Color-Tunable Luminescence and Energy Transfer Properties of

Apr 24, 2014 - Eu2+-, Mn2+-activated Ca9Mg(PO4)6F2 (CMPF) phosphors with blue to yellow color-tunable emission properties have been synthesized via ...
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Color-Tunable Luminescence and Energy Transfer Properties of Ca9Mg(PO4)6F2:Eu2+, Mn2+ Phosphors for UV-LEDs Kai Li,†,‡ Dongling Geng,†,‡ Mengmeng Shang,† Yang Zhang,†,‡ Hongzhou Lian,† and Jun Lin†,* †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: Eu2+-, Mn2+-activated Ca9Mg(PO4)6F2 (CMPF) phosphors with blue to yellow color-tunable emission properties have been synthesized via high-temperature solid-state reaction method. The crystal structure of Ca9Mg(PO4)6F2 has been identified by Rietveld refinement. The different crystallographic sites of Eu2+ in CMPF:Eu2+ phosphors have been confirmed by virtue of their fluorescence decay lifetimes. The Eu2+activated CMPF phosphors exhibit broad excitation spectra from 200 to 420 nm (which matches well with the UV-based LED chips) and emission spectra from 380 to 580 nm centered at 454 nm. Energy transfer from Eu2+ to Mn2+ ions in Eu2+, Mn2+codoped CMPF samples is possible because of the spectral overlap between Eu2+ emission and Mn2+ excitation spectra, and the constant fall of fluorescence decay lifetimes of Eu2+ ion with increasing Mn2+ concentration demonstrates the occurrence of it, which provides the color-tunable emission from blue to yellow through adjusting Mn2+ concentration. The energy transfer mechanism between Eu2+ and Mn2+ ions is verified to be electric dipole−quadrupole interaction by analyzing the experimental results. The critical distance between them calculated by concentration quenching (14.57 Å) and spectral overlap methods (14.90 Å) are consistent, which testifies the energy transfer mechanism above from Eu2+ to Mn2+ is appropriate. These results show CMPF:Eu2+, Mn2+ phosphors could be anticipated for UV-pumped white-light-emitting diodes (wLEDs).

1. INTRODUCTION In recent years, there has been great attention focused on white-light-emitting diodes (wLEDs) due to their merits of energy saving, high energy efficiency, environmental friendliness, and long operation time compared to conventional incandescent and fluorescent lamps.1−6 The traditional method used to obtain wLEDs is to combine a GaN blue LED chip with yellow phosphor YAG:Ce3+; however, the deficiency of red emission results in low color-rendering index (CRI < 80) and high correlated color temperature (CCT > 7000 K), which are important parameters for its application. In order to improve its luminescence properties, many efforts have been made,7−12 but it is still a challenge. Another useful way to achieve a high CRI includes the mixing of tricolor wLEDs consisting of ultraviolet (UV) LED with red, green, and blue (RGB) phosphors, while the high expenses, the trade-off luminescent efficiency, the fluorescence reabsorption, and the different attenuation situation of tricolor phosphors after being used for a period of time have become the problems. Therefore, to pursue efficient wLEDs, single-component phosphors assembled with UV and near-UV LEDs chips have attracted a large amount of interest for both fundamental research and practical applications. Frequently, the effective way to acquire white light is to utilize the energy transfer from a sensitizer (Eu2+, Ce3+ etc.) to a activator (Mn2+, Tb3+, Dy3+ etc.), which can be well accommodated in numerous hosts, owing to its good color © 2014 American Chemical Society

