Red-Emitting Phosphor Ba9Lu2Si6O24:Ce3+,Mn2+ with Enhanced

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A Red-Emitting Phosphor BaLuSiO :Ce , Mn with Enhanced Energy Transfer via Self-Charge Compensation Kaixin Song, Jianxin Zhang, Yongfu Liu, Changhua Zhang, Jun Jiang, Haochuan Jiang, and Hui-Bin Qin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07566 • Publication Date (Web): 02 Oct 2015 Downloaded from http://pubs.acs.org on October 5, 2015

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The Journal of Physical Chemistry

A Red-Emitting Phosphor Ba9Lu2Si6O24:Ce3+, Mn2+ with Enhanced Energy Transfer via Self-Charge Compensation Kaixin Song,† Jianxin Zhang, †,‡ Yongfu Liu,*,‡ Changhua Zhang, ‡ Jun Jiang,*, ‡ Haochuan Jiang, ‡ and Hui-Bin Qin † †

College of Electronic Information and Engineering, Hangzhou Dianzi University, Hangzhou

310018, P. R. China ‡

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

Ningbo, 315201, China *Corresponding author E-mail: [email protected]; [email protected] No. 1219 Western Zhongguan Road, Ningbo 315201, China Tel. 86-574-87619207, Fax. 86-574-86382329

ACS Paragon Plus Environment

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ABSTRACT

A new red-emitting phosphor, Ce3+-Mn2+ co-doped Ba9Lu2Si6O24 was synthesized by using solid-state reactions. The phase formation, luminescence properties, energy transfer mechanisms and

thermal

stabilities

were

investigated.

Under

the

near-ultraviolet

excitation,

Ba9Lu2Si6O24:Ce3+, xMn2+ shows a tunable emission from blue-violet to red. When x = 0.08, the photoluminescence characterized a major peak at 610 nm with an internal quantum efficiency of about 70%. The energy transfer from Ce3+ to Mn2+ was further enhanced by self-charge compensation as a result of the presence of activators at different crystallographic sites simultaneously. At 160 ℃, the quantum efficiency retained 84% of that measured at roomtemperature. The small thermal quenching and efficient luminescence suggest that Ba9Lu2Si6O24:Ce3+, Mn2+ is a promising candidate for use in ultraviolet chip pumped LEDs.

KEYWORDS: Ba9Lu2Si6O24; photoluminescence; energy transfer; self-charge compensation.


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I. INTRODUCTION White light-emitting diodes (LEDs) are penetrating the lighting market at an extremely fast rate owning to their extraordinary luminous efficiency, low power consumption, reliability, and environmental friendliness. The commercial white LED consisted of a blue LED chip and Y3Al5O12:Ce3+ (YAG:Ce3+) yellow-emitting phosphor cannot achieve a warm white light with a high color-rendering index (Ra > 80) and a low correlated color temperature (CCT < 4500K).1-5 A single phase phosphor with full-color-emitting or the blend of red, green, and blue-emitting phosphors excited by near-ultraviolet (NUV) LED chips is an effective alternate to solve those problems above.6-12 Therefore, seeking for novel NUV excitation phosphors with remarkable properties, such as high quantum efficiency (QE) and small thermal quenching is greatly in demand. Alkaline earth silicates are determined as promising matrices in the use of phosphorconverted white LEDs due to their excellent chemical stability and abundant emission colors.13-24 For example, Ce3+/Eu2+ activated Ba9RE2Si6O24 (RE = Sc, Y) exhibits tunable emissions ranging from NUV to blue-green.18-20 When Mn2+ is co-doped into these hosts, the emission can be extended to red region based on the energy transfer (ET) from Ce3+/Eu2+ to Mn2+.21-24 Maybe this ET is limited, the Mn2+ red emission intensity is not strong enough to obtain an ideal white light. In our previous work, we reported a novel green phosphor, Ba9Lu2Si6O24:Ce3+ (BLS:Ce3+) for RE = Lu, which shows a higher QE than that for RE = Sc and Y and an excellent thermal stability that is superior to most nitride phophors.25 Similarly, the strong NUV to blue-green emissions of Ce3+ in BLS should also make it to be an efficient sensitizer for Mn2+ to obtain an intense red emissions. In this work, when Ce3+ and Mn2+ are co-doped into BLS, the intensity of the


