Energy transfer and luminescent properties of Ca8MgLu(PO4)7:Tb3+/Eu3+ as a green-to-red color tunable phosphor under NUV excitation

Feiyan Xieab, Junhao Lib, Zhiyue Dongb, Dawei Wenb, Jianxin Shi*b, Jing Yanb and Mingmei Wu*b
aDepartment of Chemical Engineering, Huizhou University, Huizhou, 516007, P. R. China
bKey Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China. E-mail: cessjx@mail.sysu.edu.cn; ceswmm@mail.sysu.edu.cn; Fax: +86 20 8411 2245; Tel: +86 20 8411 2830

Received 10th May 2015 , Accepted 3rd July 2015

First published on 3rd July 2015


Abstract

Two series of single-composition Ca8MgLu(PO4)7:Tb3+ and Ca8MgTb(PO4)7:Eu3+ phosphors were prepared by a high-temperature solid-state reaction technique, and their phase structures were characterized by powder X-ray diffraction (XRD). The excitation and emission spectra, and fluorescence decays were measured and discussed in detail. The results reveal that Tb3+ can efficiently transfer excitation energy to Eu3+ via its 4f states and therefore sensitizes Eu3+ emission under NUV excitation. By adjusting the ratio of Eu3+ and Tb3+, we can tune the emission color of Ca8MgTb(PO4)7:Eu3+ from green to yellow, orange and pure red. For Ca8MgTb0.1(PO4)7:0.9Eu3+, the emission intensity at 150 °C is 87.44% of that at 25 °C, which makes it be a potential pure red phosphor for NUV LEDs.


Introduction

In the past decade, more and more interest has been focused on white light-emitting diodes (WLEDs), fabricated with a blue InGaN chip and yellow-emitting phosphor Y3Al5O12:Ce3+, due to their high brightness, long lifetime and environmental friendliness.1–4 However, these blue InGaN-based WLEDs have some disadvantages such as low color rendering index (Ra) and unsatisfactory high color temperature because of the lack of red spectral component.5,6 Moreover, the commonly used red phosphors for near ultraviolet (NUV) InGaN-based WLEDs are Y2O2S:Eu3+, which shows some drawbacks such as a lower efficiency and shorter working lifetime, due to its instability.7 Therefore, there is an urgent need to develop novel and stable red phosphors with intense emission upon NUV excitation.

In the search for red-emitting phosphors with high efficiency and proper CIE chromaticity coordinates, the Eu3+ activated phosphors are primarily considered for the reason that Eu3+ ions can emit red light with an excellent color purity. However, the low oscillator strength of 4f → 4f absorption transitions such as 7F05D4, 5L7 and 5D3 or the narrow line width of 7F05L4 transition (∼395 nm) for Eu3+ leads to a weak absorption in the NUV region or a mismatch absorption with NUV from LED chip.8 Thus, it is necessary to find sensitizers for Eu3+ luminescence.

With the development of the chip technology, the emission of InGaN LED chip extends to NUV region (350–400 nm) and the commercially available NUV LED chip from 375 to 380 nm is more and more common.9–11 Therefore, the sensitization effect of Tb3+ ions for red emitting ions, Eu3+, has attracted great attention.12–14 Tb3+ ions not only enhance the emission of Eu3+ but also broaden the absorption region due to the existence of more impurity energy levels introduced by Tb3+. It is clear that Tb3+ acts as a good sensitizer to enhance the luminescence efficiency of Eu3+ ions in K2Ln(PO4)(WO4),15 Ba3La(PO4)3,16 SrMg2La2W2O12,17 TbBO3:Eu3+,18 TbPO4:Eu3+,19 and KCaY(PO4)2:Tb3+,Eu3+ (ref. 20) phosphors. Moreover, the emitting color of the phosphors can be tuned by adjusting the ratio of Tb3+ and Eu3+ ions. The realization of tunable multicolor emission under a single excitation wavelength in phosphors is beneficial for potential application in display device.

