Increased 1D2 Red Emission of Pr3+ in NaGdTiO4:Pr3+ Due to

Jan 16, 2013 - 2013 American Chemical Society. 2216 ... exp( / ) t. 0. (1) where It and I0 are the luminescence intensities at time t and t = 0, respe...
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Increased 1D2 Red Emission of Pr3+ in NaGdTiO4:Pr3+ Due to Temperature-Assisted Host Sensitization and Its Color Variation Su Zhang, Hongbin Liang,* and Chunmeng Liu MOE Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China ABSTRACT: Temperature-dependent luminescent property of Pr3+-doped NaGdTiO4 is investigated systematically. Because of the intensive quenching of 3P0 emission, a red emission of 1D2−3H4 transition is observed in NaGdTiO4:Pr3+ at room temperature (RT). At 77 K, a broad band emission of host at about 475 nm (blue) is observed. Interestingly, the intensity of the red emission is increased with increasing temperature from 77 to 350 K and keeps nearly stable to 400 K under the host excitation, whereas a decrease of the blue host emission is presented. The results show that the increased red emission of 1D2 is associated with temperature-assisted energy transfer from host to Pr3+. Because the relative intensity of the 1 D2 and host emission is sensitive to temperature, a large variation of color is also observed from 77 K to RT. and displays, especially for Pr3+-doped titanates.12 Red emission can be obtained through quenching 3P0 emission of Pr3+ due to the intervalence charge transfer (IVCT) between Pr3+ and Ti4+,14 such as Pr3+-doped perovskite-type oxide MTiO3 (M = Ca, Sr). Because of the red emission color and the broad host excitation band, a promising application of Pr3+-doped titanate in nUV chip based wLEDs is reported.12 However, hostsensitized energy transfer process in titanate is less studied. In this article, a red emission of 1D2−3H4 transition of Pr3+ is observed at RT. Significantly, the intensity of the 1D2 red emission is increased and reaches maximum at 350−400 K. The temperature dependence of photoluminescence (PL) properties of NaGdTiO4:Pr3+ are investigated in detail. A temperatureassisted host sensitization is proposed. In addition, a large color variation (from blue to red) is observed with changing temperature from 77 K to RT. The luminescence mechanism studies are benefit for finding other novel Pr3+-activated functional fluorescence materials.

1. INTRODUCTION White light-emitting diodes (wLEDs) are a promising new generation light source due to the advantages of long lifetime, high-energy efficiency, reliability, and environment-friendly characteristics.1,2 At present, the commercial white LEDs is based on the combination of blue InGaN chip and YAG:Ce3+ yellow phosphor. Low color-rendering index (CRI) is the chief drawback.3 As an alternative, combining red, green, and blue phosphors and near-ultraviolet/ultraviolet (nUV/UV) InGaN diode chips to produce white light is favored.4 Consequently, phosphors that have broad absorption band at blue or nUV range and that can emit red light have attracted substantial attentions. However, the operating temperature of LEDs is higher than room temperature (RT), which can even reach 425 K for high-power LEDs.5 Therefore, improving the thermal quenching character of phosphors for wLEDs is another important consideration. Host sensitization via energy transfer (ET) from the excited host to Ln3+ (trivalent lanthanide ions) is an effective way to enhance the luminescence efficiency of phosphors and overcome the low absorptions of parity-forbidden f−f transitions of Ln3+.6,7 Over the past decades, host-sensitized energy transfer has been investigated in Ln3+-doped transition metal composite oxide materials, such as tungstates, vanadates, molybdates, and similar crystals, because of their importance in practical applications for cathode ray tubes, X-ray intensifying screens, laser crystals, and so forth.8−10 Recently, considering that the host sensitization charge transfer (C−T, the oxygen to transition metal) absorption bands of many transition metal composite oxides is locate at near UV range, their properties for wLEDs applications have attracted particular attentions.11−13 Among them, titanates are interesting materials for lightings © 2013 American Chemical Society