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Mn2+ red emission is much stronger than that in Ba9RE2Si6O24. The internal QE can reach as high as about 70%, which is outstanding in most NUV excited red silicate phosphors, such as NaScSi2O6:Eu2+, Mn2+ (40%),10 Ba1.3Ca0.7SiO4:Eu2+, Mn2+ (28%),16 and Ca2SiO4:Eu2+ (50%).17 Furthermore, the red emission shows a high thermal stability. At 160 ℃, 84% of the internal QE at room-temperature is still maintained. Based on these results, the red-emitting phosphor, BLS:Ce3+, Mn2+ could improve the performance of white LEDs and achieve an ideal white light. The enhanced red emission of Mn2+ could be attributed to the promoted ETs from Ce3+ to Mn2+ originating from the self-charge compensation mechanism between Ce3+ and Mn2+ at different crystallographic sites in the BLS host. This hypothesis is put forward and discussed according to the crystal structure, luminescence and fluorescence decay properties. 2. EXPERIMENTAL SECTION Synthesis.

Samples

(BLS:10%Ce3+),

with

the

nominal

(Ba8.9Mn0.1)Lu2Si6O24

xMnx)(Lu1.9Ce0.1)Si6O24

compositions

of

Ba9(Lu1.9Ce0.1)Si6O24

(BLS:10%Mn2+),

and

(Ba9-

(BLS:10%Ce3+, xMn2+) were prepared by high-temperature solid-

state reactions. The starting materials (BaCO3, 99.8%; SiO2, 99.8%; Lu2O3, 99.99%; CeO2, 99.99%; MnCO3, 99.95%) were thoroughly mixed and ground for 40 minutes in an agate mortar. The mixed powders were placed in an alumina crucible and sintered in a tube furnace at 1400 ℃ for 3h under a reductive atmosphere (5%H2 + 95%N2), then the samples were furnace-cooled to room temperature, and ground again into powders for measurement.


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Characterization. Phase purity and crystal structure were identified by a powder X-ray diffraction (XRD) analysis (Bruker D8 Advance), with Cu Kα radiation (λ = 1.54056 Å) operating at 40kV and 40mA. The morphology of the representative sample was examined using a field-emission scanning electron microscopy (FE-SEM, Hitachi S4800). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by using a Hitachi F-4600 spectrometer. The fluorescence lifetime of Ce3+ was measured by FL3-111 (Horiba Fluorolog) with a NanoLED (λex = 319 nm). The QE and temperature-dependent QE were measured by the QE-2100 spectrophotometer (Otsuka Photal Electronics). 3 RESULTS AND DISCUSSION Figure 1 displays the XRD patterns of BLS:10%Ce3+, xMn2+ with x = 0, 0.04, 0.08, 0.12, and 0.20. It is clear that all samples exhibit a single crystal phase of Ba9Sc2Si6O24 (BSS, PDF No. 82-1119) regardless of the doping concentrations.26 As shown in Figure 2, BLS crystalizes in a rhombohedral structure that is consisted of corner-shared SiO4-LuO6-SiO4 layers. The lutecium provides one crystallographic site coordinated by 6 oxygen atoms forming LuO6 octahedra. The barium provides three independent sites coordinated by 9, 10, and 12 oxygen atoms, respectively, forming three different distorted polyhedra. The morphology for this phosphor, BLS:10%Ce3+, 8%Mn2+, is also shown in Figure 3. It presents an average size less than 10 µm, exhibiting a proper particle size for uses in white LEDs. As mentioned above, the compositions based on charge balance are only nominal. We cannot declare that Ce3+ and Mn2+ only present at the Lu3+ and Ba2+ sites, respectively. Considering the ionic radii at the same coordinate environment, such as 6-coordinated,


