Ce3+ and Tb3+ doped Ca3Gd(AlO)3(BO3)4 phosphors: synthesis, tunable photoluminescence, thermal stability, and potential application in white LEDs

Novel blue-green-emitting Ca3Gd(AlO)3(BO3)4:Ce3+,Tb3+ phosphors were successfully synthesized via traditional high temperature solid reaction method. X-ray diffraction, luminescence spectroscopy, fluorescence decay time and fluorescent thermal stability tests have been used to characterize the as-prepared samples. The energy transfer from Ce3+ to Tb3+ ions in the Ca3Gd(AlO)3(BO3)4 host has been demonstrated to be by dipole–dipole interaction, and the energy transfer efficiency reached as high as 83.6% for Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+. The critical distance was calculated to be 9.44 Å according to the concentration quenching method. The emission colour of the obtained phosphors can be tuned appropriately from deep blue (0.169, 0.067) to green (0.347, 0.494) through increasing the doping concentrations of Tb3+. Moreover, the Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+ phosphor possessed excellent thermal stability at high temperature, and the emission intensity at 423 K was about 87% of that at 303 K. Finally, the fabricated prototype LED device with a BaMgAl10O7:Eu2+ blue phosphor, CaAlSiN3:Eu2+ red phosphor, Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+ green phosphor and 365 nm-emitting InGaN chip exhibited bright warm white light. The current study shows that Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+ can be used as a potential green phosphor for white LEDs.

