The Luminescence Properties and Energy Transfer from Ce³⁺ to Pr³⁺ for YAG:Ce³⁺Pr³⁺ Phosphors

Abstract

Y_2.94−xAl₅O₁₂(YAG):Ce_0.06Pr_x phosphors with various Pr³⁺ concentrations (, 0.006, 0.01, 0.03, 0.06, and 0.09) were synthesized by using a coprecipitation method. The phases, luminescent properties, and energy transfer phenomenon from Ce³⁺ to Pr³⁺ were investigated. The results indicated that the doping of Pr³⁺   did not produce any new phases but caused a slight lattice parameters increase. After Pr³⁺ doping, the YAG:CePr phosphor emits red light at 610 nm, which was regarded helpful for improving the colour rendering index of the phosphor. With Pr³⁺ concentration increase from 0.006 to 0.01 mol, the intensity of red light emission increased slightly; further increasing Pr³⁺ concentration from 0.01 to 0.09, the red light emission intensity decreased gradually. Excitation at 340, and 460 nm could not lead to the direct electronic excitation of Pr³⁺ ions; however, when YAG:CePr was excited at 340 nm a red light emission at 610 nm appeared, which implied the energy transfer phenomenon from Ce³⁺ to Pr³⁺.

1. Introduction

As the fourth generation lighting sources, white-light emitting diodes (LEDs) have the following advantages: high efficiency to convert electrical energy to light, high reliability, and long operating lifetime (about 100000 h). They have been employed in the devices such as solid-state lasers, traffic lights, and field-emission displays [1–3]. Usually, white LEDs are fabricated by the combination of blue InGaN chips and yellow-emitting phosphor (YAG:Ce) [4–6]. However, the combined InGaN chips and YAG:Ce suffers from a low colour rendering index (<80) because of red light shortage in its emission spectrum. There are two methods to solve this problem: first, add an amount of red phosphor to yellow phosphor YAG:Ce to improve its colour rendering [7]; second, dope YAG:Ce phosphors to improve the red light emission [8–11]. As the ionic radius of Pr³⁺ is similar to Y³⁺, it has been used to dope YAG:Ce phosphor to improve its red spectrum emission. Jang et al. [12] synthesized YAG:CePr phosphors with various Pr concentrations through a high temperature solid-state reaction. The obtained YAG:CePr phosphors used for white LEDs improved the colour rendering index between 80 and 83. Yang et al. [13] synthesized Pr³⁺ doped YAG:Ce nanopowder by polymer-assisted sol-gel method, which also showed a high colour rendering index of 83 in white LEDs. However, the researches on Pr³⁺ doped YAG:CePr are still limited; particularly, the energy transfer phenomenon from Ce³⁺ to Pr³⁺ is seldom researched. Here, we report the synthesis of YAG:CePr phosphors by a coprecipitation method, and the effects of Pr³⁺ doping concentrations on the properties of YAG:CePr including phases, light absorption, and luminescent properties are researched. In addition, the energy transfer from Ce³⁺ to Pr³⁺ is characterized and proved through luminescent and fluorescence lifetime measurements.

2. Experiments

YAG:Ce_0.06 ( = 0, 0.006, 0.01, 0.03, 0.06, and 0.09) phosphors were prepared by coprecipitation method. Stoichiometric amounts of source materials Y₂O₃(AR), Al(NO₃)₃·9H₂O(AR), Ce(NO₃)₃·6H₂O(AR), Pr₆O₁₁(AR), NH₄HCO₃(AR), NH₃·H₂O(AR), and HNO₃(AR) were thoroughly mixed in solution. The Y³⁺ and Pr³⁺/Pr⁴⁺ solution was obtained by dissolving Y₂O₃ and Pr₆O₁₁ in HNO₃ aqueous solution, and this solution was denoted as solution A; Al(NO₃)₃·9H₂O and Ce(NO₃)₃·6H₂O were dissolved in a suitable amount of deionized water to obtain solution B; NH₄HCO₃ was also dissolved in deionized water, and the solution was denoted as C. Solutions A and B were mixed to obtain solution D, and then solution D was added (dropwise) to solution C at a rate of 1 mL/min. After finishing the dropping process, the system was adjusted to pH of 8 by using NH₃·H₂O and stirred for 2 h, and then the obtained precipitate was aged for 12 h. The white precipitate was filtered and then washed for 3 times by using deionized water and anhydrous ethanol, respectively. The filtered product was dried at 120°C for 12 h, ground, and mixed together with NaF (6 wt%) to form the precursor. The precursor was annealed at 1600°C for 3 h in a reductive atmosphere (with 95% N₂ and 5% H₂) to get the target product.

