Mg_1.5Lu_1.5Al_3.5Si_1.5O₁₂:Ce³⁺,Mn²⁺: A novel garnet phosphor with adjustable emission color for blue light-emitting diodes

Abstract

Mg_1.5Lu_1.5Al_3.5O_1.5O₁₂:Ce³⁺,Mn²⁺, a novel garnet phosphor was synthesized by solid state reaction. The obtained samples can be excited efficiently by the blue light and exhibit varied hues from green-yellow through yellow and eventually to orange by tuning the relative proportion of Ce³⁺/Mn²⁺.

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Solid state lighting in the form of white light-emitting diodes (LEDs) has received increasing interest in recent years for the characteristic merits and promising application, which has potential to entirely replace both incandescent and fluorescent lamps in the near future.^1–3 The commercial devices in the market use InGaN blue LED in combination with a yellow phosphor, typically Ce³⁺-doped yttrium aluminum garnet (YAG:Ce³⁺). Although the path of white light realization is inexpensive and efficient, it has poor color-rendering properties due to the deficiency of the red component. Many efforts have been devoted to improve the color rendering index (CRI), which can be grouped into two categories: one is the modification of the YAG:Ce³⁺ system, including codoping the Pr³⁺ ion and incorporation the Si⁴⁺–N³⁻ chemical bond.^4,5 The problem with this method is the enhancement of the red component efficiency is not obvious; the other is exploring the red phosphors which can be added into the above-mentioned system. The obstacle in research is that the red phosphors mainly focus on the nitride system and the corresponding synthetic process is high cost and harsh.^6–8 In addition, the difference in degradation between different host phosphors will produce color aberration, an undesirable phenomenon in the real application. Accordingly, it is urgent to investigate a single-host phosphor with yellow-to red emission band for blue LEDs. Following the energy transfer mechanism, the phosphor could exhibit varied hues by codoping activators, such as Ce³⁺/Eu²⁺,⁹ Ce³⁺/Mn²⁺,¹⁰ and Eu²⁺/Mn²⁺.¹¹ The Ce³⁺ and Mn²⁺ ions possess f-d and d-d electron configuration, and they are good candidates as activators in phosphor because they can emit broadband visible light. The energy transfer between Ce³⁺ and Mn²⁺ ions has been realized in many systems, however, the corresponding research in the garnet system based on the energy transfer to increase the red component is rarely reported, perhaps the reason for this may be no suitable crystallographic site for Mn²⁺ in the existing aluminum garnet host lattice. In this communication, we substitute the Lu–Al chemical bond with the Mg–Si chemical bond in the existing garnet Lu₃Al₅O₁₂, and synthesize the compound with the formula Mg_1.5Lu_1.5Al_3.5Si_1.5O₁₂ (MLASO) which the Ce³⁺ and Mn²⁺ ions can dope into the host lattice and substitute the Lu³⁺ and Mg²⁺ crystallographic sites, respectively. The luminescent properties of the MLASO:Ce³⁺,Mn²⁺ phosphor is also investigated as well as energy transfer phenomenon between the sensitizer (Ce³⁺) and activator (Mn²⁺).

MLASO:Ce³⁺,Mn²⁺ phosphors were prepared by solid state reactions from high purity MgO, Lu₂O₃, Al₂O₃, SiO₂, CeO₂, and MnCO₃ starting materials sintered at 1350 °C under CO reducing atmosphere for 3 h. Photoluminescence (PL) and photoluminescence excitation (PLE) were measured by spectrophotometers (Hitachi, F-4500) using a xenon lamp. Crystal structure and phase were analyzed using X-ray diffraction (Bruker D8, Target Cu-Kα, 40 kV, 40 mA). Rietveld analysis of XRD profile made use of the general structure analysis system (GSAS) program. All the measurements mentioned above were performed at room temperature.