Phosphates are excellent hosts for luminescent materials because of their facile synthesis condition, good chemical stability and low cost. The Ca8MgLn(PO4)7 (Ln = Y, La, Gd or Lu) compound has whitlockite-like structure with space group R3c as β-Ca3(PO4)2. As a typical of phosphate, β-Ca3(PO4)2 has six metal sites (M1–M6) in the crystal lattice. M1 and M2 sites are coordinated by eight oxygen atoms, M3 and M5 sites are surrounded by nine and six oxygen atoms, M4 site surrounded by nine oxygen atoms is 50% occupied by Ca2+ ions, and M6 site is vacant.21–23 The special structure suggests that the lattice can accommodate other cations with similar radii and charges without significant changes in the structural framework. Moreover, the hexagonal crystal structure of Ca8MgLu(PO4)7 also favors the energy transfer from Tb3+ to Eu3+. Ca8MgLn(PO4)7:Eu2+,Mn2+ (Ln = La or Y) phosphors have been reported for WLEDs due to their outstanding luminescence properties.21 Ca8MgLn(PO4)7:Eu3+ (Ln = La, Gd or Y) phosphors have been synthesized and focused on the site-selective spectroscopy of Eu3+.24 To the best of our knowledge, the luminescence properties of Ca8MgLu(PO4)7:Tb3+ and Ca8MgTb(PO4)7:Eu3+ have not been reported. In this study, we report the synthesis and characterization of an emitting color tunable phosphor, Ca8MgTb(PO4)7:Eu3+. The mechanism of energy transfer between Tb3+ and Eu3+ in the phosphor was investigated, and the results show that the thermally stable luminescence of Ca8MgTb0.1(PO4)7:0.9Eu3+ could serve as a potential pure red phosphor for NUV LEDs.

Experimental

Two series of phosphors with the compositions of Ca8MgLu1−x(PO4)7:xTb3+ and Ca8MgTb1−y(PO4)7:yEu3+ were synthesized by a high temperature solid-state reaction method. The raw materials were CaCO3 (A. R.), NH4H2PO4 (A. R.), (MgCO3)4·Mg(OH)2·5H2O (A. R.), Lu2O3 (99.99%), Tb4O7 (99.99%) and Eu2O3 (99.99%), respectively. The raw materials with a stoichiometric ratio were mixed by grinding in an agate mortar. After mixing and grinding, the mixtures were put into crucibles and subsequently heated at 1200 °C in a chamber furnace for 3.0 h in air. Finally, the as-synthesized samples were cooled down slowly to room temperature and then ground into powder for measuring.

The structure of the samples was examined with a Rigaku D-max 2200 X-ray diffraction system with a Cu Kα radiation at 30 kV and 30 mA. The photoluminescence (PL), PL excitation (PLE) spectra and the decay curves at room temperature were measured by FLS 920-combined Time Resolved and Steady State Fluorescence Spectrometer (Edinburgh Instruments) equipped with a 450 W Xe lamp, a 60 W μF flash lamp. The temperature-dependent PL spectra were obtained on the same instrument with a temperature-controller.

Results and discussion

Crystal structures of Ca8MgLu(PO4)7:Tb3+ and Ca8MgTb(PO4)7:Eu3+

Fig. 1 shows the powder XRD patterns for Ca8MgLu(PO4)7, Ca8MgLu0.3(PO4)7:0.7Tb3+, Ca8MgTb(PO4)7, Ca8MgTb0.1(PO4)7:0.9Eu3+, and the reference diffraction lines based on the JCPDS card with no. 46-0803. The results of XRD analysis confirm that the compounds were obtained as single phase. No extra diffraction peaks related to the starting materials were observed. All the diffraction peaks of the samples can be indexed to the standard data of Ca8MgLu(PO4)7 (JCPDS card no. 46-0803). The crystal structure of Ca8MgLu(PO4)7 is hexagonal with space group of R3c, and cell parameters of a = b = 10.337 Å, c = 36.915 Å, V = 3416.2 Å3, and Z = 6. The crystal structure of Ca8MgLu(PO4)7 and coordination condition of Ca2+ is presented in Fig. 2. There are six metal sites (M1–M6) in this crystal. M1 and M2 sites are coordinated by eight oxygen atoms, M3 and M5 sites are surrounded by nine and six oxygen atoms, M4 site surrounded by nine oxygen atoms is 50% occupied by Ca2+ ions, and M6 site is vacant. For Ca8MgLu(PO4)7, Ca2+ and Lu3+ mainly occupy M1, M2 and M3, while Mg2+ occupies M5 site.21 It was found that as Lu3+ is substituted by the larger Tb3+ or Eu3+ ion, the entire diffraction profile shifts slightly towards a lower 2θ angle.
image file: c5ra08680a-f1.tif
Fig. 1 Powder XRD patterns for Ca8MgLu(PO4)7, Ca8MgLu0.3(PO4)7:0.7Tb3+, Ca8MgTb(PO4)7, Ca8MgTb0.1(PO4)7:0.9Eu3+, and the standard data of Ca8MgLu(PO4)7 (JCPDS card no. 46-0803).

image file: c5ra08680a-f2.tif
Fig. 2 The crystal structure of Ca8MgLu(PO4)7 and coordination condition of Ca2+.