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Ce3+ (r = 1.01 Å) is smaller than Ba2+ (r = 1.35 Å) while bigger than Lu3+ (r = 0.861 Å).27 Actually, Ce3+ can occupy both the Ba2+ and Lu3+ sites, which can be evidenced by the following Ce3+ luminescence properties in BLS:Ce3+. For Mn2+ (r = 0.67 Å),27 it is smaller than both Ba2+ and Lu3+. So it is also possible for Mn2+ to occupy the Ba2+ and Lu3+ sites. Thus, there are two cases for charge balance in BLS:Ce3+, Mn2+. One case is mentioned that Ce3+ and Mn2+ present at the Lu3+ and Ba2+ sites, respectively. The other is that Ce3+ and Mn2+ present at the Ba2+ and Lu3+ sites, respectively, which could be demonstrated by the Ce3+ luminescence properties in BLS:Ce3+, Mn2+. Figure 4 shows the PL and PLE spectra of Ce3+ and Mn2+ singly doped BLS. Under the 400 nm excitation (Figure 4a), BLS:10%Ce3+ exhibits a broad green emission band peaked at 490 nm with a width of 120 nm, which is attributed to the transitions of Ce3+ from the 5d to 2F5/2 and 2

F7/2.25 The asymmetric emission band can be decomposed into two Gaussian bands (dash lines)

peaked at 482 and 536 nm. The energy difference is about 2090 cm-1, which is close to the energy separation between 2F5/2 and 2F7/2 levels.28 Under the 333 nm excitation (Figure 4b), BLS:10%Ce3+ presents a different emission band with three peaks at 388, 426 and 475 nm, respectively. This phenomenon indicates that at least two kinds of Ce3+ luminescence centers exist in the BLS host. In order to simplify the discussion below, the PL spectrum with the excitation peak at 400 nm in Figure 4(a) is denoted as Ce(1), and the PLE spectrum with the UVblue emission in Figure 4(b) is denoted as Ce(2). Based on the previous researches,25 Ce(1) is assigned to the Lu3+ site with a shorter Lu-O bond length and a larger crystal-field strength, while Ce(2) is assigned to the Ba2+ site with a longer bond length and a weaker crystal-field strength. These evidences support the presence of Ce3+ at the Ba2+ and Lu3+ sites simultaneously. It is noted that the PLE band of Ce(1) overlaps with the PL band of Ce(2). This indicates that the


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ET from Ce(2) to Ce(1) should exist. Just due to this ET, the emissions of both Ce(1) and Ce(2) occur under the only excitation of Ce(2). One can also notice in Figure 4(b) that the 475 nm emission shoulder near Ce(1) is observed, differing from the Ce(1) emission peak position (490 nm). This phenomenon should result from the spectral overlap between the emissions of Ce(1) and Ce(2). Figure 4(c) shows the PL and PLE spectra of Mn2+ singly doped BLS. BLS:10%Mn2+ exhibits a red emission band peaked at 610 nm due to the 4T1(4G) → 6A1(6S) transition of Mn2+ at the Ba2+ sites. If Mn2+ occupies the Lu3+ site, its emission should locate at an infrared or a nearinfrared region. Because the crystal-field splitting for the Lu3+ site is larger than that for the Ba2+ site due to the shorter Lu-O bond length than that of Ba-O bond. However, infrared emissions are useless for white LEDs. So we did not concern about the Mn2+ emission at the Lu3+ site in this report. The PLE spectrum of BLS:10%Mn2+ consists of several bands centered at 326, 414 and 446 nm, corresponding to the transitions from the 6A1(6S) level to the 4E(4D), [4A1(4G), 4E(4G)] and 4T1(4G) levels, respectively.29 However, because the d-d transitions of Mn2+ are forbidden in spin and parity, the luminescence intensity of Mn2+ is extremely lower than that of Ce3+ in BLS. It is noted that the PLE spectrum of Mn2+ overlaps with the PL spectra of both Ce(1) and Ce(2). This provides a way to enhance the Mn2+ emission intensity through the ETs from Ce3+ to Mn2+. Herein, to simplify the discussion, we focus on the luminescence of Ce(2) and its ETs to Mn2+ because of a larger spectral overlap between the emission of Ce(2) and the excitation of Mn2+. As for the ET from Ce(1) to Mn2+ will be detailed in our following report. A series of PL spectra of BLS:10%Ce3+, xMn2+ with different Mn2+ concentrations are shown in Figure 5(a). Under the Ce(2) excitation of 333 nm, Ce3+ and Mn2+ co-doped samples exhibit strong red emissions around 610 nm. The emission intensities increase with the increasing Mn2+


