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Abstract
The discovery of single structure Ce3+ doped garnet transparent ceramics (TCs) with a broad full width at half maximum (FWHM) is essential to realize a high CRI for high-power white light emitting diodes (LEDs) and laser diodes (LDs). In this work, by utilizing the ion substitution engineering strategy, pure phase Gd3Sc2Al3O12:Ce3+ (GSAG:Ce) TC with a broad FWHM of 132.4 nm and a high CRI value of 80.7 was fabricated through the vacuum sintering technique for the first time. The optimized in-line transmittance of TCs was 58.4% @ 800 nm. Notably, the GSAG:Ce TCs exhibited a remarkable red shift from 546 nm to 582 nm, with a high internal quantum efficiency (IQE) of 46.91%. The degraded thermal stability in Ce:GSAG TCs was observed compared with that of Ce:YAG TC, owing to the narrowed band gap of GSAG. Additionally, remote excitation white LEDs/LDs were constructed by combining GSAG:Ce TCs with blue LED chips or laser sources. A tunable color hue from yellow to shinning white was achieved in white LEDs, whereas the acquired CRI and CCT of the white LDs were 69.5 and 7766 K, respectively. This work provides a new perspective to develop TCs with high CRI for their real applications in high-power white LEDs/LDs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power white light emitting diodes (LEDs) and laser diodes (LDs) employing Y3Al5O12:Ce3+ (YAG:Ce) transparent ceramics (TCs) as color convertors have attracted extensive attention currently [1–3]. Compared with phosphor powder, phosphors-in-glass, film and single crystal, YAG:Ce TC has obvious advantages of high thermal conductivity (∼10 W·m−1·K−1), excellent mechanical strength and flexibility in composite structure design [4–6]. However, the inherent limitation of red component deficiency in YAG:Ce TC results in its poor colorimetric parameters (i.e., high correlated-color temperature (CCT) and low color-rendering index (CRI)) [7–11]. Numerous strategies have been adopted to address the above problems, such as codoping red-emitting ion, [12–15] coating red phosphor, [10] desinging composite structure TC, [16,17] isovalent substitution and chemical unit co-subsitution [18–22]. Among them, modifying YAG matrix by introducing ions with large ionic radius (e.g., Gd3+, Tb3+, etc.) or ion pairs (e.g., Mg2+-Si4+, Mg2+-Ge4+) have the advantages of easy control the peak position and flexibly in expanding the full width at half maximum (FWHM) of the emissions of single structure YAG:Ce TCs [23–27]. However, if the above strategies are applied to tune the luminescence performance of YAG:Ce TC imprecisely, it would lead to the effective ionic radius mismatch between the polyhedron for ion substitution and the adjacent polyhedron to generate secondary phase, resulting in the deteriorated luminescence behavior of TC [28–30]. Therefore, for the spectral regulation purpose of YAG:Ce TC, the type and concentration of the introduced ions should be strictly controlled, and seeking for the critical concentration of doping ions is extremely necessary.
Recently, ion substitution engineering in YAG:Ce TC mainly focus on isovalent substitution, chemical unit co-subsitution and Y3+-Al3+-Al3+ co-subsitution, as well as the fabrication technics of the corresponding TCs, which have become a research hotspot for TC convertors. Based on that, the broadened FWHM, redshift emission and pure phase composition can be realized in YAG:Ce TCs to meet various requirements for white LEDs/LDs. It is worth mentioning that Gd3+ ion has been used to replace Y3+ site in YAG lattice to regulate the luminescence performance of YAG:Ce TC, since Gd3+ ion could influence the crystal field environment and local coordination environment around Ce3+ ion, resulting in the enlarged FWHM and red shift emission of YAG:Ce TC to promote its CRI. For instance, GdYAG:Ce TC with a transmittance of 80% @ 800 nm was fabricated by Nishiura et al. The emission peaks of TCs were red shifted from 530 nm to 560 nm with increasing Gd3+ ion concentration, and the CRI of the TC based white LEDs was improved from 65 to 81 [27]. YAG:10at.%Gd,Ce TC was collaborated with the InGaN/GaN LED chips on a metal-backed printed circuit board (MBPCB) chip under a forward current of 20 mA, and the optimized CRI could reach 78 [31]. Additionally, a YAG:50 at.%Gd,Ce TC based white LED device was constructed using a 454 nm blue chip, and a warm white light emission with a CRI of 67.2 was achieved under a pump power of 25 mW [32]. The detailed efforts of Gd3+ ion doping on the CRI and FWHM values of YAG:Ce TCs are shown in Table S1 of Supplement 1 [31–39]. Indeed, the luminescence performance could be regulated effectively by Gd3+ ion doping. However, the thermodynamic stability of (Gd,Y)AG matrix would be deteriorated with increasing Gd3+ ion doping concentration. Importantly, it is difficult to fabricate pure phase Gd3Al5O12:Ce (GAG:Ce) TC, because it would decompose into GdAlO3 and Al2O3 phases when the applied sintering temperature is above 1300 ° [40]. To solve this problem, Ga3+ ion has been applied to substitute the octahedral site in GAG lattice to increase its effective ionic radius to suppress the decomposition of GAG:Ce, and pure phase Gd3(Ga,Al)5O12:Ce3+ TC could be obtained when the doping amount of Ga3+ ion was higher than 10 at.% [41]. Gd3Al4GaO12:Ce3+ TCs with different Ce3+ concentrations were prepared by sintering TCs under an oxygen atmosphere. A maximum FWHM could reach 137 nm, and the optimized CRI of TC based white LED was 78.9 under a driven current of 350 mA [39]. An effective regulation of emission and excitation peaks of GAGG:Ce TC was realized, when substituting the octahedral Al3+ site by Ga3+ ion with different concentrations [42]. However, Ga2O3 is easy evaporate during sintering, leading to a stoichiometric deviation to generate secondary phase. In order to suppress the evaporation of Ga2O3, oxygen sintering is commonly applied to realize the densification of GAGG:Ce TCs, which requires relatively expensive and complicated TC preparation procedure. Therefore, it is necessary to discover an efficient approach to obtain pure phase GAG TC through the traditional sintering process.
