Chemistry Reference
In-Depth Information
maximum. This observation was supported by Sakuma
et
al
. [64].
Recently, Bachmann
. [65] provided a better understanding on
the effect of temperature quenching on Ce
et
al
emission in YAG:Ce in
the high temperature region, particularly in temperatures above 400
K. Such high temperatures have great relevance to high powered
white LEDs.
In order to study the luminescence properties of the isolated
Ce
3+
ions, greater experimental focus was placed on the lower Ce
concentration sample. This was because for higher Ce concentration
samples energy transfers and reabsorption could take place, which
would affect the temperature dependence of the luminescence
spectra and decay curves. Temperature-dependent luminescence
lifetime measurements for low Ce concentration (0.033%) showed
that the intrinsic temperature quenching Ce
3+
3+
was high at well above
700 K. The lower quenching temperatures for commercial YAG:Ce
phosphors were explained by thermal-activated concentration
quenching, as supported by Robbins
.'s work [66-68]. When
lower Ce concentration is used, YAG:Ce is a potentially a good
candidate for high power LEDs.
et al
6.4.1.2
Silicate-based phosphor
Due to some limitations of YAG:Ce yellow phosphor, other host
materials have been investigated and found to be better candidates
for yellow phosphors. Rare earth doped strontium silicate oxides
showed promising luminescence properties for use with blue or
near-UV LEDs. Strontium silicate nitrate based phosphors have also
been reported [69].
6.4.1.2.1
Luminescence properties of silicate-based
phosphors
Effect of dopants on luminescence properties
Jang
3+
+
phosphors to be used with
blue LED or near-UV LEDs to produce white light. The luminescence
excitation spectra of this phosphor showed a strong and broad PLE
peak at 415 nm; thus it could be excited by blue (460 nm) or n-UV
(405 nm) LED. Sr
et
al
. reported yellow Sr
SiO
:Ce
, Li
3
5
3+
+
based white LEDs displayed white
light ranging from natural white to cool white without any additional
phosphors. The CRI indexes of Sr
SiO
:Ce
, Li
3
5
SiO
:Ce
3+
, Li
+
based white LEDs
3
5
were larger than the R
of 60 of white fluorescent lamps. Blue-LED
a
