.26466 g/cm3 0.0798 0.0644

Figure 3 displays the crystal structure and local coordination surroundings of CYZA. Cations including Ca2+/Y3+, Zr4+, and Al3+ fully occupy the 24c, 16a, and 24d Wyckoff sites, connecting eight, six, and four surrounding O2−, respectively. As shown in Figure 3b, the [ZrO6] octahedrons and [AlO4] tetrahedrons share corners or edges with [(Ca/Y)-O8] dodecahedra, forming a three-dimensional network.12 Figure 4a exhibits the representative SEM image of the CYZA:0.03Ce3+ sample. The grains show obvious agglomeration with polyhedron shape morphology, and the size is in the range of 2−5 μm, corresponding to the low-magnification TEM image presented in Figure 4b. The EDX spectrum (Figure 4d) demonstrates that the Ca, Y, Zr, Al, and O elements constitute B


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The Journal of Physical Chemistry C Table 2. Atomic Positions and Isotropic Displacement Parameters for CYZA atom

Wyckoff

S.O.F

x

y

z

Uiso

Ca Y Zr Al O

24c 16a 24d 24c 96h

0.6667 0.3333 1 1 1

1/8 1/8 0 1/8 −0.03393(26)

0 0 0 0 0.0521(4)

0 0 0 3/4 0.15542(32)

0.0062(16) 0.0250 0.0178(6) 0.0158(15) 0.0255(14)

Figure 3. (a) Crystal structure of CYZA along the b axis and (b) coordination surroundings of Ca/Y, Zr, and Al cations in the lattice.

Figure 5. Diffuse reflectance spectra of the CYZA host and CYZA:0.03Ce3+.

that of Ce3+-doped samples is green. Hence, from the UV to visible range the undoped CYZA exhibits a high reflectance, while in the range of 260−310 nm, it exhibits a decrease, corresponding to the transition from the valence to conduction band of the host lattice. Compared with the undoped CYZA, except for the host absorption, the Ce3+-doped CYZA displays two more pronounced absorption bands peaking at about 360 and 410 nm, respectively. The two absorption bands could be attributed to the transition of Ce3+ from the 4f ground state to the spliting 5d excitation state. This is also consistent with the green body color of the Ce3+-doped CYZA. Figure 6 shows the excitation and emission spectra of CYZA:0.03Ce3+. The excitation spectrum exhibits three bands centering at 409, 340, and 277 nm. The strongest band is asymmetric and consists of two peaks, which can also be decomposed into two Gaussian sub-bands with the peaks locating at 390 and 415 nm, respectively. Among all the bands, Figure 4. (a) SEM image, (b) TEM image, (c) HRTEM image with electron diffraction pattern, and (d) EDX spectrum of CYZA:0.03Ce3+.

the sample, and no impurity elements are detected. HRTEM and selected-area diffraction patterns (SAED) were conducted to further understand the crystal structure of CYZA. Figure 4c shows the HRTEM image of CYZA with SAED pattern. The measurements show that the interplanar spacing is 0.4353 nm, corresponding well to the (022) interplanar distance of the cubic CYZA. The SAED indicates the body centered cubic (bcc) polycrystalline structure of CYZA. The rings can be indexed to the corresponding lattice planes, of which interdistances match well with the results of Rietveld refinement. These results also suggest that well-crystallized CYZA:Ce3+ powders have been obtained. Photoluminescence Properties. Figure 5 exhibits the reflectance spectra of the CYZA host and CYZA:0.03Ce3+ samples. The body color of the CYZA host is white, while