reproducibility and low costs. Recently, white emission phosphors Ca8MgY(PO4)7:Eu2+,Mn2+,13 tunable full-color emitting BaMg2Al6Si9O30:Eu2+,Tb3+,Mn2+ phosphors,14 singlephase white-emitting Ca8MgGd(PO4)7:Ln3+, Mn2+ (Ln3+ = Ce3+, Tb3+, Dy3+) phosphors,15 Ba9Y2Si6O24:Eu2+,Mn2+ phosphors for near-UV LEDs,16 and other new single-host whitelight phosphors, such as Ca2YF4PO4:Eu2+,Mn2+, Ca9MgNa(PO4)7:Ce3+/Tb3+/Mn2+, Ca5(PO4)2SiO4:Ce3+/Tb3+/Mn2+, and Ca2Ba3(PO4)3Cl:A (A = Eu2+/Ce3+/Dy3+/Tb3+) phosphors,17−20 have been prepared and reported. Some of them give good luminescence performance and have potential application in wLEDs. However, new single-host composition white-light phosphors are still highly to be explored. As an important family of compounds, apatites contain a large amount of inorganic substance with the formula M10(AO4)6X2, where M is a univalent to trivalent cation including Na+, K+, Ca2+, Sr2+, Ba2+, Cd2+, La3+, Gd3+, Y3+ and etc., while A can be P5+, V5+, Si4+, Ge4+ etc., and X often represents F−, Cl− and O2−. This kind of compounds with fluorapatite structure (space group P63/m), which have 9-fold coordinated 4f sites with C3 point symmetry and 7-fold coordinated 6h sites with Cs point symmetry, have been Received: February 25, 2014 Revised: April 11, 2014 Published: April 24, 2014 11026

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demonstrated to be suitable for rare earth ions.21−23 In 2010, Zhang etc.24 adopted simple hydrothermal method to synthesize Ce3+, Mn2+-doped Ca5(PO4)3F and deeply studied its luminescence properties, which contains the emission of CO2− radical impurities. Subsequently, a series of rare-earthdoped luminescent materials with this structure, such as Ca 4Y6(SiO4)6O:Ce3+/Tb3+, Ca6La2‑xEuxNa2(PO4)6F2:Sm3+, Ca5La5(SiO4)3(PO4)3O2:Ce3+, Mn2+, Ca8Gd2(PO4)6O2: A (A = Ce3+/Eu2+/Tb3+/Dy3+/Mn2+) have been well investigated.25−28 However, up to now, Eu2+, Mn2+-activated Ca9Mg(PO4)6F2 (CMPF) phosphors have not been reported. In this work, we successfully synthesized CMPF:Eu2+, Mn2+ phosphors with isostructure of Ca10(PO4)6F2 via high-temperature solid-state reaction method. The occupancy of crystallographic sites of Eu2+ ions and the photoluminescence of asprepared phosphors have been investigated in detail. The energy transfer from Eu2+ to Mn2+ ions and critical distances between them in this host have also been discussed. In general, the tunable emission color from blue to yellow involving white has been realized through adjusting dopant concentration of Mn2+ at fixed Eu2+ content, which shows its promising application in UV-pumped wLEDs.

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. A series of Ca9‑x‑yMg(PO4)6F2:xEu2+,yMn2+ (abbreviated as CMPF:xEu2+,yMn2+; Eu2+ and Mn2+ substitute for Ca2+, where the x and y are mole percent) powder samples were prepared by hightemperature solid-state reaction process. The doping concentrations of Eu2+ and Mn2+ were selected as 1−48 mol % and 2− 38 mol % of Ca2+ in CMPF, respectively. Typically, stoichiometric amounts of CaF2 (A.R.), CaCO3 (A.R.), SiO2 (A.R.), NH4H2PO4 (A.R.), Eu2O3 (99.99%, Shin-Etsu Chemical Co. Ltd., Tokyo, Japan), and MnCO3 (A.R.) were thoroughly mixed in an agate mortar for 40 min with an appropriate amount of ethanol and then dried at 80 °C for 1 h. After being ground for 5 min once again, the powder mixtures were sintered at 1030 °C for 5 h in a reducing atmosphere of H2 (5%) and N2 (95%) to produce the final samples. 2.2. Characterization. The X-ray diffraction (XRD) patterns were performed on a D8 Focus diffractometer at a scanning rate of 10°min−1 in the 2θ range from 10° to 120° with graphite-monochromatized Cu Kα radiation (λ = 0.15405 nm). The photoluminescence (PL) measurements were peformed with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source, and the diffuse reflectance spectra were measured and obtained using a Hitachi U-4100 spectrophotometer with the reflection of black felt (reflection 3%) and white Al2O3 (reflection 100%). The luminescence decay lifetimes were attained from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO) source. PL quantum yields (QYs) of phosphors were obtained directly by the absolute PL quantum yield measurement system (C9920−02, Hamamatsu Photonics K. K., Japan). All the measurements were performed at room temperature (RT).

