Ce3+ and Eu2+/Tb3+/Mn2+ ions codoped Ca6BaP4O17 (CBPO) phosphors have been prepared via a high-temperature solid state reaction. The structural refinement indicates that the as-prepared phosphors crystallize in monoclinic phase (C2/m) and there are two Ca sites and one Ba site in host lattice. The doping ions are determined to occupy Ca sites and the emission of Ce3+ and Eu2+ ions at different Ca sites were identified and discussed. Since bright blue and yellow emissions were observed from Ce3+ and Eu2+ ions monodoped CBPO under n-UV excitation, respectively. They were codoped into the CBPO for designing energy transfer from Ce3+ to Eu2+ to improve the luminescence efficiency of Eu2+. In addition, Tb3+ ions were added into the CBPO:Ce3+ system for realizing highly efficient green emission. The energy transfer mechanisms from Ce3+ to Eu2+/Tb3+ ions were discussed. Interestingly, the incorporation of Mn2+ ions into the CBPO:Ce3+ system enhanced the blue emission of Ce3+ ions due to the modification of crystal lattice. Finally, the thermal stability of CBPO:Ce3+, Eu2+/Tb3+/Mn2+ phosphors were investigated systematically and corresponding mechanisms were proposed. Based on these results, the as-prepared CBPO:Ce3+, Eu2+/Tb3+/Mn2+ phosphors can act as potential blue, yellow, green, and emission-tunable phosphors for n-UV based white LEDs.
© 2016 Optical Society of America
Nowdays, phosphor-converted white light-emitting diodes (pc-WLEDs) solid-state lighting (SSL) sources are burgeoning because of their extraordinary luminous efficiency, brightness, low power consumption, high reliability, long operation life, and excellent environmental friendliness . The common approach to generate white light is the combination of a 460 nm blue InGaN chip and a yellow phosphor of cerium (III)-doped yttrium aluminum garnet (YAG:Ce3+), which is still in widespread use today. However, such a device has a poor color rendering index (Ra = 70~80) and a high correlated color temperature (CCT = 4,000~7,500 K) because of lacking a red component in spectra of phosphors [2–4]. A modified way for achieving white light with high Ra (> 85) and suitable CCT (2,000~6,000 K) involves mixing the emissions of red, green, and blue (RGB) phosphors with n-UV LED chips (380–420 nm) . Unfortunately, multi-phased phosphors converted systems frequently result in the high cost and low luminescence efficiency owing to the strong reabsorption among these employed phosphors . Moreover, the problems of sedimentation and uniformity distribution of phosphors in silicon resin cannot be avoided compared with single YAG:Ce3+ phosphor . To overcome these disadvantages, many efforts have been focused on enhancing the luminescence efficiency and Ra of phosphors by designing energy transfer between activator ions in a single-phase host pumped by n-UV light [7–11]. Highly efficient and emission-tunable phosphors not only can enhance the energy efficiencies of WLEDs but also present potential advantages in color rendering index, and correlated color temperature, which enlarge their applications in vehicle lights, architecture decoration, streetlight, clinical medical lighting etc [12–15].
Among various rare earth ions, Ce3+ and Eu2+ ions in solids commonly can show efficient broad band luminescence due to the 4f–5d parity allowed electric dipole transition, which emit from blue light to red light depending on the coordination environment [16,17]. Besides using as activators, the Ce3+ and Eu2+ ions can act as efficient sensitizers, which not only helps other activator ions to emit efficiently but also forms color-tunable emission in single host [5,6]. For example, many researchers reported energy transfers from Ce3+ to Eu2+/Tb3+/Mn2+ in some phosphates, silicates, aluminates, borates and so on [5,6,18–20]. In these systems, with the introduction of Ce3+ ions, the doping ions simultaneously occupy one or more cation sites, and then the energy from the 5d1 level of Ce3+ ions is partly transferred to the 4f65d1 level of Eu2+, or the 5D3,4 level of Tb3+ or the 4G level of Mn2+, which helps Eu2+, Tb3+, and Mn2+ ions to efficiently emit and simultaneously tune the emission colors in the whole visible light range. Therefore, to realize an efficient energy transfer, the choice of hosts is a critical aspect. It is well known that phosphate phosphors have been extensively explored in view of broad band gaps, moderate phonon energies, high thermal and chemical stability, and low sintering temperatures. As a member of the phosphate family, the crystal structure of the Ca6BaP4O17 has been determined by Komuro et al. recently, and efficient blue emitting CBPO:Ce3+ and yellow emitting CBPO:Eu2+ under the excitation of n-UV or blue light were firstly reported [21,22]. Then Guo et al. realized a yellow super long-lasting emission by the codoping of Eu2+ and Ho3+ ions in the CBPO host . However, there is no report to focus on the luminescent properties of Ce3+ and Eu2+/Mn2+ co-doped Ca6BaP4O17 phosphors. Although Ce3+/Tb3+ co-doped Ca6BaP4O17 has been reported by Chen et al., the energy transfer from Ce3+ ions to Tb3+ ions appeared not efficient in view of a final blue-green emission . Herein, we synthesized a series of Ce3+, Eu2+/Tb3+/Mn2+-coactivated CBPO phosphors by a solid state reaction process. The Rietveld refinement method was used to determine the sites of doping ions in the CBPO host lattice. Energy transfer properties from Ce3+ to Eu2+/Tb3+/Mn2+ in the CBPO were systematically investigated. More importantly, the thermal stability of the studied samples were explored in details.
