Versatile phosphors BaY 2 Si 3 O 10 : RE ( RE = Ce 3 + , Tb 3 + , Eu 3 + ) for light-emitting diodes

Rare-earth-activated BaY2Si3O10 (BYSO) phosphors were synthesized via a solid-state reaction. BaY2Si3O10:Ce 3+ yields an indigo-blue emission peak at 404 nm according to excitation at 334 nm attributed to the Ce 4f→5d transition. BaY2Si3O10:Tb 3+ typically generates green emission peaks resulting from the D4→ FJ transition. BaY2Si3O10:Eu 3+ exhibits red emission peaks upon excitation at 393 nm. The quantum efficiency of these phosphors was found to be 53%, 55%, and 63% of commodity. The results in this work demonstrate that these phosphors with new compositions are good candidate luminescent materials for use in plasma display panels and light-emitting diodes, excited from VUV to UV. 2009 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; (250.5230) Photoluminescence; (300.6280) Spectroscopy, fluorescence and luminescence. References and links 1. Z. Zhang and Y. Wang, “UV-VUV excitation luminescence properties of Eu-doped Ba2MSi2O7 (M = Mg, Zn),” J. Electrochem. Soc. 154, 2, J62-J64 (2007). 2. L. Jiang, C. Chang, D. Mao, and C. Feng, “Concentration quenching of Eu in Ca2MgSi2O7:Eu phosphor,” Mat. Sci. Eng. B-Solid 103, 271-275 (2003). 3. S. H. M. Poort, H. M. Reijnhoudt, H. O. T. van der Kuip, and G. Blasse, “Luminescence of Eu in silicate host lattices with alkaline earth ions in a row,” J. Alloy. Compd. 241, 75-81 (1996). 4. S. Kubota and M. Shimada, “Sr3Al10SiO20:Eu as a blue luminescent material for plasma displays,” Appl. Phys. Lett. 81, 15, 2749-2751 (2002). 5. J. K. Park, M. A. Lim, K. J. Choi, and C. H. Kim, “Luminescence characteristics of yellow emitting Ba3SiO5:Eu phosphor,” J. Mater. Sci. 40, 15, 2069-2071 (2005). 6. J. Liu, J. Sun, and C. Shi, “A new luminescent material:Li2CaSiO4:Eu,” Mater. Lett. 60, 2830-2833 (2006). 7. M. P. Saradhi and U. V. Varadaraju, “Photoluminescence studies on Eu-activated Li2SrSiO4-a potential orange-yellow phosphor for solid-state lighting,” Chem. Mater. 18, 5267-5272 (2006). 8. X. Bai, G. Zhang, and P. Fu, “Photoluminescence properties of a novel phosphor, Na3La9O3(BO3)8:RE (RE = Eu, Tb),” J. Solid State Chem. 180, 1792-1795 (2007). 9. R. J. Xie, M. Mitomo, K. Uheda, F. F. Xu, and Y. Akimune, “Preparation and luminescence spectra of calciumand rare-earth (R = Eu, Tb, and Pr)-codoped α-SiAlON ceramics,” J. Am. Ceram. Soc. 85, 5, 1229-1234 (2002). 10. J. W. H. van Krevel, J. W. T. Van Rutten, H. Mandal, H. T. Hintzen, and R. Metselaar, “Luminescence properties of terbium-, cerium-, or europium-doped α-sialon materials,” J. Solid State Chem. 165, 19-24 (2002). 11. H. Zhang, T. Horikawa, and K. I. Machida, “Preparation, structure, and luminescence properties of Y2Si4N6C:Ce and Y2Si4N6C:Tb,” J. Electrochem. Soc. 153, 7, H151-H154 (2006). 12. W. R. Liu, Y. C. Chiu, C. Y. Tung, Y. T. Yeh, S. M. Jang, and T. M. Chen, “A study on the luminescence properties of CaAlBO4:RE (RE = Ce, Tb, and Eu),” J. Electrochem. Soc. 155, 9, J252-J255 (2008). 13. U. Kolitsch, M. Wierzbicka, and E. Tillmanns, “BaY2Si3O10: a new flux-grown trisilicate,” Acta Cryst. C62, i97-i99 (2006). 14. G. Blasse and B. C. Grabmaier, Luminescent Materials, (Springer, Berlin, German, 1994), Chap. 3. 15. R. P. Rao, “Tm activated lanthanum phosphate: a blue PDP phosphor,” J. Lumin. 113, 271-278 (2005). #112984 $15.00 USD Received 17 Jun 2009; revised 23 Jul 2009; accepted 26 Jul 2009; published 24 Sep 2009 (C) 2009 OSA 28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 18103 16. R. P. Rao, “Tb activated green phosphors for plasma display panel applications,” J. Electrochem. Soc. 150, 8, H165-H171 (2003). 17. S. Bhushan and M. V. Chukichev, “Temperature-dependent studies of cathodoluminescence of green band of ZnO crystals,” J. Mater. Sci. Lett. 9, 319-321 (1988). 18. R. J. Xie, N. Hirosaki, N. Kimura, K. Sakuma, and M. Mitomo, “2-phosphor-converted white light-emitting diodes using oxynitride/nitride phosphors,” Appl. Phys. Lett. 90, 191101-191103 (2007). 19. C. C. Lin, R. S. Liu, Y. S. Tang, and S. F. Hu, “Full-color and thermally stable KSrPO4:Ln (Ln = Eu, Tb, Sm) phosphors for white-light-emitting diodes,” J. Electrochem. Soc. 155, 9, J248-J251 (2008). 20. G. Blasse, “Energy transfer in oxidic phosphors,” Philips Res. Rep. 24, 131 (1969).

