Chemistry Reference
In-Depth Information
2+
ion. This showed that a smaller particle size resulted in the
emission spectrum at a lower energy level. This was consistent with
the results of the work from Li
than Sr
et al
. [77] where the emission peaks
2+
of M
(1 mol%) were at about 605 nm, 610 nm, and 574
nm for M = Ca, Sr, and Ba, respectively. This happens as the particle
size of the M
Si
N
:Eu
2
5
8
2+
Si
N
:Eu
compound decreases with Ca>Sr>Ba.
2
5
8
(b) Effect of dopant concentration on luminescence properties
Referring to the work of Li
et al
. [76] again, the broad red emission of
2+
6
1
Æ
7
M
Si
N
:Eu
(M = Ca, Sr, Ba) was due to the 4f
5d
4f
transition of
2
5
8
2+
the Eu
ion. This emission band was strongly dependent on the type
of M and Eu
2+
2+
concentration. For Ca
Si
N
:Eu
, the emission peak
2
5
8
2+
shifted from 605 nm to 615 nm as Eu
concentration increases.
The red shift is small due to the limited replacement of Ca
2+
ion by
2+
2+
Eu
. For Ba
Si
N
:Eu
, there was a larger red shift as compared to
2
5
8
2+
2+
Sr
Si
N
:Eu
. Ba
Si
N
:Eu
shifted from orange to the red spectral
2
5
8
2
5
8
2+
region, while Sr
Si
N
:Eu
shifted from yellow to the red spectral
2
5
8
2+
region as Eu
concentration increased.
Teng
as a promising red phosphor
and investigated the effects of Eu dopant concentration on
luminescence properties of the phosphor. With the increase in Eu
et al
. [73] proposed Sr
Si
N
2
5
8
2+
concentration, the emission band of the phosphor red-shifted from
615 nm to 638 nm under 465 nm excitation. This was due to the
increased intensity of the crystal field as more Eu
2+
ions replaced the
2+
ions.
(c) Relative luminescence intensities and thermal quenching effect
Li
Sr
2+
et al
. further reported that when M
Si
N
:Eu
(M = Ca, Sr, Ba)
2
5
8
2+
measured the
highest conversion efficiency at 75-80%. The relative luminescence
intensity was also the highest at 87%, as compared to 71% (M = Ca)
and 84 % (M = Ba) [76]. Sr
compounds were excited at 465 nm, Sr
Si
N
:Eu
2
5
8
also had the lowest thermal
quenching. At 150°C, the intensity of Ca
Si
N
:Eu
2+
2
5
8
2+
has already
dropped to 40% of that at room temperature, while Sr
Si
N
:Eu
2
5
8
2+
only decreased by a few percent. Compared with another
commercially used red phosphor CaS:Eu
Si
N
:Eu
2
5
8
2+
, at 120°C, the intensity of
CaS:Eu
2+
phosphor was only 80% of that at room temperature, while
2+
experiences a comparatively weaker thermal quenching effect and,
therefore, has the highest potential as a red-emitting conversion
phosphor for white LED applications.
2+
Sr
Si
N
:Eu
was still of 87% intensity. This shows that Sr
Si
N
:Eu
2
5
8
2
5
8
