Eu2+ Phosphors with High

Article pubs.acs.org/JPCC

Blue Emitting Ca8La2(PO4)6O2:Ce3+/Eu2+ Phosphors with High Color Purity and Brightness for White LED: Soft-Chemical Synthesis, Luminescence, and Energy Transfer Properties Mengmeng Shang,†,‡ Guogang Li,†,‡ Dongling Geng,†,‡ Dongmei Yang,†,‡ Xiaojiao Kang,†,‡ 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 ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Ce3+ and/or Eu2+ activated Ca8La2(PO4)6O2 (CLPA) oxyapatite blue phosphors have been prepared via a Pechini-type sol−gel process. X-ray diffraction (XRD), photoluminescence (PL) spectra, absolute quantum yield, as well as lifetimes were utilized to characterize samples. The emission of Ce3+ and Eu2+ ions at different lattice sites has been identified and discussed. The CLPA:0.04Ce3+ phosphor exhibits bright blue emission with higher quantum yield (67%) and excellent CIE coordinates (x = 0.160, y = 0.115) under UV excitation, and the CLPA:0.05Eu2+ phosphor also exhibits blue emission with CIE coordinates (0.187, 0.164). The energy transfer from Ce3+ to Eu2+ in CLPA:Ce3+/Eu2+ phosphors has been validated and demonstrated to be a resonant type via a dipole−dipole mechanism. The critical distance (Rc) of Ce3+ to Eu2+ ions in CLPA was calculated (by the spectral overlap method) to be 26.67 Å. The quantum yields of Ce3+ and Eu2+ coactivated CLPA phosphors are enhanced compared with that of Eu2+ activated samples due to energy transfer. The CIE coordinates of CLPA:0.04Ce3+, 0.02Eu2+ are (0.179, 0.169). The corresponding luminescence and energy transfer mechanisms have been proposed in detail. These blue phosphors might be promising for use in pc-white LEDs.

1. INTRODUCTION The past decade has seen a rapid evolution of the GaN-based light-emitting diode (LED) technology, especially focused on developing advanced solid-state lighting sources.1,2 Compared with the incandescent or fluorescent lamps, LED-based lighting can provide significant power saving, longer lifetime, higher luminous efficiency, and brightness, etc. At present, two approaches can be followed to obtain a white LED.3 On the one hand, a combination of (at least) three (red, green, blue) LEDs, namely, RGB-LEDs, with power ratios adjusted to obtain white light with a specific color temperature, which requires complicated electronics and limits its application.4,5 On the other hand, a single LED can be used in combination with one or more phosphor materials to partially or fully convert the LED emission, viz., phosphor converted LEDs (pcLEDs). The emission color of pc-LEDs only depends on the © 2012 American Chemical Society

phosphors, and so the pc-LEDs has the advantages of stable color, good color reducibility, and higher color rendering index compared with RGB-LEDs. Most of the commercially available LED-based white light sources rely on the second approach (pc-LEDs). Until recently, these are almost solely based on the combination of a blue LED and a YAG:Ce3+-based yellowemitting phosphor.6−8 However, it exhibits poor color rendering index (CRI) and high correlated color temperature due to the deficiency of red emission. One solution to this problem is to make LEDs by coating a near-ultraviolet (n-UV) emitting LED with a mixture of blue, green, and red emitting phosphors, which exhibits smoother spectral distribution over Received: March 8, 2012 Revised: April 18, 2012 Published: April 20, 2012 10222

dx.doi.org/10.1021/jp302252k | J. Phys. Chem. C 2012, 116, 10222−10231

The Journal of Physical Chemistry C

Article

earth-metal silicate is too high (thus difficult to prepare). In addition, to the best of our knowledge, the ternary rare-earthmetal silicate luminescence materials were usually prepared with a high temperature solid-state reaction, which can result in the formation of defects and then reduction of the luminescence efficiency. Apatite type rare earth phosphate Ca8La2(PO4)6O2 (CLPA) has crystal field parameters similar to its corresponding silicate, and the synthetic temperature is significantly lower. To our knowledge, up to now, no research has been carried out with Ca8La2(PO4)6O2 (CLPA) as a host doped with Ce3+ and Eu2+ ions. In the present work, we reported the synthesis of Ce3+ and/or Eu2+ doped CLPA phosphors using a Pechini-type sol−gel process (soft chemical synthesis) for the first time and determined the effect of the temperature on the formation of a pure CLPA phase. Their luminescence properties were investigated in detail. A blue light emission was observed both in Ce3+ and Eu2+ doped CLPA phosphors due to their respective 5d−4f and 4f65d1−4f7 transitions. The emission of Ce3+ and Eu2+ ions at different crystallographic sites was investigated. The absolute quantum yield of Ce3+ doped CLPA luminescence material can reach 67%. Moreover, the energy transfer properties from Ce3+ to Eu2+ under UV excitation were investigated and the critical distance (Rc) was calculated by the spectral overlap method. The corresponding luminescence mechanisms have been proposed.

the whole visible range and therefore can obtain high quality white light.9,10 In this case, a high efficient blue emitting phosphor is essential. The broad band emitting rare earth ions Ce3+ and Eu2+ are two important activators for luminescent materials, which have been studied extensively.11−14 This is primarily because of their unique emission properties, combining a broad emission spectrum (leading to good color rendering properties), relatively small Stokes shift (allowing excitation in the nearUV or blue part of the spectrum), and short decay times (avoiding saturation). Depending on the host material, high quantum efficiencies in combination with a good thermal quenching behavior can be obtained. Furthermore, the emission spectrum can be tuned from the near-UV to deep red, by appropriately choosing the host compounds. For Ce3+ ion, it is a lanthanide ion with the simplest electronic configuration, i.e., it has a 4f15d0 ground state and a 4f05d1 excited state and therefore shows typical 4f−5d transitions. The 4f ground state shows a spin orbit splitting into two levels (2F5/2 and 2F7/2), with an energy separation of about 2000 cm−1. Because the 4f−5d transitions are parity allowed, they have a large absorption cross section and appear as intense bands in spectra. Hence, the luminescent materials doped with Ce3+ ions can efficiently absorb the excitation energy. Furthermore, the 5d states of Ce3+ are outer orbitals, and the coordination surroundings have a prominent influence on their energies, so the 4f−5d transitions of Ce3+ ions appear in a large wavelength range that deeply depends on the host lattice. The position of its absorption and emission bands can be controlled by selecting a suitable matrix.15 Moreover, the Ce3+ ion also acts as a good sensitizer, transferring a part of its energy to activator ions.16 Upon doping with Eu2+, similar luminescence properties can be obtained, with the main difference being that the 4f7 ground state is a single level (8S7/2); i.e., no spin orbit splitting is present and the emission spectrum is principle characterized by a single emission band, with a typical fwhm in the range from 50 to 100 nm.17 In some compounds, multiple emission bands are present, which can lead to a broadening of the emission spectrum. This effect is observed in cases where the Eu-ions are incorporated in lattice sites with different symmetry and/or a clearly different distance with the nearest neighbor ions. Thus, the choice of host is a critical parameter in determining the optical properties of Eu2+ ions. The above characteristics of Ce3+ and Eu2+ ions meet the LED application requirements and make Ce3+ and Eu2+ become the appropriate activators in phosphor converted-LEDs (pc-LEDs).18 It is well-known that the compounds with oxyapatite structure (space group P63/m) have been effective host lattices for luminescent materials due to their applications in the medical field and solid-state lighting industry.19 Among the many synthetic oxyapatites, the ternary rare-earth-metal oxyapatite family are of practical interest as efficient host materials for the luminescence of various rare earth ions and mercury-like ions because of their high crystal field and excellent thermal, mechanical, and chemical properties.20 This oxyapatite host lattice contains two cationic sites, that is, the 9fold coordinated 4f sites with C3 point symmetry and 7-fold coordinated 6h sites with CS point symmetry. Both sites are suitable and easily accommodate a great variety of RE3+. However, most researchers focus on the investigation on the ternary rare-earth-metal silicate with the oxyapatite structure of the form M2Ln8(SiO4)6O2 (M = Ca, Mg, Sr; Ln = Y, Gd, La).19,20 However, the synthetic temperature of ternary rare-

