A comparison of morphology, structure and optical properties of ultrasmall, small and core–shell up-converting NaYF4/NaGdF4 nanocrystals co-doped with Tm3+ and Yb3+ ions

Synthesis, crystal structure, morphology and optical properties of three various NaYF4 nanoparticle systems co-doped with Tm3+ and Yb3+ ions have been investigated. Characterization of ultrasmall (size ∼8 nm), small (∼25 nm) NaYF4:2%Tm3+, 20%Yb3+ nanoparticles and core–shell structures (NaYF4:20%Yb3+–NaGdF4:2%Tm3+) synthesized according to two different protocols is presented. We have found that the up-conversion luminescence is strongly influenced by the nanoparticle size, internal structure and spatial distribution of active Tm3+ ions. Further spectroscopic studies revealed significant fluorescence rise and decay time lengthening of the 1G4 energy level of Tm3+ ions in NaYF4:20%Yb3+–NaGdF4:2%Tm3+ core–shell nanoparticles. The formation of the core–shell structures was proven indirectly through the average crystalline size increase between the bare core and core–shell nanoparticles, the observed changes in EDX composition as well as variation of Tm3+ ions luminescence properties.