An Investigation of the Chromaticity of Blue Emitting Yttrium Silicate
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with an optimum concentration of n = 0.15, improved the light output. Under I keV excitation, the luminescent emission intensity was - 40% better than a commercial standard [5] . The chromaticity was also found to be independent of the both the electron energies used and the dopant concentration [4]. A typical emission spectra of (Yi.-Ce,) 2SiO 5 is shown in Figure l(a). This phosphor has a double emission with peaks at 395 and 423 nm, arising from the splitting of the ground level energy of Ce3+ [6]. Under electron bombardment, the 4f electron in Ce is promoted to an excited state in the 5d level, as shown in Figure l(b). The spectral energy distribution of this phosphor has a large tail which extends into the red portion of the visual spectrum. Even though this double emission would normally give yttrium silicate a very good saturated blue color, the presence of the tail combined with the blue emission, gives this phosphor an overall blue-white appearance. The presence of this large emission tail (measured from 475 to 650 nm) gives chromaticity coordinates of (Y,-mCem) 2 SiO5 of x = 0.161 and y = 0.114 [4], which are undesirable. Optimally the y value should be smaller. It would be ideal if this phosphor could achieve the chromaticity of the blue TV phosphor ZnS:Ag, which has chromaticity coordinates of x =0.147 and y 0.054 [7].
5d
30E
Z% CC
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>
cF __ _
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450 500 550 Wavelength (nm) (a)
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Ce
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Figure 1. (a) (YI-mCem)2SiO5 emission spectra and (b) energy level transitions of Ce3+ in an oxide host. The large emission tail of this phosphor is due to the 5d -* 4f transitions. These types of transitions typically give a broad band emission [8]. It is not believed that since the 5d level of Ce 3 ÷becomes split by the crystal field into five different energy levels, that emission arises from all five levels, contributing to the tail. It has been stated that emission can only come from the lowest, 5d1 level of Ce 3+ when incorporated into a crystal field [9]. Ce3+ can occupy two sites in the yttrium silicate lattice [6]. It could be possible that the 5d energy levels are different between these two sites, contributing to the tail. This needs to be further investigated. To improve the chromaticity coordinates, the y coordinate needs to be reduced. If the intensity of the tail could be decreased, the chromaticity will become more desirable. Ho3+ and Er 3+ ions were chosen as co-dopants because judging from excitation and emission spectra of these dopants in Y20 3 , they could absorb some energy from the tail and re-emit that energy
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weakly at a longer wavelength. Thus, the purpose of this study was to determine whether Ho3 +, Er 3+ and Gd3+ could improve the chromaticity of this phosphor. Experimental Techniques (Yj.m.riCemGdnReq)2SiO5 (Re = rare earth ion) powders were made using combustion synthesis. Details of this synthesis technique can be found in [4,5]. Using the optimum concentration of Ce3+ (m = 0.0075) reported previously in [4], powders were doped with H
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