A series of Sr4La(PO4)3O: Ce3+, Tb3+, Mn2+ phosphors have been synthesized with a high temperature solid-state method. The luminescence properties, thermal stability, and energy transfer from Ce3+ to Tb3+ and Mn2+ in Sr4La(PO4)3O were investigated in detail. Through energy transfer, the weak green emission from Tb3+ and red emission from Mn2+ can be significantly enhanced by the introduction of the sensitizer Ce3+ ions. The emission color can be tuned by changing the ratio of Ce3+/Tb3+ and Ce3+/Mn2+ ions. White light was obtained with chromaticity coordinates of (0.3326, 0.3298) in the Sr4La(PO4)3O: 0.12Ce3+, 0.3Mn2+ sample, indicating that the Sr4La(PO4)3O: Ce3+, Tb3+, Mn2+ phosphors may have potential application in white LEDs.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Due to the benefits of long lifetime, low energy consumption, small size and eco-friendliness, the white light-emitting diodes (wLEDs) are widely used and have gained enormous commercial interest as solid state lighting sources [1,2]. At present, the most used way to generate white light is combining a blue InGaN chip with Y3Al5O12:Ce3+ (YAG: Ce) yellow phosphor . However, this kind of white emission suffers a poor color rendering index (CRI≈70-80) and a high correlated color temperature (CCT≈7500K) because of the lack of red-light component [4,5]. In recent years, many researchers have devoted to the study of red phosphors which can be excited by blue light. Such phosphors are Na5Ln(MoO4)4: Eu3+ (Ln = La, Gd, Y) , CaS: Eu2+ , CaAlSiN3:Eu2+ , and K2SiF6:Mn4+ [9,10], etc. With these phosphors, the phosphor converted wLEDs can present low CCT and high CRI values. Unfortunately, poor blue emission efficiency due to the strong re-absorption among green and red phosphors and high manufacturing cost are other challenging problems. Therefore, numerous studies have been focused on developing efficient, durable and single-phase white light emitting phosphors with the red, green and blue (RGB) components based on the mechanism of the energy transfer (ET) from sensitizes to activators [11,12].
Among various kinds of wLED phosphors, co-doping Ce3+, Tb3+ or Mn2+ ions is the common strategy for realizing white light emission through ET in the single-phase phosphors [11–13]. Ce3+ has a 4f15d0 ground state and a 4f05d1 excited state and therefore shows typical parity allowed 5d-4f electronic transitions. The distribution of Ce3+ energy level is deeply dependent on the host lattice, so the emission due to 5d-4f transition can appear in a large wavelength range. Tb3+ and Mn2+ ions have been frequently used as green and red emission components, respectively. However, the excitation bands of Tb3+ and Mn2+ ions from UV to visible region are very weak due to the forbidden 4f-4f transition of Tb3+ and 4T1-6A1 transition of Mn2+ [14,15]. In order to enhance the absorption of Tb3+ and Mn2+ in the UV region, a frequently used method is to introduce an efficient sensitizer such as Ce3+, and then the strongly absorbed excitation energy can be transferred from the 5d level of Ce3+ to the 5D3,4 level of Tb3+ or the 4G level of Mn2+ [11,16,17]. In this way, the emission-tunable single-phase phosphors can be obtained, which has advantages such as lower manufacturing cost, good color reproducibility and high luminescence efficiency [18,19].
So far, the apatites have proven to be excellent host materials for phosphors due to their good physical and chemical stability [20–24]. Sr4La(PO4)3O (SLPO) is isomorphic to the apatite compound and consists of two cationic sites, a nine-fold coordination 4f site with C3 point symmetry and a seven-fold coordination 6h site with Cs point symmetry . In our previous report, the SLPO: Eu3+/Tb3+/Ce3+ phosphors have been studied . In this work, we introduced Tb3+ or Mn2+ into SLPO: Ce3+. Through ET, a green emission peak of Tb3+ at 539 nm and a red emission band of Mn2+ with a maximum at around 605 nm can be obtained. Their photoluminescence properties and ET behaviors from Ce3+ to Tb3+ and Mn2+ were systematically studied. By adjusting the dopant concentration of Tb3+ or Mn2+, tunable emission colors from blue to green and warm white have been realized, which implies its promising application in UV-pumped wLEDs.
