Bright green-emitting, energy transfer and quantum cutting of Ba_3Ln(PO_4)_3: Tb^3+ (Ln = La, Gd) under VUV-UV excitation

Bright green-emitting, energy transfer and quantum cutting of Ba3Ln(PO4)3: Tb3+ (Ln = La, Gd) under VUV-UV excitation Dejian Hou,1 Hongbin Liang,1,* Mubiao Xie,1 Xuemei Ding,1 Jiuping Zhong,1 Qiang Su,1 Ye Tao,2 Yan Huang,2 and Zhenhua Gao2 1 KLGHEI of Environment and Energy Chemistry, MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China 2 Beijing Synchrotron Radiation Facilities, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China *[email protected]

Abstract: Tb3+ doped Ba3Gd(PO4)3 and Ba3La(PO4)3 phosphors were synthesized using the traditional high temperature solid state reaction method. The excitation, emission, and decay spectra were measured at room temperature. Efficient energy transfer (ET) from Gd 3+ to Tb3+ exists in Tb3+ doped Ba3Gd(PO4)3, and the ET efficiency increases with the increase of Tb3+ concentration. The visible quantum cutting (QC) via cross relaxation was observed upon exciting low-spin (7DJ) 5d levels of Tb3+ ions. Ba3Tb(PO4)3 sample shows relatively strong emission intensity in comparison with Zn2SiO4: Mn2+ (ZSM) upon 172 nm excitation, and with a decay time τ1/10 about 6.4 ms under 351 nm excitation, indicating the potential application of this phosphor for plasma display panels (PDPs) and Hg-free lamps. ©2011 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; Photoluminescence; (300.6280) Spectroscopy, fluorescence and luminescence.

(250.5230)

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

J. Zhong, H. Liang, B. Han, Q. Su, and Y. Tao, ―NaGd(PO3)4: Tb3+—a new promising green phosphor for PDPs application,‖ Chem. Phys. Lett. 453(4-6), 192–196 (2008). C. H. Kim, I. E. Kwon, C. H. Park, Y. J. Hwang, H. S. Bae, B. Y. Yu, C. H. Pyun, and G. Y. Hong, ―Phosphors for plasma display panels,‖ J. Alloy. Comp. 311(1), 33–39 (2000). D. Y. Wang and N. Kodama, ―Visible quantum cutting through downconversion in GdPO4: Tb3+ and Sr3Gd(PO4)3: Tb3+,‖ J. Solid State Chem. 182(8), 2219–2224 (2009). T. J. Lee, L. Y. Luo, E. W. G. Diau, T. M. Chen, B. M. Cheng, and C. Y. Tung, ―Visible quantum cutting through downconversion in green-emitting K2GdF5: Tb3+ phosphors,‖ Appl. Phys. Lett. 89(13), 131121 (2006). H. Y. Tzeng, B. M. Cheng, and T. M. Chen, ―Visible quantum cutting in green-emitting BaGdF5: Tb3+ phosphors via downconversion,‖ J. Lumin. 122–123, 917–920 (2007). B. Han, H. B. Liang, Y. Huang, Y. Tao, and Q. Su, ―Vacuum ultraviolet-visible spectroscopic properties of Tb3+ in Li(Y, Gd)(PO3)4: tunable emission, quantum cutting, and energy transfer,‖ J. Phys. Chem. C 114(14), 6770– 6777 (2010). H. J. Zhang, Y. H. Wang, Y. Tao, W. H. Li, D. K. Hu, E. X. Feng, and X. F. Nie, ―Visible quantum cutting in Tb3+-doped BaGdB9O16 via downconversion,‖ J. Electrochem. Soc. 157(8), J293–J296 (2010). H. B. Liang, Y. Tao, and Q. Su, ―The luminescent properties of Ba3Gd1xLnx(PO4)3 under synchrotron radiation VUV excitation,‖ Mater. Sci. Eng. B 119(2), 152–158 (2005). Z. F. Tian, H. B. Liang, B. Han, Q. Su, Y. Tao, G. B. Zhang, and Y. B. Fu, ―Photon cascade emission of Gd3+ in Na(Y,Gd)FPO4,‖ J. Phys. Chem. C 112(32), 12524–12529 (2008). J. Barbier, ―Structural refinements of eulytite-type Ca3Bi(PO4)3 and Ba3La(PO4)3,‖ J. Solid State Chem. 101(2), 249–256 (1992). T. Tsuboi, H. J. Seo, B. K. Moon, and J. H. Kim, ―Optical studies of Eu3+ ions in Bi4Ge3O12 crystals,‖ Physica B 270(1-2), 45–51 (1999). M. F. Hoogendorp, W. J. Schipper, and G. Blasse, ―Cerium(III) luminescence and disorder in the eulytite structure,‖ J. Alloy. Comp. 205(1-2), 249–251 (1994). E. H. Arbib, B. Elouadi, J. P. Chaminade, and J. Darriet, ―The crystal structure of the phosphate eulytite Ba3Bi(PO4)3,‖ Mater. Res. Bull. 35(5), 761–773 (2000).