Figure 6. Excitation and emission spectra of CYZA:0.03Ce3+. C


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The Journal of Physical Chemistry C the 277 nm is ascribed to the absorption of the host, and the others mainly arise from the 4f05d1 multiplets of the Ce3+ excited states (340, 390, and 415 nm). This also corresponds well to the reflectance spectra discussed above. The center of gravity of the 5d energy level, estimated by averaging the three excitation sub-bands,17 is 24 096 cm−1. In different inorganic hosts, the lowest 5d energy level for the Ce3+ ions locates variously. Two separate factors determine the energy position: on one hand, the nephelauxetic effect results in the shift in the 5d centroid from the free ion levels; on the other hand, the crystal field causes the 5d manifold split into certain widths.18 In the C site of the garnet structure, where the cubic coordinations are distorted into a dodecahedron with D2 symmetry, the 5d orbitals of Ce3+ are split into two levels, T2g and Eg. The 5d barycenter shift, εc, in CYZA can be estimated via the following equation in consideration of the (Ca/Y)−O bond lengths derived from the Rietveld refinement and the average (Ca/Y) cation electronegativity.19 n

εc = 1.79 × 1013 ∑ i=1

αsp = 0.33 +

Figure 7. Emission spectra of CYZA:xCe3+ with varying Ce3+ concentration. Inset: the relationship between the emission peak with width (fwhm) of the emission bands and the Ce3+ concentration in CYZA:xCe3+ (0 ≤ x ≤ 0.07).

αspi (R i − 0.6Δr )6

4.8 χav2

(1)

valence state of the activator Ce3+ ion; N is equivalent to the number of O2− of a surrounding Ce3+ ion; r is the ionic radius of the host cation of which the activator Ce3+ ion substitutes for; Ea represents the electron affinity of the anions around the Ce3+ ion (eV) (in this case the anions are O2−, and its value of Ea is 1.6 eV).22 When the coordination number is 8, the radii of Ca2+, Ce3+, and Y3+ are 1.12, 1.14 and 1.02 Å, respectively.25 It can be calculated that the emission peaks of Ce3+ are 19 930 cm−1 (501 nm) and 18 802 cm−1 (531 nm) when the occupancy site is the Ca2+ or Y3+ ion, respectively. Considering the ionic radii factor and the observed emission, the broad asymmetric emission band could mainly arise from the occupation of Ca2+ sites by Ce3+ ions. The increase of Ce3+ concentration also strengthens the interplay between the activator Ce3+ and the host lattice, causing the emission band to broaden.21,26 Figure 8 shows the emission intensity changes as a function of Ce3+ concentration in the CYZA: xCe3+ series under 410 nm excitation. The optimal Ce3+ dopant concentration was 3 mol %, of which the internal quantum efficiency was measured to be 56%. As the concentration of Ce3+ increases, the possibility of nonradiative energy transfer among Ce3+ ions increases.27 So

(2) 3+

where Ri is the distance of the Ce anion; n is the coordination number of anions around Ce3+; χav refers to the weighted average of the cation electronegativity in a given oxide host lattice; and Δr represents the difference of ionic radii between Ca2+/Y3+ and Ce3+. Therefore, the 5d centroid shift is estimated to be 17 643 cm−1, which is ∼3300 cm−1 larger compared to LuAG.11 Meanwhile, the value of 10Dq,10 the splitting between the barycenter of T2g and Eg, is calculated to be 15 151 cm−1, or ∼3600 cm−1 smaller than that of LuAG. This implies that the redshift of Ce3+ emission in CYZA mainly arises from the nephelauxetic effect, while in LuAG it is mostly ascribed to the crystal field inducements.20 The emission spectrum exhibits a broad band centering at 495 nm, of which the full width at half-maximum (fwhm) is 98 nm. This can be attributed to the 5d → 4f transition of the Ce3+ ion.21 Figure 7 shows the emission spectra of CYZA: xCe3+ with various Ce3+ concentrations (0 ≤ x ≤ 0.07). As the Ce3+ concentration increases, the emission maximum shifts to longer wavelength together with the width becoming larger (Figure 7, inset). The peak position of the emission band red-shifts from 484 to 503 nm. The red-shift phenomenon could arise from the energy transfer from the Ce3+ ions at higher 5d energy levels to those at the lower energy levels, causing the energy from the 5d excited state to the 4f ground state to decrease; i.e., the emission energy decreases. Thus the emission energy decreases, and the emission shifts toward longer wavelength.22,23 The emission band of the Ce3+ ions is strongly affected by its coordination environment. According to Van Uitert’s reported empirical relation,24 we can estimate the position of the Ce3+ emission via the equation as follows ⎡ ⎤ ⎛ V ⎞1/ V E = Q ⎢1 − ⎜ ⎟ 10−NEar /80⎥ ⎝4⎠ ⎢⎣ ⎥⎦