In addition to Ga3+ ion doping, Li et al. demonstrated that substituting Gd3+ ion in GAG lattice by doping 35 at.% Lu3+ ion was an effective approach to stabilize the GAG structure, and a pure phase (Gd,Lu)3Al5O12:Ce TC was fabricated by vacuum sintering at 1715 °C [43–45]. However, the emission peak of GAG TC would be blue shifted by Lu3+ incorporation, which is not conducive to its CRI improvement for the general lighting applications. The detailed investigations of ion substitution engineering regarding the match of the effective ionic radius between the dodecahedral and octahedral sites to obtain pure phase TC are shown in Supplement 1, Table S2 [30,39–41,45–55]. In general, a general rule of suppressing the decomposition of GAG through the effective ion radius match still need to investigate, which is significant to obtain a promoted CRI and a broadened FWHM in GAG TC.
In contrast, Sc3+ ion has the smallest ionic radius and largest electronegativity among rare earth ions. It has been proved that Sc3+ ion could regulate the emission spectra and FWHM of the luminescence centers (e.g., Eu3+, Yb3+ and Tm3+ ions) in YAG matrix [56–58]. Shi et al. investigated the effect of Sc3+ doping on the luminescence intensity of YAG:Ce TCs, and the highest luminescence intensity was obtained when the Sc3+ ion doping concentration was 20 at.% [59]. Recently, our group demonstrated that by introducing Sc3+ ion into the octahedral site of YAG:Ce TC, and an effective modification of the energy gap between the ground state and the 5d1 level of Ce3+ ion was realized, and a tunable color hue from yellowish white to pale-green regions was observed in TC based white LEDs [49]. It was found that Sc3+ ion could regulate the effective ion radius match between the [Lu/YO8] dodecahedron and the [Sc/AlO6] octahedron to obtain the pure garnet phase (Lu,Y)3(Sc,Al)5O12:Ce TC, and the corresponding FWHM was enlarged by a magnitude of 16% [60]. Additionally, it was worth mentioning that Dantelle et al. prepared the Gd3Sc2Al3O12:Ce3+ (GSAG:Ce) phosphor powders to pursuit a high brightness. A pure phase GSAG:Ce powders were synthesized, even when the Ce3+ ion doping concentration was as high as 18 at.% [61]. It should be noted that the Sc3+ ion doping concentration is significant to the phase evolution of garnet. Until now, however, there is few study conduct a systematic research on it. For the better application purpose of GSAG:Ce TC on the remote excitation high-power solid state lighting, the inner relationship between Sc3+ ion doping concentration and GSAG phase evolution should be investigated thoroughly. In addition, a matching relationship of the average effective ionic radius match between the [GdO8] dodecahedron and the [Sc/AlO6] octahedron of GSAG matrix, as well as the fabrication technique of GSAG:Ce TC, should be revealed.
Therefore, in this study, based on the ion substitution engineering and the inductive effect of an average effective ionic radius match between the neighboring sites, the Y3+-Al3+ sites in YAG:Ce TC were substituted by Gd3+-Sc3+ ion pairs to form the GSAG:Ce structure. Phase composition, microstructure and luminescence behaviors (i.e., photoluminescence spectra (PL), photoluminescence excitation spectra (PLE), temperature-dependent PL spectra, raman spectra, fluorescence decay and quantum efficiency) of GSAG:Ce TCs were investigated systematiclly. In addition, TC based white LED/LD devices were constructed under the remote excitation mode, and their chromaticity parameters were verified simutaneously. Finally, this work demonstrates that GSAG:Ce TC is a highly potential convertor for the future applications of high-power white LEDs/LDs.
