Chemistry Reference
In-Depth Information
3+
determined first. For Bi
doped RNbO
, the electron in the 6s orbital
4
3+
of the Bi
ion could be transferred to the low lying empty 4d orbital of
the niobium ion. This resulted in the metal-to-metal charge transfer
emitting state. Bi
3+
had two excitation bands, one at
256 nm due to 2p-4d charge transfer transition, and the other at 305
nm due to 6s-4d charge transfer transition. The excitation band at
305 nm was much stronger than the 256 nm band. For YNbO
doped RNbO
4
3+
: Bi
,
4
the luminescence band peak red-shifted to 448 nm from 410 nm,
while the emission peak for LaNbO
3+
: Bi
remained similar at 410
4
3+
showed the most intense emission peak
but did not exhibit luminescence at room temperature. Hence, for
YNbO
nm. Bi
doped GdNbO
4
3+
3+
concentration
was at 1 atom % while intensity decreased slightly as concentration
increased to more than 2 atom %.
As Bi
:Bi
, luminescence intensity peaked when Bi
4
3+
3+
was
gradually increased up to 10 atom %, blue emission from the NbO
was fixed at 0.1 atom % and concentration of Eu
4
3−
group decreased. The blue emission was consumed and resulted in
enhancement of the red luminescence at 613 nm corresponding to the
Eu
3+
3+
ion. Following this analysis, the Eu
activator ion concentration
3+
was fixed at 5%, while the Bi
concentration was gradually increased.
3+
3+
The red luminescence of Eu
concentration
increased up to 352 nm at half maximum with 15 atom % Bi
was enhanced as Bi
3+
concentrations. A higher Bi
concentration resulted in decrease in
luminescence intensity. Hence, (Y
3+
3+
Bi
NbO
:Eu
(5%) may be
0.80
0.15)
4
used as a red-emitting phosphor for white LEDs.
(b) Effect of dopants on luminescence characteristics in tantalates
Nyman
3+
(where Ln = Y, Gd, Lu) [79]
to be new promising red-emitting phosphors for white LEDs under
blue LED excitation. The PL excitation spectra showed a broad blue
excitation band for Ln = Gd especially. This is important in LED
applications as a broadband width increases the absorption cross
section of the phosphor. The PL emission spectra of the samples
showed an intense emission at 608 nm, which is a nearly ideal red
emission for high luminous efficacy white LEDs. The quantum yields
(QYs) of the KLnTa
et al
. reported KLnTa
O
:Eu
2
7
3+
phosphors were also high with Ln = Gd
close to 80%, and 65% QY peaks for the Ln = Y, Lu. QY was found and
confirmed to be dependent on crystalline structure. Nyman
O
:Eu
2
7
. [78]
also synthesized LaTa phosphors of different phases by heating the
dry powder samples at different temperatures. The LaTa-850 phase
had a QY of 55%, which is much higher than LaTa-1200 phase with
et al
