Blue-green tunable color of Ce³⁺/Tb³⁺ coactivated NaBa₃La₃Si₆O₂₀ phosphor via energy transfer

Abstract

A series of color tunable phosphors NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ were synthesized via the high-temperature solid-state method. NaBa₃La₃Si₆O₂₀ crystallizes in noncentrosymmetric space group Ama2 with the cell parameters of a = 14.9226(4) Å, b = 24.5215(5) Å and c = 5.6241(2) Å by the Rietveld refinement method. The Ce³⁺ ions doped NaBa₃La₃Si₆O₂₀ phosphors have a strong absorption band from 260 to 360 nm and show near ultraviolet emission light centered at 378 nm. The Ce³⁺ and Tb³⁺ ions coactivated phosphors exhibit color tunable emission light from deep blue to green by adjusting the concentration of the Tb³⁺ ions. An energy transfer of Ce³⁺ → Tb³⁺ investigated by the photoluminescence properties and lifetime decay, is demonstrated to be dipole–quadrupole interaction. These results indicate the NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ phosphors can be considered as potential candidates for blue-green components for white light emitting diodes.

Introduction

By virtue of the special merits of high brightness, energy-efficient, life-durable, and environmentally friendly, the white light emitting diodes (w-LEDs) made from blue or near-ultraviolet (n-UV) emitting LEDs chips coated with phosphors have the potential to overtake incandescent and fluorescent lighting types^1,2,3,4. In 1996, the w-LEDs fabricated from the blue-emitting InGaN LED chips combined with the yellow-emitting phosphors (YAG:Ce³⁺) were commercialized⁵. Unfortunately, this technology has the following problems: low color rendering index due to two-color mixing, and low color reproducibility due to the strong dependence of white color purity on the quality of phosphors^{6,7,8,9,10,11}. To solve these problems, the w-LEDs had been fabricated employing blue, green and red emitting phosphors excited by a blue or n-UV chip^12,13,14. However, the strong reabsorption of blue light by red and green phosphors reduces the luminescence efficiency in this system^15,16. To overcome these disadvantages, vigorous attentions were received to exploit the emission-tunable phosphors with strong absorption in n-UV region^17,18. Simultaneously, an energy transfer can obviously improve the luminescent efficiency and color reproducibility as well as widen the emission spectra of phosphors.

After years of efforts, a series of promising phosphors had been developed, such as fluorides¹⁹, silicates^20,21, phosphates¹², orthovanadates^22,23, borates^24,25, tungstates/molybdates^26,27, nitrides^28,29, aluminates^30,31,32, etc. Among them, the silicate compounds as luminescent hosts were intensively studied because of their remarkable stability of physical and chemical properties, flexible crystal structures and relatively easy preparation process. Among rare earth ions, the Tb³⁺ ion is the best candidate for green component due to its predominant ⁵D₄–⁷F₅ transitions peaking at around 545 nm³³. However, the electric dipole transitions within the 4f configurations of the Tb³⁺ ion is spin forbidden, resulting in the weak intensity of its absorption in the n-UV region and the narrow width. Thus, a suitable sensitizer is always necessary for the phosphors activated by the Tb³⁺ ion. It is well known that the Ce³⁺ ion is an excellent sensitizer transferring a part of its energy to an activator such as the Tb³⁺ ion depending on its lowest 5d state and broad absorption and emission bands from the allowed 4f → 5d transitions³⁴. In this work, we reported a novel silicate host phosphor, NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ for the excitation by a n-UV LED chip, and the crystal structure, luminescent properties and energy transfer mechanism between the Ce³⁺ ion and the Tb³⁺ ion had been thoroughly investigated.