2. EXPERIMENTAL SECTION NaGd0.999Pr0.001TiO4 (NGTP) and NaGdTiO4 are prepared by heating stoichiometric mixtures of Na2CO3 (analytical reagent, A.R.), Gd2O3 (99.99%), TiO2 (99.99%), and Pr(NO3 )3 solution (titrated) for 1 h at 1000 °C, followed by 3 repeated regrinding/reheating steps until the samples are found to be phase pure, as determined by powder X-ray diffraction. Samples are heated in alumina crucibles in air. A 30% excess of Na2CO3 is used to compensate for loss due to volatilization.15 Received: October 16, 2012 Revised: January 16, 2013 Published: January 16, 2013 2216

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The structure of the samples are determined with a Bruker D8 Advanced X-ray Diffractometer (XRD) with Cu Kα (λ = 1.5405 Å) radiation. The UV−vis luminescence spectra and the luminescence decay curves are recorded on an Edinburgh FLS 920 combined fluorescence lifetime and steady state spectrometer, which is equipped with a time-correlated single-photon counting (TCSPC) card. A 450W xenon lamp is used as the excitation source. For the measurements of luminescence decays, the excitation photons are provided by a 60 W μF900 microsecond flash lamp with a pulse width of 1.5 μs and a pulse repetition rate of 20 Hz. The room temperature quantum yield (QY) is measured also on the Edinburgh FLS920 fluorescence spectrometer equipped with a BaSO4coated integrating sphere. The temperature dependencies of emission and excitation spectra are measured using an Oxford OptistatDN2 nitrogen cryostat.

Figure 2. Emission and excitation spectra of NGTP at RT.

band (curve c) or 3P2 level (curve d). The red emission around 620 nm is ascribed to the transition from 1D2 to the ground state 3H4 of Pr3+ (1D2→3H4). And the emission in the vicinity of 700 nm is ascribed to 1D2→3H5. The decay curves of different emission peaks at 602, 611, 618, 637, 704, and 718 nm are measured at RT under 452 nm excitation and illustrated in Figure 3. The curves show

3. RESULTS AND DISCUSSION 3.1. Structural Characterization. High-quality XRD data for Rietveld refinement is collected over an angular 2θ range from 5° to 100° at an interval of 0.02°. Structural refinement of XRD data is performed using the TOPAS-Academic program.16 The Rietveld refinement is performed for the product using the Pbcm structure model reported by Toda et al.15 and shown in Figure 1, which converged to Rwp = 2.82% and RB = 1.40%.

Figure 3. Luminescence decay curves of NGTP at RT.

exponential decays which are nearly overlapped and can be well fitted using equation:

Figure 1. Experimental (crosses) and calculated (red solid line) XRD patterns and their difference (blue solid line) for NGTP; insert, the crystal structure of NGTP.

It = I0exp( −t /τ )

(1)

where It and I0 are the luminescence intensities at time t and t = 0, respectively; τ is the fluorescence lifetime. We obtain τ of about 90 μs for emissions from 600 to 720 nm. Because the spontaneous radiative transitions of 3P0 and 1D2 have different fluorescence lifetimes,22 the similar lifetimes of different emission wavelength give us the evidence that the red emission of NGTP is generated from the transitions of 1D2 to different ground states, that is the 3P0 emission is quenched. The τ of 1 D2 in NGTP is comparable with that in CaTiO3:Pr3+ (∼100 μs at RT).12 The low-lying IVCT state, that is the transfer of an electron from the ground 4f state of Pr3+ to Ti4+, is the most possibility for the intense quenching of 3P0. The excitation energy will easily overcome the barrier resulting in the strong emission of 1 D2-3H4 transitions.14 An empirical formula to estimate the position of IVCT is proposed by Boutinaud:21

