ARTICLE pubs.acs.org/JPCC
A Novel Single-Composition Trichromatic White-Light Ca3Y(GaO)3(BO3)4:Ce3þ,Mn2þ,Tb3þ Phosphor for UV-Light Emitting Diodes Chien-Hao Huang and Teng-Ming Chen* Phosphors Research Laboratory and Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, Republic of China ABSTRACT: A novel single-composition white-emitting phosphor Ca3Y(GaO)3(BO3)4:Ce3þ, Mn2þ,Tb3þ has been synthesized by a high-temperature solid-state reaction. The spectral overlap between the emission band of Ce3þ and the excitation band of Mn2þ, which supports the occurrence of the energy transfer from Ce3þ to Mn2þ, has been studied and demonstrated to be a resonant type via a dipole-quadrupole mechanism. Because there was no spectral overlap between the emission spectra of Ce3þ and excitation band of Tb3þ in our study, no energy transfer from Ce3þ to Tb3þ was observed, indicating that Ce3þ and Tb3þ were coexcited. Through effective resonance-type energy transfer and coexcitation, the chromaticity coordinates of Ca3Y(GaO)3(BO3)4:Ce3þ,Mn2þ,Tb3þ phosphors can be tuned from (0.152, 0.061) for Ca3Y(GaO)3(BO3)4:Ce3þ to (0.562, 0.408) for Ca3Y(GaO)3(BO3)4:Mn2þ, and eventually reaching (0.314, 0.573) for Ca3Y(GaO)3(BO3)4:Tb3þ. A white light-emitting diode (LED) was fabricated by using the white-emitting single-composition (Ca0.97)3(Y0.92)(GaO)3(BO3)4:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ pumped by a 365 nm UV-chip. Our results indicated that the CIE chromaticity coordinates and correlated color temperature (CCT) for white UV-LEDs were (0.31, 0.33) and 6524 K, respectively. Therefore, our novel white Ca3Y(GaO)3(BO3)4:Ce3þ,Mn2þ,Tb3þ can serve as a key material for phosphor-converted white-light UV-LEDs.
1. INTRODUCTION In recent years, white-light emitting diodes (white LEDs) have become a very important lighting source and are components of backlight for LCD displays. The white LEDs can be fabricated by using a blue InGaN LED chip in combination with a yellow phosphor of cerium(III)-doped yttrium aluminum garnet (YAG: Ce3þ).1 The white LED based on YAG:Ce3þ phosphor exhibits a poor color rendering index (CRI ≈ 70-80) and a high correlated color temperature (CCT ≈ 7750 K)2 because of lacking a red component.3 In recent years, white LEDs fabricated using nearultraviolet (n-UV) LED or ultraviolet LED with a single host emission color-tunable phosphor 4 have been investigated to improve the color-rendering index (Ra) and to tune the correlated color temperature (CCT) by systematically tuning the relative dopant content of sensitizer and activator.5,6 A single-composition white-light phosphor is produced by codoping sensitizer and activator into the same host matrix. The energy transfer mechanism from a sensitizer to an activator has been investigated in many inorganic hosts, such as fluorides, silicates, phosphates, and borates. For example, Caldi~ no et al.7 reported that the Ce3þf 2þ Mn energy transfer and mechanism involved with CaF2:Ce3þ, Mn2þ phosphor, the Ce3þfMn2þ energy transfer process to form small Ce3þ-Mn2þ complexes, and the Ce3þfMn2þ energy transfer was rationalized by assuming that a short-range interaction mechanism such as electric dipole-quadrupole interaction occurs in the Ce3þfMn2þ clusters. The Eu2þfMn2þ energy transfer r 2011 American Chemical Society