Materials. CaCO3, BaCO3 and CaHPO4 (≥99.99%) were purchased from Sigma-Aldrich Co. LLC. CeO2, Eu2O3 and Tb4O7 (≥99.999%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry, China. MnCO3 (≥99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai). All chemicals were used directly without further purification.
Synthesis. A series of polycrystalline Ca6(0.98-x/y/z)BaP4O17:2% Ce3+, x% Eu2+/y%Tb3+/z% Mn2+ powders were prepared by a solid state reaction process. The Ce3+/Eu2+/Tb3+/Mn2+ ions are nominally designed to substitute of Ca2+ ions in the CBPO host. The doping concentrations of Eu2+/Tb3+/Mn2+ were chosen as 0.1–15% of Ca2+ ions in the CBPO. Typically, stoichiometric amounts of CaCO3, BaCO3, CaH2PO4, CeO2, Eu2O3, Tb4O7, and MnCO3 were thoroughly mixed and ground in an agate mortar for 1 h. Then, the powder mixtures were collected in aluminum oxide crucibles and sintered in a horizontal tube furnace at 1350°C for 2 h with a reducing atmosphere of H2 (8%) and N2 (92%) atmosphere. After the furnace slowly cooled to room temperature, the sintered products were ground again, yielding the final phosphor powders.
Characterization. The X-ray diffraction (XRD) patterns were performed on a D8 Focus diffractometer (Bruker, Kalsruhe, Germany) at a scanning rate of 1° min−1 in the 2θ range from 5° to 120° with Ni-filtered Cu Kα radiation (λ = 0.15406 nm). XRD Rietveld profile refinements of the structural models and texture analysis were performed with the use of General Structure Analysis System (GSAS) software. The morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi). The photoluminescence measurements were recorded with a Fluoromax-4P spectrophotometer (Horiba Jobin Yvon, New Jersey, U.S.A.) equipped with a 450 W xenon lamp as the excitation source. Both excitation and emission spectra were recorded at 1.0 nm interval with the width of the monochromator slits adjusted as 0.50 nm. The thermal stability of the luminescence was measured by Fluoromax-4P spectrometer connected a heating equipment (TAP-02), and the samples were heated from 25°C to 250°C with a 25°C interval. After being remained at the designed temperature for two minutes, their emission spectra were recorded. Before recording the emission spectra, each sample remained two minutes at the designed temperature. The photoluminescence quantum yield (QY) was measured by absolute PL quantum yield measurement system C9920-02 (Hamamatsu photonics K.K., Japan). The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO).
3. Results and discussion
3.1 Phase identification and structure refinement
The phase purities were firstly confirmed by powder XRD. Figure 1(a) shows the representative XRD patterns of Ce3+/Eu2+/Tb3+/Mn2+ monodoped and codoped samples such as CBPO:2%Ce3+, CBPO:2%Eu2+, CBPO:2%Tb3+, CBPO:2%Ce3+, 2%Eu2+, CBPO:2%Ce3+, 8%Tb3+, CBPO:2%Ce3+, 5%Mn2+. Obviously, the diffraction peaks of all these samples were well assigned to the monoclinic Ca6BaP4O17 phase [21,22]. No impurity phase is observed, indicating the formation of single phase and the complete incorporation of doping ions into the lattice without changing the crystal structure of the host. The XRD patterns of other CBPO:Ce3+/Eu2+/Tb3+ samples with different doping concentrations also confirmed the formation of the pure CBPO phase, as shown in Fig. 1(b). The SEM image of the representative CBPO:2%Ce3+, 2%Eu2+ sample is shown in the inset of Fig. 1(a). It is found that the obtained sample consisted of aggregated bulk materials with smooth surface and particle sizes from 2 μm to 10 μm.
