Red-emitting YAG: Ce, Mn transparent ceramics for warm WLEDs application

A series of YAG:Ce,Mn transparent ceramics were prepared via a solid-state reaction-vacuum sintering method. The effects of various Mn2+–Si4+ pair doping levels on the structure, transmittance, and luminescence properties were systematically investigated. These transparent ceramics have average grain sizes of 10–16 μm, clean grain boundaries, and excellent transmittance up to 83.4% at 800 nm. Under the excitation of 460 nm, three obvious emission peaks appear at 533, 590, and 745 nm, which can be assigned to the transition 5d→4f of Ce 3+ and 4T1→6A1 of Mn2+. Thus, the Mn2+–Si4+ pairs can effectively modulate the emission spectrum by compensating broad orange-red and red spectrum component to yield high quality warm white light. After the optimized YAG:Ce,Mn transparent ceramic packaged with blue light-emitting diode (LED) chips, correlated color temperature (CCT) as low as 3723 K and luminous efficiency (LE) as high as 96.54 lm/W were achieved, implying a very promising candidate for application in white light-emitting diodes (WLEDs) industry.


Introduction
In recent decades, owing to the merits of high luminous efficiency (LE), energy saving, environmental friendliness, and long persistence, white light-emitting diodes (WLEDs) have become an essential part of our daily life [1][2][3][4]. Commercial WLEDs normally utilize 460 nm blue light to excite Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce) yellow phosphor dispersed in epoxy or silicone to generate white light, thereby high performance warm WLEDs with good thermal stability and low CCT value should be systematically studied.
Compared to phosphors, YAG transparent ceramics are one of promising fluorescent matrixes in WLEDs application with high thermal stability due to the low thermal expansion coefficient (8.4×10 -6 K -1 ) and high thermal conductivity (9-14 W·m -1 ·K -1 ) [12]. Furthermore, YAG transparent ceramics can be co-doped with rare earth or transition mental ions as activated center in Y 3+ or Al 3+ positions. Since the acquisition of warm white light requires the enrichment of red color in YAG:Ce-based WLEDs, co-doped red-activated center can be expected to yield ideal emission [13,14]. Research about introducing Cr 3+ , Pr 3+ , or Tb 3+ ion as redemitting ions to YAG matrix has been reported [15][16][17]. A classic example of this was the preparation of YAG:Ce, Pr, Cr transparent ceramic, which possesses high LE of 89.3 lm/W and CRI of 78 by the introduction of red emission band at 609, 638, 677, and 689 nm [15], along with LE of 89 lm/W was achieved by the Ce, Cr co-doped YAG transparent ceramics [16]. Besides, warm white light with a CCT value of ~3000 K and CRI value of 78.9 as well as LE of 38.5-58.9 lm/W was achieved by the transparent ceramics Gd 3 Al 4 GaO 12 :Ce 3+ (GAGG:Ce 3+ ) when light-emitting diodes (LEDs) were driven at 350 mA [18]. However, the problem of preparing GAGG:Ce 3+ ceramics is that Ga volatilizes and decomposes easily in high temperature and vacuum environment, which results in the non-uniform distribution of GAGG:Ce 3+ ceramics [19]. To prevent the Ga evaporation at high temperature, a complicated two-step sintering method-oxygen sintering and hot isostatic pressing sintering was proposed to prepare the YAGG:Ce transparent ceramics and then increased the cost [20].