Borates have been proved to be outstanding host materials for inorganic phosphors, because they have many advantages, including high chemical stability, good thermal stability, and low synthetic temperature. 43,44 Here in this work, we reported the luminescent properties of novel Ce 3+ and Tb 3+ co-activated Ca 3 -Gd(AlO) 3 (BO 3 ) 4 phosphors. The crystal structure and the site substitution were investigated to verify the purity of phosphors and clarify the effect of doping on crystal structures. The luminescent performance including excitation, emission spectra, decay time, and chromaticity diagram was discussed to elucidate the luminescent mechanism of doping ions and the energy transfer mechanism between Ce 3+ and Tb 3+ in the Ca 3 Gd(AlO) 3 (BO 3 ) 4 host. Moreover, we examined their thermal stability and LED device performance for the practical application. fabricated via a conventional high-temperature solid-state reaction technique. H 3 BO 3 (analytical reagent), CaCO 3 (analytical reagent), Gd 2 O 3 (99.99%), Al 2 O 3 (analytical reagent), Ce(NO 3 ) 3 $6H 2 O (99.99%) and Tb(NO 3 ) 3 $6H 2 O (99.99%) were used as raw materials. According to the stoichiometric ratio, these raw materials were weighted and ground in an agate mortar to achieve uniformity. In order to compensate the volatilization, the amount of H 3 BO 3 is in excess of 5 wt%. Then, these uniform mixtures were put in the alumina crucibles and sintered at 1100 C for 4 h in CO atmosphere to reduce the Ce ion into tri-valence ion. Aer that, the furnace was cooled down naturally to room temperature, and the nal products were ground and collected for further characterization.
The X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 X-ray diffractometer using Cu Ka radiation ranging with 5-80 at step rate of 0.02 . The morphology properties of the samples were obtained by a eldemission scanning electron microscope (FE-SEM; MAIA3 TES-CAN). The room-temperature photoluminescence (PL) and PL excitation (PLE) spectra and luminescence decay lifetimes of phosphors were measured by Edinburgh FS5 spectrometer equipped with a 150 W continued-wavelength Xenon lamp and a pulsed Xenon lamp, respectively. Temperature-dependent PL spectra were recorded by using the same spectrophotometer and detectors equipped with a temperature controller. The internal quantum efficiency (IQE) of the phosphors was measured on an Edinburgh FS5 spectrometer equipped with an integrating sphere coated with BaSO 4 .
The commercial blue phosphor BaMgAl 10 O 7 :Eu 2+ , red phosphor CaAlSiN 3 :Eu 2+ and our green phosphor Ca 3 Gd 0.39 -(AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,0.6Tb 3+ were mixed with silicone thoroughly, and the obtained phosphor-silicone mixture was coated on the surface of the LED chips to fabricate WLED device. The photoelectric properties of the fabricated devices were measured by using an integrating sphere spectroradiometer system (HAAS-2000, Everne). The LEDs were operated at 3 V with various driven currents of 20, 60, 120, 180, 240 and 300 mA, respectively. The spectral power distributions of LEDs were measured using a corrected spectrometer to calculate their values of correlated colour temperature (CCT) and colour rendering index (CRI). Moreover, crystallographic data and details were summarized in Table 1, and the rened lattice parameters are changed slightly through substituting Y 3+ (1.075 A) ions with Gd 3+ (1.107 A), Ce 3+ (1.196 A) and Tb 3+ (1.095 A) ions. It can be attributed to the different radii of these ions. 45 The crystal structure of Ca 3 Gd(AlO) 3 (BO 3 ) 4 and coordination (CN) environments of Ca 2+ sites (Ca1 and Ca2) are depicted in Fig. 3. Clearly, Ca2 sites are surrounded with ten oxygen atoms while Ca1 sites are coordinated with nine oxygen atoms, but the coordinative six oxygen (O4) ions of Ca2 are also shared by the adjacent four B ions. Therefore, the CN sites of Ca1 and Ca2 are 9 and 7, respectively. The above results suggest that Ln 3+ (Gd 3+ , Ce 3+ and Tb 3+ ) ions prefer to occupy the Ca1 sites. 46 Fig. 4(a) shows the representative FE-SEM image of Ca 3 -Gd 0.39 (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,0.6Tb 3+ phosphors. As can be seen, the studied sample was made up of irregular and aggregated microparticles with the size ranging from 2 to 10 mm. Besides, the elemental mapping result revealed that the components of Ca, Ga, Al, O, B, Ce and Tb were uniformly distributed over the whole range of particles, as can be seen in Fig. 4(b).  4 :0.01Ce 3+ phosphor. By monitoring at 402 nm, there were two principle excitation bands in the PLE spectrum: the rst one was located in the 250-300 nm region and the second one was located in the range of 300-380 nm with a peak at around 347 nm, originating from 4f / 5d transitions of Ce 3+ ions. 22 The sharp PLE peak at 276 nm was attributed to the 8 S 7/2 / 6 I 7/2 transition of Gd 3+ ions, indicating the energy transfer from Gd 3+ to Ce 3+ ions in the Ca 3 Gd 0.99 (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ phosphor. 47 The PL spectrum was a broad emission band centered at 402 nm, corresponding to the transition from the 5d level to the ground state of the Ce 3+ ion.

