Engineering Homogeneous Doping in Single Nanoparticle
To Enhance Upconversion Efficiency
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Abstract
Upconversion
nanoparticles (UCNPs) have shown considerable promises in many fields;
however, their low upconversion efficiency is still the most serious
limitation of their applications. Herein, we report for first time
that the homogeneous doping approach based on the successive layer-by-layer
method can greatly improve the efficiency of the UCNPs. The quantum
yield as high as 0.89 ± 0.05% is realized for the homogeneous
doping NaGdF<sub>4</sub>:Yb,Er/NaYF<sub>4</sub> UCNPs, which is nearly
2 times higher than that of the heterogeneous doping NaGdF<sub>4</sub>:Yb,Er/NaYF<sub>4</sub> UCNPs (0.47 ± 0.05%). The influences
of spatial distributions and local relative concentrations of the
dopants on the optical properties of UCNPs were investigated in the
single particle level. It was found that heterogeneous doping indeed
existed during the spontaneous growth process of the nanoparticles.
The heterogeneous doping property can further induce many negative
effects on the optical properties of UCNPs, especially the luminescent
efficiency. The spatial distributions and local relative concentrations
of the dopants can be well controlled by the successive layer-by-layer
homogeneous doping method on the monolayer level and homogeneously
distributed in the single particle level. Furthermore, by using homogeneous
doping NaGdF<sub>4</sub>:Yb,Tm as initial core, the multicolor emission
intensity of NaGdF<sub>4</sub>:Yb,Tm/NaGdF<sub>4</sub>:A (A = Tb<sup>3+</sup>, Eu<sup>3+</sup>) core/shell nanoparticles can also exhibit
20%–30% improvement. We believe that such a homogeneous doping
model can open the door to improve the upconversion optical properties
by engineering the local distribution of the sensitizer, activator,
host, etc., in a microcosmic and provide a track for engineering the
high quality UCNPs with advanced nanostructure and optical properties