The previous report has depicted that the Ce3+ and Eu2+ ions occupy Ca sites due to the similar ion sizes [21,22,25]. To further investigate the crystal structure of codoped samples and the site occupancy of the codoping ions in the CBPO host, the XRD pattern of the representative CBPO:2%Ce3+, 2%Eu2+ and CBPO:2%Ce3+, 8%Tb3+ samples were used to perform the GSAS refinement. Figures 2(a) and 2(b) shows the experimental, calculated, different XRD profiles, and Bragg positions for the Rietveld refinement of CBPO:2%Ce3+, 2%Eu2+ and CBPO:2%Ce3+, 8%Tb3+ samples, respectively. According to the refinement results, the refined two samples have monoclinic structure in space group C2/m with refined lattice parameters a = 12.3027(1) Å, b = 7.1048(1) Å, c = 11.7143(2) Å, α = γ = 90°, β = 134.4(1)o, V = 731.21(2) Å3 and Z = 2 for CBPO:2%Ce3+, 2%Eu2+, and a = 12.2925(2) Å, b = 7.1046(1) Å, c = 11.7224(2) Å, V = 730.14(2) Å3, α = γ = 90°, β = 134.4(0)o and Z = 2 for CBPO:2%Ce3+, 8%Tb3+. All atom positions, fraction factors, and thermal vibration parameters were refined by convergence and satisfied well the reflection conditions, Rwp = 6.22%, Rp = 4.32%, χ2 = 4.386 for CBPO:2%Ce3+, 2%Eu2+, and Rwp = 5.97%, Rp = 3.91%, χ2 = 4.671 for CBPO:2%Ce3+, 8%Tb3+. These results further verifies that the formation of single-phase and the crystal structure of CBPO host are unchanged with the introduction of codoping ions. A spatial view of the Ca6BaP4O17 unit cell is shown in Fig. 2(d). There are two Ca sites and one Ba site in the CBPO. It is different to the previous report that the Ca1 and Ca2 atoms in our systems are all coordinated with seven oxygen atoms to form decahedrons [21,22]. While the Ba atoms and twelve oxygen atoms form icosahedrons. However, the two Ca sites have different Ca-O bond distances, for example, the average Ca1–O and Ca2–O interatomic distances in the CBPO:2%Ce3+, 2%Eu2+ are 2.46−3.25 Å (average 2.59 Å) and 2.09−3.10 Å (average 2.47 Å), respectively. Therefore, two Ca2+ sites presents different crystal field environments. Along b axis, any two decahedrons are connected each other by sharing edges. The neighboring decahedron and icosahedron are connected through tetrahedral PO4 groups and sharing edges. By comparing the interatomic distances of Ca1-O, Ca2-O, and Ba-O in CBPO: 2%Ce3+,CBPO: 2%Ce3+, 2%Eu2+ (2.59 Å, 2.47 Å, 3.03 Å) and CBPO:2%Ce3+, 8%Tb3+ (2.51 Å, 2.50 Å, 3.05 Å), it is found that the doping of Eu2+ and Tb3+ ions just influenced the Ca-O bond length, revealing that the Eu2+, Tb3+ and Mn2+ ions should replace the Ca2+ ions in the CBPO host. This result is consistent with the previous report, which also should be attributed to similar ion sizes [21,22]. On the basis of the above structure analysis, the coordination-environmental diversity of the Ca sites in the CBPO host is benefit to the multiple 5d−4f transition emission of activators and the design of energy transfer . All the doping ions are expected to randomly occupy the Ca1 and Ca2 sites.
3.2 Photoluminescence properties and energy transfer
Figures 3(a) and 3(b) show the photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra of CBPO:2%Ce3+ (blue line, EM 471 nm; EX 378 nm) and CBPO:2%Eu2+ (orange line, EM 536 nm; EX 420 nm), respectively. EX and EM means the monitoring wavelengths for obtaining the emission spectra and excitation spectra, respectively. The same notations are applicable for the following section. Under 378 nm UV, CBPO:2%Ce3+ shows a strong blue emission due to the 5d1→4f1 transition of Ce3+ consisting of a broad band from 400 nm to 670 nm with a maximum at 471 nm. The corresponding excitation spectrum includes two broad bands: the strong one is from 330 to 450 nm (center, 378 nm) and the weak one is from 250 to 330 nm (center, 298 nm) with the maximum excitation at 378 nm. Clearly, the emission spectrum is an asymmetric emission band, implying possible spectral overlaps coming from different luminescence centers . In the CBPO host, Ca2+ ions possess two different coordination environments (Ca1 and Ca2) although all have seven-coordinated O atoms, which could be randomly occupied by Ce3+ ions. In view of a spin splitting of the ground states of Ce3+ ions to 2F7/2 and 2F5/2 states, the emission spectra of Ce3+ should be composed of two emission bands when the electron transitions happen from the lowest 5d excited state to the two ground states . Figure 3(e) confirms that the PL spectrum of CBPO:2%Ce3+ can be well-fitted with a sum of four Gaussian bands Peak 1-Peak 4, which peaks at 22,728 cm−1 (440 nm), 21,258 cm−1 (471 nm), 20,000 cm−1 (500 nm), and 18,182 cm−1 (550 nm), respectively. The energy gap between the bands Peak1 and Peak 2 is 1,502 cm−1, and that for Peak 3 and Peak 4 is1,778 cm−1, which are close to the theoretical value of 2,000 cm−1 .