The phase characterization of the synthesised materials was conducted with X-ray powder diffraction (XRD) on a D8 Advance diffractometer (Bruker) operating at 40 kV, 200 mA, using Cu Kα radiation with a scanning rate of 1°/min and scanning range of 10° ≤ 2θ ≤ 80°.

The photoluminescence spectra were measured on powder samples at room temperature using Hitachi F-4500 luminescence spectrometer with a xenon discharge lamp (150 W) as the excitation source. The excitation spectra were obtained over the range of 200 to 550 nm and emission spectra were obtained over the range of 450 to 665 nm. Both excitation and emission splits were 5.0 nm, and the scanning interval was 0.2 nm. UV-visible absorption and diffuse reflectance spectra were monitored by a SHIMADAZU Japan UV 2700, with wavelength range from 200 to 800 nm and scanning interval of 0.5 nm.

3. Results and Discussion

3.1. Phases Analysis

Figure 1 shows XRD patterns of the YAG:Ce_0.06 ( = 0, 0.006, 0.01, 0.03, 0.06, and 0.09) annealed at 1600°C for 3 h. All the diffraction peaks can be indexed to standard data of Y₃Al₅O₁₂ (JCPDS 33-0040), without the possible intermediate phases such as Y₄Al₂O₉ or YAlO₃. It means the samples were phase-pure and doping of the activator ions Pr³⁺ did not cause any observable new phases. Lattice parameters of YAG:Ce_0.06 are calculated and the results are listed in Table 1. With increase of Pr³⁺ doping concentration, diffraction peaks of the phosphors shifted to lower degree and lattice parameters increased slightly, which can be explained by the larger radius of Pr³⁺ ( pm) than that of Y³⁺ ( pm).

Table 1 

Lattice parameters of YAG:Ce_0.06Pr_x (x = 0, 0.006, 0.01, 0.03, 0.06, and 0.09).

Figure 1 

XRD patterns of YAG:Ce_0.06 ( = 0, 0.006, 0.01, 0.03, 0.06, and 0.09) samples.

3.2. Spectral Analysis

3.2.1. Ultraviolet-Visible Absorption and Diffuse Reflectance Spectra

Figure 2(a) shows the absorption spectra of YAG:Ce. The excitation (absorption) peaks at 245, 340, and 457 nm were attributed to 4f5d transitions of the Ce³⁺ ions. The strong absorption peak located at 460 nm matched well with blue-light LED chips, which means the prepared YAG:Ce phosphors can absorb blue light from LED chips efficiently.

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Figure 2 

Absorption spectra of YAG:Ce_0.06 phosphor with = 0 (a), 0.01 (b), 0.03 (c), 0.06 (d), and 0.09 (e).

Figures 2(b)–2(e) show the absorption spectra of YAG:Ce_0.06 phosphors with = 0.01, 0.03, 0.06, and 0.09 mol, respectively. With increase of Pr³⁺ concentration, the two narrow absorption peaks appeared at 239 and 289 nm which correspond to the 4f²()4f5d transition of Pr³⁺ [9] increased significantly. When part Y³⁺ sites were substituted by Pr³⁺ ions, lattice parameters of the crystal became larger, causing subtle changes around the field of Ce³⁺ luminescence centre. Therefore, it is possible that the doping of Pr³⁺ will change absorption properties of Ce³⁺. After Pr³⁺ doping, the typical absorption peaks of Ce³⁺ ions still exist in YAG:CePr phosphors, indicating the doping of Pr³⁺ has little effects on the absorption properties of Ce³⁺. Presence of absorption peak at 457 nm means that YAG:CePr can absorb blue light from LED chips.

Diffuse reflection spectra of YAG:Ce_0.06 were measured and the results were shown in Figure 3. YAG:Ce³⁺ phosphor without Pr³⁺ doping had a strong broad absorption peak at 460 nm and weak absorption band from 270 to 400 nm. With Pr³⁺ doping into the YAG:Ce_0.06 phosphors, a small absorption peak appeared at 610 nm. With increase of Pr³⁺ concentration, intensity of the absorption peak at 610 nm gradually increased. At the same time, with Pr³⁺ concentration increase, the absorption intensity of 460 nm decreased, which may be caused by the formation of defects during Pr³⁺ doping [13].

Figure 3 

UV-vis diffuse reflectivity spectra of YAG:Ce_0.06 phosphors.

