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Rare Earth-Doped Phosphors for White LEDs Chapter 278 75 1ex. emA 0.5 0 1 0.5 0 1 0.5 0 B C 300 400 500 600 Wavelength (nm) FIG. 73 transfer in Section 3.1.3) and (ii) reabsorption by Tb3+ 4f–4f transition of Ce3+ emission due to the overlap between the emission spectrum of Ce3+ (the sensitizer) and the excitation spectrum of Tb3+ (Sensitization of lumines- cence in Section 3.1.3). As shown in Fig. 73A and B, this overlap is sizeable. Under 395 nm excitation of Ce3+, the emission spectrum of Y2Si3O3N4:Ce3+, Tb3+ exhibits a broad emission centered at 480 nm caused by Ce3+ and a narrow green emission at 543 nm from Tb3+ (Fig. 73C). With increasing Tb3+ concentration, the relative intensity of blue emission due to Ce3+ decreases, whereas the relative intensity of green emission due to Tb3+ increases remark- ably as shown in Fig. 74. The calculated energy transfer efficiency from Ce3+ to Tb3+ under a near-UV excitation also increases with increasing Tb3+ concentra- tion. The highest emission intensity is obtained for (Y0.78Ce0.02Tb0.20)2Si3O3N4, with an energy transfer efficiency of $61%. The CIE chromaticity coordi- nates of Y2Si3O3N4:Ce3+,Tb3+ could be tuned from blue (0.18,0.30) to green (0.34,0.53) when the Tb3+ concentration goes up to 20% (y1⁄40.20). 4.5.2 Ba3RNa(PO4)3F:Eu2+,Tb3+ (R1⁄4Rare Earth) Phosphors Ba3RNa(PO4)3F:Eu2+,Tb3+, which can be described by the general formula (Ba3xEux)3R1yTbyNa(PO4)3F, are examples of color-tunable phosphors for white LEDs based on energy transfer from Eu2+ to Tb3+ (Jiao et al., 2013; Excitation and emission spectra of (A) (Y0.96Tb0.04)2Si3O3N4, (B) (Y0.98Ce0.02)2Si3O3N4, and (C) (Y0.94Ce0.02Tb0.04)2Si3O3N4. Redrawn from Zhu, J., Qin, S., Xia, Z., Liu, Q., 2015. Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3+,Tb3+ phosphors. Ceram. Int. 41, 12633–12637. Normalize intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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