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14. R. D. Shannon, ―Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,‖ Acta Crystallogr. A 32(5), 751–767 (1976). 15. P. Dorenbos, ―The 5d level positions of the trivalent lanthanides in inorganic compounds,‖ J. Lumin. 91(3-4), 155–176 (2000). 16. J. Zhong, H. Liang, Q. Su, J. Zhou, Y. Huang, Z. Gao, Y. Tao, and J. Wang, ―Luminescence properties of NaGd(PO3)4: Eu3+ and energy transfer from Gd3+ to Eu3+,‖ Appl. Phys. B 98(1), 139–147 (2010). 17. L. Wang, X. Zhang, Z. D. Hao, Y. S. Luo, J. H. Zhang, and X. J. Wang, ―Interionic energy transfer in Y3Al5O12:Ce3+, Pr3+ phosphor,‖ J. Appl. Phys. 108(9), 093515 (2010). 18. M. B. Xie, Y. Tao, Y. Huang, H. B. Liang, and Q. Su, ―The quantum cutting of Tb3+ in Ca6Ln2Na2(PO4)6F2 (Ln = Gd, La) under VUV-UV excitation: with and without Gd3+,‖ Inorg. Chem. 49(24), 11317–11324 (2010). 19. N. Yocom, R. S. Meltzer, K. W. Jang, and M. Grimm, ―New green phosphors for plasma displays,‖ J. Soc. Inf. Disp. 4(3), 169–172 (1996).

1. Introduction Plasma display panels (PDPs) are commercially available flat panel displays nowadays, in which the phosphors are responsible for converting the vacuum ultraviolet photons, especially the Xe resonance emission line (~147 nm) and/or the Xe2 molecular emission band (~172 nm) to tricolor emitting [1], Hg-free lamps also have such a luminescence process. Zn2SiO4: Mn2+ (ZSM) is commonly used as a green-emitting phosphor in PDPs, but it has a relatively long luminescence decay time, a green afterglow would blur the image for rapid-moving picture when ZSM is used [1,2], so a green phosphor with a proper decay time and relatively strong emission intensity is needed. Tb3+ ion meets to this application, because it usually shows strong VUV excitation bands due to the 4f-5d transition in a proper host, and has a relatively suitable decay time. Recently, many efficient Tb3+ doped phosphors, for example, GdPO4: Tb3+ [3], Sr3Gd(PO4)3: Tb3+ [3], K2GdF5: Tb3+ [4], BaGdF5: Tb3+ [5], Li(Y, Gd)(PO3)4: Tb3+ [6] have been reported. Above phosphors have high luminescence efficiency due to the occurrence of the quantum cutting process. Usually two green photons arising from Tb3+ 5D4  7FJ are emitted through a two-step ET (energy transfer) process: 5D3 + 5D4  7F6 + 7F0 and 7DJ + 5 D1,2,3  7F6 + 5D4. Theoretically, a phosphor could emit more than one visible photons when one VUV/UV photon is absorbed, so the quantum efficiency (QE) could be over 100% [7]. Ba3Ln(PO4)3 are reported to be eulytite [Bi4(SiO4)3, (BSO); Bi4(GeO4), (BGO)] structural compounds. BSO and BGO crystallizes in cubic structure with space group I 43d [8]. In this paper, Tb3+ ion doped Ba3Gd(PO4)3 and Ba3La(PO4)3 are investigated, and the quantum cutting process is confirmed. The Ba3Tb(PO4)3 phosphor shows relatively strong emission intensity under 172 nm VUV excitation and suitable decay time, which indicates the potential application of the phosphors in PDPs and Hg-free lamps. 2. Experimental section All samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0, 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5), Ba 3Gd1(x = 0.03, 0.05, 0.1), Ba3La1-xTbx(PO4)3 (x = 0.03, 0.05, 0.1) and Ba3Tb(PO4)3 were synthesized by a traditional high-temperature solid-state reaction method. The reactants BaCO3 [analytical reagent (A.R.)], NH4H2PO4 (A.R.), Na2CO3 (A.R.), Gd2O3 (99.99%), Tb4O7 (99.99%), La2O3 (99.999%) were weighed according to stoichiometric ratio. After mixing and grinding, the mixtures were first preheated at 600 °C for 2 h in air atmosphere and slowly cooled down to room temperature. Then the powders were ground and sintered at 1200 °C for 5 h in thermal carbon atmosphere. Final products were ground into white powder. The structure of the all the samples was examined by powder x-ray diffraction with Cu Kα radiation on a D8 ADVANCE X-ray diffractometer operating at 40 kV and 40 mA. The data were collected with 2θ = 10 ~90° and step size = 0.02°. The steady-state (excitation and emission) spectra in the UV range and the decay curves were measured with a FLS920 spectrometer. For steady-state spectra, a 450 W xenon lamp was used as the excitation source. For luminescence decay spectra, a 60 W μF flash lamp was used. The vacuum ultraviolet excitation spectra and corresponding emission spectra were measured at the VUV spectroscopy experimental station on beam line 4B8 of Beijing

