Green-emitting phosphor Na₂Gd₂B₂O₇:Ce³⁺, Tb³⁺ for near-UV LEDs

Abstract

An intense green-emitting phosphor Na₂Gd₂B₂O₇:Ce³⁺, Tb³⁺ is developed on the basis of the highly efficient energy transfer from Ce³⁺ to Tb³⁺. The photoluminescence emission and excitation spectra, the lifetime, and the effect of Tb³⁺ concentration are investigated in detail. Results confirm the occurrence of non-radiative energy transfer from Ce³⁺ to Tb³⁺ with an efficiency of over 80%. The intensity ratio of blue emission from Ce³⁺ and green emission from Tb³⁺can be tuned by adjusting their concentrations. The enhanced photoluminescence of Tb³⁺ with sharp emission lines could be obtained by the broad band excitation peak at about 360 nm from the allowed 4f–5d absorption of Ce³⁺ ions. The results suggest that the phosphor Na₂Gd₂B₂O₇:0.05Ce³⁺, 0.12Tb³⁺ with the optimal composition could potentially be used for near ultraviolet light-emitting diodes (n-UV LEDs).

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1. Introduction

With rapid development of the world economy and the advance of modern technology, the demand for energy is increasing at an unprecedented pace. The sustainable development of the society faces huge challenges because of the depletion of traditional fossil fuels and high pollutant emission. New clean energy sources and the energy saving devices are two desirable methods to overcome this problem. Light emitting diodes (LEDs) are considered ideal candidates for the replacement of conventional lighting applications because of their energy saving and environmental friendly nature. The world wide electricity cost for lighting would decrease by more than 50% a year if LEDs could displace general illumination light sources such as incandescent and fluorescent lamps.^1–2 Nowadays, white light emitting diodes (w-LEDs) fabricated with near-ultraviolet (n-UV, 350–410 nm) LED chips and tricolor (red, green and blue) phosphors are expected to dominate the market in the near future due to their high color rendering index (Ra) and tunable color temperatures (T_c).³ However, the range of phosphors suitable for n-UV LEDs is limited. Usually, the commercial phosphors for n-UV LED are the blue phosphor BaMgAl₁₀O₁₇:Eu²⁺, the green phosphor ZnS: Cu⁺, Al³⁺ and the red phosphor Y₂O₂S:Eu³⁺.⁴ The sulfide-based green and red phosphors suffer from poor stability, which seriously decreases the lifetime of the devices.⁵ Thus, it is important to develop novel green or red phosphors with high stability.

The Tb³⁺ ion is frequently used as an activator of green emitting luminescent materials due to its predominant ⁵D₄→⁷F₅ transition peak at around 545 nm. Moreover, the characteristic sharp emissions of Tb³⁺ originating from intra-configurational 4f–4f transitions are almost independent of the host lattice since the 4f orbital is shielded from the exterior by the filled 5s² and 5p⁶ orbitals, which leads to the excellent reproduction quality of the optical properties of the phosphor. Unfortunately, the intensities of the Tb³⁺ absorption peaks in the n-UV region are very weak and their widths are very narrow due to the strictly forbidden 4f–4f transitions,^6–7 which makes it an imperfect activator for LEDs. One of the strategies to overcome the above problem is using Ce³⁺ as a sensitizer because it has a strong excitation band and an efficient emission band originating from allowed 4f–5d transitions, which has been investigated in different hosts. However, most of their maximum excitation wavelengths do not exceed 350 nm. The maximum excitation wavelength peaked at 400 nm for sulfide-based phosphor CaLaGa₃S₆O:Ce³⁺, Tb³⁺, but its energy transfer efficiency is very low (about 11%) and the stability is poor. The above results indicate that they do not match well with the n-UV LEDs chip.^8–12

As an important family of luminescent materials, borate phosphors have great potential in the application of LEDs due to their low synthesizing temperature, environmental benignity, and excellent chemical and physical stability.^13–15 In the present study, we develop a novel borate phosphor Na₂Gd₂B₂O₇:Ce³⁺, Tb³⁺, which shows intense broad band absorption with maximum excitation at about 360 nm and excellent green emission. This suggests that Na₂Gd₂B₂O₇:Ce³⁺, Tb³⁺ (NGBO: Ce³⁺, Tb³⁺) could be a potential green phosphor candidate for n-UV LEDs.