Therefore, Peak 1-2 are assigned to the emission of Ce3+ ions at Ca1 sites, while Peak 3-4 originates from the emission of Ce3+ ions at Ca2 sites. Generally, the crystal field strength (Dq) can be determined by the following Eq [26,27]:Eq. (1) can be used as an approximation for describing crystal field splitting trends with bond distance . Obviously, the Dq is inverse proportion to the bond length (R). Therefore, the shorter the bond length and the stronger the crystal field splitting, and then the longer the emission wavelength will appear [26,27]. According to the refinement result of CBPO:2%Ce3+, the average RCa1−O and RCa2−O are 2.58 Å and 2.47 Å, respectively, indicating the crystal field strength is Dq(Ca1 sites) < Dq(Ca2 sites). Therefore, the bands peaking at 440 nm and 470 nm should be assigned to the weak crystal field of Ce3+ in Ca1 sites, while the bands peaking at 500 nm and 550 nm are indexed to Ce3+ in Ca2 sites. For CBPO:2%Eu2+, it presents an absorption band from 250 nm to 520 nm with the maximum at 420 nm . When exciting with 420 nm UV, it emits bright yellow light with CIE color coordinate (0.386, 0.563). Moreover, the emission band is also an asymmetric band ranging from 400 nm to 700 nm centered at 535 nm because two available Ca sites. By Gaussian fitting, the emission spectrum of CBPO:2%Eu2+ were well decomposed to two emission bands with the peaks at 534 nm (Ca1 sites) and 592 nm (Ca2 sites), respectively. Note that there was an obvious blue shift of emission peaks from 553 nm to 535 nm and a narrower FWHM (105 nm) with respect to the previous report. This is attributed to the longer bond length of Ca1-O (2.57 Å) and Ca2-O (2.49 Å) in our systems than that of Ca1-O (2.51 Å) and Ca2-O (2.42 Å) in the previous report, which results in the decrease of crystal field strength around Eu2+ ions . The comparison of the PL spectra for CBPO:2%Ce3+ and the PLE spectrum for CBPO:2%Eu2+ in Fig. 3(a) reveals a large spectra overlap from 400 nm to 520 nm. Accordingly, a possible resonance-typed energy transfer from Ce3+ to Eu2+ is expected. This type of energy transfer is common and has been reported in many phosphor systems . In order to confirm the energy transfer from Ce3+ to Eu2+ in the CBPO host, the PL spectra of CBPO:2%Ce3+, x%Eu2+ samples with the increase of Eu2+-doping concentrations (x) from 0 to 4 are shown in Fig. 3(b). Although the concentration of Ce3+ was fixed, the emission intensity of Ce3+ gradually decreased with increasing Eu2+ concentration, while the emission of Eu2+ increase with x. It reaches a maximum value at x = 0.1, then decreases with further increasing x due to the concentration quenching effect. In general, the concentration quenching of luminescence is due to the energy migration among the Eu2+ ions at the high concentrations. In the energy migration process the excitation energy lose at a killer or quenching site, resulting in the decrease of emission intensity. The normalized PL spectra of CBPO:2%Ce3+, x%Eu2+ in Fig. 3(c) more intuitively demonstrate a gradual transition from the emission of Ce3+ to the emission of Eu2+ with x. Of course, the CBPO:2%Eu2+ itself can be excited 378 nm-wavelength UV. However, the emission intensity of CBPO:2%Ce3+, 2%Eu2+ is 1.5 times than CBPO:2%Eu2+ as shown in the inset of Fig. 3(b). Therefore, the results can basically validate the existence of the energy transfer from Ce3+ to Eu2+. To further explore the energy transfer, we investigated the decay lifetimes of Ce3+ emission (Fig. 3(d)). Since there are two Ce3+ luminescent centers in the CBPO host, the decay curves are successfully fitted using the following two-exponential Eq :20]:Fig. 3(d)), which are 61.2, 40.4, 23.7, 19.6, and 13.2 ns, respectively. The result is a powerful evidence for the confirmation of energy transfer from Ce3+ to Eu2+ [20, 28]. The energy transfer efficiency from Ce3+ to Eu2+ can be determined by the following Eq :Figure 4(a) shows the results of energy transfer efficiency () from Ce3+ to Eu2+ calculated by Eq. (4) under 378 nm UV excitation. It is clearly seen that the energy transfer efficiency increases with increasing Eu2+ concentration. However, the increscent rate of the gradually decreases with Eu2+ doping concentration, indicating an energy transferred saturation behaviour. The maximum energy transfer efficiency can reach 82%, demonstrating that the energy transfer from Ce3+ to Eu2+ in the CBPO host is efficient.