Luminescence properties of Ca8MgLu(PO4)7:Tb3+ and Ca8MgTb(PO4)7:Eu3+

The excitation and emission spectra of Ca8MgLu0.1(PO4)7:0.9Tb3+ phosphor are presented in Fig. 3a and b, respectively. This excitation spectrum is taken at an emission wavelength of 543 nm, which reveals a series of spectral bands in the range from 280 nm to 400 nm. The f–f transitions are ascribed to the transitions of Tb3+ ions from the ground state of 7F6 to the higher energy states of 5H3 (285 nm), 5H6 (303 nm), 5D0 (319 nm), 5G2 (341 nm), 5D2 (351 nm), 5L10 (368 nm), and 5G6 (378 nm), respectively.25 The emission spectrum of the sample covers the region from 480 to 700 nm as excited at 378 nm. This spectrum consists of the characteristic transitions of Tb3+, 5D47F6 (488 nm), 5D47F5 (543 nm), 5D47F4 (585 nm) and 5D47F3 (620 nm).26 Among these characteristic peaks, the emission of the 5D47F5 transition at 543 nm is predominant and is predicted by the large values of the reduced matrix elements at J = 5 and the Judd–Ofelt theory.27
image file: c5ra08680a-f3.tif
Fig. 3 (a) Excitation and (b) emission spectra of Ca8MgLu0.1(PO4)7:0.9Tb3+, (c) the dependence of emission intensities on Tb3+ contents and (d) decay curves of Ca8MgLu1−x(PO4)7:xTb3+ (x = 0.1–1.0).

The emission intensities of Tb3+ as a function of the doping concentrations and decay curves of Tb3+ in Ca8MgLu1−x(PO4)7:xTb3+ are given in Fig. 3c and d, respectively. It can be seen that luminescence intensities of Tb3+ are enhanced with the increase of their Tb3+ concentrations in Fig. 3c. The decay curves of Tb3+ are nearly overlapped one another at different Tb3+ concentrations as shown in Fig. 3d. The decay time for all Ca8MgLu1−x(PO4)7:xTb3+ (x = 0.1–1.0) samples at room temperature were measured to be ∼2.1 ms. These observations confirm that the concentration quenching of Tb3+ does not occur in Ca8MgLu(PO4)7 host, which can be explained by the crystal structure of the host matrix. Ca8MgLu(PO4)7 has a typical whitlockite structure with a space group of R3c. In the structure of Ca8MgLu(PO4)7, the cell in the hexagonal setting can accommodate a high concentration of cations with various valence states and various sizes due to six distinct Ca2+ sites, with coordination number ranging from 6 to 9 and various Ca–O distances.23 In the structure, sites M1–M3 and M5 are fully occupied, but the site M4 is only half occupied and the M6 site is vacant. Hence, we consider that this special structure is an important factor for no concentration quenching phenomenon.

Fig. 4 illustrates the PLE and PL spectra of the typical Ca8MgTb0.7(PO4)7:0.3Eu3+. The PLE spectrum monitored at 612 nm of Eu3+ consists of 4f–4f excitation transitions of Tb3+. These results give a direct evidence of sensitizing Eu3+ by Tb3+. 5D2, 5L10 and 5G6 of Tb3+ are very close to each other. Meanwhile, 5D4, 5G2 and 5L7 of Eu3+ are close to each other. Eu3+ can make full use of those energy levels to form a combinatorial absorption, resulting from energy transfer process of Tb3+ → Eu3+. Thus, the absorption region of Eu3+ can be broadened (see Fig. S1. ESI). The excitation spectra which irradiated wavelength of 544 nm were identical to that of Tb3+ singly doped phosphor, which shows Ca8MgTb(PO4)7 can be used as green and red double-color-emitting phosphors in NUV-pumped WLEDs. On the other hand, the emission peaks of Eu3+, Tb3+ co-doped phosphor under 378 nm excitation were observed at 612 nm and 543 nm, attributed to the Eu3+ and Tb3+ ions, respectively. Therefore, the relative intensities of these two emissions can be varied by adjusting the concentrations of the two activators through the principle of energy transfer.10


image file: c5ra08680a-f4.tif
Fig. 4 PLE and PL spectra of Ca8MgTb0.7(PO4)7:0.3Eu3+ phosphor.