2. Experimental

2.1 Materials and synthesis

In the present compound Na₂Gd₂B₂O₇, we suggest that the Ce³⁺ and Tb³⁺ ions prefer to occupy the sites of Gd³⁺ ions because of the same valence and the cation size effect. Therefore the nominal formula of the present phosphors could be regarded as Na₂Gd_2−m−nB₂O₇: mCe³⁺, nTb³⁺ (NGBO: mCe³⁺, nTb³⁺). We prepared a series of green emitting Na₂Gd_1.95−nB₂O₇: 0.05Ce³⁺, nTb³⁺ (m = 0.05) phosphor powders by a conventional solid-state method under a CO reducing atmosphere. Stoichiometric amounts of the starting materials, Na₂CO₃ (A. R.), H₃BO₃ (A. R.), Gd₂O₃ (99.99%), CeO₂ (99.99%) and Tb₄O₇ (99.99%) were mixed homogeneously and placed in a small covered corundum crucible. Then the small crucible was contained in a large outer crucible and buried in carbon sticks. Afterwards, the large crucible with its contents was calcined at 980 °C for 6 h in a muffle furnace under ambient atmospheres and then cooled to room temperature. Finally, samples were crushed to fine powder.

2.2 Characterization

Powder X-ray diffractions (XRD) measurements were performed in the range of 5⁰ ≤ 2θ ≤ 50⁰ using an Rigaku–Dmax 3C powder diffractometer with Cu-Kα (λ = 1.5405 Å) radiation. The measurements of the photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded with a Hitachi F-7000 fluorescence spectrophotometer equipped with a xenon lamp as its excitation source. The decay curves were determined on an Edinburgh FLS920 spectrofluorometer, equipped with a 450 W Xe lamp and a 150 W nF900 nanosecond flash lamp. To eliminate the second-order emission of the source radiation, a cutoff filter was used in the measurements. Photoluminescence spectra of all samples were tested three times to reduce the error and all measurements were performed at room temperature.

3. Results and discussions

The crystal structure of Na₂Gd₂B₂O₇ has been refined to be monoclinic with lattice parameters a = 10.695(6) Å, b = 6.320(4) Å, c = 10.328(6) Å, β = 122.27° and the space group of P2₁/c.¹⁶ All of the as-prepared phosphor samples are identified as single-phased and are in good agreement with those reported in the JCPDS file 89-4442 regardless of the doping contents, which indicates that the introduction of activators Tb³⁺ and Ce³⁺ does not cause any significant changes in the host structures.

Fig. 1a shows the PLE and PL spectra of NGBO: 0.05Ce³⁺. The PLE spectrum monitored at 413 nm exhibits two distinct excitation bands at 246 and 358 nm, which is assigned to the 4f–5d transitions of Ce³⁺. Under excitation of n-UV light (λ_ex = 358 nm), the PL spectrum exhibits a blue emission with typical doublet bands emission peaks at about 382 and 414 nm, respectively, which corresponds to the transition of Ce³⁺ ions from the 5d¹ excited state to the ²F_5/2 and ²F_7/2 ground states. As for the Tb³⁺ singly doped NGBO sample, its PLE and PL spectra are presented in Fig. 1b. The PL spectrum under the excitation of 258 nm displays a series of sharp line emissions at 488, 543, 586 and 620 nm, due to the ⁵D₄→⁷F_J (J = 6, 5, 4, and 3) characteristic transitions of Tb³⁺ ions, and the green emission line at 543 nm from ⁵D₄→⁷F₅ transitions dominates the whole spectrum. Monitoring the emission at 543 nm, the PLE spectra include a group of weak sharp absorption peaks in the 275–400 nm longer wavelength regions (inset of Fig. 1b) and two strong absorption bands. The former is assigned to the forbidden f–f transitions of Tb³⁺, and the latter broad band absorption should be attributed to the allowed 4f–5d transition of Tb³⁺.¹¹

Fig. 1 PLE (solid curve) and PL (dash curve) spectra of NGBO: 0.05Ce³⁺, (b) NGBO: 0.06Tb³⁺, and NGBO: 0.05Ce³⁺, 0.06Tb³⁺.