The Sr4La1-x-y(PO4)3O: xCe3+, yTb3+ and Sr4-zLa1-x(PO4)3O: xCe3+, zMn2+ phosphors with different compositions were prepared by conventional high temperature solid-state reactions. The starting materials included SrCO3 (99%), (NH4)2HPO4 (99%), La2O3 (99.99%), Tb4O7 (99.99%), MnCO3 (99%) and CeO2 (99.99%). 2wt% Li2CO3 (99%) was introduced as flux. The stoichiometric starting reagents were thoroughly mixed and ground with an agate mortar. The mixture was first pre-fired in air at 600°C for 3 hours, ground again, and calcined at 1200°C for 5 hours in a reducing atmosphere (N2: H2 = 95: 5).
The phase purity was determined using an ARL X'TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV and 35 mA. Diffuse reflection spectra (DRS) were obtained with a UV/visible spectrophotometer (UV-3600, SHIMADZU) using BaSO4 as a reference in the 200-700 nm range. The morphologies of the as-prepared samples were examined by a field emission scanning electronmicroscope (FESEM, FEI, Quanta FEG). The photoluminescence excitation (PLE) spectra and photoluminescence (PL) spectra were recorded on an Edinburgh FS5 fluorescence spectrophotometer with a 150 W xenon lamp as the light source. The temperature dependent measurement was performed by the FS5 spectrofluorometer system, and the samples were mounted on a heating device that can be heated from 25 to 200 °C with the step of 1°C. The photoluminescence external quantum yields of the samples were obtained with an integrating sphere in the FS5 spectrophotometer. The fluorescence lifetime was recorded on an Edinburgh FLS 920 Fluorescence Spectrophotometer.
3. Results and discussion
3.1. Phase formation and morphology of SLPO: xCe3+, yTb3+/ xCe3+, zMn2+
The Rietveld structural refinement of the laboratory XRD data for the synthesized compound SLPO at room temperature is shown in Fig. 1(a), using the structural parameters of Sr5La(PO4)3F as an initial model . The final refinement structure parameters are listed in Table 1. The as-obtained residual factors and cell parameters are χ2 = 4.328, Rwp = 5.51%, Rp = 4.39%, and a = b = 9.721 Å, c = 7.235 Å, V = 592.135 Å3. Figure 1(b) presents the crystal structure of SLPO modeled with the acquired atomic coordinates. In the SLPO compound, there are two types of cationic sites (6h and 4f), the Sr(I) site is surrounded by nine O atoms in C3 symmetry, while the Sr(II)/La(II) position is with seven-coordinated CS symmetry . The cations in the host are connected by the isolated PO4 tetrahedrons. According to charge balance and the effective ionic radius(rSr2+, CN = 9 = 1.31 Å, rSr2+, CN = 7 = 1.21 Å, rMn2+, CN = 7 = 0.9 Å, rLa3+, CN = 7 = 1.1 Å, rTb3+, CN = 7 = 0.98 Å, rCe3+, CN = 7 = 1.07 Å) of positive ions in the host lattice , the Ce3+/Tb3+ ions are possible to enter into the La3+ sites and Mn2+ ions are expected to substitute the Sr2+ sites primarily in the SLPO lattice.
According to the previous work, for SLPO: xCe3+ phosphors, the most intense emission intensity can be obtained when x = 0.12. Here, the value of x in each SLPO: xCe3+, yTb3+/ xCe3+, zMn2+ sample is set to be 0.12. Figure 1(c) and 1(d) show the XRD patterns of SLPO: 0.12Ce3+, yTb3+ and SLPO: 0.12Ce3+, zMn2+ samples together with the standard pattern of Sr4Bi(PO4)3O (JCPDS 44-0180) as a reference. For observation, the enlarged views (2θ = 30-35 degree) of Fig. 1(c) and 1(d) are given in Fig. 1(d) and 1(f). Comparing with the standard data, it can be noticed that the diffraction peaks of SLPO: 0.12Ce3+, yTb3+ samples almost have no change when increasing the Tb3+ doping concentration y, indicating that the phase formation of SLPO is not influenced by little amount of Tb3+ or Ce3+. However, with the increase of z, an unknown impurity phase emerges when co-doping Mn2+ into SLPO: 0.12Ce3+.
Figure 2 shows the SEM morphologies of the SLPO: 0.12Ce3+, 0.1Tb3+ and SLPO: 0.12Ce3+, 0.3Mn2+ samples. It can be found that the particles exhibit relatively aggregated and irregular morphology due to the solid-state reaction preparation. In addition, the average particle sizes are about 10 μm, which can meet the requirements of w-LED phosphors.
