Luminescent characteristics of LiBaBO3:Tb3+ green phosphor for white LED

Luminescent characteristics of LiBaBO3:Tb3+ green phosphor for white LED

Journal of Alloys and Compounds 478 (2009) 813–815 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 478 (2009) 813–815

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Luminescent characteristics of LiBaBO3 :Tb3+ green phosphor for white LED Panlai Li a,∗ , Libin Pang b , Zhijun Wang a , Zhiping Yang a , Qinglin Guo a , Xu Li a a b

College of Physics Science & Technology, Hebei University, Baoding 071002, China Foreign Language Teaching & Researching Department, Hebei University, Baoding 071002, China

a r t i c l e

i n f o

Article history: Received 18 September 2008 Received in revised form 27 November 2008 Accepted 14 December 2008 Available online 24 December 2008

a b s t r a c t A novel green phosphor, LiBaBO3 :Tb3+ , has been developed for white light-emitting diodes (LEDs). The phosphor was prepared by using solid state reaction and its luminescent characteristics were investigated. The excitation and emission spectra indicate that this phosphor can be effectively excited by ultraviolet (UV) 368 and 381 nm light, and exhibit a satisfactory green performance (544 nm), nicely, fitting in with the widely applied UV LED chip. © 2009 Elsevier B.V. All rights reserved.

Keywords: Phosphor Luminescence

Within the last several years, much progress has been made in the art of high-brightness LEDs with various colors. Nowadays, a white LED device has been commendably realized using YAG:Ce as a broad band yellow phosphor coated on the blue LED chip. However, there exist at least two drawbacks in this combination. Firstly, the overall efficiency decreases rapidly when lowering the correlated color temperature of the device. Secondly, a concern with this device is that the “white” output light has an undesirable color balance for a true color rendition, viz., the output light is deficient in the red region of the visible light spectrum. In order to solve this problem, two ways were introduced. One hand, a separate red light source may have to be used to compensate LED for the red deficiency of the output light [1–3]. On the other hand, for UV (350–410 nm) LED chip coated by red, green and blue light-emitting phosphors, a promising white light generation way. Because the eyes are not sensitive to the range of 350–410 nm, therefore, the color of this mechanism depends completely on phosphor. The kinds of phosphors and the mixture ratio can be varied to adjust the chromaticity of the illuminating source according to different needs. However, there are few reports about these phosphors excited by UV chips at the present time [4–6]. Alkaline earth borate is an important luminescent material because of its excellent chemistry and thermal stabilization, facile synthesis and cheap raw material (H3 BO3 ), so it has been extensively applied to phosphor for lamps. Since it can be efficiently excited by LED chips, there have been a few reports recently about this material applied in phosphor for white LEDs [7,8]. In the

present work, we have synthesized LiBaBO3 :Tb3+ green phosphor and investigated its luminescent characteristics. The results can help the development of white LED. 1. Experimental 1.1. Sample synthesis The starting materials BaCO3 , H3 BO3 , Li2 CO3 , Na2 CO3 , K2 CO3 and Tb4 O7 (99.99% in mass) in appropriate stoichiometric ratio were mixed in the alumina crucible, then the mixed powders were calcined at 700 ◦ C for 2 h, and the LiBaBO3 :Tb3+ phosphors were obtained. 1.2. Physical measurements The structure was investigated by powder X-ray diffraction (XRD, D/max-rA, CuK␣ , 40 kV, 100 mA). The emission and excitation spectra were measured by a Shimadzu RF–540 ultraviolet spectrophotometer. The CIE was measured by PR1980B luminance apparatus. All the photoluminescence properties of these phosphors were measured at room temperature.

2. Results and discussion 2.1. X-ray diffraction analysis Fig. 1 shows the XRD pattern of LiBaBO3 :Tb3+ phosphor with 1 mol% Tb3+ , and the datum agree well with JCPDS No. 81-1808. Therefore, the host structure was not influenced by doping Tb3+ . LiBaBO3 has a monoclinic structure with P21 /n space group, and its lattice parameter is a = 0.6461 nm, b = 0.7107 nm, c = 0.7403 nm. 2.2. Luminescent characteristics

∗ Corresponding author. Tel.: +86 312 5079423; fax: +86 312 5079423. E-mail address: lipanlai [email protected] (P. Li). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.028

Fig. 2 shows the emission and excitation spectra of LiBaBO3 :Tb3+ phosphor with 1 mol% Tb3+ . Under 365 nm excitation, the emis-

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P. Li et al. / Journal of Alloys and Compounds 478 (2009) 813–815

Fig. 3. Emission intensity of LiBaBO3 :Tb3+ phosphor as function of Tb3+ concentration. Fig. 1. XRD pattern of LiBaBO3 :Tb3+ phosphor.

Effects of Tb3+ concentration on the emission spectra of LiBaBO3 :Tb3+ phosphors are also investigated. The emission spectra of the phosphors prepared at various concentrations of Tb3+

are given in Fig. 3. The emission spectra vary with the increase of Tb3+ concentration from 1 to 6 mol%. The results illuminate that the emission spectrum distribution does not vary with altering Tb3+ concentration, however, the emission intensity was obviously influenced by Tb3+ concentrations. At low Tb3+ concentrations (x < 3 mol%), because the luminescence center is not sufficient, the emission intensity is weak. However, under the condition of increasing Tb3+ concentrations, the emission intensity increases, and reaches the maximal value at 3 mol% Tb3+ , then decreases. Viz., the concentration quenching occurs when the Tb3+ concentration is beyond 3 mol%. The reason for the concentration quenching is that if the Tb3+ concentration continues to increase, the interaction of Tb3+ –Tb3+ also increases, consequently, the emission intensity becomes lower. Dexter and Schulman [9] proposed that the interaction type between sensitizers or sensitizer and activator can be determined by 1g(I/x) = c − (/3) 1g x when the concentration is high enough. Among the concentration quenching caused by the electric multipole interaction, the dipole–dipole (d–d), dipole–quadripole (d–q) and quadripole–quadripole (q–q) correspond to  = 6, 8, 10, respectively. The emission intensity of LiBaBO3 :Tb3+ phosphor is measured under the condition that Tb3+ concentration is from 3 to 6 mol%, and the concentration dependence curves (lg(I/x) ∼ lg x) are shown in Fig. 4. From the slope of the linear,  can be obtained, and  = 5.66 ≈ 6. The result indicates that the concentration self-quenching mechanisms of Tb3+ in LiBaBO3 are both the d–d interaction.

