Synthesis and Photoluminescence Characteristics of High Color Purity

J. Phys. Chem. C 2010, 114, 3645–3652

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Synthesis and Photoluminescence Characteristics of High Color Purity and Brightness Li3Ba2Gd3(MoO4)8:Eu3+ Red Phosphors Yee-Cheng Chang,† Chih-Hao Liang,† Shao-An Yan,† and Yee-Shin Chang*,‡,§ Department of Materials Science and Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan, and Department of Electronic Engineering, National Formosa UniVersity, Huwei, Yunlin 632 Taiwan ReceiVed: August 31, 2009; ReVised Manuscript ReceiVed: December 07, 2009

The object of this study is to synthesize Li3Ba2Gd3(MoO4)8 doped with Eu3+ ions and prepared with solidstate reaction technology. The results show that the dominant emission peak of Li3Ba2Gd3-x(MoO4)8:Eux phosphor is 5D0 f 7F2 (614 nm). The intensity of the emission from 5D0 to 7F2 is 3.8 times higher than that of commercial phosphors, ZnS:(Mn2+,Te2+) when the Eu3+ concentration is x ) 2.4. The CIE chromaticity coordinates of the red emission of the Li3Ba2Gd0.6Eu2.4(MoO4)8 phosphor are (0.67, 0.33), which is the NTSC system standard for red chromaticity. Because there are two regions in the excitation spectra of Li3Ba2Gd3(MoO4)8:Eu3+phosphor, one is assigned from the charge-transfer state (CTS) band at about 320 nm, and the other is from the intra-4f transitions from 350 to 500 nm. Hence, the phosphors could be strongly excited by near-UV and blue LED in solid-state lighting technology. The analysis of the lifetime, decay curves, and efficiency of the 5D0 f 7F2 emission indicates the weak energy transfer between Eu3+ pairs. The temperature dependence PL and absorption spectra study shows that thermal quenching behavior can be attributed to crossover from the 5D0 excited state to the CTS band. Introduction White-light-emitting diode (LED) technology is a potential replacement for incandescent light sources.1,2 Phosphors are one of the important materials in lighting technology and have been widely investigated.3-6 The most common white LED uses a 450-470 nm blue-emitting diode that excites a yellow-emitting YAG:Ce3+ phosphor.1 However, white light caused by blue LED and yellow phosphors has many disadvantages, such as lack of a color rendering index.7 The solution to this problem is to excite RGB phosphors by using near-UV LED in order to get excellent color rendering properties, so these phosphors require more attention.8 Recently, the molybdates Li3Ba2Gd3(MoO4)8 have been used in solid-state laser materials.9,10 Several attempts have been made to enhance the brightness of phosphors via Li doping to improve crystalline quality. In addition, Li-containing phosphors have been reported with a low synthesis temperature.11,12 The ternary molybdates Li3Ba2Gd3(MoO4)8 were synthesized, and their crystal structure was solved by Klevtsova et al.13 The Li3Ba2Gd3(MoO4)8 has monoclinic crystals with space group C2/c, a ) 5.238 (Å), b ) 12.758 (Å), c ) 19.151 (Å), and β ) 91.126°. The distribution of the Gd3+, Ba2+, and Li+ ions over the M(1) and M(2) positions leads to the chemical formula Li2M(1)2M(2)4(MoO4)8, where M(1) d Ba0.85Gd0.15, M(2) d Gd0.675Ba0.075Li0.25, and 90% of Gd3+ ions occupy the M(2) sites. The M(1) and M(2) sites are coordinated by five and eight oxygen atoms, respectively, and both sites lack inversion center symmetry.13 Therefore, a Li2M(1)2M(2)4(MoO4)8 compound could be a suitable host for a Eu-doped phosphor with high color purity and brightness by induced 5D0 f 7F2 red emission * To whom correspondence should be addressed. Tel: +886-5-6315684. Fax: +886-5-6315643. E-mail: [email protected]. † National Cheng Kung University. ‡ National Formosa University. § Address: No. 64, Wenhua Road, Huwei, Yunlin 632, Taiwan.

