Colour modulation and enhancement of upconversion emissions in K2NaScF6:Yb/Ln (Ln = Er, Ho, Tm) nanocrystals

Abstract Effective colour modulation of upconversion emissions in lanthanide-doped nanomaterials becomes even more important for fundamental and applied research. Herein, on the one hand, by rising the content of doped Yb3+ from 10 mol% to 50 mol%, a significant increase of the red/green emission ratio from 4.0 to 68.2 is observed in K2NaScF6:Yb/Er nanocrystals. This yellow to red colour change is attributed to the increased cross relaxation between Er3+ and Yb3+ caused by the increased Yb3+ amount, 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+). On the other hand, the upconversion green and red emission of K2NaScF6:Yb/Er (20/2 mol%) nanocrystals are intensified 10.6 and 8.8 folds, respectively, after an active shell (K2NaScF6:Yb) is epitaxially grown, which are more effective than the 7.4- and 6.4-fold enhancement from an inert shell (K2NaScF6) growth. Moreover, the shell thickness from 2.85 to 9.5 nm through controlling the molar ratio of shell-precursor to core from 1:2 to 3:1 can be easily realized. This study will provide more opportunities for the application of K2NaScF6:Yb/Ln nanoparticles in varied fields such as theranostics, photovoltaics, and photocatalysis.