Photoluminescence properties
The PLE and PL spectra of the representative phosphor Ca 3 Gd 0.95 (AlO) 3 (BO 3 ) 4 :0.05Tb 3+ were presented in Fig. 5(b). The PLE spectrum was recorded by monitoring with bright green emission at 544 nm, which revealed a series of excitation bands in the range of 230 to 400 nm. The broad PLE band in the range of 230-300 nm was due to the 4f-5d transitions of Tb 3+ ions. The PLE peak at 276 nm can be attributed to the 8 S 7/2 / 6 I 7/2 transition of Gd 3+ ions, indicating the occurrence of energy transfer from Gd 3+ to Tb 3+ ions. 48 In the 300-400 nm wavelength region, a series of PLE peaks were observed, which can be attributed to the 4f-4f electronic transitions of Tb 3+ ions, such as 7 F 6 / 5 H 7 at 314 nm, 7 F 6 / 5 L 7 at 343 nm, 7 F 6 / 5 D 2 at 352 nm, 7 F 6 / 5 G 6 at 370 and 7 F 6 / 5 D 3 at 380 nm. Upon 370 nm excitation, strong green emission was presented in the emission spectrum. The emission peaks at 492, 544, 586, and 622 nm can be assigned to 5 D 4 / 7 F J (J ¼ 6, 5, 4, 3) transitions, respectively. 41 By comparison of the Fig. 5(a) and (b), we can observe the overlap between the emission band of Ce 3+ and the f-f excitation of Tb 3+ , indicating the possible resonance energy transfer from Ce 3+ to Tb 3+ in Ca 3 Gd(AlO) 3 (BO 3 ) 4 host. For further identifying it, we measured the PLE and PL spectra of Ce 3+ and Tb 3+ co-doped sample Ca 3 Gd 0.85 (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,0.05Tb 3+ , as shown in Fig. 5(c). Clearly, not only the characteristic emission bands of Tb 3+ can be observed under excitation at 347 nm, but  In order to discuss the energy transfer mechanism of Ce 3+ -/ Tb 3+ , the illustration of electronic transitions of Ce 3+ and Tb 3+ is depicted in Fig. 6. Under 347 nm excitation, the electrons on Ce 3+ ions are excited from ground state ( 2 F 7/2 ) to higher excited state (5d), then some excited electrons return to ground states ( 2 F 5/2 and 2 F 7/2 ), generating blue emission of Ce 3+ . Meanwhile other excited electrons transfer part of their energy from Ce 3+ 5d level to the excited levels of Tb 3+ ( 5 D 2 ) through cross-relaxation, and subsequently, Tb 3+ relaxes non-radiatively to the 5 D 4 level from 5 D 2 and 5 D 3 states. Finally, radiative transitions take place through 5 D 4 / 7 F J (J ¼ 3-6) transitions, giving rise to the characteristic green emissions of Tb 3+ . Fig. 7 shows the PL spectra of Ca 3 Gd (0.99Àx) (AlO) 3 -(BO 3 ) 4 :0.01Ce 3+ ,xTb 3+ (x ¼ 0, 0.02, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) phosphors under excitation at 347 nm. As can be seen, the PL spectra of Ce 3+ /Tb 3+ co-doped Ca 3 Gd(AlO) 3 (BO 3 ) 4 samples consisted of blue emission band of Ce 3+ ions and the characteristic green emission peaks of Tb 3+ ions, due to the energy transfer from Ce 3+ to Tb 3+ . Moreover, with increasing Tb 3+ concentration from x ¼ 0 to x ¼ 0.6, the emission intensity of Ce 3+ ions decreased monotonically, while the emission intensity of Tb 3+ ions gradually increased without emission quenching. Interestingly, the emission colour of phosphors can be tuned through increasing Tb 3+ concentration, and the corresponding Commission Internationale de L'Eclairage (CIE) chromaticity moved from blue to green, as can be seen in Fig. 8. The digital photos depict a series of the representative Ca 3 -Gd (0.99Àx) (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,xTb 3+ (x ¼ 0, 0.1, and 0.6) phosphors under 365 nm lamp. These visual images reect the high potential of the phosphors as luminescence materials in the blue-green emission range. Besides, the IQEs of the Ca 3 -Gd (0.99Àx) (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,xTb 3+ phosphors were also listed in Fig. 8, which can be calculated based on the formula below. 49 where L S is the emission spectrum of the sample, E S and E R represent the excitation light with and without the sample in the integrating sphere, respectively. Herein, the Ca 3 Gd 0.97 (AlO) 3 -(BO 3 ) 4 :0.01Ce 3+ ,0.02Tb 3+ had the maximum value of 55.6%, and the IQE gradually decreased to be 38.2% with the increasing Tb 3+ concentration x in Ca 3 Gd (0.99Àx) (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,xTb 3+ .