2. Experimental procedure
In this work, commercial powders of α-Al2O3 (99.99%, Alfa Aesar, Ward Hill, America), Gd2O3 (99.99%, Alfa Aesar, Ward Hill, America), Y2O3 (99.99%, Alfa Aesar, Ward Hill, America), Sc2O3 (99.9%, Shandong Xiya, Shandong, China) and CeO2 (99.99%, Shandong Xiya, Shandong, China) were selected as the starting materials to fabricate GSAG:Ce TCs. They were weighted precisely in accordance to the (Y1-x-zGdxCez)3(Al1-yScy)5O12 formula (x=0, 0.75, 0.996, 0.994, 0.992, 0.99 and 0.988, respectively, y=0.2, 0.3 and 0.4, respectively, and z=0.004, 0.006, 0.008, 0.01 and 0.012, respectively), denoting as YSc00Ce04, GYSc00Ce04, GYSc20Ce04, GSc20Ce04, GSc30Ce04, GSc40Ce04, GSc40Ce06, GSc40Ce08 GSc40Ce10 and GSc40Ce12, respectively. 0.5 wt% tetraethyl orthosilicate (TEOS, 99.99%, Alfa Aesar, Ward Hill, America) was chosen as the sintering additive and charge compensator to promote sintering and regulate the valence state of Ce ion in TCs. The detailed composition design of this work is listed in Table 1. Schematic diagram of the fabrication and characterization processes of GSAG:Ce TCs is shown in Fig. 1. The mixed powders were cold isostatic pressing at 200 MPa for 10 min to form green bodies, and then vacuum sintering at 1710 °C for 8 h to obtain ceramic blocks. The sintered ceramics were machined and polished on both surfaces to obtain GSAG:Ce TCs. The detailed description could be found in Supplement 1.
Table 1. Ingredients of the GSAG:Ce TCs.
Phase composition of GSAG:Ce TCs was characterized through an X-ray diffraction device (XRD, D8 Advance, Bruker, Karlsruhe, Germany) equipped with a copper target X-ray tube with a scanning range from 10° to 80°, and the step size was 0.01°. Morphologies of the starting powders and GSAG:Ce TCs were characterized by a scanning electron microscopy (SEM, JSM-6510, JEOL, Tokyo, Japan). Elemental mapping of GSAG:Ce TCs was carried out using an energy dispersive X-ray spectroscopy (EDS, Inca X-Max, Oxford Instruments, Oxford, England). In-line transmission spectra of the polished TCs with 1.0 mm thickness were tested using an UV-Vis-NIR spectrophotometer (Lambda 950, Perkin Elmer, USA). PL, PLE and fluorescence decay spectra of GSAG:Ce TCs were measured by a fluorescence spectrophotometer (FLS920P, Edinburgh, UK) equipped with a scintillating xenon lamp. A quantum efficiency spectrometer (C11347-11, Hamamatsu, Japan) was used for the quantum efficiency measurement. An X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical, UK) with a Axis Ultra DLD was applied to verify the charge state of Ce ion using a mono-chromatized Al Kα radiation (hv=1486.6 eV, 225 W) as the X-ray source under 10−9 torr. Raman spectra of GSAG:Ce TCs were tested by a Renishaw inVia Raman spectrometer using the 785 nm laser as the excitation source. Chromaticity parameters of TC based white LED/LD devices were recorded using an integrating sphere (R98, Everfine, Hangzhou, China) under 460 nm blue chips or a 450 nm laser as the excitation sources. The power of the blue light emitted from the blue chip was 2.0 W, and the power of the emitted blue laser was 1.5 W.
3. Results and discussion
Considering the phase component of TC is closely related to its optical quality and luminescence performance, phase evolution of TCs as a function of Gd3+, Sc3+ and Ce3+ ions doping sintered at 1710 °C was evaluated. According to the chemical design in Table 1, Sc3+ ion (0.075 nm, CN = 6) substitutes the octahedral Al3+ (0.053 nm, CN = 6) site, whereas Gd3+ (0.105 nm, CN = 8) and Ce3+ (0.118 nm, CN = 8) ions occupy the dodecahedral Y3+ (0.1019 nm, CN = 8) site in YAG matrix [49,62,63]. XRD patterns of GSAG:Ce TCs with different Gd3+ and Sc3+ doping concentrations are shown in Fig. 2(a). A pure garnet phase was obtained from YSc00Ce04 and GYSc00Ce04 TCs, which was in good consistence with the diffraction peaks of GAG phase (PDF: 97-002-3849). The phase components of GYSc20Ce04, GSc20Ce04 and GSc30Ce04 TCs were composed of GAG (PDF: 97-002-3849), GdAlO3 (GAP) (PDF: 97-015-0351) and α-Al2O3 (PDF: 97-009-9783) phases. It indicated that when Gd3+ ions completely substituted the Y3+ ions and a portion of Sc3+ ions occupied the octahedral Al3+ site in YAG lattice to form GAG, an average effective ionic radius mismatch still took place between the dodecahedral [GdO8] and the octahedral [Sc/AlO6] sites. In this regard, the Sc3+ ion doping concentration should be further optimized. From Fig. 2(b) it could be found that when the Sc3+ ion doping concentration reached 40 at.%, all the TCs exhibited a pure GSAG phase under various Ce3+ doping concentrations. Therefore, a new effective ionic radius matching relationship between the [GdO8] dodecahedron and the [ScO6] octahedron in GSAG matrix was constructed. To further evaluate the ion substitution engineering in GSAG:Ce TCs, the schematic crystal structure sketch of the ion substitution process from YAG:Ce to GSAG:Ce is exhibited in Fig. 2(c). Accordingly, under the premise of realizing the pure garnet phase GSAG:Ce, the surrounding coordination environment of Ce3+ ion was modified through the ion substitution engineering strategy.
