The novel Ba2Gd2Si4O13:Ce3+,Tb3+ phosphors were systemically investigated by fluorescent method for the first time. Through an efficient energy transfer process, the obtained phosphors exhibit both a blue emission of Ce3+ and a yellowish green emission of Tb3+ with considerable intensity under near-ultraviolet excitation (300-370 nm). Tuning of the content of Tb3+ can generate the varied hues from blue to white and eventually to yellowish green. The quantum efficiency of the white phosphor Ba2(Gd0.88Ce0.02Tb0.1)2Si4O13 is 82.3% of commercial ZnS:Ag+,Cl- phosphor. Our results demonstrate that the potential application of these phosphors in solid-state lighting and (or) other areas.
©2010 Optical Society of America
White light-emitting diodes (LEDs), the next-generation of solid-state lighting, have attracted significant attention recently due to their potential application in many fields. They show lots of advantages over the existing incandescent and halogen lamps in power efficiency, reliability, long lifetime and environmental benefits .
In addition to three primary colors mixing emissions based on three individual LED, white light can be produced by coupling a blue or an ultraviolet (UV) LED with a down-converting phosphor [1–3]. Also, The commercial way has been produced by the combination of blue GaN-based LED with yellow-emitting phosphor Y3Al5O12:Ce3+ (YAG:Ce3+). However, the individual degradation rates between the blue LED and yellow phosphor can cause chromatic aberration and poor white light performance after long-time working [2,3]. Due to the remarkable development of UV diodes, the combination of an UV chip with red, green and blue phosphors pave a valid way to generate white light [4–10]. Nevertheless, in the three-converter system, the blue emission efficiency is low on account of the strong re-absorption of the blue light caused by the red or green-emitting phosphors. Therefore, the problem is still opened as to develop novel single-phased white-emitting phosphors, which are based on the luminescence and energy transfer (ET) between two activators.
In the two activators systems, ET process can take place from one co-activator that absorbs UV light and emits purple or blue light to another that emits a longer wavelength light such as yellow or red. Accordingly, white light has been obtained by doping co-activators such as Eu2+/Mn2+ [11,12], Ce3+/Eu2+  and Ce3+/Tb3+ [6,7] in a proper single host lattice. Silicates are appropriate host materials for rare earth ions (REI) due to their good physical and chemical stability and excellent optical properties. Thus, the luminescent properties REI-doped silicates phosphors, such as Sr2SiO4:Eu2+ , have been extensively investigated recently.
In this paper, a single-phased white-emitting Ba2Gd2Si4O13:Ce3+,Tb3+ phosphors with high quantum efficiency (about 82.3% of commercial ZnS:Ag+,Cl- phosphor) were prepared for the first time. The luminescent properties as well as ET phenomenon between the sensitizer and activator were investigated through 335 nm light excitation, which matches well with high-efficiency emitting at 335 nm of quaternary InAlGaN quantum-dot LEDs . After tuning the content of Tb3+, the emission color variation attributed to effective ET from Ce3+ to Tb3+ has been discussed.
The powder samples of Ba2Gd2Si4O13:Ce3+,Tb3+ were synthesized by a conventional solid state method. The raw materials were BaCO3 (A.R.), SiO2 (A.R.), Gd2O3 (99.99%), Tb4O7 (99.99%), Ce(NO3)3 (A.R.), HNO3 (A.R.). In the process of solid state reaction process, flux agents are often added to improve the crystallinity of the materials and to lower the reaction temperature. In this paper, Li2CO3 (>99.9%) was selected as flux by compared with NH4F, NH4Cl and H3BO3.
Tb4O7 was first dissolved in dilute HNO3 to prepare Tb(NO3)3 solution (1 M). Stoichiometric amounts of reactants were first weighed out and well ground, then fired at 1100 °C for 4 h in a weak reducing atmosphere (H2:N2=5:95). The resulting samples were cooled to room temperature and pulverized for further characterizations.