All of the observed peaks are consistent with the lattice constants and the reflection conditions indicating the formation of a single phase with no impurities. The crystallographic data are a = 12.4640(Å), b = 5.3336(Å), c = 5.3360(Å), and cell volume = 354.73(Å3). The ionic radius of Pr3+ is close to that of Gd3+ and far away from that of Ti4+. Thus, Pr3+ is thought to occupy Gd3+ site. 3.2. Red Emission of NGTP at RT. Figure 2 shows the photoluminescence (PL) emission and excitation spectra of NGTP at RT. The excitation spectrum is monitored at 611 nm corresponding to the red emission ascribed to 1D2→3H4 of Pr3+. As shown in Figure 2, the excitation spectrum is composed of two broad bands located at 200−300 nm (band A) and 300−400 nm (band B), and a series of sharp peaks ranging from 440 to 520 nm. Band A can be ascribed to the absorption of the NaGdTiO4 host lattice.17−19 Band B corresponds to the Pr3+−Ti4+ IVCT absorption.14,20,21 The emission spectra of NGTP (lines b, c, d in Figure 2) show a series of emission peaks in the vicinity of 590−750 nm, whether it is excited into the host band (curve b), the IVCT

IVCT(cm−1) = 58 800 − 49 800[χ (Mn +) /d(Pr 3 + − Mn +)]

(2)

Where χ(M ) is the optical electronegativity of M ; d(Pr3+ − Mn+) is the distance between Mn+ and Pr3+. Here, χ(Ti4+) is n+

2217

n+

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2.05 and the average d(Pr3+ − Ti4+) obtained from structural refinement is 3.31 Å. Thus, the IVCT is calculated to be 27 960 cm−1. Figure 2 indicates the IVCT is located at about 340 nm (≈ 29 400 cm−1). The calculation and experimental value differs by 1440 cm−1. 3.3. PL Properties of NGTP at 77 K. The normalized excitation and emission spectra of NGTP at 77 K are shown in parts a and b of Figure 4, respectively. There are several

Figure 5. Temperature-dependent emission spectrum of NGTP under 288 nm excitation.

Figure 6. Temperature dependence of intensity of 1D2 emission in NGTP upon (a) host, (b) IVCT, and (c) 3P2 excitation. The lines are drawn to guide the eye.

absent, which means that the Gd3+ ion and Pr3+ can not transfer excitation energy to host emission centers (part a of Figure 4). 3.4. Increased Red Emission of 1D2−3H4 Transitions from 77 to 350 K. Figure 5 shows the emission spectra of NGTP at different temperature under host excitation. It is obvious that the host emission is decreased along with the rise of temperature and totally quenched at about 300 K. However, it is important to mention that when gradually increasing the temperature from 77 to ∼350 K, the intensity of 1D2 emission of Pr3+ is increased. Between 350 and 400 K, the 1D2 intensity is nearly unchanging. When above 400 K, the intensity decreases with further increasing temperature due to thermalquenching. The quantum yield (QY) of NaGdTiO4 is measured to be about 20% at RT under host excitation. Figure 6 shows the temperature-dependent emission intensity of 1D2−3H4 transitions under the excitation at different wavelengths. When excited into the IVCT and 3P2 level at 340 and 452 nm (lines b and c in Figure 6), the red emission decreases continuously due to thermal-quenching. Significantly, under the excitation at host band (288 nm), however, the intensity of red 1D2 emission is increased from 77 to 350 K and keeps nearly stable between 350 and 400 K (line a in Figure 6). Because the increasing of 1D2 emission only presents under host excitation, it is possible that the intensity increasing of 1D2 emission is associated with host sensitization. Similar

Figure 4. Normalized PL excitation (a) and emission (b) spectra of NGTP at 77 K.