xTbx(PO4)3

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Synchrotron Radiation Facility (BSRF), under high-energy physics mode (1.89 GeV, 350–600 mA) at 293 K by remote access. Further measurement details can be found in our previous work [9]. 3. Results and discussion 3.1 Powder X-ray diffraction The XRD measurements for all samples were carried out at room temperature. The results of seven samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0.1, 0.5), Ba3Gd1-xTbx(PO4)3 (x = 0.05, 0.2), Ba3La1-xTbx(PO4)3 (x = 0.05, 0.2) and Ba3Tb (PO4)3 are shown in Fig. 1 as examples. All these diffractograms are similar to one another and agree well with the Joint Committee for Powder Diffraction Standard File 29-0163 [Ba3Gd(PO4)3] and 29-0175 [Ba3La(PO4)3], indicating that all samples are of single phase. 29-0163 [Ba3Gd(PO4)3] Ba3Gd1-xTbx(PO4)3: x = 0.05

Relative intensity(a.u.)

Ba3Gd1-xTbx(PO4)3: x = 0.2 Ba3La1-xTbx(PO4)3: x = 0.05 Ba3La1-xTbx(PO4)3: x = 0.2 Ba3Gd0.5La0.5-xTbx(PO4)3: x = 0.1 Ba3Gd0.5La0.5-xTbx(PO4)3: x = 0.5 Ba3Tb(PO4)3 29-0175 [Ba3La(PO4)3]

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2 Theta (degree) Fig. 1. XRD patterns of samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0.1, 0.5), Ba3Gd1-xTbx(PO4)3 (x = 0.05, 0.2, 1.0), Ba3La1-xTbx(PO4)3 (x = 0.05, 0.2) and Ba3Tb(PO4)3.

Ba3Ln(PO4)3 are reported to be eulytite [Bi4(SiO4)3 (BSO); Bi4(GeO4) (BGO)] structural compounds. BSO and BGO crystallizes in cubic structure with space group I 43d , with Bi in the 16c, Si in the 12a and O in the 48e equipoints of the space group [8,10]. The site symmetry of Bi3+ is C3 symmetry [11]. In Ba3Ln(PO4)3 the oxygen ions are found equally disordered over three (48e) sites. The composition M 3Ln(PO4)3 was found to show not only a cation disorder (M2+/La3+ on Bi3+ sites) but also an oxygen sublattice disorder [12,13]. Because Tb3+ (~109.5 pm) has near ionic radius with that of Gd 3+ (~110.7 pm) and La3+ (~121.6 pm) in nine-fold coordination environment, but shows larger difference with that of Ba2+ (~147 pm) [14]. Tb3+ ions are thought to substitute Gd3+/La3+ ions in the host.

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3.2 Luminescence of Ba3Gd0.5TbxLa0.5-x(PO4)3 in UV-vis range

x=0 x = 0.01 x = 0.03 x = 0.05 x = 0.1 x = 0.2 x = 0.3 x = 0.5

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Relative intensity(a.u.)

