1. Introduction

Researchers have recently found extensive applications for white-light-emitting diodes (WLEDs) made of a blue-emitting InGaN chip and yellow-emitting phosphor Y₃Al₅O₁₂:Ce³⁺ [1]. However, this type of combination exhibits low color rendering index (CIE Ra < 80) due to the lack of red spectral contribution [1–3]. WLEDs fabricated with UV-LED chips and the red-green-blue (RGB) emitting phosphors are considered as another way of achieving white light emission with better color rendering properties. Nevertheless, these types of WLEDs are still suffering from low emission efficiency caused by the strong reabsorption of the emitted blue light by the red and green phosphors [4,5]. Such problems could be avoided by using a single-phase white light emitting phosphor where RGB colors can be integrated in one single-phase host material, which is based on the mechanism of energy transfer from sensitizer to activator [2–22].

In recent years, various of single-phase white-light-emitting phosphors have been synthesized and investigated, including Ca₃Sc₂Si₃O₁₂:Ce³⁺, Mn²⁺ [6], Na₃LuSi₂O₇:Eu²⁺, Mn²⁺ [7], Sr₂Y₈(SiO₄)₆O₂:Bi³⁺, Eu³⁺ [8], Ca₅(PO₄)₃Cl:Ce³⁺, Eu²⁺, Tb³⁺, Mn²⁺ [9], and Ca₉Y(PO₄)₇:Eu²⁺, Mn²⁺ [10]. Among these, the phosphate system single-phase white-light-emitting phosphors, especially for phosphate phosphors with polar whitlockite-type structure, have come into focus owing to their perfect stability and good color reproducibility [10–18]. In particular, owing to the forbidden ⁴T₁→⁶A₁ transitions, the emission intensity of Mn²⁺ is weak under UV excitation. However, the emission of Mn²⁺ can be considerably improved by introducing an efficient sensitizer. Eu²⁺ has been widely used as such promising sensitizer in many Mn²⁺ doped hosts [7,9–18]. However, due to the energy transfer and the complex interaction between the donor and the acceptor, the quantum efficiencies of such single-phase white-light-emitting phosphors decreased sharply when emission centres were co-doped into the host. Needless to say, the donor plays an important role in the co-doped single-phase system. Therefore, we need to find an effective way to improve the quantum efficiency of the energy donor, in order to get the high luminescence efficiency white light in a single phase, and this will be crucial for the single-phase white light emission phosphors to find their place in practical application of WLEDs.

Topochemical reduction have been used in the development of inorganic functional materials [23–32], including the luminescent materials [23–25,31,32]. Such an effective reaction method enables us to obtain specific metastable phases bearing highly unusual oxidation states and coordination geometries, by using particular reduction agents, such as CaH₂ [23–29] and elemental Al (that is, the remote Al reduction) [30–32]. Compared with CO reduction, the remote Al reduction can avoid the raw materials from being contaminated during the reaction process. Meanwhile, the remote Al reduction method can also allow us to realize the reducing of the target sample in a wider reaction temperature and time range [31,32]. However, as far as we know, there is no report on the development or exploration of high efficiency multi-ions co-doped single-phase white-light-emitting phosphors using remote Al reduction yet. Motivated by above mentioned, in this paper, we report the preparation of Eu²⁺/Mn²⁺ co-doped Ca₉Ln(PO₄)₇ (Ln = Gd, La, Lu) phosphors through the remote Al reduction. By controlling the reaction condition of Al reduction, we achieved a significant increase in the quantum efficiency of the energy donor, and finally obtained high efficiency single-phase white-light-emitting phosphors. We hope this work can provides an enlightening reference for the preparation and development of high efficiency single-phase white-light-emitting phosphors.

2. Experiment

2.1. Sample preparation

Ca₉Ln(PO₄)₇:xEu²⁺,yMn²⁺ (Ln = Gd, La, Lu, x and y are the percentage in mol) phosphors were synthesized by a two-step solid state reaction. CaCO₃ (99%), La₂O₃ (99.99%), Lu₂O₃ (99.99%), Gd₂O₃ (99.9%), NH₄H₂PO₄ (A.R.), Eu₂O₃ (99.99%), and MnCO₃ (A.R.) were used as raw materials. Stoichiometric reagents were first mixed well, grounded in an agate mortar, and then transferred to an alumina crucible to be calcined in the muffle furnace at 1200 °C for 8 h. The obtained precursors were further reduced at 1000 °C for 2 h by remote Al reduction reaction in a furnace (the obtained precursors were transferred to an alumina crucible, together with another crucible equipped with excess Al powder were beside placed in a vacuum heating furnace and then heated to 1000 °C for 2 h under a vacuum atmosphere. In particular, the reducer and the compounds were always separated throughout the whole process of reaction). In the CO atmosphere reduction group, reference samples were calcined at 1000 °C for 8 h in an alumina crucible covered with an outer crucible filled with graphite power [13,17]. The final samples were cooled down freely to room temperature and then were re-grounded for further characterization.