2. EXPERIMENTAL SECTION 2.1. Preparation. The Ca8−yLa2−x(PO4)6O2:xCe3+, yEu2+ (abbreviated as CLPA:xCe3+, yEu2+, 0 ≤ x, y ≤ 0.1) samples were all prepared by the Pechini-type sol−gel method.21 The doping concentrations of Ce3+ and Eu2+ are 0−10 mol % La3+ or Ca2+ in Ca8La2(PO4)6O2. Stoichiometric amounts of La2O3, Eu2O3, and/or Ce2(CO3)3 (all 99.99%, Science and Technology Parent Company of Changchun Institute of Applied Chemistry) were dissolved in dilute nitric acid (HNO3) under stirring and heating, resulting in the formation of a colorless solution of Ln(NO3)3 (Ln = La, Eu, or Ce). Then, stoichiometric amounts of Ln(NO3)3 solution and Ca(NO3)2·4H2O were mixed in deionized water under stirring. The citric acid was dissolved in the above solution (citric acid/ metal ion = 2:1 in moles). The pH of the solution was adjusted to a low value with HNO3 followed by the addition of a stoichiometric amount of (NH4)H2PO4 (99%, A.R.). Finally, a certain amount of polyethylene glycol (PEG, molecular weight = 10000, A.R.) was added as a cross-linking agent. The citric acid is used to form stable metal complexes, and its polyesterification with a polyhydroxy alcohol (PEG) forms a polymeric resin. Immobilization of metal complexes in such rigid organic polymer networks reduces segregation of particular metal ions, ensuring compositional homogeneity.21 The resultant mixtures were stirred for 1 h and heated at 75 °C in a water bath until homogeneous gels formed. After being dried in an oven at 110 °C for 10 h, the gels were ground and prefired at 500 °C in air for 4 h. Then, the mixtures were fully ground and fired to the desired temperatures (1000−1300 °C) in air for 4 h. The obtained white powders were then reduced at 1000 °C for 3 h under a 10% H2/90% N2 atmosphere to produce the final samples. 2.2. Characterization. X-ray diffraction (XRD) was performed on a D8 Focus diffractometer at a scanning rate of 10° min−1 in the 2θ range from 10 to 80° with graphitemonochromatized Cu Ka radiation (λ = 0.15405 nm). The 10223



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morphology of the samples was inspected using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi). Transmission electron microscopy (TEM) images were recorded using a FEI Tecnai G2 S-Twin with a field-emission gun operating at 200 kV. The photoluminescence (PL) measurements were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Photoluminescence quantum yields (QY) were measured directly by the absolute PL quantum yield (internal quantum efficiency) measurement system (C9920-02, Hamamatsu Photonics K. K., Japan), which comprises an excitation light source of Xe lamp, monochromator, an integrating sphere capable of nitrogen gas flow, and a CCD spectrometer for detecting the whole spectral range simultaneously. The luminescence decay curves were obtained 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). All of the measurements were performed at room temperature (RT).

were all annealed at 1200 °C in air for 4 h and reduced at 1000 °C for 3 h under a 10% H2/90% N2 atmosphere. It is obvious that all the diffraction peaks of these samples can be exactly assigned to the pure hexagonal phase of Ca8La2(PO4)6O2 [space group: P63/m (176)] according to JCPDS file 330287. No other phase or impurity can be detected, indicating that the Ce3+ and Eu2+ ions were completely dissolved in the Ca8La2(PO4)6O2 host without inducing significant changes of the crystal structure. The as-prepared CLPA sample has cell parameters of a = 9.4868 Å, c = 6.9335 Å, V = 540.41 Å3, and Z = 1. The XRD patterns for CLPA samples doped with different Ce3+ and/or Eu2+ concentrations are similar to the above results in Figure 1. The ternary rare-earth-metal phosphate Ca 8 La 2 (PO 4 ) 6 O 2 is isostructural to natural oxyapatite Ca10(PO4)6F2, which has a hexagonal space group P63/m. As we know, the structure of oxyapatite is characterized by the presence of two cationic sites labeled M(I) and M(II) with the local symmetry C3 and CS, respectively.25 The crystal structure of CLPA, shown in Figure 2, also indicates that a cation located

3. RESULTS AND DISCUSSION 3.1. Crystallization Behavior, Structure, and Morphology. The crystallization behavior, composition, and phase purity of the as-prepared powder samples were first examined by XRD. Figure S1 (Supporting Information) shows the XRD patterns of the as-prepared samples annealed at various temperatures (1000−1300 °C) for 4 h. For the samples annealed at 1000 °C (a) and 1100 °C (b), an impurity phase is observed, which can be indexed as β-Ca2P2O7. However, the peak intensity of the impurity phase decreases with the increase of annealing temperature and a single phase was obtained after being annealed at 1200−1300 °C (Figure S1-c,d in the Supporting Information). All the diffraction peaks of the asprepared samples annealed at 1200 °C (or higher temperature) can be assigned exactly to the standard data of Ca8La2(PO4)6O2 (JCPDS No. 33-0287). Accordingly, the crystallization temperature of CLPA via the current sol−gel process is lower than that in the conventional solid-state reaction (T ≥ 1400 °C).22−24 The crystallinity of the CLPA phase increased upon increasing annealing temperatures, which can be confirmed by the change in the full width at half-maximum (fwhm). Figure 1 shows the representative XRD patterns of pure CLPA and Ce3+ or Eu2+ ion activated CLPA samples, which

Figure 2. (a) Crystal structure of Ca8La2(PO4)6O2 along the c-axis direction. (b) The 6h-O polyhedra along the a-axis direction. (c) The 4f-O polyhedra along the a axis direction.