3.2. Luminescence properties and energy transfer in SLPO: Ce3+, Tb3+
Figure 3 presents the diffuse reflection spectrum of the SLPO from 200 to 700 nm, which shows high reflectance in the visible range (400-700 nm), consistent with the white daylight color of the SLPO matrix.
To determine the optical bandgap value of the SLPO compound experimentally, the absorption spectrum of SLPO (see the inset of Fig. 3) was obtained from its reflection spectrum with the Kubelka-Munk (K-M) function .26]:27]. The absorption coefficient K becomes equal to 2α when the material scatters in a perfectly diffuse manner. As the scattering coefficient S can be considered as a constant with respect to the wavelength, the following expression can be obtained using Eqs. (1) and (2):Fig. 1 presents the plot of [F(R)hν] versus hν. By extrapolating the linear fit to [F(R)hν] = 0, the value of the SLPO optical bandgap (3.85eV) can be obtained.
Figure 4(a) presents the normalized PLE of SLPO: 0.1Tb3+ and PL spectra of SLPO: 0.12Ce3+ phosphors. Upon 310 nm excitation, the Ce3+ doped SLPO phosphor exhibits a broad emission band from 350 to 600 nm with the emission peak at around 469 nm, which is ascribed to the electric-dipole-allowed transition from the 5d excited state level to the 4f ground state of the Ce3+ ions . By monitoring 539 nm, the PLE spectrum of SLPO: 0.1Tb3+ contains two parts: the broad excitation band from 240 to 300 nm attributed to the 4f-5d transition of Tb3+and the sharp lines in the region of 300-400 nm, corresponding to the Tb3+ 4f-4f transitions. It can be observed that there is a distinct spectral overlap between the Ce3+ PL and Tb3+ PLE spectrum, indicating the possibility of ET from Ce3+ to Tb3+, and the emission intensity of Tb3+ can be enhanced via co-doping Ce3+ in the SLPO host.
The PLE and PL spectra of SLPO: 0.12Ce3+, 0.1Tb3+ are given in Fig. 4(b). It can be noticed that the shape of the PLE spectrum monitored at 539 nm is similar to that monitored at 469 nm. According to the previous research, the PLE spectra of the SLPO: Ce3+ phosphors vary with the monitoring emission wavelengths, probably because the Ce3+ ions will occupy more than one site in the SLPO host. The positions of the absorption bands in Fig. 4(b) are almost the same as the ones in the singly Ce3+ doped SLPO phosphors . Upon excitation of 310 nm, the PL spectrum exhibits a broadband emission from Ce3+ and a group of sharp characteristic emission lines of Tb3+. When excited upon 335nm, the intensity of Ce3+ emission center gets higher, whereas the emission intensity at 539nm from Tb3+ ions becomes weaker. These appearances reveal the occurrence of the ET from Ce3+ to Tb3+. The inset of Fig. 4(b) illustrates the possible ET (Ce3+ to Tb3+) process in the SLPO host. Electrons of Ce3+ can be pumped to the 5d level by absorbing UV light and then the ET process may occur from the excited 5d state of Ce3+ to the 5D3 level of Tb3+, which can give lower vibration frequency of phonon emission. Then the Tb3+ ions on 5D3 level incline to transits to the 5D4 level through non-radiative relaxation and shows strong emission.
To further investigate the ET process between the Ce3+ and Tb3+ ions in the host lattice, a series of SLPO: 0.12Ce3+, yTb3+ (y = 0-0.15) samples were prepared. Figure 5(a) depicts the PL spectra of the SLPO: 0.12Ce3+, yTb3+ under the excitation of 310 nm. It can be found that with the increase of Tb3+ concentration, the emission intensity of the green line at 539 nm of Tb3+ first increases till y = 0.1, and then decreases due to concentration quenching which is caused by the enhanced non-radiative ET among Tb3+ ions. Meanwhile, the intensity of the sensitizer Ce3+ declines gradually, which further supports the ET from Ce3+ to Tb3+. It can be found that when increasing the Tb3+ concentration, there is a decrease for the Ce3+ emission on the short wavelength side but an increase on the long wavelength side. This is mainly due to the overlapping of the two emission spectra from Ce3+ and Tb3+ ions in the long waveband. The emission intensities of Ce3+ ions in such side are relatively weak, and as the Tb3+ emissions strengthen, the total intensities increase.