2.2. Characterization

The phase purity of the phosphors were performed by X-ray diffraction (XRD Bruker D8 Focus) with a monochromatized source of Cu Kα radiation (λ = 0.15405 Å,) at 40 kV and 40 mA. The photoluminescence (PL) spectra were measured by using a Hitachi F-7000 fluorescence spectrophotometer equipped with a 150 W xenon lamp as an excitation source. The quantum yield measurements were performed using the same setup equipped with an integrating sphere (S-68). The luminescence decay curve was characterized by a steady state and transient state fluorescence spectrometer (FS5, Edinburgh).

3. Results and discussion

3.1. Phase analysis

The XRD patterns of the as-prepared Al reduced samples are shown in Fig. 1(a). The diffraction peaks of Eu²⁺-doped and/or Eu²⁺/Mn²⁺ codoped-Ca₉Ln(PO₄)₇ (Ln = Gd, La) samples are matched well with the standard data of JCPDS no.: 46-0402 (Ca₉Y(PO₄)₇) and the reported results [11,12,15]. The as-prepared Ca₉Lu(PO₄)₇:Eu²⁺,Mn²⁺ is consistent with the standard data of JCPDS no.: 49-1791. The results indicate that the as-prepared Al reduced single-phase Ca₉Ln(PO₄)₇:Eu²⁺,Mn²⁺ (Ln = La, Lu, Gd, marked as Ca₉Ln(PO₄)₇:Eu²⁺,Mn²⁺-Al) phosphors were successfully prepared through remote Al reduction reactions. The XRD patterns of the samples prepared through CO atmosphere are shown in Fig. 7 in the Appendix. All the diffraction peaks of the presented samples are matched well with standard data (JCPDS no.: 46-0402 and JCPDS no.: 49-1791) and the reported works [11,12,15]. The results of XRD indicate that the doped Eu²⁺, co-doped Eu²⁺/Mn²⁺ ions, remote Al reduction and CO atmosphere reduction do not cause significant changes of crystalline structure. However, the XRD intensities of all the Al reduced samples were found to be lower than that of CO reduced samples [see Fig. 1(b)]. As we all know that, the intensity of the diffraction peak is strongly depending on the crystallinity of the samples. Such difference indicates that, there seems to be some differences in the structure between the Al reduced samples and CO reduced samples. We will discuss this in combined with the spectra changes in the following section.

 

Fig. 1. (a) XRD patterns of the selected Al-reduced CLnP:Eu²⁺,Mn²⁺ (Ln = Gd, La, Lu) samples. (b) XRD patterns of the selected Al-reduced CLnP:0.01Eu²⁺ samples and CO-reduced CLnP:0.01Eu²⁺.

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As have been reported, the Ca₉Ln(PO₄)₇ has a rhombohedral crystal structure with a space group of R3C (no. 161) [33–35], and two of the Ca atoms are eight-fold coordination and the other one is nine-fold coordination. The reported studies concluded that, Eu²⁺ (1.25 Å, CN = 8; 1.3 Å, CN = 9) and Mn²⁺ (0.96 Å, CN = 8) ions substitute the site of Ca²⁺ (1.12 Å, CN = 8; 1.18 Å, CN = 9), based on the ionic radius and charge similarity [10–15,35]. Combining the XRD results, we believe that the Eu²⁺ and Mn²⁺ ions occupy the Ca²⁺ ion sites in the as-prepared Ca₉Ln(PO₄)₇:Eu²⁺,Mn²⁺.