Figure 1. (a) Rietveld refinement of powder XRD profile of Ca9Mg(PO4)6F2. (b) Crystal structure of Ca9Mg(PO4)6F2 host. (c) Representative XRD patterns of CMPF host and CMPF:Eu2+/Mn2+ samples. (d) Deviation of XRD patterns with different Eu 2+ concentration.

room temperature. The CMPF pure phase has the hexagonal system with space group P63/m and lattice parameters as a = 9.350702 Å, b = 9.350702 Å, c = 6.845183 Å, V = 518.327 Å3. The original structure model and crystallographic data are referred to Ca10(PO4)6F2 host (ICSD #240657). All atom coordinates, fraction factors, as well as thermal vibration

3. RESULTS AND DISCUSSION 3.1. Structure and Phase Purity. Figure 1a shows experimental, calculated and different XRD profiles of Ca9Mg(PO4)7F2 (CMPF) sample using GASA Rietveld refinement at 11027

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parameters were refined with convergence and fitted well the reflection condition, Rp = 3.99%, Rwp = 5.60%, and χ2 = 5.06. It is easily accepted that Mg2+ ions substitute Ca2+ sites with two different crystallographic lattices involving 9-fold-coordinated 4f sites with C3 point symmetry [denoted as Ca2+(1)] and 7fold-coordinated 6h sites with Cs point symmetry [denoted as Ca2+(2)],24 which can be clearly seen from the structure in Figure 1b. However, the casual substitution of one Mg atom to Ca atom can result in the change of lattice parameters and crystal field ligand. According to reports by Sun et al. and Jang et al., crystal field splitting (Dq) can be determined by the following equation:29−31 Dq =

1 2 r4 Ze 5 6 R

(1)

where Dq is a measure of the energy level separation, Z refers to the anion charge, e is the electron charge, r is the radius of the d wave function, and R corresponds to the bond length. Therefore, when Ca2+ is substituted by a smaller Mg2+ ion, the distance between Ca2+ and O2− becomes shorter and leads to magnitude of the crystal field. Figure 1c gives representative XRD patterns of as-synthesized samples containing CMPF host, CMPF:0.18Eu2+, CMPF:0.18Mn2+ and CMPF:0.18Eu2+, 0.18Mn2+ to judge the composition and phase purity of all the samples that were annealed at 1030 °C. All the diffraction peaks can been exactly assigned to pure hexagonal phase of Ca9Mg(PO4)6F2 (with space group P63/m) according to Rietveld refinement result. No other distinct impurities have been found, indicating the introduction of Eu2+ and Mn2+ do not arouse any phase alteration of CMPF. The corresponding substitutions of Eu2+ to Ca2+ and Mn2+ to Mg2+ are intended to be realized for the radii of Ca2+ (r = 1.18 Å for coordination number CN = 9, r = 1.07 Å for CN = 7) and Mg2+ (r = 0.92 Å for CN = 9, r = 0.75 Å for CN = 7) are close to Eu2+ (r = 1.30 Å for CN = 9, r = 1.20 Å for CN = 7) and Mn2+ (r = 0.95 Å for CN = 9, r = 0.70 Å for CN = 7), respectively, as well as the same valence state. With the increase of Eu2+ concentration, the XRD patterns shift to lower angle in Figure 1d, which is in accordance with deduction of Eu2+ occupying Ca2+ sites from ionic radii above. 3.2. Photoluminescence Properties. The photoluminescence emission and excitation spectra of the representative Eu2+ singly doped sample CMPF:0.18Eu2+ have been depicted in Figure 2a. By excited upon both 294 and 365 nm UV radiation, the emission band presents a strong and asymmetric shape which extends from 380 to 580 nm centered at 454 nm, ascribed to the 4f65d1 → 4f7 allowed transition of Eu2+. Considering two kinds of Ca2+ sites, we obtain two symmetric bands via Gaussian fit with the maximum emission intensity wavelength located at 450 and 485 nm, respectively. According to the earlier discussion by Van Uitert, the emission wavelength position of Eu2+ ions is strongly dependent on its local environment, which can be evaluated by following equation:32,33 ⎡ ⎤ ⎛ V ⎞1/ V E = Q ⎢1 − ⎜ ⎟ 10−n × Ea × r /80⎥ ⎝4⎠ ⎢⎣ ⎥⎦

Figure 2. (a) Photoluminescence emission and excitation spectra of CMPF:0.18Eu2+ sample, inset is the photograph excited under 365 nm UV lamp. (b) Decay curves of CMPF:0.18Eu2+ phosphor (monitored at 405 and 520 nm and excited at 300 nm).

