Efficient spectral regulation in Ce:Lu3(Al,Cr)5O12 and Ce:Lu3(Al,Cr)5O12/Ce:Y3Al5O12 transparent ceramics with high color rendering index for high-power white LEDs/LDs

Abstract: Realizing a high color rendering index (CRI) in Ce:LuAG transparent ceramics (TCs) with desired thermal stability is essential to their applications in white LEDs/LDs as color converters. In this study, based on the scheme of configuring the red component by Cr3+ doping, an efficient spectral regulation was realized in Ce,Cr:LuAG TCs. A unilateral shift phenomenon could be observed in both photoluminescence (PL) and photoluminescence excitation (PLE) spectra of TCs. By constructing TC-based white LED/LD devices in a remote excitation mode, luminescence properties of Ce,Cr:LuAG TCs were systematically investigated. The CRI values of Ce:LuAG TC based white LEDs could be increased by a magnitude of 46.2%. Particularly, by combining the as fabricated Ce,Cr:LuAG TCs with a 0.5 at% Ce:YAG TC, surprising CRI values of 88 and 85.5 were obtained in TC based white LEDs and LDs, respectively. Therefore, Ce,Cr:LuAG TC is a highly promising color convertor for high-power white LEDs/LDs applied in general lighting and displaying. 

Keywords: Ce,Cr:LuAG TC; spectral regulation; energy transfer; color rendering index (CRI); white LEDs/LDs 

1 Introduction

White light emitting diodes (LEDs) and laser diodes (LDs) have considerable applications in general lighting, medical treatment, plant growth, etc., thanks to their advantages such as energy saving, environmental friendliness, and high efficiency ^[1–3]. White LEDs/LDs are assembled by yellow color convertors combining blue excitation sources, and the applied color convertors include phosphor powder, phosphor in glass, single crystal, and transparent ceramic (TC) ^[4–7]. Among them, cerium doped yttrium aluminum garnet (Ce:YAG) TC has advantages of richness in doping ions, high ion doping concentration, desired mechanical property, and high thermal conductivity [8,9]. Also, Ce:YAG TC can overcome problems such as color deviation and easy aging that commonly occurs in the commercially available white LEDs/LDs, and has been becoming a research focus currently. 

However, lack of red component is the fundamental problem of Ce:YAG TC based white LEDs/LDs, resulting in their low color rendering index (CRI) and high correlated color temperature (CCT). It tremendously hinders their real applications in solid state lighting ^[10,11]. Strategies such as co-doping red ions, substituting ions with similar ionic radius, and coating red phosphors, have been put forward by researchers to obtain high quality white light emissions in white LEDs/LDs ^[12–14]. Hu et al. ^[15] altered the 5d¹ energy level of Ce³⁺ ion via substituting Y³⁺ ions by Gd³⁺ ions in Ce:YAG TCs, and the emission of Ce³⁺ ion was red shifted by 16 nm. Tang et al. ^[16] enhanced the emission properties of Ce:YAG TCs by co-doping moderate amount of Cr³⁺ ions, and found that the CRI values of the obtained TCs were increased by increasing Cr³⁺ ion doping concentration. Du et al. ^[17] observed a massive red shift from 533 to 598 nm in Ce:YAG TCs, when co-doping Mg²⁺ and Si⁴⁺ ions to substitute Al³⁺–Al³⁺ pairs in YAG lattice. Recently, our group obtained a broad emission within the orange-red region in Ce,(Pr,Mn):YAG TCs, through utilizing the “wide and narrow peak coupling effect” of Pr³⁺ and Mn²⁺ ions, and a high CRI value of 84.8 was realized ^[18]. Similar discoveries could be found elsewhere, such as Ce³⁺,Mn²⁺,Si⁴⁺:YAG ^[19], Al₂O₃–Ce,Gd:YAG ^[20], Ce:(Tb,Gd)₃Al₅O₁₂ ^[21], Ce:Gd₃Al₄GaO₁₂ ^[22], etc. Additionally, it has been found that composite structure TCs are promising due to their high saturation threshold and excellent thermal performance. Xu et al.^[23] found that light homogeneity of Al₂O₃–YAG:Ce composite ceramic was far better than that of YAG:Ce single crystal, due to intense scattering by the phase/crystal boundaries. A high thermal conductivity of 18.5 W/(m·K) and a remarkable thermal stability (only a 8% reduction in emission intensity at 200 ℃ compared with that of room temperature) were realized in Al₂O₃–YAG:Ce composite TCs. No luminescence saturation phenomenon was observed even exciting TCs under a high power density of 50 W/mm²^[24]. In general, regulating the transmission route of photon energy is significant to realize high quality luminescence in TC based white LED/LD devices.