via a dipole-quadruple mechanism was also described by Yang et al.5 in (Ca0.99-nEu0.01Mnn)Al2Si2O8, and the energy transfer efficiency was found to increase with increasing Mn2þ dopant concentration. Chang et al. later illustrated that the energy transfer from Ce3þ to Eu2þ in Sr3B2O6:Ce3þ,Eu2þ was dominated by resonance-type electric dipole-dipole interaction.6 In addition, the tunable color hues from blue through white and finally to yellow-orange could be achieved by tuning the relative ratio of Ce3þ/Eu2þ in Sr3B2O6:Ce3þ,Eu2þ. In this research, we report our recent investigation results on the luminescence and color/chromaticity tunability of a trichromatic white-emitting borate phosphor Ca 3 Y(GaO)3(BO3)4:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ by varying the relative dopant concentrations of Mn2þ and Tb3þ, in which energy transfer from Ce3þ to Mn2þ and coexcitation of Ce3þ and Tb3þ to generate white light were investigated.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A series of rare earth-doped Ca3Y(GaO)3(BO3)4:zCe3þ,xMn2þ,yTb3þ phosphors were synthesized by high temperature solid-state reaction, starting from a mixture Received: August 19, 2010 Revised: November 29, 2010 Published: January 14, 2011 2349
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Figure 2. Crystal structure of Ca3Y(GaO)3(BO3)4 (from ref 8). Figure 1. Comparison of powder XRD patterns of pristine, Ce3þ-, Mn2þ-, and Tb3þ-doped or codoped Ca3Y(GaO)3(BO3)4 (ICSD: 172155).
containing CaCO3 (A.R. 99.9%), Y2O3 (A.R. 99.9%), Ga2O3 (A. R. 99.99%), H3BO3 (A.R. 99.99%), MnO (A.R. 99.9%), Tb4O7 (A.R. 99.9%), and CeO2 (A.R. 99.9%) in the following nominal molar ratios: 3(1 - x):(1/2 - y/4 - z):3/2:4.12 = x:y/4:z (x = 0-0.2, y = 0-0.1, z = 0-0.01 mol). The powder reactants and an excess of 3 mol % of H3BO3 as a flux were blended and ground thoroughly in an agate mortar, and the homogeneous mixture was transferred to an alumina crucible and calcined in a furnace at 1273 K for 8 h under a reducing atmosphere of 15% H2/85% N2. 2.2. Sample Characterization. The phase purity of each of the samples was characterized and evaluated by powder X-ray diffraction (XRD) analysis with a Bruker AXS D8 advanced automatic diffractometer with Cu KR radiation. The photoluminescence (PL) and PL excitation (PLE) spectra of the phosphors were analyzed by using a Spex Fluorolog-3 spectrofluorometer (Instruments S.A., NJ) equipped with a 450 W Xe light source and double excitation monochromators. The powder samples were compacted and excited under a 45° incidence angle, and the emitted fluorescence was detected by a Hamamatsu Photonics R928 type photomultiplier perpendicular to the excitation beam. The Commission International de I'Eclairage (CIE) chromaticity coordinates for all phosphors were measured by a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan). The electroluminescence (EL) spectra of white LED lamps were obtained with a mixture of transparent silicon resin and white-light phosphors dropped on a UV-chip (AOT product no., DC0004CAA; Spec, 370U02C; wavelength peak, (365-370) ( 0.6 nm; chip size, 40 40 mil; forward voltage, (3.8-4.0) ( 0.02 V; power, (10-20) ( 0.21 mW) and driven with a current of 350 mA.
3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology. Shown in Figure 1 is the comparison of power XRD patterns of prestine, singly, doubly, and triply doped Ca3Y(GaO)3(BO3)4:Ce3þ,Mn2þ,Tb3þ (CYGB:Ce3þ,Mn2þ,Tb3þ) phosphors. All XRD patterns were found to agree well with those reported in the Inorganic Crystal Structure Database (ICSD 172155),8 indicating that the doped Ce3þ or codoped Ce3þ/Mn2þ or codoped Ce3þ/Mn2þ/Tb3þ ions or ion combination did not generate any impurity or induce significant changes in the host structure. Borate compounds are
Figure 3. The distribution of average particle sizes and the SEM image of the Ca3Y(GaO)3(BO3)4:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ phosphors.
