A single-phase full-color emitting Sr10[(PO4)5.5(BO4)0.5](BO2): Ce3+, Mn2+, Tb3+ phosphor was synthesized by solid-state reaction for the first time. The characteristic luminescence property of Ce3+ is investigated by Gauss fitting. Energy transfer from Ce3+ to Mn2+ and Ce3+ to Tb3+ in Sr10[(PO4)5.5(BO4)0.5](BO2) is detailedly studied by luminescence spectra, energy-transfer efficiency and lifetimes. Through effective energy transfer, the wavelength-tunable warm white light can be realized with superior chromaticity coordinates of (0.35, 0.32), high color rendering index (Ra = 89) and low correlated color temperature (CCT = 4373K) by coupling the emission bands centered at 441, 542 and 649 nm attributed to the contribution from Ce3+, Mn2+ and Tb3+, respectively. The results indicate the white Sr10[(PO4)5.5(BO4)0.5](BO2):Ce3+, Mn2+, Tb3+ phosphor can serve as a promising candidate for phosphor-converted white-light UV-LEDs.
© 2013 Optical Society of America
In the last few decades, there has been a phenomenal growth in investigation of the field of luminescence, and significant progress has been made in solid state lighting [1,2]. The breakthrough in the development of high performance light-emitting diodes (LEDs) via improvement in both internal quantum efficiency [3–7] and extraction efficiency [8,9] has resulted in practical excitation sources applicable for phosphor-converted white LEDs. Currently, phosphor-converted (pc) white light-emitting diodes (LEDs), with characteristics of high efficiency, long lifetime, and energy saving, have attracted more and more attention [10,11]. By virtue of these advantages, white LEDs are currently the mainstream in the market and are being widely used in not only point light sources, but also wide-illumination equipment, back-lighting of liquid-crystal TVs and high-power automotive headlights . In pc-white LEDs, phosphors are the key component to down-convert near-ultraviolet (n-UV)/ blue pump light from InGaN LEDs into visible light. To date, the most dominant pc-white LEDs is combination of a blue LED with a yellow phosphor ((Y,Gd)3(Al,Ga)5O12:Ce3+). Such generated light color is not true because this system lacks of a red emitting component, which restricts their use in more vivid applications [13,14]. In order to obtain a warm white light, the new technologies are based on the combination of a near-ultraviolet/ultraviolet (n-UV/UV) LED chip with three phosphors emitting red, green, and blue (RGB) colors [15,16]. However, different phosphors experience different light output degradation rates, leading to unstable white light, and poor luminous efficiency attributed to reabsorption has commonly been encountered . Therefore, this type white LEDs still face large challenges. For these reasons, people are making more efforts in developing a full-color emitting phosphor based on a single host to avoid the intrinsic color balance, device complication, and high-cost problems. One of the strategies for generating white light from single-phased phosphors is by codoping sensitizer and activator into the same host, such as Eu2+ to Mn2+ ion , Ce3+ to Tb3+ ion , or Ce3+ to Mn2+ ion .
In recent years, the luminescent properties of phosphate materials have been widely investigated as a result of certain advantages, such as excellent thermal and chemical stability , and development of optical devices based on rare earth ion doped materials is one of the most interesting fields of research. Phosphates phosphors are widely investigated because of their low cost, high stability for use in solid state lighting applications, and their important crystallographic possibilities for accommodating luminescent ions . So in this paper, we choose strontium phosphate orthoborate metaborate Sr10[(PO4)5.5(BO4)0.5](BO2) (SPB) as host and synthesized rare earth ion Ce3+, Tb3+ and Mn2+ co-doped SPB phosphors, with an aim to realize warm white light emission in this single phased phosphor. As excepted, the color tunable full-color emitting phosphor SPB: Ce3+, Mn2+, Tb3+ was obtained by adjusting the related contents of Tb3+ and Mn2+ based on the efficient resonant energy transfer. In addition, the energy transfer mechanism was also discussed in detail in SPB: Ce3+, Mn2+, Tb3+ through photoluminescence spectra and decay times. At the same time, the color rendering index (CRI) and correlated color temperature (CCT) were also studied to evaluate the quality of the white light emission.