Comparing Fig. 1a and Fig. 1b, it is clearly observed that there is an overlap between the emission spectrum of Ce³⁺ and the excitation spectrum of Tb³⁺, which indicates that the resonance-type energy transfer from Ce³⁺ to Tb³⁺ can be expected to take place in NGBO host. Moreover, the exchange interaction energy transfer behavior may occur alongside the resonant energy transfer because of the overlap between the excitation spectrum of Tb³⁺ and that of solely Ce³⁺ doped NGBO phosphors.¹⁷ As shown in Fig. 1c, the excitation spectrum of Ce³⁺ and Tb³⁺ co-doped NGBO monitored with 413 (Ce³⁺ emission) is similar to that monitored with 543 nm (Tb³⁺ emission) except for the discrepancy in relative intensity. The presence of the 4f–5d transitions from Ce³⁺ ions in the PLE spectrum monitored at the ⁵D₄→⁷F₅ transition (543 nm) proves the occurrence of energy transfer from Ce³⁺ to Tb³⁺. Under the excitation of 358 nm n-UV light from the Ce³⁺ absorption, blue bands from Ce³⁺ and green lines from Tb³⁺ appeared in the PL spectrum of NGBO: Ce³⁺, Tb³⁺. Therefore, it is expected that Tb³⁺ can serve as an activator of green emitting phosphor for n-UV LEDs through energy transfer from the Ce³⁺ intense broad band absorption in the n-UV region. In comparison with the maximum excitation wavelength of many Ce³⁺–Tb³⁺ co-doped phosphors for LEDs, such as 335 nm for Ba₂Gd₂Si₄O₁₃:Ce³⁺, Tb³⁺⁸ and 328 nm for NaCaPO₄:Ce³⁺, Tb³⁺,¹¹ the present green emitting phosphor with the optimal absorption at 358 nm is more suitable for n-UV LEDs.

Fig. 2 shows the PL spectra of NGBO: 0.05Ce³⁺, nTb³⁺ phosphors with different Tb³⁺ doping contents, n, which were recorded with an excitation wavelength of 358 nm. The inset of Fig. 2 illuminates the dependence of the intensities of Ce³⁺ and Tb³⁺ as a function of Tb³⁺ concentrations; the intensities are defined as the area under their PL curves calculated by integrating from 365 to 475 nm for Ce³⁺ and from 475 to 650 nm for Tb³⁺, respectively. It is found that the intensity of the Ce³⁺ emission becomes weaker with increasing Tb³⁺ concentration; whereas the emission intensities of Tb³⁺ gradually intensify and then begin to decline after reaching the maximum value at n = 0.12 as a result of concentration quenching. The above results indicate that efficient energy transfer takes place from Ce³⁺ to Tb³⁺; furthermore, the intensity of blue emission from Ce³⁺ or the green emission from Tb³⁺ could be tuned by appropriately adjusting the concentration of the sensitizer Ce³⁺ and the activator Tb³⁺.

Fig. 2 PL spectra (λ_ex = 358 nm) of NGBO: 0.05Ce³⁺, nTb³⁺ (n = 0, 0.06, 0.12, 0.18, 0.25). Inset: Integrated emission intensity of Ce³⁺ and Tb³⁺ with different concentration of Tb³⁺.

In order to well understand the energy transfer process, the decay curves of Ce³⁺ with different Tb³⁺ concentrations were measured, as shown in Fig. 3. It can be seen that the decay curve of the Ce³⁺ singly doped NGBO sample is well fitted with a single-exponential function with a lifetime of about 22.42 ns. However, the decay curves of Ce³⁺ ions significantly deviate from the single exponential rule with the introduction of Tb³⁺ ions, which indicates that the doping of Tb³⁺ ions modifies the fluorescent dynamics of the Ce³⁺ ions.¹⁸ The effective lifetimes are defined by eqn (1)