The ET efficiency (ƞET) from a sensitizer to an activator can be calculated with the following equation :
The fluorescence decay curves of Ce3+ monitored at 469 nm in the SLPO: 0.12Ce3+, yTb3+ phosphors under the excitation of 310 nm were measured and investigated as shown in Fig. 6(a), in order to further validate the ET behavior from Ce3+ to Tb3+ ions in the SLPO host. It can be found that the decay curves of Ce3+ deviate from the single exponential one and with the increase of Tb3+ concentration, the deviation becomes more evident. Since the decay curves are non-exponential, the decay process of these samples can be characterized by the average fluorescence lifetime calculated as follows [28,29]:Eq. (5), the average lifetime of Ce3+ in SLPO:0.12Ce3+, yTb3+ phosphors are found to be 50.3, 48.7, 45.9, 43.4, 40.0, 35.9 ns when y = 0, 0.01, 0.03, 0.07, 0.1, and 0.15, respectively. With the increase of Tb3+ concentration, Ce3+ decay time decreases monotonically, indicating the presence of ET between Ce3+ and Tb3+ ions. Furthermore, the ET efficiency (ƞET) from Ce3+ to Tb3+ can also be estimated by [29,30]:Fig. 5(b) shows the decay curves of Tb3+ (λem = 539nm, λex = 310nm) in SLPO: 0.12Ce3+, yTb3+ samples. To show clearly, the y-coordinate of each curve was set to be the logarithm of intensity, from which it can be noticed that the fluorescence lifetime of Tb3+ ion differs slightly as y varies. Similar performance of Tb3+ ion in the Ba3Ce(PO4)3 phosphor has been reported in . With Eq. (5), the decay time is calculated to be 1.47, 1.53, 1.55, 1.59, and 1.52 ms when y = 0, 0.01, 0.03, 0.07, 0.1, and 0.15, respectively. The lifetime of Tb3+ ions are affected by the ET between Ce3+ and Tb3+ as well as the interaction between Tb3+ ions. As Tb3+ content increases, the lifetime shows a bit increases at first and then decreases, which is probably attributed to the ET from Ce3+ to Tb3+ ion and the concentration quenching effect of Tb3+ ion, respectively. The distance between Tb3+ ions shortens when the concentration increases, which increases the interaction between energy traps and shortens lifetimes.
To further study the ET mechanism from Ce3+ to Tb3+ in the SLPO host, the following relation can be used, based on Dexter’s ET formula for exchange and multi-polar interactions as well as Reisfeld’s approximation .33]. Generally the value of ƞ0/ƞ can be assessed by the ratio of corresponding luminescence intensities I0/I, and then the relationship between I0/I and Cn/3 are plotted in Fig. 6(a)-6(c). It can be noticed that the optimal linear relation is obtained when n = 6 (R2 = 0.9976), indicating that the ET from Ce3+ to Tb3+ occurs through the dipole-dipole mechanism.
According to Dexter’s ET theory , the critical distance (Rc) from the sensitizer to the acceptor in the ET progress can be calculated with the spectral overlap method. For the dipole-dipole interaction, Rc can be expressed as following [35,36]:37]. E (in eV) is the energy involved in the transfer. represents the spectral overlap between the normalized shape functions of the sensitizer’s emission band fS(E) and the acceptor’s excitation band FA(E), which is obtained by letting = = 1 . In our case, it is calculated to be about 0.0188 eV−4. Accordingly, the critical distance Rc for Ce3+-Tb3+ ET is estimated to be 5.08 Å, which is consistent with those previously investigated, such as 5.8 Å reported in Ca8MgLu(PO4)7: Ce3+, Tb3+ and 7.1 Å in Ca3La6(SiO4)6: Ce3+, Tb3+ [38,39]. The result is also in accordance with the previous studies, which stated that the value of Rc is restricted to about 5-8Å as ET can only be possible for nearest neighbours in the crystal lattice if the transfer is from a broadband emitter to a narrow line absorber .
