Sub-10 nm and monodisperse β-NaYF4:Yb,Tm,Gd nanocrystals with intense ultraviolet upconversion luminescence†
Despite the fast progress in lanthanide-doped upconversion nanocrystals (UCNCs), the preparation of ultra-small, monodisperse and uniform UCNCs with intense high-order ultraviolet upconversion luminescence (UV UCL) is still a challenging issue. Here we report on a method for the controlled synthesis of β-NaYF4:18%Yb,0.5%Tm,X%Gd NCs, with sizes ranging from 8 nm to 25 nm in diameter. We show that under 980 nm excitation, the 5-photon UCL (291 nm and 345 nm) from the 1I6 level of Tm3+ is more intense than the 4-photon UCL (363 nm and 451 nm) from the 1D2 level and the 3-photon UCL (474 nm) from the 1G4 level of Tm3+. Specifically, when the particle size decreases to 8 nm, intense UV UCL from Tm3+ and high-order UV UCL from Gd3+ are still observed. Moreover, core–shell β-NaYF4:18%Yb,0.5%Tm,20%Gd@NaGdF4 NCs of 12 nm diameter were also synthesized, exhibiting more efficient UCL than the 8 nm diameter core NCs. Ultra-small UCNCs with intense UV UCL are promising for biomedical applications.