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16 Handbook on the Physics and Chemistry of Rare Earths ex. em 1 0.5 0 300 400 500 600 700 Wavelength (nm) FIG. 7 Excitation and emission spectra of YAG:Ce3+ phosphor. Data were collected from a commercial phosphor by the authors. consists of two broad bands with peaks at about 340 and 450 nm due to the 4f1 ! 4f 05d1 transition of Ce3+, and these excitation bands correspond to tran- sitions to the two lowest energy levels of the 5d orbital (Xie et al., 2011): when an electron is excited from 4f to 5d, the 5d electron of the excited 4f05d1 configuration forms a 2D term, which is split by spin-orbit coupling into two lower energy levels, 2D3/2 and 2D5/2 (Dong et al., 2006; Jacobs et al., 1978). The emission spectrum of YAG:Ce3+ shows a broad yellow emission band with a full width at half maximum (FWHM) of 120 nm (Fig. 7) assigned to the 4f05d1!4f1 transition of Ce3+. The large FWHM of YAG:Ce3+ is caused by the vibronic broadening of the emitting 2D3/2 level and by the presence of two overlapping components due to transitions to the 2F5/2 and 2F7/2 spin-orbit levels, usually separated by about 2000cm1 (Dorenbos, 2006; Feldmann et al., 2003). The strong optical absorption in the blue and the broad yellow emission of YAG:Ce3+ are the main reasons for its use in blue-LED-based white LEDs. In addition, the phosphor has a high optical absorption of $95% and an external quantum efficiency of $83% under blue light excitation (440–470 nm) (Xie et al., 2011). However, the resulting white LEDs have a low CRI and a high CCT ($5000 K), so that it is considerably difficult to generate a warm white light with YAG:Ce3+ phosphors. This is because YAG:Ce3+ has a weak emission intensity in the red. Therefore, effort has been devoted to improve the perfor- mances YAG:Ce3+. The most frequently employed approach for tuning the emission color of YAG:Ce3+ is the substitution of yttrium with other rare earth elements in the host YAG lattice. When Y3+ sites are partially substi- tuted by larger La3+, Gd3+, or Tb3+ (TAG), the emission band tends to shift Normalized intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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