Enhancement of blue upconversion luminescence in hexagonal NaYF4:Yb,Tm by using K and Sc ions

Abstract

Hexagonal (β)-NaYF4 is recognized as one of the most efficient hosts for NIR to blue and green upconversion (UC). A new method to tune the blue UC emission in β-NaYF4:Yb,Tm nanocrystals through the possible substitution of the host material with different concentrations of K+ and Sc3+ ions was investigated in detail. In this work, Na1−x K x YF4:Yb,Tm and NaY1−x Sc x F4:Yb,Tm nanocrystals were synthesized with varying Na:K and Y:Sc ratios. X-ray diffraction, transmission electron microscopy, and UC luminescence spectroscopy showed that size, morphology, and UC luminescence intensity were affected by the addition of K+ and Sc3+ ions. Substituted ions disturbed the local symmetry and also resulted in changes in the crystal field. The distance between Yb3+ and Tm3+ was affected by different concentration of K+ and Sc3+ ions, and those differences in the distance are responsible for tuning UC luminescence. This study revealed that when the concentration of K+ and Sc3+ ions were nominally increased from 20 to 100 mol% during synthesis, hexagonal NaYF4 changed to structurally different KYF4 and Na3ScF6 so that the solid solubility became difficult. We also demonstrate that the added K+ does not enter into the NaYF4 lattice, but it still plays an important role by controlling the Na/R ratio. K+ and Sc3+ ion concentration of 20 mol% during the synthesis was found to result in materials with size 30–35 nm, and shows ca. four times brighter UC emission than the previously reported lanthanide based nanocrystals. The enhancement in UC luminescence intensity makes upconversion nanophosphors versatile imaging tools for diagnosis.