Correlated color temperature tunability and energy ...

Functional Materials Letters Vol. 8, No. 6 (2015) 1550077 (5 pages) © World Scientific Publishing Company DOI: 10.1142/S1793604715500770

Correlated color temperature tunability and energy transfer phenomenon in the NaBaBO3:Dy 3+ /Eu 3+ phosphor for white light application Jianghui Zheng*, Qijin Cheng*, Cheng Zheng†, Guo Chen*, § Feng Shi† and Chao Chen*,‡, *School of Energy Research Xiamen University, Xiamen 361005, P. R. China †School

of Architecture and Civil Engineering Xiamen University, Xiamen 361005, P. R. China ‡

Funct. Mater. Lett. Downloaded from www.worldscientific.com by TONGJI UNIVERSITY on 08/11/15. For personal use only.

School of Physics and Mechanical & Electrical Engineering Xiamen University, Xiamen 361005, P. R. China § [email protected]

Received 17 April 2015; Accepted 25 June 2015; Published 12 August 2015

NaBa0:97x BO3 0.03Dy 3þ, xEu 3þ (0 • x • 0:09) single-phase white phosphors with tunable correlated color temperature (CCT) were synthesized using a conventional solid state reaction method. The phase structure and luminescence properties of the asprepared samples were investigated. The Dy 3þ, Eu 3þ doped and Dy 3þ /Eu 3þ co-doped NaBaBO3 phosphors excited by 361 nm show two blue and yellow emissions corresponding to the 4F9=2 ! 6H15=2 and 4F9=2 ! 6 H13=2 transitions of Dy 3þ ions and two red emissions due to the 5D0 ! 4 FJ (J ¼ 1; 2) transitions of Eu 3þ ions. Under 361-nm light excitation, the NaBa0:97x BO3 0.03 Dy 3þ, xEu 3þ (0 • x • 0:09) phosphors feature a white light emitting property. Through the Commission Internationale de L'Eclairage (CIE) chromaticity analysis and CCT calculation, the CIE chromaticity coordinates of the emission light are all located in the white region and can be tuned from bluish white light to reddish white light when the Eu 3þ concentration increases, and the CCT values of the obtained samples can vary from 5514.31 K to 8269.42 K. Furthermore, the energy transfer phenomenon from Dy 3þ ions to Eu 3þ ions in Dy 3þ /Eu 3þ co-doped samples was also investigated. The results indicated that, through tuning the Eu 3þ concentration of the NaBaBO3:Dy 3þ /Eu 3þ phosphors, the NaBaBO3-based phosphor can act as a potential single-phase white emitting phosphor for the application in the near-ultraviolet (NUV) white light emitting diodes. Keywords: NaBaBO3 phosphors; photoluminescence; borate; Dy 3þ /Eu 3þ co-doping.

In recent years, white light-emitting diodes (LEDs) have been treated as a next-generation green lighting sources due to their advantages such as energy saving, long life time, high luminescence efficiency and mercury free.1–3 Nowadays, there are two main ways to obtain white LEDs. The commercial method is based on the combination of the blue InGaN-based chip and the yellow-emitting YAG:Ce 3þ phosphor.4 However, the disadvantage of this method leads to the low color-rendering index and high correlated color temperature (CCT) due to the deficiency of red emission in the visible spectrum.5,6 The other way is to use nearultraviolet (NUV) chip with the tricolor (blue, green and red) phosphors. This method also suffers from the low luminous efficiency owing to the re-absorption for tricolor phosphors.7 In order to enhance the luminous efficiency, a better way, namely, to develop a single-phase white-emitting phosphor §Corresponding author.

which can be pumped by the NUV chip, has been proposed.8 Actually, the trivalent dysprosium (Dy 3þ ) ion has been treated as a typical white light ion and has attracted much attention due to its two dominant emission bands in the blue and yellow regions corresponding to the 4F9=2 ! 6 H15=2 and 4 F9=2 ! 6 H13=2 transitions, respectively.8 However, using a single doping Dy 3þ ion also suffers from the deficiency of red emission in the visible spectrum. In order to overcome this shortcoming, the Eu 3þ ion can be introduced to compensate the red emission deficiency due to the 5 D0 ! 4 FJ (J ¼ 0; 1; 2) transitions of Eu 3þ ions located in the red region. Recently, borate phosphors with general formula MNBO3 (M ¼ Li, Na, K and N ¼ Ca, Sr, Ba) doped with rare earth ions have attracted much attention due to their potential for the application in the light emitting diodes because of their low synthesis temperature and good chemical and physical stability.9–11 Tu et al. reported that the structure of NaBaBO3