and low synthesis temperature. However, few studies have investigated Li3Ba2Gd3(MoO4)8 with regard to luminescence consequently. In this paper, we discuss the luminescence behavior of Li3Ba2Gd3(MoO4)8:Eu and compare its emission intensity with that of the commercial phosphors, ZnS: (Mn2+,Te2+). The concentration and thermal quenching behavior are also investigated. Experimental Section Materials and Synthesis. Samples of Eu3+-doped Li3Ba2Gd3(MoO4)8 were synthesized by a vibrating milled solidstate reaction. The starting materials were Li2CO3, BaCO3, Gd2O3, MoO3, and Eu2O3 with a purity of 99.99% and were supplied by Aldrich Chemical Co. and Alfa Aesar. After these materials had been mechanically activated by grinding in a highenergy vibro-mill, the mixture was calcined at 900 °C in air for 12 h. Characterizations. The X-ray diffraction of the samples was examined on a Rigaku Dmax diffractometer using Cu KR radiation (λ ) 0.15406 nm) to identify the crystal phases. The morphology of the samples was inspected using a scanning electron microscope (SEM, Philips XL-40FEG). The photoluminescence (PL) spectra of these phosphors were recorded on a Hitachi F-4500 fluorescence spectrophotometer using a 150 W Xe lamp as a source at room temperature, and hightemperature (25-300 °C) PL was measured by using a heating holder with a thermal coupled and electric heater. Results and Discussion XRD and Morphology of the Phosphors. Figure 1 shows the XRD patterns of Li3Ba2Gd3-x(MoO4)8:Eux calcined at 900 °Cfor12h.BecauseallthediffractionpeaksoftheLi3Ba2Gd3-x(MoO4)8: Eux samples were in agreement with the JCPDS cards (No. 01077-0830) for the standard Li3Ba2Gd3(MoO4)8 reference, they reveal a single phase without any impurities, indicating that the Gd3+ ions were fully substituted by the Eu3+ ions.

10.1021/jp9084124  2010 American Chemical Society Published on Web 02/04/2010

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Figure 1. XRD patterns of Li3Ba2Gd3-x(MoO4)8:Eux calcined at 900 °C for 12 h.

The morphologies of the crystalline Li3Ba2Gd3-x(MoO4)8: Eux were inspected using a scanning electron microscope (SEM). Figure 2 shows SEM micrographs of Li3Ba2(Gd3-xEux)(MoO4)8 powders calcined at 900 °C for 12 h: (a) x ) 0.07, (b) x ) 0.4, (c) x ) 1.8, and (d) x ) 2.6. The morphologies show aggregated particles with sizes ranging from 3 to 5 µm, and the larger particles are attributed to the Li content in the Li3Ba2(Gd3-xEux)(MoO4)8, which enhances grain growth and improves crystallinity at a low calcination temperature.11,12 Luminescence and Absorption Properties of Eu3+-Doped Li3Ba2Gd3(MoO4)8 Phosphors. Figure 3a presents the absorption spectra of Li3Ba2Gd3(MoO4)8 and Li3Ba2Gd3-x(MoO4)8: Eux. The undoped Li3Ba2Gd3(MoO4)8 host has the broad

absorption band from 250 to 360 nm, which is attributed to the (MoO4)2- group14 and the absorption edge at 360 nm (3.4 eV). The absorption spectrum of the Li3Ba2Gd3(MoO4)8:Eux phosphors consists of two parts. One part is a broadband from 250 to 460 nm assigned to the overlap of (MoO4)2- group absorption and the Eu3+-O2- CTS band, and the other part is sharp peaks in the range from 350 to 600 nm that are associated with typical intra-4f forbidden transitions of the Eu3+ ions. It is also found from the absorption spectrum of Eu3+-doped Li3Ba2Gd3-x(MoO4)8: Eux phosphors that the absorption edge located at 455 nm (2.72 eV) shifts to a lower energy compared with that of the undoped host. The energy of the lowest charge-transfer absorption depends on the electronegativity difference between the