Figure 1. The XRD patterns of (a) CLPA, (b) CLPA:0.04Ce3+, (c) CLPA:0.05Eu2+, and (d) CLPA:0.04Ce3+, 0.06Eu2+ samples. The standard data of Ca8La2(PO4)6O2 (JCPDS No. 33-0287) is shown as a reference.

at the 4f site (C3) and its surrounding nine oxygen atoms form a tetrakaidecahedron, and these tetrakaidecahedron are connected by tetrahedral PO4 groups, while the cations located at the 6h site (CS) form decahedra with the surrounding seven oxygen atoms, and these decahedra connect each other through sharing plane, edge, and vertex. Finally, the tetrakaidecahedron and decahedron are connected through tetrahedral PO4 groups and through sharing plane, edge, and vertex. However, in fact, there are three cationic sites in phosphate apatites CLPA reported by Cohen-Adad et al., who used Eu3+ structural probe luminescence spectroscopy to show muticenters.23 Three types of Eu3+ sites have been detected from 5D0 → 7F0 transitions from both Ca2+ and La3+ substitutions due to similar ionic radii with those of Eu3+ ion and based on the existence of the two kinds of cationic sites labeled M(I) and M(II), corresponding to the 4f and 6h positions of the M10(PO4)6O2 oxyapatite structure in the hexagonal system. M(I) has trigonal point symmetry (C3) with nine oxygen anions. The second site M(II) 10224



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with CS local symmetry is in 7-fold coordination with six oxygen atoms and another O2− anion. In Ca8La2(PO4)6O2 host, it has been shown that the M(II) positions with CS symmetry could be occupied by 2 La3+ and 4 Ca2+ ions and then the M(I) sites are occupied by 4 Ca2+ cations in C3 symmetry.23 Thus, these three cationic sites are marked for Ca(I), Ca(II), and La(II) to distinguish each other, in which Ca(I) is ninecoordinated and Ca(II) and La(II) are seven-coordinated. The structure and effective ionic radii for the given coordination number (CN) are listed in Table 1.26 In view of the similar ion radius and valence, the Ce3+ and Eu2+ ions are expected to substitute the La3+ and Ca2+ sites in the Ca8La2(PO4)6O2 crystal structure, respectively. Table 1. Structure Parameters of Ca8La2(PO4)6O2 and Ionic Radii (Å) for Given Coordination Number (CN) of La3+, Ce3+, Ca2+, and Eu2+ Ions ionic radius (IR) (Å) ion

space group

La3+ Ce3+ Ca2+ Eu2+

hexagonal P63/m (176)

sites

symmetry

6h

CS

6h/4f

CS/C3

CN = 9 CN = 7 1.216 1.196 1.18 1.30

1.10 1.07 1.06 1.20

Figure 3 presents the SEM and TEM images of the CLPA:Ce3+, Eu2+ sample. The morphology for the CLPA:Ce3+

Figure 4. Typical photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra of the CLPA:0.04Ce3+ sample: (a) λex = 310 nm and λem = 415 nm; (b) λex = 274 nm and λem = 339 nm. The inset shows the digital luminescence photo of the CLPA:0.04Ce3+ sample excited at 254 nm in a UV box.

luminescence photograph under the excitation of the 254 nm UV lamp. Under 310 nm UV-light irradiation, CLPA:0.04Ce3+ shows a strong blue emission (Figure 4a, blue line) due to the 5d1 → 4f1 transition of Ce3+ consisting of a strong broad band (350−550 nm) with a maximum at 415 nm and a weak band from 320 to 340 nm. The corresponding excitation spectrum includes three absorption bands marked A (∼310 nm), B (∼280 nm), and C (∼254 nm), and the peak at 310 nm is the strongest. However, the excitation spectrum (Figure 4b, black line) recorded by monitoring the emission of 339 nm (weak band in the PL spectrum of Figure 4a) contains four obvious absorption bands at 240, 257, 274, and 310 nm and the peak at 274 nm becomes the strongest one. Upon excitation with 274 nm UV, the PL spectrum of CLPA:0.04Ce3+ consists of three bands, as shown in Figure 4b (blue line), which is obviously different from Figure 4a. Moreover, the PL intensity of CLPA:0.04Ce3+ at 322 and 339 nm had an extreme enhancement with respect to that at 415 nm. It is wellknown that the Ce3+ emission should be composed of a double band in view of the splitting of its ground state. The energy difference of this splitting between 2F7/2 and 2F5/2 of Ce3+ is about 2000 cm−1. However, the energy difference between 415 and 339 nm, 339 and 322 nm in the CLPA host is about 5402 and 1557 cm−1, respectively, which are far from 2000 cm−1. Therefore, all the three emission bands cannot be ascribed to the ground-state splitting of the single Ce3+ emission center.

Figure 3. (A) SEM image, (B) TEM image, and (C) HRTEM image of the CLPA:Ce3+, Eu2+ sample.

and CLPA:Eu2+ samples is similar to that of the CLPA:Ce3+, Eu2+ sample (Figure S2, Supporting Information). At first sight, as shown in Figure 3, it seems that the sample is composed of particles with sizes ranging from 1 to 2 μm. However, a detailed examination of the enlarged SEM micrograph from a selected region (the inset of Figure 3A) and TEM image (Figure 3B) indicate that each particle of the sample consists of many smaller crystallites (50−100 nm). The results for other samples are similar to those of the CLPA:Ce3+, Eu2+ sample and will not be shown here. The serious aggregated crystallites are attributed to the high temperature annealing process. The fine structures of CLPA:Ce3+, Eu2+ were studied by the HRTEM technique. Figure 3C is its corresponding HRTEM image, in which the lattice fringes with a d spacing of 0.29 nm correspond to the distance of the (211) plane of CLPA. The result further confirms the presence of a highly crystalline Ca8La2(PO4)6O2 phase after being annealed at 1200 °C, agreeing well with the XRD results. 3.2. Photoluminescence Properties of CLPA:xCe3+ Phosphors. Figure 4 shows the PLE and PL spectra of pure CLPA:0.04Ce3+ monitored with different wavelengths and its 10225