(3) Figure 8. Emission intensity of CYZA:xCe3+ (0 ≤ x ≤ 0.07) with different Ce3+ concentration. Inset: the curve of log(I/x) versus log(x).

Q relates to the energy position of the lower d-band edge for the free ion (Q = 50 000 cm−1 for Ce3+); V represents the D


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The Journal of Physical Chemistry C when the Ce 3+ concentration exceeds 3 mol %, the concentration quenching occurs. The critical distance of energy transfer among Ce3+ ions in CYZA can be calculated according to the following equation28 ⎡ 3V ⎤1/3 R c = 2⎢ ⎥ ⎣ 4πxc z ⎦

(4)

where V refers to the unit cell volume; xc equals the optimal concentration of Ce3+; and z represents the formula unit number in the unit cell. For the CYZA host, when z = 8, xc = 0.03, and V = 1945.00(3) Å3, the obtained Rc value is 24.92 Å, while the distance of the nearest neighbor Ca/Y obtained from the crystal structure results is a lot shorter (3.822 Å), which makes the energy transfer much easier. Thus, it seems easy to understand the reason why concentration quenching occurs at a relatively low Ce3+ concentration in this case. Energy transfer between different Ce3+ ions can occur via radiation reabsorption, multipolar interactions, or exchange. As there is little overlap between the excitation band and the emission band, it seems that the reabsorption effect could be first eliminated. The interaction type between Ce3+ centers can be identified via the following equation according to Dexter’s theory.29 I k = x 1 + β(x)θ /3

Figure 9. CIE chromaticity diagram for CYZA:xCe3+ (0 ≤ x ≤ 0.07) phosphors with different Ce3+ dopant concentrations.

(5)

Here, x represents the concentration of the Ce3+ ion, and both β and k are constants for the given host under the same excitation conditions. On the basis of the results reported by Van Uitert, θ = 6, 8, and 10 stand for dipole−dipole, dipole− quadrupole, and quadrupole−quadrupole interactions, respectively, while θ = 3 corresponds to the exchange interaction.30 The inset of Figure 8 illustrates the relation between I/x and x. The curve of log (I/x) versus log (x) is found to be nearly linear with the fitted slope of −1.06. This corresponds to θ = 3, suggesting that the exchange interaction, i.e., the energy transfer among the nearest-neighbor ions, dominates the process of Ce3+ concentration quenching. The changes of CIE chromaticity coordinates of the CYZA:xCe3+ phosphors with various Ce3+ contents are listed in Table 3 and also presented in Figure 9. Table 3. Emission, fwhm, Normalized Emission Intensity, and the CIE Coordinates for CYZA:xCe3+ (0 ≤ x ≤ 0.07) Ce3+ concentration x x x x x x

= = = = = =

0.01 0.02 0.03 0.04 0.05 0.07

λem (nm)

fwhm of emission (nm)

normalized PL intensity (%)

484 493 495 497 498 503

95 96 98 100 101 105

88.75 96.74 100.0 92.16 85.59 78.47

CIE (x, y) (0.1739, (0.1842, (0.1957, (0.2063, (0.2114, (0.2312,

0.3007) 0.328) 0.3554) 0.3781) 0.3857) 0.4154)

Figure 10. (a) Temperature-dependent emission spectra of CYZA:0.03Ce3+ phosphor. (b) The emission intensity with peak position as a function of temperature. The commercial Ba2SiO4:Eu2+ was exhibited for comparison.

