Rare-earth-activated BaY2Si3O10 (BYSO) phosphors were synthesized via a solid-state reaction. BaY2Si3O10:Ce3+ yields an indigo-blue emission peak at 404 nm according to excitation at 334 nm attributed to the Ce3+ 4f→5d transition. BaY2Si3O10:Tb3+ typically generates green emission peaks resulting from the 5D4→7FJ transition. BaY2Si3O10:Eu3+ 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
Considerable interest in phosphors has led recently to the rapid development of promising display and illumination technologies. In particular, full-color photoluminescent materials are required for use in plasma display panels (PDPs) and ultraviolet light-emitting diodes (UV-LEDs). Silicates are good candidates for use in stable host structures because they have high physical-chemical stability, water-resistant properties, stable crystal structures, and excellent optical properties [1,2]. Blasse et al. reported on the fluorescent properties of Eu2+- activated binary and ternary silicates, including (Ca,Sr)2MgSi2O7, MSiO3 (M=Ba, Sr, Ca), BaSi2O5, BaMgSiO4, CaMgSiO4, and Sr2LiSiO4F. The systemization and luminescent properties of blue-excitable or UV-excitable Eu2+-activated new silicates, such as Sr3Al10SiO20 , Ba3SiO5 , Li2CaSiO4 , and Li2SrSiO4  have also been elucidated. The results of these studies reveal these silicate phosphors are potential candidate in blue-pumping or UV-pumping LED phosphors. Not only phosphors doped with divalent europium ions but also those doped trivalent rare-earth ions, such as Ce3+, Tb3+, and Eu3+, have been investigated: examples include Na3La9O3(BO3)8:RE3+ (RE=Eu, Tb) , Ca-α-SiAlON:RE (RE=Eu2+, Tb3+, and Pr3+) , (Ca,Y)SiAlON:RE (RE=Eu2+, Tb3+, and Ce3+) , and Y2Si4N6C:Ce3+ or Y2Si4N4C:Tb3+ .
Our previous work investigated the luminescent properties of novel blue-(CaAlBO4:Ce3+), green-(CaAlBO4:Tb3+), and red-(CaAlBO4:Eu3+) emitting phosphors . To the best of our knowledge, the luminescent properties of rare-earth ions – activated BaY2Si3O10 have not yet been reported upon. The aim of this study is to elucidate the synthesis, photoluminescence, thermal-stability, and color chromaticity of the new blue (BaY2Si3O10:Ce3+), green (BaY2Si3O10:Tb3+), and red phosphors (BaY2Si3O10:Eu3+) and present their corresponding spectroscopic properties under VUV and UV excitation.
BaY2Si3O10:xCe3+ (x=1, 3, 5, 10, and 15 mol%), BaY2Si3O10:yTb3+ (y=5, 10, 20, 30, 40, and 60 mol%), and BaY2Si3O10:zEu3+ (z=5, 10, 20, 30, 40, 50, 60, and 70 mol%) were synthesized via a solid-state reaction. The starting materials that were used (to prepare OR in the preparation of) these phosphors were BaCO3 (99.98%, Aldrich), Y2O3 (99.99%, Aldrich), SiO2 (>99.9%, Strem Chemicals), Eu2O3 (99.9%, Aldrich), Tb4O7 (99.9%, Strem Chemicals), and CeO2 (99.998%, Strem Chemicals). The raw materials were weighed out in stoichiometric proportions and the mixtures were then fired at 1350°C for 10 h in a 15%H2/85%N2 atmosphere (Ce3+, Tb3+) or an air atmosphere (Eu3+). The resulting powder was cooled to room temperature in a furnace, ground, and pulverized for further measurements.
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 double-beam 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-Al2O3 was used as a standard in the measurements.
3. Results and discussion
3.1. XRD patterns and atom structure of synthesized BaY2Si3O10
BaY2Si3O10 is a new silicate structure that was discovered by Kolitsch et al.. The structure is based on zigzag chains, parallel to , of edge-sharing distorted YO6 octahedra, linked by horseshoe-shaped trisilicate groups and Ba atoms in irregular eight-coordination. The mean bond lengths of Y-O, Si1-O, Si2-O, and Ba-O bond lengths were 2.268, 1.626, 1.633, and 2.872Å, respectively.
Figure 1(a) presents the XRD patterns of the BaY2Si3O10 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 BaY2Si3O10 was obtained in this study. BaY2Si3O10 has a monoclinic structure with space group P21/m and lattice constants of a=5.399(1) Å, b=12.179(2) Å, c=6.852(1) Å, β=106.37(3)o, V=432.28(14) Å3, and Z=2. The ionic radii (r) of Ba2+ (CN=8), Y3+ (CN=6), and Si4+ (CN=4) are 1.42 Å, 0.90 Å, and 0.26 Å, respectively . The radii of ions of the rare-earth dopant elements, Ce3+ (CN=6, r=1.01 Å), Tb3+ (CN=6, r=0.923 Å), and Eu3+ (CN=6, r=0.947 Å) are such that these rare-earth ions were expected to occupy the Y3+ sites in the BaY2Si3O10 host.
3.2. The VUV-UV excitation spectrum and emission spectra under various activators
Figure 2(a) shows the PL and PLE spectra of Ba(Y0.975Ce0.025)2Si3O10. 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 Ce3+. The PL spectrum has an emission peak at 404 nm, attributed to the transition of 5d to 4f transition. The Stokes shift for Ce3+ in the BaY2Si3O10 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 2F5/2 and 2F2/7, 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 .