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Considering the result that different excitation wavelengths yield different emission spectra, we can deduce that three types of Ce3+ luminescent centers exist in the Ca8La2(PO4)6O2 host. This is related to the crystal structure of Ca8La2(PO4)6O2, which provides three different sites for the cations in it as discussed earlier, i.e., Ca(I) located at the 4f (C3) site and Ca(II) and La(II) at the 6h (CS) site.23 According to Uitert’s report, the emission position of the Ce3+ ion is strongly dependent on its local environment, which has been suggested to obey an empirical relation between the energetic position of the Ce3+ emission and the local structure in various compounds by Van Uitert as obeying27 ⎡ ⎤ ⎛ V ⎞1/ V × 10−(nEar)/80⎥ E (cm−1) = Q *⎢1 − ⎜ ⎟ ⎝4⎠ ⎢⎣ ⎥⎦

(1)

Here E is the position for the Ce3+ ion emission peak, Q* is the position in energy for the lower d-band edge for the free Ce3+ ion (Q* = 50000 cm−1), V is the valence of the Ce3+ ion (V = 3), n is the number of anions in the immediate shell about the Ce3+ ion, Ea is the electron affinity of the atoms that form anions (eV), and r is the radius of the host cation replaced by the Ce3+ ion (Å). For Ea is constant in the same host, V = 3, Q* = 50000 cm−1, the value of E is directly proportional to the product of n and r. According to ref 26, r is calculated to 118 pm for Ca(I) n = 9 and 106 pm for Ca(II) and 110 pm for La(II) n = 7, respectively. Therefore, we can draw a conclusion that the first band centered at 322 nm is attributed to the 5d−4f emission of Ce3+ ions occupying the Ca(I) site with C3 symmetry and nine-coordination, the second band centered at 339 nm is due to the 5d−4f emission of Ce3+ ions occupying the La(II) site with CS symmetry and seven-coordination, and the third band with peak at 415 nm can be assigned to another Ca2+ site [Ca(II)] with CS symmetry. The PL intensity of Ce3+ as a function of its doping concentration (x) in CLPA:xCe3+ samples is shown in Figure S3 (Supporting Information). At first, the PL emission intensity of Ce3+ ions increases with the increase of its concentration (x), reaching a maximum value at x = 0.04, then decreasing with further increase of its concentration (x) due to the concentration quenching effect. Thus, the optimum doping concentration for Ce3+ is 4 mol % in the Ca8La2(PO4)6O2 host. In general, the concentration quenching of luminescence is due to the energy migration among the activator ions at the high concentrations. In the energy migration process, the excitation energy will be lost at a killer or quenching site, resulting in the decrease of luminescence intensity.28 In addition, with increasing Ce3+ ion concentration, the emission peaks of Ce3+ ions show a slight shift to the long wavelength side. This may be a result from the entrance of more Ce3+ ions into 6h or 4f sites and the change of CLPA crystal field at a high doping concentration. Figure 5 shows the CIE chromaticity diagram for the CLPA:0.04Ce3+ sample (point 1). It can be seen that the CLPA:0.04Ce3+ sample emits bright blue light with high color purity. Quantum yields and chromaticity coordinates (x, y) of the different concentrations of Ce3+ doped CLPA samples are summarized in Table 2. The CIE coordinates of all samples locate in the blue region, and the highest quantum yield of CLPA:Ce3+ samples can reach 67%. These results are consistent with those of photoluminescence spectra. 3.3. Photoluminescence Properties of CLPA:yEu2+ Phosphors. Similar to Ce3+-doped CLPA samples, Eu2+ activated CLPA samples also show blue emitting under the

Figure 5. The CIE chromaticity diagram for the CLPA:0.04Ce3+ (point 1) and CLPA:0.05Eu2+ (point 2) samples.

Table 2. Quantum Yields (QYs) and Chromaticity Coordinates (x, y) of the Ce3+ and/or Eu2+ Doped CLPA Samples under UV Excitation Ca8La2(PO4)O2 0.02Ce3+ 0.04Ce3+ 0.06Ce3+ 0.08Ce3+ 0.02Eu2+ 0.05Eu2+ 0.06Eu2+ 0.08Eu2+ 0.04Ce3+, 0.04Ce3+, 0.04Ce3+, 0.04Ce3+,

λex

quantum yields (%)

CIE coordinates (x, y)

310

59 67 47 44 13 14 9 7 43 31 19 13

(0.157, 0.123) (0.160, 0.115) (0.168, 0.158) (0.171, 0.159) (0.184, 0.156) (0.187, 0.164) (0.196, 0.185) (0.199, 0.192) (0.179, 0.169) (0184, 0.181) (0.190, 0.192) (0.198, 0.201)

318

0.02Eu2+ 0.05Eu2+ 0.06Eu2+ 0.08Eu2+

310

excitation of UV light. Figure 6 shows the PLE and PL spectra of the CLPA:0.05Eu2+ sample. Monitored with 453 nm, the excitation spectrum of the CLPA:0.05Eu2+ sample includes three absorption bands, as shown in Figure 6a, which are mainly due to transitions of Eu2+ from the 4f7 ground state to the 4f65d1 excited state.29 The Eu2+ ions have many excited states, and consequently, they show unresolved broad excitation spectra. The excitation spectrum shows that the excitation wavelength can extend from 200 to 420 nm, which nearly covers the whole UV region. Thus, it can be pumped by an InGaN-based UVchip for the white-LED and might be used in white-LEDs as a blue phosphor. Among the three absorption bands in the excitation spectrum, the peak at 318 nm is the strongest one. Under 318 nm UV-light irradiation, CLPA:0.05Eu2+ shows a strong asymmetry broad band from 400 to 600 nm with a maximum wavelength at about 453 nm (Figure 6a), which all can be ascribed to the 4f65d1 → 4f7 allowed transition of Eu2+. The asymmetric emission band from 400 to 600 nm can be deconvoluted into two Gaussian components peaked at 455 nm (I1) and 489 nm (I2), respectively, which are shown in Figure 6b. The full width at half maxima (fwhm) of the Gaussian components is 45.4 nm 10226