3.2. Luminescence properties of Eu²⁺ and Eu²⁺/Mn²⁺ activated CGP

The PL spectra of Al reduced Ca₉Gd(PO₄)₇:xEu²⁺ with different Eu²⁺ concentrations (x = 0.01, 0.015, 0.02, 0.025, 0.03) under the excitation of 327 nm are shown in Fig. 2(a) (Samples prepared by Al reduction and CO reduction with same components are obtained from one same precursor). Upon the increasing of x, there is always one dominating emission band centred at 491 nm with a broad bandwidth which is attributed to the 4f⁶5d¹→4f⁷ of the Eu²⁺ ions. The emission intensities have an obvious increasing trend with increasing Eu²⁺ concentration, and then the emission intensity declines dramatically when Eu²⁺ dopant content reached at x = 0.025. The PL spectra of Ca₉Ln(PO₄)₇:Eu²⁺ prepared through CO reduction method (marked as Ca₉Ln(PO₄)₇:Eu²⁺-CO) were shown in Fig. 8 in the Appendix). The optimal Eu²⁺ dopant content in CO reduced Ca₉Ln(PO₄)₇ was found to be x’ = 0.01. It can be clearly observed that the PL intensity of Al reduced Ca₉Gd(PO₄)₇:xEu²⁺ phosphors is greatly enhanced compared with the CO reduced samples in Fig. 2(a) and Fig. 2(b). The PL intensity of the CGP:0.025Eu²⁺-Al has a clearer advantage than that of CGP:x’Eu²⁺-CO and is 4.3 times and 12.5 times of the reference samples when x’ is 0.01 and 0.025, respectively (It should be noted that, the Al reduced sample and CO reduced sample with the same composition are derived from the same precursor. The PLE and PL spectra of precursors CLnP:Eu³⁺ (Ln = La, Lu, Gd) are presented in Fig. 9 in the Appendix. In particular, new broad band emission covering the whole red region and centred at 630 nm from the remote Al reduced Ca₉Ln(PO₄)₇:xEu²⁺ phosphors was observed. Considering the Ca²⁺ sites for Eu²⁺ occupancy, we plot the emission spectrum of CGP:0.03Eu²⁺ under 327 nm excitation by Gaussian fitting in Fig. 2(c). As we can see, the emission spectrum can be decomposed into four broad emission bands, centred at 462, 491, 546 and 630 nm, respectively, which are attributed to the different coordination sites of Ca²⁺ in the CGP, related to one nine-coordinated Ca²⁺, two eight-coordination Ca²⁺ and six-coordination Ca²⁺ [11,35].

 

Fig. 2. (a) PL spectra of CGP:xEu²⁺-Al with different Eu²⁺ doping contents and CGP:0.01Eu²⁺-CO. The inset shows the relation between PL intensity and Eu²⁺ content. (b) PL spectra of CGP:0.025Eu²⁺-Al (λ_ex = 327 nm), CGP:0.025Eu²⁺-CO (λ_ex = 329 nm). (c) Experimental spectrum (solid line), fitted curve (red dashed line), and deconvoluted Gaussian components (greed lines) of Al-reduced CGP:0.03Eu²⁺ sample. (d) PL spectra of CGP:0.025Eu²⁺,yMn²⁺-Al with different Mn²⁺ doping contents.

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As we know, the d-f transitions of Eu²⁺ ions are sensitive to their local crystal field environment. Therefore, both the enhancement of PL intensity and the appearance of new emission band suggest that obvious changes happened in the crystals field environment of Eu²⁺ ions after being treated by the remote Al reaction. In fact, the mechanism of topochemical reactions effected on the target cations has been extensively studied. All these reports suggested that randomly distributed oxygen vacancies have been created in the reduced phases prepared by such reactions [23–32]. Thus, associated with the XRD results, things become understandable: the remote Al reduction treatment altered the local environments around the Eu²⁺ ion crystal field by creating oxygen vacancies in the host, and the affected sensitive Eu²⁺ then show their response of changes in both PL intensity and spectra morphology. The PL spectra of Ca₉Ln(PO₄)₇:xEu²⁺-Al (Ln = La, Lu) and the corresponding reference samples are shown in Fig. 10 in the Appendix, and the results proves that remote Al reduction reaction has the same effect on the PL intensity and spectra morphology of Ca₉Ln(PO₄)₇:xEu²⁺-Al (Ln = La, Lu).

After determining the optimal doping concentration of Eu²⁺ ion, we co-doped Mn²⁺ into the host, aiming to achieve the white light in the single phase. Figure 2(d) shows the emission spectra of CGP:0.025Eu²⁺,yMn²⁺ phosphors (y = 0, 0.005, 0.01, 0.015, 0.02, 0.025) under 328 nm excitation (The PL spectra of Ca₉Ln(PO₄)_7:Eu²⁺,Mn²⁺-Al (Ln = La, Lu) are shown in Fig. 11 in the Appendix). The PL intensity of Eu²⁺ at 490 nm decreases with increasing Mn²⁺ content. Furthermore, the PL intensity of the broad red emission band at 650 nm was observed to first increase with the content of Mn²⁺ and reach a maximum at y = 0.02, then decrease with higher Mn²⁺ concentration.