Fig. 2. XRD patterns of GSAG:Ce TCs: (a) with different Gd3+ and Sc3+ doping concentrations, (b) with different Ce3+ doping concentrations, (c) schematic crystal structure sketch of GSAG:Ce TC.
To further confirm the effect of Gd3+ and Sc3+ doping on the phase composition and crystal structure parameter of GSAG:Ce TCs, crystal structure refinement was carried out by means of the Rietveld method, using the General Structure Analysis System (GSAS) software [64,65]. Fig. 3(a-d) display the detailed refinement results of GYSc00Ce04, GYSc20Ce04, GSc20Ce04 and GSc40Ce04 TCs. The Rietveld refinement results of the rest TCs are shown in Supplement 1, Fig. S1, and the detailed refinement parameters are listed in Supplement 1, Table S3. The refinement results of TCs were in consistent with that of the XRD analysis in Fig. 2. Additionally, Fig. 3(e) shows the variation trend of the lattice constant and cell volume of the corresponding TCs. Both the lattice constant and cell volume were increased from YSc00Ce04 to GYSc00Ce04 TCs, owing to the larger ionic radius of Gd3+ ion compared with that of Y3+ ion. The variation of the lattice constant and cell volume from GYSc20Ce04 to GSc30Ce04 TCs was nonlinear, attributing to the appearance of the GAP and α-Al2O3 secondary phases. With increasing Sc3+ ion doping concentration, the ratio of GAP and α-Al2O3 secondary phases was decreased. Pure GSAG phase was presented among TCs when the doping concentration of Sc3+ ion was 40 at.%, and their lattice constant and cell volume were far larger than that of the YSc00Ce04 TC without Sc3+ and Gd3+ doping. Meanwhile, two groups of dodecahedral Ce-O bond lengths (denoted as d(Ce1-O) (the shorter bond length) and d(Ce2-O) (the longer bond length), respectively) and the dodecahedral distortion ratio D of TCs are presented in Fig. 3(f). The dodecahedral distortion ratio D of TCs can be evaluated through utilizing the corresponding bond length parameters according to the Eq. (1): [28,48]
where Li is the distance from the cation to the “i” coordinated oxygen atom, and Lav is the average bond length. For YSc00Ce04, GYSc00Ce04 and GSc40Ce04 TCs with a pure garnet phase, their bond lengths were increased with increasing Gd3+ and Sc3+ ion doping concentration. The calculated distortion ratio D values from YSc00Ce04 to GSc40Ce04 TCs were 1.5%, 3.7%, 2.98%, 2.65%, 2.471 and 2.413%, respectively. Meanwhile, for GYSc20Ce04, GSc20Ce04 and GSc30Ce04 TCs, their crystal splitting effect (CFS) was decreased, owing to the decreased dodecahedral distortion ratio D by Sc3+ incorporating. As a result, the crystal splitting effect around Ce3+ ion of TCs was regulated effectively, and the complexity of the surrounding coordination environment of Ce3+ ion was increased in garnet matrix with the introduction of Gd3+ and Sc3+ ions.Fig. 3. Rietveld refinement patterns of (a) GYSc00Ce04, (b) GYSc20Ce04, (c) GSc20Ce04 and (d) GSc40Ce04, (e) variations of the lattice parameter and lattice volume from YSc00Ce04 to GSc40Ce04 TCs, (f) the corresponding d(Ce1-O) and d(Ce2-O) bond lengths and the dodecahedral distortion ratio D.
In order to evaluate the optical qualities of the GSAG:Ce TCs as color convertors for white LEDs/LDs, their optical properties were investigated. From the appearances of YSc00Ce04 and GYSc00Ce04 TCs shown in Fig. 4(a), it was obvious that Gd3+ ion incorporation significantly affected their visual colors, indicating that the emission peak of GYSc00Ce04 TC was adjusted effectively. GYSc20Ce04, GSc20Ce04 and GSc30Ce04 TCs were opaque, owing to the presented GdAlO3 and Al2O3 impurities. When the doping amount of Sc3+ ion reached 40 at.% (from GSc40Ce04 to GSc40Ce12), the color of the TCs was gradually tuned from yellow to orange-red, and their transparency was increased with increasing Ce3+ doping concentration. In-line transmission spectra of all TCs are displayed in Fig. 4(b). The YSc00Ce04 TC without Sc3+ and Gd3+ doping owned the highest transmittance, and the optimized transmittance of GSAG:Ce TCs with Sc3+ and Gd3+ doping was 58.4% @ 800 nm (GSc40Ce12). It could be seen that transmittances of TCs were declined with increasing Sc3+ and Gd3+ concentrations from YSc00Ce04 to GSc30Ce04, and then increased with elevating Ce3+ ion concentration from GSc40Ce04 to GSc40Ce12. The detailed transmittances of all TCs at 800 nm and 400 nm are displayed in Fig. 4(c). This variation trend could be explained as follows: the incorporated 75 at.% Gd3+ ion altered the refractive index and melting point of GYSc00Ce04 TC, and its optimized sintering temperature was changed accordingly. The decreased transmittance from GYSc20Ce04 to GSc30Ce04 TCs could be attributed to two main reasons. One is that the refractive index difference between the main crystalline phase and secondary phase. The other factor is the secondary phase itself that caused light scattering. Both effects lead to a declined transmittance of TCs [66,67]. The increased transmittance from GSc40Ce04 to GSc40Ce12 TCs was attributed to the sintering promotion effect of CeO2, which affected the densification behavior of TCs during sintering to affect their optical qualities. Additionally, the absorption band around 275 nm could be assigned to the 8S7/2→5IJ transition of Gd3+ ion. The typical absorption bands centered at 340 nm and 460 nm were caused by the 4f→5d2 and 4f→5d1 transitions of Ce3+ ion, respectively. The absorption at 460 nm of Ce3+ ion could be excited effectively by the blue LED and LD excitation sources. Reflection spectra measurement was carried out to evaluate the absorption properties as a function of Sc3+ ion concentration of the opaque TCs (from GYSc20Ce04 to GSc30Ce04 TCs), as is shown in Fig. 4(d). It is obvious that the typical absorption bands centered at 275nm, 340 nm and 460 nm observed from the reflection spectra were attributed to the absorptions of Gd3+ and Ce3+ ions, respectively.