Despite the red component could be increased by utilizing the above strategies, thermal stability of TC is decreased simultaneously, owing to the relatively large ionic mismatch between doping ions and Y³⁺/Al³⁺ ions in YAG lattice. On the contrary, thermal stability of a color convertor would be enhanced significantly, if Y³⁺ ion in YAG were completely substituted by Lu³⁺ ion with a smaller ionic radius to form LuAG. Xu et al. ^[25] proved that thermal stability of Ce:LuAG TC was far superior to that of Ce:YAG TC with identical Ce³⁺ concentration at 200 ℃. Additionally, only a 10% decreased PL intensity at 450 K compared to that of room temperature in Ce:LuAG TC was demonstrated by Zhang et al. ^[26]. Ce,Pr:LuAG TCs were fabricated by Zhou et al. ^[27] using the co-precipitated powders, and their luminescence properties at 550 nm were promoted effectively, thanks to the effective energy transfer between Pr³⁺ and Ce³⁺ ions. In general, luminescence properties of TC based white LEDs/LDs could be further optimized, if applying LuAG TC as the color convertor. 

In addition, current strategies regarding the promotion of luminescence properties of TCs mainly focus on adjusting their emissions within the orange region (550–650 nm, which is crucial with application importance), because the wavelength range that human eyes can perceive is between 400 and 700 nm ^[14,19]. However, it should be noted that regulating the red region between 650 and 750 nm is significant to realize a standard white light emission, and it would be interesting and necessary to conduct a systematic research on regulating the red emission in Ce:LuAG TC. Until now, however, there is few study providing a clear insight on it. 

Considering the emission band of Cr³⁺ ion in garnet material covers 650–800 nm region under 440 or 590 nm excitation, and a desired spectral regulation is expected, if doping Cr³⁺ ion into Ce:LuAG TC. Although the Cr³⁺ emission in garnet has no obvious contribution to the emission within the orange range, it should be pointed out that owing to the highly overlapped Ce³⁺ emission and Cr³⁺ excitation bands, an effective energy transfer between Ce³⁺ and Cr³⁺ ions with high efficiency is expected. Both the trajectory of photon energy and the energy transfer between Ce³⁺ and Cr³⁺ ions determine the red component of the emitted light of Ce,Cr:LuAG TC directly. Additionally, Cr₂O₃ can hardly evaporate during high temperature treatment, and Cr³⁺ ion can absorb the blue light emitted from the LED chip directly, which is beneficial to the utilization rate of the incident blue light. Therefore, it can be deduced that the scheme of Cr³⁺ ion doping is an effective approach to obtain high quality solid state lighting for white LEDs/LDs. 

In this study, high quality Ce,Cr:LuAG TCs were fabricated by a solid state reaction sintering method under vacuum. Microstructural and spectral properties, as well as the energy transfer between Ce³⁺ and Cr³⁺ ions in LuAG TCs were investigated systematically. White LED/LD devices were also assembled by combining the as prepared TCs with blue LED chips/laser sources to evaluate their luminescence properties, with the purpose of providing a reference approach towards the spectral optimization within the red region in TC convertors. Finally, this work provides an effective strategy to advance the development and application of LuAG TC for high power white LEDs/LDs. 