functionally versatile and can serve as excellent host matrices for efficient luminescence, due to their large band gaps. Recently, a new borate Ca3Y(GaO)3(BO3)4 (CYGB) was reported by Yu et al.,9 and its crystal structure was analyzed and refined on the basis of powder diffraction data, which revealed a hexagonal space group P63/m and cell parameters of a = 10.5167(3) Å, c = 5.8146 (2) Å, V = 556.94 Å3, and N = 2. The crystal structure of CYGB indicates that GaO6 octahedra share edges to form chains along the c-axis, which are connected by triangular BO3 groups, forming a Kagome-type lattice in the ab plane. Two kinds of tunnels exist in the Kagome lattice, in which Y3þ/Ca2þ ions occupy a smaller trigonal tunnel site and Ca2þ/Y3þ ions occupy a larger apatite-like hexagonal tunnel site (Figure 2). Each cation has several different coordination environments: Ca2þ/Y3þ(1) is nine-coordinated, Ca2þ/Y3þ(2) is seven-coordinated, Ga3þ is six-coordinated, and B(1) and B(2) are three-coordinated to oxygen atoms. The ionic radii for nine- and seven-coordinated Ca2þ and Y3þ are 1.06, 0.96 Å and 1.18, 1.075 Å, respectively; however, the ionic radii for nine- and seven-coordinated Ce3þ are 1.196 and 1.07 Å, those for Tb3þ are 1.095 and 0.98 Å, and that for seven-coordinated Mn2þ is 0.9 Å. Therefore, Ce3þ, Tb3þ, and Mn2þ are expected to randomly occupy the Ca2þ/ Y3þ sites in the CYGB crystal structure. Figure 3 shows the particle size and the SEM image of the CYGB:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ 2350
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Figure 5. PL spectra of CYGB:0.01Ce3þ,xMn2þ, with x equal to 0, 0.01, 0.03, 0.05, 0.10, and 0.20, respectively. Figure 4. (a) Spectral overlap between PL spectrum of CYGB:Ce3þ (solid line) and PLE spectrum of CYGB:Mn2þ (dash line). (b) PLE/PL spectra of CYGB:Ce3þ (black solid and dash line) and PLE spectrum of CYGB:Tb3þ (blue solid line), indicating absence of overlap.
phosphor. The particle sizes of CYGB:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ phosphor were found to vary from 2 to 40 μm, and the average particle size is about 17 μm. The inset of Figure 3 shows the scanning electron microscopy (SEM) morphology of the CYGB:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ phosphor, which consists of particles that are aggregated and irregular. 3.2. Investigation on Spectral Overlap between Ce3þ and Mn2þ. Significant spectral overlap between the broad Ce3þ emission band (Figure 4a, solid line) ascribed to the 5d1 f 4f1 transition10 and several excitation bands of Mn2þ centered at 347, 364, 407, 418, and 466 nm (Figure 4a, dash line), which corresponded to the transitions from the 6A1g ground state to the excited states 4 Eg, 4T2g, [4Eg, 4A1g], 4T2g, and 4T1g levels, as shown in Figure 4a, were observed. These observations have been observed and reported by Huang et al.11,12 It has been reported that the excitation spectrum of Mn2þ 6A1g f [4Eg, 4A1g] centered at 407 nm exhibits substantial overlap with the emission band of Ce3þ 5d1 f 4f1 centered at 409 nm, indicating a favorable condition for possible energy transfer between the sensitizer Ce3þ and the activator Mn2þ. Under