Li3Ba2Gd3(MoO4)8:Eu3+ Red Phosphors

J. Phys. Chem. C, Vol. 114, No. 8, 2010 3647

Figure 2. SEM micrographs of Li3Ba2Gd3-x(MoO4)8:Eux powders calcined at 900 °C for 12 h: (a) x ) 0.07, (b) x ) 0.4, (c) x ) 1.8, and (d) x ) 2.6.

Eu3+-O2- ligand15,16 and the Eu3+-O2- CTS, with the lower absorption edge causing a small electronegativity difference between Eu3+-O2-.16 The lower absorption edge of Eu-doped Li3Ba2Gd3-x(MoO4)8:Eux phosphors is attributed to the Eu3+-O2- CTS band, which may effect the thermal quenching behavior. Figure 3b shows the excitation spectra of Li3Ba2Gd3-x(MoO4)8: Eux, which is monitored at 614 nm emission (5D0 f 7F2). The shape difference between the absorption and excitation spectra in the CTS band of Eu3+-O2- and Mo5+-O2- is because part of the CTS absorption does not contribute to the 5D0 f 7F2 emission. There is a series of sharp excitation bands present between 350 and 550 nm that are associated with the typical intra-4f transitions of the Eu3+ ions that centered at 364, 384, 395, 415, 466, and 532 nm and were attributed to the 7F0 f 5D , 7F f 5L , 7F f 5L , 7F f 5D , 7F f 5D , and 7F f 4 0 7 0 0 0 3 0 2 0 5D , respectively. The strongest excitation peaks at 465 nm 1 contribute to the 7F0 f 5D2 transition in the blue light region, and the Li3Ba2Gd3-x(MoO4)8:Eux phosphor is thus suitable to be used for near-UV and blue LED exciting red phosphor for white lighting devices. Figure 4 shows the emission spectra of Li3Ba2Gd3-x(MoO4)8: Eu3+ with different Eu3+ concentrations under 465 nm excitation. The strongest emission peak of Li3Ba2Gd3-x(MoO4)8:Eu3+, located at 614 nm, is attributed to 5D0 f 7F2. Other peaks at 532-540, 551-563, and 579-598 nm are assigned to the 5D1 f 7F1, 5D1 f 7F2, and 5D0 f 7F1 transitions of the Eu3+ ions,

respectively. The line shape of emission does not change with Eu3+ ion concentrations. Most of the valence electrons of trivalent rare-earth elements are shielded by the 5s and 5p outer electrons, and so, the f-f transitions of trivalent lanthanides are weakly affected by ligand ions in the crystals.17 Therefore, the optical spectra of most phosphors doped with trivalent rareearth elements are similar to those expected for free ions. However, a few transitions are sensitive to the environment of the crystal, and these have been called hypersensitive transitions. The electric dipole transition 5D0 f 7F2 is hypersensitive, and the emission intensity is strongly influenced by ligand ions in the crystals.17 When the Eu3+ ions are located at a low symmetry site, the 5D0 f 7F2 emission transition often dominates in the emission spectrum. The magnetic dipole 5D0 f 7F1 transition regardless of environment, according to the Judd-Ofelt theory.17 In the Eu3+ ions doped in Li3Ba2Gd3(MoO4)8 phosphors, the emission intensity of 5D0 f 7F2 is more sensitive than the emission intensity of 5D0 f 7F1, which indicates that the Eu3+ ions have no inversion center. The (5D0 f 7F2)/(5D0 f 7F1) emission ratio (i.e., the asymmetry ratio) can be used as an index to measure the site symmetry of Eu3+ ions.18,19 Figure 5 shows the dependence of the (5D0 f 7F2)/(5D0 f 7F1) emission ratio on Eu3+ ion concentrations in Li3Ba2Gd3-x(MoO4)8:Eux under excitation at 465 nm. The ratio is independent of Eu3+ concentration, which indicates that the symmetry of Eu3+ ions does not change with this concentration. The asymmetry ratio of Li3Ba2Gd3-x(MoO4)8:Eux is equal to 8.1, which is very large