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concentrations (process ① in Figure 5(b)) and have a maximal intensity at x = 0.15. After that, the intensity decreases because of the Mn2+ concentration quenching effect. In addition, unlike other deep-red-emitting phosphors, BLS:10%Ce3+, xMn2+ concentrates the red light in the visible range where the human eye’s sensitivity is still high.30 Therefore, if this red emission intensity is enhanced, the efficiency can also be improved. The inset in Figure 5(a) shows the PLE spectrum of the optimum sample, BLS:10%Ce3+, 15%Mn2+, monitored at the Mn2+ emission around 610 nm. It exhibits a mainly excitation band around 333 nm, which is similar to the PLE spectrum of Ce(2) in Figure 4(b). This means that the intense red emission in Ce3+-Mn2+ co-doped BLS is mainly from the excitation of Ce(2) and effective ETs take place from Ce(2) to Mn2+. Interestingly but abnormally, the Ce(2) emission is enhanced during the ET process other than decreasing monotonically as increasing Mn2+ concentration. This phenomenon could be related to the occupation preference of activators under the modulation of self-charge compensation. As demonstrated above, Ce3+ can occupy both the Ba2+ and Lu3+ sites. Although Ce3+ is smaller than Ba2+, the replacement of Ce3+ for Ba2+ could be restricted by the charge difference between Ce3+ and Ba2+ in the Ce3+ singly doped BLS (for x = 0). If Mn2+ is co-doped into BLS (for x > 0), the charge difference can be compensated by the replacement of Mn2+ for Lu3+. As the schematic diagram in Figure 5(b) shows, more and more Ce3+ ions could present at the Ba2+ sites with increasing Mn2+ concentration and then intensifies the Ce(2) emission, resulting an enhanced ET and a strong red emission (process ②). Here, it is worth noting that the emission intensity of Ce(2) is closely related to the population of Ce3+ at the Ba2+ sites and the ETs between Ce(2) and Mn2+. Increasing Mn2+ concentrations not only improves the population of Ce3+ at the Ba2+ sites that enhances the emission intensity of Ce(2), but also promotes the ETs that decrease the Ce(2) emission. The competition between the Ce(2) emission and its ETs to Mn2+ is difficult to be