3.2.2. Excitation and Emission Spectra

Figure 4 shows the emission spectra of YAG:Ce_0.06 ( = 0.006, 0.01, 0.03, 0.06, and 0.09) phosphors excited at wavelengths of 340 nm and 460 nm, respectively. As shown in the emission spectra, they both show a broad peak around 534 nm and a sharp peak around 610 nm. The electron configuration of 4f¹ for Ce³⁺ can transfer to its excited state of and split into two spectroscopic terms and by spin coupling. The broad peak located at 534 nm in the yellow-green region can be attributed to the ,²F_5/2 transition. The appearance of 610 nm, which is caused by transition of the Pr³⁺ ion [9, 13], indicates that Pr³⁺ doping increased the red light emission for YAG:Ce_0.06. With Pr³⁺ doping concentration increasing, the intensity of yellow light emission at 534 nm decreased. Specifically, the intensity of the yellow light emission peak decreased from 632.48 () to 183.24 () under excitation at 340 nm (a decrease of 449.24); and the intensity decreased from 616.47 () to 246.21 () under excitation at 460 nm (a decrease of 370.26).

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Figure 4 

PL emission spectra of YAG:Ce_0.06 phosphors excited at 340 nm (a) and 460 nm (b).

For the red light from Pr³⁺ emission at 610 nm, the intensity firstly increased to the maximum at and then decreased monotonically. Insets of Figures 4(a) and 4(b) present the relation of the Pr³⁺ doping concentration and the PL emission intensity of 610 nm at excitation at 340 nm and 460 nm, respectively. Specifically, under excitation at 340 nm, the peak intensity increased gradually from 106.36 to 157.54 when increased from 0.006 to 0.01 and then decreased from 157.54 to 21.16 when further increased from 0.01 to 0.09. A similar trend for the spectra that are excited at 460 nm is shown. Firstly, with Pr³⁺ concentration increase (from 0.006 to 0.01), part Y³⁺ was substituted by Pr³⁺, and thus intensity of red light which caused by Pr³⁺ increased. However, further increase of Pr³⁺ concentration (from 0.01 to 0.09) decreased the distance between Pr³⁺ and Pr³⁺, which induced the self-quenching phenomenon, and thus decreased the light intensity.

Figure 5 shows the excitation and emission spectra of YAG:Ce_0.06, YAG:Pr_0.01, and YAG:Ce_0.06Pr_0.01, respectively. The sample with singly doped Ce³⁺ showed a strong yellow light emission at 528 nm arising from the 5d4f transitions of Ce³⁺ ions, and the excitation spectra included a blue absorption at 460 nm and two UV absorptions at 230 nm and 340 nm. For the singly doped Pr³⁺, at an excitation at 288 nm, the emission spectra can be divided into three groups. The group in the UV region consisted of two broad emission bands at 317 nm and 381 nm, which were attributed to the 4f5d ( = 4, 5, and 6) and 4f5d ( = 2, 3, and 4) transitions of Pr³⁺, respectively. The emission group in the range of 450 nm to 600 nm was attributed to the transition, and the main peak at 488 nm was due to transition. The other group was a red light emission at 610 nm, which was attributed to transitions. As shown in the excitation spectra of singly doped Pr³⁺, two UV absorptions were located at 239 nm and 289 nm but no absorption peaks at 340 nm. However, under excitation at 340 nm, the emission spectra of Ce³⁺ and Pr³⁺ codoped YAG:Ce_0.06Pr_0.01 exhibit not only the yellow emission caused by Ce³⁺ but also the red emission caused by Pr³⁺ (Figure 5(c)). Figures 5(d) and 5(e) show the emission spectra excited at 337 nm and 343 nm respectively, and they also show similar emission spectra to that excited at 340 nm. As the excitation at 340 nm cannot lead to the direct electronic excitation of Pr³⁺ ions, the appearance of 610 nm red line emission from Pr³⁺ indicates the energy transfer from the excited Ce³⁺ to its neighbouring Pr³⁺ ions. To further prove the energy transfer phenomenon, YAG:Ce_0.06Pr_0.01 was excited at 460 and 465 nm, and its emission spectra were shown in Figures 5(f) and 5(g). It can be seen that the red light emission at 610 nm also exists under excitation at 460 and 465 nm, which further confirmed the energy transfer from Ce³⁺ to Pr³⁺.