Results and Discussion

Crystal structure and phase formation

Figure 1(a) demonstrates the observed and calculated XRD patterns as well as their difference for the Rietveld refinement of NaBa₃La₃Si₆O₂₀. In the refinement, an initial structure model and atomic positions of NaBa₃Eu₃Si₆O₂₀ were adopted for the structure refinement^35,36. NaBa₃La₃Si₆O₂₀ crystallizes in the noncentrosymmetric space group Ama2 and unit cell parameters are obtained as a = 14.9226(4) Å, b = 24.5215(5) Å and c = 5.6241(2) Å, which are slightly larger than those of NaBa₃Eu₃Si₆O₂₀ due to large ionic radius of La³⁺ ion³⁷. As shown in Fig. 1(c), the basic structural units are distorted (SiO₄)⁴⁻ tetrahedra which are further linked by the Ba, La, and Na atoms to build a complex three-dimensional framework. The Na atoms which are surrounded by six oxygen adopt distorted pentagonal-pyramidal geometry, the Ba1 and Ba2 atoms coordinated to seven and eight oxygen are in distorted trigonal prism and cube configuration. In the structure of NaBa₃La₃Si₆O₂₀, there are two kinds of La sites, implying that there are two possible types of Ce³⁺ ions in the NaBa₃La₃Si₆O₂₀:Ce³⁺ samples. The La1 atoms are coordinated to seven oxygen atoms to form pentagonal bipyramid while the La2 surrounded by eight oxygen atoms are in square anti-prism environment (Fig. 1(d)).

Figure 1

(a) The XRD profiles for the Rietveld refinement of NaBa₃La₃Si₆O₂₀. (b) The XRD patterns of the NaBa₃La₃Si₆O₂₀, NaBa₃La₃Si₆O₂₀:0.007Ce³⁺ and NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors. (c) The structure of unit cell of NaBa₃La₃Si₆O₂₀ along the c axis. (d) The coordination environments of La1 and La2 in NaBa₃La₃Si₆O₂₀.

Figure 1(b) shows the selected XRD patterns of the as-synthesized representative samples of NaBa₃La₃Si₆O₂₀ and NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ (0 ≤ y ≤ 0.30) and the quantitative analysis of all the samples illustrate that the doping of Ce³⁺ or/and Tb³⁺ are successful (Supplementary Table S1). Also, it can be seen that all the diffraction peaks of the selected phosphors match well with the NaBa₃La₃Si₆O₂₀ phase. Even at high doping concentration of the Tb³⁺ ion (30%), the XRD patterns of phosphors are almost same with that of undoped phase, which illustrates the excellent stability and accommodation capacity for doped ions of crystal structure of the NaBa₃La₃Si₆O₂₀ host. The XRD profiles for the Rietveld refinement of the single element doped and co-doped samples and the coordination, occupancy and isotropic displacement parameter for all samples are listed (Supplementary Figs S1–S7, Tables S2–S9).

Photoluminescence properties and energy transfer

As shown in Fig. 2(a), the PLE spectra of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺ sample consist of three absorption bands centered at around 254, 282 and 331 nm, which arise from the electronic transitions between the ground state (²F_5/2 and ²F_7/2) and the levels of 5d excited split by crystal field of the Ce³⁺ ion³⁸. Under the excitation wavelength of 331 nm, the Ce³⁺ ion doped NaBa₃La₃Si₆O₂₀ sample shows an asymmetric emission band extending from 340 to 500 nm with the maximum at 378 nm, indicating a possible spectral overlap originating from different luminescence centers. It is obvious that one type of Ce³⁺ ions gives rise to two emission band due to the transitions from the lowest 5d excited states to two ground states (²F_7/2 and ²F_5/2) respectively³⁹. However, the emission band of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺ sample can be decomposed into four Gaussian components A–D peaking at 364, 380, 394 and 410 nm with the energy gaps between A and C is 2092 cm⁻¹, that of B and D is 1926 cm⁻¹, which are close to the theoretical value of 2000 cm^−1 40,41. These results imply that there should be two kinds of Ce³⁺ ions, which is consistent with the previous investigation on the crystal structure that there are two kinds of different chemical environment of La³⁺ ions in the NaBa₃La₃Si₆O₂₀ host.

Figure 2

(a) The PLE (dark solid line) and PL spectra (red solid line) of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺ sample and the Gaussian peaks fitting (the green dashed lines of the Ce³⁺ (1) and the blue dashed lines of the Ce³⁺ (2)). (b) The PL spectra of the NaBa₃La₃Si₆O₂₀:xCe³⁺ samples with varying concentration of the Ce³⁺ ions.

As given in Fig. 2(b), the PL intensity of the NaBa₃La₃Si₆O₂₀:xCe³⁺ samples increases gradually with the increase of the doping concentration of the Ce³⁺ ions and reaches the maximum when the x value is 0.007, and then begins to decrease due to concentration quenching⁴². It is also indicated that the Ce³⁺ ion is a sensitizer for the Tb³⁺ ion and an energy transfer of Ce³⁺ → Tb³⁺ is crucial to enhance green emission of the Tb³⁺ ion and achieve color tunable emission light. Therefore, the optimal concentration of the Ce³⁺ ion in the NaBa₃La₃Si₆O₂₀:xCe³⁺ samples is confirmed to be 0.007.