Previous research revealed that Mn 2+ can emit red light in a strong crystal field by the 4 T 1 → 6 A 1 transition [21,22]. Consequently, the Mn 2+ -doped YAG:Ce phosphors showed the improvement of CRI and CCT performance of WLEDs, through the combination of three emission bands centered at 560, 593, and 745 nm, when it is excited by 460 nm blue light [23,24]. Moreover, the Y 3-m-n Ce m Mn n Al 5-n Si n O 12 phosphors were also designed by the incorporation of Mn 2+ -Si 4+ pairs and then realized color point tuning for white light generation [25]. Similarly, the LE of 119.93 lm/W, internal quantum yield of 81.0%, and CRI value of 73.6 as well as CCT value of 5674 K were achieved in a Mn-doped Y 3 Al 5 O 12 :Ce 3+ single crystal for WLEDs application [26]. However, it is difficult to prepare large-sized YAG:Ce,Mn single crystal for long production cycle and high cost. Furthermore, the YAG:Ce 3+ ,Mn 2+ ,Si 4+ phosphors embedded in glass ceramics (PIGs) were also prepared as a chromaticallytunable luminescent material, in which the CCT and LE reach 4954-3753 K and 80.8-60.3 lm/w, respectively [27]. Moreover, Lu 3 Al 5 O 12 :Mn (LuAG:Mn) transparent ceramics with red-emitting at 600-700 nm have been reported [28]. In summary, optimized emission with low CCT value can be achieved by the strategy of co-doping of Mn 2+ in YAG:Ce 3+ system. In comparison to emission band of Pr 3+ and Cr 3+ , Mn 2+ helps to obtain broad orange-red and red emission band, which is beneficial to reach full-luminescence in the visible range. Although the Mn 2+ ions have been widely used in inorganic luminescent materials, to the best of our knowledge, the Mn 2+ doped YAG transparent ceramics, as a more efficient fluorescent conversion media for WLEDs application, have not yet been investigated.
In this work, red-emitting YAG:Ce,Mn transparent ceramics with various Mn 2+ -Si 4+ pair doping concentration were successfully fabricated via a solid-state reaction sintering method. Mn 2+ ions were introduced to compensate broad orange-red and red spectrum component and achieve full emission band in the visible range under the excitation of 460 nm. Besides, Si 4+ ions were introduced to make the charge balance. A combination of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), fluorescent spectrum, lifetime, and transmittance analysis has been carried out to investigate the microstructure and luminescence properties of YAG:Ce,Mn transparent ceramics in detail. Meanwhile, the effects of Mn 2+ -Si 4+ pair doping concentration on the structure, transmittance, and luminescence properties, and the energy transfer mechanism were discussed. The YAG:Ce,Mn transparent ceramics assembled with blue LED chips have tunable emission from cold white to warm white as well as high LE and low CCT, which are promising candidates for warm WLEDs application. , and SiO 2 (99.9%) were used as raw materials. Among them, Si 4+ from SiO 2 was introduced to make the charge balance. 0.05 wt% MgO and 0.5 wt% tetraethyl orthosilicate (TEOS) were used as sintering aids. According to the composition listed in Table 1, raw materials were ball milled for 24 h in ethanol, and then the ethanol slurry was dried at 60 ℃ for 12 h and sieved through a 200-mesh screen. The fine powder was pressed into pellets (Φ = 40 mm) followed by cold isostatic pressed at 200 MPa. After removing the residual organic at 900 ℃ for 8 h, the compacted pellets were sintered at 1750 ℃ for 8 h under vacuum environment. All the samples were polished to 1 mm for characterization. Besides, Mn 2+ singly doped transparent ceramic Y 3 Al 4.76 O 12 :0.12Mn 2+ , 0.12Si 4+ (YAG:Mn) was also prepared as a comparison.

2 Characterization