interesting differences for the PL spectra between 77 K (Figure 4) and RT (Figure 2). First, for the excitation spectrum of 611 nm emission, the intensity of band A (host excitation band) is weaker than that of band B (IVCT band) at 77 K (part a of Figure 4), but the reversed case is found at RT (part a of Figure 2). The decreasing of host excitation intensity indicates that at low temperature the energy transfer from the host to Pr3+ is less effective, this standpoint is directly supported by the 1D2 emission intensity dependency on the temperature (line a in Figure 6). Second, excitation at 285 nm produced a broad emission band located around 475 nm at 77 K (part b of Figure 4). This blue emission band is ascribed to the titanate host emission. Many other titanate phosphors also show host luminescence, such as SrTiO3, CaTiO3 and NaYTiO4. It can be ascribed to the recombination of self-trapped excitons (STEs),17,23−25 which generally have large half-width of emission peak and big stokes shift (1−2 eV).25−28 The insert of part b of Figure 4 shows the 77 K emission spectrum of undoped NaGdTiO4, which is consistent with the band emission of NGTP. When monitored at the band emission (∼475 nm) for NGTP, only the host absorption is observed (part a of Figure 4). The IVCT and the Gd3+ absorptions are 2218

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phenomenon is reported in Pr3+-doped YNbO4,29 in which an increasing of the 1D2 emission is observed and accompanied by a significant decrease of niobate luminescence intensity. Temperature-assisted host sensitization is considered as a reasonable interpretation.29 However, the host emission of undoped niobate is not shown in ref 29. However, fluorescence decay curves of NGTP are measured under host excitation at different temperatures. Using eq 1, we calculate the lifetimes of 1D2 emission at different excitation conditions and exhibit them in Figure 7. Figure 7 reveals that

For describing the excited state populations of host and activator, the rate equations are given by9,10,31 dnH /dt = WH − βHnH − ωHA nH − ωnrnH

(3)

dnA /dt = ωHA nH − βA nA − ω′nr nA

(4)

where, WH is the pumping rate of the host; nH and nA are the concentrations of excited states of host and activators; βH and βA are the fluorescence decay rates of the host and activator, respectively. For this case, any direct pumping of the activator ions can be neglected and WH can be treated as a delta function. The ωHA is the energy transfer rate between host and activators. ωnr and ω′nr are the nonradiation transfer rate from host and activators to quenching centers. Because of the large difference in energy between the activator emission and the host absorption, no back transfer from activator to host is present. Accordingly, in NGTP the increased emission intensity and lifetime of 1D2 is thought to be originated from an enhanced ωHA. Before the temperature reaches the point when 1D2 can be nonradiative de-excitation by crossover to IVCT, the radiation transition of 1D2 is dominated by a competitive process of receiving energy from host and transferring energy to quenching centers. The higher the ωHA is, the longer the lifetime and the stronger the emission intensity will be. Therefore, it is reasonable that in NGTP the competitive process reaches balance at 350−400 K resulting in the maximum emission intensity and lifetime of 1D2 at this range of temperature. If the remarkable increasing of the intensity and the lifetimes of 1D2 emission is related to the thermally activated host sensitization of the Pr3+ in NGTP, it will be connected to the emission properties of the host. The fluorescence decay curves of host emission in both NGTP and undoped NaGdTiO4 are measured and shown in Figure 8. On the one hand, it is obvious that at the initial portion of the host decay curves in NGTP, there is a faster-decay process (inserts of part a of

Figure 7. Temperature dependence of the lifetimes of 1D2 emission (611 nm) in NGTP under host excitation. The line is drawn to guide the eye.