Fig. 1 shows a Rietveld refinement of the XRD patterns of MLASO. The initial structural model which approximates the actual structure of MLASO was constructed with crystallographic data previously reported for Lu₃Al₅O₁₂ (JCPDS card No.73-1368). All of the observed peaks satisfy the reflection condition, confirming that the single-phase nature of the MLASO. Successful formation of the single phase can attribute to the fact that the end members Lu₃Al₅O₁₂ and M₃Al₂Si₃O₁₂ (M = Mg, Mn) possess similar garnet crystal structure with the general formula {R₃}[C₂](D₃)O₁₂, where {}, [] and () denote dodecahedral, octahedral and tetrahedral co-ordination, respectively. Therefore, it is reasonably inferred that both Lu³⁺ and Mg²⁺ ions occupy the dodecahedral sites in our sample. This inference can be supported by the XRD result of our sample. The XRD peaks of MLASO samples shift towards higher 2θ angles, compared with the XRD peaks of Lu₃Al₅O₁₂, indicating the shrinkage of the unit cell. This can be expected from the substitution of smaller Mg²⁺ ions (89 pm) for larger Lu³⁺ ions (97 pm) on dodecahedral sites, and smaller Si⁴⁺ ions (40 pm) for larger Al³⁺ ions (67.5 pm) on the octahedral sites.¹² Similarly, the activators Ce³⁺ and Mn²⁺ have distributed into the host lattice by replacing the Lu³⁺ and Mg²⁺ ions, respectively, and the related XRD profiles are shown in Fig. S1.†

Fig. 1 Experimental (crosses), calculated (solid line), and difference (bottom) results of XRD refinement of MLASO.