Although the energy transfer is efficient, a luminescent decrease happens when the doping concentration of sensitizer and activator ions reaches a critical value. The corresponding critical distance RC between Ce3+ ions and Eu2+ ions can be estimated using the Eq. given as follow :29]:20,29]:Figs. 4(a)–4(c).The linear relationship reaches the optimal one for by comparing the fitting factors of R2 values. Therefore, the energy transfer mechanism from Ce3+ to Eu2+ ions in the CBPO host is determined to be the electric dipole−dipole interaction.
For dipole-dipole interaction, the energy transfer probability (in s−1) from a sensitizer to an acceptor is given by the following formula [20,24]:20,29]:20]. Using the Eq. (9), the critical distance RC was estimated to be 23.6 Å, which is basically consistent with the previous result calculated by the concentration quenching method (19.1 Å), such as 25 Å reported in BaLiF3:Ce3+, Eu2+ and 26.67 Å in Ca8La2(PO4)6O2:Ce3+/Eu2+ [20,28,30]. Except for the energy transfer between Ce3+ and Eu2+ ions, the other energy transfer properties in the CBPO host such as Ce3+→Tb3+ and Ce3+→Mn2+ are also discussed. Figure 5(a) shows the PLE and PL spectra of CBPO:2%Ce3+ (blue line) and CBPO:2%Tb3+ (green line). The excitation spectrum of CBPO:2%Tb3+ shows a broad band from 230 nm to 275 nm centered at 254 nm (the strongest excitation) and some narrow line transitions from 275 nm to 450 nm. The former is related to the 4f8–4f75d transition of Tb3+, and the latter is due to its intra-(4f) transitions. Under 254 nm UV, the as-prepared CBPO:2%Tb3+ gives green emission, and the obtained emission spectrum consists of the f-f transition lines within the Tb3+ 4f8 electron configuration, i.e., 5D3 → 7F6 (380 nm), 5D3 → 7F5 (420 nm), 5D3 → 7F4 (438 nm), 5D4 → 7F6 (491 nm), 5D4 → 7F5 (542 nm, the strongest one), 5D4 → 7F4 (588 nm), and 5D4 → 7F3 (625 nm), respectively. However, the luminescence efficiency of CBPO:Tb3+ in n-UV region is low due to weak 4f-4f transition lines at n-UV region, it need to be enhanced or optimized for a potential application in n-UV based WLEDs. In view of a spectral overlap between the PL spectra of CBPO:2%Ce3+ and the 4f-4f excitation transitions of CBPO:2%Tb3+, there should be a resonance-type energy transfer from Ce3+ to Tb3+. The appearance of the excitation spectrum of Ce3+ in CBPO:2%Ce3+, 8%Tb3+ monitoring with the characteristic emission of Tb3+ at 542 nm also demonstrates the existence of Ce3+→Tb3+ energy transfer, as shown in Fig. 5(b). This is consistent with the previous result in , Chen et al. When fixingthe Ce3+ content, the emission intensity of Ce3+ continuously decreases with increasing Tb3+ concentration, while the emission intensity of Tb3+ gradually increases, as shown in Fig. 5(c). Therefore, efficient energy transfers could occur when Ce3+, Tb3+ are codoped into the CBPO host. Moreover, the relative emission intensity of Tb3+ to Ce3+ is higher than that in , Chen et al. According to the Eq. (2), the average decay lifetimes (Fig. 5(d)) of the CBPO:2%Ce3+, y%Tb3+ (x = 0, 0.5, 1, 2, 3, 4, 8, 15) samples are calculated to be 61.2 ns, 54.1 ns, 47.9 ns, 41.6 ns, 40.3 ns, 39.2 ns, 32.8 ns, and 27.8 ns, respectively. Obviously, it gradually decrease with increasing Tb3+ concentration, which further validate the energy transfer from Ce3+ to Tb3+ in the CBPO. The and RC for the Ce3+→Tb3+ energy transfer in the CBPO host are respectively calculated using the Eqs. (3) and (4) to be 55% (Fig. 4(a)) and 12.1 Å, respectively. Moreover, the electric multipolar interaction mechanism for the Ce3+→Tb3+ energy transfer is also dipole−dipole interaction, as disclosed in Figs. 4(e)-4(g).