2.1 Materials and synthesis
All the powder samples were synthesized by traditional solid-state reaction method. The starting materials were SrCO3 (A.R. 99.9%), (NH4)2HPO4 (A.R. 99.9%), H3BO3 (A.R. 99.9%), MnCO3 (A.R. 99.9%), CeO2 (A.R. 99.99%) and Tb4O7 (A.R. 99.99%). The stoichiometric raw materials were ground thoroughly in an agate mortar and then heated to 773 K in air for 3h. Subsequently the preheated mixture was ground again and fired to 1423 K for 8 h in an alumina crucible under N2-H2 (8%) atmosphere in horizontal tube furnaces. Finally the as-synthesized samples were slowly cooled to room temperature inside the tube furnace under H2-N2 flow.
2.2 Measurements and characterization
The crystal structure of the synthesized samples was identified by using a Rigaku D/Max-2400 X-ray diffractometer (XRD) with Nifiltered Cu Kα radiation. The diffuse-reflectance spectra (DRS) were obtained by a UV-visible spectrophotometer (PE Lambda 950) using BaSO4 as a reference. The photoluminescence (PL) and PL excitation (PLE) spectra of the samples were measured by using an FLS-920T fluorescence spectrophotometer equipped with a 450W Xe light source and double excitation monochromators. The PL decay curves were measured by an FLS-920T fluorescence spectrophotometer with nF900 nanosecond Flashlamp as the light source. The quantum efficiency (QE) was measured by a Fluorlog-3 spectrofluorometer equipped with 450W xenon lamp (Horiba Jobin Yvon). Thermal quenching was tested using a heating apparatus (TAP-02) in combination with PL equipment. All of the measurements were performed at room temperature.
3. Results and discussion
3.1 Phase identification of as-prepared phosphors
Figures 1(a) and 1(b) display the results of Rietveld refinement of the XRD data profiles of SPB host by GSAS program  (Rwp = 7.24, χ2 = 2.946) and the representative XRD patterns of SPB: 0.03Ce3+, SPB: 0.03Ce3+, 0.05Mn2+ and SPB: 0.03Ce3+, 0.05Mn2+, 0.03Tb3+ samples. No detectable impurity phase is observed in the obtained samples even at high doping concentration. The XRD profiles are well fitted with the calculated data according to Chen’s report , indicating that the obtained samples are single phase and the rare earth ions have been successfully incorporated in the SPB host lattices without changing the crystal structure.
3.2 Photoluminescence properties of SPB: Ce3+ and SPB: Mn2+
Figure 2 shows the PL and PLE spectra for SPB: Ce3+ (a) and SPB: 0.01Mn2+ (b). The PLE spectrum of SPB: Ce3+ at 354 nm excitation extends from 240 to 420 nm with two distinct bands peaking at 297 and 354 nm attributed to the 4f–5d transition of Ce3+ ions, which indicates that the phosphors can be effectively excited by UV light. The emission spectra show asymmetric broadband characteristic of Ce3+ ions, but the FWHM value amounts to 5030 cm−1, which is not comparable to that typically observed for Ce3+ activated phosphors containing a single type of luminescent center (~4000cm−1) . Considering that there are two different cationic sites in SPB (as illustrated in Fig. 2(a)), then we take Gaussian fitting algorithm and find that the curve can be well fitted into four emission bands, which centered at 419 and 454 nm, with an energy difference of 1840 cm−1 and the other two bands peaked at 509 and 562 nm with an energy difference of 1850 cm−1, respectively, which is close to the theoretical energy difference value of Ce3+ (~2000 cm−1) . So the four peaks can be justifiably assigned to the 5d-4f (2F5/2, 2F7/2) emissions of Ce3+ ions occupying two Sr sites (Sr1/2, Sr3). Due to a slight difference between crystal fields of Sr1 and Sr2 sites, the bands are not resolved distinctly. Generally, the bond length (R) of Sr-O affects the crystal field strength (Dq) significantly, i.e., Dq is proportional to 1/R5 . So the bands peaking at 419 and 454 nm are assigned to Ce3+ ions occupying Sr1/2 sites due to weak crystal field (average distance Sr1/2–O≈2.749 Å) and the bands peaking at 509 and 562 nm are attributed to Ce3+ ions occupying Sr3 sites due to strong crystal field (Sr3–O≈2.635 Å). Meanwhile, the PL intensity is found to increase gradually with increasing Ce3+ ions concentration until it reaches concentration quenching when x = 0.03. In Fig. 2(b), the red emission band of 649 nm is attributed to the 4T1→6A1 transition of 3d5 level of Mn2+ ions and the corresponding PLE spectrum shows typical d-d transitions of Mn2+ ions. Based on the observed significant overlap between the PLE spectrum of Mn2+ and the PL spectrum of Ce3+ ions (inset in Fig. 2(b)), the effective resonance-type energy transfer is expected to take place from Ce3+ to Mn2+ ions. It can be seen that the Ce3+ or Mn2+ ions singly doped SPB emits blue or red light, so it is possible to obtain color-tunable white light emission by adjusting the concentration of Ce3+ and Mn2+ ions in SPB based on the energy transfer.