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Chang et al. centers becomes small enough to allow a resonant energy transfer, so the energy can be easily transferred from one luminescent center to another. As a result, if Eu3+ ion concentrations are sufficiently high, the higher level (5D1) emission can be easily quenched via cross-relaxation and the 5D emission becomes dominant. In addition, at low Eu3+ doping 0 concentration (x ) 1%), the cross-relaxation can be neglected .The intensity of the higher excited state (5D1) emission is very weak compared with that of very low phonon energy host materials, such as BaR2ZnO5:Eu (R ) Y, Gd) phosphors.6,21 The nonraditation multiphonon relaxation rate of the 4fn configuration is given by17

WNR ) WNR(0)exp(Rnpωp)

Figure 3. (a) Absorption and (b) excitation spectra of Li3Ba2Gd3-x(MoO4)8: Eux calcined at 900 °C for 12 h (λem ) 614 nm).

compared with the high site symmetry of trivalent rare-earth element phosphors, such as BaLa2ZnO5:Eu.19 The CIE chromaticity coordinates of the red emission of the Li3Ba2Gd0.6Eu2.4(MoO4)8 phosphor is (0.67, 0.33), which follows the NTSC system standard red chromaticity. ZnS:(Mn2+,Te2+) is used as the standard for the Li3Ba2Gd3-x(MoO4)8:Eux phosphors. The intensity of the emission of the 5D0 f 7F2 transition for Li3Ba2Gd0.6(MoO4)8:Eu2.4 is 3.8 times higher than that of the commercial phosphor red sulfide phosphors. The lack of an inversion center around the Eu3+ ions can provide a high color purity and brightness red phosphor. The intensity of the 5D emission increases with the Eu3+ ion concentration, and as J the critical concentration is reached, so the intensity of the 5DJ emission gradually decreases. This concentration quenching behavior will be discussed in more detail in the next section. Concentration Quenching Effect of 5DJ)0,1 Emission. The insetofFigure4showsthe5D1emissionspectrumofLi3Ba2Gd3-x(MoO4)8: Eux phosphors as a function of Eu doping concentration. The intensity of the emission from 5D1 f 7F1 increases with increasing Eu3+ ion concentrations and gradually decreases as the doping concentration becomes higher than x ) 0.1. This is because the higher-energy level 5D1 is quenched by the crossrelaxation mechanism.20 This is a nonradiative process whereby excitation energy from an ion decaying from a higher excited state (5D3.2.1) of Eu3+ promotes a neighboring ion from the ground state to the metastable state level, such as 5D1(Eu1) + 7F (Eu ) f 5D (Eu ) + 7F (Eu ).20 As the concentration of 0 2 0 1 3 2 luminescent centers increases, the distance between luminescent

(1)