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The highest quantum yield of CLPA:Eu2+ samples only reaches 14%, as shown in Table 2. Figure S4 (Supporting Information) shows the variation of PL spectra of CLPA:yEu2+ samples with the Eu2+ concentration (y) under 318 nm UV excitation. At first, the PL emission intensity of Eu2+ increases with the increase of its concentration (y), reaching a maximum value at y = 0.05 and then decreasing with further increase of its concentration (y) due to the concentration quenching effect. This is similar to that of Ce3+ ions.28 It was found that the energy transfer between inequivalent Eu2+ centers occurred at a Eu2+ concentration above 1%,31 which is consistent with the fact that the optimum concentration of Eu2+ is 5%. Additionally, it is worth mentioning that the CLPA:Eu2+ samples were prepared by calcining CLPA:Eu3+ in reducing atmosphere (90% N2/10% H2 gas mixture). Before reducing, the CLPA:Eu3+ samples exhibit red emission resulting from the 5 D0−7F2 transition under the excitation of 266 nm due to the Eu−O charge transfer band (CTB), as shown in Figure S5 (Supporting Information). 3.4. Photoluminescence Properties and Energy Transfer in CLPA:0.04Ce3+, yEu2+ Samples. On the basis of the above PLE spectra of the Ce3+ and Eu2+ single-doped samples, it can be observed that Eu2+ activated CLPA samples have a broader absorption range in the UV region than that of Ce3+ activated samples and so Eu2+ activated CLPA sample can better satisfy the requirements of white-LEDs. However, Eu2+ activated CLPA sample has a lower luminescence intensity and quantum yield than that of Ce3+ activated sample. Moreover, a significant spectral overlap was observed between the emission band of Ce3+ and the excitation band of Eu2+, as shown in Figure 7a, with the result that the effective resonance energy transfer from Ce3+ to Eu2+ is expected. Consequently, Ce3+ and Eu2+ are codoped into the CLPA lattice as energy donor and energy acceptor, respectively, expecting that Ce3+ and Eu2+ coactivated samples can exhibit excellent luminescence properties.32 The energy transfer evidence from Ce3+ to Eu2+ is shown in Figure S6 (Supporting Information) and Figure 7b. When monitoring with the emission of Eu2+ in the CLPA lattice at 453 nm (Figure S6 in the Supporting Information), the excitation spectrum of CLPA:Ce3+, Eu2+ shows a broad absorption band ascribed to the 4f → 5d transition of both Ce3+ and Eu2+ ions. Figure 7b shows the PL spectra for CLPA:0.05Eu2+ and CLPA:0.04Ce3+, 0.05Eu2+ excited at 310 nm. The blue-emission intensity of Ce3+ and Eu2+ codoped CLPA sample is higher than that of Eu2+ single doped one. In addition, Figure 8 shows the PL spectra of CLPA:0.04Ce3+, yEu2+ (y = 0−0.08) samples. With increasing Eu2+ concentration, the emission peaks of CLPA:Ce3+, Eu2+ samples are found to shift to long wavelength. All above results confirm that an energy transfer from Ce3+ to Eu2+ occurred in Ce3+ and Eu2+ codoped CLPA samples. In order to further validate the energy transfer from Ce3+ to Eu2+, we investigated the decay curves of Ce3+ and Eu2+. Figure 9a shows the representative decay curves of Ce3+ emission in CLPA:0.04Ce3+, yEu2+ (y = 0.02, 0.06) samples excited at 310 nm and monitored at 415 nm. The decay curve of CLPA:0.04Ce3+ has been analyzed by curve fitting, and it can be well fitted through a triple-exponential function (Figure S7 in the Supporting Information), which is attributed to the Ce3+ substitution for three different crystallographic sites. These results are also supported by the crystal structure and spectral characterization. With the increase of the Eu2+ doping content,

Figure 6. (a) Typical PLE and PL spectra of the CLPA:0.05Eu2+ sample. (b) The emission spectra of CLPA:0.05Eu2+ under λex = 318 nm and its Gaussian components at 455 nm (I1) and 489 nm (I2), respectively. The inset shows its digital luminescence photo excited at 254 nm in a UV box.

(I1) and 105.5 nm (I2), respectively. The two emission bands are due to the fact that there is more than one emitting center in the Ca8La2(PO4)6O2 lattice. As discussed earlier, there are also three cations available for Eu2+: the Ca(I) site or the Ca(II) site or the La(II) site in CLPA:Eu2+.23 In general, Eu2+ emission can be observed when the Eu2+ is either at a monovalent or a divalent cation site.30 Therefore, the blue emission of Eu2+ ion would not be observed if it occupies the La(II) site. This means that Eu2+ ions only occupy the Ca(I) or Ca(II) site in the Ca8La2(PO4)6O2 lattice. Similar to the luminescence properties of Ce3+, the emission position of Eu2+ ion is also strongly dependent on its local environment. According to eq 1, similar results can be obtained: the first band centered at 455 nm is attributed to the 4f65d1 → 4f7 emission of Eu2+ ion occupying the Ca(I) site with C3 symmetry and nine-coordination, and the second band centered at 489 nm is due to the 4f65d1 → 4f7 emission of Eu2+ ion occupying the Ca(II) site with CS symmetry and seven-coordination. The CIE chromaticity diagram for the CLPA:0.05Eu2+ sample under 318 nm UV excitation is shown in Figure 5 (point 2). It can be concluded from PL spectra (Figures 4 and 6) of Ce3+ and Eu2+ activated CLPA phosphor and Table 2 that CLPA:Eu2+ samples show blue emission but the luminescence intensity and the quantum yield are much lower than that of Ce3+ doped CLPA samples. 10227



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Figure 7. (a) Spectral overlap between the PLE spectrum of CLPA:Eu2+ (black line) and the PL spectrum of CLPA:Ce3+ (blue line). (b) PL spectra for CLPA:0.05Eu2+ and CLPA:0.04Ce3+, 0.05Eu2+ excited at 310 nm.

Figure 9. (a) Decay curves of Ce3+ emission in CLPA:0.04Ce3+, yEu2+ (y = 0.02, 0.06) samples excited at 310 nm and monitored at 415 nm. (b) The dependence of decay time of Ce3+ on different Eu2+ concentrations (y) and energy transfer efficiencies from Ce3+ to Eu2+ in CLPA:0.04Ce3+, yEu2+ (0−0.08) samples (λex = 310 nm).

where A1, A2, and A3 are constants. τ1, τ2, and τ3 are the threeexponential components of the decay time. The average lifetimes of Ce3+ as a function of different Eu2+ concentrations were calculated, as shown in Figure 9b (black line). It can be seen that the lifetime of the Ce3+ emission shortens with increasing doping content of Eu2+, which strongly supports the energy transfer from the Ce3+ to Eu2+ ions. In addition, the energy transfer efficiency from Ce3+ to Eu2+ was also investigated. Generally, the energy transfer efficiency from a sensitizer to activator can be expressed as the following equation34−36 τ ηT = 1 − S τS0 (3)

Figure 8. PL spectra for CLPA:0.04Ce3+, yEu2+ (y = 0−0.08) samples excited at 310 nm. The inset shows the corresponding CIE chromaticity diagram.

where ηT is the energy transfer efficiency and τS0 and τS are the lifetime of the Ce3+ sensitizer in the absence and presence of Eu2+ ions, respectively. As a consequence, the ηT values from Ce3+ to Eu2+ in CLPA:0.04Ce3+, yEu2+ samples were calculated as a function of y and represented in Figure 9b (blue line). It can be observed that the energy transfer efficiency increases with increasing Eu2+ concentration. However, the increscent rate of the energy transfer efficiency gradually decreases with the increase of Eu2+ concentration. These results reveal that the energy transfer from Ce3+ to Eu2+ has a trend to saturation with a continuous increase of Eu2+ concentration due to the fixed Ce3+ concentration. The maximum energy transfer efficiency

the decay of the Ce3+ ions becomes faster and faster attributed to the energy transfer from the Ce3+ to Eu2+ ions (Figure 9a). The average fluorescence lifetime was defined as the following formula33 τavg =

A1τ12 + A 2 τ2 2 + A3τ32 A1τ1 + A 2 τ2 + A3τ3

(2) 10228



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Figure 10. (a) Dependence of ln(τS0/τS) of Ce3+ on C and τS0/τS of Ce3+ on (b) C6/3, (c) C8/3, and (d) C10/3.