X-ray diffraction (XRD) was performed using a PHILIPS X'pert PRO diffractometer with CuKα (1.5418Å) radiation.The photoexcitation (PLE) and emission (PL) spectra were obtained at room temperature using a Spex Fluorolog-3 spectrophotometer with 450W Xe light sources.All of the spectra were obtained at a scan rate of 150 nm min -1 .The VUV photoluminescence (PL) and photoluminescent excitation (PLE) spectra were obtained at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan using the BL03A beam line.The PLE spectra were collected by scanning a 6 m in length cylindrical grating monochromator with a grating at 450 l/min, over a wavelength range of 100-350 nm.The Commission International de I'Eclairage (CIE) chromaticity coordinates were measured using a Laiko DT-101 color analyzer that was equipped with a CCD detector (Laiko Co., Tokyo, Japan).The reflectance spectra of the samples were obtained using a Hitachi 3010 doublebeam UV-vis spectrometer (Hitachi Co., Tokyo, Japan) that was equipped with an Ø60-mm integrating sphere whose inner face was coated with Spectralon® (polytetrafluoroethylene, PTFE); α-Al 2 O 3 was used as a standard in the measurements.[14].The radii of ions of the rare-earth dopant elements, Ce 3+ (CN = 6, r = 1.01 Å), Tb 3+ (CN = 6, r = 0.923 Å), and Eu 3+ (CN = 6, r = 0.947 Å) are such that these rare-earth ions were expected to occupy the Y 3+ sites in the BaY 2 Si 3 O 10 host.The PLE spectrum included excitation humps at 296, 334, and 357 nm.A stronger excitation hump between 300 and 360 nm was observed to correspond to the 4f-5d transition of Ce 3+ .The PL spectrum has an emission peak at 404 nm, attributed to the transition of 5d to 4f transition.The Stokes shift for Ce 3+ in the BaY 2 Si 3 O 10 host was determined to be ~5188 cm -1 .The PL spectrum was further deconvoluted by assuming a Gaussian profile of two emission peaks at 400 and 444 nm, attributed to the transitions of 5d to 2 F 5/2 and 2 F 2/7 , respectively.The energy difference between 400 and 444 nm was calculated to be ~2478 cm -1 , which is close to theoretical value of ~2000 cm -1 [14].
Figure 2(c) shows the PL and PLE spectra of composition-optimized Ba(Y 0.4 Eu 0.6 ) 2 Si 3 O 10 phosphor.The broad band at ~262 nm were attributed to the charge transfer transition O 2-→ Eu 3+ , and the sharp lines between 300 and 450 nm were resulted from the f-f transition of Eu 3+ ions.The PL spectrum exhibited typical emission lines assigned to the transitions 5 D 0 to 7 F J (J = 1, 2, 3, 4).The highly intense line at 590 nm is well know to be associated with the magnetic dipole 5 D 0 → 7 F 1 transition and the strong line at 611 nm is corresponds to the electric dipole transition.In this work, the dominant emission peaks of BaY 2 Si 3 O 10 :Eu 3+ located at 611 nm are caused by the electric dipole transition, indicating that the Eu 3+ ions occupied the sites of non-inversion symmetry [11].The emission peak of Eu 3+ at ~579 nm originated from the 5 D 0 → 7 F 0 transition, which is a forbidden transition.The 5 D 0 → 7 F 0 transition is observed when Eu 3+ occupies a lattice site with C v , C nv or C s symmetry [12].In this investigation, a single emission peak at 579 nm indicates that Eu 3+ occupied only one Y 3+ site, which observation is consistent with the site symmetry of Y 3+ and the crystalline structure of BaY 2 Si 3 O 10 .Additionally, the series of BYSO samples is determined to be suitable for VUV excitation (λ = 172 nm), as displayed in Fig. 3.The excitation bands of BYSO:Ce 3+ , BYSO:Tb 3+ , and BYSO:Eu 3+ at ~172 nm were attributed to host absorption.A small hump observed at ~220 nm (Fig. 3a) resulted from the f-d excitation of Ce 3+ [15].The excitation band at around 230 nm (Fig. 3b) was caused by the 7 D J transition of Tb 3+ [16].The band between 200 and 280 nm was the CT band of Eu 3+ -O 2-.The emission peak at ~460 nm may be caused by the occupation by Ce 3+ of Ba 2+ sites at the high-resolution synchrotron radiation beam line.The results indicate that these BYSO with the new compositions are good candidate luminescent materials for excitation under VUV and UV.