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Fig. 2. (a) Emission spectra of Gd ion in samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0, 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) (λex = 274 nm); (b) Emission intensity of Gd 3+ (312 nm) as a function of Tb3+ concentration (x value) in samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0, 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) (λex = 274 nm); (c) Emission and (d) Excitation spectra of sample Ba3Gd0.5Tb0.1La0.4(PO4)3 (λex = 274 nm, λem = 541 nm).

Under 274 nm excitation, emission line at 312 nm was observed in Fig. 2(a), which is due to the 6PJ  8S7/2 transition of Gd3+ ions in the host. The emission intensity of band at 312 nm decreases with the increasing of Tb3+ concentration as shown in Fig. 2(a) and Fig. 2(b), indicating Gd3+Tb3+ energy transfer (ET) efficiency increases with the x value in series samples Ba3Gd0.5TbxLa0.5-x(PO4)3. Emission lines also observed in the range 360 nm ~700 nm upon 274 nm excitation as in Fig. 2(c), which are assign to 5D3  7FJ (J = 0, 1, 2, 3, 4, 5, 6) and 5D4  7FJ (J = 3, 4, 5, 6) transitions of Tb3+ ions. Figure 2(d) shows the excitation spectrum of sample Ba3Gd0.5Tb0.1La0.4(PO4)3 by monitoring emission of Tb3+ at 541 nm. For Tb3+ ion, it has 4f8 configuration, and the ground state is 7FJ. When an electron is excited to 5d orbital, two 4f75d1 states with different energies can be observed: low spin level 7DJ with higher energy and high spin level 9DJ with lower energy. Both 7DJ and 9DJ states can be further split by the crystal field. High energetic 7FJ 7DJ transition is spin-allowed and therefore is expected to with higher probability; low energetic 7FJ 9DJ transition is spin-forbidden and should be with lower probability. In the excitation spectrum (d), two broad bands at about 232 nm and 260 nm were observed. As a simple estimate, the excitation at about 232 nm is attributed to the lowest spin-allowed 5d transition absorption band, and that at about 260 nm is from the lowest spin-forbidden transition absorption. The lowest 4f5d state for Ba3Gd(PO4)3: Ce3+ was reported to be ~(31.25  0.48) × 103 cm1 [8]. Accordingly, the energy difference between the spin-allowed f-d transitions of Tb3+ and Ce3+ is ~(11.9  0.48) × 103 cm1, which is near the average energy difference between Tb3+ and Ce3+ in the host [15]. Usually, the lowest spin-forbidden 4f  5d absorption band is ~(6300  900 cm1) lower than the lowest spin-allowed 4f  5d absorption band [15]. When we marked 260 nm as the lowest spin-forbidden 4f  5d absorption band, the energy difference between the two bands is ~4641 cm 1. The observed position is about 10 nm higher than the estimated one (~272 nm), showing a little difference

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from the reference. In addition, the excitation lines at 312 nm and 274 nm due to transitions from 8S7/2 to 6PJ and 6IJ of Gd3+ ion can be observed, which confirms energy transfer from Gd3+ to Tb3+ in the host. Other weak excitation lines in 300 ~400 nm belong to the f-f transitions of Tb3+ ion. 3.3 Gd3+Tb3+ ET in Ba3Gd0.5TbxLa0.5-x(PO4)3 Decay curves of Ba3Gd0.5TbxLa0.5-x(PO4)3

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Logarithmic Intensity(a.u.)

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Time (ms) Fig. 3. Luminescence decays of Tb3+ 5D4 in samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) at room temperature (λex = 274 nm, λem = 541 nm).

Decay curves for samples Ba3Gd0.5TbxLa0.5-x(PO4)3 with different doping concentration (x = 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) were measured at room temperature (λ ex = 274 nm, λem = 541 nm) and shown in Fig. 3. When Gd3+ ions were excited at 274 nm, two different processes can be observed for Tb3+ emission: decay process and build-up process. Gd3+ first absorbed energy by 8S7/2  6I7/2 transition, and then transferred energy to Tb 3+ ions in build-up process. Two processes are significantly influenced by Tb 3+ concentration, which become faster and faster with the increasing of the Tb3+ doping concentration, indicating Gd3+Tb3+ and Tb3+Tb3+ energy transfer processes are more efficient with the increase of Tb 3+ doping concentration.

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Logarithmic Intensity(a.u.)

Theoretical results

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Time(ms) Fig. 4. The theoretical luminescence decay curves of Ba 3Gd0.5TbxLa0.5-x(PO4)3 (x = 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) samples at room temperature (λex = 274 nm, λem = 541 nm).