Fig. 4. (a) Appearances, (b) in-line transmission spectra and (c) transmittances at 800 nm and 400 nm of the mirror-polished GSAG:Ce TCs (1.0 mm thickness), (d) reflection spectra of GYSc20Ce04, GSc20Ce04 and GSc30Ce04 TCs.
Figure 5 shows the SEM images of the thermal etched surfaces and fracture surfaces of GSAG:Ce TCs sintered at 1710 °C for 8 h. It was obvious that the introduction of Gd2O3, Sc2O3 and CeO2 had a remarkable influence on the generation of impurity phases and residual pores inside ceramic bulks. The YSc00Ce04 TC without Gd3+ doping exhibited a homogeneous microstructure, and there was no pore or secondary phase observed, indicating its high optical transmittance (Fig. 5(a) and (a’)). Secondary phases could be clearly observed from GYSc20Ce04 to GSc30Ce04 TCs with Sc3+ ion doping concentrations lower than 40 at.% (Fig. 5(c)-(e) and (c’)-(e’)). It has been confirmed from the XRD patterns in Fig. 2 that the chemical compositions of the secondary phases were GdAlO3 and Al2O3, respectively. It should be noted that clean grain boundaries were exhibited in TCs with the Sc3+ ion doping concentration of 40 at.% (Fig. 5(f)-(j) and (f’)-(j’)), and their grain size distribution was uniform. Residual pores could be observed from all the TCs with 40 at.% Sc3+ doping, and the amount of residual pores was reduced with increasing Ce3+ ion concentration, illustrating the sintering of TCs was effectively promoted by CeO2. From the fracture surfaces of TCs shown in Fig. 5(a')-(j’), it was obvious that the fracture mode of all TCs was characteristiced mainly by transgranular. Additionally, to further reveal the elemental distribution of GSAG:Ce TCs, an EDS analysis of GSc40Ce12 TC was recorded, and the corresponding results are displayed in Supplement 1, Fig. S2. It could be seen that all Gd, Sc, Ce, Al and O elements were distributed homogenously in GSc40Ce12 TC.
Fig. 5. SEM images of thermal etched surfaces and fracture surfaces of (a) and (a’) YSc00Ce04, (b) and (b’) GYSc00Ce04, (c) and (c’) GYSc20Ce04, (d) and (d’)GSc20Ce04, (e) and (e’)GSc30Ce04, (f) and (f’) GSc40Ce04, (g) and (g’) GSc40Ce06, (h) and (h’) GSc40Ce08, (i) and (i’) GSc40Ce10, (j) and (j’) GSc40Ce12.
An XPS study was carried out on the GSc40Ce12 TC to evaluate the valence state of Ce ion in GSAG matrix. Figure 6(a) exhibits the total survey scan of the GSc40Ce12 TC, and Fig. 6(b)-(e) show the high resolution XPS spectra of Gd 4d, Al 2p, Sc 2p and Ce 3d, respectively. It was obvious that the spectral peak at 142.1 eV was attributed to the characteristic peak of the dodecahedral coordinated Gd3+ ion in GSAG (Fig. 6(b)). The Al 2p peak with the binding energy of 74.1 eV was belonged to [AlO4] tetrahedron (Fig. 6(c)), while the Sc 2p peaked at 402.3 eV was assigned to [ScO6] octahedron in GSAG matrix (Fig. 6(d)) [68–70]. From Fig. 6(e) it was worth mentioning that the spectral peak of 885 eV was ascribed to the characteristic peak of Ce3+ ion, indicating that the valence state of Ce ion in the as-prepared GSAG:Ce TC was trivalent [71,72]. Two reasons were responsible to the trivalent state of Ce3+ ion. First, TEOS (SiO2) was applied as the charge compensator, which could effectively suppress the oxidation of Ce ion into tetravalent. Second, the applied vacuum sintering condition provided a weak reducing environment during the densification process of TCs, which was conducive to maintain the trivalent charge state of Ce3+ ion.
Fig. 6. (a) XPS survey scan of GSc40Ce12 TC; high-resolution XPS spectra of (b) Gd 4d, (c) Al 2p, (d) Sc 2p and (e) Ce 3d.