The crystal structure was characterized by X-ray diffraction (XRD, Miniflex600, Rigaku, Japan) with Cu Kα radiation. Microstructures were observed by field emission scanning electron microscope (FESEM, SU-8010, Hitachi, Japan). In-line transmittance curves were measured using a UV/VIS/NIR spectrophotometer (Lambda950, Perkin Elmer, USA). The valance state of Mn ions was demonstrated via X-ray photoelectron spectroscopy (XPS, Escalab250Xi, Thermo Fisher, USA). Room-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectra were tested using a fluorescence spectrophotometer (FLS920, Edinburgh Instruments, UK) equipped with a xenon lamp as the excitation source. Fluorescence decay curves were recorded using a fluorescence spectrophotometer (FLS980, Edinburgh Instruments, UK) equipped with 470 nm laser as the excitation source. These transparent ceramics were packaged with commercial 460 nm blue chips into LED chip-on board (COB) modules and further LED lamps in Zhongkexinyuan Photoelectric Technology Co. (Fuzhou, China). The related roomtemperature CCT, LE, and Commission International de L'Eclairage (CIE) chromaticity color coordinates of YAG:Ce,Mn transparent ceramics were measured using an integrating sphere (Everfine PMS-50 system, EVERFINE Corporation, Hangzhou, China). Figure 1 shows the optical pictures of YAG:Ce,Mn transparent ceramics CM0, CM1, CM2, CM3, CM4, and CM5 (y = 0, 0.01, 0.03, 0.06, 0.09, 0.12), respectively. These ceramic samples were doublepolished and displayed homogeneous appearance. The size of these samples is 30 mm in diameter and 1.0 mm in thickness, through which the words on back can be seen clearly. After the introduction of more Mn 2+ -Si 4+ pairs, YAG:Ce,Mn transparent ceramics' color gradually changes from yellow to red. Figure 2 shows the XRD patterns of transparent ceramics CM0-CM5 with various Mn 2+ -Si 4+ pair concentration. It can be seen from Fig. 2 that diffraction peaks of ceramics CM0-CM3 perfectly match with the standard YAG (PDF#33-0040) phase and no secondary phase is detected, indicating Ce 3+ and Mn 2+ -Si 4+ pairs have been completely incorporated into YAG crystal lattice when y = 0, 0.01, 0.03, and 0.06. However, several weak diffraction peaks aside from the standard YAG phase appear when y = 0.09 and 0.12, indicating that secondary phase exists in ceramics CM4 and CM5. These peaks match with the standard Mg 2 SiO 4 (PDF#74-1681) phase. The existence of secondary phase would result in inhomogeneous microstructure. Besides, diffraction peaks slightly shift toward to lower angle   range with increasing Mn 2+ -Si 4+ pair concentration, suggesting that YAG:Ce,Mn transparent ceramics obtain expanding unit cell. The expanding unit cell may be attributed to bigger ion size of Ce 3+ (r = 0.1143 nm) and Mn 2+ (r = 0.096 nm) in comparison with Y 3+ (r = 0.1019 nm) and Al 3+ (r = 0.054 nm), although the ion size of Si 4+ (r = 0.026 nm) is smaller [15,26].  reasonably concluded that the coexisting Mg 2 SiO 4 forms a liquid phase during the sintering process and is distributed around grain boundaries due to the excess of Si 4+ . The reason why that grain boundaries broaden and Mg 2 SiO 4 phase appears is that SiO 2 may play the dual role of charge balancer and sintering aids, which would cause the deviation in composition from the stoichiometry of YAG:Ce,Mn ceramics. Additional TEOS as sintering aids was introduced; however, redundant Si 4+ would induce secondary phase formation when y = 0.09 and 0.12. Seriously, residual pores are observed in ceramics CM4 and CM5 (Figs. 3(e) and 3(f)), which would reduce the densification and result in lower transmittance. In order to see clearly grain boundaries and pores, the enlarged view of ceramics CM4 and CM5 are shown in Figs. 3(e') and 3(f'), respectively. One reason why the wider grain boundaries can be seen in Figs. 3(e) and 3(e') is that the thermal corrosion time is too long and residual pores mainly appear in grain boundaries. This result suggests that the Mn 2+ -Si 4+ pair doping concentration should be optimized to avoid the existence of pores and impurity phase as well as help to improve the density of YAG:Ce,Mn transparent ceramics.  Fig. 4(a), ceramics CM0-CM5 obtain extremely high optical transmittance from 300 to 800 nm. The ceramic CM0 obtains best transmittance of 83.4% at 800 nm, which almost reaches the theoretical limit (84%) of YAG transparent ceramics. As Mn 2+ -Si 4+ pairs were introduced into the YAG:Ce structure, the transmittance of ceramics CM1, CM2, CM3, CM4, and CM5 reaches 83.3%, 82.5%, 82.3%, 78.4%, and 78.0% at 800 nm, respectively. These results are consistent with SEM results shown in Fig. 3. The lower transmittance of ceramics CM4 and CM5 can be ascribed to inhomogeneous microstructure and residual pores. On the other hand, similar absorption bands around 340 nm and 440-470 nm are observed in ceramics