the quenching temperature of 1D2 is about 400 K, which is generated from the nonradiative de-excitation of 1D2 by crossover to IVCT and has been reported in detail.14,20,21 The higher the IVCT position, the higher the quenching temperature of 1D2 emission. For instance, the IVCT band is located at about 26 700 and 35 000 cm−1 in CaTiO3 and YNbO4, respectively.20,29,30 Hence, the quenching temperature of 1D2 emission in CaTiO3:Pr3+ and YNbO4:Pr3+ is about 300 and 500 K, respectively.20,29,30 For NGTP, the IVCT is about 29 400 cm−1 and the quenching temperature is nearly 400 K. Importantly, for temperature below 400 K, the lifetimes of 1 D2 presents an upward tendency under host excitation (Figure 7). This phenomenon is consistent with the changing trend of the intensity of 1D2 emission (Figure 6). The mechanism of the host sensitization process is investigated in the following section. 3.5. Host Sensitization for 1D2 Emission in NGTP. The energy transfer between host and Ln3+ activators are studied in many transition metal compounds, such as tungstate, vanadate, and molybdates.8−10 Thermally activated exciton model has been proposed to characterize host-sensitized energy transfer in similar crystals.8−10,31 For these compounds, at low temperatures the excitons become self-trapped with an activation energy E. At high temperatures, the excitons can overcome the activation energy and undergo a thermally activated hopping migration. During this migration, the excitons can be trapped by quenching centers or by activators, such as in Ln3+-doped CaWO4 and YVO4.8,9,31 In more detail, the introduction of Ln3+ into the host crystal can either enhance the quenching of host emission if the ions act as additional quenching centers or decrease the quenching if the ions act as scattering or recombination centers to inhibit the exciton migration.10 For example, in Nd3+-doped CaMoO4 increasing the temperature will allow excitons to migrate closer to the Nd3+ before transfer to quenching center.10

Figure 8. Temperature-dependent decay curves of host emission at 475 nm under 288 nm excitation in (a) NGTP and (b) undoped NaGdTiO4. 2219

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(∼50 K).25 From above analysis, one can see that, although crude, doping Pr3+ can decrease the activated energy of STEs. Furthermore, the temperature-dependent PL excitation spectra of NGTP are measured and shown in Figure 10.

Figure 8), which indicates the existence of energy transfer from the host to Pr3+.33 On the other hand, the decays become faster with increasing temperature (part a of Figure 8). When the temperature is over 200 K, the decay of host emission in NGTP is much faster than that in undoped NaGdTiO4 (part b of Figure 8, and Figure 9) providing the evidence that at higher

Figure 10. Temperature-dependent PL excitation spectra of NGTP. Figure 9. Temperature-dependent fluorescence lifetime of host emission in (a) NGTP and (b) undoped NaGdTiO4. Lines are the fit of the experimental data obtained using eq 6.

Obviously, the intensity of the excitation band of host (band A) is increased with temperature and reaches maximum at 350 K. The variety is consistent with that of emission intensity, which means that a temperature-dependent host sensitization process may exist. Additionally, the host excitation band presents a red shift of about 850 cm−1 (from 284 to 291 nm) when heating from 77 to 500 K (Figure 10) indicating a decrease in bandgap. Similar shift was observed in Pr3+-doped CaTiO3.37 3.6. Large Color Variation from 77 K to RT. Because of the existence of blue emission of host at low temperature, a large color variation is expected at different temperature. It is obvious that the host emission (located around 475 nm) is intensively decreased along with the rise of temperature and totally quenched at 300 K (shown in Figure 5), whereas the red emission of 1D2 transitions is increased. Hence, because of the change of the relative intensity of host emission (blue) and 1D2 emission (red), NGTP has a temperature sensitive emission color. Figure 11 demonstrated the International Commission on Illumination (CIE) chromaticity coordinate and the photographs of NGTP at different temperature. It is shown

temperature the exciton will be easier transfer to Pr3+. Similarly, in Gd3+-doped ScPO4 at low temperatures, energy transfer from STEs to initial quenching centers is dominated; at room temperature, however, the transfer to Gd3+ is dominated.33 For Figure 8, the average fluorescence lifetime can be calculated by equation:32 τ=

∫0

∞

Ι(t ) dt Ι0

(5)