2 Materials and methods 

2. 1 Material preparation 

High purity Lu₂O₃ (99.99%, Alfa Aesar, Ward Hill, USA), α-Al₂O₃ (99.999%, Alfa Aesar, Ward Hill, USA), CeO₂ (99.99%, Alfa Aesar, Ward Hill, USA), Cr₂O₃ (99.9%, Alfa Aesar, Ward Hill, USA), and Y₂O₃(99.99%, Alfa Aesar, Ward Hill, USA) powders were selected as the starting materials. They were weighted precisely using an analytical balance, and the detailed formula design of LuAG TCs was displayed in Table 1. Also, a 0.5 at% Ce:YAG TC was fabricated to further optimize the luminescence performances of the constructed TC based LEDs/LDs. 0.5 wt% tetraethyl orthosilicate (TEOS, 99.99%, Alfa Aesar, Ward Hill, USA) was selected as the sintering additive to promote the densification of TCs during sintering.

Table 1 Ingredients of the Ce,Cr:LuAG TCs 

The powder mixtures were placed in high purity nylon ball milling jars with anhydrous ethyl alcohol as the milling agent, and planetary ball milled for 20 h under 150 rpm. The milled slurry was dried at 80 ℃ in an oven for 12 h in air, and then sieved though a 100-mesh screen. The sieved powders were uniaxially pressed into pellets with a diameter of 22 mm using a stainless-steel mold, and then placed into a highpressure water tank and cold isostatic pressed (CIPed) at 200 MPa for 10 min to further increase their densities. The CIPed green bodies were calcined at 1000 ℃ for 3 h in a muffle furnace in air to remove residual volatile organic compounds. The sintering process was carried out at 1800 ℃ for 8 h under 10^-3 Pa in a tungsten mesh heated vacuum furnace, and the applied heating and cooling rates were 2 and 10 ℃/min, respectively. Finally, all the sintered TCs were annealed at 1400 ℃ for 25 h in air to remove the oxygen vacancies generated during vacuum sintering, and then grinded and mirror polished on both surfaces into 1 and 0.4 mm thickness for LuAG and YAG TCs, respectively. 

2. 2 Characterization 

Phase compositions of the obtained TCs were identified by an X-ray diffraction machine (XRD; D8 Advance, Bruker, Karlsruhe, Germany) equipped with a copper target X-ray tube, and the selected scanning range (2θ) was 10°–80° with a step size of 0.01°. In-line transmission spectra of the polished TCs were tested using a UV–VIS–NIR spectrophotometer (Lambda 950, Perkin Elmer, Waltham, MA, USA) with a standard, dual light beam arrangement with adjustable slit width, and the applied scanning speed was 500 nm/min. Microstructural investigation of the sintered TCs was carried out by a scanning electron microscope (SEM; JSM-6510, JEOL, Kariya, Japan). For the characterization of polished surfaces of TCs, they were thermal etched at 1450 ℃ for 2 h in air in a muffle furnace before SEM measurement. An energy dispersive X-ray spectrometer (EDS, Inca X-Max, Oxford Instruments, Oxford, UK) connected with the SEM was utilized to recognize the element distribution of TCs. Photoluminescence (PL), photoluminescence excitation (PLE), and fluorescence decay spectra were detected by a spectrophotometer (FLS 980, Edinburgh Photonics, Edinburgh, UK) with a scintillating xenon lamp. Chromaticity parameters of the prepared TCs were characterized using an integrating sphere (R98, Everfine, Hangzhou, China) assembled with 460 nm blue chips or a 450 nm laser source as the excitation sources, and the obtained spectroscopic data were processed by the system owned CAS-200 software. All the above measurements were carried out at room temperature. 