In order to further investigate the effect of Gd3+ and Sc3+ doping on the spectral property of TCs, as well as the spectral characteristics of pure garnet phase GSAG:Ce, normalized PLE and PL spectra and their FWHM values of GSAG:Ce TCs are shown in Fig. 7. Normalized PL and PLE spectra of YSc00Ce04 TC without Sc3+ and Gd3+ doping are shown in Fig. 7(a). Two broad excitation bands peaked at 340 and 460 nm could be observed simultaneously from the PLE spectra of TC, attributing to the 4f to 5d2 and 5d1 transitions of Ce3+ ion respectively. The emission peak centered a 552 nm was detected from the PL spectrum (λex=460 nm) of YSc00Ce04 TC, corresponding to the 5d−4f transition of Ce3+ ion. Meanwhile, normalized PLE and PL spectra of GYSc00Ce04, GYSc20Ce04, GSc20Ce04, GSc30Ce04 and GSc40Ce04 TCs are displayed in Fig. 7(b) and (c), respectively. The emission peaks centered at 275 nm, 312 nm, 340 nm and 460 nm could be clearly observed from the PL spectra of all the TCs. Among them, the narrow excitation bands at 275 nm and 312 nm were originated from the 8S7/2→6IJ and 8S7/2→6PJ transitions of Gd3+ ion, while the bands centered at 340 nm and 460 nm were stemmed from the 4f→5d2 and 4f→5d1 of Ce3+ ion, respectively. It should be noted that an obvious red shift phenomenon could be observed from the PL spectra of TCs with Gd3+ doping compared with that of the YS00Ce04 TC, indicating that the distortion of [CeO8] dodecahedron was enlarged. The emission peaks from GYSc00Ce04 to GSc40Ce04 TCs were blue shifted with increasing Sc3+ doping concentration, owing to the decreased crystal field splitting effect. Similar observation has been reported in our previous work [49]. The detailed peak positions of the PL spectra are displayed in Fig. 7(f). Additionally, with increasing the Gd3+ ion doping concentration (from 0 at.% to 100 at.%) and Sc3+ ion doping concentration (from 20 at.% to 40 at.%), the FWHM values of the 4f-5d2 transition in the PLE spectra of TCs were increased from 61.7 nm to 85 nm, while that of the 5d−4f transition in the PL spectra of TCs were broadened from 99.5 nm to 132.4 nm (Fig. 7(h)). The mechanism of the FWHM expansion could be explained as follows: one is that the distortion of the dodecahedron was increased thanks to the Gd3+ and Sc3+ ion substitution, leading to the degraded Ce3+ local symmetry [73,74]. The other factor is the local complexity around Ce3+ ion was aggravated, since the evolution of the dodecahedron was initially altered from [YO8] to [Gd/YO8], and then transformed from [Gd/YO8] to [GdO8]. Similarly, the evolution of the octahedron was changed from [AlO6] to [Sc/AlO6], and then transformed to [ScO6] [75,76].
Fig. 7. (a) Normalized PLE and PL spectra of YSc00Ce04 TC (λex=460 nm, λem=544 nm); (b) and (c) normalized PLE and PL spectra of GYSc00Ce04, GYSc20Ce04, GSc20Ce04, GSc30Ce04 and GSc40Ce04 TCs, (d) and (e) normalized PLE and PL spectra of GSc40Ce04, GSc40Ce06, GSc40Ce08, GSc40Ce10 and GSc40Ce12 TCs, (f) and (g) the peak positions of PL spectra, (h) and (i) the FWHM values of PLE and PL spectra.
With respect to the PLE and PL spectra of pure phase TCs from GSc40Ce04 to GSc40Ce12, the absorption bands originated from the Gd3+ and Ce3+ ions could be detected from their PLE spectra (Fig. 7(d)). This phenomenon was similar to the observations in Fig. 7(b). The PL spectra of the corresponding TCs provided a yellow-orange emission characteristic, and the observed emissions peaks were red shifted from 577 nm to 583 nm with increasing Ce3+ doping concentration from 0.4 at.% to 1.2 at.% (Fig. 7(e)). The detailed emission peaks of the TCs from GSc40Ce04 to GSc40Ce12 are exhibited in Fig. 7(g). The broadened PLE spectra and the red-shift PL spectra were stemmed from the enhanced CFS effect around Ce3+ ion. Similar behavior has been previously described by Jiang et al., in which a narrowed FWHM (from 136.1 nm to 133.3 nm) with increasing Ce3+ doping concentration was observed in GAGG:Ce TCs [39]. In general, the broadened PLE spectra of GSAG:Ce TCs is conducive to match the blue light emitted from LED/LD excitation sources. Also, the extension of PL spectra of GSAG:Ce TCs is beneficial for realizing the warm white light emission with high CRI in white LEDs/LDs.