excitation at 365 nm, the Ce3þ emission was found to center at 409 nm (Figure 4b, black dash line), and Mn2þ emission was centered at 589 nm. Furthermore, several excitation bands of Tb3þ were also observed to appear at 256, 272, 284, 295, 303, 316, 325, 341, 351, 354, 368, and 379 nm (Figure 4b, blue solid line), which could be referred to as the transitions from the 7F6 ground state to the excited states of Tb3þ 5KJ, 5IJ, 5HJ, 5D1,0, 5GJ, 5L10, and 5D3 levels, as reported by Li et al.13 This observation revealed that they did not exhibit spectral overlap and there was no energy transfer from Ce3þ to Tb3þ as expected. Therefore, Ce3þ and Tb3þ could be coexcited (Figure 4b). As a result, there was a resonance-type energy transfer from Ce3þ to Mn2þ and coexcitation in CYGB:Ce3þ,Mn2þ, Tb3þ hosts. 3.3. Energy Transfer Mechanism and Luminescence Decay. Figure 5 illustrates the dependence of PL spectra on the Mn2þ dopant content for CYGB:0.01Ce3þ,xMn2þ phosphors (x = 0, 0.01, 0.03, 0.05, 0.10, and 0.20). The PL intensity of Ce3þ at 409 nm
Figure 6. Decay curves of Ce3þ emission monitored at 409 nm for CYGB:0.01Ce3þ,xMn2þ phosphors (x = 0-0.2) under excitation at 365 nm. The inset shows the dependence of energy transfer efficiency (ηT) on the Mn2þ content.
was found to decrease with the increasing Mn2þ content (x), whereas the PL intensity of Mn2þ at 589 nm was observed to increase with the increasing Mn2þ content (x) until the emission intensity of Mn2þ is saturated when x is above 0.1, and then the concentration quenching appears when the Mn2þ dopant content (x) is greater than 0.1, which is related to the energy transfer probability occurring from Ce3þ to Mn2þ. The decay curves of CYGB:0.01Ce3þ,xMn2þ phosphors (x = 0, 0.01, 0.03, 0.05, 0.10, and 0.20) excited at 365 nm and monitored at 409 nm are shown in Figure 6. The decay curves were well fitted with a second-order exponential decay mode by the following equation:11,12 I ¼ A1 expð - t=τ1 Þ þ A2 expð - t=τ2 Þ
ð1Þ
where I is the luminescence intensity; A1 and A2 are constants; t is the time, and τ1 and τ2 are rapid and slow lifetimes for exponential components, respectively. The values of A1, τ1, A2, and τ2 are obtained as shown in Table 1. Using these parameters, the average decay times (τ*) can be determined by 2351
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the formula as follows:11,12 τ ¼ ðA1 τ21 þ A2 τ22 Þ=ðA1 τ1 þ A2 τ2 Þ
ð2Þ
The average decay times (τ*) were calculated to be 51.2, 40.4, 29.8, 25.5, 16.6, and 7.44 ns for CYGB:0.01Ce3þ,xMn2þ with x = 0, 0.01, 0.03, 0.05, 0.1, and 0.2, respectively. The energy transfer efficiency (ηT) can be calculated using the following equation by Paulose et al.:14 τS ηT ¼ 1 ð3Þ τS0 Table 1. Decay Times of CYGB:0.01Ce3þ,xMn2þ Phosphors (x = 0-0.2) Excited at 365 nm with Emission Monitored at 409 nm sample
A1
τ1
A2 -8
τ2
τ* (ns)a -7
x = 0.00
0.1011
1.89 10
0.0246
1.84 10
51.2
x = 0.01
0.1123
1.84 10-8
0.0221
1.52 10-7
40.4
x = 0.03
0.1252
1.24 10-8
0.0203
1.37 10-7
29.8
x = 0.05
0.1297
1.12 10-8
0.0194
1.21 10-7
25.5
x = 0.10
0.1772
7.68 10-9
0.0181
1.04 10-7
16.6
x = 0.20
0.4211
4.68 10-9
0.0168
7.65 10-8
a
τ* = (A1τ12 þ A2τ22)/(A1τ1 þ A2τ2).