3.3 Photoluminescence properties and ET in SLPO: Ce3+, Mn2+
As a transition metal ion with 3d5 electronic configurations, Mn2+ generally exhibits a broad emission band due to 4T1-6A1 transitions, and its position strongly depends on the host lattice, i.e. green emission in weak crystal field and red emission in strong crystal field [4, 41]. However, the intensities of Mn2+ absorption and emission bands are very weak because the d-d transitions are spin and parity forbidden. To enhance the absorption and emission intensities of Mn2+ in SLPO, Ce3+ ions were co-doped and tunable emission colors were obtained through ET processes. Figure 7(a) presents the normalized PLE and PL spectra of SLPO: 0.4Mn2+ and SLPO: 0.12Ce3+. The main excitation peaks of the SLPO: 0.4Mn2+ spectrum in the range of 250-490 nm could be ascribed to the transitions from the 6A1(6S) level to the 4T1(4P), 4E(4D), 4T2(4D), [4A1(4G), 4E(4G)], and 4T2(4G) energy levels of Mn2+, respectively [18,33,38,42,43]. It can be noticed that the emission band of SLPO: 0.12Ce3+ is overlapped with the absorption band of SLPO: 0.4Mn2+, which implies that the ET is probably to exist in the Ce3+ and Mn2+ co-doped SLPO phosphor. From the view of Ce3+ to Mn2+ transition in the SLPO host, the ET behavior between Ce3+ and Mn2+ ions can be explained by the similar value of the excited 5d state energy level of Ce3+ and the 3d state energy levels of Mn2+ ions, as shown in the inset of Fig. 7(a). In order to further investigate the Ce3+ to Mn2+ ET in SLPO, a series of SLPO: 0.12Ce3+, zMn2+ (z = 0, 0.2, 0.3, 0.35, and 0.4) phosphors have been prepared and their PL spectra under the excitation of 310nm UV are displayed in Fig. 7(b). Two emission bands can be noticed in the spectra: a broad blue band due to Ce3+ luminescence centers and an orange-red emission band (550-700nm, 4T1-6A1 transition) of the Mn2+ ions. With the increase of Mn2+ content, the PL intensity of Ce3+ decreases gradually, while that of Mn2+ increases. This result indicates the presence of the ET from Ce3+ to Mn2+. According to Eq. (4), the ET efficiency gradually increases from 8.46% to 22.30% when z increases from 0.2 to 0.4.
Furthermore, the decay curves and lifetimes of Ce3+ and Mn2+ ions in SLPO: 0.12Ce3+, zMn2+ phosphors under the excitation of 310 nm were investigated. Figure 8(a) presents the decay curves of SLPO: 0.12 Ce3+, zMn2+ phosphors with 310 nm excitation and 469 nm emissions. According to Eq. (5), the calculated lifetimes of Ce3+ for SLPO: 0.12Ce3+, zMn2+ (z = 0.2, 0.3, 0.35, and 0.4) samples are 46.3, 42.6, 40.6, and 37.5 ns, respectively. The inset of Fig. 8(a) shows the decay curves of Mn2+ (λem = 610nm, λex = 310nm) in SLPO: 0.12Ce3+, zMn2+ samples. As Mn2+ content increases, the lifetime shows obvious increase, which may be mainly originating from the ET between Ce3+ and Mn2+ ions. the lifetimes of Mn2+ ions can be calculated as 29.1, 43.5, 46.8, and 47.8 ms for z = 0.2, 0.3, 0.35, and 0.4, respectively. The ƞET values of ET behavior from Ce3+ to Mn2+ are also estimated using Eq. (6), with the results of 7.82%, 15.16%, 19.14%, and 25.34%, respectively. Like SLPO: Ce3+, Tb3+, the ET from Ce3+ to Mn2+ in the SLPO host is also inefficient. According to Eq. (7), when n = 6, the I0/I and plot shows the optimal linear relationship, which indicates that the ET from Ce3+ to Mn2+ ions in SLPO can be determined to be the electric dipole-dipole interaction, as shown in Fig. 8(b), 8(c) and 8(d). The critical distance (Rc) calculated with Eq. (8) is 4.97 Å (fd = 10−7 when Mn2+ is the acceptor ion ).
3.4 CIE chromaticity diagram, external quantum yield, and thermal stability
The chromaticity coordinates of SLPO: 0.12Ce, yTb3+ (0≤y≤0.15) and SLPO: 0.12Ce, zMn2+ (0≤z≤0.4) phosphors were calculated based on the corresponding PL spectra with 310 nm excitation, which are represented in Table 2, and the corresponding CIE chromaticity diagram is presented in Fig. 9. With the increasing y or z value, the CIE values of SLPO: 0.12Ce3+, yTb3+ and SLPO: 0.12Ce3+, zMn2+ can be tuned from greenish blue (0.2310, 0.2742) to green (0.2906, 0.3705) region (arrow A, point 1 to 6) and from greenish blue to warm white (0.3488, 0.336) (arrow B, point 1, 7, 8, 9, 10) region, respectively. White light emission can be achieved by co-doping Ce3+ and Mn2+ ions in SLPO under the excitation of 310 nm UV-light, with the composition of SLPO: 0.12Ce3+, 0.3Mn2+ or 0.35Mn2+ and CIE value of (0.3326, 0.3298) or (0.3417, 0.3336) as shown in Fig. 9 (point 8, 9). The external quantum yields of the samples excited at 310nm were also measured and listed in Table 2. It can be noted that the performances on quantum efficiencies are not so good and further studies are necessary to improve such properties.