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quantitatively analyzed in a low Mn2+ concentration (x < 0.12). For instance, the Ce(2) emission does not increase when x = 0.04 and 0.06 in BLS:10%Ce3+, xMn2+. It may be contributed to the fact that the ET takes the dominant role. Maybe some uncertain mechanisms exist in this system and still need to be investigated. As for a higher Mn2+ concentration (x > 0.12) the Ce(2) emission intensities drop continuously, which should be related to the saturation of Ce3+ at the Ba2+ site and the predomination of ETs from Ce(2) to Mn2+. In addition, it is to be noted that the emission band of Ce(2) changes apparently with the increasing Mn2+ concentrations. Only the 388 nm emission peak is remained when x = 0.2. As previously discussed in Figure 4, the emission shoulder of Ce(2) around 475 nm origins from the ET from Ce(2) to Ce(1). It drops rapidly as increasing Mn2+ concentrations because of the decreasing population of Ce3+ at the Lu3+ sites under the modulation of the self-charge compensation. In the meanwhile, the ET from Ce(2) to Mn2+ declines the 426 nm emission. Therefore, only one peak at 388 nm is remained when x reaches 0.2. These phenomena above also give evidence of the charge balance case mentioned in the structure discussion that Ce3+ and Mn2+ present at the Ba2+ and Lu3+ sites, respectively. Just due to the existence of the self-charge compensation between Ce3+ and Mn2+, the ET is enhanced greatly and outstanding red emissions of Mn2+ are achieved in the BLS host. To further investigate the ET efficiency between Ce(2) and Mn2+, the fluorescence decay curves of Ce3+ were measured. As Figure 6 shows, the fluorescence of Ce(2) at 380 nm decays faster and tends to multi-exponential with increasing Mn2+ concentrations, which provides a convincing evidence for the ETs from Ce(2) to Mn2+. The fluorescence lifetimes (τ) were obtained by integrating the normalized decay curves. In BLS:10%Ce3+, xMn2+, the Ce3+ lifetimes are 26, 21, 18 and 16 ns with x = 0, 0.04, 0.12, and 0.20, respectively.


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The ET efficiency, ηT, can be calculated by the following equation: ηT = 1－τ/τ0 where τ and τ0 are the lifetime of Ce(2) in the presence and absence of Mn2+ ions. As can be seen in Figure 7, ηT grows in direct proportion to the Mn2+ concentration and reaches 37% at x = 0.20. The efficiencies also include the ETs from Ce(2) to Ce(1) and is difficult to be extracted from that between Ce(2) and Mn2+. However, it does not prevent the achievement of the strong red Mn2+ emissions through the ETs from Ce(2) to Mn2+. Figure 8 displays the internal QEs of BLS:10%Ce3+, xMn2+ as a function of Mn2+ concentrations. The stars denote the internal QEs of red emissions detected from 550 to 750 nm. A maximum was determined as 49% for x = 0.15, which is consistent with the optimal concentration of the red emission in Figure 5. The circles denote the internal QEs detected in a full emission range from 360 to 750 nm that includes the Ce(2) emission. The according internal QE can achieve as high as 70% when x = 0.08. Compared with most NUV-excited red silicate phosphors (28-50%),10, 16, 17 BLS:10%Ce3+, xMn2+ exhibits an excellent internal QE. As is well known, the phosphor-converted LEDs generally work at a high temperature at about 150 ℃. Thus, the thermal quenching is an important technological parameter for phosphors. Figure 9(a) illustrates the temperature-dependent relative QEs of BLS:10%Ce3+, 8%Mn2+. Only 16% of the room-temperature QE is lost at 160 ℃ (433 K), exhibiting a smaller thermal quenching than YAG:Ce3+ (~80%) 3 even on par with some nitride phosphors (80-90%).31-33 In Figure 9(b), a slight shift of the emission spectra towards higher energies is observed with increasing temperature. It may be caused by the expansion of lattice parameters at high temperature that decrease the crystal-field splitting.