Generally, the critical distance R_C between the Ce³⁺ ions can be calculated with the following equation given by Blasse⁴³:

where V is the volume of unit cell, x is the critical concentration of doped ions, where the emission intensity of phosphors reaches the maximum, N is the number of host cations per unit cell. For the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺ sample, N = 12, V = 2057.989 ų, R_C is calculated to be about 25.00 Å. Dexter noted a non-radiative energy transfer usually was attributed to exchange or multipole – multipole interaction in oxide phosphors and the exchange interaction was valid only when the Rc was shorter than 5 Å⁴⁴. In consequence, the concentration quenching mechanism of the Ce³⁺ ions in the NaBa₃La₃Si₆O₂₀:xCe³⁺ samples is dominated by the multipole – multipole interaction.

Figure 3(a) depicts the PLE and PL spectra of the NaBa₃La₃Si₆O₂₀:0.20Tb³⁺ sample. The PLE spectrum monitored at 542 nm exhibits a broad absorption band centered at 268 nm from 200 to 300 nm and several peaks within the scope of 300 to 400 nm. The former excited peak is ascribed to 4f⁸–4f⁷5d transition of the Tb³⁺ ion, while the latter peaks are from the intra-4f⁸ transitions^45,46. Under the excitation wavelength of 268 nm or 378 nm, the NaBa₃La₃Si₆O₂₀:0.20 Tb³⁺ sample emits green light with main peaks at 412, 435, 457, 488, 542, 581 and 622 nm, which can be ascribed to the ⁵D₄–⁷F_J (J = 6, 5, 4 and 3) transitions of the Tb³⁺ ion. However, because the f–f absorption is a forbidden transition, only some narrow f–f transition lines locate in the excitation range of n-UV LED in spite of difficultly bumping the Tb³⁺ ion⁴⁷. There is an overlap between the emission band (magenta line in Fig. 3(a)) of the Ce³⁺ ions and the f–f transition (olive line in Fig. 3(a)) absorption band of the Tb³⁺ ions, therefore, it is potential that the Ce³⁺ ions can be sensitizers to transfer energy to the Tb³⁺ ions to enhance their absorption. As shown in Fig. 3(b), the PL spectrum of the NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ phosphors exhibits broad emission bands corresponding to the allowed f–d transition of the Ce³⁺ ions and the ⁵D₄–⁷F_J characteristic transitions of the Tb³⁺ ions. The emission intensity of the NaBa₃La₃Si₆O₂₀:Tb³⁺ samples under excitation wavelength of 268 nm is larger than that under 378 nm, because the intensity of the absorption peak centered at 268 nm is more intense than that at 378 nm. However, the emission light intensity monitored at 268 nm is less than that at 374 nm in the NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ phosphors. These results verify that it is the overlap between f-f transition (peaking at 374 nm) but not f-d transition (peaking at 268 nm) of the Tb³⁺ ions and the emission band of the Ce³⁺ ions induce the energy transfer. Figure 3(b) also shows the excitation spectrum of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, 0.20Tb³⁺ phosphor monitored at 378 nm (the Ce³⁺ ions emission) is similar to that of at 542 nm (the Tb³⁺ ions emission) except the difference of luminous intensity, which provides another evidence for energy transfer of Ce³⁺ → Tb³⁺.

Figure 3

The PLE and PL spectra of the NaBa₃La₃Si₆O₂₀:xCe³⁺, yTb³⁺ phosphors (a), and the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, 0.20Tb³⁺ phosphor (b).

To further investigate the sensitized luminescence of the Tb³⁺ ions by the Ce³⁺ ions, the emission spectra of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors were measured (Fig. 4). Although the amount of the Ce³⁺ ions is fixed, their emission intensity gradually decreases along with the increase of the concentration of the Tb³⁺ ions. The result indicates that a lot of Tb³⁺ ions as acceptors accelerate energy diffusion of donors, which speeds up the average transfer rate of Ce³⁺ → Tb³⁺.

Figure 4

The PL spectra of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors.