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Fig. 1. XRD patterns of the NaBa0:97 BO3:0.03Eu 3þ and NaBa0:97x BO3:0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and 0.09) phosphors. The standard data card ICSD 80110 for NaBaBO3 is provided as a reference.

was monoclinic with a space group of C2/m for the first time in 1995.12 The latest research work about the Ce 3þ , Eu 3þ and Sm 3þ doped NaBaBO3 phosphors was reported.3,13,14 However, there are no reports on the luminescence properties of Dy 3þ /Eu 3þ co-doped NaBaBO3 materials. In this work, we studied the Dy 3þ /Eu 3þ co-doped NaBaBO3 phosphors which were prepared using a solid state reaction method. The structrual properties of NaBaBO3: Dy 3þ /Eu 3þ were studied by X-ray diffraction (XRD), and the Commission Internationale de L'Eclairage (CIE) value and the CCT tunability of NaBaBO3:Dy 3þ /Eu 3þ were investigated. Moreover, the energy transfer efficiency from Dy 3þ ions to Eu 3þ ions was also investigated. Our results indicate that the NaBaBO3:Dy 3þ /Eu 3þ phosphor exhibits a great

potential application as a single phase white-emitting phosphor for white NUV LEDs. In our experimental work, we have systematically investigated luminescence properties of the Dy 3þ doped NaBaBO3 phosphors and found that, at a Dy 3þ concentration of 0.03, the highest luminescence intensity could be observed. Hence, in this work, the concentration of Dy 3þ was fixed at 0.03 and the chemical formula for the NaBaBO3:Dy 3þ /Eu 3þ phosphors was set as NaBa0:97x BO3:0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and 0.09). The Dy 3þ /Eu 3þ co-doped NaBaBO3 phosphors were prepared by solid-state reactions in air. Besides, the NaBa0:97 BO3:0.03Eu 3þ phosphor was prepared to obtain the single Eu 3þ ion-doped NaBaBO3. BaCO3(AR), H3BO3(AR), Na2CO3(AR), Eu2O3(4N) and Dy2O3(3N) were used as starting materials. The stoichiometric materials were weighed and then ground together in an agate mortar. Thereafter, the mixture was taken into a corundum crucible and precalcined at 400 ○ C for 1 h, and subsequently further sintered at 850 ○ C for 3 h in air. Finally, the obtained powders were cooled down naturally to room temperature. X-ray diffractometer (XRD) (Panalytical X-Pert PRO) with CuKα (40.0 kV, 30.0 mA, λ ¼ 1:5418 Å) radiation was carried out to verify the crystalline phase of the samples. Photoluminescence excitation (PLE) and emission (PL) spectra were measured using the Hitachi F-7000 spectrofluorometer. XRD patterns of the NaBa0:97 BO3:0.03Eu 3þ and NaBa0:97x BO3:0.03Dy 3þ, xEu 3þ (x ¼ 0; 0.03, 0.05, 0.07 and 0.09) phosphors were presented in Fig. 1. It can be found that all of the diffraction peaks from the samples can be well indexed to the standard ICSD 80110 card, consistent with a NaBaBO3 phase of a monoclinic structure. There is no impurity peak detected in the experimental range, indicating that a pure crystalline compound was obtained.

(a)

(b)

Fig. 2. (a) PLE spectrum (λem ¼ 576 nm) and PL spectrum (λex ¼ 361 nm) of the NaBa0:97 BO3:0.03Dy 3þ phosphor. (b) PLE spectrum (λem ¼ 614 nm) and PL spectrum (λex ¼ 361 nm) of the NaBa0:97 BO3 :0.03Eu 3þ phosphor.

