Lanthanide-doped upconversion nanocrystals of β-NaYF 4:18%Yb,0.5%Tm, X%Gd and core–shell structured β-NaYF 4:18%Yb,0.5%Tm,20%Gd@NaGdF 4 as small as 8 nm and 12 nm in diameter respectively were synthesized. They display intense and high-order ultraviolet (UV) emissions upon 980 nm excitation.
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-NaYF 4:18%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 1I 6 level of Tm 3+ is more intense than the 4-photon UCL (363 nm and 451 nm) from the 1D 2 level and the 3-photon UCL (474 nm) from the 1G 4 level of Tm 3+. Specifically, when the particle size decreases to 8 nm, intense UV UCL from Tm 3+ and high-order UV UCL from Gd 3+ are still observed. Moreover, core–shell β-NaYF 4:18%Yb,0.5%Tm,20%Gd@NaGdF 4 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.