1. Introduction

Recently, Ce³⁺-doped yttrium aluminum garnet (Y₃Al₅O₁₂) is widely applied as scintillators, afterglow materials and color converters in white light emitting diodes (WLEDs) [1,2]. The most common WLED strategy is to combine Ce³⁺-doped Y₃Al₅O₁₂ phosphors and blue InGaN chips. WLED fabricated with near ultraviolet chip (300 to 410 nm) is a potential substitute for current lighting sources [3–6]. However, Ce³⁺-doped Y₃Al₅O₁₂ can hardly be excited by 350-400 nm light, making it unsuitable for near ultraviolet LED. Solid solution phosphor is an efficient tool for exploring the single-activator-doped phosphors. For solid solution phosphor, the variable composition can simultaneously alter multiple parameters of the host lattice and thereby change the local environment surrounding the activator [7]. By utilizing this strategy, some novel phosphors and new luminescence phenomena have been identified, such as tunable spectrum, efficient luminous output, and improved thermal stability. Denault et al. developed Sr_1.975Ce_0.025Ba(AlO₄F)_1-x(SiO₅)x solid solution phosphor with the maximum emission wavelength tuned from green (λ_em = 523 nm) to yellow (λ_em = 552 nm) by tuning the composition [8]. Xia et al. reported a series of solid solution phosphors such as Ca₂(Al_1−xMg_x)(Al_1−xSi_1+x)O₇:Eu²⁺ [9] and (CaMg)_x(NaSc)_1−xSi₂O₆ [10], and provided a theoretical understanding of the tunable luminescent properties by chemical unit cosubstitution [11,12].

As a typical garnet possessing a cubic structure with Ia3d symmetry, Mg₃Al₂Si₃O₁₂ is isostructural with Y₃Al₅O₁₂, which make it possible to introduce Mg₃Al₂Si₃O₁₂ into Ce³⁺-doped Y₃Al₅O₁₂ for forming solid solution phosphors. The introduction can be considered as (Y, Al)⁶⁺ pair substituted by (Mg, Si)⁶⁺ pair, which will vary the electronic and crystal structure of the host crystals, and affect the local environment surrounding Ce³⁺. Thus, the excitation spectrum of the single Ce³⁺-doped garnet is potentially anticipated to be broadened and expanded from blue light to near ultraviolet light.

In this work, (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:Ce³⁺ solid solution phosphors were synthesized via introducing the isostructural Mg₃Al₂(SiO₄)₃ into Y₃Al₅O₁₂:Ce³⁺. The anomalous spectral broadening is mainly subjected to statistical composition fluctuation because of the random distribution of Y, Mg, Al and Si at the same crystallographic site. By controlling the content of (Y, Al)⁶⁺ substituted by (Mg, Si)⁶⁺, the excitation spectrum of (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂: Ce³⁺ solid solution phosphors can be tuned to near ultraviolet region. A near ultraviolet LED fabricated with Y_1.5Mg_1.5Al₂Al_1.5Si_1.5O₁₂:Ce³⁺ phosphors and a 400 nm chip can emit strong white light. This work provides a strategy for developing a near ultraviolet light phosphor from a single Ce³⁺-doped garnet.

2. Experimental

(Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:0.06Ce³⁺ (x = 0-0.6) was prepared by conventional solid state reaction method. Typically, the stoichiometric amounts of Y₂O₃ (99.99%), CeO₂ (99.99%), Al₂O₃ (A. R.), SiO₂ (A. R.) and MgO (A. R.) were thoroughly mixed and then preheated at 1000°C for 2 h in air. Next, the precursors were calcined at 1600 °C for 6 h in a reductive atmosphere (H₂ 10%/N₂ 90%). After cooling down to room temperature, the obtained products were crushed and washed for use.