Fig. 2. Emission and excitation spectra of LiBaBO3 :Tb3+ phosphor.

Fig. 4. Relation between the lg(I/x) and lg x of Tb3+ .

sion spectrum exhibits four major emission bands at 488, 544, 593 and 616 nm, which are attributed to the 5 D4 → 7 F6 , 5 D4 → 7 F5 , 5 D → 7 F , and 5 D → 7 F typical transitions of Tb3+ ions, respec4 4 4 3 tively. The strongest one appears at 544 nm. The four typical emission peaks are split in different ways. The energy level transition 5 D4 → 7 F6 is split into 488 and 493 nm emission peaks; 5 D → 7 F is split into 544 and 549 nm emission peaks; 5 D → 7 F is 5 4 4 4 split into 583, 590, 593 and 596 nm emission peaks; and 5 D4 → 7 F3 is split into 616, 620 and 623 nm emission peaks. These splits were resulted in the crystal field effects, and their extents are related to the structure characteristic of LiBaBO3 crystal field. The excitation spectrum for 544 nm emission consists of several band emissions including 242–277 nm and 368–381 nm. The two excitation bands correspond to the weak 4f7 5d1 absorbability and the strong 4f → 4f absorbability, respectively. The excitation and emission spectra indicate that this phosphor can be effectively excited by UVLED, and emit green light. And the CIE chromaticity is (x = 0.249, y = 0.578). Therefore, LiBaBO3 :Tb3+ phosphor is a promising phosphor for white LED. 2.3. Effects of Tb3+ concentration on the emission spectra of LiBaBO3 :Tb3+ phosphor

P. Li et al. / Journal of Alloys and Compounds 478 (2009) 813–815

Fig. 5. Emission spectrum of LiBaBO3 :Tb3+ phosphor as function of Li+ concentration.

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Na+ or K+ was introduced, the evolvement trend is sameness as Li+ with different charge compensation. However, the charge compensation concentration corresponding to the maximum emission intensity is different with different charge, and the concentration is 4 and 3 mol% corresponding to Na+ and K+ , respectively. We compared with the maximum emission intensity with doping Li+ , Na+ and K+ . Fig. 6 exhibits the result. And the results show that the maximal emission intensity of doping Li+ is higher than that of Na+ or K+ , the result is well agreement with Ref. [10]. The results can be explained by the following reasons: when the charge compensation is incorporated into the host lattice, the aberration was brought in the crystal lattice, which induce the probability of transition emission and enhance the emission spectrum intensity of LiBaBO3 :Tb3+ phosphor. However, the emission spectrum intensity of LiBaBO3 :Tb3+ does not increase all along with the increasing charge compensation concentration. This means that only portion charge compensation is incorporated into a host lattice, when the doping concentration is higher than the Tb3+ concentration, the excrescent part will substitute for the Ba2+ site, and the excrescent negative charge will engender, which makes the emission spectrum intensity decrease [11]. The difference of charge radii can explain that the charge compensation concentration corresponding to the maximum emission intensity is different with different charges. The radius of Ba2+ in the host lattice is 0.118 nm, and the radii of Li+ , Na+ and K+ is 0.059, 0.116 and 0.133 nm, respectively. Comparing with K+ , Li+ and Na+ are easy incorporated into the host lattice, so the doping concentration is higher than K+ , and the doping concentration is 4 and 4 mol%, respectively. 3. Conclusions

Fig. 6. Effect of Li+ , Na+ and K+ on emission spectrum of LiBaBO3 :Tb3+ phosphor.

2.4. Effect of Li+ , Na+ and K+ on the emission spectrum of LiBaBO3 :Tb3+ phosphor Tb3+ ,

is incorporated into When a trivalent metallic ion, such as a host lattice and substitutes for a divalent metallic ion, the charge balancing is necessarily required. For LiBaBO3 :Tb3+ , the incorporation of alkali metal ions can neutralize the charge generated by Tb3+ substitution for Ba2+ , and thus stabilize the structure and enhance the luminescence. In order to keep the charge balance, we regard Li+ (Li2 CO3 ), Na+ (Na2 CO3 ) and K+ (K2 CO3 ) as charge compensations. Li+ , Na+ and K+ concentrations are all from 1 to 6 mol%, and the Tb3+ concentration is 3 mol% in this research. Under the condition of doping Li+ , the influence of Li+ concentrations on the emission spectrum of LiBaBO3 :Tb3+ phosphor was studied, and the result is shown in Fig. 5. The results show that the emission intensity increases with increasing Li+ concentration, then decreases, and reaches the maximum value at 4 mol% Li+ . Under the condition that

In conclusions, the excitation and emission spectra of LiBaBO3 :Tb3+ phosphor indicate that it can be effectively excited by UV LED, and emit 544 nm green light. And the CIE chromaticity is x = 0.249, y = 0.578. Therefore, LiBaBO3 :Tb3+ phosphor is a promising phosphor for white LED. Acknowledgement This work was financial supported by Hebei Developing Foundation of Science & Technology (5121510b). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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