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Figure 10 shows the chromaticity coordinates of BLS:10%Ce3+, xMn2+ based on the corresponding emission under excitation with 333 nm. It can be seen that the color evolves from the blue region to the orange-red region with the coordinates changing from (0.219, 0.241) to (0.521, 0.311). White light zone is passed as different emission components of Ce3+ and Mn2+. The luminescence photographs excited by the 365 nm-NUV lamp exhibits a tunable color emission from blue-green to orangered. These results means that BLS:10%Ce3+, xMn2+ can act as a promising orangered phosphor for possible applications in solid-state lighting and displays. In addition, it can be promising to get a single-phase white light-emitting phosphor with high colorrendering index and tunable color temperature via introducing an efficient green activator, such as Tb3+, which is concentrated in our further study. 4. CONCLUSIONS In this work, a series of Ce3+-Mn2+ co-doped BLS samples were synthesized by solid-state reactions. The XRD patterns suggest that BLS crystallizes in BSS structure with corner-shared SiO4-LuO6-SiO4 layers. In the BLS host, Ce3+/Mn2+ can occupy both Ba2+ and Lu3+ sites simultaneously, resulting enhanced ETs from Ce3+ to Mn2+ via self-charge compensation. A strong red emission peaked at 610 nm, with an internal QE of about 70%, was obtained in BLS:10%Ce3+, xMn2+ (x = 0.08) under the NUV excitation. This efficient red-emitting concentrates the emitted light in visible spectral region, which can effectively improve the performance of White LEDs. In addition, BLS:Ce3+, Mn2+ exhibits an excellent thermal stability. At 160 ℃, it retained 84% of the initial QE that measured at room-temperature, performing a smaller thermal quenching than YAG:Ce3+ and some nitride phosphors. These outstanding features make BLS:Ce3+, Mn2+ a promising red phosphor for white LEDs.


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5. ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (NSFC11404351, 51202051), Ningbo Municipal Natural Science Foundation (2014A610122), Ningbo Science and Technology Innovation Team (2014B82004), and the China Postdoctoral Science Foundation (2014M560497, 2015T80638). REFERENCES (1) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science. 2005, 308, 1274-1278. (2) Setlur, A. Phosphors for LED-Based Solid-State Lighting. Electrochem. Soc. Interface. 2009, 16, 32-36. (3) Bachmann, V.; Ronda, C.; Meijerink, A. Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce. Chem. Mater. 2009, 21, 2077-2084. (4) Liu, Y.; Zhang, X.; Hao, Z.; Wang, X.; Zhang, J. Generation of Broadband Emission by Incorporating N3- into Ca3Sc2Si3O12:Ce3+ Garnet for High Rendering White LEDs. J. Mater. Chem. 2011, 21, 6354-6358. (5) Zhu, H.; Lin, C. C.; Luo, W.; Shu, S.; Liu, Z.; Liu, Y.; Kong, J.; Ma, E.; Cao, Y.; Liu, R. S.; et al. Highly Efficient Non-Rare-Earth Red Emitting Phosphor for Warm White Light-Emitting Diodes. Nat. Commun. 2014, 5, 1-10. (6) Li, G.; Geng, D.; Shang, M.; Zhang, Y.; Peng, C.; Cheng, Z.; Lin, J. Color Tuning Luminescence of Ce3+/Mn2+/Tb3+-Triactivated Mg2Y8(SiO4)6O2 via Energy Transfer: Potential Single-Phase White-Light-Emitting Phosphor. J. Phys. Chem. C 2011, 115, 21882–21892.