where R depends on the character of the phonon, WNR is the relaxation rate, ∆E is the energy difference between the levels, pωp is the highest available vibrational phonon energy, and n ) ∆E/pωp is the number of phonons to fill the energy gap. The BaR2ZnO5:Eu (R ) Y, Gd) host has a maximum available phonon energy of about 500 cm-1,21 and the Li3Ba2Gd3(MoO4)8 host has a maximum phonon energy of around 1000 cm-1.13 According to eq 1, a high maximum available phonon energy can reduce the number of phonons and thus fill the energy gap and enhance multiphonon relaxation to bridge the energy level between 5D1 and 5D0. Hence, the 5D1 emission intensity of Li3Ba2Gd3(MoO4)8:Eu is very weak compared with that of BaR2ZnO5:Eu (R ) Y, Gd) phosphors. The relative emission intensity and decay time of 5D0 f 7F2 with Eu3+ ions are presented in Figure 6. Before concentration quenching is reached, the intensity of emission increases with the Eu3+ ion concentration until x ) 2.4. This phenomenon is caused by the energy transfer between luminescent centers, and these energy-transfer chains trigger energy migration to crystalline defects or trace ions.22 In addition, decay time does not significantly change with increasing Eu3+ ion concentration until x ) 2.4, and then it slightly decreases as the Eu3+ ion concentration continues to rise. The concentration quenching is due to energy transfer from one activator (donor) to another until the energy sink (acceptor) in the lattice is reached. Hence, the energy transfer will strongly depend on the distance (R) between the Eu3+ ions, which can be obtained using the following equation23,24

(

R)2

3V 4πxN

)

1/

3

(2)

where x is the concentration, N is the number of trivalence of rare-earth ions in the Li3Ba2Gd3(MoO4)8 unit cell [N ) 6 in Li3Ba2Gd3(MoO4)8], and V is the volume of the unit cell (V ) 1.279 × 10-27 m3 in this case).13 For the 5D0 to 7F1 transition, the critical concentration is estimated to be about x ) 2.4, where the measured emission intensity and decay time begin to decrease. The critical distance (Rc) between the donor and acceptor can be calculated from the critical concentration, for which the nonradiative transfer rate equals the internal decay rate (radiative rate). Blasse23 assumed that, for the critical concentration, the average shortest distance between the nearest activator ions is equal to the critical distance. In addition, the radiation lifetime of the donor center (τD) is affected by the energy-transfer process between the donor and acceptor. For simplicity, τD can be expressed as follows25

1 1 ) + Pt τD (τD)0

(3)

where (τD)0 is the radiative lifetime of donor ions and Pt is the



Figure 4. Dependence of emission spectra of Li3Ba2Gd3-x(MoO4)8:Eux on Eu3+ ion concentrations calcined at 900 °C for 12 h (λex ) 465 nm) compared with ZnS:(Mn2+,Te2+) commercial phosphors (λex ) 466 nm). The inset shows the 5D1 emission spectrum of Li3Ba2Gd3-x(MoO4)8:Eux phosphors as a function of Eu doping concentration.

TABLE 1: Lifetime (τ) of the 5D0 f 7F2 Transition, Efficiency (η), and Distance between the Eu3+-Eu3+ Pair of Li3Ba2Gd3-xEux(MoO4)8 Phosphors

Figure 5. Dependence of the asymmetry ratio on Eu3+ ion concentration in Li3Ba2Gd3-x(MoO4)8:Eux under an excitation of 465 nm.

average probability of transfer to the energy sink. For simplicity, we assume that the quantum efficiency ) l for the low concentration (x ) 1%) and the (τD)0 ) 0.48 ms. To estimate the reduction in efficiency due to the Eu3+-Eu3+ pair transfer to the killer (i.e., Pt), the efficiency (η) can be simplified as follows:25

η)

1/(τD)0 1/τ

(4)

The lifetime (τD) of the 5D0 f 7F2 transition, efficiency (η), and distance between the Eu3+-Eu3+ pair of Li3Ba2Gd3(MoO4)8 phosphors are presented in Table 1, in which it can be seen that the efficiency starts to decrease significantly before the critical concentration and that, when the concentration is up to x ) 3, the efficiency falls slightly to 85.0%. This reduces REu-Eu and enhances the average probability of transfer to the energy sink. The critical distance (Rc) is equal to 5.53 Å, which is close to the distance between the Gd3+ sites, which is around 5 Å.13 As the REu-Eu is smaller than the critical distance (Rc), the energy transfer can be along the porous corrugated layer RE3+ sites.