7.44
As a consequence, the ηT values from Ce3þ to Mn2þ for CYGB:0.01Ce3þ,xMn2þ were calculated to be 0, 21.1%, 41.8%, 50.2%, 67.6%, and 85.5% for CYGB:0.01Ce3þ,xMn2þ phosphors and plotted as a function of x (x = 0, 0.01, 0.03, 0.05, 0.1, and 0.2) as represented in the inset of Figure 6. These results indicated that the ηT of CYGB:0.01Ce3þ,xMn2þ increased with increasing Mn2þ dopant content and the largest energy transfer efficiency from Ce3þ to Mn2þ observed for CYGB:0.01Ce3þ, 0.2Mn2þ was 85.5%. On the basis of Dexter's energy transfer formula for exchange and multipolar interactions, the following relation can be obtained:15 η η ln 0 µ C and 0 µ CR=3 ð4Þ η η where ηo and η are the luminescence quantum efficiency of Ce3þ in the absence and presence of Mn2þ, respectively; the values of ηo/η can be estimated approximately by the ratio of relative luminescence intensity ratio (IS0/IS); and C is the concentration of Mn2þ. On the other hand, ln (ηo/η)RC corresponds to the exchange interaction, and (τS0/τS)RCR/3 with R = 6, 8, and 10 corresponds to dipole-dipole, dipole-quadrupole, and quadrupolequadrupole interactions, respectively. The relationships between ln(IS0/IS) and C as well as IS0/IS and CR/3 are illustrated in Figure 7a-d, and a linear behavior was observed only when
Figure 7. Dependence of ln(ISO/IS) of Ce3þ on (a) C and ISO/IS of Ce3þ, (b) C6/3, (c) C8/3, and (d) C10/3. 2352
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Figure 8. Dependence of PL and PLE spectra of CYGB:0.01Ce3þ, 0.03Mn2þ,yTb3þ (y = 0, 0.01, 0.03, 0.05, 0.07, and 0.10) on y, and the inset shows the relative PL intensity as a function of y at λex = 365 nm.
R = 8, implying that energy transfer from Ce3þ to Mn2þ occurs via the dipole-quadrupole mechanism, which is similar to those previously observed by Caldi~ no et al.16 and Martínez-Martínez 17 et al. The critical distance of energy transfer Rc was calculated by using the concentration quenching method where the critical distance RCe-Mn between Ce3þ and Mn2þ can be estimated by the following formula suggested by Blasse:18 3V 1=3 RCe- Mn ¼ 2 ð5Þ 4πxc N In the equation, V is the volume of the unit cell, and N is the number of host cations in the unit cell. The acquired values of V and N are 556.94 Å3 and 2, respectively, based on the crystal structure analysis of CYGB.9 The critical distance of Ce3þ f Mn2þ energy transfer (RCe-Mn) of CYGB:0.01Ce3þ,xMn2þ was determined to be 23.69, 17.45, 14.92, 11.97, and 9.55 Å when x equals 0.01, 0.03, 0.05, 0.1, and 0.2, respectively. The critical concentration (xc), at which the luminescence intensity of Ce3þ is one-half of that in the sample in the absence of Mn2þ, is 0.4255. Therefore, the critical distance (Rc) was calculated to be 10.77 Å. The critical distance Rc of ETCefMn for the dipole-quadrupole mechanism for the spectral overlap method can be calculated by using the following equation:19,20 Z fq λ 2 Q A Rc8 ¼ 0:63 1028 s 4 FS ðEÞFA ðEÞ dE ð6Þ fd ES where QA is the absorption cross-section of Mn2þ that is equal to 4.8 10-16 fd; fd = 10-7 and fq = 10-10 are the oscillator strengths of the activator (Mn2þ) dipole and quadrupole electric transitions; λs (Å) and E (eV) are the emissionRwavelength and the emission energy of Ce3þ, respectively; and Fs(E)FA(E) dE expresses the spectral overlap between the normalized Ce3þ emission Fs(E) and the Mn2þ excitation FA(E), and it was estimated to be 2 eV-1. The estimated Rc between Ce3þ and Mn2þ ions in CYGB host was approximately 10.3 Å, which was in good agreement with that obtained by concentration quenching (Rc = 10.77 Å) and spectral overlap (Rc = 10.3 Å) methods. These results imply
Figure 9. Representation of the CIE chromaticity coordinates for CYGB:0.01Ce3þ,xMn2þ (point nos. 1-5), CYGB:0.01Ce3þ, and 0.03Mn2þ,yTb3þ (point nos. 6-10).
