4 research outputs found
Redistribution of Activator Tuning of Photoluminescence by Isovalent and Aliovalent Cation Substitutions in Whitlockite Phosphors
Many
strategies, including double substitution, addition of charge
compensation, cation-size-mismatch and neighboring-cation substitution,
have contributed to tuning photoluminescence of phosphors for white
light-emitting diodes. These strategies generally involve modification
of a certain special site where the activator occupies; tuning strategy
based on multiple cation sites is very rare and desirable. Here we
report that isovalent (Sr<sup>2+</sup>) and aliovalent (Gd<sup>3+</sup>) substitutions for Ca<sup>2+</sup> tune the photoluminescence from
one band to multiple bands in whitlockite β-Ca<sub>3–<i>x</i></sub>Sr<sub><i>x</i></sub>(PO<sub>4</sub>)<sub>2</sub>:Eu<sup>2+</sup> and β-Ca<sub>3–3<i>y</i>/7</sub>Gd<sub>2<i>y</i>/7</sub>(PO<sub>4</sub>)<sub>2</sub>:Eu<sup>2+</sup> phosphors. The saltatory variation of the emission
spectra is caused by the removal of Eu<sup>2+</sup> from the site
M(4) to other sites. Moreover, we found the mechanisms of dopant redistribution
tuning the luminescence are different. The incorporation of Gd<sup>3+</sup> makes the site M(4) empty according to the scheme 3Ca<sup>2+</sup> = 2Gd<sup>3+</sup> + â–¡, while Sr<sup>2+</sup> substitution
causes the cation sites to be enlarged due to cation size mismatch.
Additionally, the influence of the cation substitutions on the photoluminescence
thermal stability of phosphors is researched. The strategies, emptying
and enlarging sites, developed herein are expected to provide a general
route for tuning luminescence of phosphors with multiple sites in
the future
Changing Ce<sup>3+</sup> Content and Codoping Mn<sup>2+</sup> Induced Tunable Emission and Energy Transfer in Ca<sub>2.5</sub>Sr<sub>0.5</sub>Al<sub>2</sub>O<sub>6</sub>:Ce<sup>3+</sup>,Mn<sup>2+</sup>
A series
of color-tunable Ce<sup>3+</sup> single-doped and Ce<sup>3+</sup>,
Mn<sup>2+</sup> codoped Ca<sub>2.5</sub>Sr<sub>0.5</sub>Al<sub>2</sub>O<sub>6</sub> phosphors were synthesized by a high-temperature solid-state
reaction. The crystal structure, luminescent properties, and energy
transfer were studied. For Ca<sub>2.5</sub>Sr<sub>0.5</sub>Al<sub>2</sub>O<sub>6</sub>:Ce<sup>3+</sup> phosphors obtained with AlÂ(OH)<sub>3</sub> as the raw material, three emission profiles were observed.
The peak of photoluminescence (PL) spectra excited at ∼360
nm shifts from 470 to 420 nm, while that of the PL spectra excited
at 305 nm stays unchanged at 470 nm with the increase of Ce<sup>3+</sup> content. Furthermore, the peak of PL spectra is situated at 500
nm under excitation at ∼400 nm. The relationship between the
luminescent properties and crystal structure was studied in detail.
Ce<sup>3+</sup>, Mn<sup>2+</sup> codoped Ca<sub>2.5</sub>Sr<sub>0.5</sub>Al<sub>2</sub>O<sub>6</sub> phosphors also showed interesting luminescent
properties when focused on the PL spectra excited at 365 nm. Obvious
different decreasing trends of blue and cyan emission components were
observed in Ca<sub>2.5</sub>Sr<sub>0.5</sub>Al<sub>2</sub>O<sub>6</sub>:0.11Ce<sup>3+</sup>,<i>x</i>Mn<sup>2+</sup> phosphors
with the increase in Mn<sup>2+</sup> content, suggesting different
energy transfer efficiencies from blue- and cyan-emitting Ce<sup>3+</sup> to Mn<sup>2+</sup>. Phosphors with high color-rendering index (CRI)
values are realized by adjusting the doping content of both Ce<sup>3+</sup> and Mn<sup>2+</sup>. Studies suggest that the Ca<sub>2.5</sub>Sr<sub>0.5</sub>Al<sub>2</sub>O<sub>6</sub>:Ce<sup>3+</sup>,Mn<sup>2+</sup> phosphor is a promising candidate for near UV-excited w-LEDs
Composition Screening in Blue-Emitting Li<sub>4</sub>Sr<sub>1+<i>x</i></sub>Ca<sub>0.97–<i>x</i></sub>(SiO<sub>4</sub>)<sub>2</sub>:Ce<sup>3+</sup> Phosphors for High Quantum Efficiency and Thermally Stable Photoluminescence
Photoluminescence
quantum efficiency (QE) and thermal stability are important for phosphors
used in phosphor-converted light-emitting diodes (pc-LEDs). Li<sub>4</sub>Sr<sub>1+<i>x</i></sub>ÂCa<sub>0.97–<i>x</i></sub>Â(SiO<sub>4</sub>)<sub>2</sub>:​0.03Ce<sup>3+</sup> (−0.7 ≤ <i>x</i> ≤ 1.0) phosphors
were designed from the initial model of Li<sub>4</sub>SrCaÂ(SiO<sub>4</sub>)<sub>2</sub>:​Ce<sup>3+</sup>, and their single-phased
crystal structures were found to be located in the composition range
of −0.4 ≤ <i>x</i> ≤ 0.7. Depending
on the substitution of Sr<sup>2+</sup> for Ca<sup>2+</sup> ions, the
absolute QE value of blue-emitting composition-optimized Li<sub>4</sub>Sr<sub>1.4</sub>ÂCa<sub>0.57</sub>Â(SiO<sub>4</sub>)<sub>2</sub>:​0.03Ce<sup>3+</sup> reaches ∼94%, and the
emission intensity at 200 °C remains 95% of that at room temperature.
Rietveld refinements and Raman spectral analyses suggest the increase
of crystal rigidity, increase of force constant in CeO<sub>6</sub>, and decrease of vibrational frequency by increasing Sr<sup>2+</sup> content, which are responsible for the enhanced quantum efficiency
and thermal stability. The present study points to a new strategy
for future development of the pc-LEDs phosphors based on local structures
correlation via composition screening