CYZA under 410 nm excitation. The emission intensity decreases with the temperature increasing from 25 to 250 °C. In the thermal quenching process, the activation energy can be expressed via the Arrhenius formula31

It can be observed clearly that the emission color of CYZA:xCe3+ phosphors changes from cyan to green when the Ce3+ content increases from 1% to 7%, while the chromaticity index varies from (0.1739, 0.3007) to (0.2312, 0.4154). Thermal quenching property of phosphors is an important index for the application in LEDs. Figure 10 presents the temperature-dependent photoluminescence of Ce3+-doped

⎛I ⎞ ΔE ln⎜ 0 ⎟ = ln A − ⎝I⎠ kT E

(6) DOI: 10.1021/acs.jpcc.5b01552 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

(5) Setlur, A. Phosphors for Led-Based Solid-State Lighting. Electrochem. Soc. Interface 2009, 16, 32. (6) Huang, C.-H.; Chiu, Y.-C.; Yeh, Y.-T.; Chan, T.-S.; Chen, T.-M. Eu2+-Activated Sr8znsc (Po4) 7: A Novel near-Ultraviolet Converting Yellow-Emitting Phosphor for White Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2012, 4, 6661−6668. (7) Lu, C. H.; Hong, H. C.; Jagannathan, R. Sol-Gel Synthesis and Photoluminescent Properties of Cerium-Ion Doped Yttrium Aluminium Garnet Powders. J. Mater. Chem. 2002, 12, 2525−2530. (8) Shimomura, Y.; Honma, T.; Shigeiwa, M.; Akai, T.; Okamoto, K.; Kijima, N. Photoluminescence and Crystal Structure of GreenEmitting Ca3sc2si3o12: Ce3+ Phosphor for White Light Emitting Diodes. J. Electrochem. Soc. 2007, 154, J35−J38. (9) Setlur, A.; Srivastava, A. On the Relationship between Emission Color and Ce 3+ Concentration in Garnet Phosphors. Opt. Mater. 2007, 29, 1647−1652. (10) Setlur, A. A.; Heward, W. J.; Gao, Y.; Srivastava, A. M.; Chandran, R. G.; Shankar, M. V. Crystal Chemistry and Luminescence of Ce3+-Doped Lu2camg2 (Si, Ge) 3o12 and Its Use in Led Based Lighting. Chem. Mater. 2006, 18, 3314−3322. (11) Gong, X.; Huang, J.; Chen, Y.; Lin, Y.; Luo, Z.; Huang, Y. Novel Garnet-Structure Ca2gdzr2 (Alo4) 3: Ce3+ Phosphor and Its Structural Tuning of Optical Properties. Inorg. Chem. 2014, 53, 6607−6614. (12) Zhong, J.; Zhuang, W.; Xing, X.; Liu, R.; Li, Y.; Liu, Y.; Hu, Y. Synthesis, Crystal Structures, and Photoluminescence Properties of Ce3+-Doped Ca2lazr2ga3o12: New Garnet Green-Emitting Phosphors for White Leds. J. Phys. Chem. C 2014, 119, 5562. (13) Rietveld, H. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (14) Larson, A. C.; Von Dreele, R. B. Gsas. General Structure Analysis System; LANSCE: MS-H805, Los Alamos, NM, 1994. (15) Toby, B. H. Expgui, a Graphical User Interface for Gsas. J. Appl. Crystallogr. 2001, 34, 210−213. (16) Berry, F. J.; Dávalos, J. Z.; Gancedo, J. R.; Greaves, C.; Marco, J. F.; Slater, P.; Vithal, M. Cation Distribution and Magnetic Interactions in Substituted Iron-Containing Garnets: Characterization by Iron-57 Mössbauer Spectroscopy. J. Solid State Chem. 1996, 122, 118−129. (17) Li, Y. Q.; De With, G.; Hintzen, H. The Effect of Replacement of Sr by Ca on the Structural and Luminescence Properties of the RedEmitting Sr 2 Si 5 N 8: Eu 2+ Led Conversion Phosphor. J. Solid State Chem. 2008, 181, 515−524. (18) Dorenbos, P. 5 D-Level Energies of Ce 3+ and the Crystalline Environment. Iii. Oxides Containing Ionic Complexes. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 125117. (19) Dorenbos, P. Energy of the First 4f 7→ 4f 6 5d Transition of Eu 2+ in Inorganic Compounds. J. Lumin. 2003, 104, 239−260. (20) Zhang, L.; Zhang, J.; Zhang, X.; Hao, Z.; Zhao, H.; Luo, Y. New Yellow-Emitting Nitride Phosphor Sralsi4n7: Ce3+ and Important Role of Excessive Aln in Material Synthesis. ACS Appl. Mater. Interfaces 2013, 5, 12839−12846. (21) Li, Y. Q.; de With, G.; Hintzen, H. Synthesis, Structure, and Luminescence Properties of Eu 2+ and Ce 3+ Activated Baysi 4 N 7. J. Alloys Compd. 2004, 385, 1−11. (22) Huang, C.-H.; Lai, Y.-T.; Chan, T.-S.; Yeh, Y.-T.; Liu, W.-R. A Novel Green-Emitting Srcasial 2 O 7: Eu 2+ Phosphor for White Leds. RSC Adv. 2014, 4, 7811−7817. (23) Wang, X.; Zhao, Z.; Wu, Q.; Li, Y.; Wang, C.; Mao, A.; Wang, Y. Synthesis, Structure, and Luminescence Properties of SrSiAl2O3N2: Eu2+ Phosphors for Light-Emitting Devices and Field Emission Displays. Dalton. Trans. 2015, 44, 11057. (24) van Uitert, L. G. An Empirical Relation Fitting the Position in Energy of the Lower D-Band Edge for Eu2+ or Ce3+ in Various Compounds. J. Lumin. 1984, 29, 1−9. (25) Shannon, R. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta

where I0 and I refer to PL intensity at the initial temperature and testing temperature T, respectively. ΔE is the activation energy; A is a constant; and k equals the Boltzmann’s constant. The inset of Figure 10a shows that the ΔE was obtained to be 0.22 eV. Figure 10b displays the comparison of the thermal quenching between CYZA:Ce 3+ and the Ba 2 SiO 4 :Eu 2+ commercial phosphor. The CYZA:Ce3+ exhibits superior thermal stability to the commercial Ba2SiO4:Eu2+ phosphor. When the temperature increased up to 150 °C, the emission intensity remains 62% of the initial value; meanwhile, the intensity of the commercial Ba2SiO4:Eu2+ decreases to 48%. Moreover, it is worth noticing that the emission peaks change little (in the range of 491−497 nm) when temperature varied, indicating a good color stability under different temperatures, which is good for LED application.

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CONCLUSIONS In summary, a novel garnet phosphor CYZA:Ce3+ has been successfully identified via the solid-state method. These results from XRD refinement and TEM analysis indicate that this compound belongs to the Ia3̅d space group of the cubic crystal system. The samples exhibit polyhedron shape morphology, and the size is in the range of 2−5 μm. The CYZA:Ce3+ shows intense luminescence under n-UV excitation. As the Ce3+ concentration increases, the emission colors vary from cyan to green (484−503 nm) because of the energy transfer from the Ce3+ ions at higher 5d energy levels to those at the lower energy levels. The optimal doping concentration of Ce3+ was 3 mol %, of which the internal quantum efficiency is 56%. The concentration quenching of Ce3+ emission occurs via the energy transfer among the nearest-neighbor ions. The thermal stability is superior to the Ba2SiO4:Eu2+commercial phosphor. All these results imply that the CYZA:Ce3+ is a potential candidate applied in white LEDs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-931-8912772 (office). Fax: +86-931-8913554 (office). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Gansu Industry and Information Technology Committee, Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003), and the National Natural Science Funds of China (Grant No. 51372105).

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REFERENCES

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A Novel Garnet Phosphor for White LEDs - ACS Publications

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