The excitation behavior of the terbium ion (Tb3+) 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 4f8→4f75d1 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 Tb3+ emission associated with the 5D4→7FJ (J=6, 5, 4, 3) transitions, which are 5D4→ 7F6 (483, 490, 493nm), 5D4→7F5 (543, 555nm), 5D4→7F4 (585, 590nm) and 5D4→7F3 (621, 627nm). The dominant green emission peak for BaY2Si3O10:Tb3+ is at 543 nm.
Figure 2(c) shows the PL and PLE spectra of composition-optimized Ba(Y0.4Eu0.6)2Si3O10 phosphor. The broad band at ~262 nm were attributed to the charge transfer transition O2-→ Eu3+, and the sharp lines between 300 and 450 nm were resulted from the f-f transition of Eu3+ ions. The PL spectrum exhibited typical emission lines assigned to the transitions 5D0 to 7FJ (J=1, 2, 3, 4). The highly intense line at 590 nm is well know to be associated with the magnetic dipole 5D0→7F1 transition and the strong line at 611 nm is corresponds to the electric dipole transition. In this work, the dominant emission peaks of BaY2Si3O10:Eu3+ located at 611 nm are caused by the electric dipole transition, indicating that the Eu3+ ions occupied the sites of non-inversion symmetry . The emission peak of Eu3+ at ~579 nm originated from the 5D0→7F0 transition, which is a forbidden transition. The 5D0→7F0 transition is observed when Eu3+ occupies a lattice site with C v, C nv or C s symmetry . In this investigation, a single emission peak at 579 nm indicates that Eu3+ occupied only one Y3+ site, which observation is consistent with the site symmetry of Y3+ and the crystalline structure of BaY2Si3O10.
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:Ce3+, BYSO:Tb3+, and BYSO:Eu3+ at ~172 nm were attributed to host absorption. A small hump observed at ~220 nm (Fig. 3a) resulted from the f–d excitation of Ce3+ . The excitation band at around 230 nm (Fig. 3b) was caused by the 7DJ transition of Tb3+ . The band between 200 and 280 nm was the CT band of Eu3+-O2-. The emission peak at ~460 nm may be caused by the occupation by Ce3+ of Ba2+ 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.
3.3 UV-Vis diffuse reflectance spectra and relative emission intensity dependence of temperature effect
Figure 4 presents the reflection spectra of BYSO, BYSO:5%Ce3+, BYSO:40%Tb3+, and BYSO:60%Eu3+. The spectrum of pristine BaY2Si3O10 included a host absorption edge at ~230 nm from which the optical band gap was estimated to be ~5.39 eV. The Ce3+-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 Ce3+, which is consistent with the PLE spectra of BYSO:Ce3+, shown in Fig. 2(a). The third peak was attributed mostly to host absorption. The BYSO:Tb3+ yields a weak, broad band between 260 and 380 nm, attributable the f–f transition of Tb3+, whereas Eu3+-doped BYSO has an absorption peak at ~394 nm, typically attributed to the f–f transition of Eu3+. These results reveal that the emissions of Ce3+, Tb3+, and Eu3+ that are doped in a BaY2Si3O10 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 [17–19]:
where I0 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 Ce3+, Tb3+, and Eu3+, respectively, as given in Table 1. The luminescent performance of BaY2Si3O10:RE3+ was optimized by varying the respective dopant content. Table 1 presents the PL intensity for various Ce3+, Tb3+, and Eu3+ dopant concentrations in Ba(Y1-xCex)2Si3O10, Ba(Y1-xTbx)2Si3O10, and Ba(Y1-xEux)2Si3O10, respectively. The optimal doping concentrations of Ce3+, Tb3+, and Eu3+ 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 (Rc) between these activators in the host, since the BaY2Si3O10 lattice contains only one crystallographically distinct Y3+ site with 6-coordination . As a result, the critical energy transfer distances between RE3+ ions for Eu3+, Tb3+, and Ce3+ in the three phosphors are calculated using the following equation :
where xc 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=432.28 Å3, Z=2 and the critical doping concentrations of Ce3+, Tb3+, and Eu3+ in BaY2Si3O10 host were 0.05, 0.4, and 0.6, respectively. Therefore, the Rc values of Ce3+, Tb3+, and Eu3+ 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 BaY2Si3O10:RE3+ phosphors can also be calculated by using these following equations:
where Ei(λ) is the integrated luminescence of the powder upon direct excitation, and Eo(λ) is the integrated luminescence of the powder excited by indirect illumination from the sphere. The term Le(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 BaMgAl10O17:Eu2+ (blue), LaPO4:Ce3+,Tb3+ (green), and La2O2S:Eu3+ (red), respectively. Finally, Table 1 presents the Commission International de I’Eclairage (CIE) chromaticity, decay time, critical distance, and activation energy of BYSO:RE3+.
In summary, BaY2Si3O10:RE (RE=Ce3+, Tb3+, Eu3+) phosphors were synthesized successfully via a solid-state reaction and investigated using X-ray diffraction, photoluminescence, reflectance, and activation energy. BaY2Si3O10:Ce3+ exhibits an indigo-blue emission and good thermal stability for luminescence performance. Green-emitting phosphor BaY2Si3O10:Tb3+ shows a stronger luminescence and a longer decay time than others. The chromaticity coordinate of the BYSO:Eu3+ 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.
The authors would like to thank for the financial supports from Industrial Technology Research Institute under contract no. 8301XS1751, the National Science Council (Contract nos. NSC 97-2113-M-002-012-MY3 and 97-3114-M-002-005), and the Economic Affair (Contract no. 97-EC-17-A-07-S1-043).
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