According to Dexter’s energy transfer theory,37 the energy transfer process through multipolar interaction depends on the extent of overlap of the emission spectrum of the sensitizer with the absorption spectrum of the activator, the relative orientation of interacting dipoles, and the distance between the sensitizer and the activator. For a dipole−dipole interaction, the energy transfer probability (PSA) from a sensitizer to an activator is given by the following formula:

can reach 70%. The above results indicate that the energy transfer from Ce3+ to Eu2+ is quite efficient. On the basis of Dexter’s energy transfer expressions of multipolar interaction and Reisfeld’s approximation, the following relation can be given as37,38 ⎛η ⎞ ln⎜⎜ S0 ⎟⎟ ∝ C ⎝ ηS ⎠

ηS0 ηS

∝C

(4)

PSA(dd) =

n /3

(5)

where ηS0 and ηS are the luminescence quantum efficiencies of Ce3+ in the absence and presence of Eu2+, respectively; C is the total doping concentration of the Ce3+ and Eu2+ ions; eq 4 corresponds to the exchange interaction and eq 5 with n = 6, 8, and 10 corresponds to dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole interactions, respectively. The value of ηS0/ηS can be approximately estimated from the related lifetime’s ratio (τS0/τS). Thus, eqs 4 and 5 can be represented by the following equation

⎛τ ⎞ ln⎜ S0 ⎟ ∝ C ⎝ τS ⎠ τS0 ∝ C n /3 τS

3 × 1012fd 6

R τS

∫

fS (E)FA(E) E4

dE (8)

where fd is the oscillator strength of the involved dipole absorption transition of the activator, τS is the radiative decay time of the sensitizer, R is the sensitizer−activator average distance, f S(E) represents the normalized emission shape function of the sensitizer, FA(E) represents the normalized absorption shape function of the activator, and E is the energy involved in the transfer (eV). The critical distance (Rc) of the energy transfer from the sensitizer to activator is defined as the distance for which the probability of transfer equals the probability of radiative emission of the sensitizer, i.e., the distance for which PSA·τS = 1. Therefore, Rc can be obtained from the formula

(6)

R c 6 = 3 × 1012fd

(7)

∫

fS (E)FA(E) E4

dE

(9)

where ∫ f S(E)FA(E) dE/E4 represents the spectral overlap between the normalized shapes of the Ce3+ emission f S(E) and the Eu2+ excitation FA(E), and in our case it is calculated to be about 0.0059 eV−4. Using eq 9, the critical distance Rc was estimated to be 26.67 Å, which is consistent with those

The relationships of ln(τS0/τS) ∝ C and (τS0/τS) ∝ C n/3 are illustrated in Figure 10, in which a linear behavior was observed only when n = 6, implying that energy transfer from Ce3+ to Eu2+ occurs via a dipole−dipole mechanism, which is similar to those previously investigated.39−43 10229



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previously investigated, such as 25 Å reported in BaLiF3:Ce3+, Eu2+ and 32.7 Å in Ba2ZnS3:Ce3+, Eu2+.40,43 The CIE chromaticity coordinates and quantum yield for the CLPA:0.04Ce3+, yEu2+ phosphors with different y values were summarized in Table 2. It can be obtained that the chromaticity coordinates of Ce3+ and Eu2+ coactivated CLPA phosphors still locate in the blue light area (inset in Figure 8). More importantly, compared with the quantum yields of Eu2+ single doped CLPA phosphors, the quantum yields of CLPA:0.04Ce3+, yEu2+ (y = 0.02, 0.05, 0.06, 0.08) phosphors are enhanced greatly. Taking example for the 2% Eu3+ doped CLPA sample, its quantum yield is only 13%; however, the quantum yield increases to 43% by Ce3+ transferring its energy to Eu2+ in Ce3+ and Eu2+ coactivated CLPA samples.

CLPA:yEu2+ samples with the Eu2+ concentration (y) under 318 nm UV excitation (Figure S4); the PLE and PL spectra of the CLPA:Eu3+ sample (Figure S5); the PL excitation spectrum of the Ce3+ and Eu2+ codoped CLPA sample (Figure S6); and the photoluminescence decay curve of Ce3+ emission in CLPA:0.04Ce3+ and curve-fitting under excitation at 310 nm, monitored at 415 nm (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

■

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

4. CONCLUSIONS In conclusion, Ce3+ and/or Eu2+ activated Ca8La2(PO4)6O2 (CLPA) blue-emitting materials have been prepared via a Pechini-type sol−gel method. The CLPA phase formed after annealing at 1200 °C in air for 4 h, which is lower than a high temperature solid state reaction. The prepared samples are composed of aggregated nanoparticles with sizes ranging from 50 to 100 nm. The Ce3+ ion simultaneously occupies three kinds of sites in the Ca8La2(PO4)6O2 host and gives different emission under UV excitation: the first emission band centered at 322 nm is attributed to the 5d−4f emission of Ce3+ ions occupying the Ca(I) site with C3 symmetry and ninecoordination; the second band centered at 339 nm is due to the 5d−4f emission of Ce3+ ions occupying the La(II) site with CS symmetry and seven-coordination; and the third band with a peak at 415 nm can be assigned to another Ca2+ site [Ca(II)] with CS symmetry. Thus, the Ce3+ activated CLPA samples exhibit bright blue emission with higher quantum yield (67%) and excellent CIE coordinates (x = 0.160, y = 0.115) under UV excitation. Different from the luminescence mechanism of Ce3+ activated CLPA samples, the Eu2+ ion occupies two sites in the Ca8La2(PO4)6O2 host, which are the Ca(I) site with C3 symmetry and Ca(II) sites with CS symmetry, respectively. Their emission spectra only exist of one broad band centered at 453 nm and also show blue emission. In addition, an energy transfer from Ce3+ to Eu2+ occurred in Ce3+ and Eu2+ ion coactivated CLPA host and has been demonstrated to be a resonant type via a dipole−dipole mechanism. The critical distance (Rc) of Ce3+ to Eu2+ ions in CLPA was calculated by spectral overlap methods to be 26.67 Å. The quantum yields of Ce3+ and Eu2+ coactivated CLPA samples are enhanced compared with that of the corresponding Eu2+ single-activated samples through energy transfer interaction. The CIE coordinate of CLPA:0.04Ce3+, 0.02Eu2+ locates at (x = 0.179, y = 0.169) with high color purity. More importantly, the excitation band was extended due to the energy transfer and nearly covered the whole UV region, which contributes to its application in UV-LEDs. The results indicate that the CLPA:Ce3+/Eu2+ are promising blue phosphors for application in pc-white LEDs.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS This project is financially supported by National Basic Research Program of China (2010CB327704) and the National Natural Science Foundation of China (NSFC 51172227, 20921002).

