Journal of The Electrochemical Society, 156 共8兲 J221-J225 共2009兲
J221
0013-4651/2009/156共8兲/J221/5/$25.00 © The Electrochemical Society
Luminescent Properties and Energy Transfer of Green-Emitting Ca3Y2„Si3O9…2:Ce3+,Tb3+ Phosphor Yi-Chen Chiu,a,z Wei-Ren Liu,a Yao-Tsung Yeh,a Shyue-Ming Jang,a and Teng-Ming Chenb,z a
Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan b Phosphors Research Laboratory and Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan Green-emitting phosphors of Ca3Y2共Si3O9兲2:Ce3+,Tb3+ have been synthesized by solid-state reaction. The luminescence properties were characterized by photoluminescence 共PL兲 and photoluminescence excitation spectra, reflection spectra, and a detection system using an integrating sphere equipped with a spectrofluorometer. With Ce3+ co-doping as a sensitizer, the PL intensity of Ca3Y2共Si3O9兲2:Ce3+,Tb3+ was found to increase dramatically by a factor of 5.8. For comparison, the emission intensity of composition-optimized Ca3Y2共Si3O9兲2:Ce3+,Tb3+ is 126% of green commodity, ZnS:Cu,Al. The quantum efficiency of Ca3Y2共Si3O9兲2:Ce3+,Tb3+ is as high as 77% compared to ZnS:Cu,Al 共53%兲. The energy transfer from Ce3+ to Tb3+ is also of resonant type via a dipole–dipole mechanism with the energy-transfer critical distance of 6.78 Å. By utilizing the principle of energy transfer and appropriate tuning of activator contents, we are currently evaluating the potential application of Ca3Y2共Si3O9兲2:Ce3+,Tb3+ as a highly efficient UV-convertible phosphor. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3148242兴 All rights reserved. Manuscript submitted April 3, 2009; revised manuscript received May 6, 2009. Published June 12, 2009.
Ce3+ may act as an efficient sensitizer transferring energy to Tb3+ in several host lattices.1-7 For example, Zhao et al.5 studied the Ce3+–Tb3+ interactions by nanosecond techniques at different temperatures and calculated the probability and efficiency of energy transfer from Ce3+ to Tb3+. Similar energy transfer was also observed by Srivastava et al.6 in the K3La共PO4兲2:Ce3+,Tb3+ phosphors. They clarified the mechanism of energy transfer and quantitatively evaluated this transfer in the host, indicating that the Ce3+ → Tb3+ energy transfer can be explained by a dipole–dipole interaction mechanism with a critical distance of 6.5 Å. Recently, Lu et al.7 reported the luminescence and energy-transfer characteristics of BaBPO5 and confirmed the effect of sodium ions in BaBPO5:Ce3+,Tb3+ phosphors. According to Dexter’s theory,8 the efficiency of energy transfer is mainly determined by the overlapping between the Ce3+ emission and Tb3+ excitation spectra. Neither the investigations regarding the luminescence in Ca3Y2共Si3O9兲2 nor the energy transfer between Ce3+ and Tb3+ in the host of Ca3Y2共Si3O9兲2 has been reported. In the present study, not only the luminescence properties and energy transfer of Ca3Y2共Si3O9兲3:Ce3+,Tb3+ but also the calculation of the energy-transfer critical distance between Ce3+ and Tb3+ ions based on the model proposed by Blasse are also investigated in this work.9 The preliminary data demonstrated that the phosphors can emit blue to yellow-greenish light under UV excitation by systematically tuning the Ce3+ /Tb3+ ratio in the host. The energy-transfer mechanism and the critical distance of energy transfer are of resonant type via a dipole–dipole interaction and are 6.78 Å, respectively.
