The R, Ce:YAG (R: Pr³⁺ , Eu³⁺ , Gd³⁺ ) single crystals were grown by the Czochralski method. The high-purity yttrium oxide powder (99.999%), aluminum oxide (99.999%), cerium oxide (99.99%) were uniformly mixed at the stoichiometric ratio and then pressed into blocks that placed in an iridium crucible. To grow the R, Ce:YAG (R: Pr³⁺ , Eu³⁺ , Gd³⁺ ) single crystal, the oxide of R (praseodymium oxide, europium oxide, and gadolinium oxide) as a plus was added into the mixture. In our experiment, the concentrations of R³⁺ and Ce³⁺ were set as 0.8  mol% and 0.8  mol%, respectively. After molten at 1200  ° C for 10  h in the Ar atmosphere to rule out the absorbent water and other impurities, the (111) oriented YAG single crystal used as a seed was gradually lifted up from the surface of the melting liquid. Through the periods of necking down, shoulder pulling, fixed-neck growing, and growing ending, a crystal was obtained. During fixed-neck growing, the lifting speed of the YAG single crystal seed was 1– 2.5  mm/h with a spinning rate of 12  r/min.

The crystalline phases of the samples were identified by x-ray diffraction (XRD) measurement (Bruker D8, Germany) using Cu K_α radiation. The absorption and transmittance spectra of all samples were tested with a spectrophotometer (Lambda950, Perkin Elmer). The emission and excitation spectra were recorded at the wavelength range of 200– 750  nm, respectively, with a spectrophotometer equipped with a xenon lamp (JASCO FP6500, Japan).

The luminescent properties of all samples were tested by using a photo-electric testing system (PMS-80V1, EVERFINE). PMS-80V1 is an optical measurement system which contains an integrated sphere and a spectrometer, and controlled by a PC program. The LED chip was placed at the center of a reflector cup. The integrated sphere equipped with an optical fiber and a CCD detector was used to measure the optical spectrum of the WLED. When testing, the Ce:YAG single crystal plates were covered on the LED chip at the center of the integrated sphere. The scanning range was from 380  nm to 800  nm with the scanning interval of 5  nm and the input voltage of 3  V. The electroluminescence (EL) spectra, color coordinates, color rendering indexes, color temperatures, and luminous efficiencies of the specimens were tested.

The XRD patterns of the Ce:YAG and R, Ce:YAG (R: Pr³⁺ , Eu³⁺ , Gd³⁺ ) single crystals are shown in Fig.  1. The YAG single phase is identified, and weak peaks are observed which do not belong to the YAG phase, it is speculated that they are induced by crystal field splitting after the R³⁺ ions are codoped. The XRD pattern of the Ce:YAG single crystal is consistent with JCPDS card (No.  33-0040), indicating that the Ce³⁺ dopants are incorporated into the YAG host lattice without altering the original structure of YAG. Though the codoped ions induce weak peaks, they do not change the whole crystal structure and impact the following optical properties.

Fig.  1.

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	Fig.  1. XRD patterns of R, Ce:YAG (R: Pr3+ , Eu3+ , Gd3+ ) single crystals.

The optical transmittances of the R, Ce:YAG (R: Pr, Eu, Gd) single crystals are shown in Fig.  2. The transmittance of the Eu, Ce:YAG single crystal sample is 79.66%, and those of the Gd, Ce:YAG and the Pr, Ce:YAG single crystal samples are 74.22% and 60.67% at 800  nm, respectively. The transmittance of the R, Ce:YAG (R: Gd³⁺ , Eu³⁺ ) single crystal is obviously larger than that of the Pr, Ce:YAG single crystal. The absorptions at nearly 340  nm and 460  nm of all samples are generated by the absorption due to the corresponding Ce³⁺ transitions of 2F_5/2 → 5d and 2F_7/2 → 5d, and those at 288  nm and 236  nm originate from the Pr³⁺ transitions of 4f² → 4f5d. The peak at 224  nm is attributed to the charge transfer band and the absorption at 275  nm corresponds to ⁸S_7/2 → ⁶I_J of Gd³⁺ .