The emission and excitation spectra of the MLASO:Ce³⁺ and MLASO:Mn²⁺ are represented in Fig. 2. As shown in Fig. 2(a), under blue of 430 nm excitation, the Ce³⁺ singly doped sample exhibits a broad asymmetric green-yellow emission centered at 525 nm, which is attributed to the 5d¹→4f¹ transition of the Ce³⁺ ions. The asymmetric emission band can be deconvoluted into two Gaussians centering at 517 and 577 nm, as indicated in Fig. S2.† The energy difference between spin–orbit splitting of 4f ground state in MLASO:Ce³⁺ is about 2010 cm⁻¹, and is in agreement with the theoretical value of 2000 cm⁻¹, indicating that the emission originated from the lowest 5d level to the ²F_5/2 and ²F_7/2 levels, respectively. Compared with the luminescence properties of Lu₃Al₅O₁₂:Ce³⁺ phosphor, those of MLASO:Ce³⁺ exhibit some differences: the PLE and PL spectra shift to the short wavelength; the absorption sub-band at short-wavelength becomes wider. The blue-shift of Ce³⁺ luminescence is opposite to the work reported by Katelnikovas' group:¹³ the red-shift of Ce³⁺ luminescence can be realized in the LuAG system by the substitution Al³⁺–Al³⁺→Mg²⁺–Si⁴⁺, and the related reason is analyzed as below. In Katelnikovas' samples Lu₃Al₃MgSiO₁₂:Ce³⁺, the Mg²⁺–Si⁴⁺ couple on the octahedrally coordinated Al³⁺ sites and tetrahedrally coordinated Al³⁺ sites. We infer that the reason for the red-shift of the Ce³⁺ luminescence can be ascribed to two factors: First, the Si⁴⁺ ions substituted tetrahedral Al³⁺ crystallographic sites. The Si⁴⁺ ions possess the higher charge and smaller size in comparison to Al³⁺ ions, which lead to the O–{Lu}–O bond angle for the dodecahedral-tetrahedral shared edge becoming smaller in the Lu₃Al₃MgSiO₁₂ in comparison to typical aluminate garnet (Lu₃Al₅O₁₂), resulting in the red-shift of Ce³⁺ luminescence.⁵ Second, the Mg²⁺ ions substituted octahedral Al³⁺ crystallographic sites. The Mg²⁺ ions possess the smaller charge (+2) and bigger size (72 pm) in comparison to Al³⁺ ions (+3, 53.5 pm), which lead to the lower charge density of the ions located onto the octahedral sites. The lower charge density results in a decrease covalent interaction between the oxygen anions and the ions located at octahedral sites, but the increased covalent interaction between the oxygen anions and the ions located at dodecahedral sites, and a red-shift of Ce³⁺ luminescence can be expected. In our sample Lu_1.5Mg_1.5Al_3.5Si_1.5O₁₂:Ce³⁺, the Mg²⁺ and Si⁴⁺ ions occupy the dodecahedrally coordinated Lu³⁺ sites and tetrahedrally coordinated Al³⁺ sites, respectively. The substitution of tetrahedral Al³⁺ crystallographic sites by Si⁴⁺ would make a similar effect on luminescence properties as the above analysis. However, it is worth noting the occupancy of Mg²⁺ on the dodecahedral Lu³⁺ crystallographic sites. Taken into consideration the smaller size of Mg²⁺ (89 pm) compared to Lu³⁺ (97 pm), a blue-shift of Ce³⁺ luminescence is expected. The variation of Ce³⁺ luminescence in our samples can be attributed to the interaction of the two factors, and the experimental phenomenon shows that the latter one dominates the effect. Although the analysis above can explain the luminescent properties of Ce³⁺ ions in these garnet, further work is needed to get a better understanding for the structure-property correlation in Ce³⁺-doped garnet phosphors. Beside the blue-shift of the Ce³⁺ luminescence in MLASO:0.05Ce³⁺, the excitation bands in the deeper UV additional excitation bands, unlike the UV excitation bands in Lu₃Al₅O₁₂:Ce³⁺. This additional excitation bands in the UV for these garnet could be due to silicate 2nd phases (e.g., apatites or Lu₂SiO₅/Lu₂Si₂O₇) which contain Ce³⁺ ions, these 2nd phases could be below XRD detection limits. Similar result has been previously noted in other work for the Ce³⁺ luminescence in silicate garnets.¹⁴ As depicted in Fig. 2(b), the PL spectrum of MLASO:Mn²⁺ shows a broad red emission band centering at 620 nm ascribed to the spin-forbidden ⁴T₁(⁴G)→⁶A₁(⁶S) transition of the Mn²⁺ ions. The PLE spectra thereof cover a wide range of 200–500 nm. However, the relative intensity of the red emission from the Mn²⁺ is very weak, and this is attributed to the spin and parity forbidden d→d transition of Mn²⁺ ion. Moreover, a conspicuous spectral overlap between the emission band of the Ce³⁺ ions and the excitation of band the Mn²⁺ ions was observed between Fig. 2(a) and 2(b). Therefore, it is expected that an efficient energy transfer can occur from the Ce³⁺ to Mn²⁺ ions.

Fig. 2 Excitation and emission spectra for phosphors with composition of MLASO:0.05Ce³⁺ (a), MLASO:0.15Mn²⁺ (b).

Fig. 3 shows the emission spectra for MLASO:0.05Ce³⁺,xMn²⁺ phosphors with different x of 0, 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, 0.35, 0.45 and 0.50, which were measured at excitation wavelength of 430 nm, corresponding to the optimal excitation wavelength of the energy donor Ce³⁺. The PL spectrum of MLASO:0.05Ce³⁺,xMn²⁺ appear not only as a green-yellow band of the Ce³⁺ ions but also as a red band of the Mn²⁺ ions. With increasing Mn²⁺ content (x), the emission intensity of the Mn²⁺ ions increases systematically and reaches saturation as x equals to about 0.10, whereas the intensity of the Ce³⁺ sensitizer was found to decrease remarkably from x = 0.02 to 0.50. The observed saturation in Mn²⁺ intensity is attributed to the concentration quenching. In addition, it is noticeable that the emission peak of the Mn²⁺ ions shifts to long wavelength direction with increasing the concentration thereof, which might be due to the enhancement of crystal field strength surrounding Mn²⁺ ions.