From eq 5 we obtain the lifetimes of host emission in NGTP and undoped NaGdTiO4 at different temperature and show them in Figure 9. Between 77 and 150 K the lifetime of host is higher in NGTP than in undoped NaGdTiO4. This finding is consistent with our above proposal that Pr 3+ acts as recombination center; it can decrease the quenching of host emission at low temperature, while efficient energy transfer from host to Pr3+ ions occurs when temperature is raised, presenting as an enhanced 1D2 red emission. Similar host sensitization process is also observed in Nd3+-doped TiO2 that the emission of Nd3+ is increased and reaches maximum at about 200 K, which is accompanied by a strong quenching of STEs emission at 100 K.34,35 If one assumes that, for irradiation transition of STEs there is a competing deexcitation process, which may include nonradiative recombination and energy transfer to activator and it has an activation energy E, one can write25,36 τ0 τ (T ) = 1 + A exp( −E /kT ) (6) where τ(T) is the decay time of host at different temperature, k is the Boltzmann constant, τ0 is the lifetime at a sufficiently low temperature, and A is the frequency factor. The fitted results are plotted in Figure 9 as solid lines. We obtained the values of E are approximately 1920 cm−1 (≈ 0.24 eV) and 1150 cm−1 (≈ 0.14 eV) for undoped NaGdTiO4 and NGTP, respectively. These values are larger than the E of STEs emission in SrTiO4 (about 0.07 eV), which has a lower quenching temperature

Figure 11. CIE chromaticity diagram and photographs of NGTP at different temperature. The line is a guide for the eye. 2220

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(19) Zhong, H.; Li, X.; Shen, R.; Zhang, J.; Sun, J.; Zhong, H.; Cheng, L.; Tian, Y.; Chen, B. J. Alloys Compd. 2012, 517, 170. (20) Boutinaud, P.; Mahiou, R.; Cavalli, E.; Bettinelli, M. Chem. Phys. Lett. 2006, 418, 185. (21) Boutinaud, P.; Sarakha, L.; Mahiou, R.; Dorenbos, P.; Inaguma, Y. J. Lumin. 2010, 130, 1725. (22) Malinowski, M.; Wolinski, W.; Wolski, R.; Strcek, W. J. Lumin. 1991, 48−49, 235. (23) Blasse, G. Mater. Res. Bull. 1983, 18, 525. (24) Boutinaud, P.; Pinel, E.; Dubois, M.; Vink, A. P.; Mahiou, R. J. Lumin. 2005, 111, 69. (25) Leonelli, R.; Brebner, J. L. Phys. Rev. B 1986, 33, 8649. (26) Blasse, G.; Verhaar, H. C. G.; Lammers, M. J. J.; Wingefeld, G.; Hoppe, R.; De Maayer, P. J. Lumin. 1984, 29, 497. (27) Blasse, G. J. Chem. Phys. 1968, 48, 3108. (28) Blasse, G. Struct. Bond. 1980, 42, 1. (29) Boutinaud, P.; Cavalli, E.; Bettinelli, M. J. Phys.: Condens. Matter 2007, 19, 386230. (30) Boutinaud, P.; Putaj, P.; Mahiou, R.; Cavalli, E.; Speghini, A.; Bettinelli, M. Spectrosc. Lett. 2007, 40, 209. (31) Neikirk, D. P.; Powell, R. C. J. Lumin. 1979, 20, 261. (32) Hou, D.; Liang, H.; Xie, M.; Ding, X.; Zhong, J.; Su, Q.; Tao, Y.; Huang, Y.; Gao, Z. Opt. Express 2011, 19, 11071. (33) Zhou, Y.; Feofilov, S. P.; Seo, H. J.; Jeong, J. Y.; Keszler, D. A.; Meltzer, R. S. Phys. Rev. B 2008, 77, 075129. (34) Le Boulbar, E.; Millon, E.; Ntsoenzok, E.; Hakim, B.; Seiler, W.; Boulmer-Leborgne, C.; Perrière, J. Opt. Mater. 2012, 34, 1419. (35) Luo, W.; Li, R.; Chen, X. J. Phys. Chem. C 2009, 113, 8772. (36) Struck, C. W.; Fonger, W. H. J. Appl. Phys. 1971, 42, 4515. (37) Boutinaud, P.; Sarakha, L.; Cavalli, E.; Bettinelli, M.; Dorenbos, P.; Mahiou, R. J. Phys. D: Appl. Phys. 2009, 42, 045106.

that NGTP has a large variation in hue (from blue to red) between 77 to 300 K.

