1. Introduction

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 [1].

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⁺ [5] 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⁺ [2], 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 [13]. After tuning the content of Tb³⁺, the emission color variation attributed to effective ET from Ce³⁺ to Tb³⁺ has been discussed.

2. Experimental

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. [14] 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) ų, 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.

 

Fig. 1 XRD patterns of un-doped, Ce-doped samples and calculated patterns from ICSD file of Ba₂Gd₂Si₄O₁₃.

Download Full Size | PDF

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(Gd_0.5Tb_0.5)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 ⁸S_7/2-⁶I_J, ⁸S_7/2-⁶P_J transition of Gd³⁺ [15]. Meanwhile, the Tb³⁺ doped sample exhibits a strong green emission excited by short UV light. The typical sharp emission peaks of emission spectra (λ_ex=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) [15]. 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 ⁵D₃-⁷F_J transitions often be quenched for high Tb³⁺ concentration doped samples due to the cross relaxation ⁵D₃ + ⁷F₆ → ⁵D₄ + ⁷F₀ [15].

 

Fig. 2 Excitation and emission spectra of the single-doped and co-doped samples, (a) Ba₂(Gd_0.5Tb_0.5)₂Si₄O₁₃, (b) Ba₂(Gd_0.98Ce_0.02)₂Si₄O₁₃ and (c) Ba₂(Gd_0.88Ce_0.02Tb_0.1)₂Si₄O₁₃; (d) is the corresponding energy level scheme.

Download Full Size | PDF

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 Ce^{3+ 2}F_5/2-5d 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 ⁸S_7/2-⁶I_J transition of Gd³⁺ [15]. 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 ²F_5/2 and ²F_7/2 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 ²F_5/2 and ²F_7/2 levels of Ce³⁺ (~2000 cm⁻¹) [15].

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(Gd_0.88Ce_0.02Tb_0.1)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 ²F_5/2-5d 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 ²F_5/2 and ²F_7/2 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⁺ (⁵D₄-⁷F_J). 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(Gd_1-x-yCe_xTb_y)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(Gd_0.98Ce_0.02)₂Si₄O₁₃ (0.159, 0.03) with that of Ba₂(Gd_0.5Tb_0.5)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(Gd_0.88Ce_0.02Tb_0.1)₂Si₄O₁₃ sample (Fig. 2(c)) is calculated to be (0.247, 0.290), i.e., white light is obtained in the Ba₂(Gd_0.88Ce_0.02Tb_0.1)₂Si₄O₁₃ sample excited at UV light of 335 nm.

 

Fig. 3 (a) CIE chromaticity diagram for Ba₂(Gd_1-x-yCe_xTb_y)₂Si₄O₁₃ phosphors. (1) x = 0.02, y = 0; (2) x = 0.02, y = 0.04; (3) x = 0.02, y = 0.1; (4) x = 0.02, y = 0.2; (5) x = 0.02, y = 0.3; (6) x = 0.02, y = 0.4; (7) x = 0.02, y = 0.5; (8) x = 0.02, y = 0.6; (9) x = 0, y = 0.5. The inset shows the photos of Ba₂(Gd_0.5Tb_0.5)₂Si₄O₁₃ (green), Ba₂(Gd_0.98Ce_0.02)₂Si₄O₁₃ (blue) and Ba₂(Gd_0.88Ce_0.02Tb_0.1)₂Si₄O₁₃ (white) taken under 365 nm excitation in dark. (b) Emission spectra (λ_ex = 335 nm) of Ba₂(Gd_0.98-yCe_0.02Tb_y)₂Si₄O₁₃ phosphors (y = 0, 0.04, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6).

Download Full Size | PDF

The inset of Fig. 3(a) just gives the intense yellowish green, blue and white output photos taken from Ba₂(Gd_0.5Tb_0.5)₂Si₄O₁₃, Ba₂(Gd_0.98Ce_0.02)₂Si₄O₁₃ and Ba₂(Gd_0.88Ce_0.02Tb_0.1)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(Gd_1-x-yCe_xTb_y)₂Si₄O₁₃ and commercial ZnS:Ag⁺,Cl^- phosphor, the quantum efficiency was estimated by comparing the sample with this commercial phosphor. For Ba₂(Gd_0.88Ce_0.02Tb_0.1)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(Gd_0.98-yCe_0.02Tb_y)₂Si₄O₁₃ phosphors were systemically investigated.

Figure 3(b) describes the emission spectra of Ba₂(Gd_0.98-yCe_0.02Tb_y)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(Gd_0.98-yCe_0.02Tb_y)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(Gd_0.98-yCe_0.02Tb_y)₂Si₄O₁₃ with y = 0, 0.04, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6, respectively.

 

Fig. 4 (a) Decay curves of Ce³⁺ emission (400 nm), (b) Lifetime of Ce³⁺, energy transfer efficiency (η_ET) of Ba₂(Gd_0.98-yCe_0.02Tb_y)₂Si₄O₁₃ phosphors (y = 0, 0.04, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6).

Download Full Size | PDF

The energy transfer efficiency of Ce³⁺ η_ET,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 η_ET,yTb can be expressed by [11],

4. Conclusion

A series of single-phased color-tunable Ba₂(Gd_1-x-yCe_xTb_y)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(Gd_0.88Ce_0.02Tb_0.1)₂Si₄O₁₃ was estimated to be 82.3% of commercial ZnS:Ag⁺,Cl^- phosphor. Our results demonstrate that Ba₂(Gd_1-x-yCe_xTb_y)₂Si₄O₁₃ phosphors are promising candidates for solid-state lighting and other areas.