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The PLE spectrum (λem ¼ 576 nm) and PL spectrum (λex ¼ 361 nm) of the NaBa0:97 BO3:0.03Dy 3þ phosphor are shown in Fig. 2(a), whereas, Fig. 2(b) shows the PLE spectrum (λem ¼ 614 nm) and PL spectrum (λex ¼ 361 nm) of the NaBa0:97 BO3:0.03Eu 3þ phosphor. The excitation spectrum of NaBa0:97 BO3:0.03Dy 3þ consists of a group of sharp peaks around 300–400 nm located at 324 nm, 348 nm, 363 nm and 386 nm corresponding to the transitions from 6H15=2 to 4 K15=2 , 4M15=2 , 4P3=2 and 4M21=2 of Dy 3þ ions, respectively.15 The excitation spectrum of NaBa0:97 BO3:0.03Eu 3þ consists of a broad band from 250 nm to 300 nm and also several sharp peaks around 310–420 nm. The broad band is attributed to the transition between the charge transfer band (CTB) of O 2 ! Eu 3þ and the sharp peaks located at 321, 361, 382, 394 and 417 nm can be assigned to the transitions of 7 F0 ! 5 H3, 7F0 ! 7 D4, 7F0 ! 5 G4, 7F0 ! 5 L6 and 7F0 ! 5 D3 of Eu 3þ ions, respectively.16 From the PL spectra of NaBa0:97 BO3:0.03Dy 3þ and NaBa0:97 BO3:0.03Eu 3þ (λex ¼ 361 nm), it can be found that the Dy 3þ doped NaBaBO3 sample shows two emission bands with peaks located at blue (481 nm) and yellow (576 nm) regions corresponding to the 4F9=2 ! 6 H13=2 and 4F9=2 ! 6 H15=2 transitions of Dy 3þ ions, respectively.17 Whereas, the NaBaBO3:Eu 3þ sample features a red emission property with peaks located at 595 nm and 614 nm due to the 5D0 ! 7 F1 and 5D0 ! 7 F2 transitions of Eu 3þ , respectively.17 Figure 3 shows the PL spectra of NaBa0:97x BO3: 0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and 0.09) phosphors, which were excited by 361 nm NUV light. One can notice that, when the concentration of the Eu 3þ ions increases from 0 to 0.07, the emission intensities with peaks located at 595 nm and 614 nm (corresponding to the 5 D0 ! 7 F1 and 5D0 ! 7 F1 transitions of Eu 3þ ions, respectively) increase while the emission intensities with peaks

Fig. 3. PL spectra of NaBa0:97x BO3:0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and 0.09) (λex ¼ 361 nm).

located at 481 nm and 576 nm (corresponding to the 4 F9=2 ! 6 H13=2 and 4F9=2 ! 6 H15=2 transitions of Dy 3þ ions, respectively) decrease. This phenomenon can be explained by the energy transfer from Dy 3þ ions to Eu 3þ ions, which can be also confirmed by the spectral overlap between the PL spectrum of the NaBa0:97 BO3:0.03Dy 3þ phosphor and the PLE spectrum of the NaBa0:97 BO3:0.03Eu 3þ phosphor shown in the gray area of Fig. 2(b). However, the intensities of 5D0 ! 7 F1 and 5D0 ! 7 F2 transitions decrease with a further increase of the concentration of the Eu 3þ ions to 0.09. This may be caused by the concentration quenching of the Eu 3þ ions.18 In addition, since the defects have an influence in luminescence property as reported by Jakes et al. in the case of Eu 2þ doped CsBr system,19,20 the decreased luminescent intensities of the 5D0! 7 F1 and 5D0! 7 F2 transitions of Eu 3þ ions may be also affected by the defects in the NaBaBO3 host when Eu 3þ concentration increases to 0.09. This can be illustrated by the following formula: NaBaBO3

00

Eu2 O3 ! 2Eu Ba þ 3O O þ V Ba :

(1)

The energy level diagrams of Dy 3þ and Eu 3þ ions and their possible energy transfer processes in the NaBaBO3: Dy 3þ,Eu 3þ phosphors are shown in Fig. 4. Under the NUV light excitation, Dy 3þ ions can be excited from the ground level 3H15=2 to the excited level 4P3=2. Then Dy 3þ ions can make non-radiative (NR) transition from 4P3=2 down to 4F9=2, and thereafter the transition is radiative and emits blue light centered at 481 nm and yellow light centered at 575 nm corresponding to 4F9=2 ! 6 H13=2 and 4F9=2 ! 6 H15=2 transitions of Dy 3þ ions, respectively. At the same time, the energy transfer from Dy 3þ ions to Eu 3þ ions occurs, and thus the emitting intensities decrease with the increase of the concentration of Eu 3þ ions.

Fig. 4. Energy level diagrams of Dy 3þ and Eu 3þ ions and their possible energy transfer processes in NaBaBO3:Dy 3þ, Eu 3þ phosphors (λex ¼ 361 nm).