When the Ba3Gd0.5TbxLa0.5-x(PO4)3 samples were excited by 274 nm, the rate equations for the population densities in the 6P7/2 level of Gd3+ ion and 5D4 level of Tb3+ ion can be written as follows: *

*

P dN Gd NP P*   PGd  KGd Tb N Gd , dt  Gd *

*

dN TbD ND P*   Tb  KGd Tb N Gd , dt  TbD *

(1)

(2)

*

P where the N Gd and N TbD are the population densities in the 6P7/2 level of Gd3+ ion and 5D4

level of Tb3+ ion, KGd Tb is the nonradiative energy transfer rate from 6P7/2 state of Gd3+ to 5D4 state of Tb3+ ions in the samples. Then the fluorescence intensity I (t) of Tb3+ ions at 541 nm under 274 nm excitation can be given as following:

I (t )  N TbD* (t ) 

P* KGd  TbN Gd (0)  t t  exp(  P )  exp(  D )  ,  1 1  Gd  Tb    P D

(3)

 Tb  Gd

P* Here N Gd (0) is the initial value of the population density in the excited 6P7/2 state within Gd3+ ion immediately after the excitation and we ignore the time nonradiative energy transfer from

IJ to 6PJ state within Gd3+.  Gd is the decay time of Gd3+ ion under 274 nm excitation; the decay time for Tb3+ ion under 5DJ excitation [16]. 6

P

 TbD is

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Logarithmic intensity(arb.units)

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Fig. 5. Luminescence decay of Gd P7/2 in samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0, 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) (λex = 274 nm, λem = 312 nm) at room temperature. P Using the measured  TbD and  Gd , we got theoretical decay curves for samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) under 274 nm excitation and monitoring at 541 nm in Fig. 4. The theoretical results agree well with the experimental observations. Figure 5 shows the Tb3+ ion concentration dependence of the luminescence decay curves of Gd3+ emission for all the powder samples Ba3Gd0.5TbxLa0.5-x(PO4)3 (x = 0, 0.01, 0.03, 0.05, 0.1, 0.2, 0.3, 0.5) under 274 nm excitation at room temperature. Without Tb 3+ ion, the luminescence of Gd3+ in sample Ba3Gd0.5La0.5(PO4)3 shows a nearly exponential decay. When Tb3+ ions are introduced, the decays deviate from exponential. This deviation is more evident with the increase of the Tb3+ doping concentration, and the decay becomes faster and faster due to the energy transfer from Gd3+ to Tb3+. Because of the non-exponential decay of Gd3+ in the presence of Tb3+, we define an average fluorescence lifetime of the Gd3+ as follows [17]:



 Gd   I Gd (t )dt,

(4)

0

where IGd(t) is normalized to its initial intensity. For an approximate estimation, we use the decay constant 0 of sample Ba3Gd0.5La0.5(PO4)3 as the intrinsic lifetime of Gd3+ fluorescence. Then, the efficiency of Gd3+Tb3+ ET ( GdTb ) might be evaluated with formula (5). The results are listed in Table 1.

GdTb  1   Gd /  Gd 0 .

(5)

We can observe that the energy transfer efficiencies of Gd Tb increase from 13% for x = 0.01 to 97% for x = 0.5, indicating more efficient energy transfer process occurred in the sample with a high Tb3+ ion concentration. 3+

3+

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Table 1. The Lifetime Values of the 6P7/2 State Within Gd3+ ions (τ) in Ba3Gd0.5TbxLa0.53+ 3+ x(PO4)3 Samples Under 274 nm Excitation and the Energy Transfer Efficiency (ŋGd Tb ) x value 0 0.01 0.03 0.05 0.1 0.2 0.3 0.5