Experimental Materials and synthesis.— Polycrystalline phosphors with compositions of Ca3共Y1−x−yCexTby兲2共Si3O9兲2 described in this work were prepared by solid-state reaction. Briefly, the constituent carbonates and oxides CaCO3共99.99%兲, Y2O3共99.99%兲, SiO2共99.9%兲, CeO2共99.9%兲, and Tb4O7共99.99%兲 共all from Aldrich Chemicals, Milwaukee, WI兲 were first well ground and intimately mixed in the requisite proportions; all powder samples were sintered under a reducing atmosphere at 1350°C for 8 h with one intermittent regrinding to avoid possible incomplete reaction.
z
E-mail:
[email protected];
[email protected]
Materials characterizations.— The phase purity of the asprepared samples was checked by powder X-ray diffraction 共XRD兲 analysis with a Bruker AXS D8 advanced automatic diffractometer with Cu K␣ radiation 共 = 1.5418 Å兲 operating at 40 kV and 20 mA. The XRD profiles were collected in the range of 10° ⬍ 2 ⬍ 80°. The measurements of photoluminescence 共PL兲 and PL excitation 共PLE兲 spectra were performed by using a Spex Fluorolog-3 spectrofluorometer 共Instruments S.A., Edison, NJ兲 equipped with a 450 W Xe light source and double-excitation monochromators. The powder samples were compacted and excited, and emitted fluorescence was detected by a Hamamatsu Photonics R928-type photomultiplier perpendicular to the excitation beam. The spectral response of the measurement system was calibrated automatically on startup. To eliminate the second-order emission of the source radiation, a cutoff filter was used in the measurements. All PL measurements in the present work were manipulated at room temperature. The reflectance spectra of the phosphor samples were measured with a Hitachi 3010 double-beam UV/visible spectrometer 共Hitachi Co., Tokyo, Japan兲 equipped with a 쏗60 mm integrating sphere whose inner face was coated with BaSO4 or Spectralon, and poly共tetrafluoroethylene兲 was used as a standard in the measurements. The Commission International de I’Eclairage 共CIE兲 chromaticity coordinates for all samples were determined by a Laiko DT-100 color analyzer equipped with a charge-coupled device detector 共Laiko Co., Tokyo, Japan兲. The quantum efficiency 共QE兲 was measured by a detection system using an integrating sphere whose inner face was coated with Spectralon equipped with a spectrofluorometer 共Horiba Jobin-Yvon Fluorolog 3-22 Tau-3兲. The device was based on a Labsphere optical Spectralon integrating sphere 共diameter of 100 mm兲, which provided a reflectance of 99% over a 400–1500 nm range 共⬎95% within 250–2500 nm兲. The sphere accessories were made from Teflon rod and sample holders or Spectralon 共baffle兲. The measurement procedures and correlation theorem were clearly described in Ref. 10. Results and Discussion Crystal structure of Ca3Y 2共 Si3O9兲2.— Yamane et al.11 reported the X-ray single-crystal structure of Ca3Y2共Si3O9兲2. Ca3Y2共Si3O9兲2 crystallizes in a monoclinic crystal system with a space group of C2/c. Ca/Y atoms are in eight-, seven-, and sixfold coordination sites, and Y are substituted at the Ca site as a substitutional disorder situation. The Ca/Y atom layers and Si3O9 ring layers are stacked along the 关101兴 direction. The three atom sites and one vacant site are arranged in the Ca/Y layer 共Ca1/Y1:Ca2/Y2:Ca3/Y3:vacant = 2:2:1:1兲. Based on the effective ionic radii 共r兲 of cations with
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Journal of The Electrochemical Society, 156 共8兲 J221-J225 共2009兲
J222
d transition 5d
2
2
=329nm =387nm em
F7/2, F5/2
f
ex
Intensity(a.u.)