When codoping Ce3+ and Mn2+ ions into the CBPO, the PLE and PL spectra of CBPO:2%Ce3+, z%Mn2+ (z = 0.5, 1, 2, 3, 4) have the same profiles to that of CBPO:2%Ce3+, which emits highly efficient blue light, as shown in Fig. 6. The basically same CIE color coordinates of CBPO:2%Ce3+ (0.18, 0.27) and CBPO:2%Ce3+, 3%Mn2+ (0.19, 0.27) also confirms this result. Therefore, the Ca sites of the CBPO is adverse to the luminescence of Mn2+ and the energy transfer of Ce3+→Mn2+. Interestingly, the PL intensity increased firstly with the increase of Mn2+ content, reaching a maximum emission at z = 2%, as shown by the inset of Fig. 6. Then the emission intensity gradually decreased with z values due to the concentration quenching effect. The improvement for the PL intensity of the CBPO:2%Ce3+, z%Mn2+ system is possibly attributed to the modification of CBPO crystal lattice. Because the Ce3+ ion (1.07 Å, CN = 7) has a larger ion radius than Ca2+ ion (1.06 Å,CN = 7), the doping of Ce3+ ions generates a slight distortion of CBPO crystal lattice, which increases the nonradiative transition. While the Mn2+ ion (0.90 Å, CN = 7) has a smaller size than Ca2+ ion, the incorporation of Mn2+ ions possibly relieves the lattice tension and promotes the crystalline, and thus enhance the emission intensity, as the shrink in Ca-O bond lengths RCa1-O = 2.55Å, RCa2-O = 2.47Å in CBPO:2%Ce3+, 5%Mn2+ than RCa1-O = 2.58Å, RCa2-O = 2.46Å in CBPO:2%Ce3+.
In order to improve the performance of WLED devices, it is necessary to develop phosphors with high quantum yields (QYs). Table 1 summarizes the QYs of CBPO:Ce3+, Eu2+/Tb3+/Mn2+ excited at 378 nm UV. It is evidently found that the QYs of codoped CBPO:Ce3+, Eu2+/Tb3+ samples are higher than that of monodoped CBPO:Eu2+/Tb3+ samples. Therefore, these as-prepared phosphors can be more efficiently excited by n-UV light through designing efficient energy transfers (Ce3+→Eu2+/Tb3+) to generate yellow emission of Eu2+ and green emission of Tb3+. The similar improvements appear in the CBPO:Ce3+, Mn2+ series although the absence of Ce3+→Mn2+ energy transfer, reavealing another possible ways to enhance the luminescence efficiency by modifing the crystal lattice of host. Except for improving luminescence efficiency, efficient energy transfer from sensitizer to activator is a feasible route to tune emission color [5,28,29]. The CIE color coordinates of CBPO:Ce3+, Eu2+/Tb3+/Mn2+ (monitoring at 378 nm) are also listed in Table 1. In our case, the Ce3+, Eu2+, and Tb3+ ions in the CBPO can give bright blue emission and yellow emission as well as green emission, respectively, as shown in Table 1. Furthermore, the efficient energy transfers from Ce3+ to Eu2+/Tb3+ in the CBPO host have been validated. Thus, it is reasonable to predict a color-tunable emission from blue light to yellow light in Ce3+ and Eu2+-coactivated CBPO systems and a color-tunable emission from blue light to green light in Ce3+ and Tb3+-coactivated CBPO systems. Our experiments have confirmed the above situations by changing the relative doping contents of Ce3+ and Eu2+/Tb3+ ions. Figure 7 depicts the color coordinates for CBPO:2%Ce3+, x%Eu2+ (black dots) and CBPO:2%Ce3+, y%Tb3+ (red dots). It is seen that the color coordinates of the CBPO:2%Ce3+, x%Eu2+ phosphors shift gradually from blue region (0.176, 0.271) to bluish-while region (0.242, 0.364) and eventually to yellow region (0.410, 0.550) with an increase of the doping content of Eu2+ (Table 1). The corresponding luminescence photos confirms the blue-to-yellow tunable emission colors. Similarly, the blue-to-green tunable lights were obtained by continuously changing the Tb3+ concentration (Fig. 7) in CBPO:2%Ce3+, y%Tb3+ (y = 0-15). The corresponding color coordinates were shifted from blue light for y = 0 to green light (0.283, 0.411) for y = 15 upon exciting with 378 nm UV (Table 1). These results demonstrate that the as-prepared CBPO:Ce3+, Eu2+/Tb3+/Mn2+ phosphors by designing efficient energy transfers can act as the potential emission-tunable phosphors for applications in n-UV (360−400 nm) based WLED devices.