4. CONCLUSIONS Deep understanding of the temperature-dependent luminescent properties of Pr3+ activated NaGdTiO4 is achieved. A redemission is observed at RT originating from the strong quenching of 3P0 transition. Importantly, the red emission of 1 D2 has a higher intensity at 350−400 K than at RT, which is due to the temperature-assisted energy transfer from host to Pr3+. These findings give us a possible approach of improving the temperature performance of phosphors used for wLEDs as well as for other applications, which also should be operated at high temperature, such as high pressure mercury lamp. However, a large color variety is obtained along with the increasing of temperature from 77 to 300 K because of the change of relative intensity of the blue host emission and red 1 D2 emission. The chromaticity coordinates gradually shift from blue-whitish to red region. Therefore, NGTP may also have a promising application as a fluorescence thermometer for temperature between 77 K and RT.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Tel.: +86-20-84111038, Fax: +86-20-84111038. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Natural Science Foundation of China (Grant Nos. 10979027, 21171176, and U1232108).

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REFERENCES

(1) Lin, C. C.; Xiao, Z. R.; Guo, G.-Y.; Chan, T.-S.; Liu, R.-S. J. Am. Chem. Soc. 2010, 132, 3020. (2) Bachmann, V.; Ronda, C.; Meijerink, A. Chem. Mater. 2009, 21, 2077. (3) Kang, D.; Yoo, H. S.; Jung, S. H.; Kim, H.; Jeon, D. Y. J. Phys. Chem. C 2011, 115, 24334. (4) Huang, C.-H.; Chen, T.-M. Inorg. Chem. 2011, 50, 5725. (5) Tsao, J. Y. IEEE Circ. Dev. Mag. 2004, 20, 28. (6) Yamamoto, H.; Urabe, K. J. Electrochem. Soc. 1982, 129, 2069. (7) Wang, D.-Y.; Chen, T.-M.; Cheng, B.-M. Inorg. Chem. 2012, 51, 2961. (8) Treadaway, M. J.; Powell, R. C. Phys. Rev. B 1975, 11, 862. (9) Sardar, D.; Powell, R. C. J. Appl. Phys. 1980, 51, 2829. (10) Elouadi, B.; Powell, R. C.; Holt, S. L. J. Solid State Chem. 1987, 69, 369. (11) Yang, X.; Liu, J.; Yang, H.; Yu, X.; Guo, Y.; Zhou, Y.; Liu, J. J. Mater. Chem. 2009, 19, 3771. (12) Boutinaud, P.; Cavalli, E.; Mahiou, R. J. Phys.: Condens. Matter 2012, 24, 295502. (13) Inaguma, Y.; Muronoi, T.; Sano, K.; Tsuchiya, T.; Mori, Y.; Katsumata, T.; Mori, D. Inorg. Chem. 2011, 50, 5389. (14) Boutinaud, P.; Pinel, E.; Oubaha, M.; Mahiou, R.; Cavalli, E.; Bettinelli, M. Opt. Mater. 2006, 28, 9. (15) Toda, K.; Kameo, Y.; Kurita, S.; Sato, M. J. Alloys Compd. 1996, 234, 19. (16) Coelho, A. A. TOPAS ACADEMIC; Brisbane, Australia, 2005; Vol. version 4. (17) Blasse, G.; Bril, A. J. Chem. Phys. 1968, 48, 3652. (18) Dorenbos, P.; Krumpel, A. H.; van der Kolk, E.; Boutinaud, P.; Bettinelli, M.; Cavalli, E. Opt. Mater. 2010, 32, 1681. 2221

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