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54 Handbook on the Physics and Chemistry of Rare Earths 1 0.5 0 ex. x = 0.01 x = 0.02 x = 0.10 300 400 Wavelength (nm) Excitation and emission spectra of (Y1xCex)4Si2O7N2 (x 1⁄4 0.01, 0.02 and 0.10). Redrawn from Wu, Q., Yang, Z., Zhao, Z., Que, M., Wang, X., Wang, Y., 2014. Synthesis, crystal structure and luminescence properties of a Y4Si2O7N2:Ce3+ phosphor for near-UV white LEDs. J. Mater. Chem. C 2, 4967–4973. Fig. 50 shows excitation and emission spectra of (Y1xCex)4Si2O7N2:Ce3+ (x1⁄40.001, 0.02, and 0.10). The excitation spectrum of the phosphor with x1⁄40.001 shows a broad absorption band ranging from 280 to 425nm with the largest optical absorption at 355 nm. With increasing Ce3+ concentration, the absorption intensity in the near-UV region around 400 nm increases and, in correlation with this, the emission color also shifts from blue to green, sug- gesting that this red shift is correlated with the population of the four crystallo- graphically independent Y sites available for Ce3+. The phosphor with x1⁄40.10 exhibits a broad green emission with a maximum at 515 nm under 398 nm exci- tation. Under the same excitation the Y4Si2O7N2:Ce3+ (x1⁄40.02) phosphor has quantum efficiency of about 47%. With increasing temperature up to 200°C, the emission intensity gradually decreases. The relative emission intensity measured at 200°C is approximately 10% of that at room temperature (Wu et al., 2014). The isostructural Ce3+-activated Lu4Si2O7N2 phosphors show much higher green emission intensity, about 180% that of Y4Si2O7N2:Ce3+ (Xu et al., 2009). Besides Y4Si2O7N2 and Y2Si3O3N4, Y6Si3O9N4 and Y5(SiO4)3N, two of the members of the Y–Si–O–N quaternary system are also suitable host materials. The Ce3+-activated Y6Si3O9N4 and Y5(SiO4)3N phosphors exhibit a blue or green emission under a near-UV excitation (Deng et al., 2011; van Krevel et al., 1998). 4.2.4 Red-Emitting Phosphors 4.2.4.1 CaAlSiN3:Eu2+ Phosphors As pointed out in Section 4.2.1, CaAlSiN3:Eu2+ (CASN) is one of the notorious commercial red-emitting nitride phosphors for blue LEDs. The luminescence FIG. 50 em 500 600 700 Normalized intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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