Ultrasmall Monodisperse NaYF 4 :Yb 3+ /Tm 3+ Nanocrystals with Enhanced Near-Infrared to Near-Infrared Upconversion Photoluminescence

Photoluminescent NaYF(4):Yb(3+)/Tm(3+) nanocrystals are ideally suited for in vitro and in vivo photoluminescence (PL) bioimaging due to their virtue of near-infrared to near-infrared (NIR-to-NIR) upconversion (UC); they display PL with a peak at approximately 800 nm if excited at approximately 980 nm. Here, we report the synthesis of monodisperse NaYF(4):Yb(3+)/Tm(3+) nanocrystals of ultrasmall size (7-10 nm) with high UC efficiency. The intensity of their NIR UC emission was demonstrated to increase by up to 43 times along with an increase in the relative content of Yb(3+) ions from 20 to 100%, with a corresponding decrease in the Y(3+) content from 80 to 0%. The achieved ultrasmall NaYbF(4):2% Tm(3+)…