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(14) Liu, Y.; Zhang, X.; Hao, Z.; Wang, X.; Zhang, J. Tunable Full-Color-Emitting Ca3Sc3Si3O12:Ce3+, Mn2+ Phosphor Via Charge Compensation and Energy Transfer. Chem. Commun. 2011, 47, 10677-10679. (15) Liu, Y.; Zhang, X.; Hao, Z.; Luo, Y.; Wang, X.; Ma, L.; Zhang, J. Luminescence and Energy Transfer in Ca3Sc2Si3O12:Ce3+, Mn2+ White LED Phosphor. J. Lumin. 2013, 133, 21-24. (16) Lv, W.; Jiao, M.; Zhao, Q.; Shao, B.; Lü, W.; You, H. Ba1.3Ca0.7SiO4:Eu2+, Mn2+: A Promising Single-Phase, Color-Tunable Phosphor for Near-Ultraviolet White-Light-Emitting Diodes. Inorg. Chem. 2014, 53, 11007–11014. (17) Sato, Y.; Kato, H.; Kobayashi, M.; Masaki, T.; Yoon, D. H.; Kakihana, M. Tailoring of Deep-Red Luminescence in Ca2SiO4:Eu2+. Angew. Chem. 2014, 126, 7890–7893. (18) Nakano, T.; Kawakami, Y.; Uematsu, K.; Ishigaki, T.; Toda, K.; Sato, M. Novel Ba-Sc-SiOxide and Oxynitride Phosphor for White LED. J. Lumin. 2009, 129, 1654–1657. (19) Brgoch, J.; Borg, C. K. H.; Denult, K. A.; DenBaars, S. P.; Seshadri, R. Tuning Luminescent Properties Through Solid-Solution in (Ba1-xSrx)9Sc2Si6O24:Ce3+, Li+. Solid State Sci. 2013, 18, 149-154. (20) Brgoch, J.; Borg, C. K. H.; Denault, K. A.; Mikhailovsky, A.; DenBaars, S. P.; Seshadri, R. An Efficient, Thermally Stable Cerium-Based Silicate Phosphor for Solid State White Lighting. Inorg. Chem. 2013, 52, 8010-8016. (21) Zhang, X.; Liu, Y.; Lin, J.; Hao, Z.; Luo, Y.; Liu, Q.; Zhang, J. Optical Properties and Energy Transfers of Ce3+ and Mn2+ in Ba9Sc2(SiO4)6. J. Lumin. 2014, 146, 321–324. (22) Kim, Y.; Park, S. Eu2+, Mn2+ Co-Doped Ba9Y2Si6O24 Phosphors Based on Near-UVExcitable LED lights. Mater. Res. Bull. 2014, 49, 469–474.


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(23) Park, S. Ce3+-Mn2+ Cooperative Ba9Y2Si6O24 Orthosilicate Phosphor. Mater. Lett., 2014, 135, 59-62. (24) Bian, L.; Wang, T.; Liu, S.; Yang, S.; Liu, Q. L. The Crystal Structure and Luminescence of Phosphor Ba9Sc2Si6O24:Eu2+, Mn2+ for White Light Emitting Diode. Mater. Res. Bull., 2015, 64, 279-282. (25) Liu, Y.; Zhang, J.; Zhang, C.; Xu, J.; Liu, G.; Jiang, J.; Jiang, H. Ba9Lu2Si6O24:Ce3+: An Efficient Green Phosphor with High Thermal and Radiation Stability for Solid-State Lighting. Adv. Opt. Mater. 2015, 3, 1096-1011. (26) Wang, L. H.; Schneemeyer, L. F.; Cava, R. J.; Siegrist, T. A New Barium Scandium Silicate: Ba9Sc2(SiO4)6. J. Solid. State. Chem. 1994, 113, 211-214. (27) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta. Cryst. 1976, A32, 751-767. (28) Holloway, W. W.; JR; Kestigian M. K. Optical Properties of Cerium-Activated Garnet Crystals. J. Opt. Soc. Am. 1969, 59, 60-63. (29) Palumbo, D. T.; Brown, J. J.; Jr. Electronic States of Mn2+-Activated Phosphors. J. Electrochem. Soc. 1970, 117, 1184-1188. (30) Vos, J. J. Colorimetric and Photometric Properties of a 2-deg Fundamental Observer,” Color Res. Appl. 1978, 3, 125-128. (31) Xie, R. J.; Li, Y. Q.; Hirosaki, N.; Yamamoto, H. Nitride Phosphors and Solid-State Lighting; CRC Press: Boca Raton, 2010. (32) Zhang, Z.; Kate, O. M. T.; Delsing, A. C. A.; Stevens, M. J. H.; Zhao, J.; Notten, P. H. L.; Dorenbos, P.; Hintzen, H. T. Photoluminescence Properties of Yb2+ in CaAlSiN3 as A Novel Red-Emitting Phosphor for White LEDs. J. Mater. Chem. 2012, 22, 23871-23876.