in which I(t) represents the luminescence intensity at a time t. On the basis of eqn (1), the effective lifetime values were calculated to be about 9.32, 4.15, 3.95 and 3.63 ns for NGBO: 0.05Ce³⁺, nTb³⁺ with n = 0.06, 0.12, 0.18 and 0.25, respectively. It is clearly observed that the decay times of the Ce³⁺ ions decrease with an increase of the Tb³⁺ doping concentrations, which also supports the theory that the energy transfer mechanism from Ce³⁺ to Tb³⁺ is non-radiative.⁷ Furthermore, the energy transfer efficiency (η_T) can be expressed by eqn (2)^8,10,11,18where τ_s0 and τ_s are the lifetimes of Ce³⁺ in the absence and presence of Tb³⁺ ions , respectively. On the basis of the above calculated results of the lifetime, the energy transfer efficiency from the sensitizer Ce³⁺ to the activator Tb³⁺ was calculated according eqn 2 and plotted as a function of the Tb³⁺ content n in the inset of Fig. 3. It is found that the energy transfer efficiency increases steeply from 0 to 81.5% as n = 0 to 0.12. As the concentration of nTb³⁺ further increases up to 0.18 and 0.25, the values of η_T are estimated to be 82.4% and 83.8% within experimental error and keep nearly constant, which suggests that the energy transfer is close to saturation.

Fig. 3 Decay curves of the emission at 385 nm for Ce³⁺ in phosphors NGBO: 0.05Ce³⁺, nTb³⁺ with different contents of Tb³⁺ (λ_ex = 358 nm, n = 0, 0.06, 0.12, 0.18, 0.25). Inset: Dependence of energy transfer efficiency on concentration of Tb³⁺.

From the corresponding PL spectra upon 358 nm excitation, the commission International de I'Eclairage chromaticity coordination of the phosphors NBGO: 0.05Ce³⁺, nTb³⁺ (n = 0, 0.06, 0.12, 0.18 and 0.25) have been calculated and presented in Fig. 4. With the increase of Tb³⁺ concentration, the color hue of the phosphor gradually moves forward to the green region from the deep blue region under the excitation of n-UV light with 358 nm wavelength. The inset in Fig. 4 demonstrates a group of photos of the NBGO: 0.05Ce³⁺, nTb³⁺ phosphors with different Tb³⁺ contents under 365 nm excitation in a UV box. As consistent with Fig. 2, the results also indicate that the phosphor NBGO: 0.05Ce³⁺, 0.12Tb³⁺ shows the strongest green light, which suggest that the present phosphor could be used as a potential candidate as a green emitting phosphor for n-UV LEDs.

Fig. 4 CIE diagram of NBGO: 0.05Ce³⁺, nTb³⁺ phosphors (n = 0, 0.06, 0.12, 0.18 and 0.25) excited at 358 nm. Inset: phosphors under 365 nm excitation.

4. Conclusion

In summary, a series of single-phased NBGO: Ce³⁺, Tb³⁺ phosphors with tunable color were developed in this paper. The emission color of the obtained phosphors can be tuned appropriately from deep blue to green through simply adjusting the concentration of Tb³⁺ because of the different emission compositions of Ce³⁺ and Tb³⁺. The decrease of the Ce³⁺ transition decay time with the introduction of Tb³⁺ confirms the occurrence of an efficient non-radiative energy transfer from Ce³⁺ to Tb³⁺. Tb³⁺ can give a bright green emitting light by a strong excitation band of Ce³⁺, centered at about 358 nm near-UV light, perfectly matching with n-UV LED chips. The phosphor NBGO: 0.05Ce³⁺, 0.12Tb³⁺ shows the brightest green light and its energy transfer efficiency from Ce³⁺ to Tb³⁺ is over 80%, which indicates that it can serve as a potential green emitting phosphor for n-UV LEDs.

Acknowledgements

This work was supported by the high-level talent project of Northwest University, National Natural Science Foundation of China (No.50802031) and Ph.D Programs Foundation of Ministry of Education of China (No.20070487076), Natural Science Foundation of Hubei Province (2010CDB01607) and Foundation of Shaanxi Educational Committee (11JK0528).

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