The crystal structure of samples was analyzed by X-ray diffractometer (XRD, TD-3500, Dandong, China) with Cu Kα radiation (λ = 1.5406 Å). Powder XRD data were collected in scanning mode for a 2θ range of 10-80° with a step of 0.02° and a rate of 2.0° min⁻¹. Unit cell refinements were accomplished using JADE software. The emission and excitation spectra were measured by using a fluorescent spectrophotometer (F-7000, Hitachi, Japan) with a 150 w Xe lamp as the excitation light source. The morphologies and selected area electron diffraction (SAED) images of samples were acquired using a transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses (JEM-21000, JEOL, Japan). The temperature quenching property was detected by thermocouples inside the plaque and was controlled with a standard high-temperature fluorescence controller (TAP-02, Orient KOJI, UK). The luminous efficiency, color-rendering index, and the Commission international de I’Eclairage (CIE) color coordinates of the fabricated LEDs were characterized using a high accuracy LED photo-color and electron test system (HSP3000, Hangzhou Hongpu Optoelectronics Technology, China) and were evaluated under a current of 350 mA.

3. Results and discussion

Figure 1(a) shows the XRD patterns of (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:Ce³⁺ (x = 0-0.6). All the diffraction peaks are in agreement with the standard data of Y₃Al₅O₁₂ (JCPDS card No. 33-0040). No evidence of impurity was detected, indicating the solid solutions were obtained. The XRD peaks located at 33.3°correspond to (420) crystal plane and shift slightly toward the higher scattering angle with an increase of x (Fig. 1 (b)). This observation demonstrates the cosubstitution of the (Mg, Si)⁶⁺ pair for the (Y, Al)⁶⁺ pair changes the lattice parameters in Y₃Al₅O₁₂ host. As shown in Fig. 1(c), the calculated lattice constant has a decline as x increases. Hence, these results agree with the lattice shrinkage owing to replacement of Y³⁺ (0.1019 nm, CN = 8) ions in dodecahedral sites and Al³⁺ (0.039 nm, CN = 4) ions in tetrahedral sites by the smaller Mg²⁺ (0.089 nm, CN = 8) ions and Si⁴⁺ (0.026 nm, CN = 4) ions (Fig. 1(d)), which refer to the previous report [13].

 

Fig. 1 (a) Full range (10-80°) XRD patterns, (b) selected diffraction peaks in the region between 32.5 and 34° and (c) lattice parameter of (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:0.06Ce³⁺ (x = 0-0.6). (d) Crystal structures of Y₃Al₂Al₃O₁₂.

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To confirm that the isostructural solid solution was formed, the HRTEM image and EDS mapping of a single Y_2.1Mg_0.9Al_4.1Si_0.9O₁₂:0.06Ce³⁺ particle were measured. As shown in Fig. 2, the interplanar spacing of 0.268 nm can be assigned to the (420) crystal plane, which have decreased slightly because of lattice shrinkage. Selected-area electron diffraction pattern along a [00-1] direction shows the single crystal character of the particle. Energy-dispersive spectroscopy (EDS) mapping of a single grain shows that Al, Y, Ce, Mg and Si are homogeneous distribution in a macroscopical particle.

 

Fig. 2 (a) TEM image and (b) HRTEM image of a single Y_2.1Mg_0.9Al_4.1Si_0.9O₁₂:0.06Ce³⁺ particle. (c) Selected-area electron diffraction pattern along a [00-1] direction. EDS mapping of (d) a single Y_2.1Mg_0.9Al_4.1Si_0.9O₁₂:0.06Ce³ particle for (e) Al, (f) Y, (g) Ce, (h) Mg and (i) Si elements.

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Figures 3(a) and 3(b) show excitation spectra and emission spectra of (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂: Ce³⁺ (x = 0-0.6). Under 530 nm emission, the excitation spectra are composed of two broad bands with the peaks at ~340 nm and ~450 nm, corresponding to the 5d–4f electronic excitation of Ce³⁺. With increasing x, the two bands show a distinct broadening. In particular for the band peaking at ~450 nm which is suitable for blue LED chips, its left wing is obviously raised and blue-shifted, whereas the right wing is slightly red-shifted. The full width at half maximum (FWHM) increases monotonically from 65 nm (x = 0) to 94 nm (x = 0.6), containing 23 nm blue-shift of left wing and 6 nm red-shift of right wing. The anomalous spectral broadening can be attributed to reduced structure symmetry, enhanced structural disorder and statistical composition fluctuation caused by the random distribution of Y, Mg, Al and Si at the same crystallographic sites [14]. The change of spectral width is dependent on evolution of the local environment surrounding Ce³⁺ in Y₃Al₅O₁₂.