A large amount of heat would be generated during high-power LED/LD operation, since the electro-optical conversion efficiencies of blue LED/LD excitation sources are quite low. Hence, it is important to evaluate the thermal stability of TCs for their better applications in the field of solid state lighting. Temperature-dependent PL spectra of GdYSc00Ce04, GSc20Ce04, GSc30Ce04 and GSc40Ce04 TCs are displayed in Fig. 8. The emission intensities of all TCs were gradually declined with increasing the operating temperatures from 298 K to 498 K. It was found that the luminescence intensities of TCs at 423 K were maintained 47.7%, 27.6%, 23.4% and 22.1%, respectively, compared with their the intensities at 298 K. The deteriorated thermal stability of TCs should be assigned to the narrowed band gap caused by the Gd3+ and Sc3+ incorporation [77]. According to the previous report, the band gap of GSAG material is approximately 6.3 eV, which is smaller than that of YAG (6.5 eV) [61]. In general, a changed band gap of a garnet TC is inevitable, if a high dose ion substitution was applied to obtain the pure garnet phase TC with excellent spectral property. Meanwhile, incorporating Gd3+ and Sc3+ ions into YAG matrix enriched the diversity of lattice vibration modes, and then enriched the phonon vibration diversity accordingly. Therefore, this incorporation might increase the collision probability between the phonon lattice waves produced by phonons and the electromagnetic wave generated by the electron transition of Ce3+ ion. In this regard, a portion energy could be released in the form of heat dissipation, and the thermal stability of TCs was reduced. Additionally, the activation energies (ΔE) of TCs for their thermal quenching behaviors were calculated by the Arrhenius Eq. (2):[64,78]
Fig. 8. Temperature-dependent PL spectra of (a) and (a’) GdYSc00Ce04, (b) and (b’) GSc20Ce04, (c) and (c’) GSc30Ce04, (d) and (d’) GSc40Ce04 in the temperature range of 298 K-498 K (λex=460 nm).
In order to provide a clear insight regarding the effect of Gd3+ and Sc3+ doping on the nonradiative transition behavior of TCs, fluorescence decay curves of all GSAG:Ce TCs were investigated by exciting TCs under 460 nm, as is shown in Fig. 9(a)–(c). Monitoring at 570 nm, all the decay curves could be well fitted by the single exponential decay function. It illustrated that Ce3+ ions occupied only one crystallographic site. The fitted lifetimes from YSc00Ce04 to GYSc00Ce04 TCs were declined, owing to the fact that the incorporated Gd3+ ion increased the probability of non-radiative transition. The variation of lifetimes from GYSc20Ce04 to GSc30Ce04 TCs was fluctuated slightly, owing to the segregated secondary phase in ceramics. Under the normal stoichiometric ratio, a portion of Ce3+ ions could be soluted into the matrix of secondary phase, resulting in a decreased amount of the luminous Ce3+ ions. Besides, the appearance of secondary phase might cause energy consumption among ceramic system, and generate traps in ceramic matrix. For the pure phase GSAG:Ce TCs (from GSc40Ce04 to GSc40Ce12), their lifetimes were moderately decreased with increasing Ce3+ doping concentration, due to the inter-activation among Ce3+ ions.
Fig. 9. (a)–(c) Fluorescence decay curves of all TCs, (d) IQE values from YSc00Ce04 to GSc40Ce04 TCs, (e)–(f) Raman spectra of YSc00Ce04, GYSc00Ce04 and GSc40Ce04 TCs within the frequency range of 100–1000 cm−1.
Internal quantum efficiency (IQE) measurement was implemented to lucubrate the luminescence behavior of TCs, and the IQE values of YSc00Ce04, GYSc00Ce04, GYSc20Ce04, GSc20Ce04, GSc30Ce04and GSc40Ce04 TCs (λex=460 nm) are presented in Fig. 9(d). It could be seen that the IQE values were decreased from 86.42% to 29.55% (from YSc00Ce04 to GSc30Ce04 TCs), and then increased to 46.91% (GSc40Ce04). The low IQE values from GYSc20Ce04 to GSc30Ce04 was attributed to the secondary phases in TCs. Compared with that of YSc00Ce04 TC, the pure phase GSc40Ce04 TC exhibited a lower IQE value, since the matching degree between Ce3+ ion and matrix affected the IQE value of ceramic [61]. The obtained IQE of 46.91% in GSc40Ce04 TC was suitable for the practical applications in solid-state lighting. In order to further verify the structural development of TCs with Gd3+ and Sc3+ doping, Raman spectra of YSc00Ce04, GYSc00Ce04 and GSc40Ce04 TCs within the frequency range of 100–1000 cm−1 were displayed in Fig. 9(e)–(f). It was obvious that the vibration of Raman peaks was strengthened as incorporating Gd3+ and Sc3+ ions into TCs, illustrating the structural symmetry of GSAG matrix was altered by Gd3+ and Sc3+ doping [73,74]. The observed phenomenon was in accordance with that of the thermal stability performance of TCs (Fig. 8).