3 Results and discussion 

Figure 1(a) shows the XRD patterns of LuAG TCs. All the diffraction peaks could be well indexed as the pure LuAG phase (PDF 97-018-2354), and there was no impurity phase (e.g., LuAM, LuAP, CeO₂, and Cr₂O₃) observed, indicating that the solid-state reaction between Lu₂O₃ and Al₂O₃ were completed during vacuum sintering. Increasing Cr³⁺ doping concentration hardly altered the phase composition of TCs, and there was no obvious peak shift observed from the main diffraction peaks of XRD patterns, owing to the low doping concentration of Cr³⁺ ion. Generally, Cr³⁺ ion (0.615 Å, CN = 6) would occupy the octahedral Al³⁺ site (0.53 Å, CN = 6), and Ce³⁺ ion (1.143 Å, CN = 8) would substitute the dodecahedral Lu³⁺ site (0.977 Å, CN = 8) in LuAG lattice ^[8,28], owing to their similar ionic radius. The detailed schematic crystal structure sketch concerning the ion substitution process in Ce,Cr:LuAG TC is shown in Fig. 1(b).

Fig. 1 (a) XRD patterns and (b) schematic crystal structure sketch of Ce,Cr:LuAG TCs. 

Appearances and in-line transmission spectra of the polished LuAG TCs are shown in Fig. 2. All the samples exhibited a transparent appearance, and the words behind them could be clearly recognized by the naked eyes. The color of the TC without Cr³⁺ doping (Ce01Cr0) was yellowish green, i.e., the intrinsic color of Ce³⁺ ion. With increasing Cr³⁺ doping concentration, the color of the TCs was changed from yellowish green to light green, indicating that their emissions were tuned effectively by Cr³⁺ ion incorporation. From the transmission spectra it could be seen that moderate amounts of Ce³⁺ and Cr³⁺ doping hardly affected the transparency of TCs, and their transmittances at 800 nm were close to 70%. Increasing Cr³⁺ doping concentration deteriorated the transparency of TCs. The variation trend of the transmittance at 800 and 400 nm of the prepared TCs could be found in Fig. 2(b). Two broad absorption bands centered at 340 and 445 nm were ascribed to the 4f–5d¹ and 4f–5d₂ transitions of Ce³⁺ ion, respectively ^[29,30]. Besides the intrinsic absorption bands of Ce³⁺ ion, the absorption centered at 430 and 596 nm could be observed from all the Cr³⁺ ion doped samples, corresponding to the ⁴A₂–⁴T₁ and ⁴A₂–⁴T₂ transitions of Cr³⁺ ion, respectively ^[31]. 

Fig. 2 (a) Appearances and in-line transmission spectra of the Ce,Cr:LuAG TCs and (b) variation trend of the detailed transmittances at 800 and 400 nm of the prepared TCs. 

Figure 3 shows the SEM micrographs of the sintered LuAG TCs. From the fracture surfaces of the samples (Figs. 3(a)–3(h)) it could be seen that the fracture modes of all the TCs were characterized by both intergranular and transgranular, and residual pores could be observed simultaneously, providing a quasidensified microstructure. As can be seen from the polished surfaces of TCs (Figs. 3(a′)–3(h′)), the amount of residual pores was increased with increasing Cr³⁺ doping concentration, indicating that Cr₂O₃ affected the densification behavior of LuAG TCs during sintering. Also, feature of the residual pores was characterized by intergranular pores. The applied sintering temperature was 1800 ℃ for all TCs, which was lower than the ideal sintering temperature of LuAG TC, resulting in the intergranular pores in ceramic bulks. Generally, for the real application of TC convertor for white LEDs/LDs, residual pores could act as light scattering centers to increase the light extraction rate ^[32–35]. 

Fig. 3 Fracture surfaces and polished surfaces of (a, a′) Ce0Cr01, (b, b′) Ce01Cr0, (c, c′) Ce01Cr01, (d, d′) Ce01Cr02, (e, e′) Ce01Cr03, (f, f′) Ce01Cr04, (g, g′) Ce01Cr05, and (h, h′) Ce01Cr06. 