the “active” cation, Ea is the electron affinity of the atoms that form anions (eV), which is different when Eu2+ is introduced into different anion complexes with different coordination numbers and is a constant for the same host, r is the radius of the host cation (Ca2+) replaced by the “active” cation (Eu2+). From the equation, we can readily deduced that the value of E is proportional to the quantity of n and r. On the basis of the above effective ionic radii of Ca2+, we can conclude that the band peaked at 450 nm can be attributed to the 4f65d1 → 4f7 transition of Eu2+ ions occupying the Ca(1) sites with ninecoordination, while the other band attributes to Eu2+ ions replacing Ca(2) ions with seven-coordination. The corresponding lighting photograph of sample excited under 365 nm UV lamp is shown in Figure 2a, which is in agreement with its emission spectrum. Monitored at 454 nm, the unresolved broad excitation spectrum consists of a broad band from 200 to 420 nm peaked at 250, 294, 365 nm originated from Eu2+ different excited states, indicating that it can be well pumped by UV LED chips. While monitored at 420 and 495 nm, the excitation spectra have the shape differences, which may be ascribed to the different crystal lattice sites of Eu2+ ions. In order to further certify the existence of two kinds of Eu2+ emission centers, the decay curves of CMPF:0.18Eu2+ monitored at 405 and 520 nm are shown in Figure 2b. The lifetime can be assessed by the simple equation:34

(2)

τ=

where E is the position of the d-band edge in energy for the rare-earth ion (cm−1), Q represents the position in energy for the lower d-band edge for the free ion (Q = 34000 cm−1 for Eu2+), V is the valence of the “active” cation (V = 2 for Eu2+), n refers to the number of anions in the immediate shell around

∫0

∞

I (t ) d t

(3)

where τ is calculated lifetime value, and I(t) is the normalized intensity of emission spectra. The lifetimes are evaluated to be 726.46 and 393.31 ns monitored at 405 and 520 nm, 11028

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respectively. The obvious differences in fluorescent lifetime indicate two kinds of Eu2+ emission centers. Figure 3 demonstrates the diffuse reflectance spectra of CMPF host and CMPF: xEu2+ phosphors with different x. The

between the excitation and emission spectra of activator and sensitizer. As far as this kind of phosphors is concerned, this mechanism hardly takes place. Therefore, the nonradiative energy transfer mode among different Eu2+ ions in this host is considered to be an exchange coupling or via a multipolar interaction, which can be examined by following expression proposed by Van Uitert, Ozawa, and Jaffe35−40 I = [1 + β(x)θ /3 ]−1 (4) x where I is the emission intensity, x is the activator ion concentration, and β is a constant for the same host crystal under the same excitation conditions. The type of nonradiative energy transfer can be estimated by analyzing the constant θ from this formula. The value of θ is 3, 6, 8, 10, corresponding to energy transfer mechanism of exchange coupling, electric dipole−dipole, dipole−quadrupole or quadrupole−quadrupole interactions, respectively. According to the formula, the linear fitting of log(x) versus log(I/x) for the different peaks at 450 and 485 nm, as exhibited in Figure 5, shows the slope of two

Figure 3. Reflection spectra of the CMPF:xEu2+ (x = 0−0.48) samples.

CMPF host has an energy absorption band ranging from 270 to 325 nm. When Eu2+ ions are introduced into host, strong broad absorption spectra from 240 to 430 nm in UV region appear, originating from 4f7−4f65d1 absorption of Eu2+, which are different from CMPF host. The constantly increasing intensity in these phosphors with increasing Eu2+ concentration further indicates that the absorption derives from Eu2+ ions. Figure 4a shows the dependence of Eu2+ emission intensity (the total and the two Gaussian fitting bands) excited at 294 nm on Eu2+ concentration. With gradual increase of Eu2+ doping concentration, we can see that the emission intensity of Eu2+ rises first until the maximum x at 0.18 and then drops, which can be explained by the increase of activator centers and then the appearance of concentration quenching effect when the distance between activators is close enough at high Eu2+ content, respectively. The variations of emission intensity of two Gauss fitting bands are presented in Figure 4b, which are consistent with that of the total band. Concentration quenching mainly occurs because of the energy transfer among Eu2+ ions in the hosts. Generally, the nonradiative energy transfer is dominant from one Eu2+ ion to another, including reabsorption of radiation, an exchange coupling, or via a multipolar interaction. The reabsorption of radiation can predominate only when there is a broad overlap