Fig. 3 Emission spectra of MLASO:0.05Ce³⁺, xMn²⁺ phosphors.

The energy transfer between the Ce³⁺ and Mn²⁺ ions in MLASO:Ce³⁺,Mn²⁺ can be confirmed by the above-described observation and calculated energy transfer efficiency (η_T) can be expressed by¹⁵

where I_S0 and I_S and are the luminescence intensities of the sensitizer (Ce³⁺) without and with activator (Mn²⁺) present. The η_T was calculated as a function of x and is represented in Fig. S3.† The energy transfer efficiency increases with increasing Mn²⁺ content and reached 96% when x is about 0.50. The high energy efficiency indicates that the energy transfer from the Ce³⁺ to Mn²⁺ ions is the dominant reason for the increase in the Mn²⁺ emission intensity, instead of the energy absorption by the Mn²⁺ ions themselves. On the basis of Dexter's energy transfer of exchange and multipolar interactions, the following relation can be obtained:^11,16where C is the concentration of Mn²⁺. ln(I_S0/I_S)∝C corresponds to the exchange interaction and (I_S0/I_S))∝C^n/3 with n = 6, 8, and 10 corresponding to electric dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The plots are represented in Fig. 4. Linear behavior was observed only when n = 8, implying that energy transfer from the Ce³⁺ to Mn²⁺ ions occurred via the dipole–quadrupole mechanism. Therefore, the critical distance (Rc) for the energy transfer can be expressed by:¹⁷

Fig. 4 (a) Dependence of ln(I_S0/I_S) of Ce³⁺ on C; and I_S0/I_S of Ce³⁺ on (b)C^6/3; (c) C^8/3 and (d)C^10/3.

where the oscillator strength of the Mn²⁺electric quadrupole transition (f_q) is about 10⁻¹⁰, and λ_S is the emission wavelength of the Ce³⁺ ions. ∫F_S(E)F_A(E)/E⁴dE represents the spectral overlap between the normalized shapes of the Ce³⁺F_S(E) emission and the Mn²⁺ excitation F_A(E), and in our case it is calculated to be about 0.00696 eV⁻⁵. The R_c of the energy transfer was calculated to be about 9.3 Å. It should be noted that this estimates do not account for Ce³⁺–Ce³⁺ energy migration, which would tend to increase our estimates for the critical radius for energy transfer.¹⁸

The Commission International de I′Eclairage (CIE) chromaticity coordinates for MLASO:0.05Ce³⁺, xMn²⁺ are represented in Fig. 5. With increasing Mn²⁺ content, the chromaticity coordinates (x, y) vary systematically from (0.310, 0.498) to (0.447, 0.434) and ultimately to (0.512, 0.385); corresponding color tone of the sample changes gradually from green-yellow to yellow and eventually to orange. Various white lights are expected to obtain when the tunable emission of MLASO:Ce³⁺,Mn²⁺ couple with blue LEDs. The obtained samples have more emission intensity in the red region than the commercial phosphor Y₃Al₅O₁₂:Ce³⁺, thus, the white light with higher CRI is expected to be generated. These results indicate that this single-phased tunable-emitting phosphor may be useful for the development of white LEDs, due to the stronger emission in the red region. Although the phosphors may suffer the system-level limitation for using Mn²⁺ phosphor in LEDs based upon the slow decay times of Mn²⁺ that lead to intensity saturation, it can be overcome through reducing the incident flux on Ce³⁺–Mn²⁺ phosphors.¹⁹

Fig. 5 CIE chromaticity diagram for MLASO:0.05Ce³⁺,xMn²⁺ excited at 430 nm. (a) x = 0; (b) x = 0.05; (c) x = 0.10; (d) x = 0.15; (e) x = 0.20; (f) x = 0.25; (g) x = 0.35.