Eu concentration (x)

τ (ms)

η (%)

REu-Eu (Å)

2 2.2 2.4 2.6 2.8 3

0.471 0.459 0.448 0.437 0.413 0.408

98.1 95.6 93.3 91.0 86.0 85.0

5.88 5.70 5.53 5.39 5.26 5.14

Figure 7 shows the decay curves of the 5D0 f 7F2 transition for various Eu3+ ion concentrations in Li3Ba2(Gd3-xEux)(MoO4)8 under excitation at 465 nm. All of the decay curves were fitted by a single exponential decay I ) I0 exp(-t/τ), where I is intensity, I0 is initial intensity, and τ is lifetime. In general, the distance between activator ions decreases as the activator concentration increases. Subsequently, the energy-transfer process between activator ions becomes more frequent, providing an extra decay channel. This extra decay channel possibly has a different relaxation rate that leads to the decay curve showing nonsingle exponential decay.26-28 However, the curve shows a single exponential decay when the Gd3+ sites are fully substituted by Eu3+ ions in Li3Ba2(Gd3-xEux)(MoO4)8 phosphor. This result corresponds to the fact that the efficiency is only reduced by a factor of 15% in the Li3Ba2Eu3(MoO4)8 phosphor. According to efficiency and decay curve analyses, there is a weak energy transfer between Eu3+ pairs, which may be caused by the Eu3+ being shielded by the Li, Mo, and Ba polyhedral in the Li3Ba2Eu3(MoO4)8 structure. Thermal Quench Behavior. Figure 8 shows the thermal stability of Li3Ba2Gd0.6Eu2.4(MoO4)8 from a temperature of 25-300 °C. The relative peak intensity of Li3Ba2Gd0.6Eu2.4(MoO4)8 had decreasing dependence on the temperature and fell by a factor of 85% from the initial intensity at 300 °C. The absorption of Li3Ba2Gd3-x(MoO4)8:Eux indicated a lower Eu3+-O2- CTS band, and the low-lying O2- to Eu3+ CTS band can provide a path to relaxation of the excited state via a nonradivite process. Many studies have examined the thermal quenching mechanism of 5D0 f 7F2 emission, which can cross over the 5D0 state to the CTS band.29-31 The crossover

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Figure 6. Relationship of emission intensity of 5D0 to 7F2 and decay time with different Eu3+ ion concentrations in Li3Ba2Gd3-x(MoO4)8:Eux under an excitation of 465 nm.

Figure 7. Decay curves of the 5D0 f 7F2 transition for various Eu3+ ion concentrations in Li3Ba2(Gd3-xEux)(MoO4)8 under an excitation of 465 nm.

quenching from the 5D0 excited state to the CTS band is a thermal activitation process that can be described by the following equation30

I(T) ) I0[1 + A exp(-E/kT)]-1

(5)

where A is a constant and E is the activation energy from the 5D state to the CTS band. 0 TheactivationenergyforthermalquenchingofLi3Ba2Gd3-x(MoO4)8: Eu3+ can be obtained by ln[ I/I(T)] versus 1/kT, as shown in

Figure 8b, and the activation energy is equal to 0.283 eV (2282.5 cm-1). The crossover mechanism is a possible pathway to thermal quenching, compared with the mutiphonon model that requires 16.3 phonons at 1000 cm-1 to bridge the energy between the 5D0 and 7F2 levels and is thus too difficult to obtain. According to the absorption spectra and the temperature dependence PL spectra, possible pathways for thermal quench are presented in Figure 9. The 5D2 and 5D1 are relaxed by multiphonon emission or cross-relaxation to the 5D0 state. The


Figure 8. (a) Temperature dependence of emission spectra for Li3Ba2Gd3-x(MoO4)8:Eux phosphors. The inset in (a) shows the relative intensity emission of Li3Ba2Gd3-x(MoO4)8:Eux phosphors as a function of temperature. (b) Plot of activation energy for thermal quenching of Li3Ba2Gd3-x(MoO4)8:Eu3+.