that the emission intensity (peaking at 589 nm) of Mn2þ ion increases with decreasing Ce3þ-Mn2þ distance (or increasing Mn2þ content) until reaching saturation; furthermore, when the Mn2þ content exceeded 0.01 (i.e., Ce3þ-Mn2þ distance shorter than Rc), the Mn2þ emission intensity began to decrease, which was attributed to the occurrence of energy reabsorption among the nearest Mn2þ ions, as shown in Figure 7. 3.4. Luminescence and Chromaticity of Ce3þ, Mn2þ, and Tb3þ. Figure 8 showed the PL and PLE spectra of CYGB: 0.01Ce3þ,0.03Mn2þ,yTb3þ phosphors with different percent of dopant contents (y) of 0, 0.01, 0.03, 0.05, 0.07, and 0.10. The PL spectrum under excitation at 365 nm was found to consist of six emission bands in the visible-wavelength region, one at 409 nm (Ce3þ 5d1 f 4f1 transition), four green-emitting Tb3þ peaks located at about 488, 544, 581, and 620 nm (due to 5D4 to 7F6, 7F5, 7F4, 7 F3 transitions),21-23 and a broad orange red-emitting band at 589 nm (due to a Mn2þ d-level spin-forbidden transition).24 With the increasing Tb3þ dopant concentration, the PL intensity of Tb3þ emission was found to increase, and that of Ce3þ emission was observed to decrease, yet the PL intensity of Mn2þ did not increase nor decrease with the increasing Tb3þ dopant concentration. Thus, there could not be any energy transfer between Mn2þ and Tb3þ as expected. The x and y values of CIE chromaticity coordinates for CYGB:Ce3þ,xMn2þ,yTb3þ phosphors with different dopant contents were measured and presented in Figure 9 and Table 2, respectively. The representing features of chromaticity coordinates for CYGB:0.01Ce3þ,xMn2þ phosphors could be tuned from ultraviolet (0.152, 0.061) to orange-red (0.507, 0.371) position by controlling the Mn2þ concentration. For the phosphors with CYGB:0.01Ce3þ,0.03Mn2þ,yTb3þ compositions, the chromaticity coordinates varying from (0.286, 0.192) to (0.316, 0.331) and eventually to (0.327, 0.390) clearly indicated that the color was tunable from violet to white, even to green in the visible spectral region by controlling the Tb3þ concentration. Therefore, the chromaticity of Ca3Y(GaO)3(BO3)4:Ce3þ,Mn2þ,Tb3þ phosphors could be tuned from (0.152, 0.061) for Ca3Y(GaO)3(BO3)4:Ce3þ to (0.562, 0.408) for Ca3Y(GaO)3(BO3)4:Mn2þ and eventually to (0.314, 0.573) for Ca3Y(GaO)3(BO3)4:Tb3þ 2353
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Table 2. Comparison of CIE Chromaticity Coordinates for CYGB:0.01Ce3þ,xMn2þ,yTb3þ (λex = 365 nm) and Simulated White Light Using Y3Al5O12:Ce3þ Commodity (λex = 460 nm) no. of points in CIE diagram A
sample compositions 3þ
CYGB:Ce
2þ
combination with a UV-chip was superior to those ((0.292, 0.325), CCT = 7756 K) of a InGaN-based white LED, which relied on a blue LED coated with YAG:Ce3þ because the former showed higher color rendering index and lower CCT value.2
CIE (x, y) (0.152, 0.061)
B
CYGB:Mn
(0.562, 0.408)
C
CYGB:Tb3þ
(0.314, 0.573)
1 2
x = 0.01, y = 0.00 x = 0.03, y = 0.00
(0.208, 0.079) (0.277, 0.162)
3
x = 0.05, y = 0.00
(0.345, 0.221)
4
x = 0.10, y = 0.00
(0.475, 0.342)
5
x = 0.20, y = 0.00
