Journal of Nanomaterials
Volume 2015 (2015), Article ID 615863, 5 pages
http://dx.doi.org/10.1155/2015/615863

Research Article

The Influence of Codopant Aluminum Ions (Al³⁺) on the Optical Characteristics of YBO₃:Sm³⁺ Phosphors

Hao-Ying Lu and Yi-Shao Chen

Department of Electronic Engineering, National Quemoy University, Kinmen 89250, Taiwan

Received 29 September 2014; Accepted 24 November 2014

Academic Editor: Silvia Licoccia

Copyright © 2015 Hao-Ying Lu and Yi-Shao Chen. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The yttrium borate (YBO₃) phosphors with codopants Al³⁺ and Sm³⁺ ions were prepared via the chemical coprecipitation method with one-hour thermal treatment at 1200°C. From the XRD patterns, the codopant Al³⁺ does not change the crystal structures of YBO₃:Sm³⁺ and these patterns indicate that the phosphors crystallize as the hexagonal structure. Besides, the codopant Al³⁺ does not affect the wavelengths of emission bands but enhances the PL intensities of emission bands. Under the wavelength 406 nm excitation source, the emission peaks locating at wavelengths 571 nm, 611 nm, and 657 nm are assigned to the electronic transitions ⁴G5/2→⁶H5/2, ⁴G5/2→⁶H7/2, and ⁴G5/2→⁶H9/2, respectively. The PL intensities of phosphors Sm0.01AlxY0.99−xBO₃ increase with the Al³⁺ ion concentration. As the concentration of Al³⁺ ions increases to 3%, the PL intensity of Sm0.01AlxY0.99−xBO₃ phosphor reaches its maximum intensity. When the concentration of Al³⁺ ions is above 3%, the PL intensity of phosphor Sm0.01AlxY0.99−xBO₃ decreases. Comparing with the Sm0.01Y0.99BO₃ phosphor, the PL intensity locating at wavelength 571 nm of Sm0.01Al0.03Y0.96BO₃ phosphor is about 1.8 times stronger than the Sm0.01Y0.99BO₃ phosphor. It is believed that the codopant Al³⁺ can improve the luminescent characteristics of YBO₃:Sm³⁺ phosphors.

1. Introduction

In the past, plasma display panels (PDPs) were applied to full-color large-area flat panel displays because of their high performance and scalability [1, 2]. Phosphors play an important role in PDPs and they are usually stimulated by vacuum ultraviolet (VUV) light (145–180 nm) generated by the discharge of Xe or Ne gas [3]. In recent years, the phosphors have attracted more attentions due to their extensive applications in light emitting diodes (LEDs). Compared with the PDPs, the LEDs possess several advantages, such as the longer lifetime, lower energy consumption, higher efficiency, and higher brightness [4–6]. The advantages above make LEDs be important lighting devices in recent years. Owing to the excellent power-saving property (compared with the traditional lighting devices, such as the incandescent bulbs and fluorescent lamps), people put much effort to fabricate the white-light emitting diodes (WLEDs). Combining the emissions of red-light, green-light, and blue-light phosphors is a method to fabricate the WLEDs [7]. However, it is very difficult to control the color purity accurately with three phosphors. Combination of a blue-light LED chip and a yellow-light phosphor is most widely used in fabricating WLEDs [8] and this method reveals the importance of yellow-light phosphors.

In the present studies, oxides, including aluminates, borates, and silicates, could be used as the host materials due to the strong absorption of the VUV light [9]. The strong absorption can enhance the PL intensity effectively. In our previous work, the YBO₃:Sm³⁺ phosphors were synthesized to replace the YAG phosphors to fabricate the WLEDs and in fact they can emit orange-yellow light. But the emitting intensities of YBO₃:Sm³⁺ phosphors are too weak to be used in the WLEDs. In order to enhance the emitting intensity, the sensitizers are usually used to codope with the host materials [10]. In Kwon et al.’s research, the addition of sensitizer is an effective method to improve the optical properties of phosphor [11, 12]. In this paper, the Al³⁺ is chosen as the sensitizer to codope with the YBO₃:Sm³⁺ phosphors to form the phosphors ( starts from 0 to 0.05). As the concentration of Al³⁺ increases to 3% (), the PL intensity is about 1.8 times stronger than the phosphors.