Color-tunable emission of Ca8MgTb(PO4)7:Eu3+ under NUV excitation

Fig. 5 shows the emission spectra of Ca8MgTb1−y(PO4)7:yEu3+ with different Eu3+ contents under 378 nm excitation. In Ca8MgTb(PO4)7 sample with no Eu3+-doping, the characteristic emissions of Tb3+ are observed. With the doping of Eu3+ (y = 0.1), besides the emission of Tb3+, we can also observe the emission of Eu3+. With the increase of Eu3+ concentration, the luminescence of Tb3+ begins to decrease and that of Eu3+ increases, which are the results of the enhancing probability of energy transfer from Tb3+ to Eu3+. Therefore, Ca8MgTb1−y(PO4)7:yEu3+ (y = 0–0.7) samples show a tunable emission from green to red, depending on the ratio of Tb3+ to Eu3+. As shown in the CIE chromaticity diagram of Fig. 6 and Table 1, the CIE chromaticity coordinates of the corresponding samples are shifting from (0.331, 0.592) to (0.644, 0.352). More importantly, the coordinate of the red emission sample Ca8MgTb0.1(PO4)7:0.9Eu3+ is (0.644, 0.352), which is very close to the National Television System Committee (NTSC) standard for red subpixels (0.67, 0.33).28 The digital photos of Ca8MgTb(PO4)7:Eu3+ in Fig. 5 also support the spectral results. Ca8MgTb(PO4)7:Eu3+ can be used as green-red double color phosphors for NUV-based WLEDs.
image file: c5ra08680a-f5.tif
Fig. 5 PL spectra (λex = 378 nm) and luminescence photographs (λex = 365 nm) of Ca8MgTb1−y(PO4)7:yEu3+ (y = 0, 0.1, 0.3, 0.5, 0.7).

image file: c5ra08680a-f6.tif
Fig. 6 CIE chromaticity diagram for Ca8MgTb1−y(PO4)7:yEu3+ (y = 0–0.9) under 378 nm excitation.
Table 1 Comparison of the CIE chromaticity coordinates for Ca8MgTb1−y(PO4)7:yEu3+ excited at 378 nm
Sample no. %Eu3+ CIE coordinates (x, y)
1 0 (0.331, 0.592)
2 5 (0.395, 0.542)
3 10 (0.432, 0.514)
4 20 (0.480, 0.476)
5 30 (0.524, 0.443)
6 50 (0.582, 0.399)
7 70 (0.623, 0.368)
8 90 (0.644, 0.352)


Energy transfer mechanism of Tb3+ → Eu3+ in Ca8MgTb(PO4)7:Eu3+

Luminescence decay time measurements are performed to further analyze the energy transfer phenomenon. Fluorescence decays of samples Ca8MgTb1−y(PO4)7:yEu3+ with different doping concentration (y = 0–0.9) at room temperature are shown in Fig. 7a. A single-exponential decay process was observed with different y values in Ca8MgTb1−y(PO4)7:yEu3+, the curves were well fitted by the following equation,25
 
It = I0[thin space (1/6-em)]exp(−t/τ) (1)
where It and I0 are the luminescence intensities at time t and t = 0, respectively, and τ is the decay time. The values of τ were calculated to be about 2.05, 1.92, 1.75, 1.49, 1.32, 1.14, 1.00 and 0.98 ms for Ca8MgTb1−y(PO4)7:yEu3+ with y = 0.0, 0.05, 0.1, 0.2, 0.3, 0.5, 0.7 and 0.9, respectively, as shown in Fig. 7b. It can be observed that the decay time of Tb3+ ions decreases with increasing the contents of Eu3+, due to the energy transfer from Tb3+ to Eu3+. Eqn (2) can be used to estimate the energy transfer probability (PTb→Eu) from Tb3+ to Eu3+,29
 
image file: c5ra08680a-t1.tif(2)
where τ0 and τ are the corresponding lifetimes of the donor Tb3+ in the absence and presence of the acceptor Eu3+ for the same donor concentration, respectively. The energy transfer efficiency (ηTb→Eu) is also evaluated from eqn (3),
 
image file: c5ra08680a-t2.tif(3)
According to the above eqn (2) and (3), the values of PTb→Eu and ηTb→Eu can be calculated in Table 2 and Fig. 8, respectively. The results indicate that the energy transfer efficiency from Tb3+ to Eu3+ is effective and strongly depends on the doping concentration of Eu3+ in Ca8MgTb(PO4)7 host. Clearly, it is known from Fig. 8 that the energy transfer efficiency from Tb3+ to Eu3+ increases gradually with the increase in Eu3+ concentration.