3.3 Photoluminescence properties of SPB: Ce3+, Mn2+ and luminescence dynamics
In SPB: 0.03Ce3+, xMn2+(0.005≤x≤0.05) (Fig. 3(a)), the PL spectra upon Ce3+ excitation at 354nm not only exhibit the Ce3+ blue emission at 441 nm but also the Mn2+ red emission at 649 nm. Meanwhile, as an increase of Mn2+ at fixed Ce3+ concentration, the intensity of the blue band decreases gradually while the red component increases (see Fig. 3(b)). These results further support the occurrence of energy transfer from Ce3+ to Mn2+. To further understand the process of energy transfer, the PL decay curves of Ce3+ in SPB: 0.03Ce3+, xMn2+ excited at 354 nm are measured and depicted in Fig. 3(c). The corresponding luminescent decay times can be fitted by a second-order exponential mode and the average decay time τ can be determined by the formula Fig. 3(c). According to Dexter’s formulation , the energy transfer rate is given byEq. (2), it becomes clear that the energy transfer rate P is in inverse proportion to the decay time τD. It can be seen that the decay lifetime of the Ce3+ decreases monotonically with an add up of the Mn2+ doping concentration, which also supports the energy transfer from the Ce3+ to Mn2+. Moreover, the energy transfer efficiency ηCe→Mn can be expressed by Fig. 3(d). It can be seen that the energy transfer efficiency ηT is found to increase gradually with the increase of Mn2+ content. The critical distance Rc for energy transfer from the Ce3+ to Mn2+ is calculated using the concentration quenching method. The critical distance was estimated according to the equation 31, 32].
3.4 Photoluminescence properties of SPB: Ce3+, Mn2+, Tb3+
To enhance the low color rendering property due to a green-region deficiency as seen in Fig. 3(a) and obtain warm white emission, we also prepared SPB: Tb3+ and SPB: 0.03Ce3+, 0.05Mn2+, yTb3+ (0.005≤y≤0.03). Figure 4(a) shows the PL spectrum of SPB: Ce3+ and the PLE spectrum of Tb3+. It can be seen that there nearly no spectral overlap between them, which indicates that there may not exist energy transfer between Ce3+ to Tb3+. The DRS of the SPB, SPB: 0.03Ce3+, 0.05Mn2+ and SPB: 0.03Ce3+, 0.05Mn2+, 0.01Tb3+ samples are shown in Fig. 4(b). It is clear that the spectrum of the SPB host exhibits a high reflection in the visible range. For SPB: Ce3+, Mn2+ sample, it displays strong absorption bands between 250 and 450 nm attributed to the 4f→5d electronic transitions of the Ce3+ and d-d transitions absorption of Mn2+. For Ce3+, Mn2+ and Tb3+ co-doped sample SPB, it also shows a strong absorption band in the wavelength range of 250-450 nm except for the enhanced absorption intensity by f-f transition absorption of Tb3+, indicating that the trebly doped phosphor may be suitable for UV LED excitation. The PL spectra of SPB: 0.03Ce3+, 0.05Mn2+, yTb3+ (0.005≤y≤0.03) are illustrated in Fig. 4(c) and the inset indicates the intensity change of different emission bands. Upon 354 nm excitation, the PL spectrum is found to consist of three major emission bands in the entire visible spectral region, one is the blue emission arose from the 5d-4f transitions of Ce3+, another is the green emission due to the 5D4 -7FJ (J = 3,4,5,6) transitions and the third part is a broad red-emitting band caused by Mn2+ d-level spin-forbidden transitions. Along with increasing Tb3+ concentration, the PL intensity of Tb3+ emission is found to increase, and that of Ce3+ emission is observed to decrease. This is because there is competition between absorption of Tb3+ and Ce3+. Both Ce3+ and Tb3+ have the same chance to absorb the photons. The intensity depends on the number of emission center. Therefore, the emission intensity of Tb3+ will increase and that of Ce3 + will decrease with the increase in the number of Tb3+. The emission intensity of Mn2+ will have the same tendency as that of Ce3+. As a result, the emission intensity of Mn2+ also decrease accordingly with Tb3+ concentration increasing, which is in good agreement with experimental result as shown in Fig. 4(c).