 

Fig. 3 (a) Normalized excitation spectra and (b) emission spectra of (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:0.06Ce³⁺ (x = 0-0.6). (c) Excitation spectra and (d) emission spectra of Y₃Al₅O₁₂:0.06Ce³⁺ with doping different elements (Mg²⁺, Si⁴⁺, and (Mg, Si)⁶⁺).

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Figures 4(b) and 4(c) show a CeO₈ polyhedron is simultaneously coordinated by six (Al/Si)O₄ tetrahedra through nodes and edges, and by four (Mg/Y)O₈ square antiprisms through edges [15]. With varying x, the evolution of local environment surrounding Ce³⁺ from Al-rich site to Si-rich site and Y-rich site to Mg-rich site because of introducing Mg₃Al₃Si₂O₁₂ (Figs. 4(a) and 4(d)). The variation of the Si/Al and Mg/Y ratios play crucial roles in the change of the spectral shape and the spectral width. Analyzed via Gaussian fitting (Figs. 4(e)-4(g)), the band peaking at ~450 nm was divided into three sub-bands peaking at ~413 nm (Ce(I)), ~450 nm (Ce(II)) and ~488 nm (Ce(III)), respectively. Considering Si/Al ratio has a more important effect on the spectral width than that of Mg/Y ratio (Fig. 3(c)), Ce(I) is believed to reside at a Si-rich site, whereas Ce(II) resides at an Al-rich site; Ce(III) is believed to reside at a Mg-rich site, whereas Ce(II) resides at an Y-rich site. When x = 0.3, 0.4 and 0.5, the luminescence intensity ratios of Ce(I)/Ce(II)/Ce(III) are 0.09/0.96/0.20, 0.13/0.96/0.23 and 0.36/0.95/0.25, respectively, which well match with the sharply enhanced left wing and slightly enhanced right wing, that can be contributed to broadened excitation spectra.

 

Fig. 4 Local environment of CeO₈ surrounded by (Si/Al)O₄ (a), AlO₄ (b), YO₈ (c), and (Mg/Y)O₈ (d)in the crystal structure. Gaussian fitting of the excitation spectra of (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂: Ce³⁺ with x = 0.3 (e), 0.4 (f) and 0.6 (g)

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In addition to the excitation spectral broadening, the red-shift of emission spectra is also an interesting feature of the solid solution phosphors. The peak emission is red-shifted by 10 nm, moving from 530 nm (x = 0) to 540 nm (x = 0.60). Owing to the substitution of Mg for Y and Si for Al, the decrease of structure symmetry and the lattice shrinkage, and self-absorption increase usually induce to yield large crystal field splitting which red-shift the photoluminescence spectra. However, Fig. 3(c) and d show the excitation spectral broadening is weakened and emission spectra have no obvious shift, as the single substitution of Mg for Y, or Si for Al in host lattice. During the process of doping Mg²⁺ or Si⁴⁺ into the Y₃Al₅O₁₂ lattice, cation vacancies and oxygen vacancies will appear. These vacancies, namely crystal lattice defects, increase the probability of non-radiative transitions, and thus decrease Ce (I) or Ce (III) luminescent efficiency, which may reduce the excitation spectral broadening and spectral shift [16].