image file: c5ra08680a-f7.tif
Fig. 7 (a) Decay curves and (b) fluorescence lifetimes of Tb3+ under 378 nm excitation as a function of y values in Ca8MgTb1−y(PO4)7:yEu3+ (y = 0–0.9).
Table 2 Energy transfer probabilities and efficiencies of Tb3+ → Eu3+ in Ca8MgTb1−y (PO4)7:yEu3+
%Eu3+ Decay time (ms) PTb→Eu (ms−1) ηTb→Eu
0 2.05 0 0
5 1.92 0.033 0.06
10 1.75 0.084 0.14
20 1.49 0.183 0.27
30 1.32 0.270 0.36
40 1.22 0.332 0.40
50 1.14 0.389 0.44
60 1.05 0.465 0.49
70 1.00 0.512 0.51
80 0.99 0.522 0.52
90 0.98 0.533 0.52



image file: c5ra08680a-f8.tif
Fig. 8 Dependence of (a) energy transfer probability PTb→Eu and (b) efficiency ηTb→Eu on Eu3+ concentration (y = 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) in Ca8MgTb1−y(PO4)7:yEu3+.

Fig. 9 illustrates the decay curves of Ca8MgTb1−y(PO4)7:yEu3+ (λex = 378 nm, λem = 612 nm). Two different processes can be observed for Eu3+ emission, decay process and build-up process. In the build-up process, the energy absorbed by the 7F65G6 transition in Tb3+ ions is transferred to Eu3+ ions. The build-up process is significantly influenced by the content of Eu3+. As shown in Fig. 9a, the build-up process becomes faster and faster with increasing the content of Eu3+ ions, which indicates that the energy transfer process from Tb3+ to Eu3+ becomes more efficient with the increase of Eu3+. When the Ca8MgTb1−y(PO4)7:yEu3+ samples are excited by 378 nm, the rate equations for the population densities in the 5D4 level of Tb3+ ion and 5D0 of Eu3+ ion can be expressed as follows25,30

 
image file: c5ra08680a-t3.tif(4)
 
image file: c5ra08680a-t4.tif(5)
where the NTb and NEu are the population intensities of the 5D4 level of Tb3+ and 5D0 of Eu3+, respectively. KTb−Eu is the non-radiative energy transfer rate from the 5D4 state of Tb3+ to 5D0 of Eu3+. Then the fluorescence intensity It of Eu3+ ions at 612 nm under 378 nm excitation can be given as following,
 
image file: c5ra08680a-t5.tif(6)
Using the measured values of τTb and τEu, the theoretical curves for the Ca8MgTb1−y(PO4)7:yEu3+ samples are obtained as presented in Fig. 9b, which show two processes for Eu3+ emission, being similar to the measured curves. That is to say, the theoretical curves are consistent with the experimental ones.


image file: c5ra08680a-f9.tif
Fig. 9 (a) Experimental decay curves and (b) the corresponding theoretical curves for Ca8MgTb1−y (PO4)7:yEu3+.

The energy transfer scheme of Tb3+–Eu3+ in Ca8MgTb(PO4)7 host is shown in Fig. 10. The Tb3+ ions can be excited from the ground state (7F6) to the excited states by UV light, and then relax to the lowest excited state 5D4 through multiphonon relaxation, then radiatively return to ground states (7FJ). When co-doped with Eu3+, the energy from 5D47FJ transitions of Tb3+ will be transferred to Eu3+ through cross-relaxation, which will relax to the 5D0 (Eu3+) level, where the orange-red emission (5D07FJ) takes place. Because the 5D47FJ emission of Tb3+ is effectively overlapped with the 7F0,15D0,1,2 absorption of Eu3+, the energy transfer from Tb3+ to Eu3+ is efficient.18 Exchange interactions and multipolar interactions are two main aspects of the resonant energy-transfer mechanism. The energy transfer between the Tb3+ and Eu3+ ions mainly takes place by exchange interactions.8,18,19


image file: c5ra08680a-f10.tif
Fig. 10 Schematic energy–level diagram of Tb3+ and Eu3+ in Ca8MgTb(PO4)7 and energy transfer from Tb3+ to Eu3+.