UV-Vis diffuse reflectance spectra and relative emission intensity dependence of temperature effect
Figure 4 presents the reflection spectra of BYSO, BYSO:5%Ce 3+ , BYSO:40%Tb 3+ , and BYSO:60%Eu 3+ .The spectrum of pristine BaY 2 Si 3 O 10 included a host absorption edge at ~230 nm from which the optical band gap was estimated to be ~5.39 eV.The Ce 3+ -doped BYSO displays a broad hump with peaks at ~360, 300, and 260 nm.The first two peaks were due to the 4f-5d transition of Ce 3+ , which is consistent with the PLE spectra of BYSO:Ce 3+ , shown in Fig. 2(a).The third peak was attributed mostly to host absorption.The BYSO:Tb 3+ yields a weak, broad band between 260 and 380 nm, attributable the f-f transition of Tb 3+ , whereas Eu 3+ -doped BYSO has an absorption peak at ~394 nm, typically attributed to the f-f transition of Eu 3+ .These results reveal that the emissions of Ce 3+ , Tb 3+ , and Eu 3+ that are doped in a BaY 2 Si 3 O 10 host correspond to the absorption of activators.With respect to the relationship between emission intensity and surrounding temperature, plotted in Fig. 5, the photoluminescence strength increased gradually as the temperature declined, because the number of phonons decreased.To determine the activation energy, the Arrhenius equation was fitted to the thermal quenching data [17][18][19]: where I 0 denotes the initial integrated peak area; I(T) is the integrated peak area at a given temperature T; c is a constant; E is the activation energy for thermal quenching, and k is Boltzman's constant.The activation energies for thermal quenching were found to be 0.25, 0.15, and 0.11 eV for Ce 3+ , Tb 3+ , and Eu 3+ , respectively, as given in Table 1.The luminescent performance of BaY 2 Si 3 O 10 :RE 3+ was optimized by varying the respective dopant content.Table 1 presents the PL intensity for various Ce 3+ , Tb 3+ , and Eu 3+ dopant concentrations in Ba(Y 1-x Ce x ) 2 Si 3 O 10 , Ba(Y 1-x Tb x ) 2 Si 3 O 10 , and Ba(Y 1-x Eu x ) 2 Si 3 O 10 , respectively.The optimal doping concentrations of Ce 3+ , Tb 3+ , and Eu 3+ were found to be 5, 40, and 60 mol.%, respectively.As the concentration increased beyond the critical concentration, the emission intensity began to decrease because of concentration quenching of the activators.
The following equation can be used to estimate the critical energy transfer distance (R c ) between these activators in the host, since the BaY 2 Si 3 O 10 lattice contains only one crystallographically distinct Y 3+ site with 6-coordination [13].As a result, the critical energy transfer distances between RE 3+ ions for Eu 3+ , Tb 3+ , and Ce 3+ in the three phosphors are calculated using the following equation [20]: where x c is the critical concentration; Z is the number of cation sites (per OR in the ) unit cell, and V is the volume of the unit cell.In this case, V = 432.28Å 3 , Z = 2 and the critical doping concentrations of Ce 3+ , Tb 3+ , and Eu 3+ in BaY 2 Si 3 O 10 host were 0.05, 0.4, and 0.6, respectively.Therefore, the R c values of Ce 3+ , Tb 3+ , and Eu 3+ were 34, 16, and 8 Å, respectively.In order to further determine the quantum efficiency of photo-conversion for these novel phosphors, herein we have used integrated sphere method for the measurements of quantum efficiency (Φ) of phosphor samples.The quantum efficiencies of BaY 2 Si 3 O 10 :RE 3+ phosphors can also be calculated by using these following equations: where E i (λ) is the integrated luminescence of the powder upon direct excitation, and E o (λ) is the integrated luminescence of the powder excited by indirect illumination from the sphere.The term L e (λ) is the integrated excitation profile obtained from the empty integrated sphere (without the sample present).The corresponding QE was found to be 53%, 55%, and 63% of BaMgAl 10 O 17 :Eu 2+ (blue), LaPO 4 :Ce 3+ ,Tb 3+ (green), and La 2 O 2 S:Eu 3+ (red), respectively.Finally, Table 1 presents the Commission International de I'Eclairage (CIE) chromaticity, decay time, critical distance, and activation energy of BYSO:RE 3+ .