CM0-CM5, which can be attributed to the transition of Ce 3+ ions from its 4f ground state to 5d excited state [29]. That indicates YAG:Ce,Mn ceramics can be effectively excited under the 460 nm excitation. Nevertheless, the absorption intensity around 340 nm and 440-470 nm gradually declines with increasing Mn 2+ -Si 4+ pair doping concentration. That would be related to expanding unit cell in YAG:Ce,Mn ceramics and the energy transfers from Ce 3+ to Mn 2+ . Mn 2+ singly doped transparent ceramic M5 was also prepared via the same process. As shown in Fig. 4(b), the in-line transmittance of 1.0 mm thick ceramic M5 (the inset photo in Fig. 4(b)) is up to 74.3% at 800 nm. No obviously apparent absorption band can be found, especially at 460 nm, indicating that Mn 2+ is hard to be directly excited by blue light. Figure 5 shows the XPS spectra of YAG:Ce,Mn ceramics. Figure 5(a) shows the survey scan of XPS, and it indicates the existence of Y, Al, O, Si, and Mn. Figure 5(b) presents the high-resolution XPS spectrum of Mn 2p of ceramic CM5. Two separate peaks with the binding energy of 641.5 and 653.1 eV belong to the Mn 2p 3/2 and Mn 2p 1/2 , respectively, with energy difference of 11.6 eV. These values reveal that Mn 2+ exists in YAG:Ce,Mn ceramic samples, which is consistent with that of Mn 2+ in MgY 2 Al 4 SiO 12 :Ce 3+ ,Mn 2+ phosphor [30]. Figure 6 presents the PL and PLE spectra of ceramic CM0, CM5, and M5. The PL spectrum of ceramic  CM0 exhibits a typical Ce 3+ yellow-green broad band centered at 533 nm ( Fig. 6(a)), which can be attributed to the 5d→4f transition of Ce 3+ in YAG matrix [5,6,15]. These two excitation bands centered at 340 and 460 nm are corresponding to absorption bands in transmittance spectrum, respectively. And the excitation intensity at 460 nm is stronger than that of 340 nm for the reason that YAG:Ce,Mn ceramics obtain higher absorption for blue light. After Mn 2+ -Si 4+ pairs were introduced into the YAG:Ce ceramics, it can be seen from Fig. 6(b) that two obvious emission bands aside from Ce 3+ emission band are observed at 590 and 745 nm, which is similar to the PL spectrum of Mn 2+ -doped YAG phosphors and PIGs phosphor [27,28,31]. As for the emission band of Mn 2+ at 528 nm, its intensity is so weak in comparison to strong Ce 3+ emission intensity that it cannot be observed. Besides, it is commonly accepted that Mn with different valences emit different luminescence properties. Aside from the aforementioned emission wavelength of Mn 2+ ions, Mn 3+ emission bands are observed peaked at 608 and 760 nm attributed to 5 T 2 → 5 E transition and Mn 4+ emission bands are observed peaked at 668 nm attributed to 2 E→ 4 A 2 [26,28]. In this work, the luminescence wavelength of Mn 3+ and Mn 4+ can not be observed and it is worth noting that Mn 2+ is oxidized little or nothing in the vacuum environment, and thus almost Mn 2+ ions exist in YAG:Ce,Mn transparent ceramics. That is consistent with the XPS results. These PLE spectra of ceramic CM5 monitored at 533, 590, and 745 nm show a similar shape, further supporting the energy transfers from Ce 3+ to Mn 2+ . Moreover, several weak bands are observed in PLE spectrum in the wavelength from 300 to 500 nm. Figure 6(c) shows the PL and PLE spectrum of ceramic M5 and its PL spectrum exhibits three emission bands of Mn 2+ around 528, 590, and 745 nm, further supporting the existence of Mn 2+ ions. In addition, the same excitation bands centered at 375, 408, 425, 450, and 468 nm are observed via monitored Mn 2+ emission wavelength at 590 nm, corresponding to transition of 6 A 1 ( 6 S) ground state to 4 T 2 ( 4 D), [ 4 A 1 ( 4 G), 4 E( 4 G)], 4 T 2 ( 4 G), and 4 T 1 ( 4 G) excited state, respectively [23,25,32]. Figure 7 shows the PL spectra of ceramics CM0-CM5. All samples achieve a broad band of Ce 3+ ions centered at 533 nm under the excitation of 460 nm. After the introduction of Mn 2+ -Si 4+ pairs, two emission bands of Mn 2+ are maximized at 590 and 745 nm. Furthermore, with increasing Mn 2+ -Si 4+ pair concentration, the emission intensity of Ce 3+ declines, but that of www.springer.com/journal/40145 Mn 2+ increases monotonously. These results are related to the reduced absorption of Ce 3+ around 440-470 nm with increasing Mn 2+ -Si 4+ pair content, which