Intensity(a.u.)
f
f
f transition
=377nm =541nm em
b
ex
d transition
a: D4
5
7
5
b: D4
7
c: D4
5
7
5
7
250
300
350
400
450
500
550
600
200
300
Luminescence spectra of Ca3共 Y,Ce兲2共 Si3O9兲2.— The Ce3+ ions show efficient UV to violet emission in the host lattice. Figure 1 shows the PL 共exc = 329 nm兲 and PLE 共em = 402 nm兲 spectra of Ca3共Y0.99Ce0.01兲2共Si3O9兲2. The excitation spectrum consists of three absorption peaks consisting of 300, 329, and 350 nm, which correspond to the transitions from the ground state of Ce3+ to its fieldsplitting levels of the 5d1 state. Excitation into this band yields the emission spectrum 5d → 4f共 2F7/2, 2F5/2,兲, which shows the absence of the characteristic doublet. Random distribution of Ca2+ /Y3+ sites in different unit cells in Ca3Y2共Si3O9兲2 generated variations in the 5d energy level. Hence, the Ce3+ activators suffer a complex crystal field in the host which caused the spectra broadening of the Ce3+ emission. Consequently, the absence of the doublet splitting may be due to the inhomogeneous broadening of the Ce3+ emission band resulting from the multiple random distribution site occupancy. Luminescence spectra of Ca3共 Y,Tb兲2共 Si3O9兲2.— Figure 2 shows the excitation and emission spectra of Tb3+ in Ca3Y2共Si3O9兲2. The excitation peaks consist of a number of lines in the region from 300 to 390 nm and a broad band peaking at about 270 nm. The broad band is ascribed to the spin to allow transition from the 4f to the 5d state of the Tb3+ ion. The lines correspond to absorption of the forbidden f-f transition of the Tb3+ ion. The Tb3+ emission peaks are 485, 541, 580, and 620 nm, which are assigned to the 5D4 to 7FJ 共J = 6,5,4,3兲 transitions, respectively. The blue emission from the 5 D3 level of Tb3+ was observed at a low doping concentration of Tb3+. In fact, many Tb3+-activated materials show a blue emission from the 5D3 level and a green emission from the 5D4 level. The energy difference between 5D3 and 5D4 is the same as that between 7 F0 and 7F6, which sometimes correspond to the energy transfer of identical centers 5D3共Tb3+兲 + 7F6共Tb3+兲 → 5D4共Tb3+兲 + 7F0共Tb3+兲. At higher Tb3+ concentrations, considerable quenching of the 5D3
400
500
d
600
700
Wavelength(nm) Figure 2. 共Color online兲 PLE and PL spectra of Ca3Y2共Si3O9兲2:Tb3+ phosphor. 共ex = 378 nm, em = 541 nm兲.
emission is observed, which is quenched by the phenomenon of the cross relaxation process mentioned above and thus only 5D4 to 7FJ emissions can be observed.13 Energy transfer in Ca3Y 2共 Si3O9兲2:Ce,Tb phosphor.— One strategy to enhance the luminescence of Tb3+ is to use the co-doping of the Ce3+ ion as a sensitizer, such as LaPO4:Ce3+,Tb3+. In this study, we attempt to verify whether Ce3+ can further enhance the luminescence of Tb3+. The corresponding energy-transfer mechanism of Ce3+ → Tb3+ is also investigated in this work. The PL and PLE spectra for Ca3Y2共Si3O9兲2:Tb3+ and Ca3Y2共Si3O9兲2:Ce3+ phosphors reveal a significant spectral overlap between the emission band of Ce3+ and Tb3+ excitation. Therefore, the effective resonance-type energy transfer from Ce3+ to Tb3+ is expected. Figure 3 shows the PL spectra for Ce3+- and Tb3+-coactivated Ca3共Y0.99−nCe0.01Tbn兲2共Si3O9兲2 phosphors with different dopant contents n of 0, 0.05, 0.10, 0.20, 0.25, and 0.30, respectively. The PL intensity of the Tb3+ activator 共or energy acceptor兲 increased, whereas that of the Ce3+ sensitizer 共or energy donor兲 simultaneously decreased monotonically. The CIE chromaticity coordinates for
Intensity(a.u.)