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To further investigate the energy transfer efficiency (ηT ) from Dy 3þ ions to Eu 3þ ions, the ηT can be calculated by the following equation21:


ηT ¼ 1

I ; Is

(2)

where I and Is are the PL emission intensities of Dy 3þ ions in the absence and presence of Eu 3þ ions, respectively. By using Eq. (2), the calculated results of ηT as a function of Eu 3þ ion concentration are shown in Fig. 5. The ηT gradually increases with the increase of Eu 3þ ion concentration and the maximum value reaches 41.4% when x is 0.09. This result demonstrates that the energy transfer from Dy 3þ ions to Eu 3þ ions is efficient. Since the CIE color coordinate value is one of the important factors for evaluating the performance of phosphors, the CIE color coordinates for these samples were calculated though the use of the PL emission spectra data excited by 361 nm, by using the CIE 1931 color matching functions.5,22,23 The CIE chromaticity coordinate diagram of the NaBa0:97x BO3:0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and 0.09) phosphors is shown in Fig. 6. As shown in Fig. 6, the chromaticity coordinates are all located in the white region and can be tuned from bluish white light to reddish white light when the Eu 3þ concentration increases. Moreover, the CCT was also calculated to characterize the white light property. The CCT can be calculated using the McCamy empirical equation24: CCT ¼ 437n 3 þ 3601n 2 6861n þ 5514:31;

Fig. 6. CIE chromaticity coordinate diagram of the NaBa0:97x BO3: 0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and 0.09) phosphors (λex ¼ 361 nm).

Table 1. CIE chromaticity coordinate and CCT values of NaBa0:97x BO3:0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and 0.09) phosphors. NaBa0:97x BO3 : 0.03Dy 3þ, xEu 3þ x¼0 x ¼ 0:03 x ¼ 0:05 x ¼ 0:07 x ¼ 0:09

(3)

where n stands for (x xe )/(y ye ). Here, xe is equal to 0.3320 and ye is equal to 0.1858. Table 1 summarizes the calculated CIE color coordinates and CCT values of the NaBa0:97x BO3:0.03Dy 3þ, xEu 3þ (x ¼ 0, 0.03, 0.05, 0.07 and

Fig. 5. Energy transfer efficiency (ηT ) as a function of Eu 3þ ion concentration.

Chormaticity coordinate (x; y) (0.295, (0.313, (0.329, (0.332, (0.324,

0.295) 0.302) 0.313) 0.315) 0.309)

CCT (K) 8269.42 6734.34 5876.14 5514.31 6034.91

0.09) phosphors. The results demonstrate that it is possible to tailor the white emission color and the value of CCT by changing the Eu 3þ ion concentration in the Dy 3þ /Eu 3þ codoping system. The CCT values of the obtained samples can vary from 5514.31 K to 8269.42 K. In general, the white light is suitable for the cold white commercial application when the CCT value of white light is greater than 5000 K.25,26 Note that, as the CCT value of standard daylight at noon is 6500 K (D65),22,26 a suitable white light with the color coordinates of (0.313, 0.302) and the CCT of 6734.34 K was obtained in our samples when the Eu 3þ concentration is 0.09. In this paper, Dy 3þ /Eu 3þ co-doped NaBaBO3 white emitting phosphors with tunable CCT were synthesized using a conventional solid state reaction method. Their luminescence properties were investigated. Under 361 nm light excitation, the NaBa0:97x BO3 0.03Dy 3þ, xEu 3þ (0 • x • 0:09) phosphors feature a white light emitting property. The CIE

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chromaticity coordinate analysis shows that the emission peaks are all located in the white region and can be tuned from bluish white light to reddish white light when the Eu 3þ concentration increases. The CCT calculation results indicated that the CCT can vary from 5514.31 K to 8269.42 K for the obtained samples. The energy transfer phenomenon in the NaBaBO3:Dy 3þ /Eu 3þ phosphors was investigated and it is found that the energy transfer from Dy 3þ ions to Eu 3þ ions is efficient. The results indicate that the NaBaBO3:Dy 3þ /Eu 3þ phosphor is a potential candidate as a cold white light phosphor for the NUV white LEDs.


Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 61076056), the Fundamental Research Funds for the Central Universities (Grant Nos. 2013121031 and 2013SH004), Fujian Provincial Department of Science & Technology (Grant No. 2015H0036), Program for New Century Excellent Talents in Fujian Province University (NCETFJ), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, China.

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