(0.507, 0.371)
6
x = 0.03, y = 0.01
(0.286, 0.192)
7
x = 0.03, y = 0.03
(0.294, 0.259)
8
x = 0.03, y = 0.05
(0.306, 0.298)
9 10
x = 0.03, y = 0.07 x = 0.03, y = 0.10
(0.316, 0.331) (0.327, 0.390)
a
blue InGaN chip
(0.144, 0.030)
b
white light Y3Al5O12:Ce3þ
(0.292, 0.325)
c
Y3Al5O12:Ce3þ phosphors
(0.429, 0.553)
4. CONCLUSIONS We have synthesized and investigated the PL and EL performance of a series of single-composition trichromatic white-light Ca3Y(GaO)3(BO3)4:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ phosphors, with which we have demonstrated that the generation of white light can be achieved, and this is attributed to the effective resonance-type Ce3þ to Mn2þ energy transfer via a dipolequadrupole mechanism and coexcitation of Ce3þ and Tb3þ simultaneously. The RCe-Mn has also been evaluated by both the concentration quenching and the spectral overlap methods. Because of its properties such as resonance-type effectiveness, capability of energy transfer from Ce3þ to Mn2þ, and coexcitation of Ce3þ and Tb3þ, the chromaticity of Ca3Y(GaO)3(BO3)4: Ce3þ,Mn2þ,Tb3þ phosphors can be tuned from (0.152, 0.061) for Ca3Y(GaO)3(BO3)4:Ce3þ, (0.562, 0.408) for Ca3Y(GaO)3(BO3)4:Mn2þ, and (0.314, 0.573) for Ca3Y(GaO)3(BO3)4:Tb3þ. A white LED lamp with CCT = 6524 K and CIE = (0.31, 0.33) that are close to ideal white light was realized by pumping a singlehost white light-emitting phosphor (Ca0.97)3(Y0.92)(GaO)3(BO3)4: 0.01Ce3þ,0.03Mn2þ,0.07Tb3þ with a 365 nm UV-chip. Our results indicate that Ca3Y(GaO)3(BO3)4:Ce3þ,Mn2þ,Tb3þ phosphor exhibits great potential to serve as a key material for single-composition phosphor converted white-emitting LEDs. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ 886-35731695. E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by the National Science Council of Taiwan, ROC, under contract no. NSC98-2113-M-009-005MY3. ’ REFERENCES Figure 10. EL spectrum of a pc-LED lamp fabricated with a 365 nm LED chip and a white-emitting CYGB:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ driven by a 350 mA current.
by systematically adjusting the relative Mn2þ and Tb3þ dopant concentrations. 3.5. EL Spectrum of pc-LED Lamp Fabricated with CYGB: Ce3þ,Mn2þ,Tb3þ. To demonstrate the potential application of CYGB:Ce3þ,Mn2þ,Tb3þ phosphors, a LED lamp was fabricated through use of a 365 nm UV-chip and the white-emitting compositionoptimized CYGB:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ phosphor driven by a 350 mA current. The electroluminescence (EL) spectrum of the white-light LED based on a 365 nm UV-chip and a singlecomposition CYGB:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ phosphor is shown in Figure 10. The CIE color coordinates and correlated color temperature of the white-LEDs were found to be (0.31, 0.33) and 6524 K, respectively. The inset shows the appearance of a well-packaged trichromatic LED lamp in operation. These results demonstrated that the performance of the white LED based on the CYGB:0.01Ce3þ,0.03Mn2þ,0.07Tb3þ phosphor in
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