3.3 Thermal stability of luminescence
The excellent luminescence stability of phosphors with temperatures is an essential requirement in practical WLED lighting applications, which influences the luminescent efficiency and lighting quality of the lighting devices [31–34]. In order to improve the thermal stability of phosphors, a comprehensive understanding of the thermal quenching of phosphors is necessary. Therefore, the temperature-dependent luminescence properties of the CBPO:Ce3+, Eu2+/Tb3+/Mn2+ samples in air from 25°C to 250°C were systematically investigated. Figures 8(a) and 8(b) show the PL spectra of CBPO:2%Ce3+ and CBPO:2%Eu2+ samples at different temperatures, respectively. With increasing temperature from 25°C to 250°C, the emission peaks of CBPO:2%Ce3+ are unchanged, while the PL spectra of CBPO:2%Eu2+ appear a blue shift. Their emission intensity all clearly decreases because of nonradiative transitions from the excited state to the ground state, which can retain 83% for CBPO:2%Ce3+ and 44% for CBPO:2%Eu2+ of the original value (RT) at 150°C (Fig. 8(e)) . In general, the temperature-dependent luminescence properties of CBPO:2%Ce3+ are basically consistent with the previous results reported by Komuro et al. . However, the thermal quenching of the CBPO:2%Eu2+ sample is much lower than Komuro’s result, which should be attributed to the smaller stokes shifts (SS) [21,22,35].
The corresponding mechanism could be explained by the configuration coordinate diagram in Fig. 8(c) [36,37]. In the simple schematic model, the ground and excited states of Eu2+ can be described with parabolas, and overhead parabolas present two different excited states. As suggested, the electrons at excited states can be thermally excited with thermal energy ΔE1, 2 to reach the crossing point C1, 2 of the two parabolas, respectively. These electrons then return to ground states, and the transition energy is released through heat dissipation rather than radiation emission, accompanied by the creation of a large number of phonons. Consequently, thermal quenching of luminescence occurs. Thermal energy increases (ΔE1<ΔE2) with the decrease in Stokes shift (SS2<SS1), suggesting that the probability of nonradiative transition can be weakened by thermal activation. Thus, a blue-shifted yellow emission in our CBPO:2%Eu2+ (the maximum at 535 nm) relative to the 553 nm in , Komuro et al. reduced itself thermal quenching, that is, smaller Stokes shift results in lower thermal luminescence quenching [36,37]. This is a possible reason for the better thermal stability with temperatures of CBPO:2%Ce3+ than CBPO:2%Eu2+. Different to the stable emission peaks at 471 nm in CBPO:2%Ce3+, a slight blue shift was observed with temperatures in CBPO:2%Eu2+,which originated from the simultaneous blue shift emission of Eu2+ ions at Ca1 and Ca2 sites. According to the previous report, the crystal lattice expansion around Eu2+ ions at heating conditions should be responsible for the blue shift emission [38,39]. Since a very close size of Ce3+ (1.07 Å) and Ca2+ (1.06 Å) ions at seven-coordination environments, the substitution of Ce3+ ions for Ca2+ ions did not change the environment of crystal lattice around Ce3+ ions. Thus, the CBPO:2%Ce3+ presented a fixed emission position at 471 nm and a better thermal stability than CBPO:2%Eu2+. The improvement of thermal stability of CBPO:2%Ce3+, 3%Mn2+ relative to CBPO:2%Ce3+ might be assigned to the increasing rigidity of crystal lattice by doping a smaller Mn2+ (0.96 Å) ions than the Ce3+ and Ca2+ ions (Fig. 8(e)), which is consistent with the PL results [38, 39]. Since Ce3+ ions have a lower thermal quenching than Eu2+ ions in the CBPO, the thermal stability of Ce3+ and Eu2+-codoped CBPO samples were expected to be between CBPO:Ce3+ and CBPO:Eu2+. Our experiment result confirms this determination, as shown in Figs. 8(d) and 8(e). Compared to CBPO:2%Eu2+, the luminescence quenching of CBPO:2%Ce3+, 2%Eu2+ decreased 13% at 150°C (Fig. 8(e)). Therefore, the thermal stability of CBPO:Ce3+, Eu2+ could be improved by designing efficient energy transfer, which increasing their practical application in n-UV WLED devices.