Average lifetime (ms) 2.72 2.36 1.64 1.05 0.70 0.32 0.19 0.07

ŋGd  Tb(%) 0 13 40 62 74 88 93 97

3.4 Cross-relaxation and quantum cutting in the system Tb3+ ion is one of the typical ions that show cross-relaxation, it is known that Tb3+ ion shows blue emission when emitting from 5D3 is domination, Tb3+ ion doped phosphors are more often used as green-emitting materials if the emission is mainly from 5D4 level. Usually, when concentration is low, it mainly shows blue emission from 5D3, but with the increasing of doping concentrations, the 5D3 emission will be gradually quenched, resulting in strong green emission from 5D4 [4–6]. Samples Ba3Gd1-xTbx(PO4)3 (x = 0.03, 0.05, 0.1), Ba3La1-xTbx(PO4)3 (x = 0.03, 0.05, 0.1) and Ba3Tb(PO4)3 were excited under 232 nm, and the emission spectra are shown in Fig. 6. For Ba3Gd1-xTbx(PO4)3 (x = 0.03, 0.05, 0.1) samples, emission from 5D3 at 378 nm can be observed clearly for sample x = 0.03, when x value increases to 0.05 and 0.1, emission band at 378 nm decreases. This emission band can hardly be observed for x = 1.0 (sample Ba3Tb(PO4)3), which is attributed to the enhancement of the Tb 3+ concentration dependent cross-relaxation process (5D3 + 5D4  7F6 + 7F0) between Tb3+ ions. Ba3La1-xTbx(PO4)3 (x = 0.03, 0.05, 0.1) samples show the same trend, When Tb 3+ content increases, blue emission from 5D3 levels decreases and green emission from 5D4 levels increases. In our system, the 5D4 emission intensity increases with increasing Tb3+ concentration, the concentration quenching has not been evidently observed.

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750

Wavelength(nm) Fig. 6. Emission spectra of Ba3Gd1-xTbx(PO4)3(x = 0.03, 0.05, 0.1), Ba3La1-xTbx(PO4)3 (x = 0.03, 0.05, 0.1) and Ba3Tb(PO4)3 under 232 nm excitation.

The quantum efficiency can be improved through quantum cutting by another crossrelaxation (7DJ + 5D1,2,3  7F6 + 5D4) process. Quantum cutting for Tb3+ ion has been achieved in fluoride and phosphate [3,4]. All the systems that can achieve quantum cutting process share the two characteristics: first, almost all systems contain Gd3+ ion; second, the process was achieved under high energy such as VUV photon and lowest spin-allowed 5d (7DJ) band excitation. According to our above analysis, the band at 232 nm is attributed to the lowest spinallowed 5d transition absorption. For Ba3Gd0.95Tb0.05(PO4)3 sample, under different wavelengths (172, 232, 274 and 351 nm) excitation, the emission spectra are shown in Fig. 7(a). All the curves were normalized to the 5D3  7F6 emission intensity at 378 nm of Tb3+. To clearly display the experimental errors under different excitations, the inset figures c and d show the normalized emission spectra range from 373 nm to 400 nm.

#144206 - $15.00 USD Received 18 Mar 2011; revised 29 Apr 2011; accepted 14 May 2011; published 23 May 2011

(C) 2011 OSA

6 June 2011 / Vol. 19, No. 12 / OPTICS EXPRESS 11079

4

c

Ba3Gd0.95Tb0.05(PO4)3

a

ex = 172 nm

Relative intensity(a.u.)

3

ex = 232 nm

2 370

380

390

ex = 274 nm

400

ex = 351 nm

1 0 4

d

b

Ba3La0.95Tb0.05(PO4)3

3

ex = 172 nm ex = 232 nm

2 370

380

390

ex = 351nm

400

1 0 400

450

500

550

600

650

Wavelength(nm) Fig. 7. Samples Ba3Gd0.95Tb0.05(PO4)3(a) and Ba3La0.95Tb0.05(PO4)3(b) emission spectra under different wavelength excitation(all the curves were normalized by the 5D3  7F6 emission at 378 nm of Tb3+); the inset figure c and d are the magnified 5D3  7F6 emission band.

In previous studies, after normalized the spectrum to 5D3  7F6 intensity, the 5D4 emission intensity under Gd3+ 6IJ excitation is usually as a reference to prove quantum cutting of Tb 3+ [18]. At that moment, when an electron is excited to 6IJ level of Gd3+, it can transfer to 5DJ levels of Tb3+ ion, then yield 5D3/5D4 emission after the multiphonon relaxation and crossrelaxation. The emission intensity ratio of 5D4 to 5D3 has a fixed value at a specific doping concentration and temperature. In other cases, if the 5D4/5D3 emission intensity ratio is higher when the 5d levels of Tb3+ or the host are excited than that when the 6IJ level of Gd3+ is excited, one think the occurrence of the quantum cutting process [18]. According to above viewpoint, the emission intensity ratio of 5D4/5D3 is expected to be same for an exact sample respectively under 351 or 274 nm excitations. But in our experiment, we found that, direct (351 nm excitation) and indirect (274 nm excitation) excitation are different, the blue curve (under 351 nm excitation) in Fig. 7 exhibits somewhat higher 5D4/5D3 emission than the brown curve (under 274 nm excitation), maybe due to other unknown mechanism during the energy transfer process from Gd3+ to Tb3+ and within Tb3+ ions or due to the experimental error. Here we choose the emission intensity under direct excitation of Tb 3+ at 351 nm as reference to confirm quantum cutting. The energy difference between the lowest low-spin 5d (7DJ) and the 5D3 level is ~16.8 × 3 10 cm1 in present case, which is near the difference between 7F6 and 5D4 levels, so the following resonant type cross-relaxation can occur: Tb3  7 D J   Tb3 ( 7 F6 )Tb 3  5 D 4   Tb 3  5 D 3  .