To further evaluate the as-prepared TCs as color convertors for the general lighting applications, GSAG:Ce TC based white LEDs were assembled under the remote excitation mode. Figure 10 displays the normalized EL spectra, chromaticity parameters (CRI and CCT) and CIE chromaticity diagrams of the constructed white LED devices. Obviously, compared with that of YSc00Ce04 TC with a yellow light emission, the color hue of the rest lighting white LEDs was altered from yellowish pink to shinning white as increasing Gd3+ and Sc3+ doping concentrations, see the inserts of Fig. 10(a)–(f). The CIE color coordinates from GYSc00Ce04 to GSc40Ce06 TCs were changed from (0.3587, 0.4151) to (0.2192, 0.2269), which was in consistent with that the observed color hue of white LEDs (Fig. 10(h)). The variation trend of CRI and CCT values of white LEDs is exhibited in Fig. 10(g), and it was obvious that the CRI values were promoted from 61.6 to 80.7 with an increment of 31%, and the lowest CCT value was 3073 K. The detailed CRI, CCT and LER values, as well as the color coordinates of all TC-based white LEDs are displayed in Supplement 1, Table S4. The optimized chromaticity parameters of the white LEDs should be attributed to the more appropriate red/green/blue ratio of the emitted light, thanks to the enlarged FWHM. As a result, GSAG:Ce TC is a promising color convertor for white LEDs with high CRI applied in general lighting.
Fig. 10. EL spectra of (a) GYSc00Ce04, (b) GYSc20Ce04, (c) GSc20Ce04, (d) GSc30Ce04, (e) GSc40Ce04, (f) GSc40Ce06 TCs and photographs of the lighting TC-based white LED devices (inserts), (g) CRI and CCT of TCs, (h) CIE color coordinates of TCs under 350 mA drving current.
The heat dissipation behavior of TCs should be thoroughly considered to prevent the thermal quenching during the operation of TC based white LDs. Consequently, a aluminum alloy support frame was applied to fix the GSc40Ce04 TC to eliminate the possible thermal induced luminescence decay. Appearances of the designed apparatus for the white LD measurement are shown in Fig. 11 (a)–(b). The power of the emitted blue laser was 1.5W, and the applied excitation wavelength of the laser source was 450 nm. Considering the GSc40Ce04 TC owned the highest CRI of 80.7 among all TCs for white LEDs. In this regard, GSc40Ce04 TC was chosen as the typical color convertor for the white LD measurement. The corresponding EL spectrum of the GSc40Ce04 TC based white LD is shown in Fig. 11(e). Notably, the color hue of the white LD was yellowish white (Fig. 11(c) and (d)), and the corresponding CRI was as high as 69.5 with a CCT of 7766 K. Additionally, the detailed color coordinates, CRI and CCT values of all the constructed TC based white LDs are exhibited in Supplement 1, Table S5. It was found that the CRI values of the white LDs were promoted effectively when Gd3+ and Sc3+ ions were incorporated into GSAG:Ce TCs. With increasing Ce3+ ion doping concentration, the CRI values of GSAG:Ce TCs presented a decreasing trend, owing to the proportional mismatch among the emitted red/green/blue light. These results indicate that GSAG:Ce TC is a promising color convertor for the remote excitation white LDs.
Fig. 11. (a) and (b) appearances of the constructed TC based white LDs, (c) and (d) visual effect of the lighting white LD device, (e) the measured EL spectra of GSc40Ce04 TC based white LD.
4. Conclusion
In this study, by utilizing the ion substitution engineering strategy, pure garnet phase GSAG:Ce TCs were fabricated via incorporating Gd3+ and Sc3+ ions into YAG:Ce matrix through vacuum sintering for the first time. The optimized GSAG:Ce TC exhibited a high CRI of 80.7 and a broad FWHM of 132.4 nm, as well as a high IQE of 46.91%. More importantly, a distinct red shift from 546 nm to 582 nm was obtained, and the optimal in-line transmittance of GSAG:Ce TC was 58.4% @ 800 nm. The broadened FWHM of TCs was ascribed to the altered local symmetry around Ce3+ ion, and the narrowed band gap leaded to the degraded thermal stability of TCs. By combining GSAG:Ce TCs with blue LED chips, white LEDs with a remote excitation mode were constructed, and the color hue of the lighting LEDs was altered from yellow to shinning white as increasing Gd3+ and Sc3+ ion doping concentrations. Additionally, by exciting GSAG:Ce TCs under a blue laser with the emitted power of 1.5 W, the obtained CRI and CCT of the white LD device were 69.5 and 7766 K, respectively. This work not only provides a promising GSAG:Ce TC for high-power white LEDs/LDs, but also proves that ion substitution engineering is conducive to extend the FWHM, while maintaining the pure garnet structure of TC.
Funding
National Natural Science Foundation of China (51902143, 61775088, 61971207, 61975070); Open Project of State Key Laboratory of Advanced Materials and Electronic Components (FHR-JS-202011017); Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX20_0922); Special Project for Technology Innovation of Xuzhou City (KC19250, KC20201, KC20244); Natural Science Foundation of the Jiangsu Higher Education Institutes of China (19KJB430018, 20KJA430003); International Science and Technology Cooperation Program of Jiangsu Province (BZ2019063, BZ2020030, BZ2020045); Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_2096, KYCX18_2097, KYCX18_2098, KYCX18_2099); Natural Science Foundation of Jiangsu Province (BK20191467); Key Research and Development Project of Jiangsu Province (BE2018062, BE2019033).
Acknowledgments
Special thanks to Xuzhou All-to Optoelectronics Co., Ltd. (Xuzhou, China) for the electro-luminescence characterization of the white LED/LD devices.
Disclosures
The authors declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Supplemental document
See Supplement 1 for supporting content.
References
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