Energy transfer mechanism
The decay curves of Ca 3 Gd (0.99Àx) (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,xTb 3+ samples were measured for identifying the energy transfer mechanism, as shown in Fig. 9. The uorescence average lifetimes s can be obtained from following formula: where I 1 and I 2 refer to intensities at different time, and s 1 and s 2 represent the corresponding decay time for the exponential components, respectively.
where I S0 and I S represent the luminescence intensity of According to the dependence of the intensities of the 5d-4f transition of Ce 3+ at 402 nm, the energy transfer efficiency of Ce 3+ / Tb 3+ was calculated and shown in Fig. 10. At the beginning of the process, the energy transfer efficiency raised rashly with increasing Tb 3+ concentration. Subsequently, it grew slowly aer x ¼ 0.4, and eventually reached up to 83.6% when x ¼ 0.6. As it is known, the mechanism of energy transfer from Ce 3+ to Tb 3+ ions can be attributed to exchange interaction or electric multipolar interaction. To gure out which interaction dominated in the energy transfer process, the average distance (R c ) between the Ce 3+ donors and Tb 3+ acceptor ions in Ca 3 -Gd 0.39 (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,0.6Tb 3+ phosphor was evaluated by using the following equation: 21 where C is the total concentration of Ce 3+ and Tb 3+ ions, N is coordination number, and V is the cell volume. For the Ca 3 -Gd 0.39 (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,0.6Tb 3+ phosphor, the V, C and N are 536.75 A 3 , 0.61 and 2, respectively. Accordingly, R c was determined to be 9.44Å. Generally, exchange interaction   requires a smaller R c value (<5 A), 15 and consequently the energy transfer from Ce 3+ to Tb 3+ in Ca 3 Gd 0.39 (AlO) 3 (BO 3 ) 4 :0.01-Ce 3+ ,0.6Tb 3+ phosphor would take place via electric multipolar interaction.
As discussed by many researchers, the multipolar interaction can be further conrmed by using the following formula: 38 where h 0 and h are the QEs of the Ce 3+ in the absence and presence of Tb 3+ , respectively. The value of h 0 /h can be approximately calculated by the ratio of relative luminescence intensity ratio (I S0 /I); C is the concentration of the sum of Ce 3+ and Tb 3+ . By using eqn (5), the electric multipolar interaction parameter n taking the values 6 (dipole-dipole), 8 (dipole-quadrupole), and 10 (quadrupole-quadrupole) were compared by the dependence of I S0 /I of Ce 3+ on C n/3 , as demonstrated in Fig. 11. A line relation was well-tted at n ¼ 6, so the energytransfer mechanism of Ce 3+ / Tb 3+ could be the dipoledipole interaction.

Thermal stability
As a very important index, the thermal stability can inuence the colour output and brightness of phosphor. Therefore, we measured the temperature-dependent emission spectra of Ca 3 Gd 0.39 (AlO) 3 (BO 3 ) 4 :0.01Ce 3+ ,0.6Tb 3+ sample and the curves were shown in Fig. 12(a). Clearly, with temperature increased from 303 K to 523 K, the emission intensity of Ca 3 Gd 0.39 - in which I 0 is the initial emission intensity, I T is the intensity at temperature T, E a is the activation energy, C is a constant for a certain host and k is the Boltzmann constant, respectively. Fig. 12(b) shows the plot of ln(I 0 /I À 1) vs. 1/kT, and the experimental data can be linear tted with a slope of À0.167. Thus, the activation energy of thermal quenching was 0.167 eV.  Fig. 13(a). The bright warm white light can be seen in the inset of Fig. 13(a). The CCT, CRI (Ra), and CIE chromaticity coordinate of this WLED device was measured to be 3158 K, 79.7, and (0.427, 0.401), respectively. The above values of the WLED device under various currents were also measured (see Fig. 13(b)) and there was little variation, which conrmed the stable white light output in the device.

Conclusions
In summary, we have successfully developed a novel green emitting phosphor for near UV-pumped WLEDs.

Conflicts of interest
There are no conicts to declare.