The thermal stability, which will induce luminescence quenching or even degradation of phosphors, plays an important role for the application. Figure 5 shows the temperature-dependence of the photoluminescence properties of the synthesized (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:0.06Ce³⁺ (x = 0-0.4). The relative photoluminescence intensity of the sample decreases with increasing temperature over the range of 25 to 250 °C due to the larger nonradiative transition probability at higher temperatures. The thermal quenching is reduced and then progressively incresed with increasing x. At 140 °C (413 K), the emission intensity is reduced by 20%, 17%, 24%, 29%, 31% for x = 0-0.4. Of these, Lu_2.7Mg_0.3Al_4.7Si_0.3O₁₂:Ce³⁺ exhibits the best thermal ability of emission. The electron density around O^2- anions is essentially competed by activator ion and the neighboring cations, so the M-O^2- (M = Mg²⁺, Si⁴⁺, Y³⁺, and Al³⁺) bonding will mainly control the degree of covalence in the Ce³⁺−O^2- bonding. Compared with the larger (Y, Al)⁶⁺ pairs, the bonds connecting between smaller (Mg, Si) ⁶⁺ pairs and O²⁻ anions are relatively covalent, so the covalence of Ce³⁺−O^2- bonds is weakened. Herein, the Ce³⁺−O^2- bonds become less covalent as x increases. The covalence of Ce³⁺−O^2- bond is progressively decreased by the neighbor cation control, resulting in the reduction of the thermal quenching activation energy [17].

 

Fig. 5 Temperature-dependent emission intensity of (Lu_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:Ce³⁺ (x = 0-0.4).

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In this work, (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:0.06Ce³⁺ solid solution phosphors exhibit broadened excitation spectra and red-shifted emission spectra. The obtained phosphors can suit well not only the blue chips but also near ultraviolet chips. As mentioned in Fig. 3(c), compared with x = 0, as x = 0.5 the excitation intensity remarkably rises by 50%. Two LEDs were fabricated with Y_1.5Mg_1.5Al_3.5Si_1.5O₁₂: Ce³⁺ phosphors by using 450 nm and 400 nm chips, respectively (Fig. 6). The chromaticity coordinate of the LEDs fabricated with 450 nm and 400 nm chips are (x = 0.2843 y = 0.2687) and (x = 0.3771 y = 0.4356), respectively. The luminous efficacies of these LEDs are 103.9 and 55.5 lm/W under 350-mA drive current. It is very interesting to find that the luminous efficacies are well consistent with the excitation intensity at different wavelength. These findings show this series of phosphors can promote application in white light-emitting diodes.

 

Fig. 6 (a) Electroluminescence spectra of wLEDs fabricated with different chips and (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂: Ce³⁺ phosphors. Images of Y_1.5Mg_1.5Al_3.5Si_1.5O₁₂:Ce³⁺ under (b) natural light and (c) 400 nm light. (d) Image of a wLED using 400 nm chip driven under 350 mA.

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4. Conclusions

In conclusion, the (Y_1-xMg_x)₃Al₂(Al_1-xSi_x)₃O₁₂:Ce³⁺ solid solution phosphors were synthesized by conventional solid state reaction method. The obtained particle shows the single crystal character and the cations (Al, Y, Ce, Mg and Si) are homogeneous distribution. With increasing x, the band peaking at ~450 nm has a distinct broadening and the FWHM increases monotonically from 65 nm (x = 0) to 94 nm (x = 0.6). The cosubstitution of the (Mg, Si)⁶⁺ pair for the (Y, Al)⁶⁺ pair causes lattice shrinkage and changes the spectral shape width. The band with the peak of ~450 nm can be divided into three sub-bands peaking at ~413 nm (Ce(I)), ~450 nm (Ce(II)) and ~488 nm (Ce(III)), respectively. When x = 0.3, 0.4 and 0.5, the luminescence intensity ratio of Ce (I)/Ce (II)/Ce (III) is 0.09/0.96/0.20, 0.13/0.96/0.23 and 0.36/0.95/0.25, respectively, which is well consistent with the definitely enhanced left wing and slightly enhanced right wing. The intensity of excitation light (x = 0.5) is enhanced by 50% at 400 nm compared with x = 0. The luminous efficacies of the LEDs using 450 nm and 400 nm LED chips are 103.9 and 55.5 lm/W under 350-mA drive current. The broadened excitation spectrum of the obtained solid solution phosphors will extend the applied range from blue light to near ultraviolet light in white light-emitting diodes.