The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) on a Philips X’Pert PRO SUPER X-ray diffraction apparatus with Cu Kα radiation (λ=0.154056 nm) as the incident radiation. The excitation spectra and emission spectra were measured by using an FLS920 spectrofluorometer (Edinburgh Instruments) with a CW Xe lamp (450 W) as the excitation light source and an RR928P photomultiplier for signal detection. Luminescent decay curves of Ce3+ emissions were measured by using a nanosecond flashlamp (nF900) as the excitation source. All the measurements were carried out at room temperature.
3. Results and discussion
Ba2Gd2Si4O13 is a new silicate structure first reported by Wierzbicka et al.  in 2010. It represents a new structure type and contains finite zigzag-shaped C2-symmetric Si4O13 chains and Gd2O12 dimers consisting of edge-sharing GdO7 polyhedra. The [9+1]-coordinated Ba atoms are located in voids in the atomic arrangement. It has a monoclinic structure with space group C2/c and lattice constants of a=12.896(3) Å, b=5.212(1) Å, c=17.549(4) Å, β=104.08(3) o, V=1144.1(5) Å3, and Z=4. The ionic radii of Ba2+ (CN=8), Gd3+ (CN=6), Si4+ (CN=4), Ce3+ (CN=6) and Tb3+ (CN=6) are 1.42, 0.94, 0.26, 1.01 and 0.92 Å, respectively. In view of the effective ionic radii of cations and different coordination numbers, the REI dopants (Ce3+ and Tb3+) were expected to replace the Gd3+ sites.
Figure 1 illustrates the XRD patterns of the un-doped, Ce-doped samples and reference calculated patterns from ICSD file of Ba2Gd2Si4O13. The diffraction peaks of the samples agree quite well with the calculated patterns, indicating not only are the obtained samples be pure monoclinic phase, but also the dopants never cause any significant change.
Figure 2 presents the excitation and emission spectra of the single-doped and co-doped samples as well as the corresponding energy level scheme. The excitation spectra of Ba2(Gd0.5Tb0.5)2Si4O13 powder (Fig. 2(a)) show two absorption bands at 220-290 nm and 300-400 nm, respectively. The former is due to the spin-allowed 4f8-4f75d transition of Tb3+, and the latter describes the 4f-4f transitions of Tb3+, where the sharp lines observed at 273 and 311 nm over Tb3+ f-d excitation band is just related to the 8S7/2-6IJ, 8S7/2-6PJ transition of Gd3+ . Meanwhile, the Tb3+ doped sample exhibits a strong green emission excited by short UV light. The typical sharp emission peaks of emission spectra (λex=236 nm) in Fig. 2(a) originate from 4f-4f transitions of Tb3+ ions: 5D4-7F6 (488 nm), 5D4-7F5 (543 nm), 5D4-7F4 (582 nm) and 5D4-7F3 (619 nm) . There does not exist the emissions in the blue region that comes from the higher energy 5D3 level. As the 5D3-5D4 transition is resonant with 7F6-7F0 transition, the emission of 5D3-7FJ transitions often be quenched for high Tb3+ concentration doped samples due to the cross relaxation 5D3 + 7F6 → 5D4 + 7F0 .
The excitation spectra of Ce3+ doped powder (Fig. 2(b)) consist of three strong bands with maximum at 335 nm, 310 nm and 270 nm, which can be assigned to the Ce3+ 2F5/2-5d transition separated by the crystal-field splitting of the 5d state. Also, an odd weak sharp line can be observed at 273 nm, which is ascribed to the 8S7/2-6IJ transition of Gd3+ . Besides, within the emission spectra of Ce3+ doped sample, an UV excitation at 335 nm leads to a broad asymmetric blue emission band at ca. 400 nm, which is the result of parity-allowed transitions of the lowest component of the 5d state to 2F5/2 and 2F7/2 levels of Ce3+. Furthermore, the asymmetry peak can be deconvoluted into two Gaussian profiles centering at 388 and 420 nm with an energy difference of about 1964 cm−1, and this energy difference is fairly well in agreement with the theoretical difference between the 2F5/2 and 2F7/2 levels of Ce3+ (~2000 cm−1) .
