The effect of intentional potassium co-doping on the luminescent properties of Yb3+ and Tm3+ doped α-NaYF4 core and core-shell nanoparticles

Simple and effective ways to circumvent limited luminescence efficiency of up-converting nanoparticles (UCNPs) are sought. One of the methods relays on distorting the crystallographic structure of host material by co-doping the nanocrystals with optically inactive co-dopants. Here we study the influence of K+ doping and surface passivation on the up-converting properties of the α-NaYF4 nanocrystals co-doped with 20% Yb3+ and 0.1 or 2% Tm3+. The intentionally chosen concentrations of K+ ions, which were meant to replaced sodium ions were fixed to 0, 5, 10, 20 to 30%. Potassium ions modified the spectroscopic properties of both core and core-shell NPs, but the differences were noticed between samples doped with 0.1% Tm3+ and 2% Tm3+ ions. Replacement of sodium by potassium ions decreased up-conversion luminescence intensity as well as shortened thulium excited states lifetimes in the samples doped with 0.1% Tm3+, while the opposite behavior was found in the samples co-doped with higher 2% thulium concentration.