The main physicochemical properties of as-synthesized SPB serial phosphors are summarized in Fig. 4(d) and Table 1. The correlated color temperature (CCT) was calculated by using McCamy’s approximate formula  and Quantum efficiencies are measured by the integrated sphere method and can be calculated according to the equationEqs. (5) and (6), the quantum efficiencies of representative phosphors as well as the commercial phosphors YAG are calculated and shown in Table 1. We can see that the emission color of SPB: 0.03Ce3+, xMn2+, yTb3+ phosphors can be tuned from blue to green or red based on the energy transfer from Ce3+ to Mn2+ and Ce3+ to Tb3+, which demonstrates controllable emitting colors as a function of Mn2+ and Tb3+ contents. Hence, the genial warm white light can be realized with excellent CIE (0.35, 0.32), low CCT (4373K) and high CRI (Ra = 89) by appropriately tuning the ratio of the Mn2+ to Tb3+ under the radiation of UV light, which is better than that (CIE≈(0.291, 0.300), CCT≈8500K) of a LED based on YAG: Ce3+ with blue InGaN chip  and suitable for the application of warm white light LED. Depending on all above studies and experiments, it can be concluded that the full-color-emitting and color-tunable phosphor Sr10[(PO4)5.5(BO4)0.5](BO2): Ce3+, Mn2+, Tb3+ can serve as a great potential and flexible phosphor for using in warm white-light LED devices.
3.5 Thermal quenching properties of SPB: 0.03Ce3+, 0.05Mn2+, 0.02Tb3+
The thermal quenching property is one of the important technological parameters for phosphors used in solid-state lighting for it has considerable influence on the light output and color rendering index. The temperature dependent emission spectra for SPB: 0.03Ce3+, 0.05Mn2+, 0.02Tb3+ under excitation at 352 nm were measured and illustrated in Fig. 5. The inset displays a comparison of the thermal luminescence quenching of our phosphor with that of the commercial yellow phosphor YAG: Ce3+ (Mitsubishi Chemical Corporation, Japan). As can be seen in Fig. 5, the thermal stability of SPB: 0.03Ce3+, 0.05Mn2+, 0.02Tb3+ is inferior to that of YAG:Ce3+ when the temperature is above 50 °C, though the intensity of both of them dropped to about 63% of its initial value at a temperature of 200 °C. The observation can be rationalized by the fact that increasing temperature has increased the population of higher vibration levels, the density of phonons and the probability of non-radiative transfer (energy migration to defects). For the application in high power LEDs, thermal quenching property has to be enhanced.
In conclusion, a series of single-phase novel emission tunable Sr10[(PO4)5.5(BO4)0.5](BO2): Ce3+, Mn2+, Tb3+ phosphors have been synthesized and the luminescence properties as well as energy transfer from Ce3+ to Mn2+ and Ce3+ to Tb3+ are investigated for the first time. The energy transfer efficiency and critical distance Rc are also calculated. The emission color of SPB: 0.03Ce3+, xMn2+, yTb3+ phosphors can be tuned from blue to green or red based on the effective energy transfer. More importantly, the wavelength-tunable warm white light can be realized with a superior CIE (0.35, 0.32), high CRI (Ra = 89) and low CCT (CCT = 4373K). Our results indicate that the full-color-emitting and color-tunable phosphor Sr10[(PO4)5.5(BO4)0.5](BO2): Ce3+, Mn2+, Tb3+ can be a promising candidate for single-composition phosphor converted white-emitting LEDs.
This work is supported by National Science Foundation for Distinguished Young Scholars (No. 50925206) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120211130003).
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