1. Introduction

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.[3] 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 [4], Ba₃SiO₅ [5], Li₂CaSiO₄ [6], and Li₂SrSiO₄ [7] 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) [8], Ca-α-SiAlON:RE (RE=Eu²⁺, Tb³⁺, and Pr³⁺) [9], (Ca,Y)SiAlON:RE (RE=Eu²⁺, Tb³⁺, and Ce³⁺) [10], and Y₂Si₄N₆C:Ce³⁺ or Y₂Si₄N₄C:Tb³⁺ [11].

Our previous work investigated the luminescent properties of novel blue-(CaAlBO₄:Ce³⁺), green-(CaAlBO₄:Tb³⁺), and red-(CaAlBO₄:Eu³⁺) emitting phosphors [12]. 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.

2. Experimental

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.[13]. The structure is based on zigzag chains, parallel to [010], 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.

 

Fig. 1. (a) XRD patterns of synthesized BaY₂Si₃O₁₀ at different temperatures (I) 1100°C; (II) 1200°C; (III) 1300°C; (IV) 1350°C, and (V) pure BaY₂Si₃O₁₀ (ICSD file no.240470). (b) The atom structure of unit cell in this chemical compound.

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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) ų, 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 [14]. 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

 

Fig. 2. Photoluminescence spectra of BaY₂Si₃O₁₀ samples which prepared under various activators by (a) RE=Ce³⁺, (b) RE=Tb³⁺, and (c) RE=Eu³⁺ measured at room temperature, respectively.

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Figure 2(a) shows the PL and PLE spectra of Ba(Y_0.975Ce_0.025)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 ²F_5/2 and ²F_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 (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→⁷F_J (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(Y_0.4Eu_0.6)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 ⁷F_J (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 [11]. 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 [12]. 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.

 

Fig. 3. VUV PL and PLE spectra of BaY₂Si₃O₁₀:RE involving in (a) RE=Ce³⁺, (b) RE=Tb³⁺, and (c) RE=Eu³⁺ measured at room temperature, respectively.

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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⁺ [15]. The excitation band at around 230 nm (Fig. 3b) was caused by the 7DJ transition of Tb³⁺ [16]. The band between 200 and 280 nm was the CT band of Eu³⁺-O^2-. 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.

 

Fig. 4. Comparison of UV-Vis diffuse reflectance spectra of BaY₂Si₃O₁₀ (Host) and assynthesized BaY₂Si₃O₁₀:RE (RE=Ce³⁺, Tb³⁺, Eu³⁺) phosphors.

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Fig. 5. Relative emission intensity dependence of temperature effect of as-synthesized BaY₂Si₃O₁₀:RE (RE=5%Ce³⁺, 40%Tb³⁺, 60%Eu³⁺) phosphors.

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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(Y_1-xCe_x)₂Si₃O₁₀, Ba(Y_1-xTb_x)₂Si₃O₁₀, and Ba(Y_1-xEu_x)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 (R_c) between these activators in the host, since the BaY₂Si₃O₁₀ lattice contains only one crystallographically distinct Y³⁺ site with 6-coordination [13]. 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 [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=432.28 ų, 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 R_c 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 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 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⁺.

Table. 1. Variation performances of BaY₂Si₃O₁₀:RE (RE=Ce³⁺, Tb³⁺, Eu³⁺) measured at different condition.

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4. Conclusion

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.