The variation trend of grain size as a function of Cr³⁺ doping concentration could be found in Fig. S1 in the Electronic Supplementary Material (ESM), and it is obvious that the grain size of TCs was moderately increased with increasing Cr³⁺ doping concentration. However, the grain size of the fabricated TCs was only increased from 2.79 to 3.78 μm, owing to the low sintering temperature. Additionally, EDS mapping of the sintered Ce01Cr04 sample is shown in Fig. S2 in the ESM to investigate its elemental distribution, and it could be found that all the adopted elements (Lu, Al, O, Ce, and Cr) were distributed homogeneously inside ceramic bulk, indicating that both Ce³⁺ and Cr³⁺ ions were solid soluted into LuAG lattice without segregation ^[32–35].

PL and PLE spectra of Ce0Cr01 TC are shown in Fig. 4(a). Recorded at 687 nm, two broad absorption bands centered at 428 and 593 nm could be observed, originating from the ⁴A₂–⁴T₁ and ⁴A₂–⁴T₂ transitions of Cr³⁺ ion, respectively. These absorptions leaded to the characterized green color of the Cr³⁺ doped LuAG TCs. By exciting Ce0Cr01 TC under 428 nm, an intensive broad emission band covering the orange-red and red regions centered at 710 nm was found in its PL spectra. This emission was corresponded to the spin-allowed ⁴T₂→⁴A₂ transition of Cr³⁺ ion. Simultaneously, a sharp R line (zero-phonon line) due to the spin-forbidden ²E→ ⁴A₂ transition could be detected at 690 nm. Similar observation has been reported by researchers in Cr³⁺ doped garnet structured materials ^[36]. 

Figure 4(b) shows the normalized PLE spectra of Ce,Cr:LuAG TCs with different Cr³⁺ doping concentrations (λ_em = 523 nm). Two excitation bands centered at around 340 and 455 nm could be observed, corresponding to the 4f–5d¹ and 4f–5d² transitions of Ce³⁺ ion, respectively. Notably, a unilateral red shift phenomenon was observed from the left wing of the 410–500 nm band with increasing Cr³⁺ doping concentration, whereas the right wing of this band was not influenced by Cr³⁺ doping. In other words, the peak positions of the PLE peaks were not influenced by Cr³⁺ ion doping, and only a local red shift occurred in the shape of the peaks of the PLE spectra, when doping Cr³⁺ ion into Ce:LuAG host. Therefore, it could be deduced that a portion of the emitted photon of Ce³⁺ ion was absorbed by Cr³⁺ ion in Ce,Cr:LuAG TCs, resulting in the unilateral red shift of their PLE spectra ^[37]. Besides, it was speculated that the complicated local crystal environment around Ce³⁺ ion by doping Cr³⁺ ion into the [AlO6] octahedron might be another reason that caused the unilateral red shift phenomenon ^[38]. 

Fig. 4 (a) PL and PLE spectra of Ce0Cr01 sample, (b) PLE and (c) PL spectra of Ce:LuAG and Ce,Cr:LuAG TCs, and (d) detailed full width at half maximum (FWHM) values of PL and PLE spectra as a function of Cr³⁺ doping concentration. 