It is known that the 4f−4f transitions of rare earth ions are strongly shielded by the outside 5s and 5p electrons, its excitation and emission spectra are not basically affected by the crystal environments, and thus has a low thermal quenching with temperature . Therefore, the Tb3+˗doped CBPO sample shows an high thermal stability, which only quenches about 6% of the original intensity at 150°C, as shown in Fig. 9(a). When co-doped Ce3+ and Tb3+ ions into the CBPO host, their temperature-dependent luminescence properties simultaneously presented the characteristic thermal quenching effect of Ce3+ and Tb3+-monodoped sample (Figs. 9(b)-9(d)). At a 2%Tb3+ doping, the energy transfer efficiency from Ce3+ to Tb3+ was low in the CBPO, and the luminescence originates from Ce3+ and Tb3+ ions were basically equivalent. Therefore, the thermal quenching of CBPO:2%Ce3+, 2%Tb3+ lied between CBPO:2%Ce3+ and CBPO:2%Tb3+ (Fig. 9(e)). With adding Tb3+ content to 4%, the ratio of emission of Tb3+/Ce3+ increased, the thermal stability of CBPO:2%Ce3+, 4%Tb3+ were enhanced to be close to that of CBPO:2%Tb3+. Interestingly, the thermal stability of CBPO:2%Ce3+, 10%Tb3+ even exceeded CBPO:2%Tb3+, as shown in Fig. 9(e). A possible mechanism as follow: In CBPO:2%Ce3+, 10%Tb3+, the emission of Tb3+ dominates due to the Ce3+→Tb3+ energy transfer, and thus the effect of Ce3+-thermal quenching on the thermal stability of phosphors are weak. Although the energy transfer efficiency is efficient in CBPO:2%Ce3+, 10%Tb3+, it is not complete, and thus there is residual emission of Ce3+ ions. The increasing temperatures induce more excitation energy of Ce3+ ions to transfer to the 5D4 energy level of Tb3+ ions, decreasing the emission intensity of Ce3+ ions gradually, as shown by the inset in Fig. 9(c). Although Tb3+ ions itself have a thermal quenching, the thermal quenching rate in the beginning is lower than the energy transfer rate of Ce3+ ions in the CBPO, resulting in the emission of Tb3+ ions exceed its original intensity at RT. Until to 250°C, the thermal quenching of Tb3+ prevail the obtained excitation energy from Ce3+, leading a decreased emission. In general, the luminescence efficiencies and thermal stabilities of CBPO:Ce3+, Eu2+/Tb3+/Mn2+ can be obviously improved by designing efficient energy transfers or modifing crystal lattice, which are promising blue/yellow/green phosphors for n-UV WLEDs lighting.
A series of Ce3+ and Eu2+/Tb3+/Mn2+ codoped CBPO phosphors were prepared by solid state reaction process. The structure refinement indicates that the Ce3+ and Eu2+/Tb3+/Mn2+ ions entered into two Ca sites. Under 378 nm UV, the CBPO:Ce3+ and CBPO:Eu2+ present blue emission (471 nm) and yellow emission (535 nm), respectively. When codoping Eu2+/Tb3+ ions into CBPO:Ce3+, efficient energy transfers from Ce3+ to Eu2+/Tb3+ were realized, and thus the luminescence efficiencies of Eu2+ and Tb3+ in the CBPO were obviously improved. Moreover, blue-to-yellow tunable emission for CBPO:Ce3+, Eu2+ and blue-to-green tunable emission for CBPO:Ce3+, Tb3+ were obtained. The corresponding energy transfer mechanisms from Ce3+ ions to Eu2+/Tb3+ ions in the CBPO host were determined to be all dipole−dipole interactions. By incorporating the Mn2+ ions to modify the crystal lattice of phosphors, the blue emission of Ce3+ was also improved. Finally, the thermal stability of CBPO:Ce3+, Eu2+/Tb3+/Mn2+ phosphors were systematically investigated. In view of a lower thermal quenching of Ce3+ than Eu2+ in the CBPO, the thermal stability of yellow-emitting CBPO:Ce3+, Eu2+ was better than the CBPO:Eu2+. Interestingly, the green-emitting CBPO:Ce3+, Tb3+ show high thermal stability due to the higher energy transfer rate than the thermal quenching rate at a high Tb3+ level with temperatures. In addition, the incorporation of Mn2+ ions in CBPO:Ce3+ systems possibly relieved the distortion of crystal lattice resulted from the mismatch of ion radius between Ca2+ ions and Ce3+ ions, which also improve their thermal stability. Based on the experimental results, the as-prepared single-phased multicolor-emitting CBPO:Ce3+, Eu2+/Tb3+/Mn2+ phosphors with improved luminescence efficiency and thermal stabilities show promising applications in n-UV based WLEDs.
This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 21301162, 21171152), and the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2917-I-564-060).
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