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REFERENCES

(1) Nakamura, S.; Mukai, T.; Senoh, M. Appl. Phys. Lett. 1994, 64, 1687. (2) Hashimoto, T.; Wu, F.; Speck, J. S.; Nakamura, S. Nat. Mater. 2007, 6, 568−571. (3) Steigerwald, D. A.; Bhat, J. C.; Collins, D.; Fletcher, R. M.; Holcomb, M. O.; Ludowise, M. J.; Martin, P. S.; Rudaz, S. L. IEEE J. Sel. Top. Quantum Electron. 2002, 8, 310−320. (4) Muthu, S.; Schuurmans, F. J. P.; Pashley, M. D. IEEE J. Sel. Top. Quantum Electron. 2002, 8, 333−338. (5) Huh, Y. D.; Shim, J. H.; Kim, Y.; Rag Do, Y. J. Electrochem. Soc. 2003, 150, H57−H60. (6) Haranath, D.; Chander, H.; Sharma, P.; Singh, S. Appl. Phys. Lett. 2006, 89, 173118. (7) Lu, C.-H.; Jagannathan, R. Appl. Phys. Lett. 2002, 80, 3608. (8) Kasuya, R.; Kawano, A.; Isobe, T.; Kuma, H.; Katano, J. Appl. Phys. Lett. 2007, 91, 111916. (9) Jung, K. Y.; Lee, H. W.; Jung, H. Chem. Mater. 2006, 18, 2249− 2255. (10) Sheu, J. K.; Chang, S. J.; Kuo, C. H.; Su, Y. K.; Wu, L. W.; Lin, Y. C.; Lai, W. C.; Tsai, J. M.; Chi, G. C.; Wu, R. K. IEEE J. Sel. Top. Quantum Electron. 2003, 15, 18−20. (11) (a) Im, W. B.; Brinkley, S.; Hu, J. Chem. Mater. 2010, 22, 2842− 2849. (b) Chen, W. P.; Liang, H. B.; Ni, H. Y. J. Electrochem. Soc. 2010, 157, J159−J163. (c) Lin, H. H.; Liang, H. B.; Han, B.; Zhong, J. P.; Su, Q.; Dorenbos, P.; Birowosuto, M. D.; Zhang, G. B.; Fu, Y. B.; Wu, W. Q. Phys. Rev. B 2007, 76, 035117. (d) Suehiro, T.; Hirosaki, N.; Xie, R. J.; Sato, T. Appl. Phys. Lett. 2009, 95, 051903. (12) (a) Dierre, B.; Xie, R. J.; Hirosaki, N.; Sekiguchi, T. J. Mater. Res. 2007, 22, 1933−1941. (b) Liang, H. B.; Lin, H. H.; Zhang, G. B.; Dorenbos, P.; Su, Q. A. J. Lumin. 2011, 131, 194−198. (c) AlouiLebbou, O.; Goutaudier, C.; Kubota, S.; Dujardin, C.; Cohen-Adad, M. T.; Pedrini, C.; Florian, P.; Massiot, D. Opt. Mater. 2001, 16, 77−86. (d) Mahalingam, V.; Tan, M.; Munusamy, P.; Gilroy, J. B.; Raudsepp, M.; van Veggel, F. C. J. M. Adv. Funct. Mater. 2007, 17, 3462−3469. (13) (a) Kimura, N.; Sakuma, K.; Hirafune, S.; Asano, K.; Hirosaki, N.; Xie, R. J. Appl. Phys. Lett. 2007, 90, 051109. (b) Huang, C. H.; Chen, T. M. Opt. Express 2010, 18, 5089−5099. (c) Bachmann, V.; Ronda, C.; Oeckler, O.; Schnick, W.; Meijerink, A. Chem. Mater. 2009, 21, 316−325. (d) Xie, R. J.; Hirosaki, N.; Sakuma, K.; Kimura, N. J. Phys. D: Appl. Phys. 2008, 41, 144013. (14) (a) Chartier, C.; Barthou, C.; Benalloul, P.; Frigerio, J. M. J. Lumin. 2005, 111, 147−158. (b) Datao, Tu, D. T.; Liu, L. Q.; Ju, Q.; Liu, Y. S.; Zhu, H. M.; Li, R. F.; Chen., X. Y. Angew. Chem., Int. Ed. 2011, 50, 6306−6310. (c) Park, J. K.; Kim, C. H.; Park, S. H.; Park, H.

ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of CLPA samples annealed at different temperatures (Figure S1); the SEM image of the CLPA:Ce3+ sample (Figure S2); the variation of PL spectra of CLPA:xCe3+ samples with the Ce3+ concentration (x) under 310 nm UV excitation (Figure S3); the variation of PL spectra of 10230