Normalized PL spectra of Ce,Cr:LuAG TCs are displayed in Fig. 4(c). The characteristic broad emission band centered at 523 nm could be clearly resolved, corresponding to the 5d–4f transition of Ce³⁺ ion. The emission of Cr³⁺ ion could be detected simultaneously from all the Cr³⁺ doped samples. It could be clearly seen that the emission band of Ce³⁺ ion overlapped with the absorption band of Cr³⁺ ion. Therefore, Ce³⁺ ions in Ce,Cr:LuAG matrix not only acted as luminescence centers, but also as sensitizers proceeding the energy transfer from Ce³⁺ to Cr³⁺ ions. A portion of the photons emitted from Ce³⁺ ions could be absorbed by Cr³⁺ ions to realize red light emission. The energy transfer from Ce³⁺ to Cr³⁺ ions could be processed through two means, i.e., radiative transition and non-radiative transition. The detailed schematic diagram of the energy transfer process from Ce³⁺ to Cr³⁺ ions is shown in Fig. S3 in the ESM ^[16]. The emission intensity of Cr³⁺ ions was increased with increasing Cr³⁺ doping concentration, and reached the maximum when the Cr³⁺ concentration was 0.5 at% (Fig. 4(c)). Further increasing Cr³⁺ doping concentration decreased the emission intensity of TC, thanks to the concentration quenching effect. Besides, the right wing of the Ce³⁺ emission band was blue shifted as increasing Cr³⁺ concentration, whereas there was no obvious shift observed from the left wing of this band. This unilateral blue shift was owing to the increased absorption at around 593 nm that overlapped the PL spectra of Ce³⁺ ion as increasing Cr³⁺ ion doping concentration. It was similar to that of the observed unilateral red shift from the PLE spectra shown in Fig. 4(b). The detailed full width at half maximum (FWHM) values of both PL and PLE spectra of Ce,Cr:LuAG TCs are presented in Fig. 4(d), illustrating that Cr³⁺ ion doping could regulate the luminescence performance of Ce:LuAG TCs effectively. 

In order to further validate the availability of the prepared TCs as fluorescent convertors, TC based white LED devices were constructed using the remote excitation mode. The operating power and the emitting wavelength of the blue LED chips were 20 W and 460 nm, respectively. Figure 5(a) shows the electroluminescent (EL) spectra of the TCs. It was evident that the sample without Ce³⁺ doping had a strong blue light emission, since the absorption ability of Cr³⁺ ion at 460 nm was far more inferior than that of Ce³⁺ ion. Besides, the green component corresponding to the Ce³⁺ emission was decreased with increasing Cr³⁺ doping concentration, thanks to the enhanced energy transfer from Ce³⁺ to Cr³⁺ ions. The variation trend of the chromaticity parameters is displayed in Fig. 5(b), and it could be found that with the increasing Cr³⁺ doping concentration, the luminescence characteristic of the TC based white LEDs was changed from greenish to blueish. 

Figure 5(c) shows the CRI values of Ce,Cr:LuAG TCs. It was obvious that the CRI values were increased with increasing Cr³⁺ doping concentration, and reached the maximum value of 75.7 in Ce01Cr03 TC. Further increasing Cr³⁺ doping concentration decreased the CRI values. The deteriorated CRI was due to the proportional mismatch among red/green/blue light. Though the optimized CRI value of 75.7 was not very ideal, it was much higher than that of the TC without Cr³⁺ doping. It should be noted that the increment of CRI was as high as 46.2%. 

Fig. 5 (a) EL spectra, (b) chromaticity parameters, (c) detailed CRI values, and (d) variation trend of light proportion of the TC based white LEDs driven by 350 mA current.

The detailed variation trend of the red, green, and blue light proportion is shown in Fig. 5(d). According to the setting of CAS-200 software, the spectral bands of red/green/blue light were 600–780 nm, 500–600 nm, and 380–500 nm, respectively. The corresponding light proportion was obtained by calculating the ratio of the luminous flux of the emitted light to the total luminous flux. It could be seen from Fig. 5(d) that the red component was increased monotonously, whereas the greenish yellow light component was descended simultaneously. Because the blue light emitted from the chip was absorbed by the entire surface of TC under the remote excitation mode, the Cr³⁺ ions in TCs could not reach their saturation status, resulting in the continuously increased red light proportion in Ce,Cr:LuAG TC based white LEDs. 