3.3. Energy transfer between Eu²⁺ and Mn²⁺

The PL decay curves of Al-reduced CGP:0.025Eu²⁺,yMn²⁺ were measured and presented in Fig. 3 (excited at 328 nm and monitored at 490 nm) to identify the energy transfer process between the Eu²⁺ and Mn²⁺. The decay time <τ> can be best fitted with a second-order exponential equation, given as the following [21,36–39]:

 

Fig. 3. PL decay curves of the Al-reduced CGP:0.025Eu²⁺,yMn²⁺ samples (y = 0, 0.005, 0.01, 0.015, 0.02, 0.025, excited at 328 nm and monitored at 490 nm). The inset shows the average decay time of Eu²⁺ in CGP:0.025Eu²⁺,yMn²⁺ as a function of Mn²⁺ content y.

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The energy transfer efficiency (η_ET) from Eu²⁺ to Mn²⁺ can be calculated by formula [36–39]:

According to Dexter’s energy transfer expressions of multipolar exchange and interaction, the following relation can be given [21,40]:

 

Fig. 4. Dependence of I_S0/I_S of Eu²⁺ on (I) $C_{\textrm{M}{\textrm{n}^{\textrm{2 + }}}}^{\textrm{6/3}}$; (II) $C_{\textrm{M}{\textrm{n}^{\textrm{2 + }}}}^{\textrm{8/3}}$ and (III) $C_{\textrm{M}{\textrm{n}^{\textrm{2 + }}}}^{\textrm{10/3}}$ in CGP:Eu²⁺,Mn²⁺-Al.

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However, we noticed that, the R² we obtained is a little bit lower than the result of the reported Eu²⁺/Mn²⁺ co-doped phosphor of single phase (dependence of I_S0/I_S of Eu²⁺ in Ca₉Ln(PO₄)₇:Eu²⁺,Mn²⁺ (Ln = La, Lu) also can be seen in Fig. 13 in the Appendix) [11,12,14,15]. We also noticed that the concentration for Mn²⁺ ions to reach the highest emission intensity is much lower. It seems that something is affecting the energy transfer between Eu²⁺ and Mn²⁺. One possible reason is that the ⁴T₁→⁶A₁ transition of Mn²⁺ is also affected by the remote Al reduction treatment (see Fig. 14 in the Appendix). Thus, similar with the response of Eu²⁺, the PL intensity of Mn²⁺ was also improved. Another possible reason is that the sensitizer Eu²⁺ who get reinforced after being treated by the topochemical reaction can provide more energy to the activator Mn²⁺. What we can’t forget is that the new broad band red emission centred at 630 nm from Eu²⁺ is overlapped with those of Mn²⁺. We believe that, it is the combination of the above three reasons that lead to the unusual phenomenon of low fitting accuracy and low quenching concentration of Mn²⁺.

It can be clearly observed that the corresponding color tone of CLnP:0.025Eu²⁺,yMn²⁺ (Ln = Gd, Lu) phosphors shifts gradually from green to white and eventually to red with the addition of Mn²⁺ concentration. White light was realized by tuning the ratio of the Eu²⁺ to Mn²⁺ ions in CGP:0.025Eu²⁺,0.015Mn²⁺ (W1) and CLuP:0.025Eu²⁺,0.015Mn²⁺ (W2) samples, respectively. The color hue of CLP:0.03Eu²⁺,yMn²⁺ phosphors is tunable from green to yellow and, eventually, to red in the visible spectral region by changing the doped Mn²⁺ content. Yellow light was realized in CLP:0.03Eu²⁺,0.015Mn²⁺ (W3) phosphor. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of CLnP:Eu²⁺,Mn²⁺ (Ln = La, Lu, Gd) with different Mn²⁺ content y are presented in Fig. 5 and Table 1.

 

Fig. 5. CIE chromaticity diagram for Ca₉Ln(PO₄)₇:Eu²⁺,yMn²⁺-Al (Ln = Gd, La, Lu) phosphors excited at 340 nm.

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Table 1. PL quantum yield of Ca₉Ln(PO₄)₇:Eu²⁺,Mn²⁺ (Ln = La, Lu, Gd) and the references at different excitation wavelength.