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Figure 5 

PL excitation and emission spectra of YAG:Ce_0.06 (a), YAG:Pr_0.01 (b), and YAG:Ce_0.06Pr_0.01 (c–g) (black line is excitation spectra, and red line is emission spectra. For Figure 6(b), the emission spectrum of YAG:Pr_0.01 was encircled with a red box).

Figure 6 

Energy level diagram and energy transfer of YAG:Ce³⁺Pr³⁺.

Figure 6 shows the energy level diagram and energy transfer behaviour of YAG:Ce³⁺Pr³⁺. It shows that the energy can be transferred from the 5d relaxed state of Ce³⁺ ion to energy level of the Pr³⁺ ion through a radiative transition process. And the energy can also be transferred from the lowest 5d band of Ce³⁺ to energy level of Pr³⁺ through a nonradiative process. The relaxed lowest 5d band of the Ce³⁺ ion is located at around 18,500 cm⁻¹, and therefore the energy transfer may occur radioactively at the D single state and the transition may lead to red light emission. If the energy transfers from the 5d energy band of Ce³⁺ to the energy band of Pr³⁺ through a nonradiative transition, a green light emission () and a red light emission () should appear simultaneously. However, the green light emission () did not appear which meant that the nonradiative transition from the lowest 5d band of Ce³⁺ to energy level of Pr³⁺ did not happen [9]. As shown in the above emission spectra, there was only a weak red emission (610 nm) due to the transition of Pr³⁺, indicating the existence of radiative energy transfer from the 5d relaxed state of Ce³⁺ to the energy level of Pr³⁺.

3.2.3. Fluorescence Lifetime

The fluorescence decay curves of the samples were measured, and they were exponentially fitted to obtain the fluorescence decay lifetime [14]. Figures 7(a) and 7(b) show the decay curves of yellow light emission under excitation at 340 and 460 nm, which were monitored at 534 nm. Figure 7(c) shows the decay curves of the red emission for the ³H₅ transition of Pr³⁺ under excitation at 340 nm, which were monitored at 610 nm. The fluorescence lifetimes of Ce³⁺ and Pr³⁺ are listed in Table 2. When doping Pr³⁺ into YAG:Ce³⁺, the lifetime of yellow light decreased, and it further decreased as Pr³⁺ concentrations increase. Specifically, at excitation at 340 nm, fluorescence lifetimes of the yellow light emission decreased from 34.6 ns to 7.5 ns; and at excitation at 460 nm the fluorescence lifetimes decreased from 35.6 ns to 7.3 ns. In YAG:Ce³⁺, only the energy transfer between two Ce³⁺ can happen, while in the codoped YAG:Ce³⁺Pr³⁺ there can be energy transfer between Pr³⁺ and Ce³⁺. It may be the reason for lifetime decrease of yellow light. In the case of energy transfer from Ce³⁺ to Pr³⁺, the decrease time from Ce³⁺ should be equal to the rise time from Pr³⁺. However, the rise time from Pr³⁺ was not found here; oppositely, the lifetime of red light also decreased with Pr³⁺ doping. We guess the self-quenching between Pr³⁺ and Pr³⁺ maybe caused the lifetime decrease of red light.

Table 2 

Fluorescence lifetime of yellow and red light in Al₅O₁₂:Pr_x.

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Figure 7 

(a, b) Fluorescence lifetime of yellow light (534 nm) excited at wavelengths 340 nm (a) and 460 nm (b). (c) Fluorescence lifetime of red light (610 nm) excited at wavelength 340 nm.

4. Conclusions

A series of YAG:Ce_0.06 phosphors with various Pr³⁺ concentrations ( = 0, 0.006, 0.01, 0.03, 0.06, and 0.09) were synthesised by coprecipitation method. Doping of Pr³⁺ did not change the phase of the YAG but induced a slight lattice parameters increase. Under excitation at either 340 nm or 460 nm, they both show a broad emission peak at 534 nm and a narrow emission peak at 610 nm. Appearance of the red emission peak at 610 nm was beneficial for improving the colour rendering of YAG:Ce³⁺. As Pr³⁺ concentration increased from 0.006 to 0.01 mol, the intensity of red light emission increased slightly, and then it decreased gradually when Pr³⁺ concentration further increased from 0.01 to 0.09. Emission properties of YAG:CePr at excitation at 340 and 460 nm proved the energy transfer from Ce³⁺ to Pr³⁺.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation (Grant no. 21276220), the National Key Technology Support Programme (Grant no. 2013BAC13B00), Yancheng Institute Of Technology Talent Programme (Grant no. KJC2013007), and Jiangsu Province Qing Lan Project and The Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (Grant no. AE201124).