Upconversion Effective Enhancement by Producing Various Coordination Surroundings of Rare-Earth Ions

In this manuscript, we present a
simple route to enhance upconversion
(UC) emission by producing two different coordination sites of trivalent
cations in a matrix material and adjusting crystal field asymmetry
by Hf4+ co-doping. A cubic phase, Y3.2Al0.32Yb0.4Er0.08F12, with these
structural characteristics was synthesized successfully by introducing
a small ion (Al3+) into YF3. X-ray diffraction
(XRD), nuclear magnetic resonance (NMR), transmission electron microscopy
(TEM), X-ray spectroscopy (XPS), and fluorescence spectrophotometry
(FS) were employed for its crystalline structure and luminescent property
analysis. As a result, the coordination environments of the rare-earth
ions were varied more obviously than a hexagonal NaYF4 matrix
with the same Hf4+ co-doping concentration, with vertical
comparison, UC luminescent intensities of cubic Y3.2Al0.32Yb0.4Er0.08F12 were largely
enhanced (∼32–80 times greater than that of different
band emissions), while the maximum enhancement of hexagonal NaYF4 was by a factor of ∼12. According to our experimental
results, the mechanism has been demonstrated involving the crystalline
structure, crystal field asymmetry, luminescence lifetime, hypersensitive
transition, and so on. The study may be helpful for the design and
fabrication of high-performance UC materials.