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3.4. PL quantum yield of CGP:xEu²⁺,yMn²⁺ phosphors

PL quantum yield (PLQY) of selected samples were calculated according to the method described by L.A. Moreno [41]. The sample PLQY can be given as following:

Where φ_d is the measured internal quantum yield using direct excitation, which refers to the ratio of amount of fluorescence to the excitation beam absorbed by sample. Repeat the step to calculate quantum yield and absorptance for indirect excitation, A_d is the absorptance for direct excitation which is calculation as the ratio of amount of excitation beam absorbed by sample, φ_i is the internal quantum yield using indirect excitation. The PLQY of the selected Ca₉Ln(PO₄)₇:0.025Eu²⁺,yMn²⁺ (Ln = La, Lu, Gd, y = 0, 0.005, 0.01, 0.015, 0.02, 0.025) together with the reference samples under relative optimal excitation band are shown in Table 1. Consistent with the PL spectra results, the PLQY of the remote Al reduced sample is found to be greatly enhanced. The PLQY of CGP:0.025Eu²⁺ is 3.9 times higher than that of optimized CGP:0.01Eu²⁺ sample prepared by CO reduction. The PLQY of remote Al reduced white light emitting CGP:0.025Eu²⁺,0.025Mn²⁺ phosphors can reach as high as 54.8%. In particular, the PLQY of all the Al reduced phosphors does not show any sign of decrease, but were even found to be gradually increased after Mn²⁺ were co-doped with Eu²⁺ into the host. This phenomenon is significantly different from that of the reported single-phase white light emitting phosphors of co-doped systems [42,43], which confirms again the contribution of Al reduction to the enhancement of PL intensity and PLQY of the luminescence centres, especially for the donors Eu²⁺.

3.5. LED lamps fabrication and EL spectra

Three LED lamps were fabricated by using the 340 nm chips with the as-prepared CGP:0.025Eu²⁺,0.015Mn²⁺ (W1), CLuP:0.025Eu²⁺,0.015Mn²⁺ (W2), CLP:0.03Eu²⁺,0.015Mn²⁺ (W3) phosphors, respectively. It can be seen clearly that the as-fabricated LEDs show bright light emission after driven by the 300 mA current. The electroluminescent (EL) spectra of the as-fabricated LEDs driven by the 300 mA current are shown in Fig. 6. W1, W2 and W3 show correlated color temperature (abbreviated as CCT) of 5292 K, 5101 K, 3012 K with the CIE color coordinates of (0.337, 0.346), (0.325, 0.361) and (0.409, 0.408), respectively.

 

Fig. 6. The electroluminescent spectra of white LED lamps fabricated using a near-UV 340 nm chip combined with a single-phased phosphor CLnP:Eu²⁺,Mn²⁺ (Ln = Gd, Lu, La) driven by a 300 mA current.

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4. Conclusion

In summary, a series of single-phase Ca₉Ln(PO₄)₇:Eu²⁺,Mn²⁺ (Ln = La, Lu, Gd) phosphors with enhanced quantum yield were successfully developed through a topochemical reduction reaction strategy by using elemental aluminum as reduction agent. Due to the effect on the surrounding crystal field environment of Eu²⁺ by introducing oxygen vacancy in the host, new broad band emission covering the whole red region and centered at 630 nm in Ca₉Ln(PO₄)₇:Eu²⁺ were created, and the PL intensities were found to be greatly enhanced. Under UV light excitation, the CGP:Eu²⁺ presents 4.3 times higher PL intensity than the optimized phosphor prepared by traditional CO reduction method. Through an effective improvement on the quantum efficiency of the energy donors, high PLQY white light were obtained in CGP:0.025Eu²⁺,0.015Mn²⁺ (52.1%) and CLuP:0.025Eu²⁺,0.015Mn²⁺ (43.3%) samples. The PLQY of all the Al reduced phosphors was even found to be gradually increased after Mn²⁺ were co-doped with Eu²⁺ into the host due to the comprehensive improvements of the crystal field environment of the luminescence centers and the way of energy transfer between the donors (Eu²⁺) and acceptors (Mn²⁺). Finally, white LED lamps were fabricated by using the 340 nm chips with the as-prepared single-phase white light emitting phosphors, the as-fabricated LEDs can give bright white light emission driven by the 300 mA current. The result of this work can also provide an enlightening reference for the preparation and development of high efficiency single-phase white light emitting phosphors.