The distance between Tb3+ and Eu3+ ions can be estimated by using the equation pointed out by Blasse,31

 
image file: c5ra08680a-t6.tif(7)
where Rc corresponds to the mean separation between the nearest Tb3+and Eu3+ ions at xc, V is the volume of the unit cell, N is the number of available sites for the dopant in the unit cell and xc is the concentration. In this case, V is estimated to be 3447.1 Å3, xc is 1 and N is 6 due to 6 divalent metal sites in the unit cell.21 The distance RTb−Eu is calculated to be 10.31 Å. This value is much longer than 5 Å, the critical distance of the exchange interaction.8,32 So the concentration quenching of Tb3+ does not occur in Ca8MgLu(PO4)7 host, which is consistent with the previous conclusion obtained from the point of crystal structure.

Thermally stable luminescence of Ca8MgTb(PO4)7:Eu3+

The thermal quenching property is one of the important technological parameters for phosphors used in solid-state lighting because it has a considerable influence on the light output and color rendering index. The temperature dependence of the integrated emission intensity of Ca8MgTb0.1(PO4)7:0.9Eu3+ excited with 378 nm is illustrated in Fig. 11a upon heating the phosphor in a temperature range from 25 to 200 °C. When the temperature is increased up to 150 °C and 200 °C, the emission integrated intensity is 87.44% and 79.18% of that at 25 °C. The results are better than those of some red phosphors, such as NaGd(WO4)2:Tm3+,Dy3+,Eu3+, 51.60% at 200 °C,33 and LaMgAl11O19:Sm3+,Eu3+, 56% at 150 °C.34 The CIE chromaticity coordinates of Ca8MgTb0.1(PO4)7:0.9Eu3+ sample at different temperatures are also shown (see Table S1 ESI), which show that the phosphor has a good color stability. Therefore, Ca8MgTb(PO4)7:Eu3+ phosphor has an excellent thermal stability and could be potential for high-powered LED applications. In order to understand the temperature dependence of emission intensity and to determine the activation energy for thermal quenching, the Arrhenius equation was used to fit the thermal quenching data of the Ca8MgTb0.1(PO4)7:0.9Eu3+ phosphor,35
 
image file: c5ra08680a-t7.tif(8)
where I0 is the initial emission intensity, I is the intensity at different temperatures, ΔE is activation energy of thermal quenching, A is a constant for a certain host and k is Boltzmann constant (8.617 × 10−5 eV). Eqn (8) can be revised as,
 
image file: c5ra08680a-t8.tif(9)

image file: c5ra08680a-f11.tif
Fig. 11 (a) Temperature dependence of the emission integrated intensity of Ca8MgTb0.1(PO4)7:0.9Eu3+ and (b) the Arrhenius fitting result.

As shown in Fig. 11b, the experimentally calculated activation energy ΔE was 0.15 eV for Ca8MgTb0.1(PO4)7:0.9Eu3+.

Conclusions

In summary, Ca8MgLu(PO4)7:Tb3+ and Ca8MgTb(PO4)7:Eu3+ phosphors were prepared using a high-temperature solid-state reaction technique. The energy transfer process of Tb3+ → Eu3+ has been investigated by the photoluminescence emission and excitation spectra, the decay curves, and the effect of the ratio of Tb3+ to Eu3+. It is demonstrated that Tb3+ can efficiently sensitize the luminescence of Eu3+ under NUV excitation due to effective energy transfer from Tb3+ to Eu3+, and the energy transfer efficiency increases with increasing the concentration of Eu3+. Ca8MgTb1−y(PO4)7:yEu3+ (y = 0–0.9) phosphors exhibit bright emission under 378 nm excitation and the emission color can be tunable from green, to yellowish-green and red region. Moreover, the temperature-dependence of luminescence shows that Ca8MgTb(PO4)7:Eu3+ phosphor has an excellent thermal stability. These results indicate that Ca8MgTb(PO4)7:Eu3+ can be promising as a potential candidate for the application in NUV-based WLEDs.

Acknowledgements

This work was financially supported by grants from the Joint Funds of the National Natural Science Foundation of China (NNSFC) and Guangdong Province (No. U1301242), NNSFC (21271190), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No. 20130171130001), the Government of Guangdong Province for Industrial Applications of Rare Earth Photoluminescence Materials (2012B091000026 and 2013A090100010), the Natural Science Foundation of Guangdong Province (No. 9151027501000047 and S2012020011113), and the Natural Science Foundation of Huizhou university (No. hzuxl201301).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08680a

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