Figure 5. Linear fitting of log(x) versus log(I/x) for the different peaks at 450 and 485 nm in various Eu2+ concentration CMPF phosphors beyond the concentration quenching.

fitting line are −1.919 and −1.645, respectively. Hence, the value of θ are calculated to be 5.757 and 4.935, approximated as 6,39,40 indicating that the dominant concentration quenching mechanisms of Eu2+ in two emission centers in the CMPF are both dipole−dipole interactions. In order to investigate the concentration quenching phenomenon, the critical distance can be evaluated by the equation suggested by Blasse:41

Figure 4. (a) Variation of emission spectra for CMPF:xEu2+ samples excited at 294 nm. (b) Dependence of emission intensities of two Gauss fitting bands on Eu2+ content in CMPF:xEu2+ samples. 11029

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⎡ 3V ⎤1/3 Rc ≈ 2⎢ ⎥ ⎣ 4πXcN ⎦

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color can be obtained via adjusting Mn2+ concentration. And the emission intensity of Eu2+ conspicuously decreases compared to that in Figure 6a, and emission intensity of Mn2+ enhances evidently relative to that in Figure 6b, which also testifies the energy transfer from Eu2+ to Mn2+ ions. The energy transfer between Eu2+ to Mn2+ ions can be further proved by fluorescent decay curves of Eu2+ (monitored at 454 nm and excited at 300 nm) in different Mn2+ doping concentration samples of CMPF:0.18Eu2+,yMn2+(y = 0, 0.02, 0.06, 0.10, 0.18, 0.26). The representative calculated results of lifetimes using above equation3 decreases from 495.93 to 274.36 ns, corresponding to y = 0 to 0.26, which can be acquired in Figure 7, respectively. Decrease of Eu2+ lifetime with increase of Mn2+ concentration provides a further confirmation of energy transfer from Eu2+ to Mn2+ ions.

(5)

where V is the volume of the unit cell, N is the number of host cations in the unit cell, and Xc is the critical concentration of dopant ions. For the GMPF host, N = 1, V = 518.327 Å3, and Xc is 18% for Eu2+. Accordingly, the critical distance (Rc) was estimated to be about 17.65 Å. For the electric dipole−dipole mechanism, the Rc for energy transfer between Eu2+ also can be expressed by42 Rc = 0.63 × 1028Q A /E 4

∫ fS (E)FA(E) dE

(6)

−16

where QA = 4.8 × 10 fd is the absorption coefficient of Eu2+, fd is the oscillator strength of the transition, which is taken as 0.01 for Eu2+. E (in eV) is the maximum energy of spectral overlap as 3.076 eV. ∫ f S(E) fA(E) dE represents the spectral overlap between the normalized profiles of the Eu2+ emission f S(E) and excitation fA(E), respectively, which is determined to be 0.065 eV−1. Therefore, the calculated value Rc is about 16.73 Å. This result further confirms that the energy transfer mechanism between Eu 2+ occurs via a dipole−dipole interaction. 3.3. Energy Transfer in Eu2+, Mn2+ Codoped Phosphors. Parts a and b of Figure 6 show the excitation and

Figure 7. Decay curves and lifetimes of Eu2+ in representative samples CMPF:0.18Eu2+,yMn2+ (monitored at 454 nm excited at 300 nm).

Figure 8a displays the dependence of emission intensity excited at 365 nm on different Mn2+ doping concentration in CMPF:0.18Eu2+,yMn2+ (y = 0−0.38) phosphors. The emission spectra shows two broad bands centered at 454 and 565 nm, which corresponds to the emission of Eu2+ and Mn2+, respectively. Therefore, the tunable emission color of phosphors can be obtained via trimming the ratio of Eu2+ and Mn2+ doping concentration. With increasing of Mn2+ doping content y, the relative emission intensity of Eu2+ ion monotonously descends, while the emission intensity of Mn2+ ion first increases gradually until y = 0.34 and then decreases, which can be clearly seen from Figure 8b. The drop of emission intensity of Eu2+ ion and first raise of Mn2+ ion can be attributed to the energy transfer from Eu2+ to Mn2+ ions. Subsequently, the decrease of Mn2+ emission intensity may be ascribed to concentration quenching between Mn2+ ions when y > 0.34, which often occurs in many other hosts.43−46 The CIE chromaticity coordinates and quantum yields (QYs) for CMPF:0.18Eu2+,yMn2+ (y = 0, 0.02, 0.06, 0.10, 0.18, 0.26, 0.34, 0.38) phosphors excited under 365 and 294 nm UV, respectively, are presented in Table 1. The variety of the CIE chromaticity coordinates can be seen from A to H with increase of Mn2+ doping concentration in Figure 9. It can be found that the tunable color occurs with increasing Mn2+ concentration y from 0 to 0.38. This is also easily observed on the right of Figure 9, where the photographs of luminescent phosphors excited under a 365 nm UV lamp are displayed. With the increasing of Mn2+ concentration, the absolute quantum yield for the samples keeps at about 20%. Moreover, the QY can be improved by controlling the particle size,