Appendix

The XRD patterns of the selected samples are shown in Fig. 7 in the Appendix. The diffraction peaks of Ca₉Ln(PO₄)₇ (Ln = Gd, La) are matched well with the standard data of JCPDS no.: 46-0402 (Ca₉Y(PO₄)₇) and the reported results. The as-prepared Ca₉Lu(PO₄)₇ is consistent with the standard data of JCPDS no.: 49-1791. The results of XRD indicate that the doped Eu²⁺, co-doped Eu²⁺/Mn²⁺ ions, CO atmosphere reduction do not cause significant changes in crystalline structure.

 

Fig. 7. XRD patterns of pure phase CLnP:Eu²⁺ phosphors prepared by CO atmosphere reduction.

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Fig. 8. PLE (λ_em = 482 nm) / PL (λ_ex = 324 nm) spectra of the CO-reduced CGP:xEu²⁺ samples (x = 0.005, 0.01, 0.015, 0.02, 0.025, 0.03).

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Fig. 9. (a) PL (λ_ex = 395 nm) and (b) PLE (λ_em = 613 nm) spectra of CGP:xEu³⁺. (c) PL (λ_ex = 395 nm) and (d) PLE (λ_em = 613 nm) spectra of CLP:xEu³⁺. (e) PL (λ_ex = 395 nm) and (f) PLE (λ_em = 613 nm) spectra of CLuP:xEu³⁺.

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In Fig. 10 in the Appendix, the emission intensities of CLnP:xEu²⁺-Al (Ln = La, Lu) have an obvious increasing trend with increasing Eu²⁺ concentration, and then the emission intensity declines dramatically when Eu²⁺ dopant content reached at x = 0.03 and x = 0.025, for CLP:xEu²⁺-Al and CLuP:xEu²⁺-Al respectively. The PL spectral intensity of CLP:0.01Eu²⁺-Al is 5.7 times of that of CLP:0.01Eu²⁺-CO, and the PL spectral intensity of CLuP:0.01Eu²⁺-Al is 4.6 times of that of CLuP:0.01Eu²⁺-CO. In particular, new broad band emission covering the whole red region and centered at 630 nm from the remote Al reduced CLP:xEu²⁺-Al phosphors was observed. The phenomenon that the energy donor's luminous intensity of CLnP:xEu²⁺-Al (Ln = La, Lu) is greatly increased is also observed.

 

Fig. 10. PL spectra of (a) CLP:xEu²⁺-Al sample (λ_ex = 319 nm) and CLP:0.01Eu²⁺-CO sample (λ_ex = 324 nm). (b) CLuP:xEu²⁺-Al (x = 0.01, 0.015, 0.02, 0.025, 0.03, 0.04; λ_ex = 328 nm) and CLuP:0.01Eu²⁺-CO (λ_ex = 325 nm). The inset shows the PL intensity of the Al reduced samples as a function of the Eu²⁺ content x.

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Fig. 11. (a) PLE (λ_em = 500 nm) / PL (λ_ex = 311 nm) spectra of CLP:0.03Eu²⁺,yMn²⁺-Al and (b) PLE (λ_em = 488 nm) / PL (λ_ex = 331 nm) spectra of CLuP:0.025Eu²⁺,yMn²⁺-Al.

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Fig. 12. PL decay curves of the Al-reduced (a) CLP:0.03Eu²⁺,yMn²⁺ and (b) CLuP:0.025Eu²⁺,yMn²⁺ samples. The inset shows the average decay time of Eu²⁺ as a function of Mn²⁺ content y.

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Fig. 13. Dependence of I_S0/I_S of Eu²⁺ on (I) $C_{\textrm{M}{\textrm{n}^{\textrm{2 + }}}}^{\textrm{6/3}}$; (II) $C_{\textrm{M}{\textrm{n}^{\textrm{2 + }}}}^{\textrm{8/3}}$; (III) $C_{\textrm{M}{\textrm{n}^{\textrm{2 + }}}}^{\textrm{10/3}}$ in (a) CLP:Eu²⁺,Mn²⁺-Al and (b) CLuP:Eu²⁺,Mn²⁺-Al.

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Fig. 14. PL (λ_ex = 275 nm) spectra of CGP:0.02Mn²⁺ prepared by remote Al reduction reaction and CO atmosphere, respectively.

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Funding

National Natural Science Foundation of China (51902203, 51672177); Program of Shanghai Academic Research Leader (19XD1434700); Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (TP2014062).

Acknowledgments

Yongzheng Fang and Yalan Huang have equally contributed to this work.

Disclosures

The authors declare no conflicts of interest.

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