Figure 5 and Table 1 show the variation of Commission International deL’Eclairage (CIE) chromaticity coordinates of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors (y = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) under excitation wavelength at 331 nm. The insets of Fig. 5 are the photographs of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors with different amount Tb³⁺ ions in a 365 nm n-UV lamp box. These results indicate that the emission light color can be modulated from deep blue to green only by varying the contention of the Tb³⁺ ions. Therefore the NaBa₃La₃Si₆O₂₀:Ce³⁺, yTb³⁺ samples can be potential color-tunable phosphors for application in n-UV based WLED devices.

Table 1 The comparison of the CIE chromaticity coordinates of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors (λ_ex = 331 nm).

Figure 5

The CIE chromaticity coordinates of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors (0 ≤ y ≤ 0.3) excited under 331 nm and the pictures of the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors (0 ≤ y ≤ 0.3) in a 365 nm UV box.

Energy transfer mechanism

In general, the energy transfer from a sensitizer to an activator in oxide may take place via exchange interaction or electric multipolar interaction⁴⁸. The separation distance R_Ce–Tb can be also estimated from equation (1). Here, x is the total concentration of the Ce³⁺ and Tb³⁺ ions, where the luminescence intensity of sensitizer is half of that in samples lack of activator. For the NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ phosphors, the value of and is about 0.021 and 0.75 respectively, thus R_Ce–Tb is calculated to be about 7.5 Å. Since exchange interaction was restricted to distances of about 4 Å, the energy transfer mechanism of Ce³⁺ → Tb³⁺ should may be electric multipolar interaction^43,44.

According to Dexter’s energy transfer expressions of multipolar interaction and Reisfeld’s approximation, the following relation can be given as^{42,48,49,50,51}:

where η₀ and η are the luminescence quantum efficiency of the Ce³⁺ ions in absence and presence of the Tb³⁺ ions, n = 6, 8 and 10 are corresponding to dipole – dipole, dipole – quadrupole and quadrupole – quadrupole interactions, respectively. The value ƞ_0/ ƞ is approximately estimated by the ratio of related luminescence intensity I₀/I, I₀ is the intrinsic luminescence intensity of the Ce³⁺ ions, and I is the luminescence intensity of the Ce³⁺ ions in presence of the Tb³⁺ ions. Figure 6(a–d) illustrates the relationships between I₀/I and as well as I₀/I and . The R² value is reasonable in Fig. 6(b,c), implying the energy transfer of Ce³⁺ → Tb³⁺ may occur via dipole–dipole or dipole–quadrupole interaction. However, Sommerdijk stated the probability of energy transfer of Ce³⁺ → Tb³⁺ via electric dipole-dipole interaction was less likely, therefore, dipole – quadrupole interaction should mainly contribute to energy transfer of Ce³⁺ → Tb^3+ 52.

Figure 6

The dependence of I₀/I of the Ce³⁺ ions on (a) (b) (c) (d) .

In order to further validate the energy transfer process, the room temperature decay curves for the 4f-5d (centered at 378 nm) transition of the Ce³⁺ ions in NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ (y = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) excited at 330 nm are shown in Fig. 7 For existing two types of Ce³⁺ ions in topic phosphors, the decay curves should be well fitted with a typical two exponential function⁵³:

Figure 7

The decay curves for the emission of the Ce³⁺ ions in the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors excited under 330 nm and monitored at 378 nm.

where I(t) and I₀ are the luminescence intensity at time t, A₁ and A₂ are the fitting constants, τ₁ and τ₂ represent the decay time for the exponential components. Then the average lifetime (τ*) can be calculated to be 24.5, 22.6, 21.7, 20.5, 20.0, 19.2 and 17.3 by the following formula^40,54:

The decay time of the Ce³⁺ ions decreases as increase of the concentration of the Tb³⁺ ions, which strongly demonstrates the energy transfer of Ce³⁺ → Tb³⁺.

Subsequently the energy levels model for the energy transfer processes of Ce³⁺ → Tb³⁺ was investigated. As given in Fig. 8(a), the Ce³⁺ ion absorbs light firstly, then it jumps from the ground states (²F_5/2) to the excited states (5d energy levels), subsequently the excited state Ce³⁺ ion returns to the lowest level of 5d levels by giving off excess energy to its surroundings, eventually goes back to the ²F_7/2 or ²F_5/2 ground states by a radiative process. The energy transfer efficiency of Ce³⁺ → Tb³⁺ should increase as the increase of the concentration of the Tb³⁺ ions due to more neighboring Tb³⁺ ions around the Ce³⁺ ions. Finally the energy level transitions of ⁵D₄ to ⁷F_J (J = 3, 4, 5 and 6) produce the characteristic emission of the Tb³⁺ ions.