Figure 6. Excitation and emission spectra of CMPF:0.18Eu2+ (a), CMPF:0.18Mn2+(b), and CMPF:0.18Eu2+,0.18Mn2+ (c) phosphors. The corresponding spectral overlap is also presented in the color area.

emission spectra of CMPF:0.18Eu2+ and CMPF:0.18Mn2+ samples, respectively. The emission spectrum of CMPF:0.18Mn2+ presents a broad band extending from 500 to 650 nm excited at 404 nm, peaking at 565 nm. However, the emission intensity is much weaker than that of CMPF:0.18Eu2+ excited at 294 nm in Figure 6a, which is ascribed to the 4 T1g(G)−6A1g(S) spin-forbidden transition of Mn2+ ions. The excitation spectrum monitored at 565 nm consists of several peaks at 280, 335, 404, 495 nm, corresponding to the transitions of Mn2+ ion from 6A1g(S) to 4Eg(D), 4T2g(D), [4A1g, 4E1g(G)], and 4T1g(G) excited levels, respectively. The overlap of Eu2+ emission and Mn2+ excitation spectra can be readily observed from the color area in Figure 6, parts a and b, which demonstrates the possibility of resonance type energy transfer from Eu2+ to Mn2+ ions in this host. This can be further verified by the similar excitation spectra monitored at 454 and 565 nm except for the intensity in CMPF:0.18Eu2+,0.18Mn2+ phosphor from Figure 6c. The emission spectrum covers the emission band of Eu2+ and Mn2+ ions, therefore, the tunable 11030

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Figure 9. CIE chromaticity coordination of CMPF:0.18Eu2+,yMn2+ samples (A−H corresponds to y = 0, 0.02, 0.06, 0.10, 0.18, 0.26, 0.34, 0.38, respectively.). The luminescence photographs of corresponding phosphors excited under a 365 nm UV lamp are shown on the right of the picture.

Figure 8. (a) Variation of emission intensity for CMPF:0.18Eu2+,yMn2+ (y = 0−0.38) phosphors on Mn2+ doping content excited at 365 nm. (b) Relative emission intensity of Eu2+ and Mn2+ in CMPF:0.18Eu2+,yMn2+ phosphors.

Table 1. Variation of CIE Chromaticity Coordinates (x, y) and Quantum Yields (QYs) for CMPF:0.18Eu2+,yMn2+ Phosphors Excited under 365 and 294 nm UV, Respectively sample no. Mn2+ concentration (y mol) A B C D E F G H

0 0.02 0.06 0.10 0.18 0.26 0.34 0.38

CIE coordinates (x, y) (0.155, (0.176, (0.197, (0.209, (0.255, (0.284, (0.308, (0.338,

0.121) 0.161) 0.196) 0.216) 0.290) 0.334) 0.368) 0.409)

QY (%) 18 14 18 19 22 21 18 21

Figure 10. Dependence of energy efficiency, ηT on Mn2+ doping concentration (y mol).