2. Materials and Methods

The YBO₃:Sm³⁺ and YBO₃:Sm³⁺, Al³⁺ phosphors were synthesized via the chemical coprecipitation method with one-hour thermal treatment at 1200°C. Samarium nitrate hexahydrate (99.9%, Alfa Aesar), yttrium nitrate hexahydrate (99.9%, Alfa Aesar), aluminum nitrate nonahydrate (>98%, Panreac), and boric acid H₃BO₃ (99.8%, Panreac) were used as the starting materials to prepare the precursor solutions. , , , and H₃BO₃ were dissolved into distilled water separately according to the stoichiometric ratio. Then, the solutions above were mixed together and stirred for an hour at room temperature. After stirring, 1% ammonia solution was used as the precipitant and added to the solution mentioned above. With the addition of ammonia, the pH value of mixed solution was adjusted to 9.0, and this solution was placed for an hour. After an hour, the white precipitation could be observed and separated directly by a centrifuge. This white precipitation was washed repeatedly with distilled water and ethanol to remove the impurities and then dried at 70°C for 12 hours. Finally, the dried precipitation was grinded for 15 minutes and used as the precursor to synthesize the and phosphors via the suitable thermal treatment in the air atmosphere for an hour.

The crystal structures of phosphors were characterized by Rigaku Miniflex II desktop X-ray diffractometer with CuKα radiation (: 10° to 40°, step: 0.01°). The size of the particle and morphology of prepared phosphors were investigated by using a field-emission scanning electron microscope (FE-SEM, Hitachi S4100) with 15 KV accelerating voltage. And the PL spectra and the PLE spectra were examined with a fluorescence spectrophotometer (Hitachi F-2700). All the measurements were carried out at room temperature.

3. Results and Discussion

The XRD patterns of the ( starts from 0 to 0.05) phosphors annealed at 1200°C are shown in Figure 1. Comparing with the JCPDS card, all peaks are consistent with the number 16-0277 pattern and they exhibit a pure hexagonal phase with vaterite-type structure. These patterns prove that the codopant Al³⁺ does not cause the transformation of the crystal structure. According to the Scherrer formula, the average size of the particle in diameter can be calculated by the FWHM of (1 0 0) peak. Here, the Scherrer formula is shown as follows:

where is the average particle size, is the wavelength of X-ray radiation (1.54Å), is the full width at half maximum (FWHM), and is the diffraction peak angle. The average particle sizes calculated by the Scherrer formula are shown in Table 1. Referring to Figure 2, the calculated results are similar to the exact sizes of particle.

Table 1: The average particle sizes of phosphors with different concentrations.

The average particle size of phosphors

concentration
0%
0.5%
1%
2%
3%
5%

Particle size (nm)
96
107
112
106
99
102

Figure 1: The XRD patterns of the phosphors () annealed at 1200°C for 1 hour.

(a)

(b)

Figure 2: The FE-SEM images of (a) and (b) phosphors annealed at 1200°C for 1 hour.

Figure 2 shows the images of surface morphology of the and phosphors. From Figures 2(a) and 2(b), the codopant Al³⁺ does not really influence the particle sizes and shapes. The FE-SEM images show that the prepared phosphors are spherical and the sizes of the particle are between 100 and 200 nm. These sizes of particle are consistent with the XRD results. In addition, the spherical morphology of phosphors synthesized by the method in this paper has several advantages, including the high packing densities, good slurry properties, and smoother light intensity distributions [13].

In our previous work, the YBO₃ phosphor with 1% Sm³⁺ possesses the strongest PL intensity. In order to enhance the PL intensity of YBO₃:Sm³⁺ phosphors and prevent the concentration-quenching effect of activator, the sensitizer Al³⁺ is used to codope with the YBO₃:Sm³⁺ phosphors. The emission spectra of the () phosphors with the excitation wavelength 406 nm are shown in Figure 3. The emission peaks locating at wavelengths 571, 611, and 657 nm appear because of the following transitions ⁴G5/2 → ⁶H5/2, ⁴G5/2 → ⁶H7/2, and ⁴G5/2 → ⁶H9/2, respectively [14]. The peak locating at 657 nm is too weak to dominate the color coordinates and these phosphors can yield the orange-yellow light. Besides, from these emission spectra, the codopant Al³⁺ does not actually affect the emission wavelength but it really influences the intensity of PL emission. From Figure 4, the emission intensity of phosphor increases with the Al³⁺ concentration. As the Al³⁺ concentration increases to 3%, the phosphor possesses the strongest PL intensity. The addition of sensitizers Al³⁺ leads to a significant increment of the PL emission intensity because the sensitizer Al³⁺ ions absorb the excitation energy and transfer the energy to the Sm³⁺ ions through the host lattice [15]. Moreover, the excitation energy absorbed by the Y³⁺ ions and groups may be transferred nonradiatively to the Al³⁺ ions and then to the Sm³⁺ ions. Another research team believes that the nonradiative transfer mechanism, the resonance between absorber and emitter, dominates the enhancement as well [16]. When the concentration of Al³⁺ is above 3%, the PL intensity decreases. This phenomenon reveals that the energy transfer not only occurs between the sensitizers and activators but also occurs within the sensitizers. As the concentration of Al³⁺ ions increases over 3%, the distance between Al³⁺ ions would be small enough, and the excitation energy absorbed by the Al³⁺ ions tends to transfer within the Al³⁺ ions rather than transfer between Al³⁺ and Sm³⁺ [17]. Except the radiative relaxation, the energy of the activator Sm³⁺ can release through a nonradiation transition instead of orange emission [15]. Figure 5 shows the mechanism mentioned above. The probability of path A increases with the concentration of Al³⁺ ions and dominates the mechanism of energy transfer till the concentration of Al³⁺ increases to 3%. When the concentration of Al³⁺ ions is above 3%, the path B dominates the energy transfer and results in the decrease of PL intensity.