different coordination numbers 共CNs兲 reported by Shannon,12 the ionic radii of Ce3+ 共r = 1.010 Å when CN = 6, r = 1.070 Å when CN = 7, and r = 1.143 Å when CN = 8兲 and Tb3+ 共r = 0.923 Å when CN = 6, r = 0.98 Å when CN = 7, and r = 1.04 Å when CN = 8兲 are close to that of Ca2+ 共r = 1.000 Å when CN = 6, r = 1.060 Å when CN = 7, and r = 1.120 Å when CN = 8兲 and Y3+ 共r = 0.900 Å when CN = 6, r = 0.960 Å when CN = 7, and r = 1.019 Å when CN = 8兲. For the considerations of ionic radius matching and charge balance, we thereby conclude that Ce3+ and Tb3+ tend to occupy three different Y sites.
F3
a
Wavelength(nm) Figure 1. 共Color online兲 PLE and PL spectra of Ca3Y2共Si3O9兲2:Ce3+ phosphor 共ex = 329 nm, em = 387 nm兲.
F5
F4
d: D4
c
200
F6
1 2 3 4 5 6 7
1 2 3 4 5 6
n=0 n=0.01 n=0.05 n=0.10 n=0.20 n=0.25 n=0.30
7
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450
500
550
600
Wavelength(nm) Figure 3. 共Color online兲 PL spectra for Ca3共Y0.99−nCe0.01Tbn兲2共Si3O9兲2 phosphors 共PL excited at 329 nm兲.
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Journal of The Electrochemical Society, 156 共8兲 J221-J225 共2009兲 0.6
Table I. Comparison of CIE chromaticity coordinates for Ca3„Y0.99−nCe0.01Tbn…2„Si3O9…2 „ex = 329 nm…. Sample no. 1 2 3 4 5 6 7
Sample composition 共n兲 0 0.01 0.05 0.10 0.20 0.25 0.30
共x,y兲 共0.15, 共0.16, 共0.21, 共0.25, 共0.28, 共0.29, 共0.30,
0.03兲 0.08兲 0.23兲 0.34兲 0.46兲 0.48兲 0.50兲
J223
PL intensity at 541 nm
0.4
— 1.0 3.4 4.7 5.2 5.5 5.8
0.2
0.0
Ca3共Y0.99−nCe0.01Tbn兲2共Si3O9兲2 phosphors with different dopant contents are measured and summarized in Table I and the CIE coordinates are also represented in Fig. 4. The energy-transfer efficiency 共T兲 of Ce3+ → Tb3+ can be expressed by14 IS T = 1 − IS0
关1兴
where IS0 and IS are the luminescence intensities of the sensitizer 共Ce3+兲 in the absence and presence of the activator 共Tb3+兲, respectively. The energy-transfer efficiency from Ce3+ to Tb3+ in Ca3共Y0.99−nCe0.01Tbn兲2共Si3O9兲2 is calculated and illustrated in Fig. 5. With increasing Tb3+ dopant content, T increased gradually. The energy transfer from a sensitizer to an activator may take place via exchange interaction and electric multipolar interaction.6 Exchange interaction needs a large direct or indirect overlapping between donor and acceptor orbitals leading to easy electronic exchange. Because both Ce3+ and Tb3+ are reducing ions, such an exchange would require too high energies. Therefore energy transfer via exchange interaction is impossible. Thus the energy transfer in Ca3Y2共Si3O9兲2 takes place via electric multipolar interaction. Based
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Tb3+ content (n) Figure 5. 共Color online兲 Dependence of the energy-transfer efficiency T in Ca3共Y0.99−nCe0.01Tbn兲2共Si3O9兲2 on Ce3+.