According to the Dexter’s energy transfer expression of multipolar interaction and Reisfeld’s approximation, the energy transfer mechanism from Eu2+ to Mn2+ ions in this host should occur via electric multipole-multipole interaction. The following relation can be adopted:49,50 η0 ∞C α /3 η (8)

morphology, and crystalline defects via optimizing the synthesis processing and chemical composition. With regard to this host, the energy transfer efficiency ηT from Eu2+ to Mn2+ ions in CMPF:0.18Eu2+,yMn2+ phosphors can be expressed through the following equation:47,48 I ηT = 1 − S IS0 (7)

where η0 and η are the luminescence quantum efficiencies of Eu2+ in the absence and presence of Mn2+, respectively; C is the concentration of Mn2+; and α = 6, 8, and 10 corresponds to dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole interactions, respectively. In order to conduct simple assess, the value of η0/η can be approximately estimated from the luminescence intensity ratio (IS0/IS) as follows: IS0 ∞C α /3 IS (9)

where ηT is the energy transfer efficiency and IS0 and IS are the luminescence intensity of Eu2+ ions in the absence and presence of Mn2+ ions, respectively. The trend of decreasing emission intensity of Eu2+ and increasing emission intensity of Mn2+ have been shown in Figure 8. As seen from Figure 10, the energy transfer efficiency monotonously ascends with continuous increase in Mn2+ concentration while the increasing rate decreases because the fixed Eu2+ concentration restricts the energy transfer from Eu2+ ions to Mn2+ ions. 11031

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where IS0 and IS are emission intensity of Eu2+ in the absence and presence of Mn2+, respectively. The least-squares fittings of IS0/IS versus CMn2+α/3 (α = 6, 8, 10 respectively), as exhibited in Figure 11, show that the linear dependence of the electric

blue to yellow can be realized by adjusting concentration variation of Eu2+ and Mn2+. The energy transfer mechanism between them is demonstrated to be dipole−quadrupole interaction, and the critical distance is calculated to be 14.90 Å via spectral overlap method, which agrees well with that of 14.57 Å obtained through concentration quenching way. As depicted, Eu2+-, Mn2+-activated CMPF phosphors can be potentially applied for UV-pumped wLEDs.

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AUTHOR INFORMATION

Corresponding Author

*(J.L.) E-mail: [email protected]. Telephone: +86-431-85698041. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by the National Natural Science Foundation of China (NSFC 51332008, 51172227, 21221061), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (U1301242), and the National Basic Research Program of China (2010CB327704, 2014CB643803).

Figure 11. Dependence of IS0/IS of Eu2+ on(a), CMn2+8/3 (b), and CMn2+10/3 (c).

dipole−quadrupole interaction is the best one among the fitting results, which implies that energy transfer from Eu2+ to Mn2+ ions in the Ca9Mg(PO4)7F2 host is dominated by the electric dipole−quadrupole interaction. The energy transfer critical distance(Rc) between Eu2+ and Mn2+ ions can be calculated by concentration quenching method using eq 5, in which the Xc, the total concentration of Eu2+ and Mn2+, where the luminescence intensity of Eu2+ is half of that in the absence of Mn2+, is different from abovementioned in Eu2+ single doped samples and considered to be 0.32 here. Consquently, the Rc value is evaluated to be 14.57 Å, which also can be obtained through spectral overlap way, and the expression can be given by following equation:51,52 Rc 8 = 3.024 × 1012λS 2fq

∫ fS (E)fA (E)/E 4 dE

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REFERENCES

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where fq = 10−10 is the oscillator strength of Mn2+ dipole and quadrupole electrical absorption transitions; λS = 4540 Å is the wavelength of strongest intensity of Eu2+; E is the energy involved in the transfer (in eV); ∫ f S (E) fA (E)/E4 dE represents the spectral overlap between the normalized shapes of the Eu2+ emission f S(E) and the Mn2+ excitation fA(E), and in our case it is calculated to be about 0.40038 eV−1. Accordingly, the Rc value is calculated to be 14.90 Å, which is almost consistent with that obtained above by concentration method. This can further certify the energy transfer mechanism from Eu2+ to Mn2+ ions in this host is electric dipole− quadrupole interaction.

4. CONCLUSION In summary, Eu2+-, Mn2+-activated Ca9Mg(PO4)6F2 (CMPF) phosphors have been successfully prepared via high-temperature solid-state reaction process. In Eu2+ singly doped samples, the energy transfer mechanisms of two Eu2+ ions emission centers are confirmed to be both dipole−dipole interactions. The optimum concentration of Eu2+ is 18 mol %. For Eu2+, Mn2+ codoped phosphors, energy transfer from Eu2+ to Mn2+ ions can be deduced by overlap of Eu2+ emission and Mn2+ excitation spectra and demonstrated by the decrease of Eu2+ fluorescence decay lifetimes with increasing Mn2+ concentration, under UV radiation. The tunable emission color from 11032

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