gives strong evidence of the energy transfer from Ce 3+ to Mn 2+ ions. Although the doping value y is up to 0.12, the emission intensity of Mn 2+ at around 745 nm is still weak. It is acknowledged that the wavelength of 745 nm is insensitive to human eyes due to adjacent infrared region. Hence, after optimizing Mn 2+ doping concentration, the Mn 2+ doping can enrich red light component and help tune the color point toward red region via emission maximized at 590 nm, which is greatly beneficial to obtain desirable CCT value and then generate warm white light. Figure 8 presents the fluorescence decay curves of ceramics CM0-CM5. Under 470 nm laser light excitation, decay curves were recorded via monitoring Ce 3+ emission peak at 533 nm. Their average lifetime of ceramics CM0-CM5 were calculated to be 74, 63, 58, 53, 48, and 46 ns, respectively, and therefore a fluorescence decay process takes place quickly in Ce 3+ and Mn 2+ -Si 4+ pair co-doped system, which reveals the mechanism of energy transfer between Ce 3+ and Mn 2+ and it is similar to Ref. [27]. In addition, the energy transfer efficiency (η T ) of Ce 3+ →Mn 2+ in YAG:Ce,Mn transparent ceramic should be taken into consideration. According to the average lifetime, η T can be obtained from the formula [25,33]:

Results and discussion
(1) where τ is the average lifetime of the donor Ce 3+ ions in the presence of the acceptor Mn 2+ ions and τ 0 is the average lifetime of the donor Ce 3+ ions without acceptor Mn 2+ ions. Then, the η T of various YAG:Ce,Mn transparent ceramics was calculated and listed in Table 2. In this work, the η T shows an enhancement as the  According to the PL spectra and fluorescence decay curves as well as Refs. [23,26,27], the energy transfer process in YAG:Ce,Mn transparent ceramics can be illustrated as Fig. 9. After Ce 3+ replaced Y 3+ position, its 4f ground state splits to 2 F 5/2 and 2 F 7/2 energy level due to spin-coupling. Also, energy splitting of 5d excited state occurs. Under the excitation of 460 nm, the electronic state of Ce 3+ is excited to upper 5d state. Next, a rapid electron relaxation process takes place from 5d state to 2 F 5/2 and 2 F 7/2 energy level, which emits a broad yellow-green band centered at 533 nm. Because the lowest 5d 1 energy level of Ce 3+ is a little higher than the 4 T 1 of Mn 2+ , the energy transfer process takes place easily from Ce 3+ to neighboring Mn 2+ via non-radiation transition. And then the 4 T 1 excited electrons transfer back to 6 A 1 ground state, which emits three bands at 528, 590, and 745 nm since the local environment around the Mn 2+ ions differ. Such spin-forbidden transition with broad bands would be beneficial for the full emission spectrum in the visible range.
In order to evaluate the illumination performance of these transparent ceramics, ceramics CM0-CM5 were combined with commercial blue LED chips. Table 3 shows the value of CCT, LE, and CIE chromaticity color coordinates. Ceramic CM0 shows the highest CCT (4044 K) due to the deficiency of red spectrum, although the LE is the highest. As Mn 2+ -Si 4+ pairs were introduced, the CCT and LE simultaneously decrease. Among them, transparent ceramics CM1-CM3 with proper CCT of 3933-3402 K and LE of 103.46-68.79 lm/W are promising fluorescent materials for warm WLEDs application. In comparison to YAG:Ce ceramics, the LE of YAG:Ce,Mn fluorescent ceramics inevitably decreases on account of energy transfer between Ce 3+ and Mn 2+ . However, YAG:Ce,Mn ceramics obtain low CCT value and achieve warm white light. That is good for the health of human eyes. Figure 10(a) shows CIE chromaticity color coordinates diagram of ceramics CM0-CM5 as these transparent ceramics were excited by 460 nm blue light. With increasing Mn 2+ -Si 4+ pair doping concentration, their color coordinates ranging from (0.425, 0.552) to  (0.474, 0.507) mean emission color can shift from yellowgreen to orange and the enrichment of red spectrum component. As a result, it is promising to achieve high quality warm white light via YAG:Ce,Mn ceramics when combined with blue LEDs. Figure 10(b) presents the optical picture of transparent ceramics CM0 and CM2 fabricated in COB modules. After encapsulating lampshade, Fig. 10 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.