on Dexter’s energy-transfer formula of exchange interaction, the following relation can be given as15-17
冉 冊
IS0 ⬀ CCe+Tb IS
ln
关2兴
where CCe+Tb is the total dopant concentration. The ln共IS0 /IS兲 − C plot exhibits a nonlinear relationship, which further indicates that the exchange interaction is not a dominant energy-transfer mechanism in Ca3Y2共Si3O9兲2:Ce3+,Tb3+. The result is identical to the previous prediction. On the basis of Dexter’s energy-transfer formula of multipolar interaction and Reisfeld’s approximation the following relation can be obtained15-17 0 n/3 ⬀ CCe+Tb
关3兴
where 0 and are the luminescence QEs of Ce3+ in the absence and presence of Tb3+, respectively; the values 0 / can be approximately replaced by the ratio of related luminescence intensities 共IS0 /IS兲; CCe+Tb is the total dopant concentration; and n = 6, 8, and 10, corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole, respectively. The IS0 /IS − Cn/3 plot is exhibited in Fig. 6a and b, giving a linear relation in both dipole–dipole and dipole–quadrupole interac-
2.2
IS0/IS of Ce3+
2.0
1.8
R=0.9971
R=0.9896
1.6
1.4
1.2
1.0 0.00
Figure 4. 共Color online兲 CIE chromaticity diagram for Ca3共Y0.99−nCe0.01Tbn兲2共Si3O9兲2 phosphors with different Tb3+ dopant contents 共ex = 329 nm兲.
0.02
0.04
0.06
C6/3 Ce+Tb
0.08
0.10
0.00
0.01
0.02
0.03
C8/3 (Ce+Tb)
0.04
0.05
Figure 6. 共Color online兲 Dependence of IS0 /IS of Ce3+ on C6/3 and C8/3.
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Journal of The Electrochemical Society, 156 共8兲 J221-J225 共2009兲
J224 80
3+
C3Y2 Si3O9 2:Ce,Tb
3+
Ca3Y2 Si3O9 2:15%Ce ,40%Tb
60
C3Y2 Si3O9 2:Tb ZnS:Cu,Al
80 60
Intensity (a.u.)
Reflectance (%)
40
3+
Ca3Y2 Si3O9 2:15%Ce
40 20 100 80 60 40 20 80
3+
Ca3Y2 Si3O9 2:40%Tb Ca3Y2 Si3O9
60
2
40 20 300
400
500
600
Wavelength(nm)
200
Figure 7. Reflectance spectra for Ce3+- and Tb3+-activated and Ce3+ /Tb3+-coactivated Ca3Y2共Si3O9兲2 phosphors.
tions. The electric dipole–dipole interaction usually accompanies the electric dipole–quadrupole interaction because the coulombic effect of the former is larger than that of the latter. Therefore, the energy transfer from Ce3+ to Tb3+ is demonstrated by the dipole–dipole mechanism, which is similar to that examined in Ref. 18 and 19. Considering the dipole–dipole interaction, the energy-transfer probability PSA 共in s−1兲 from a sensitizer to an acceptor is given by the following formula8,9 DD PCe−Tb = 0.63 ⫻ 1028
QA 6 S0RCe−Tb ES4
冕
FS共E兲FA共E兲dE
关4兴
where QA = 3.5 ⫻ 10 cm2 eV is the absorption cross section of Tb3+,20 R is the donor–acceptor distance 共in angstroms兲, ES is the energy involved in the transfer 共in eV兲, 0 is the donor radiative lifetime 共in seconds兲, and 兰FS共E兲FA共E兲dE is the overlap integral between normalized cerium emission 关FS共E兲兴 and terbium excitation 关FA共E兲兴 spectra, and it is estimated at 0.521 eV−1. The critical distance 共Rc兲 of energy transfer from the sensitizer to the acceptor is defined as the distance for which the probability of transfer equals the probability of radiative emission of donor, i.e., the distance for which PCe–TbS0 = 1. Hence, Rc can be expressed by 6 RCe−Tb = 0.63 ⫻ 1028
QA ES4
冕
FS共E兲FA共E兲dE
关5兴