Figure 8

(a) The energy levels model for the energy transfer processes of Ce³⁺ → Tb³⁺. (b) The dependence of the emission of the Ce³⁺ ions and Tb³⁺ ions, and the energy transfer of Ce³⁺ → Tb³⁺ on the doping concentration of the Tb³⁺ ions in the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ phosphors.

The energy transfer efficiency ƞ_T from the Ce³⁺ ions to the Tb³⁺ ions can be calculated according to the following equation⁵⁵:

where I₀ and I_y are the emission light intensity of the sensitizer with and without an activator, respectively. In the NaBa₃La₃Si₆O₂₀:0.007Ce³⁺, yTb³⁺ samples, the Ce³⁺ ion is a sensitizer and the Tb³⁺ ion is an activator. The ƞ_T values can be calculated as 7.06%, 16.71%, 28.95%, 37.04%, 48.95%, 78.31%, as a function of y (y = 0.15, 0.30, 0.45, 0.60, 0.75, 0.90), respectively (Fig. 8(b)). The energy transfer of Ce³⁺ → Tb³⁺ is consistent with the conclusion that the energy transfer efficiency increases as the increase of the concentration of the Tb³⁺ ions due to more neighboring Tb³⁺ ions around the Ce³⁺ ions and is equivalent to that of the reported K₂MgSiO₄:Ce³⁺, Tb³⁺ silicate phosphor⁵⁶.

Conclusion

A series of novel NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ phosphors were prepared by solid state method. The energy transfer processe of Ce³⁺ → Tb³⁺ has been demonstrated to be dipole – quadrupole interaction. The tunable colors from deep blue to green can be realized by varying the doping concentration of the Tb³⁺ ions under the irradiation of 331 nm. These results demonstrate the as-prepared NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ samples can act as potential n-UV based w-LED phosphors.

Experimental Section

Compounds synthesis

The NaBa₃La₃Si₆O₂₀:Ce³⁺, Tb³⁺ phosphors were synthesized by high temperature solid state method. Na₂CO₃ (A.R.), BaCO₃ (A.R.), SiO₂ (A.R.), La₂O₃ (99.99%), CeO₂ (99.99%) and Tb₄O₇ (99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the initial chemicals were used without further purification. Stoichiometric amounts of the above-mentioned chemicals were ground thoroughly by an agate mortar, packed tightly in an alumina crucible. The temperature of the furnace was heated up to 500 °C at a rate of 60 °C/h, then held for 24 h to preheat the mixture in air atmosphere. After the mixture was ground once again, the temperature was increased to 930 °C at a rate of 60 °C/h and held for 100 h with four intermittent grindings. Finally, the prepared phosphors were cooled to room temperature and reground into resulting phosphors. For convenient expression, NaBa₃La_3−3x–3yCe_3xTb_3ySi₆O₂₀ is abbreviated as NaBa₃La₃Si₆O₂₀:xCe³⁺, yTb³⁺. For example, NaBa₃La_2.55Ce_0.15Tb_0.30Si₆O₂₀ is denoted as NaBa₃La₃Si₆O₂₀:0.05Ce³⁺, 0.10Tb³⁺.

Material characterization

The powder XRD measurements were taken on a Bruker D8 X-ray diffractometer with a Cu Kα source (λ = 1.5418 Å) in the angular range from 5° to 80° with a scanning step of 0.15. The structure refinement was carried out with the General Structure Analysis (GSAS) and EXPGUI software^57,58. XRD Rietveld profile refinements of the structural models were performed using the General Structure Analysis (GSAS) software. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained by an FLS-980 fluorescence spectrophotometer equipped with a 450 W Xe light source. The photoluminescence lifetime curves were measured on an FLS-920 fluorescence spectrophotometer equipped with a laser as light source. All measurements were performed at room temperature. The element analyses of samples were performed by the (X-ray fluorescence) XRF method on a thermo ARL ADVANTXP+ apparatus.