As shown above, Ce3+ doped sample exhibits broad band emission at 350-500 nm, while Tb3+ doped sample shows an absorption band ranging from 300 to 400 nm. It means that, there is a strong overlap between Ce3+ emission and Tb3+ absorption in the range of 350- 400 nm. Therefore, it is expected that an efficient energy transfer can occur from Ce3+ to Tb3+.
Figure 2(c) depicts the excitation and emission spectra of Ba2(Gd0.88Ce0.02Tb0.1)2Si4O13 sample. 335 nm light excitation, as the optimal excitation wavelength for Ce3+ but not for Tb3+, not only does Ce3+ of the phosphor exhibit the blue emission band, but also Tb3+ radiates the yellowish green emission band. The excitation spectra monitored at 400 nm show several broad excitation bands agreement with the Ce3+ solely doped system (Fig. 2(b)). Besides, the excitation spectra monitored at 543 nm show both the broad band at 236 nm owing to the 4f8-4f75d transition of Tb3+ and the three broad bands ranging from 260 to 380 nm due to the 2F5/2-5d transitions of Ce3+. Both excitation and emission spectra in Fig. 2 indicate the high light output of Tb3+ actually comes from the ET process from Ce3+ to Tb3+.
The corresponding energy levels scheme of Ba2Gd2Si4O13:Ce3+,Tb3+ with optical transitions and energy transfer processes is displayed in Fig. 2(d). After Ce3+ firstly absorbs UV light (300-370 nm), electron is pumped to 5d level, and then non-radiatively relaxes to the lowest component of 5d level, finally decays to 2F5/2 and 2F7/2 levels by radiative process, and emitting photons (388, 420 nm). Because the value of energy level of excited 5d state of Ce3+ is close to the 5D3 and other levels of Tb3+ ions, it is highly possible that energy transfers from Ce3+ to Tb3+ ions, promoting it from 7F6 ground state to 5D3 and other levels (labeled as ET in Fig. 2(d)). Then the excited Tb3+ relaxes to the 5D4 levels non-radiatively and gives the strong emission of Tb3+ (5D4-7FJ). As mentioned above, due to the cross relaxation 5D3 + 7F6 → 5D4 + 7F0 (labeled as CR in Fig. 2(d)), no emission from 5D3 level can be observed.
It is worthwhile to notice that the emission band of Ce3+ ions at blue region and the emission band of Tb3+ ions at yellowish green emission region. Combining both of them, white light emission may be achieved.
Therefore, luminescence colors of Ba2(Gd1-x-yCexTby)2Si4O13 phosphors excited at 335 nm are characterized by Commission International de I’Eclairage (CIE) chromaticity diagram and shown in Fig. 3(a) . The line connecting the chromaticity point of Ba2(Gd0.98Ce0.02)2Si4O13 (0.159, 0.03) with that of Ba2(Gd0.5Tb0.5)2Si4O13 (0.348, 0.569) crosses the white region. It proves that white light can be yielded by combining the emission of Ce3+ and Tb3+ in Ba2Gd2Si4O13 host. The chromaticity coordinate Ba2(Gd0.88Ce0.02Tb0.1)2Si4O13 sample (Fig. 2(c)) is calculated to be (0.247, 0.290), i.e., white light is obtained in the Ba2(Gd0.88Ce0.02Tb0.1)2Si4O13 sample excited at UV light of 335 nm.
The inset of Fig. 3(a) just gives the intense yellowish green, blue and white output photos taken from Ba2(Gd0.5Tb0.5)2Si4O13, Ba2(Gd0.98Ce0.02)2Si4O13 and Ba2(Gd0.88Ce0.02Tb0.1)2Si4O13 under 365 nm excitation in dark, respectively. Considering the excitation efficiency was very high in the range from 300 to 370 nm with a center at 335 nm, our results show that these blue-white-green phosphors may be used as the converting phosphors for UV-LED.
