Chemistry Reference
In-Depth Information
3+
1
1
level
then proceeds in a photon cascade process, with the emission of
two or more photons:
lying 4f state in Pr
,
S
. The radiative decay from this
S
0
0
1
3
1
3
3
. However, this situation
is unfavorable in some host systems like YF
S
-
P
,
I
;
P
-
H
0
1
6
0
4
due to two
main reasons. First, the first step in the cascade process resulted in
UV emission. Secondly, there was evidence of direct recombination
from the
and LaF
3
3
1
3
H states [53]. These transitions are
not useful and should be avoided.
Besides rare earth doping being an important factor in improving
luminescence efficiency of phosphors, the reduction of size of
phosphors is also important. This is proven by the use of nanoparticles
as host lattices in phosphors that showed remarkable luminescence
such as in recent studies on Pr
S
state to the ground
0
3+
nanoparticles [54].
Furthermore, rare earth doped materials in the nanosize range
were found to enhance the rate of radiative decay in phosphors as
reported by Dicke
ions in ZrO
2
. [55]. It was hypothesized that nanoparticles
are involved in a process known as superradiance, which refers to
the appearance of a fast component of PL decay accompanied by a PL
enhancement. This is expected only in the initial stages of PL, when
there is a high concentration on excited ions. After which, normal
PL decay is observed. Rare earth ions emit at resonant energy when
heavily excited, exhibiting superradiance.
In a study, Eu
et al
3+
doped nanoparticles with two different dopant
concentrations (5% and 15%) were investigated. The ultrafast decay
process was observed under the same excitation condition. Results
indicated that the ultrafast components of the PL decay were identical
in both systems, which was most likely heavily dependent on the
radiative PL decay in the nanosize phosphor, possibly superradiance
[56].
From the discussion above, it is clear that both rare earth doping
and the reduction of particle size are important factors influencing
the luminescence efficiency in phosphors. Therefore, in the next
section where rare earth doped phosphors in white LEDs will be
reviewed, the effects of particle size and rare earth dopants on the
luminescence characteristics will be emphasized.
In the next sections, recent works on various types of rare earth
doped phosphors for white LED applications will be discussed.
These phosphors include yellow-, red-, green-, and blue-emitting
phosphors. The luminescence property of phosphors is the main
factor for determining their applications in LED. Luminescence
properties of phosphors are strongly dependent on crystalline
properties and particle size, surface morphology, host lattice, as