Thermal quenching performance of phosphors is an important index for LED application. Generally, the emission intensity gradually decreases as the probability of non-radioactive transition increases when the phosphor is heated. Figure 10(a) and 10(b) show the temperature-dependent emission spectra of the SLPO: 0.12Ce3+, 0.1Tb3+ and SLPO: 0.12Ce3+, 0.3Mn2+ samples, respectively. The emission intensity decreases as the temperature increases from 25 to 200 °C. It can be noticed that both the emission intensities of Tb3+ and Mn2+ decrease more slowly than those of Ce3+ when heated. Besides, the Ce3+ emission shows a blue-shift, like the thermal property of SLPO: 0.12Ce3+ phosphor, which should be ascribed to thermally-active phonon-assisted tunneling from the lower-level excited states to the higher-level excited states of Ce3+ [22, 43].
Setting the emission intensities at 25 °C as the normalized standard values, the relative PL emission intensities of Ce3+, Tb3+ and Mn2+ via temperature in SLPO: 0.12Ce3+, 0.1Tb3+ and SLPO: 0.12Ce3+, 0.3Mn2+ phosphors are presented in the insets of Fig. 10(a) and Fig. 10(b), respectively. The values are calculated according to the peak emission intensities of the Ce3+, Tb3+, and Mn2+ ions [29, 31, 44]. The PL intensities of the Ce3+ and Tb3+ emission centers at 150 °C of SLPO: 0.12Ce3+, 0.1Tb3+ decrease to only 40% and 49% of the initial intensities, respectively. For the SLPO: 0.12Ce3+, 0.3Mn2+ phosphor, the values become 39% and 79%. It has been reported that the Ce3+ doped SLPO phosphor had a poor thermal stability, but the Tb3+ singly doped sample had a relatively good performance . The intensity of the Tb3+ emission center in the SLPO: 0.07Tb3+ phosphor at 150 °C decreased to 79% of the initial intensity, which was more stable than that in the SLPO: 0.12Ce3+, 0.1Tb3+ sample. This phenomenon is most probably caused by the wide range overlap between the PL spectra of Ce3+ and Tb3+ ions in SLPO, so the emission intensity at 539 nm depends on both Tb3+ and Ce3+ emission centers. It’s worth noting that different thermal quenching rates of Ce3+ and Mn2+ ions in SLPO might cause unstable color output. Under 310nm excitation, the color coordinates of the SLPO: 0.12Ce3+, 0.3Mn2+ phosphor shifts from (0.3326, 0.3298) in 25 °C to (0.4204, 0.3514) in 150 °C. Therefore, SLPO: 0.12Ce3+, 0.3Mn2+ phosphor is not applicable for some highly-stable color-output lighting fields. Fortunately, the white emission will gradually turn into a “warm white light” region as the thermal quenching rate of Mn2+ is lower than that of Ce3+ ions in SLPO. Thus the use in CRI-tunable wLED devices with temperatures might be explored .
A series of SLPO: 0.12Ce3+, yTb3+ and SLPO: 0.12Ce3+, zMn2+ phosphors have been synthesized by the traditional solid state reaction. The ETs from Ce3+ to Tb3+ and from Ce3+ to Mn2+ were studied in detail. The mechanisms of the two ETs were predominated by dipole-dipole interaction. Varied emitting colors from greenish blue to green or warm white can be achieved by proper tuning of the concentration of Tb3+ or Mn2+ ions. White light emission with CIE value of (0.3326, 0.3298) can be achieved in SLPO: 0.12Ce3+, 0.3Mn2+ phosphor under the excitation of 310nm UV-light. With the development of semiconductor technology, the chips for fabricating with phosphors in LED can be produced with different emitting wavelengths, which indicates that the SLPO: Ce3+/Tb3+/Mn2+ is a promising single-composition phosphor for application involving white LEDs.
National Natural Science Foundation of China (11604115, 11547023); the Educational Commission of Jiangsu Province of China (17KJA460004); Huaian Science and Technology Funds (HAC201701).
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