Fig.  2.

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	Fig.  2. Optical transmittances of R, Ce:YAG (R: Pr3+ , Eu3+ , Gd3+ ) single crystals.

Eu³⁺ and Ce³⁺ simultaneously exist in the YAG crystal structure and the new ions system must be considered and discussed because of the interaction between the two ions. On one hand, the Ce³⁺ ion ([Xe]⁴f₁) is likely to form the stable Xe electron configuration after losing one electron. On the other hand, the half-filled-shell effect will occur in the Eu³⁺ ion ([Xe]⁴f₆) if an electron is obtained, which is the relatively stable electron configuration. Therefore, the chemical reaction Eu³⁺ + Ce³⁺ → Eu²⁺ + Ce⁴⁺ is possible and there is doubt whether the Eu²⁺ ions are dominant in the YAG crystal structure. Fadlalla et al. studied the crystalline YAG:Eu²⁺ phosphors and the samples exhibited blue emission spectra within the range of 447– 503  nm with the main broad-band emission peak at 480  nm, belonging to the characteristic emission of the transition of Eu²⁺ ions in YAG.^[10] However, it is not observed in the PL emission spectra of the Eu, Ce:YAG single crystal. So the effect of Eu²⁺ could be ignored. The PL emission spectra of the Eu, Ce:YAG single crystal under the excitation of 400  nm and 460  nm are shown in Fig.  3. Like the Ce:YAG single crystal, strong emission is observed around 527  nm, characteristic of the 5d→ 4f transition of the Ce³⁺ ion. With the detected wavelength of 530  nm, broad excited peaks at 466  nm and 341  nm are also shown along with the sharp excited peak at 263  nm in Fig.  4. The broad peaks centered at 466  nm and 341  nm are originating for the Ce³⁺ transitions of 2F_5/2 → 5d and 2F_7/2 → 5d. However, the strong-intensity light of wavelength 263  nm cannot directly excite the Ce³⁺ ions and we speculate that it belongs to the Eu³⁺ excitation from ⁷F₀ to upper F levels. Hence it is inferred that the energy transfer of Eu³⁺ → Ce³⁺ may take place in the Eu, Ce:YAG single crystal system. Enhancing the 528  nm emission band may be realized in two ways. In one way, the emission peak around 528  nm related to the Eu³⁺ transition from the ⁵D_1.2 level to the ⁷F_0.1.2 level directly appears after the Eu³⁺ non-radiative energy transition from the upper F levels to the ⁵D_1.2 level. In the other way, under the excitation of wavelength 263  nm, the Eu³⁺ ion is excited to the upper F level by optical excitation, then all of the energy is transferred to the 5d level of Ce³⁺ . The ²D_5/2 → ²F_5/2.7/2 transitions occur after the Ce³⁺ non-radiative transition 5d→ ²D_5/2.

Fig.  3.

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	Fig.  3. PL emission spectrum of Eu, Ce:YAG single crystal after 460  nm excitation. The inset shows the PL emission spectrum corresponding to 400  nm excitation.

Fig.  4.

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	Fig.  4. PL excitation spectrum of Eu, Ce:YAG single crystal (detected wavelength of 590  nm). The inset shows PL excitation spectrum corresponding to detected wavelength of 530  nm.