Fluorescence decay curves of Ce,Cr:LuAG TCs are plotted to further explore the energy transfer process from Ce³⁺ to Cr³⁺ ions under 460 nm excitation, as is shown in Fig. 6. The decay behavior of all the TCs could be fitted well by the single exponential decay function. From Fig. 6 it was obvious that with increasing Cr³⁺ doping concentration, lifetime of Ce,Cr:LuAG TCs at 523 nm presented a monotonic decreasing trend. It was ranged from 55.52 ns (Ce01Cr00) to 33.47 ns (Ce01Cr04), illustrating an effective energy transfer from Ce³⁺ to Cr³⁺ ions. 

Fig. 6 Fluorescence decay curves of Ce,Cr:LuAG TCs. 

Energy transfer efficiency was determined according to Eq. (1) ^[19,39]: 
η_T = 1 − τ/τ₀     (1) 

where τ and τ₀ are the average lifetime of the donor Ce³⁺ ions in the presence and without Cr³⁺ ions, respectively. With increasing Cr3+ doping concentration, the calculated ηT of the Ce,Cr:LuAG TCs were 12.76%, 22.42%, 31.93%, and 39.82%. It revealed that the energy transfer efficiency of TCs was promoted effectively by Cr³⁺ ion doping, demonstrating that Cr³⁺ ion doping is an effective approach to regulate the luminescence behavior of Ce:LuAG TC. 

Judging from the EL spectra shown in Fig. 5, it was evident that the greenish yellow component of the single structure Ce,Cr:LuAG TC was inadequate. Therefore, scheme of combining Ce,Cr:LuAG TCs with a 0.5 at% Ce:YAG TC was performed to further improve the luminescence performance of TC based white LEDs. Appearance and transmission spectrum of the applied 0.5 at% Ce:YAG TC could be found in Fig. S4 in the ESM. Figure 7(a) presents the schematic sketch of the TC based white LED device, in which Ce,Cr:LuAG TC was placed at the top of the LED device, and Ce:YAG TC was placed between the Ce,Cr:LuAG TC and the blue LED chip. From the chromaticity parameters shown in Fig. 7(b), it could be clearly seen that the color hue was regulated effectively by using the “ceramic combination strategy”, indicating the luminescence performance of white LEDs was further optimized. 

Fig. 7 Schematic sketch of the white LED constructed with Ce:YAG and Ce,Cr:LuAG TCs and (b) the corresponding chromaticity parameters. 

Figure 8 indicates the EL spectra and the corresponding CRI values of the white LEDs using Ce:YAG and Ce,Cr:LuAG TCs as color convertors. It was noteworthy to see that the obtained CRI values of the white LEDs were drastically promoted by using the “ceramic combination strategy”, which was in consistence with the optimized chromaticity parameters shown in Fig. 7(b). Surprisingly, by combining Ce:YAG TC with Ce01Cr04 TC, the obtained CRI of the corresponding white LED was as high as 88.0. Ratios of the red/green/blue emissions of the white LEDs constructed with the combined TCs were more reasonable, compared with that of the LEDs constructed with the single structure Ce,Cr:LuAG TCs. From the insets of Fig. 8, it is obvious that the real sense emitting color of the white LEDs was adjusted effectively by doping Cr³⁺ ion, as well as by combining Ce,Cr:LuAG and Ce:YAG TCs. Luminescence efficiencies of Ce:Cr:LuAG and Ce:YAG/Ce,Cr:LuAG TCs based white LEDs are plotted in Fig. S5 in the ESM. Furthermore, the applied Ce³⁺ doping concentration in Ce,Cr:LuAG TCs could be further optimized, in order to obtain white light emission with a considerable CRI.

Fig. 8 EL spectra and the corresponding CRI values of the white LED constructed with Ce:YAG combined with (a) Ce01Cr01, (b) Ce01Cr02, (c) Ce01Cr03, (d) Ce01Cr04, (e) Ce01Cr05, and (f) Ce01Cr06 TCs and their real sense visual renderings during LED operation (inset). 