So under low-spin 5d excitation, the 5D4 emission results from two processes  and , while when excited to the 5DJ states of Tb3+ (351 nm excitation), emission just comes from process  in Fig. 8. Therefore, two emitting photons will be generated through the crossrelaxation process; the quantum efficiency of Tb3+ can exceed 100% upon excitation to its low-spin 5d states in theory. The intensity of the 5D4 emission can be enhanced upon 5d excitation relative to that upon 351 nm excitation for a sample with a specific doping concentration.

#144206 - $15.00 USD Received 18 Mar 2011; revised 29 Apr 2011; accepted 14 May 2011; published 23 May 2011

(C) 2011 OSA

6 June 2011 / Vol. 19, No. 12 / OPTICS EXPRESS 11080

50000

7

45000

low-spin 5d ( DJ)

40000

Energy(cm-1)

35000

①

30000 5

5

D3

D3

25000

5

5

D4

20000

D4

15000

① ②

10000 5000

7

7

F6

F6

0

Tb3+

Tb3+

Fig. 8. The quantum cutting process of Tb3+ion in the host.

Another Tb3+ doped sample Ba3La0.95Tb0.05(PO4)3 was also investigated to compared with Ba3Gd0.95Tb0.05(PO4)3, the results are shown in Fig. 7(b). The emission intensity ratio of 5D4 to 5 D3 under 232 nm and 172 nm excitation are a little higher than under 351 nm excitation, indicating quantum cutting process also can occur without Gd 3+ ion in the system. However, the 5D4/5D3 emission intensity ratio of sample Ba3Gd0.95Tb0.05(PO4)3 is larger than that of sample Ba3La0.95Tb0.05(PO4)3 under the same excitation wavelengths, showing that the quantum cutting is inefficient without Gd3+. The emission curve under 351 nm is used as reference. Then following equation was used to estimate the quantum cutting efficiency [4]:

=

P(QC ) 5 D4  P( NQC ) 5 D4 , P( NQC ) 5 D4  1

(6)

where P(QC )5 D4 is the emission intensity ratio of 5D4 to 5D3 when the quantum cutting occurs, P( NQC )5 D4 is the emission intensity ratio of 5D4 to 5D3 when the quantum cutting process does not occur. Here we assume there is no possible nonradiative loss during the process. The results are displayed in Table 2. Table 2. The Emission Intensity Ratio of 5D4 to 5D3 and the Quantum Cutting Efficiency λex(nm) 172 232 351

Ba3Gd0.95Tb0.05(PO4)3 If (5D4/5D3) 2.67 3.52 2.18

ŋ(QC) 22% 42% 0

λex(nm) 172 232 351

Ba3La0.95Tb0.05(PO4)3 If (5D4/5D3) 2.31 2.39 2.20

ŋ(QC) 5% 6% 0

From Table 2, we can see that the quantum cutting efficiencies are different with Gd3+ and without Gd3+ in the system. When Gd3+ exists in the host, the quantum efficiency is roughly calculated to be 22% and 42%. Without Gd3+ ion, the efficiencies are just about 5% and 6%. But what the real role Gd3+ ion played in the system, it still needs further investigation.