With codoped Eu³⁺ ions, we expect to see the Eu³⁺ contribution to enhance the red light wave band. Though the broad peak around 528  nm is obtained at the excitation of 460  nm, but this is the expected result. Therefore 400  nm is used as the excited wavelength, and the appearance of peaks at 528  nm, 594  nm, 609  nm, and 629  nm is depicted in the inset spectrum of Fig.  3. The intense emission peak at 594  nm, characteristic of Eu³⁺ magnetic dipole transition ⁵D₀ → ⁷F₁, is found along with two relatively weak emissions around 609  nm and 629  nm, which are originating from Eu³⁺ electric dipole transitions of ⁵D₀ → ⁷F₂ and ⁵D⁰ → ⁷F₃, respectively. The excitation spectrum corresponding to 590  nm emission is shown in Fig.  4. Except for the Ce³⁺ characteristic peaks, a tense peak centered at 294  nm can be clearly observed from the PL excitation spectrum. At the same time, the wavelengths around 341  nm and 460  nm cannot directly excite the Eu³⁺ ions but several red emission peaks are observed. The above various factors are taken into consideration, the energy transfer of Ce³⁺ → Eu³⁺ may happen in the the Eu, Ce:YAG single crystal system. That is to say, the excitation of Eu³⁺ occurs via the Ce³⁺ absorption, which means that the Ce³⁺ ion acts as a sensitizer for the Eu³⁺ luminescence. The energy transfer route of Ce³⁺ → Eu³⁺ can be deduced from the energy level diagrams for Ce³⁺ and R³⁺ ions (Fig.  7). The Ce³⁺ ion is excited to the ²D₀ (5d) state by optical excitation, then all of the energy is transferred to the ⁵D₁ level of Eu³⁺ . The ⁵D₀ → ⁷F_1.2.3 transitions occur after the non-radiative transition ⁵D₁ → ⁵D₀.

Due to the Pr³⁺ abundant wave band for absorption and emission, it is believed that the Pr³⁺ ions are appropriate to be added to the Ce:YAG single crystal as a co-doping element for approving the emission light. Heesun Yang et al. reported the PL emission spectra of Pr-codoped YAG:Ce nanocrystalline samples with excitation wavelengths of 287  nm and 440  nm and the energy transfer between Pr³⁺ and Ce³⁺ was described.^{[6, 7]} Kim et al. introduced the single phase full-color-emitting phosphor for near-UV pumped white LED, but few studies on the codoped Ce:YAG single crystal using rare-earth ions have been reported.^[11] The theoretical investigations on the codoped YAG:Ce nanocrystalline is almost based on the crystal structure; to some degree, those theories can be applied to the codoped YAG:Ce single crystal.

Since excitation wavelengths of 287  nm and 440  nm have been studied, those of 385  nm, 405  nm, and 460  nm are chosen for the broad absorption in the Pr, CeYAG single crystal system in this paper. In order to investigate the formation of the emission peaks, wavelengths of 560  nm, 609  nm, and 696  nm are detected (520  nm and 608  nm corresponding excitation spectra in Ref.  [6]). With the excitation of 385  nm, the Pr, CeYAG single crystal sample shows a sharp emission peak at 579  nm, which is related to the relaxation of ³P₀ → ³H₅ of the Pr³⁺ ions. A broad emission peak at 529  nm and a sharp emission peak at 609  nm are observed under the excitation of 405  nm and 460  nm, but stronger in the former and weaker in the latter in comparison to the results under 440  nm in Ref.  [6]. The appearance of the emission peaks at 529  nm and 609  nm is closely related to the Ce³⁺ → Pr³⁺ energy transfer. The Ce³⁺ → Pr³⁺ energy transfer can be depicted by a non-radiative relaxation and the radiative relaxation of the excited 5d electrons of the Ce³⁺ ion. After the electrons of the Ce³⁺ ion are excited from the 4f state to the 5d level, most of them return to the ground state of the Ce³⁺ ion, a small amount of them can transfer energy in the form of radiative relaxation from the 5d level to ¹D₂ level of the Pr³⁺ ions. Another small amount of electrons can transfer nonradiative energy to the ³P₀ energy level of the Pr³⁺ ions. Consequently, 608  nm emission appears due to the radiative energy transition from the ¹D₂ energy level to the ³H₄ energy level of the Pr³⁺ ions. The nonradiative way strengthens the emission in the range of 525– 600  nm. Because 288  nm excitation cannot directly excite the Ce³⁺ ion existing in the Pr, Ce:YAG single crystal sample, the 4f₂ → 4f5d transition is generated in the Pr³⁺ ions. Then the Pr³⁺ → Ce³⁺ energy transfer is realized in a nonradiative way which is the relaxation from the ³P₀ energy level of the Pr³⁺ ions to the 5d state of the Ce³⁺ ions, followed by the relaxation of the excited 5d electrons of the Ce³⁺ ions.

