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Rare Earth-Doped Phosphors for White LEDs Chapter 1.2 278 17 R noncodoped R = Gd3+ R = La3+ R = Tb3+ 1 0.8 0.6 0.4 0.2 0 500 600 700 800 Wavelength (nm) Emission spectra of R codoped YAG:Ce3+ phosphors (R1⁄4La, Gd, and Tb). Redrawn from Shao, Q., Li, H., Dong, Y., Jiang, J., Liang, C., He, J., 2010. Temperature-dependent photoluminescence studies on Y2.93xLnxAl5O12:Ce0.07 (Ln1⁄4Gd, La) phosphors for white LEDs application. J. Alloy Compd. 498, 199–202; Jang, H.S., Im, W.B., Lee, D.C., Jeon, D.Y., Kim, S.S., 2007. Enhancement of red spectral emission intensity of Y3Al5O12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs. J. Lumin. 126, 371–377. to a longer wavelength (Fig. 8), and nowadays, Gd3+-doped YAG:Ce3+ is commonly used in white LEDs. On the other hand, the partial substitution of Al3+ sites with larger Ga3+ ions results in a shift of the emission band to a shorter wavelength (blue shift; Kanke and Navrotsky, 1998; Pavese et al., 1995). In addition, the emission band also shifts to a shorter wavelength when smaller Lu3+ ions substitute the Y3+ site (Bachmann et al., 2009a; Chenavas et al., 1978; Meagher, 1975; Wu et al., 2007a). As mentioned earlier, the emission wavelength of Ce3+-activated YAG is strongly dependent on the crystal field. From the viewpoint of crystal-field theory, the emission band corresponding to 5d!4f transition generally shifts to a longer wavelength by doping smaller cations into the host lattice because the crystal-field strength around the activator is increased by lattice shrin- kage. The enhancement of the crystal-field strength contributes to increase the crystal-field splitting of the 5d excited level, and the energy gap between the lowest 5d excited level and 4f ground level becomes smaller; conse- quently, the emission bands shift to a longer wavelength. However, the emis- sion band of YAG:Ce3+ is blue shifted by doping smaller Lu3+ into the Y3+ site, and Lu3Al5O12:Ce3+ (LuAG:Ce3+) shows a broad green emission with a peak at 530 nm. LuAG is now a commercially available green emission phosphor for white LEDs. Wu et al. (2007a,b) reported a surprising emission band shift in a series of Mg3Y2yGdyGe3zSizO12:Ce (0 y 2, z 1⁄4 0, 1) garnets with respect to FIG. 8 Normalized intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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