Conclusion
In summary, BaY 2 Si 3 O 10 :RE (RE = Ce 3+ , Tb 3+ , Eu 3+ ) phosphors were synthesized successfully via a solid-state reaction and investigated using X-ray diffraction, photoluminescence, reflectance, and activation energy.BaY 2 Si 3 O 10 :Ce 3+ exhibits an indigo-blue emission and good thermal stability for luminescence performance.Green-emitting phosphor BaY 2 Si 3 O 10 :Tb 3+ shows a stronger luminescence and a longer decay time than others.The chromaticity coordinate of the BYSO:Eu 3+ were found to be (0.64, 0.36) in higher red color purity region.The results in this work demonstrate that this series of phosphors is expected to be promising candidates for application in PDPs and UV-LEDs.

3. 1 .
XRD patterns and atom structure of synthesized BaY 2 Si 3 O 10 BaY 2 Si 3 O 10 is a new silicate structure that was discovered by Kolitsch et al.[13].The structure is based on zigzag chains, parallel to [010], of edge-sharing distorted YO 6 octahedra, linked by horseshoe-shaped trisilicate groups and Ba atoms in irregular eight-coordination.The mean bond lengths of Y-O, Si 1 -O, Si 2 -O, and Ba-O bond lengths were 2.268, 1.626, 1.633, and 2.872Å, respectively.

Figure 1 (
Figure 1(a) presents the XRD patterns of the BaY 2 Si 3 O 10 host synthesized at different temperatures and Fig. 1(b) depicts the atomic structure of the unit cell.These figures are consistent with 240470-ICSD, revealing that a pure and highly crystalline phase of BaY 2 Si 3 O 10 was obtained in this study.BaY 2 Si 3 O 10 has a monoclinic structure with space group P2 1 /m and lattice constants of a = 5.399(1) Å, b = 12.179(2) Å, c = 6.852(1)Å, β =

Figure 2 (
Figure 2(a) shows the PL and PLE spectra of Ba(Y 0.975 Ce 0.025 ) 2 Si 3 O 10 .The PLE spectrum included excitation humps at 296, 334, and 357 nm.A stronger excitation hump between 300 and 360 nm was observed to correspond to the 4f-5d transition of Ce 3+ .The PL spectrum has an emission peak at 404 nm, attributed to the transition of 5d to 4f transition.The Stokes shift for Ce 3+ in the BaY 2 Si 3 O 10 host was determined to be ~5188 cm -1 .The PL spectrum was further deconvoluted by assuming a Gaussian profile of two emission peaks at 400 and 444 nm, attributed to the transitions of 5d to 2 F 5/2 and 2 F 2/7 , respectively.The energy difference between 400 and 444 nm was calculated to be ~2478 cm -1 , which is close to theoretical value of ~2000 cm -1[14].The excitation behavior of the terbium ion (Tb 3+ ) yielded sharp emission lines at 272, 284, 303, 317, 326, 341, 351, 354, 369, 374, and 378 nm in Fig.2(b), among which the bands in the range of 230-300 nm were attributed to the 4f 8 →4f 7 5d 1 transition and those in the range 300-400 nm were due to the 4f→4f transition.The PL spectrum on the right-hand side reveals typical Tb 3+ emission associated with the 5 D 4 → 7 F J (J = 6, 5, 4, 3) transitions, which are 5 D 4 →