In conclusion, a novel garnet-based phosphor Mg_1.5Lu_1.5Al_3.5Si_1.5O₁₂:Ce³⁺,Mn²⁺ has been synthesized by solid state reaction. The phosphor shows intense blue absorption, and two emission bands centered at about 525 and 620 nm, which are attributed to Ce³⁺ and Mn²⁺ ions, respectively. We have demonstrated that the energy transfer from the Ce³⁺ to Mn²⁺ ions is dominated by the electric dipole-quadrupole interaction. Preliminary studies showed that this phosphor might have potential applications as a blue radiation-converting phosphor for white LEDs with high color rendering index.

This work is financially supported by the National Natural Science Foundation of China (grant no. 20771098) and the Fund for Creative Research Groups (grant no. 20921002), and the National Basic Research Program of China (973 Program, grant no. 2007CB935502).

References

1. E. F. Schubert, J. K. Kim, H. Luo and J. Q. Xi, Rep. Prog. Phys., 2006, 69, 3069–3099.
2. H. A. Hoppe, Angew. Chem., Int. Ed., 2009, 48, 3572–3582.
3. R. J. Xie and N. Hirosaki, Sci. Technol. Adv. Mater., 2007, 8, 588–600.
4. H. S. Jang, W. Bin Im, D. C. Lee, D. Y. Jeon and S. S. Kim, J. Lumin., 2007, 126, 371–377.
5. A. A. Setlur, W. J. Heward, M. E. Hannah and U. Happek, Chem. Mater., 2008, 20, 6277–6283.
6. C. Hecht, F. Stadler, P. J. Schmidt, J. S. A. der Guenne, V. Baumann and W. Schnick, Chem. Mater., 2009, 21, 1595–1601.
7. Y. Q. Li, J. E. J. van Steen, J. W. H. van Krevel, G. Botty, A. C. A. Delsing, F. J. DiSalvo, G. de With and H. T. Hintzen, J. Alloys Compd., 2006, 417, 273–279.
8. C. J. Duan, X. J. Wang, W. M. Otten, A. C. A. Delsing, J. T. Zhao and H. T. Hintzen, Chem. Mater., 2008, 20, 1597–1605.
9. Y. H. Song, G. Jia, M. Yang, Y. J. Huang, H. P. You and H. J. Zhang, Appl. Phys. Lett., 2009, 94.
10. C. H. Huang, T. W. Kuo and T. M. Chen, ACS Appl. Mater. Interfaces, 2010, 2, 1395–1399.
11. N. Guo, Y. J. Huang, H. P. You, M. Yang, Y. H. Song, K. Liu and Y. H. Zheng, Inorg. Chem., 2010, 49, 10907–10913.
12. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751–767.
13. A. Katelnikovas, H. Winkler, A. Kareiva and T. Jüstel, Opt. Mater., 2011, 33, 992–995.
14. A. A. Setlur, W. J. Heward, Y. Gao, A. M. Srivastava, R. G. Chandram and M. V. Shankar, Chem. Mater., 2006, 18, 3314–3322.
15. H. P. You, G. Y. Hong, X. Y. Wu, J. K. Tang and H. P. Hu, Chem. Mater., 2003, 15, 2000–2004.
16. T. F. Soules and C. B. Duke, Phys. Rev. B: Solid State, 1971, 3, 262–274.
17. H. P. You, J. L. Zhang, G. Y. Hong and H. J. Zhang, J. Phys. Chem. C, 2007, 111, 10657–10661.
18. A. A. Setlur, J. J. Shiang and U. Happek, J. Phys. Chem. C, 2011, 115, 3475–3480.
19. A. A. Setlur, J. J. Shiang and U. Happek, Appl. Phys. Lett., 2008, 92, 081104.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra00894g

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