Figure 3: The emission spectra of the () phosphors annealed at 1200°C for 1 hour.

Figure 4: The peak intensities of phosphors with different Al³⁺ concentrations.

Figure 5: The mechanism of energy transfer in the YBO₃:Sm³⁺, Al³⁺ phosphor. A means activator and S means sensitizer.

The excitation spectra of () phosphors are shown in Figures 6 and 7. Figure 6 shows the PLE spectra of phosphor with different emission wavelengths. The wavelengths of all absorption peaks are the same, and the intensity ratios of absorption peaks are similar as well. Ten absorption peaks can be observed in Figure 7 and these ten peaks overlap with each other to form four absorption bands. The 406 nm absorption peak locating at the third absorption band is the strongest peak of all and is chosen as the excitation wavelength for the PL measurements. This strongest absorption is the result of the following transition ⁶H5/2→⁴ K11/2 [18]. From the PLE spectra, the YBO₃:Sm³⁺ phosphor with 3% Al³⁺ can absorb the more energy and this phenomenon is consistent with the PL spectra.

Figure 6: The excitation spectra of phosphor with different emission wavelengths.

Figure 7: The excitation spectra of () phosphors annealed at 1200°C for 1 hour.

Figure 8 shows the Commission Internationale de L’Eclairage (CIE) chromaticity diagram of the and phosphors. The color coordinates of YBO₃:Sm³⁺ phosphor and YBO₃:Sm³⁺, Al³⁺ phosphor are (0.542, 0.457) and (0.531, 0.467), respectively. From the CIE diagram, the emission colors of these two phosphors locate at the orange-yellow light area. With the addition of Al³⁺ ions, the color coordinate shifts toward the yellow-light area. Referring to the PL emission spectra, the sensitizer Al³⁺ enhances the emission intensity of 571 nm peak mostly and changes the intensity ratio of 571 nm peak and 611 nm peak. This mechanism leads to the change of chromaticity.

Figure 8: The CIE chromaticity diagram of (a) and (b) phosphors annealed at 1200°C for 1 hour.

4. Conclusion

The nanosized YBO₃:Sm³⁺, Al³⁺ phosphors can be obtained via the chemical coprecipitation method. With the thermal treatment at 1200°C for 1 hour, the average size of particle is between 100 and 200 nm and it is consistent with the result calculated by the Scherrer formula. The synthesized phosphors exhibit the spherical morphology and can emit the orange-yellow light. From the XRD patterns, the addition of sensitizer Al³⁺ does not affect the crystal structure and all the phosphors crystallize as the hexagonal phase with vaterite-type structure. With the excitation wavelength 406 nm, the phosphors can emit three emission bands. The emission peaks locating at wavelengths 571, 611, and 657 nm are the results of the following transitions ⁴G5/2 → ⁶H5/2, ⁴G5/2 → ⁶H7/2, and ⁴G5/2 → ⁶H9/2, respectively. From the PL spectra, the emission intensity of phosphor increases with the Al³⁺ concentration. As the Al³⁺ concentration increases to 3%, the phosphor possesses the strongest PL intensity. The addition of sensitizer Al³⁺ does not influence the emission wavelengths but enhances the emission intensity of 571 nm peak. Comparing with the phosphor, the PL intensity locating at wavelength 571 nm of phosphor is about 1.8 times stronger than the Al³⁺-free phosphor. This phenomenon leads to the change of intensity ratio and shifts the CIE coordinate to the yellow-light area. This result indicates that the codopant Al³⁺ can improve the optical characteristics of YBO₃:Sm³⁺ phosphors effectively.

Conflict of Interests

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

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