pathways of the thermal quenching of the 5D0 state are through a Eu3+-O2- CTS band. Some electrons then overcome the activation energy assisted by phonons as the temperature increases and feed to the 7FJ state, which provides the nonraditative process, and the remaining electrons are contributed to the 5D0 f 7F1 emission. Conclusions A novel phosphor Eu3+-doped Li3Ba2Gd3(MoO4)8 was synthesized by a solid-state reaction, and the photoluminescence properties were investigated. The emission intensity of Li3Ba2Gd0.6Eu2.4(MoO4)8 is 3.8 times more than that of the commercial phosphors, ZnS:(Mn2+,Te2+) and has high color purity CIE chromaticity coordinates located at (0.67, 0.33), which is just at the NTSC system standard for red chromaticity. Hence, the phosphor could be excited by not only near-UV but also blue LED in solid-state lighting technology. Study of the Eu3+ concentration variation indicated weak energy transfer between Eu3+-Eu3+ pairs that could be caused by the Eu3+ being shielded by the Li, Mo, and Ba polyhedral in the Li3Ba2Eu3(MoO4)8 structure, and thermal quenching is due to the crossover quenching from the 5D0 excited state to the CTS band.


Figure 9. Plot of pathways for the thermal quenching of the 5D0 state through a CTS band.

Acknowledgment. The authors thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 98-2622-E-150-065-CC3. References and Notes (1) Shimizu, Y.; Sakano, K.; Noguchi, Y.; Moriguchi, T. U.S. Patent 5,998,925, 1998. (2) Sakuma, K.; Hirosaki, N.; Kimura, N.; Ohashi, M.; Xie, R. J.; Yamamoto, Y.; Suehiro, T.; Asano, K.; Tanaka, D. IEICE Trans. Electron. 2005, E88-C, 2057–2064. (3) Chang, C. K.; Chen, T. M. Appl. Phys. Lett. 2007, 90, 161901. (4) Piao, X. Q.; Machida, K. I.; Horikawa, T.; Hanzawa, H. Appl. Phys. Lett. 2007, 91, 041908. (5) Allen, S. C.; Steckl, A. J. Appl. Phys. Lett. 2008, 92, 143309. (6) Liang, C. H.; Chang, Y. C.; Chang, Y. S. Appl. Phys. Lett. 2008, 93, 211902. (7) Toda, K.; Kawakami, Y.; Kousaka, S. I.; Ito, Y.; Komeno, A.; Uematsu, K.; Sato, M. IEICE Trans. Electron. 2006, E89-C, 1406–1412. (8) Nishida, T.; Ban, T.; Kobayashi, N. Appl. Phys. Lett. 2003, 82, 3817. (9) Song, M. J.; Wang, G. J.; Zhang, L. H.; Lin, Z. B.; Wang, G. F. J. Alloys Compd. 2009, 478, 423–426. (10) Song, M. J.; Zhang, L. H.; Wang, G. F. J. Alloys Compd. 2009, 480, 839–842. (11) Yi, S. S.; Bae, J. S.; Moon, B. K.; Jeong, J. H.; Park, J. C.; Kim, W., III Appl. Phys. Lett. 2002, 81, 3344. (12) Cho, J. Y.; Do, Y. R.; Huh, Y. D. Appl. Phys. Lett. 2006, 89, 131915. (13) Klevtsova, R. F.; Vasil’ev, A. D.; Glinskaya, L. A.; Kruglik, A. I.; Kozhevnikova, N. M.; Korsun, V. P. J. Struct. Chem. 1992, 33, 443–447. (14) Haque, Md. M.; Lee, H. I.; Kim, D. K. J. Alloys Compd. 2009, 481, 792–796.

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Synthesis and Photoluminescence Characteristics of High Color Purity

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