Considering 335 nm is the optimal excitation wavelength for Ba2(Gd1-x-yCexTby)2Si4O13 and commercial ZnS:Ag+,Cl- phosphor, the quantum efficiency was estimated by comparing the sample with this commercial phosphor. For Ba2(Gd0.88Ce0.02Tb0.1)2Si4O13 white phosphor, the quantum efficiency was estimated to be 82.3% of mentioned commercial phosphor. The results prove that these color tunable phosphors with high efficiency can find potential application in solid-state lighting and other areas.
In order to give a convincing evidence for the presence of energy transfer from Ce3+ to Tb3+, the luminescence of Ce3+ and Tb3+ and the decay curves of Ce3+ in Ba2(Gd0.98-yCe0.02Tby)2Si4O13 phosphors were systemically investigated.
Figure 3(b) describes the emission spectra of Ba2(Gd0.98-yCe0.02Tby)2Si4O13 phosphors (y = 0, 0.04, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6). When the Ce3+ doping concentration is fixed, with the increase of Tb3+, the intensity of Ce3+ emission decreases monotonously, while the intensity of Tb3+ emission improves rapidly, even reaches a maximum at y = 0.5, and then remarkably decreases when Tb3+ content further increases due to concentration quenching. Such phenomena indicate that the quenching concentration of Tb3+ is 50% (y = 0.5).
Figure 4(a) gives the decay curves of Ce3+ emission (400 nm) of Ba2(Gd0.98-yCe0.02Tby)2Si4O13 phosphors (y = 0, 0.04, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6). For all samples, nearly single exponential luminescence decay curves are observed. The lifetime of Ce3+ single-doped sample is about 36.3 ns. The lifetime of nanosecond order is one of the specific characteristics of Ce3+ electric-dipole allowed 5d-4f transition. The increase of Tb3+ doping leads to faster decay, which is attributed to ET from Ce3+ to neighboring Tb3+. The decay times (τ, shown in Fig. 4(b)) were calculated to be about 36.3, 27.6, 23, 18.1, 15.1, 13.6, 11.6 and 10.9 ns for Ba2(Gd0.98-yCe0.02Tby)2Si4O13 with y = 0, 0.04, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6, respectively.
The energy transfer efficiency of Ce3+ ηET,yTb can be obtained experimentally by dividing the decay lifetimes of the Ce3+, Tb3+ co-doped phosphors to the decay lifetime of the Ce3+ solely doped one, meanwhile, the energy transfer efficiency ηET,yTb can be expressed by ,Figure 4(b) clearly shows the relation between energy transfer efficiency (ηET) and Tb3+ concentration. Along with the increase of Tb3+ content from 0 to 0.6, ηET was determined to be 0%, 23.8%, 36.6%, 50.2%, 58.4%, 62.5%, 68% and 70%, respectively. The luminescent spectra, decay times of Ce3+ ions and energy transfer efficiency prove that the ET process from Ce3+ to Tb3+ is very efficient.
A series of single-phased color-tunable Ba2(Gd1-x-yCexTby)2Si4O13 phosphors were first investigated systemically. White-light emission is obtained under UV (300-370 nm) excitation by combining the blue emission of Ce3+ and the yellowish green emission of Tb3+. The quantum efficiency of the white phosphor Ba2(Gd0.88Ce0.02Tb0.1)2Si4O13 was estimated to be 82.3% of commercial ZnS:Ag+,Cl- phosphor. Our results demonstrate that Ba2(Gd1-x-yCexTby)2Si4O13 phosphors are promising candidates for solid-state lighting and other areas.
This work was supported by the National Natural Science Foundation of China (No. 10904131) and Foundation of Jinhua Science and Technology Bureau (No. 2008-1-151). The authors also thank Prof. Sheng Li for the helpful discussion.
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