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

: Kale V, Soukka T, Hölsä J, Lastusaari M

Publisher: SPRINGER

: 2013

: Journal of Nanoparticle Research

: JOURNAL OF NANOPARTICLE RESEARCH

: J NANOPART RES

: UNSP 1850

: 8

: 15

: 8

: 1850

: 1861

: 12

: 1388-0764

DOI: https://doi.org/10.1007/s11051-013-1850-8

Hexagonal (beta)-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 beta-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-xKxYF4:Yb, Tm and NaY1-xScxF4: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.