#144206 - $15.00 USD Received 18 Mar 2011; revised 29 Apr 2011; accepted 14 May 2011; published 23 May 2011

(C) 2011 OSA

6 June 2011 / Vol. 19, No. 12 / OPTICS EXPRESS 11081

3.5 Potential applications The VUV excitation spectrum and 172 nm VUV excited emission spectrum of the commercial green phosphor Zn2SiO4: Mn2+ (ZSM), which were measured at the same conditions as the sample Ba3Tb(PO4)3 (BTP) at room temperature are plotted in Fig. 9. ZSM shows a broad emission band and the emission peak is at about 524 nm, the excitation spectrum shows several excitation bands due to the host related absorption and the electronic transitions of Mn2+. All the emission bands observed in the BTP emission spectrum (black curve) were attributed to the 5D4  7FJ transitions of Tb3+, and three excitation bands were shown in the range 125-360 nm in Fig. 9(a), the band around 174 nm may mainly due to the host-related absorption, 230 nm and 258 nm are the lowest spin-allowed (7FJ 7DJ) and spin-forbidden (7FJ 9DJ) absorptions, respectively. The integrated emission intensity of Ba 3Tb(PO4)3 (BTP) is about 34% and 51% of Zn2SiO4: Mn2+ (ZSM) under 147 nm and 172 nm excitation, respectively. The decay curves of Ba3Tb (PO4)3 (BTP) and Zn2SiO4: Mn2+ (ZSM) were measured at room temperature as in Fig. 10, sample BTP can be well fitted using a single exponential equation and the decay constant τ1/10 6.4 ms. Zn2SiO4: Mn2+ (ZSM) shows has a relatively long luminescence decay time with τ1/10 about 8.0 ms. Though the above curves are obtained under 351 nm and 250 nm excitation but not under VUV (147 and172 nm) excitation, it is considered that the activator decay under VUV excitation will be around these values in the present case [16,19]. ex = 147 nm, BTP

em = 541 nm, BTP

ex = 147 nm, ZSM

Relative intensity(a.u.)

em = 524 nm, ZSM

ex = 172 nm, BTP

230 nm

174 nm

ex = 172 nm, ZSM

258 nm

a

b

125 150 175 200 225 250 275 300 325 350300 350 400 450 500 550 600 650 700

Wavelength(nm) Fig. 9. The excitation and emission spectra of Ba 3Tb(PO4)3(BTP) and Zn2SiO4: Mn2+ (ZSM) samples at RT.

Ba3Tb(PO4)3 (BTP) shows relatively strong emission bands under 172 nm excitation and exhibits proper decay time under 351 nm excitation, which indicates Ba 3Tb (PO4)3 (BTP) may be potential application in plasma display panels (PDPs) and Hg-free lamps.

#144206 - $15.00 USD Received 18 Mar 2011; revised 29 Apr 2011; accepted 14 May 2011; published 23 May 2011

(C) 2011 OSA

6 June 2011 / Vol. 19, No. 12 / OPTICS EXPRESS 11082

1000

Logarithmic Intensity(a.u.)

BTP (ex = 351 nm, em = 541 nm) ZSM (ex = 250 nm, em = 524 nm)

100

10 0

2

4

6

8

10

12

14

16

18

20

Time(ms) Fig. 10. Decay curves of Ba3Tb(PO4)3 (BTP) together with the decay curve of commercial phosphor Zn2SiO4: Mn2+ (ZSM) at room temperature.

4. Conclusions The luminescence properties of Tb3+ doped Ba3Gd(PO4)3 and Ba3La(PO4)3 samples were investigated at room temperature. The lowest spin-allowed and spin-forbidden 5d absorption for Tb3+ in the host are located at 232 nm and 260 nm. The energy transfer efficiency from Gd3+ to Tb3+ increases from 13% to 97% with the increasing of Tb 3+ concentration from 1% to 50%. Visible quantum cutting through cross-relaxation (7DJ + 5D1,2,3  7F6 + 5D4) was observed for Ba3Gd0.95Tb0.05(PO4)3 and Ba3La0.95Tb0.05(PO4)3 samples under 232 nm and 172 nm excitation. Ba3Tb (PO4)3 (BTP) sample shows strong emission intensity and a proper decay time, which may make it a potential phosphor for PDPs and Hg-free lamps. Acknowledgments The work is financially supported by National Basic Research Program of China (973 Program, Grant No. 2007CB935502), National Natural Science Foundation of China (Grant Nos. 20871121 and 10979027), and Natural Science Foundation of Guangdong Province (Grant No. 9151027501000003).

#144206 - $15.00 USD Received 18 Mar 2011; revised 29 Apr 2011; accepted 14 May 2011; published 23 May 2011

(C) 2011 OSA

6 June 2011 / Vol. 19, No. 12 / OPTICS EXPRESS 11083