With respect to laser lighting test, the as-fabricated Ce,Cr:LuAG/Ce:YAG TCs were fixed on an aluminium alloy support frame with excellent heat dissipation performance to eliminate the possible thermal induced luminescence attenuation. Appearance of the apparatus for the LD measurement is shown in Fig. 9(a). It was composed of an integrating sphere and the constructed LD device. The detailed structure of the applied LD device could be found in our previous work [40]. A 0.5 at% Ce:YAG TC was placed underneath the Ce,Cr:LuAG TCs, in order to evaluate the luminescence performance of the constructed white LDs. The applied excitation wavelength of the laser source was 450 nm. Also, the selected pump power of the laser source was 1 W, in order to avoid excessive local temperature that could deteriorate the luminescence performance of TCs during LD operation. 

Fig. 9 (a) Appearance of the apparatus for the LD measurement, (b) EL spectra, (c) CRI values, and (d) luminous flux of the combined Ce,Cr:LuAG/Ce:YAG TC based LDs under 455 nm excitation.

EL spectra of the combined Ce,Cr:LuAG/Ce:YAG TC based white LDs are shown in Fig. 9(b). The sharp emission peak located at 450 nm was corresponded to the radiation of the laser source with a narrow emission band. The emission bands of both Ce³⁺ and Cr³⁺ ions were clearly resolved, and a distinct energy transfer from Ce³⁺ to Cr³⁺ ions could be observed simultaneously. Therefore, in addition to LED sources, the prepared LuAG TCs could also be excited effectively by blue lasers with high energy density and tiny radiation area. 

Figure 9(c) indicates the CRI values of the combined Ce,Cr:LuAG/Ce:YAG TCs as a function of Cr³⁺ doping concentration. Considering a portion of the transmitted blue light is highly directional compared to the emitted light from the TC, in this regard, a 1 W blue laser was applied as the exciting source, in order to avoid the possible side effect induced by the laser source that could deteriorate the CRI performance. It was obvious that the obtained CRI values were regulated effectively by doping Cr³⁺ ions into Ce:LuAG TC. With increasing Cr³⁺ doping concentration, the yellow component of the emitted light was reduced, indicating a portion of the yellow light emitted from Ce³⁺ ions was absorbed by Cr³⁺ ions to increase the red component of white LDs. It should be noted that the optimized CRI value was as high as 85.5, which was almost identical to that of the TC based white LEDs shown in Fig. 8. 

However, from Fig. 9(d) it was found that the luminous flux of the white LDs was moderately decreased from 257.3 to 218.5 lm with increasing Cr³⁺ doping concentration. Therefore, there existed a tradeoff between CRI and luminous flux. The decreased luminous flux should be attributed to the energy loss originated from the increased yellow light absorption when increasing Cr³⁺ ion doping concentration. Nevertheless, from Fig. 9(c) it was noteworthy to see that the increment of CRI of white LDs was as high as 59.2%. Consequently, it was worthwhile to sacrifice a small fraction of luminous flux to realize a dramatic increased CRI in Ce,Cr:LuAG/Ce:YAG TC based white LDs. Finally, in addition to white LEDs, Ce,Cr:LuAG TC is also a considerable color convertor for white LDs in the future. 

4 Conclusions 

This work illustrates that the luminescence performance of the Ce:LuAG TC convertor for white LEDs/LDs could be improved drastically by Cr3+ ion doping. The vacuum sintered Ce,Cr:LuAG TCs exhibited a pure phase microstructure with a homogeneous elemental distribution. Unilateral shift phenomenon was observed in both PLE and PL spectra of Ce,Cr:LuAG TCs. Notably, CRI of the prepared Ce:LuAG TC was increased by a magnitude of 46.2% by Cr³⁺ doping, owing to the optimized emitting light component. It was noteworthy that by combining Ce,Cr:LuAG TCs with a 0.5 at% Ce:YAG TC, high CRI values of 88/85.5 for white LEDs/LDs were obtained, thanks to the further optimized blue/green/red light proportion. In general, this study confirmed that Ce,Cr:LuAG TC is a potential light convertor for white LEDs/LDs, and its application prospect is foreseen in the future. 

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