In addition, the small emission peak at 467  nm under the excitation of 405  nm and 692  nm are showed in Figs.  5(b) and 5(c). the former is originated from the transition of ³P₀ → ³H₅ of the Pr³⁺ ions and the latter is corresponding to the radiative energy transition of Ce³⁺ → Pr³⁺ , followed by the ¹D₂ level to ²F level of the Pr³⁺ ions. Figure  6(c) shows the PL excitation with detected wavelength of 695  nm, and only an excited peak at 460  nm is observed. As the Pr³⁺ ions cannot be directly excited by the excitation of wavelength 460  nm, the result reflects the Ce³⁺ → Pr³⁺ energy transfer.

Fig.  5.

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	Fig.  5. PL emission spectra of Pr, Ce:YAG single crystal under excitation of various wavelengths: (a) 385  nm, (b) 405  nm, (c) 460  nm.

Fig.  6.

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	Fig.  6. PL excitation spectra of Pr, Ce:YAG single crystal with various detected wavelengths: (a) 560  nm, (b) 609  nm, (c) 695  nm.

As can be seen in the schematic energy level diagrams of Ce³⁺ and Gd³⁺ ions (Fig.  7), the 6P level of the Gd³⁺ ion is partly overlapped with the 5d level of the Ce³⁺ ion. The energy transfer is able to occur between Gd³⁺ and Ce³⁺ ions in theory. However, most researchers did not pay attention to the energy level transition, because the Gd³⁺ ion did not induce other longer emission wave bands. In recent years, the Gd³⁺ -substituted Ce:YAG phosphors have been intensively studied due to its special spectral variation. Yang et al. reported that the Ce³⁺ emission was red-shifted by Gd³⁺ substitution due to a modified crystal field splitting in the Gd, Ce:YAG nanocrystalline system.^[7] Nishiura et al. studied the high power application in white LEDs using transparent Gd, Ce:YAG ceramics.^[12] While for the single crystal, only Latynina et al. reported the properties of Czochralski grown Ce, Gd:Y₃Al₅O₁₂ single crystal for white light-emitting diode and the corresponding wafers showed better values and improved stability under high temperatures in comparison to the commercial phosphor powders.^[13] In this paper, compared with those of Ce:YAG, Eu, Ce:YAG, and Pr, Ce:YAG single crystals, the PL emission of Gd, Ce:YAG under the excitation of 340  nm and 460  nm is depicted in Figs.  8 and 9. The emission spectra of Ce:YAG, Eu, Ce:YAG, and Pr, Ce:YAG single crystals at 528  nm are almost overlapped in Fig.  8, and with the codoped Gd³⁺ ion, the related emission spectra are red-shifted 13  nm to the emission peak at 541  nm. In Fig.  9, for the PL emission of the Pr³⁺ and Eu³⁺ codoped Ce:YAG single crystal, the red-shift spectral peak changes from 528  nm to 531  nm and it is not obvious. However, the width of the emission band is enlarged and it is beneficial to improve the luminescent behaviors of the corresponding devices. For the PL emission of the Gd, Ce:YAG single crystal, the broad band emission is red-shifted from 528  nm to 540  nm. It coincides with Nishiura et al.’ s report. According to Ref.  [14], the 5d state of the Ce³⁺ ion can be adjusted by partially substituting Y³⁺ or Al³⁺ site with different types of cations. Because the ionic radius of Gd³⁺ (1.05  Å ) is larger than that of Y³⁺ (1.02  Å ) occupying a dodecahedral site in the host lattice of the YAG single crystal, the substitution of the Gd³⁺ ion causes the Ce– O distance to be shortened, leading to an increased crystal field splitting. It is noted that the crystal field splitting (D_q) is proportional to ∼ 1/r⁵, where r is the bond length of the activator cation– ligand anion. This larger crystal field splitting further lowers the lowest energy level of the Ce 5d state band and consequently induces a red-shift of the Ce emission. The electroluminescent behaviors are investigated in the following.

Fig.  7.

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	Fig.  7. Schematic energy level diagrams of Ce3+ , Eu3+ , Pr3+ , and Gd3+ ions.

Fig.  8.

