Chemistry Reference
In-Depth Information
100°C for 12 h. The average particle size was between 12 nm and 16
nm depending on Sm concentration [40].
2+,
3+
phosphors
using the hydrothermal method. Starting raw materials were AIN,
Sr(NO
Ding
et al
. also synthesized SrSi
N
O
:Yb
, Nd
2
2
2
, mixed and stirred. The
mixture was then poured into an autoclave and heated to a soaking
temperature at 150°C with a heating rate of 10°C/min, followed by
cooling, washing, and drying. After the dried products were obtained,
the products were fired for 2-6 times at 800-1200°C under reduced
atmosphere of N
)
, Na
SiO
, Yb(NO
)
, and Nd(NO
)
3
2
2
3
3
3
3
3
(2.5-7.5%) in a tube furnace. Analysis showed
that the phosphor sintered at 1000°C for 4 h showed the highest
PL intensity and is a good host material for yellow-green-emitting
phosphor, which generates white light when combined with blue
LED [42]. The comparison of various rare earth phosphor synthesis
methods and their advantages/disadvantages are summarized in
Table 6.1.
-H
2
2
6.4
Factors Affecting the Luminescence
Properties of Rare Earth Phosphors
In order to improve the luminescence properties of the phosphors,
the efficiencies of energy transfer among the dopant ions should be
enhanced, while non-radiative recombination should be minimized
[48]. These factors are influenced by the energy transfer and
excitation mechanisms of the host materials. Godlewski [48] reported
that in order to achieve narrow spectra emissions, phosphors were
usually doped with transition metals (TM) or rare earth elements.
Transition metals doping will not be discussed here.
Rare earth ions can be excited through various mechanisms,
such as successive absorption of the pumped photons by a single
ion or transferring the absorbed energy to the neighboring ions.
For energy transfer, an energy overlap between the energy donor
and the energy acceptor is required. In case of rare earth doped
phosphors, rare earth ion acts as the energy acceptor. However, the
excitation mechanism in rare earth doped mechanism also has its
limitations. The 4f-4f intra-shell transitions in rare earth ions are
inefficient due to the parity and spin selection rules [49, 50]. Since
4f-4f excitation is hard to take place in rare earth doped materials,
4f-5d charge transfer or excitonic mechanism must be used. [51] In
another study, Yatsenenko
et al
. [52] reported efficient luminescence