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D.; Choi, S. Y. Appl. Phys. Lett. 2004, 84, 1647. (d) Smet, P. F.; Avci, N.; Poelman, D. J. Electrochem. Soc. 2009, 156, H243−H248. (15) (a) Im, W. B.; Fellows, N. N.; DenBaars, S. P.; Seshadri, R.; Kim, Y. Chem. Mater. 2009, 21, 2957−2966. (b) Ghosh, P.; Kar, A.; Patra, A. J. Phys. Chem. C 2010, 114, 715−722. (c) Park, W.; Singh, P. S.; Sohn, K. S. J. Electrochem. Soc. 2011, 158, J184−J188. (16) (a) Shang, M. M.; Li, G. G.; Kang, X. J.; Yang, D. M.; Geng, D. L.; Lin, J. ACS Appl. Mater. Interfaces 2011, 3, 2738−2746. (b) Liu, W. R.; Chen, T. M. J. Phys. Chem. C 2010, 114, 18698−18701. (c) Zhang, G.; Wang, J.; Yan Chen, Y.; Su, Q. Opt. Lett. 2010, 35, 2382−2384. (d) Ghosh, P.; Kar, A.; Patra, A. Nanoscale 2010, 2, 1196−1202. (17) Dorenbos, P. J. Lumin. 2003, 104, 239−260. (18) (a) Takahashi, K.; Hirosaki, N.; Xie, R. J.; Harada, M.; Yoshimura, K.; Tomomura, Y. Appl. Phys. Lett. 2007, 91, 091923. (b) Song, Y.; Jia, G.; Yang, M.; Huang, Y.; You, H.; Zhang, H. Appl. Phys. Lett. 2009, 94, 091902. (c) Guo, C. F.; Luan, L.; Shi, F. G.; Xu, D. J. Electrochem. Soc. 2009, 156, J125−J128. (d) Chen, Y.; Gong, M.; Cheah, K. Mater. Sci. Eng., B 2010, 166, 24−27. (19) (a) Raju, G. S. R.; Park, J. Y.; Jung, H. C.; Moon, B. K.; Jeong, J. H.; Kim, J. H. J. Electrochem. Soc. 2011, 158, J20−J26. (b) Sun, J. M.; Skorupa, W.; Dekorsy, T.; Helm, M.; Rebohle, L.; Gebel, T. J. Appl. Phys. 2005, 97, 123513. (c) Shen, Y. Q.; Chen, R.; Xiao, F.; Sun, H. D.; Tok, A.; Dong, Z. L. J. Solid State Chem. 2010, 183, 3093−3099. (d) Lin, J.; Su, Q. J. Alloys Compd. 1994, 210, 159−163. (20) (a) Chambers, M. D.; Rousseve, P. A.; Clarke, D. R. J. Lumin. 2009, 129, 263−269. (b) Raju, G. S. R.; Jung, H. C.; Park, J. Y.; Moon, B. K.; Balakrishnaiah, R.; Jeong, J. H.; Kim, J. H. Sens. Actuators, B 2010, 146, 395−405. (c) Lin, J.; Su, Q. J. Alloys Compd. 1995, 225, 120−123. (d) Li, G. G.; Zhang, Y.; Geng, D. L.; Shang, M. M.; Peng, C.; Cheng, Z. Y.; Lin, J. ACS Appl. Mater. Interfaces 2012, 4, 296−305. (21) (a) Lin, J.; Yu, M.; Lin, C. K.; Liu, X. M. J. Phys. Chem. C 2007, 111, 5835. (b) Serra, O. A.; Severino, V. P.; Calefi, P. S.; Cicillini, S. A. J. Alloys Compd. 2001, 323−324, 667−669. (c) Sousa Filho de, P. C.; Serra, O. A. J. Fluoresc. 2008, 18, 329−337. (22) Yoshikawa, A.; Kochurikhin, V. V.; Futagawa, N.; Shimamura, K.; Fukuda, T. J. Cryst. Growth 1999, 204, 302−306. (23) Ouenzerfi, R.; El; Panczer, G.; Goutaudier, C.; Cohen-Adad, M. T.; Boulon, G.; Trabelsi-Ayedi, M.; Kbir-Ariguib, N. Opt. Mater. 2001, 16, 301−310. (24) Boulon, G.; Collombet, A.; Brenier, A.; Cohen-Adad, M.-T.; Yoshikawa, A.; Lebbou, K.; Lee, J. H.; Fukuda, T. Adv. Funct. Mater. 2001, 11, 263−270. (25) Ryan, F. M.; Warren, R. W.; Hopkins, R. H.; Murphy, J. J. Electrochem. Soc. 1978, 125, 1493−1498. (26) Shannon, R. D. Acta Crystallogr. 1976, A32, 751−767. (27) Van Uitert, L. G. J. Lumin. 1984, 29, 1−9. (28) (a) Lin, C. C.; Liu, R. S.; Tang, Y. S.; Hu, S. F. J. Electrochem. Soc. 2008, 155, J248−J251. (b) Chan, T. S.; Lin, C. C.; Liu, R. S.; Xie, R. J.; Hirosaki, N.; Cheng, B. M. J. Electrochem. Soc. 2009, 156, J189− J191. (c) Liu, X. M.; Lin, J. J. Mater. Chem. 2008, 18, 221−228. (29) Blasse, G.; Wanmaker, W. L.; Tervrugt, J. W.; Bril, A. Philips Res. Rep. 1968, 23, 189−200. (30) (a) Lee, B.; Lee, S.; Jeong, H. G.; Sohn, K. S. ACS Comb. Sci. 2011, 13, 154−158. (b) Chen, Y.; Wang, J.; Zhang, X.; Zhang, G.; Gong, M.; Su, Q. Sens. Actuators, B 2010, 148, 259−263. (c) Smet, P. F.; Parmentier, A. B.; Poelman, D. J. Electrochem. Soc. 2011, 158, R37− R54. (d) Kang, J.-G.; Sohn, Y. G.; Nah, M.-K.; Youn-Doo Kim, Y.-D.; Ogryzlo, E. A J. Phys.: Condens. Matter 2000, 12, 3485−3495. (31) Blasse, G. J. Solid State Chem. 1986, 62, 207−211. (32) (a) Lakshminarasimhan, N.; Varadaraju, U. V. J. Electrochem. Soc. 2005, 152 (9), H152−H156. (b) Chang, C. K.; Chen, T. M. Appl. Phys. Lett. 2007, 91, 081902. (c) Guo, C. F.; Luan, L.; Shi, F. G.; Ding, X. J. Electrochem. Soc. 2009, 156, J125−J128. (33) (a) Shi, L.; Huang, Y.; Seo, H. J. J. Phys. Chem. A 2010, 114, 6927−6934. (b) Zhu, G.; Wang, Y. H.; Ci, Z. P.; Liu, B. T.; Yurong Shi, Y. R.; Xin, S. Y. J. Electrochem. Soc. 2011, 158, J236−J242. (34) (a) Yang, W. J.; Chen, T. M. Appl. Phys. Lett. 2006, 88, 101903. (b) Kar, A.; Datta, A.; Patra, A. J. Mater. Chem. 2010, 20, 916−922.

(35) Kwon, K. H.; Im, W. B.; Jang, H. S.; Yoo, H. S.; Jeon, D. Y. Inorg. Chem. 2009, 48, 11525−11532. (36) Paulose, P. I.; Jose, G.; Thomas, V.; Unnikrishnan, N. V.; Warrier, K. R. M. J. Phys. Chem. Solids 2003, 64, 841−846. (37) Dexter, D. L. J. Chem. Phys. 1953, 21, 836−850. (38) Blasse, G. Philips Res. Rep. 1969, 24, 131−144. (39) Caldino, U. G. J. Phys.: Condens. Matter 2003, 15, 7127−7137. (40) Tan, Y.; Shi, C. J. Phys. Chem. Solids 1999, 60, 1805−1810. (41) Lin, H.; Liu, X. R.; Pun, E. Y. B. Opt. Mater. 2002, 18, 397−401. (42) Najafov, H.; Kato, A.; Toyota, H.; Iwai, K.; Bayramov, A.; Iida, S. Jpn. J. Appl. Phys. 2002, 41, 1424−1430. (43) Yang, W. J.; Chen, T. M. Appl. Phys. Lett. 2007, 90, 171908.

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