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FIGURE CAPTIONS Figure 1. XRD patterns of BLS:Ce3+, xMn2+ with different Mn2+ concentrations (x = 0, 0.04, 0.08, 0.12, and 0.20). Figure 2. Crystal structure diagrams of Ba9Lu2Si6O24 viewed in b and c-direction and the coordination environment of cation sites for activators. Figure 3. Typical SEM image of BLS:10%Ce3+, 8%Mn2+ prepared at 1400 ℃. Figure 4. PLE and PL spectra for BLS:10%Ce3+ ((a) excited by 333 nm and (b) excited by 400 nm) and BLS:10%Mn2+ (c). Figure 5. (a) PL spectra for BLS:10%Ce3+, xMn2+ (x = 0-0.20) excited at 333 nm and the according PLE spectrum for x = 0.15 (the inset). (b) Schematic diagram of Self-charge compensation assisted energy transfers. Figure 6. Fluorescence decay curves of Ce3+ in BLS:10%Ce3+, xMn2+ samples. The samples were excited at 333 nm and monitored at 380 nm. Figure 7. Energy transfer efficiency (ηT) of BLS:10%Ce3+, xMn2+ as a function of Mn2+ concentrations. Figure 8. Internal QEs of BLS:10%Ce3+, xMn2+ detected in the red emission of 550-750 nm (stars) and the full emission of 360-750 nm (circles) with x = 0-0.20. Figure 9. (a) Temperature dependence of the relative QEs for BLS:10%Ce3+, 8%Mn2+. (b) Temperature-dependent spectral emissions, showing a minimal shift towards higher energies and decrease in intensity with increasing temperature. Figure 10. Evolution of CIE chromaticity coordinates for BLS:10%Ce3+, xMn2+ with x from 0 to 0.20 and the luminescence photographs of BLS:10%Ce3+, xMn2+ (x = 0, 0.08, 0.15, 0.20) excited at 365 nm.


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Figure 1. XRD patterns of BLS:Ce3+, xMn2+ with different Mn2+ concentrations (x = 0, 0.04, 0.08, 0.12, and 0.20).


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Figure 2. Crystal structure diagrams of Ba9Lu2Si6O24 viewed in b and c-direction and the coordination environment of cation sites for activators.


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Figure 3. Typical SEM image of BLS:10%Ce3+, 8%Mn2+ prepared at 1400 ℃.


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Figure 4. PLE and PL spectra for BLS:10%Ce3+ ((a) excited by 333 nm and (b) excited by 400 nm) and BLS:10%Mn2+ (c).


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Figure 5. (a) PL spectra for BLS:10%Ce3+, xMn2+ (x = 0-0.20) excited at 333 nm and the according PLE spectrum for x = 0.15 (the inset). (b) Schematic diagram of Self-charge compensation assisted energy transfers.


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Figure 6. Fluorescence decay curves of Ce3+ in BLS:10%Ce3+, xMn2+ samples. The samples were excited at 333 nm and monitored at 380 nm.


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Figure 7. Energy transfer efficiency (ηT) of BLS:10%Ce3+, xMn2+ as a function of Mn2+ concentrations.


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Figure 8. Internal QEs of BLS:10%Ce3+, xMn2+ detected in the red emission of 550-750 nm (stars) and the full emission of 360-750 nm (circles) with x = 0-0.20.


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Figure 9. (a) Temperature dependence of the relative QEs for BLS:10%Ce3+, 8%Mn2+. (b) Temperature-dependent spectral emissions, showing a minimal shift towards higher energies and decrease in intensity with increasing temperature.


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Figure 10. Evolution of CIE chromaticity coordinates for BLS:10%Ce3+, xMn2+ with x from 0 to 0.20 and the luminescence photographs of BLS:10%Ce3+, xMn2+ (x = 0, 0.08, 0.15, 0.20) excited at 365 nm.


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TOC GRAPHIC


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Red-Emitting Phosphor Ba9Lu2Si6O24:Ce3+,Mn2+ with Enhanced

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