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	Fig.  8. The PL emission spectra of R, Ce:YAG (R: Pr3+ , Eu3+ , Gd3+ ) single crystals under 340  nm excitation.

Fig.  9.

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	Fig.  9. The PL emission spectra of R, Ce:YAG (R: Pr3+ , Eu3+ , Gd3+ ) single crystals under 460  nm excitation.

The excitation band covering from 400  nm to 500  nm is most intense and provides a background for the application of these phosphors in blue InGaN chips. The broad band located from 500  nm to 600  nm provides an ideal yellow light complementary to the blue light emitted by InGaN LED chips to produce white light. Here we choose the 460  nm-LED chips as a basis to improve the electroluminescent properties of WLEDs utilizing R, Ce:YAG (R: Pr³⁺ , Eu³⁺ , Gd³⁺ ) in comparison to the Ce:YAG-based WLED.

When the thickness of the sample is 1  mm, the Ce:YAG-single-crystal-based WLED shows a high luminous efficiency (42.45l  m/W) and a color rendering index of 62.8 (Table  1). And it produces a natural white light (5030  K). The Eu, Ce:YAG-single-crystal-based WLED displays a relatively lower luminous efficiency (22.11l  m/W) and a color rendering index of 72.9, which gives a cold white light (6777  K). At the same time, the Pr, Ce:YAG-single-crystal-based WLED gives a higher luminous efficiency of 45.86 lm/W, which is almost 2.5 times of that in Ref.  [7], and shows a warm white light (4420  K). With the Gd³⁺ codoped Ce:YAG single crystal, the corresponding WLED shows a luminous efficiency of 41.29l  m/W and produces the warmest white light (3663  K). These statistics indicates that via codoping R³⁺ (Eu³⁺ , Pr³⁺ , Gd³⁺ ) ions in the Ce:YAG single crystal system, the electroluminescent properties of the related WLEDs can be improved with higher color rendering index and warmer white light.

Table 1.

Table 1.

Table 1. Optical properties of white LEDs using 1.0-mm-thickness R, Ce:YAG (R: Pr3+ , Eu3+ , Gd3+ ) single crystals at the current of 250  mA. Sample CRI LE/lm· W− 1 CCT CIE(x, y) Ce:YAG 62.8 42.45 5030 (0.3525, 0.4483) Eu, Ce:YAG 72.9 22.11 6777 (0.3039, 0.3511) Pr, Ce:YAG 62.6 45.86 4420 (0.3875, 0.4900) Ce, Gd:YAG 60.5 41.29 3663 (0.4252, 0.4707)

Table 1. Optical properties of white LEDs using 1.0-mm-thickness R, Ce:YAG (R: Pr3+ , Eu3+ , Gd3+ ) single crystals at the current of 250  mA.

The same result can be concluded from Fig.  10. Figure  10 exhibits the thickness-dependent CIE color coordinates of all as-prepared samples. With the thickness increasing, the CIE x and y values increase and the corresponding points move greatly from the blue region to the yellow-greenish region. For certain kinds of materials (such as Ce:YAG single crystal), the corresponding points are approximately linearly distributed. So we use lines a– d respectively to represent the CIE color coordinates related to all thicknesses of R, Ce:YAG (R: 0, Eu³⁺ , Pr³⁺ , Gd³⁺ ). The lines b– d shift to the more deeper yellow region than that of the Ce:YAG single crystal, which is caused by the strengthened red component light with the codoping ions. In particular, the point of the Gd³⁺ codoped Ce:YAG single crystal moves the deepest in the yellow region and causes a higher color temperature (warmer white light). This result is attributed to the red-shift phenomenon, the emission peak shifts to the longer wavelength with the codoping Gd³⁺ ions. The enhanced performance of Eu, Ce:YAG and Pr, Ce:YAG single crystals is due to the broadened emission peak at 528  nm and the additional red wave band.

Fig.  10.

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	Fig.  10. The CIE color coordinates of WLEDs using R, Ce:YAG (R: Pr3+ , Eu3+ , Gd3+ ) single crystals of different thicknesses (0.5  mm, 1.0  mm, 2  mm).