Using the value and the calculated spectral overlap, the Rc of energy transfer from Ce3+ to Tb3+ in Ca3Y2共Si3O9兲2:Ce3+,Tb3+ was about 6.78 Å, which agrees with that 共i.e., 6.5 Å兲 reported for K3La共PO4兲3:Ce3+,Tb3+.6 The PLE spectrum is comparable to an absorption spectrum. To investigate the energy absorption of the silicate phosphors, diffuse reflectance spectra for parent and doped Ca3Y2共Si3O9兲2 are measured and shown in Fig. 7. As indicated in Fig. 7, for parent Ca3Y2共Si3O9兲2 a decrease in reflectance from 250 to 350 nm was noted. The middle points at ca. 288 nm may be used to estimate the approximate bandgap of host material Ca3Y2共Si3O9兲2. Several absorption bands are in the UV region of Ca3Y2共Si3O9兲2:Tb3+, which could be attributed to the f → f, f → d transitions of Tb3+ and host absorption. Reflectance spectra for the Ca3Y2共Si3O9兲2:Ce3+ phase exhibit two absorption bands, which may be attributed to the Ce3+ 4f → 5d transition. When co-doping Ce3+ and Tb3+ ions, the reflectance spectra are noted that combine with Ce3+- and Tb3+-ion absorptions. Figure 8 shows the comparison between the luminescence intensity of the Ca3Y2共Si3O9兲2:40%Tb3+ and
300
400
500
600
700
800
Wavelength (nm) Figure 8. 共Color online兲 The luminescence properties of Ca3Y2共Si3O9兲2:Tb3+ and Ca3Y2共Si3O9兲2:Ce3+,Tb3+ phosphors compared to those of the commercial phosphor ZnS:Cu,Al under the same measurement conditions.
Ca3Y2共Si3O9兲2:15%Ce3+,40%Tb3+ phosphors and those of the commercial green phosphor under the same measurement conditions. The emission intensity is about 17% for Ca3Y2共Si3O9兲2:Tb3+ opposite to that of ZnS:Cu,Al. After co-doping Ce3+ and Tb3+ in Ca3Y2共Si3O9兲2, the emission intensity is increased by 126% as compared to that of ZnS:Cu,Al. The result demonstrates that energy transfer from Ce3+ to Tb3+ efficiently enhances the PL intensity of Ca3Y2共Si3O9兲2:Tb3+ remarkably. The QEs of Ca3Y2共Si3O9兲2:Tb3+, Ca3Y2共Si3O9兲2:Ce3+,Tb3+, and ZnS:Cu,Al are about 10, 77, and 53%, respectively. The QE of phosphors is already higher than that of commercial phosphor. Conclusions In summary, the luminescence properties of Ce3+ and Tb3+ ions were investigated on the host Ca3Y2共Si3O9兲2. The spectroscopic data indicate that the Ce3+ → Tb3+ energy-transfer process takes place in the host matrix of Ca3Y2共Si3O9兲2. The energy transfer from Ce3+ to Tb3+ has a dipole–dipole mechanism. The critical energytransfer distance was roughly estimated at 6.78 Å. The Ca3Y2共Si3O9兲2:Ce3+,Tb3+ phosphors have a higher luminescence intensity and QE than those of commercial phosphors. Furthermore, we have demonstrated that the Ca3共Y0.99−nCe0.01Tbn兲2共Si3O9兲2 can be systematically tuned to generate blue light to yellow-greenish light under UV radiation and it has been shown to exhibit the potential to act as a UV-convertible phosphor. Acknowledgment This research was supported by the Industrial Technology Research Institute under contract no. 8301XS1751 and in part by the National Science Council of Taiwan under contract no. NSC952113-M-009-024-MY3 共T.-M.C.兲. Industrial Technology Research Institute assisted in meeting the publication costs of this article.
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